INFLUENCE OF CALCIUM IMPURITIES ON THE PROPERTIES OF STEATITE CERAMICS

P. G. Usov, V. I. Vereshchagin

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

Steatite ceramics belong to porcelain-like materials with 30–35% glass phase. Such a phase ratio is obtained when, in preparing the batch, 6–10% of special fluxes \((\mathrm{BaCO_3}, \mathrm{CaCO_3}\) or \(\mathrm{SrCO_3})\) are added to talc along with clay components [1]. Most magnesium-silicate systems (cordierite ceramics, steatite ceramics) are characterized by a narrow sintering interval and a narrow interval of the sintered state [2]. To create more favorable sintering conditions for steatite ceramics, the addition of \(\mathrm{BaCO_3}\) is better than \(\mathrm{CaCO_3}\). During firing of ceramics, calcium oxide forms a more mobile silicate melt, which narrows the firing interval of ceramic products and promotes the growth of magnesium metasilicate crystals. Therefore, in developing steatite compositions, partial or complete replacement of calcium oxide by barium oxide is pursued [3] (Table 1). However, because of its low cost and lesser scarcity, calcium carbonate continues to be one of the components of steatite bodies [4].

In the present work, the influence of calcium on the properties of steatite materials is considered from different positions, namely from the standpoint of its influence on the stability of the crystalline phase of the ceramic (magnesium metasilicate). According to the data currently available [5, 6], magnesium metasilicate can exist in several polymorphic modifications, the stability of which depends on conditions. The instability of individual forms of magnesium metasilicate leads to a highly undesirable phenomenon—the aging of steatite dielectrics. Our studies have shown that the cause of ceramic aging is the transition of the proto-form (protoenstatite) into the clino-form (clinoenstatite) of magnesium metasilicate at the storage and service temperatures of the products. This process is accelerated in larger crystals of \(\mathrm{MgSiO_3}\) [7].

It is known [8] that clinoenstatite \(\mathrm{Mg_2[Si_2O_6]}\) and diopside \(\mathrm{CaMg[Si_2O_6]}\) form a continuous series of monoclinic minerals with a \(\mathrm{CaO}\) content from 0.0% to 26.0%. In order to investigate how small amounts of \(\mathrm{CaO}\) affect the structure of magnesium metasilicate formed during the synthesis of steatite material, talc was fired together with additions of \(\mathrm{CaO}\) at \(1300\text{–}1320^\circ\). (Preliminary firing of talc at these temperatures is one of the technological operations in the production of steatite ceramics.) The amount of \(\mathrm{CaO}\) was varied from 0.1% to 10.0%; it was introduced as an aqueous solution of the nitrate salt \(\mathrm{Ca(NO_3)_2}\) into talc calcined at \(1000^\circ\mathrm{C}\). This achieved maximum dispersion and uniformity in the distribution of the additives throughout the entire mass of the material. Talc

Table 1

Chemical compositions of steatite materials

were processed in the ground state to a particle size of less than 0.5 mm (sieve No. 0.5).

The talc treated in this way was pressed, at a specific pressure of about 900 kg/cm², into flat specimens, after which it was fired at 1300–1320°C.

The principal product of firing pure varieties of natural talcs, for example those from the Onot and Alguy deposits, is protoenstatite—the stable high-temperature modification of MgSiO₃ (see Table 2). The effect of additions on the phase composition of the firing products of Onot talc is detected beginning with 0.5% CaO: on the

Table 2

X-ray analysis data for modifications of magnesium metasilicate
and firing products of Onot talc at 1300°C (anode—Cu)

radiograph, lines of clinoenstatite appear (Table 3). With an addition of 5% CaO, practically all the MgSiO₃ in the talc firing products (at 1300°C) appears as clinoenstatite, with a crystal size of about 10 microns. Protoenstatite is retained in the talc firing products when CaO from 1% to 3% is introduced, its stability here being much lower (Tables 3, 4) than after firing natural talc. These data enabled us to draw very important conclusions concerning calcium in the compositions of steatite bodies.

Table 3

Intensities of the maxima of protoenstatite and clinoenstatite on X-ray diffraction patterns of firing products of Onot talc with CaO additions
(firing at 1300°C) (absolute intensities in mm)

The overwhelming majority of steatite materials currently produced, in terms of their phase composition, are protoenstatite materials. The glassy phase of the ceramic prevents the transformation of the proto-form of $\mathrm{MgSiO_3}$ into the clino-form, but the stabilization is not complete [7].

In practice, the instability of protoenstatite (its tendency to transform) is detected when the body of the product is ground, by the method of X-ray phase analysis. On the X-ray diffraction pattern of steatite material ground to a fine powder, the maxima of the X-ray reflections of clinoenstatite are sharply intensified:

\[ d — 2.95;\quad d — 2.10;\quad d — 1.37\ \text{Å} \]

and the maxima of the reflections of protoenstatite are weakened:

\[ d — 1.97;\quad d — 1.49;\quad d — 1.31\ \text{Å}\quad (\text{Tables 4, 1}). \]

Table 4

X-ray analysis data for steatite ceramic products with different CaO contents (anode — Cu)

With this research methodology, the following fact is revealed: with the same phase composition of intact articles and the same microstructure, the stability of the protoenstatite of ceramics K-1 and (LK-1) is much lower than the stability of the protoenstatite of ceramics P-5. The difference between these materials lies in their chemical composition (Table 1); in particular, K-1 contains 1.25% CaO. In order to verify the assumption that the cause of the decrease in the stability of the crystalline phase of steatite materials is CaO impurities, ceramic P-5 articles were obtained, but the talc was preliminarily fired with the addition of 2.5% CaO introduced by the method already described in this work. The experiment confirmed the assumptions—the stability of the protoenstatite worsened (see Table 4, II); lines of clinoenstatite are also observed in the intact specimen, and their intensity increases considerably after grinding. Such a state of the structure of steatite materials is undesirable.

The influence of calcium on changes in the chemical composition and the state of the spatial lattice of the crystalline phase of talc ceramics is also manifested when calcium is introduced into the batch in the usual way, in the form of carbonate as a flux. The degree of incorporation of calcium into the structure of $MgSiO_3$ depends on its amount in the mass and on the temperature regime of treatment. The crystalline phase of talc ceramics, where, besides clay, $CaCO_3$ serves as the flux (chemical composition in Table 1), is represented by minerals of the clinoenstatite–diopside series. On the X-ray diffraction pattern of the intact article, maxima close to the reflections of clinoenstatite and diopside are recorded (Table 4, III); the X-ray pattern cannot be unambiguously interpreted as clinoenstatite, protoenstatite, or diopside.

The conclusions of this work are as follows:

1. The negative influence of calcium on the sintering process of steatite ceramics and on the properties of finished articles consists not only in the creation of unfavorable properties of the cementing melt (a mobile melt), but also results from physicochemical interaction with the lattice of magnesium metasilicate.

2. The result of such interaction may be a decrease in the stability of the protoenstatite modification of $MgSiO_3$ in the ceramic body, as well as deeper changes in the chemical composition and structure of the crystalline phase of the ceramics, depending on the amount of calcium impurity.

3. Impurities of calcium oxide (especially in amounts of 1–5%) are undesirable in steatite bodies both as contaminants of the main component, talc, and when deliberately introduced into the batch.

4. The introduction of calcium carbonate as a flux-forming component is not dangerous in those ceramic compositions where it is intended to obtain, as the crystalline phase, clinopyroxenes of the clinoenstatite–diopside series, which do not undergo polymorphic transformations.

REFERENCES

1. V. G. Avstnikov, E. I. Zin’ko. Physicochemical Principles of Steatite Electrical-Insulating Ceramics. In: Physicochemical Principles of Ceramics. Collection of articles, Promstroiizdat, Moscow, 1956.

2. U. D. Kingery. Introduction to Ceramics, 196. Promstroiizdat, Moscow, 1964.

3. G. I. Berdov. Steatite ceramics for metal-ceramic lamps. Scientific-technical collection Elektronika, 4, 1959.

4. D. D. Churabo. New Nonmetallic Materials for Radio Equipment. Gosenergoizdat, Moscow, 1961.

5. N. A. Toropov et al. Phase Diagrams of Silicate Systems. Handbook 41–48, Izd. “Nauka,” Moscow–Leningrad, 1965.

6. A. Palatskii. Technical ceramics, 24–28. Gosenergoizdat, Moscow–Leningrad, 1959.

7. P. S. Usov, Yu. V. Reshetina. On the polymorphism of magnesium metasilicate in steatite articles at low temperatures. Proceedings of TPI, No. 196, Tomsk, 1968.

8. G. V. Kukolev. Chemistry of Silicon and Physical Chemistry of Silicates. Izd. “Vysshaya shkola,” Moscow, 1966.