MATERIALS iEnE& EGMEERNG A SEVIER laterals Science and Engineering A328(2002)267-276 www.elsevier.comflocate/msea Stress-induced阝B→∝- cristobalite phase transformation in (Na2O+Al2O3)-codoped silica Chin-Hsiao Chao, Hong-Yang Lu* Institute of Materials Science and Engineering, National Sun Yat-Sen Unicersity, Kaohsiung 80424, Taiwan Received 9 April 2001: received in revised form 25 June 2001 Abstract Colloidal gel-derived silica(SiO,) powder codoped with(Na, 0+Al,O3 was sintered at 1100C. The crystalline phase content and phase transformation of the sintered ceramics have been studied via X-ray diffractometry and scanning electron microscopy The amount of B-cristobalite retained metastably in the mixture to room temperature is found to depend on the level of additives Samples codoped with Na, O and Al,O3, both of 6.30 mol%o, were found to contain only B-cristobalite in the crystalline mixture. and which is known as the chemically stabilized cristobalite(CSC). Multiple liquid phase separation is also observed in the codoped samples. The lattice spacing dola of a-cristobalite increases with the doping level while dib of the B-phase remains almost unchanged in all compositions studied. Surface grinding or pulverizing of the sintered samples into powder induces the B-a-cristobalite phase transformation. The mechanism of the high temperature B-cristobalite stabilization to room temperature associated with both the chemical and mechanical terms is discussed. o 2002 Elsevier Science B.v. all rights reserved Keywords: Sintering: Nucleation and growth; Phase transformation 1. Introduction perature B-cristobalite is a truly dynamic disordered phase [3]. Others [4, 5]. however, proposed models Crystalline silica(SiO2)experiences a series of poly- which suggested that the p-structure is composed of morphic phase transformation from cristobalite to low-symmetry domains tridymite, and to quartz upon cooling to room temper The stabilization of B-cristobalite can occur [6,7 by ature in atmospheric pressure. Other crystalline phases (a) altering chemical composition, and/or(b)imposing of stoichovite and coesite also exist under high pres- mechanical constraints In the first case, it appears that sures. Both the higher temperature polymorphs of acceptor impurities in solid solution with silica stabilize cristobalite and tridymite are known to exist metastably the p-phase in a manner similar [6] to that of the fully to room temperature in atmospheric pressure, although stabilized cubic (c)-ZrO2 [8]. The former named accord quartz is believed to be the thermodynamically most ingly the chemically stabilized cristobalite(CSC)has stable phase under such conditions. It remains uncer been synthesized [9-ll] by incorporating stuffing tain if the stabilization of cristobalite and tridymite in catons of Ca+ cu+ and sr+ into the tectosilicate natural minerals, which often incorporate impurity ox framework, or alternatively [ll] by directly replacing ides, is imparted by cation substitutions for silicon. The the Sio4 tetrahedra with AIPOa. However, the fact that B-o-cristobalite phase transformation is accompanied the chemically modified silica did not result in any with a volume contraction of &5 vol. when the significant change in the lattice parameters [4, 6, 7] had crystal changes from the cubic Fd3m to tetragonal gued against the stuffed B-cristobalite structure and P4,2,2 symmetry [1]. Recent studies supported by so the chemical stabilization. It suggested [7 that molecular simulations suggested [2] that the high-tem- any impurities would have affected the lattice parame ters determined experimentally. Contradictorily, it was ding author.Tel.:+886-7-525-4052;fax:+886-7-525- also reported [12] that the lattice constant of a-cristo balite decreased with heating the silicic-acid-derived E-mail address: hyl@mailnsysu. edu. tw(H.-Y. Lu) powders from 1080 to 1420C, in which the 101- -peak 0921-5093/02/s.see front matter c 2002 Elsevier Science B.V. All rights reserved PI:S0921-5093(01)01703-8
Materials Science and Engineering A328 (2002) 267–276 Stress-induced -cristobalite phase transformation in (Na2O+Al2O3)-codoped silica Chin-Hsiao Chao, Hong-Yang Lu * Institute of Materials Science and Engineering, National Sun Yat-Sen Uniersity, Kaohsiung 80424, Taiwan Received 9 April 2001; received in revised form 25 June 2001 Abstract Colloidal gel-derived silica (SiO2) powder codoped with (Na2O+Al2O3) was sintered at 1100 °C. The crystalline phase content and phase transformation of the sintered ceramics have been studied via X-ray diffractometry and scanning electron microscopy. The amount of -cristobalite retained metastably in the mixture to room temperature is found to depend on the level of additives. Samples codoped with Na2O and Al2O3, both of 6.30 mol%, were found to contain only -cristobalite in the crystalline mixture, and which is known as the chemically stabilized cristobalite (CSC). Multiple liquid phase separation is also observed in the codoped samples. The lattice spacing d101 of -cristobalite increases with the doping level while d111 of the -phase remains almost unchanged in all compositions studied. Surface grinding or pulverizing of the sintered samples into powder induces the -cristobalite phase transformation. The mechanism of the high temperature -cristobalite stabilization to room temperature associated with both the chemical and mechanical terms is discussed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sintering; Nucleation and growth; Phase transformation www.elsevier.com/locate/msea 1. Introduction Crystalline silica (SiO2) experiences a series of polymorphic phase transformation from cristobalite to tridymite, and to quartz upon cooling to room temperature in atmospheric pressure. Other crystalline phases of stoichovite and coesite also exist under high pressures. Both the higher temperature polymorphs of cristobalite and tridymite are known to exist metastably to room temperature in atmospheric pressure, although quartz is believed to be the thermodynamically most stable phase under such conditions. It remains uncertain if the stabilization of cristobalite and tridymite in natural minerals, which often incorporate impurity oxides, is imparted by cation substitutions for silicon. The -cristobalite phase transformation is accompanied with a volume contraction of 5 vol.% when the crystal changes from the cubic Fd3m to tetragonal P41212 symmetry [1]. Recent studies supported by molecular simulations suggested [2] that the high-temperature -cristobalite is a truly dynamic disordered phase [3]. Others [4,5], however, proposed models which suggested that the -structure is composed of low-symmetry domains. The stabilization of -cristobalite can occur [6,7] by: (a) altering chemical composition, and/or (b) imposing mechanical constraints. In the first case, it appears that acceptor impurities in solid solution with silica stabilize the -phase in a manner similar [6] to that of the fully stabilized cubic (c)-ZrO2 [8]. The former named accordingly the chemically stabilized cristobalite (CSC) has been synthesized [9–11] by incorporating ‘stuffing’ cations of Ca2+, Cu2+ and Sr2+ into the tectosilicate framework, or alternatively [11] by directly replacing the SiO4 tetrahedra with AlPO4. However, the fact that the chemically modified silica did not result in any significant change in the lattice parameters [4,6,7] had argued against the stuffed -cristobalite structure and so the chemical stabilization. It was suggested [7] that any impurities would have affected the lattice parameters determined experimentally. Contradictorily, it was also reported [12] that the lattice constant of -cristobalite decreased with heating the silicic-acid-derived powders from 1080 to 1420 °C, in which the 101-peak * Corresponding author. Tel.: +886-7-525-4052; fax: +886-7-525- 6030. E-mail address: hyl@mail.nsysu.edu.tw (H.-Y. Lu). 0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 7 0 3 - 8
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 quantities
268 C.-H. Chao, H.-Y. Lu / Materials Science and Engineering A328 (2002) 267–276 shifted towards higher 2-angles. Second, the transformation of -cristobalite to may be hindered mechanically by matrix constraint when the volume change accompanied with the phase transformation is suppressed. 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 stabilization 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 pressureless-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 -cristobalite and those of 49–51° for -quartz were used for both the crystalline phase identification and quantitative 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 electron 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 grainboundaries and other microstructural features. 3. Results 3.1. Identification of crystalline phases The (Na2O+Al2O3)–ySiO2 powders of four compositions 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 reground 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 represent 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
C -H. Chao, H.Y. Lu/Materials Science and Engineering 4328(2002)267-276 temperature polymorph of crystalline silica, B-cristo 19.5 balite, has apparently been stabilized and retained 1100℃/48h metastably to room temperature in the y= 19.5 samples with 4.65 mol% of both Na,O and Al,O3(Table 1) The lattice spacing of d=0.4069 nm(C-1)does not match exactly with that of either the a- or B-phase given by the JCPDS files, but falls between dIor C-2C- 0.4039 nm and d11B=0.4110 nm. However, the C 102aC4 reflection in Fig. l(a)can be deconvoluted to two peaks corresponding to the lattice spacings of 0.4084 and 0.4058 nm and representing(101)a and(lID),respec- 20(deg) tively. This is shown in Fig. 1(b). The co-existence of a-cristobalite and the untransformed B-cristobalite can be discerned. It is also noted that no peak splitting(of the C-l reflection in Fig. I(c) was detected before the 20 1100℃/48h sintered samples had been pulverized again to powders a-cristobalite (C-l(a) in Fig. I(c). The C-4 peak(Fig. I(a))at 26 35.80 corresponding to a lattice spacing of d=0. 2506 nm again lies amongst d12a =0.2467 nm, d200 =0.2487 nm and d220B=0.2530 nm(JCPDS 27-605 for B-cristo balite). Similarly, it implies the co-existence of a-and B-cristobalite in the sintered mixture. The later is appar 20.521.021.5 022523.023 ently the untransformed high temperature B-phase 20(dee) which has been retained metastably to room 35 temperature. Sintered samples re-ground to powder were passed 1100℃/48h through a 45 um sieve prior to XRD analysis. The characteristic feature of the re-ground samples is the peak splitting of the initially Gaussian-type reflections of C-1(also given in Fig. I(c))and C-4. The C-l peak 20=21.8%(of Fig. 1(a)) has splitted of dup=0.4086 nm at 20=21.75 and d1ola=0.4058 nm at 20=21.90. They are again not completely in 20.521.021.522.022.52 3.524.0 accordance with the d-spacings(of d1B=0.4110 nm 20(deg) and dIola=0.4039 nm)given by the JCPDS files. The 111)B reflection shown in Fig. 1(c)for both as-sintered ng. I. XRD traces of (a)as-sintered surface of the y= 19.5 sample and re-ground samples remains almost unchanged in C for 24 h,(b) deconvoluded peak, and (c)re- position. However, the (101), reflection shifting to- higher 20-angles) from that (of <5 wt %)and beyond the detection by XRD, if of the as-sintered sample surface can be easily discerned they exist at all in the mixture from Fig. I(c) The reflections of C-l at 20=21.8 and C4 at 35.8 3. 1.2. Composition with y=24.6 may include both a- and B-cristobalite since the respec For powder compacts containing a smaller amount tive 20-angles are very close to each other(as indicated of Al,Oa and Na,o(e.g. y=24.6 of 3. 76 mol% as given in Fig. I(a). Investigating the peak areas of C-I and in Table 1), sintering at 1100C for 24 h results in C-4 in Fig. I(a)reveals that the sample may also mixture of a-and B-cristobalite. The XRd trace resem contain B-cristobalite. The relative intensities of C-l to bles that of the y=19.5 samples (of Fig. I()). The C-4 by integrating the peak areas are 1045: 24: 41: 236 20=21.8 reflection approximated to a Gaussian-type Normalizing them on the basis of the C-2 peak (i.e. peak locating between(101)a and(Ill)e is again ob (lID)a) intensity following the JCPDS file 39-1425, the tained. and which is also designated to C-1 in Fig. 2 mixture containing only a-cristobalite would give a Similar peak splitting and shift are observed from the peak ratio of 300: 24: 27: 51. The discrepancy of excess y=246 samples re-ground to 45-38 Hm, shown by intensities by A=745 for the C-l peak and 4=185 for C-1(a)in Fig. 2. When the same sample was pulverized the C-4 peak indicates the co-existence of B-cristobalite further down to particle size of 38 um, not only that with the a-phase in the crystalline mixture. The high the peak splitting had become more distinctive, the
C.-H. Chao, H.-Y. Lu / Materials Science and Engineering A328 (2002) 267–276 269 Fig. 1. XRD traces of (a) as-sintered surface of the y=19.5 sample sintered at 1100 °C for 24 h, (b) deconvoluded peak, and (c) reground powder showing -cristobalite. temperature polymorph of crystalline silica, -cristobalite, has apparently been stabilized and retained metastably to room temperature in the y=19.5 samples with 4.65 mol% of both Na2O and Al2O3 (Table 1). The lattice spacing of d=0.4069 nm (C-1) does not match exactly with that of either the - or -phase given by the JCPDS files, but falls between d101= 0.4039 nm and d111=0.4110 nm. However, the C-1 reflection in Fig. 1(a) can be deconvoluted to two peaks corresponding to the lattice spacings of 0.4084 and 0.4058 nm and representing (101) and (111), respectively. This is shown in Fig. 1(b). The co-existence of -cristobalite and the ‘untransformed’ -cristobalite can be discerned. It is also noted that no peak splitting (of the C-1 reflection in Fig. 1(c)) was detected before the sintered samples had been pulverized again to powders (C-1(a) in Fig. 1(c)). The C-4 peak (Fig. 1(a)) at 2= 35.8° corresponding to a lattice spacing of d=0.2506 nm again lies amongst d112=0.2467 nm, d200=0.2487 nm and d220=0.2530 nm (JCPDS 27-605 for -cristobalite). Similarly, it implies the co-existence of - and -cristobalite in the sintered mixture. The later is apparently the untransformed high temperature -phase which has been retained metastably to room temperature. Sintered samples re-ground to powder were passed through a 45 m sieve prior to XRD analysis. The characteristic feature of the re-ground samples is the peak splitting of the initially Gaussian-type reflections of C-1 (also given in Fig. 1(c)) and C-4. The C-1 peak at 2=21.8° (of Fig. 1(a)) has splitted into two peaks of d111=0.4086 nm at 2=21.75° and d101=0.4058 nm at 2=21.90°. They are again not completely in accordance with the d-spacings (of d111=0.4110 nm and d101=0.4039 nm) given by the JCPDS files. The (111) reflection shown in Fig. 1(c) for both as-sintered and re-ground samples remains almost unchanged in position. However, the (101) reflection shifting towards smaller d-spacings (higher 2-angles) from that of the as-sintered sample surface can be easily discerned from Fig. 1(c). 3.1.2. Composition with y=24.6 For powder compacts containing a smaller amount of Al2O3 and Na2O (e.g. y=24.6 of 3.76 mol% as given in Table 1), sintering at 1100 °C for 24 h results in a mixture of - and -cristobalite. The XRD trace resembles that of the y=19.5 samples (of Fig. 1(a)). The 2=21.8° reflection approximated to a Gaussian-type peak locating between (101) and (111) is again obtained, and which is also designated to C-1 in Fig. 2. Similar peak splitting and shift are observed from the y=24.6 samples re-ground to 45–38 m, shown by C-1(a) in Fig. 2. When the same sample was pulverized further down to particle size of 38 m, not only that the peak splitting had become more distinctive, the (of 5 wt.%) and beyond the detection by XRD, if they exist at all in the mixture. The reflections of C-1 at 2=21.8° and C-4 at 35.8° may include both - and -cristobalite since the respective 2-angles are very close to each other (as indicated in Fig. 1(a)). Investigating the peak areas of C-1 and C-4 in Fig. 1(a) reveals that the sample may also contain -cristobalite. The relative intensities of C-1 to C-4 by integrating the peak areas are 1045:24:41:236. Normalizing them on the basis of the C-2 peak (i.e. (111)) intensity following the JCPDS file 39-1425, the mixture containing only -cristobalite would give a peak ratio of 300:24:27:51. The discrepancy of excess intensities by =745 for the C-1 peak and =185 for the C-4 peak indicates the co-existence of -cristobalite with the -phase in the crystalline mixture. The high
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 hand
270 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 apparently been encouraged in the mixture by the reduction 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 powder particle size is further reduced. The temperature of the cubictetragonal 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), sintering 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 temperature of 1100 °C) to room temperature. The high-temperature -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 mixture, 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.
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 discerned
C.-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 contents of Al2O3 and Na2O in the initial silica sols produce 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 contains 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 temperature -cristobalite. The -cristobalite in samples with y=13.9 stabilized and retained metastably to room temperature has also sustained the phase transformation 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 content is increased in the phase assemblage. This has become more conspicuous as the (Al2O3+Na2O)-content 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 preparing the polished sections for microstructure analysis. The characteristic microstructures of samples containing (a) both (+)-cristobalite and (b) only -cristobalite 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 disintegrated 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 distinguishable 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 -cristobalite (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 characteristic 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 mechanical 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 cubic symmetry. These -cristobalite particles of 300–500 nm unyielding to mechanical grinding would have been fully-stabilized by the doping solutes and become ‘nontransformable’ 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.