Overfiring of Barium Steatite Ceramic Products and Its Causes

P. G. Usov, V. I. Vereshchagin

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

Steatite ceramics are used as a high-frequency dielectric. The availability of natural raw material (talc), which during firing relatively readily yields the crystalline phase of the ceramic—magnesium metasilicate (\(\mathrm{MgSiO_3}\))—favorably distinguishes steatite ceramics from dielectrics of a similar class. To lower the sintering temperature of steatite ceramic products and to increase the stability of its crystalline component, clay additives and other special fluxes are introduced into the batch \([1, 2, 3]\). The most suitable flux is barium oxide; it has a favorable effect on the dielectric properties of the ceramic and increases the stability of protoenstatite, in which \(\mathrm{MgSiO_3}\) is chiefly represented in the ceramic structure. Barium is introduced into the batch as a carbonate salt. From the technological point of view, steatite bodies have such disadvantages as a narrow sintering interval and a narrow interval of the sintered state (firing interval).

The firing interval of steatite products is affected by another undesirable phenomenon—overfiring, reducing it to \(15\)—\(20^\circ\).

Overfiring is characterized by the fact that, after complete sintering of the steatite material has been reached, a further increase in the firing temperature leads to an increase in the dimensions of the specimen, a decrease in its bulk density, and the rather frequent appearance of bubbles and swellings on the surface of the specimens (Fig. 1). Overfiring becomes more pronounced as the treatment temperature rises relative to the normal firing temperature. At a firing temperature of P-5 ceramic products (component composition in Table 1) \(80^\circ\) above the normal temperature, they increase in size by \(3\)—\(5\%\), reducing mechanical strength by \(25\)—\(30\%\) to values \((\sigma = 1150\)—\(1300\ \mathrm{kg/cm^2})\) that are not permissible for steatite materials. The electrical characteristics of products after overfiring also decrease; the specific volume resistance decreases from \(10^{15}\) to \(10^9\ \mathrm{ohm \cdot cm}\).

It is known that the mere presence of closed pores in a fired material does not lead to intense deformation \([4]\). For this, it is necessary that chemical reactions occur inside the closed pores, accompanied by the release of gases capable of increasing the pore volume. At the same time, the melt must have a certain sufficiently low viscosity, allowing the gases to expand.

During sintering of steatite materials, a rapid increase in the liquid phase and a rapid decrease in its viscosity \([5]\) are characteristic, which should strongly promote overfiring. The probability of formation of a gas phase in steatite ceramics as a result of volatilization of the oxides included in its composition (\(\mathrm{MgO}\), \(\mathrm{SiO_2}\), \(\mathrm{Al_2O_3}\), \(\mathrm{BaO}\)) is excluded because of their low volatility at ceramic firing temperatures \([6]\).

Fig. 1. External signs of overfiring of steatite products (fourfold magnification)

The assumption that the cause of gas formation is the burning out of residues of the organic binder in the products is not confirmed. The binder in them burns out completely during the biscuit firing at a temperature of about 750°C; the biscuit firing itself is carried out to 960°C. The products enter the final firing absolutely clean on the surface and without any traces of coloration by coke in the fracture. Under reducing firing conditions, the conversion of ferric oxide to ferrous oxide may be a cause of formation of the gas phase [7]. In radio-ceramic bodies, the iron content is low and is strictly regulated.

A more probable source of gas formation during firing of steatite products is barium carbonate, which has not completely reacted with the constituents of the batch during firing of the body. Barium carbonate (BaCO₃) dissociates at atmospheric pressure at 1450° [1]; its decomposition in ceramic bodies proceeds at lower temperatures. According to data from complex thermal analysis (using a derivatograph), intensive decomposition of BaCO₃ in body P-5 occurs at 600–850°C. According to our studies [8], the completeness and rate of decomposition of barium carbonate in the batch are affected by the fineness of grinding of the batch and the thoroughness of its mixing. It was found that in finely ground (to 60 μm), well-mixed bodies, the reaction of barium carbonate with the components of the batch practically goes to completion at a temperature of 900°C. However, in more coarsely ground bodies (to passage through sieves No. 0075 and No. 0088), the decomposition reaction of BaCO₃ proceeds with greater difficulty. Unreacted BaCO₃ at temperatures above 1100°C cakes and enters into reaction with the components of the batch only with great difficulty.

At the same time, the possibility of formation of more complex compounds of the type BaO·BaCO₃ [9] is not excluded. In petrographic analysis, small grains (1–2 μm) of a mineral are encountered in the body of ceramic products P-5 and B-17 (compositions in Table 1), distinguished by high birefringence and pronounced pseudoabsorption characteristic of carbonates. The refractive indices of the mineral lie within the limits of the refractive indices of magnesium metasilicate, 1.660, and the glass phase, $n = 1.530$.

Table 1

Component composition of the batch of steatite bodies P-5 and B-17

(it is not possible to determine the optical constants precisely because of the small size of the mineral crystals). The refractive index of witherite ($\mathrm{BaCO_3}$) also lies within these limits; for it $N_p = 1.529$ and $N_g = 1.677$. In all probability, regions of unreacted $\mathrm{BaCO_3}$, or of a more complex carbonate mineral, are the cause of the formation of internal cavities and bubbles on the surface of steatite-ceramic parts at firing temperatures above the optimum.

In overfiring of steatite materials, not only blistering is observed, but also a uniform increase in volume, reaching up to 15%, which is associated with the simultaneous expansion of all pores uniformly distributed throughout the entire volume of the material (Fig. 2). In this case it is assumed

Fig. 2. Pores in the body of steatite ceramic after overfiring. Reflected light, ×230

that there is simultaneous evolution of $\mathrm{CO_2}$, connected in some way with firing of the raw body. A delay in the decomposition of barium carbonate at temperatures above $900^\circ\mathrm{C}$ leads to the fact that the glass phase, which forms at temperatures of about $1100^\circ\mathrm{C}$, hampers the escape of $\mathrm{CO_2}$ and, with further temperature increase, isolates it in the pores. It is known that $\mathrm{CO_2}$ has the greatest solubility of all gases in glass melts and, under certain conditions, can be released [7, 9]. According to Vargin [9], at temperatures of about $1300^\circ\mathrm{C}$ a maximum of $\mathrm{CO_2}$ solubility in glass is observed, and at $1200^\circ$ a minimum of this process. Firing of the raw steatite body is carried out at temperatures of $1300$–$1350^\circ\mathrm{C}$, whereas firing of articles made from synthesized body is carried out at temperatures $100^\circ$ lower. When barium carbonate in the batch of the steatite body P-5 is replaced by oxide, the phenomenon of overfiring is no longer observed at temperatures $100^\circ$ above the optimum. Further, our investigations showed that steatite bodies containing calcium carbonate as a flux, which decomposes much more readily than $\mathrm{BaCO_3}$, are practically not prone to overfiring.

When replacing barium carbonate by oxide in the batch of barium steatite ceramic, it should be remembered that barium oxide is characterized by considerably greater solubility in water than the carbonate; therefore, dewatering of the body after wet grinding should be carried out not on filter presses, but by another method, for example by evaporation. If barium carbonate is used, another practical recommendation may be useful: when firing a raw body in granules or articles formed from it, additional holds should be made in the temperature interval $700$–$900^\circ$, something to which no attention is currently paid.

The results of our investigations lead to the following conclusions:

1. The cause of overfiring of barium steatite ceramic articles is the expansion of the pores of the material due to $\mathrm{CO_2}$ that has not been completely evolved at temperatures below $900^\circ$. Overfiring of articles formed from a raw body is more pronounced.

2. To ensure complete assimilation of $\mathrm{BaCO_3}$ by the batch of the steatite body during firing, grinding must be carried out until passage through

SITO No. 0060 with good-quality mixing, which increases the reactivity of the batch in the solid state.

1. When barium carbonate is replaced by oxide in the steatite-body batch, overfiring of the products is not observed.

REFERENCES

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