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

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Alkali Metal Corrosion of Alumina : Thermodynamics, Phase Diagrams and Testing Alkali metal corrosion of alumina : thermodynamics, phase diagrams and testing Citation for published version (APA): Hoek, van, J. A. M. (1990). Alkali metal 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 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. 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 door 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 metals in combination with aluminium oxide 12 1.2 Alkali metal corrosion of aluminium oxide 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 caesium oxide - aluminium oxide system 36 3.1.2 The potassium oxide - aluminium oxide system 37 3.1.3 The calcium oxide - aluminium oxide system 38 3.1.4 The magnesium oxide - aluminium oxide system 42 3.1.5 Alkali metal oxide - calcium oxide systems 44 3.1.6 Alkali metal oxide - magnesium oxide systems 44 3.2 Quasi-ternary systems 44 3.2.1 The Cs 0 - alkaline earth metal oxide - Al 0 systems 44 2 2 3 3.2.2 The K 0 - alkaline earth metal oxide - Al 0 systems 45 2 2 3 Literature 50 6 4 GENERAL EXPERIMENTAL SET UP FOR PHASE DIAGRAM 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 ALUMINATE 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 Ion 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 solid 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 melting point. 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 pressure sodium discharge lamps. Because of the high pressure (about lOs Pa), needed to obtain a broad spectrum of light, the temperature 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 temperatures. 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 (plasma) 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 electron 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.
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