Cold-sintering Preparation And Electrical Properties Of Zinc Oxide-based Ceramics

ZnO ceramic is widely used in the field of thermoelectricity,gas sensors,solar cells and varistors,because of its broad band gap and large excitation binding energy.However,the conventional high sintering temperature results large grain size,decreases the electrical conductivity,and consumes a large amount of energy.Therefore,it is of great importance to reducing the sintering temperature and grain sizes.Cold sintering process can prepare high densified ceramics at low temperature,and the grain size hardly grows during the sintering process.In this work,ZnO-based ceramics were successfully prepared by cold sintering process,the principle of cold sintering and the effect of heat treatment on the microsturecture and electrical properties were systematically studied.When acetic acid was used as solvent during the cold sintering process,the effect of the acetic acid concentration,sintering temperature and holding time on the characteristics of zinc oxide ceramics were studied.Highly densed ZnO ceramics with relative density of 97.5%and grain size less than 1?m were achieved by cold sintering,at the acetic acid concentration of 1 mol·L-1,sintering temperature of 260? and holding time of 1 hour.The mechanism of cold sintering was further explored by changing the size and reaction activity of raw ZnO powders through high temperature treatment,as well as altering the type of liquid solvents.When the zinc acetate was used as solvent during the cold sintering process,it was found that the density and microstructure were similar to that of acetic acid,indicating that the key point of realizing cold sintering is to provide a suitable source for precipitation.The raw ZnO powders were pre-heat-treated at 800?,100?and 1200?,the density of ceramics prepared by cold sintering decreased with the increase of grain size,which indicated that the particle size was also important for realizing cold sintering.The as-cold-sintered ZnO ceramics were heat-treated at different temperatures and atmospheres.It was found that the crystallinity of the heat-treated ceramics was improved and the grain size increased little,still lower than 2?m.It should be noted that high electrical conductivity of 16.4 S/cm was obtained at 500? in Argon atmosphere,with Seebeck coefficient of-370?V·K-1 and power factor of 230?W m-1K'2.The high performance was attributing to the improved crystallinity and the production of a large number of oxygen vacancies.The Al-doped-ZnO(ZAO)powders with various Al content(0.5%,1%,2%and 3%)were synthesized by co-precipitation method.High densed ZAO ceramics with density of 96.0%and grain size aound 200 nm were achieved by cold sintering and using acetic acid as solvent.High electrical conductivity of 458.7 S/cm was obtained at Al content of 0.5%after heat-treating at 900? in argon atmosphere for the as-cold-sintered ZAO ceramics.The results indicate high performance ZnO-based ceramics can be prepared by cold sintering method.