CRYSTALLIZATION AND STRUCTURE FORMATION OF ARTICLES MADE FROM GLASS OF STEATITE-BODY COMPOSITION WITH TiO₂ ADDITIONS DURING FIRING

P. T. Usov, Yu. I. Nesterov

(Presented by the chemical section of the annual scientific conference of TPI on the results of scientific research for 1965)

It is known that the crystallization of glass is improved with the participation of catalysts and can have a directed character. With their aid it is possible to obtain an enormous number of crystallization centers in the glass mass. For this purpose, additions are used that are less soluble in the glass and whose crystal-lattice structure corresponds, to a certain degree, to the structure of the crystals of the separating phases. The greater the crystallographic affinity of the nuclei of the new phase with the addition, the less supersaturation is required for their formation; conversely, as the structural difference from the crystallizing substance increases, greater supersaturation is required for a nucleus of the new phase to arise on this addition.

Crystallization and structure formation were studied by us on articles manufactured by pressing powder from glass of the following composition: SiO₂—56.70%, Al₂O₃—2.06%, Fe₂O₃—0.24%, BaO—11.95%, K₂O—0.18%, melted with an addition of titanium dioxide in amounts of: 0.02, 0.04, 0.06, 0.08, 0.10, 0.125, 0.25, 0.50, 1.0, 2.0, 3.0, 4.0%.

A higher content of titanium dioxide colors the glass and worsens the dielectric properties of the articles. The glasses were melted in a kryptol furnace in corundum crucibles at temperatures of 1540–1580°C, with holding at the final temperature for about 30 min. They were then ground until they passed completely through a 0.063 mm sieve. From such powders, articles were formed on a paraffin binder. After biscuit firing, the formed articles were fired at identical temperatures up to complete sintering according to standard schedules.

Differential thermal analysis of glasses with TiO₂ additions shows that titanium dioxide has little effect on their crystallization temperature (Fig. 1). But it turns out that an addition of TiO₂, acting as a catalyst, must not only have similarity in the structure of the crystal lattices, but also be present in the glass in an optimum amount; its accumulations, smaller than certain critical dimensions, cannot induce crystallization. Additions of the catalyst in amounts above the critical value also have a negative effect on crystallization.

The results of the investigations show (Tables 1, 2) that the optimum addition is TiO₂ in an amount of 0.125%. Articles made from such a body, already during biscuit firing, give shrinkage of up to 9%, which amounts to 75% of the total shrinkage of the specimen upon attainment of an absolutely dense structure. Water absorption also changes. As a result of this, the volumetric

Change in water absorption of samples with temperature (sinterability)

Table 1

Water absorption of samples made from masses with various titanium dioxide contents, %

Change in shrinkage of samples with temperature

Table 2

Shrinkage of samples from masses 1–5 with various titanium dioxide contents, %

Table 3

Indices of X-ray diffraction patterns of glasses with an addition of 0.125% TiO₂, crystallized at different temperatures

\(d\) — interplanar distance of crystals in Å.
\(I\) — absolute intensity of the lines in cm.

Fig. 1. Differential-thermal curves. Amount of TiO₂ addition, %: 1 — without addition; 2 — 0.02; 3 — 0.01; 4 — 0.06; 5 — 0.08; 6 — 0.1; 7 — 0.125; 8 — 0.25; 9 — 1; 10 — 2; 11 — 3; 12 — 4.

Changes in the dimensions of the articles during the final firing are small. The possibility of their deformation during firing with this method of preparing the body is completely eliminated. In this case, no blistering of the articles during overfiring is observed either. A special feature of the crystallization of glasses with an addition of 0.125% TiO₂ is that the crystalline phase of the firing product before complete sintering consists of grains of very small size, less than one micron. The grains are uniformly distributed throughout the volume and are uniformly surrounded by glass. The total refractive index of the fine-grained aggregates corresponds to protoenstatite. Such a structure is highly desirable; it ensures greater stability of the properties of the articles and improves their dielectric and mechanical properties. X-ray study data (Table 3) indicate that at low firing temperatures of the articles enstatite is formed from the glass, which is not detected at all in the products of glass firing at higher temperatures. Cristobalite is also not found in these compositions.