Heat conduction sintering

Copper, with its high heat conductivity, resists frictional heat during service and is readily moldable. It is generally used as a base metal, at 60—75 wt %, whereas tin or zinc powders are present at 5—10 wt %. Tin and zinc are soluble in the copper, and strengthen the matrix through the formation of a soHd solution during sintering. 

All types of catalytic reactors with the catalyst in a fixed bed have some common drawbacks, which are characteristic of stationary beds (Mukhlyonov et al., 1979). First, only comparatively large-grain catalysts, not less that 4 mm in diameter, can be used in a filtering bed, since smaller particles cause increased pressure drop. Second, the area of the inner surface of large particles is utilized poorly and this results in a decrease in the utilization (capacity) of the catalyst. Moreover, the particles of a stationary bed tend to sinter and cake, which results in an increased pressure drop, uneven distribution of the gas, and lower catalyst activity. Finally, porous catalyst pellets exhibit low heat conductivity and as a result the rate of heat transfer from the bed to the heat exchanger surface is very low. Intensive heat removal and a uniform temperature distribution over the cross-section of a stationary bed cannot, therefore, be achieved. The poor conditions of heat transfer within... 

As sintering proceeds and coalescence and densification occur, the overall heat conduction problem does not remain unaffected. Clearly, the effective thermophysical properties change, thereby influencing the overall temperature distribution and the local sintering problem as well. 

Various high conductivity materials have been alloyed with metal hydrides to form enhanced heat transport composite materials. Eaton et al. [31] experimented with various alloyed metal additives including copper, aluminum, lead, and lead-tin. The samples were alloyed at elevated temperature (200-600 °C) and cycled. In many samples, cycling resulted in the separation and fracture of the alloy and thus a reduction in composite thermal conductivity. Sintered aluminum structures of 20% solid fraction have been integrated with LaNis hydride materials with success, resulting in effective thermal conductivities of 10-33 W/mK [32-34]. Temperatures required for this process and added mass and volume may exclude application to some complex hydrides. 

In these experiments, the positive effect of sintering was achieved first of all due to high heat conductivity of metal powders. On the contrary, ceramic powders have low heat conductivity. Therefore, it was rather difficult to sinter these powders when they were similar to metal ones in particle size (about 0.5 pm). To overcome the problem ceramic powders with smaller particles were used. As it is known, the fine powders are susceptible to sintering due to greatly reduced melting point. 

If the distribution of the microwave power is uniform or follows certain trend, e.g., exponentially decaying from the surface into the bulk of the materials, heat conduction equation alone is sufihcient for the modeling of the process. This is especially tme forhighloss materials. Simulation methods, including finite elements and finite differences, have been used to analyze the microwave sintering process based on heat conduction equation [71, 72]. 

Ceramic foams are produced from organic precursor foams such as polyurethane or polyolefins. Their pores are then filled with an aqueous slurry of the ceramic typically containing 20 wt.% of ceramic particles in the size range of from 0.1 to 10 pm [461]. Wetting agents, dispersion stabilisers and viscosity modifiers are added to the slurry. Suitable ceramics are alumina, alumina silicates, zirconia, stabilised zirconia and titania, amongst others. The pores ofthe precursor foam may be filled completely or only coated on their surface by the ceramic particles. The foam is then dried and calcined at 1000 °C, which removes the polymer and sinters the ceramic. Metallic foams have similar properties compared with ceramic foams, but superior mechanical stability and improved heat conductivity. 

Beryllium oxide is the most heat conductive dielectric material and is frequently used in sintered form as a heat sink, but it is generally not used to make heat-conductive adhesives because of the toxicity and high cost of powdered beryllia. 

Johnson and co-workers at NorthMestem University in the USA, used a circular cross-section applicator, operating in the TE- mode and with the inner surfaces of the walls given a smooth, silver coating to reflect heat to conduct sintering experiments on Si3N, SiC, AI2O3 and A1 3-TiC composites ... 

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