STUDY OF THE STABILITY OF PROTOENSTATITE IN STEATITE MATERIALS

P. G. USOV, V. I. VERESHCHAGIN

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

For steatite materials, such an undesirable phenomenon as aging is characteristic [1]. One of the causes of aging is considered to be the instability of the crystalline phase of steatite ceramics [2]. The crystalline phase of steatite materials currently produced is represented by protoenstatite, which is the equilibrium high-temperature modification of magnesium metasilicate [3]. The low-temperature modification of $\mathrm{MgSiO_3}$ is clinoenstatite. It has been established that stabilization of protoenstatite in the body of steatite ceramics is promoted by the glass phase, of which there is about 30%; however, from the standpoint of theory and practice, it is of interest how stable the metastable state of protoenstatite in ceramics is and what factors influence its stability. This paper presents a method for determining the instability of protoenstatite in a steatite material on the basis of X-ray structural analysis. The use of X-ray analysis in recognizing the modification of $\mathrm{MgSiO_3}$ has major advantages in comparison with the petrographic method previously used for this purpose, namely:

1) the fine structure of the material and the close intergrowth of $\mathrm{MgSiO_3}$ crystals with glass do not interfere;

2) it is not necessary to grind the specimen (which is sometimes very important);

3) the objectivity of the analytical results is higher.

For a long time the existence of protoenstatite as an independent phase of magnesium metasilicate was denied. This was proved by means of X-ray structural analysis, after which the optical constants of protoenstatite were determined. In our work, petrographic analysis was used to determine the microstructure of the materials under investigation.

The X-ray constants of the pure modifications are given in Table 1 (URS-50-I diffractometer, anticathode—Cu).

According to X-ray analysis data, the investigated ceramic specimens were characterized by the degree of imperfection (defectiveness) of the protoenstatite structure. For this purpose we used the half-width of the X-ray diffraction of the protoenstatite lines $d - 2.70$; $d - 1.96$; $d - 1.49$ and $d - 1.31\,\text{\AA}$, which practically do not interact with the maxima of the diffraction reflections of clinoenstatite. Such a method was applied by V. A. Bronom [4] in studying the structure of periclase. The half-width of the X-ray interference was calculated as the quotient obtained by dividing the area of the peak by its height. Quantitative evaluation of the forming-

Table 1

X-ray constants of pure modifications of MgSiO₃
(anticathode — Cu)

…formed as a result of transformations of clinoenstatite, was determined from the ratio of the areas of the diffraction maxima $d = 1.96\ \text{Å}$ and $d = 2.10\ \text{Å}$. These maxima are of medium intensity, but they have the advantage over more intense reflections that they do not interfere with one another when the specimen is a mixture of MgSiO₃ modifications. From X-ray analysis data for mixtures with known ratios of the modifications, the dependence of the amount of protoenstatite in the specimen on the ratio of the areas $Sd = 1.96$ and $Sd = 2.10$ was obtained (Fig. 1); this dependence was used in our studies as a standard.

Fig. 1. Dependence of the ratio of the areas of X-ray diffraction for $d = 1.96\ \text{Å}$ and $d = 2.10\ \text{Å}$ on the quantitative ratios of protoenstatite and clinoenstatite in the specimen.

A certain qualitative assessment of the ratio of the modifications of magnesium metasilicate is given by the interval of angles \(2\Theta\) on the X-ray pattern between the principal reflections \(d=3.17\ \text{\AA};\ d=2.90\text{–}2.86\ \text{\AA}\). Thus, for 100% protoenstatite this interval is \(\Delta 2\Theta = 2^\circ 38' \pm 12'\), and for clinoenstatite \(\Delta 2\Theta = 3^\circ 00'\). In specimens with a mixture of these modifications, the maxima \(d=2.90\ \text{\AA}\) and \(d=2.86\ \text{\AA}\) interact with one another, and the distance of the total-index line from the line \(d=3.17\ \text{\AA}\) to some extent characterizes the ratio of the modifications of \(\mathrm{MgSiO_3}\). All these indices were used as a whole. The instability of protoenstatite in the body of steatite ceramics is revealed when the body is ground, namely: part of the protoenstatite transforms into clinoenstatite; the imperfection (defectiveness) of the structure of the preserved protoenstatite increases, which suggests the possibility of its further transformations. (The specimens were ground until they passed through sieve No. 0060; particle size less than 60 microns.)

Our investigations showed that the stability of protoenstatite in the body of steatite ceramics is directly dependent on the microstructure of the material, being a function of the size of the \(\mathrm{MgSiO_3}\) crystals. Several specimens with different microstructures were studied (Table 2). In the coarser-grained material (specimen 1), grinding promotes the transformation of a larger amount of protoenstatite, and the imperfection of the structure of that which remains is more pronounced. Grinding a steatite material with a fine structure, where the size of the \(\mathrm{MgSiO_3}\) crystals is about one micron, does not lead to a change in the phase composition of the specimen and does not affect the state of the internal structure of protoenstatite (specimen 3). The half-width of the X-ray interference of the maxima characteristic of protoenstatite remains constant.

Conclusions

1. Comparing the results of X-ray analysis of steatite ceramics from an undestroyed sample and from a ground specimen makes it possible to evaluate the material with respect to the stability of its crystalline phase, which is of great importance in studying the phenomenon of ceramic aging.

2. The proposed method can be successfully used for quality control of the material of finished steatite products.

REFERENCES

1. N. P. Bogoroditskii et al. Radioceramics. Gosenergoizdat, Moscow, 1963.
2. A. Palatskii. Technical Ceramics. Gosenergoizdat, 1959.
3. N. A. Toropov, V. P. Barzakovskii et al. Phase Diagrams of Silicate Systems. Handbook. “Nauka,” Moscow–Leningrad, 1965.
4. V. A. Bron, M. I. Disterova, L. P. Sudakova. Effect of heating temperature on the defectiveness of the crystal lattice of magnesium oxide and its ability to sinter. In: Inorganic Materials. Proc. Acad. Sci. USSR, “Nauka,” 1965.

Table 6

Data from processing X-ray diffraction patterns of steatite materials of different microstructure