Refractory linear thermal expansion

By definition, refractory metals exhibit low thermal and electrical conductivities and have equally low thermal expansion properties (Table 3.6). As a relative benchmark, common metals such as iron and copper have coefficients of linear thermal expansion on the order of 12.1 and 17.7 p,m m K , respectively. Also for comparative purposes, the electrical/thermal conductivities for Fe and Cu are 9.71 j,Qcm V78-2 Wm and 1.67p,Q cm V397W m K , respectively. 

Material Refractoriness ("C) Temperature of initial deformation under load (°Q Porosity (%) Bulk density (g cm ) Compression strength (MPa) Linear thermal expansion a X lO" (20-1200 )... 

With any material used in high-temperature applications, the effect of linear thermal expansion, and especially the permanent linear change, must be considered. Shrinkage of castables is less than that of plastic refractories therefore, permanent linear change is less. Castable refractories are significantly superior to firebrick in permeability resistance and spalling resistance. Plastic refractories have better spalling resistance than either firebrick or castables. 

Celsian. Barium feldspar, Ba0.Al203.2Si02 m.p. 1780 °C. There are two crystalline forms, resulting in a non-linear thermal expansion curve. Celsian refractory bricks have been made and have found some use in electric tunnel kilns. 

With a low thermal conductivity of 0.06 W cm-1 K 1 and a low thermal expansion coefficient a 4.5 x 10 6oC 1, mullite is useful for many refractory applications [49], According to Schneider, most mullites display low and nonlinear thermal expansions below, but larger and linear expansion above, 300°C. The volume thermal expansion... 

Thermal Expansion. The reversible increase in dimensions of a material when it is heated. Normally, the linear expansion is quoted, either as a percentage or as a coefficient, in either case over a stated temperature range for example, the thermal expansion of a silica refractory may be quoted as 12%... 

Tables 1.8 and 1.9 and Fig. 1.21 give some reference data on the values of the thermal coefficient of linear expansion for oxides, refractory, and ceramic materials [100-102]. Crystals with a cubic lattice (CaO, MgO) have equal values of linear coefficients of expansion along aU axes. The typical linear coefficients of thermal expansion for such materials are 6-8 x 10 and increase with the temperature up to 10-15 X 10 K . Anisotropic crystals with low symmetry have different values of linear coefficients of thermal expansion along different axes, but with a temperature increase, this difference becomes smaller. Materials with strong chemical bonds (silicon carbide, titanium diboride, diamond) have low values of linear coefficients of thermal expansion. However, these materials have high values of Debye characteristic temperature (values of the linear coefficients of thermal expansion grow below the Debye temperature and are almost constant above it).

For unheated heat insulation and refractory materials, the temperature dependence of thermal expansion is essential. Figure 2.90 in Sect. 2.7 shows the temperature dependencies of linear coefficients of thermal expansion for several vermiculite-based materials. The materials cannot be recommended for use in the high-temperature devices due to high-volume increases upon heating. The excep-ti(Mis are materials 2 and 3, which have uniform temperature dependencies of linear coefficients of thermal expansion. 

Thermal Expansion of MgO. The coefficient of thermal expansion of an essentially pure MgO refractory is very high for example, at 1425°C, the linear expansion of a fused MgO or isostatically pressed and fired MgO of 99 wt. % minimum purity with a minimum bulk density of 3180kg/m is about 2%. Of course, incorporating other refractory raw materials with magnesia in a refractory body will alter this value. Care is required in using these materials to account for this property in an engineering sense. 

Figure 9 illustrates the radial brick joint compression-only behavior. In this example the stress-strain behavior of the refractory material was assumed to remain totally elastic. As shown, a portion of the joint on the hot face end of the radial brick joint is in compression, and a portion on the cold face end of the brick is separated. The circumferential loading is a maximum at the lining hot face and decreases linearly to zero at an internal location of the brick joint. For actual elastic/plastic refractory behavior, the circumferential loading would be nonlinear. The internal location is where the joint begins to separate. This joint behavior can be explained fundamentally by considering the temperature of the various locations in the brick joint compared to the steel shell temperature and the coefficient of thermal expansion of the brick material and the steel shell. 

Table 1.9 Thermal linear coefficient of expansion for refractory and heat insulation materials (x 10 [100-102]...

---