STUDY OF THE EFFECT OF THE DISPERSION OF THE STARTING MATERIAL ON THE SINTERABILITY OF MICA CERAMICS

P. G. Usov, E. N. Belomestnova, V. I. Vereshchagin

(Presented by the scientific-methodological seminar of the CTF)

Mica ceramics is a material obtained on the basis of fine-crystalline synthetic mica. Mica-ceramic products are used as heat-resistant insulation and vacuum-tight parts in various radio-electronic apparatus [1]. Interest in mica ceramics is associated above all with such properties as high heat resistance (above 700°C), good machinability in the fired state, and the ability to form strong joints with metals. Initially, the technology for producing mica ceramics was developed with the aim of utilizing fine-crystalline waste obtained in the production of mica single crystals. However, the valuable properties determined the independent significance of this material. Fine-crystalline mica can be obtained either by crystallization from a melt or by solid-phase synthesis. The questions of mica-ceramics technology have not been studied sufficiently. We investigated the effect of powder dispersion and the amount of binder on sinterability. The necessity of this kind of research becomes clear if one takes into account that, according to the known technology, a material with water absorption of 1–2% is obtained [2]. It is recommended that the starting material be ground until it passes through sieve No. 015. It is generally accepted that increasing the degree of dispersion has little effect on the properties of the sintered material.

When using mica obtained by crystallization from a melt, finer grinding proves irrational because of the difficulty of grinding mica crystals. In the sinter, however, the crystal sizes do not exceed 20 μm; the material, as a rule, is easily ground. Therefore, when using mica obtained in the process of solid-phase synthesis, the use of comparatively coarse-dispersed (−150 μm) mica powder appears unjustified.

We synthesized normal fluorophlogopite \((\mathrm{KMg}_3\mathrm{AlSi}_3\mathrm{O}_{10})\) by firing at a temperature of \(1000^\circ\mathrm{C}\) from a batch based on dehydrated talc. The synthesis product consisted by 90–95% of fine-grained \((5\text{–}15\ \mu\mathrm{m})\) mica. An 85% orthophosphoric acid was used as the binder. The sinters were crushed, then ground in a porcelain drum until they passed through sieves No. 020, No. 015, and No. 012, and were used to prepare a pressing powder. The amount of binder was varied from 10 to 15%. The pressing powder was prepared according to the technology proposed by the All-Union Scientific Research Institute of Asbestos Cement. Samples dried at \(120^\circ\mathrm{C}\) were fired in closed fireclay capsules at temperatures of 1000, 1100, and \(1150^\circ\mathrm{C}\). Water absorption, shrinkage, and mechanical strength were measured for the fired samples.

Comparison of the results showed that the degree of dispersion has a very substantial effect on the sinterability of the samples. With a constant amount of binder, the sinterability of the samples, as the grain size of the starting material decreases, first improves somewhat and then becomes worse (Table 1). This phenomenon may be explained by the fact that, as dispersion increases, a deficiency of binder arises, i.e., not the entire surface of the grains proves to be wetted and reactive.

Table 1

Effect of the amount of binder and the particle size of the starting material on the degree of sinterability of mica ceramics \((1150^\circ\mathrm{C})\)

Increasing the amount of binder promotes an increase in the mechanical strength of the samples; this effect is especially evident in samples based on more finely dispersed mica: the compressive strength of mica ceramics based on mica powder with a grain size \(<120\ \mu\mathrm{m}\), at a binder content of 10%, is \(700\ \mathrm{kg}/\mathrm{cm}^2\), while when the amount of binder is increased to 15% it rises to \(1100\text{–}1200\ \mathrm{kg}/\mathrm{cm}^2\). Evidently, in order to substantially improve the sinterability of mica ceramics when using fine-grained material, it is necessary to increase the amount of binder somewhat and to distribute it more uniformly. Increasing the pressing pressure also has a substantial effect on the properties of the material. On the basis of fluorophlogopite sinter ground until it passed completely through a 060 sieve, samples were pressed \((P = 3000\ \mathrm{kg}/\mathrm{cm}^2)\) which, after firing at a temperature of \(1150^\circ\mathrm{C}\), exhibited zero water absorption and a mechanical strength of \(1500\ \mathrm{kg}/\mathrm{cm}^2\).

Conclusions

Increasing the degree of dispersion of mica proves effective in cases where, along with this, the amount of binder is increased, the uniformity of its distribution is improved, and the pressing pressure is increased.

Literature

1. M. S. Laizerson. Radio Engineering and Electrical Engineering, vol. 1077–1081, 1958.
2. I. E. Comeforo, J. Amer. Ceram. Soc., 37, 427–432, 1954.