Oxidizing

The most widely used simple oxide is Al2O3. It has moderate thermal shock resistance, good stability to various atmospheres, and is a good electrical insulator at high temperatures.

• The strength of ceramics is affected by trace impurities and microstructural features. In general, polycrystalline alumina has fairly good and almost constant strength below 1000–1100°C; at higher temperatures, the strength drops to less than half the room temperature value in temperature increments of 400°C.

• Single crystal alumina is stronger than polycrystalline Al2O3 and the strength actually increases between 1000 and 1100°C.

• Molten silica glass has excellent thermal shock properties, but when heated above 1100°C for a long time, it will undergo devitrification and lose most of its thermal shock properties.

• Beryllium oxide and magnesium oxide are stable to very high temperatures in oxidizing environments. Above 1700°C, MgO is highly volatile under reducing conditions and vacuum, while BeO shows better resistance to volatility, but is easily volatilized by water vapor above 1650°C. Beryllium oxide has good electrical insulation properties and high thermal conductivity; however, its high toxicity limits its use.

• Calcium oxide and uranium oxide also hydrate to a lesser extent. The latter, UO2, can be oxidized to the low melting point U3O8.

• Pure zirconia is rarely used in ceramic bodies; however, stable or partially stabilized cubic ZrO2 is the most useful simple oxide for operation above 1900°C.

• Thorium oxide has good high temperature properties but is expensive and radioactive. Titanium oxide is easily reduced to low-valent oxides and cannot be used in neutral or reducing atmospheres.

Carbon, Carbides and Nitrides

Carbon and graphite are used individually to make refractory products for the lower linings of blast furnaces, as well as electrodes for steel and aluminum production. They are also often used in combination with other refractory materials. These materials are highly refractory non-wetting materials and are useful refractories in non-oxidizing environments. Metal-deposited carbon has high strength, reaching 280 MPa (40,600 psi). Oxidizes at high temperatures but can be used up to 2760°C for short periods under neutral or reducing conditions. New composite materials made from carbon fiber are promising, especially in the field of aerospace structures.

When heated under oxidizing conditions, silicon carbide and silicon nitride, Si3N4, form a protective layer of SiO2 on their surfaces and can be used up to approximately 1700°C. Silicon carbide has very high thermal conductivity and can withstand thermal shock cycles without damage. It is also an electrical conductor and is used in electric heating elements.

Other carbides have relatively poor oxidation resistance. Under neutral or reducing conditions, several carbides have potential use as technical ceramics in aerospace applications, such as carbides of B, Nb, Hf, Ta, Zr, Ti, V, Mo and Cr. Ba, Be, Ca and Sr carbides will be hydrolyzed by water vapor. Silicon nitride maintains good strength at high temperatures and is the most oxidation-resistant nitride. Boron nitride has excellent thermal shock properties and is similar to graphite in many ways, except that it is not an electrical conductor.

Refractory Physical Properties

The bulk density of bricks depends on the specific gravity and porosity of the components. The latter is controlled by the porosity of the raw material and the structure of the bricks. Coarse, medium and fine aggregates all affect the degree of particle accumulation. Usually the highest possible density is desired. During the firing process, the particles and matrix form a glassy, direct or solid ceramic bond. Sintering is usually accompanied by shrinkage unless new components are formed causing expansion. Stresses may arise due to differences in volume changes between coarse and fine fractions caused by different sintering rates and additive phase formation. Particle size distribution, molding method and firing process influence structure, while permeability is related to porosity, which in turn depends on structure.