Kinetics and mechanism of anatase-to-rutile phase transformation in the presence of borosilicate glass
- PDF / 681,475 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 70 Downloads / 201 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Kinetics and mechanism of anatase-to-rutile phase transformation in the presence of borosilicate glass Jau-Ho Jean and Shih-Chun Lin Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China (Received 16 July 1998; accepted 20 April 1999)
The effects of borosilicate glass (BSG) on the kinetics and mechanism of anataseto-rutile phase transformation have been investigated. The displacive anatase-to-rutile phase transformation kinetics of TiO2 were greatly enhanced in the presence of BSG. The transformation kinetics followed the Avrami equation, and the results showed an apparent activation energy of 260–370 kJ/mol, which was close to the bond strength of Ti–O, suggesting a reaction-controlling mechanism. The values of the Avrami exponent were in the range of 1.4–2.3, which could be interpreted as two-dimensional reaction-controlled growth at zero nucleation rate. The rutile particles obtained by the phase transformation were always much larger than the starting anatase powders, which was explained by a mechanism of phase-transformation–induced particle coalescence.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
TiO2 has three crystalline phases including brookite (orthorhombic), anatase (tetragonal), and rutile (tetragonal), among which rutile is the most stable phase.1,2 Phase transformation of anatase-to-rutile is thermodynamically favorable and can take place thermally or mechanically.1–6 Reports have shown that the transformation is very sensitive to the presence of impurities2,7–15 and to the firing atmosphere,16,17 which was explained by the types of defects generated during firing. Since the rupture and formation of Ti–O bonds was identified to be the rate-controlling step during anatase-torutile phase transformation,2,18 the transformation rate could be accelerated if the impurities or atmosphere could enhance its breakage and generation. It was believed that the interstitial titanium could hinder, whereas the oxygen vacancies could promote, the rearrangement of Ti–O bonds. Impurities including Li2O, CuO, CoO, MnO2, Sb2O3, and V2O5 enhanced the transformation rate, but impurities such as Nb2O5, WO3, SiO2, Cl−1, −2 PO−3 4 , and SO4 retarded the transformation rate. Moreover, the transformation rate was greatly increased in H216 and Ar–Cl217 but decreased in vacuum,16 compared with that in air. In this study, effects of a low-softening-point borosilicate glass (BSG) on the kinetics and mechanism of the anatase-to-rutile transformation were investigated. The BSG was chosen because it wets anatase well, resulting in good densification of BSG + TiO2 at temperatures 20–30 min at 875 °C. As firing proceeds, the size of the faceted crystals shows no significant change but their number increases dramatically. Comparing microstructural results in Fig. 11 with XRD results in Figs. 4 and 5, it is concluded that the anatase-to-rutile transformation also involves microstructural coarsening, forming faceted rutile crystals.
Data Loading...
