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Alkali Metal Corrosion of Alumina : Thermodynamics, Phase Diagrams and Testing

Alkali Metal Corrosion of Alumina : Thermodynamics, Phase Diagrams and Testing

Alkali of alumina : thermodynamics, diagrams and testing

Citation for published version (APA): Hoek, van, J. A. M. (1990). corrosion of alumina : thermodynamics, phase diagrams and testing. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR338654

DOI: 10.6100/IR338654

Document status and date: Published: 01/01/1990

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Download date: 24. Sep. 2021 ALKALI METAL CORROSION OF ALUMINA

Thermodynamics, phase diagrams and testing ALKALI METAL CORROSION OF ALUMINA

Thermodynamics, phase diagrams and testing

PROEFSCHRIFf

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof. ir. M. Tels, voor een comrnissie aangewezen het College van Dekanen in het openbaar te verdedigen op dinsdag 30 oktober 1990 te 16.00 uur.

door

JOHANNES ADRIANUS MARIA VAN HOEK

geboren te Breda Dit proefschrift is goedgekeurd door de promotor Prof. Dr. R. Metselaar en de copromotor Dr. FJ.J. van 1.00 CONTENTS

1 INTRODUCTION 11

1.1 Applications of alkali in combination with 12 1.2 Alkali metal corrosion of 12 1.3 The use of phase diagrams in the investigations of corrosion phenomena 13 1.4 Outline of this thesis 15 Literature 17

2 THERMODYNAMIC ASPECTS OF ALKALI METAL CORROSION OF ALUMINIUM OXIDE 19

2.1 Predictions of corrosion reactions based on thermodynamics 20 2.2 Estimation of thermodynamic data 27 List of symbols 32 Literature 33

3 LITERATURE REVIEW ON PHASE DIAGRAMS 35

3.1 Quasi-binary systems 36 3.1.1 The oxide - aluminium oxide system 36 3.1.2 The oxide - aluminium oxide system 37 3.1.3 The oxide - aluminium oxide system 38 3.1.4 The oxide - aluminium oxide system 42 3.1.5 - systems 44 3.1.6 Alkali metal oxide - systems 44 3.2 Quasi-ternary systems 44 3.2.1 The Cs 0 - alkaline metal oxide - Al 0 systems 44 2 2 3 3.2.2 The K 0 - oxide - Al 0 systems 45 2 2 3 Literature 50

6 4 GENERAL EXPERIMENTAL SET UP FOR DETERMINATIONS 53

4.1 Method 54 4.2 Materials 54 4.3 Preparation of the mixtures 55 4.4 Heat treatment 55 4.5 Characterization of the phases 57 Literature 57

5 PHASE RELATIONS IN THE CaO-Cs 0-AI 0 SYSTEM 59 2 2 3

5.1 A method to determine the composition of an unknown compound 60 5.2 A new quasi-ternary compound: Cs 0·2CaO·4Alp3 62 2 5.3 Subsolidus phase relations 65 5.4 The role of caesium-B-alumina 67 5.5 Discussion 67 Literature 69

6 PHASE RELATIONS IN THE CaO-K 0-AI 0 SYSTEM 71 2 2 3

6.1 Experimental 72 6.2 Results 72 6.2.1 Subsolidus phase relations 72 6.2.2 2K 0·3CaO·5Al 0 : a new quasi-ternary compound 74 2 2 3 6.2.3 The CaO·Alp3 - KP·Al 0 - AlP3 composition area 78 2 3 6.3 Thermodynamic evaluation of the phase diagram 78 6.4 Discussion 84 List of symbols 85 Literature 86

7 7 PHASE RELATIONS BETWEEN THE COMPOUNDS K-B-ALUMINA, Ca-B-ALUMINA AND CALCIUM HEXA 87

7.1 Literature review 88 7.1.1 The structure of calcium hexa aluminate 88 7.1.2 The structure and properties of B-alumina 91 7.1.3 exchange properties of B-alumina 93 7.2 Experimental procedure 98 7.3 Results and discussion 99 7.3.1 Pure KO melt 99 7.3.2 Pure Ca0 melt: a new Ca-B-alumina 99 2 7.3.3 KCVCa0 melts 106 2 7.4 Stability of Ca-B-alumina and CaIK-B-alumina solutions 110 7.5 Conclusions 112 Literature 114

8.1 Experimental 118 8.2 The CsP-MgO-AlP3 system 118 8.3 The KP-MgO-AlP3 system 119 8.4 Discussion 125 Literature 126

9 POTASSIUM CORROSION TESTS OF ALUMINIUM OXIDE 127

9.1 Literature review 128 9.2 Experimental 130 9.2.1 Method 130 9.2.2 Materials 131 9.3 Method evaluation 132 9.4 Results and discussion 134 9.5 Conclusions 141 Literature 142

8 10 GENERAL CONCLUSIONS 143

Summary 146 Samenvatting 148 Curriculum Vitae 150 Dankwoord 151

9 CHAPTER 1

INTRODUCTION 1 INTRODUCTION

1.1 Applications of alkali metals in combination with aluminium oxide

Aluminium oxide or alumina (Al 0 ) is an extremely stable compound with a high 2 3 . Because of its combination of mechanical, electrical, optical and chemical properties (1), it is used in many applications. Although many more examples could be mentioned (2), in this introduction we will only discuss three applications where it is used in combination with an alkali metal. The ftrst application is in high discharge lamps. Because of the high pressure (about lOs Pa), needed to obtain a broad spectrum of , the of the envelope will also raise (1100-1500 K). The material of the envelope must therefore have mechanical strength, be translucent and be resistant against sodium vapour at higher . Translucent gastight sintered alumina (TGA) is a common material in these types of discharge lamps. In a MagnetoHydroDynamic (MHD) electrical generator an electrically conducting gas () is flushed with high speed through a strong magnetic fteld. The positive and negative charged particles are subjected to forces in opposite directions and are collected at two electrically isolated electrodes. The gas often consists of inert gases in combination with an alkali metal vapour, to make it electrically conductive. The temperature of the inside of the generator may exceed 1900 K and is often constructed of alumina. In our own laboratory, work is performed on thermionic energy converters (3-8). Thermal energy is converted into electric energy by emission from a high temperature electrode (emitter) to a low temperature electrode (collector). In order to avoid space charge effects caesium vapour is introduced in the interelectrode space. Caesium is chosen here because of its low ionisation potential. The electrodes are separated by aluminium oxide because of its high electrical resistivity and because of its chemical inertness.

1.2 Alkali metal corrosion of alumina

In the applications we mentioned above, materials are exposed to high temperatures in contact with an alkali metal vapour. Under these circumstances the alkali metals are highly agressive and only few materials have a sufftcient

12 corrosion resistance (9). Alumma seems to be quite resistant but with increasing requirements the corrosion resistance becomes a limiting factor for the lifetime of the materials. In the high pressure sodium discharge lamps, slow reaction of the sodium with the alumina envelope starts to take place at temperatures of about 1500 K. Blackening of the walls in combination with loss of sodium causes a decreasing performance of the lamp (10). The corrosion product, B-alumma (11), is an ionic conductor and therefore the isolating properties of alumina are lost. Similar problems will also arise in the other applications. The corrosion resistance, however, is largely determilled by the sinter additives and the grain boundary phases. For instance, silica additions have a detrimental effect (12-16). Other well known sinter aids for the densification of alumina are magnesia and calcia. The influence of these additives on the sodium corrosion is studied by several authors (17-20) but still not fully understood. No systematic studies are performed on the influence of these additives on the potassium or caesium resistance of alumina. In this thesis we want to present the results of such a study.

1.3 The use of phase diagrams in the investigations of corrosion phenomena

In order to understand corrosion, it is very important to know the related phase diagrams. Consider a binary system and assume a compound A is brought in contact with a compound B. From the phase diagram it is immediately clear whether corrosion will occur or not. When A and B are in equilibrium, no reaction will take place. On the other hand, when from the phase diagram it is known that between A and B in equilibrium a compound AB is formed, corrosion will occur and the corrosion product will be the compound AB. Corrosion by compound B of a material A containing a dope D can be understood by examming the ternary phase diagram A-B-D. Suppose, the binary phase diagram A-D contains one binary compound AD and assume there is no . In that case, the dopes will be present in the material as a phase AD. There are several possibilities for the ternary phase diagram, depending on whether it contains one or more stable ternary phases. Some of these possibilities are given in figure 1.1. In the phase diagram of figure 1.1 a, no ternary phase is stable and therefore only one possibility is left. Both A and AD are in equilibrium with B and therefore no corrosion will occur. When a ternary compound is stable, there

13 B a

B b

D AD A

B B c

D AD A D AD A

Figure 1.1 Four fictive phase diagrams of the A-B-D system with a. no ternary phase, b. ternary phase T not in equilibrium with phase A and c. ternary phase T in equilibrium with phase A.

14 are two possibilities as given in figure 1.1 b and 1.1 c. In the frrst possibility the ternary compound is in equilibrium with D, AD and B but not with A (figure 1.1 b) and therefore no reaction between doped material A and B will occur. On the other hand, when the ternary compound is in equilibrium with B, AD and A or with B, AD, A and D (figure 1.1 c), the doped material will react with B to form the ternary compound. The above example clearly shows the need for the examination and understanding of the phase relations in the relevant systems. For this reason, most of this thesis is about phase diagrams and their meaning for alkali metal corrosion of alumina. The reaction between alkali metal and alumina results in mixed and aluminium. For this reason we examined the oxide phase diagrams because from these the corrosion products can be read.

1.4 Outline of this thesis

Thermodynamical aspects of the possible corrosion reactions are given in chapter 2. With the results of this chapter we are able to predict the compatibility of alumina with the alkali metals at all temperatures. The main part of this thesis is about phase diagram investigations. As pointed out in the previous paragraph, these diagrams are essential for understanding the corrosion behaviour of alumina. In chapter 3 the literature data about the relevant phase diagrams and in chapter 4 the methods and materials used in the phase diagram studies are given. The results of the study on the phase relations in the CaO-Cs 0-AI 0 system 2 2 3 are described in chapter 5. We developed a new method to determine the composition of a hitherto unknown quasi-ternary compound present in this system and with this method we were able to fmd and characterize this new compound. The phase relations in this system provide a lot of information on the influence of CaO on the corrosion resistance of alumina against alkali metals. The investigations on the CaO-K O-AI 0 system were more extended and are 2 2 3 described in chapter 6 and 7. The phase relations found were not clear at frrst but by a rigorous thermodynamic evaluation the problems in this system were solved. Together with the experimental results we are able to propose a consistent phase diagram which is presented in chapter 6. The part of this diagram including the K-6-alumina compound and calcium hexa aluminate is of special interest. K-6-alumina is thought to be a corrosion product and because of

15 its close resemblance to calcium hexa aluminate, which is believed to reside at the grain boundaries, the phase relations in this area are very important as described in chapter 7. A metastable calcium aluminate with the B-alumina structure was found and therefore also a phase diagram representing metastable equilibria is given. In chapter 8 the influence of MgO is examined by the investigation of the MgO-Cs O-AI 0 and MgO-K O-AI 0 systems. 2 23 2 23 Finally, to verify the theoretical predictions based on the previous chapters, the potassium resistance of alumina is examined. These results are described in chapter 9 and clear conclusions can be drawn. In the concluding chapter 10 the main conclusions from this thesis are presented.

16 Literature

1. W. H. Gitzen, "Alumina as a Material", American Ceramic Society (Columbus, Ohio 1970). 2. H. U. Borgstedt and C. K. Mathews, "Applied of the Alkali Metals" (Plenum Press, New York and London 1987). 3. G. H. M. Gubbels, L. R. Wolff and R. Metselaar, Solid State lonies, 16 (1985) 47-54. 4. L. R. Wolff, J. M. Hermans, J. Adriaansen and G. H. M. Gubbels, High Tech. , Ed. P. Vincenzini (Elsevier, Amsterdam, 1987) 2397-2406. 5. G. H. M. Gubbels, L. R. Wolff and R. Metselaar, J. Appl. Phys., 64 (1988) 1508-1512 6. G. H. M. Gubbels, L. R. Wolff and R. Metselaar, Appl. Surf. Sci., 40 (1989) 193-199. 7. G. H. M. Gubbels, L. R. Wollf and R. Metselaar, Appl. Surf. Sci., 40 (1989) 201-207. 8. G. H. M. Gubbels and R. Metselaar, Surf. Sci., 226 (1990) 407-411. 9. J. F. Fink, Rev. Coat. Corros., l (1978) 1-47. 10. E. F. Wyner, J. Illum. Eng. Soc., ~ (1979) 166-173. 11. P. Hing, J. Mater. Sci., 11 (1976) 1919-1926. 12. R. G. Smith, F. Hargreaves, G. T. J. Mayo and A. G. Thomas, J. Nucl. Mater., 10 (1963) 191-200 13. J. K. Higgens, Trans. J. Br. Ceram. Soc., 65 (1966) 643-659. 14. L. N. Grossman, Rev. Int. hautes Temper. Refract., 16 (1979) 255-261. 15. M. G. Barker and T. K. Leung, Mater. Sci. Monogr., Q(1980) 870-878. 16. J. Jung, A. Reck and R. Ziegler, J. Nucl. Mater., 119 (1983) 339-350. 17. P. Hing, J. Illum. Eng. Soc., 10 (1981) 194-203. 18. G. de With, P. J. Vrugt and A. J. C. van de Yen, J. Mater. Sci., 20 (1985) 1215-1221. 19. J. D. Hodge, Proc. Electrochem. Soc., 85 (1985) 261-270. 20. R. K. Datta and L. N. Grossman, Proc. Electrochem. Soc., 85 (1985) 271-290.

17 CHAPTER 2

THERMODYNAMIC ASPECTS OF ALKALI METAL CORROSION OF ALUMINA 2 THERMODYNAMIC ASPECTS OF ALKALI METAL CORROSION OF ALUMINA

Thennodynamic calculations are a powerful tool in predicting corrosion behaviour of materials. With these calculations one can in principle predict the compatibility of various materials in all kinds of environments. In this chapter we want to predict whether a reaction is thennodynamically allowed between Al 0 2 3 and the gases of alkali metals in an free environment. We do this by calculating the changes in Gibbs energy of the system as a result of possible reactions. When this change is negative there is a possibility the reaction will occur under standard conditions.

2.1 Predictions of corrosion reactions based on thermodynamics

When we want to predict wether or not corrosion will occur, it is important to consider all possible reactions. 3 Suppose the ftrst step of the reaction is the reduction of Al + and oxidation of the alkali metal M:

(2.1)

In this reaction M can be all kinds of alkali oxides, for example MO, M 0, v°W 2 M 0 or M0 • For AlyO one should read Al, Al 0, Al 0 or AIO. The reaction with 2 2 2 z 2 2 2 . the most favourable change in Gibbs energy for all alkali metals is:

---.7) 3 M 0 + 2 Al (2.2) 2

The changes in Gibbs energy for reaction 2.2 are given in ftgure 2.1 and for comparison also the curve when Cs 0 is fonned instead of Cs O. The fact that the 2 2 2 change in Gibbs energy of reaction 2.2 for caesium is less than for a reaction where CS 0 2 is fonned, might suprise the reader. One can calculate the change of 2 Gibbs energy of the reaction

(2.3)

The change in Gibbs energy of this reaction is negative and therefore one might

20 1400

1200 C..202 Cs20

1000 Rb20 K20 Qi "6 800 ....E J Na20 ~ 600 19 ro +- Q) 400 "0

200

0 LI

-200 0 300 600 900 1200 1500 1800

Temperature (K)

figure 2.1 Change in standard Gibbs energy as a function of temperature for the reaction: AI 0 + 6 M ) 3 Mp + 2 AI 2 3 and for comparison AI 0 + 3 Cs ) 3/2 CSP2 + 2 AI 2 3

21 conclude that Cs222° is more stable than Cs 0. But this conclusion is only true when the compounds are under one atmosphere oxygen pressure. In our case there is no oxygen in the system and for this reason when CS2°2 is formed instead of CS2° in reaction 2.1, more Al must be reduced. Because this reduction costs a lot 2° 3 of energy, the total change in Gibbs energy is higher when Cs 0 is formed. 2 2 It also can be seen in figure 2.1 that the change in Gibbs energy at temperatures up to 1500 K in case of is negative and for the other alkali metals positive. In his thermodynamic studies, Singh (1) developed a criterion for predicting the compatibility of oxide ceramics with lithium and sodium. His criterion is based on the reaction 2.2 and is formulated as: when ArGo> 0, reaction will not occur; when ArGo ::: 0 reactants and products remain in equilibrium; and when ArGo < 0, spontaneous reaction occurs to completion. His conclusion is therefore that under standard conditions, sodium is compatible with alumina and lithium is not. However, further reaction of the reaction products with Al 0 or alkali metal 2 3 might occur. For instance, for all alkali metals mixed oxide compounds of M2° and Al are known. Mixed oxide compounds between Al and the other alkali metal 2° 3 2° 3 oxides like MO, M2°2 and M02 are not known. These oxides are for this reason not considered as possible reaction products anymore and· not because the first step of the reaction (2.1) has a higher change in Gibbs energy. The compounds with highest Al23°-content can be described as M2O·l1Al23° (B-alumina), M2O·5Al23° (B"-alumina) or M20·AlP3. Thermodynamic data for Na20·11A120 3, Kp.l1AlP3' Li 0·Al 0 , Nap.Al 0 and K 0·Al 0 are available in literature (2-6). 2 2 3 2 3 2 2 3 We will first consider the reaction:

(2.4)

For M is Li, Na or K the change in Gibbs energy is in all cases negative. This means that for the combination of reaction 2.2 and 2.4:

-----73 M 0 + 2 Al (2.2) 2 (2.4) -----7 3 M 20·Al2°3 + + 6 M (2.5) the change in Gibbs energy might be negative. Because no data are available for

22 Rb 0·Al and Cs 0·Al estimates must be made which are explained in the next 2 23° 2 23°, paragraph. The results of these estimates are used to calculate the change in Gibbs energy for reactions 2.4 and 2.5, and are shown in figures 2.2 and 2.3. From figure 2.3 it can be concluded that under standard conditions Al is 2°3 not stable in the presence of alkali metals. In the case of sodium the change in

Gibbs energy becomes positive above =:: 1200 K and, therefore, above this temperature sodium and Al are compatible. With the aid of figure 2.3 we can 2° 3 determine at each temperature the maximum pressure of each alkali metal which is allowed to be in contact with Al in order to avoid corrosion. These 2° 3 can be calculated by

(2.6)

/irGo is the change in Gibbs energy of the reaction 2.5 under standard conditions (J/mole), R is the gas constant (J/mole K), T is the temperature (K), K is the equilibrium constant which is equal to p;.,t and PM is the partial pressure of the alkali metal M (Pa). This equation is processed in figure 2.3 in a way the alkali metal pressure can be read in the same way as the partial pressure of oxygen can be read from an Ellingham plot (7). This means that we should draw a line through the origin (0 K and 0 kJ/mole) and through the curve of the alkali metal at the temperature we are interested in. The maximum pressure can be read on the right axis of the plot on the PM scale. For example, if we want to know the maximum pressure of potassium one can use in contact with Al 0 2 3 at 1400 K without a serious chance on corrosion, we must draw a line through the origin and the potassium curve at 1400 K. At the right axis the maximum pressure of about 104 Pa is found. This means that at higher pressures of potassium at this temperature one can expect corrosion of Al 3' 2° In thermionic energy converters, for example, the caesium pressure is typically between 1 x 102 and 1 x 103 Pa at temperatures of Al 0 up to 1300 K. 2 3 From figure 2.3 we learn that the maximum pressure at 1300 K that can be used without corrosion of Al 0 is in the same order of magnitude. Because of the 2 3 inaccuracy of the estimates it is, therefore, dangerous to predict the corrosion behaviour of Al 0 in these conditions. 2 3 In the considerations thus far, only M 0.Al 0 type of compounds were taken 2 2 3 into account. Other mixed oxides M O·l1Al and M O·SAl were already 2 23° 2 23° mentioned. When these compounds are formed the reactions will be:

23 o

t-- t--- Li -500 - Na n; - <5 .....E J ¢ -1000 19 K ....,ro f::::: r- Qj ------r--- Rb l) ~ ~-- ~ Rb -- -, , Cs -1500 , -- .. - , --, , - -'- --- Cs

-2000 o 300 600 900 1200 1500 1800

Temperature (K)

figure 2.2 Change in standard Gibbs energy for the reactions

3 M 0 + 3 Al 0 ) 3 M 0·Al 0 0 2 2 3 2 2 3 The curves of and caesium are based on our estimates of d " at 298 K of f the M 0·Al 0 compounds. For the dotted lines we used the estimated values from 2 2 3 Kazin et aI. (15).

24 200

100 Na a

-]00 ,-.... J< Q) (3 -200 Rb E ""-.. ~ -300 Cs ---... C) Rb / / en -400 / ...... > , Q) , Li / 'U / -500 / ------.-"/'

-600 .,­ .,­ Cs .,- ., .,- --- /' -700 --- / ---...... ,// .....,- -800

-900 0 300 600 900 1200 1500 Temperalure (K)

Figure 2.3 Change ill standard Gibbs energy (left axis) and the equilibrium alkali metal vapour pressure (right axis) for the reaction 4 AIP3 + 6 M ~ 3 M 0·AI 0 + 2 AI 2 2 3 The thermodynamic values for Rb 0·AI and Cs 0·AI were estimated. The dashed 2 23° 2 23° lines are calculated with the estimates of Kazin et al. (15).

25 (2.7)

(2.8)

Only data on Na20-l1Al23° and K20-l1Al23° are known from literature (2,3). For this reason no estimates of the absolute entropies (So) and enthalpies of formation (~fIo) of this kind of compounds can be made in the same way we did for M 0-Al 0 . It is not necessary to know these values, however, because when 2 2 3 M20-5AlP3 and Mp-l1Al20 3 are stable with respect to AlP3 and M20-AlP3 it is sure that the changes in Gibbs energy ~ GO of reactions 2.7 and 2.8 are more r negative than ~rGo of reaction 2.5. For this reason the end products of the corrosion reaction will depend on whether one has an excess alumina or alkali metal. We will pay less attention to the aluminium which is a product of all reactions described thus far. In a pure Al -alkali metal interaction, the only 2° 3 interaction of aluminium that might occur is with the alkali metal. The enthalpy of solution of liquid alkali metal M and liquid aluminium is for all alkali metals positive except for lithium for which it is slightly negative (8) and for this reason this reaction will be neglected. It is very rare, however, to have in an experimental setup only Al 0 and alkali metal. In many practical cases 2 3 another metal is present in the environment, for instance capsules in our own corrosion experiments or in high pressure Na-lamps. Aluminium can react with this metal to form compounds or a solid solution. The change in Gibbs energy for this reaction must be added to the change of Gibbs energy of the corrosion reaction 2.5 (9,10). Because of this, the maximum pressure of the alkali metal that can be used in combination with A1 3 without 2° corrosion will be lower. An other point we would like to mention here is the role of oxygen. Thus far we considered an oxygen free atmosphere. If, however, there is for any reason oxygen in the system the situation will change drastically. Because there is no need anymore for the alkali metal to reduce the Al 0, which is energetically 2 3 very unfavourable, the change of the Gibbs free energy of the overall reaction will be much more negative. The main part of the reaction is now the formation of the mixed oxide, described by reaction 2.4. As can be seen in figure 2.2 these reactions contribute a lot of energy to the overall reaction. For this reason oxygen has a detrimental effect on the alkali metal resistance of alumina.

26 2.2 Estimates of thermodynamic data

We need to estimate the thermodynamic data of the rubidium and caesium (Rbp·Al20 3 and CsP·AlP3)' We will do this by a graphical method based on what is called a homologous series. In the past it was demonstrated several (11,12) that the heat of formation can be estimated from consideration of the of elements. For example Roth et al. (13) demonstrated a relationship between the heat of formation of metal oxides and the of the metal in the compound. For this estimate we take the fIrst column of the periodic table as a homologous series and we extrapolate the thermodynamic data of the Li, Na and K aluminates to data for the Rb and Cs aluminates. When the absolute entropies (So) of Lip.AlP3' Nap·Al 0 and K 0·Al 0 are 2 3 2 2 3 plotted as a function of temperature as is done in figure 2.4, one sees a gradual increase in entropy when going to higher temperatures but also when going from lithium to potassium. As rubidium and caesium are the next alkali metals in the periodic table of the elements, it is reasonable to estimate for the entropies of Rb 0·Al 0 and CS 0·Al 0 the curves shown in fIgure 2.4. 2 2 3 2 2 3 The measured enthalpies of formation (dfHo) of Li 0·Al 0 , Na 0.Al 0 and 2 2 3 2 2 3 K 0·Al 0 as a function of temperature are given in figure Here we see no 2 2 3 2.5. gradual change in the same direction when going from Li, Na to K. The change of enthalpy of formation as a function of temperature, however, is the same for these three compounds. For this reason the only values we need are the enthalpies of formation at 298 K. With the aid of figure 2.5 we can extrapolate these values to higher temperatures. Two estimated values for dH of Mp·Alp3 at 298 K are available in form o literature. Lindemer et a1. (14) gave as estimate of dll at 298 K for K 0·Al 0 , 2 2 3 Rb 0·Al 0 and CS 0·Al 0 (at that the value for K 0·Al 0 had not been 2 2 3 2 2 3 2 2 3 measured yet) respectively -2266, -2256 and -2270 kJ/mole. Although they give several ways in which such an estimate can be made, they do not tell how they o actually arrived at their results. A few years later, when dfH of K 0·Al 0 was 2 2 3 o measured, estimates of dfR for Rb2O·Al23° and Cs2O·Al23° were given by Kazin et al. (15). They calculated the enthalpies of formation at 298 K for several hundreds of crystalline substances by an empirical method based on the effective charges on the or . The values they calculated for Rb20·Al 3 and 2° CS20·AlP3 are respectively -2406 and -2448 kJ/mole at 298 K. o An other way dll values can be estimated is again with the use of

27 600 Cs Rb 500 K Na 400 Li ID "0 ...... E 300 ::]

(/)

200

100

O'----'------'-----'---'---...... L..----'------J 200 400 600 800 1000120014001600

Temperature (K)

Figure 2.4 The standard absolute entropies (So) as a function of temperature of the alkali metal aluminates M20oAl203' The curves for Cs and Rb are estimated.

28 -2000

Na

-2100 Q) g K/Rb/Cs "- J ~ Li E -0 -2200 I ....ro Qj "0 -2300

- 2400 L-----'L-----.JL-----.J_----.J_----l_----l_----l_----I 200 400 600 80010001200140016001800

Temperature (K)

Figure 2.5 The enthalpies of formation for the alkali metal aluminates M20oAlP3 from the elements as a function of temperature. The values for the rubidium and caesium compounds are estimated to coincide with the curve of K20oAl2030

29 homologous series. In figure 2.6 homologous series for alkali metal mixed oxides are given. The criteria for choosing the compounds were that enough data should be available and that the form of the first part of the curve was identical to o that of Mz0·Alz°3, This last criterion means in practice that ali for the lithium and potassium compound were lower than for the sodium compound. In figure 2.6 one can see that the change in the values for alio of alkali metal mixed oxides when going from potassium to caesium is for all mixed oxides very small o (e.g. the slope of the curves is zero). We therefore assume that the ali of Kp.Al 0 , Rbp.AlP3 and Cs O·AlP3 at 298 K are all equal to -2361 kllmole, Z 3 Z which is the measured value for K 0·Al 0 . In figure 2.6 the values of Kazin et 2 2 3 al. are also indicated. When these values are correct, the homologous series of the M 0·Al compounds would be an exception with regard to the other alkali 2 2°3 metal mixed oxides. The same arguments can be used against the absolute values estimated by Lindemer et al., although the nearly equal values for the potassium, rubidium and caesium compounds predicted by them agree with our ideas. We, therefore, think our estimates to be the most likely. When we assume that the change in enthalpy of formation as a function of temperature is the same for Li O.Al 0 , Na O'Al 0 ,K O·Al 0 , Rb O·Al 0 and Z 2 3 Z 2 3 Z 2 3 z z 3 Cs O'Al 0 , we now can easily estimate the enthalpies of formation of Rb 0·AI 0 Z 2 3 2 2 3 and Cs 0·Al 3 as a function of temperature because these curves will coincide 2 2° with the curve of the . These data were used in the previous paragraph to calculate the change in Gibbs energy of reaction 2.5. We now will give a critical evaluation of the estimates to see how reliable they are. o The biggest error is probably made in the estimate of afH at 298 K. In figure 2.6 the estimates of Kazin et al. are compared with our own results. In the calculations of the change of Gibbs energy of the corrosion reaction 2.5 we also used the values of Kazin et al. for comparison. The results of these calculations are also given in figure 2.3. It can be seen in this figure that the curves of the change in Gibbs energy are shifted to more negative values and therefore

Al2°3 becomes less stable. This means that the alkali metal pressure needed to corrode Al 0 is lower. On the other hand, our estimates might also be too low 2 3 (too negative) if one compares the homologous series in figure 2.6. If the slope of the second part of the M O·Al curve is the same as for the M O·B 2 23° 2 23° compounds, the values for Rb2O·Al23° and Cs2O·Al23° would be -2337 and -2313 kllmole respectively as indicated in figure 2.6 by circles. This means that the changes in Gibbs energy of the corrosion reaction 2.5 for rubidium and

30 -800

-1000 1 -1200 ----- OJ 2 "0 -1400 ~ = 3 .....E 4 J 6 -1600 ~ 5 E L- ---- 0 I -1800 ro +' 6 Qj -2000 ~ 0 u

-2200 ~ 0 -2400 X °7X

Na K 30 Rb Cs 60

atomic number

Figure 2.6 Homologous series for alkali metal mixed oxides. The enthalpies of formation at 298 K of several mixed oxides are given as a function of the atomic number of the alkali metal. 1 M O·SO , 2 M O·SO , 3 M O·CrO , 4 M O·MoO , = 2 z = Z 3 = ZZZ= 3 5 Mp'SiO ' 6 Mp.B 0 and 7 Mp·Al 0 " = z = z 3 = Z 3 0: the dfHo of Rb O·B 0 is not known, z z 3 X: estimated values of Kazin (15) for RbzO·AlP3 and CsP·AlP3' 0: our estimates of maximum values for Rb O·Al 0 and CszO.AlzO z Z 3 f r

31 caesium will shift about 72 and 144 kJ/mole respectively to less negative values. This means that under standard conditions the changes in Gibbs energy remain negative for both rubidium and caesium. One must be aware, however, that such a shift in the change in Gibbs energy causes a great shift in the alkali metal pressure that is needed for corrosion. As a conclusion, with the aid of the estimates we can give some predictions o for the corrosion reactions although the importance of good measurements of LlfH has been shown.

List of symbols

R gas constant = 8.314 J mole- 1 K 1

M alkali metal

PM : partial pressure of alkali metal M in . vi number of moles of compound i taking part in a reaction. This has a negative sign if the amount of compound i is less after the reaction.

S? absolute entropy of a compound i. o o LlrS standard entropy of a reaction, defmed as LlrS = Tvi S? LlfH? standard enthalpy of formation of a compound i from elements. o o LlrH the standard enthalpy of a reaction, defmed by LlrH =TVi LlfH? LlP? standard Gibbs energy of formation of compound i from elements. Llpo the standard Gibbs energy of a reaction, defined by Llpo = TVi LlP? K equilibrium constant

32 Literature

1. R. N. Singh, J. Am. Ceram. Soc., 59 (1976) 112-115. 2. J. T. Kummer, Progress Solid State Chern., I (1972) 141-175. 3. G. M. Kale and K. T. Jacob, Metall. Trans. B, 20B (1989) 687-691. 4. R. P. Beyer, M. J. Ferrante and R. R. Brown, J. Chern. Thermodyn., 12 (1980) 985-991. 5. 1. Barin, O. Knacke and O. Kubaschewski, "Thermochemical Properties of Inorganic Substances" (Springer Verlag, Berlin, 1977). 6. D. R. Stull and H. Prophet, "JANAF Thermochemical Tables", NSRDS-NBS 37, 2nd edition (US Department of Commerce, , 1971). 7. R. A. Swalin, "Thermodynamics of ", 2nd edition (John Wiley and Sons, New York, 1972). 8. F. R. de Boer, R. Boom, W. C. M. Mattens, A. R. Miedema and A. K. Niessen, "Cohesion in Metals, Transition Metal Alloys", volume 1 (North-Holland, Amsterdam, 1989). 9. J. T. Klomp, in Fundamentals of Diffusion Bonding, editor Y. Ishida. Elsevier, Amsterdam, (1987), 3. 10. J. T. Klomp and A. J. v. d. Yen, Ferrite-Metal interfacial reaction. Paper presented at Acta Met. conference. S. Barbara, Jan. 1989. 11. F. Trombe, Comptes Rend., 218 (1944) 457-458. 12. P. Sue, J. Chim. Phys., 42 (1945) 45-60. 13. W. A. Roth, U. Wolff and O. Fritz, Z. Elektrochem., 46 (1940) 45-46. 14. T. B. Lindemer, T. M. Besmann and C. E. Johnson, J. Nucl. Mater., 100 (1981) 178-226. 15. 1. V. Kazin, V. 1. Kyskin, S. M. Petrova and D. S. Kaganyuk, Russ. J. Phys. Chern. (Eng. Transl.), 58 (1984) 19-21.

33 CHAPTER 3

LITERATURE REVIEW ON PHASE DIAGRAMS 3 LITERATURE REVIEW ON PHASE DIAGRAMS

The major part of this thesis is about the determination of four phase diagrams important for understanding the corrosion behaviour of alumina. From the previous chapter we know that the corrosion products of a reaction between an alkali metal and aluminium oxide will probably be mixed oxides. Because we want to study the influence of CaO and MgO on the reactions, we have taken the systems

Cs2O-CaO-Al230, K2O-CaO-Al230, Cs2O-MgO-Al23° and K2O-MgO-Al23° under investigation. In this chapter we want to review the literature concerning these systems.

3.1 Quasi-binary systems

3.1.1 The - aluminium oxide system The Cs 0-Al 0 system has never been investigated intensively. To our 2 2 3 knowledge no phase diagram has ever been published. Two compounds in this quasi-binary system have been reported so far (1,2).

CsP·AlP3 can be made by reacting CsN03 or Cs2C03with AlP3·3Hp at 973 K to 1173 K. The is cubic with a = 8.10 A (3). It reacts with to form hydrates (4). Above 823 K this reaction will proceed in the opposite direction. When Cs O·Al ° was heated at 1273 K for 40 hours in an open 2 2 3 crucible, 50 % of the CS2° had already evaporated (2). The second phase mentioned in this system is CS2O·11Al2°3' It is reported to be formed as a pure compound with a hexagonal p-alumina structure (2) by heating

CS20·Al20 3 for one year at a temperature of 973 K. A detailed description of this 13-alumina structure is given in chapter 7, from which it follows that this structure type is very unlikely because of the large radius of the caesium ion. The lattice parameters given by Semenov (2), a = 5.584 A and c = 22.83 A are not in agreement with the X-ray diffraction data of Langlet (1) from which the c-axis can be calculated to be c = 23.1 A. Both these values are smaller than one can expect on the basis of the variation of the c-lattice parameter of 13-alumina with the radius of the metal ion (5). From these data one would expect a c-axis of at least c = 23.3 A. When one immerses Na-p-alumina in a melt of CsCl to try to exchange the cations, the exchange is very slow and never complete (6). From all

36 these literature data we can conclude that it is very doubtful whether Cs-B-alumina can be made and is stable in its pure form.

3.1.2 The - aluminium oxide system In the past, the Na O-Al system attracted much more attention than the 2 2° 3 K2O-Al23° system. The Na2O-Al23° system is important because of the Na-B-alumina phase that is used as a solid electrolyte in Na/S batteries. Since K-B-alumina is not an as good , less work was done on the K 0-Al 0 system. 2 2 3 The last decade this system received more attention because of its importance for the understanding and controlling of the high temperature corrosion of the walls in a magneto hydrodynamics (MHD) electric generator. Potassium is used to enhance the electrical conductivity of the plasma inside the MAD generator. Because of the similarity between the systems, much of what was found for the Na 0-Al 0 systems will also be valid for the KP-Al 0 system. 2 2 3 2 3 The fIrst compound ever reported in the K 0-Al 0 system was potassium 2 2 3 monoaluminate, K 0·Al Brownmiller (7) reported the optical properties and the 2 2°. 3 X-ray diffraction pattern of this extremely hygroscopic compound which is stable to at least 1923 K. The most recent powder diffraction fIle found in the JCPDS system (8) is still the pattern reported by Brownmiller. However, Otsubo et al. (9) and Brumakin et al. (10) found an endothermic effect at 810 K which was due to a reversible transition. At 298 K the cellparameter of the cubic lattice is a = 15.41 A. Above 810 K this cellparameter is halved to a = 7.798 A at 873 K which is close to the value deducted by Barth (11) from Brownmillers data. Therefore, the compound investigated by Brownmiller was the high temperature modifIcation. This must be due to stabilizing additives because Brumakin et al. (10) found that the high temperature modifIcation could not be found at lower temperatures after rapid cooling. In quasi-ternary systems such as K 0-Al 0 -Si0 , however, the high temperature form can also be found at room 2 2 3 2 temperature (12) due to the stabilizing effect of SiO. No literature was found 2 on the effect of CaO or MgO on the stability of the high temperature form. A study on the subsystem K O-K O·AI by Bon et aI. (13) revealed a compound 2 2 2° 3 with the formula 3K 0·Al 0 . This monoclinic compound is not stable at 2 2 3 temperatures above 773 K and decomposes into K 0 and K 0·Al 0 . 2 2 2 3 In 1978, Eliezer and Howald (14) accumulated the data known for the K20'Al203-Al203 subsystem. A thermodynamic analysis in combination with

comparison with the Na2O·Al 2° 3-Al 2° 3 system led to the presentation of possible phase diagrams. The data were rather scarce at that time and therefore the

37 diagrams are of limited value. The experimental studies on the KZO·AlZ03-AlZ03 subsystem by Roth (15) and Moya et al. (16) led to the phase diagrams given in figure 3.1 (17). The theoretical formula of 6-alumina based on its crystal structure (18) is K O·llAl 0 .A detailed description of this crystal structure is given in chapter z Z 3 7. Moya et al. (16) found for the limits of the 6-alumina phase Kp·lOAlP3 and

K O·4.75Al 0 • The solid solution found by Roth was less extended. One of the z z 3 limits found for 6-alumina is close to the theoretical formula of 6"-alumina

(K O·5.3Al 0 ). The crystal structure of 6"-alumina is almost the same as the z z 3 B-alumina structure. The unit cell, however, contains three -type blocks instead of two. As in the Na O-Al 0 system (19) problems arose concerning the stability of Z Z 3 6-alumina and 6"-alumina. For many years contradictory results were published about the stability of these phases in the Na O-Al 0 system because both phases Z Z 3 were often found together and the transformations were very slow. Alden (20) reported 6- and 6"-alumina to be stable at temperatures as high as 1950 K. The experiments of Hodge (21), however, show clearly transformation of Na-B"-alumina to Na O-Al 0 and B-alumina at temperatures as low as 1673 K and therefore Z Z 3 proves Na-6"-alumina to be at least metastable at this temperature. At lower temperatures the transformation becomes infmitely slow and therefore in practice these compounds are almost always found together. This is also found for the K O-Al 0 system (15,16). To our knowledge no proof for the stability of the Z Z 3 various phases in this system has been published up to now and therefore we assume of the K-6"-alumina phase. In the rest of this thesis we will treat B"-alumina and B-alumina as one solid solution between the theoretical limits Kp·5.3AlZ0 3 and KZO.llAlP3' Besides the above mentioned 6- and 13" -alumina compounds, Yamaguchi and Suzuki (22) reported a third compound with resembling crystal structure. This compound was designated B'-Al 0 and the ideal formula should be K O.7.22Al 0 • This z 3 z z 3 compound was only reported by these authors and is believed to be actually 6-alumina (23). Therefore this compound is not taken into account but is mentioned here for the sake of completeness.

3.1.3 The calcium oxide - aluminium oxide system The quasi-binary system CaO-Al 0 has been the subject of numerous studies, z 3 mainly because of its importance to the . As can be seen in figure 3.2 the two most recently published phase diagrams are very much alike (24,25)

38 a

2600

2'00

noo " \ \ \ 2000 \ , \ \ ,,------*---, ,(91%) \ , , I -'-(B_)--,~) ~Dh ::::~::: '60°J--_--:..:=-'- ..min)

1400 (62%) IS

1200

1000

Uo! %

Figure 3.1 Phase diagrams of the system K a·AI a -AI a as found by a. Roth (15) and b. Moya 2 2 3 2 3 et al. (16)

39 a 2100

2000

1900

CoO 184 %.~l'---""'r-=-'-"--l 1-2600·) 1800 \ Liquid CA •• LiQ. , (78'%,) 1762 ~ 5° \ 1700 \

1600 CA.' CA, AI,O,. I~OO CA.

CoO' C,A I~OO

(~O.6~%) C,A • CA 1300 20 CoO

b

2000r--~---,r------r------,---"""'7"'1------'

1800

1600

1400 6'" .. ~ 0

Figure 3.2 Phase diagrams of the system CaO-Al 0 when a. l2CaO·7Al 0 is not stable (24) 2 3 2 3 and b. l2CaO·7Al 0 is stable (25) in an moisture free atmosphere. 2 3

40 and contain beside CaO and AlP3 also 3CaO·Al 0 , CaO·Alp3' CaO·2Alp3 and 2 3 CaO·6Al 0 . In the past other compounds were reported from which 5CaO·3Al 0 2 3 2 3 appeared to be metastable (24) or was actually 12CaO·7Alp3 (26), 2CaO·Al 0 (27) 2 3 was never conftrmed to exist and 3CaO·16 Al 0 (28) and 3CaO·5Al 0 (29,30) were 2 3 2 3 later found to be respectively CaO·6Alp3 and CaO.2Al 0 (31). 2 3 The difference between the reported diagrams is mainly the 12CaO·7Al2°3 compound. Nurse et al. (24,32) reported that in an air and moisture free atmosphere this compound is not stable with respect to 3CaO.Al 0 and CaO·Al 0 . 2 3 2 3 Their conclusion was that the formula of the stable phase was Ca12Al14032(OH)2' For this reason they omitted the primary fteld of 12CaO.7Al 0 in the CaO-AI 0 2 3 2 3 phase diagram. Later, this phase has been a subject for many studies and much controversy still exists (33-36). Recently it has been reported by Singh and Glasser (36) that 12CaO·7Al ° can be formed in a dry atmosphere although its 2 3 formation kinetics are more favourable in moist atmospheres. It can take up water reversibly in the temperature range 1173-1573 K with a maximum of", 1.4 % which is corresponding with the formula suggested earlier by Nurse (24,32). A second compound in this CaO-Al system, about which still some 2° 3 uncertainties exist, is CaO·6Al 0 . An electrochemical study of Allibert et al. 2 3 (37) showed CaO·6Alp3 to be the most stable compound in this system at temperatures as low as 923 K. In diffusion couple studies (38-40) between 1473 and 1833 K, CaO·6Alp3 is never formed at temperatures below 1603 K. Weisweiler (39) found that the CaO·6Al 0 phase is the most easily formed calcium aluminate 2 3 at 1673 K at the expense of CaO·Al and CaO·2Al which were more easily 2° 3 2°, 3 formed at lower temperatures. Singh and MandaI (40), however, found the opposite. At temperatures higher than 1683 K the CaO·2Al 0 phase grew at the expense of 2 3 the CaO·6Alp3 phase. The experiments of Kahatsu (38) at 1603 K resulted in the observation of the various calcium aluminate phases in the order 12CaO·7Al 0 2 3 » 3CaO·Alp3 > CaO·6Alp3'> CaO·2Alp3 '" CaO·Alp3' Scian et al. (41) did not fmd any formation ofCaO·6Alp3 in a 1:2 CaCO/Al(OH)3 mixture heated at 1373-1673 K for 5 minutes to 5 hours. The kinetics and mechanism of the calcium hexa aluminate phase was also studied by Semin et al. (42) in the 1370-1870 K temperature range. They stated that CaO·6Al formation should be possible on 2° 3 thermodynamic grounds in the whole temperature range studied by them. They only give, however, the results of temperatures higher than 1470 K and CaO·6Al 0 is 2 3 formed at all these temperatures. The formation of calcium aluminate phases was studied by MacKenzie et al. (43-45) in 3:1 and 1:1 CaO:Alp3 mixtures at temperatures between 1173 and 1573 K. They reported the formation of the

41 CaO·6Al 0 phase after 4 hours at temperatures as low as 1173 K in the 3:1 2 3 mixture but not in the 1:1 mixture, which is very unlikely. The amount of CaO·6Al 0 phase after 4 hours heating at a certain temperature was constant up 2 3 to 1373 K. At 1473 K the amount of CaO·6Al 0 started to increase. In a phase 2 3 formation study of Verwey and Saris (46) no formation of CaO·6Al20 3 from CaC0 -Al 0 or CaO.2Alp3-Al203 mixtures was found after 100 hours at 1473 K 3 2 3 including repeated grinding. When this compound was synthesized at higher temperatures, however, the compound did not show any tendency to decompose after 100 hours at 1473 K. Although CaO·6Al 0 is generally believed to be stable at al 2 3 temperatures up to its melting point, from all these experiments it does not become clear whether this phase is not stable at lower temperatures or whether the kinetics of the formation are very slow. All calcium aluminates are thermodynamically metastable in the presence of water. They may interact with water to form all kinds of hydrates from which the cubic tri calcium aluminate hydrate 3CaO·Alp3·6H20 may be regarded as the most stable one (27). The development of mechanical strength in cement is based on these hydratation reactions. For this reason Buttler et al (47) and Ohta et al. (48) studied hydratation of calcium aluminates. From these studies, which were conducted with water/solid ratios of 0.6 to 100, one can conclude that these hydratation reactions are very slow and reversible at higher temperatures (>473 K) except for the hydratation of 12CaO·7Alp3 which was already discussed. For this reason we do not need to worry that these reactions may disturb our investigations because our samples are only short times in contact with water beneath this temperature. From all the calcium aluminates, the crystal structures were determined (26,49-54) and powder data are reported in the JCPDS powder data file (55). Because of its resemblance to the 6-alumina structure a detailed description of the CaO·6Al 0 structure is given in chapter 7. 2 3

3.1.4 The magnesium oxide - aluminium oxide system The most recently published phase diagram of the MgO-Al 0 system is reported 2 3 by Colin (56) and is given in figure 3.3. The main features of the diagram are the same as previously reported by Alper et al. (57) and Roy et al. (58). Spinel, MgO·Al 0 , is the only stable quasi-binary compound in this system. 2 3 Though Alper et al. (57) reported solubility of MgO in MgO·AI 0 , other authors 2 3 did not fmd evidence for this (56,58-60). It is well established that at higher temperatures a considerable amount of alumina can dissolve in the spinel

42 ,, Liquid .,.,.. -~-:::..---- " ...... 1- 2000 ------~----

y

1500

1000

MgO MOl %

Figure 3.3 Phase diagram of the system MgO-Al 0 as determined by Colin (56) 2 3

43 structure. At lower temperatures, however, spinel can be treated as a line compound. In the crystal structure of MgO·AI 0 (MgAI 0 ), the oxygen atoms are face 2 3 2 4 centered cubic closed packed with magnesium and aluminium at the interstices. More oxides with the general formula AB 0 have the same spinel-type crystal 2 4 structure. The cell parameter of MgO·AI 0 is a = 8.083, which is very close to 2 3 the value found for CsAI 0 • This compound, however, does not have the 2 4 spinel-type structure. The formation of spinel from AI 0 and MgO is very sluggish (60). Therefore 2 3 reactive powders and intensive mixing are needed.

3.1.5 Alkali metal oxide - calcium oxide systems No quasi-binary compounds are reported in the K O-CaO and Cs O-CaO systems 2 2 and no information on these phase diagrams was found.

3.1.6 Alkali metal oxide - magnesium oxide systems No quasi-binary compounds are known in the Cs 0-MgO system. More than that, no 2 literature on this system could be found at al. The K 0-MgO-Si0 system was investigated by Roedder (61,62) and he noted that 2 2 no phases appeared in the quasi-binary K O-MgO system. On the other hand, Bardin 2 et al. (63) were able to synthesize 3Kp.MgO at 1073 K from MgO and Kp. Nothing is stated on the stability of this compound and therefore it might be a metastable phase.

3.2 Quasi-ternary systems

3.2.1 The Cs 0 - alkaline earth metal oxide - Al 0 systems 2 2 3 Only work on one quasi-ternary phase diagram involving Cs 0 and AI 0 has been 2 2 3 done in the past, namely on the Cs O-SiO -AI 0 diagram (64,65) This was done 2 2 2 3 because of its importance for nuclear technology. No similarity between this system and the MgO-Cs O-AI 0 or CaO-Cs O-AI 0 systems can be expected in 2 23 2 23 advance. The great stability of the quasi-ternary SiO -Cs O-AI 0 compounds are 2 2 2 3 the reason for the severe caesium corrosion of alumina with Si0 in it (66-68). 2

44 3.2.2 The K 0 - alkaline earth metal oxide - Al 0 systems 2 2 3 In the early years of this century it was recognized that an intensive study and knowledge of the phase diagrams associated with the CaO-Si0 -Al 0 system was 2 2 3 of vital importance to the cement industry. For this reason many quasi-ternary systems were studied at that time. Brownmiller (7) did a study on the CaO-K 0-Al 0 system with special attention to the portion with smaller 2 2 3 percentages of K 0. No quasi-ternary compounds were found in equilibrium with the 2 liquid phase. The diagram as found by Brownmiller is given in figure 3.4. From this it can be seen that K O·Al is in equilibrium with all the calcium 2 2° 3 aluminates. Although Brownmiller found B-alumina to crystallize in the KP-Al20 3 system, it was not found in the quasi-ternary system. In those days, the knowledge of B-alumina was very poor and from the paper it is not clear whether Brownmiller already knew that B-alumina is in fact a potassium aluminate or not. Furthermore, calcium hexa aluminate, CaO·6Al 0, was not mentioned at all. This 2 3 compound and B-alumina are very much alike and therefore it is important to know the phase relations with regard to these compounds. No other literature about this quasi-ternary system was found and therefore it was decided to reinvestigate it.

Schraier studied in 1954 the KP-MgO-AlP3-Si02 system in the high silica portion (69). However, no results of the K 0-MgO-Al 0 system are given. Many 2 2 3 years later, Roth and coworkers (70) did some phase equilibria research on parts of the KP-MgO-Fep3-Alp3-Si02 system. They found MgO and KP·Al20 3to be in equilibrium according to reaction 3.1.

~ (3.1) MgO·AlO23 + KO2 MgO + KO·AlO2 23

This reaction was found to be the corrosion mechanism of the MgO·~03 insulation in a MHD system. Further preliminary results of the K2O-MgO-Al2°3 system showed it to be analogous to the Na 0-MgO-Al 0 system in its compounds formation. For 2 2 3 this reason we will also consider the Na 0-MgO-Al 0 system in our discussions. 2 2 3 The phase diagram of this system in the high alumina range obtained by Weber and Venaro (71) is shown in figure 3.5. They found three quasi-ternary compounds in this system, all with structures similar to B-alumina. The chemical composition of stabilized ternary B"-alumina is assumed to be represented by the formula Na Mg Al ° (72). In this ternary B"-alumina, l+x x ll-x 17 the charge of the excess sodium in the conduction plane is compensated by a substitution of an Al3+-ion from the spinel type by a Mg2+-ion. The

45 CaO A1 2 0 3

Figure 3.4 The diagram of the quasi-ternary system CaO-K O-AI 0 . The diagram shows the 2 2 3 fmal products of from the melt proposed by Brownmiller (7). C = CaO, K = K 0 and A = AI 0 2 2 3

46 ll101e%

A1 2 0 3 / \ mole%

Na 2 0

10/ )' \ .. O:..:./ ~~ .. ;:i:::::::==7'i~

30 20 10 ~<_mole% A1 2 0 3 MgO

Figure 3.5 The phase diagram ofthe system CaO-Na O-AI 0 according to Weber and Venaro 2 2 3 (71).

47 homogeneity range of stabilized B"-alumina is often investigated and mostly it is found that the maximum value of x is around 2/3 (20,73,74). The formula Na Mg AI 0 is attributed to a second quasi- ternary 1.36 1.88 14.96 25 compound with a structure similar to B-alumina (75) and which therefore is named B'''-alumina. The difference is that the spinel-type blocks contain six oxygen layers instead of four. The potassium analogue has been identified also and is described by the same formula (76,77). Its space is hexagonal P63/mmc with a = 5.62 A and c = 32.0 A. The third compound in the Na O-MgO-AI 0 system is assumed to be the larger 2 2 3 spinel block analogue of B"-alumina with the formula Na Mg AI 0 and 1.67 2.67 14.33 25 is called B""-alumina. However, no X-ray diffraction data or properties are reported and the potassium analogue has never been found. The problem about the stability and homogeneity ranges of the sodium B- and B"-alumina phases is not yet completely solved. In practice it will be very difficult to solve these problems because of the very slow kinetics. The two quasi ternary compounds B'" - and B""-alumina are the large spinel block analogues of respectively B- and B"-alumina. Because of the similarity between the small and large spinel block compounds, one can expect the same problems with the determination of the stability for the large block compounds. However, no such problems are reported in literature thus far. Furthermore, it is highly improbable that the B"'- and B""-alumina compounds have a fixed as suggested by the formulas given. If both these compounds do exist, we would expect a phase diagram as given in figure 3.6 with a solid solution area between B'''- and B""-alumina as also reported for B- and B"-alumina.

48 mole%

A1 2 0 3 I \ mole%

Na 2 0

10[ :.", _.:;,!.IJc.: ..:\':+.. _._ };O~. ~-=:.

30 20 10 A1 0 < mole% 2 3 MgO

Figure 3.6 Our proposal for a more probable phase diagram of the system CaO-Na 0-Al 0 (see 2 2 3 text).

49 Literature

1. G. Langlet, Centre d'Etudes Nucleaires de Saclay, Rapport CEA-R-3853 (1969). 2. N. N. Semenov, A. G. Merkulov and I. A. Vorsina, Ser. Khim. Nauk., 1 (1967) 75-83. 3. G. Langlet and G. Chaudron, C. R. Acad. Sci., 259 (1964) 3769-3770. 4. F. F. Barkova, N. E. Pysina and N. A. Strukulenko, Sb. Dokl. naIl. Vsesojuznom SOy. Novosibirsk Nauka (1967). 5. R. A. Huggins, Mater. Sci. Res., 2 (1975) 155-175. 6. Yung-Fang Yu Yao and J. T. Kummer, J. Inorg. Nucl. Chern., 29 (1967) 2453-2475. 7. L. T. Brownmiller, Am. J. Sci., 29 (1935) 260-277. 8. JCPDS Powder Diffraction File, American Society for Testing and Materials, Philadelphia, # 2-0897. 9. Y. Otsubo, K. Yamaguchi and Y. Kawamura, Nippon Kagaku Zasshi, 83 (1962) 352-353. 10. E. I. Brumakin, G. V. Burov, I. G. Rozanov and G. Sh. Shekhtman, Russ. J. Inorg. Chern., 23 (1978) 1868-1870. 11. T. F. W. Barth, J. Chern. Phys., J. (1935) 322-. 12. A. Yamaguchi, Yogyo Kyokai Shi, 78 (1970) 74-75. 13. A. Bon, C. Gleitzer, A. Courtois and J. Protas, C. R. Acad. Sci., Ser. C, 278 (1974) 785-788. 14. I. Eliezer and R. A. Howald, High. Temp. Sci., 10 (1978) 1-16. 15. R. S. Roth, Adv. Chern. Res., 186 (1980) 391-408. 16. J. S. Moya, E. Criado and S. De Aza, J. Mater. Sci., 17 (1982) 2213-2217. 17. R. S. Roth, J. R. Dennis and H. F. McMurdie, Phase Diagrams for Ceramists, Vol. VI, (1987) edited by M. A. Clevinger and H. M. Ondik, American Ceramic Society, Westerville, Ohio; Figures 6349 and 6350. 18. N. Iyi, Z. Inoue and S. Kimura, J. Solid State Chern., 61 (1986) 81-89. 19. R. C. De Vries and W. L. Roth, J. Am. Ceram. Soc., 52 (1969) 364-369. 20. M. Alden, Solid State Ionics, 20 (1986) 17-23. 21. J. D. Hodge, J. Am. Ceram. Soc., 66 (1983) 166-169. 22. G. Yamaguchi and K. Suzuki, Bull. Chern. Soc. Jpn., 41 (1968) 93-99. 23. J. T. Kummer, Prog. Solid State Chern., 1 (1972) 141-175. 24. R. W. Nurse, J. H. Welch and A. J. Majumdar, Trans. J. Br. Ceram. Soc., 64 (1965) 409-418. 25. A. K. Chatterjee and G. I. Zhmoidin, J. Mater. Sci., 1 (1972) 93-97.

50 26. H. Bartl and T. Scheller, Neues Jahrb. . Abh., 35 (1970) 547-552. 27. V.E. Kaushanskii, Zhumal Prikladnoi Khimii, 52 (1979) 1712-1718. 28. K. V. Lagerqvist, S. Wallmark and A. Westgren, Z. Anorg. Chern., 234 (1937) 1-16. 29. G. A. Rankin and F. E. Wright. Am. J. Sci., 39 (1915) 1-79. 30. H. Von Wartenberg, H. J. Reusch and E. Saran, Z. Anorg. Allg. Chern., 230 (1937) 257-276. 31. M. Rolin and P. H. Thanh, Rev. Hautes Temp. Refract., ~ (1965) 179-182. 32. R. W. Nurse, J. H. Welch and A. J. Majumdar, Trans. J. Br. Ceram. Soc.,64 (1965) 323-332. 33. H. Bartt and T. Scheller, Neues Jahrb. Mineral. Abh., 35 (1970) 547-552. 34. T. Yu. Chemekova and Yu. p. Udalov, Izv. Akad. Nauk SSSR, Neorg. Mater., 10 (1974) 2191-2193. 35. V. N. Gladkiy, 1. S. Kulikov, 1. V. Ostrovskaya and A. A. Telegin, Izv. Akad. Nauk SSSR Met., 2. (1980) 78-82. 36. V. K. Singh and F. P. Glasser, Ceram. Int., 14 (1988) 59-62. 37. M. Allibert and C. Chattillon, J. Am. Ceram. Soc., 64 (1981) 307-314. 38. 1. Kohatsu and G. W. Brindley, Z. Phys. Chern., 60 (1968) 79-89. 39. W. Weisweiler and S. J. Ahmed, Zem.-Kalk-Gips, 33 (1980) 84-89. 40. V. K. Singh and U. K. MandaI, Trans J. Br. Ceram. Soc., 81 (1982) 112-113. 41. A. N. Scian, J. M. Porto LOpez and E. Pereira, Cern. Concr. Res., 17 (1987) 198-204. 42. E. G. Semin, T. 1. Berbenitskaya, L. P. Benderskaya, E. A. Ivanova and N. F. Fedorov, Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, 19 (1983) 1122-1125. 43. K. J. D. MacKenzie, R. K. Banerjee and M. R. Kasaai, J. Mater. Sci., 14 (1979) 333-338. 44. K. J. D. MacKenzie and R. K. Banerjee, J. Mater. Sci., 14 (1979) 339-344. 45. K. J. D. MacKenzie and R. K. Banerjee, Trans. J. Br. Ceram. Soc., 77 (1978) 88-92. 46. H. Verwey and C. M. P. M. Saris, J. Am. Ceram. Soc., 69 (1986) 94-98. 47. F. G. Buttler and H. F. W. Taylor, J. Appl. Chern., 2 (1959) 616-621. 48. M Ohta, T. Kimura and H. Yanagida, J. Ceram. Assoc. Jpn. Int. Ed., 96 (1988) 36-,40. 49. P. Mondal and J. W. Jeffery, Acta. Crystallogr. Sect. B: Struct. Sci., B11 (1975) 689-697. 50. W. Horknerund Hk. Mtiller-Buschbaum, J. Inorg. Nucl. Chern., 38 (1976)

51 983-984. 51. V. 1. Ponomarev, D. M. Kheiker and N. V. Belov, Sov. Phys. Crystallogr. (Eng. Transl.), 15 (1971) 995-998. 52. K. Kato and H. Saalfeld, Neues Jahrb. Mineral. Abh., 109 (1968) 192-200. 53. M. W. Dougill, , 180 (1957) 292-293. 54. E. R. Boyko and L. G. Wisnyi, Acta. Crystal1ogr., 11 (1958) 444-445. 55. JCPDS Powder Difraction File, American Society for Testing and Materials, Philadelphia, # 9-413, 23-1036, 23-1037, 32-148, 32-149, 33-253. 56. F. Colin, Rev. Int. Hautes Temp. Refract., J. (1968) 267-283. 57. A. M. Alper, R. N. McNally, P. H. Ribbe and R. C. Donan, J. Am. Ceram. Soc., 45 (1962) 263-268. 58. D. M. Roy, R. Roy and E. F. Osborn, Am. J. Sci., 251 (1953) 337-361. 59. W. T. Bakker and J. G. Lindsay, Am. Ceram. Soc. Bull., 46 (1967) 1094-1097. 60. J. T. Bailey and R. Russell, Am. Ceram. Soc. Bull., 50 (1971) 493-496. 61. E. W. Roedder, Am. J. Sci., 249 (1951) 81-130. 62. E. W. Roedder, Am. J. Sci., 249 (1951) 224-248. 63. J-C. Bardin, M. Avallet and M. Cassou, C. R. Acad. Sci. Paris, 278 (1974) 709-712. 64. T. V. Solov'eva, 1. Kh. Moroz and G. A. Vydrik, Russ. J. Inorg. Chern., 15 (1970) 909-911. 65. R. Odoj and K. Hilpert, High Temp.- High Pressures, 12 (1980) 93-98. 66. R. G. Smith, F. Hargreaves, G. T. J. Mayo and A. G. Thomas, J. Nucl. Mater., 10 (1963) 191-200. 67. J. K. Higgens, Trans. J. Br. Ceram. Soc., 65 (1966) 643-659. 68. L. N. Grossman, Rev. Int. hautes Temp. Refract., 16 (1979) 255-261. 69. J. F. Schraier, J. Am. Ceram. Soc., 37 (1954) 501-533. 70. R. S. Roth, Adv. Chern. Ser., 186 (1980) 391-408. 71. N. Weber and A. F. Venero, Am. Ceram. Soc. Meeting, (1970) I-JV-70. 72. M. Bettman and C. R. Peters, J. Phys. Chern., 73 (1969) 1774-1780. 73. J. P. Boilot, G. Collin, Ph. Colomban and R. Comes, Phys. Rev., B22 (1980) 5912-5923. 74. J. B. Bates, H. Engstrom, J. C. Wang, B. C. Larson, N. J. Dudney and W. E. Brundage, Solid State lonics, J. (1981) 159-162. 75. M. Bettman and L. Terner, J. Inorg. Chern., 10 (1971) 1442-1446. 76. M. Blanc, A. Mocellin and J. L. Strudel, J. Am. Ceram. Soc., 60 (1977) 403-409. 77. K. J. Morrissey and C. B. Carter, Mater. Sci. Res., 15 (1983) 297-308.

52 CHAPTER 4

GENERAL EXPERIMENTAL

SET UP FOR PHASE DIAGRAMS DETERMINATIONS 4 GENERAL EXPERIMENTAL SET UP FOR PHASE DIAGRAM DETERMINATIONS

4.1 Method

The phase diagrams were determined according to a static method. A mixture of the compounds is heated until equilibrium is reached. After cooling, the phases present in the sample are determined by X-ray diffraction (XRD). By analyzing samples of various compositions the isothermal cross section of the phase diagram can be determined.

4.2 Materials

The materials used in the phase diagram experiments are listed in table 4.1. Cs C0 was handled always in a glovebox in a dry N atmosphere because it is 2 3 2 deliquescent. In this way it could be prevented that CS C0 attracted water. The 2 3 water content was measured several times by thermogravimetric analysis but never exceeded 1 wt%. Although the reaction with water is less severe, K CO3 was also handled in the 2 glovebox.

Table 4.1 Materials used in the phase diagram examinations.

material purity (%) supplier

Al(N03)3·9Hp zur analyse > 98.5 E Merck AG > 99.995 Les Rubis Synthetique des Alpes Al2°3 extrapure CaC0 zur analyse E Merck AG 3 > 99 Cs C0 FO Optipur > 99.8 E Merck AG 2 3 K CO3 zur analyse > 99 E Merck AG 2 MgO zur analyse > 97 E Merck AG Mg(NO ) ·6H zur analyse > 99 E Merck AG 3 2 2° Emulsifyer Span 80 Fluka Chemie AG

54 4.3 Preparation of the mixtures

The alkaline were weighed in a dry atmosphere. Once the amount of was determined it was not important anymore whether or not it attracts water, because later on the mixture was heated at 473 K to remove the water. Therefore, for practical reasons the further mixture preparation was not done in the glovebox although contact with water containing atmospheres was avoided as much as possible. Once the amount of alkali metal carbonate was determined, the other compounds were added as to give the appropriate mixtures. The mixing was done on a roller bench in polyethene flasks with ertalon balls in it. Rolling the mixture for 10 hours was in most cases sufficient. In the experiments on the phase diagrams of the type X 0-MgO-AI 0 however, 2 2 3 sometimes problems arose and equilibrium could not be reached. This was caused by

insufficient mixing of MgO and Al 0 • In those cases hot drying (1,2) 2 3 was used to avoid this problem. To this end, solutions of Al(N0 )3 and Mg(N0 )2 3 3 3 2 were mixed to get the desired ratio of Al + and Mg +. This solution was added to kerosene with a between 413 and 433 K. The volume ratio kerosene/water was about 6/5. To stabilize the , 1 g of emulsifying agent, "Span 80", was added to every 100 ml of kerosene. Stirring with an "Ultra-turrax" stirring apparatus produced a water in oil emulsion. This emulsion was added in small drops to kerosene at a temperature of ::: 423 K in a normal distillation equipment. Because the small water droplets from the emulsion were dispersed immediately in the whole volume of hot petroleum, the evaporation of the water was extremely fast. The precipitated well mixed in small particles. The powder was filtered and washed with pentane. After drying the precipitate was heated at 600°C for 10 hours in air to decompose the nitrates.

The result was a white, very well mixed mixture of MgO and Al 0 • 2 3 Finally, the appropriate amount of Cs CO or K CO was added and mixed with 2 3 2 3 the MgO-Al 0 mixture on a roller bench. 2 3

4.4 Heat treatment

When the mixing was done, a pellet of about 250-500 mg was pressed. This was done in a perspex mould to avoid contamination. Some material may be picked up during the pressing but then this will bum out during the heat

55 treatment. After drying at 473 K and weighing, the samples were heated at the desired temperature. With the aid of thermogravimetric analyses (Netzsch STA 409) it was checked whether the decomposition of the carbonates was complete and whether the evaporation of the alkali metal was not too fast at that temperature. In order to reach equilibrium the samples were heated in air for 24 hours up to 500 hours in a horizontal furnace. The furnace tube is made of alumina (Degussa 99.7%) which was heated electrically from the outside. In this tube the alumina crucibles (Dyko-Sint Al99GB, 99.7%) containing the pellets were placed in such a way that contact between the pellet and the crucible was minimized to two points. No visible corrosion of the crucibles took place during the experiments. The crucibles were taken out of the oven while still hot. They were transferred to a glovebox to cool down in a water free atmosphere, because some reaction products were hygroscopic. When these compounds react with water and their crystal structure is destroyed, they will not be detected by XRD. For this reason all samples were handled in a dry atmosphere once they had had their heat treatment. During the calcination of the pellets alkali metal oxide losses were often considerable. This means that the composition changed during the heat treatment. By measuring the weight before and after the heat treatment, however, we could calculate the resulting composition. In this calculation we assume that the weight loss is only due to CO and alkali metal oxide evaporation. Since we know 2 the amount of CO , we can calculate the evaporated amount of alkali metal oxide. 2 The accuracy of this method is good enough for our purpose, but one has to bear in mind that the real composition may differ slightly. In the case of the MgO-Cs O-Al 0 phase diagram, equilibrium was difficult to 2 2 3 obtain. For this reason long heating periods were needed. In order to avoid complete evaporation of Cs 0 before equilibrium could be reached, the heat 2 treatments took place in sealed molybdenum capsules. To make this possible, the pellets were fIrst given a heat treatment at 1123 K for 4 hours to decompose

Cs C0 • From thermogravimetric analysis we learned that this should be sufficient 2 3 to decompose Cs C0 completely. Before and after the heat treatment the pellets 2 3 were weighed. In this way the decomposition and evaporation of CO could be 2 checked. Sometimes the weight loss was not enough after one treatment. In those cases the heat treatment was repeated as many times as needed. After the decomposition was complete the pellet was sealed in the molybdenum capsule for the fmal heat treatment at 1173 K.

56 4.5 Characterization of the phases

Powder X-ray diffraction was used to identify the various phases. When hygroscopic compounds react with water, their crystal structures will change or are destroyed. For this reason all XRD measurements were conducted in an evacuated diffractometer. For this purpose a high temperature diffractometer was used. In this home built apparatus (3) CuKa. radiation was used in combination with a curved monochromator. The sample chamber is evacuated with a turbomolecular pump to a pressure lower than 10-3 Pa. Under these conditions reaction with water could be prevented.

Literature

1. P. Reynen, H. Bastius, M. Faizullah and H. v. Kamptz, Ber. Dtsch. Keram. Ges., 54, (1977), 63-67. 2. P. Reynen, H. Bastius and M. Fiedler, in Ceramic Powders, editor P. Vincenzini, Elsevier, Amsterdam, (1983), 499-504. 3. L. Boon, H. de Jonge Baas and R. Metselaar, Accepted for publication in Powder Diffr.

57 CHAPTER 5

PHASE RELATIONS IN THE

CaO-Cs20-A120 3 SYSTEM 5 PHASE RELATIONS IN THE CaO-Cs 0-AI 0 SYSTEM 2 2 3

The principle of investigation of the CaO-Cs 0-Al 0 system is described in 2 2 3 chapter 4. The temperature used was 1173 K with heating times between 24 and 200 hours. Thermogravimetric analyses showed complete dissociation of the carbonates between 923 K and 1173 K. In the ftrst experiments (no. 1 to 3 in ftgure 5.1), we found from XRD measurements that together with Al ° and Cs 0·Al ° an unknown compound was 2 3 2 2 3 always formed. We developed a new method to locate this compound and to establish at the same time the phase relations in this system.

5.1 A method to determine the composition of an unknown compound

The principle of our approach is based on the fact that CS 0·Al 3 decomposes 2 2° upon heating at 1173 K according to reaction 5.1:

(5.1)

So when we heat a series of mixtures of Cs 0·Al 0 , Al 0 and the unknown 2 2 3 2 3 compound for long times, Cs will evaporate out of Cs 0·Al until no 2° 2 2° 3 CS 0·Al 0 is present anymore. Assuming that no Cs 0 evaporates from the unknown 2 2 3 2 compound and assuming no other quasi-ternary compound to be present in this system, the resulting compositions will lie on a straight line connecting Al 0 2 3 and the unknown compound. On the other hand, when we heat a series of mixtures of CaO, CS and the unknown compound, CS will again evaporate out of the pellets. 2° 2° The result will be mixtures of the unknown compound and CaO. The point or region where the end composition of the two series of mixtures will cross is the composition of the unknown compound. Two assumptions were made. The ftrst is that only one quasi-ternary phase exists in the CaO-Cs O-Al ° system. When other quasi-ternary compounds are 2 2 3 stable in this system, they must show up in the XRD diagrams. Only one new compound was found during all the experiments. The second assumption is that the unknown quasi-ternary phase is stable at 1173 K and no CS evaporates from it. 2° As can be seen in ftgure 5.1, all end compositions lie on two straight lines

ending in CaO and Al 3' This is an indication that the assumption is correct. 2°

60 mole% A1 2 03 \ !

80 60 40 20 CaO l'- N co A1 0

Figure 5.1. Representation of some illustrative experiments. • starting compositions (CsCO3-CaCO3-Al 3 mixtures) 2° o CS 0·Al 0 , Al 0 and Cs 0·2CaO·4Al 0 (samples heated 2 2 3 2 3 2 2 3 20 hours, weights had not stabilized yet)

f:::, CaO and Cs 0·2CaO·4Al 0 2 2 3 o Al 0 and Csp.2CaO·4Alp3 2 3 x pure Cs 0.2CaO·4Al 0 2 2 3 One should bear in mind that the end compositions are calculated and could differ a little from reality.

61 Thennogravimetric analyses afterwards showed that indeed no Cs 0 evaporates from 2 the unknown compound. As already described elswhere (chapter 4) the composition can be calculated during the experiments by measuring the weight before and after the heat treatment. The accuracy is good enough for our purpose but one has to bear in mind that the real composition might differ slightly. The composition of the quasi-ternary phase must therefore be checked by chemical analyses.

5.2 A new quasi-ternary compound: Cs 0.2CaO·4AI 0 2 2 3

The new method was used to establish the composition of the unknown compound. Once the composition of the compound was roughly located, we heated mixtures 19 to 27 (see figure 5.1) at 1173 K until their weights had stabilized. The diffraction pattern of mixture 23 showed only peaks of the unknown, white compound. Chemical analyses of the sample resulted in a somewhat higher Cs 0 2 content than calculated (34 wt% Cs, 10.2 wt% Ca and 28.6 wt% AI). This corresponds best to Cs O·2CaO·4Al 0 . The diffraction pattern was measured more 2 2 3 accurately using a Guinier camera and from these data the cell parameters were determined (see table 5.1). Cs O·2CaO·4Al 0 was found to be monoclinic with 223 a = 8.2197 A, b = 5.2782 A, c = 7.7639 A, ~ = 93.487° and the spacegroup P2! or P21/m. Thennogravimetric analysis showed no weight loss up to 1403 K (see figure 5.2). At this temperature Cs 0 starts to evaporate very slowly until no Cs 0 is 2 2 present anymore at 1773 K. When Cs O·2CaO·4AI 0 is left in humid air, it picks 2 2 3 up water and loses the crystallinity at the same time. Therefore it is essential that the determination of the phase diagram is conducted in a water free environment. In the Na 0-CaO-Al 0 system, two quasi-ternary compounds are known (1-4) but 2 2 3 a comparable compound Na O·2Ca0-4Al 0 is not found. Also the crystal types are 2 2 3 different from monoclinic and hence no similarity is present between the two systems with regard to the quasi-ternary compounds.

62 Table 5.1 X-ray powder diffraction data of Cs O·2CaO·4Al 0 223

Symmetry monoclinic a = 8.2197 A c = 7.7639 A Spacegroup P2l or Pl2/m b = 5.2782 A ~ = 93.4870

d (A) dObs(A) IIIl HKL deal/A) obs IIIl HKL dealc(A)

8.204 1.3 100 8.205 1.8985 2.5 320,222 1.8991 7.747 20 001 7.750 1.8811 5 4 1 I 1.8812 5.476 14 101 5.470 1.8618 8 321 1.8628 4.438 35 1 1 0 4.439 1.8186 6 014,313- 1.8189 4.362 20 011 4.363 1.7963 2.0 1 r4 1.7974 4.103 8 200 4.102 1.7841 8 123 1.7845 3.908 50 111 3.908 1.7689 1.0 402 1.7689 3.875 35 002 3.875 1.7541 3 412,114 1.7545 3.803 2.5 111 3.798 1.7336 2.5 322 1.7345 3.723 14 201 3.720 1.7232 6 313 1.7234 3.588 5 102 3.589 1.7205 6 130 1.7203 3.4251 45 102 3.4242 1.7117 8 223,204 1.7118 3.2391 50 210 3.2390 1.6838 6 131 1.6840 3.1216 25 012 3.1235 1.6754 1.6 131 1.6749 3.0404 45 211 3.0409 1.6562 2.0 403,223 1.6574 2.9678 70 1 1 2 2.9679 1.6192 12 420 1.6195 2.9391 50 211 2.9387 1.6168 12 230 1.6170 2.9063 8 202 2.9065 1.6012 6 421,032 1.6014 2.8728 14 112 2.8726 1.5901 6 231 1.5905 2.7349 6 202,300 2.7350 1.5797 8 132,413 1.5800 2.6396 100 020 2.6391 1.5753 10 231 1.5753 2.5311 3 301 2.5311 1.5670 2.0 510 1.5669 2.5113 10 120 2.5123 1.5617 2.5 024,323+ 1.5618 2.4981 25 021 2.4982 1.5565 4 314 1.5559 2.4233 8 103 2.4221 1.5484 3 124 1.5482 2.3773 8 1 2 1 2.3769 1.5401 1.0 105 1.5401 2.3200 12 013 2.3202 1.5206 2.0 124,422 1.5205 2.2819 1.3 311 2.2823 1.4842 2.5 224 1.4839 2.2651 5 113 2.2653 1.4789 5 502,115+ 1.4788 2.2489 6 203 2.2484 1.4694 1.6 422 1.4693 2.2195 12 220 2.2195 1.4540 2.5 033,404 1.4537 2.2003 2.0 113 2.2014 1.4485 4 115 1.4485 2.1811 20 022 2.1812 1.4405 3 133 1.4403 2.1519 2.5 221 2.1525 1.4244 2.5 512,503+ 1.4245 2.1282 1.0 203 2.1283 1.3676 2.0 404,600+ 1.3675 2.0902 12 122 2.0903 1.3403 2.0 315 1.3397 2.0683 2.5 213 2.0686 1.3305 2.5 125 1.3302 2.0511 8 400 2.0511 1.3248 2.0 431 1.3248 2.0093 2.5 312 2.0093 1.3200 4 040 1.3196 1.9737 2.5 213 1.9739 1.3014 4 041 1.3008 1.9537 8 401,22"2 1.9538 1.2910 2.0 225,504+ 1.2912 1.9370 10 004,303 1.9376 1.2785 3 134,432 1.2783 1.9117 6 104,410 1.9118 1.2660 2.0 333,602 1.2658

63 110

100 I------ol--.+++++ +

90 f- +

80 f- ~ ~ ~ +

70 f- + +

60 f-

50 -

40 IIIIIIIIIIIIII 400 600 800 1000 1200 1400 1600 Temperature (K)

Figure 5.2. Thennogravimetric plot of Cs 0·2CaO·4Al 0 • Heating rate is 5 Klmin. Sample is 2 2 3 heated from 293 to 1123 K and held 10 hours at that temperature. Then it is heated to 1173 K, held for ten hours etc. The marks are the weights after 10 hours at that temperature.

64 5.3 Subsolidus phase relations

The experiments discussed so far produced information about the Cs 0-rich part 2 of the phase diagram. In order to get information about the

CaO-Al2° 3-Cs 20·2CaO·4Al 2° 3 triangle, we set up an other type of experiment. All known calcium aluminates were synthesized in pure form by high temperature syntheses. Samples of these compounds were placed in an alumina crucible also containing CS2CO3 and heated for 60 hours at a temperature of 1173 K. 3CaO·Al 0 , 7CaO·12Alp3 and CaO·Al 0 react with a large excess Cs 0 (50 % 2 3 2 3 2 Cs20 in overall composition) forming CaO and Cs20·2CaO·4Al20 3. When CaO.2Al 0 , on the other hand, reacts with a large excess Cs 0, Cs 0·2Ca0-4Al 0 2 3 2 2 2 3 is the only phase detected by XRD. The reactions of the calcium aluminates with smaller amounts Cs 0 can be written by (CaO = C, Cs 0 = Cs, Al 0 = A): 2 2 2 3

C A + 1% Cs -----7 C + CA + CsC A (5.2) 3 3 2 4

CA + 1% Cs -----7 CsC A + CA + CA (5.3) 12 7 2 4 12 7 3

CA + 5% Cs -----7 CsC A + CA + CA (5.4) 2 4 12 7

CA + 5% Cs -----7 CA + CsC A (5.4) 2 2 2 4

All results are in good agreement with the diagram proposed by us in figure

5.3. The dashed lines between Cs2O·Al23° and Cs20·2CaO·4Al23° are used to indicate that, although Cs 0·Al is under the experimental conditions not 2 2° 3 stable at 1173 K, CsP·AlP3 and Cs 0·2CaO·4Al 0 were found together at the 2 2 3 beginning of the experiments. Cs2° will evaporate out of Cs20·Al 2° 3 when heated for longer times in an open system, leaving Al 0 and evaporated Cs 0. 2 3 2 In the Al2°3-rich part of the diagram, some problems arose. The results of the CaO·6Al 0 + Cs C0 experiment could not be understood on the basis of the phase 2 3 2 3 diagram we constructed. When CaO·6Al 0 was heated together with Cs C0 ' we 2 3 2 3 found that an amorphous material had been formed. This experiment neither confmns nor contradicts the proposed diagram. In order to verify the stability of CaO·6Al 0 , this compound was heated for 1000 hours at 1173 K. No signs of 2 3 decomposition were found. On the other hand CaO·6AI 0 could not be formed within 2 3 1000 hours at 1173 K from a CaCO -Al mixture. Therefore, no conclusion about 3 2° 3 the stability of CaO·6Al 0 at 1173 K could be drawn from these experiments. This 2 3 part of the diagram is not completely understood and should therefore be studied more closely.

65 CaO A1 2 0 3 u

Figure 5.3. Phase diagram at 1173 K in the CS 0-CaO-AI 0 system. The shaded area is not 2 2 3 filled in because it is not completely clarified. C =CaO, Cs =Cs 0 and A =AI 0 ; CsC A =Cs 0·2Ca0-4AI 0 2 23 24 2 23

66 5.4 The role of caesium-B-alumina

In all the experiments done to establish the phase diagram, we never found lines in the XRD spectra that could be attributed to Cs20·llAlP3 (Cs-B-alumina) as reported by Langlet (6) or Semenov et al. (7). As already pointed out in chapter 3, Cs-B-alumina is unlikely to exist but to make sure we did not miss this compound we tried to synthesize it in two different ways. The fIrst is by mixing Cs CO and Al or Al(OH) with Cs/Al ratios varying Z 3 Z°3 3 between 5:1 to 1:5. These mixtures were heated for 5 to 360 hours at temperatures K. between 1073 K and 1373 The only phases found in these experiments were Alz0 3 and CSZO·AlP3' A second manner to synthesize B-alumina is by ion exchange like done in chapter 7 to make K- and Ca-B-alumina from Na-B-alumina. For this purpose Na-B-alumina was immersed in a ten-fold excess CsCl melt for four periods of one hour at 1173 K. It was possible to exchange the sodium partly for caesium but the maximum amount of caesium was already reached after one . The next treatments did not add any more caesium. These results are in agreement with the results of Yao and Kummer (8). Our conclusion from these experiments is that pure Cs-B-alumina (CszO.11Alz0 3) is not stable and therefore the phase diagram as proposed by us in fIgure 5.3 without this compound is correct.

5.5 Discussion

All the experimental results we obtained during this study are in good agreement with the phase diagram proposed by us. In spite of this, we should also make some critical remarks. In the Alz°3-rich part of the diagram, two compounds are reported in literature which we did not fmd during our own investigations. Several attempts were made to make Cs O·llAl and all of these failed. We z z°3 also never found it during our investigations of the quasi-ternary phase diagram. In our opinion this compound does not exist. The problem of the formation of CaO·6Al 0 is discussed in chapter 3. It was Z 3 never reported to be formed at 1173 K but it is believed to be stable at this temperature. It is known that CaO segregates to the grain boundaries of polycristalline Al (5) probably leading to the formation of CaO·6Al This z°3 Z°. 3

67 is the reason why it is important to know the behaviour of this compound when it is exposed to Cs 0. For this reason we did not want to leave CaO·6Al 3 out of 2 2° the diagram, although we do not know the exact phase relations. During the experiments, CS ° evaporates continuous. Because of this one can 2 argue that equilibrium is never reached and therefore the diagram is a compatibility diagram instead of a phase diagram.

As mentioned above, CaO segregates to the grain boundaries of Al 0 - As shown Z 3 in the experiments, all the calcium aluminates react with CS ° to form 2 Cs 0-2CaO·4Al 0 • This means that even without knowing in which form (phase) CaO 2 2 3 is present on the grain boundaries, we can predict from the diagram proposed by us that the grain boundaries will be corroded by Cs 0. This is due to the 2 stabilisation of the corrosion product by CaO in comparison to the pure Cs 0-Al 0 interaction were the only compound that can be formed is Cs 0-Al 0 • 2 2 3 2 2 3 The result of the increasing stability of the corrosion product is an increasing driving force for the corrosion reaction. This thermodynamic argument is the only reason why CaO enhances the Cs 0 corrosion of Al 0 • Kinetic effects because of 2 2 3 the resemblance of CaO·Al 3 and the corrosion product as can be used in the case 2° of sodium (2) do not hold in the caesium case. CaO·6Al 0 is very similar to 2 3 Na-B-alumina and therefore is thought to act as a nucleation centre for Na-B-alumina and by this way to enhance the reaction between sodium and alumina. Cs-B-alumina, however, was never found during our experiments and we therefore think that this compound does not exist. From the results in this chapter we may conclude that it is better to avoid CaO as a sinter aid for alumina, when this is used in caesium containing atmospheres.

Acknowledgement The author gratefully acknowledge E. J. Sonneveld, Technisch Physische Dienst, PO Box 155, Delft, The Netherlands, for determining the cell parameters of

Cs 0.2CaO·4Al 0 • 2 2 3

68 Literature

1. L. T. Brownmiller and R. H. Bogue, J. Res. Natl. Bur. Stand. (U.S.), ~ (1932) 289-307. 2. J. D. Hodge, Proc. Electrochem. Soc., 85 (1985) 261-270. 3. G. 1. Zaldat, M. S. Kupriyanova and A. L. Tubolev, Izv. Akad. Nauk. SSSR, Neorg. Mater., 21 (1985) 51-53. 4. H. Verwey and C. M. P. M. Saris, J. Am. Ceram. Soc., 69 (1986) 94-98. 5. P.E.C. Franken and A.P. Gehring, J. Mater. Sci., 16 (1981) 384-388. 6. G. Langlet, Centre d'Edutes Nucleaires de Saclay, Rapport CEA-R-3853 (1969). 7. N. N. Semenov, A. G. Merkulov and 1. A. Vorsina, Ser. Khim. Nauk., 1 (1967) 75-83. 8. Y. Y. Yao and J. T. Kummer, J. Inorg. Nucl. Chern., 29 (1967) 2453-2475.

69 CHAPTER 6

PHASE RELATIONS IN THE

CaO-IS,0-A120 3 SYSTEM 6 PHASE RELATIONS IN THE CaO.K 0.AI 0 SYSTEM 2 2 3

6.1 Experimental

The method used to detennine this phase diagram is fully described in chapter 4. No serious deviations from the outlined method were needed in this system; some small modifications are described in the concerning part. The temperature used was 1373 K and heating times varied between 50 and 600 hours, depending on the composition. Thermogravimetric analyses (figure 6.1) showed complete dissociation of the carbonates at 1240 K. In the range from 1240 K to 1400 K, including a period of 10 hours at 1340 K, the weight was constant and hence no K 0 was lost. 2 Of all the samples for which the K 0 content was critical, the K 0 losses 2 2 could be minimized by a heat treatment of 20 hours at 1173 K before the [mal heating at 1373 K, to make sure the sample did not contain free K 0 anymore. 2 The phase diagram is partly checked by reactions of potassium with CaO· Al 0 3 2 and CaO·6A1 0 • These experiments were conducted in molybdenum sealed crucibles 2 3 as is described in detail in chapter 10.

6.2 Results

In all samples were B-alumina was found, B"-alumina was also present. This means that four compounds were sometimes observed. This is in conflict with the phase rule. As discussed in chapter 3, the stability and the composition range of 13/13"-alumina are not yet clear. No evidence for the stability/metastability of the B-aluminas was found during our own experiments. Indications for the exact composition range were also not found and therefore these compounds are treated as one phase with the composition range between the two empirical formulas K O·5.3A1 0 and K Q.llA1 0 . 2 23 2 23

6.2.1 Subsolidus phase relations The results of the experiments are given in the form of a phase diagram in figure 6.2. The reactions of potassium with CaO·A1 0 and CaO·6A1 0 can be 2 3 2 3 described by:

72 110

100

90

80 .....,~

~ I I I L- 70 I I I ,--, I 60 •

50

40 400 600 800 1000 1200 1400 1600 Temperature (K)

Figure 6.1 Thermogravimetric plot of a 1:1:8 mixture of CaC0 IK CO/A1(OH)3' Heating rate is 3 2 5 K/min. Sample is heated to 1140 K and held for a period of 10 hours. These periods are repeated every 100 K up to 1640 K.

73 ------7 ss( 3!K CaO·Alz°3+K CaO·Alz°3+ 12CaO·7Alz° Z0) + 2K O·3CaO·SAl 0 + Al (6.1) z Z 3

CaO·6Al 0 + K ------7 CaO·6Al + CaO·2Al + 6-alumina + Al (6.2) Z 3 z°3 z°3

These reactions therefore are in agreement with the phase diagram. In the CaO-Kz° rich part some results were obtained which could not be fitted in the phase diagram as proposed. Repeated experiments and longer heating times in both open alumina crucibles and sealed molybdenum capsules fmally led to the phase diagram as proposed. In paragraph 6.3 we will account for the different results in some of these experiments. The 12CaO·7Al 0 compound shows a small solubility for KzO. Brownmiller (1) z 3 reported in his study of the "lime--alumina" system a solid solution of SCaO·3Al and K 0. Since SCaO·3Al was later reported to be metastable or z°3 z Z° 3 appeared to be actually 12CaO·7Al (2,3), we think Brownmiller meant the same z°3 solid solution. We did not measure the maximum amount ofKz°in 12CaO·7Alz°3but assumed the measurements of Brownmiller were good at this point (about 2%). 12CaO·7Al 0 withoutKzO is cubic with a cell parameter a = 11.982 (2,3). We z 3 A found that with increasing amount of K the cell parameter is increased to z° a = 12.0S A according to the XRD powder pattern. The solid solution with the • maximum amount of Kz° (with the largest cell) is in equilibrium with 2Kp·3CaO·SAl 0 , 3CaO·Alp3 and CaO·Alp3' Z 3

6.2.2 2K20.3CaO·SAl20 3: a new quasi-ternary compound In almost all mixtures that were heated, unidentified peaks were found in the XRD powder pattern. By looking at the intensities of these peaks, we could roughly locate the unknown compound in the phase diagram. By making several mixtures in that region we were able to synthesize the new compound in the pure form. Its formula appeared to be 2K 0·3CaO·SAl 0. The most intense lines of its z z 3 powder diffraction pattern are given in table 6.1. Unfortunately, up till now we were not able to determine the crystal symmetry and the lattice parameters. In the Na O-CaO-Al 0 system, two quasi-ternary compounds are known. One of z Z 3 these used to be denoted as 2Nap·3CaO·SAlp3 (4-6) but Verwey and Saris (7) found the right formula should be Na O·CaO·2Al 0 . The reason why this was not z z 3 recognized for a long time is the overlapping of X-ray reflections of "2Na O-3CaO·SAl 0 " and Na O·Al 0 and the solid solution formed between these z Z 3 Z Z 3 compounds. A closer look at the diffraction patterns of K O·Al 0 (8-10) and Z Z 3

74 1~--:\:~?1\······-?;;fY'X,·~~~~;A KA 5 .~z .,(/. /- ~~.;yc:::.. /...... KA 11

CaO A1 2 0 3 u

Figure 6.2 Phase diagram at 1373 K of the ISO-CaO-A1 0 system. 2 3 K ISO, C CaO and A Al 0 : K C A 2K 0·3CaO·5Al 0 = = = 2 3 2 3 5 = 2 2 3

75 2Kp·3CaO·5Al 0 (table 6.1) does not indicate any overlap and does not give any 2 3 reason to doubt about the interpretation of the results. Comparison of the X-ray diffraction patterns of 2K O·3CaO·5Al and Na O·CaO·2Al does not show any 2 23° 2 23° resemblance and therefore an other explanation must be sought to explain for the problems we have with the determination of the cell parameters. The fact that 2K 0·3CaO·5Al 0 is hygroscopic may play a role in this respect. 2 2 3 Thermogravimetric analyses showed that water could be picked up from humid air. X-ray diffraction analyses of samples that were left outside the dry atmosphere of the glovebox for a few hours to several days showed a slowly changing crystal structure. In figure 6.3 a typical diffraction pattern of a "wet sample" is compared with a freshly prepared sample. High temperature x-ray diffraction of the wet sample showed a slow change of the pattern at higher temperatures. Back at room temperature, after a period of several hours at 1373 K, the pattern of the "dry form" remained. The difficulties we had with characterizing this compound with X-ray diffraction techniques is probably due to this phenomenon of water pick up in combination with a change of the crystal structure. We were not able yet to solve this problem.

Table 6.1 X-ray powder diffraction data of 2~O'3CaO'~03' Only the strongest lines are given.

dobs(A) 1/1 1 dobs(A) 1/1 1 dobs(A) 1/1 1

8.08 60 3.044 80 2.012 10 7.715 100 2.958 95 1.956 10 5.913 5 2.893 30 1.919 15 5.214 5 2.836 65 1.876 15 4.420 15 2.604 75 1.849 10 4.275 10 2.578 50 1.823 10 3.903 10 2.397 20 1.685 5 3.680 15 2.263 15 1.651 5 3.580 15 2.208 10 1.543 20 3.471 20 2.188 10 1.498 10 3.292 35

76 a 100 90 80 70 60 50 40 30 20 10 f- a I I,JLuJII 10 14 18 22 26 30 34 38 2-Theta

b 100 90 80 70 60 50 40 l- 30 f- 20 10 a I III I I I I I 10 14 18 22 26 30 34 38

2-Theta

Figure 6.3 Comparison of the X-ray diffraction patterns (CuKa) of 2CaO·3Kp.SAl 0 2 3 a. freshly prepared and b. after several days in humid air.

77 6.2.3 The CaO·Al20TK20·Al203-Al203 composition area The equilibria in this part were ftrst established in the usual way, e.g. mixing and heating at 1373 K. Under these conditions, however, CaO·6Al 0 was 2 3 never fonned within the experimental times. From this, one might conclude that there is a three phase equilibrium between CaO·2Al 0 , Al 0 and K 0·11Al 0 . 2 3 2 3 2 2 3 However, at this temperature, CaO·6Al 0 is believed to be stable and the fact 2 3 that it is never fonned at this temperature is supposed to be a kinetic effect. The experiments were therefore repeated in a modifted way. The mixtures were ftrst heated for 50 hours at 1673 K, which was sufftcient to reach equilibrium. This temperature is high enough to fonn the CaO·6Al 0 phase. After this frrst 2 3 heat treatment, the pellets were reheated at 1373 K for several periods with a total time of 500 hours until equilibrium had been reached again. The development of the phases was followed using XRD. The phase equilibria found in this way were checked once more by heating mixtures of CaO.2Al 0 , CaO·6Al 0 and K C0 or 2 3 2 3 2 3 CaO·6Al 0 , Al 0 and K C0 . All results lead to the phase diagram proposed by us 2 3 2 3 2 3 in ftgure 6.2. As discussed in chapter 3 it is not clear whether K20·5Al203-K20·11Al203 is one solid solution or whether these are two phases. Here we consider it as one phase. According to the phase diagram, CaO·6Al 0 is in equilibrium with this 2 3 phase. This part of the diagram has been subject to a closer look of which the results are described in chapter 7. The results of this more detailed investigation confrrm the phase diagram at this point. The way the phase diagram is drawn suggests CaO·2Al 0 is in equilibrium with 2 3 a very small composition range of the 6-alumina composition range. In fact, this composition range might be slightly extended, because we were not able to determine this experimentally. CaO.6AI 0 is, however, in equilibrium with the 2 3 largest part of the composition range, as is drawn in the phase diagram.

6.3 Thermodynamic evaluation of the phase diagram

As already mentioned earlier, some results obtained were not in agreement with the phase diagram as proposed by us in ftgure 6.2. In this paragraph we will give a thennodynamic evaluation of the phase diagram which will account for these disagreements in the experimental results and the same time will provide estimations of some thennodynamic values. Throughout this calculation we will treat the pure oxides K 0, CaO and Al 0 2 2 3

78 as if they were elements. In this way we can deal with this system as a ternary system and hence the calculations are simplified without loosing much of their value. For clarity, the symbols used for the oxides are also simplified as K = K 0, C = CaO and A = Al 0 .A phase diagram in this notation is given in 2 2 3 figure 6.4. All thermodynamic data were taken from references 11-14. No value for the Gibbs energy of formation of K 0 at 1373 K is known because at this temperature 2 it is not stable and decomposes. For this reason we extrapolate the values from lower temperatures to 1373 K. This can be done because in all compounds potassium is indeed present as K and hence we can use an estimated value which K would 2° 2° have had if it was stable at this temperature. The error we make by using this extrapolated value, however, does not effect the results seriously anyway. The standard Gibbs free energy of formation of K-6-alumina is reported several times (15-18) but the disagreement in the values is large. For our calculations, however, it is not necessary to use it and therefore we do not take it into account. Thermodynamic data of KA ' K C A and of the C AfKP solid solution S 2 3 s 12 (C 2A K ) are not known and are, therefore, variables in our calculations. 1 703 . 9 With the phase diagram given in figure 6.4 as a basis, we attacked the problem in two ways. In the first approach we calculated the changes in Gibbs energy of a series of possible reactions. For example

(6.3)

From the phase diagram we know that the change in Gibbs energy !l3GO of this reaction must be negative:

O o o o o !lG 3 !lG + 2 !lG - !lG - 8 !lG < 0 (6.4) 3 = fCA fKAS fK2C3AS fA

By doing this for all possible reactions we are able to create a frrst series of inequalities with as variables the Gibbs energy of formation of KCA, KA 2 3 S S and the C solid solution. 12AfKP A second series of inequalities can be obtained as follows. The Gibbs energies of formation of the calcium aluminates are all known from literature. We can therefore calculate the activities of CaO (a) and Al (a) in the phase c' 2° 3 A triangles a to f by reactions similar to reaction 6.5:

79 K

KA 5

KA ll a d e N co <:C

Figure 6.4 Schematic of the K O-CaO-AI 0 system. The small letters are used to 2 2 3 indicate the various areas referred to in paragraph 6.3.

80 CA + A r-- CA (6.5) ~ 2

In compositions that lie between CA and CA the activities of these compounds 2 are equal to one and therefore:

~ GO = -RTln (6.6) s

From this equation we can calculate, in area d from the phase diagram in figure 6.4, the activity of A (ad). On the other hand, in the triangle CA, KCA A 2 3 S and KAs (j) the activities of these compounds are equal to one and hence with the reaction 6.3 this to

(6.7)

With this equation we have a relation between the change in Gibbs energy of reaction 6.3 (~PO) and the activity of A in triangle j (a~). We know that the activity of A is getting lower when going from pure A (triangle f) to the other ends of the phase diagram (the triangles a and 1). From the same phase diagram we can determine the order of the activities in the areas d, j, i, and c to be a~ > a~ > a~ > a:. In this way we can enclose the Gibbs energy of reaction 6.3 between a maximum and minimum value. The same can also be done for the activities of K 0 and CaO. Working through the phase diagram in this way gives a second set 2 of inequalities with the same variables as in the first series of inequalities. Together with the first set we have 72 inequalities with 3 variables. With this it is possible to calculate ranges in which the values of the Gibbs energies of K C A ' KA and the C12AjK20 solid solution should fit in. 2 3 s s The results of these calculations are a set of Gibbs energies of formation for K C A ' KA and the C12AjK20 solid solution which are consistent with the values 2 3 s s taken from literature for the other compounds and with the phase diagram proposed by us in figure 6.2. The values found are listed in table 6.2, together with the values used in the calculations. The accuracy of the calculated Gibbs energies is not as good as suggested by the precision of the calculated values given in table 6.2. If, however, we take for example for the Gibbs energy of formation of KCA -13539 kJ/mole instead of 2 3 s -13531 kJ/mole (a difference of less than 0.06 % !) we can not create consistency

81 Table 6.2 Standard Gibbs energies of formation at 1373 K of the compounds in the system

CaO-K2O-Al2°3'

compound l!.pO (kJ/mole) l!.po (kJ/gramatom)

Al 0 -1858.5 -371.7 2 3 CaO·6Alp3 -11950.0 -373.4 CaO·2Alp3 -4510.8 -375.9 CaO·Alp3 -2641.8 -377.4 12CaO·7Alp3 -22233.8 -376.8 From literature 3CaO·Al 0 -4134.2 -375.8 (11-14) 2 3 CaO -742.2 -371.1 K 0·AlP3 -2794.9 -349.4 2 K 0 -602 -200.7 estimated value 2 Kp·5AlP3 -10334 -369.0 3CaO·2Kp·5Al 0 -13531 -365.7 Results of 2 3 }- 12CaO·7Al 3·0.39K 0-22594 -375.5 calculations 2° 2

with the other values and with a possible phase diagram. This does not mean that the real value is within 0.06 % of the value calculated because the values used for the calculation are also much less accurate. The fact that we needed such a high precision to get consistency is, however, of great importance. The best way to understand this is to build a three dimensional model of the phase diagram. In this model, the third dimension is the standard Gibbs energy of formation per of all compounds. From this model it can be seen that only small changes in the unknown Gibbs energy of formation of KCA are allowed because otherwise 2 3 S either this compound is not stable or an other compound becomes unstable. This is caused by the fact that differences in the Gibbs energies of formation per atom are very small (see table 6.2). The consequence of this is that the energy profit of reactions involved in reaching equilibrium in certain composition areas is very low and, accordingly, the driving force of the reaction is very low. This is the explanation for the disagreements we found during the experiments. When we have for example a mixture

82 of KA and CAK ,they should react according to reaction 6.8: 12 7 0.39

2.22 KA + C AK (6.8) 12 7 0.39

The result is only a very small energy gain of -0.946 kJ. Because of this, only small disturbances like tiny amounts of contaminations, surface and temperature effects are sufficient to make other equilibria possible. This is illustrated in table 6.3, in which for each triangle the activities of A1 0 are given. Especially the values in the triangles a, g and h are very close 2 3 together and therefore problems in these triangles can be expected. We assumed in the calculations a maximium solubility of K 0 in C A of 2 % 2 12 7 which corresponds to the formula CAK. Deviation of this maximum (super 12 7 0.39 saturation) will also have great influence on the calculations and on the equilibria.

Table 6.3

The activity of AlP3 (a), CaO (ad and (a ) in the three phase areas of Kp K the phase diagram as given in figure 6.4. Activities given by - can not be calculated with the data from table 6.2.

triangle a a a A c K 11 a 0.0149 1.000 2.3 x 10- b 0.0310 0.784 c 0.0495 0.596 15 d 0.407 0.0725 3.6 x 10- 17 e 0.905 0.0146 6.7 x 10- f 1.000 0.0081 11 g 0.0222 0.876 1.5 x 10- 11 h 0.0210 0.892 1.7 x 10- 12 0.0538 0.549 3.3 x 10- 12 j 0.123 0.240 1.4 x 10- 12 k 0.104 0.180 3.4 x 10- 13 1 4.2 x 10- 1.000 1.000

83 6.4 Discussion

As was the case in the Cs223O-CaO-Al ° system, problems arose when we tried to establish the phase relations in the area close to CaO·6Al 0 . In the 2 3 K O-CaO-Al 03 system we were able to solve this problem because of two 2 2 reasons. The fIrst reason is that the mixtures could be heated to higher temperatures without complete loss of K20. By heating the mixtures at 1673 K, CaO·6Al2°3could be formed in the K2°containing mixtures where it should be formed according to the phase diagram. With a second heat treatment at 1373 K for over 500 hours we were able to reach equilibrium and to determine the phase relations at this temperature. The second and most important reason is found in the difference between the Cs 0-Al 0 and K 0-Al 0 systems. No B-alumina compound was found in the 2 2 3 2 2 3 Cs 0-Al 0 system. In the K 0-Al 0 system, however, this compound is known and 2 2 3 2 2 3 was indeed found during our investigations. Because of the similarity of CaO·6Alp3 and K-B-alumina we were able to form CaO·6Alp3 in equilibrium with Kp·11AlP3 at 1373 K, starting with a mixed CaIK-B-alumina. This is described in detail in chapter 7. TheformationofCaO·6Al20 3 andK20·11AlP3 at 1173 Kproves that the phase relations found in this chapter and given in the phase diagram of fIgure 6.2 are correct. From the phase diagram it can be seen that CaO·6Al 0 and 2 3 K-6-alumina can coexist and do not form an extended solid solution. When we look closer at the K O-CaO-Al phase diagram with regard to the 2 2° 3 potassium resistance of Al 0 , it is not clear whether CaO will have a negative 2 3 effect. Although a stable quasi ternary compound was found in this system, formation of this compound will not occur from the CaO·6Al 0 phase, supposedly 2 3 present at the grain boundaries (19,20), unless the K 0 fraction exceeds 2 15 mole%. This means that the ternary compound only plays a role when the corrosion reaction is in progress and when the damage is already done. The close similarity of CaO·6Al 0 and K-B-alumina, however, might have a negative effect 2 3 on the corrosion resistance of Al , This compound could act as a nucleation 2°3 centre and therefore might promote the formation of 6-alumina and with this the corrosion process of Al 0 as was found for the sodium corrosion of High Pressure 2 3 Sodium (HPS) lamp envelopes (5,21). Investigations on the kinetics of the K-B-alumina formation could result in more clarity about this.

84 List of symbols

-1 -1 R : gas constant = 8.314 J mole K ~p? : standard Gibbs energy of formation of compound i. vi : number of moles of compound i taking part in a reaction. This has a

negative sign if the amount of compound i is less after the reaction. ~po : the standard Gibbs energy of a reaction, defmed by ~po =Tvi ~p? ~ : activity of compound i in the three phase area marked j. A: AlO 2 3 K: KO 2 C: CaO

85 Literature

1. L. T. Brownmiller, Am. J. of Sci., 29 (1935) 260-277. 2. R. W. Nurse, J. H. Welch and A. J. Majumdar, Trans. J. Br. Ceram. Soc., 64 (1965) 409-418. 3. H. Bartl and T. Scheller, Neues Jahrb. Mineral. Abh., 35 (1970) 547-552. 4. L. T. Brownmiller and R. H. Bogue, J. Res. Nat!. Stand. (U.S.), ~ (1932) 289-307. 5. J. D. Hodge, Proc. Electrochem. Soc., 85 (1985) 261-270. 6. G. I. Zaldat, M. S. Kupriyanova and A. L. Tubolev, Izv. Akad. Nauk.. SSSR, Neorg. Mater., 21 (1985) 51-53. 7. H. Verwey and C. M. P. M. Saris, J. Am. Ceram. Soc., 69 (1986) 94-98. 8. A. Yamaguchi, Yogyo Kyokai Shi, 78 (1970) 74-75. 9. E. I. Brumakin, G. V. Burov, I. G. Rozanov and G. Sh. Shekhtman, Russ. J. Inorg. Chern., 23 (1978) 1868-1870. 10. JCPDS Powder Diffraction File, American Society for Testing and Materials, Philadelphia, # 2-0897 11. I. Barin, O. Knacke and O. Kubaschewski, "Thermochemical Properties of Inorganic Substances" (Springer Verlag, Berlin, 1977). 12. D. R. Stull and H. Prophet, "JANAF Thermochemical Tables", NSRDS-NBS 37, 2nd edition (US Department of Commerce, Washington, 1971). 13. R. P. Beyer, M. J. Ferrante and R. R. Brown, J. Chern. Thermodynam., 12 (1980) 985-991. 14. B. Hallstedt, J. Am. Ceram. Soc., 73 (1990) 15-23. 15. Y. Y. Yao and J. T. Kummer, J. Inorg. Nucl. Chern., 29 (1967) 2453-2475. 16. R. V. Kumar and D. A. R. Kay, Metall. Trans. B, 16B (1985) 295-301. 17. M. Itoh and Z. Kozuka, J. Am. Ceram. Soc., 71 (1988) C36-C39. 18. G. M. Kale and K. T. Jacob, Metall. Trans. B, 20B (1989) 687-691. 19. R. I. Taylor, J. P. Coad and A. E. Hughes, J. Am. Ceram. Soc., 59 (1976) 374-375. 20. P. E. C. Franken and A. P. Gehring, J. Mater. Sci., 16 (1981) 384-388. 21. R. K. Datta and L. N. Grossman, Proc. Electrochem. Soc., 85 (1985) 271-290.

86 CHAPTER 7

PHASE RELATIONS BETWEEN THE COMPOUNDS

K-~-ALUMINA, Ca-~-ALUMINA

AND CALCIUM HEXA ALUMINATE 7 PHASE RELATIONS BETWEEN THE COMPOUNDS K·.B-ALUMINA, Ca-6-ALUMINA AND CALCIUM HEXA ALUMINATE

In the quasi-ternary phase diagram K O-CaO-Al 0 two compounds with 223 compositions close to Al 0 are very much alike: K 0·llAl 0 and CaO·6Al 0 . In 2 3 2 2 3 2 3 the phase diagram as proposed by us in chapter 6 these two compounds do not form a solid solution. Because of their similarity this is an unexpected result. In this chapter, the results of a closer look at these two compounds are described.

7.1 Literature review

7.1.1 The structure of calcium hexa aluminate CaO·6Al 0 is a stoichiometric line compound. Its structure has been refmed 2 3 by Kato and Saalfeld (1) in 1968 and found to be isostructural with the mineral magnetoplumbite. This structure is given in figure 7.1. It is hexagonal with space group P63/mmc (no. 194) with lattice constants a = 5.559 A and c = 21.893 A. One unit cell is built up from two blocks which are isostructural with spinel. One block contains, perpendicular to the c-axis, four oxygen layers which are cubic close packed with the aluminium ions in three crystallographically different octahedral sites and one tetrahedral site. These spinel type blocks are separated by a plane which contains calcium, aluminium and oxygen. The aluminium in this layer is five coordinated by oxygen in a trigonal bipyramid. Recently, Utsunomiya et a1. (2) refmed the structure with special attention to this five coordinated aluminium. They suggest two models for the trigonal bipyramid. In the so called central atom model, the five coordinated aluminium atom is located on the mirror plane of the trigonal bipyramid. In the split atom model the aluminium atom is distributed between two equivalent sites slightly displaced from the mirror plane. They found indications for the split atom model to be correct. Furthermore they suggest that the aluminium atom is not at rest but dynamically jumps between the two sites. The structure as refined by Utsunomiya et al. is in agreement with the structure as refmed by Kato and Saalfeld if a mean position is assumed for the split aluminium atom. The atomic positions given by Kato and Saalfeld are

88 IC ~

Figure 7.1

The crystal strUcture of CaO·6Al20 3" o = Aluminium o = Oxygen @ = Calcium

89 Table 7.1 Distances between atoms in CaO·6Al according to Kato et al. (1) and 2° 3 calculated by us from the atomic positions given by Kato et al.

Atoms Kato et al. recalculated coordination distance (A) distance (A) of aluminium

Al(I)-O(I) 1.995 1.995 octahedral Al(I)-0(2) 1.818 1.818 Al(I)-0(3) 1.970 1.970 Al(I)-0(4) 1.860 1.860 Al(2)-0(1) 1.801 1.801 tetrahedral Al(2)-0(3) 1.828 1.828 Al(3)-0(2) 1.865 1.865 octahedral Al(3)-0(7) 1.896 1.967 Al(4)-0(1) 1.883 1.883 octahedral Al(5)-0(6) 2.187 2.187 pentahedral Al(5)-0(7) 1.839 1.740 Ca-O(6) 2.694 2.694 Ca-O(7) 2.848 2.785

correct. The aluminium-oxygen distances, however, are not calculated correctly. In table 7.1 the values given by them are compared with the values calculated by us. Because the mistakes are all in the distances between 0(7) and other atoms we think Kato and Saalfeld took a wrong position of the 0(7) atom when they calculated the distances. We want to mention here that assuming great similarity between the CaO·6AI 0 2 3 and BaO·6Al 0 structures (3,4) will lead to errors. BaO·6Al 0 is actually a 2 3 2 3 mixture of two phases (5) which were named as Phase I and Phase II (6). The compositions of these phases are not completely clear but can probably described as BaO.7.3Al 0 (7,8) and BaO·4.5Al (9) respectively. Both these phases have a 2 3 2° 3 B-alumina structure or a B-alumina superstructure (9,10). Although calcium and are in the same column of the periodic table of the elements, up till now no indication has been found that a similar phenomenon is true for CaO·6Al 0 and 2 3 therefore the properties of "BaO·6Alp3" and CaO·6Alp3 will differ.

90 7.1.2 The structure and properties of B-alumina In the B-alumina structure an extremely fast ion transport is possible. This high ion conductivity is very useful in places were a solid electrolyte is needed, for example in an electrochemical Na/S battery. For this reason the B-alumina structure has been subject of many studies. Reviews of several aspects of these studies have been published (11-16). We do not have the intention to be complete in this review because that is nearly impossible. Only those aspects of the B-alumina structure that might be important to our subject are discussed here. The name B-alumina was ftrst used by Rankin and Merwin (17) in 1916. They thought it was an isomorph of Al 0 because the presence of sodium was not 2 3 detected. Several years later it was recognized that sodium is necessary for the stability of the structure. With X-ray diffraction experiments Bragg et al. (18) and Beevers et al. (19,20) determined the main features of the structure. They came to the empirical formula Na 0·11Al 0 , for which the crystal structure is 2 2 3 described. This description is by no means complete and many studies have been undertaken to clarify the details of the structure (for example 21-23). Besides , oxides of other elements can react with Al 3 to form 2° B-alumina. Potassium oxide is one of these compounds (24). Because sodium-B-alumina is the best known B-alumina, the discussion is mainly based on knowledge of this compound. Most properties, however, will be very similar for the other B-alumina compounds. The B-alumina structure is hexagonal with the space group P63/mmc (no. 194). The lattice constants are about a = 5.6 A and c = 22.5 A, depending on the cation. The cell is built from two so called spinel type blocks (see ftgure 7.2) in the same way as in the magnetoplumbite structure. These spinel blocks are held together in the B-alumina structure by layers which are very loosely packed with sodium and oxygen. These layers do not contain any aluminium. This is the major difference between B-alumina and the magnetoplumbite type structure of CaO·6Al 0 . It is therefore wrong and confusing to use the name Ca-B-alumina for 2 3 the magnetoplumbite type structure of CaO·6Al as is often done, not only in 2° 3 the older (25) but also in recent publications (26-28). It is regretful that in the JCPDS ftle, which concerns the crystal structure, CaO·6Al °3 is called 2 Ca-B-alumina (28). In this thesis, the name B-alumina is used only for structures similar to that of Na-B-alumina given in ftgure 7.2 and described by Peters et al. (21). The loosely packed layers are responsible for the fast ionic transport

91 IC ~

Figure 7.2 The crystal structure of 6-alumina. o = Aluminium o = Oxygen ~ = Sodium, Potassium and so on.

92 properties of this structure. The possible sites and distribution of sodium and oxygen in this so called conduction layer are given in figure 7.3. Three possible types of sites for the sodium ions are represented. Sodium is distributed between the anti Beever and Ross (a-BR), the Beever and Ross (BR) and the mid oxygen (MO) sites in a way as given in figure 3b (30). In stoichiometric Na 0·llAl 0 , one 2 2 3 sodium is distributed between these sites. In fact, this compound can accommodate considerably more sodium than the empirical formula suggests. To compensate for 3 the excess charge, Frenkel defects of Al + ions are introduced in the structure (31). Two of those aluminium ions from the spinel blocks are moved to interstitial sites on both sides of the conduction layer. Between these aluminium ions an extra interstitial oxygen can be placed. The result is an oxygen between two Frenkel defects which in the Kroger and Vink notation is written as VAl'" Al:"- O~' - Al:" VAl"'. The effective charge of this defect is minus two 1 1 1 and thus compensates for two extra sodium ions. This mechanism is called a Reidinger defect. With these Reidinger defects it appeared to be possible to compensate for a 100 % excess compared to the empirical formula Na 0·1lA1 0 2 2 3 leading to the formula Na O·5.5Al ° (16). On the other hand, Newsam (32) 223 predicted the upper limit of excess cations to be 57% on the basis of ion-ion interaction analyses and therefore it is not completely clear how much cations B-alumina can possibly contain. In the quasi binary Na O-Al system one more compound with a similar 2 2° 3 structure exists. The empirical formula is Na O·5Al ° and this compound is 2 2 3 called B"-alumina. The structure is built up of three spinel blocks instead of two and the ordering of sodium in the conducting layers is also somewhat different. The main features of the structures, however, are the same. The extra 3 sodium is not only balanced by interstitial oxygen but also by Al + vacancies in 2 the spinel blocks. B"-alumina can be stabilized by Li+ or Mg + in these vacancies. In chapter 3 some more details about the relative stability of B- and B"-alumina are given.

7.1.3 Ion exchange properties of B-alumina As described in the previous paragraph, the sodium ions in B-alumina are extremely mobile. This mobility creates the possibility to exchange these sodium ions with ions from a melt. This possibility was already recognized by Toropov et a1. (3,33) in 1939. Since this first article on the exchange reactions of B-alumina, many studies have been conducted on this subject. In this review we concentrate on the possibility of exchanging potassium and calcium and on a

93 aBR aBR a. Oxygen rnO 0 e BR

rnO rnO

aBR aBR

0.05 0.05 b. Oxygen 0.18 0 0.59

0.18 0.18

0.05 0.05

Figure 7.3 Views of the conduction layer of Na-8-alumina along the c-axes. a. The possible sites available for sodium are the Beever-Ross site (BR), the mid-Oxygen position (mO) and the anti Beever-Ross site (aBR). b. The average occupation factor for B-alumina with the formula Na 25 Al 0 2S' 1. 11 17.1

94 possible mixed CaIK-B-alumina phase. Toropov et al. describe in their several exchange reactions of two types, e.g. exchanging divalent ions for monovalent ions and vice versa:

The compositions as given in these reactions were determined by chemical analyses. Note that the mass balance of reaction 7.2 is not right and can only be consistent if the oxygen contents has changed or when A1 0 is also a product of 2 3 the reaction. From the X-ray studies they conclude that there is a "considerable similarity" of the crystalline structures between the products and starting materials. It is not clear, however, what they exactly mean with this statement because magnetoplumbite and B-alumina are also considerably similar. Ion exchange reactions of BaO·6AI 0 as done by Toropov et al. (3) have been 2 3 reinvestigated recently by Iyi et al. (7). They were able to exchange barium with potassium in a ratio 1:2 e.g. for every Bl+ ion two K+ ions were introduced into the structure. As already pointed out, comparison of BaO·6A1 0 and CaO·6A1 0 is 2 3 2 3 very misleading because BaO·6A1 is not a single phase (9) and has the 2° 3 B-alumina type structure in contrast to the magnetoplumbite type structure of CaO·6A1 0 · 2 3 Yao and Kummer (34) conducted an extensive study on the ion exchange properties of B-alumina. The work can be summarized by two main conclusions. The fIrst conclusion is that exchange of monovalent ions is in most cases very fast and complete. For caesium the exchange is incomplete and slow because of the large . As a second conclusion, no exchange of divalent ions was observed. This may be due to unfavourable equilibrium or to a low diffusion rate (too low temperature). On the other hand, the exchange of divalent ions can be complete or nearly complete in magnesia stabilized J3"-alumina (35-37). Seevers et al. (36) found the 2 2 cl+, sl+, Bl+, Cd + and Pb + B"-aluminas to be stable. In their study on the relationship between structure and stability of B-alumina type phases, Petric et al. (38) found decomposition of Ca- and Sr-B"-alumina into the magnetoplumbite type structure after heating at 1873 K. They classifIed the structures of the equilibrium phases according to the valencies and ionic sizes. Elements with lower valencies and large ionic radii have the tendency to form the B- or

95 B"-alumina structure. This is in good agreement with the studies of Stevels and Verstegen (39,40). They studied a series of hexagonal aluminates because of their ability to serve as host lattices for ions with luminescence properties like Eu2+. They mixed Al 3and MC0 (M = Ca, Sr and so on) and heated this mixture at 2° 3 1473 K to 1873 K. In a second series MgO was added in an attempt to stabilize the B-alumina structure. The results of these experiments are given in figure 7.4. As can be seen in this figure they were not able to prepare Ca- or CaMg-B-alumina. There were no indications of structures with alternating B-alumina and magnetoplumbite layers as a sort of intermediate between these two structure types as is found for Na~u2+_ and Na~d3+-aluminates (41). Dyson and Johnson (42) tried to distinguish several B-alumina forms by X-ray diffraction and for this purpose synthesized a series of B-aluminas. In a mixture of Nap, CaO and Al 0 heated at 1373 K to 1673 K, both CaO·6Alp3 and 2 3 sodium-B-alumina occurred. Their conclusion was therefore that no solid solution occurs between CaO·6Al 0 and sodium-B-alumina. The same conclusion was drawn 2 3 from similar experiments by Hodge (28). According to Dyson and Johnson this is in contradiction to what Greene and Bogue (43) found in their phase equilibrium study on the Na 0-CaO-Al 0 -Si0 system. Greene and Bogue, however, did not 2 2 3 2 mention such a solid solution and therefore there seems to be no contradiction between these results. Moroz et al. (25) observed the influence of small amounts of Na2° and CaO on the micro structure of sintered Al 0 . They found with XRD both CaO·6Al 0 and 2 3 2 3 B-alumina but on the micrographs only one phase identified as B-alumina could be found. This phenomenon is explained by mixed layer crystals consisting of "calcium- and sodium-B-aluminas". The 23Na-NMR studies of Roth et al. (44) on the sodium mobility in sintered B-alumina in relation to the sinter additives as CaO revealed a strong decrease of mobility. This is explained by assuming a solid solution of the conductor B-alumina with the insulator CaO·6Al 0. Buechele and De Jonghe (45) came to the 2 3 same conclusion on basis of very small changes in the lattice parameters of B-alumina. From this literature review on the exchange properties of B-alumina it is not clear whether CaO·6Alp3 and Kp·l1AlP3 will form a solid solution at 1373 K or not. A closer look at these two compounds with regard to this question is therefore necessary.

96 IoniC ,adius

0.99 CaAI 0 CaM9xAI12-2/3x019 12 19 Magnetoplumbite

1.12 S,A1 0 12 19 S,MgAI 100 17

1.13 EuAI 0 12 19 EuMgAI100 17

1.20 PbAI 12 °19 PbMgAI100 17 B-alumina 1.33 Ba 1-xAI11017.5-x BaMgAI 10°17

Figure 7.4 A schematic representation of the relation between the ionic radius of metals and the crystal structure of the metal aluminate.

97 7.2 Experimental procedure

The starting materials for these experiments were KCI and CaCl from E. Merck 2 AG (zur analyse) and sodium-l3-alumina from Alfa (99%). The X-Ray diffraction pattern of the l3-alurnina showed a small amount of Al 0 . Wet chemical analyses 2 3 gave a sodium to aluminium ratio which corresponds to Na O·1OA1 3 or 2 2°

NaI.lAl11°17.05 . The small B-alurnina crystals were treated in a KCl-CaCl melt for 15 minutes 2 to one hour at 1173 K. The ratio to B-alumina was 10:1 to make sure an excess of potassium and calcium was present in the system to avoid only partial exchange. The composition of the melt was varied from pure KCI to pure CaCl in 2 steps of 25 mole%. The was washed away afterwards with . With X-ray energy dispersive (EDS) conducted on a Jeol JSM-840 A scanning electron it was checked whether still sodium or chloride was present. If so, the exchange or washing step was not completed and hence was repeated once more. To determine the stability of the phases formed, the products were heated in air at 1173 K and 1373 K for 100 to 600 hours. For comparison, CaO·6AI °3was made by "standard procedure". This means in our 2 case mixing AlP3 (Rubis des synthetique des Alpes) and CaC0 (E Merck AG) in a 3 6:1 composition and heating this at several temperatures to a maximum of 1923 K. The XRD diffraction pattern was identical with that given in the JCPDS file of CaO·6Alp3 (29). Phase formation and transformations were examined by chemical analyses, X-Ray 7 diffraction and 27 Al solid state nuclear magnetic resonance eAl MAS NMR). For the chemical analyses the samples were dissolved in a melt of LiB0 • Mter 2 cooling down, the melt was dissolved in HN0 and the chemical composition 3 examined by atomic absorption spectroscopy (A.A.S.). The routine powder X-ray diffraction measurements were conducted on a Rigaku D/max-IIC configuration. This is a wide angle goniometer equipped with a graphite curved crystal monochromator. To distinguish between the different l3-aluminas, the length of the c-axis is a good parameter to look at. This length was calculated from the 2 0.14, 3 0.12, 0 0.14, 2 0.13 and 2 0.7 reflections with the assumption that the a-axis is constant for all B-alurninas and was taken in the calculations to be 5.595 A. AIO was used as an internal standard. For the 2 3 precise measurements of diffraction patterns a Guinier camera was used of the Johansson type (Enraf-Nonius BV). The intensities were measured with a

98 densitometer. The 27AI MAS (magic angle spinning) N.M.R. spectra were measured on a Broker CXP 300 spectrometer equipped with a 7 T , on a Bruker AM 500 spectrometer with a 11.7 T magnet and on a Broker AM 600 spectrometer with a 14 T magnet. The spectra were recorded with spinning rates ranging between 5 kHz and 15 kHz. Pulse angles of about rc/6 were obtained, using relatively strong r.f. fields ('tTC/2 ~ 2 Ils). The pulse interval time was 1 s. In all cases between 60 and 500 free induction decays (FID's) were sufficient to obtain a fair signal to noise ratio. Chemical shifts (0) were measured relative to an external standard of aqueous AI0 solution (0 ppm). 3

7.3 Results and discussion

7.3.1 Pure KCI melt As expected, complete exchange of potassium and sodium occurred. An exchange time of 15 minutes at 1173 K was sufficient to accomplish this. The cell parameters were determined to be a = 5.592 A and c = 22.70 A, which is in agreement with the values found in literature (7,42,46).

7.3.2 Pure CaCl2 melt: a new Ca-B-alumina Complete exchange of the sodium ions within 30 minutes was observed. Chemical analyses of the reaction product are in agreement with a 2:1 exchange of sodium and calcium according to

Na Al + 0.55 CaCI ------7 Ca AI + 1.1 NaCI (7.3) 1.1 11°17.05 2 0.55 11°17.05

This formula is identical with CaO·lOAI The formula and the exchange 2°. 3 properties suggest a B-alumina type compound. With XRD and N.M.R. measurements this was confirmed. The crystals have a hexagonal symmetry with the lattice parameters a = 5.5902 A and c = 22.455 A. With a Guinier camera both line positions and intensities of the diffraction pattern were measured very accurately. The diffraction data as will be published in the JCPDS powder data file are given in table 7.2. With these data a structure refinement was undertaken in the space group P63/mmc (no. 194). As starting model, the atomic parameters from CaO·6AI 0 2 3

99 Table 7.2 X-ray powder diffraction data of Ca-6-alumina, CaO.5SAll1017.0S.

3 Symmetry Hexagonal a = 5.5902 A V = 607.6 A Space group P63/mmc (194) c = 22.455 A Z=2

d (A) HKL dobs(A) III1 HKL dca1c(A) obs III1 dcalc(A)

11.26 100 002 11.23 1.8240 2 121 1.8238 5.61 50 004 5.61 1.8058 1 122 1.8060 4.84 1 010 4.84 1.7778 1 123 1.7775 4.73 2 o 1 1 4.73 1.7509 1 1 1 10 1.7506 4.44 30 012 4.45 1.6459 6 0210 1.6462 4.06 10 o 1 3 4.07 1.6265 2 o 1 13 1.6269 3.664 5 014 3.666 1.6138 3 030 1.6138 2.960 2 016 2.961 1.6040 3 0014 1.6039 2.807 5 008 2.807 1.5975 1 032 1.5973 2.795 25 1 1 0 2.795 1.5895 30 127 1.5894 2.711 2 112 2.712 1.5605 18 0211 1.5605 2.674 60 o 1 7 2.674 1.5549 3 1 1 12 1.5550 2.502 50 114 2.502 1.5510 8 034 1.5509 2.428 6 018 2.428 1.5329 6 128 1.5329 2.421 6 020 2.421 1.4819 10 036 1.4819 2.407 45 021 2.407 1.4755 3 129 1.4755 2.366 16 022 2.366 1.4187 3 1 2 10 1.4185 2.303 2 023 2.303 1.4059 25 0213 1.4060 2.246 10 0010 2.246 1.3977 75 220 1.3976 2.239 25 116 2.240 1.3871 8 222 1.3868 2.223 12 024 2.223 1.3564 8 224 1.3562 2.131 35 025 2.131 1.3371 20 0214 1.3370 2.033 50 026 2.033 1.3218 2 133 1.3216 1.9322 20 027 1.9322 1.3091 1 226 1.3092 1.8330 10 028 1.8331 1.2384 8 137 1.2386

100 as given by Kato and Saalfeld (1) were used. From this model the aluminium in the layers between the spinel blocks were skipped while only half of the oxygen positions in this layer were occupied. In figure 7.5 a comparison between the calculated intensities of the model, the calculated intensities of CaO·6Alp3 and the measured intensities of the powder X-ray diffraction pattern is made. From this figure it can be seen that the model is a good starting point for a structure refmement. In the further refmement we decided therefore to leave the spinel blocks practically unchanged and to concentrate on the layer between these blocks. However, the possibilities to determine a structure with only powder diffraction data are limited. A few solutions to the problem, with only small differences, are possible. The best atomic position parameters in our view are given in table 7.3.

Table 7.3 The positional parameters of Ca-B-alumina, Ca Al 0 . o.ss II 17.05 Overall thermal parameter is fixed at 0.500.

Atom Multiplicity Number per x z Wyckoff letter unit cell

1 Ca(l) 6h 1.1 0.250 0.750 2 Al(I) 2 a 2 0.000 0.000 3 Al(2) 4 f 4 0.333 0.028 4 Al(3) 4f 4 0.333 0.180 5 Al(4) 12 k 11.16 0.169 0.891 6 Al(5)* 12 k 0.84 0.838 0.175 7 0(1) 4 e 4 0.000 0.148 8 0(2) 4f 4 0.333 0.950 9 0(3) 2 c 1.6 0.333 0.250 10 0(4) 12 k 12 0.155 0.052 11 0(5) 12 k 12 0.504 0.150 12 0(6)* 6h 0.5 0.799 0.250

*Interstitial atoms for excess charge compensation, a so called Reidinger defect.

101 a 100

50

- - ~ a I I I b 100

50

a I I 1111 c 100

50

a I II I I 1,11 5 10 20 30 40 2-theta

Figure 7.5 Comparison of the measured X-ray diffraction pattern of CaO·lOAl 0 (a) with the 2 3 calculated patterns of Ca-13-alumina (b) and CaO·6Al (c). 2° 3

102 Going from Na-B-alumina (Na Al to Ca-B-alumina (Ca .5SAlII0 17 os), 1.1 11°17.0S ) 0 • the number of cations in the conduction layer is halved. We have therefore 0.55 calcium ions for each Beever-Ross site (x = 0.333, y = 0.667 and z = 0.75). Despite of this, the calcium ion is displaced from this position and smeared out over three positions round the Beever-Ross site, similar to what was found for ternary Ca-B"-alumina (47) and Na-B-alumina (30) (figure 3b). The fact that the calcium ion is not exactly positioned at the Beever-Ross site can be explained by the mobility of the ion in the conduction plane. As can be seen in the positional parameters in table 7.3, we introduced so called Reidinger defects to compensate for the excess charge due to the excess calcium in comparison to the ideal B-alumina formula. This improved the fit between the experimental and the calculated intensities slightly. On bases of the powder diffraction data we cannot decide whether this model is indeed better, but because it is an accepted model for B-aluminas (24) we introduce it for Ca-B-alumina also. The most important result of the XRD pattern refmement is that no aluminium is found in the conduction layer on the x = 0.0, y = 0.0 and z = 0.25 position. This is a proof that the structure is of the B-alumina type instead of the magnetoplumbite type. For extra proof of the Ca-B-alumina structure m contrast to the magnetoplumbite type structure, solid state 27Al-N.M.R. spectra were measured. The great difference between the two possible structures is the pentahedrally coordinated aluminium. Magic angle spinning (MAS) N.M.R. techniques at high field have proved to be excellent tools for the determination of coordination states of aluminium in oxygen lattices (48 and refs. herein). Although several times NMR signals for penta coordinated aluminium were reported (49), thus far N.M.R. studies of only one well defmed compound led to the observation of a five coordinated aluminium at 35 ppm (50,51). In CaO·6Al 0 , 8 % of the aluminium 2 3 atoms are in pentahedral sites. In spite of this, Miiller et al. (4) could not observe a signal in the range of 30 to 40 ppm and thought it was obvious that this is caused by a strong quadrupolar broadening. In figure 7.6 the MAS-27Al-N.M.R. spectra of CaO·6Al 0 as found by us are 2 3 given. The spectra of CaO·6Al 0 have a high didactical value because they 2 3 clearly show how different contributions to the spectra can be influenced by the experimental conditions. At 7 T, two signals, 7 and 16 ppm, are detected which can be attributed to aluminium with an octahedral oxygen coordination. At high spinning rates (about

103 f

e

d c -----....r----* * b *

* * * a

100 50 o -50 PPM

Figure 7.6 MAS 27Al-N.M.R. spectra of CaO·6Alp3 recorded at 7 T with (a) 5 kHz MAS, (b) 10 kHz MAS, (c) 15 kHz MAS and 11.7 T with (d) 5 kHz MAS, (e) 10 kHz MAS and (1) at 14 T with 11 kHz MAS. Spinning side bands are indicated by *

104 15 kHz), a splitting of the 7 ppm signal is revealed. At higher fields, however, a singlet shifted to 9 ppm remains and therefore it can be concluded that the splitting observed at 7 T and 15 kHz MAS is due to a quadrupolar coupling and not due to two slightly different octahedra. The 16 ppm signal is not shifted at higher field in comparison to the spectra measured at 7 T. This was also found by Muller et al. (4) who therefore assigned the 9 ppm signal to the 12 more distorted Al0 octahedra (Al(1) in table 7.1). The 16 ppm signal was assigned to 6 the remaining two crystallographically different octahedra because according to Kato and Saalfeld (1) in these octahedra the Al-O distances are identical or nearly identical (see Al(3) and Al(4), table 7.1). As one can see from the distances calculated by us in table 7.1, one of these octahedra (Al(3» is not as symmetric as one would expect from the distances given by Kato and Saalfeld (1). For the assignment of the N.M.R. signals, however, it appears that the mistake made by Kato and Saalfeld seems to have no serious consequences. The 16 ppm signal is broadened by a dipolar coupling considerably larger than 15 kHz. In addition to the octahedra signals, the spectra at 7 T exhibit at MAS spinning larger than 10 kHz another two maxima at 54 and 64 ppm. Both signals are within the region of the AlO tetrahedra. The spectra measured at higher 4 fields show clearly that the two maxima are due to a very strongly quadrupolar coupled signal and thus are both due to the one tetrahedral site. Moreover, the 27Al nuclei responsible for this resonance again show a dipolar coupling of < 10 kHz. Results obtained at 14 T (AM 600) at 11 kHz MAS are comparable with those at 11.7 T except for the relative areas of the 9 ppm and 16 ppm signals. Unfortunately, like Muller et ai. (4) we were not able to detect a signal which could be attributed to the five coordinated aluminium atoms. In addition to the explanation of Muller et aI., we think that this might be due to what Utsunomiya et al. (2) called the split atom model. In this model the aluminium atom jumps dynamically between two sites within the trigonal bipyramid. If the movement of the aluminium atom is slow on the N.M.R. time scale, the result of this is a series of very distorted pentahedral sites with very large quadrupolar couplings. As a consequence of this, one can expect a dispersion of chemical shifts causing a number of overlapping, small and broadened lines. The result is a small (only 8 % of the aluminium atoms occupy this site) and very broad signal which will be in practice undetectable with simple pulsed 27Al MAS NMR. Although we originally intended to distinguish between the B-alumina and the magnetoplumbite type structure by means of the pentacoordinated aluminium, one

105 can still benefit a lot from the MAS-N.M.R. spectra of CaO·lOAl 0 , Na-B-alumina 2 3 and K-B-alumina given in figures 7.7 to 7.9. If one compares the spectra of CaO·lOAl Na-B-alumina and K-B-alumina with CaO·6Al the most striking 2°, 3 2° 3 difference is the much sharper lines of the CaO·6Al 0 spectra. A closer look 2 3 also reveals that the three different octahedral sites in the B-aluminas are much more similar to one another than to the octahedral sites in CaO·6Al 0 . Most 2 3 clearly this can be seen in the Ca-B-alumina spectrum at 14 T, where only one signal from the three octahedral sites remains. The explanation must be sought in a combination of two phenomena, namely quadrupolar broadening and chemical shift dispersion. The quadrupole coupling is revealed by the decreasing line width and the shift of the signal when going from 7 T to higher fields. The quadrupole broadening in the Na- and K-B-alumina spectra is smaller than in Ca-B-alumina spectra. This can be understood if one realizes that the quadrupolar broadening is due to an electric field gradient. The charge on the calcium ion is twice the charge on the sodium and potassium ions and therefore the electric field gradient is also larger. The quadrupolar broadening, however, is seen for both calcium aluminates and there is no reason to assume a larger quadrupolar coupling in Ca-B-alumina. We therefore think the broad lines in the B-aluminas also to be caused by the distribution of the cations over a number of different sites as was found with XRD for B-aluminas (30,47) and as can be concluded from our own XRD measurements. Because of this distribution, the same crystallographical aluminium sites will differ in their magnetic resonance behaviour anyway. This will cause a dispersion in both chemical shifts and in quadrupole couplings. The very close resemblance between the spectra of CaO·10Al 3' Na-B-alumina 2° and K-B-alumina in contrast to the spectra of CaO·6Alp3 strongly indicates that the aluminium atoms are in very similar positions in the first three compounds and thus is an extra proof that CaO.lOAl has the B-alumina type structure. 2°3

7.3.3 KCIICaCl melts 2 During one hour the sodium B-alumina crystals were treated with 25%(75%, 50%/50% and 75%/25% KCl/CaCl melts at 1173 K. The compositions after the 2 treatment can be given respectively as 0.71Kp·0.29CaO·10.0Al 0 , 2 3 0.56Kp·0.44CaO·9.9AlP3 and 0.19Kp·0.81CaO.9.6Al 0 . The resulting crystals 2 3 were of the B-alumina type with cell parameters between the values of pure Ca-B-alumina and pure K-B-alumina. These results point out that a solid solution of calcium- and potassium-B-alumina is formed.

106 * e

d

c

b a_-----

100 50 o -50 PPM

Figure 7.7 MAS 27Al-N.M.R. spectra of CaO·1OAl 0 recorded at 2 3 7 T with (a) 5 kHz MAS, (b) 10 kHz MAS, (c) 15 kHz MAS and 11.7 T with (d) 5 kHz MAS and at 14 T with 8.8 kHz MAS. Spinning side bands are indicated by *.

107 d *

c *

b * a

100 50 o -50 PPM

Figure 7.8

MAS 27Al-N.M.R. spectra of Na-B-alumina recorded at 7 T with (a) 5 kHz MAS, (b) 10 kHz MAS and (c) at 11.7 T with 5 kHz MAS and (d) at 14 T with 8 kHz MAS. Spinning side bands are indicated by *.

108 d *

* c

100 50 o -50 PPM

Figure 7.9

MAS 27Al-N.M.R. spectra of K-B-alumina recorded at 7 T with (a) 5 kHz MAS, (b) 10 kHz MAS and (c) at 11.7 T with 5 kHz MAS and (d) at 14 T with 8 kHz MAS. Spinning side bands are indicated by *.

109 Experiments to try to exchange partly the calcium ions from CaO·6Al 0 for 2 3 potassium ions from a K SO melt, all had no result. Mter 4 hours at 1173 K, 2 4 still no potassium was found at all in the crystals and the XRD pattern was also not changed. This means that the ion exchange properties of 13-alumina and CaO·6Al 0 are not the same, despite the crystal structure similarity. 2 3

7.4 Stability of Ca-13-alumina and CaIK-13-alumina solid solutions

To determine the stability of Ca-13-alumina and of the CaIK-13-alumina solid solutions, crystals of these were heated at 1173 and 1373 K. We found decomposition into the magnetoplumbite type structure and Al 0 3 according to 2

CaO·lOAl 0 ~ CaO·6Al 0 + 4 Al 0 (7.4) 23 23 23

The decomposition of the solid solutions at 1373 K was monitored as a function of time by measuring the length of the c-axis of the resulting 13-alumina. The results are given in figure 7.10. From this figure it can be seen that during heat treatment the c-axes of all three solid solutions tend to become equal to the value found for pure K-13-alumina. This can be explained by a decreasing amount of CaO in the CaIK-13-alumina solid solution. This is caused by the formation of CaO·6Al 0 . 2 3 The method used is not accurate enough to measure the small differences in length of the c-axis of pure K-13-alumina and the series of the 75%/25% melt. Formation of CaO·6Al 0 and Al 0 , however, was clearly observed. 2 3 2 3 The heat treatments of the solid solutions and of Ca-13-alumina at 1173 K had the same results. The changes after 600 hours heating, however, are not as pronounced as at 1373 K.

110 22.8 -,------,

75/25 22.7 --======KAlJ 50/50 - S 0 22.6 - ;; 25/75 '"on V <:: ~ '";< 22.5 '"1 c.>

22.4 -

22.3 +-----.,-,---,..--,------,,----.,-'---,------.------1 o 200 400 600 Time (hours)

Figure 7.10 The length of the c-axes of K/Ca-B-alumina solid solutions as a function of the time if the solid solutions are heated at 1373 K. The dashed lines are the values found for pure Ca-B-alumina and K-B-alumina. The three solid solutions are made by immersing Na-B-alumina in 25n5, 50/50 and 75/25 mole %/% KCl/CaC1 melts. 2

111 7.5 Conclusions

With exchange experiments we were able to synthesize a regular Ca-B-alumina with the formula CaO·lOAl , This is the result of a 2: I exchange of sodium and 2°3 calcium. This 2: 1 exchange was also found in the often quoted exchange experiments of Toropov et al. (3,33). They started with "Na 0·12Al 0 " to get 2 2 3 CaO·6Al 0 . At the time these experiments were done, the knowledge of B-alumina 2 3 was very poor and the descriptions of the experiments in the papers are therefore not very clear. The chemical analyses, however, leave no doubt and differ clearly from our results. From the poorly measured X-ray diffraction pattern it cannot be concluded what the starting materials and reaction product at that time were. Whatever it was Toropov et al. found in the past, our exchange experiments clearly show the exchange properties of the Na-B-alumina with calcium ions. As for Ca-B"-alumina was found by Petrie et al. (38), Ca-B-alumina is transformed to the magnetoplumbite type structure during a heat treatment. This fits in the empirical rule that elements with lower valencies and large ionic radii have the tendency to form B-alumina type structures instead of magnetoplumbite type structures and vice versa (38,40). The Ca-B-alumina/K-B-alumina solid solution made by the exchange method behaves in the same way during a heat treatment as Ca-B-alumina. Hence, this solid solution is also not stable at 1373 K. This was also found by other authors (22,28,42,52) for the Na O-CaO-Al system, but such a clear evidence as we 2 2° 3 found was never delivered. In fact, the conclusion that the solid solution is not stable was drawn by the other authors from their inability to make the solid solution. The decomposition of the solid solution, however, means that the phase diagram as proposed by us in chapter 6 is correct. We could for completion and for clarity, construct from our own results in combination with data from literature a phase diagram representing metastable equilibria as given in figure 7.11. In this phase diagram there are two solid solutions: Ca/K-B"-alumina and Ca/K-B-alumina. The triangle marked with the question mark might also be filled up with a solid solution when the composition range of Ca-B-alumina is taken from CaO·6Alp3 to CaO·l0Alp3 similar to all other B-aluminas. This diagram is for the alkali corrosion of alumina not of direct interest but stresses the similarity between the binary sides of the phase diagram. The fact, however, that we can not exchange the calcium ions for potassium ions in a K SO melt reveals 2 4 that CaO·6Al 0 , present on grain boundaries of alumina doped with CaO, will not 2 3 be transformed easily to K-B-alumina.

112 A1 2 0 3

Figure 7.11 The phase diagram representing the metastable equilibria of a part of the K 0-CaO-Al 0 system. 2 2 3

Acknowledgement The author gratefully acknowledges A. J. van de Berg and E. J. Sonneveld (Laboratory for Technical , Technical University Delft, The Netherlands) for conducting the precise XRD measurements and for the structural refmement. We also are grateful to J. L. C. Daams of Philips Research Laboratories for his advice and usefull discussion on the XRD measurements. Furthermore, we also are grateful to J. W. de Haan and L. J. M. van de Yen (Laboratory for Instrumental Analysis, Technical University Eindhoven, The Netherlands) for their help with and discussions about the 27Al-NMR spectra. Finally we want to thank SON/NWO for the use of the HF-NMR Facility (Toemooiveld, 6525 ED Nijmegen) for the measurements of the NMR spectra at 11.7 T and 14 T.

113 Literature

1. K. Kato and H. Saalfeld, Neues Jahrb. Mineral. Abh., 109 (1968) 192-200. 2. A. Utsunomiya, K. Tanaka, H. Morikawa and F. Marurno, J. Solid State Chern., 75 (1988) 197-200. 3. N. A. Toropov and M. M. Stukalova, C. R. Acad. Sci. URSS, 27 (1940) 974-977. 4. D. MUller, W. Gessner, A. Sarnoson, E. Lipprnaa and G. Scheller, Polyhedron ~ (1986) 779-785. 5. F. Haberey, G. Oehlschlegel and K. Sahl, Ber. Dtsch. Kerarn. Ges., 54 (1977) 373-378. 6. S. Kimura, E. Bannai and I. Shindo, Mater. Res. Bull., 17 (1982) 209-215. 7. N. Iyi, S. Takekawa and S. Kimura, J. Solid State Chern., 59 (1985) 250-253. 8. F. P. F. van Berkel, H. W. Zandbergen, G. C. Verschoor and D. J. W. Udo, Acta Crysta1logr. Sect. C: Cryst. Struct. Commun., C40 (1984) 1124-1127. 9. N. Iyi, S. Takekawa, Y. Bando and S. Kimura, J. Solid State Chern. 47 (1983) 34-40. 10. N. Iyi, Z. Inoue, S. Takekawa and S. Kimura, J. Solid State Chern., 60 (1985) 41-50. 11. R. C. DeVries and W. L. Roth, J. Amer. Cer. Soc., 52 (1969) 364-369. 12. J. T. Kummer, Progr. Solid State Chern., I (1972) 141-175. 13. R. A. Huggins, Mater. Sci. Res., .2 (1975) 15-175. 14. G. Lucazeau, Solid State Ionics, ft (1983) 1-25. 15. R. Collongues, D. Gourier, A. Kahn, J. P. Boilot, Ph. Colornban and A. Wicker, J. Phys. Chern. Solids, 45 (1984) 981-1013. 16. R. Stevens and J. G. P. Binner, J. Mater. Sci., 19 (1984) 695-715. 17. G. A. Rankin and H. E. Merwin, J. Am. Chern. Soc., 38 (1916) 568-588. 18. W. L. Bragg, C. Gottfried and J. West, Z. Kristallogr., 77 (1931) 255-274. 19. C. A. Beevers and S. Brohult, Z. Krista1logr., 95 (1936) 472-474. 20. C. A. Beevers and M. A. S. Ross, Z. Kristallogr., 97 (1937) 59-66. 21. C. R. Peeters, M. Bettrnan, J. W. Moore and M. D. Glick, Acta Cryst. Sect. B: Struct. Sci., B27 (1971) 1826-1834. 22. W. L. Roth, J. Solid State Chern., ~ (1972) 60-75. 23. G. Collin, J. P. Boilot, A. Kahn, T. Thery and R. Comes, J. Solid State Chern., 21 (1977) 283-292. 24. N. Iyi, Z. Inoue and S. Kimura, J. Solid State Chern., 61 (1986) 81-89. 25. I. Kh. Moroz, G. A. Vydrik, I. V. Fedina and Kh. S. Valeev, Izv. Akad. Nauk.

114 SSSR, Neorg. Mat., 13 (1977) 1868-1871. 26. R. V. Kumar and D. A. R. Kay, Metall. Trans. B, 16B (1985) 107-112 and 295-301. 27. Y. Hong, D. Hong, Y. Peng, L. Li and S. Wei, Solid State Ionics, 25 (1987) 301-305. 28. J. D. Hodge, Proc. Electrochem. Soc., 85 (1985) 261-270. 29. JCPDS Powder Diffraction File, American Society for Testing and Materials, Philadelphia, card # 33-253. 30. N. Fanjat, G. Lucazeau, J. Bates and A. J. Dianoux, Physica B, 156-157 (1989) 342-345. 31. W. L. Roth, F. Reidinger and S. La Place, Superionic Conduct. [Proc. Conf.] (1976) 223-241. 32. J. M. Newsam, Solid State Ionics, Q(1982) 129-134. 33. N. A. Toropov and M. M. Stukalova, C. R. Acad. Sci. URSS, 24 (1939) 459-461. 34. Y. F. Y. Yao and J. T. Kummer, J. Inorg. Nucl. Chern., 29 (1967) 2453-2475. 35. G. C. Farrington and B. Dunn, Solid State Ionics, 1 (1982) 267-281. 36. R. Seevers, J. DeNuzzio and G. C. Farrington, J. Solid State Chern., 50 (1983) 146-152. 37. M. Alden, J. O. Thomas and G. C. Farrington, Acta Crystallogr. Sect. C: Cryst. Struct. Commun., C40 (1984) 1763-1766. 38. A. Petrie, J. Kirchnerova, C. W. Bale and A. D. Pelton, Mater. Res. Soc. Symp. Proc., 135 (1989) 497-504. 39. J. M. P. J. Verstegen and A. L. N. Stevels, J. Lumin., 2 (1974) 406-414. 40. A. L. N. Stevels and A. D. M. Schrama-de Pauw, J. Electrochem. Soc., 123 (1976) 691-697. 41. P. Valerga, X. Jin, F. Vozzo and W. Roth, Mater. Res. Soc. Symp. Proc., 135 (1989) 529-534. 42. D. J. Dyson and W. Johnson, Trans. J. Br. Ceram. Soc., 72 (1973) 49-55. 43. K. T. Greene and R. H. Bogue, J. Res. Natl. Bur. Stand. (U.S.), 36 (1946) 185-207. 44. W. L. Roth, I. Chung and H. S. Story, J. Am. Ceram. Soc., 60 (1977) 311-315. 45. A. C. Buechele and L. C. De Jonghe, Am. Ceram. Soc. Bull., 58 (1979) 861-864. 46. J. S. Moya, E. Criado and S. De Aza, J. Mater. Sci., 17 (1982) 2213-2217. 47. M. Alden and J. O. Thomas, Acta Crystallogr. Sect. C: Cryst. Struct. Commun.,

115 C40 (1984) 1763-1766. 48. D. Milller, W. Gessner, A. Samoson, E. Lipprnaa and G. Scheler, J. Chern. Soc. Dalton Trans., (1986) 1277-1281. 49. J. P. Gilson, G. C. Edwards, A. W. Peters, K. Rajagopalan, R. F. Wormsbecher, T. G. Roberie and M. P. Shatlock, J. Chern. Soc., Chern. Commun., (1987) 91-92. G. L. Turner, R. J. Kirkpatrick, S. H. Risbud and E. Oldfield, Am. Cerarn. Bull.,66 (1987) 656-663. S. H. Risbud, R. J. Kirkpatrick, A. P. Taglialavore and B. Montez, J. Am. Ceram. Soc., 70 (1987) C10-C12. 50. L. B. Alernany and G. W. Kirker, J. Am. Chern. Soc., 108 (1986) 6158-6162. 51. E. Lipprnaa, A. Samoson and M. Magi, J. Am. Chern. Soc., 108 (1986) 1730-1735. 52. H. Verwey and C. M. P. M. Saris, J. Am. Ceram. Soc., 69 (1986) 94-98.

116 CHAPTER 8

THE M 0-MgO-A1 0 2 2 3 SYSTEMS 8.1 Experimental

The method and details of the experimental techniques are described in chapter 4. Because the formation of MgO·Al 0 is very slow, hot kerosene drying was used 2 3 in all experiments. The temperature used in the Cs 0-MgO-Al 0 system was 1173 K. The main 2 2 3 problem at this temperature was the slow formation of the phases in combination with the fast evaporation of Cs 0. To make sure that equilibrium could be 2 reached, the pellets were also heated for 400 hours in sealed molybdenum capsules. The results of these experiments are compared with the results obtained in an open system. A difficulty in the Cs O-MgO-Al 0 system is the close similarity of the 2 2 3 powder XRD patterns of MgO·Al and Cs 0·Al 3' Both compounds have a cubic 2°3 2 2° crystal structure with parameters respectively a = 8.0831 A and a = 8.098 A (1,2). For this reason most lines of the XRD patterns overlap. There are a few lines with low intensities, however, which are different. For identification of

MgO·Al20 3 we used the lines with indices 5 3 1 and 5 3 3 with d = 1.3662 Aand d = 1.2330 A. For CsP·Al 0 we used the 3 3 1 reflection with d = 1.858 A. 2 3 The temperatures used in the K 0-MgO-Al 0 system were 1373 K and 1673 K 2 2 3 with heating times varying from 20 to 1000 hours. In this system, some experiments were also conducted in a sealed molybdenum capsule. The only thermodynamically stable oxide of aluminium is the a-Al 0 or 2 3 form. In addition to this stable form, a number of metastable phases are known (3). The preparation and heat treatment determines the sequence by which the material is fmally transformed to a-Alp3 (4-6). When starting with Al(NO)3' one has to bear in mind that beside a-Al 0 other aluminium oxide forms may show 2 3 up in the XRD pattern.

We did not fmd a magnesium stabilized Cs-6"-alumina. This is in agreement with our earlier conclusion that Cs-6-alumina does not exist. No other quasi-ternary or new quasi-binary compounds were found either in the

118 Cs O-MgO-Al system. There are therefore only two possibilities left for the 2 2° 3 phase diagram. Either MgO and CS 0·Al 0 or Cs 0 and MgO·AlP3 are in 2 2 3 2 equilibrium. With the estimated values of the Gibbs energy of CS 0·Al 0 of 2 2 3 chapter 2 and with data for the other compounds from literature (7) we can calculate the change in standard Gibbs energy of reaction 8.1:

(8.1)

At 1173 K the change in standard Gibbs energy of this reaction ('\GO) is about -415 kJ/mole and thus MgO and Cs O·AI ° should be in equilibrium. In an open 2 2 3 system, however, we found evaporation of CS out of CS 0·Al 3until no CS was 2° 2 2° 2° present anymore. Mter long heating periods we found always MgO·Al °3with MgO or 2 Al 0 , depending on the starting composition. We therefore conducted the 2 3 experiments once more in closed molybdenum crucibles to prevent complete evaporation of Cs 0. In these experiments we found indeed MgO in equilibrium with 2 CS 0·Al 0 as we expected from the thermodynamic considerations. We also did an 2 2 3 experiment with a mixture of MgO·Al and Cs in a molybdenum capsule. The 232° ° MgO·Al 0 was partly transformed to MgO and Cs 0·Al 0 and therefore this is in 2 3 2 2 3 agreement with the other results. In the three phase area MgO·Al -Cs O·Al -Al long heating times (300 2°3 2' 2°3 2°3 hours) were needed to form a.-Al 0 . No other crystal form of aluminium oxide was 2 3 ever found. The results of shorter heat treatments were always mixtures of Csp·AlP3' MgO·AlP3 and probably amorphous AlP3' With the results of these experiments we can construct two diagrams, one for an open and one for a closed system. Both these diagrams are given in figures 8.1 and 8.2.

In the K 0-MgO rich part with Al 0 contents of less than 50%, no 2 2 3 quasi-ternary compounds were found. The quasi-binary compound 3K 0·MgO reported 2 by Bardin et a1. (8) was not detected at 1373 K and therefore we think it is not stable. MgO and Kp.AI 0 are in equilibrium. This is in agreement with the results of 2 3 Roth and coworkers (9) who found degradation of the MgO·Al insulation in a MHO 2° 3 system according to reaction 8.2.

119 MgO A1 20 3

Figure 8.1 Phase diagram of the Cs 0-MgO-Al 0 system at 1173 K for a closed system. 2 2 3

120 Cs 20 (gas)

MgO A1 20 3

Figure 8.2 Behaviour diagram of the Cs 0-MgO-Al system at 1173 K for an open system 2 2° 3 (Cs 0·Al 0 decomposes). 2 2 3

121 (8.2)

The change of standard Gibbs energy of this reaction (A GO) is about -350 kJ/mole 2 (7,10) and therefore conftrms our experimental results. In the Al 0 rich part, problems arose. As we discussed in chapter 3, we may 2 3 expect two quasi-ternary compounds in this area. K-B"'-alumina was found and described several times (11,12) and K-B""-alumina was expected because of the similarity with the Nap-MgO-AlP3 system (9,13). We only found sometimes small traces of B"'-alumina but never found B""-alumina within 500 hours at 1373 K. For this reason we decided to do experiments at a higher temperature. At 1673 K, K-B'''-alumina was indeed formed. The kinetics even at this temperature were very slow and because of the problems arising from this we did not determine the exact stoichiometry of this compound but assume the formula from literature (11,12) is correct. The K-B""-alumina compound, however, was also at this temperature never found. In the triangle MgO·Al 0 ,K 0·Al 0 and 2 3 2 2 3 ternary B"-alumina, these compounds were the only compounds formed. The composition of B''''-alumina as reported for sodium is within this triangle. This means that the potassium compound either has an other composition or does not exist. In not one of our experiments we found indications for this compound to exist. The results of the experiments at 1673 K are given in ftgure 8.3. Even at this higher temperature, the kinetics of the formation and transformation of the B-, B" and B"'-alumina compounds are very slow. For example, once 6"-alumina is formed, it needs very long heating times to be transformed to other compounds. For this reason it is very difftcult to obtain equilibrium and to be sure equilibrium is reached. The diagram given in ftgure 8.3, however, is in agreement with most experiments. For the study of the potassium corrosion of alumina at lower temperatures, however, the diagram at 1673 K is less interesting. The K-6"'-alumina compound found at 1673 K is not or very slowly formed at 1373 K and therefore probably not a corrosion product at this lower temperature. For this reason we have studied this system at 1373 K too. One has to bear in mind that at this temperature equilibrium is hard or impossible to obtain within experimental time and therefore the diagram presented by us is a behaviour diagram instead of an equilibrium phase diagram. The results of this study are given in ftgure 8.4. At this temperature, in only a few experiments small traces of B"'-alumina were found. We therefore omitted this compound from the behaviour diagram. Three two-phase regions were found in this system. The exact compositions of

122 MgO MA

Figure 8.3 Phase diagram of the K O-MgO-Al 0 system at 1673 K. 2 2 3

123 MgO MA

Figure 8.4 Behaviour diagram of the K 0-MgO-Al 0 system at 1373 K. 2 2 3

124 B"-alumina which are in equilibrium with MgO·AIP3-KP·AIP3 and MgO.AI 0 -AI 0 , however, could not be determined. 2 3 2 3

8.4 Discussion

The diagrams for the Cs O-MgO-AI ° system are both important from corrosion 2 2 3 point of view. From the diagram of the closed system (figure 8.1) it can be seen that MgO·AI will be corroded by Cs 0. In an open system, however, the 2° 3 2 corrosion product (Cs O·AI is not stable because of evaporation of Cs 0. This 2 2°) 3 2 does not mean that no corrosion will occur in an open system. In both an open and a closed system, at first Csp.AIP3 will be formed. In an open system (figure 8.2) it can, depending on the CS ° vapour pressure, decompose after some time. At 2 that time, however, the damage has already been done in the form of microcracks because of the difference in volume of CS 0.Al 0 and AI 0 • 2 2 3 2 3 From both diagrams we can conclude that MgO dopes in a sintered alumina material will have no negative influence on the caesium resistivity. In the Cs 0-MgO-Al 0 system, no quasi-ternary phases were found and therefore MgO has 2 2 3 no stabilizing effect on the corrosion products. For the K 0-MgO-AI 0 system, no equilibrium phase diagram could be 2 2 3 constructed from the results of the experiments. The temperatures (1373 K and 1673 K) are too low to reach equilibrium within experimental time. From the behaviour diagram of figure 8.4, however, valuable information can be obtained. Although K-B"'-alumina is probably stable at 1373 K, it will not be found as a corrosion product or only after very long time from a reaction of MgO doped alumina with potassium. For this reason K-B"'-alumina is of less interest in relation to the corrosion problem. No other ternary compounds besides MgO stabilized B"-alumina are found at this temperature. The influence of MgO on the corrosion reaction is therefore a stabilization of the corrosion product because ternary B" -alumina will be formed instead of B-alumina. From the diagram alone, however, we can not conclude what influence this will have on the total corrosion reaction.

125 Acknowledgement

The author gratefully acknowledges all the students who helped during their practical course with the experiments described in this chapter. Special thanks are for Johan Vreugedenhil and Ineke Oomens who contributed a lot.

Literature

1. JCPDS Powder Diffraction File, American Society for Testing and Materials, Philadelphia, # 21-1152 and # 23-882. 2. G. Langlet, Centre d'Edutes NucIeaires de Saclay, Rapport CEA-R-3853 (1969). 3. W. H. Gitzen, "Alumina as a Ceramic Material" (Am. Ceram. Soc., 1970). 4. A. L. Dragoo and J. J. , J. Am. Ceram. Soc., 50 (1967) 568-574. 5. J. A. Thornton and J. Chin, Ceram. Bull., 56 (1977) 504-512. 6. A. Ayral, J. Phalippou and J. C. Droguet, Mater. Res. Soc. Symp. Proc., 121 (1988) 239-244. 7. I. Barin, O. Knacke and O. Kubaschewski, "Thermochemical Properties of Inorganic Substances" (Springer Verlag, Berlin, 1977). 8. J-C. Bardin, M. Avallet and M. Cassou, C. R. Acad. Sci. Paris, 278 (1974) 709-712. 9. R. S. Roth, Adv. Chern. Ser., 186 (1980) 391-408. 10. R. P. Beyer, M. J. Ferrante and R. R. Brown, J, Chern. Thermodynam., 12 (1980) 985-991. 11. M. Blanc, A. Mocellin and J. L. Strudel, J. Am. Ceram. Soc., 60 (1977) 403-409. 12. K. J. Morrissey and C. B. Carter, Mater. Sci. Res., 15 (1983) 297-308. 13. N. Weber and A. F. Venera, Am. Ceram. Soc. Meeting, (1970) 1-JV-70.

126 CHAPTER 9

POTASSIUM CORROSION TESTS OF ALUMINIUM OXIDE 9 POTASSIUM CORROSION TESTS OF ALUMINIUM OXIDE

Little is known about the potassium corrosion of aluminium oxide. We therefore tested AI 0 in potassium vapour and tried to clarify the influence of CaO dopes 2 3 on the reaction rate. The results of these experiments are described in this chapter.

9.1 Literature review

The alkali corrosion of ceramic materials is examined for many reasons. Most of the investigations are concerning sodium because of the widespread use in for example Magneto HydroDynamic (MHO) systems, High Pressure Sodium (HPS) lamps, fast breeder reactors and heat exchangers. Fink (1) in his review on experimental work on the compatibility of sodium with refractory ceramics concluded that high purity alumina (>99%) is compatible with sodium. Additives as Si0 and Cr 0 , however, appeared to have a detrimental effect on the corrosion 2 2 3 resistance (2). The influence of other additives as MgO and CaO has been studied many times in the last decade, mainly because of its importance to lighting industries. Hing (3) reported a greater percentage fallout in HPS lamps with calcia containing envelopes. In these envelopes, 6-alumina was formed on the grainboundaries. Diffusion of sodium along these grainboundaries caused a loss of sodium. The same was found by Prud'homme van Reine (4), but in calcia free aluminium oxide no sodium diffusion was found. De With et al. (5) in their corrosion tests found that reacts with sodium at temperatures between 1073 K and 1273 K to form Na 0·AI °.They also found a large difference in 2 2 3 reaction rate between the different crystal faces of a sapphire single crystal. This confIrms the conclusion of Fink (1) that the compatibility of alumina and sodium is a of kinetics rather than thermodynamics. According to Hodge (6) the enhancement of the alkali corrosion by calcium and magnesium can be explained in two ways. First, the incorporation of CaO and MgO into quasi-ternary compounds with greater stability than the quasi-binary alkali metal oxide-aluminium oxide compounds. Formation of such compounds leads to an increasing driving force of the overall corrosion reaction. The second explanation is based on kinetics. Spinel (MgO·AI 0) or calcium hexa aluminate 2 3

128 (CaO·6Al on the grainboundaries of alumina might act as nucleation centre and 2°)3 therefore promote the formation of 6-alumina. Hodge (6,7) concluded from his phase diagram and reaction kinetic studies that the enhancement of 6-alumina formation in the presence of calcia and/or magnesia is a purely kinetic. effect. This is in agreement with the conclusions of Datta and Grossman (8) who studied the influence of CaO·6Al 0 on the 6-alumina formation. They, however, explained 2 3 the enhancement of the 6-alumina formation by assuming greater mobility of sodium in CaO·6Alp3 than in AlP3' It is, therefore, generally believed that the corrosion of Al by sodium is 2°3 controlled by kinetics rather than by thermodynamics, although the stabilizing effect of magnesium on the 6-alumina structure must not be forgotten (6). The high temperature compatibility of caesium with Al 0 is studied often 2 3 (9-13) and the general result is that AlP3 is not corroded by caesium. This is in contrast with the predictions made in chapter 2. The test conditions in the experiments were quite different, however, and not always clear. Dopes and impurities, together with the porosity of the tested material, again play an important role. For example, Si0 has a detrimental effect because of the 2 formation of very stable quasi-ternary compounds (10-12). Little is known about the potassium resistance of alumina. Anderson (14) studied the of sapphire in an arc discharge lamp ftlled with potassium. They found formation of 6-alumina on the cracks. They could explain this by the residual oxygen inside the lamp:

--~) Kp.12AlP3 (6-alumina) (9.1) 2 K + 1/2 °2 + 12 Al2°3

They reported a decrease of this reaction when oxygen were used and therefore no oxidation/reduction reaction is involved. In their paper· on the properties of AlP3 (15), Susnitzky and Carter stated that potassium reacts above 1473 K with Al 0 to form 6-alumina. No experiments or literature 2 3 references are given and therefore this statement is of little value. The

reaction of K2°and Na2°with a Al23°/CaO·6Al23° system was described by Criado et al. (16). In both cases they found formation of B-alumina, but less severe corrosion was found with Na O. Other corrosion tests were done by Sata and 2 Kasukawa (17), who studied reactions of several materials with K SO and 2 4 K2S04!K2C03 melts. Al 0 showed fairly good results in the K S0 test, but poor 2 3 2 4 results were obtained in the K SO !K CO melt. Addition of CaO to Al improved 2 4 2 3 2°3 the corrosion resistance markedly. No explanations for the phenomena are given.

129 From the few experiments reported in literature it is not clear whether alumina will be corroded by potassium or not. On purely thermodynamic bases (chapter 2, figure 2.3) one would expect a similar corrosion behaviour as in the caesium-alumina system. The difference in atomic size of caesium and potassium in combination with the difference in driving force, however, might cause different corrosion behaviour.

9.2 Experimental

9.2.1 Method The corrosion tests were conducted in sealed molybdenum crucibles. In these crucibles, alumina is heated in a potassium vapour atmosphere and afterwards examined for corrosion. The crucibles were loaded with alumina and potassium in a way as drawn in figure 9.1. In this set up, contact between alumina and the potassium melt could be prevented.

alumina • molybdenum D potassium I I

Figure 9.1 Experimental set up for potassium corrosion tests.

130 Before use, the molybdenum was cleaned and outgassed in a of about 1 x 10-3 Pa at 1473 K, well above the test temperatures. To clean the surface, the alumina samples were treated for 10 minutes with boiling HN0 , rinsed with 3 demi-water and heated in air for one hour at 1473 K. The molybdenum cups were filled and sealed by in an atmosphere with P02 = 0.1 Pa. In this way, the amount of oxygen inside the crucible is negligible and the reaction studied is indeed a reaction of alumina with potassium instead of potassium oxide. During the welding, the bottom of the crucible must be cooled heavily to prevent evaporation of potassium. The sealed molybdenum crucibles were placed in a capsule for safety. When the crucible is heated, the pressure will rise and might possibly cause rupture of the crucible. Heating was performed in a vertical, electrically heated oven. The atmosphere was either vacuum or nitrogen/. Experiments were conducted at 1173 K and 1373 K with holding times of 1 to 100 hours. At these temperatures, the potassium pressures are 3 x lOs Pa and 10 x lOs Pa respectively (18-20). After the heating period, the crucibles were opened in air by sawing off the top. As fast as possible, the crucibles were filled with petroleum to prevent reaction with ambient air. The excess potassium was destroyed by slowly adding . The samples were taken out and kept in a dry atmosphere of a glovebox. The corrosion products were studied using the following methods: - Change in visual appearance. - Change of weight. - X-ray diffraction was used to identify the reaction products. These measurements were conducted in a home built vacuum powder diffractometer (21) or with a Rigaku D/max-IIC configuration, depending on the hygroscopicity of the products. - With a Jeol JSM-840 A scanning electron microscope (SEM) the were examined. On the same apparatus, the change in chemical composition and distribution of the elements was determined with energy dispersive analysis (EDS).

9.2.2 Materials The potassium corrosion of aluminium oxide is tested with four different kinds of polycrystalline materials. The main features of the corrosion behaviour were studied on commercially available high-purity, high- grade sintered alumina, manufactured by

131 Degussa. This "Degussit Al23" has a purity of 99.7% (>99.5%) with the main impurities MgO «0.3%), CaO«0.05%), Na 0 «0.1%), Fep3 «0.04%) and Si0 2 2 «0.1%) and has no open pores. The influence of CaO on the corrosion rate was studied on three series of alumina made in different ways and with CaO dope levels from 0 to 1000 wtppm. For the ftrst series, pellets were pressed from Al 0 powder (Les Rubis 2 3 Synthetique des Alpes) with less than 50 ppm impurities. These pellets were presintered in air at 1473 K for 10 hours to of about 50%. The CaO dopes were added to the pellets by immersion in solutions with varying well known concentrations of Ca(N0 )2 (E. Merck AG, zur analyse >98%). Samples were doped 3 with 0, 8, 60, 250 and 1000 ppm CaO in this way. After drying and decomposition of the nitrates, the fmal sintering step was done in air at 2123 K for 10 hours. Relative densities after sintering were all higher than 97.5%. The other two series were made by continuous hot pressing (22) and vacuum sintering (5,23). Details of the preparations can be found in the literature cited. The hot pressed materials are doped with 0 to 250 ppm CaO in steps of 50 ppm and the vacuum sintered samples are doped with 100 ppm MgO and 2, 6, 16 and 41 ppm CaO. Relative densities are higher than 98.5% and 99.8% respectively. The vacuum sintered samples were all sintered to translucency.

9.3 Method evaluation

In the ftrst experiments at 1373 K, severe corrosion of all samples occurred. The molybdenum of the crucibles, however, contained considerable amounts of aluminium, either in solution or as intermetallic compounds like AlM0 and 3 Al M0 . The question is whether the formation of the solution or intermetallic g 3 compounds has any influence on the corrosion reaction because the overall change in Gibbs energy is lowered. It might be possible that in absence of molybdenum the reaction will not occur. In order to prevent reaction of aluminium with molybdenum, the set up was changed as drawn in ftgure 9.2. In this way, only the thin molybdenum wire of 15 Ilm is in direct contact with the alumina test ring. An experiment at 1373 K for 10 hours showed corrosion of the sample with only a small amount of aluminium dissolved in the molybdenum wire. The change of

132 weight of the test sample was in agreement with the reaction products found with XRD:

(9.2)

The interaction of aluminium with molybdenum found earlier is therefore not necessary for the corrosion reaction to occur. In the following tests we do not take into account the aluminium interaction with the crucibles anymore.

alumina • molybdenum D potassium

Figure 9.2 Modified corrosion test set up to study the influence of the molybdenum crucible on the reaction. Contact between AI 0 and molybdenum is minimalized to only a 2 3 thin wire of 15 ~m.

133 9.4 Results and discussion

All samples tested at 1373 K with a potassium pressure of 10 x lOs Pa were corroded and therefore the results of these experiments are in agreement with the thermodynamic predictions of chapter 2. The interaction of potassium with Al varied from only blackening of the 2° 3 surface (1 hour) to complete destruction and conversion of alumina (20 to 100 hours) as can be seen in figures 9.3 and 9.4. The weight change accompanying this corrosion is difficult to interpret. It is a combination of weight gain due to potassium pick up and a weight loss because some aluminium dissolves in the molybdenum crucible. The general tendency, however, is a weight gain. X-ray diffraction analysis showed in most cases B-alumina as corrosion product but sometimes traces of K 0·Al were also found. In samples from which only the 2 2° 3 outside was blackened, Al 0 was the only phase found. The weight of a sample 2 3 which was not in contact with molybdenum during the test (see figure 9.2) was increased with 14%. This is in agreement with reaction 9.2 where Al 0 is 2 3 completely converted in B-alumina and aluminium. Because no aluminium was found in the molybdenum crucible, the sample must contain metallic aluminium. From reaction equation 9.2 one can calculate that the sample should contain about 2.8 wt% aluminium. With a very slow XRD step scan, however, we could not detect this. Samples in an intermediate stage between starting and complete corrosion showed besides blackening of the surface also degeneration of the grainboundaries (see figure 9.5a) in the whole sample of 3 x 3 x 9 mm. An X-ray image could not be made because of the fast evaporation of potassium in the electron microscope. The X-ray EDS semi quantitative linescan across several grains given in figure 9.5b, however, clearly show that potassium is mainly present at the grainboundaries. This proves a higher diffusion rate of potassium along the grainboundaries than in the bulk of Al 2°. 3 One alumina sample was subjected to K at 1173 K for 5 hours by heating it in 2° contact with K CO3 in an electrically heated horizontal tube furnace. The sample 2 was partly transformed to B-alumina but no blackening occurred at the surface or the grainboundaries. This shows that the blackening of the samples tested in a potassium vapour is probably caused by metallic aluminium which is formed when Al 3 is reduced. Blackening of the grainboundaries therefore means the presence 2° of metallic aluminium on these grainboundaries. We can explain this as follows. The Al 0 on the surface is first reduced by potassium with the formation of 2 3

134 Figure 9.3 Total destruction of hot pressed Ai 0 without dopes. Secondary electron images 2 3 at two magnifications of a sample that was tested in a potassium atmosphere for 20 hours at 1373 K.

135 Figure 9.4 Total destruction of vacuum sintered Al 0 3 doped with 6 ppm CaO. Secondary 2 electron images at two magnifications of a sample that was tested in a potassium atmosphere for 20 hours at 1373 K.

136 a

b

A T o M I C

Distance ().lrn)

Figure 9.5 Potassium corrosion of a "Degussit AI "sample showing only grainboundary 23 attack. Sample was tested in a potassium atmosphere for 10 hours at 1373 K. a. Secondary electron image and b. EDS measurements across several grains

137 J3-alumina. Potassium ions will diffuse along the grainboundaries into the material and form J3-alumina. The transformation of A1 to J3-alumina is 2°3 accompanied with a volume increase of 28%. This will cause tensions and micro cracks on the grainboundaries. Potassium vapour now can penetrate along these cracks and reduce A1 inside the material with formation of metallic 2° 3 aluminium. X-ray diffraction analysis of the surface of a completely corroded sample revealed a very important phenomenon. The only phase present was K-J3-alumina but with only very low intensities of the normally very strong 0 0 1 lines. This is due to preferential orientation of the J3-alumina crystals: the c-axis of the crystals are all oriented parallel to the surface. When the XRD pattern is measured of a powdered sample, the 0 0 1 lines are strong again. From figure 9.6 the preferential orientation also can clearly be seen. The needle like crystals on the surface are actually hexagonal crystals seen perpendicular to the c-axis. The preferential orientation of J3-alumina with the c-axis parallel to the surface has strong influence on the corrosion behaviour of alumina. As discussed earlier, J3-alumina is an ionic conductor. The potassium ions are very mobile in a layer between the spinel type blocks. This so called conduction layer lies perpendicular to the c-axis. This means that if B-alumina is oriented with the c-axis parallel to the surface, the conduction layer is perpendicular to the surface. This creates a perfect diffusion path for potassium from the surface to the inside of the sample. The corrosion layer will therefore not inhibit further reaction and thus the corrosion reaction will continue. When the orientation of the formed crystals could be changed in a way that the conduction layer is parallel to the surface, the corrosion rate probably would be reduced. For changing the orientation, however, the reason for the preferential orientation must be understood first. A strikingly similar result was obtained by Kohatsu and Brindley (24) in their diffusion couple reactions between CaO and alumina. CaO·6A1 0 , with the 2 3 magnetoplumbite type structure, was found with the c-axis parallel to the reaction surface. No explanation was given for it. The starting A1 0 in both our 2 3 own and in the experiments of Kohatsu and Brindley, was randomly arranged,

138 a

b

Figure 9.6

Total corrosion of a "Degussit AI II sample after 20 hours in a potassium 23 atmosphere at 1373 K. a. Secondary electron image from surface, showing needle like crystals. b. Secondary electron image from fracture plane with hexagonal 8-alumina crystals.

139 present, the longest crystallographic axis is found perpendicular to the direction of diffusion. A second resemblance in these systems is that only one of the species is mobile. Both these conditions are also true for the potassium aluminium oxide system studied by us. A possible explanation for the preferential orientation is the faster growth of nuclei and crystals with the c-axis perpendicular to the reaction surface. When a crystal is oriented with the c-axis perpendicular to the surface, the so called conduction plane is parallel to the surface and therefore the fast diffusion path for the potassium ions going into the material is blocked. The growth of such a crystal will be very slow. On the other hand, when a nucleus or crystal is oriented with the conduction plane perpendicular to the surface, fast diffusion of potassium and fast growth of the crystal is possible. This will result in the preferential orientation found by us in the tested alumina samples. The samples tested at 1173 K with a potassium pressure of 10 x 105 Pa showed, as expected, much less corrosion. In most cases the corrosion was restricted to only blackening of the surface and of the grainboundaries. The CaO doped materials were all heated at 1173 K for 20 hours. The hot pressed samples showed no weight change and AI 0 3 was the only phase detected. 2 The outside and grainboundaries were coloured black, the bulk was grey. No significant difference between the samples with varying dope levels could be detected. Considering the colour, the air sintered samples were less corroded. Again no influence of the calcia dope level was noticed. The vacuum sintered samples did not show any sign of corrosion at all. The samples retained their translucency and only very little potassium could be detected on the samples. It appears that the CaO dope level has no influence on the corrosion rate. The sintering step of the alumina samples, however, seems to be much more important. The vacuum sintered materials have a greater corrosion resistance against potassium attack than the other samples. The same was found by de With et al. (5) for the sodium resistance of alumina. aluminium (Y AI 0 , YAG) is a material which can be sintered 3 5 12 to translucency. It is reported (27,28) that YAG exhibits a greater lithium and sodium resistance than alumina. For this reason we also tested this material for its potassium resistance. The holding time at 1373 K was 20 hours. The sample was very slightly grey coloured and had a weight gain of 0.3%. XRD revealed mainly

YAG but also Y20 3 and unidentified peaks were found. YAG is therefore not stable in an atmosphere of 10 x 105 Pa potassium at 1373 K. Under similar conditions, however, the corrosion of alumina was much more severe.

140 9.5 Conclusions

Under the experimental conditions used in this study, alumina is not stable in the presence of potassium. This is in agreement with the calculations of chapter 2. The need of free oxygen was proposed by Anderson (14) to explain 8-alumina formation in potassium arc lamps. We showed that potassium is able to reduce alumina and therefore the corrosion is not depending on the presence of oxygen in the system. The formation of 6-alumina with the ion conducting layer preferentially oriented perpendicular to the surface, promotes further reaction of the bulk material. Since no protective corrosion layer is formed, the corrosion rate does not decrease with time. No influence of the CaO dope level on the corrosion rate was found. The sintering atmosphere, on the other hand, has great influence on the corrosion resistance. Vacuum sintering of alumina gives the best results in our experiments. Comparison of YAG with alumina with respect to the potassium corrosion resistance showed a significantly greater resistance of YAG. The YAG sample is, however, also corroded.

Acknowledgement The author gratefully acknowledges H. T. Munsters of Philips Research Laboratories for sealing the molybdenum crucibles

141 Literature

1. J. K. Fink, Rev. Coat. Corros., ~ (1978) 1-47. 2. J. Jung, A. Reck and R. Ziegler, J. Nucl. Mater., 119 (1983) 339-350. 3. P. Ring, J. Mater. Sci., 11 (1976) 1919-1926. 4. P. R. Prud'hornrne van Reine, Sci. Ceram., 12 (1984) 741-749. 5. G. de With, PJ. Vrugt and AJ.C. van de Yen, J. Mater. Sci., 20 (1985) 1215. 6. J. D. Hodge, Proc. Electrochem. Soc., 85 (1985) 261-270. 7. J. D. Hodge, J. Electrochem. Soc., 133 (1986) 833-836. 8. R. K. Datta and L. N. Grossman, Proc. Electrochem. Soc., 85 (1985) 271-290. 9. P. Wagner and S. R. Coriell, Rev. Sci. Instr., 30 (1959) 937-938. 10. R. G. Smith, F. Hargreaves, G. T. J. Mayo and A G. Thomas, J. Nucl. Mater., 10 (1963) 191-200. 11. J. K. Higgens, Trans. J. Brit. Ceram. Soc., 65 (1966) 643-659. 12. L. N. Grossman, Rev. Int. hautes Temper. Refract., 16 (1979) 255-261. 13. E. J. Britt, J. L. Desplat, T. Gulden, J. Chin and V. Cone, Proc. 20th IECEC, Miami Beach (1985) 3.499-3.511. 14. N. C. Anderson, J. Am. Ceram. Soc., 62 (1979) 108-109. 15. D. W. Susnitzky and C. B. Carter, J. Am. Ceram. Soc., 68 (1985) 569-574. 16. E. Criado, J. S. Moya and S. De Aza, Ceram. Int., I (1981) 19-21. 17. T. Sata and K. Kasukawa, Rev. Int. hautes Temper. Refract. 17 (1980) 174. 18. J. P. Stone, C. T. Ewing, J. R. Spann, E. W. Steinkuller, D. D. Williams and R. R. Miller, J. Chem. Eng. Data, 11 (1966) 315-320. 19. N.B. Vargaftik, L.D. Volyak, V.M. Anisimov, V.G. Stepanov, V.F. Kozhevnikov and A K. Chelebaev, Inzhenemo-Fizicheskii Zhumal, 39 (1980) 986-992. 20. H. U. Borgstedt and C. K. Mathews, "Applied Chemistry of the Alkali metals" (Plenum Press, New York, 1987) 21. L. Boon, H. de Jonge Baas and R. Metselaar, Powder Diffr., Accepted for publication. 22. G. de With and N. Hattu, J. Mater. Sci. Lett., 16 (1981) 841-844. 23. G. de With, J. Mater. Sci., 19 (1984) 2195-2202. 24. I. Kohatsu and G. W. Brindley, Z. Phys. Chem., 60 (1968) 79-89. 25. J.A. van Beek, S.A. Stalk and FJ.J. van Loo, Z. Metallkde., 73 (1982) 439. 26. G. F. Bastin, J. H. Maas, F. J. J. van Loo and R. Metselaar, Proc. 11th Plansee-Semin. (1985) 513-526. 27. E. J. Cairns and R. A Murie, J. Electrochem. Soc., 121 (1974) 3-19. 28. G. de With, Mater. Sci. Monogr., 38C (1987) 2063-2075.

142 CHAPTER 10

GENERAL CONCLUSIONS 10 GENERAL CONCLUSIONS

From the results of the thermodynamic calculations, which are given in an Ellingham-like diagram in figure 2.3, it is clear that under most conditions Al ° is in principle not stable in contact with lithium, potassium, rubidium and Z 3 caesium. Sodium is less corrosive from this point of view. Our study of the Cs O-CaO-Al 0 system reveals that CaO will have a negative z Z 3 influence on the caesium resistivity of alumina. The quasi-ternary phase found in this system (CszO·2CaO.4Alp3) appears to be very stable and will be be formed after corrosion. From the phase relations in the diagram of figure 5.3 it is obvious that this compound will be formed readily upon interaction between CaO containing alumina ceramics with caesium and therefore will influence the corrosion reaction from the beginning. In the K O-CaO-Al 0 system we also found a quasi-ternary phase z Z 3 (2Kp·3CaO·5Alp3)' The phase diagram depicted in figure 6.2 shows, however, that this compound will only be formed when the K content exceeds a value of about z° 15 mole% and therefore this compound will not play an important role in the

corrosion reaction at first. In this respect is the Alz°3 rich part of the phase diagram much more important. CaO is believed to be present on the grainboundaries as CaO·6Al 0 . It is thermodynamically less favourable to reduce CaO·6Al 0 than Z 3 Z 3 to reduce Al 0 . For these reasons, the possible negative thermodynamic effect, Z 3 e.g. a higher energy gain resulting from the corrosion reaction by formation of a more stable quasi-ternary reaction product, will have low influence. On the other hand, the phase diagram representing metastable equilibria in the Kp-CaO-Alp3 system (figure 7.11) as well as the easy transformation of Ca-B-alumina into calcium hexa aluminate show a strong similarity of the phases in the alumina rich region of the phase diagram. A kinetic effect resulting in the easy formation of the B-alumina corrosion product, will therefore be of greater influence. MgO as a sintering aid in alumina will have· no negative influence on the potassium and caesium resistivity. In the Cs O-MgO-Al 0 system (figures 8.1 and z Z 3 8.2), no quasi-ternary phases were found at all and for this reason the reaction product of caesium with alumina, whether pure or doped with MgO, will in all cases be the same. Thermodynamically it is easier to reduce Al 0 than MgO·Al 0 Z 3 Z 3 and from this point of view, adding a MgO dope will have a positive influence on the caesium resistance of alumina. The equilibrium phase diagram of the K O-MgO-Al 0 system could not be established. From the behaviour diagram z Z 3 presented in figure 8.4, however, it is obvious that MgO stabilized B"-alumina

144 will be formed at 1373 K instead of 6-alumina, which is the corrosion product of undoped alumina. This will probably not have a pronounced effect on the corrosion rate and therefore MgO can be used as a sintering aid in the densification of alumina which is to be used in potassium or caesium containing atmospheres. As a result of our phase diagram studies we found three new phases;

CszO·2CaO·4Alp3' 2Kp·3CaO·5Alp3 and the metastable CaO·lOAlz0 3 phase. Without the thorough investigations of these phase diagrams these compounds and the phase relations would have remained unknown and as a result one would have expected other corrosion products. Sound knowledge of the related phase diagrams is for this reason essential in the understanding of corrosion phenomena. It is rather risky to extrapolate the properties and behaviour of one alkali metal to all others. The enthalpies of formation of the sodium containing compounds in figure 2.6 are all exceptions in the homologous series (or are the lithium compounds the exceptions ?). For this reason the sodium corrosion of alumina is thermodynamically different from the other alkali metals as can be seen in the Ellingham-like diagram of figure 2.3. From the same figure one would expect similar corrosion behaviour of alumina in potassium and caesium containing atmospheres. The fast reaction of potassium with Alz03 found by us, however, is in contradiction with this expectation because the good caesium resistivity of

Alz°3 is well known from literature. Furthermore, the fact that CaO has a negative influence on the sodium resistivity and possibly also on the potassium and caesium resistivity, might suggest that this is the same for all alkali metals. The possible negative influence of CaO, however, is a kinetic effect in the case of potassium (resemblance of compounds) and is a thermodynamic effect (stabilisation of corrosion product) in the case of caesium and therefore it is more a coincidence that CaO has a negative effect on the corrosion resistance of alumina for both alkali metals. As already predicted by our thermodynamic calculations, we can conclude from our corrosion experiments that alumina, under the conditions used in our experiments, is not stable in the presence of potassium. K-6-alumina is formed with a preferential crystallografic orientation in such a way that the ionic conduction layer in this compound is perpendicular to the surface. This promotes further reaction with the bulk of alumina and thus corrosion will continue after formation of a corrosion layer. No influence of CaO on the corrosion rate could be monitored. Dope levels varying between 0 and 1000 ppm did not result in a difference in reaction rate. The sintering conditions on the other hand clearly influence the reaction rates. Vacuum sintered alumina showed the best corrosion resistance.

145 Summary

Alkali metal corrosion of alumina is a problem often met in a number of applications of these materials. This thesis deals with the thermodynamics, related phase diagrams and testing of some materials. In order to predict under what circumstances a reaction will occur, the change in standard Gibbs energy is calculated for possible reactions between alumina (AlP3) and the alkali metals. To be complete for all alkali metals, we had to estimate the thermodynamic data for some possible corrosion products. These estimations were made with the use· of homologous series and are thought to be more reliable than the estimations found in literature. With the use of the estimated values, an Ellingham like diagram is constructed, from which the maximum temperature and alkali metal vapour pressure with low risk on corrosion, can be read. From literature it is known that the sinter dopes of alumina play an important role in the corrosion. To understand this, a sound knowledge of the phase diagrams is essential. Phase diagram investigations on the Cs 0-CaO-Al 0 system 2 2 3 at 1173 K revealed a new very stable quasi-ternary phase. A new method was developed to obtain the composition of this compound (Cs 0·2CaO·4Al 0 ). With 2 2 3 powder X-ray diffraction, chemical- and thermogravimetric analysis this compound was characterized. From the phase diagram it can be seen that this phase will be formed when CS is in contact with alumina doped with CaO and therefore it is 2° better to avoid CaO as a sinter aid. The Cs 0-MgO-Al 0 system, on the other 2 2 3 hand, does not contain a quasi-ternary compound and from the phase diagram it can be concluded that MgO will not have a negative influence on the corrosion of alumina. The subsolidus phase relations in the K 0-CaO-Al 0 system are determined at 2 2 3 1373 K with a combination of experimental work and thermodynamic calculations. The experiments revealed a quasi-ternary phase (2K 0·3CaO·5Al 0 ) but the phase 2 2 3 relations were not always clear. By calculating the activities of the components in the three phase areas and by calculating the change in standard Gibbs energy of all possible reactions, the phase relations and the standard Gibbs energy of formation of the quasi-ternary compound could be determined. From these results it can be seen that reaching equilibrium is difficult because of the low energy gain resulting from it. The from corrosion point of view most interesting part, the high alumina comer, is examined furthermore by exchanging the mobile cations from sodium-B-alumina (theoretical Na O·l1Al 0) for potassium and/or calcium 2 2 3 ions. Exchanging sodium for calcium produces an unknown calcium aluminate

146 (CaO·l0Al 0 ). The structure was determined by using 27Al-NMR measurements and 2 3 powder X-ray diffraction analysis and turned out to be a 6-alumina type one. A solid solution of K-6-alumina and Ca-B-alumina is formed in the whole composition range but, similar with the pure Ca-6-alumina, is metastable. The K O-CaO-Al system was investigated at 1373 K but the kinetics are much 2 2° 3 too slow at this temperature. For this reason a behaviour diagram is presented from which possible corrosion products can be read. From the K 0-CaO-Al 0 phase 2 2 3 diagram and the K O-MgO-Al behaviour diagram, however, it is not clear what 2 2° 3 the influence of these dopes will be on the potassium resistance of alumina. In order to check the thermodynamic predictions and to clarify the corrosion mechanism, several kinds of aluminium oxide materials were tested at high temperatures in a closed system containing potassium. Alumina is corroded badly under the experimental conditions. The corrosion clearly starts at the grainboundaries but the bulk is also transformed fast. The formed B-alumina shows preferential orientation such that the fast diffusion path in B-alumina is perpendicular to the surface. For this reason, the corrosion layer does not have a protective character. No influence of the CaO dope level on the corrosion rate was found. The sintering conditions of alumina appear to be much more relevant.

147 Samenvatting

Alkalimetaal corrosie van alurniniumoxide is een probleem dat vaak optreedt in veel toepassingen van deze materialen. Dit proefschrift handelt over de therrnodynarnica, gerelateerde fasendiagrarnrnen en tests van een aantal materialen. Om een voorspelling te kuooen doen onder welke omstandigheden er een reactie op zal treden, is de verandering in de standaard Gibbs energie berekend van een aantal mogelijke reacties tussen alurniniumoxide (Al 0 ) en de alkali metalen. Om Z 3 volledig te kunnen zijn voor alle alkalimetalen was het noodzakelijk om een schatting te maken van de therrnodynarnische gegevens van sornrnige reactie producten. Deze schattingen werden gemaakt met behulp van homologe reeksen en zijn waarschijnlijk betrouwbaarder dan de geschatte waarden uit de literatuur. Met behulp van de geschatte waarden is een Ellingharn-achtig diagram geconstrueerd. Uit dit diagram is het mogelijk de maximale temperatuur en de maximale alkalimetaal dampdruk af te lezen waarbij de kans op corrosie nog klein is. Uit de literatuur is het bekend dat de sinterhulprniddelen een belangrijke rol spelen bij de corrosie van alurniniumoxide. Om dit te kuooen begrijpen is een gedegen keoois van de relevante fasendiagrarnrnen noodzakelijk. Door het fasendiagram onderzoek aan het CS O-CaO-Al 0 systeem bij 1173 K, werd een z Z 3 nieuwe quasi-ternaire fase bekend. Ben nieuwe methode werd ontwikkeld om de samenstelling te kuooen bepalen van deze verbinding (Cs O·2CaO·4Al 0 ) welke z z 3 vervolgens met behulp van poeder rontgendiffractie, chernische en therrnische analyse is gekarakteriseerd. Uit het fasendiagram kan afgelezen worden dat deze verbinding gevorrnd zal worden als Csz° en alurniniumoxide in contact staan met elkaar en daarom is het beter om CaO niet als sinterhulprniddel te gebruiken. In tegenstelling tot het CS O-CaO-Al 0 systeem, bevat het C\O-MgO-Al 0 systeem z Z 3 z 3 geen quasi-ternaire verbinding en uit het fasendiagrarn kan geconcludeerd worden dat MgO geen negatieve invloed zal hebben op de cesium corrosie van alurniniumoxide. Door een combinatie van experimenten en therrnodynarnische berekeningen zijn de subsolidus fasenrelaties in het Kp-CaO-Alp3 systeem bij 1373 K bepaald. Een nieuwe quasi-ternaire fase 2K O·3CaO·5Al ° werd gevonden maar de fasenrelaties Z Z 3 waren niet altijd duidelijk. Door de activiteiten van de verschillende componenten in alle driefasengebieden en de verandering in Gibbs energie van mogelijke reacties te berekenen, was het mogelijk om de fasenrelaties en om de vorrnings Gibbs energie van de nieuwe fase te bepalen. Uit deze therrnodynarnische berekeningen is het duidelijk dat het instellen van evenwicht moeilijk is door de

148 geringe energiewinst ennee gepaard gaat. Het gedeelte van het fasendiagram dat uit het oogpunt van corrosie het meest interessant is (met een hoog aluminiumoxide gehalte) is verder ook nog onderzocht door de mobiele kationen van natrium-6-aluminiumoxide (theoretisch Na O·11Al uit te wisselen met kalium 2 2°) 3 en/of calcium ionen. Door natrium met calcium uit te wisselen ontstond een voorheen onbekend calciumaluminaat (CaO·lOAl 0 ). De structuur is bepaald door 2 3 een combinatie van poeder rongtendiffractie en 27Al-NMR metingen en bleek van het 6-aluminiumoxide type te zijn. Ben vaste oplossing K-l3-aluminiumoxide en Ca-6-aluminiumoxide over het hele compositiegebied bleek, evenals het zuivere Ca-6-aluminiumoxide, metastabiel te zijn. Het ISO-MgO-AlP3 systeem is onderwcht bij 1373 K maar de kinetiek bij deze temperatuur is te langzaam om fasenevenwichten te bepalen. Om deze reden is een gedragsdiagram bepaald waaruit de mogelijke corrosieproducten kuonen worden afgelezen. Uit het Kp-CaO-Al 0 fasendiagram en het K 0-MgO-Al 0 2 3 2 2 3 gedragsdiagram wordt niet duidelijk wat de invloed van CaO en MgO toevoegingen zal zijn op de kaliumresistentie van aluminiumoxide. Om de thennodynamische voorspellingen te controleren en om het corrosie mechanisme te verhelderen zijn een aantal soorten aluminium oxide getest bij hoge temperaturen in een kaliumatmosfeer. Aluminiumoxide corrodeert hevig onder de .experimentele omstandigheden. De korrelgrenzen worden eerst aangetast, maar al snel is ook de bulk van het materiaal getransfonneerd. Het gevonnde 6-aluminiumoxide heeft een preferente orientatie met het snelle diffusie pad loodrecht op het oppervlak. Hierdoor heeft de corrosielaag geen beschennend karakter. Het CaO gehalte blijkt geen invloed te hebben op de corrosiesnelheid. De sintercondities van het aluminiumoxide daarentegen zijn weI van invloed op het corrosiegedrag.

149 Curriculum Vitae

lohan van Hoek werd op 26 mei 1962 geboren te Breda. Hij behaalde zijn Atheneum B diploma in 1980 aan het Theresia Lyceum te Tilburg. In hetzelfde jaar is hij begonnen aan de studie scheikunde aan de Katholieke Universiteit te Nijmegen. Van augustus 1984 tot februari 1985 verkreeg hij aan de afdeling didactiek der scheikunde de Ie graads lesbevoegdheid. Het doctoraalexamen werd afgelegd op 27 september 1986, met als bijvakken Kristallografie en Moleculaire Spectroscopie en met als hoofdvak Algemene en Anorganische Chemie. In november 1986 trad hij in dienst van de Technische Universiteit Eindhoven. Bij de vakgroep Vaste Slof Chemie en Materialen werd oJ.v. Prof. Dr. R. Metselaar en Dr. FJJ. van Loo het in dit proefschrift beschreven onderzoek uitgevoerd.

150 Dankwoord

Ben koers win je niet alleen. Daarvoor is een heel peloton "waterdragers" nodig. Zo is dat ook bij het schrijven van een proefschrift. Velen hebben me geholpen over de bergen heen te komen. Iedereen wil ik daarvoor danken. Speciale dank, zander de andere te kort te doen, gaat echter uit naar twee super knechten zander wie ik nooit de fmish zou hebben bereikt. Annette heeft me altijd weer in het zadel gekregen na een valpartij en heeft me na elke zware etappe de tijd gegeven te recupereren. Nooit heeft ze het vertrouwen in mij opgezegd. Frans van Loo ben ik bijzondere dank verschuldigd voor zijn zeer inspirerende discussies en zijn altijd bereidwillige oor. Hij heeft me vanuit minder goede posities weer naar voren weten te manoevreren en heeft me door zijn raad en tips het benodigde koersinzicht bijgebracht.

151 Stellingen behorend bij het proefschrift

ALKALI METAL CORROSION OF ALUMINA Thermodynamics, phase diagrams and testing

Johan van Hoek Eindhoven 30 oktober 1990

1 De juistheid van het Ti-Al-O fasendiagram zoals voorgesteld wordt door Tressler, Moore en Crane is onzeker omdat niet aangetoond is dat de laag welke TiO genoemd wordt, in werkelijkheid geen TiAl(O) is. R.E. Tressler, T.L. Moore en R.L. Crane, J. Mater. Sci., 8 (1973) 151-161.

2 De waamemingen van Toropov en Stukalova aan hun ion uitwisselings experimen­ ten zijn onvolledig en/of fout. Toch hebben deze experimenten vele jaren gediend als leidraad voor soortgelijke experimenten en zijn daarom waardevol. N.A. Toropov en M.M. Stukalova. C. R. Akad. Sci. URSS., 27 (1940) 974-977. Dit proefschrift. hoofdstuk 7

3 De aanname van Sommer, Mui en Smith dat de desorptiecoefficient van waterstof aan grafiet 20% bedraagt, heeft geen fysische ondergrond en moet meer gezien worden als rekenkundige parameter om de vorming van diamant in CVD reactoren te verklaren. M. Sommer, K. Mui en F.W. Smith. Solid State Commun., 69 (1989) 775-778.

4 De conclusie van Risbud e.a. dat het door hun onderzochte Si0 -Al 0 -glas 2 2 3 zeker vijf-gecoordineerd aluminium bevat, is voorbarig. S.H. Risbud, RJ. Kirkpatrick, A.P. Taglialavore en B. Montez, J. Am. Ceram. Soc., 70 (1987) ClO-C12.

5 "The good news: anything is possible on your computer. The bad news: nothing is easy" (T.H. Nelson) Het eerste deel van deze uitdrukking is in ieder geval onjuist. Zoals hij zelf al zegt: All simulations are political" (computer­ simulaties laten sommige feiten zien, andere feiten weg en het probleem wordt altijd vereenvoudigd). T.H. Nelson "Computer LibIDream Machines" (revised edition of Computer Lib, 1974) Tempus Books of Microsoft Press. Redmond. Washington. 1987. 6 Hong e.a. gebruiken de naam Ca-6-alumina voor de verbinding CaO·6Al 0 • Dit 2 3 is af te raden daar hiervan de suggestie uitgaat dat deze verbinding een 6-alumina type structuur heeft in plaats van een magnetoplumbiet type structuur. Y. Hong, D. Hong, Y. Peng, L. Li and S. Wei, Solid State Ionics, 25 (/987) 301-305.

7 Men moet zich goed realiseren dat de waarnemingen van Becher e.a. gedaan zijn aan TEM-samples welke nagenoeg tweedimensionaal zijn. Extrapolatie naar drie dimensies om de whisker versterking van keramische composieten te verklaren moet daarom met de nodige scepsis bekeken worden. P. F. Becher, C-H. Hsueh, P. Angelini and T. N. Tiegs, J. Am. Ceram. Soc., 71 (/988) 1050-1061.

8 Het door Okutani e.a. voorgestelde reactiemechanisme voor de vorming van SiC uit Si N en C bij bestraling, correspondeert niet met de door hen waargenomen 3 4 vorming van SiC whiskers. T. Okutani, . Nakata, M Suzuki, M. Yamaguchi and J. Watanabe, Proceedings of the i h CIMTEC, Montecatini Terme, Italy, 1990.

9 Het wordt in toenemende mate een probleem dat wetenschappers meer interesse hebben in het gereedschap (computers) dan in het werk waarvoor het gebruikt dient te worden (de wetenschap). R.P. Feynman, "Surely You're Joking, Mr. Feynman", Bantam Books, Toronto, 1985, p 109.

10 De term "spin quantum getal" wekt verkeerde associaties op en kan om deze reden beter vervangen worden door een term waarbij men zich niets voor kan stellen. o.a. in M Alonso and E. Finn, "Fundamentele natuurkunde, 4 quantumfysica", Elsevier, Amsterdam, 1979, p 167. and in M. Karplus and R. N. Porter, "Atoms and : An Introduction for students of Physical Chemistry", Menlo Park, California, 1970, p 152.

11 Ben mogelijkheid om de kosten van het universitair onderwijs nog verder te reduceren zou zijn om na alle technici en ander ondersteunend personeel vervolgens ook de universitair docenten en hoogleraren te vervangen door aio- of oio-ers. Of de kwaliteit van het onderwijs hierdoor bevorderd wordt, is echter zeer twijfelachtig.

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