Recycling Photovoltaic Silicon Waste For Fabricating Porous Mullite Ceramics

Solar energy is a clean and renewable source of energy.Photovoltaic technology,which converts solar energy directly into electricity,is booming.However,in the process of making silicon solar cells,a large amount of solid waste is produced.The photovoltaic waste contains solid particles with very small particle size and water-soluble organic additives,which can easily cause serious pollution to the environment.Therefore,in order to promote the healthy development of the photovoltaic industry,it is very important to recycle the valuable resources in the photovoltaic waste.In this paper,mullite porous ceramics were prepared using photovoltaic waste as the main raw material.The research contents of this paper are as follows:（1）Porous mullite ceramics with coral-like structure were prepared by solid state reaction sintering using PSW as silicon source andγ-Al₂O₃as aluminum source.The phase change,linear shrinkage,volume density,microstructure,porosity,mechanical strength,pore size distribution and gas flux of the sintered samples were characterized,and the reaction mechanism of mullitization at low temperatures was studied.Using ammonium molybdate tetrahydrate as an additive,the initial temperature of mullitization of porous ceramics could be reduced from 1000℃ to less than 900℃,and the final temperature of mullitization could be reduced from more than 1100℃ to 900℃.In the microstructure,mullite grains were cross-linked and interlocked,which made the samples with 5%ammonium molybdate tetrahydrate had the bending strength of 53.18 MPa when sintered at 900℃.The porosity of mullite ceramics prepared under different conditions varied little,ranging from 40.93 to 43.38%.As the sintering temperature increased from 900℃ to1100℃,the average pore diameter and nitrogen flow of the samples decreased.（2）Porous mullite ceramics were prepared at 800-1100℃ using PSW and alumina hydroxide treated at different temperatures as silicon and aluminum sources,respectively.Ammonium molybdate tetrahydrate and ammonium tungstate are used as additives respectively.The effects of the types of additives,the calcination temperature of aluminum hydroxide and the sintering temperature on the properties of mullite ceramics,such as microstructure,phase composition,strength,porosity,linear expansion rate and pore size distribution,were studied.The results showed that the mullitization reaction of samples with ammonium molybdate tetrahydrate and ammonium tungstate as additives occurred at800℃ and 1000℃,respectively.With the increase of heat treatment temperature from400℃ to 800℃,the strength of mullite porous ceramics increased from 28.86 MPa to37.71 MPa,and the porosity of mullite porous ceramics decreased from 52.58%to 44.39%.As the sintering temperature increases from 800℃ to 1000℃,the average pore size of the sample increased.（3）Mullite/Si C porous ceramics were prepared by two-step sintering using silicon waste produced by cutting silicon ingot with Si C mortar in photovoltaic industry as silicon source and Al（OH）₃as aluminum source.The effects of sintering system,additive types and sintering temperature on the structure and properties of porous ceramics were studied.The study of sintering method showed that mullite whiskers with better crystallinity could be obtained by two-step sintering.The study of additive types showed that a large number of mullite whiskers could be produced in porous ceramics by adding molybdenum trioxide and ammonium molybdate tetrahydrate.However,compared with molybdenum trioxide,when ammonium molybdate tetrahydrate was used as an additive,the ratio of length to diameter of mullite whiskers in the porous ceramics was higher.Mullite/Si C porous ceramics with porosity between 67.12-73.06%and volume density between 0.77-0.97g·cm^-3could be prepared by two-step sintering process.