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 sol id-state phase transformations. Both NPG and AMPL have monoclinic low temperature alpha-phase and cubic high temperature gamma-phase structu res. These polymorphic changes in structure occur at 41 and 80-degrees -C, for NPG and AMPL, respectively. Below the alpha --> gamma transiti on, these compounds are dielectric materials with conductivities on th e order of 10(-8) S/m (at 22-degrees-C) and 10(-9) S/m (at 25-degrees- C), for NPG and AMPL, respectively. The lack of appreciable conductivi ty in the low temperature phase is consistent with charge transport vi a low temperature structural reorientation. There is a significant inc rease in the conductivity through the alpha --> gamma phase transition with a maximum of 10(-6) S/m (at 82.05-degrees-) for NPG and 10(-5) S /m (at 133.25-degrees-C) for AMPL in the high temperature phase. The c onductivities in both the alpha- and gamma-phases have been found to b e thermally activated. The temperature dependent conductivity follows the Arrhenius equation with the activation energy of the alpha --> gam ma transition being larger in the case of NPG. Remarkably, the activat ion energies of the high temperature gamma-phases are nearly equivalen t, suggestive of similar conduction mechanisms and charge carriers. Th e charge carrier diffusion coefficients, D (t) are in the range of 1 a nd 3 x 10(-10) CM2/S (45 < T < 56-degrees-C) for NPG, and between 6.9 and 7.3 x 10(-10) cm2/s (85 < T < 92-degrees-C) for AMPL. The ac condu ctivity in the alpha-phase exhibits behavior representative of a doubl e well potential energy profile where the low and high frequency condu ctivities are constant with a Debye-like transition at intermediate fr equencies. The gamma-phase ac conductivity is frequency independent as is the case in simple hopping conductivity processes. AC impedance sp ectroscopic measurements on NPG and AMPL have been made and frequency and temperature effects on the conductivities, relaxation times, and d iffusional parameters of NPG and AMPL are presented.