An Experimental Phase Diagram Study of Ternary Pentaerythritol-Pentaglycerine-Neopentylglycol Orieintationally Disordered Plastic Crystals
Chemical and Materials Engineering
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This dissertation is on the experimental determination of Ternary Solid Solution Phase Diagrams of C5H12O4-C5H12O3-C5H12O2 (PE-PG-NPG) Orientationally Disordered "Plastic Crystal" Compounds. "Plastic crystals' are energy storage materials with layer or chain type low temperature crystal structures and transform to orientationally disordered phases at certain temperature; storing large amount of latent heat in addition to heat capacity. There are few pure compounds commercially available so to vary the phase transition temperatures, at which the thermal energy is stored, binary compounds have been made from these limited compounds. In order to further extend the range, ternary solid solution polyalcohol compounds have been made and characterized by in-situ differential scanning calorimetery, in-situ x-ray diffraction, and in-situ NMR methods. The PE-PG and PG-NPG binaries are well established, but there were discrepancies in PE-NPG binary phase diagram which was re-determined in this study. Phase stability studies of solid solutions have been made and ternary isothermal sections are derived using binary phase diagram data and the ternary experimental data. Results show single, two phase solid solution regions of α and γ, as well as high temperature γ and γT form in their parent low and high temperature crystal structures and no intermediate or line compounds are formed. The α phase has BCT structure and the β phase is monoclinic. All the γ phases have FCC type of structure that exhibit molecular O-H bond rotation/oscillation. Ternary isothermal sections are shown at 303K, 318K, 333K, 353K, 413K, 433K, 463K, 478K, 493K, 513K and 553K. The ternary tie triangles are established. Typically in the pure and binary polyalcohol's the solid-liquid phase transition enthalpies are significantly lower than the solid-solid ones, but in these ternaries the majority of these transitions show high enthalpy of transitions. For example, 45PE-45PG-10NPG mol% has 23.025 J/g of solid to liquid transition enthalpy. From the isothermal section at 353K we propose small α, α+γ, and γ +γT region with a large α+ γT phase region. The tie triangles α+ γ + γT and L + γ + γT are generally deduced based on phase rules. Liquid phases begin to appear at higher temperatures, for example at 413K, it was found that &alpha, &alpha+&gamma, &gamma+Liq., &alpha+ &gammaT, &gamma + &gammaT, &gammaT + L , and single liquid phase region coexist, along with 2 tie triangles, namely α + γ + γT , L + γT +γ regions. To our knowledge this is the first time PE-PG-NPG ternary phase equilibrium has been developed and that will widen the energy storage capabilities. Details of the thermal and crystallographic behavior of binary PE-NPG, and ternary PE-PG-NPG at different temperatures based on XRD, DSC and NMR results are presented.