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

DK4027_book.fm Page 305 Monday, May 16, 2005 2:01 PMNano/microstructure and Multiphase Materials 305Polymerized alkoxidesChemical gel from gel-formersColloidal solPhysical gelMIXINGliquidSilanesSilazanes,...Salt powders or solubles salts(a)Al 2 O 34ZrO 2·6SiO 2ZrO 2·4SiO 24ZrO 2·3P 2 O 5ZrO 2·P 2 O 5Na 2 OSiO 2SiO 2Na 2 ONa 2 O·CaO·2Al 2 O 3Na 2 OZrO 2P 2 O 5 SiO 2P 2 O 5(b)ZrO 2·Na 2 OFIGURE 12.1 (a) Schematic flowchart of hybrid routes. (b) Sol-gel forming compositions(alkoxide route) in the SiO 2 -Al 2 O 3 -ZrO 2 -P 2 O 5 , 4ZrO 2·6SiO 2 -Na 2 O-4ZrO 2·3P 2 O 5 , ZrO 2 -Na 2 O-SiO 2 , ZrO 2 -P 2 O 5 , ZrO 2·Na 2 O-SiO 2 , ZrO 2·4SiO 2 -Na 2 O-P 2 O 5 , and SiO 2 -P 2 O 5 -Na 2 O·CaO·2Al 2 O 3 phase diagrams. The representation corresponds to the final oxidecomposition. Solid line indicates the limit of optically clear gel by slow hydrolysis.Dashed line indicates the limit of easy synthesis of (translucent) monolithic gels.(Reprinted from Colomban, P., Gel technology in ceramics, glass-ceramics and ceramicceramiccomposites, Ceram. Int., 15, 23, 1989. With permission from Elsevier.)II. PRECURSORS AND ANALYSIS METHODSPrecursors must have different properties: 4–14 (1) a high content of the finalelements (mostly aluminum, silicon, zirconium, titanium, phosphorus), (2) a lowcontent of health hazardous elements and elements that corrode the equipment(e.g., chlorine, sulfur), (3) a viscosity adapted to the process: low viscosity forpreform infiltration, medium viscosity for spinning and coating, (4) a controlledprecursor-ceramic transformation (bubbling is researched for foams but not fordense parts), (5) the ability to be mixed with other precursors or to be processed(“good” hydrolysis rate), and (6) low cost.Regarding sol-gel routes, the most used reagents are those made with gelformercompositions: aluminium-s-butoxide [Al(OC 4 H 9 ) 3 ], tetraethoxysilane[Si(OC 2 H 5 ) 4 ] (and homologous methoxysilane), aluminum-silicon ester