Potential Thermal Energy Storage Materials Research Articles

AC Conductivities of Neopentylglycol and 2‐Amino‐2‐Methyl‐1,3‐Propanediol

Neopentylglycol (NPG) and 2‐amino‐2‐methyl‐1,3‐propanediol (AMPL) are potential thermal energy storage materials which undergo energetic solid‐state phase transformations. Both NPG and AMPL have monoclinic low temperature α‐phase and cubic high temperature γ‐phase structures. These polymorphic changes in structure occur at 41 and 80°C, for NPG and AMPL, respectively. Below the α→γ transition, these compounds are dielectric materials with conductivities on the order of 10−8 S/m (at 22°C) and 10−9 S/m (at 25°C),. 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 α→γ phase transition with a maximum of 10−6 S/m (at 82.05°C) for NPG and 10−5 S/m (at 133.25°C) for AMPL in the high temperature phase. The conductivities in both the α‐ and γ‐phases have been found to be thermally activated. The temperature dependent conductivity follows the Arrhenius equation with the activation energy of the α→γ transition being larger in the case of NPG. Remarkably, the activation energies of the high temperature γ‐phases are nearly equivalent, suggestive of similar conduction mechanisms and charge carriers. The charge carrier diffusion coefficients, D(t) are in the range of for NPG, and between for AMPL. The ac conductivity in the α‐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 γ‐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.