INCREASING THE REACTIVITY OF STEATITE BODIES DURING FIRING

P. G. USOV, Yu. I. NESTEROV

(Presented by the scientific seminar of the Department of Silicate Technology)

Steatite products are manufactured from synthesized bodies. First, magnesium metasilicate is synthesized by firing talc, and then, on its basis or with the addition of small amounts of raw talc, a body is prepared that is likewise subjected to firing. From the ground synthesized body, with an organic binder, products are formed by the hot pressure-casting method or by pressing; their final properties are also formed by firing. Talc, or a body based on it, is fired during synthesis at higher temperatures than those used for firing the shaped products. For different bodies this difference amounts to from 50 to 120°C. The firing of talc and of the synthesized body based on it is carried out at higher temperatures so that the complex phase composition of the body remains unchanged when products are made from it. The firing of steatite bodies is carried out according to regimes that ensure the most complete course of phase-formation reactions, bringing the system close to the equilibrium state, and creating favorable conditions for the formation of the crystals of the resulting phases with a minimum number of lattice defects. The noncrystallizing part of the body, upon cooling, is completely annealed, solidifying in the form of glass. Such a body has a minimum reserve of free energy and low reactivity. A distinctive feature of the sintering process of products during firing from such bodies is the absence of the chemical reactions that occur when products made from mineral raw material not previously treated by firing are fired.

According to P. O. Gribovskii [1], the use of bodies synthesized by this method for manufacturing products stabilizes the sintering process, makes it possible to shorten the firing duration, and reduces the volume changes (shrinkage) undergone by the semifinished product during firing. An analysis of the fusibility of steatite bodies, as well as of products made from them, shows that during firing they sinter within a very narrow temperature interval. The existing regime for the synthesis of bodies does not promote an increase in their reactivity or a widening of the sintering interval. To obtain a body with increased reactivity during firing, it is necessary to achieve the greatest possible disordering of the lattices of its crystalline phase, so that the crystals are as far as possible from perfection and have the greatest number of defects. It is known that the reactivity of solid

of substances depends to a very great extent on their dispersity and degree of crystallinity, since these properties largely determine structurally sensitive diffusion processes, through which the reactions taking place are usually carried out. P. P. Budnikov and A. M. Ginstling [2] write that the rate of sintering depends on the degree of perfection of the crystalline structure of the body. The sintering rate of a crystalline body having a lattice structure far from equilibrium is significantly—hundreds of times—greater than the sintering rate of slightly distorted crystals. It is probable that disordering the structure of crystals can be used as a method for increasing the reactivity of bodies during firing. At the same time it is fully possible to preserve the positive properties of the synthesized body: constancy of composition, the possibility of stabilizing the sintering process, small volume changes of the products during firing, and others.

Disordering the structure of crystalline phases can in practice be obtained by various methods: mechanically—by grinding to a finely divided state, thermally—by heat treatment under various regimes, and by others. Some data in this direction are presented in the present work. The investigations were carried out with a steatite body of the following composition:

1) talc from the Onot deposit, fired at 1320°C—48%,
2) talc from the Onot deposit, raw—32%;
3) barium carbonate —15%,
4) Chasov-Yar clay — 5%.

The firing schedule for the products was standard.

The effect of the fineness of grinding of a body prepared in the normal manner on the sinterability of the products is given in Table 1. It is seen that coarsely dispersed bodies sinter with greater difficulty. Products made from a body with particle size finer than 0.063 mm attain complete sintering at a temperature of 1250°C, approximately 30°C lower than the sintering temperature of products made from a body with particle size finer than 0.088 mm; products made from a body ground to complete passage through a sieve with 1 mm openings sinter with still greater difficulty.

It is important that, when products made from finely ground bodies (−0.063 mm) are fired, during sintering—especially at temperatures close to complete sintering—there is a small difference between true and apparent porosity (Table 2), which is favorable for obtaining dense structures. When products made from bodies of coarser grinding are fired, at temperatures close to complete sintering, the gap between true and apparent porosity increases as the dispersity of grinding decreases, which leads to difficulty, or to the impossibility, of obtaining dense structures during firing, since the apparent porosity reaches zero values earlier. The experimental data also show that, in products prepared from a body of the same composition but of different grinding fineness, it is impossible to obtain the same structural density when firing according to a single schedule. It is of interest to determine the effect of low-temperature firing of the steatite body on its reactivity, since talc acquires a high degree of disordering of the structure of crystalline phases when fired to the temperatures of its decomposition—1000–1200°C. For the study, a body was prepared using raw talc and fired at temperatures of 1000, 1100, and 1200°C. After grinding to a fineness of minus 0.063 mm, products were formed from them by pressing with a paraffin binder under the same parameters as from the ordinary body.

Products made from bodies fired at temperatures of 1000 and 1100°C have a lower temperature at the onset of sintering, but are more difficult to form, have high initial porosity, and during firing give larg—

Table 1

Change in the properties of articles during firing and behavior of articles made from masses of different fineness of grinding

Table 2

Change in the true and apparent porosity of articles during firing
from bodies of different grinding fineness

total shrinkage. Articles made from such bodies more often acquire a distorted shape and can hardly be used in practice. Articles made from bodies based on raw talc, fired at a temperature of 1200°C, behave during firing practically the same as those made from the usual, normally prepared body.

Behavior during firing of articles made by the glass-ceramic method

Probably, a body obtained in the form of glass with an “acclimatized” or “conditioned” structure [2] will have a higher reactivity. Such a body is capable of crystallizing by itself, without the introduction of crystallization nuclei.

During firing of a steatite body of the composition under study, the predominant, and often the only, crystalline phase formed is magnesium metasilicate. In this case, the formation of magnesium metasilicate occurs at temperatures lower than the melting temperature of the composition. This same compound is in equilibrium with the melt of the system MgO—BaO—Al₂O₃—SiO₂ of the composition studied. Therefore, in order to obtain glass with a “conditioned” structure, the body was melted by us at temperatures close to the crystallization temperature of the composition—1560–1580°C—and with a short hold in the molten state (about 10 min). Under such a regime, the ordered structure of the melt in the form of sibotaxic groups is preserved to some extent, and the glass obtained crystallizes readily upon reheating without the introduction of a catalyst.

From the ground glass, articles are made by ordinary methods: either by hot pressure casting, or by pressing with a paraffin binder. What is important is that the heat of crystallization of magnesium metasilicate, with this method of preparing the body, is used to form the structure of the article, which is useful and technologically more convenient, since the self-heating of the body ensures uniform heating of the articles throughout their entire volume without the formation of internal stresses and, consequently, deformations.

Table 3

Change in the properties of products during firing from a body based on glass

Figure 1 gives the DTA curves of glass and of a body of the same composition prepared by the ceramic method. The crystallization curve of the glass is typical for such compositions (3, 4).

The sinterability of products made from a body obtained in the form of glass and ground to 0.063 mm is given in Table 3.

Discussion of results

The temperature at the end of sintering of the products (complete sintering) does not depend on the method of preparing the body. For one body composition and the same fineness of grinding, the temperature of complete sintering of the products does not change. Increasing the reactivity of bodies with disordering of the structure during preparation, along with changes in other properties, lengthens the sintering interval as a result of lowering the temperature at which sintering begins. Bodies based on raw talc, prepared by firing at 1000–1100°C, have a longer sintering interval in comparison with a body prepared in the normal way. To use them, however, is hardly possible—

Fig. 1. DTA curves of a steatite body with different methods of preparation.
I—P-5 body obtained by the glass method.
II—P-5 body obtained by the ceramic method.

however. They are more difficult to form into a product. With the same batch composition and pressing parameters, products made from these batches have an initial porosity 2–2.5 times higher and, during firing, exhibit greater shrinkage, as a result of which the probability of deformation of the products increases. At the same time, the greater shrinkage is due not only to the loose structure of the pressed body, but also to the change in the specific gravity of the material during firing.

When the batch is fired to a temperature of 1200°C, the defectiveness of the lattices of the crystalline phase decreases. When products made from it are fired, no increase in reactivity is observed. A general drawback of batches prepared by the ceramic method is the tendency of products made from them to bloat during overfiring; moreover, different variations in batch preparation do not eliminate this drawback.

The batch obtained in the form of glass behaves quite differently. It crystallizes readily at a temperature of about 900°C without catalysts. Products made from it show the least shrinkage during final firing and develop the best structure. At an overfiring temperature 100°C above the normal one, no signs of blistering are observed in products made from this batch. The dielectric characteristics of products based on glass of steatite composition are more constant, with a smaller scatter in the values, and meet the requirements of the GOST standard.

REFERENCES

1. P. O. Gribovskii. Hot Casting of Ceramic Products, Gosenergoizdat, Moscow—Leningrad, 1961.

2. P. P. Budnikov, A. M. Ginstling. Reactions in Mixtures of Solid Substances, Stroiizdat, Moscow, 1965.

3. I. I. Kitaigorodskii, R. Ya. Khodakovskaya. The precrystallization period in glass and its significance. In: The Glassy State, issue 1, Publishing House of the USSR Academy of Sciences, Moscow—Leningrad, 1963.

4. I. I. Kitaigorodskii, R. Ya. Khodakovskaya. Certain regularities of glass ceramization in the system SiO₂—Al₂O₃—MgO. Proceedings of the USSR Academy of Sciences, Inorganic Materials, vol. 1, No. 5, 1965.

5. P. G. Usov, Yu. I. Nesterov. Proceedings of the Tomsk Polytechnic Institute, issue 151, 1965.