Thermal conductivity of powders

Calcination. Calcination involves a low (<1000° C) temperature soHd-state chemical reaction of the raw materials to form the desired final composition and stmcture such as perovskite for BaTiO and PZT. It can be carried out by placing the mixed powders in cmcibles in a batch or continuous kiln. A rotary kiln also can be used for this purpose to process continuously. A sufficiendy uniform temperature has to be provided for the mixed oxides, because the thermal conductivity of powdered materials is always low. 

The apparent thermal conductivity of powder insulation at cryogenic temperatures is generally obtained from... 

Calcination can be carried out by placing the mixed powders in shallow saggers in a batch or continuous kiln. The saggers may need to be closed if any of the constituents are volatile, as is the case with lead oxide. The container surfaces in immediate contact with the powder must not react with it both to avoid contamination and to permit reuse of the sagger. The thermal conductivity of powdered materials is always low, so that a sufficiently uniform temperature can only be obtained through a depth of a few centimetres when the period at maximum temperature is, as is usual, only 1 or 2h. 

To increase thermal conductivity of powder layer metal powders of copper, aluminium are added. Composites are compacted in pellets, which can be sintered in addition. Their main characteristics are coefficient of effective thermal conductivity and coefficient of gas-permeability. The weight fraction of powder in such compacts serves as the controlled parameter, and it has the optimum, when gas-permeability does not worsen sharply at acceptable thermal conductivity. Encapsulation of hydride powder by material with high thermal conductivity followed by compaction of pellets and their sintering is also used. 

By results of our experiments mean coefficient of effective thermal conductivity of powder LaNi5 is Xe l.25 0.05 W/(m-K). The porosity of powder layer was 0.5-0.55. Effective thermal conductivity of ZrCrFei.2 powder was 0.5 W/(m-K). 

Values of effective thermal conductivity for a cell modeling tube sorber A,ef=5 0.5 W/(m-K) with a corrugated foil and 1.0-1.5 W/(m K) without it were found in experiments. Corrugated aluminium foil increases effective thermal conductivity of powder bed approximately 3-5 times. Value of effective thermal conductivity in mathematical model for calculations of tube sorbers was assumed to be f=5.8 W/(m-K). 

When determined from the physical constants, the values of the time constant are found to vary because of the appreciable change of the specific heat with temperature. The precision is limited by the uncertainties in the values of the thermal conductivity of powders. Large single diamond crystals have a high thermal conductivity (see Table I), being of the same order of magnitude as for silver... 

K. Tajiri, H. Matsubara, T. Asahina and M. Kosaka, Thermal Conduction of Powders ( I ) - Some Metal Oxide Powders - (in Japanese), Energy Resources, 7, No. 5, 79-83 (1886). 

Suissa E, Jacob 1, Hadari Z (1984), Experimental measurements and general conclusions on the effective thermal conductivity of powdered metal hydrides , J. Less-Common Metals, 104, 287-295. 

Powder material Thermal conductivity of powder material W-m- K for different plasma gases ... 

For heterogeneous materials, the effect of geometry must be considered using structural models. Utilizing Maxwell s and Eucken s work in the field of electricity, Luikov et al. [105] initially used the idea of an elementary cell, as representative of the model structure of materials, to calculate the effective thermal conductivity of powdered systems and solid porous materials. In the same paper, a method is proposed for the estimation of the effective thermal conductivity of mixtures of powdered and solid porous materials. 

Since the technique was first employed in 1931 (Stahlane and Pyk, 1931) to measure the thermal conductivity of powders, there have been significant improvements in the practical realization of the technique. In modem instm-ments the wire sensor aets both as the heat source and as a thermometer. Rapid development of analogue and digital equipment as well as of eomputer-driven data-acquisition systems, have meant that precise measurements of transient electrical signals can be made quickly. Thus, it has become possible to measure the resistance change taking place in the hot wire as a consequence of its temperature rise with a... 

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