Isostatic pressure densification

Gel synthesis has often been described as a process ofparticle and cluster-cluster aggregation (Brinker, 1990). Indeed this model leads to structures similar to those observed. Beurroies et al. describe the gel densification phenomenon imder isostatic pressure as a conpaction and interpenetration of clusters, which can be considered as fractal objects (Beurroies et al., 1998). This model explains the volume variation as a fimction of applied pressure and the decrease of the cluster center to center distance observed by SAXS. 

The densification of aerogels under isostatic mercury pressure can be totally irreversible after pressurization-depressurization, as is the case for silica-zirconia aerogels (Pirard, 1995). But most of the time there is a partial reversibihty, which can be relatively important if the sample is maintained at moderate pressure for a short time (Alaoui, 1998). Anyway, the sample volume is smaller after densification, even after a possible relaxation. Therefore, it is possible to study the effect of isostatic pressure on aerogels by characterizing the texture of materials first densified to a known value of isostatic pressure. 

When submitted to mercury porosimetry, the structure ofthese materials is compressed under isostatic pressure, before mercury can penetrate by intmsion in the largest pores. The curves obtained show a progressive volume reduction as a function ofpressure and this volume reduction is due to material densification and not to mercury intmsion inside the pore volume. Consequently, Washburn s equation (11-1) should not be used to analyze the... 

DENSIFICATION BY ISOSTATIC PRESSURE Structure and Properties Evolution... 

Figure 5-12. Room temperature densification of a base catalysed silica aerogel as a function of isostatic pressure levels.

The die-and-punch method of compact imparts an anisotropic pressure to the compact. It is possible to apply pressure equally in all directions using a process called isostatic pressing, in which the powder is compacted with a fluid, such as hydraulic fluid, water, or an inert gas. Isostatic pressing is carried out using formed, flexible molds, called cans, that contain the powder and allow the pressure from the fluid to be transferred to the powder. The flexible molds are then stripped off the compact after densification. Isostatic pressing eliminates the die-wall friction inherent in die compaction and allows for consolidation of large components. However, isostatic pressing... 

Densification method Low gas pressure sintering Gas pressure sintering Hot pressing Hot isostatic pressing (HIP) Reaction bonding Reaction bonding and postsintering... 

A new type of liquid-phase sintered SiC using yttria [1314-36-9J, Y20, as the oxide additive and submicrometer SiC powder for enhanced densification, produces a material which can be densified without the application of pressure (13). This material, sintered from cold isostatically pressed billets, appears to be comparable to silicon nitride in strength and fracture toughness. 

A promising method for densification of combustion-synthesized products is a combination of SHS with hot isostatic pressing (HIP). This idea was first applied to the synthesis of TiB2 ceramics under a pressure of 3 GPa, which was provided by a cubic anvil press, resulting in 95% dense material (Miyamoto et al, 1984 Yamada et al., 1987). The relatively low exothermic reaction of SiC from elemental powders was also carried out under these conditions (Yamada et al, 1985), and 96% conversion to 8-SiC was achieved as compared to 36% conversion when the reaction was initiated locally. 

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