No abstract

Pentaglycerine (PG) and tris-hydroxy-aminomethane (Tris) are potential thermal energy storage materials which undergo energetic solid-state phase transformations. The low temperature a-phase structure of PG is body-centered tetragonal and that of Tris is orthorhombic. The high-temperature phase structure of both compounds is face-centered cubic. Polymorphic changes in the structure occur at 89~ for PG and 135~ for Tris. These compounds are dielectrics at low temperature with conductivities on the order of i0 -8 ~-~ cm -~ (at 21~ and I0 -9 ~-~ cm ~ (at 25~ for PG and Tris, respectively. Heating samples from room temperature leads to an initial decrease in a-phase conductivities followed by an increase for both of these compounds as a result of proton hopping. Continuous increases in the conductivity were observed at the transition temperature, with discontinuity evident in the case of PG. The conductivity variations in the a-and ~-phases have been found to be thermally activated. The activation energies are nearly equal suggesting similar conduction mechanisms and charge carriers. From the relaxation time data, an equation has been obtained for the charge carrier self-diffusion coefficient for the ~-phase of PG, just above the transition temperature; D (~) = 0.085e ~7,1~7~T. The conductivity in the ~]-phases are independent of the frequency in the range of i0 to i03 Hz. The temperature and frequency effects on the conductivities, relaxation times, and diffusional parameters of PG and Tris are presented.Molecular "plastic crystals" which undergo solid-state transitions are potential candidates for thermal energy storage applications.

Neopentylglycol (NPG) and 2-amino-2-methyl-l,3-propanediol (AMPL) are potential thermal energy storage materials which undergo energetic solid-state phase transformations. Both NPG and AMPL have monoclinic low temperature a-phase and cubic high temperature ~-phase structures. These polymorphic changes in structure occur at 41 and 80~ for NPG and AMPL, respectively. Below the a --> ~/transition, these compounds are dielectric materials with conductivities on the order of 10 -8 S/m (at 22~ and 10 -9 S/m (at 25~ for NPG and AMPL, respectively. The lack of appreciable conductivity in the low temperature phase is consistent with charge transport via low temperature structural reorientation. There is a significant increase in the conductivity through the a --> ~ phase transition with a maximum of 10 6 S/m (at 82.05~ for NPG and 10 -~ S/m (at 133.25~ for AMPL in the high temperature phase. The conductivities in both the a-and ~/-phases have been found to be thermally activated. The temperature dependent conductivity follows the Arrhenius equation with the activation energy of the a ---> ~ transition being larger in the case of NPG. Remarkably, the activation energies of the high temperature ~-phases are nearly equivalent, suggestive of similar conduction mechanisms and charge carriers. The charge carrier diffusion coefficients, D(t) are in the range of 1 and 3 • 10 10 cm2/s (45 < T < 56~ for NPG, and between 6.9 and 7.3 X 10 10 cm2/s (85 < T < 92~ for AMPL. The ac conductivity in the a-phase exhibits behavior representative of a double well potential energy profile where the low and high frequency conductivities are constant with a Debye-like transition at intermediate frequencies. The ~-phase ac conductivity is frequency independent as is the case in simple hopping conductivity processes. AC impedance spectroscopic measurements on NPG and AMPL have been made and frequency and temperature effects on the conductivities, relaxation times, and diffusional parameters of NPG and AMPL are presented.