12 Nano/microstructure - institut de chimie et des materiaux paris-est

DK4027_book.fm Page 311 Monday, May 16, 2005 2:01 PMNano/microstructure and Multiphase Materials 311content) and the sintering temperature. The nanostructure and the porosity (andhence the densification and many other properties) can therefore be tailored bythe chemical parameters controlling the inorganic polymerization and thehydroxyl content: relative proportion of reagents, additions of complexing liquids,temperature, sonification, sintering atmosphere, and heating rate. 7,25,28,30–33 Theseremarks are general: for instance, the chain length of SiC (or C) polymericprecursor determines the silicon/carbon and carbon/hydrogen stoichiometry,because terminal branches contain more carbon and hydrogen.Like many hydrates, gels present a unique challenge when densified underpressure at room temperature. 34–37 Due to the liquid (water generally) present atthe boundary, viscous rearrangement under pressure allows densification andoptimization of the contact between polymeric grains, approaching the “perfect”packing obtained by sedimentation: true sintering can be achieved under thecombined action of the pressure and the liquid phase soaked in the pores (watergenerally, but also alcohol). Gel sintering is associated with an increase in opticalclarity. 36 On heating, the grains formed retain a memory of the packing of primarymoieties. The grain growth does not destroy this arrangement. Figure 12.4b showsgel pellets of Nasicon solid solutions prepared by different sol-gel routes. Translucentor even optically clear samples are easily obtained. The plasticity of thegel offers the possibility of facilitating the compaction of any kind of particle,especially those with an elongated shape (whiskers, platelets), by coating eachparticle with a gel film. 37 The gel coating lubricates the particle and promoteshomogeneity of the applied pressure. Plasticity and shapability disappear withthe loss of the water adsorbed at the “grain” surface, above ~200°C. Dependingon the composition, the shrinkage rate then decreases up to 500 (zirconia) to1000°C (aluminosilicates), where drastic shrinkage takes place. The latter resultsfrom the backbone rearrangement due to the departure of the last stabilizing OHgroups (Figure 12.2 and Figure 12.3). Note that because of the ability of the gelto be deformed, a memory effect very similar to the well-known paste memoryof kaolin-rich porcelain paste can be observed. An example is shown inFigure 12.5, with a compacted pellet of zirconia gel fired at 1200°C.Gels transform more or less gradually into a meso/microporous glass. 24–26 Alogarithmic plot of the weight loss vs. the inverse of the temperature allows theonset determination of water and hydroxyl departures, and hence the measurementFIGURE 12.5 Self-wrapped zirconia pellet after firing at 1200°C (diameter ~10 mm).