Wen-Ming Chien

Heat Capacities of ‘Plastic Crystal’ Solid-state Thermal Energy Storage Materials

The heat capacities of polyalcohol “Plastic Crystals,” such as pentaerythritol [PE, C–(CH2OH)4], pentaglycerine [PG, CH3–C–(CH2OH)3], neopentylglycol [NPG, (CH3)2–C–(CH2OH)2] and neopentylalcohol [NPA, (CH3)3–C–(CH2OH)] have been measured over a temperature range encompassing solid-solid and solid-liquid phase changes. Other amine plastic crystals such as tris(hydroxymethyl)-aminomethane [Tris, H2NC(CH2OH)3] and 2-amino-2-methyl-1,3-propanediol [AMPL, (CH3)(NH2)–C–(CH2OH)2] have also been evaluated. The heat capacities of the α phase of the above compounds range between 188 to 294 J/mol K and of the γ phase between 246 and 474 J/mol K. There is a significant increase in the heat capacity when solid-state phase transition occurs, because of orientational disorder. Heat capacity equations for α, γ and liquid phases of these six materials are reported.

Solid-solid phase transition in trimethylolpropane (TRMP)

An orientationally disordered crystalline (ODIC) plastic phase (γ) was observed in Trimethylolpropane (TRMP) during heating by high resolution thermal and X-ray diffraction analyses. TRMP is a potential thermal energy storage material. The enthalpies of solid-solid (α → γ at 327.8 K) and fusion (γ → liquid at 332.7 K) transitions are 16.36 kJ/mol and 0.9 kJ/mol, respectively. Supercooling was observed during solidification of melts, and this supercooled γ phase began to transform to a metastable crystalline phase, designated as α′, after 20 minutes at room temperature. The lattice parameters of the monoclinic α phase, obtained from this study, are: a = 0.8427(5), b = 0.9580(5), c = 0.9185(6) nm, β = 98.958(3)° @ 298 K with Z = 4. The ODIC high temperature FCC γ phase has a lattice parameter: a = 0.9274(3) nm @ 328 K with Z = 4. Differential scanning calorimetry, in-situ Bragg-Brentano X-ray diffractometry, and high temperature and resolution Guinier X-ray diffraction systems were used to determine these phase transitions.

High-Pressure Raman Spectroscopy of Tris(hydroxymethyl)aminomethane

High-pressure Raman spectroscopy has been used to study tris(hydroxymethyl)aminomethane (C(CH(2)OH)(3)NH(2), Tris). Molecules with globular shapes such as Tris have been studied thoroughly as a function of temperature and are of fundamental interest because of the presence of thermal transitions from orientational order to disorder. In contrast, relatively little is known about their high-pressure behavior. Diamond anvil cell techniques were used to generate pressures in Tris samples up to approximately 10 GPa. A phase transition was observed at a pressure of approximately 2 GPa that exhibited relatively slow kinetics and considerable hysteresis, indicative of a first-order transition. The Raman spectrum becomes significantly more complex in the high-pressure phase, indicating increased correlation splitting and significant enhancement in the intensity of some weak, low-pressure phase Raman-active modes.

X-ray diffractometry studies and lattice parameter calculation on KNO3-NH4NO3 solid solutions

The solid-state phase transitions of the KNO3-NH4NO3 solid solutions have been determined by high temperature X-ray diffractometry, and lattice parameter calculation has also been performed. Ammonium nitrate (AN) is of great use for gas generators of automobile air bag systems. The X-ray diffraction results showed the single (AN) phase III from 5% to 20% KNO3 in NH4NO3 and up to 373 K, which is the important temperature range for the air bag gas generator applications. The X-ray diffraction patterns of the low temperature KNO3 phase (KN II) are from 92%-100% KNO3 composition range and up to 393 K temperature. The high temperature KNO3 phase (KN I) showed very broad composition range from 20% up to 100% KNO3 at various temperature ranges. The lattice parameters of the NH4NO3-rich (AN III) and KNO3-rich (KN II and KN I) solid solutions have been calculated at different temperature range. The volumes of AN III phase decrease from 0.3201(4) to 0.3166(1) nm 3 at the room temperature and from 0.3250(6) to 0.3215(3) nm 3 at 373 K as the compositions increase from 5% to 20%KNO3. The lattice constants of the hexagonal KN I phase show that there is no significant change in a-direction when the temperature increases. Details of X-ray results, lattice expansions and equations during heating are presented.

Experimental Determination of NH 4 NO 3 -KNO 3 Binary Phase Diagram

The solid-state phase transitions in ammonium nitrate (AN)-potassium nitrate (KN) system, andthe equilibrium AN-KN phase diagram have been determined by using differential scanningcalorimetry and high-temperature in situ x-ray diffractometry. Sample preparation was per-formed in a special “dry room” with very low humidity. A single phase region (AN III) with nophase transitions to 373 K was observed in the composition range 5 to 20% KN; this is criticalfor use in air bag gas generators. The high-temperature KN phase (KN I) has a wide range ofstability from 20 to 100 wt.% KN. There are one eutectic, two eutectoid, three peritectoid, andone congruent transformations in this phase diagram. Two new nonstoichiometric phases werefound at lower temperatures in the mid-composition range between the AN and KN terminalsolid solutions. Details of the phase equilibria are presented.

Amorphous Alloy Membranes Prepared by Melt-Spin methods for Long-Term use in Hydrogen Separation Applications

Amorphous Ni-based alloy membranes show great promise as inexpensive, hydrogenselective membrane materials. In this study, we developed membranes based on nonprecious Ni-Nb-Zr alloys by adjusting the alloying content and using additives. Several studies on crystallization of the amorphous ribbons, in-situ x-ray diffraction, SEM and TEM, hydrogen permeation, hydrogen solubility, hydrogen deuterium exchange, and electrochemical studies were conducted. An important part of the study was to completely eliminate Palladium coatings of the NiNbZr alloys by hydrogen heattreatment. The amorphous alloy (Ni0.6Nb0.4)80Zr20 membrane appears to be the best with high hydrogen permeability and good thermal stability.

Effect of Gaseous Impurities on Long-Term Thermal Cycling and Aging Properties of Complex Hydrides for Hydrogen Storage

Hydrogen is one of the alternate fuels for vehicular applications where the exhaust is mainly water vapor when used with either fuel cells or internal combustion engines. In the transportation sector, a proton exchange membrane (PEM) fuel cell, that electrochemically combines inputs of pure hydrogen and oxygen (from air), may be used to directly generate electricity for an electric motor drive with water vapor and heat produced as byproducts. However, hydrogen fuel cell technologies must be competitive in this venue with the ubiquitous internal combustion engine using gasoline and diesel fuels, both of which offer an established production and distribution infrastructure as well as a familiar and commonly accepted form of high density and stable energy storage. The United States Department of Energy (DOE) is addressing the standards that a hydrogen fuel system would have to meet as part of their FreedomCAR initiative. Conventionally, hydrogen is compressed in high pressure cylinders with obvious safety issues. The authors believe that hydrogen may be stored in metal hydrides at low pressures. Hydrides may be classified as (a) classical hydrides (heavy hydrides with low wt.%H), and (b) complex hydrides (light weight hydrides with high H-wt.%). In this study, the focus was on practical aspects of complex hydrides related to refilling hydrogen in vehicles at hydrogen gas stations, where one expects ppm levels of impurities. These gaseous impurities affect the performance of the complex hydrides as a medium for storing hydrogen. The main focus of the project was to understand the effect of trace element gases, mixed with hydrogen, on the long-term cycling and aging properties of complex hydrides of Li3N-H2. Another secondary aspect was to test complex hydrides developed by MHCoE partners. In addition, they also conducted thermodynamic and crystallographic research of these hydrides to understand the reaction pathways.