An introduction to heat transfer

Emerging challenges and materials for thermal management of electronics

The recent developments and trends in combustion science towards the synthesis of nanomaterials are discussed. Different modifications made to conventional combustion approaches for preparation of nanomaterials are critically analyzed. Special attention is paid to various applications of combustion synthesized nanosized products.

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Combustion synthesis and nanomaterials

Flame aerosol synthesis of ceramic powders

Combustion synthesis has emerged as a facile and economically viable technique for the preparation of advanced ceramics, catalysts and nanomaterials. Recent innovations in the combustion and processing parameters have resulted in a better understanding of combustion phenomena and control of microstructure and property of the products.

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Combustion synthesis: an update

The effect of the addition of aluminum on the ignition and self-propagating high-temperature synthesis (SHS) reaction between titanium and carbon was experimentally investigated. Although TiC was the only product compound in the final product, the reaction between titanium and aluminum was believed to occur before the ignition of the reaction between titanium and carbon, as evidenced by a hump that appeared in the temperature profile during heating. The ignition temperature of the reaction between titanium and carbon was significantly decreased by the addition of aluminum. This was explained by the fact that the aluminum provides an easier route for reactant mass transfer, thus significantly increasing the reaction rate of the reaction between titanium and carbon. As the temperature was increased to beyond the melting point of aluminum during heating, aluminum melted and titanium dissolved into it. As the temperature was increased further (1050°C), the titanium-containing aluminum spread over the carbon particles. Ignition is believed to have occurred by the reaction of titanium and carbon at the interface by the diffusion of titanium through the aluminum melt to the interface. The rate of heating and the density of the reactant compact also affected the ignition temperature. These were explained by their influence on the extent of the capillary spreading of the titanium-containing aluminum melt on the carbon particles or by the formation of a TiAlx layer at the interface between the aluminum melt and the titanium particles. Fracture surfaces of the product showed three different types of morphology, i.e., discrete particles, groups of grains similar to sintered bodies, and unreacted reactant particles. A possible reaction mechanism was proposed that describes the ignition process and explains the formation of each type of product.

Ignition phenomena and reaction mechanisms of the self-propagating high-temperature synthesis reaction in the titanium-carbon-aluminum system

The thermal conductivity of epoxy resin composites filled with combustion-synthesized hexagonal boron nitride (h-BN) particles was investigated. The mixing of the composite constituents was carried out by either a dry method (involving no use of solvent) for low filler loadings or a solvent method (using acetone as solvent) for higher filler loadings. It was found that surface treatment of the h-BN particles using the silane 3-glycidoxypropyltrimethoxysilane (GPTMS) increases the thermal conductivity of the resultant composites in a lesser amount compared to the values reported by other studies. This was explained by the fact that the combustion synthesized h-BN particles contain less -OH or active sites on the surface, thus adsorbing less amounts of GPTMS. However, the thermal conductivity of the composites filled with the combustion synthesized h-BN was found to be comparable to that with commercially available h-BN reported in other studies. The thermal conductivity of the composites was found to be higher when larger h-BN particles were used. The thermal conductivity was also found to increase with increasing filler content to a maximum and then begin to decrease with further increases in this content. In addition to the effect of higher porosity at higher filler contents, more horizontally oriented h-BN particles formed at higher filler loadings (perhaps due to pressing during formation of the composites) were suggested to be a factor causing this decrease of the thermal conductivity. The measured thermal conductivities were compared to theoretical predictions based on the Nielsen and Lewis theory. The theoretical predictions were found to be lower than the experimental values at low filler contents ( 60 vol %).

Thermal Conductivity of Epoxy Resin Composites Filled with Combustion Synthesized h-BN Particles

The counterflow diffusion flame burner: A new tool for the study of the nucleation of refractory compounds

A combustion synthesis (SHS) process has been developed for the synthesis of Si3N4 powder under low nitrogen pressures. Si and NaN3 powders were used as the reactants, and NH4Cl powder was added as a catalytic agent. These powders were mixed and pressed into a cylindrical compact. The compact was wrapped up with an igniting agent (i.e., Ti + C), and the synthesis reaction was triggered by the combustion of the igniting agent. Addition of NH4Cl was found necessary for the combustion synthesis reaction under low nitrogen pressures (< 1.2 MPa). The product as synthesized is mostly in the form of agglomerated fine particles (0.1–1 μm in diameter) and is composed mainly of α-phase and a minor amount of β-phase. Effects of various experimental parameters (N2 pressure, NaN3, NH4Cl, and Si3N4 contents) on the product conversion and the combustion temperature were investigated. A possible reaction mechanism was proposed that explains the effects of the experimental parameters on the synthesis reaction.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400039121

Combustion synthesis of Si3N4 powder

Aluminum nitride powder was synthesized by a combustion synthesis method using various additives. Each additive was mixed with Al powder, and the powder mixture was then pressed into a compact. The combustion reaction was ignited by heating the compact under N2 atmosphere of 0.4 MPa. Additives containing halogens were found to have a catalytic effect on the combustion reaction. High product yields were obtained when using additives of NH4X, CO(NH2)2, CH3(CH2)16COOH, and CO2H(CH2)2CO2H. In all these cases, eggshell-like skins were observed to form on the Al particles at the early stage of combustion. The catalytic effect, formation of the eggshell-like skins, and their effects on the combustion process were investigated and discussed.

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Shyan Lung Chung

Papers

Ignition phenomena and reaction mechanisms of the self-propagating high-temperature synthesis reaction in the titanium-carbon-aluminum system

Thermal Conductivity of Epoxy Resin Composites Filled with Combustion Synthesized h-BN Particles

The counterflow diffusion flame burner: A new tool for the study of the nucleation of refractory compounds

Combustion synthesis of Si3N4 powder

Combustion synthesis of aluminum nitride powder using additives