Power compensated thin film calorimetry at fast heating rates

Abstract

With the goal of measuring the heat released or absorbed during phase transitions occurring in small samples, we have developed a power-compensated membrane-based calorimeter that can maintain linear heating rates spanning 1–1000 K/s under nonadiabatic conditions. The device works in the intermediate range of heating rates between conventional, β < 10 K/s, and thin film, β > 10 4 K/s, scanning calorimeters. Active control in real time during heating/cooling experiments is achieved using the NI-7831 card, which includes a 1 M field programmable array with a control loop timer of 20 μs. The performance capabilities of the instrument are demonstrated using a case study: the melting of 100-nm-thick In films. A dynamic sensitivity below 1 μJ is currently achieved. We show that under the present development, heating rates above 200–300 K/s in ambient gas result in a widening of the melting peak because of uncompensated thermal effects. Power loss corrections are applied to obtain the heat capacity of the sample and therefore correct enthalpy values of the transition.