EFFECT OF THE DISPERSITY OF MAGNESIUM METASILICATE CRYSTALS ON THE STABILIZATION OF PROTOENSTATITE

P. G. Usov, V. N. Gurina

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

Magnesium metasilicate is a chemical compound with complex polymorphism. The question of phase relationships and the stability of modifications is of great practical importance as applied to the technology of production and operation of steatite ceramic materials, which contain up to 70% magnesium metasilicate. Possessing high physicomechanical and dielectric properties, talc-based ceramics are prone to aging.

According to current views, aging is associated with the transformation of metastable protoenstatite into clinoenstatite, or with the instability of the amorphous cementing bond.

Analysis of the literature data [1—4] shows that there is a definite relationship between the microstructure of the steatite body and its tendency toward aging: ceramics with grain sizes smaller than 3 µm, composed of protoenstatite that is metastable under normal conditions, possess high and constant service properties; steatite materials with a coarse-grained structure, represented by metasilicate in the form of proto- and clinoenstatite, have a clearly pronounced tendency toward aging.

The stability of ceramics with a fine-grained structure is explained by the formation of a protective glassy film on protoenstatite crystals. However, protoenstatite may also be stable in the absence of glass. The study of the conditions for the stability of protoenstatite is an important problem for steatite ceramics.

The present article gives the results of investigations on the effect of crystal size on the course of phase transitions of magnesium metasilicate. Clinoenstatite was selected as the object of study; the method of obtaining it assumes the formation of large crystals, with an average size greater than 0.5 mm: synthesis from a melt of stoichiometric composition based on enriched Altai talc. To obtain specimens with a specified particle size, the material was dispersed and separated into fractions. The study of phase transformations was carried out by means of differential-thermal and high-temperature X-ray phase analyses.

The transformation of clinoenstatite during heating is recorded by an endothermic effect on the DTA curve at 1100°C (Fig. 1). The observed change in the diffraction pattern of the X-ray reflection shows that the thermal effect is associated with the transformation of clinoenstatite into protoenstatite.

Fig. 1. DTA heating curves of clinoenstatite

1 — particle size less than 5 μm;
2 — particle size 5–10 μm;
3 — particle size 10–50 μm;
4 — particle size 50–100 μm.

On the basis of DTA data, the heat of transformation (0.22 kcal/mole) and the activation energy (1.5 kcal/mole) were determined. The low energy values characterizing the transformation indicate the closeness of the internal energy of the lattices of protoenstatite and clinoenstatite and determine the high probability of phase transitions in kinetic terms.

In view of the data presented, the transformation upon heating may, in its manner of occurrence, be assigned to first-order phase transitions.

The transformation of protoenstatite into clinoenstatite upon cooling has a number of essential features. For the transformation temperature, hysteresis is observed, so that the transition of the high-temperature form to the low-temperature form may occur 40–300° lower than the transformation upon heating. The temperature of the reverse transition is determined to a substantial degree by the preceding thermal history of the specimen and by the size of the crystals (Table 1).

Distinct loops of thermal effects on the DTA cooling curves (Fig. 2) and the prolonged preservation of the diffraction lines of protoenstatite on the corresponding X-ray patterns make it possible to conclude that the limiting factor of the transformation is elements of a reconstructive type (it is known that in transitions of protoensta-

Table 1

Temperature of the onset of transformation as a function of particle size and treatment temperature

Fig. 2. DTA cooling curves of protoenstatite

1 — particle size less than 5 μm;
2 — particle size 5–10 μm;
3 — particle size 10–50 μm;
4 — particle size 50–100 μm.

t — clinoenstatite—is destroyed to 50% of the Mg–O bonds. The variability of the temperature at which the transformation begins as a function of crystal size is connected with the features of the structural rearrangement, which requires a small activation energy. Since the free energy includes strain energy and surface energy, the degree of dispersity of the crystals influences the total reserve of free energy. Overcoming the energy barrier during the transformation of a finely dispersed material may be due to the nonuniform distribution of the energy of thermal vibrations of the atoms. In a coarse-crystalline material, the distribution of the energy of thermal motion of the atoms is more uniform and does not create an excess of energy sufficient to overcome the energy barrier. At the same time, because of the low activation energy, the phase transition is possible at a lower temperature. The energy barrier is overcome through the appearance of additional energy. Apparently, the transformation at 860°C is associated with the appearance of an excess of energy provided by the occurrence, in protoenstatite crystals, of thermal stresses due to nonuniform cooling through the volume.

The irreversible transformation into protoenstatite of the initial clinoenstatite with a crystal size of less than 5 μm after firing at 1250°C is achieved by sintering of the material, which leads to a decrease in the free energy of the system and counteracts the occurrence of transformations with a change in particle volume, since such a transformation must involve disruption of contacts between particles and an increase in surface energy.

Mechanical destruction of contacts between protoenstatite grains can cause transformation [4, 5]; however, in order to observe a noticeable transition into clinoenstatite of the fine-grained protoenstatite obtained by us, it was necessary to subject the dispersed material to additional thermal treatment for at least 50 hours at 860°C. In the unground specimen, with such heat treatment, no change in phase composition was observed.

Because of the high probability of supercooling and stabilization, at normal temperature, of the pure mineral protoenstatite, the possibility of supercooling will increase in ceramic compositions under the stabilizing action of glass. The stresses arising in this case may lead to the appearance of microcracks and to a loss of the mechanical strength and vacuum tightness of the ceramic.

Table 2 presents the results of a study of the effect of magnesium metasilicate particle size on phase formation and the stability of the properties of model steatite-ceramic structures obtained on the basis of separately synthesized crystalline (clinoenstatite of different fractional composition) and amorphous components.

Table 2

Compositions of the crystalline phase of steatite materials with different particle sizes

From the data presented it follows that magnesium metasilicate particles up to 10 $\mu$ in size may occur in the form of completely or partially stabilized protoenstatite. Larger crystals, under the cooling regimes adopted in steatite-ceramic technology, transform into clinoenstatite. Ceramics with a coarse-grained structure have reduced physicomechanical properties. With crystal sizes of 5–10 $\mu$, protoenstatite is not a completely stabilized phase in the ceramic composition. Its transition to clinoenstatite is accompanied by weakening of the structure and causes aging of the articles.

Conclusions

1. The transformation of protoenstatite into clinoenstatite is accompanied by anomalous phenomena: variability in the temperature at which the phase transition begins and relatively irreversible stabilization of fine-grained protoenstatite.

2. Polymorphic processes in the crystalline component of steatite ceramics are a significant cause of aging.

3. The probability of aging of articles is determined by the size of crystalline formations of magnesium metasilicate in the body composition.

4. Methods for obtaining steatite articles with high and stable properties must provide for the absence of phase processes in the material during cooling, i.e., the formation of a fine-grained structure with a stabilized protoenstatite composition.

References

1. G. A. Smolenskii, A. S. Berkman, A. M. Eidelkind. Steklo i keramika, No. 4, 1949.

2. A. I. Blyumen. In: Talc, 112, Promstroiizdat, Moscow, 1952.

3. V. V. Lapin. In: Talc, 59, Promstroiizdat, Moscow, 1952.

4. W. Bussem, C. Schusterius, K. Stuckardt, Wissenschaftliche Veröff. d. Siemens Werke, 17, 59, 1938.

5. M. Költermann. Tonindustrie-Zeitung und Keramische Rundschau, No. 6, 121; No. 7/8, 159, 1964.