Sol-gel matrices of BAs, NAS and CAs 741 (anorthite, CAS); NaAISi3O(albite, NAS); and a (350C)and NaCl (950C) for various times between mixed CNAS composition (7CAS+ 3NAS). Gel 6 h powders were prepared by instant hydrolysis of a mixture of alkoxides diluted in 2-propanol(volume 3 RESULTS AND DISCUSSION ratio 1: 20) by an aqueous solution and dispersion of alkali and alkaline earth reagents. Silicon and 3.1 Densification and mechanical properties aluminium were introduced by using TEOS and Figure 1 compares the plots of shrinkage versus aluminium-s-butoxide, respectively. Sodium is intro- temperatur of the different matrices: an initial duced by using NaOH solution (and lithium doping shrinkage occurs below 300C and is associated with from LiNO, solution) whereas barium and calcium orous glass (or xerogel) were introduced by using a milky dispersion of Cao transformation. The main shrinkage takes place and Bao powder in CO2-free water. A large excess of the dehydroxylation-(nucleation)-densification water (volume ratio of water:alkoxide= 20: 1) was tion at about 650C (NAS),800C(CNAS), used, and there was vigorous mixing during and after(BAS5 %Li and CAS)and 900C(BAS). A strong the hydrolysis. Alternatively, the use of alkaline earth viscous flow is observed for the low refractory NAs nitrates or salicylates dissolved in the alkoxide- and CNAs compositions. The lowering of the propanol mixture leads to gels exhibiting shrinkage densification temperature arising from the use of behaviour(e. g. expansion after a first shrinkage step, sol-gel precursor is thus not significant for NAS and related to the dehydroxylation-nucleation reaction) CNAS compositions. On the other hand, CAS and which are not compatible with a good densification. BAS fired compositions which are geometrically stable The addition of small amounts of lithium is however up to about 1500C seem to be, a priori, the most possible by the use of a three-liquid mixture(alcoholic suitable matrices for the preparation of low solution, base aqueous solution, lithium nitrate temperature-sinterable refractory composites aqueous solution) The highest mechanical strength was measured for leads to a gel dispersion in water. The BAS-matrix composites. The ultimate three-point water and alcohol were removed by drying under Ir flexural strength reaches 250 MPa for composites bulbs. We obtained a fine white, submicronic, exhibiting open porosity close to 5%(Table 1). This amorphous gel powder which was fired for 2 h at value is higher than the mechanical strength of 700C in air. The resulting powder remained amorphous and mesoporous. By heating above hexagonal celsian(BAS), monoclinic celsian (BASS %Li, BAS10%Li), anorthite (CAS, CNAS)and glass(NAS)were obtained, respectively 2.3 Techniques The shrinkage measurements were made by using an BAS Adamel Lhormergy DI24 apparatus(Instrument SA) with alumina rod and support. Curves were drawn in BAS coLi air at heating rate of 5C/min. Fracture surfaces and sliced or polished sections of the composites were observed in a 200 kV Cambridge scanning electron microscope. Flexural strengths were measured by CAS three-point bending tests on bar specimens(35 mm in length)at a cross-head speed of 0.1 mm/min at room temperature. Typically, three samples were broken for each composite and the mean value is given. X-ray infraction powder patterns were recorded on CNAS powdered samples. Micro-Raman spectra wer orded at the 514. 5 nm exciting wavelength of an Ar laser with an XY Dilor multichannel microprobe NAS equipped with a liquid nitrogen cooled Wright CCD (1 300)array measured by the Archimedean water impregnation thod 1. Linear of BaAl2Si2O8 (BAS), Chemical attacks were performed on 0.,o. 0-05 S5%Li), CaAl Si2O8(CAS) sections of composites immersed in boiling (CNAS) and NaAlSi,O (NAS) gels versus are(at room temperature, gels trated sulphuric acid (340C) or in molten contain about 40 wt% water)Sol-gel matrices of BAS, NAS and CAS 741 (anorthite, CAS); NaAlS&Oa (albite, NAS); and a mixed CNAS composition (7CAS + 3NAS). Gel powders were prepared by instant hydrolysis of a mixture of alkoxides diluted in 2-propanol (volume ratio 1:20) by an aqueous solution and dispersion of alkali and alkaline earth reagents. Silicon and aluminium were introduced by using TEOS and aluminium-s-butoxide, respectively. Sodium is intro￾duced by using NaOH solution (and lithium doping from LiN03 solution) whereas barium and calcium were introduced by using a milky dispersion of CaO and BaO powder in COz-free water. A large excess of water (volume ratio of water:alkoxide = 2O:l) was used, and there was vigorous mixing during and after the hydrolysis. Alternatively, the use of alkaline earth nitrates or salicylates dissolved in the alkoxide￾propanol mixture leads to gels exhibiting shrinkage behaviour (e.g. expansion after a first shrinkage step, related to the dehydroxylation-nucleation reaction) which are not compatible with a good densification. The addition of small amounts of lithium is however possible by the use of a three-liquid mixture (alcoholic solution, base aqueous solution, lithium nitrate aqueous solution). The process leads to a gel dispersion in water. The water and alcohol were removed by drying under IR bulbs. We obtained a fine white, submicronic, amorphous gel powder which was fired for 2 h at 700°C in air. The resulting powder remained amorphous and mesoporous. By heating above lOOO”C, in air, hexagonal celsian (BAS), monoclinic celsian (BASS%Li, BASlO%Li), anorthite (CAS, CNAS) and glass (NAS) were obtained, respectively. 2.3 Techniques The shrinkage measurements were made by using an Adamel Lhormergy D124 apparatus (Instrument SA) with alumina rod and support. Curves were drawn in air at heating rate of S”C/min. Fracture surfaces and sliced or polished sections of the composites were observed in a 200 kV Cambridge scanning electron microscope. Flexural strengths were measured by three-point bending tests on bar specimens (35 mm in length) at a cross-head speed of O-1 mm/min at room temperature. Typically, three samples were broken for each composite and the mean value is given. X-ray diffraction powder patterns were recorded on powdered samples. Micro-Raman spectra were re￾corded at the 514.5 nm exciting wavelength of an Ar+ laser with an XY Dilor multichannel microprobe equipped with a liquid nitrogen cooled Wright CCD (1200-300) array detector. The open porosity was measured by the Archimedean water impregnation method. Chemical attacks were performed on polished sections of composites immersed in boiling concen￾trated sulphuric acid (340°C) or in molten NaNO, (350°C) and NaCl (950°C) for various times between 15 min and 6 h. 3 RESULTS AND DISCUSSION 3.1 Densikation and mechanical properties Figure 1 compares the plots of shrinkage versus temperature of the different matrices: an initial shrinkage occurs below 300°C and is associated with the (aqua) gel to mesoporous glass (or xerogel) transformation. The main shrinkage takes place with the dehydroxylation-(nucleation)-densification reac￾tion at about 650°C (NAS), 800°C (CNAS), 850°C (BASS%Li and CAS) and 900°C (BAS). A strong viscous flow is observed for the low refractory NAS and CNAS compositions. The lowering of the densification temperature arising from the use of sol-gel precursor is thus not significant for NAS and CNAS compositions. On the other hand, CAS and BAS fired compositions which are geometrically stable up to about 1500°C seem to be, a priori, the most suitable matrices for the preparation of low￾temperature-sinterable refractory composites. The highest mechanical strength was measured for BAS-matrix composites. The ultimate three-point flexural strength reaches 250MPa for composites exhibiting open porosity close to 5% (Table 1). This value is higher than the mechanical strength of I I I I I%* 1. Linear shrinkage of BaA1,Si,OB (BAS), 0.95BaAl,Si,O, O.OSLi,O (BASS%Li), CaAl&O, (CAS), i 0.7CaAl,OSi,O, 0*3NaAlSi,O, (CNAS) and NaAlSi,O, (NAS) gels versus temperature (at room temperature, gels contain about 40 wt% water)