ELECTROPORCELAIN BODIES BASED ON ALGUY TALC

P. G. USOV, G. N. POPOVA

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

Electrical-insulating ceramic materials containing talc in their batch composition as the principal component are finding ever wider application in electrical and radio engineering owing to their high mechanical and dielectric properties.

Among all types of talc ceramics, steatite bodies have become the most widespread; they contain 60–80% talc and are used for manufacturing both small installation parts and large antenna insulators of various configurations. The raw materials for producing these articles are high-quality talcs with additions of plastic refractory clays and fluxes.

We investigated steatite bodies based on low-iron talc from the recently discovered Alguy deposit. The purpose of the investigation was to study the dependence of the technological and technical characteristics of steatite bodies on the type and amount of fluxes introduced into their composition. At the same time, the task was set of selecting optimal compositions that could be recommended for the production of steatite electrical-insulating articles. The following types of raw materials were used as starting materials for synthesizing experimental bodies: talc from the Alguy deposit, natural and beneficiated; Chasov-Yar clay; nepheline from the Goryachegorsk deposit of Krasnoyarsk Krai, beneficiated by triple magnetic separation; and feldspar. Data from the chemical analysis of the starting materials are given in Table 1.

Alguy talc was introduced into the composition of the bodies in the raw state, and also after preliminary firing at a temperature of 1300–1320°. The raw and fired talc, as well as the feldspar and nepheline, were subjected to preliminary fine grinding in a ball mill until they passed completely through a sieve of 10000 openings/cm². The experimental bodies were prepared by weight batching and mixed in the presence of water in a ball mill with flint pebbles. The specimens were formed by semi-dry pressing on a manual hydraulic press at a pressure of 750 kg/cm². All firing characteristics and compressive mechanical strength were determined on cylindrical specimens with diameter equal to height = 14.5 mm. Whiteness was determined on disks 2.5 cm in diameter. The configuration and dimensions of the specimens for electrical tests met the requirements of GOST 5458—50 for “high-frequency ceramic” materials. Firing of the experimental specimens was carried out in a la—

in a laboratory silite furnace to temperatures from 1100 to 1330°C with an interval of 50–20° and holding at the final temperature for 2 hours. The total firing time was 6–9 hours.

Table 1

Four series of experimental steatite bodies were investigated; their composition is given in Table 2*.

Table 2

Batch composition of the experimental bodies, in weight %

The bodies of the first series (1, 2, 3) were intended for studying the effect of feldspar content in them on their technological properties.

The bodies of the second series (4, 5, 6), into which nepheline was introduced as a flux in the same amounts as feldspar in the bodies of the first series, were intended for studying the effect of this replacement on their properties.

In the bodies of the third series (7, 8, 9), a combined flux was used, with different ratios of feldspar and nepheline. The total amount of flux in these bodies was kept the same, equal to 10%. The last two bodies, 2a and 5a, corresponded in composition to

* Note: When selecting approximate compositions of the experimental bodies, the results of investigations of steatite bodies based on Onot talc with alkaline-earth fluxes, previously carried out by GIEKI [1], were taken into account.

bodies 2 and 5, with the sole difference that the Algui natural talc, containing a considerable amount of quartz, was replaced in them by talc enriched by flotation and almost completely corresponding, in its chemical composition (Table 2), to the composition of the talc mineral. The amount of fluxes in the bodies of the first and second series was varied by changing the amount of calcined talc. Raw talc and clay were introduced into all experimental bodies in equal amounts. The ceramic properties of the bodies studied are given in Table 3.

As can be seen from the table, all the bodies sinter in the temperature range 1200–1300°C. Increasing the feldspar content in the bodies of the first series from 5 to 10% lowers the sintering temperature by 40 to 50°, but at the same time increases the firing shrinkage of the articles and somewhat lowers their strength characteristics. A further increase in the amount of feldspar in the bodies is entirely inexpedient, since, without improving their technological properties, it leads to an even greater decrease in the compressive strength of specimens made from these bodies, evidently causing a considerable increase in the amount of glass phase. Of the bodies of the second series, the best technological characteristics were recorded for body five, with 10% nepheline. Its sintering interval is sufficiently wide—50°. Body four of this series, having a very narrow sintering interval, did not sinter at a temperature of 1280°; at a temperature of 1300° the specimens made from it deformed. Increasing the nepheline content to 15% worsened the firing characteristics of the body, narrowing the sintering interval, lowering the compressive strength of the fired specimens, and increasing their shrinkage.

Partial replacement of feldspar by nepheline (bodies 7, 8, 9) had an adverse effect on all technological characteristics. Of the three bodies in this series, at a temperature of 1280° only one sintered, body nine, with a feldspar-to-nepheline ratio of 1 : 3, but its water absorption remained considerable—0.051%; the sintering interval was very narrow, less than 20°, and the compressive strength of the sintered specimens was less than 2000 kg/cm².

Bodies 2a and 5a, containing enriched talc, sinter at a higher temperature in comparison with the analogous bodies 2 and 5 containing unenriched talc. Specimens from body 2a had a considerably higher strength in comparison with the second body; in body 5a no advantages over its analogous body 5 were recorded. The disadvantage of both bodies based on enriched talc is the very narrow sintering interval (about 20°), which will make it difficult to work with them in the production of large-sized articles. Thus, of the eleven bodies studied, bodies 1, 2, and 5 possess the most favorable technological characteristics. Taking into account that in our country there is experience in firing steatite articles with a narrow (10–15°) sintering interval, it was decided to include body 2a among the bodies selected for more detailed study. The sintering behavior of the four selected bodies is reflected in Figures 1 and 2. As can be seen from Fig. 1, shrinkage for all bodies has rather close values. The greatest shrinkage is shown by the body with 10% feldspar, the smallest by that with 10% nepheline. For all bodies, shrinkage continues to increase even after considerable overheating of the specimens (30–50°) relative to the sintering temperature, which indicates their continuing densification. This is also confirmed by the increase in bulk density (Fig. 2) with overheating. The only exception is body 2a, in which, with overheating above the sintering temperature by 30°, growth of the specimens and a decrease in bulk density are observed, although this is not reflected in the water absorption (Fig. 2),

Table 3

Comparative table of the ceramic properties of experimental bodies

Fig. 1. Dependence of shrinkage of experimental bodies on firing temperature:
1 — shrinkage by diameter of body 1;
2 — the same for body 2; 3 — the same for body 5;
4 — the same for body 2a; 5 — shrinkage by height of body 1; 6 — the same for body 2;
7 — the same for body 5; 8 — the same for body 2a.

Fig. 2. Dependence of water absorption and bulk density of experimental bodies on firing temperature:
1 — water absorption of body 1; 2 — the same for body 2;
3 — the same for body 5; 4 — the same for body 2a;
5 — bulk density of body 1;
6 — the same for body 2; 7 — the same for body 5; 8 — the same for body 2a.

evidently, due to the considerable vitrification of the surface of the specimens. In body 2 only a very slight increase in height is observed when overheated by 50° above the sintering temperature; in diameter the specimens decrease; the bulk density increases. Under the microscope, in polished sections of the ceramic materials under study, a fine-grained mass is recorded, composed of grains of magnesium metasilicate 1–2 microns in size and glass. The total refractive index of these aggregates is \(n_{\mathrm{av}} = 1.603 \pm 0.005\). Larger grains of magnesium silicate, 5–7 microns in size, have \(n_{\mathrm{av}} — 1.656\), which closely corresponds to clinoenstatite. Quartz grains are encountered, comprising about 1%, and isolated grains with \(n — 1.529\), which approximately corresponds to cordierite. The pores are isometric, more rarely regular in shape, 0.01–0.05 mm in size, and constitute 3–5% of the entire area of the polished sections. The dielectric properties of the materials studied are characterized by the following values: dielectric permittivity \(\varepsilon — 4.6—5.3\); tangent of the dielectric loss angle under normal conditions \(—60—100 \cdot 10^4\); specific volume resistivity \(—10^{13}—10^{14}\ \Omega \cdot \mathrm{cm}\); electrical strength \(E — 28—36\ \dfrac{\mathrm{kV}}{\mathrm{mm}}\).

Conclusions

1. The optimal bodies for producing steatite electroporcelain based on Alguy talc with an alkaline flux are bodies containing, in their batch composition, 20% Chasov Yar clay; 20% raw natural or beneficiated Alguy talc; 50–55% Alguy talc (natural or beneficiated), fired at a temperature of 1300–1320°; 5–10% feldspar or 10% nepheline.

2. The indicated steatite electrical-insulating bodies substantially surpass ordinary electroporcelain in dielectric properties, while not being inferior to it in strength.

Consequently, inexpensive Alguy talc can evidently replace the traditional, often costly, aluminous raw material in the production of electroporcelain and improve the quality of the products manufactured.

LITERATURE

1. V. G. Avetikov, G. V. Belinskaya, E. I. Zin’ko. Investigation of the properties of plastic steatite bodies for the production of insulators. Proceedings of the State Research Electrokeramic Institute, issue 1, Gosenergoizdat, Moscow–Leningrad, 92–108, 1956.