Densification factor

Ceramics which, when indented, undergo the mechanisms listed above, with the exception of cracking, can be grouped as Mohr-Coulomb materials for which five parameters—E, a-y, v, H, and a—are needed to calculate hardness. Here E, a-y, and i> have their usual meaning while H is the stress hardening rate and a is the densification factor. Clearly if indentation causes densification then a and H must be closely linked. 

Y. Murata and R. Smoak in S. Somiya and S. Saito, eds., Proc. Int. Sjmp. of Factors in Densification and Sintering of Oxide and Nonoxide Ceramics, Gakujutsu Biinken Fukyu-Kai, Tokyo, 1979, p. 382. 

Hydrolysis and Polycondensation. As shown in Figure 1, at gel time (step C), events related to the growth of polymeric chains and interaction between coUoids slow down considerably and the stmcture of the material is frozen. Post-gelation treatments, ie, steps D—G (aging, drying, stabilization, and densification), alter the stmcture of the original gel but the resultant stmctures aU depend on the initial stmcture. Relative rates, of hydrolysis, (eq. 2), and condensation, (eq. 3), determine the stmcture of the gel. Many factors influence the kinetics of hydrolysis and... 

Thermal-Gradient Infiltration. The principle of thermal-gradient infiltration is illustrated in Fig. 5.15b. The porous structure is heated on one side only. The gaseous reactants diffuse from the cold side and deposition occurs only in the hot zone. Infiltration then proceeds from the hot surface toward the cold surface. There is no need to machine any skin and densification can be almost complete. Although the process is slow since diffusion is the controlling factor, it has been used extensively for the fabrication of carbon-carbon composites, including large reentry nose cones. 

Researchers who have focused more on understanding cause-effect relationships in solution processing have given attention to film drying and pyrolysis behavior, densification processes, and nucleation and growth into the desired crystalline state. Both thermodynamic and kinetic factors associated with the phase transformation from the amorphous state to the crystalline state have been considered.11 119 Control of these factors can lead to improvements in the ability to influence the microstructure. It is noted that in the previous sentence, influence has been carefully chosen, since the ability to manipulate the factors that govern the nature of the phase transformation to the extent that full control of the microstructure is possible remains to be demonstrated. However, trends in characteristics such as film orientation and columnar versus uniaxial grains have certainly already been achieved.120... 

Intermetallic compound formation may be observed as the result from the diffusion across an interface between the two solids. The transient formation of a liquid phase may aid the synthesis and densification processes. A further aid to the reaction speed and completeness may come from the non-negligible volatility of the component(s). An important factor influencing the feasibility of the reactions between mixed powders is represented by the heat of formation of the desired alloy the reaction will be easier if it is more exothermic. Heat must generally be supplied to start the reaction but then an exothermic reaction can become self-sustaining. Such reactions are also known as combustion synthesis, reactive synthesis, self-propagating high-temperature synthesis. 

T Shimohira, A Makishima, K Kotani, M Wakakuwa. In S Somiya, S Saito, eds. Proc Int Symp Factors in Densification and Sintering of Oxide and Non-Oxide Ceramics. Tokyo Tokyo Institute of Technology, 1978, pp 119-127. 

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

In contrast, increasing the gap width between the dry granulator rollers decreases the density of the compacted ribbon or briquette (also possible when other factors, such as feed rate or roll force, are changed). This reduced densification process results in a weaker binding of API to carrier excipients, and reduced overall granulation particle size distribution and increased bypass. 

While the observed dependence of the densification rate on particle size, applied pressure, and temperature follows qualitatively the predictions of the sintering models, factors such as particle packing, sintering atmosphere, and heating schedule can have a decisive effect on the sintering process. 

Jamming of material within the extrusion die has been the single most limiting factor to continuous low cost densification processing. We have experimented in a number of ways in attempts to quantify the relationships among critical parameters associated with the jamming phenomena (8, 9). 

An important result of this study is the finding that the work and pressure of compression or extrusion can be reduced by a factor of about two by preheating the feedstock to 200-225 C before densification, This requires extra thermal energy for complete drying and to heat the biomass (heat capacity about 1.8 J/g-C) to the higher temperature however, these are offset by lower electrical power costs, lower equipment costs because of the lower pressure requirements, possibly reduced die wear due to improved lubricity of the biomass at increased temperatures, and increased fuel value due to complete water removal and prepyrolysis. These factors must be tested at the commercial scale before any conclusions can be drawn on the desirability of preheating feedstock. 

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