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 (ε_r) 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 (ε_r ∼ 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.