A dynamic technique for measuring thermal conductivity in cylindrical geometry designed for use in radiation damage studies

A dynamic technique for measuring thermal conductivity in cylindrical geometry has been developed for eventual use in radiation damage studies. This technique does not require a precise knowledge of either the thermal diffusivity or specific heat of the material under study in order to obtain the thermal conductivity. Moreover, cylindrical specimens are not assumed infinite in the radial dimension. Conductivities may be calculated from a simple relationship between measured heat inputs and temperature differences. The technique could be used as developed with almost no modification in a low temperature irradiation facility such as that of the Argonne CP-5. Relatively little on-site equipment is required within the reactor cryostat. This is essential due to the small cooling rates typically available. (100 watts at very low temperatures in the CP-5). Temperature differences are measured using a phase sensitive amplifier in conjunction with a mechanical chopper. These combine to produce an extremely sensitive and stable d.c. amplifier with widely variable response times. The accuracy to which a d.c. signal can be measured with this system is mainly a function of the stability of the signal. For example, a 0.1 microvolt signal which is stable enough to permit the use of a 1.12 second time constant on the amplifier output can be easily measured to an accuracy of 1 percent. This sensitivity permits the measurement of extremely small temperature differences between thermocouple junctions. Thermal conductivity measurements were made on silicon and aluminum in the temperature range of 100°K to 300°K. Silicon was used for three reasons: (1) its strong temperature dependence of thermal conductivity and diffusivity over this temperature range, (2) the availability of previous measurements by other experimenters which would serve as a check on the validity of the technique, and (3) the susceptibility of silicon to damage by fast neutron bombardment, which is exhibited in a reduction in thermal conductivity. Aluminum was measured as an alternate material to further establish the validity of the technique. The mathematical development of the technique is given along with a description of equipment and the results of measurements on silicon and aluminum samples.U of I Onlythesi