Magnesia ceramic is a typical new type of ceramic and also a traditional refractory. Magnesium oxide itself has strong corrosion resistance to alkaline metal solutions. The prepared magnesium oxide ceramic crucible has excellent chemical properties and stability against metal corrosion, and has no effect on magnesium, nickel, uranium, aluminum, molybdenum, etc. Magnesium oxide ceramics can work stably to 2400 ℃ under the protection of oxidizing atmosphere or nitrogen, so magnesium oxide is a key material in the advanced process of modern metallurgical industry.
01 Properties and Characteristics of Magnesia Ceramics
Magnesium oxide ceramics belong to cubic crystal system with sodium chloride structure. The main crystal phase of magnesium oxide ceramics is magnesium oxide, the theoretical density is 3.85g/cm, the melting point is 2800 ℃, and the Mohs hardness is 5-6. Magnesia is a kind of weak alkaline substance, which has strong corrosion resistance to alkaline metal solution. It is difficult to sinter magnesium oxide because of its compact ion packing density, high symmetry of ion arrangement, few lattice defects. Therefore, in the preparation process of magnesium oxide ceramics, in order to reduce the sintering temperature, it is necessary to add a certain amount of sintering aids. Generally, mixed rare earth metal oxides (RxOY), TiO2, ZrO2, SiO2 are used as sintering aids. The high temperature stability and corrosion resistance of magnesium oxide ceramics are superior to those of aluminum oxide ceramics, which can be used in the corrosive environment of strong alkali metals.
02 Preparation of magnesia ceramics
(1) Preparation of magnesia raw materials
Magnesium containing compounds are abundant in nature, which exist in the crust and ocean in various mineral forms, such as magnesite (MgCO3), dolomite (CaMg (CO3) 2), brucite (Mg (OH) 2), talc (Mg [Si4O10] (OH) 2), etc. In industry, magnesium oxide is mainly extracted from the above minerals, and recently, it is extracted from seawater.
Most of the magnesium oxide extracted from minerals or sea water is first made into magnesium hydroxide or magnesium carbonate, and then calcined and decomposed into magnesium oxide, which can be further chemically or thermally treated to obtain high-purity magnesium oxide.
(2) Preparation of magnesium oxide ceramics
During preparation, magnesium oxide raw materials are processed and batched according to composition. In order to promote sintering and make grains grow slightly, and to reduce the hydration tendency of preparation, some additives, such as TiO2, Al2O3, V2O3, etc., can be added. Table 1 lists the effects of some additives on the grain size and sinterability of magnesium oxide. If high-purity magnesium oxide ceramics are required, the method of adding additives cannot be used to promote sintering and grain growth, but the method of activated sintering is used, that is, Mg (OH) 2 is calcined at an appropriate temperature to obtain active magnesium oxide with many lattice defects, which is used to manufacture sintered magnesium oxide ceramics. It is generally appropriate to calcine and decompose Mg (OH) 2 at 1200 ℃.
Magnesium oxide has strong chemical activity, is easy to dissolve in acid, and has large hydration capacity. Therefore, the manufacturing process of magnesium oxide ceramics must consider this characteristic of raw materials. In order to reduce the activity of magnesium oxide, it should be prefired to 1100~1300 ℃. The grinding lining and grinding ball used for grinding raw materials shall be made of ceramic materials instead of steel balls. Dry grinding is used to make it waterproof. It is better to use vibration mill to finely grind, so that more fine particles can be obtained. The forming methods can be semi dry pressing method, grouting method, hot pressing casting method and hot pressing method. The binder used in the semi dry pressing method is glycerin, polyvinyl alcohol, carbon tetrachloride solution of beeswax, etc., which is made of fine particles and molded under the pressure of 50~70MPa. The preparation of grout for injection molding requires anhydrous ethanol as the medium to avoid the hydration and expansion of magnesium oxide. It is better to add 2%~3% oleic acid (generally about 14%) to the raw materials for hot die casting during fine grinding. The fine grinding time should not be too long to prevent the agglomeration of magnesium oxide particles. The bulk density of the product obtained by hot pressing method can be close to the theoretical density value. The general pressure is 20~30MPa, the temperature is 1300~1400 ℃, and the pressurization time is 20~40min.
Magnesia ceramics are mostly produced by grouting method. However, due to the easy hydration of magnesium oxide, its process is more complex. When preparing the slurry, in order not to contact magnesium oxide with water, it is necessary to use anhydrous ethanol and other organic liquids as the suspension medium. The widely used process is as follows: magnesium oxide raw materials are mixed with enough distilled water to form a paste and fully hydrated into Mg (OH) 2. After being stored for a certain time, it shall be dried under 100 ℃, calcined under sealed conditions at 1450~1600 ℃, and kept for 8h, so that magnesium hydroxide can be decomposed into magnesium oxide again. Then ball mill for 45~90h, then add water (50%~60% water) and continue to mill for 70~90min. The suspension slurry is formed for casting molding. In order to improve the slurry performance, the pH value can be adjusted to 7~8. After demoulding, the billet is dried at 70 ℃. In order to quickly remove the moisture in the billet, the moist air should be removed as soon as possible. The ceramics are first fired at 1250 ℃, then put into a corundum porcelain sagger and sintered at 1750~1800 ℃ for 2h.
03 Application of magnesia ceramics
The theoretical service temperature of magnesium oxide ceramics is up to 2200 ℃, which can be used for a long time at 1600 ℃~1800 ℃. Its high temperature stability and corrosion resistance are better than alumina ceramics, and it does not work with magnesium oxide with Fe, Ni, U, Th, Zn, Al, Mo, Mg, Cu, Pt, etc., so its application scope can include: crucibles or other refractory materials under corrosive conditions in steel, glass and other smelting industries.
Magnesia ceramics can be used as crucibles for smelting metals, and also suitable for smelting high-purity uranium and thorium in the atomic energy industry; It can also be used as a thermowell. It can be used as a crucible for radome and infrared radiation projection window materials, smelting metals and alloys, such as nickel alloy, radioactive metal uranium, thorium alloy, iron and its alloys, etc. Piezoelectric, superconducting materials and other raw materials, and pollution-free, lead corrosion resistant; It can also be used as ceramic sintering carrier, especially β- Sintering protection of ceramic products with corrosive and volatile substances under high temperature, such as Al2O3.