An adiabatic calorimeter for heat capacity measurements of small samples -The heat capacity of nonlinear optical materials KTiOPO4 and RbTiOAsO4 crystals

A small sample size automated adiabatic calorimeter for measuring heat capa cities in the temperature range of 60-360 K has been constructed. The sampl e cell of the calorimeter was made of thin-walled gold-plated copper with a n internal volume of 6 cm(3), equipped with a miniature platinum thermomete r, surrounded by two adiabatic shields and housed in a high vacuum can. Two sets of six-junction chromel-constantan thermocouples were mounted between the cell and shields to indicate the temperature difference between them. The lid of the cell was sealed to the main body with a special sealant cycl eweld. The adiabatic conditions of the calorimeter cell were automatically controlled by two units of temperature controller. A mechanical pump was us ed to pump out the vapor of liquid nitrogen in the cryostat to obtain 60 K or even lower temperature. All the experimental data, including the electri cal energy and temperature were collected and processed automatically by a personal computer with a predetermined program. The performance of the calo rimeter was examined by measuring the heat capacities of alpha-Al2O3. The d eviations of experimental results from the smoothed curve lie within +/-0.2 %, while the inaccuracy is within +/-0.5% compared with the recommended ref erence data. The heat capacities of two important nonlinear optical crystals KTiOPO4 and RbTiOAsO4 have been measured by using this calorimeter in the range of 60- 360 K, No thermal anomaly was observed from the heat capacity measurements for the two materials in the above-mentioned range. (C) 2000 Elsevier Scien ce B.V. All rights reserved.