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

2007 AIChE Annual Meeting

Recent lunar spacecraft missions have confirmed the presence of water and miscellaneous volatiles within the craters of the moon’s south pole. If harvested, these resources have the potential to provide incredible cost savings to the space program. Challenges include extreme conditions and lack of solar power in the permanently shadowed areas. The proposed concept involves the transfer of beamed power from solar concentrators to extract the volatiles from the regolith within the shadowed craters of the moon. The present paper focuses on the analysis of heat transfer from the regolith surface, irradiated by the light beam. This analysis is needed for the determination of solar power required for the extraction of volatiles. To demonstrate the feasibility of the proposed concept and validate the heat transfer analysis, experiments involving laser irradiation of JSC-1A lunar regolith simulant mixed with water have also been conducted.

/pdf/extraction-of-volatiles-from-lunar-regolith-using-solar-4swkt6kvln.pdf

Extraction of Volatiles from Lunar Regolith Using Solar Power

Combustible mixtures of lunar regolith with magnesium could be used for the production of construction materials on the Moon. The present paper focuses on decreasing Mg content in these mixtures and increasing the density and strength of the combustion products. It has been shown that preheating to 100°C allows combustion of JSC-1A/Mg mixtures at 8 wt% Mg. Combustion of pellets submerged in silica led to a noticeable increase in the product strength. Significantly higher density and strength were achieved using SHS compaction. I. Introduction ELF-PROPAGATING high-temperature synthesis (SHS) has been envisioned for the production of construction materials on the Moon. This technique involves self-sustained propagation of the combustion wave over the mixture due to exothermic reactions between the mixture components. SHS has been used for synthesis of numerous ceramic compounds and other materials. Recently, SHS in the mixtures of JSC-1A lunar regolith simulant with magnesium has been investigated. 1-3 Thermodynamic calculations of the adiabatic flame temperatures and combustion products over a wide range of regolith/metal mixture ratios have shown that magnesium provides higher adiabatic temperatures as compared to aluminum. 1 Experiments on the combustion of Mg/JSC-1A mixture pellets in argon environment determined the combustion characteristics at under different conditions. 1-3 For stable combustion, the minimum concentration of magnesium was 13 wt%. 3 These experiments have shown, however, that the strength of the combustion products is not sufficiently high for using them as construction materials. The present paper focuses on two goals: to decrease the content of magnesium in the JSC-1A/Mg reaction and to obtain denser and stronger products. To minimize the magnesium content, a preheating step was added before ignition of the JSC-1A/Mg mixture. It is common in many SHS systems to raise the initial temperature of the green mixture for increasing the combustion temperature and front velocity. To produce dense and stronger materials, a technique called SHS compaction was used. In this method, the green mixture is located inside some inert powder (e.g., sand) placed under a hydraulic press. The mixture is ignited and after the combustion process, the product mixture is compacted by the press. This method allows for the production of denser products and items that have complicated shape and can be used with no subsequent treatment. As a preparation for SHS compaction, experiments with pellets submerged in silica powder were initially conducted. Next, an apparatus for quasi-isostatic pressing was constructed and SHS compaction tests with JSC1A/Mg mixtures were performed.

Combustion of Lunar Regolith/Magnesium Mixtures for the Fabrication of Construction Materials

Molybdenum silicides and borosilicides are promising structural materials for advanced power plants. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at high temperatures. For example, molybdenum disilicide (MoSi2) has excellent oxidation resistance and poor mechanical properties, while Morich silicides such as Mo5Si3 (called T1) have much better mechanical properties but poor oxidation resistance. One approach is based on the fabrication of MoSi2-T1 composites that combine high oxidation resistance of MoSi2 and good mechanical properties of T1. Another approach involves the addition of boron to Mo-rich silicides for improving their oxidation resistance through the formation of a borosilicate surface layer. In particular, Mo5SiB2 (called T2) phase and alloys based on this phase are promising materials. In the present paper, MoSi2-T1 composites and materials based on T2 phase are obtained by mechanically activated self-propagating high-temperature synthesis (MASHS). To obtain denser products, the so-called SHS compaction (quasi-isostatic pressing of hot combustion products) has been employed. Thermal analysis has shown that SHS compaction significantly improves the oxidation resistance. Self-sustained combustion of Mo/Si/B mixtures for the formation of T2 phase becomes possible if the composition is designed for adding a more exothermic reaction of MoB formation. These mixtures exhibit spin combustion. Oxidation resistance of the obtained multi-phase Mo-Si-B materials is independent on the concentration of Mo phase in the products. The “chemical oven” technique has been used to obtain a single Mo5SiB2 phase and an alloy consisting of α-Mo, Mo5SiB2, and Mo3Si phases.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427414009646

Evgeny Shafirovich

Papers

2007 AIChE Annual Meeting

Extraction of Volatiles from Lunar Regolith Using Solar Power

Combustion of Lunar Regolith/Magnesium Mixtures for the Fabrication of Construction Materials

Mechanically Activated Combustion Synthesis of Molybdenum Silicides and Borosilicides

Thermoanalytical Study of Hydroxylammonium Nitrate Decomposition at High Pressures