NEWS OF THE TOMSK POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV, ORDER OF THE OCTOBER REVOLUTION AND ORDER OF THE RED BANNER OF LABOR

Vol. 259　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1975

NEW CERAMICS

FOR HIGH-FREQUENCY TECHNOLOGY

P. G. USOV, Z. S. KONOVALOVA, V. I. VERESHCHAGIN
(Presented by the scientific seminar of the inorganic departments)

The development of radio engineering and electronics is steadily increasing the demand for ceramic materials with special properties. Domestic and world practice has extensive experience in the use of natural nonplastic raw material—talc—for the production of high-frequency ceramics of steatite and forsterite composition \((\mathrm{MgSiO_3}\) and \(\mathrm{Mg_2SiO_4})\).

In addition to magnesium metasilicate (steatite ceramics), diopside is also suitable for practical purposes. Diopside is readily synthesized from various products, has no polymorphic modifications, and has a coefficient of thermal expansion that is closely matched to the coefficient of thermal expansion of the glass phase of barium aluminosilicate composition. The use of diopside as the crystalline phase in ceramics has its advantages: aging is excluded, and the ceramic is heat-resistant.

In our practical work we investigated the clinoenstatite—diopside system. Minerals of the clinoenstatite—diopside series were synthesized with the following molar ratios of calcium and magnesium oxides:

1. \(0 : 1\ \mathrm{MgSiO_3}\)—clinoenstatite,

2. \(0.1 : 0.9\ \mathrm{Ca_{0.1}Mg_{0.9}SiO_3}\),

3. \(0.2 : 0.8\ \mathrm{Ca_{0.2}Mg_{0.8}SiO_3}\),

4. \(0.3 : 0.7\ \mathrm{Ca_{0.3}Mg_{0.7}SiO_3}\),

5. \(0.4 : 0.6\ \mathrm{Ca_{0.4}Mg_{0.6}SiO_3}\),

6. \(0.5 : 0.5\ \mathrm{Ca_{0.5}Mg_{0.5}SiO_3}\)—diopside,

7. \(1 : 1\ \mathrm{CaMgSi_2O_6}\).

As starting materials for the synthesis of these compositions, both pure chemical reagents—\(\mathrm{SiO_2}\), \(4\mathrm{MgCO_3}\cdot\mathrm{Mg(OH)_2}\cdot6\mathrm{H_2O}\), \(\mathrm{CaCO_3}\)—and natural products—talc and tremolite—were used. The component compositions for the synthesis of the minerals are given in Tables 1 and 2.

For the synthesis of clinoenstatite (1) and of the mineral following it (2), floated talc from the Algui deposit of the following chemical composition was used:

\[ \mathrm{SiO_2}—64.16\%;\quad \mathrm{MgO}—30.23\%;\quad \mathrm{CaO}—0.22\%; \]

\[ \mathrm{Al_2O_3}—0.35\%;\quad \mathrm{Fe_2O_3}—0.04\%;\quad \text{loss on ignition}—4.68\%. \]

Table 1

Component composition of mixtures for the synthesis of minerals from chemical reagents

Table 2

Component composition of batches for minerals of the clinoenstatite—diopside series based on natural minerals

For the synthesis of diopside (6) and the other minerals, tremolite, likewise from the Alguy deposit, of the following chemical composition was used as the main component of the batch:

$$ \mathrm{SiO_2} — 61.32\%;\quad \mathrm{MgO} — 25.5\%;\quad \mathrm{CaO} — 10.64\%; $$

$$ \mathrm{Al_2O_3} — 1.17\%;\quad \mathrm{Fe_2O_3} — 0.18\%;\quad \text{loss on ignition} — 1.72\%. $$

In addition, diopside (7) was also obtained on the basis of talc from the Alguy deposit.

The synthesis of the minerals was carried out both in the solid phase and by melting a fritted batch with subsequent slow crystallization of the melt. Firing of the briquetted batch after grinding (sieve No. 0060) was carried out in an electric furnace with silite heating elements at final temperatures from 1300 to 1400°C. The prefired briquettes were melted in corundum crucibles in a kryptol furnace at final temperatures of 1420–1600°C. Cooling of the melt proceeded as the furnace cooled over the course of a day.

Repeated experiments show that, of all the minerals (under various conditions), diopside is synthesized most readily. The synthesis of minerals based on pure reagents proceeds with difficulty, with by-products, does not give unambiguous results, and requires great care in carrying out the investigations with respect to dosing of the batch, grinding, mixing, and the firing regime. It should also be noted that, on the basis of natural materials—talc and tremolite—the synthesis of minerals is accomplished more easily both in the solid phase and by crystallization of the melt. The phase composition of the minerals was determined by X-ray and petrographic analyses. Analysis of the radiographs showed that, for minerals of compositions 2–4, by-products are still recorded ($d$ — 4.05–4.08 Å — cristobalite), and the maxima are strongly broadened owing to the imperfect internal structure of the new formations.

By crystallization of the melt it is possible to obtain minerals with well-formed crystals. The radiographs of the diopside synthesized by us (from different products) exhibit complete analogy with the reference data.

Since the CTE determines the thermal stability of ceramic articles and changes substantially with temperature, we determined the values of the coefficients of thermal expansion of minerals of the clinoenstatite—diopside series (Table 3) in the temperature interval 20–900°C. Measurements for determining the CTE were carried out on a quartz vertical dilatometer. Of the CTE values obtained, the value closest to the CTE of the glass phase of barium aluminosilicate composition is possessed by mineral (6), diopside.

Table 3

Table of CTE values for clinopyroxene–diopside minerals based on natural minerals

Thus, taking into account that the synthesis of this mineral is easily carried out in comparison with the other minerals, the agreement of its CTE with that of the glass phase, and also the absence of polymorphic transformations, diopside may be recommended as the crystalline phase of the ceramic. The use of diopside makes it possible to obtain a new high-frequency ceramic with increased thermal resistance and good electrical and mechanical properties.

REFERENCES

1. W. Eitel, Physical Chemistry of Silicates. Moscow, Foreign Literature Publishing House, 1962.
2. V. I. Mikheev, X-ray Determinative Tables for Minerals. Moscow, State Scientific and Technical Publishing House, 1957.