Mechanical Properties Of TiO₂ Nanoceramic Through High Pressure Sintering

Titanium dioxide（TiO₂）has a wide range of applications in photocatalysis,nanoceramics,gas sensors and electronic devices due to its unique physical and chemical properties.Obviously,the mechanical properties of TiO₂ materials are the basis for supporting the devices to work well,so the TiO₂ compacts with higher density and better mechanical properties are expected to be synthesized at industrial synthesis conditions.Until now,the traditional methods for synthesis of nano-TiO₂ ceramic in bulk form are hot-press or spark-plasma sinterings by using TiO₂ nanopowder as a precursor.However,the nanopower precursor easily absorbs the impurities due to the high specific surface area,which results in the high porosity,low density,and inhomogeneous microstructures in the sintered bulk materials of nano-TiO₂ ceramic.In addition,the nanocrystals are easily grown during the sintering process.Therefore,the sintered TiO₂ ceramic usually has poor mechanical properties,and it is urgent to develop new synthesis methods to solve these problems.Recently,the successful synthesis of nano-twinned cubic boron nitride and diamond demonstrates that ultrafine microstructure obtained by using high-pressure and high-temperature（HPHT）methods with a suitable precursor can improve the overall performances of materials.In this work,the hard and dense fine-grained nano-TiO₂composite ceramic materials are obtained by the HPHT method with the submicron anatase-TiO₂ as precursor.The effects of sintering temperature and holding time on the microstructures and properties of nano-TiO₂ composite ceramics were further investigated.Compared with the traditional sintering methods,the HPHT method can induce the phase transformation of anatase-TiO₂ to form columbite-TiO₂,and increase the nucleation rate but inhibit the grain growth.The multiphase nano-TiO₂ ceramics with a grain size less than 30 nm were fabricated through HPHT method.The high pressure sintering can promote the density of ceramics,and thus improves the hardness of samples.The sample synthesized from 6 GPa and 450℃has the highest hardness of 13.5±0.3 GPa,fracture toughness of 2.4MPa·m^0.5,and relative density of 97.28%.Further research found that the optimum holding time was 60 min at the HPHT condition of 6 GPa and 450°C,and the densification was not significant with further increasing the holding time.The sample with the highest relative density of 97.56%and high compressive strength of 750 MPa was synthesized at 450℃with holding time of 120min.With the holding time was prolonged,the sample hardness increased first and then decreased,but the fracture toughness first decreased and then tended to be consistent.