Porosity of refractories

The porosity of refractory bricks has a direct bearing on the thermal conductivity. The densest and the least porous bricks have the highest thermal conductivity owing to the absence of air voids. On the other hand, in porous bricks the entrapped air in the pores acts as a nonconducting material. 

Thermal conductivity Thermal conductivity is increased when the porosity of refractory bricks decreases and the air is entrapped in refractory decreases. 

Table 1.2 Porosity of refractory and heat insulatirai materials, used in alnminimn industry...

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

The work of immersion Wj is a thermodynamic quantity that describes any process of infiltration of liquids into porous media, for instance fabrication of composites by liquid routes, liquid state sintering or infiltration of refractories by molten metals or salts. In the example of Figure 1.36, at a depth z, any porosity (assumed cylindrical and open) of radius r larger than (—2<7Lvcos0)/(pgz) will be infiltrated by the non-wetting liquid, while for smaller porosities no infiltration will occur. 

Total porosity is determined most conveniently by weighing and determining the specimen volume with a mercury volumeter or by indirect methods (see p. 292, Section IV.4.2.). The total porosity of most refractories varies over the range of 15-30 %. Most of the pores are open. 

The principal properties of silica refractories are listed in Table 33. The porosity is relatively high, up to 30% with coke-oven silica. Attainment of values lower than those given in the table is rendered difficult by the volume changes involved in polymorphic inversions. Silica is typical by its very low content of closed pores, only 0.1 to 0.5%, whereas with fireclay their content amounts to 2 — 6%. There is no significant difference between the total porosities of standard fireclay and silica refractories however, substantially lower porosities can be attained with special grades of fireclay (less than 10%). 

Compact insulating materials in the form of bricks or blocks are usually preferred in industrial applications. They are made of standard types of refractory materials with artificially induced porosity reaching 70%. This raised porosity can be obtained in various ways ... 

Similar to the change in volume the porosities of a material fired to different temperatures may be plotted against the temperature. The curves thus obtained are equally instructive and valuable, for the comparison of the pyro-technical properties of refractories. The lower curve of Fig. 3 gives the results obtained with a fireclay material. Here again the rate of porosity decrease, the temperature at which the structure has become dense, shown by the approach to zero porosity, and the evidence or overfiring offer data of practical importance. The accuracy of the results obtained depends of course upon the accuracy of the temperature measurements. Too much attention cannot be given to the calibration of the pyrometers. 

Resistance to Slagging Action.—According to the porosity of the refractories, the temperature, viscosity and the chemical nature of the slag or glass the absorp-... 

From previous considerations it is apparent that this type of refractory canno b( . as heat-resisting as the previous class. However, for first-class materials the soft-( iiing point should not he lower than that of cone 28, about 1,635°C. or 2,975°F. The M rmanont contraction or expansion upon reheating to 1,400°C. is, as a rule, low and should not exceed 0.5 per cent. The porosity may vary between 20-28 per cent, liie rc sist ance to compression at temperatures of 1,350°C. is high and the contraction should in no case exceed 4 per cent at the temperature given and under a load of 40 lb. per s(iuare inch. 

In the center of the lining Van Vlack [42] detected the absorption of 8% alkali. The high porosity of this region was attributed to carbon disintegration of the refractory. For the inside of the bosh area a coating of carbon and alkalies was observed with the sample containing as much as 42% alkali. The refractories involved are in the 40% to 45% alumina class. 

In most cases the furnace is made of graphite, which has good thermal and corrosion resistance. As a result of its porosity, graphite can take up the sample without formation of appreciable salt deposits at the surface. However, apart from graphite, atomizers made of refractory metals such as tungsten have also been used (Fig. 57) (see e.g. Refs. [175, 175a]). 

Parts with an open coherent porosity of >20% are used in vacuum tubes and aeronautics because of refractoriness, shape stability, and chemical resistance. Examples are porous cathodes impregnated with alkaline earth oxides as electron-emitting sources in special tubes, thermoionic converters heated by nuclear energy, and ionic propulsion units with porous tungsten plates as ionic sources for Cs vapor of high temperature. Those emitter plates are made of globular tungsten powder (7 pm) density 75-85%, pore size 2-30 pm, and pore number 1.4 x 10 -8 x 10 cm . ... 

It is not possible to fabricate typical refractory ceramics to 100% of their theoretical density. Therefore, methods of controlling the porosity of the final product must be considered. The role of this initial porosity as sites for fission gas, as well as its effects on thermal conductivity and mechanical strength, is a significant factor in the design. 

Gas-fired radiators These radiators consist of a perforated plate (metal or refractory), which is heated by gas flames in one of the surfaces so the plate raises its temperature and anits radiant energy. The porosity of the plate determines the temperature of the other surface so as to ensure a safe process. Figure 19.6b shows a sketch of this type of radiator (van t Land, 1991, p. 250). The temperature of such a radiator is generally between 1500°C and 1700°C with wavelengths from 2.7 to 2.3 pm (van t Land, 1991, p. 249). The radiant efficiency of such radiators is typically about 60%. 

Intrinsic sources of porosity are the molar volume change (due to the formation of the product) and the porosity generated by the thermal migration. Figure 28 shows the relationship between the molar volume of the reactants and the product for selected refractory materials. The lines represent a value of x in the relationship Fp = jcFr, where Fp and Fr are the molar volumes of the product and reactants, respectively. The figure shows that the formation of TaC, B4C, for example results in a porosity of about 10%, while the formation of SiC, MoSi2, results in a porosity of about 30%. The other intrinsic source porosity is thermal migration (the Soret... 

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