Sintering pressure

As with cold pressing, HIP resolves the problems of density variation inside the ceramic, which is prevalent in hot pressed ceramics. When solid state sintering is performed before hot isostatic pressing, the combined process is called sinter-HIP or post-HIP. 

As discussed in Section 16.3.2.1, pressure alters the driving force 

TABLE 16.12 Hot Isostatic Pressing Conditions for Various Ceramics  

Working of High Performance Ceramics, p. 251. Blackie, London, 1988. 

TABLE 16.13 Surface Stresses and Temperature Differences for Various Siapes 

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During pressure sintering, interiDarticle compressive stress, approximated by the externally applied stress and nonnalized by the relative density of the compact p, supplements the surface tension driving force for pore shrinkage ... 

PTFE mouldings and extrudates may be machined without difficulty. Film may be obtained by peeling from a pressure sintered ring and this may be welded to similar film by heat sealing under light pressure at about 350°C. 

The homopolymer is difficult to fabricate and has been shaped by hammering (like a metal), impact moulding and pressure sintering at 420°C and 35 MPa pressure. The copolymers are somewhat easier to fabricate. The difficulty in fabrication has severely limited the development of these polymers. 

Binder nature particle pressure Sintering Holding vacuum V Relative ... 

Hot pressing (or pressure sintering) is the simultaneous application of pressure and heat to a powder mass enclosed in a die. This technique allows the use of lower temperatures and pressures and shorter processing times than those for conventional sintering and thus permits the production of bodies with finer grain size, lower porosity and higher purity. 

Kressin AM, Doan VV, Klein JD, Sailor MJ (1991) Synthesis of stoichiometric cadmium selenide films via sequential monolayer electrodeposition. Chem Mater 3 1015-1020 Miller B, Heller A, Robbins M, Menezes S, Chang KC, Thomson JJ (1977) Solar conversion efficiency of pressure sintered cadmium selenide liquid junction cells. J. Electrochem Soc 124 1019-1021. 

It has been illustrated that polycrystalline materials can be operated in regenerative electrolytic solar cells yielding substantial fractions of the respectable energy conversion efficiency obtained by using single crystals. Pressure-sintered electrodes of CdSe subsequently doped with Cd vapor have presented solar conversion efficiencies approaching 3/4 of those exhibited by single-crystal CdSe electrodes in alkaline polysulfide PEC [84]. 

The consolidated titanate waste pellets are similar in appearance to their glass counterparts, i.e., both are dense, black and apparently homogeneous. Microscopic analyses, however, reveal important differences between these two waste forms. While little definitive work has been done with glassy waste forms, it is apparent that several readily soluble oxide particulates of various nuclides are simply encapsulated in the glass matrix. The titanate waste form has undergone extensive analyses which includes optical microscopy, x-ray, scanning electron microscopy, microprobe, and transmission electron microscopy (l ) The samples of titanate examined were prepared by pressure sintering and consisted of material from a fully loaded titanate column. Zeolite and silicon additions were also present in the samples. 

Figure 2. Transmission electron photomicrograph of a ceramic titanate waste form. The sample was prepared by pressure sintering a titanate fully loaded with fission waste oxides and includes zeolite and silicon additions.

The waste forms were a fully loaded, pressure sintered titanate waste containing zeolite and silicon additions, a copper boro-silicate glass prepared from Frit 199 supplied by Battelle Northwest Laboratories, and a copper borosUicate glass prepared from the same frit and containing 30% by weight of waste oxides. 

After loading, the materials are dried at lOO C and pressure sintered. Optimum conditions were established as llOO C and 6.9 MPa, as no improvement in the product was achieved by increasing either temperature or pressure. Cylindrical pellets up to 5 cm in diameter were produced and the feasibility of a semi-continuous pressure sintering process was demonstrated. Temperatures could be decreased as far as by the addition of a glass... 

The baseline process, including the pressure sintering step, was demonstrated with both simulated high level waste and under hot cell conditions using a waste solution prepared from typical spent light water reactor fuel. A batch contacting method using sodium titanate was also evaluated, but the overall decontamination factor was much lower than obtained in the column process. 

The use of inorganic ion exchangers to solidify liquid radioactive waste followed by pressure sintering to produce a ceramic waste form appears to be a viable alternative to calcina-tion/vitrification processes. Both the process and waste form are relatively insensitive to changes in the composition of the waste feed. The stability of the ceramic waste form has been shown to be superior to vitrified wastes in leaching studies at elevated temperatures. Further studies on the effects of radiation and associated transmutation and the influence of temperature regimes associated with potential geologic repositories are needed for a more definitive comparison of crystalline and amorphous waste forms. 

Post sintered RBSN Gas pressure Sintering (GPSN)... 

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