268 C -H. Chao, H.Y. Lu/Materials Science and Engineering 4328(2002)267-276 shifted towards higher 20-angles. Second, the transfor- hot zone. The estimated cooling rate(of air-cooling) mation of B-cristobalite to a may be hindered mechani- was x 110C min- for pressureless-sintering in a cally by matrix constraint when the volume change conventional box furnace equipped with Sic heating accompanied with the phase transformation is sup- elements pressed. Indeed, tensile stresses generated across the Crystalline phases were analysed in the 20 range of interface of glass and B-cristobalite during cooling was 20-80 using X-ray diffractometry(XRD)(Siemens thought [13] to have triggered off the transformation of D5000, Karlsruhle, Germany) operating at 40 kV and the metastable B-cristobalite to o in crystallized silica. 30 mA using CuKo radiation with Ni filter; a scanning We have investigated the crystalline phases in the speed of 0.6 20 min was adopted. A scanning speed intered samples prepared from colloidal gel-derived of the 20-angle at 0.005- and a time constant of 10 SiO2 powders co-doped with Na2O and Al2O3. The s were always ensured for step-scanning. Reflections sintered samples re-ground to fine powders are found to between the 20 angles of 20.5-23.5 for a-and B-cristo- have induced the phase transformation of metastable balite and those of 49-51 for a-quartz were used for B-cristobalite to a-phase. Besides the chemical stabiliza- both the crystalline phase identification and quantita- tion of B-cristobalite, the likelihood of particle size tive analysis. The relative content of a-to B-cristobalite effects on the B-0-cristobalite transformation is also in sintered discs was determined by measuring the peak discussed areas of (lll and(101)x using a calibrated curve established by a modified external standard method [4] 2. Experimental procedures Surface morphology and polished sections of the sintered samples were observed using a scanning elec Chemically stabilized B-cristobalite was synthesized tron microscope (SEM) (JEM6400, JEOL, Tokyo, in the Na2O-AlO3-SiO2 system adopting the powder Japan) operating at 20 kV. Sintered samples were compositions reported by Perrotta et al. [9]. The solute ground and polished mechanically with Sic papers cations of Al+ and Na+ were added in the equal successively before diamond lapping to I um surface concentration I molar ratio. The Na2O-doped roughness. Polished sections were then chemicall colloidal silica sols of Ludox HS-30 containing 2 30 etched using HF-HNO3 solution to delineate grain wt% of silica particles were supplied by E I du Pont de boundaries and other microstructural features Nemours(Wellington, DE). Appropriate quantities of Al, (SO4)3.13H,o and Na, CO,(reagent grade, Katayama, Osaka, Japan) dissolved in de-ionized water 3. Results was blended in the colloidal silica sols according to the oppositions of (Na,0+Al,O3)-ySiO2 given in 3. 1. Identification of crystalline phases Table 1. the mixed solution contained in a beaker was dried in an oven at 120 oC for 12 h to a cake of The(Na2O+Al,O3)-ySiO2 powders of four compo- amorphous powder. It was then deagglomerated using sitions with y= 13.9, 19.5, 24.6 and 32. 4 were sintered an agate mortar and pestle before passing through a and determined for their crystalline phases. The 177 um sieve. The ground and sieved powder was Na,O+Al2O3 contents of the initial compositions are lry-pressed at x 50 MPa in a wC-inserted steel die listed in Table I giving the dopant concentrations in assembly to discs of 10 mm()x 1.5 mm. The green both mol% and wt % compacts accommodated in an alumina boat were pres sureless-sintered at 1100C before cooling to room 3. 1. Composition with y= 19.5 temperature by rapid withdrawal of the boat from the As-sintered sample surface and sintered samples re- round to powder have both been analyzed. Fig. 1(a) Table I presents the representative XRD trace of the as-sintered Na,O-AL,O, contents of the initial colloidal silica compositions surface of the y= 19.5 samples, which have been fired at 1100C for 48 h. The reflection peaks in the le range AL,O -,, of 20= 17-37 are designated to C-l at 20= 21.8, C-2 13.9 at27.3°,C-3at31.2°andC-4at35.8°. The lattice spacings calculated from C-2 at 20=27.3 and C-3 at 3.76 4.65 6.29 31.20 agree very well with those of (lll)a and (102)B 4.65 6.29 respectively, as compared with those given by JCPDS 2 mol% 87.42 Al-O, wt% 39-1425 for a-cristobalite. These two reflections repre- 64 sent characteristically that the sintered mixture consist SiO, wt 87.73 83.59 predominantly of a-cristobalite. They may also contain other silica polymorphs but only of negligible quantities268 C.-H. Chao, H.-Y. Lu / Materials Science and Engineering A328 (2002) 267–276 shifted towards higher 2
-angles. Second, the transfor￾mation of
-cristobalite to may be hindered mechani￾cally by matrix constraint when the volume change accompanied with the phase transformation is sup￾pressed. Indeed, tensile stresses generated across the interface of glass and
-cristobalite during cooling was thought [13] to have triggered off the transformation of the metastable
-cristobalite to in crystallized silica. We have investigated the crystalline phases in the sintered samples prepared from colloidal gel-derived SiO2 powders co-doped with Na2O and Al2O3. The sintered samples re-ground to fine powders are found to have induced the phase transformation of metastable
-cristobalite to -phase. Besides the chemical stabiliza￾tion of
-cristobalite, the likelihood of particle size effects on the

-cristobalite transformation is also discussed. 2. Experimental procedures Chemically stabilized
-cristobalite was synthesized in the Na2O–Al2O3 –SiO2 system adopting the powder compositions reported by Perrotta et al. [9]. The solute cations of Al3+ and Na+ were added in the equal concentration of 1:1 molar ratio. The Na2O-doped colloidal silica sols of Ludox® HS-30 containing 30 wt.% of silica particles were supplied by E.I. du Pont de Nemours (Wellington, DE). Appropriate quantities of Al2(SO4)3 · 13H2O and Na2CO3 (reagent grade, Katayama, Osaka, Japan) dissolved in de-ionized water was blended in the colloidal silica sols according to the compositions of (Na2O+Al2O3)−ySiO2 given in Table 1. The mixed solution contained in a beaker was dried in an oven at 120 °C for 12 h to a cake of amorphous powder. It was then deagglomerated using an agate mortar and pestle before passing through a 177 m sieve. The ground and sieved powder was dry-pressed at 50 MPa in a WC-inserted steel die assembly to discs of 10 mm()×1.5 mm. The green compacts accommodated in an alumina boat were pres￾sureless-sintered at 1100 °C before cooling to room temperature by rapid withdrawal of the boat from the hot zone. The estimated cooling rate (of air-cooling) was 110 °C min−1 for pressureless-sintering in a conventional box furnace equipped with SiC heating elements. Crystalline phases were analysed in the 2
range of 20–80° using X-ray diffractometry (XRD) (Siemens D5000, Karlsruhle, Germany) operating at 40 kV and 30 mA using CuK radiation with Ni filter; a scanning speed of 0.6° 2
min−1 was adopted. A scanning speed of the 2
-angle at 0.005° s−1 and a time constant of 10 s were always ensured for step-scanning. Reflections between the 2
angles of 20.5–23.5° for - and
-cristo￾balite and those of 49–51° for -quartz were used for both the crystalline phase identification and quantita￾tive analysis. The relative content of - to
-cristobalite in sintered discs was determined by measuring the peak areas of (111)
and (101) using a calibrated curve established by a modified external standard method [14]. Surface morphology and polished sections of the sintered samples were observed using a scanning elec￾tron microscope (SEM) (JEM6400, JEOL, Tokyo, Japan) operating at 20 kV. Sintered samples were ground and polished mechanically with SiC papers successively before diamond lapping to 1 m surface roughness. Polished sections were then chemically etched using HF-HNO3 solution to delineate grain￾boundaries and other microstructural features. 3. Results 3.1. Identification of crystalline phases The (Na2O+Al2O3)–ySiO2 powders of four compo￾sitions with y=13.9, 19.5, 24.6 and 32.4 were sintered and determined for their crystalline phases. The Na2O+Al2O3 contents of the initial compositions are listed in Table 1 giving the dopant concentrations in both mol% and wt.%. 3.1.1. Composition with y=19.5 As-sintered sample surface and sintered samples re￾ground to powder have both been analyzed. Fig. 1(a) presents the representative XRD trace of the as-sintered surface of the y=19.5 samples, which have been fired at 1100 °C for 48 h. The reflection peaks in the range of 2
=17–37° are designated to C-1 at 2
=21.8°, C-2 at 27.3°, C-3 at 31.2° and C-4 at 35.8°. The lattice spacings calculated from C-2 at 2
=27.3° and C-3 at 31.2° agree very well with those of (111) and (102)
respectively, as compared with those given by JCPDS 39-1425 for -cristobalite. These two reflections repre￾sent characteristically that the sintered mixture consist predominantly of -cristobalite. They may also contain other silica polymorphs but only of negligible quantities Table 1 Na2O–Al2O3 contents of the initial colloidal silica compositions Al2O3–Na2O–ySiO2 y 32.4 24.6 19.5 13.9 Al2O3 mol% 3.76 6.29 2.91 4.65 Na2O mol% 6.29 2.91 3.76 4.65 SiO2 mol% 94.19 92.48 90.70 87.42 Al2O3 wt.% 4.83 6.21 7.63 10.21 Na2O wt.% 6.20 2.94 4.64 3.77 SiO2 wt.% 92.23 90.02 83.59 87.73