Effect of Additives on the Crystallization of Forsterite during Its Production from Talc
Abstract Generated abstract
This paper examines methods for limiting crystal growth in forsterite ceramics produced from talc and magnesium oxide, with the aim of improving properties associated with fine crystalline structure. Experimental batches based on two talcs were prepared with magnesium oxide and 1 percent additions of cerium or thorium oxides, then wet milled, pressed, fired at 1000 to 1350°C, and studied petrographically. Forsterite formation began at 1000 to 1100°C, while thorium oxide produced finer crystals and maintained a fine crystalline structure over a broader 1200 to 1300°C interval than cerium oxide. The study concludes that 1 percent thorium oxide can slow forsterite crystal growth up to the final firing temperature, without requiring a strontium oxide modifier.
Full Text
of excess thermodynamic functions in these systems and, in this way, confirms the correctness of the thermodynamic approach to polarography with accumulation. The concentration dependences in the system [[unclear: system components]] are considerably more complex. From our experimental data, the dependence of the height of the anodic lead peak, at its constant concentration in solution and with the thallium concentration changing in the direction of increase, represents an oscillating curve with a damped amplitude, on which a monotonically decreasing component can be distinguished. It may be assumed that this character of the dependence is associated with the oscillatory character of the interaction energy in this system, which can be represented as
\[ W \sim W_0 + A \frac{\cos aK}{aK}, \]
where \(K\) is the radius vector of the Fermi surface.
The experimental data obtained by us confirm the fundamental possibility of using literature data on the thermodynamic properties of binary and more complex systems. It is possible to distinguish the solution of such problems as predicting concentration and other dependences in the method of polarography with accumulation, using the mutual influence of elements to increase sensitivity and resolving power, and estimating the errors introduced by a component, especially at concentrations significantly lower than the detection limit under the experimental conditions in physicochemical and analytical studies. This indicates that the effect of mutual influence of elements (a particular case of which is the influence of the substrate in the PPN method) represents a promising means of increasing sensitivity and resolving power.
REFERENCES
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A. A. Kaplin, in: Advances in Polarography with Accumulation, TTU Publishing House, Tomsk, 1973, 136.
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A. A. Kaplin, T. F. Ryamontseva, in: Advances in Polarography with Accumulation, TTU Publishing House, Tomsk, 1973, 137.
EFFECT OF ADDITIVES ON THE CRYSTALLIZATION OF FORSTERITE DURING ITS PRODUCTION FROM TALC
P. G. Usov, V. I. Vereshchagin, N. K. Glushkova
Forsterite, \(Mg_2SiO_4\), is the principal crystalline phase of forsterite ceramics. Forsterite ceramics are among the high-frequency dielectrics currently in widespread use. Forsterite materials are resistant to electrical ...
aging. The CTE of forsterite, \((10–10.5)\cdot 10^{-6}\ \mathrm{deg}^{-1}\), agrees well with the CTE of titanium, \((9–9.5)\cdot 10^{-6}\ \mathrm{deg}^{-1}\), which is used in making products with a strong metal–ceramic bond /1/.
However, articles made of forsterite ceramics have low thermal-shock resistance. Increasing thermal-shock resistance and improving other electrical and mechanical characteristics is a primary task.
The raw materials for producing forsterite are talc and magnesium oxide. During firing, talc decomposes \((870–960^\circ\mathrm{C})\), with the formation of magnesium metasilicate \((\mathrm{MgSiO_3})\). The introduced magnesium oxide (the calculated amount) binds the free silica into metasilicate, and then forsterite \((\mathrm{Mg_2SiO_4})\) is formed.
One of the probable ways of solving the problem posed is to obtain a fine-crystalline structure of the forsterite body /1,3/. It has been observed that if the ceramic body is obtained from finely ground starting products, the crystallization process will proceed within the same limits of grain sizes. It has been established that the addition of oxides of elements of the second group of D. I. Mendeleev’s periodic system in an amount of 0.7–1.0% \((\mathrm{BaO}, \mathrm{SrO}, \mathrm{BeO})\) promotes the formation of fine-crystalline magnesium metasilicate /5/. But for forsterite a recrystallization process is characteristic. Forsterite with a fine-crystalline structure was obtained, but when the temperature was raised to complete sintering of the ceramic body, the grain size of the forsterite increased by a factor of 4–5.
One of the probable ways to retard crystal growth is to obtain, in the ceramic body, glass phases with increased viscosity. For this purpose, additions of a strontium oxide modifier and a zirconium oxide stabilizer were introduced, and the influence of these additions on the crystalline structure of the body was investigated. Fine-crystalline forsterite was obtained; moreover, with increasing temperature the crystal size increases not by a factor of 4–5, but by a factor of 2.
Using the results obtained earlier, experiments were carried out to obtain forsterite with additions of cerium and thorium oxides. Several batches were prepared on the basis of Alui and Onot talcs with additions of the calculated amount of magnesium oxide and 1% cerium and thorium oxides.
The raw material was batched and ground by wet milling in ball mills until it passed through sieve No. 0060; the specimens were pressed and fired in a kiln for 6 hours to temperatures of 1000, 1100, 1200, 1250, 1300, and 1350°C, with holding at the final temperatures. Petrographic studies of the specimens were carried out. The studies showed that in all specimens the formation of forsterite begins at 1000–1100°C; in specimens with an addition of cerium oxide, forsterite formation begins immediately with small crystals and is not accompanied by their growth when the temperature is raised to 1300°C; the pattern of forsterite formation on the basis of Alguyskii and Onotskii talcs is very similar. In specimens with an addition of thorium oxide the crystals are finer, and the interval of fine-crystalline structure is larger (1200–1300°C) than with the addition of cerium oxide. In specimens with an addition of cerium oxide the crystals are considerably larger, the interval of fine-crystalline structure is narrow (1280–1300°C), and crystal growth is more significant. When strontium oxide is introduced into the batch containing thorium oxide, the strontium oxide additive does not produce a substantial improvement in the structure.
Conclusion
The addition of 1% thorium oxide promotes the formation of fine-crystalline forsterite even in the absence of the $\mathrm{SrO}$ modifier over a wide temperature range (1200–1300°C) and slows crystal growth when the temperature is raised to the final firing temperature of the products (1350°C).
Literature
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