ON THE SYNTHESIS AND SINTERING OF MAGNESIUM ALUMINATE SPINEL

V. A. Amelina, V. I. Vereshchagin, P. G. Usov
(Presented by the scientific seminar of the inorganic departments)

Magnesium aluminate spinel can be obtained by melting the oxides and by synthesis in the solid phase. Recently greater importance has been attached to the second method, since it does not require high temperatures. Our work consisted in studying the influence of different component ratios, different methods of mixing, and different dispersity on the amount of spinel formed during firing and on its sintering.

Mixtures with the molar ratio
$\mathrm{MgO}:\mathrm{Al_2O_3}=1.2:1;\ 1.1:1;\ 1.05:1;\ 1:1;\ 1:1.05;\ 1:1.1;\ 1:1.2$
were prepared:

1) by coprecipitation of the hydroxides $\mathrm{Mg(OH)_2}$ and $\mathrm{Al(OH)_3}$, calculated as oxides, from solutions of nitrate salts $\mathrm{Mg(NO_3)_2\cdot 6H_2O}$, $\mathrm{Al(NO_3)_3\cdot 9H_2O}$ with ammonium hydroxide at $\mathrm{pH}=8$;

2) by mechanical mixing of oxides: $\mathrm{MgO}$ of special grade, loss on ignition 15%, grain size along the length 30–40 μm, and $\mathrm{Al_2O_3}$, loss on ignition 1.5%, grain size 6–8 μm;

3) by mechanical mixing of oxides: $\mathrm{MgO}$, loss on ignition 10%, grain size by diameter 6–8 μm, and $\mathrm{Al_2O_3}$, loss on ignition 1.5%, grain size 6–8 μm.

In the mechanically mixed batches only the grain size of $\mathrm{MgO}$ was varied, since high dispersity is desirable for the less mobile component, namely the magnesium component [1, 2]. The specimens were pressed by the dry method at a pressure of 750 kg/cm². The amount of spinel was recorded from the change in the ratios of the areas of X-ray reflections for spinel $d=1.533\ \text{Å}$ (reflection plane—511,333) and for $\mathrm{MgO}$ $d=1.485\ \text{Å}$ (reflection plane 220). Traces of spinel obtained from coprecipitated hydrates could be recorded after firing at 400°C. Quantitatively, however, up to 700°C it was not possible to characterize the spinel because of the indistinctness of the reflections.

In specimens made from special-grade $\mathrm{MgO}$ and $\mathrm{Al_2O_3}$ (6–8 μm), traces could be recorded at 700°C, and the spinel phase could be quantitatively characterized from 1100°C. In specimens made from $\mathrm{MgO}$ (6–8 μm) and $\mathrm{Al_2O_3}$ (6–8 μm), traces appeared at 1000°C, and it proved possible to characterize the phase quantitatively from 1300°C. Data on the quantitative change of spinel are given in Tables 1 and 2.

The sintering of the specimens was judged from the change in shrinkage and water absorption of the specimens. Maximum shrinkage and minimum water absorption are observed in specimens from coprecipitated mixtures with the molecular ratio $\mathrm{MgO}:\mathrm{Al_2O_3}=1.05:1;\ 1:1$ (Table 3). Data on shrinkage and water absorption indicate that sintering of specimens from coarse-grained components proceeds considerably more weakly than in fine-dis…

Table 1

Change in the amount of spinel as a function of dispersion, mixing method, and different component ratios at a temperature of 1400°C

Table 2

Increase in the spinel phase with increasing firing temperature, in %

Table 3

Change in shrinkage and water absorption \((T = 1500^\circ\text{C})\)

…dispersed ones. In coprecipitated materials, sintering approaches completion at 1500°C; in mechanically mixed materials it is still insignificant, and up to 1500°C proceeds very weakly. The best ratios for sintering are the compositions \( \mathrm{MgO}:\mathrm{Al}_2\mathrm{O}_3 = 1.05:1;\ 1:1 \).

Spinel ceramics are finding ever wider application in radio electronics. Recently, work has been under way involving their use in titanium-ceramic seals. The ceramic currently used, with a forsterite crystalline phase, does not meet the requirements of industry, since it promotes poisoning of the oxide cathode. A necessary condition for the use of ceramics in titanium-ceramic lamps is matching the CTE of titanium and the ceramic. The composition of spinel ceramic \( \mathrm{MgO}:\mathrm{Al}_2\mathrm{O}_3 = 1.4:1 \) is well matched in CTE (it is equal to \(100\text{—}110 \cdot 10^{-7}\,^\circ\mathrm{C}^{-1}\) in the temperature range…

100–1000°C) with titanium. This well-known body was designated Sh-64, according to its alumina content, \(Al_2O_3 = 63.9\%\). The coefficient of thermal expansion of MgO in the interval 20–800°C ranges from \(13 \cdot 10^{-6}\ ^\circ C^{-1}\) to \(14 \cdot 10^{-6}\ ^\circ C^{-1}\) [3, 4, 5]. The CTE of magnesium aluminate spinel ranges between \(6.7 \cdot 10^{-6}\ ^\circ C^{-1}\) and \(11.0 \cdot 10^{-6}\ ^\circ C^{-1}\) [3, 4, 5]; for spinel of stoichiometric composition it is \(8.7 \cdot 10^{-6}\ ^\circ C^{-1}\). The question of the use of magnesium aluminate spinel in seals with Ti has already been considered in the literature [6–9]. The CTE of Ti is \(10 \cdot 10^{-6}\ ^\circ C^{-1}\). With an increase in the amount of free MgO in the spinel, its CTE increases.

The second part of the present work is devoted to the possibility of sintering composition Sh-64 at reduced temperatures, on the order of 1400–1450°C. For this purpose, microadditives of certain elements were used. A total of 16 compositions were investigated. Starting materials: MgO, extra-light, loss on ignition 15%, elongated crystal form, size along the length 30–40 mm; \(Al_2O_3\), ChDA grade, loss on ignition 1.5%, grain size 6–8 μm.

The specimens were formed by the dry method at a specific pressure of \(750\ \text{kg}/\text{cm}^2\), and firing was carried out in the interval 1200–1600°C. With certain additives, vacuum density is reached at 1450°C when firing in a silite furnace, which is accompanied by shrinkage of 23–23.2%. Firing to higher temperatures in a hydrogen atmosphere—1500 and 1600°C—is not effective and does not always lead to zero water absorption.

According to the research data, it may be asserted that, by using various combinations of additives, it is possible to sinter material Sh-64 to vacuum density already at 1400–1450°C in an oxidizing atmosphere. From previously published works [10, 11] it is known that synthesis of spinel precedes its recrystallization; i.e., the earlier spinel synthesis is completed, the earlier the sintering processes begin. The change in the phase composition of the specimens after firing was determined by X-ray analysis from the change in the areas of X-ray reflections: \(d = 1.533\) (reflection plane 333,511) for spinel, \(d = 2.085\) (reflection plane 200) for periclase.

In the absence of microadditives, the spinel-formation process is not completed even at 1450°C, since the maximum reflections of corundum remain. The spinel-formation process must be completed with the complete binding of alumina, and the excess of free periclase should amount to 11%.

When firing in an oxidizing medium, complete correspondence is observed in the change of the phase ratios; however, when firing in hydrogen up to temperatures above 1450°C, only the spinel content remains constant, while the content of magnesium oxide decreases until it disappears. Possible causes of this may be the formation of a solid solution with spinel or volatilization of MgO.

In addition to the change in phase composition, the change in the lattice constant was studied as the firing temperature of the specimens increased. Spinel crystallizes in the cubic system with a unit-cell parameter of 7.978–8.081 Å. The latter value is for natural spinel in the presence of impurities. The concentration of temperature defects increases with rising temperature; this accounts for the sintering of crystalline substances in the solid phase. Impurities introduce additional defects and affect sintering; diffusion of ions in the crystal occurs by means of defects. The maximum increase in the unit-cell parameter upon introduction of additives occurs at a temperature of 1400°C; after firing at 1450°C the lattice constant decreases to a value characteristic of reference spinel. Firing in a hydrogen atmosphere promotes a slower attainment of the normal values of the lattice parameter of \(MgAl_2O_4\).

Additives, entering the spinel lattice, form solid solutions.

of interstitial or substitutional solid solubility, thereby promote the formation of new defects and accelerate sintering of the material.

Conclusions

1. The formation of spinel without additives from co-precipitated oxide hydrates begins already at 400°C. In specimens prepared by mechanical mixing of MgO that has been milled and \( \mathrm{Al_2O_3} \) (6–8 μm), synthesis begins at 500°C, while from MgO (6–8 μm) and \( \mathrm{Al_2O_3} \) (6–8 μm) it begins at 1000°C. In specimens made from amorphous MgO and \( \mathrm{Al_2O_3} \), at 900°C as much spinel was recorded as at 1400°C in mechanically mixed coarse-grained specimens.

2. The process of spinel formation from the amorphous phase is completed by 1300–1400°C. The optimum ratio of the components, both according to synthesis data and according to sintering data, corresponds to the molar ratio \( \mathrm{MgO}:\mathrm{Al_2O_3}=1:1 \).

3. The method of mixing plays an enormous role in synthesis and sintering. Coprecipitation achieves the best mixing of the components, and the spinel-formation reaction proceeds more intensively than in a mixture obtained by mechanical mixing of the components.

4. An increase in the amount of spinel with temperature entails a proportional decrease in MgO.

5. By introducing additives of certain elements, it was possible to lower the temperature of synthesis and sintering of spinel of composition Sh-64 from 1700 to 1450°C.

6. The advantages of firing ceramics of composition Sh-64 with additives in an oxidizing atmosphere have been substantiated.

7. The unit-cell parameter of Sh-64 after firing at a temperature of 1450°C corresponds to the parameter for standard spinel.

References

1. G. Wagner. Angem. Chem. 49, 735, 1936.

2. A. S. Berezhnoy, E. Z. Slonimskaya. Collection of Works of the Kharkov Institute of Refractories. Kharkov, Metallurgizdat, 45, 1939.

3. P. P. Galkin, R. Ya. Popil’skii. Refractories, 1965, No. 8.

4. T. N. Nilsen, U. H. Leipold. J. Amer. Ceram. Soc. 1963, 46, No. 8.

5. High-Temperature Techniques. Edited by Kembel, 1959.

6. Information reference sheet 012777 of the Ministry of the Electronics Industry of the USSR, TsNIITEIN, UDC 666.59321.

7. French Patent No. 3735/1.358.776, class CO4B-HO1J.

8. U.S. Patent 3.113.846.

9. British Patent No. 1.155.418, class C1A.

10. P. P. Budnikov, K. M. Zlochevskaya. Refractories, 1958, No. 3.

11. E. V. Degtyareva et al. Refractories, 1966, No. 8.