Sol-gel control of the matrix net-shape sintering Among commercially available reagents, we have at 150oC leads to gels with the following typical selected titanium butoxide (Ti(OCHs)4), aluminium formula: TiO,5 (OH).5H,O, ZrO198(OHDo04 4H2O S-butoxide(Al(oC4H53, TEOS (Si(OC, Hs)4), alu- and Al,O2(OH)0-6.5H2O minium-silicon ester (OC4H5)x-Al-O-Si(oC3 H,)3) and zirconium i-propoxide(zr(oC3 H,)4); the high 3.2 Alkoxide infiltration and pyrolysis melting temperature of the corresponding oxides The infiltration has been detailed in our patent may exclude a strong reaction with alumina below Samples(i.e. the strengthened powder compacts, 1400C. Furthermore, their viscosity can be adjusted stage 2 of Fig. 1) are carefully dried at 200oC, at about 0.7 poise by heating between 40 and then immersed in liquid alkoxides(under atmo 80C. 0 Hydrolysis-polycondensation and drying spheric pressure or I MPa)for 30 min with the help of ult immersed in boiling water at 120C for 3 h(stage 4) then finally dried at -120C and pyrolysed at various temperatures from 1000 to 1400C (stage 2). Typically 2-3% of the porosity is filled by oxide after one cycle. 2 Figure 4 shows an example of the deposition on the fibre, which was obtained from the alkoxide hydrolysis-polycon densation inside the composite. The small agglom erates(mean diameter so.5 um) after pyrolysis are visible. Analysis of the zirconium concentration across the monolith thickness after four cycles of infiltration and pyrolysis reveals a higher concen tration zone in the vicinity of the monolith surface. This gives a laminar behaviour to the monolith(Fig. 5). The location of high zirconia P 4. Zirconia deposit at surface of a C fibre( diameter -g content regions after infiltration seems to be posite after eight infiltration cycles or -0ooc heat-treated com- )observed in the fracture of a 120 related to the pore heterogeneity resulting fr rom slip-casting Fig. 5. Back-scattered electron micrograph of an alumina monolith after four cycles of zirconium i-propoxide infiltration-hydrolysis firing and final heat treatment at 1400 C in air(a, bar: 10 mm). Detail at larger magnitude is given in(b, bar: 20 um)and(c 600 nm). a SEM micrograph of the frontier between the dense core and the porous contour is given in(d, bar: 4oAn'bar *This reagent must be handled in a glove box(s10 pp. m. H2O)and careful drying of the porous materials to be infiltrated is requiredSol-gel control of the matrix net-shape sintering Among commercially available reagents, we have selected titanium butoxide (Ti(OC4H5)J, aluminium s-butoxide (Al(OC,H& *, TEOS (Si(OC,H,),), alu￾minium-silicon ester ((OC,+H,)2-Al-O-Si(OC3H5)3) and zirconium i-propoxide (Zr(OC,H,),); the high melting temperature of the corresponding oxides may exclude a strong reaction with alumina below 1400°C. Furthermore, their viscosity can be adjusted at about 0.7 poise by heating between 40 and 80”C.‘” Hydrolysis-polycondensation and drying Fig. 4. Zirconia deposit at surface of a C fibre (diameter -9 pm) observed in the fracture of a 1200°C heat-treated com￾posite after eight infiltration cycles of zirconium i-propoxide. at 150°C leads to gels with formula: TiO,.,(OH).SH,O, and Al,0,.,(OH)o.,.5H20.17~18 1479 the following typical Z~,.,8(OH)o.,.4H20 3.2 Alkoxide infiltration and pyrolysis The infiltration has been detailed in our patent.” Samples (i.e. the strengthened powder compacts, stage 2 of Fig. 1) are carefully dried at 2OO”C, then immersed in liquid alkoxides (under atmo￾spheric pressure or 1 MPa) for 30 min with the help of ultrasonic stirring (stage 3); next they are immersed in boiling water at 120°C for 3 h (stage 4) then finally dried at -120°C and pyrolysed at various temperatures from 1000 to 1400°C (stage 2). Typically 2-3% of the porosity is filled by oxide after one cycle.‘* Figure 4 shows an example of the deposition on the fibre, which was obtained from the alkoxide hydrolysis-polycon￾densation inside the composite. The small agglom￾erates (mean diameter 10.5 pm) after pyrolysis are visible. Analysis of the zirconium concentration across the monolith thickness after four cycles of infiltration and pyrolysis reveals a higher concen￾tration zone in the vicinity of the monolith surface. This gives a laminar behaviour to the monolith (Fig. 5). The location of high zirconia content regions after infiltration seems to be related to the pore heterogeneity resulting from slip-casting. Fig. 5. Back-scattered electron mbcrograph of an alumina monolith after four cycles of zirconium i-propoxide infiltration-hydrolysis firing and final heat treatment at 1400°C in air (a, bar: 10 mm). Detail at larger magnitude is given in (b, bar: 20 pm) and (c, bar: 500 nm). A SEM micrograph of the frontier between the dense core and the porous contour is given in (d, bar: 200 pm). *This reagent must be handled in a glove box (510 p.p.m. H,O) and careful drying of the porous materials to be infiltrated is required