Papers by Keyword: β-Si₃N₄

Abstract: α-Si₃N₄ possesses excellent sintering activity, which is used to prepare high performance Si₃N₄-based ceramics and composite refractory. Si₃N₄ powder is always synthesized by nitriding silicon in controlled-atmosphere furnace whose furnace volume is very small(effective volume: 1840×1420×1660mm), the extreme reaction heat is difficult to diffuse, which leads to high reaction temperature and conversion of α-Si₃N₄ to β-Si₃N₄, thus α-Si₃N₄ is difficult to be obtained in controlled-atmosphere furnace. While flame-isolation nitridation shuttle kiln has much larger furnace volume to conduct reaction heat (effective volume: 11500×4190×1684mm), so it owns homogeneous temperature field and stable low-temperature environment which benefits the preparation of α-Si₃N₄. Thermodynamic analysis of Si-N system is shown that Si₃N₄ can be formed by two formats: direct nitridation of Si(s) and indirect nitridation of SiO(g); to ensure completely nitridation, the particle size of silicon powder should be less than 88μm. With reclaimed powder from polysilicon cutting slurry as starting materials, both reactive α-Si₃N₄ and SiC mixed powder were successfully prepared in flame-isolation nitridation shuttle kiln. Because of the gas-gas reaction between SiO(g) and N₂(g), α-Si₃N₄ is fiber-like and in favor of processing high quality Si₃N₄-based materials.

Abstract: In this paper, α-Si₃N₄ was mixed with Y₂O₃ and MgO by ball milling. Spray granulation was adopted to fabricate spherical powder. The influence of the sintering temperature and the holding time on the thermal conductivity of the powder was studied. The results show that α-Si₃N₄ raw powder was transformed into β-phase after sintering. Spherical powder, of which the diameter is about 20 μm, was fabricated. Some compounds such as YMgSi₂O₅N, Y₄Si₂N₂O₇ and Y₂Si₃N₄O₃ were generated in the samples. The thermal conductivity of the pressed powder first increased then decreased with the sintering temperature rising, showing a maximum at 1800°C. Also the thermal conductivity first increased then decreased as the holding time getting longer, showing a maximum at a sintering time of 2 hours.

Abstract: This paper presents the results of synthesizing rod-like -Si3N4 crystals with Y2O3 as additives by combustion synthesis under different nitrogen pressure. The effect of nitrogen pressure on the final morphology of rod-like -Si3N4 was investigated and the crystal growth mechanism was discussed in detail. The results reveal that the final morphology of products is depended on the nitrogen pressure. With the increasing nitrogen pressure, the ratio of rod-like crystals is increasing.

Abstract: It is an effective way to decrease dielectric constant of microwave transparent materials by improving porosity rate. Network -Si3N4 porous transparent materials were prepared by gas pressure sintering (GPS) with Y2O3 additions. Porosity rate of silicon nitride using pore former with different content starch, polytetrafluoroethylene (PTEE) powder respectively and their compounded component were discussed in this paper. It is concluded that porosity rate can be improved by all the three, starch is most favorable to improve porosity but great volume rebound after cold iso-static pressing were happened and easily lead to crack of green body. Method of pore-making by step on principle of pore formers discharging in different temperature range used to get high strength and porosity green body. Series of porous silicon nitride microwave transparent materials were prepared with porosity rate from 48% to 67%, bending strength from 162MPa to 59MPa and dielectric constant from 3.41 to 2.37.

Abstract: It is necessary to know the synthesis mechanism of Fe-Si3N4 by flash-combusting FeSi75 in order to control the phase composition and microstructure of Fe-Si3N4 composites. In this paper, Fe-Si3N4 and starting material FeSi75 were analyzed with XRD, SEM and EDS. The results show that Fe-Si3N4 synthesized by flash-combusting FeSi75 (≤0.074 mm) is composed of β-Si3N4, α-Si3N4, FexSi, and SiO2, in which β-Si3N4 and α-Si3N4 are from the nitridation of metal silicon and part of the silicon in ξ phase while FexSi is from the nitridation of ξ phase; during the nitridation of ξ phase, Si content declines gradually, when Fe: Si is close to 3, the nitriding reaction tends to balance; the loose accumulation of nitriding products results in the slow heat release, which makes α-Si3N4 transform to β-Si3N4 and β-Si3N4 grow further to form rod-like β-Si3N4 crystals of high slenderness ratios; the rapid quenching of nitriding products helps to keep the proportion between α-Si3N4 and β-Si3N4 small particles, and to reduce the crystallization of rod-like β-Si3N4; The grain size and distribution of FexSi in Fe-Si3N4 are related to the particle size and distribution of ξ phase in the starting material.

Abstract: Silicon nitride ceramics with MgSiN2 as additives were sintered by hot pressing at 1600° ~ 1750 °C for 1-12 h under uniaxial pressure of 20 MPa. The specimens were characterized by x-ray diffraction, scanning electron microscopy, transmission electron microscopy and photothermal deflection spectroscopy. After sintered at 1750°C for 1 h, the thermal conductivity of the material was 90 W·m-1·K-1. The thermal conductivity could remarkably increase to 120 W·m-1·K-1 by prolonging the dwell time from 1 h to 12 h. The present work demonstrated that MgSiN2 additives were effective to improve the thermal conductivity of β-Si3N4 ceramic.

Abstract: This paper reviews the most important results by the authors on the processing of textured -Si3N4 and -sialon by slip casting in a strong magnetic filed of 12 T and reaction-sintering. The a, b-axis textured -Si3N4 and -sialon have been obtained using the static magnetic field because of the magnetic susceptibility of ca, b > c c for -Si3N4. However, the c-axis textured -Si3N4 has also been successfully obtained using a rotating magnetic field. The -Si3N4 crystal was found to exhibit substantially stronger orientation ability than the a-Si3N4 crystal regardless of the Si3N4 raw powders. It reveals that the -Si3N4 nuclei play a key role in the texture development in -Si3N4/-sialon.

Abstract: β-Si3N4 and β-SiAlON powders were prepared by combustion synthesis with SrCO3 and NH4F used as additives. The resultant β-Si3N4 and β-SiAlON powders consisted of elongated prismatic microcrystals. By adding SrCO3, the anisotropic growth of β-Si3N4 and β-SiAlON crystals is improved and their aspect ratios increase. The addition of NH4F enhanced nitridation reactions and reduced the residual Si in combustion products. It was proposed that the elongated prismatic β-Si3N4 and β-SiAlON crystals grew from liquid phase and the composition and property of this liquid was affected by the addition of SrCO3.

Abstract: New electric molding composites were fabricated with hybridizing epoxy and several fillers. The examined fillers were β-Si3N4, BN and fused silica or their combinations. The thermal conductivity of β-Si3N4 filler was compared with BN as filler for advanced epoxy molding compounds. The influence of pure resin matrix and mixture matrix on the thermal conductivity of EMC was discussed. The results were explained with Maxwell equation.

Abstract: An effect of nano-scale TiN grains on the mechanical properties and microstructure of Si3N4 based ceramic tool materials is investigated at the different sintering temperature. Compared to monolithic Si3N4 ceramic tool materials, the sintering temperature is decreased and mechanical properties is enhanced when only one percent of nano-scale TiN in term of mass is added into the Si3N4 matrix. The optimum mechanical properties are achieved when Si3N4/TiN nanocomposites tool materials were sintered at the sintering conditions of 1650
, 30MPa and holding time of 40min. The flexural strength, fracture toughness and hardness are 1018.2MPa, 8.62MPa⋅m1/2 and 14.58GPa respectively. SEM micrographs indicate that microstructure is composed of the elongated and equiaxed β-Si3N4 grains, and some nano-scale TiN grains are enveloped into matrix grains.