STUDY OF THE CAUSE OF BLISTERING IN STEATITE PRODUCTS DURING FIRING

P. G. USOV, Yu. I. NESTEROV, L. V. MITINA

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

In the production of ceramic products from steatite bodies with the participation of barium carbonate, the phenomenon of blistering during firing is observed. The presence of a blistering process is extremely undesirable, since it causes a decrease in the density of the products, and blisters located close to the surface are exposed during grinding and polishing, as a result of which the products are rejected. In addition, blistering has an adverse effect on the mechanical and electrical properties of the products. Therefore, the question of the causes of blistering occupies one of the important places on the path toward improving the quality of steatite products.

The phase composition of steatite ceramics is characterized by the presence of a crystalline phase of magnesium metasilicate in one of its modifications and of glass, which contains SiO₂, BaO, Al₂O₃, and a small amount of impurities of other oxides. The oxides included in steatite ceramics are distinguished by very high thermal stability. According to U. D. Kingery [1], magnesium oxide is stable in air, nitrogen, carbon oxides, argon, hydrogen, ammonia, and vacuum up to temperatures on the order of 1700°C.

Aluminum oxide is inert with respect to air, water vapor, hydrogen, argon, carbon oxides, nitrogen, and vacuum up to temperatures on the order of 1700°C. At temperatures below its boiling point, aluminum oxide is characterized by extremely low volatility [2]. Therefore, the oxides Al₂O₃ and MgO cannot be the cause of blistering. Some investigators [3, 4] consider iron oxides to be the cause of blistering in ceramic bodies. However, we cannot regard iron oxides as the cause of blistering in steatite ceramics, since they are present in its composition in a very small amount (up to 1.0%); moreover, P. P. Budnikov, in studying the influence of iron oxides on the properties of steatite ceramics, worked with body compositions containing up to 7% iron oxides and did not note any increase in its tendency toward blistering [5].

A more probable source of gas formation during the firing of steatite products is barium carbonate that has not fully reacted with the constituent parts of the batch during firing of the steatite body. On this question, N. P. Bogoroditskii indicates [6] that the evolution of CO₂ during the decomposition of barium carbonate is often the cause of the formation of internal cavities and pimples on radio components, owing to the difficulty of removing CO₂ from the ceramic body during the firing process.

We have observed that, in bodies with barium carbonate of identical composition, the formation of blisters in the structure of products during the firing process

does not always occur. More intense blistering of the bodies occurs at temperatures above the optimum. The aim of our work was to study the rate and completeness of assimilation of barium carbonate by the individual constituent parts of the steatite body and by the batch during firing, and the influence of fineness of grinding and mixing time on these processes.

The investigation was carried out by the gravithermometric method on torsion balances. Specimens made from mixtures were heated from 25 to 1000°C in an air atmosphere at a rate of 10° per minute. The change in weight was recorded at every 20°. Before batching, the starting components were crushed until they passed completely through a sieve with 0.5 mm openings, after which a mixture was prepared in weight ratios of 1 : 1 of the components studied with BaCO₃. The dry mixture was ground in an agate mortar until it passed through a sieve with 60 μ openings. The ground powder was briquetted into disks of the following dimensions: diameter 16 mm, height 2 mm, weight 400 mg, at a specific pressure of 180 kg/cm². Before the study the specimens were dried to constant weight at a temperature of 200°C. The study was performed on materials used for the synthesis of steatite bodies in the radioceramics industry (Table 1). Figures 1–6 show the weight changes on heating of the materials used and of their mixtures with barium carbonate.

Table 1

* Note: Quartz glass was taken for the study because, in industrial compositions of steatite bodies, the silica phase is closer to the composition of technical quartz glass.

The data of our experiment on the assimilation of barium carbonate by quartz glass, shown in Fig. 1, indicate that the reaction begins at approximately 540°C and that the end of the reaction corresponds to 900°C.

These data are in full agreement with the results of investigations carried out earlier by V. B. Glushkova [8]. Barium carbonate in a mixture with raw kaolin (Fig. 2) interacts on heating at lower temperatures than with quartz glass and has no effect on the decomposition temperature of kaolin. The process of their interaction during heating consists of two successive reactions. The first reaction proceeds up to a temperature of 450°C and consists in the decomposition of kaolin with the elimination of chemically bound water. The second reaction begins at a temperature of 450°C and is characterized by interaction of the kaolin residue with barium carbonate. The product of the thermal decomposition of kaolin interacts with BaCO₃ in the mixture

Fig. 1. 1. Behavior of quartz glass during heating. 2. Behavior of $\mathrm{BaCO_3}$ during heating. 3. $\mathrm{BaCO_3} + \mathrm{SiO_2}$ (quartz glass).

Fig. 2. 1. Kaolin. 2. $\mathrm{BaCO_3}$. 3. $\mathrm{BaCO_3} +$ raw kaolin. 4. $\mathrm{BaCO_3} +$ kaolin calcined at $800^\circ\mathrm{C}$.

at lower temperatures than the same residue obtained by calcining kaolin at a temperature of 800°C.

It is likely that this reflects the increased chemical activity of kaolin anhydride at temperatures close to the temperature of its formation [3], and the catalytic action of water vapor [9].

Barium carbonate with Chasov-Yar clay (Fig. 3) interacts in an analogous manner.

In contrast to kaolin and clay, talc in the raw state, containing chemically bound water, reacts with \( \mathrm{BaCO_3} \). This reaction

Fig. 3. 1. \( \mathrm{BaCO_3} \) + raw clay.
2. \( \mathrm{BaCO_3} \) + clay calcined at 800°C.
3. Chasov-Yar clay.
4. \( \mathrm{BaCO_3} \).

proceeds at temperatures (Fig. 4) approximately 150° below the temperature of thermal decomposition of talc. The reaction begins at a temperature of

Fig. 4. 1. Onot talc. 2. \( \mathrm{BaCO_3} \).
3. \( \mathrm{BaCO_3} \) + raw talc. 4. \( \mathrm{BaCO_3} \) + talc calcined at 1000°C.
5. \( \mathrm{BaCO_3} \) + talc calcined at 1350°C.

550°C, and the change in weight of the mixture of raw talc with barium carbonate during heating occurs as a result of the simultaneous release of chemi-

chemically bound water and CO₂. The curve of weight change upon heating of such a mixture has no sharp bends and lies in the region of lower temperatures than the decomposition curves of pure talc and BaCO₃.

Barium carbonate with talc, preliminarily calcined at a temperature of 1000°C, interacts upon heating at the same rate as with raw talc and in the same temperature interval. With talc preliminarily calcined at a temperature of 1350°C, the BaCO₃ reaction shifts into a higher-temperature region, with a slight increase in the temperature at the end of the reaction.

The interaction of barium carbonate with the batch upon heating begins at the same temperatures as with the components separately, but because the relative BaCO₃ content in the batch is smaller than in tests with the components, its complete assimilation ends at lower temperatures—about 800°C (Fig. 5),

Fig. 5. Interaction of barium carbonate with the batch of composition P-5 mass.

Fig. 6. Interaction of barium carbonate with the batch of composition P-5 mass at different fineness of grinding of the batch.

(for the study, mass P-5 of the following composition was used: raw talc—32%, calcined talc—48%, Chasov-Yar clay—5%, BaCO₃—15%).

The results of the study show that BaCO₃ interacts with all constituent parts of the steatite-mass batch, at identical ratios and heating rates, in approximately one and the same temperature interval (Table 2).

We also investigated the influence of the fineness of grinding of the components on the completeness of the reaction with barium carbonate. Mixtures of barium carbonate and quartz glass, prepared in a ratio of 1 : 1, were ground until complete passage through sieves of 6400 openings/cm² and 4900 openings/cm².

The first mixture, upon heating to a temperature of 900°C, reacts to 95%; the second mixture, under the same conditions, reacted only to 93% (Fig. 6). At the same time, a further increase in temperature to 1100°C has no appreciable effect on the completeness of the reaction. At temperatures above 1100°C, barium carbonate sinters intensively and dissociates into oxides with great difficulty. It is precisely this barium carbonate that causes the blistering process in the structure of the finished product during firing.

Conclusions

As a result of the investigations carried out, it has been established:

Table 2

1. The reaction of $\mathrm{BaCO_3}$ with silica begins at 540°C.

2. The product of the thermal decomposition of kaolin reacts with $\mathrm{BaCO_3}$ at lower temperatures than the same residue obtained by calcining kaolin at a temperature of 800°C.

3. Talc reacts with $\mathrm{BaCO_3}$ at temperatures approximately 150°C lower than the temperature of its thermal decomposition.

4. $\mathrm{BaCO_3}$ reacts with the batch at the same temperatures as with the components, but its complete assimilation ends at lower temperatures.

5. To ensure the complete assimilation of $\mathrm{BaCO_3}$ by the batch of the steatite body during firing, it is necessary to grind it until it passes completely through a sieve with 10,000 openings/cm², with good-quality mixing, which increases the reactivity of the batch in the solid state and lowers the temperature of the beginning and end of the interaction.

Literature

1. Collection of articles Research at High Temperatures, translated from English by V. A. Karelina, Izd-vo IL, 1962.

2. Thermodynamic Properties of Substances, vol. 1, edited by Academician Glushko, Izd-vo AN SSSR, 1962.

3. R. D. Hill, D. N. Grook. Some causes of blistering of clays and shales, RZhKh, vol. 10, Izd-vo AN SSSR, 1961.

4. G. P. Budnikov, A. M. Ginstling. Reactions in Mixtures of Solid Substances, Gosstroiizdat, 1961.

5. P. P. Budnikov. The influence of iron oxides on certain properties of steatite materials. Proceedings of GIEKI, issue IV, Gosenergoizdat, 1960.

6. V. M. Kokabadze, G. D. Chachanidze. On certain possible reactions in the barium carbonate—silica system. Transactions of the Georgian Polytechnic Institute, Tbilisi, 1961.

7. N. P. Bogoroditskii et al. Radio Ceramics, Gosenergoizdat, 1963.

8. Journal of Inorganic Chemistry, vol. II, issue 10, Izd-vo AN SSSR, 1957.

9. Solid-State Chemistry, edited by Garner, Izd-vo IL, 1961.