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)01703-8Materials 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 Uni
ersity, 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 poly￾morphic phase transformation from cristobalite to tridymite, and to quartz upon cooling to room temper￾ature in atmospheric pressure. Other crystalline phases of stoichovite and coesite also exist under high pres￾sures. 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 uncer￾tain if the stabilization of cristobalite and tridymite in natural minerals, which often incorporate impurity ox￾ides, 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-tem￾perature
-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 accord￾ingly 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 parame￾ters determined experimentally. Contradictorily, it was also reported [12] that the lattice constant of -cristo￾balite 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