C -H. Chao, H.Y. Lu/Materials Science and Engineering 4328(2002)267-276 diola of a-cristobalite increases monotonically with presented in Fig. 5(b)(for feature-l in Fig 5(a). Some higher dopant levels. of these particles have coalesced to form doublets and The peak shift of (101)a from 20=21.9(of y= 19.5) triplets. Second, extensive microcracks, feature-2, can to 20= 220(of y= 32. 4)for the three samples is also not be differentiated for its trans- or inter-granular an indication of different solid solubilities (of both nature with certainty, since the matrix has been disinte- AP+ and Na*), as shown in Fig. 4(a). The lattice grated after grinding and polishing and grains are no spacings exhibit an increasing trend from the lower longer discernible from Fig. 5(b). Third, the residual (Al,O3+ Na2O)-doping levels of y= 32. 4 to y= 19.5 of glassy phase contains two distinctive types of crystalline higher doping levels. It suggests that the higher con- particles, dendritic and spherical in sha The SeM tents of Al,O3 and Na,O in the initial silica sols pro- backscattered electron image(BED) provides a distin duce sintered samples containing more a-cristobalite guishable contrast. It represents the nucleation at th with the larger dyol. Nevertheless, dug of B-cristobalite early stage as shown by higher magnification in F remains unchanged at 20=21.75 of d11B=0.4086 nm. 5(c) for feature-3. The indication is that two types of It is demonstrated unambiguously that the y= 13.9 nucleation have taken place from the melts. Some of samples with 6.29 mol%(Al,O3+Na2O)-doping con- the spherical particles have also formed multiplets as tains solely of B-cristobalite(Fig 4(a)in the crystalline indicated in Fig. 5(c). Here, the atomic contrast(bright mixture. It appears that silica samples grown in stilbite versus grey) revealed by SEM-BEI suggests that these NaCa2AlSSi13O36)[6] could have dissolved Na,O and two types of crystals may have different chemical Al,, of sufficient quantities to render the stabilization compositions ofβ- cristobalite The cracked matrix contains both a- and B-cristo- The results suggest that the content of Al2O3 and balite(Fig. 5(a) and(b)as evidenced from XRD(for Na,o determine the crystalline phases of the sintered y=19.5 shown in Fig. 4(a)where the (101) and (IID colloidal gel-derived powder. Higher additive content peaks are almost equal in height). It is of course not appears to favour the formation of the high tempera- known from SEM which are the B-cristobalite crystals ture B-cristobalite. The B-cristobalite in samples with Nevertheless, microstructure analysis using TEM [19] y=13.9 stabilized and retained metastably to room has identified B-phase in the sintered(Al,O3+Na2O)- temperature has also sustained the phase transforma- codoped samples, and it will appear in another tion induced by re-grinding of the sintered samples to manuscript. Along the glass-matrix interface(indicated powder. With higher(Al,O3+ Na2O)-doping levels, e. g. by arrow in Fig. 5(a)and shown also by higher magnifi y=13.9, not only the transformation to a-tridymite is cation in Fig. 5(d)), it can be seen that cracking has not completely suppressed, but also the B-cristobalite con- extended into the residual glass. The indication is that tent is increased in the phase assemblage. This ha the cracks in Fig. 5(b) are likely to have been generated become more conspicuous as the(Al,O3+ Na2O)-con- by grinding(or indeed the applied stress) because of the tent increases from y= 24.6 (i.e. 3.76 mol%)to y= 13.9 volume contraction accompanying with the induced (i.e. 6.29 mol%)and also for longer sintering hours(of B-a-cristobalite phase transformation. The character- 48 h as in Fig 4(a)) istic feature of the a-cristobalite twin lamellae [20] better ned [19 by TEM has been smeared out 3. 2. Microstructure after grinding and polishing after grinding. Consequently, unlike those in Fig 6(b), they can no longer be identified by SEM. The Phase transformation has been induced on sintered spherical particles, however, remain intact (and still sample surfaces by grinding and polishing when prepar- attached to the cracked matrix) when allowing cracks ing the polished sections for microstructure analysis. to deflect into the matrix containing predominantly the The characteristic microstructures of samples contain- transformed a-phase(Fig. 5(b). It suggests that me- ing(a) both(a+ B)-cristobalite and(b) only B-cristo- chanical grinding induces the B-+a-cristobalite phase balite in the crystalline mixture, as evidenced by XRD transformation, but some of the B-cristobalite in the ( Fig. 4(a)), are described in the following form of spherical particles, successfully resisted the transformation, have retained its high temperature cu 3.2.1. y=19.5 samples containing (+B)-cristobalite bic symmetry. These B-cristobalite particles of 300-500 a typical microstructure of samples with y=19, e unyielding to mechanical grinding would have been stabilized by the doping solutes and become'non containing Na,O and Al,O3, each of 4.65 mol%, and transformable at room temperature adopting the term sintered at 1100C for 48 h, is shown by SEM-SEl in used for the Y2O3-doped Zro2[8] Fig. 5(a) where three specific features(labelled as 1, 2 Dendritic and cellular growth(Fig. 6(a)) are both and 3)are discernible at low magnifications. First of all observed in one of the residual pores from the sintered spherical particles of 300-500 nm are found to disperse and polished sample. No induced cracks such as those biquitously in the porous and cracked matrix, occurred in Fig. 5(b)can immediately be discernedC.-H. Chao, H.-Y. Lu / Materials Science and Engineering A328 (2002) 267–276 271 d101 of -cristobalite increases monotonically with higher dopant levels. The peak shift of (101) from 2
=21.9° (of y=19.5) to 2
=22.0° (of y=32.4) for the three samples is also an indication of different solid solubilities (of both Al3+ and Na+), as shown in Fig. 4(a). The lattice spacings exhibit an increasing trend from the lower (Al2O3+Na2O)-doping levels of y=32.4 to y=19.5 of higher doping levels. It suggests that the higher con￾tents of Al2O3 and Na2O in the initial silica sols pro￾duce sintered samples containing more -cristobalite with the larger d101. Nevertheless, d111
of
-cristobalite remains unchanged at 2
=21.75° of d111
=0.4086 nm. It is demonstrated unambiguously that the y=13.9 samples with 6.29 mol% (Al2O3+Na2O)-doping con￾tains solely of
-cristobalite (Fig. 4(a)) in the crystalline mixture. It appears that silica samples grown in stilbite (NaCa2Al5Si13O36) [6] could have dissolved Na2O and Al2O3 of sufficient quantities to render the stabilization of
-cristobalite. The results suggest that the content of Al2O3 and Na2O determine the crystalline phases of the sintered colloidal gel-derived powder. Higher additive content appears to favour the formation of the high tempera￾ture
-cristobalite. The
-cristobalite in samples with y=13.9 stabilized and retained metastably to room temperature has also sustained the phase transforma￾tion induced by re-grinding of the sintered samples to powder. With higher (Al2O3+Na2O)-doping levels, e.g. y=13.9, not only the transformation to -tridymite is completely suppressed, but also the
-cristobalite con￾tent is increased in the phase assemblage. This has become more conspicuous as the (Al2O3+Na2O)-con￾tent increases from y=24.6 (i.e. 3.76 mol%) to y=13.9 (i.e. 6.29 mol%) and also for longer sintering hours (of 48 h as in Fig. 4(a)). 3.2. Microstructure after grinding and polishing Phase transformation has been induced on sintered sample surfaces by grinding and polishing when prepar￾ing the polished sections for microstructure analysis. The characteristic microstructures of samples contain￾ing (a) both (+
)-cristobalite and (b) only
-cristo￾balite in the crystalline mixture, as evidenced by XRD (Fig. 4(a)), are described in the following: 3.2.1. y=19.5 samples containing (+)-cristobalite mixture A typical microstructure of samples with y=19.5 containing Na2O and Al2O3, each of 4.65 mol%, and sintered at 1100 °C for 48 h, is shown by SEM-SEI in Fig. 5(a) where three specific features (labelled as 1, 2 and 3) are discernible at low magnifications. First of all, spherical particles of 300–500 nm are found to disperse ubiquitously in the porous and cracked matrix, as presented in Fig. 5(b) (for feature-1 in Fig. 5(a)). Some of these particles have coalesced to form doublets and triplets. Second, extensive microcracks, feature-2, can not be differentiated for its trans- or inter-granular nature with certainty, since the matrix has been disinte￾grated after grinding and polishing and grains are no longer discernible from Fig. 5(b). Third, the residual glassy phase contains two distinctive types of crystalline particles, dendritic and spherical in shape. The SEM backscattered electron image (BEI) provides a distin￾guishable contrast. It represents the nucleation at the early stage as shown by higher magnification in Fig. 5(c) for feature-3. The indication is that two types of nucleation have taken place from the melts. Some of the spherical particles have also formed multiplets as indicated in Fig. 5(c). Here, the atomic contrast (bright versus grey) revealed by SEM-BEI suggests that these two types of crystals may have different chemical compositions. The cracked matrix contains both - and
-cristo￾balite (Fig. 5(a) and (b)) as evidenced from XRD (for y=19.5 shown in Fig. 4(a) where the (101) and (111)
peaks are almost equal in height). It is of course not known from SEM which are the
-cristobalite crystals. Nevertheless, microstructure analysis using TEM [19] has identified
-phase in the sintered (Al2O3+Na2O)- codoped samples, and it will appear in another manuscript. Along the glass-matrix interface (indicated by arrow in Fig. 5(a) and shown also by higher magnifi- cation in Fig. 5(d)), it can be seen that cracking has not extended into the residual glass. The indication is that the cracks in Fig. 5(b) are likely to have been generated by grinding (or indeed the applied stress) because of the volume contraction accompanying with the induced

-cristobalite phase transformation. The character￾istic feature of the -cristobalite twin lamellae [20] better discerned [19] by TEM has been smeared out after sample grinding. Consequently, unlike those in Fig. 6(b), they can no longer be identified by SEM. The spherical particles, however, remain intact (and still attached to the cracked matrix) when allowing cracks to deflect into the matrix containing predominantly the transformed -phase (Fig. 5(b)). It suggests that me￾chanical grinding induces the

-cristobalite phase transformation, but some of the
-cristobalite in the form of spherical particles, successfully resisted the transformation, have retained its high temperature cu￾bic symmetry. These
-cristobalite particles of 300–500 nm unyielding to mechanical grinding would have been fully-stabilized by the doping solutes and become ‘non￾transformable’ at room temperature adopting the term used for the Y2O3-doped ZrO2 [8]. Dendritic and cellular growth (Fig. 6(a)) are both observed in one of the residual pores from the sintered and polished sample. No induced cracks such as those occurred in Fig. 5(b) can immediately be discerned.