Defect Evolution Of Zirconia Ceramics During Flash Sintering

Flash sintering is a novel sintering method developed in the recent,with a unique features of power surge,sharply increased electric conductivity and fast densification.Flash sintering has the advantages of low sintering temperature and fast densification,which fundamentally solves the problems of slow diffusion speed and high sintering temperature for a long time in traditional sintering methods,and is highly valued by researchers.However,due to the transient characteristic of flash sintering process(generally in a few seconds),the mechanism of flash sintering occurrence and the defect generation and rapid migration are still unknown.It is significant to study the defect evolution process of flash sintering and to understand the flash sintering mechanism deeply.This thesis focused on 3 mol% yttria stabilized zirconia(3YSZ)ceramics.Effect of furnace temperature,electric field strength,current density and holding time on 3YSZ defect concentration,microstructure and electrical properties were studied.The formation and evolution of defects and their influence on electrical properties were revealed by theoretical model calculation and microstructure analysis.The thesis is mainly on the following subjects:First,the effect of current density and furnace temperature on the conductivity of 3YSZ during flash sintering steady stage was studied.The difference of conductivity between with and without electric field was compared and the extra oxygen vacancies formed by electric field were calculated.The influence and evolution of defect modified by sintering parameters were investigated and the effect of extra oxygen vacancies on 3YSZ conduction behavior and mechanism was illuminated.Second,the effect of oxygen partial pressure on incubation stage,defect evolution and conduction was investigated.Third,the electrical properties differences of flash-sintered 3YSZ in different positons were analyzed by impedance.Calculated the oxygen vacancy concentration and discussed the evolution.The effect of oxygen vacancy on the grain and grain boundary electrical properties was indicated.Fourth,the number and evolution of stacking fault were analyzed by SEM and TEM to reveal the mechanism of oxygen vacancy on high dimensional defects during the flash sintering.The main conclusions include the following:(1)Adjusting the current density of extra electric field can change conductivity,power dissipation and sample temperature of 3YSZ in flash sintering steady stage.The conductivity of 3YSZ in steady stage was higher than 3YSZ with no extra electric field,this is due to the extra oxygen vacancy formed by extra electric field.The result of extra oxygen vacancy calculation shown the effective oxygen vacancy concentration which participate in conduction would first experience an increase,then decreased with the increasing current density.This is because high current density made the aggregation and nucleation of oxygen vacancies into high dimensional defects,the confinement ability to oxygen vacancies increased and conductivity decreased.The same trend was observed in different furnace temperature flash sintering.However,when the furnace temperature is 600? and below,the effective oxygen vacancies have no obvious regularity.It is inferred that the low temperature oxygen vacancy may be produced less and led an error influence.(2)The incubation time,steady stage power and resistance of flash sintering in anaerobic environment were all less than those of aerobic condition.At 700?,oxygen vacancies and negative carriers combine to form neutral oxygen vacancy space charge layer under low oxygen conduction(0.05-0.1).Oxygen vacancy is consumed and the conductivity decreased,resulting in a longer incubation time.When oxygen partial pressure exceeds 0.25,the incubation time increased with the increasing oxygen partial pressure from 225 s to 250 s,which inhibited the formation of oxygen vacancy and led to the prolongation of incubation time.(3)The impedance results shown that the electrical properties of flash-sintered 3YSZ in different regions differed greatly.The oxygen vacancy concentration in different regions was quantitatively calculated by dc bias impedance analysis and double Schottky barrier model.The results shown that oxygen vacancy concentration in the anode and middle regions is about 4 times higher than that in the cathode and no electric field regions.Combined with the different of electrical properties,it is shown that there are a large number of neutral oxygen vacancies in the cathode region,which increased the grain boundary impedance.The increase of current density would accelerate the carrier migration and increased the grain boundary conductivity.Prolonging the holding time can increase the density,thus reducing the material resistance.(4)The large number of high dimensional defects(twins and pores)were observed in the cathode by SEM and TEM and the defect density is related to the sintering parameters.The current density is the key factor in the formation of high dimensional defects.When current density is higher than a certain critical value,the oxygen vacancy defects can aggregate to form pores.Increasing the electric field strength can increase the pore size and quantity.In contrast,the holding time has no obvious effect on pore formation and morphology.