Primary and secondary phase separation of sodium silicate glasses

JOURNAL OF NON-CRYSTALLINE SOLIDS 1 (1968) 29--38 © North-Holland Publishing Co., A m s t e r d a m

PRIMARY

AND SECONDARY SODIUM

PHASE

SEPARATION

OF

SILICATE GLASSES

E. A. PORAI-KOSHITS and V. I. AVERJANOV Grebenshchiko v Institute of Silicate Chemistry of the Academy of Sciences of the U.S.S.R., Nab. Makarova, 2, Leningrad B-164, U.S.S.R. Received 4 July 1968 The immiscibility curve in the system Na20-SiOz shows that the amount of Na20 content in the silica rich phase is small at 500°C and hardly changes with temperature up to 800°C but increases sharply in the interval 800-865 °C. The silica content of the alkali-rich phase increases continuously with temperature. The curve is thus extremely asymmetric. The electron microscope photographs show that the separate phases (primary phase separation) separate again into two phases during cooling or during a second heating of the glass at a lower temperature. This secondary phase separation does not depend upon the duration of the initial heating of the glass. It is connected with the asymmetry of the immiscibility curve. The alkali-rich phase separates more easily but with the help of a repeated treatment the silica-rich phase can also be caused to phase separate. This secondary phase separation is a general phenomenon and must be given attention when discussing the number of phases in the glass. Certainly the existence of triple-phase glasses cannot be excluded. Phase separation of higher orders can be achieved with the help of heat treatments in several steps. 1. Introduction S o m e years ago we investigated the phase s e p a r a t i o n o f glasses a n d discovered, b y e l e c t r o n - m i c r o s c o p e a n d small-angle X - r a y scattering m e t h o d , the possibility t h a t besides the m a i n phases some " h i g h dispersive" structure m a y be f o r m e d l ) . Three s u p p o s i t i o n s were m a d e as to the origin o f such " f i n e " structure: a) t h a t it has a fluctuation nature2); b) t h a t there is a special type o f phase separation, the so-called micros e p a r a t i o n 3); c) t h a t such fine structure arises in the process o f quenching d o w n to r o o m t e m p e r a t u r e as a result o f o v e r s a t u r a t i o n o f one o f the phases a n d o f its s e c o n d a r y phase separation4). A little later, d u r i n g his investigation o f the phase s e p a r a t i o n o f s o d i u m silicate glasses, T r a n T h a c h - L a n f o u n d t h a t all phases are h o m o g e n e o u s ; t h a t is to say, he d i d n o t notice the existence o f fine structureS). N o w there is no d o u b t a b o u t the existence a n d n a t u r e o f this fine structure 6-8). M o r i y a , W a r r i n g t o n a n d D o u g l a s called it the " b a c k g r o u n d structure"8), b u t we 29

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E.A. PORAI-KOSHITS A N D V. I. AVERJANOV

think that it is more correct to speak about the "secondary separation" 7) to emphasize its nature. In the present paper the results of more detailed investigation of the secondary phase separation by electron-microscopy are presented. 2. Experimental The binodal curve o f the metastable immiscibility o f sodium silicate glasses showed in fig. 1 was plotted in the following way: T~C I 1700

1600

150O

14001 900

800

70C

50C

0

4

8

B. . 12

. . . 16

20

24

Na20, mole% 28

Fig. 1. The binodal curve of immiscibility for sodium silicate glasses (T1). AB is the line of equal volume phases, TL the liquidus line.

PRIMARY AND SECONDARY PHASE SEPARATION

31

The right branch of this curve (for compositions from 20 to 5 mole % Na20 ) was obtained by using the temperatures of transparency (liquation) determined by us earlierg). To obtain the left branch we used the so-called "rule of lever". According to this rule, the following equation can be written for any glass (e.g. " b " glass) under the cupola of immiscibility:

WNa abdsi = bcdNa Ws~' where dNa and dsi are macroscopic values of glass densities enriched in sodium oxide and in silica respectively (in this case with 1.3 and 17 mole % Na20); WNa and Wsl are volume fractions of these glasses. The latter were determined from the total area of droplets and matrix in electron microscopic pictures of the glass " b " by the linear method. Thus with the help of electron micrographs of glasses (the composition and temperature of which are represented in fig. 1 by crosses), the values marked with black circles were found and used to build the left branch of the binodal curve. Absolute values of the mean square difference between the electron densities of phases enriched in sodium oxide and silica, (Ap)2caL, can be calculated with the help of the binodal curve for all glasses under the cupola of immiscibility. These values can also be calculated experimentally by the smallangle X-ray scattering intensity curves without any information on the composition and volume of laminated glass phases being used. Experimental values of(Ap)2exp can also be determined in absolute units by comparing the intensities of primary and scattered radiations, or by standard specimen 9,10). In fig. 2, the comparison of plots (Ap)2~L and (Ap)2exp. versus the composition of glasses is given; the latter were determined (as in ref. 10) in absoluted units11). Some discrepancy of the upper curves may be easily explained by the lack of heating time for the specimens at a rather low temperature to reach an equilibrium state - the experimental points would rise after longer heating. But the general agreement between calculated and experimental curves proves the absolute correctness of the small-angle scattering data. Besides it makes possible the use of the binodal curve in fig. 1 for obtaining separate phase composition and its alteration during the heat treatment or its alteration as a result of changes of the general glass composition. The most striking feature of the binodal curve is its asymmetry which points to a different temperature dependence of the mutual solubility of the components. The solubility of alkali in silica (the left branch of the curve) is almost unchanged up to 800°C approximately, when it begins gradually to increase till the critical immiscibility temperature (860°C for glass with

32

E.A. PORAI-KOSHITS AND V.I. AVERJANOV

about 7.5 mole O//oN a 2 0 ) and reaches its top value. But the solubility of silica in alkali (the right branch of the curve) increases continuously with the rise of temperature. The asymmetry of the binodal curve should be borne in mind in the process of considering the separate stages of phase separation. (a?) 2

. . . . Calc. ---.-o---Exp.

ld4(~) 2

4

./.~/,f ~

~.~ ~.~\~..

'\

0

10

20 Na20,mole%

Fig. 2. Comparison of calculated and experimental (Ap) 2 curves (in absolute units) versus sodium silicate glass composition at: (1) 580°C, (2) 660°C, (3) 685°C, (4) 715°C and (5) T~> T~.

The secondary phase separation of a primary separated phase takes place as a result of oversaturation of the latter with silica or alkali under the lowering of temperature - either during the process of cooling (quenching) the glass or under the secondary heat treatment at lower temperature. The most favourable conditions for the secondary phase separation during normal cooling are when the phase composition is above the spinodal curve and the phase intersects it during lowering of the temperature. If we disregard the influence of the viscosity on the secondary phase separation, we shall find out that the phase enriched in alkali is easier exposed to the secondary phase separation owing to the asymmetry of the immiscibility cupola. The composition of this phase changes greatly and the oversaturation occurs more rapidly. One can see it from the comparison of photos (figs. 3 and 4). In glass with 7.5 mole ~o N a 2 0 (this composition is found somewhere to the left of the equal phase volumes line AB) the drops are enriched in alkali. In glass with 12.5 mole ~o N a 2 0 the matrix is enriched in alkali*). Let us pay attention to an interesting peculiarity of the photos shown on * The technique of glass preparation for the electron-microscope investigation is given in refs. 4 and 12.

33

PRIMARY AND SECONDARY PHASE SEP/~RATION

(a)

(c)

(b)

(d)

(e) Fig. 3. The secondary phase separation of the droplet phase enriched in alkali in glasses with 7.5 mole ~ Na~O: (a) before heating; and after heating at 836 °C for (b) 0.5 hr, (c) 1 hr, (d) 2 hr and (e) 4 hr. x 18900.

(a)

(b)

Fig. 4. The secondary phase separation of the matrix phase enriched in alkali in glasses with 12.5 mole ~ Na20 heated for 2 hr, (a) at 766°C, and (b) 797°C. × 18900.

34

E.A. PORAI-KOSHITSAND V. I. AVERJANOV

fig. 3. The regions of large sizes (of a primary liquation origin) grow as the time of heating at 836 °C grows, but the region of small sizes remains constant. The former is normal for the usual phase separation. But the secondary phase separation (in the case when it occurs in the phase the composition of which reached the binodal) does not depend on the time of heating at 836 °C because

(a)

(e)

(b)

(f) S7

(c)

(g)

(d)

(h) Fig. 5. The secondary phase separation in glasses with 12.5 mole ~ Na20 heated for 2 h r at: (a) 766°C, (b) 777°C, (c) 809°C, (d) 816°C, and then heated at 668°C for 2 hr (e)-(h). x 18900.

PRIMARYAND SECONDARYPHASESEPARATION

35

the secondary drops do not appear at that temperature, but only during the cooling from 836 °C. At the same time the secondary phase separation must obviously depend on the temperature difference of primary and secondary heat treatment, because this difference influences the degree of phase oversaturation with one of the components. Fig. 5 shows the photos of the glass with 12.5 mole ~ Na20. The specimens were heated twice: first at different temperatures under the immiscibility cupola (that is to say they had different primary phase composition, see fig. 1) and then at the same temperature plus a lower temperature of 668 °C to intensify the secondary phase separation process. The sizes of the secondary drops in the glassy matrix and their total volume increase noticeably as the difference of temperature between the primary and secondary heat treatment increases, because in this case the primary phase (matrix) appears to be more oversaturated with silica. As to the phase enriched in silica it can also be exposed to the secondary phase separation during the usual cooling process, but only after the heat treatment of the glass at high temperature or after the secondary heat treatment because this phase has higher viscosity and smaller oversaturation; the secondary heat treatment can increase the secondary phase separation if

(a)

(b)

(c) Fig. 6. The secondary separation of the phase enriched in silica. Glasses with 7.5 mole Na20. (a) heated at 836 °C for 4 hr, only the phase enriched in alkali separates; (b)heated at 849 °C for 4 hr, the phase enriched in silica (matrix) begins to separate; (c) double heating, at 849°C, for 4 hr, and at 720°C for 2 hr, the phase enriched in silica has separated distinctly. × 18900.

36

E.A. PORAI-KOSHITS AND V. I. AVERJANOV

it had already occurred once. Such cases are shown by the photos on fig. 6. In this case the volume of the separated phase is small which corresponds to the form of the left branch of the immiscibility curve. Thus the properties of the secondary inhomogeneous regions are fully determined by the form of the binodal curve and consequently have the same liquation nature as the primary phases. During the repeated heat treatment of the separated glass at the lower temperature the secondary drops can disappear from a primary phase. They can be removed to another phase which has the same composition as the secondary drops. Fig. 7 shows the gradual clearing of alkali phase from the small drops enriched in silica.

(a)

(b)

(c)

(d)

Fig. 7. The glass with 7.5 mole ~ Na~O. Secondary droplets disappear gradually as additional heat treatment goes on. The primary heat treatment is at 849°C for 4 hr (a). The secondary heat treatment is at 720 °C, (b) for 5 rain, (c) for 10 rain, and (d) for 60 rain. The general character of the secondary phase separation should be noted. It has been observed by us in lithium silicate glasses (fig. 8), and by Galakhov in alkali-earth-silicate systemsl~). It should also be borne in mind during the investigation of two or more component glasses. In this connection we may suppose that sodium-borosilicate glasses have the two-phase structure. The small-drop properties (which were mistaken for the third phase) are similar to the properties of the secondary drops in sodium silicate glasses. Finally, if the glass was exposed to the secondary phase separation at the temperature 7"1 and then to stepwise heat treatment (decreasing the tempera-

PRIMARY AND SECONDARY PHASE SEPARATION

37

ture each time), phase separation of the highest orders can be produced. A t the t e m p e r a t u r e T2 < T1, secondary phase separation appears. T h e n o n h e a t i n g once more at T 3 < T2, phase separation within the secondary separated phase will be of the third order etc. These processes are well controlled. T h a t is why the p r e p a r a t i o n of glasses with very different structure becomes possible.

Fig. 8. The secondary phase separation of the glass matrix with 23 mole ~ LieO and 77 mole ~ SiO2 heated at 720 °C, and then heated at a lower temperature. × 18900.

The p r o b l e m of the i n t e r n a l structure of individual phases a n d that of the glasses the c o m p o s i t i o n of which is outside the b i n o d a l curve is n o t considered in this work, t h o u g h some experimental data give evidence of their i n h o m o g e n e o u s structure. References

1) v. I. Averjanov, N.S. Andreev and E. A. Porai-Koshits, Critical phenomena in sodium silicate glasses, in: Physics of Non-Crystalline Solids, Ed. J. A. Prins (NorthHolland Publ. Co., Amsterdam, 1965) p. 580; E. A. Porai-Koshits, D.A. Goganov and V. I. Averjanov, A study of the supermolecular structure of silicate glasses by direct methods, in: Physics of Non-Crystalline Solids, Ed. J. A. Prins (North-Holland Publ. Co., Amsterdam, 1965) p. 117. 2) V. N. Filipovich, Some aspects of the formation of new phases in melt and glasses, in: The Structure of Glass, Vol. 5, Structural Transformations in Glass at High Temperature (Consultants Bureau, New York, 1965) p. 39. 3) Ya. Galakhov and S. Ya. Konovalova, Dokl. Akad. Nauk SSSR 155 (1964) 122. 4) V. I. Averjanov, N. S. Andreev and E. A. Porai-Koshits, in: Physics o f Non-Crystalline Solids, Ed. J. A. Prins (North-Holland Publ. Co., Amsterdam, 1965) p. 580; V. I. Averjanov and E. A. Porai-Koshits, Electron microscope investigation of the heterogeneous structure and initial crystallization stages of glasses in the system Li20-SiO2, in: The Structure of Glass, Vol. 5, Structural Transformations in Glass at High Temperature (Consultants Bureau, New York, 1965) p. 63. 5) Tran Thach-Lan, Verres et Refractaires 20 (1966) 8. 6) D. A. Gaganov and E. A. Porai-Koshits, Dokl. Akad. Nauk SSSR 165 (1965) 1037. 7) V. |. Averjanov and E. A. Porai-Koshits, in: VIth All-Union Congr. on Electron Microscopy, Novosibirsk, 1967. 8) Y. Moriya, D. H. Warrington and R. W. Douglas, Phys. Chem. Glasses 8 (1967) 19. 9) N. S. Andreev, D. A. Goganov, E. A. Porai-Koshits and Yu. G. Sokolov, in: Chemically heterogeneous structure of two component sodium silicate glasses, in: The

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10) 11) 12)

13)

E.A. PORAI-KOSHITS AND V. 1. AVERJANOV

Structure of Glass, Vol. 3, Catalyzed Crystallization of Glass (Consultants Bureau, New York, 1964) p. 47. N. S. Andreev and T. I. Ershova, Dokl. Akad. Nauk SSSR 172 (1967) 1299. D. A. Goganov and E. A. Porai-Koshits, Dokl. Akad. Nauk SSSR 167 (1967) 1266. N. S. Andreev, V. I. Averjanov and E. A. Porai-Koshits, Investigation of the structure of low-alkali sodium-silicate glasses by the scattering of visible light and by electron microscopy, in: The Structure of Glass, Vol. 5, Structural Transformations in Glass at High Temperature (Consultants Bureau, New York, 1965) p. 49. F. Ya. Galakhov, Microseparation in two-component silicate systems, in: The Structure of Glass, Vol. 5, Structural Transformations in Glass at High Temperature (Consultants Bureau, New York, 1965) p. 92.