The thermal conductivity of KV fused silica at 2?300�K

Abstract

The thermal conductivity I for the material is determined in two stages on specimens of cross section S and thickness s (as is the thermal resistance of the contact R): first one measures the temperatures and derives the difference AT I from the readings with the heater switched off, and then the difference AT and the heater power Q are measured with the heater on. The sink temperature was the same in both cases (difference not more than 0.05~ The conductivity is l ~= S[(AT--AT~)/Q-RI The TSPN-4 platinum thermometers were calibrated on IPTS-68, while the germanium TSG-2 ones were calibrated on the VNIIFTRI scale. Maker's error for the thermometers 0.01~ The discrepancies between the readings during calibration did not exceed 5 mK, and the values were reproduced with an error of !0.2 mK on all the specimens. The temperature drift did not exceed 2-3 mK/h, while AT in most of the experiments was I~ There is a difficulty in implementing our method at cryogenic temperatures on account of providing reliable thermal contact between the specimens and the sink or body of the heater. In the plane-layer method, the contact resistance enters into the measured thermal resistance and distorts the result considerably (particularly under vacuum conditions). The contact resistance may be reduced and stabilized at room temperature by liquid contact media, which also somewhat eases the specifications for finish in the contacting surface and eliminates the condition for a high compressive force. However, the usual liquids (vacuum oil or organosilicon liquids) solidify at cryogenic temperatures and become brittle, and the contact layer is disrupted by the temperature-depe ndent stresses and strains. Consequently, the contact resistance increases by several orders of magnitude. It is therefore necessary