PRODUCTION OF CERAMICS WITH A DIOPSIDE CRYSTALLINE PHASE BASED ON TREMOLITE

P. G. USOV, Z. S. KONOVALOVA, V. I. VERESHCHAGIN

(Submitted by the seminar of the departments of inorganic chemistry, KhTF)

The present work considers the question of producing ceramics with a diopside crystalline phase.

The domestic and foreign scientific literature contains only fragmentary information in this area. In Japan, sintered diopside porcelain synthesized from seawater has been investigated. At the same time, world practice has considerable experience in the use of magnesium metasilicate in steatite ceramics.

Magnesium metasilicate (clinoenstatite) \( \mathrm{MgSiO_3} \)—diopside \( \mathrm{CaMg[Si_2O_6]} \) are the end members of a series of solid solutions with a CaO content from 0.0 to 26 wt.%. Small impurities of calcium oxide in steatite ceramics are highly undesirable, since they stimulate transformations of one polymorphic form into another at low temperatures, which leads to aging of the ceramics. Diopside, however, has no polymorphic forms, and thus aging of ceramics with this crystalline phase is eliminated. Diopside has a low coefficient of thermal expansion \((60\text{—}65 \cdot 10^{-7}\,^{\circ}\mathrm{C}^{-1})\); consequently, ceramics with a diopside crystalline phase should be heat-resistant.

In our practical work, to obtain high-frequency ceramics with a diopside crystalline phase, tremolite from the Alguy deposit was used.

Tremolite is a mineral with a natural combination of calcium and magnesium oxides; therefore, the synthesis of diopside from this mineral is carried out more simply. The chemical composition of tremolite satisfies the requirements for raw materials for the production of high-frequency dielectrics (Table 1).

The ratio of magnesium and calcium oxides in tremolite—\(2\mathrm{CaO} \times 5\mathrm{MgO} \times 8\mathrm{SiO_2} \times \mathrm{H_2O}\)—does not correspond to the composition of diopside; therefore, a correction was made. The required amount of calcium was introduced as carbonate.

The ceramic was prepared in parallel both from diopside preliminarily synthesized from tremolite and from a batch mixture into which, in addition to tremolite with calcium carbonate, fluxes—bentonite and barium carbonate—were introduced, calculated to obtain about 30% of a glassy phase of barium aluminosilicate composition.

The component composition of the body was as follows: Alguy tremolite—46.2%; bentonite—14.6%; barium carbonate—12.4%; calcium carbonate—20.8%; sand from the Tugan deposit—6.2%.

Chemical composition of the body: \( \mathrm{SiO_2} \)—53.6%; \( \mathrm{CaO} \)—18.9%; \( \mathrm{MgO} \)—13.5%; \( \mathrm{BaO} \)—11.3%; \( \mathrm{Al_2O_3} \)—2.7%.

Ceramics for the investigations were prepared by sintering technology, with the parts shaped by hot pressure casting on production semi-automatic machines. Analyzing the experimental material obtained, the following may be noted: sintering takes place at temperatures of 1230–1250°C.

Table 1

Chemical composition of tremolite

The articles were fired in two stages—the preliminary biscuit firing was carried out in backfill according to the biscuit-firing schedule in a muffle, and the final firing—in an electric furnace with a silicon-carbide heating element, with a final two-hour temperature hold (Table 2). The sintering temperature was determined on the basis of the dependence of shrinkage, water absorption, and compressive strength on temperature (Fig. 1). A smoother sintering curve is obtained by ceramics synthesized from raw tremolite.

Table 2

Temperature schedule for firing the articles

BISCUIT FIRING

FINAL FIRING

These ceramics show no change in phase composition either after mechanical or after thermal effects.

The phase composition was studied by means of X-ray diffraction analysis, performed on a URS-50-I diffraction unit with a copper anticathode. The tests showed increased thermal stability in compar-

...with steatite ceramics. Thermal shock resistance was evaluated by the heat-change method, the essence of which consists in rapid heating of the specimens to a temperature of 300°C followed by cooling in running water (Fig. 2).

Fig. 1. Dependence of water absorption and shrinkage on temperature:
a) body based on synthesized diopside;
b) body based on raw tremolite

Table 3

Chemical resistance

Table 4

Dielectric permittivity
and dielectric loss tangent
at a temperature of 20°C; $f = 10^5$ Hz

$\varepsilon_1$ and $\mathrm{tg}\,\delta_1$ — for specimens based on synthesized diopside.
$\varepsilon_2$ and $\mathrm{tg}\,\delta_2$ — for specimens based on raw tremolite.

The new ceramic has high mechanical strength; the flexural strength is 1800–1900 kg/cm². The chemical resistance of diopside ceramic to water, acids, and alkalis is higher than that of similar steatite ceramic. (Chemical resistance was determined by the Turner method) (Table 3).

The average coefficient of linear thermal expansion in the temperature range 20–800°C is 70–75 · 10⁻⁷ °C⁻¹.

Articles with a diopside crystalline phase showed high dielectric properties (Table 4). The electrical resistivity, ohm·cm, as a function of temperature is expressed by the following data: at 20°C—1.5 · 10¹⁴, 100°C—1.5 · 10¹⁴, 400°C—2.1 · 10¹⁴.

The results of the studies show the possibility of using tremolite from the Alguy deposit to obtain a new type of high-frequency ceramic with good dielectric and mechanical properties and not prone to aging.

Fig. 2. Dependence of $\sigma_{\text{comp}}$ on the number of heat changes: upper curve—specimens based on raw tremolite; middle curve—specimens based on synthesized diopside; lower curve—specimens of steatite ceramic P-5.

REFERENCES

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4. P. P. Budnikov, I. A. Bulavin et al. New Ceramics. Publishing House of Literature on Construction, Moscow, 1969.