Densification degree

Some examples of the rich information provided by IR spectroscopy, described in this chapter, include the material structural evolution during the different stages of sol-gel processing, the detection of OH species and porosity, the analysis of structural homogeneity, phase separation and densification degree and the study of structural differences between thin films, thicker multilayer films and bulk sol-gel samples, as well as between purely inorganic and sol-gel derived hybrid materials. 

At the optimized laser energy ot 96 kj the laser metal deposition-processed layer showed the highest densification degree and was free of any pores and cracks. Also, a relatively high microhardness and significantly improved tribological properties were found (30). 

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. 

Along with this process of densification, there is also a decrease in degree of crystallinity. The extent of change in crystallinity is highly dependent on the species of the specimen and on the previous NH3-treatment but usually lies between 5 and 18% (45-47). 

It is clear from the preceding discussion that collapse, densification and increased amorphous character can be minimized by single gaseous treatments of short duration. The degree of relaxation is less than that under full swelling conditions, but can be in the range of practical working conditions as has been demonstrated in Zurich and is discussed later. 

Next we assume that the particulate solids at the beginning of the mixing chamber fill the rolling pool until the point of the minimum rotor diameter. The maximum degree of particulate bed densification and compaction, as it is forced through... 

If an equilibrated glass is subjected to only a few-degree change in temperature AT, the glass structure will not be far from equilibrium even immediately after the temperature change and the process of relaxation toward equilibrium can be described by a linear spectrum of relaxation times. The shape of the relaxation spectrum governing densification... 

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. 

---