Renormalized field theory for the static crossover in dipolar ferromagnets

A field theoretical description for the static crossover in dipolar ferromagnets is presented. New non leading critical exponents for the longitudinal static susceptibility are identified and the existence and magnitude of the dip in the effective critical exponent of the transverse susceptibility found by matching techniques is scrutinized

Space Resources and Space Settlements

The technical papers from the five tasks groups that took part in the 1977 Ames Summer Study on Space Settlements and Industrialization Using Nonterrestrial Materials are presented. The papers are presented under the following general topics: (1) research needs for regenerative life-support systems; (2) habitat design; (3) dynamics and design of electromagnetic mass drivers; (4) asteroids as resources for space manufacturing; and (5) processing of nonterrestrial materials

Low-Cost Magnesium-Based Thermoelectric Materials: Progress, Challenges, and Enhancements

Magnesium-based thermoelectric (TE) materials have attracted considerable interest due to their high ZT values, coupled with their low cost, widespread availability, nontoxicity, and low density. In this review, we provide a succinct overview of the advances and strategies pertaining to the development of Mg-based materials aimed at enhancing their performance. Following this, we delve into the major challenges posed by the severe working conditions, such as high temperature and thermal cycling, which adversely impact the behavior and long-term stability of the TE modules. Challenges include issues like the lack of mechanical strength, chemical instability, and unreliable contact. Subsequently, we focus on the key methodologies aimed at addressing these challenges to facilitate the broader application of the TE modules. These include boosting the mechanical strength, especially the toughness, through grain refining and additions of second phases. Furthermore, strategies targeted at enhancing the chemical stability through coatings and modifying the microstructure, as well as improving the contact design and materials, are discussed. In the end, we highlight the perspectives for boosting the practical applications of Mg-based TE materials in the future

Electrocatalytic Properties of Ni-Fe BasedAlloys Toward Oxygen Evolution Reaction

In this dissertation, laser-based manufacturing was applied to develop NiFe-based alloys, with focus on their electrochemical characteristics toward oxygen evolution reaction during water electrocatalysis. The targeted electrocatalysts were designed by combining catalysis principles of absorption energy, electronic effect and geometry effect. Firstly, a one-step laser-based manufacturing method was applied to prepare a thin layer of NiFe-based electrodes with varied Ni to Fe ratios to replace high-cost noble metals as the electrocatalyst for oxygen evolution reaction (OER). Since each Ni has 0.6 d-vacancy and each Fe atom has 2.2 d-vacancies. The alloying of Ni-Fe allows electrons flowing from Ni to Fe, causing the increase of d-vacancy, resulting in a catalyst matching better to the reaction electron transfer coordination. The formation of ultra-small secondary FeNi3 phases effectively increased active sites, as well as the average lattice spacing to match better with the geometries of species involved in OER reactions. Secondly, laser processed alloys usually experience special thermal cycles that could produce anisotropic and heterogeneous microstructures significantly different from parts made by traditional casting. The electrocatalytic performances of laser-processed alloys differ significantly from traditionally processed alloys and provide interesting insights on novel electrocatalyst. The laser processed Ni-Fe alloys are prepared by varied laser scanning speeds as the electrocatalyst toward OER. In addition Ni-Fe alloys prepared by arc melting and spark plasma sintering are also prepared for comparison. Thirdly, monolithic nanoporous NiFe electrocatalyst is developed by dealloying Ni6Fe4Al10 prepared by laser-based manufacturing for the first time. The resulted nanoscale pores provided high surface areas and more active sites for catalytic reactions, while the microscale pores provided sufficient channels for gas and ion diffusion. Compared to the bulk NiFeelectrode, the nanoporous NiFe electrode exhibits an improved electrocatalytic activity. At last, Ni, Fe based high entropy alloys (HEAs), including FeNiMnCrCu and FeCoNiCrAl, were synthesized via arc melting and their electrochemical performance was studied. Due to the varied atomic radius and structure configurations of the elements in an HEA, the lattice parameters in the crystalline are usually seriously distorted and severe defects exist in the lattice with changed valence electron concentration, which are advantageous properties for catalyst performance

Perovskite-silicon tandem based photoelectrochemical systems for efficient solar hydrogen generation

Direct solar hydrogen generation(DSTH) is a promising method for renewable hydrogen generation, where solar energy drives the generation of hydrogen and oxygen by molecular dissociation of water on a catalytic or semiconductor surface. Despite an enormous amount of work over the last few decades, DSTH has not yet been implemented on large scale due to low efficiency and high costs. This thesis investigates perovskite-silicon tandem PV and Earth abundant catalysts-based system for low-cost and high-efficiency direct-solar-hydrogen-generation. The thesis starts by identifying performance limitations and conceptualising practical device designs to improve the STH efficiency towards 20%. We develop a new theoretical framework to quantify and compare different loss mechanisms in photovoltaic(PV) based solar hydrogen generation systems and evaluate the potential of different loss mitigation techniques to improve the solar to hydrogen generation efficiency. Our analysis shows that the two largest losses in an ideal system are energy lost as heat in the photovoltaic component, and current and voltage mismatch between the PV and electrochemical (EC) components due to sub-optimal system configuration. Employing loss mitigation techniques targeting the two major efficiency losses results in predicted STH efficiencies above 20%, without the need for further improvement of the photovoltaic devices or catalyst. These results demonstrate where best to target interventions to mitigate the biggest losses and ensure maximum improvement in the performance. To achieve the STH target set by the DOE for the year 2020, we develop a system with perovskite-silicon tandem PV and Ni based Earth abundant catalysts with a record STH efficiency of 20%. The NiMo electrodes with a high density of NiMo active sites are fabricated in a flower-stem morphology and exhibit an exceptional HER performance. In addition, an improved perovskite top cell with a record open circuit voltage is achieved using n-dodecylammonium bromide. Further analysis is performed to assess the potential for improvement in the STH efficiency. With Si solar cell performance already close to theoretical limits, perovskite solar cell offer opportunities of improvement to improve the tandem performance. Improving the performance of perovskite solar cell alone could improve the STH efficiency to 25%. A technoeconomic analysis is performed to assess the cost competitiveness of the developed system. The levelized cost of hydrogen (LCOH) of DSTH system is calculated as $4.12/Kg. Combining efficiency improvements with projected cost reduction could further reduce the LCOH to$2.3/Kg, presenting a remarkable opportunity to realise cheap renewable hydrogen. Moving towards the aim of a fully integrated system for direct solar hydrogen generation, we develop a photoelectrochemical system based on perovskite PV and Si photocathode system in tandem configuration. We demonstrate a high-performance Si photocathode, incorporating state-of-the-art charge selective passivation and Earth abundant catalysts, with an ABPE of over 10%. Charge selective passivation layers improve the Si photocathode performance by roughly 70% compared to Si photocathode without any passivation, highlighting the importance of the efficiency loss due to recombination at the Si/catalyst interface. An overall water splitting efficiency of 17% is achieved for the photoelectrochemical system when combined with a previously reported Earth abundant OER catalyst and a wide bandgap perovskite solar cell in tandem. These results show that perovskite-silicon tandem based photoelectrochemical systems have the potential to make large scale direct solar hydrogen generation a reality. This work will encourage dedicated research with a clear awareness of components and system design that needs to be focused on, towards achieving the aim of low-cost, large scale solar hydrogen generation

Hybrid spintronic materials:Growth, structure and properties

10.1016/j.pmatsci.2018.08.001Progress in Materials Science9927-10

Ultima Replicated Optics Research

Designs are reviewed incorporating processes suitable for replication of precision spherical segments of very large (greater than 20 meter diameter) telescopes combining ultra-lightweight and high precision. These designs must be amenable to assembly and alignment after deployment . The methods considered lie outside the present scope of fabrication, deployment and alignment considered to date. Design guidelines for reducing the weight and low frequency resonance in low G environment were given by The Serius Group, Dr. Glenn Zeiders, and are considered baseline for this activity. The goal of a rigid design of 10 Kg/sq M is being persued for the Next Generation Space Telescope (NGST) and is not likely adequate for advanced efforts. Flexures have been considered for maintaining the figure of many lightweight structures by control loop processes. This adds to the complexity and weight to the extent that it becomes difficult to recover the benefits. Two fabrication guidelines lead to a stiffer and concurrently lighter structure. First the use of thin vertical wall triangular structural reinforcements to increase the resistance to bending is preferred over hexagonal or square similar sections. Secondly, the incorporation of a similar back sheet on a cellular structure markedly improves the geometric stiffness. Neither improves the short range stiffness. Also often overlooked is that selected material properties must include high microyield and low hysteresis in addition to high elastic modulus to weight (stiffness). The fabrication steps can easily exceed the strain requirement