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 capacities in the temperature range of 60–360 K has been constructed. The sample cell of the calorimeter was made of thin-walled gold-plated copper with an internal volume of 6 cm3, equipped with a miniature platinum thermometer, 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 cycleweld. The adiabatic conditions of the calorimeter cell were automatically controlled by two units of temperature controller. A mechanical pump was used 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 electrical energy and temperature were collected and processed automatically by a personal computer with a predetermined program. The performance of the calorimeter was examined by measuring the heat capacities of α-Al2O3. The deviations of experimental results from the smoothed curve lie within ±0.2%, while the inaccuracy is within ±0.5% compared with the recommended reference data.