Ceramic powder processing heat treatment

Many kinds of artificial hip joints are available commercially, but they all consist of the same parts, i.e. a metal stem or shaft, usually made of a titanium alloy and a ceramic head of aluminium or zirconium oxide. The production of the ceramic head starts with a powder and ends with the sintering process. The heat treatment will cause the head to shrink. After production, the head is thoroughly tested, e.g. on its spherical shape and surface roughness. 

As an example of the property changes induced by microwave processing, in the sintering process a porous ceramic powder compact evolves toward a dense body during heat treatment at elevated temperatures. During densification, changes in s, e", and k can be large in amplitude and complex in nature. The lack... 

Additives such as pigments are added to the glass powder. A sintering process is initiated during heat treatment at temperatures of approximately 500°-800°C to produce a raw glass-ceramic in the form of a cylindrical ingot. 

Vitrification - heat treatment which produces enough viscous liquid at the firing temperature to completely fill the porous spaces in the original powder compact. This process is relatively inexpensive and is of particular importance in the production of porcelain and clay-based ceramics. 

Crystal size is usually less sensitive and is independent of environment. Heat treatment does, of course, cause an increase in crystal size, and crystal growth may be followed all the way through a complete ceramic fabrication process. Thus while a particular oxide or metal powder produced by a chemical process may have a crystallite size of 10-100 A, subsequent heat treatment (calcination) by which the powder is prepared for slip-casting, dry pressing, etc., may increase the crystal size to about 0.01-0.1 //, and from the final sintering process a crystal size of about 100 // may result. 

As discussed in Sections 13.11 and 13.12, for some ceramic fabrication techniques, the precursor material is in the form of a powder. Subsequent to compaction or forming of these powder particles into the desired shape, pores or void spaces exist between the powder particles. During the ensuing heat treatment, much of this porosity will be eliminated however, often this pore elimination process is incomplete and some residual porosity will remain (Figure 13.22). Any residual porosity will have a deleterious influence on both the elastic properties and strength. For example, for some ceramic materials the magnitude of the modulus of elasticity E decreases with volume fraction porosity P according to... 

What is tme for ceramics can be transposed, partly, to glasses and hydraulic binders these three types of materials are interrelated and we can say that all three bring into play ceramic compounds. But if we consider that a ceramic object is made of ceramic compounds processed by ceramic techniques, we can differentiate these materials by the order in which the three fundamental steps of the process take place powders (P), forming of the object (F) and heat treatments of drying and sintering (HT) ... 

Nuclear Applications. Powder metallurgy is used in the fabrication of fuel elements as well as control, shielding, moderator, and other components of nuclear-power reactors (63) (see Nuclearreactors). The materials for fuel, moderator, and control parts of a reactor are thermodynamically unstable if heated to melting temperatures. These same materials are stable under P/M process conditions. It is possible, for example, to incorporate uranium or ceramic compounds in a metallic matrix, or to produce parts that are similar in the size and shape desired without effecting drastic changes in either the stmcture or surface conditions. OnlyHttle post-sintering treatment is necessary. 

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