Preparation And Study On Microstructure And Electrical Properties Of ZnO-based Varistor Ceramics With High Voltage-gradient

The influence of high-energy ball milling (HEBM) and rare-earth oxide of Y₂O₃, Dy₂O₃ and Er₂O₃ on the electrical properties of ZnO-based varistor ceramics sintered at 800-1000℃ was researched. Using nanocrystalline ZnO-based varistor powders obtained by HEBM, advanced ZnO-based varistor ceramics sintered at 1000℃, with voltage gradient above 400V/mm, energy absorption more than 230J/cm³ and residual voltage ratio less than 1.65, were made the first time.The influence of sintering temperature and ball milling tune on the electrical properties, density and microstructures of high-energy ball milled varistor ceramics, and the influence of ball milling time on powder particle size, structure and the contamination from steel balls, were researched firstly. The results showed that the varistor ceramics' voltage gradient, nonlinear coefficient and density decreased and leakage current increased with increasing sintering temperature. Very little Bi-rich phase or vanishment resulted from vaporizing leads to the decrease in density, the steep decrease in nonlinear coefficient and the rapid increase in leakage current when samples sintered at 1100-1300℃. HEBM made a severe lattice distortion, following that the large-radius ions' solubility increased within ZnO grain and barrier height (Φ_b), donor concentration (N_d), density of interface states (N_s) increased and barrier width( ω )decreased, and hence the electrical properties were improved. With the time of HEBM increasing, this effect becomes more notable. Varistor ceramic density increasing with increasing ball milling time comes mainly from two reasons. First, HEBM caused the refinement of the grain sizes and enhanced lattice distortion energy in the powders, which are responsible for the increase in the sintering driving force and kinetics factor and the shortening of the elemental diffusion distance. Second, liquid phases increased the contacted area between reactant phases, accelerated the transmission speed, promoted the reaction among reactant phases. The refinement of the grain size caused by HEBM is prominent at the stage of milling for 0-5h and this effect becomes more distinct with decreasing sintering temperature. Particle size was reduced from 1.50 to 0.42μm after HEBM for 5h, enhancing the sinterability of mixed powders. Average ZnO grain size was reduced from 120 to 59 run after HEBM for 20h. Phase transformation did not generate after HEBM, but diffraction peaks were very wide. The Fe contamination from steel balls is not detrimental under our...