AN INVESTIGATION OF RELATIONSHIP BETWEEN PHASE SEPARATION AND CRYSTALLIZATION OF ZnO-Al2O3-SiO2 GLASSES

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AN INVESTIGATION OF RELATIONSHIP BETWEEN PHASE SEPARATION AND

CRYSTALLIZATION OF ZnO-Al2O3-SiO2 GLASSES

Li Jiazhi, Shan Ying, Hu Guanging

To cite this version:

Li Jiazhi, Shan Ying, Hu Guanging. AN INVESTIGATION OF RELATIONSHIP BETWEEN

PHASE SEPARATION AND CRYSTALLIZATION OF ZnO-Al2O3-SiO2 GLASSES. Journal de

Physique Colloques, 1982, 43 (C9), pp.C9-231-C9-234. �10.1051/jphyscol:1982941�. �jpa-00222470�

Col2oque C9, suppllmont au

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AN INVESTIGATION OF RELATIONSHIP BETWEEN PHASE SEPARATION AND CRYSTAL- LIZATION OF Zn0-A1203-Si02 GLASSES

Li Jiazhi, Shan Ying and Hu Guanging

Shanghai I n s t i t u t e

Ceramics, ~ c a d e m i a

China

RQsum6.- Les rdsultats de la microscopic glectronique, le 1'ATD et de la dif- fraction des rayons X montrent qu'il y a une assez forte tendance de la s6pa- ration de phase dans les verres de systBme Zn0-A1203-Si02 (ZAS). Pendant le processus de cristallisation, la solution solide de B-quartz pourra Stre obte- nue comme phase primaire mstastable, si le degrQ de sQparation de phase est petit dans le verre. La solution solide de B-quartz et celle de 6-Willemite seront obtenues si la ssparation de phase est assez complSte. Non seulement, la sdparation de phase dans le verre influence la composition des phases cristallines primaires, mais encore accGl2re la cristallisation du verre ZAS.

Abstract.- TEM, DTA, X-ray diffraction results reveal that there is a strong tendency to phase separation into two immiscible liquid phases in the Zn0-A1203- Si02 (ZAS) glasses. 6-quartz solid solution would precipitate as metastable initial crystalline phase, when the degree of phase separation of the original glass is low, whereas 8-quartz solid solution and 6-Willemite solid solution would both be ~recipitate when the degree of phase separation is more complete.

It was found that degree of phase separation not only determines the initial crystalline phase, but also accelerates the crystallization of the ZAS glasses.

I. Introduction.- Phase separation and its relation to crystallization is one of the most interesting problems in modern glass science. Results of investigation on phase separation and crystallization of Mg0-A1203-Si02-Ti02 glasses show that oxides com- position and thermal history are the main factors affecting phase separation. There is also some evidence demonstrating that the extent of phase separation has some influence on the type of crystalline phase and properties of glass-ceramic /I/. It has been shown that phase separation was found in ZnO-Si02 system and Zn0-A1203- Si02-Zr02 glasses /2-5/. In the present paper, relationship between phase separation and crystallization of Zn0-A1203-Si02 glasses were studied through DTA, TEM and X- ray diffraction analyses.

11. Experimental results.-

(1) Compositional range of glasses.- Compositional range of transparent ZAS glasses melted at 1600°C and formed by quenching in air was found to be Si02 40-60 %, A1 O3 10-20 %, ZnO 20-50 %. Three glasses were prepared, their oxides composition are ?is- ted in Table I.

glass ^NO.

65 25

Table I. Composition of glasses (mol %)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982941

C9-232 JOURNAL DE PHYSIQUE

(2) Selection of thermal history conditions.- In order to obtain glasses with va- rying extent of phase separation, samples with different thermal history were pre- pared : (a) The glass melt was quenched from 1600°C in water. (b) Annealed at 650°C for 2 hours. (c) Quenched in air then annealed at 700°C for 10 or 100 hours.

(3) Transmission electron microscopic examination.- TEM photographs of three glasses quenched in water directly from melt and annealed for 2 hours at 650°C are shown in Fig. 1 and Fig. 2. Amounts of liquid droplets phase presented in samples of glass No. 1 and No. 2 annealed at 650'~ for 2 hours were greater than those in water quen-

ched samples. For glass No.3 phase separation was found, no matter whether it was water quenched or annealed. Fig. 3 illustrates the phase separation of glass No.1 with various thermal history. Raising the annealing temperature or prolonging the annealing time of glass No. 1 growth of liquid droplets phase occurred, thus its amount was increasing.

No. 1 No. 2 Ne. 3

Fig. 1 : TEM photographs of three glasses quenched In water directly from melt.

No. 1 No. 2 No. 3

Fig. 2 : TEM photographs of three glasses annealed at 6 5 0 ' ~ for 2 hours

(4) DTA curve and identification of crystalline phase.- DTA curves were at a heating rate of 10°C per minute for three glasses annealed for 2 hours at 650°C and are shown in Fig.4. Fig.5 illustrates the DTA curves and crystalline phases corresponding to various exothermic reactions for samples of glass No.1 with different thermal his- tory. They show that as the duration of annealing prolongs to 100 hours, 20°C lowe- ring of the ower limit of crystallization results, the rate of crystallization will be accelerated too.

Fig. 3 : TEM photographs of glass No. I with various thermal histories.

(a) Quenched in a i r . ( b ) Quenched in air, annealed at 700°C for 10 hours.

(c) Quenched in air, annealed at 70O0C for 100 hours.

W dm go0 ~ a a 'c

Fig. 5 :DTA curves and corresponding crystalline phases of glass No. 1 with various thermal histories. I-ouenched in air. 2-quenched in air, annealed for 10 hrs. at 700°C. 3-quenched in air, annealed for 100 hrs. at 700°~.

B-Qsas. --8-quartz solid solution E-Q,.

,

W.-- ~illemite, G . --gahnite, C. --cristobalite.

111. Discussion.- The composition of glass No. 3 is located very near to an immis- cible region in the high-silica side of Si02-A1 03-ZnO ternary phase diagram, so that the glass sample water quenched from 1600°C woufd retain the two immiscible phases.

As to the composition of these two immiscible phases, we tried to predict them from the following experimental results. It was shown from the TEM protographs, that the liquid droplets protrude over the matrix phase, which indicates that the etching rate of matrix phase by hydrofluorid acid is larger than that of liquid droplet phase. Besides, from the results of electrical conductivity measurements of glass samples No. 2 with different thermal annealing history (650°C/2 hr. 2.4~10-10

(a-

cm)-I, 75O0~/10 hr. 9.8~10-12 (a-cm)-I), it can be seen that by raising or prolong- ing the annealing temperature or time, their electrical conductivity would decrease with the coarsening and increasing the amount of liquid droplets phase. That points to the decreasing of electrical conducting carrier-zinc ions in the continuous ma- trix phase or increasing of zinc ions in the isolated liquid droplet phase. These results allocj us to predict that the matrix phase is a silica-rich phase, while the liquid droplet phase is zinc-rich.

C9-234 JOURNAL DE PHYSIQUE

Atoms or atomic groups in the glass will rearrange correspondingly when it trans- forms into crystalline state. The appearence of metastable initial crystalline phase demonstrating that the chemical composition of one phase in glass is very close to that of this crystalline phase. In our experiments, B-quartz,.,. (the composition of which is very near to that of silica rich matrix phase) will ~recipitate as a meta- stable initial crystalline phase, if thereis only a small amount of liquid droplet phase present in the glass. As the amount of Zn-rich liquid droplet phase in the glass increases, then B-Willemites

,.

Since the boundary interfaces formed around the liquid droplets will lower the work of formation of critical crystallites and so decrease the required energy of crys- tallization, the lower limit of crystallizations will shift to lower temperature.

IV. Conclusions.-

1. ZAS glass has a strong tendency to phase separation into two liquid phases. Trans- parent glasses could be formed in the compositional range of Si02 40-60 % Al2O3 10- 20 %, ZnO 20-50 %, when they were melted at 1600 OC and quenched in air.

2. During crystallization process, the type of initial crystallized phase is depen- dent on the different degree of phase separation.

3. Phase separation will accelerate the crystallization of ZAS glasses and lower the lower limit of crystallization temperature of the initial crystalline phase.

References.

/ I / Shan Ying, Li Jiazhi, Deng Zequn, Zhou Xueqin, J. of the Chinese Silicate Soc.,

9 (1981)403.

/2/ Syosi Yokoish, Hazine Saito, J. of the Ceramic Society of Japan, 86 (1978)308.

/3/ G. F. Neilson, Disc. Farad Soc., 50 (1970)145.

/4/

/5/ George H. Beall, Francis W. Martin, U.S. Pat. 3681097.