The Design And Research Of Strong Binding And Dense Coating On Inner Wall Of Quartz Crucible

In recent years, solar energy has received extensive attention and been hailed as astrategic alternative energy sources because of its abundance, wide radiation, safetyand non-pollution, etc.. Polycrystalline silicon becomes the main raw material ofphotovoltaic market with advantages of lower cost, simpler preparation process andrelatively higher conversion efficiency. The most common method of preparationduring industrial production is the directional solidification which realizes purificationof polycrystalline silicon through segregation coefficients of impurities. Thedirectional solidification is performed in a quartz crucible due to its relativeavailability in high purity and high melting point. A reaction between silica and liquidsilicon occurs when the temperature is close to or exceeds the melting point of silicon,leading to adhension. Consequently, intrinsic impurities of crucible especially metalimpurities which are fatal contaminations to influence the quality of silicon even atthe ppm and ppb range will diffuse into silicon ingot through the juncture. The majorsolution to these problems encountered in the process is to select a material with hightemperature resistance, high chemical stability and no influence on quality of siliconas an isolation layer between ingot and crucible.Si3N4meets these requirements completely as an emerging material withhigh-performance. However, the phenomenon that coating sheds from crucible duringthe directional solidification seriously affects the quality of silicon. This paper aimedat improving the binding strength by co-sintering of coating and green substrate,introducing nanosilica, H3BO3or mixture into Si3N4slurry and adjusting the contentsof the components. The experimental results indicated that:(1) PVA and PVP solution had better dispersion of Si3N4powders and had animprovement for the stability of slurry compared with water as dispersion medium.Slurries with3wt%PVA and7wt%PVP as dispersion media,30wt%、35wt%Si3N4,respectively, were selected as the base slurries.(2) Compared with the sintered one, the unsintered silica substrate was morelikely to be permeated by Si3N4slurry. After pretreatment Si3N4was partly oxidatedto cristobalite, and then this crystal tended to be amorphous after treatment at1500oCin Ar atmosphere. After heat treatment binding strength of coating brushed onunsintered substrate was greater than that of coating brushed on sintered one. With the addition of nanosilica, the permeation rate increased at beginning and then decreased;the content of cristobalite increased after pretreatment and binding strength betweencoating and substrate increased.(3) H3BO3was introduced into Si3N4slurry. The more the ratio of H3BO3andSi3N4was, the more obvious the connection of particles was. Different contents ofnanosilica was added with the fixed content of Si3N4and H3BO3. By the increasing ofratio of H3BO3, binding strength increased, and coating was denser.