Preparation and Properties of SiC Based Electrical Conductive Ceramics

Author:Chen Jun Jun

Supervisor:huang zheng ren


Degree Year:2018





A series excellent properties of silicon carbide(SiC)ceramics support wide applications in various industrial fields.However,the extremely high hardness ranked behind only diamond gives a difficult problem for conventional grinding process.And the monolithic SiC ceramics with good properties may exhibit immutable semi-conductive properties,which may harm the application in electric field.But as the electrical resistivity of SiC ceramic is adjusted under 100Ω·cm,the machining process will be easy,rapid,effective and accurate via electrical discharge machining(EDM).And the EDM is more suitable to prepare complex surface of SiC ceramics compared with grinding process.For now,study on adjusting the electrical properties of SiC ceramics is still incomplete and the involving mechanisms are not clear.So it is significant to prepare SiC ceramics with high conductivity for large and industrialized fabrication and extended application.In this paper,we used different compositions to design SiC based ceramics.The sintering methods contained pressureless solid phase sintering,pressureless liquid phase sintering and hot press liquid phase sintering.The mutual relationships of relative density,composition,microstructure,and electrical properties were studied.And the relative mechanisms were investigated.Firstly,20wt%Zr B2/SiC composite ceramics were fabricated via pressureless solid phase sintering in argon atmosphere at different sintering temperature.The microstructure of ceramics could be controlled by sintering temperature.Then the electrical and mechanical properties could be adjusted.The details are as follows:The samples sintered at 1900°C have low relative density and the pores are interpenetrating;When the sintering temperature is increased to 2000°C,the relative density increases,the pores are isolated and distributed,and the resistivity is determined by grain boundary instead of the continuous pores;When the sintering temperature is further increased to 2100°C,the relative density further decreases,and the SiC grains gradually grow and the grain boundary phase decreases,resulting in a continuous decline in the resistivity;When the sintering temperature is 2200°C,the crystallinity of the carbon in the sintered sample increases,the amorphous layer enveloping the ZrB2 crystal grains disappears,the grain boundary becomes clear and clean,and the SiC crystal grains continue to grow and grow into columnar crystals,resulting in a significant decrease in resistivity again.However,if the sintering temperature is too high,excessive growth of Si C grains will occur,resulting in a decrease in the flexural strength.Therefore,in order to ensure low resistivity and high strength,the best sintering temperature is 2200°C.Secondly,2200°C was chosen as sintering temperature to investigate the effect of ZrB2 content on phase composition,microstructure,mechanical properties and electrical properties.The result show that all the samples have the relative density above 99%that is close to fully dense.The addition of ZrB2 improved fracture toughness remarkably.Importantly,the introduction of Zr B2 weakened varistor nonlinear characteristic of composite and reduced its resistivity.The reason is the evolution of grain boundary in conductive paths.The sharp decrease of resistivity indicates the formation of percolation paths.The percolation threshold obtained via percolation model is 10.80 vol%(19.71 wt%).This value is much less than conventional composites(33vol%),because the percolation path originates from grain boundary breakdown other than continuous conductor chains.Thirdly,the densification technology of atmospheric pressure liquid phase sintering SiC ceramics under argon atmosphere was discussed.The influences of dispersion medium,dispersant,buried powder conventional sintering process and sintering temperature on density,composition,microstructure,electrical properties and mechanical properties of ceramics were investigated.The results show as below:Water as a dispersion medium and dispersant can activate the dispersed powder and reduce the sintering activation energy of the sintered powder.During the liquid phase sintering,the liquid phase sintering aid will lose due to volatilization at high temperature.But buried powder sintering method can compensate for this loss.Samples sintered at 1950°C-2000°C have the highest relative density(>97%),and the samples sintered at 2000°C have the highest bending strength.The resistivity of the samples sintered at different temperatures are low as about 101Ω·cm,which is caused by the doping of Si C grains by Al atoms during high temperature sintering.Finally,the graphene oxide(GO)was used as the second phase in the hot-press liquid-phase sintering process.The SiC-based multiphase ceramics were prepared under the argon atmosphere.The results show that when GO is well dispersed in inorganic solvent water.Because the functional groups of GO such as-OH or-COOH can provide solvation power,so it is easy to form a hydration film on GO surface,which can ensure good dispersion of GO flakes in inorganic solvents.However,there is no solvation film in organic solvent ethanol,so the GO is easy to agglomerate.GO increases the bending strength of the ceramics but reduce the fracture toughness.In the sintering process,the GO sheets in the sample were distributed along the direction perpendicular to the pressure,resulting enhanced electrical anisotropy.In order to examine the influence of grain size on electrical properties,the samples were annealed at 1800°C.The results showed that the grain size of the sample without annealing was increased,and the resistivity decreased and the electrical anisotropy was weakened.