Porosity refractory ceramics

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. 

Ceramic filter elements have a particular application for higher fluid temperatures and for handling corrosive fluids. Elements made from vitreous bonded refractory ceramic aggregate also have a high resistance to physical and thermal shock. Ceramic elements can be cast or moulded in various element forms with varying degrees of porosity and uniformity of structure. Void volumes are typically up to 50%. 

Ceramic materials are divided according to various aspects, for example, according to chemical and phase composition, porosity (dense and porous ceramics) and structure (coarse and fine ceramics). The group of fine ceramics includes, for instance, porcelain and technical ceramics, whilst the coarse ceramic materials are represented by brickmaking products and by refractories for the construction of industrial furnaces. According to the purpose and fields of application, ceramics are also divided into building ceramics, technical ceramics, domestic ware, refractories, etc. 

Calcination of washed aluminum hydroxide at 1,100-1,300°C causes shrinkage of the particles as they dehydrate and yields a denser, harder product, a-alumina, useful as a constituent of abrasives. The low porosity and high temperature stability (up to approximately 1,800°C) of this product also make it useful for refractories either as bricks or as a loose fill. Ceramics with an alumina content of 85-95% and fired for up to 2 days at 1,450°C possess superior strength, hardness, and electrical and thermal insulating... 

This value of density obtained is actually better than a comparable commercial Refractory tube, sheath or rod made of mullite-bonded porous SiC, Anderman Ceramics , having a composition Sic = 90, AljOj = 6.5 Si02 = 3.5 wt.%, density = 2.2gm/cc, open porosity = 25%... 

Permeability. The rate of flow of a fluid (usually air) through a porous ceramic material per unit area and unit pressure gradient. This property gives some idea of the size of the pores in a body -whereas the measurement of porosity (q.v.) evaluates only the total pore volume. From the permeability of a compacted powder the specific surface (q.v.) of the powder can be deduced. B.S. 1902 Pt. 3.9 describes a gas permeability test applicable to refractory materials, as does ASTM C577. The ASTM C866 test for the filtration rate of whiteware clays depends on their water permeability. See blaine test carman equation lea and nurse... 

The definition for an unshaped refractory is given in ISO 1927 (5) and EN 1402-1 It is a mixture, which consists of an aggregate and a bond or bonds, prepared ready for use either directly in the condition in which it is supplied or after the addition of a liquid that satisfies the requirements on refractoriness given in ISO R 836. They may contain metallic, organic, or ceramic fiber material. This mixture is either dense or insulating. An insulating mixture is one whose true porosity is not less than 45% when determined in accordance with EN 1094-4, using a test piece prefired to specified conditions. 

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