Refractories porosity

Models for the corrosion process point to the fact that hot face temperature is the most important variable in controlling refractory life. To reduce corrosion loss, the usual approach is to employ higher-purity refractories or lower-porosity refractories (such as fusion cast brick) because precise limits on hot face temperature are not practical given process goals. In new furnace designs, consideration should be given to hot face temperature control as a strategy to attain the most economical refractory cost. 

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. 

Thermal Conductivity. The refractory thermal conductivity depends on the chemical and mineral composition of the material and increases with decreasing porosity. The thermal conductivities of some common refractories are shown in Figure 2. 

Isostatic pressing gives a highly uniform product, although the production rate is somewhat low. It typically contains very small grains and Uttle or no porosity. In this process, a mbber sock or bag of the desked shape is filled with the refractory mix. The sock is then subjected to extremely high pressure in a hydrauUc pressure chamber. 

Uses. Hot-pressed hBN is useful for high temperature electric or thermal insulation, vessels, etc, especially in inert or reducing atmospheres, and for special materials such as IITV semiconductors (qv). Its low thermal expansion makes it resistant to thermal shock. The powder can be used as a mold release agent or as thermal insulation. Boron nitride is also available in fiber form (19). BN deposited pyrolyticaHy on refractory substrates at 1200—1800°C has a turbostratic stmcture and low porosity it has greater chemical resistance and is impervious to helium. 

The use of PM products in Army materiel fulfills a variety of special applications. These products would include magnetic materials, refractory metals, controlled-porosity materials,... 

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. 

Porosity bears but little connection to permeability. The permeability is a measure of the rate of diffusion of liquids and gases through the refractory. It is understandable that the... 

Full catalyst formulations consist of zeolite, metal and a binder, which provides a matrix to contain the metal and zeolite, as well as allowing the composite to be shaped and have strength for handling. The catalyst particle shape, size and porosity can impact the diffusion properties. These can be important in facile reactions such as xylene isomerization, where diffusion of reactants and products may become rate-limiting. The binder properties and chemistry are also key features, as the binder may supply sites for metal clusters and affect coke formation during the process. The binders often used for these catalysts include alumina, silica and mixtures of other refractory oxides. 

Another chondrite component is an optically opaque (in thin section) assortment of very fine-grained minerals that fills the spaces between the larger chondrules, refractory inclusions, and metal grains. This material is called matrix. Characterization of matrix minerals is hampered by their tiny particle sizes (as small as 50-100 nm). Moreover, the fine grain sizes, high porosity, and permeability of matrix make it especially susceptible to alteration during later heating or exposure to aqueous fluids. 

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