Phase Equilibria and Properties of Glasses in the Al2o3-Yb2o3-Sio2 System

Paper Journal of the Ceramic Society of Japan 101 [10] 1101-1106 (1993)

Phase Equilibria and Properties of Glasses in the Al2O3-Yb2O3-SiO2 System

Yuichiro MURAKAMI and Hirokazu YAMAMOTO AdvancedTechnology Research Center, Mitsubishi Heavy Industries, Ltd., 1-8-1, Sachiura,Kanazawa-ku, Yokohama-shi 236

Al2O3-Yb2O3-SiO2系の相平衡とガラスの性質

村上勇一郎・山本博一三菱重工業 (株)基盤技術研究所, 236横浜市金沢区幸浦1-81

[Received May 10, 1993; Accepted July 13, 1993]

The phase diagram and properties of glasses of the highest softening temperature in the rare-earth-con Al2O3-Yb2O3-SiO2 system were studied by X-ray diffrac taining aluminosilicate glasses. However, the glass tion, differential thermal analysis, scanning electron forming region and details of the properties of this microscopy and infrared absorption spectroscopy. The glass is still unknown. lowest solidus temperature in this system was 1500•Ž. Regarding the phase diagram of the 3-component The glass-forming region was determined by quench Al2O3-Ln2O3-SiO2 systems, phase diagram of the ing specimens after melting in an infrared image fur Al2O3-Y2O3-SiO2 system has been already repor nace. The glass transition temperature, the onset and ted.9)-11) Phase diagrams of the systems containing peak temperatures for crystallization were investigat ed as a function of composition and found to increase other rare-earth oxides have not been studied. with increase in SiO2 concentration. The activation Although phase diagrams of the rare-earth-containg energy for crystal growth in glasses tended to attain a systems are suggested to be similar to each other, maximum at the eutectic composition. It is suggested the liqudus and solidus temperatures of them are ex that Yb ion is mainly a network modifier in glasses. pected to be different depending on the kinds of rare The solidus temeperature of this system was compared earth elements. with those of the systems containing other rare-earth In this study, phase equilibria of the Al2O3-Yb2O3 oxides. - SiO2 system and the properties of glasses in this sys tem have been investigated, and the solidus tempera Key-words: Phase diagram, Al2O3-Yb2O3-SiO2 system, ture of this system has been compared with those of Glass, Glass forming region, Crystallization, Infrared spec tra, Solidus temperature, Rare-earth the other rare-earth-containing systems to clarify the difference of phase diagrams among them.

2. Experimental procedures 1. Introduction 2.1 Preparation of specimen and phase dia

The Al2O3-Ln2O3-SiO2 systems (Ln=rare-earth gram element) produce the rare-earth-containing alumino The starting materials used were Al2O3 (purity: silicate glasses with high softening temperature, 99.9%, averaged particle size: 0.96ƒÊm) , Yb2O3

high elastic modulus and high alkaline urability,1)-7) (99.9%, 1.0ƒÊm) and SiO2 (99.99%, 0.94ƒÊm). and have attracted attention as a possible application These powders were mixed in ethanol to a given com

to laser glass,1) glass fiber for fiber reinforcement,3) position. This mixture was dried and pressed into a radiotherapy,5) etc. compact of 10•~10•~40mm under a pressure of 70 Shelby and Kohli4) have studied the properties of MPa and then calcined at 1400•Ž for 1h. The cal these glasses and shown that the glass transition tem cined products were melted in an infrared image fur

perature and the softening temperature tend to rise nace with a halogen lamp as the light source and as the radius of rare-earth ion decreases. As an ori quenched to prepare specimens.

gin of this phenomenon, since the rare-earth ion is The liquidus and the solidus temperatures of each suggested to act as a network modifier in glasses, it specimen were determined by using a self-made is considered that the field strength of rare-earth ion differential thermal analyzer (DTA) during heating increases with decrease in ionic radius and that the at a rate of 5•Ž/min up to the maximum heating tem

bond strength between rare-earth ion and surround perature of about 1700•Ž. In this analyzer, specimen ing oxygen may increase.4),7) In another glass system was embedded in a high purity alumina crucible with containing rare-earth element, Li et al.8) have report 10mm in diameter and 30mm in length and the tem ed that the rare-earth ion acts as a network modifier perature of specimen was measured directly by the in the BaO-Y2O3-SiO2 glasses. Pt 40%Re-Pt20%Re thermocouple. To estimate Because the ionic radius of Yb is smaller than the error of measuring temperature, the eutectic tem those of other rare-earth elements, the Al2O3-Yb2O3 perature of MgO-SiO2 and Al2O3-Y2O3-SiO2 sys SiO2 system is expected to produce glasses with the tems10),11) was analyzed by this analyzer and it was

1101 1102 Phase Equilibria and Properties of Glasses in the Al2O3-Yb2O3-SiO2 System

found that the accuracy of temperature was •}2•Ž.

In order to determine the phase diagram at 1550•Ž, specimen was heat-treated at 1550•Ž for 0.5h in a high purity alumina crucible and then quen ched to prepare specimen for X-ray analysis. The specimen, which had small quantity of liquid phase and no softening due to melting at 1550•Ž, was fur ther heat-treated at the same temperature for a maxi mum of 10h until the phase equilibrium is confirmed to be attained by the X-ray analysis. These specimen were cut and polished and then analyzed y the X ray diffraction to determine the crystal structure and to identify the phases. The phases at other tempera Fig. 1. Liquidus temperature of the 2-component system as a ture were analyzed by the same method using speci function of Al2O3 content; men quenched from each temperature. The micros (a) (Yb2O3)37.9(SiO2)62.1-(Al2O3)70.2(SiQ2)29.8 tructures of specimens were analyzed y scanning ,(b) (Yb2O3)33.3(SiO2)66.7-(Al2O3)60(SiO2)40, (c) (Yb2O3)26.2(SiO2)73.8-(Al2O3)57.9(SiO2)42.1, electron microscopy (SEM) and the X-ray microanal (d) (Yb2O3)22.1(SiO2)77.9-(Al2O3)52.3(SiO2)47.7, ysis (EPMA). (e) (Yb2O3)19.9(SiO2)80.1-(Al2O3)49(SiO2)51. To compare the solidus temperatures of the (f) (Yb2O3)18.6(SiO2)81.4-(Al2O3)46.9(SiO2)53.1.

present system to those of the other rare-earth con (g) (Yb2O3)15.7(SiO2)84.3-(Al2O3)41.9(SiO2)58.1, taining systems, the solidus temperature of speci (h) (Yb2O3)13.2(SiO2)86.8-(Al2O3)37.1(SiO2)62.9, (i) (Yb2O3)12.3(SiO2)87.7-(Al2O3)35.2(SiO2)64.8, mens with (Al2O3)20(Ln2O3)20(Si2O3)60 composition (j) (Yb2O3)9.2(SiO2)90.8-(Al2O3)28.2(SiO2)71.8. were also determined y DTA, where Ln represents Dy, Er, G, La, Nd, Sm, Y and Yb. Purity and averaged particle size of the rare-earth oxides are Al2O3 (A) , Yb2O3 (Y) and liquid phase (L). 99.9% and 1ƒÊm, respectively. The solidus tempera Figure 1 represents the composition dependence tures of the specimens below 1400•Ž were deter of liquidus temperature in the ten types of 2-compo

mined y a Rigaku TAS200 system differential ther nent systems (Yb2O3)100-p(SiO2)p-(Al2O3)100 -q

mal analyzer (accuracy of temperature: •}1•Ž). (SiO2)q determined by DTA. These results can clari 2.2 Preparation of glasses and properties fy the region of the existence of liquid phase at high The calcined products described above were melt temperature. The composition dependence of liqui ed in an infrared image furnace and quenched to pre dus temperature in each system attained a minimum.

pare glasses and then the glass-forming region was An arrow in Fig. 1 depicts a composition with the determined. The glass transition temperature (Tg) lowest liquidus temperature in this system, i.e. the

and the onset and peak temperatures of crystalliza ternary eutectic composition (Al2O3)23 .6(Yb2O3)10.3 tion (Tc and T0, respectively) were determined by (SiO2)66.1), where its eutectic temperature is DTA (Rigaku TAS200 system) at a heating rate of 1500•Ž. 5•Ž/min as a function of composition. The heating Phase boundary of the liquid phase at 1550•Ž was rate of DTA was varied in a range from 3 to 10•Ž/ determined by plotting the compositions where the li min and the activation energy for crystal growth in quidus temperature becomes 1550•Ž in the liquidus glasses was determined by analyzing the heating temperature vs. composition curves shown in Figs. rate dependence of T0. The Fourier transform in 1, 4, 5 and 6, and the phase diagram was determined frared absorption spectra of glasses were also meas by analyzing the crystal structure of specimens quen uredure d to clarify the role of rare-earth ion in glasses y ched from 1550•Ž by X-ray diffraction. The phase di a JEOL FT-IR spectrophpotometer. agram of the Al2O3-Yb2O3-SiO2 system at 1550•Ž o btained in this work is shown in Fig. 2. Liquid phase 3. Results and discussion does not appear in a region of higher Yb2O3 composi 3.1 Phase equilibria in Al2O3-Yb2O3-SiO2 sys tion than that of the straight line between D and G in tem Fig. 2. The AY phase (Al2Yb4O9) was confirmed to The phase diagrams established in this study were exist at 1550•Ž by the X-ray analysis, which is in of the 3-component Al2O3-Yb2O3-SiO2 system at agreement with the phase diagram of the 2-compo 1550•Ž and of the three types of 2-component sys nent Al2O3-Yb2O3 system by Mizuno and Nogu tems Yb2Si2O7-Al6Si2O13, Al1 .25Yb0.75O3-SiO2 and chi,12) Al1 .44Yb0.56O3-Si02. The phases appeared in these di The SEM micrographs (backscattered electron im agrams are SiO2 (cristobalite, abbreviated to C, ap age) of specimens quenched from 1550•Ž were peared at 1470-1713•Ž), SiO2 (tridymite, T, 870 shown in Fig. 3. White portions in the photograph 1470•Ž) , Yb2Si2O7 (ytterbium disilicates, D), show the crystals which are rich in the elements with

Yb2SiO5 (YS), Al2Yb4O9 (AY), Al1 .25Yb0.75O3 (ytter large atomic number such as Yb. The crystals were bium aluminium garnet, G), Al6Si2O13 (mullite, M), also identified by analyzing an X-ray image using Yuichiro MURAKAMI et al. Journal of the Ceramic Society of Japan 101 [101] 1993 1103

EPMA. The cross sections of the observed crystal solidus temperature is 1500•Ž. In the temperature lites in Fig. 3 were polyhedral in the case of D, G and range of 1500-1505•Ž, coexistence of the three

M phases, whereas massive in the case of C phase. In phases L+D+M is expected. However, this phase a region shown by "L" in Fig. 3 (b), there appear the equilibrium has not been confirmed. These unestab microstructures with the contrast between light and lished phase boundaries are shown by broken lines in shade, suggesting the occurrence of phase separa the figure. The liquidus temperature above 1700•Ž tion in the glassy phase, which was formed by cool was estimated by extrapolating the experimental ing the liquid phase. Because the cooling rate of value of liquidus temperature to the melting points specimen was about 60•Ž/min in this experiment, of Yb2Si2O7 and Al6Si2O1311) in Fig. 4. the glassy phase formed during cooling the liquid Phase diagrams of the Al1 .25Yb0.75O3-SiO2 and phase might be separated into compositions of high Al1.44Yb0.56O3-SiO2 systems are shown in Figs. 5 and and low Yb2O3 concentrations. This phenomenon is 6. In the region of lower SiO2 concentration (<54 similar to the phase separation reported in the Al2O3 mol% in Fig. 5 and <51mol% in Fig. 6), the solidus

Y2O3-SiO2 glasses.10) temperature is 1505•Ž. In the region of higher SiO2 Figure 4 presents the phase diagram of the 2-com ponent Yb2Si2O7-Al6Si2O13 system. In this system,

Fig. 3. SEM micrographs (back scattered electron image) of Fig. 2. Phase diagram of the Al3O3-Yb2O3-SiO2 system at the specimen quenched from 1550•Ž. 1550•Ž. (a) (Al2O3)47(Yb2O3)18(SiO2)35 with L+A+G phases, L: represents liquid phase, C: SiO2 (cristobalite), D: Yb2Si2O7, (b) (Al2O3)10(Yb2O3)10(SiO2)80 with L+C+D phases, YS: Yb2SiO5, Y: Yb2O3, AY: Al2Yb4O9, G: Al1 .25Yb0.75O3, A: Al3O3, (c) (Al2O3)25(Yb2O3)25(Si02)50 with L+D+G phases, M: Al6Si2O13. (d) (Al3O3)7(Yb2O3)33(SiO2)60 with L+M phases.

Fig. 4 Fig. 5 Fig. 6

Fig. 4. Phase diagram of the Yb2Si2O7-Al6Si2O13system. Symbols of phases show the same phases as those in Fig. 2. Fig. 5. Phase diagram of the Al1.25Yb0.75O3-SiO2system. T: represents SiO2 (tridymite). Symbols of phases show the same phases as those in Figs. 2 and 4. Fig. 6. Phase diagram of the Al1.44Yb0.55O3-SiO2system. Symbols of phases show the same phases as those in Figs. 2, 4 and 5. 1104 Phase Equilibria and Properties of Glasses in the Al2O3-Yb2O3-SiO2 System

concentration (•†54mol% in Fig. 5 and •†51mol% content, 33.2mol%), suggesting that the glass tends in Fig. 6), the solidus temperature is 1500•Ž, which to become stable as the composition approaches the is the lowest temperature of the appearance of liquid eutectic composition. phase in the Al2O3-Yb2O3-SiO2 system. In these The glass transition temperature (Tg) and the on figures, the regions c and f are considered to be set and peak temperatures for crystallization (TC L+D+M and L+A+M, respectively, although and T0, respectively) of glasses in the 2-component their phase equilibria have not been confirmed at Al2-xYbxO3-SiO2 systems (x=1.0, 0.75 and 0.56) present. The transformation temperature from are shown in Fig. 9 as a function of SiO2 content. Tg cristobalite to tridymite has been reported to be tends to increase slightly as the SiO2 concentration 1470•Ž;11) this could not be detected clearly by DTA. increases. Tc and T0 tend also to increase as the SiO2 In a specimen with the ternary eutectic composi concentration increases. This may come from the tion shown by an arrow in Fig. 1, it was found by the fact that the increase of SiO2 concentration increases -ray analysis that the specimen was of the coexis the number of network former, which may make the tence of three phases Yb2Si2O7+Al6Si2O13+SiO2 glass stabler against crystallization. These are simi (the same phases with the area shown by D+M+C lar to those reported in the Al2O3-Y2O3-SiO2 or D+M+T in Figs. 5 and 6) at a temperature glasses.10),13) range below solidus temperature. Thereby, this ter In the previous paper,10) the activation energy for nary eutectic composition is considered to be of the crystal growth (ƒ¢E) has been shown to become max 3-component Yb2Si2O7-Al6Si2O13-SiO2 system. imum at the eutectic composition in the Al2O3-Y2O3- 3.2 Properties of glasses SiO2 glasses. In order to clarify the possibility of The glass-forming region of the Al2O3-Yb2O3 such a phenomenon in the present glasses, ƒ¢E was SiO2 system was determined by quenching specimen determined by analyzing the heating rate (ƒ¿) depen after melting in an infrared image furnace. Figure 7 illustrates the compositions from which the glasses were obtained (•›), the compositions which were partially crystallized (•¢), and the compositions which were almost crystallized (•œ), when the speci men was quenched from the melt. The real line represents the glass-forming region of the Al2O3- Yb2O3-SiO2 system. Although the exact tempera ture of melting specimen in an infrared image fur nace is uncertain, it is assumed to be about 1750•Ž as suggested in the previous paper.10) Figure 3 shows the composition dependence of the glass transition temperature (Tg) and the onset and peak temperatures for crystallization (TC and To, respectively) of glasses in the 2-component Y2Si207 Al6Si2O13 system. Dependence of Tg on the Al2O3 concentration is small. T0 attained a maximum at a composition near the eutectic composition (Al2O3 Fig. 7. Glass-forming region of the Al2O3-Yb2O3-SiO2 system.

Fig. 8 Fig. 9 Fig. 10

Fig. 8. Composition dependence of the glass transition temperature (Tg) and the onset and peak temperatures for crystallization (Tc

and T0, respectively) of glasses in the Yb2Si2O7-Al6Si2O13 system. Fig. 9. Composition dependence of the glass transition temperature (Tg) and the onset and peak temperatures for crystallization (Tc and T0, respectively) of glasses in the system; (a) AlYO3-SiO2 , (b) Al1.25Yb0.75O3-SiO2 and (c) Al1.44Yb0.56O3-SiO2• Fig. 10. Activation energy for crystal growth (ƒ¢E) in glasses of the Yb2Si2O7-Al6Si2O13 system as a function of Al2O3 content . Yuichiro MURAKAMI et al. Journal of the Ceramic Society of Japan 101 [10] 1993 1105

deuce of T0 by DTA, varying the heating rate in a due to bending vibration of SiO4 tetrahedron in Fig. range from 3 to 10•Ž/mm. The following modified 11(b) is weaker than those in Figs, 11 (a) and (c), Kissinger equation14) was used to determine ƒ¢E. because the SiO2 concentration of a specimen in Fig. ln(ƒ¿n/T02)=-mƒ¢E/(RT0)+const. (1) 11(b) is smaller than those of others. The present where m and n are constants dependent on the result suggests that the Y ion acts mainly as a net mechanism of crystal growth. The values m=3 and work modifier in glasses, which is consistent with n=4 were used, based on the proposal by Matusita the prediction of Shelby and Kohli4) and Tanabe et and Sakka,14) since the crystal growth is suggested al.7) to occur by the 3-dimensional volumetric crystalliza 3.3 Solidus temperature of rare earth-contain tion in the glassy specimen for DTA in the present ing systems system. Figure 10 shows the activation energy for Phase diagram of the Al2O3-Yb2O3-SiO2 system crystal growth (ƒ¢E) of glasses in the Yb2Si2O7- obtained in this work is similar to that of the Al2O3 Al6Si2O13 system as a function of composition. The - Y2O3-SiO2 system.9),10) However, the solidus temper ƒ¢ E value attained a maximum at a composition near ature in the former is higher than that in the latter. the eutectic composition in this system. This result Phase diagram of the rare-earth-containing Al2O3 suggests that the growth rate of crystals in glasses - Ln2O3-SiO2 systems is considered to e similar to decreases and the stability of glasses increases as the each other and the solidus temperatures of them are composition of glass approaches the eutectic compo dependent on the kinds of rare-earth elements. As sition. The ƒ¢E values observed in the present glass described above, the lowest solidus temperature in es are in the same order with those (540-770kJ/ the present system is that (1500•Ž) of the Yb25i207 mol) in the Al2O3-Y2O3-SiO2 glasses.10 Al6Si2O13-SiO2 system. In order to compare briefly ) In the rare-earth-containing glasses, the rare-earth the phase diagram of the Al2O3-Yb2O3-SiO2 system ions have been reported to act as a network modifier with those of the other rare-earth-containing sys in glasses.4)-8) To confirm the role of Yb ion in the tems, solidus temperature of the Ln2Si2O7-Al6Si2O13

present glasses, the infrared absorption spectra of SiO2 systems (Ln=Dy, Er, Gd, La, Nd, Sm, Y, Yb) the Al2O3-Yb2O3-SiO2 glasses were measured. The was determined by DTA using specimens with the results are shown in Fig. 11. There appear the three (Al2O3)20(Ln2O3)20 (SiO2)60 composition. Although types of absorption bands; the first band near 1100 Y is not rare-earth, Y-containing system is also pre

cm-1 is considered to be due to the stretching vibra pared for comparison. tion of bridging oxygen (O-Si-O) in the network, Figure 12 shows a relation between the solidus the second one near 920cm-1 to that of non-bridging temperature and the radius of rare-earth ion in the oxygen (Si-O-) and the third one near 460 cm-1 to Ln2Si2O7-Al6Si2O13-SiO2 systems. Solidus tempera the bending vibration of O-Si-O bond in the Si4 ture tends to increase as the radius of rare-earth ion tetrahedron.13),15) As the Yb2O3 concentration in decreases in these systems. In Fig. 12, the Yb-con creases in Fig. 11, absorption of a band near 1100 taining system has the highest solidus temperature cm-1 due to bridging oxygen becomes weaker and among them, where Yb has the smallest ionic radius. that near 920cm-1 due to non-bridging oxygen Similar phenomena have been reported in the becomes stronger. This result indicates that the num other rare-earth-containing systems. Anderson and ber of bridging oxygen tends to decrease and the Eatton16) have shown that the eutectic temperature number of non-bridging oxygen tends to increase as of the Si3N4-Ln2O3-SiO2 system tends to increase as

Al2O3 is replaced y Yb2O3. A band near 460cm-1 the radius of rare-earth ion decreases. Shelby et al.4),7) have shown that the glass transition tempera ture and the softening temperature of Al2O3-Ln2O3 -SiO2 glasses tend to increase as the radius of rare earth ion decreases. As an origin of this phenomenon in these glasses, it is considered that the increase in field strength of rare-earth ion due to the decrease in

Fig. 11. Fourier transform infrared absorption spectra of the glasses; (a) (Al2O3)32(Yb2O3)5.9(SiO2)62.1, (b) (Al2O3)35.8(Yb2 Fig. 12. Solidus temperature of the 3-component Ln2Si207-A16 O3)18.6(SiO2)45.6and (c) (Al2O3)15 (Yb2O3)19.4(SiO2)65.6. Arrows Si2013-Si02 system (Ln = Dy, Er, Gd, La, Nd, Sm, Y, Yb) as a show position of absorption bands. function of the radius of rare-earth ion. 1106 Phase Equilibria and Properties of Glasses in the Al2O3-Yb2O3-SiO2 System

ionic radius may increase the binding energy of glass activation energy for crystal growth attain a maxi es. Thereby, the properties of rare-earth-containing mum at a composition near the eutectic composition. glasses are considered to be affected by the radius of (5) Analyzing the infrared spectra of glasses, it rare-earth ion. is suggested that Yb ion is mainly a network Origin of the relationship between solidus tempera modifier in glassses. ture and ionic radius in the 3-component systems in (6) Solidus temperature of the Ln2Si207 Fig. 12 is not clarified at present. Because this soli Al6Si2O13-SiO2 system (Ln=Dy, Er, Gd, La, Nd, dus temperature is the lowest temperature where Sm, Y, Yb) tend to increase as the radius of rare the three solid phases (Yb2Si2O7, Al6Si2O13, SiO2) earth ion decreases. The system with Yb has the and the liquid phase coexist, there might be some highest solidus temperature among them. possibility that the difference of formation energy (ƒ¢H) among these phases is dependent on the References radius of rare-earth ion. Therefore, the free energy 1) T. Izumitani, H. Toratani and H. Kuroda,J. Non-Cryst. of these phases should be analyzed in future to clari Solids,47, 87-100 (1982). fy the origin of this phenomenon. This is an interest 2) M. Imaokaand T. Yamazaki,Yogyo-Kyokai-Shi, 70, 115-23 (1962). ing problem from the view point of the materials de 3) A. Makishimaand T. Shiohira, J. Non-Cryst.Solids, 38, sign of ceramics. 661-66 (1980). 4) J. E. Shelbyand J. T. Kohli,J. Am. Ceram.Soc., 73, 39-42 4. Summary (1990). Phase diagrams of the Al2O3-Yb2O3-SiO2 system 5) E. M. Erbe and D. E. Day,J. Am. Ceram.Soc., 73, 2708-13 (1990). and the properties of glasses in this system were in 6) A. Makishima,Y. Tamuraand T. Sakaino,J. Am. Ceram. vestigated. The results are summarized as follows: Soc.,61, 247-49 (1978). (1) Phase diagram of the 3-component Al2O3- 7) S. Tanabe, K. Hirao and N. Soga,J. Am. Ceram.Soc., 75, Yb2O3-SiO2 system at 1550•Ž and those of the three 503-06 (1992). 8) Y. Li, H. Kozukaand S. Sakka, Yogyo-Kyokai-Shi,95, 538 types of 2-component Yb2Si2O7-Al6Si2O13, Al1 .25 -44 (1987). Yb0 .75O3-SiO2 and Al1.44Yb0.56O3-SiO2 systems were 9) I. A. Bondarand F. A. Garakhov,Izv. Akad . NaukSSSR Ser. established and the region where the liquid phase ex Khim., 1963, 1325-26 (1963). ist at a high temperature was clarified. The lowest 10) Y. Murakamiand H. Yamamoto,Seramikkusu Ronbunshi , solidus temperature in the present system is 1500•Ž. 99, 215-21 (1991) .11) "Phase Diagramfor Ceramist",Am. Ceram. Soc., Vol. 1 (2) The glass-forming region of the Al2O3 (1964) p. 123; Vol.2 (1969)p. 108; Vol.3 (1975) p. 132. - Yb2O3-SiO2 system was clarified by quenching the 12) M. Mizunoand T. Noguchi,Yogyo-Kyokai-Shi , 88, 322-27 ( melt in an infrared image furnace 1980). (3) The glass transition temperature of the 13) K. Odaand T. Yoshio,Seramikkusu Ronbunshi , 97, 1493-97 ( present glasses is in a range of 880-895•Ž. The onset 1989). and peak temperatures for crystallization are in a 14) K. Matusita and S. Sakka,J. Non-Cryst.Solids , 38, 741-46 ( 1980). range of 975-1040•Ž and 1070-1230•Ž, respectively. 15) H. Toyuki, Yogyo-Kyokai-Shi,85, 263-68 (1977). They tend to increase as the SiO2 concentration is in 16) C. A. Andersonand R. Batton,Ceramic Materials for High creased. TemperatureTurbines, Final Tech. Rep.,U.S. EnergyRes . (4) In the 2-component Yb2Si2O7-Al6Si2O13 sys Dev., No. EY76C055210(1977). tem, the peak temperature for crystallization and the