Preparation Of Silicon Carbide-aluminum Nitride Ceramic Composites Using Photovoltaic Silicon Waste

As a result of developments in the photovoltaic industry,more and more photovoltaic silicon waste（PSW）has been generated as a solid contaminant.This added contamination augments the burden of environmental protection for enterprises and results in the waste of valuable resources.PSW is mainly composed of polyethylene glycol（PEG）cutting fluid,silicon carbide abrasive,high purity silicon powder,a small amount of scrap iron and water.However,it is difficult to separate Si powder from PSW because of its fine granularity and the similar particle size range of SiC powder.Therefore,indirect use of Si powder is the second-best method in the current status of direct recycling.SiC-AlN composites is an advanced ceramic material,which has good application in aerospace and electronic substrate.However,the thermal conductivity of SiC-AlN composite ceramics is lacking,which seriously restricts the application of this material.In this paper,SiC-AlN composite powders were synthesized by mixing purified photovoltaic silicon waste with AlN,and SiC-AlN ceramic composites were prepared by spark plasma sintering.The main researches are as follows:Firstly,solid PSW was purified as SiC source,mixed with AlN powder,and Si-C was synthesized in argon atmosphere in vacuum sintering furnace.The effects of different carbon content,temperature and AlN content on the product phase and microstructure of composite powders were studied.The best effect compound powder was prepared at1400℃.With the increase of temperature,SiC whiskers formation was gradually observed with the Si-C reaction was gradually carried out.Subsequently,Y₂O₃was added as sintering additives to the optimal powder,and the SiC-AlN ceramic composites were prepared by spark plasma sintering.The influence of sintering temperature and AlN content on composite ceramics was studied.The results show that the mechanical properties of ceramic composites are greatly improved by Y₂O₃.The ceramic composites with 5 wt%AlN content sintered at 1900℃attended the best thermal conductivity of124.7 w/（m·K）,while the ceramic composites with 15 wt%AlN content reached the best bending strength of 571.6 MPa.Secondly,Al powder was used as AlN source to prepare SiC-AlN composite powders by Si-C reaction with nitrogen reaction of Al.The results show that SiC-AlN composite powders with SiC whiskers were obtained,which is the same as the composite powder prepared from AlN.SiC with particle size is uniformly distributed in the composite powder,and there is a certain amount of SiC whiskers.In addition,the intermediate product Si₅Al ON₇is formed at low reaction temperature and converted to SiC-AlN composite powder when the temperature reaches 1600 to obtain.The results showed that the thermal conductivity of the composite ceramics sintered at 1900 with 5wt%AlN content was122.6W/（m K）,and the 15wt%AlN content sample had the best flexural strength of467.9MPa.With the increase of AlN content,the solid solution of the sample was obvious,and the fracture mode of composites changed from crystal fracture to transgranular cleavage fracture grandually.Finally,the purified PSW and AlN were used as raw materials for reactive sintering.The influence of sintering temperature and different carbon content on one-step sintering was studied.The results showed that the density reached the maximum value of 95%at1900℃under normal carbon content,and the flexural strength was 246MPa at this time.The two-step sintering with different holding temperature was analyzed.It is found that a small amount of silicon exudates from the composite ceramics,resulting in the density decrease to about 84%.The performance of ceramic composites obtained by one-step sintering is higher than that obtained by two-step sintering.However,the performance of ceramic composites obtained by one-step sintering is lower than that sintered by SiC-AlN composite powders.