C -H. Chao, H.Y. Lu/Materials Science and Engineering 4328(2002)267-276 (101) reflection at 20=21.95 designated to C-1(b) also appeared to have higher intensity than(IlDB- The suggestion is that re-grinding the sintered samples to powder and to finer particles favours the formation of C-1(b)(<38um) a-cristobalite. The B-+al-phase transformation has ap parently been encouraged in the mixture by the reduc C1a)(4538um) tion of particle sizes from pulverizing or simply the relief of the matrix constraints. It is clear that the(101) eflection has shifted towards smaller d-spacing as pow 21,021.5 2.523.023.5 der particle size is further reduced. The temperature of the cubic-tetragonal phase transformation dep upon the particle size is not uncommon in aces of surface(C-1), re-ground to The retention of high-temperature polymorphs has powders of different sizes(C-1(a)and C-l(b)of y=24. 6 sintered at been reported for various ferroelastic and ferroelectric 00°cfor24h. ceramics of, e.g. BaTiO3, [15, 16] ZrO2[17] and indeed SO2[6,15,18] 1100℃/48h 3.1.3. Composition with y= 13.9 For samples with higher Al2O3- and NayO-contents (e.g. y= 13.9 of 6.29 mol% as given in Table 1), sinter ing at 1100C for 48 h has produced predominantly B-cristobalite even after samples are re-ground to fine powders of <45-38 um. It is shown clearly in Fig 3 where the characteristic a-cristobalite reflections of 18212427303336394245 (1l1) and (102), are absent from the XRD trace Judging from the peak intensity, the air-cooled sample Fig. 3 ith y-3. 9 sintered at l100" C for 48 h showing The indication is that crystallization of B-cristobalite from the sintered gel-derived powder compact is still continuing upon cooling(from the sintering tempera- 1100℃/4 ture of 1100C) to room temperature. The high-tem (20=22.00 perature B-cristobalite in the sintered mixture has been retained metastably to room temperature by the higher doping level of 6.29 mol% The XRD trace for the y= 13.9 sample pulverized to 45-38 um powder did not show any peak splitting at 20=21 all(as evidenced form Fig 4(a)). In fact, it is the fully stabilized sample in which the B-a-cristobalite phase transformation has been prevented all together 21,0 23.0 Sintering at 1100C for 48 h, samples with y=19.5, 20(deg) 24.6 and 32.4 always yielded the crystalline mixture of (a+ B)-cristobalite. The XRD traces in the proximity of Cristobalite (d 20=21.75 for the three compositions and for y= 13.9 are given in Fig. 4(a). Taking the peak height to represent the amount of crystalline phases in the mix ture, it appears that the quantity of B-cristobalite(of 0.406 20=21.75) remains almost unaltered when that of a-cristobalite has increased very appreciably e.g. by N1.6 times from samples containing Al,O3 and Na2O y=324 of 4.65 mol%(=19.5)to 2.91 mol%(=32. 4). It is illustrated in Fig. 4(b) that the lattice spacing of B- cristobalite(duB) als increasing Al,O3- and Na2O-content. The data point for the y= 32. 4 sample is missing from Fig. 4(b) since Fig4.(a) Peak shift registered by(111)B and (101)a, and (b) change its B-cristobalite content is negligible from the XRD of the lattice spacings of dola and dinp in samples of four dopant levels sintered at 1100 C for 48 h trace(as indicated in Fig. 4(a)). On the other hand270 C.-H. Chao, H.-Y. Lu / Materials Science and Engineering A328 (2002) 267–276 Fig. 2. XRD traces of the as-sintered surface (C-1), re-ground to powders of different sizes (C-1(a) and C-1(b)) of y=24.6 sintered at 1100 °C for 24 h. (101) reflection at 2
=21.95° designated to C-1(b) also appeared to have higher intensity than (111)
. The suggestion is that re-grinding the sintered samples to powder and to finer particles favours the formation of -cristobalite. The

-phase transformation has ap￾parently been encouraged in the mixture by the reduc￾tion of particle sizes from pulverizing or simply the relief of the matrix constraints. It is clear that the (101) reflection has shifted towards smaller d-spacing as pow￾der particle size is further reduced. The temperature of the cubic
tetragonal phase transformation depending upon the particle size is not uncommon in ceramics. The retention of high-temperature polymorphs has been reported for various ferroelastic and ferroelectric ceramics of, e.g. BaTiO3, [15,16] ZrO2 [17] and indeed SiO2 [6,15,18]. 3.1.3. Composition with y=13.9 For samples with higher Al2O3- and Na2O-contents (e.g. y=13.9 of 6.29 mol% as given in Table 1), sinter￾ing at 1100 °C for 48 h has produced predominantly
-cristobalite even after samples are re-ground to fine powders of 45–38 m. It is shown clearly in Fig. 3 where the characteristic -cristobalite reflections of (111) and (102) are absent from the XRD trace. Judging from the peak intensity, the air-cooled sample contains more
-cristobalite than the water-quenched. The indication is that crystallization of
-cristobalite from the sintered gel-derived powder compact is still continuing upon cooling (from the sintering tempera￾ture of 1100 °C) to room temperature. The high-tem￾perature
-cristobalite in the sintered mixture has been retained metastably to room temperature by the higher doping level of 6.29 mol%. The XRD trace for the y=13.9 sample pulverized to 45–38 m powder did not show any peak splitting at all (as evidenced form Fig. 4(a)). In fact, it is the fully stabilized sample in which the

-cristobalite phase transformation has been prevented all together. Sintering at 1100 °C for 48 h, samples with y=19.5, 24.6 and 32.4 always yielded the crystalline mixture of (+
)-cristobalite. The XRD traces in the proximity of 2
=21.75° for the three compositions and for y=13.9 are given in Fig. 4(a). Taking the peak height to represent the amount of crystalline phases in the mix￾ture, it appears that the quantity of
-cristobalite (of 2
=21.75°) remains almost unaltered when that of -cristobalite has increased very appreciably e.g. by 1.6 times from samples containing Al2O3 and Na2O of 4.65 mol% (y=19.5) to 2.91 mol% (y=32.4). It is illustrated in Fig. 4(b) that the lattice spacing of
- cristobalite (d111
) also remains unchanged with the increasing Al2O3- and Na2O-content. The data point for the y=32.4 sample is missing from Fig. 4(b) since its
-cristobalite content is negligible from the XRD trace (as indicated in Fig. 4(a)). On the other hand, Fig. 3. Samples with y=13.9 sintered at 1100 °C for 48 h showing predominantly
-cristobalite by water-quenching and air-cooling. Fig. 4. (a) Peak shift registered by (111)
and (101), and (b) change of the lattice spacings of d101 and d111
in samples of four dopant levels sintered at 1100 °C for 48 h.