STUDY OF THE PROCESSES FOR PRODUCING HEAT-RESISTANT AND NON-AGING STEATITE CERAMICS

P. G. USOV

Steatite ceramics include ceramic dielectrics whose raw material for production is talc. This ceramic is characterized by low dielectric losses at high frequencies, has low dielectric permittivity and high electrical and mechanical strength. It is used as mounting parts operating in high-frequency fields, high-voltage insulators, certain types of capacitors, and others.

The disadvantages of the ceramic are: low thermal resistance and a tendency to age during service. Regardless of the method of manufacture, the crystalline phases of the ceramic have a constant composition, formed by the products of thermal decomposition of talc—magnesium metasilicate, one or two of its modifications, and cristobalite. The compositions of the glassy phase are variable; they depend on the compositions of the batches of steatite bodies and on the chemical purity of the initial raw materials. In the batches of steatite bodies at some of our plants, the talc component amounts to from 70 to 90%, and the amount of metasilicate phase in the composition of the ceramic body correspondingly varies from 56 to 71%. The glass phase most often has a barium aluminosilicate composition, more rarely with additions of zinc oxide, zirconium dioxide, calcium oxide, alumina, and others. The basicity of the glasses is different; the coefficient of the molecular ratio of their components varies over wide limits: from 0.11 to 0.60. A substantiated explanation of the necessity for such a wide variation in the compositions of the glass phase of the ceramic has not yet been given in the literature. Normally manufactured steatite ceramics are chemically stable. Aging of the ceramic manifests itself in changes over time in a number of its physical properties: the appearance of secondary porosity, loss of vacuum tightness, reduction of mechanical strength, and others. The cause of ceramic aging is its low thermal stability and the non-equilibrium phase composition of the body. The coefficient of thermal expansion of the glass phase of steatite ceramic compositions synthesized by domestic plants varies from \(3.0\) to \(7.0 \cdot 10^{-6}\). With coefficients of thermal expansion of protoenstatite and clinoenstatite equal to \(9.8\) and \(13.5 \cdot 10^{-6}\), respectively, their absolute mismatch with the glass phase is: for protoenstatite from 40 to 226% and for clinoenstatite from 93 to 350%.

Assuming the separate existence of the crystalline and glassy phases of the ceramic in the form of two plates joined

between them a contact layer; upon cooling, a stress arises between them equal to (1) [1]:

\[ \sigma=\frac{\varepsilon}{1-\mu}\cdot V\cdot \Delta\alpha\cdot \Delta T\ \text{kg/cm}^2, \tag{1} \]

where
\(\sigma\) — stress in the contact layer, kg/cm\(^2\);
\(\varepsilon\) — modulus of elasticity, kg/cm\(^2\);
\(\mu\) — Poisson’s ratio;
\(V\) — volume fraction of the phase in fractions of unity;
\(\Delta\alpha\) — difference in the CTE of the crystalline phase and the glass;
\(\Delta T\) — temperature interval in which stresses arise under the action of \(\Delta\alpha\).

The quantities \(V\) and \(\Delta T\) in equation (1), for one type of ceramic (steatite), are constant. The term \(\frac{\varepsilon}{1-\mu}\) varies insignificantly within the range of ceramic compositions used. With an inconstant composition of the glassy phase of the ceramic, the quantity that undergoes the greatest change is the mismatch of the coefficients of thermal expansion of the glassy and crystalline phases—\(\Delta\alpha\). Combining the constant terms of the equation and multiplying them by \(1\cdot 10^{-6}\), we obtain the coefficient at \(\Delta\alpha\). In this case \(\Delta\alpha\) is expressed as the difference of the coefficients of thermal expansion of the crystalline phase and glass without the multiplier \(1\cdot 10^{-6}\). Then equation (1) is written in the form

\[ \sigma=A'\cdot \Delta\alpha\ \frac{\text{kg}}{\text{cm}^2}, \tag{2} \]

where

\[ A'=\frac{\varepsilon}{1-\mu}\cdot V\cdot \Delta T\cdot 1\cdot 10^{-6}. \]

In three-dimensional structures with grains of the crystalline phase of identical size, the fraction of the total force transmitted by tangential shear stresses at the grain boundary is smaller than in plates, since normal stresses also act here. The total contact area of crystals in a three-dimensional system is proportional to \(3/d\), and the average tangential and normal stresses at the crystal boundary are approximately equal to:

\[ \sigma_{\text{total}}=\sigma_{\text{shear}}+\sigma_{\text{normal}}=\frac{1}{3}\cdot \sigma\cdot v\cdot d \]

or

\[ \sigma_{\text{total}}=A\cdot \Delta\alpha\cdot d, \]

where \(d\) is the size of the crystals, and the coefficient \(A=\frac{1}{3}A'\cdot v\).

For steatite ceramics, at the values: \(\varepsilon=0.7\cdot 10^6\) kg/cm\(^2\); \(M=0.25\); \(V_{\text{cr.ph}}=0.7\); \(\Delta T=1000\) (from 1020 to 20°C), the coefficients \(A'\) and \(A\) will respectively be equal to 650 and 150. Then we obtain the stress in the contact layer of plates, arising as a result of the mismatch of the coefficients of thermal expansion upon cooling to normal temperature,

\[ \sigma=650\cdot \Delta\alpha. \]

And the total stress (tangential and normal) at the crystal boundary in the volume of the body will correspondingly be equal to:

\[ \sigma_{\text{total}}=150\cdot \Delta\alpha\cdot d, \]

$\Delta \alpha$ is the mismatch of the coefficients of thermal expansion, taken without the power factor. For example: the CTE of protoenstatite is \(9.8 \cdot 10^{-6}\), that of glass is \(7.0 \cdot 10^{-6}\); \(\Delta \alpha = 9.8 - 7.0 = 2.8\).

It is not yet known how tangential and normal stresses are distributed in a ceramic body, but it is clear that the total stress depends on two of its physical characteristics: on the magnitude of the mismatch between the coefficients of thermal expansion of the crystalline phases and the glass, and on the size of the crystals of the crystalline phase. Both of these factors act in the same direction. With the same chemical composition of the glass phase and equal grain sizes of the crystalline phase, the total stress in the ceramic body, for example from body II–5, will be 2.5 times greater for a clinoenstatite composition of the crystalline phase than for a protoenstatite one. In terms of the magnitude of the mismatch between the coefficients of thermal expansion of the crystalline and glassy phases, the “heaviest” bodies are LK-1 from the television plant of the city of Krasnoyarsk. The phase composition of steatite ceramics during heat treatment does not reach an equilibrium state. Protoenstatite, which constitutes the principal crystalline phase of the body, is in a metastable state at normal temperature. Under favorable conditions it transforms into clinoenstatite. The glass phase is also nonequilibrium. During firing, microregions are formed in it with a structure close to cristobalite; in such regions cristobalite crystallizes during service of the articles, especially under thermal action. Cristobalite crystallization from glass proceeds at the highest rate at temperatures of \(300\text{–}350^\circ\mathrm{C}\), at the temperatures of its modification transformation from the \(\beta\) to the \(\alpha\) form and back. In this case the process of cristobalite crystallization is always accompanied by the polymorphic transformation of protoenstatite into clinoenstatite. The transformation of protoenstatite into clinoenstatite is accompanied by a slight rearrangement of the crystal lattice and proceeds with a small change in the energy balance of the reaction. The heat effect of the transformation is about \(0.5\ \mathrm{kcal/mol}\), and the activation energy is about \(1.7\ \mathrm{kcal/mol}\). Such reactions are very sensitive to external conditions. In accordance with the Le Chatelier principle, cooling of the articles during firing stimulates the transformation of protoenstatite into clinoenstatite, since this process is accompanied by a decrease in volume of about 3%. The glassy phase of the body, with a lower coefficient of thermal expansion, resists the volume changes of protoenstatite crystals and externally stabilizes them. At the contact layer crystal–glass phase, stresses arise whose magnitude, as shown above, depends on the mismatch of the coefficients of thermal expansion of the crystalline phase and the glass and on the grain size of the crystalline phase. In articles made from a body with a coefficient-of-thermal-expansion mismatch of 30%, the limiting size of crystals stabilized by glass in the form of protoenstatite, under standard production regimes, is within \(3\text{–}5\ \mu\). Above this size, the magnesium metasilicate crystals in ceramics are not stabilized by the glass phase, and at normal temperature they are in the form of clinoenstatite. During service, slowly—and under thermal action more rapidly—cristobalite crystallizes from the glass phase. This process is accompanied by disruption of the continuity of the glass phase in the contact layer crystal–glass; the inhibiting action of the glass phase weakens, and the polymorphic transformation of protoenstatite into clinoenstatite and a change in the physical properties of the articles occur.

In ceramics with stable properties there must be no secondary crystallization of the glass phase. The stability of protoenstatite in the meta-

the metastable state depends to a greater extent on the size of the crystals than on external stabilization by glass. The transformation of clinoenstatite into protoenstatite on heating occurs at a temperature of 1100°C irrespective of the crystal size, whereas the reverse transition clearly depends on this factor and on the degree of overheating of the material during firing. The crystal sizes affect the rate of transformation of clinoenstatite into protoenstatite on heating and the rate of the reverse transition of protoenstatite into clinoenstatite on cooling. Crystals smaller than 5 μm after firing at 1100°C with a hold of up to one hour transform in the forward and reverse directions practically reversibly. The reverse transition begins at a temperature of 1100°C and is almost completely finished at 1060°C. The low-temperature form of metasilicate with this crystal size, after firing at 1150°C, is completely restored upon cooling together with the furnace and upon quenching in air. After firing at 1200°C, about 10–15% protoenstatite remains in specimens quenched in air, in the form of fine-grained aggregates, with an average refractive index \(N_{\mathrm{av}} = 1.620\). In specimens cooled together with the furnace, the high-temperature form is completely absent. At a firing temperature of 1250°C, protoenstatite is stabilized and, upon cooling by quenching or together with the furnace, does not transform into clinoenstatite. Its transformation also does not occur during storage for two years.

It has been observed that during firing at 1250°C the transformation of clinoenstatite into protoenstatite is accompanied by a change in crystal morphology. Large metasilicate crystals become dispersed, as a result of which the limiting grain size of protoenstatite becomes 1–2 μm instead of 5 μm in the starting material. Crystals 5 to 10 μm in size are not stabilized by firing at 1250°C. After firing at 1100–1200°C they behave similarly to crystals smaller than 5 μm, transforming completely into clinoenstatite upon cooling. In the products fired at 1250°C, the protoenstatite phase is retained down to a temperature of 860°C; below this temperature it transforms into clinoenstatite. The rate of the transition is low; as a result, the transformation is completed in specimens cooled to room temperature. The higher the firing temperature of clinoenstatite, the greater the percentage of protoenstatite retained in cooled specimens; the latter gradually, during storage, transforms into clinoenstatite. The product of firing clinoenstatite at 1500°C, with a grain size of 5 to 10 μm, contains up to 60% protoenstatite. With storage time and upon grinding, its amount in the material decreases at first rapidly and then slowly, down to complete disappearance, approximately over the course of one year. Protoenstatite obtained by firing clinoenstatite with crystal sizes from 10 to 50 μm at 1250°C cannot be brought to room temperature. Its reverse transition begins at a temperature of 860°C and, upon cooling to room temperature, is completed fully. By firing at a high temperature (about 1500°C), part of the protoenstatite can be brought to room temperature, but within one day of storage it completely transforms into the monoclinic variety. A distinctive feature of coarse-crystalline metasilicate with grain sizes from 50 to 100 microns is its rapid, almost instantaneous phase transformation. During cooling, the transition of protoenstatite into clinoenstatite occurs in the temperature interval 800–780°C. Within a twenty-degree range the transformation is completed fully in the course of 2 min. It proved impossible to improve the stability of protoenstatite in the metastable state with this crystal size by increasing the firing temperature.

It was shown above that, in order to reduce thermal stresses in the body of steatite ceramics, the formation of the clinoenstatite form during firing is undesirable. Consequently, crystallization of the metasilicate phase in grains larger than 5 microns must not be allowed.

To obtain steatite ceramic articles with a fine-grained structure of the crystalline phase of protoenstatite composition, two production methods are proposed.

The glass-ceramic method—by directed crystallization of glass. Glass of the composition of a steatite body crystallizes well without nucleators. It is known that, in the melt at temperatures close to the crystallization temperature (to the liquidus line), there exist structural groups having the composition of the future crystalline phase. Being fixed in the glass, they can serve as ready-made crystallization centers upon heating. A feature of obtaining glass of the composition of a steatite body with structural groups of magnesium metasilicate is that, at the temperatures at which it is obtained, 1540–1560°C, the metasilicate melts incongruently with the formation of forsterite. Prolonged holding at the melting temperature, or increasing the melting temperature, leads to disruption of the elements of long-range order in the structural groups of $\mathrm{MgSiO_3}$ and to the formation of new groups of composition $\mathrm{Mg_2SiO_4}$, crystallizing in the form of forsterite, which is highly undesirable in the phase composition of the steatite body. Heat treatment of the steatite batch to melting stimulates the formation of metasilicate groups in the glass. In order to ensure complete crystallization of metasilicate from the glass, it is necessary to preserve its structural groups during the melting process; for this purpose the glass must be melted at a temperature close to the melting temperature of the batch, with a minimum holding time in the molten state.

The sizes of the crystals of magnesium metasilicate separating from glass of the composition of a steatite body do not possess the desired uniformity. In the crystallization products, crystals up to 4 microns in size are observed, with the refractive index of clinoenstatite $N_{\mathrm{cp}} = 1.658$. Therefore, in order to increase the intensity of crystallization, it is necessary to introduce nucleators $\mathrm{TiO_2}$ or $\mathrm{ZrO_2}$ into the batch during glass melting in optimal amounts, equal to 0.12–0.2%, selected experimentally for the glass compositions under study. With nucleators, crystallization of the glasses proceeds within a narrow temperature interval, with a maximum at 980°C. After final firing, the crystalline phase of the articles consists of protoenstatite with grain sizes smaller than one micron.

In contrast to the usual technology, articles made from glass are sintered during firing in two stages. During the biscuit firing, whose temperature coincides with the crystallization temperature of the glass, the heat of crystallization is released, heating the material uniformly throughout its volume. As a result, the articles undergo shrinkage approximately equal to half the total shrinkage during firing of articles to zero water absorption. During final firing of articles made from glass, for every 10°C of temperature in the interval of intensive sintering, the shrinkage is half that of an article made by the usual manufacturing method. When articles are manufactured by the glass-ceramic method, the phenomena of bloating during normal firing and overfiring are not observed; as a result, the interval of the sintered state increases, and the firing process becomes more technologically reliable.

The second method is the conventional one, but with the introduction of modifying additives. Protoenstatite is a representative of the minerals of the pyroxene series. Its structure consists of silicate-oxygen pyroxene chains, cross-linked by $\mathrm{Mg^{2+}}$ cations. Chain polymerization of the silicate-oxygen ion causes the formation of elongated metasilicate crystals—

… elongated or even fibrous form. The maximum stresses in such crystals develop along the axis of preferential growth, as a result of which, at elevated firing temperatures, they rupture, forming fine-grained aggregates. With the existing method of heat treatment of the steatite body, the required degree of crushing of magnesium metasilicate is not achieved and, consequently, the desired dispersion of the crystalline phase is not obtained. The $\mathrm{Mg}^{2+}$ ions in the metasilicate have octahedral coordination with oxygen; they are readily replaced by other cations having a similar ionic radius. Thus solid solutions—diopside and pigeonite—are formed. Using this property, we introduced additions of cations with chemical properties related to magnesium but with considerably larger or smaller ionic radii, which do not form solid solutions with magnesium and do not give sixfold coordination with oxygen. Such cations, occupying the position of magnesium in the structure, break the pyroxene chains into terminal segments of small size, as a result of which fine-grained crystals of protoenstatite are formed. The modifiers act not only to change the morphology of the crystals; they also influence the perfection of the structure of the protoenstatite crystal lattice. The compositions and characteristics of the tested modifiers are given in Table 1.

Table 1

* $F$ is the ionic-field strength. This is a criterion of the ordering action of the introduced cation on neighboring oxygen ions. It is equal to $F = \dfrac{Z}{r^2}$, where $Z$ is the valence and $r$ is the ionic radius.

The activity of the modifiers is in complete agreement with the magnitude of the ionic-field strength, and their optimum amounts in the bodies are from 0.4 to 0.8%—calculated as oxides—in the order of increasing molecular weight from BeO to BaO. With such additions, the protoenstatite phase of the ceramic has a very fine-grained structure and a perfect crystal-lattice structure. Table 2 gives the half-width of the principal reflections of protoenstatite and its resistance to mechanical and thermal effects.

The thermal shock resistance of products made by different methods, when tested within the temperature ranges $20 \div -195.8^\circ\mathrm{C}$ (liquid nitrogen) and $20 \div 900^\circ\mathrm{C}$, is presented graphically in Figs. 1, 2, and 3. The density of the body structure of ceramics made by the ordinary method changes immediately after one or two thermal cycles; in ceramics with a fine-grained crystalline phase it remains unchanged throughout all twenty thermal cycles. The modifiers BeO, SrO, and BaO ensure not only the crystallization of protoenstatite in fine-grained form in steatite bodies, but also stimulate the liberation of the glass phase from free-

...structural groups of silica, crystallizing in the form of cristobalite during firing of the articles. In such articles, secondary crystallization of the glass phase does not occur.

Table 2

Note: P-500°C denotes powder of 60 µm fineness, calcined at a temperature of 500°C.

Table 3

Mechanical strength of the articles

Conclusions

With the establishment of methods for controlling the crystallization of magnesium metasilicate during firing of steatite bodies, it has become possible to produce heat-resistant and non-aging ceramics with improved technical properties.

Fig. 1. Thermal-shock resistance of articles during testing in the temperature range 20–195.8°C: ob — ceramic made by the conventional method; SrO, BaO, BeO — ceramics with modifiers; \(C_T\) — ceramic produced by the glass-ceramic method.

Fig. 2. Thermal-shock resistance of articles during testing in the temperature range 20–195.8°C: ob — ceramic made by the conventional method; SrO, BaO, BeO — ceramics with modifiers; \(C_T\) — ceramic produced by the glass-ceramic method.

Fig. 3. Thermal-shock resistance of articles during testing in the temperature range 20–900°C: ob — ceramic made by the conventional method; SrO, BaO, BeO — ceramics with modifiers; \(C_T\) — ceramic produced by the glass-ceramic method.

REFERENCES

1. U. D. Kingery. Introduction to Ceramics. Publishing House for Construction Literature, Moscow, 140, 1964.

2. P. G. Usov. Aging of steatite ceramics. Eleventh Regional Scientific-Technical Conference Dedicated to Radio Day. Section VI, Radio Electronics, Novosibirsk, 1968.

3. P. G. Usov, V. I. Vereshchagin. Investigation of low-temperature processes in steatite ceramics by DTA and X-ray analysis. Eleventh Regional Scientific-Technical Conference Dedicated to Radio Day. Section IV, Radio Electronics. Novosibirsk, 1968.

4. P. G. Usov, V. N. Gurina. Influence of the size of magnesium metasilicate crystals on stabilization of protoenstatite at low temperature. Eleventh Regional Scientific-Technical Conference Dedicated to Radio Day. Section VI, Radio Electronics. Novosibirsk, 1968.

5. P. G. Usov, N. V. Sobora. Kinetics of decomposition and the influence of additions on phase formation of talc during firing. Eleventh Regional Scientific-Technical Conference Dedicated to Radio Day. Section VI, Radio Electronics. Novosibirsk, 1968.

6. P. G. Usov, S. F. Pankova, V. V. Pankov, V. V. Serebrennikov. Investigation of the electrical conductivity of steatite ceramics. Eleventh Regional Scientific-Technical Conference Dedicated to Radio Day. Section VI, Radio Ceramics. Novosibirsk, 1968.

7. P. G. Usov, S. F. Pankova, V. V. Serebrennikov. Influence of irradiation on the structure and phase composition of steatite ceramics. Problems of Chemistry. Proceedings of Tomsk State University, issue 4, 192, 1968.

8. P. G. Usov, S. F. Pankova, V. V. Serebrennikov. Influence of irradiation on the thermal stability of steatite ceramics. Problems of Chemistry. Proceedings of Tomsk State University, issue 4, 192, 1968.

9. P. G. Usov, S. F. Pankova, V. V. Serebrennikov. Influence of irradiation on the technological properties of a steatite body with an addition of 4% cerium dioxide. Abstracts of the Conference on Radiation Physics. TSU Publishing House, Tomsk, 1967 (Materials of the Conference on Radiation Physics of Dielectrics, 2, 1968, in press).

10. P. G. Usov, S. F. Pankova, V. V. Pankov, V. V. Serebrennikov. On the electrical conductivity of a steatite body and articles at various temperatures. Abstracts of the Scientific-Technical Conference Dedicated to the 50th Anniversary of October, issue 1, 20, 21, Tomsk, 1967.

11. P. G. Usov, V. I. Vereshchagin. Low-temperature processes in steatite ceramics. Collection of Materials of the Jubilee Scientific-Technical Conference Dedicated to the 50th Anniversary of October. Tomsk, 1967.

12. P. G. Usov, V. I. Vereshchagin. Production of steatite articles through glass with microadditions of ZrO₂. Collection of Materials of the Scientific-Technical Conference Dedicated to the 50th Anniversary of October. Tomsk, 1967.

13. P. G. Usov, Yu. I. Nesterov, V. I. Vereshchagin. Author’s certificate No. 225755, 1968.

14. P. G. Usov, V. I. Vereshchagin. Application for an author’s certificate for “A method of producing steatite ceramics with an extremely fine body structure and improved technical properties.” Decision on granting an author’s certificate dated January 2, 1969.

15. P. G. Usov, Yu. I. Nesterov. Investigation of the influence of the method of preparing a steatite body on its technological properties. Proceedings of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov, vol. 151, Publishing House of Tomsk University, Tomsk, 1966.

16. P. G. Usov, Yu. I. Nesterov, L. V. Mitina. Investigation of the cause of blistering of steatite articles during firing. Proceedings of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov, vol. 151, Publishing House of Tomsk University, Tomsk, 1966.

17. P. G. Usov, E. P. Solomatina. Interaction of steatite bodies B-5 and B-17 with a technological binder. Proceedings of the First Scientific Conference of the Tomsk Branch of the All-Union Chemical Society named after D. I. Mendeleev. Publishing House of Tomsk University, Tomsk, 1969.

18. P. G. Usov, V. N. Gurina. Influence of the size of magnesium metasilicate crystals on the phase transitions of polymorphic forms. Collection of Materials of the Jubilee Scientific-Technical Conference Dedicated to the 50th Anniversary of October. Tomsk, 1967.

19. P. G. Usov, S. F. Pankova, V. V. Pankov, V. V. Serebrennikov. Properties of articles made of steatite ceramics impregnated with cerium nitrate salts. Reports at the Symposium on Theoretical Studies of Rare-Earth Elements, Alloys, and Compounds. Moscow, 1968 (in press).

20. P. G. Usov, S. F. Pankova, V. V. Pankov, V. V. Serebrennikov. Influence of cerium additions on the properties of steatite bodies. Problems of Chemistry, issue 7 (in press).

1. P. G. Usov, V. N. Gurina. On phase formation of \(MgSiO_3\) during synthesis from oxides. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

2. P. G. Usov, V. N. Gurina, N. V. Sobora. On the polymorphism of magnesium metasilicate in steatite ceramics. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

3. P. G. Usov, V. N. Gurina. On the polymorphism of magnesium metasilicate at high temperature. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

4. P. G. Usov, V. N. Gurina. Study of the processes occurring during crystallization of a melt of composition \(MgSiO_3\). Izvestiya Tomskogo politekhnicheskogo instituta (in press).

5. P. G. Usov, V. I. Vereshchagin, N. V. Sobora. Study of the crystallization products of molten talc. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

6. P. G. Usov, N. V. Sobora, E. N. Belomestnova. Phase transformations of talc during heating. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

7. P. G. Usov, N. V. Sobora. Kinetics of the thermal decomposition of talc. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

8. P. G. Usov, N. V. Sobora. Influence of additions of alkali-metal oxides on the phase formation of talc during firing. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

9. P. G. Usov, N. V. Sobora. Kinetics of the improvement of the structure of magnesium metasilicate formed as a result of talc decomposition. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

10. P. G. Usov, V. I. Vereshchagin, N. V. Sobora. Study of the crystallization products of glass of talc composition. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

11. P. G. Usov, V. I. Vereshchagin. Low-temperature DTA effect in steatite ceramics. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

12. P. G. Usov, V. I. Vereshchagin. On the polymorphism of magnesium metasilicate in steatite articles at low temperatures. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

13. P. G. Usov, V. I. Vereshchagin. Study of the stability of protoenstatite in steatite materials. Izvestiya Tomskogo politekhnicheskogo instituta (in press).

14. P. G. Usov, V. I. Vereshchagin. Influence of calcium impurities on the properties of steatite ceramics. Izvestiya Tomskogo politekhnicheskogo instituta (in press).