Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

Ammonia-borane and related compounds as dihydrogen sources.

Hydrogen storage for transportation applications requires high volumetric and gravimetric storage capacity. B-N compounds are well suited as storage materials due to their light weight and propensity for bearing multiple protic (N-H) and hydridic (B-H) hydrogens. This critical review briefly covers the various methods of hydrogen storage, and then concentrates on chemical hydrogen storage using B-N compounds. The simplest B-N compound, ammonia borane (H3NBH3), which has a potential 19.6 wt% hydrogen storage capacity, will be emphasised (127 references).

B–N compounds for chemical hydrogen storage

The recent developments and trends in combustion science towards the synthesis of nanomaterials are discussed. Different modifications made to conventional combustion approaches for preparation of nanomaterials are critically analyzed. Special attention is paid to various applications of combustion synthesized nanosized products.

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Combustion synthesis and nanomaterials

In the search for future energy supplies, the application of hydrogen as an energy carrier is seen as a prospective issue. However, the implementation of a hydrogen economy is suffering from several unsolved problems. Particularly challenging is the storage of appropriate amounts of hydrogen. In this context one of the promising hydrogen storage techniques relies on liquid-phase chemical hydrogen storage materials, in particular, aqueous sodium borohydride, ammonia borane, hydrazine, hydrazine borane and formic acid. The use of these materials in hydrogen storage provides high gravimetric and volumetric hydrogen densities, low potential risk, and low capital investment because it is largely compatible with the current transport infrastructure. In this review, we survey the research progresses in hydrogen generation from these liquid-phase chemical hydrogen storage materials and their regeneration.

Liquid-phase chemical hydrogen storage materials

Combustion of magnesium particles in carbon bioxide and monoxide

In aluminized solid propellants, the use of Al particles coated by certain transition metals may improve engine performance characteristics, owing to decreased agglomeration and lower ignition temperature of the particles. For this application, a detailed knowledge of particle ignition is required. Earlier, we reported the ignition mechanism of single ∼2.5-mm Ni-coated Al particles heated by a laser in Ar and CO 2 atmospheres. In the present work, using the same apparatus, we investigate the ignition mechanism of Fe-coated Al particles. The results show that intermetallic reactions contribute to the heating rate upon Al melting at 660°C, but the particles ignite at 1350-1500°C under normal-gravity conditions. The ignition mechanism includes the formation of a solid Fe 2 Al 5 phase at the interface between liquid Al and the coating, with subsequent melting of Fe 2 Al 5 , formation of a solid FeAl layer, and conversion of the Fe coating to a Fe-Al solid solution. Microgravity (10 -3 -10 -2 g) experiments with Fe-coated and Ni-coated Al particles were conducted to reduce convection inside the particles, which may influence phase mixing during ignition. In microgravity, both Ni-coated and Fe-coated Al particles ignited at 1250-1400°C. The significantly lower ignition temperature, compared with conventional oxide-coated Al, suggests that Fe-coated and Ni-coated Al particles are promising candidates for propulsion applications.

Ignition of Iron-Coated and Nickel-Coated Aluminum Particles Under Normal- and Reduced-Gravity Conditions

The preliminary feasability assessment analyses the potential for underground coal gasification within Indiana. A review of existing worldwide operations and geological requirements demonstrates that the application of UCG practices in Indiana has very significant potential benefits, but careful analysis of the specific geological conditions, physical and chemical properties of coals, water resources, coupled with an assessment of the state-of-the-art technologies must be conducted to identify potential UCG sites and to determine the feasibility of employing this technology in Indiana. Of particular importance is the relatively small number of active and successful operators of UCG projects around the world and that collaborations with one or two among them could be beneficial for all concerned. There are significant opportunities for economic development that will provide dividends for first movers in the Illinois basin. The report recommends nine 'promising zones' for UCG in two large coal deposits (the Springfield and Seelyville coal beds) in Knox, Gibson, Vanderburgh, Warrick and Posey counties. 69 refs., 10 figs., 6 tabs., 1 app.

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The potential for underground coal gasification in indiana

Synthesis of one-dimensional Ga2O3 nanostructures via high-energy ball milling and annealing of GaN

*† Metal-CO2 propulsion is less known than other Mars in-situ resource utilization (ISRU) technologies. This concept, based on using Martian carbon dioxide as an oxidizer in jet or rocket engines, offers the advantage of no chemical processing for CO2 and thus requires less power consumption than ISRU alternatives. In this paper, we review various aspects of metal-CO2 propulsion, such as combustion of magnesium and aluminum in CO2, engine types and characteristics, production of liquid CO2 and metal fuel on Mars, and potential missions. Lunar and terrestrial applications are also discussed. I. Introduction N-SITU Resource Utilization (ISRU) is recognized as an enabling technology for exploration of Mars, which can significantly reduce the mass, cost, and risk of robotic and human missions. The critical element in future missions is the large mass of propellant for a Mars ascent vehicle, while power and propellant to accommodate a long stay and mobility on Mars are also important. Transportation of propellant from Earth to Mars requires tremendous increase in the initial mass of hardware in low Earth orbit (and hence mission cost) as compared to prior no-return missions. Fortunately, Mars possesses resources that can be used for propellant production. The Martian atmosphere consisting of 95% CO2 is the obvious and most promising in-situ resource. I

Evgeny Shafirovich

Papers

Combustion of magnesium particles in carbon bioxide and monoxide

Ignition of Iron-Coated and Nickel-Coated Aluminum Particles Under Normal- and Reduced-Gravity Conditions

The potential for underground coal gasification in indiana

Synthesis of one-dimensional Ga2O3 nanostructures via high-energy ball milling and annealing of GaN

Metal-CO2 propulsion for mars missions: Current status and opportunities