Powder compaction compact density

Shock-synthesis experiments were carried out over a range of peak shock pressures and a range of mean-bulk temperatures. The shock conditions are summarized in Fig. 8.1, in which a marker is indicated at each pressure-temperature pair at which an experiment has been conducted with the Sandia shock-recovery system. In each case the driving explosive is indicated, as the initial incident pressure depends upon explosive. It should be observed that pressures were varied from 7.5 to 27 GPa with the use of different fixtures and different driving explosives. Mean-bulk temperatures were varied from 50 to 700 °C with the use of powder compact densities of from 35% to 65% of solid density. In furnace-synthesis experiments, reaction is incipient at about 550 °C. The melt temperatures of zinc oxide and hematite are >1800 and 1.565 °C, respectively. Under high pressure conditions, it is expected that the melt temperatures will substantially Increase. Thus, the shock conditions are not expected to result in reactant melting phenomena, but overlap the furnace synthesis conditions. 

Figure 7.22 Nonuniformities in the powder compact density will lead to uneven shrinkage during sintering and distortion of pressed shape.

Figure 9.35 Stereographic pair of a replica of the pore space in a partially densified ZnO powder compact (density 73% of the theoretical). (Courtesy of M-Y. Chu.)...

Miscellaneous Processes. Metal strip for cladding can be produced by cold pressing metal powder into alow density green strip, foUowed by sintering to compact the powder. AHoy powders can be made into strip, along with specialized strip with one powder bonded to a different powder on the opposite side. 

Liquid-Ph se Sintering. Sintering ia the Hquid state refers to the sintering of a powder mixture of two or more components, of which at least one has a melting temperature lower than the others. The sintering temperature is then selected ia such a manner that a Hquid phase is formed ia which the soHd powder particles of the other components rearrange. A high density powder compact is the result. 

Both zirconium hydride and zirconium metal powders compact to fairly high densities at conventional pressures. During sintering the zirconium hydride decomposes and at the temperature of decomposition, zirconium particles start to bond. Sintered zirconium is ductile and can be worked without difficulty. Pure zirconium is seldom used in reactor engineering, but the powder is used in conjunction with uranium powder to form uranium—zirconium aUoys by soHd-state diffusion. These aUoys are important in reactor design because they change less under irradiation and are more resistant to corrosion. 

To further characterize the event it is first necessary to identify critical features of the initial configuration that will strongly influence the process. For powder compacts, the most obvious features are the morphological characteristics of the powders, their microstructures, and the porosity of the compact. For solid density samples, the grain structure, grain boundaries, defect level, impurities, and inclusions are critical features. 

Fig. 6.4. At relatively low pressures the shock speeds observed for stress waves in low density powder compacts are dominated by the crush-up of the powder toward solid density. The figure shows measured wavespeeds for a range of densities and fits to the data based on Herrmann s P-a model on Fe. Note the unusually low wavespeeds compared to solid density (after Herrmann [69H02]).

In addition to these micromechanical considerations, low pressure shock compression of porous powder compacts has distinctive features not encountered in low pressure solid density samples. Basically, the sample is dominated by the pores, and the wavespeed at pressures less than those required to crush the sample to solid density is unusually low and is little dependent on the properties of the solid. 

The author s work has included the development of the Sandia Bear and Bertha explosive recovery fixtures, that provide a standardized set of fixtures in which recovery experiments can be routinely carried out at peak shock pressures from 4 to 500 GPa. Shock-induced, mean-bulk temperatures from 50 to 1200°C are achieved with variation in the density of the powder compacts under study. 

It should be observed that every element except the powder system in the recovery system is chosen for favorable shock properties which can be confidently simulated numerically. The precise sample assembly procedures assure that the conditions calculated in the numerical simulations are actually achieved in the experiments. The influence of various powder compacts in influencing the shock pressure and mean-bulk temperature must be determined in computer experiments in which various material descriptions are used. Fortunately, the large porosity (densities from 35% to 75% of solid density) leads to a great simplification in that the various porous samples respond in the same manner due to the radial loading introduced from the porous inclusion in the copper capsule. 

For the two explosive loading systems used, the initial pressure wave into the powder is relatively low, varying from perhaps 1.5-4 GPa. In such cases the most relevant compression characteristic of the powder compact is its crush strength , i.e., the pressure required to compress the porous compact to solid density. In the simulations, this strength can be varied over a wide range with the P-a model. The wavespeed of the initial waves was modeled on the basis of shock-compression data on rutile at densities from 44% to 61% of solid density [74T02]. 

Fig. 6.7. The predicted, one-dimensional, mean-bulk temperatures versus location at various times are shown for a typical powder compact subjected to the same loading as in Fig. 6.5. It should be observed that the early, low pressure causes the largest increase in temperature due to the crush-up of the powder to densities approaching solid density. The "spike in the temperature shown on the profiles at the interfaces of the powder and copper is an artifact due to numerical instabilities (after Graham [87G03]).

Mean-bulk temperatures are representative of a powder compact at a density of 55% of solid density. 

A. Gupta, G.E. Peck, R.W. Miller and K.R. Morris, Real-time near-infrared monitoring of content uniformity, moisture content, compact density, tensile strength, and Young s modulus of roller compacted powder blends, J. Pharm. Sci., 94(7), 1589-1597 (2005). 

The density-pressure relationship for powder compaction at room temperature typically increases from the apparent density at zero pressnre to values that approach the theoretical density at high pressures, as illustrated in Figure 7.16. A compact with 100% theoretical density would indicate that it contains no porosity. Soft powders are more easily densified than hard powders at a given pressnre, and irregularly shaped powders have lower densities than spherical powders in the low-pressure regime. 

The conclusions drawn from this trial indicated vacuum deaeration, employed in the described equipment design, increased the compaction rate, reduced the fines (non-compacted powder), and increased the compact density (R.W. Miller, unpublished notes, June 1996). 

Powders are porous materials and their bulk and relative densities can change with consolidation (6). However, a powder s true density is the density of its solid phase only and thus is independent of the state of consolidation. The true density of organic excipients typically ranges from 1.0 to 1.6g/cm3 while inorganic excipients (e.g., calcium phosphate) show values greater than 2g/cm3. True density is used to determine powder or compact solid fraction (SF) (see below) and it may be a consideration when selecting excipients if segregation is a concern. True density is often determined by gas pycnometry. 

Figure 7.21 Density variations in single- and double-action pressed powder compacts. Double-action pressing gives a denser and more uniform powder compact. (From Thompson, 1981.)...

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 ... 

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