| Quartz（Si O2）crucible for crystallization of high-purity silicon in the photovoltaic industry has been facing the problems of crystallization fracture and adhesion of silicon ingots for a long time,which affects the quality of silicon ingots.Reusable crucible materials such as Si3N4,Si C and Al2O3 have been developed around the world,but they are difficult to produce and will contaminate the Si ingot.Therefore,it is urgent to develop high-performance Si O2 based crucible materials to meet the requirements of solar grade high-purity silicon materials for the rapid development of photovoltaic industry.In order to inhibit the crystallization of Si O2 and avoid contamination,silicon nitride（Si3N4）could be used as additive material and introduced in to fused Si O2.However,the research on the effect of inhibiting crystallization during high temperature service（>1410°C）is insufficient and the mechanism of inhibiting crystallization is not clear.For the adhension between Si melt and quartz crucible,α-Si3N4 coating is applied as diffuse barrier due to its non-wetting property and hard infiltration.However,the barrier effect ofα-Si3N4coating is limited and Si melt can easily penetrate the coating.Then,reactive infiltration occurs between Si melt and Si O2 and Si melt is wetted with Si O2,which results in adhension between Si and Si O2.To solve the above problems,this work intends to prepare a new type of Si O2 based composite with high Si3N4 content.First,the crystallization behavior of Si O2during sintering preparation（1250°C）and service（1450°C-1550°C）should be effectively inhibited in the composite to improve the volume stability of the material.Second,reinforced non-wettability between Si melt and crucible material is required to ensure that the composite has good resistance to Si melt adhesion and infiltration when Si melt penetrates the coating and contacts with crucible material.According to Cassie-Baxter theory,the contact angle between Si melt and multicomponent materials is closely related to the relative proportion of each component and their intrinsic contact angle.For Si3N4-Si O2 composite,Si3N4,Si O2 and pore are contained and the contact angles between the above components and Si melt are 49°,85°and 180°respectively.It can be considered that the interface characteristics between Si melt and Si3N4-Si O2 composite can be controlled by optimizing the content of Si3N4（used as crystallization inhibitor）and pore structure of the material.The research content includes the following points:firstly,Si3N4-Si O2 composites with adjustable porosity and different content of Si3N4（2-20 wt%）were prepared usingα-Si3N4as crystallization inhibitor by gel casting method.Secondly,crystallization behavior of Si3N4-Si O2 composites at sintering temperature and service temperature were studied and the inhibiting crystallization mechanism of large content Si3N4 was revealed.The effects ofα-Si3N4 content and porosity on the wetting behavior of high temperature interface between Si melt and composites were investigated.The infiltration conditions and mechanism of Si melt on composites surface were revealed.Finally,low costβ-Si3N4 was introduced into composite and coating.Then crystallization ofβ-Si3N4-Si O2 composite and interfacial behavior betweenβ-Si3N4 containing“coating/composite”systems and Si melt were systematically investigated for further reducing the manufacturing cost of Si3N4-Si O2composites.Based on the above research works,the following conclusions can be drawn:1.During gel-casting process,optimized slurry was prepared by controlling the content of dispersant AA/AMPS and the p H value of the pre-solution.When the dispersant content is 0.3 wt%and the p H value is 3,the slurry has the lowest viscosity and the powder has the best suspension stability in the slurry,leading to homogeneous spacing.After sintering,ceramic prepared from above slurry has uniform structure and the highest Weber modulus.Forα-Si3N4-Si O2 composite,small amount ofα-Si3N4（≤10 wt%）would fill the gaps between directly contacted Si O2 particles,which improved the density of composite and promoted sintering.However,20 wt%α-Si3N4 would completely separate Si O2 particles.The difficult sintering characteristics betweenα-Si3N4 particles would hinder the sintering of composite.High performanceα-Si3N4-Si O2 composites could be achieved by adjustingα-Si3N4 content and sintering temperature.The porosities of composites variy from 26.3%to 34.1%.And the flexural strengthes change from 39.1 MPa to 47.3 MPa,which are much higher than that of industrial Si O2 material（20 MPa）.2.During high temperature service（1450°C-1550°C）,the crystallization amount inα-Si3N4-Si O2 composites decreases gradually from 29%-69%（Si O2 ceramic）to 2-10%（composites with 20 wt%α-Si3N4）.On the one hand,the introduction ofα-Si3N4 could reduce the direct contact between Si O2,which inhibits the diffusion and mass transfer between Si O2 particles and further inhibits the fracture and rearrangement of Si-O bond.On the other hand,the O-Si-N chemical bond with high binding energy（535.5e V）could be formed in situ,which would increase the crystallization activation energy from 409.6 k J/mol for pure Si O2 material to 931.2 k J/mol forα-Si3N4-Si O2 composite with 20 wt%α-Si3N4.3.The wettability of Si melt on substrate when Si completely melts is related to porosity.Non-wettability with contact angle of higher than 120°could be obtained on porous substrate,while wettability with contact angle of 70°is achieved on densed substrate.The interfacial behavior between Si melt and substrates after complete melting is related with composition and porosity.With regard to infiltration,the interfacial reaction is gradually enhanced with addition ofα-Si3N4,leading to formation of Si/Si3N4 composite layers with depth of 4μm-16μm.For wettability,there is a transformation from non-wetting to wetting for porous due to the decrease of porosity caused by sintering.In addition,the contact angle between Si melt and subtrates decrease from 88°for Si O2 ceramic to below 25°forα-Si3N4-Si O2 composites.However,the contact angle of Si melt on composite with 20 wt%α-Si3N4reaches 130°due to the synergy effect of high porosity and pinning effect.On the one hand,high porosity benefited the non wettability when Si completely melted.On the other hand,severe interfacial reaction led to the in-situ formation of grooves at the triple line.The pinning effect of groove wall would limit the spread of Si,resulting in higher apparent contact angle and non-wetting behavior.4.The introduction ofα-Si3N4 into the substrate material can significantly improve the interfacial behavior between the Si melt and theα-Si3N4 coated substrate.For wettability,the transition time from non-wetting to wetting is prolonged from 320 s for pure Si O2substrate to 4320 s for composite with 2 wt%α-Si3N4.And non-wettability with contant angle of 102°-123°could be obtained with the further increase ofα-Si3N4 content.With regard to infiltration,a porous Si3N4 layer would form when Si penetrates the coating and contacts with substrate,which plays a role of secondary isolation and improves the resistance to Si adhesion.5.Compared withα-Si3N4,β-Si3N4 can also inhibit the high temperature crystallization of fused silica.Under high temperature service conditions（1450°C-1550°C）,the crystallizing amount ofβ-Si3N4-Si O2 composite material gradually decreases from 5%-40.9%（pure Si O2）to 0.1%-6.3%（composite with 20 wt%β-Si3N4）.The mechanism of inhibiting crystallization is the same as that ofα-Si3N4.6.The introduction ofβ-Si3N4 into the substrate material can also improve the interfacial behavior between Si melt andα-Si3N4 coated composite substrate.The contact angle is higher than 122°and a porous silicon nitride layer is formed after infiltration,which improves resistance to Si adhesion.Whenβ-Si3N4 is used as the coating material,the contact angle of Si melt onβ-Si3N4 coatedα-Si3N4-Si O2 composite substrate andβ-Si3N4 coatedβ-Si3N4-Si O2 composite substrates are 129°and 138°,respectively.And a porous Si3N4secondary isolation layer would be in-situ formed when Si melt penetrates through theβ-Si3N4 coating,which could improve the resistance to Si adhesion. |
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