SINTERING OF DENSE CERAMICS

Sintering of un-calcined powders produces lower shrinkage upon heating. The high surface area of the submicron crystallites leads to a lower fluidity and powder compaction is not very effective, but densification is greater on heating (Juang and Hon 1996, Landi et al. 2000). Densification occurs at the nanoscale between the individual crystallites within the particles and between the individual particles. 

A partial vapor pressure of water is needed to retain the structural OH at temperatures greater than 900°C (Riboud 1973) however this has also been noted to decrease the densification rate. Dehydroxylation in HAp produces an oxy-hydroxylapatite (Kijima and Tsutsumi 1979). The inclusion of peroxide ions (Zhao et al. 2000) is expected to decrease the diffusion and, hence, also slow densification at high temperatures. 

Stoichiometry, described by the Ca/P molar ratio, has a major influence on the physical properties of the apatite and, hence, its application. For calcium-deficient hydroxylapatite, a decrease in surface area starting at 700°C leads to a lower shrinkage (8% less for a powder with Ca/P of 1.64 relative to stoichiometric HAp at 1200°C) (Raynaud et al. 2002b). Low shrinkage in the final sintered product is usually accom- 

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Tartaj J., Moure C, Lascano L., Duran P. Sintering of dense ceramics bodies of pure lead titanate obtained by seeding-assisted chemical sol-gel. Mater. Res. Bnll. 2001 36 2301—2310 Traversa E., Di Vona M.L., Licoccia S., Sacerdoti M., Carotta M.C., Gallana M., Martinelli G. J. Sol-Gel Sci. Technol. 2000 19 193-196... 

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