KINETICS OF THE IMPROVEMENT OF THE STRUCTURE OF MAGNESIUM METASILICATE FORMED AS A RESULT OF TALC DECOMPOSITION

P. G. Usov, N. V. Sobora

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

As a result of the thermal decomposition of talc, magnesium metasilicate is formed in the form of protoenstatite and amorphous silica. The latter is regarded as a by-product of the reaction.

One of its principal properties—low dielectric losses—depends on the perfection of magnesium metasilicate, the crystalline phase of steatite ceramics. Conductivity, or dielectric losses, in steatite ceramics arise from a combination of the following causes: 1) the existence of structural defects on the surface of crystals, 2) the motion of ions in the lattice of the crystalline phase, and 3) the motion of intermediate ions in the glass phase.

In cracks or defects of crystals, ions are held by a smaller force restricting their motion.

Therefore, the process of ordering the structure of protoenstatite as a function of the temperature and duration of heat treatment is of considerable interest. Ultimately, the properties of the finished products depend on the completeness of the reaction during the period in which defects are eliminated.

In the process of rearrangement of the talc lattice into the pyroxene lattice, certain layers of atoms simultaneously belong to both the old and the new lattices. Since, both during the formation and during the disappearance of a crystalline phase, the displacement of the particles composing it occurs not statically at random, but along definite directions depending on the crystallographic form of the initial phase [2]. Thus, an elastic connection exists between the initial and the new lattices. As a result, shear stresses and the corresponding elastic deformations arise.

The presence of dislocations and defects in a crystal causes a disturbance of the periodic structure, which is reflected in X-ray diffraction patterns. The presence of disorder is characteristically manifested in the integral intensity of the diffraction lines. In perfect crystals no reflections occur except reflections at Bragg angles. In this case, narrow single peaks are observed on radiographs [3]. In deformed, defective crystals the diffraction lines are strongly broadened, which is usually associated with two causes: 1) distortions (defects) of the crystal lattice, which give rise to inhomogeneous elastic deformations, and 2) the presence of submicroinhomogeneities [4]. However, it is experimentally very difficult to separate the influence of deformations and submicroinhomogeneities on line broadening, since both

effects are caused by dislocations and therefore are closely connected with one another [3]. In the present work the two factors of structural defectiveness were not separated and were considered together.

The process of improvement of the crystalline structure of protoenstatite was monitored by X-ray diffraction analysis by measuring the half-width of X-ray interference (line broadening) of reflections with angle \(2\Theta = 46^\circ\). The half-width of the X-ray interference was calculated as the quotient obtained by dividing the area of the maximum by its height.

High-quality samples of two morphological varieties of talc from the Onot and Algui deposits served as the initial object of study. Test specimens 2.5 cm in diameter and 0.3 cm high were pressed by the dry method. Firing was carried out at temperatures of \(1200\)–\(1380^\circ\mathrm{C}\) in \(50^\circ\mathrm{C}\) increments, with holding at the final temperature from 0 to 6 h, at 30-min intervals during the first 3 h, and then at one-hour and two-hour intervals. The rate of improvement of the structure of protoenstatite was judged from the decrease with time in the half-width of the X-ray interference of the characteristic maximum \(d = 1.97\).

As the experiments showed, the duration of heat treatment of the initial talcs at temperatures below \(1200^\circ\mathrm{C}\) and at \(1200^\circ\mathrm{C}\) has little effect on the elimination of defects in the internal structure. Disorder continues to persist. Only an increase in temperature to \(1250^\circ\mathrm{C}\) and above revealed a decrease in the half-width of the X-ray interference with increasing exposure time under isothermal conditions.

The selection of numerous kinetic equations to describe the process of improvement of the crystal lattice did not yield positive results. A clearer solution was obtained by graphical interpretation of the experimental data. The dependence of the corresponding logarithms of the half-width of the X-ray interference on holding time at constant temperatures is shown in Figs. 1 and 2. As is evident from the graphs, this dependence is a broken line consisting of two straight-line segments. The first straight-line segment corresponds to the rate of improvement of defective protoenstatite, and from its slope numerical values of the reaction-rate constants were found (Table 1).

Fig. 1. Kinetic curves for the improvement of the structure of protoenstatite based on Onot talc

Fig. 2. Kinetic curves for the improvement of the structure of protoenstatite based on Algui talc

Table 1

Values of the rate constants for the ordering of the structure of protoenstatite as a function of firing temperature and the initial talc

The second segment indicates that further ordering of the internal structure of the mineral ceases and a certain equilibrium concentration of defects is established. Increasing the firing temperature shifts this equilibrium.

The process of ordering the crystal lattice proceeds continuously over a wide temperature interval, from 1250°C up to 1400°C. However, ordering of the protoenstatite structure does not go to completion, as indicated by the diffraction patterns, which do not correspond to the standard X-ray diffraction pattern of protoenstatite. This is due to the fact that in the firing products of pure talcs, along with further ordering of the structure, recrystallizational growth of protoenstatite grains takes place. The lattice of the latter, in turn, carries defects and is more readily transformed into clinoenstatite. Clinoenstatite is observed under the microscope in all firing products of Onot and Alguy talcs, beginning at 1200°C and higher.

The process of improving the structure of protoenstatite requires a sufficient amount of activation energy, which was determined graphically and amounts to about 10 kcal/mole.

Conclusions

1. The change in the half-width of X-ray interference with time satisfactorily characterizes the rate of ordering of the crystal structure.

2. The process of improving the structure of protoenstatite formed as a result of talc decomposition is a slow process and proceeds over a wide temperature interval. However, the ordering reaction of the protoenstatite structure based on pure talcs does not go to completion.

3. Numerical values have been determined for the reaction-rate constant and for the activation energy of the process of improving protoenstatite.

Literature

1. M. D. Pigtceriik. Jour. Amer. Ceram. Soc. 7, 217—218, 1947.
2. P. P. Budnikov, A. M. Ginstling. Reaction in Mixtures of Solid Substances, Gosstroiizdat, Moscow, 1961.
3. Van Bueren. Defects in Crystals. IL, Moscow, 1962.
4. G. V. Kurdyumov, V. K. Kritskaya, V. A. Ilyina, L. I. Lysak. Izvestiya AN SSSR, 17, 3, Moscow, 1953.