Magnetic and interface microstructure contribution to bulk specific heat of nanocrystalline Ni–P alloy

Abstract

The bulk specific heat of fully dense nanocrystalline Ni–P electrodeposits with essentially constant P content (about 4 at%) and varying average grain sizes from 6.9 to 28.9 nm was investigated using modulated differential scanning calorimetry. In the lower temperature range from room temperature to 120 °C, at which the as-deposited sample microstructure was thermally stable, the bulk specific heat varied only within ~2 % despite the substantial variation of interface volume fractions from 0.11 to 0.39 for this series of samples. Moreover, the measured bulk specific heat values of the Ni–P samples were all located within the reported specific heat value range for conventional polycrystalline Ni. Evidently, the contribution due to grain size-related interface excess free volume is negligible and the bulk specific heat for the materials can be characterized as a structure-insensitive property. In the elevated temperature range from 150 °C to the Curie temperature of 357 °C, the magnetic contribution to the specific heat was significantly influenced by the chemical environment of P in the Ni–P samples. When P atoms were in the form of supersaturated solution in the nickel matrix, a complete suppression of the characteristic λ peak of the magnetic contribution in the specific heat curves was observed for all materials. The λ peak re-appeared in the specific heat curve after the Ni–P sample underwent a transformation to a two-phase microstructure consisting of Ni and Ni3P grains. It can be concluded that at a given P content, paramagnetic phosphorus atoms in the form of solutes are more effective in reducing the magnetic contributions to the specific heat than the form of paramagnetic Ni3P second-phase particles for the nanocrystalline Ni–P alloy.