INFLUENCE OF MICROADDITIVES ON THE SINTERING OF ALUMINUM OXIDE

P. G. Usov, A. D. Shiltsina, V. I. Vereshchagin

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

At present, interest is steadily increasing in alumina materials with an $\mathrm{Al_2O_3}$ content of more than 99%, which is associated with the task of reducing dielectric losses and ensuring the stability of the properties of ceramic dielectrics over a wide temperature range.

The main obstacle in obtaining materials with a high $\mathrm{Al_2O_3}$ content that are suitable for use is the difficulty of achieving the densest and most homogeneous possible structure of the ceramic body during heat treatment of the starting powder. Therefore, considerable attention has been, and continues to be, devoted to studies of the sintering process of aluminum oxide.

The works of Frenkel, Pines, and other researchers established the diffusion mechanism of aluminum oxide sintering [1]. However, for the development of the process of self-diffusion (or exchange of ions by places), which determines sintering in the solid phase, it is necessary to overcome a certain energy barrier. For the sintering of corundum, which has a stable crystal lattice with dense hexagonal packing of ions (lattice energy $-3600$–$4000$ kcal/mol [2]), high temperatures are required. The sintering temperature of $\alpha$-$\mathrm{Al_2O_3}$ from material of technical purity ($\mathrm{Al_2O_3}$—99–99.5%) and with a particle size of 1–2 microns lies within 1700–1750°C. At this temperature a density of 3.75–3.85 g/cm³ is achieved. A further increase in temperature to 1800–1850°C and prolonged holding do not lead to additional densification [3].

From the theory and practice of solid-phase synthesis it is known that sintering of powder-like materials can be achieved by introducing additions of foreign substances through the formation of solid solutions and free vacancies in the lattice [4].

From publications on this question it is known that some additives greatly accelerate the sintering of $\alpha$-$\mathrm{Al_2O_3}$, lowering the sintering temperature ($\mathrm{TiO_2}$, $\mathrm{MnO}$, $\mathrm{Fe_2O_3}$), while others regulate the crystallization process of corundum ($\mathrm{MgO}$ [4]) or improve individual properties of ceramics. Thus, $\mathrm{ZrO_2}$ increases the thermal resistance of corundum ceramics [5], and $\mathrm{Cr_2O_3}$, $\mathrm{Fe_2O_3}$, $\mathrm{V_2O_5}$ increase mechanical strength and hardness [6]. Additions of $\mathrm{SrO}$, $\mathrm{BaO}$, $\mathrm{CaO}$ are recommended for reducing gas inclusions [6, 7].

In the present work, results are given of studies of the influence of additions of $\mathrm{MgO}$, $\mathrm{SrO}$, $\mathrm{Y_2O_3}$, $\mathrm{Cr_2O_3}$, and $\mathrm{ZrO_2}$ on the sintering of aluminum oxide in amounts from 0.012 to 1.2 mol% (Table 1). The choice of additives was determined by...

Table 1

Content of introduced oxides, wt %

from the calculation of improving the sintering of Al₂O₃, achieving a favorable fine-grained structure, and improving the properties of the material, while excluding any possibility of a reduction in dielectric characteristics. The work did not set itself the task of studying the influence of alumina dispersion, the temperature regime, and the firing atmosphere on the sintering of aluminum oxide. All experiments were carried out in parallel on the same ground starting material; only the type and amount of additives were varied.

GA-85 grade alumina according to GOST 6912-64 was used as the starting material for the investigations. The chemical composition of the alumina (in wt %) was: Al₂O₃—98.96, Fe₂O₃—0.03, SiO₂—0.03, Na₂O—0.3, loss on ignition—0.68. The additives were introduced as aqueous solutions of chloride and nitrate salts. Simultaneously with wet grinding of the alumina, it was mixed with additives in a ball mill for 14 hours. The average particle size after grinding was 2 μm. After grinding, the charge was dried, and pressing powder was prepared from it by mixing with a plasticizing additive (8% paraffin) and rubbing through sieve 05. Specimens 18 mm in diameter and 2–3 mm high were pressed at a specific pressure of 350 kg/cm², after which they were fired in a hydrogen furnace EP1776, in the temperature range 1450–1700°C. The hold at the specified temperature was 1.5–2.0 hours. The degree of sintering of the specimens—

Fig. 1. Dependence of the bulk density of specimens fired at a temperature of 1700°C on the amount of introduced additives.

tives was judged from the water absorption, bulk density, and shrinkage of the specimens after firing.

When considering the dependence of the bulk density of specimens fired at \(1700^\circ\mathrm{C}\) on the amount of additives introduced (Fig. 1), the following regularities are observed. The effect of the additives on the sintering of alumina is manifested at the very smallest amounts (\(0.012\)—\(0.25\ \mathrm{mol}\%\)).

For all additives, with the exception of \(\mathrm{Cr_2O_3}\), over all or most of the concentration range a linear increase in bulk density with the amount of additive is observed. At low concentrations, in the case of \(\mathrm{ZrO_2}\) up to \(0.12\ \mathrm{mol}\%\), and in the case of \(\mathrm{Y_2O_3}\) up to \(0.25\ \mathrm{mol}\%\), a sharply increasing nonlinear dependence of bulk density on the amount of additives introduced is observed.

The dependence of the bulk density of specimens with chromium oxide added on the additive concentration has a maximum at \(0.25\ \mathrm{mol}\%\) (\(0.38\ \mathrm{wt}\%\)) \(\mathrm{Cr_2O_3}\).

The change in water absorption of the specimens as a function of the amount of additives (Fig. 2) is not strictly linear. Each additive is characterized by its own

In the figure: vertical axis—Water absorption, %; horizontal axis—Oxide content, \(\mathrm{mol}\%\). Curves are labeled \(\mathrm{Cr_2O_3}\), \(\mathrm{ZrO_2}\), \(\mathrm{MgO}\), \(\mathrm{SnO}\), and \(\mathrm{Y_2O_3}\).

Fig. 2. Dependence of water absorption of specimens fired at a temperature of \(1700^\circ\mathrm{C}\) on the amount of additives introduced.

concentration interval in which a sharp change in water absorption is observed, after which the change in water absorption with increasing amount of additive is insignificant. These data make it possible to limit the upper level of additives introduced in obtaining ceramics to \(0.25\ \mathrm{mol}\%\). With this amount of additives, the densification process proceeds most intensively at temperatures of \(1550\)—\(1600^\circ\mathrm{C}\), with shrinkage of \(17\)—\(18\%\). At higher temperatures the shrinkage slows down and by \(1700^\circ\mathrm{C}\) amounts to \(19\)—\(22\%\) (Fig. 3).

Taking into account the views of well-known researchers on the action of additives on the sintering of corundum [4—7], considering the recent information on the corresponding physicochemical systems [8, 9], and relying on the experimental data we obtained, we adhere to the following interpretation of the mechanism of action of the additives under consideration. First of all, the nature of the interaction of the additives with the main phase—corundum—will depend on the chemical nature of the additive. Thus,

Fig. 3. Temperature dependence of the shrinkage of alumina samples with various additives (content of introduced oxides, 0.25 mol%).

In the case of MgO and SrO, which readily form a spinel and the compound \(SrO \cdot 6Al_2O_3\) with \(Al_2O_3\), a thin layer of newly formed phases with lower surface energy is produced on the corundum grains. This leads to improved sintering without recrystallization grain growth. Partial incorporation of \(Mg^{2+}\) and \(Sr^{2+}\) ions into the corundum lattice is either absent or plays a secondary role. Hence the linear dependence of the action of these additives on the densification of aluminum oxide specimens during heat treatment is observed (Fig. 1).

Additions of \(ZrO_2\) and \(Y_2O_3\) have very limited solubility in aluminum oxide (for \(Y_2O_3\) this is even an assumption); therefore, up to the solubility limit of the additive in aluminum oxide, a nonlinear dependence is observed in the densification of the specimens. Subsequently, the mechanism of action of these additives is analogous to the mechanism of action of the MgO additive.

The extremum in the action of the \(Cr_2O_3\) additive can be explained by the fact that, in the solid phase, along with the dissolution of \(Cr_2O_3\) in aluminum oxide, at certain additive contents a solid solution of \(Al_2O_3\) in chromium oxide is formed. This new phase may possibly also hinder the sintering of corundum. However, to confirm such an explanation, additional studies using more precise methods are necessary.

The effectiveness of the introduced additives is to a large extent associated with the radius of the corresponding ions (Table 2). At the same charge of the impurity ions, their action is better in the case of a larger radius, as is seen from a comparison of the action of SrO and MgO, \(Y_2O_3\) and \(Cr_2O_3\).

Using the results of these studies, corundum ceramics with additions of MgO, SrO, and \(Y_2O_3\) together with MgO were obtained by sintering technology. The ceramic products had zero water absorption (vacuum density), a bulk density equal to \(3.92 \text{ g/cm}^3\), strength—

...strength in static bending on the order of 3700–3900 kg/cm² and good dielectric characteristics.

Table 2

Ionic radii (Å) according to Pauling [2]

Conclusions

1. Additions of Y₂O₃, MgO, SrO, Cr₂O₃, and ZrO₂ in small amounts markedly improve the sinterability of powdered alumina. The effectiveness of an addition is directly dependent on the chemical nature and ionic radius of the cation introduced.

2. The most promising for practical use are additions of Y₂O₃, SrO, and MgO in amounts not exceeding 0.25 mol%. The effect of the Cr₂O₃ addition on alumina sintering requires further study.

3. Corundum ceramics obtained from technical alumina with the recommended additions (MgO, SrO, Y₂O₃) have high density, mechanical strength, and good dielectric characteristics.

Literature

1. P. P. Budnikov, A. M. Ginstling. Reactions in Mixtures of Solid Substances. Moscow, Gosstroiizdat, 1961.

2. W. D. Kingery. Introduction to Ceramics. Publishing House of Literature on Construction, Moscow, 1967.

3. V. L. Balkevich. Technical Ceramics. Moscow, Gosstroiizdat, 1968.

4. E. V. Degtyareva, I. S. Kainarskii. Sintering of corundum with additions. Proceedings of the MKhTI, issue 24, 1957.

5. N. N. Silina. Investigation of the influence of additions of ZrO₂, CrO₃, TiO₂, MgO on certain most important properties of corundum ceramics. Author’s abstract of dissertation, 1955.

6. N. M. Pavlushkin. Sintered Corundum. Moscow, 1961.

7. N. M. Pavlushkin. Influence of additions of Group II elements on the properties of sintered alumina. Studies in the Field of Chemical Technology of Glass and Ceramics. Collected papers. Moscow, 1962.

8. A. S. Berezhnoi. Multicomponent Oxide Systems. Kiev, “Naukova Dumka,” 1970.

9. N. A. Toropov et al. Phase Diagrams of Silicate Systems. Leningrad, “Nauka,” 1969.