High temperature thermal conductivity of platinum microwire by 3ω method.

The 3ω method for thermal conductivity measurement has emerged as an effective technique applicable to micro/nanowires and thin films. This paper describes the adaptation of the method to temperatures as high as 725 K enabling reliable thermal conductivity measurements on such samples for which previously published methods have been found inadequate. In the technique, a sample wire is heated by applying a sinusoidal current at an angular frequency ω, which causes a temperature and resistance variation at an angular frequency, 2ω, leading to a voltage signal at 3ω. The sample is connected as a four-terminal resistor to a digital lock-in amplifier, which is used to detect the in-phase and out-of-phase 3ω voltages resulting from the applied 1ω current. The data are fitted by varying the values of the thermal resistance and diffusion time, both of which are functions of thermal conductivity. Measurements are made at steady state temperatures between 300 and 725 K. Meaningful measurements at elevated temperatures require that thermal losses be understood and minimized. Conduction losses are prevented by suspending the sample above the mounting substrate. Convection losses are minimized by maintaining a vacuum of ~10(-5) torr inside the sample chamber. To minimize radiation losses, an appropriately sized sample is shrouded with a double heat-shield, with the inner shield temperature near that of the sample. Using the 3ω method, the thermal conductivity of platinum was determined to vary between 71.8 and 80.7 Wm(-1) K(-1) over the temperature range of 300 to 725 K, in agreement with published values measured for bulk samples.

Temperature Dependence of Photocurrents Produced by X and Gamma Rays in Silicon Radiation Detectors.

The temperature dependence of dc photocurrents produced by x and gamma rays in silicon radiation detectors of the n-p, p-i-n, and surface-barrier type was investigated in a temperature range between 20 and 50 °C. Photodiode photocurrents, assumed as being equal to the generated currents I g, showed a positive temperature dependence in all detectors investigated. Their temperature coefficient at 25 °C varied between +0.004 per °C and + 0.002 per °C. The temperature dependence of short-circuit currents I sc measured by a compensation method, was positive and nearly linear for n-p type detectors but nonlinear and negative for p-i-n and surface-barrier type detectors. It is shown, that this different behavior of individual detectors is due to the influence of the strongly temperature-dependent junction current I j which under the short-circuit mode is drained off the generated current I g . The junction current is a function of the internal series resistance R s and the junction resistance R j of the irradiated detector (I j = I sc R s /R j ). With increasing resistance ratio R s /R j , the current ratio I j /I g increases and the temperature coefficient αsc of the short-circuit current decreases. Temperature coefficients (αsc)25 measured in the different detectors at 25 °C and a current density 6 × 10-7 A/cm2 decreased with increasing resistance ratio from + 0.004 per °C to - 0.005 per °C. Resistance ratios R s /R j of the detectors investigated ranged between 0.01 and 0.24 approximately. Thus, (αsc)25 measured in an individual detector can be changed by changing its effective series resistance. A decrease of (αsc)25 with increasing I sc was observed in detectors with larger resistance ratios. This was apparently due to the voltage dependence of R j at higher junction voltages produced by larger short-circuit currents.