Pores, porosity powder compacts

When a hydrostatic pressure pa is applied to the external surface of a powder compact, it can be represented by using a model, as shown schematically in Fig. 5.25a [1]. In this case, the applied pressure exerts a load on the surface of the solid, which is Fa = AxPa, where Aj is the total external cross-sectional area of the solid, including areas pores. Due to presence of porosity at the grain boundaries, the actual grain boundary area Ag is smaller than the total external area. If a force balance is maintained across any plane of the solid, the following expression is valid ... 

The PVS mixture mle is also applied to study the effect of residual porosity on sintering behaviors of the Zr02-TiN composite powders, by incorporating porosity as a third phase with zero conductivity. With this assumption, Eqs. (6.19) and (6.20) are still applicable, by using a reduced volume concentration of the secondary phase, y, due to the presence of the pores. The reduced volume concentration can be related to relative density (D) of the sintering powder compacts, given by V = D — y, where y is the volume fraction of the matrix phase in partially sintered compacts. 

Consolidated structure The consolidated structure (i.e, powder compact) that forms after the liquid is removed from the suspension depends on the state of the dispersion. Well-dispersed suspensions tend to form consolidated structures with a higher particle packing density (lower total porosity) when compared to samples prepared from suspensions with flocs/agglomerates (Figure 10.26) (12). In addition, the samples prepared from well-dispersed suspensions tend to have a smaller average pore size and a narrower pore size distribution. 

Plot the limiting porosity in a powder compact as a function of the grain size (in the range of 0.1 to 100 p.m) when sintering is carried out in an insoluble gas at atmospheric pressure. Assume that the pore size is one third the grain... 

Mercury porosimetry provides a more quantitative method of characterizing the porosity in a particle compact. Bulk density, pore volume, mean pore size, and the pore size distribution of a powder compact can all be determined by mercury porosimetry. ... 

The mercury porosimetry results presented in Figure 5.6 illustrate the influence of particle size and size distribution on the size and size distribution of the porosity in green ceramic filled glass (CFG) composites consisting of 65 vol % of 0.4—1.5-)lm median particle size alumina and 35% borosilicate glass. The porosimetry results reveal that a relatively broad size distribution of pores exist within the green powder compacts and that pore size distribution and mean pore radius decreases with the substitution of fine alumina for the coarse in the CFG composites. The mean equivalent cylindrical pore radius, r, and surface-volume mean equivalent spherical particle radius, of a powder compact consisting of >l- J,m particles are proportional to one another and related by the fractional porosity, e, of the powder compact by... 

Consequently, for the same fractional porosity (i.e., relative density), pore size increases proportionally with increasing particle size, and finer pores are expected in compacts composed of finer particles.Using Equation 5.4, one can approximate the mean pore size in a powder compact solely on the basis of the mean size of the particles in the compact and its relative density. 

This technique has also been used in combination with nitrogen absorption to study the pore structure of some excipients, particularly MCC in both the powdered and compacted state. The intraparticulate porosity of MCC has been shown to be unaffected by tableting the interparticular pores, however, are gradually reduced in size [38]. Recently this method has been used to evaluate the internal structure of tablets prepared from microcapsules [150]. 

Based on the definition of density, two new terms are defined. Porosity is defined as the proportion of a powder bed or compact that is occupied by pores and is a measure of the packing efficiency of a powder and relative density is the ratio of the measured bulk density and the true density ... 

Sintering involving a chemical reaction is typical for the manufacture of some carbides and nitrides. For example, dense SiC is obtained by firing carbon compacts in silicon vapours. A similar example is provided by the manufacture of densified Si3N4 w here the nitride is formed in the powdered metal compact by reaction with gaseous nitrogen the product fills up the pores and thus gives low porosity. 

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