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Refinement of Nanoscale Grain Structure in Bulk Titania via a Transformation‐Assisted Consolidation (TAC) Method
Author(s) -
Liao ShihChieh,
Colaizzi James,
Chen Yijia,
Kear Bernard H.,
Mayo William E.
Publication year - 2000
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.2000.tb01530.x
Subject(s) - rutile , anatase , materials science , sintering , grain size , grain growth , nucleation , nanocrystalline material , phase (matter) , chemical engineering , mineralogy , analytical chemistry (journal) , metallurgy , nanotechnology , thermodynamics , chemistry , chromatography , organic chemistry , physics , photocatalysis , engineering , catalysis
Bulk nanocrystalline TiO 2 samples (100% rutile) with a relative density as high as 97% and a grain size of <20 nm have been produced via high‐pressure (up to 8 GPa)/low‐temperature (∼0.3 T m , where T m is the melting temperature) sintering, using a toroidal‐type high‐pressure apparatus. Nanophase TiO 2 powder with a metastable anatase structure and an initial grain size of ∼38 nm was used as the starting material. During sintering, the anatase phase transformed to either the rutile or srilankite phase, depending on the pressure–temperature ( P – T ) combination. The starting temperature of the anatase‐to‐rutile phase transformation decreased from ∼550°C at ambient pressure to ∼150°C at 2.5 GPa. Grain growth was limited by the low sintering temperature and the multiple nucleation events in the parent phase. The grain size of the transformed rutile decreased as the sintering pressure increased, which can be explained by the combined effect of increasing the nucleation rate and decreasing the growth rate with high pressure. We have demonstrated that it is possible to produce a dense sintered compact with a grain size even smaller than that of the starting powder. The high‐pressure srilankite phase was observed at P – T conditions as low as 4.75 GPa and 250°C, respectively; however, unlike the anatase‐to‐rutile phase transformation, the rutile‐to‐srilankite phase‐transformation temperature increased as the pressure increased. Also, in contrast to the irreversible anatase‐to‐rutile phase transformation, the srilankite will reversibly transform to rutile under the appropriate circumstances. This observation provides an opportunity to further refine the TiO 2 grain structure by switching the sintering conditions (temperature and pressure) between the regions in which the rutile or srilankite phase are stable.