Preparation Of Fused Silica Support And Its Properties

Photocatalytic membrane reactors（PMRs）has the function to separate water from polluted water under ultraviolet light.It is environmental-friendly,sustainable in continuous operation and is one of the most promising methods for wastewater treatment.However,the efficiency of photocatalytic reaction in current configuration of PMRs would be reduced as ultraviolet is blocked by membrane fouling.In this case,a configuration that arrange the UV light source on the opposite side of the ceramic membrane layer is proposed and has well prospect to solve the problem theoretically.Fused silica,commonly used in functional optical components,is a unique material possessing low thermal expansion,high chemical stability while high transparency within UV and visible light range.Ceramic membranes with UV transparent fused silica support would be an ideal component for the new configuration of PMRs.The key work of this thesis is to develop a method for the preparation of fused silica ceramic membrane support with high UV transparency,high water permeability and sufficient mechanical strength.The main conclusions are as follows:Firstly,crystallization mechanism of fused silica and the effect of crystallization inhibitors on fused silica were studied to solve the crystallization problem while sintering.Crystallization activation energy of the as-received fused silica was calculated via JMA equation to be 831kJ·mol^-1·K^-1.The crystallization activation energy for fused silica with 3%of Nd₂O₃,H₃BO₃and 5%of H₃BO₃ increased to 1218 kJ·mol^-1·K^-1,1045 kJ·mol^-1·K^-1,1239 kJ·mol^-1·K^-1,respectively.By adding 5%of H₃BO₃,the crystallization of fused silica was hindered through liquid phase mechanism,showing the best efficiency in inhibiting crystallization.Secondly,the effects of sintering temperature,holding time and boron acid content on porosity,sintering shrinkage,UV transmittance and crystallization were studied by orthogonal experiments.Results shows that the properties of fused silica support were significantly influenced by boron acid content.Boron acid can dehydrate into boron acid which reacts with fused silica at high temperature to form liquid phase,both inhibiting crystallization and assisting the sinter process.According to the range analysis of each factors,optimal fused silica support was prepared by adding 5%boron acid and sintering at 1150℃for 1 h.The as-prepared support did not crystallize,showing a mean pore size of 29μm,open porosity of 42.3%,UV transmittance of 33.9%,bending strength of 14.9 MPa and pure water flux of 764.32 m³·m^-2·h^-1.The support possesses both basic properties for the application in ceramic support and superior UV light transmittance.Finally,the effects of porosity,thickness and pore medium on the UV transmittance of fused silica support were studied.Besides,P25 powder was immobilized on the support and photocatalytic experiments with UV light source on each side of the support were conducted.The porosity and thickness mainly affect the UV transmittance by altering the scattering centers with the support,while pore medium affects the scattering efficiency by altering the difference of refractive index and thus affects the UV transmittance.It is noteworthy that the UV transmittance of the support submersed in water was as high as 64.2%,much higher than that of the present reports.In this case,the degradation rate of methyl orange solution was 84.9%with UV light on the front side and 68.5%on the opposite for support loaded with 8 mg·L^-1 P25sol.The photocatalytic reaction rate for samples with UV light source on the back was 61.2%of the front side on average,indicating great potential for the application in the new configuration of PMRs.