Flash Sintering Fabrication And Its Mechanism Of Zirconia

Ceramics are widely used due to their high temperature resistance,acid resistance,alkali resistance,and high strength.However,sintering of ceramics requires high temperature.Therefore,it consumes a lot of energy and increases its cost for production,which limits their application.A series of measures have been taken to reduce the cost of ceramics and improve their properties.On the one hand,the addition of sintering aids which enhances diffusion rate or creates a liquid phase allows for accelerated sintering;on the other hand,new sintering technologies are continuously developing,such as hot isostatic pressing,microwave sintering,spark plasma sintering,ect.These new sintering technologies not only reduce the energy consumption,but also greatly improve the properties of the ceramics.The objective of the thesis is to understand the phenomenological behavior of flash sintering and use it for broad ceramic sintering.Flash sintering was first reported by Rishi Raj's team of University of Colorado Boulder in 2010,which refers to the rapid densification of the green body under a certain temperature and a critical field.Usually,the densification of the green body in flash sintering is completed in a few seconds.Flash sintering presents two typical phenomena in the sintering process:a rapid densification and an abrupt nonlinear increase in power dissipation.Compared to other sintering methods,flash sintering presents the advantages of ultralow processing temperature and superfast sintering process.Although several possible mechanisms were proposed to explain the phenomena of flash sintering,underlying mechanism remains unknown.Based on this,we have carried out the research on sintering mechanisms of flash sintering from the following aspects:?1?Effects of sintering parameters,such as applied field,current density,furnace temperature,were studied on the onset temperature,incubation time,densification and microstructure of green body and dense one of ZrO₂.Results reveal that the estimated grain boundary thickness and specific conductivity increase with the applied field.The reason is that the applied field is in favor of the nucleation of oxygen vacancy,and the higher strength of the applied field leads to higher oxygen-vacancy concentration.?2?Effects of the oxygen partial pressure on the onset temperature or incubation time of flash sintering of ZrO₂ were studied in oxygen-enriched environment.Results indicate the onset temperature and the incubation time increase with the oxygen pressure.In oxygen-enriched environment,the onset flashing temperature increases with the increase of oxygen partial pressure by affecting the incubation time.Based on the present research,we find electron conduction plays an important role in the incubation stage.?3?Effects of sintering parameters,such as applied field,holding time,were studied on the onset temperature,incubation time,densification and microstructure of green body of ZrO₂.This may be explained by the fact that the nucleation of oxygen vacancy is related with the holding time.With the increase of the holding time,the oxygen vacancy concentration increases.?4?Effects of sintering parameters,such as applied field,current density,furnace temperature,were discussed on the onset temperature,incubation time,densification and microstructure of green body of 10 wt%SiC_w-ZrO₂ composite.The density of the composite is close to the theoretical value and the whiskers are strongly bonded with the ZrO₂ matrix.The hardness and fracture toughness of the composite are measured by indentation technique to be 11.0±0.3 GPa and 9.7±0.3 MPa.m^1/2,respectively.This method effectively obviates constrained sintering in whisker reinforced composite,which clearly demonstrates the flash sintering could be a good method to prepare whisker reinforced composite.?5?Effects of sintering parameters,such as applied field,current,were studied on the onset temperature,densification and microstructure of green body of Al₂O₃-Y₃Al₅O₁₂-ZrO₂.As-prepared ceramics well preserve the phase composition and microstructure of the original eutectic powder.The fabricated ceramic shows reasonably high hardness and very good fracture toughness,being 13.91±1.43 GPa and 6.20±0.34 MPa m^1/2,respectively.These results clearly demonstrate the feasibility of employing the flash-sintering technique to fabricate high-performance eutectic ceramics.