Abstract
Low-temperature heat capacities (cp) for milligram(mg)-sized samples of corundum, fayalite and sanidine were measured with a recently available commercial calorimeter (the heat capacity option of the Physical Properties Measurement System (PPMS), produced by Quantum Design®, which operates on the basis of a heat-pulse technique. The measurements were performed between 5 and 300 K on synthetic single-crystals and powders. Since cp-data for the above minerals are known from low-temperature adiabatic calorimetry (low-TAC) these data were used to constrain the accuracy of the PPMS-calorimeter in measuring their cp on mg-sized samples.
A relative uncertainty (100*σcp/cp), as a measure of precision, of ~0.3% for T > 50 K, increasing to ~0.5% for T < 50 K, was obtained in our heat capacity measurements using single-crystals weighing 10-50 mg and sintered powder weighing 48 mg. The precision was lower when the cp was measured for sample powders that had been sealed in Al-pans (0.5 ± 0.2% relative uncertainty at T > 100 K, 2-3% at T < 100 K for 10-20 mg powders, and ~ 5% for 5 mg powders).
The accuracy was highest for cp measurements on single-crystals and sintered-powder. For these, PPMS heat capacities were in excellent agreement with comparable low-TAC data (deviating from these by a maximum of 0.5 ± 0.8% in the temperature range 100–300 K). Heat capacity measurements on sealed powders did not achieve such a high degree of accuracy, but were systematically lower than low-TAC data by 1-2%. Reference entropies were reproduced with a relative error of ≤ 0.5% using single-crystal and sintered-powder samples, and were 1-2% too low for sealed-powder samples.
Our measurements demonstrate that the PPMS-calorimeter is a promising new tool for obtaining low-temperature cp-data and calorimetrically determined standard entropies for mg-sized mineralogical samples which are only available in limited amounts.
