Compression–Expansion Processes for Chemical Energy Storage: Thermodynamic Optimization for Methane, Ethane and Hydrogen

Several methods for chemical energy storage have been discussed recently in the context of fluctuating energy sources, such as wind and solar energy conversion. Here a compression–expansion process, as also used in piston engines or compressors, is investigated to evaluate its potential for the conversion of mechanical energy to chemical energy, or more correctly, exergy. A thermodynamically limiting adiabatic compression–chemical equilibration–expansion cycle is modeled and optimized for the amount of stored energy with realistic parameter bounds of initial temperature, pressure, compression ratio and composition. As an example of the method, initial mixture compositions of methane, ethane, hydrogen and argon are optimized and the results discussed. In addition to the stored exergy, the main products (acetylene, benzene, and hydrogen) and exergetic losses of this thermodynamically limiting cycle are also analyzed, and the volumetric and specific work are discussed as objective functions. It was found that the optimal mixtures are binary methane argon mixtures with high argon content. The predicted exergy losses due to chemical equilibration are generally below 10%, and the chemical exergy of the initial mixture can be increased or chemically up-converted due to the work input by approximately 11% in such a thermodynamically limiting process, which appears promising.

Effect of Argon during Diamond Deposition by Hot Filament Chemical Vapor Deposition

The effect of argon content upon the growth rate and the properties of diamond thin films grown with different grains sizes are explored. An argon-free and argon-rich gas mixture of methane and hydrogen is used in a hot filament chemical vapor deposition reactor. Characterization of the films is accomplished by scanning electron microscopy, Raman spectroscopy and high-resolution x-ray diffraction. An extensive comparison of the growth rate values and films morphologies obtained in this study with those found in the literature suggests that there are distinct common trends for microcrystalline and nanocrystalline diamond growth, despite a large variation in the gas mixture composition. Included is a discussion of the possible reasons for these observations.

Argon incorporation on silicon carbide thin films deposited by bias co-sputtering technique

ABSTRACTThe influence of negative substrate bias on the chemical, electrical and mechanical properties of silicon carbide (SiC) thin films deposited onto (100) silicon substrate by dc magnetron cosputtering without external substrate heating is reported. These studies were performed by using the following techniques: Rutherford backscattering spectroscopy (RBS), profilometry, Raman spectroscopy, four-point probe method and nanoindentation. The results indicate that there is a good correlation between the substrate bias voltage and the argon incorporation into SiC film, namely, the SiC films deposited under substrate bias of –200 V and –300 V have higher argon content and higher elastic modulus and hardness than those deposited at 0 V. An opposite behavior was found for electrical resistivity: the SiC deposited at –300 V has resistivity of 0.45 Ω.cm whereas the deposited at 0 V has 7.0 Ω.cm.