The characterisation of aqueous silicon slips

The optimum conditions of aqueous silicon slips for slip casting was achieved by employing various experimental techniques like zeta potential, particle/floc size distribution, sedimentation, viscosity and rheological measurements as a function of pH of the slips in the pH range of 2–11. Silicon suspensions display a maximum in zeta potential values and a minimum in sedimentation height and viscosity in two pH regions of 4–5 and 8. The slips behaved as near-newtonian at these pH values up to a solid loading of 52 wt% and as non-newtonian with thixotropic behaviour above this solid loading. Samples with highest green density (68%) were produced from 72 wt% solid loaded slips which were conditioned at a pH of 8 and milled for 26 h. The cast bodies of these slips were nitrided to obtain reaction bonded silicon nitride products which were also characterised.     

Application of polypropylene carbonate for the tape casting of silicon nitride ceramics

QPAC40 (polypropylene carbonate), with a little decomposition residue, is commonly used as a binder in aluminum nitride (AlN) tape casting. In this paper, we tried to explore its application in silicon nitride (Si₃N₄) tape casting. By studying the influence of dispersant, binder, plasticizer/binder ratio, and solid loading on slurry and green tape properties, the optimum formulation of the tape casting of Si₃N₄ slurry was determined, and the green tape with a uniform structure and relative density up to 63.16% was prepared. Si₃N₄ ceramics were obtained by debinding at 600°C for 1 h in vacuum and gas-pressure sintering at 1830°C for 2 h in N₂. The thermal conductivity and flexural strength of Si₃N₄ ceramics were 56.28 ± 1.21 W/(m·K) and 1130.67 ± 23.58 MPa, respectively. These results indicated that QPAC40 can be used to prepare Si₃N₄ sheets through tape casting.     

晶体硅铸锭用氮化硅涂层失效机理及改性研究

氮化硅涂层是多晶硅铸锭必不可少的脱模剂,新型高效多晶硅铸锭对氮化硅涂层提出更高的要求。重点研究了氮化硅涂层在新型高效多晶硅铸锭过程中的失效机理,结果显示,氮化硅的碳化是导致氮化硅涂层与硅熔体间非浸润性降低,以及氮化硅涂层失效的主要原因。采用改进的溶胶-凝胶法（Sol-Gel）结合表面无碳处理制备增强型氮化硅涂层,该涂层在铸锭实验中显示出较高的强度及与硅熔体间良好的非浸润性,能够很好地满足新型高效多晶硅对氮化硅涂层的要求。     

氮化硅流延膜的制备

流延成型是一种制备高质量陶瓷基片的成型方法.氮化硅是一种高热导率的材料,有望在电子基片领域获得应用.本文利用流延成型制备了具有较好柔韧性和一定强度的氮化硅流延素坯膜.研究了无水乙醇、无水乙醇/丁酮作为溶剂时对浆料粘度的影响.通过优化流延浆料添加剂的各种配比,得出了适合氮化硅粉体(SN-E10)流延的最佳配方.     

Direct coagulation casting of silicon nitride suspension via a dispersant reaction method

A direct coagulation casting method for silicon nitride suspension via dispersant reaction was reported. Tetramethylammonium hydroxide (TMAOH) was used as dispersant to prepare silicon nitride suspension with high solid loading and low viscosity. Influences of TMAOH and pH value on the dispersion of silicon nitride powder were investigated. Glycerol diacetate (GDA) was used to coagulate the silicon nitride suspension. Influences of the concentration of glycerol diacetate on the viscosity and pH value of the suspension were investigated. It was indicated that high viscosity sufficient to coagulate the suspension was achieved by adding 1.0–2.0 vol% glycerol diacetate at 40–70 °C. The coagulation mechanism was proposed that the silicon nitride suspension was destabilized by dispersant reacting with acetic acid which was hydrolyzed from glycerol diacetate at elevated temperature. Coagulated samples could be demolded without deformation by treating 50 vol% silicon nitride suspensions with 0.2 wt% tetramethylammonium hydroxide and 1.0–2.0 vol% glycerol diacetate at different temperatures. Dense silicon nitride ceramics with relative density above 98.8% had been prepared by this method using glycerol diacetate as coagulating agent sintered by different methods.     

Vacuum heat-treatment of MgO-densified silicon nitride ceramics and their compatibility with molten aluminium and copper

Silicon nitride with 3% MgO powder mix was compacted with cold isostatic pressing followed by uniaxial pressing. Pressureless sintering of the compacted silicon nitride (Si₃N₄) crucibles was at 1600 °C for 30 min in the carbon furnace. The densities achieved after this process are 3140 kg/m³. One of these crucibles was vacuum heat treated at 1575 °C for 5 h to remove grain boundary glass. Both this crucible and the as-sintered crucibles were used for melting aluminium and copper by heating in, air atmospheres to 700 °C and 1100 °C, respectively. Vacuum heat-treated glass free ceramic crucible was successfully used for the molten aluminium and copper handling. On the other hand, the chemical bond occurred for as sintered Si₃N₄ crucible. No adherence was observed after examining of interface between Al and the heat-treated ceramic crucible with SEM and EDX analysis.     

多晶硅生产中硅芯氧化问题探究

改良西门子法生产多晶硅中硅芯是多晶硅生长的载体,硅芯氧化会降低多晶硅产品品质,硅芯氧化的产品对下游铸锭、直拉均有不同程度的影响。目前硅芯氧化是影响多晶硅产品质量的一大问题。分析了硅芯氧化机理,针对相关因素开展了试验,提出了降低硅芯氧化率的措施。     

Thermal and corrosion properties of silicon nitride for copper die casting components

Due to the high melting temperature of copper and copper alloys, conventional die-steel components used in pressure die casting these materials exhibit short service lifetimes and undergo thermal fatigue. Thermal and corrosion properties of silicon nitride were studied to assess the material's applicability in substituting conventional die-steels in casting copper and copper alloys. In this study, experiments were conducted to test the thermal shock resistance and corrosion behaviour of a commercial silicon nitride in contact with molten pure copper. The results did not indicate any corrosive reaction between silicon nitride and pure copper. However, the presence of copper oxides at high temperatures accelerated the oxidation of the ceramic resulting in considerable loss of material. The thermal shock behaviour of silicon nitride proved to be adequate for the application.     

Zn/N共掺杂碳全包覆切割废硅料用于锂离子电池负极材料

为使光伏切割废硅料再生用于锂离子电池负极材料,设计了一种一步法实现Zn/N共掺杂碳全包覆切割废硅料的复合结构电极材料。利用PDDA络合剂“桥连”作用,解决了酚醛树脂无法在尺寸较大、形貌不规则的切割废硅料表面成核生长的问题。设计的复合结构缓解了硅在充放电循环过程中巨大的体积膨胀,同时提高了材料的导电性。获得的wSi@NC/Zn-2电极在0.2 A/g的电流密度下循环300圈后具有1392 mAh/g的容量保持,在0.5 A/g大电流密度下循环300圈仍有1082 mAh/g的可逆容量,还具有优异的倍率性能。     

Chemical stability of silicon nitride coatings used in the crystallization of photovoltaic silicon ingots. Part I: Stability in vacuum

Photovoltaic silicon is currently grown in silica crucibles coated with an oxidized silicon nitride powder, which acts as an interface releasing agent between the silicon and the crucible. A series of experiments was performed to study the reactions between coating components under high vacuum, varying the temperature, the holding time and the oxygen content in the coating. The results are discussed with the help of a simple analytical model taking into account the diffusive transport of reaction species from the inside of the porous coating to its surface and then their evaporation into the vapour phase.     

Recycling of silicon kerf waste for preparation of porous SiCw/SiC membrane supports by in situ synthesis

Recycling has enormous economic benefits and practical significance under the context of gradually increasing solid wastes. In order to recycle and reuse the silicon kerf waste, in this work, porous SiC_w/SiC ceramics were successfully prepared by in situ synthesis from silicon kerf waste and fired at 1400-1500°C for 4 hours. The results showed that these porous ceramics, reinforced by the interlocking whisker, revealed high apparent porosity (48.02%-51.76%) and cold compressive strength (5.68-9.54 MPa). Furthermore, the practicable pore size (2.09-2.53 μm) and decent durability showed the potential of these porous ceramics as membrane supports. This work verified the possibility of the SiC-based ceramics prepared from the silicon kerf waste.     

Porous Si3N4 ceramics prepared via slip casting of Si and reaction bonded silicon nitride

Slip casting process combined with reaction bonded silicon nitride (RBSN) was used to prepare porous Si₃N₄ ceramic with near-net and complex shape. A butyl stearate (BS) coated process was introduced to restrain the hydrolysis of Si, and ammonium polyacrylate (NH₄PAA) was used to enhance the dispersion of coated Si. The measured oxygen content showed that the hydrolysis of Si was strongly prohibited by BS coating, and relatively low viscosity was obtained with the addition of 0.25-1.5 wt% NH₄PAA to the 60 wt% solid load slurry. 40-60 wt% solid load slurries were used for slip casting in the experiment. After vacuum degassing, slip casting, debindering and nitridation, a density of 1.57-1.92 g/cm³ (porosity 50.9-40%) and a flexural strength of 47-108 MPa were obtained. The samples without vacuum degassing showed a large number of nanowires grown in the large pores.     

Chemical stability of silicon nitride coatings used in the crystallization of photovoltaic silicon ingots. Part II: Stability under argon flow

Processing of photovoltaic silicon by solidification is currently carried out under argon flow in silica crucibles coated with an oxidized silicon nitride powder. A series of experiments was performed to study the reactions between coating components under argon flow by varying the temperature, the holding time and the oxygen content in the coating. The results are discussed with the help of a simple analytical model taking into account the diffusive transport of gaseous reaction species from the inside of the porous coating to the flowing argon. The conclusions drawn are used to discuss different practical aspects of the photovoltaic silicon crystallization process.     

Microstructure and mechanical properties of silicon nitride with tri-laminate structures

Silicon nitride ceramics with tri-laminate structures were prepared using two kinds of layers; layer with the aligned silicon nitride whisker seeds (named as “S” layer) and layer without the seed (“N” layer). The fracture toughness values on the casting surface of N layer of sample with a tri-laminate structure (N–S–N structure) showed an anisotropy, and this is contrary to the isotropic fracture toughness observed from the casting surface of sample consisting of only N layers. The fracture toughness anisotropy observed from N layer of the former sample is explained in terms of the microstructural anisotropy induced by the sintering shrinkage anisotropy within the casting plane.     

Development of a tape casting process for making thin layers of Si3N4 and Si3N4 + TiN

A process for the tape casting of silicon nitride ceramics has been developed and is described in detail. A solvent (ethanol) based recipe was developed using polyvinyl butyral and polyethylene glycol as the binder and plasticizer, respectively. The effect, of milling times, dispersant, solvent, plasticizer and binder contents were all investigated as well as that of the binder to plasticizer ratio. In addition the beneficial effect of multi-stage milling of the slip was evaluated. The removal of the ceramic tape from the carrier film is described. In addition the recipe and process used for producing silicon nitride tape was successfully adapted for the production of two different silicon nitride + titanium nitride composite based tapes. From the tapes produced it was possible to hot press dense multi-layer laminate structures with the thinnest layers being 45 μm thick.     

Direct coagulation casting of silicon carbide suspension via polyelectrolyte dispersant crosslink reaction

A direct coagulation casting method for silicon carbide ceramic suspension using dispersant crosslink reaction is reported. Polymer electrolyte (polyethyleneimine, PEI) was used as dispersant to prepare silicon carbide suspension. Common food additives (carboxymethyl cellulose, CMC) were used to coagulate the electrosteric stabilized silicon carbide suspension. There was a well disperse silicon carbide suspension with 0.2 wt% PEI at pH = 5-6. Influence of coagulant on viscosity and zeta potential of the silicon carbide suspension was investigated. It indicates that the high solid loading silicon carbide suspension can be destabilized and coagulated at elevated temperature. It can be attribute to the gradual decrease of electrosteric force due to the crosslink reaction between PEI and CMC. Silicon carbide wet green body with compressive strength of 1.99 MPa could be demolded at 70°C which is higher than that prepared by traditional DCC and dispersant reaction method for nonoxide ceramics. Dense silicon carbide ceramics with relative density above 98.8% and 99.3% had been prepared by liquid phase pressureless and hot pressed sintering, respectively.     

Modeling for particle size prediction and mechanism of silicon nitride nanoparticle synthesis by chemical vapor deposition

Particle size is a vital characterization for silicon nitride nanoparticle as its scale determines its application area. Particle size prediction for synthesis of silicon nitride nanoparticle by chemical vapor deposition (CVD) is much needed. In this study, a model is proposed for particle growth during silicon nitride nanoparticle synthesis by CVD in order to predict particle size. Comparison between modeling and experimental results validated the model. The modeling results showed that lower pressure in the condensation room would be an effective way of obtaining silicon nitride nanoparticles with smaller particle size. An expression is established to reveal the relation between the mean particle diameter of silicon nitride nanoparticle and pressure in the condensation room based on the modeling. The modeling method is capable of predicting the mean particle size of ultrafine silicon nitride powder to within 3.6% accuracy. Corresponding manufacturing thermal parameters are recommended for silicon nitride nanoparticle production with different mean particle sizes. Modeling and analysis in this article may provide theoretical guidance for production of silicon nitride nanoparticle by CVD.

© 2017 American Association for Aerosol Research     

Direct nucleation of cubic boron nitride on silicon substrate

Cubic boron nitride thin films were deposited on silicon substrates by plasma chemical vapor deposition, and the nanostructure of the interface between the film and substrate was investigated by cross-sectional high-resolution transmission electron microscopy. Cubic boron nitride was found to nucleate directly on the properly pretreated silicon substrates in some local areas; in particular, cBN nuclei with a size of approximately 3 nm are nucleated on (111)–(100) step surfaces with an epitaxial relationship. This suggests the possibility of direct growth of cubic boron nitride in special surface environments of silicon substrates.     

Polishing behaviors of ceria abrasives on silicon dioxide and silicon nitride CMP

The effects of ceria (CeO₂) abrasives in chemical mechanical polishing (CMP) slurries were investigated on silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) polishing process. The ceria abrasives were prepared by the flux method, using potassium hydroxide (KOH) as the grain growth accelerator. The primary particle size of the ceria abrasives was controlled in the range of ~ 84-417 nm by changing the concentration of potassium hydroxide and the calcination temperature without mechanical milling process. The removal rate of silicon dioxide film strongly depended upon abrasive size up to an optimum abrasive size (295 nm) after CMP process. However, the surface uniformity deteriorated as abrasive size increases. The observed polishing results confirmed that there exists an optimum abrasive size (295 nm) for maximum removal selectivity between oxide and nitride films. In this study, polishing behaviors of the ceria abrasives were discussed in terms of morphological characteristics.     

Fabrication and properties of in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites

The in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites were fabricated via gelcasting followed by pressureless sintering. SNNWs were well distributed in the porous silicon nitride matrix. The tip-body appearance suggested a VLS growth mechanism. The flexural strength and elastic modulus of the prepared composites can achieve 84.3?±?3.9?MPa and 23.3?±?2.0?GPa respectively (25?°C), while the corresponding porosity was 40.7?vol.%. Remarkably, the strength retention rate of the composites at 1400?°C was up to 66.1%. This is due to the excellent thermal stability of SNNWs and silicon nitride matrix. Also, the fracture toughness of the composites was improved to ～42% larger than pure porous silicon nitride ceramics because of the bridging effect of the NWs and the interlocking effect of β-Si₃N₄ crystals. In addition, a good thermal shock resistance and dielectric properties were indicated. The good overall performance made SNNWs/SN composites promising candidate for advanced high-temperature applications.