Microwave dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

TiO2 bulk ceramics were fabricated by using both spark plasma sintering (SPS) and the conventional sintering method (CSM). Starting materials were ultra fine rutile powders (<50 nm) prepared via the sol–gel process. CSM achieved the relative sintering density of 99.2% at 1300 °C. The grain size of 1300 °C sintered specimen was 6.5 μm. However, the sintering temperature of SPS for the density of 99.1% was as low as 760 °C, where the grain size was only 300 nm. In order to re-oxidize the Ti3+ ions due to the reducing atmosphere of the SPS process and the high temperature of the CSM process, the prepared TiO2 specimens were annealed in an oxygen atmosphere. The dielectric constant (ɛr) and quality factor (Q × f) of SPS-TiO2 re-oxidized specimens in a microwave regime were 112.6 and 26,000, respectively. These properties were comparable to those of 1300 °C sintered CSM specimens (ɛr ∼ 101.3, Q × f ∼ 41,600). These microwave dielectric properties of nanocrystalline TiO2 specimens prepared using SPS were discussed in terms of grain size variation and Ti4+ reduction.