INVESTIGATION OF THE SINTERING OF MAGNESIUM ALUMINATE SPINEL WITH THE INTRODUCTION OF MICROADDITIVES

P. G. USOV, V. A. AMELINA, V. I. VERESHCHAGIN

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

Ceramics with a spinel crystalline phase are among the promising radio materials in terms of their electrical and mechanical properties. In connection with the growing demands of industry, an ever wider circle of researchers is turning to the problem of the synthesis and sintering of spinel ceramics.

The object of study in investigating the sintering process is spinel composition Sh-64, i.e., Al₂O₃—63.9%, MgO—36.1%, proposed by Koifman [7]. This ceramic is well matched in coefficient of thermal expansion with Ti and therefore can be used in Ti-ceramic seals.

It is known that the reactivity of the starting materials depends to a considerable extent on the initial raw material and the production temperature. In the experiment the following were used: precipitated MgO, with a specific surface area of 19000 cm²/g, measured on the PSKh-4, the crystal form elongated, grain length 30 μm, LOD 15%; Al₂O₃ of grade GA-85, preliminarily ground, specific surface area determined on the PSKh-4 = 8000—8100 cm²/g, grain size 5 μm, LOD 1.5%.

The sintering rate is determined by the nature of the compound, the degree of perfection of the crystalline structure, dispersion, pressing pressure, temperature and duration of the process, and a number of other factors [6].

In order to enhance the activity of the crystal lattice, expressed in the distortion of its parameters, an extensive group of microadditives was investigated. In the present work the discussion will concern the influence on the densification process of the most promising additives, such as BeO, Y₂O₃, La₂O₃, ZrO₂, and SrO.

The objective of the study was to select the necessary holding time and the optimal amount of additives in the sintering of the selected material of composition Sh-64.

At present, a wide circle of scientists is working in the field of sintering theory. The Frenkel—Pines theory forms the basis of the doctrine of sintering. The basic propositions set forth in the works of these authors determined the direction of research in the field of sintering.

According to Frenkel [2], the densification of a porous body during heat treatment is caused by the tendency to decrease the free surface energy. This process is based on viscous flow, and it occurs under the influence of capillary pressures.

In Pines’s work [1], a mechanism was proposed for the diffusional healing of closed porosity in a crystalline solid.

Under real conditions, sintering is a very complex process and is accompanied by simultaneous improvement of the crystal lattice [3]. On the basis of the work of Pines and other researchers, it may be considered that the densification process occurs mainly through volume diffusion and partly through surface diffusion, which manifests itself only in the early stages of sintering [4, 5].

On the basis of previous studies it was shown that the best method of introducing additives into the Sh-64 mixture is their preliminary firing into one of the components before mixing the batch, in order to activate the sublattice of the component.

The additives were introduced as aqueous solutions of salts, mainly nitrates. The effect of the additives was studied on specimens 2.5 mm high and 25 mm in diameter, pressed under a pressure of 750 kg/cm². Sintering was monitored from shrinkage and water-absorption data, beginning at a temperature of 1200° C, when, according to X-ray data, spinel synthesis was practically complete.

Firing of specimens based on synthesized spinel was carried out in furnaces with silicon-carbide heaters, with different holding times at the final temperature: 0, 30, 60, 120, 240, and 360 min.

The shrinkage and water-absorption data are given in Tables 1 and 2. From the tabulated data it may be noted that, for all compositions, the values of water absorption

Table 1

Change in shrinkage with holding time at a temperature of 1450° C

decrease considerably, and the shrinkage values increase up to a two-hour holding time at this temperature. A further increase in holding time does not have a substantial effect on sintering and, consequently, is inexpedient.

Comparing the water-absorption and shrinkage data for mixtures with different additives, we observe that, with a two-hour exposure, the greatest sintering is found in specimens with a BeO additive.

To clarify the question of the optimum amount of additives on the basis of

Table 2

Change in water absorption with holding time at a temperature of 1450°C

On the basis of Tables 1 and 2, graphs were constructed showing the change in shrinkage as a function of the amount of additive introduced at the previously selected holding time of 2 hours.

From graph 1 and Tables 1 and 2 it is evident that additives affect the sintering rate of ceramic composition Sh-64 differently, depending on their nature.

In the case of introducing Be²⁺ ions into the MgO sublattice, Be²⁺ ions enter the interstitial spaces, since the ionic radius of Be²⁺ (0.35 Å) is considerably smaller than the ionic radius of Mg²⁺ (0.66 Å). As Be²⁺ accumulates in the MgO lattice, its stability decreases, the bonds weaken, and, accordingly, the lattice is distorted and its capacity for reaction interaction with alumina increases. However, the BeO addition cannot be increased indefinitely, since beryllium oxide may dissolve only to a limited extent, and the remaining free BeO may promote the formation of a glass phase and form low-melting eutectics.

The sintering rate, with increasing BeO content (Fig. 1), steadily increases over the investigated concentration range, and it is not possible to reach a limiting value. However, an additive introduced in amounts above 0.6 at % increases the sintering rate only very slightly.

Additives of Y₂O₃ and La₂O₃ (Fig. 1) apparently influence the Al₂O₃ sublattice, taking into account the nature of these compounds. However, RY³⁺ (1.06 Å) and RLa³⁺ (1.14 Å) considerably exceed the size of the Al³⁺ ion (0.57 Å). Consequently, the matter can only be the formation of solid solutions with very limited solubility. From the graphical data it is evident that the sintering rate increases with increasing amounts of Y₂O₃ and La₂O₃ additives up to a certain limit, after which a sharp decrease is recorded.

Additives of ZrO₂ and SrO (Fig. 1) increase the sintering rate in proportion to their amount.

It follows from the above that additions of BeO, Y₂O₃, and La₂O₃ should be introduced in amounts corresponding to 0.6; 0.15; 0.2 at %, calculated per cation.

![Figure 1 graph]

Fig. 1. Dependence of shrinkage on the amount of additives during sintering of MgAl₂O₄ of composition Sh-64 \((T_{\text{firing}} = 1450^\circ\text{C})\): 1 — BeO additive, 2 — Y₂O₃ additive, 3 — La₂O₃ additive, 4 — ZrO₂ additive, 5 — SrO additive.

ZrO₂ and SrO additives are capable of forming new phases. To prevent the formation of new phases, the content should be limited, respectively, to 0.2 and 0.5 at.% Zr and Sr.

Owing to the considerable discrepancy between the radii of the impurity ions and the radii of Mg and Al ions, the effect of the additives is manifested in small quantities. The solubility limit is explained by the chemical affinity, valence, and radius of the introduced ions.

The next important point is to determine the causes of the intensive densification of this material when microadditives are introduced. One of the factors affecting sintering, as noted earlier, is the state of the crystal lattice. A qualitative characteristic of the structure is given by the half-width of the maxima of X-ray reflection in combination with the unit-cell parameters [8]. The experimental data are presented in Figs. 2, 3, and 4.

The half-width of the maxima of X-ray reflection of spinel and periclase changes during firing of composition Sh-64 without additives smoothly, without inflections: for spinel — with decreasing rate, for periclase—

![Figure 2 graph]

Fig. 2. Change in the half-width of the maxima of X-ray diffraction from the \((511,333)\) plane of spinel, \(d = 1.552\ \text{Å}\), during firing of composition Sh-64 with various microadditives: 1 — 0.5% ZrO₂ additive, 2 — 1.0% SrO additive, 3 — 1.0% BeO additive, 4 — 0.2% Y₂O₃ additive, 5 — reference.

![Figure 3 graph]

Fig. 3. Change in the half-width of the maxima of X-ray diffraction from the \((200)\) plane of periclase, \(d = 1.108\ \text{Å}\), during firing of composition Sh-64 with various microadditives: 1 — 0.5% ZrO₂ additive, 2 — 1.0% SrO additive, 3 — 1.0% BeO additive, 4 — 0.2% Y₂O₃ additive, 5 — reference.

with an increasing rate (Figs. 2, 3). The introduction of microadditives changes the character of the variation in the half-width of the X-ray diffraction line. At temperatures of 1300–1400°C, bends in the curves are observed, depending on the type of additive and the crystalline phase. These same temperatures correspond to points of abrupt change in the lattice constant of the spinel (Fig. 4).

Fig. 4. Dependence of the lattice constant of spinel on the firing temperature of a batch of composition Sh-64 with certain microadditives: 1 — addition of 0.5% ZrO₂, 2 — addition of 1.0% SrO, 3 — addition of 1.0% BeO, 4 — addition of 0.2% Y₂O₃, 5 — standard addition.

On the basis of the regularities in the change of the half-width of the X-ray reflection of spinel during firing of the Sh-64 batch with additives, we note that up to a temperature of 1400°C (in the case of 0.6 at% Be, up to 1300°C) the crystal lattice of the spinel has more defects than the spinel lattice in the Sh-64 composition without additives. In the temperature interval 1400–1500°C, accelerated improvement of the lattice occurs to a state better than that of the standard. Sintering of the investigated batches proceeds in this same temperature region. For the addition of 0.6 at% Be, all the effects described begin earlier, at a temperature of 1300°C; by a temperature of 1450°C, the batch with this additive sinters practically to the vacuum density.

Conclusions

1. The investigated additives BeO, Y₂O₃, La₂O₃, ZrO₂, and SrO accelerate the sintering of spinel; despite their small amounts, they introduce defects into the lattice and thereby accelerate sintering.

2. The microadditives Y₂O₃ and La₂O₃ form solid solutions of very limited solubility. Even small amounts of admixtures substantially distort the crystal lattice and activate the densification process.

3. The necessary holding time has been established—2 hours—at the final firing temperature.

4. The optimal amounts of additives, calculated in at% of the cation, have been established as follows: Be — 0.6 at%, Y — 0.02 at%, La — 0.15 at%, Zr — 0.2 at%, Sr — 0.5 at%.

References

1. B. Ya. Pines. ZhTF, 16, 6, 137, 1946.
2. Ya. I. Frenkel. ZhETF, 16, 1, 29, 1946.
3. Ya. E. Geguzin. Physics of Sintering. “Nauka,” 1967.
4. B. Ya. Pines. Uspekhi fiz. nauk., 62, 501, 1954.
5. B. Ya. Pines. ZhTF, 16, 737, 1946.
6. P. P. Budnikov, A. M. Ginstling. Reactions in Mixtures of Solid Substances. Stroiizdat, 1971.
7. N. I. Koifman. Information reference sheet 012777. Ministry of the Electronics Industry of the USSR, TsNIITEIN, UDC 66659321.
8. I. A. Shishakov. Basic Concepts of Structural Analysis. USSR Academy of Sciences, 1961.