Specific heat of apiezon T grease from 1 to 350° K

Abstract

We have measured the magnetic susceptibility and heat capacity of CuO nanoparticles (16 nm) from 2 K to 400 K using a Quantum Design Physical Properties Measurement System (PPMS). The magnetization curves, acquired at various field strengths from 0 Oe to 50 kOe, are similar to literature data of bulk and nanoscale CuO showing a minimum at about 150 K, an upturn as temperature approaches 0 K, and a broad maximum at high temperatures extending beyond 400 K. The heat capacity data between 200 K and 400 K show several broad peaks. The number of these peaks and the temperatures at which they occur differ significantly from the magnetic transitions known to exist in bulk CuO. To further investigate these transitions in nanoscale CuO, we performed temperature dependent x-ray diffraction (XRD) at temperatures from 90 K to 700 K from which lattice parameters as a function of temperature were derived using a Rietveld refinement. Although no phase transitions were observed in these data, changes in the slopes of the lattice parameters are apparent at the transition temperatures observed in the heat capacity and susceptibility measurements. The various transitions in the heat capacity data are attributed to competing ferromagnetic and antiferromagnetic interactions caused by structural properties that are unique to nanophase CuO, and the temperature range of multiferroicity in nanoscale CuO is shown to extend to temperatures higher than those observed for bulk CuO. The heat capacity and temperature dependent XRD measurements are the first of their kind to be reported for CuO nanoparticles.