Sintering, Structure And Properties Of Nanosized ZnO Under Ambient And Ultra-High Pressure

In recent years, nanosized ZnO has been paid special attention because of its important semiconductive characteristic. In this paper, compaction and sintering of ZnO nanopowders under ambient and high pressure are investigated. Bulk samples of nanocrystalline ZnO with high density are successfully prepared. Microhardness, electrical properties and light absorption properties of the samples are also measured. ZnO nanopowders are prepared by the homogeneous precipitation method. The influences of density of the green disks, sintering temperature and time on microstructure, density and grain sizes during the process of densification are systematically discussed. The apparent activation energy for grain growth, the value of pre-exponential constant and the grain growth kinetic exponents are calculated using the phenomenological kinetic grain growth equation. Study on evolution of ZnO nanocrystalline grains is carried out on a cubic high-pressure apparatus. The structures, component and morphology of the samples are characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and differential scanning calorimeter. Microhardness, electrical properties and light absorption are measured using durometer,MY-1 three-parameter durometer and UV-vis spectrophotometer respectively. Influence of microstructures on the properties of the bulk nano-materials ZnO is studied. The purity of ZnO nanopowders prepared is better than 99.5%. The morphologies of the particles tend to be rounded. Nanopowders exhibit good dispersion properties and narrower distribution in size. The average grain size is about 20nm and the largest of them is about 35nm, which agree to the requirement for sintering Dmax<2Daverage. Optimum method for preparing bulk nanosized ZnO is obtained. The sample sintered at 700℃ for 2 hours by repeat compaction process is 50 - 60nm in size and relative density 91.2%. Kinetic grain growth equation of nanosized ZnO sintered at ambient pressure is expressed as: G⁶ =1.52×10¹²t exp(-6.4×10⁴/RT) nm⁶·h^-1 Obvious pressure-induced grain refinement is observed during the compacting of nanosized ZnO under high pressure. The sample prepared at 300℃under the pressure of 6GPa is 40-50nm in size and relative density 99%. The activation volume at 300℃ from 1 to 3GPa and from 4 to 6GPa are calculated, which is about -5.82cm3/mol, 9.66cm3/mol, respectively. Microhardness of samples sintered at ambient pressure increases first and then decreases with increasing the sintering temperature. Microhardness of the sample sintered under high pressure is higher than that of the one prepared at ambient pressure. The bulk nanosized ZnO with high density has the non-linear current-voltage characteristics. The higher the relative density and the smaller the grain sizes, the better the non-linear current-voltage characteristics. UV-vis absorption spectra of nanosized ZnO all shift to the red at 200nm and shift to the blue at 400nm. The intensity is greater than that of the coarse crystalline ZnO and decreases with increasing grain sizes.