Densification process

Fig. 7. Thermal conductivity of composites obtained using various densification processes.

Figure 2.9. Approximate temperature ranges for densification processes contributing to structural evolution (non-isothermal processing with moderate ramp rate).

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... 

An example where one metal melts before the densification process, is the formation of bronze from a 90 10 weight percentage mixture of copper and tin. The tin melts at a temperature of 505 K, and the liquid immediately wets the copper particles, leaving voids in the compact. The tin then diffuses into the copper particles, leaving further voids due to the Kirkendall effect. The compact is therefore seen to swell before the final sintering temperature of 1080 K is reached. After a period of homogenization dictated by the criterion above, the alloy shrinks on cooling to leave a net dilatation on alloy formation of about 1%. 

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. 

Eventually, the top of the pore wiU close off, and reactants will not be able to find their way to the bottom of the pore to complete the densification process. 

The thus obtained high-density Mn-Zn ferrite was investigated in detail from the view of physical and mechanical properties, that is, the relationships between the composition of metals (a,) ) and <5 the magnetic properties such as temperature and frequency dependence of initial permeability, magnetic hysteresis loss and disaccommodation and the mechanical properties such as modulus of elasticity, hardness, strength, and workability. Figures 3.13(a) and (b) show the optical micrographs of the samples prepared by the processes depicted in Fig. 3.12(a) and (b), respectively. The density of the sample shown in Fig. 3.13(a) reached up to 99.8 per cent of the theoretical value, whereas the sample shown in Fig. 3.13(b) which was prepared without a densification process, has many voids. 

Tablet porosity vs. force curve to visualize the densification process Heckel plots to quantitate the densification process and characterize materi-...

The experimental results described above show that the gas-permeability properties of thin glassy polymer films (submicrometer in thickness) are more time- or history-dependent than much thicker films (the bulk state for example, 50 pm or thicker) seem to be. This is manifested in terms of physical aging over a period of 1 year and more. The observed permeability values for the current thin films are all initially greater than the reported bulk values but approach or become less than these values after a few days or weeks, depending on the thickness. After a year, the thin films may be as much as four times less permeable than the reported bulk values. Selectivity increases with aging time, as might be expected from a densification process. 

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. 

Figure 26 shows the thermal expansion behavior of requenched epoxies. Upon reaging, the densification process was again measurable. Data shown in Figure 26, therefore, supports the notion that physical aging processes are thermoreversible. 

Methane, the most stable hydrocarbon, and some others, in the absence of oxidants can endure temperatures of 1000°C and higher. With some mantle and lower crustal rock porosity, gas law behavior would require that during planet earth formation and the long gradient-densification process, most of the primordial methane component (which) escaped oxidation, should have diffused upward to be trapped in the sedimentary crustal reservoirs and likewise for ethane and... 

As formed, carbon black is a fluffy powder possessing low density. The densification process involves the removal of occluded air by agitation and followed by dry or wet process pelletization. In both the dry and wet pelletization process, nearly spherical pellets or beads will form that are typically composed of >99% carbon black and trace impurities such as sulfur. Thus, carbon black is sold as a low density powder or as a pelleted form in pigmenting and other end uses. The choice of a fluffy or pelleted carbon black for dispersion in a given system depends upon the dispersion and handling equipment and end use. For example, pelleted carbon blacks are used most frequently in production of black masterbatch carbon black powders are typically used to tint chromatic compounds. 

Rotary tableting machines work with a number of punch and die sets which move in a circle. The dies are fixed in a round die table and the die table circulates. Together with the dies the lower and upper punches circulate on tracks. The lower punches close the dies. The densification process is bilateral since both punches pass the compression wheels and the force is evolving on the upper as well as on the lower side of the powder bed. The produced tablets show the same hardness on the upper and lower surfaces. 

The Heckel equation describes the densification process with first-order kinetics. A linear equation is obtained with a slope which is inversely proportional to the yield strength. The slope of the Heckel equation provides information on the plastic deformation of the powder. It has also been published that the slope of the Heckel equation can be correlated with the elastic modulus (Young s modulus). 

Silica structure normally densifies above 800X. The presence of water, however, may have accelerated the densification process. Silica-containing membranes are not thermally stable if water is present even at 600 C. They are subject to permanent structural densification. This smictural change causes reduction in the membrane permeability. 

FIGURE 12.3 Sketch of the nucleation-densification process (b) involved with the dehy-droxylation of a gel surface (a). (After Reference 27.)... 

Compression force is the major driving force in the powder densification process. The rate and extent of the applied force on the powder bed not only affects the way panicles physically deform but also determines the integrity of the compact formed (crushing strength/tensile strength). 

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