Thermal expansion of refractories

FIG. 201. Thermal expansion of refractory oxides (from Campbell and Sherwood, 1967). Depends on degree of stabilization, -f- Depends on crystalline forms. 

The observations on thermal expansion of refractory carbides in Sec. 2.0 of Ch. 4 are applicable to the refractory nitrides. Table 11.3 shows that generally the higher the bond energy of the compound, the lower the expansion. 

Thermal Expansion of Refractory and Heat Insulation Materials Elements of Theory... 

In the A1 industry, it is necessary to take into account the thermal expansion of refractories in the bottom part of holding and melting furnaces in the cast house and in anode baking furnaces. The thermal expansion of carbon cathode is relatively small however, it might be sufficient to take it into account. Yet the sodium expansion of cathode blocks is several times higher compared to the values of thermal expansion. In the design of the reduction cells, the value of the thermal expansion of carbon cathode blocks is only the reference data. 

The thermal expansion of refractory constructions should be calculated and taken into account during design of the furnaces in order to avoid excessive thermal strains and consequent chippings and spallings. The thermal expansion of a 6-m-long roof of a furnace at a temperature of 800-900 °C will be 28-30 mm if the thermal expansion coefficient of the refractory is 5-6 x 10 K. ... 

A. M. Belikov, Ya. S. Umanskii. Nauch. Doklady VyS hei Shkoly 1 192, 1958 see also author s abstract of dissertation of Belikov s X-Ray Determination of the Constants of the Quasi-Electric Force of Thermal Vibrations and the Coefficients of Thermal Expansion of Refractory Metal Phases, Moscow, 1958. 

Thermal Properties. Refractories, like most other soHds, expand upon heating, but much less than most metals. The degree of expansion depends on the chemical composition. A diagram of the thermal expansion of the most common refractories is shown in Figure 1. 

Vitreous silica has a wide range of commercial and scientific applications. Its unique combination of physical properties includes good chemical resistance, minimal thermal expansion, high refractoriness, and excellent optical transmission from the ultraviolet to the near-infrared. 

In this way, it is possible to explain the low thermal expansion of silica refractories in the medium temperature range and a considerable expansion at low temperatures. This i.s also why silica differs considerably in thermal shock resistance in these temperature ranges. For this type of refractory, it is thus inadequate to specify the usual value of mean thermal expansion coefficient for a certain temperature range (e.g. 20—1000 C). The polymorphic inversion also results in an expansion anomaly of stabilized Z1O2 and of fusion-cast corundum-baddeleyite refractories around 1000 C (cf also Fig. 201). 

Thermal Properties.—The thermal qualities of refractories, specific heat, conductivity and expansion are determined according to the established physical methods. It is evident that these properties are of considerable practical importance. The data available, however, on these subjects are quite meager, especially if it is considered that the structure of the manufactured product, irrespective of its chemical nature, is of paramount importance. Furthermore, these properties are subject to change with temperature and comparatively few constants are at hand to illustrate the character of these relations. It is known that the specific heat and thermal conductivity increase with temperature but the fundamental laws governing these changes have not been established. Furthermore, it must be realized that the structure of all these materials is certain to undergo physical changes which affect the thermal qualities. 

Thermal Expansion The thermal expansion of periclase is the greatest of all pure refractory oxides and approaches the expansion of metals. Exact expansion measurements have been carried out on single crystals of periclase and high-purity sintered magnesia, the results of which are tabulated in Tables 8.5 and 8.6. 

Warm-up times can be considerably less than dryout times, if no moisture needs to be driven off. Some warm-up time is important even for previously dried-out furnaces to minimize refractory spalling because of too-rapid or uneven thermal expansion of the dry, solid refractory. 

In any refractory lined system, it is required to determine the thermal expansion of the system in order to determine the stress, support points and restraint points. In order to do this, the designer must accurately determine the skin temperature of the various components. From this data, the thermal expansion of the components is computed, and from this, the stress, depending on support and restraint points. 

Baudran Expansion Apparatus. A device for measuring the thermal expansion of ceramics up to 1500 C expansion of the test-piece is directly transmitted by a lever system to an inductive displacement detector. This procedure is used for testing refractories (B.S. 1902, Pt. lA) it was devised by A. Baudran (Bull. Soc. Franc. Ceram. (27), 13, 1955). Baume Degrees (°Be). A system, introduced by a Frenchman, A. Baume, in 1768 for designating the specific... 

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%... 

The elimination or at least minimization of the positive coolant density component of the temperature reactivity effect is a favourable factor in limitating the consequences of ULOF and UTOP. The negative reactivity feedback caused by a thermal expansion of the control rod drive lines also plays an important role. Analysis of a ULOF accident shows that sodium boiling and fuel melt are excluded because the core outlet temperature does not exceed 800 C. Nevertheless a refractory sodium-cooled tray beneath the core is provided to prevent release of corium beyond the reactor vessel boundary and formation of a critical mass. Preliminary safety analysis for the BN-1600M reactor plant shows that ... 

The gaps between the rammed paste and the cathode blocks appear due to problems with the binder (which may be caused by too low a temperature of the ramming paste or cathode blocks). More rarely, it is caused from problems with the grain size composition. Also, it might be caused by preheating and problems in a mismatch of the thermal expansion of the ramming paste and the cathode blocks. It is likely the most serious defect, which causes immediate leakage of the electrolyte in the refractory layer. 

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. 

Figure 8 Thermal expansion of selected refractory materials. (From CERAM data.)...

The refractory industry has found chromite useful for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stability of crystalline structure. 

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