Papers by Author: Ke Xin Chen

Abstract: Thermally conducting, but electrically insulating, polymer-matrix composites exhibiting low dielectric constant are needed for electronic packaging. For developing such composites, this work used silicon nitride particles as fillers and epoxy as matrix. The thermal conductivity of Si3N4 particles epoxy-matrix composites was increased by up to 31.4 times than that of neat polymer by silane surface treatment of the particles prior to composites fabrication. The increase in thermal conductivity is due to decrease in the filler-matrix thermal contact resistance through the improvement of the interface between matrix and particles. At 45.4 vol. % silane-treated Si3N4 particles only, the thermal conductivity of epoxy-matrix composites reached 9.72W/ (m*K). The dielectric constant was also low (up to 5.0 at 1 MHz). However, Si3N4 addition caused the flexural strength and ductility to decrease from the values of the neat polymer.

Abstract: Si3N4-filled epoxy resin composite was fabricated employing liquid press molding method. Properties, such as thermal conductivity, dielectric constant of Si3N4-filled epoxy resin composite were evaluated, the effect of the content of Si3N4 and surface treatment of the filler was also considered. A silane coupling agent, namely NH2−(CH2)3Si−(OC2H5)3, was applied to functionalize the surface of Si3N4 filler. Experimental results showed that the thermal conductivity of the composites is strongly dependent on the filler and is dominated by the interface of epoxy resin and Si3N4 particles. As the Si3N4 volume fraction increasing, thermal conductance of Si3N4-filled composite was improved obviously, especially for that of silane-treated Si3N4 powder filled composite. Dielectric constant of the composite increases linearly, however, it still remains at a relatively low level (<5, at 1MHz).

Abstract: In present work, ultra-fine powders with alpha phase content higher than 95 wt% and specific surface area of 15.33 m2/g were prepared by mechanical activated combustion synthesis (MACS) process. The sinterability of as-fabricated Si3N4 as well as the microstructure and mechanical properties of the sintered bulk were investigated by comparing with a kind of commercial available Si3N4 powders used as diluents in MACS process. Employing hot-pressing method, both powders were sintered equally by using Y2O3 and Al2O3 as sintering aids. Results showed that smaller particle size and higher specific surface area were obtained by MACS process when compared with the commercial one. Bulk Si3N4 appeared approximately the same relative density, hardness, strength and fracture toughness, however, proved to be higher while using MACS powders.

Abstract: In this paper, α-SiAlON powders co-doped with (Ca+M) (M=Mg, Yb, Sr) were prepared by combustion synthesis. The effect of Ca incorporation on the phase composition and grain morphology of reaction products was discussed. The experimental results showed that Ca incorporation promoted the growth of rod-like α-SiAlON crystals in (Ca+Mg) and (Ca+Yb) systems. For (Ca+Sr) system, the addition of Ca assisted Sr into α-SiAlON crystal lattice and increased the relative content of α-SiAlON in reaction product.

Abstract: In the study, the spark plasma sintering (SPS) method was adopted to fabricate Si3N4 ceramics with Y2O3-MgO additives. Specimens with different grain dimensions, grain shapes, α/β phase ratios, densities were obtained by changing the heating rate and dwell time of SPS. The relationship between the microstructure and the thermal conductivity was studied. Results show that the heating rate and the dwell time have great influence on the microstructure and properties of Si3N4 ceramics. Both equiaxed and columnar β- Si3N4 grains are formed during sintering, but the thermal conductivity of Si3N4 ceramics is affected only by columnar grains. The thermal conductivity of the ceramics increases together with the formation and the growth of the β- Si3N4 columnar grains.

Abstract: In this paper, b-SiAlON whiskers were synthesized by combustion of silicon and aluminium powders with SiO2 and Y2O3 as additives and a-Si3N4 or b-SiAlON powders as diluents. SEM observation of the products showed that the crystals with an aspect ratio higher that 5, which could be assigned to almost pure and well crystallized b-SiAlON. The influence of the amount of NH4F additive, the type of diluent on the phase formation and on the morphology of products was investigated.

Abstract: In this paper, Yb a-sialon powders with elongated grains have been prepared by combustion synthesis. The effects of diluents, nitrogen pressure, adding a-sialon seeds and NH4F on the phase transformation and microstructure development of the samples are discussed systematically. The experimental results show that the addition of diluents, proper nitrogen pressure and additive NH4F can facilitate the nitridation of Si. It was also found that the added a-sialon seeds and NH4F are beneficial to the nucleation and growth of elongated grains.

Abstract: In present works, the low cost TiO2 powders were partially introduced to raw mixture as playing roles of both reactant and dilution. The experimental results showed that increasing the ratio of TiO2 to titanium powders up to 70wt%, the combustion synthesis reaction could still proceed by self-sustained mode. Single phase of TiNxC1-x with ultra fine particle size could be prepared by using TiO2 as dilution. Influences of TiO2 addition on phase formation and microstructure of product were also investigated. It was found that the combustion temperature and as-synthesized TiNxC1-x particle size decreased as increasing the amount of TiO2 addition.

Abstract: Titanium carbonitride powders were synthesized directly by a combustion synthesis process between titanium and carbon in a nitrogen atmosphere. The relationships between properties of the final product and the combustion reaction parameters were systematically investigated. Especially, the effects of nitrogen pressure on the phase formation and microstructure of the as-synthesized products were experimentally investigated. The reaction mechanism of Ti(C,N) was proposed through quench experiment, the variation of combustion temperature on time and thermodynamics analysis.

Abstract: AlN whiskers have been successfully synthesized by carbothermal reduction. The thermodynamics and growth kinetics of AlN whiskers were studied at 1600°C using CaCO3 as a catalyst. The research indicated that AlN whiskers are more easily nucleated from the liquid phase than at the surface of solid phase. AlN whiskers are nucleated by VLS mechanism and the liquid, which plays a dominant role in the VLS mechanism, is formed by Al-Ca interphases, such as CaO×2Al2O3 and CaO×6Al2O3. Kinetic studies suggest that the catalyst reacts with Al2O3 to form a low melting point eutectic (1390°C). The liquid phase formed at this low melting point eutectic provides good conditions for nucleation of AlN whiskers. At the synthesis temperature, the liquid phase vaporizes, thus creating suitable conditions for the subsequent growing of whiskers by the VLS mechanism. This growing mechanism conforms to thermodynamics and a lot of proof indicates that screw dislocations play an important role in the process of the whiskers' formation.