LOW-TEMPERATURE DTA EFFECT IN STEATITE CERAMICS

P. G. USOV, V. I. VERESHCHAGIN

(Presented by the scientific seminar of the Department of Silicate Technology, Faculty of Chemical Technology)

Steatite ceramics—special-purpose ceramics—retain good dielectric characteristics at high frequencies and are used in radio engineering as high-frequency and vacuum-tight ceramics. The main drawback of these ceramics, which calls into question the expediency of their production, is aging, i.e., the loss of valuable properties and vacuum tightness during storage and operation of ceramic products. The crystalline phase of steatite ceramics is represented by magnesium metasilicate \((\mathrm{Mg}\cdot \mathrm{SiO}_3)\). Magnesium metasilicate as a chemical compound can exist in three polymorphic modifications: enstatite, clinoenstatite, and protoenstatite.

The glass phase of most steatite-ceramic compositions consists of barium aluminosilicate glasses with small additions of calcium oxides and alkali metals, which are introduced through the raw-material components.

There are still few works devoted to the aging of steatite-ceramic products. The views of different authors on this process are often contradictory and are presented in the form of assumptions. Thus, Büssem, Schusterius, and Stuttgart [3] proposed that the cause of aging is the transition of protoenstatite into clinoenstatite, which is accompanied by volume changes. This view is supported by Foster [4]. L. M. Blyumen associates aging with the poor chemical resistance of barium glasses [1]. V. V. Lapin, rejecting the preceding points of view, draws attention to the structural features of the body of steatite ceramics [2].

Proceeding from the theoretical and practical interest in the aging process of steatite ceramics, we carried out extensive investigations from the standpoint of its stability at low temperatures (below \(500^\circ\))—the temperatures of storage and service of these ceramics. We established a low-temperature exothermic effect on the DTA curve of steatite products, with a maximum at temperatures of \(320—350^\circ\). The effect is detected at the limiting sensitivities of the instruments. Thus, when working with a Kurnakov pyrometer, a ballast resistance of 10 ohms was introduced into the Pt—PtRh thermocouple circuit; when working with a derivatograph, the analysis was carried out with a DTA sensitivity of \(1/1.5\). The magnitude of the effect varies depending on the composition of the ceramics, the technology of its manufacture, and the firing schedule of the mass and products. Ceramic parts of one and the same composition, but from different batches, have on the DTA curve the above-mentioned effect differing in magnitude (Fig. 1). The effect is 3–5 times smaller in magnitude than the effect of the polymorphic transformation of cristobalite (Fig. 3).

An exothermic effect in such a system as steatite ceramic, where there is both a crystalline phase and an amorphous phase—glass—suggests the possibility of two different processes:

1. The transition of a metastable modification of the crystalline phase into a stable one with a denser space lattice.

2. The process of transition of particles of the amorphous phase into a crystalline one, i.e., the process of crystallization of the glass.

At first, when working with a Kurnakov pyrometer at heating rates of 15°/min, the effect appeared at a temperature of 350°C, and we explained it by the polymorphism of magnesium metasilicate. But it then turned out that glasses of compositions close to the compositions of the glass phase of steatite ceramics (Table 1) can give analogous thermal effects on the DTA curve at temperatures of 300–310°. This occurred in the case of insufficient holding of these glasses in the molten state at a temperature of 1540°. As the holding time of the glass at 1540° was increased, the effect on the DTA curve disappeared (Fig. 2).

Table 1

Approximate composition of glasses of the glass phase of steatite ceramics

Fig. 1. DTA curves of B-17 ceramic parts from different batches. (Heating rate of the samples 15°/min. The samples were ground until passing through sieve No. 0060).

Fig. 2. DTA curves of glasses of the SiO₂—Al₂O₃—BaO system. (Glasses 1, 2, 3, 4 were held at a temperature of 1540° for 1 hour; glass M—for 2 hours. The samples were ground until passing through sieve No. 0060; heating rate of the samples 15°/min.)

The low-temperature exothermic effect on the DTA curve of the indicated glasses is a consequence of their microinhomogeneity, pri-

the cause of which lies in insufficient homogenization of the glass mass in the molten state, which ultimately leads to crystallization of accumulations of amorphous silica into cristobalite. Petrographic analysis of these glasses after DTA up to a temperature of 500° reveals about 1% cristobalite. When working with the derivatograph, the heating rate was set at 5°/min, and the rotation speed of the photographic recording drum was increased twofold. In this way it was possible to obtain, for some specimens, two separate effects with maxima at temperatures of 260° and 320°, respectively, and for others—a bend in the effect curve at a temperature of 320° (Fig. 3). Thus, the occurrence of two fundamentally different relaxation processes (i.e., processes of transition to a more stable state) in the temperature range 250–330° has been established for steatite materials. With the aid of petrographic and X-ray methods of investigation, the data from which the limited length of the article does not permit us to present, we were able to determine the nature of the low-temperature processes that are one of the principal causes of aging in steatite ceramics. The effect at 320–350° on the DTA curves of steatite-ceramic products is associated with the transition of the metastable form of magnesium metasilicate—protoenstatite—into the stable form—clinoenstatite. But this process depends to a great extent on the following causes:

Fig. 3. DTA curves: 1—cristobalite, 2 and 3—of steatite-ceramic articles (specimens ground until passing through sieve No. 0060, specimen heating rate 5°/min.).

Fig. 4. DTA curves of articles: 1—B-17 ceramics, 2—P-5 ceramics, 3—glass-ceramics of composition B-17, 4—glass-ceramics of composition P-5 with addition of 0.12% ZrO₂ (specimens ground until passing through sieve No. 0060, specimen heating rate 15°/min.).

1. How homogeneous the glass phase of the ceramics is. Thus, microheterogeneity of the glass phase of B-17 ceramics is detected by the optical method; its refractive index varies within the limits 1.534–1.560.

2. How fine the structure of the ceramics is, i.e., how small the grains of the crystalline phase are and to what degree they are uniformly distributed in the glass phase. From Fig. 4 it is evident that ceramics obtained by the technology for producing glass-ceramics—crystallization of glasses of steatite compositions—are more stable, despite the fact that the crystalline phase of these articles is represented almost entirely by the metastable phase—protoenstatite.

The following conclusions follow from the data of our investigations:

1. In steatite ceramic articles, during service, complex relaxation processes occur that impair their properties and are, by their nature:

a) polymorphism of the crystalline phase of the ceramic,

b) microinhomogeneity of the glass phase of the ceramic.

1. The quality of steatite ceramic articles, from the standpoint of their stability, can be judged by the magnitude of the effect at temperatures of 300—350° on the DTA curve of the material.

2. The presently existing technology for manufacturing steatite ceramic articles does not make it possible reliably to obtain a stable, non-aging ceramic.

3. The glass-ceramic method of manufacturing articles of steatite composition ensures the production of a stable structure of the articles, together with sharp improvements in all their technical properties.

REFERENCES

1. L. M. Blyumen. “The role of the glassy phase in talc ceramics.” Talc (collection of articles), Promstroiizdat. Moscow, 1952.

2. V. V. Lapin. “Investigation of the structure of talc and talc ceramics.” Talc (collection of articles), Promstroiizdat. Moscow, 1952.

3. A. Palatsky. Technical Ceramics. 4—44. (Translated from German.) Gosenergoizdat. Moscow, 1959.

4. W. R. Foster. “High-Temperature X-Ray Diffraction of the Polymorphism of MgSiO₃.” Journal of the American Ceramic Society, vol. 34, No. 9, pp. 255—259, 1951.