Fast Sintering Of Zirconia Ceramics

Zirconium oxide is a structural ceramic material suitable for several applications including dental crown,thermal insulation coatings,refractory materials and many other fields because of its hardness,high melting point,and low thermal conductivity.The sintering processing of zirconia ceramics is greatly retarded by the low self-diffusion coefficient of Zr ions,which require high temperature（1300-1600°C）and long soaking time（1-4 h）in conventional sintering（CS）system.The latter is an energy intensive process,leading to grain coarsening with detrimental effects on the mechanical properties of ceramics（i.e.,flexural strength）.This text introduced two kinds of innovative techniques Flash Sintering（FS）and Ultra-fast High temperature Sintering（UHS）and explored them:ⅰ)the contactless flash sintering using Dielectric Barrier Discharge（DBD） technique;ⅱ)the athermal field effect in flash sintering,cause by a notch,to study how temperature,and current density influence final microstructure;ⅲ)thermally insulated UHS technique.In this thesis,3YSZ（commercial TZ-3YSB-E,Tosoh,90 nm particle size,https://www.tosoh.com/our-products/advanced-materials/zirconia-powders）was used as starting material.In the third chapter,the contactless discharge sintering technology was employed and the effects of discharge time,the types of plasma and current were explored.Herein,a disk shape 3YSZ specimens was treated by various discharge time（0,3,10,30 s）and plasma resource（oxygen,argon）to figure out the mechanism of sintering.In this case,Scanning Electron Microscopy（SEM）confirmed that under a cold plasma discharge time for 30 s and O₂ plasma source,a relative density of～99%was obtained;conversely,when the arc plasma was attempted,the samples lost their integrity and high current concentration onto sample was recorded（>100m A）compared with oxygen（15m A）causing hot spots and uneven densification.The study confirmed that the sample did not sinter by the contact with cold plasma but by the current flowing across the samples.The fourth chapter used flash sintering and conventional sintering to cure macroscopic defects in nano-zirconia（i.e.notch）and studied the shrinkage of defects by adjusting the current density and holding time in flash sintering,which revealed how temperature and current density affect the final microstructure.From SEM observation,the grain size of the FS sample grew up in range of 0.4-8.6μm and presented a certain gradient between-500 and+500μm area where in vicinity of notch.In comparison with CS process,homogeneous microstructure was presented in these areas.The assisted COMSOL simulation point out grain growth not not directly correlated with temperature increasing but for current concentration.High current density weekly retarded grain growth and accelerated densification,while porous and large grain size showed under low current density.This work solved the problem of long-term athermal field effect in FS and provided a path for healing and joining of material.The mechanical properties of zirconia are closely related to the grain size,especially the fracture toughness and bending strength values are particularly sensitive to it.In the fifth chapter,sintering of zirconia sample was carried out using Ultra-fast High-temperature Sintering（UHS）.Herein,we altered power output,heating rate,sintering time and controlled temperature factors to study the microstructure evolution and densification.At the same time,the effect of thermally insulated sintering was also studied by adding alumina insulator（80×30×30 mm³）on the surface of carbon felt.Experimental result pointed out that at similar power output,the final density of thermally insulated UHSed zirconia is 99%versus 68%（no insulator applied）;when the current output was 35 A and sintering time was 60 s,a homogeneous bulk zirconia sample with a relative density of 99%with a fine grain size of 178±18 nm was obtained.By cross-linking finite element simulations with master sintering curve calculations,a good fitting between experimental and predicted density was obtained,allowing the development of predictive models for UHS densification.