INVESTIGATION OF FORSTERITE CERAMICS WITH A FINE-CRYSTALLINE STRUCTURE

P. G. Usov, V. I. Vereshchagin, N. K. Glushkova

(Presented by the scientific seminar of the inorganic departments)

Forsterite ceramics are currently among the widely used high-frequency dielectrics. The principal crystalline phase of the ceramics is magnesium orthosilicate \((\mathrm{Mg}_2\mathrm{SiO}_4)\), which is not subject to polymorphic transformations. Forsterite is stable at high temperatures and in a reducing atmosphere; it melts without decomposition and is decomposed by hydrochloric acid, releasing silicic acid in the form of studnia [1]. Table 1 gives the typical physical properties of forsterite as a ceramic dielectric [1]. Forsterite materials are not subject to electrical aging. The coefficient of thermal expansion of forsterite \((10—10.5)\cdot 10^{-6}\ \mathrm{deg}^{-1}\) agrees well with the coefficient of thermal expansion of titanium \((9—9.5)\cdot 10^{-6}\ \mathrm{deg}^{-1}\), which is used in producing articles with a strong metal—ceramic bond [2]. However, the resistance of forsterite ceramics to thermal shocks, their thermal conductivity, and mechanical strength are insufficient for producing reliable seals with rigid metallic structures.

The task arose of finding ways to increase the thermal resistance and strength of forsterite ceramics in combination with high electrical characteristics.

Thermal shock resistance depends on many factors; it increases with increasing thermal conductivity and mechanical strength [2]. The latter depends strongly on the structure of the material and its porosity [3]. Fine-grained ceramics possess greater strength than coarse-grained ceramics at the same phase composition [3].

Thus, one of the most likely ways of improving the thermal shock resistance and mechanical strength of forsterite ceramics is to obtain a fine-grained crystalline phase of forsterite and the minimum necessary amount of glass phase with high viscosity and improved characteristics. The glass phase of the ceramic used has a barium aluminosilicate composition. The task was set to investigate the processes of formation of forsterite with a fine-grained body structure from talc, talc products, and magnesium oxide.

During firing, first the decomposition of talc occurs \((870—960^\circ\mathrm{C})\), then the formation of magnesium metasilicate \((\mathrm{MgSiO}_3)\); afterward, in the presence of the calculated amount of \(\mathrm{MgO}\), free silica is bound into metasilicate and forsterite is formed from magnesium metasilicate [2]. Consequently, if magnesium metasilicate is obtained during the firing of talc in a finely crystalline state, then, possibly, the formation of forsterite from it will proceed within the same grain-size range. Of-

Table 1

It follows from this that the crystallization of MgSiO₃ during the firing of talc must be directed toward achieving a homogeneous fine-grained structure, influencing the process by small additions of certain elements. It is known that the addition of elements of the second group of D. I. Mendeleev’s periodic system in an amount of 0.7—1.0% (BaO, SrO, BeO) promotes the formation of fine-crystalline magnesium metasilicate [4].

However, the forsterite subsequently obtained (beginning at 1000°C and above) is very susceptible to recrystallization.

It is necessary to create conditions under which the formation of forsterite throughout the mass as the temperature is increased would not be accompanied by crystal growth. For this purpose, additions of elements—or, more precisely, their oxides—were introduced that promote an increase in the viscosity of the glass phase. Up to 1% ZrO₂ was introduced. Several batches were prepared, and studies were carried out.

Petrographic analysis showed that in all specimens from batches with additions of 0.7% SrO and 1% ZrO₂, the body has a uniform structure. The crystalline phase is forsterite; the crystal size is about 1 micron over a wide temperature interval (from 1200 to 1300°C). The glass occurs as films on the crystals. In specimens from batches without additions the structure is somewhat nonuniform; the minimum crystal size is 1—2 μm at a temperature of 1280—1300°C. In all specimens, beginning at 1300°C, crystal growth occurs.

X-ray analysis showed that the formation of forsterite from batches with additions and without additions begins already at a temperature of 1000°C; only in the former do we have more characteristic maxima of forsterite, and their intensity is somewhat higher. When the temperature is raised to 1200—1300°C, all characteristic maxima of forsterite are present (d = 2.753, d = 2.250, d = 1.74). Moreover, their intensity is higher for the batches with additions [5].

On the basis of the results of these investigations, ceramic bodies were prepared (their component composition is presented in Table 2), as well as reference bodies based on Alguisk and Onotsk talcs.

The batches were ground until passing through sieve No. 0060 (10,000 openings/cm²), the bodies were granulated, and the optimal firing temperature was determined on small disks. At this temperature, in all bodies the crystal sizes of forsterite are minimal and the entire crystalline phase of the specimens is represented by forsterite (petrographic and X-ray studies). The granules were fired, ground by dry milling with oleic acid, and specimens were pressed with the addition, as a plastic binder, of 11.2% paraffin and 0.5% wax. Then followed a preliminary firing in packing up to 960°C and a final firing up to 1350°C, since only at this temperature is a densely sintered body obtained.

Table 2

Tests of the products showed that the most favorable structure is found in products based on Altai talc with additions of a modifier (SrO) and a stabilizer (ZrO₂), compared with products made from talc without additions; the average crystal size is 5–6 µm; the glass phase amounts to 10–12% for Altai talc and 20–15% for Onot talc. Products made from a batch (with additions) based on Altai talc are not inferior in their properties to products made from Onot talc, and in some cases surpass them: they are more heat-resistant, have a higher volume resistivity, a low dielectric loss tangent, and exhibit stable shrinkage.

To achieve the best results, it is necessary to reduce the final firing temperature of the product to 1300°C, since up to this temperature the forsterite grains have minimum sizes.

REFERENCES

1. Dielectrics and Their Applications. Gosenergoizdat, 1959.
2. V. L. Balkevich. Technical Ceramics. Moscow, 1968.
3. P. A. Rebinder, I. S. Lipman. Research in the Field of Surface Phenomena. ONTI, 1936.
4. U. D. Kingery. Introduction to Ceramics. Moscow, 1967.
5. V. I. Mikheev. X-ray Determinative Guide to Minerals. Gosgeoltekhizdat, 1957.