Degree of densification

Values are indicative only. Precise values depend on sintering conditions and degree of densification. 

CVI is a special CVD process in which the gaseous reactants penetrate (or infiltrate) a porous structure which acts as a substrate and which can be an inorganic open foam or a fibrous mat or weave. The deposition occurs on the fiber (or the foam) and the structure isgradually densified to form a composite.The chemistry and thermodynamics of CVT are essentially the same as CVD but the kinetics is different, since the reactants have to diffuse inward through the porous structure and the by-products have to diffuse out.f l Thus, maximum penetration and degree of densification are attained in the kinetically limited low-temperature regime. 

In the case of oxides with variable valence state, the degree of densification may depend further on partial oxygen pressure in the atmosphere. For instance, spinel MgCr204 requires for its densification an O2 pressure lower than 10 Pa (Anderson, 1974). Maximum densities are attained at an oxygen pressure beyond which the spinel becomes unstable and volatilizes. 

It takes 0.2 h for the relative density of a 0.1 pm average diameter powder to increase from 60 to 65 percent. Estimate the time it would take for a powder of 10pm average diameter to achieve the same degree of densification if the rate-controlling mechanism were (a) lattice diffusion and (b) viscous flow. 

The idea that the elastic moduli of ceramic powder compacts are close to zero has also led to the idea that the change in the elastic constants is simply given by the degree of densification. Thus, it has been suggested that... 

Papers are available in many forms varying in thickness, degree of densification, and composition (additive type or floe to fibrid ratio). Pressboards, which differ from paper in thickness and rigidity, are likewise available in several thicknesses and degrees of densification. The product of choice will depend on many factors including end-use thermal and mechanical performance requirements, formability or ease of fabrication, and the desired degree of saturability. 

Equation (5.224) reveals that F is a function of the interparticle distance h for a given volume fraction of the liquid, which can be used to identify the inter-relationship between the contact angle and the efficiency of the particle rearrangement processes. It is found that a small contact angle is always desirable to achieve high degree of densification. 

The densification rate by diffusion may be estimated following the derivation of a scaling law (see Section 4.4.1). In the case of lattice diffusion, for example, the time needed to get the same degree of densification, t, is expressed as... 

As in the case of flexural strength, variation of the Young s modulus with the degree of densification increases during the initial stages of sintering. 

The desired, in-use properties (i.e., mechanical, electrical, dielectric, optical, thermal, physical, and magnetic) of a ceramic are generally only realized after a dense, cohesive body is produced by thermal consolidation. The properties of a finished ceramic body are largely dependent on the degree of densification achieved and the microstructure produced during thermal consolidation consequently, thermal consolidation is one of the most critical steps in ceramic processing. 

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