The apparatus for high precision measurements of the thermal conductivity

G. Guseinov1 and E. Guseinov1

1Institute of Physics, Daghestan Scientific Center of the RAS, Makhachkala, Russia

Keywords: thermal conductivity, apparatus, porous media
property: thermal conductivity
material: porous glass, fluids

Currently, a new kind of materials with special physical and chemical properties – nanomaterials - have attracted significant interest. At the same time, the thermo-physical properties of these materials, and in particular the thermal conductivity coefficient, still are almost unknown. One of the reasons of this lacuna in our knowledge is the lack of reliable technique for the measurement of the thermal parameters.

The device described in our work is based on the method of parallel horizontal sheets with the heat protection, which allows one to investigate the thermal conductivity of the substances in the temperature range of 100–650K at different pressures, including the region of the phase transitions and the critical point vicinity. It contains the measuring part and the sampling cell. The measuring part is made of copper, constantan and porous ceramics soaked with thermo-electrical material. Due to a protection system inside the device, the directed flow of heat from the inner heater down to the cooler is created through the layer of investigated substance. The heat losses from the inner heater are controlled by the three protective cylinders: two of which are made of constantan and the main one, positioned between the two coaxially, is made as heat and electric isolating. Such a system allows one not only to control the losses, but also to reduce them through protective cap made of a porous material, saturated with metal or semiconductor. The chosen materials have simultaneously the high thermo-power (being a metal or semiconductor) and the low thermal conductivity (porous ceramics). In the work it is shown, that the implementing this design allows one to increase additionally the sensitivity and thermoelectric figure of merit of the protective cap. The last one covers the internal heating part, while differential superficial thermocouples are formed by internal heating, external compensating part, and the protective cap itself. The electric signal from the surface differential thermocouple is proportional to the temperature difference between the surfaces of the main protective cylinder, so that a zero value of signal means the absence of the flow through the cylinder. The sampling cell is consisted of two discs made of homogeneous metal (the stainless steel), and supplied by thermocouples T and ΔT. The upper disk is called conditionally as a hot layer, and the lower one – as a cool layer. The investigated substance is suited completely hermetically in the gap between layers and jumper when measured. The sampling cell, being positioned under the measuring part and brought to close contact with it by pressing, is placed into the autoclave. The inaccuracy of the thermal conductivity measurement is within 1.2%. Using the described apparatus, the thermal conductivity has been investigated in the wide range of the parameters of the state including the critical region for: etalon system – distilled water and microporous media, saturated by fluids. The apparatus has not any analogues in our country or abroad.

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