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ホーム アーカイブ Thermal Letter 役員 今後の会合 国際伝熱会議の集会

ISBN : 0-85295-345-3

International Heat Transfer Conference 10
1994, 14-18 August, Brighton, UK

INFLUENCE OF SEGREGATION AND BULK AND SURFACE DIFFUSION ON THE HEAT TRANSFER IN POROUS CERAMICS

Get access DOI: 10.1615/IHTC10.4230
pages 267-272

要約

Heat transfer in porous ceramic materials is accompanied by diffusion of various kinds of impurities, normally present in the solid crystalline phase. These impurities tend to concentrate (segregate) from the bulk region onto surfaces of small pores, existing in the region separating two sintered crystalline granules. Under the influence of an externally applied temperature gradient the impurities diffuse in the bulk and along the pores' surfaces. This process is accompanied by heat release and absorption occurring on the opposite pore sides, and the concomitant distortion of the temperature field. The cumulative effect of these distortions is to increase the effective thermal conductivity λeff of ceramic materials.
Estimates, performed on the basis of several known segregation-diffusion isotherms, show that the described heat transfer mechanism has the most profound influence on λeff of materials measured in vacuum. However, it is normally negligible for atmospheric pressure, where its influence is dominated by thermal conductivity of gas within the pores. The above estimates are used to explain the temperature dependence of refractory oxides at various gas pressures.
There is ample experimental evidence that the thermal conductivity and thermal diffusivity of some refractories depend upon the thermal prehistory of the samples tested. The temporal dependence of the thermophysical properties can be attributed to the influence of the kinetics of surface segregation processes.
A model describing the temporal dependence of effective thermal conductivity of ceramic materials, caused by changes of their temperature, is developed. This model is based on the surface segregation kinetics analysis performed for various surface reactions and diffusion rates. The solution exhibits simple asymptotic behaviors For long times the segregation kinetics curve reduces to the well-known McLean's expression. For short times it coincides with the reaction rate limited model suggested by Rawlings et. al.
The temporal dependence of the effective thermal conductivity of the pore phase is estimated on the basis of the segregation kinetics curves. It is shown that the temporal behavior of thermal conductivity of porous ceramics is attributable to the time variations of surface concentration after the sudden temperature change.
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