Calorimetric studies of some energy-related materials

Abstract

Low-temperature heat capacities were measured for two kinds of energy-related materials. The first is a series of OH-substituted neopentane, C(CH 3) 4− n (CH 2OH) n with n=2 and 3. Each of them undergoes a first-order phase transition with a large enthalpy change. The transition occurred at 314.5 K with Δ trs H=(12.50±0.02) kJ mol −1 for n=2, at 358.2 K with Δ trs H=(21.24±0.05) kJ mol −1 for n=3, respectively. Both of them prove to be useful as an energy reservoir which stores and releases the latent heat while keeping their transition temperatures. The second is the clathrate hydrates. For the type II clathrate hydrates encaging polar guest molecules (tetrahydrofuran THF, acetone Ac and trimethylene oxide TMO), two kinds of dielectric relaxation are observed in relation to the freezing out of reorientational motion of the host water molecules and of the guest dipoles. Doping of a minute amount of KOH into the samples turns out to accelerate dramatically the motion to release the immobilized state and to induce their ordering transitions. A phase transition occurred at 61.9 K for THF hydrate, 46.6 K for Ac hydrate, and 34.5 K for TMO hydrate, respectively. The associated entropy change is of the order of 2.4 J K −1(H 2O mol) −1 and similar in magnitude to that of dopant-induced transition in ordinary ice I h. Nature of dynamic disorder of the host and guest species as studied by dielectric and thermal conductivity measurements are reviewed briefly in relation to the ordering processes.