ON THE POSSIBILITY OF USING PRODUCTS OF THE INTERACTION OF ZIRCON WITH CALCIUM CARBONATE IN THE PRODUCTION OF CAPACITOR CERAMICS

P. G. Usov, E. P. Solomatina

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

The use of zircon has opened up a new direction in the search for new materials for electrical engineering. Zircon ceramics with a low dielectric permittivity, with $\operatorname{tg}\delta = 0.008$–$0.01$ at radio frequencies, are promising; they are intended mainly for the manufacture of various installation-type electrical-insulating parts and are used in capacitor production for low-capacitance capacitors.

Zirconium-containing compounds possess a number of specific properties, which has attracted the attention of many researchers [1–6]. However, further studies aimed at obtaining broader information on the properties of these compounds are also of interest, in order to outline an expansion of the areas of their application.

This article presents the results of a study on the possibility of using calcium zirconates and zirconosilicates in the production of capacitor ceramics.

For the synthesis of the compounds, an industrial method was chosen for opening zircon concentrate by sintering it with chalk. The work used zircon from the sands of the Tugansk deposit; the optimal conditions for opening it were recommended by the Basic Laboratory of Mineral Raw Materials at Tomsk Polytechnic Institute [7]. To identify the mechanism of interaction of zircon with calcium oxide and to establish the phase composition of the reaction products at stoichiometric ratios in the mixture $ZrSiO_4 : CaO = 1 : 1,\ 1 : 2,\ 1 : 3$, thermodynamic, kinetic, and X-ray methods of analysis were used.

Thermodynamic analysis of reactions in the system $CaCO_3 — CaO — ZrSiO_4$ showed that the interaction between zircon and calcium oxide is theoretically possible from $560^\circ K$, when the formation of zirconium dioxide is thermodynamically probable. To assess the thermodynamic sequence of formation and the stability of compounds at high temperatures in the system $CaCO_3 — CaO — ZrSiO_4$, the dependence of $\Delta Z^0$ of the reactions on the composition of the starting mixtures was considered. The results of calculations of $\Delta Z^0 = f(T)$ for the compositions made it possible to compile series for the thermodynamic sequence of formation and stability of compounds in this system (Tables 1, 2).

The sequence of reactions and the phase composition of the products of interaction of zircon with calcium carbonate as a function of the component ratio in the mixture were established by high-temperature X-ray analysis. X-ray patterns were recorded on a diffractometer.

Table 1

Thermodynamic sequence of formation of compounds in the system
$\mathrm{CaCO_3—CaO—ZrSiO_4}$

Table 2

Thermodynamic sequence of stability of compounds of the system
$\mathrm{CaCO_3—CaO—ZrSiO_4}$

URS-50I with a high-temperature attachment in the angular interval $2\theta = 24—34^\circ$, in which, at the initial moment, two diffraction maxima corresponding to the starting substances are recorded. With increasing temperature, additional diffraction maxima appear, the positions of which change differently for mixtures of zircon with 1, 2, and 3 moles of calcium oxide.

As a result of indexing the radiographs it was established that the final products of the interaction of zircon with calcium oxide at their molar ratio are zirconium dioxide and calcium metasilicate, which form successively through a series of intermediate reactions. When zircon is sintered with two and three moles of calcium oxide, the final products are $\mathrm{CaZrO_3}$ and $\mathrm{Ca_3ZrSi_2O_9}$. The kinetics of the interaction of zircon with calcium carbonate was also studied as a function of the ratio of the components in the mixture in the temperature interval 1000—1300°C at 100° intervals. In view of the difficulty of quantitatively identifying all reaction products, the kinetics of zircon decomposition and calcium oxide binding as a whole was studied, irrespective of the intermediate reactions by which the interaction proceeds. The reaction rates and the reaction-rate constants were calculated from the equations:

\[ l^{\frac{a}{a-x}} = B \cdot Z^n \quad \text{or} \]

\[ \frac{dx}{dZ} = \frac{n(a-x)^2}{2{,}3 \cdot a \cdot Z} \quad \text{and} \]

\[ K = \frac{dx}{dZ} \cdot Z, \]
in which:

$d$ — amount of decomposed zircon or bound $\mathrm{CaO}$ by the beginning of the holding period, %;

$x$ — amount of decomposed zircon or bound $\mathrm{CaO}$ over a definite time interval during the course of the reaction, %.

The data for carrying out the kinetic treatment were obtained as a result of chemical analysis of the samples after the corresponding thermal treatment.

The rectilinear dependence of the logarithm of the reaction-rate constant on the reciprocal value of the absolute temperature confirms the correct-

ness of the selected kinetic equation for describing the experimental data (Fig. 1). The obtained data for the reaction-rate constants at several temperature values made it possible to calculate the activation energy by the Arrhenius equation. It is, at a ratio \( \mathrm{ZrSiO_4:CaO}=1:1 \), \(10.5\ \mathrm{kcal/mol}\); at \( \mathrm{ZrSiO_4:CaO}=1:2 \), \(5.6\ \mathrm{kcal/mol}\); at \( \mathrm{ZrSiO_4:CaO}=1:3 \), \(7.2\ \mathrm{kcal/mol}\).

The curves in Fig. 2 illustrate the change—a sharp decrease—in the reaction rate with time. This indicates that, at the maximum

Fig. 1. Dependence of the reaction-rate constant on absolute temperature:
1 — \( \mathrm{ZrSiO_4:CaO}=1:1 \);
2 — » \(=1:2\);
3 — » \(=1:3\)

rate, the reactions proceed at the initial moment, and then, owing to their diffusion character, their course slows down.

The diffusion character of the process of zircon decomposition by calcium oxide is confirmed by microscopic studies, in which a definite sequence of the process was revealed. The interaction of zircon with calcium oxide proceeds in the solid phase layer by layer from the surface of each zircon crystal; on its faces, at first, small crystals of the newly formed phase appear, creating a reaction rim, which subsequently spalls off from the zircon grains. Loosening and spalling of the reaction product occur because the compounds formed have a larger molecular volume than zircon. This also explains the reason for the high rate of the reaction in the relatively low-temperature region, namely that the reaction product does not form a dense layer around the grains and, continuously spalling off,

Fig. 2. Change in the reaction rate with time at a temperature of \(1200^\circ\mathrm{C}\):
1 — \( \mathrm{ZrSiO_4:CaO}=1:1 \);
2 — » \(=1:2\);
3 — » \(=1:3\)

Fig. 3. Change in the properties of specimens as a function of firing temperature:
1 — linear dimensions, %;
2 — water absorption, %;
3 — bulk density, g/cm³.

it exposes clean surfaces of the zircon crystal. As a result of a comprehensive study it was established that the interaction proceeds at the highest rate at a ratio \(\mathrm{ZrSiO_4} : \mathrm{CaO} = 1 : 2\), with the formation of \(\mathrm{CaZrO_3}\) and \(\mathrm{Ca_3ZrSi_2O_9}\) as the final products.

These facts were taken as the basis for selecting the crystalline phase of the experimental ceramic body. In view of the incompatibility of zirconate synthesis with the manufacture of articles from them, due to volume changes, synthesis of the crystalline phase is provided for as a separate stage of the technological process. Synthesis of the crystalline phase, combined with preparation of the ceramic body, was carried out in a silite furnace at a temperature of \(1320^\circ\mathrm{C}\) with a 2-hour hold. The synthesized product, with a content of free calcium oxide not exceeding 2%, is a loose mass; therefore grinding is not associated with difficulties. The possibility of manufacturing articles by pressing and by hot pressure casting was investigated. To study the technological properties of the body, specimens were pressed on a laboratory hydraulic press under a pressure of \(750\ \mathrm{kg/cm^2}\). Firing shrinkage, bulk density, water absorption, and compressive strength were determined on specimens—cylinders with a diameter and height of 14 mm. The results of the study are presented in Fig. 3. The ultimate compressive strength is \(1800\text{–}2000\ \mathrm{kg/cm^2}\). At present, studies are continuing on the technical and electrical characteristics of the body. This ceramic should be promising because of the simplicity of its composition, the uncomplicated technological process (low firing temperature with a sufficiently wide interval of the sintered state), and the availability of local raw materials—Tugan zircon and chalk deposits of Rozhdestvenskoe and Kuendatskoe.

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