| As a new generation of catalyst support materials,porous silicon carbide(SiC)ceramics can significantly improve catalysis performance due to the excellent physical and chemical properties.Recently,mullite oxide-bonding is developed to realize a low-temperature and low-cost sintering process for porous SiC.However,there are still two main challenges limiting this technique.(1)Increasing the sintering temperature accompanies enhanced oxidation of SiC resulting in the mass formation of cristobalite/silica phase without being fully transformed to mullite.While reducing the sintering temperature compresses the formation of the cristobalite phase but also limits the mullite densification.Therefore,the mechanical property of mullite-bonded SiC porous ceramic still needs further improvement.(2)As a low-cost,environmentally friendly pore-forming technique,the freeze-casting method has been applied in the fabrication of porous SiC ceramics.Thus fabricated porous ceramics exhibits high porosity and good connectivity.However,the pore structure is generally lamellar,which is not suitable for catalyst support applications.In this paper,it was realized to control the performance of the mullite oxide-bonded SiC porous ceramics by combining the properly design of the bonding agents and control the sintering conditions by the freeze-casting method.The main results are as follows:(1)Based on the mullite oxide-bonded method,a composite bonding agent system was proposed to realize both liquid-phase and gas-phase mass transfer.The mullite oxide-bonded SiC porous ceramics sintered at 1450-1550℃ exhibited ’a high amount of mullite’,’good mullite densification’ and ’no detectable cristobalite phase by XRD analysis’.There were a large number of mullite whiskers in samples sintered at 1550℃for 2 h,which significantly improved the flexural strength reaching 135 MPa,33%higher than that fabricated by the SPS technique.In addition,bonding agents also affected the pore structure.With addition Al(OH)3,Y2O3 and CaF2,the pore shape transformed from strip-like to continuous channel-like and irregular shape,respectively.It is feasible to control the performances of mullite oxide-bonded SiC porous by the proper design of the bonding agent.(2)Based on the freeze-casting method with water as the freezing vehicle,effects of the freezing temperature on the ice growth behaviour and the pore structure were studied.It was achieved that the pore structure transformation from lamellar(with the distance between lamella-10 μm,lamella thickness-4 μm)to cellular(with the pore diametre-25 μm)by increasing the freezing temperature from-80℃ to-15℃,which could solve the problem of the monotonous pore structure fabricated by the water-base freeze-casting method.The reason for the pore structure transformation is that the decreased freezing temperature weakens the driving force of the ice growth during freezing.Thus,the solid particles in front of the growing ice crystal cannot be efficiently repelled.Then,they are entrapped by the ice crystal,and ’entrapment’ phenomena occurred.Based on this,the ’entrapment index’ was proposed to qualitatively describe the likelihood of entrapment.Since the experimental phenomena reported in these paper and other articles can be well explained by the ’entrapment index’,it is reasonable and universally valid,providing theoretical guidance on the pore structure design for the fabrication of the porous ceramics by the freeze-casting method. |
