铀基非晶合金的发展现状

铀基非晶合金是非晶家族中的特殊成员,受限于铀元素的高活性与放射性特点,目前这类非晶材料的研究极不充分.本文结合非晶合金的最新发展动态简要介绍了铀基非晶发展历史,较系统地总结了本团队的最新铀基非晶研究工作:首先较详细地介绍了新型铀基非晶的制备技术、成分体系、形成规律与晶化行为,澄清了其形成机制与热稳定性;结合高分辨电镜分析展示了其微观结构特点;采用纳米压痕技术揭示了这类非晶的微纳力学性能;利用电化学测试方法评估了其耐腐蚀性能.这些结果丰富了非晶材料的内涵,有助于深化对非晶物理基础科学问题的理解,并推动新型铀合金材料的发展,为这种材料的潜在工程应用奠定了基础.     

Narrow band gap group III-nitride alloys

Recent results on the properties of narrow gap group III-nitrides and their alloys are reviewed. It is shown that InN with the energy gap of 0.7 eV exhibits classical characteristics of a narrow gap semiconductor with strongly nonparabolic conduction band and an energy dependent electron effective mass. With the new discovery, the direct band gaps of the group III-nitride alloys span an extremely wide energy range from near infrared in InN to deep ultraviolet in AlN offering possibilities for new device applications of these materials. We also discuss properties of dilute group III-N–V alloys in which incorporation of a small amount of nitrogen results in a dramatic band gap reduction. All the unusual properties of the alloys are well described by a band anticrossing model that considers an interaction between localized nitrogen states and the extended states of the conduction band.     

Giant magnetic anisotropy in tetragonal FeCo alloys

In order to further increase the recording density in hard disk drives, new media materials are required. Two essential parameters of future recording media are a large uniaxial magnetic anisotropy energy (MAE) K(u) and a large saturation magnetization M(s). Based on first-principles theory, we predict that very specific structural distortions of FeCo alloys possess these desired properties. The discovered alloy has a saturation magnetization that is about 50% larger than that of FePt--a compound that has received considerable attention lately-with a uniaxial MAE that can easily be tailored reaching a maximum value that is 50% larger than that of FePt.     

Modeling of high entropy alloys of refractory elements

Reverting the traditional process of developing new alloys based on one or two single elements with minority additions, the study of high entropy alloys (HEA) (equimolar combinations of many elements) has become a relevant and interesting new field of research due to their tendency to form solid solutions with particular properties in the absence of intermetallic phases. Theoretical or modeling studies at the atomic level on specific HEA, describing the formation, structure, and properties of these alloys are limited due to the large number of constituents involved. In this work we focus on HEA with refractory elements showing atomistic modeling results for W–Nb–Mo–Ta and W–Nb–Mo–Ta–V HEA, for which experimental background exists. An atomistic modeling approach is applied for the determination of the role of each element and identification of the interactions and features responsible for the transition to the high entropy regime. Results for equimolar alloys of 4 and 5 refractory elements, for which experimental results exist, are shown. A straightforward algorithm is introduced to interpret the transition to the high entropy regime.     

非晶态合金与氢相互作用的研究进展

非晶态合金在力学性能、耐磨耐蚀性、磁性等方面比传统晶态合金具有显著优势,是一类有优良应用前景的新型结构与功能材料.非晶态合金与氢相互作用可以产生很多有趣的物理化学现象和应用.本文从物理基础和材料应用两个方面评述非晶态合金和氢相互作用的研究进展,在物理基础研究方面,从氢在非晶态合金中的存在状态出发,讨论氢在非晶态合金中的溶解、分布、占位和扩散等相关物理问题,进而分析氢对非晶态合金的热稳定性、磁性、内耗、氢脆等的影响.在材料应用研究方面,对非晶态储氢合金、非晶态合金氢功能膜、吸氢改善非晶态合金的塑性和玻璃形成能力、氢致非晶化、利用非晶态合金制备纳米储氢材料等方面的研究进展进行评述.最后总结并展望有关非晶态合金与氢相互作用的研究和应用.     

Magnetic properties of amorphous alloys

The magnetic properties of existing amorphous iron based materials, i.e., Fe--metalloid, Fe--early transition metals and Fe--rare earth alloys, are briefly discussed for some representative alloys. The spin orientation of amorphous Fe--metalloid alloys has been determined by the angular dependence of hyperfine interactions. It is shown that in iron--early transition metals ferromagnetic order is not long-ranged, but determined by magnetic clusters. The magnetic hyperfine field distributions of Fe-rich iron--early transition metals consist of a high and a low field tail. The magnetic structure has been investigated for two representative Fe--RE (RE = Er, Ce) amorphous alloys. For the first time, the magnetic coupling phenomenon in amorphous/crystalline multilayers has been discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photoemissive properties of binary magnesium-barium and aluminium-lithium metallic alloys

The photoemissive properties of binary Mg-Ba and Al-Li metallic alloys, suitable as high-current photocathodes, were studied. The photoelectron energy-distribution curves and the quantum yield's spectral characteristics are presented for these materials. The concept of forming the advanced photoemissive properties for metallic solid solutions and heterophase alloys is suggested.     

Growth of normally-immiscible materials (NIMs), binary alloys, and metallic fibers by hyperbaric laser chemical vapor deposition

This work demonstrates that two or more elements of negligible solubility (and no known phase diagram) can be co-deposited in fiber form by hyperbaric-pressure laser chemical vapor deposition (HP-LCVD). For the first time, Hg-W alloys were grown as fibers from mixtures of tungsten hexafluoride, mercury vapor, and hydrogen. This new class of materials is termed normally-immiscible materials (NIMs), and includes not only immiscible materials, but also those elemental combinations that have liquid states at exclusive temperatures. This work also demonstrates that a wide variety of other binary and ternary alloys, intermetallics, and mixtures can be grown as fibers, e.g. silicon-tungsten, aluminum-silicon, boron-carbon-silicon, and titanium-carbon-nitride. In addition, pure metallic fibers of aluminum, titanium, and tungsten were deposited, demonstrating that materials of high thermal conductivity can indeed be grown in three-dimensions, provided sufficient vapor pressures are employed. A wide variety of fiber properties and microstructures resulted depending on process conditions; for example, single crystals, fine-grained alloys, and glassy metals could be deposited. PACS 81.15.Fg; 81.05.Bx; 81.05.Je; 81.15.Gh     

Anomalous resistivity in Nb-Ti alloys

The anomalous resistivity behaviour which we have observed in crystalline NbTi alloys is understood in the framework of Two Level Systems model proposed for amorphous materials. It is shown that the fluctuations associated with the ω phase instability is the most dominant scattering mechanism in Ti rich transition metal alloys.     

Combinatorial approaches for the design of metallic alloys

The design of new metallic alloys is faced with the challenge of an increasing complexity of the alloys composition, processing and resulting microstructures necessary to answer to multiple property targets, together with a requirement that the design stage be faster and less expensive. This paper shows that combinatorial methods, combining numerical and experimental approaches, can be applied to the specific requirements of alloy design and lead to improved understanding of fundamental processes of physical metallurgy, such as precipitation, together with improved alloy compositions and processing.     

Strategies for optimizing the thermoelectricity of PbTe alloys

The thermoelectric materials have been considered as a potential candidate for the new power generation technology based on their reversible heat and electricity conversion.Lead telluride(Pb Te) is regarded as an excellent mid-temperature thermoelectric material due to its suitable intrinsic thermoelectric properties.So tremendous efforts have been done to improve the thermoelectric performance of Pb Te,and figures of merit,z_T 2.0,have been reported.Main strategies for optimizing the thermoelectric performance have been focused as the main line of this review.The band engineering and phonon scattering engineering as two main effective strategies are systemically summarized here.The band engineering,like band convergence,resonant levels,and band flatting have been addressed in improving the power factor.Additionally,phonon scattering engineerings,such as atomic-scale,nano-scale,meso-scale,and multi-scale phonon scatterings have been applied to reduce the thermal conductivity.Besides,some successful synergistic effects based on band engineerings and phonon scatterings are illustrated as a simultaneous way to optimize both the power factor and thermal conductivity.Summarizing the above three main parts,we point out that the synergistic effects should be effectively exploited,and these may further boost the thermoelectric performance of Pb Te alloys and can be extended to other thermoelectric materials.     

Anisotropic lattice distortions in random alloys from first-principles theory

Within the framework of the exact muffin-tin orbitals (EMTO) theory we have developed a new method to calculate the total energy for random substitutional alloys. The problem of disorder is treated within the coherent potential approximation (CPA), and the total energy is obtained using the full charge density (FCD) technique. The FCD-EMTO-CPA method is suitable for determination of energy changes due to anisotropic lattice distortions in random alloys. In particular, we calculate the elastic constants of the Cu-rich face centered cubic Cu-Zn alloys ( alpha-brass) and optimize the c/a ratio for the hexagonal Zn-rich alloys for both the epsilon and eta phases.     

Catalytic applications of amorphous alloys: Expectations, achievements, and disappointments

This review intends to summarize the major achievements in the application of amorphous alloys as precursors of catalyst materials. This non-traditional catalyst preparation method may provide supported catalysts with novel chemical and structural properties. Selected examples for both glassy alloy precursors and those fabricated by mechanochemistry include CO oxidation over binary and ternary alloys, dehydrogenation over Cu-M (M = Ti, Zr or Hf), one-step synthesis of methyl isobutyl ketone, and selective hydrogenation of unsaturated carbonyl compounds. Ni alloys for methanation developed for the project to solve global warming by recycling carbon dioxide are also discussed.     

Significant correlation between macroscopic and microscopic parameters for the description of localized plastic flow auto-waves in deforming alloys

Understanding of mechanical properties of materials and a possibility to predicting them from ab initio calculations have fundamental importance for solid state theory. In this work we establish a significant correlation between the product of the macroscopic parameters of localized plastic flow auto-waves in deforming alloys, their length and propagation rate and the product of the microscopic (lattice) parameters of these materials, the spacing between close-packed planes of the lattice and the rate of transverse elastic waves. Thus, these products can be regard as invariants of plastic and elastic deformation processes, respectively. Moreover, the established regularity suggests that the elastic and the plastic processes simultaneously involved in the deformation are closely related. Our work also demonstrates that ab initio simulations can be used for the prediction of parameters of localized plastic flow auto-waves in deforming alloys.     

Strain rate sensitivity of ultra-fine and nanocrystaline metals and alloys

The mechanical behavior of metals and alloys is strongly related to grain size. In particular, the grain refining leads to the increase in yield strength in the ultra-fine grain (<1 μm) and nanocrystalline (<100 nm) regimes.Instrumented nanoindentation measurements allow a rapid evaluation of mechanical properties of materials, and the possibility to perform tests in a very wide range of loads. The strain rate sensitivity of ultra-fine and nanocrystalline metals can be derived by changing loading rates. The present paper presents the results on the strain rate sensitivity of ultra-fine grain metals produced by equa-channel angular pressing and nanocrystalline materials produced via electrodeposition. The results were obtained by systematic experiments performed at different loading rates (3, 30 and 300 mN/s) showing broad ranges of variations for the investigated metals. Also, the strain rate sensitivity of the studied materials was derived from the load vs. depth curves.     

Charge transfer and magnetic properties in amorphous alloys of Fe with Y,Th or Zr

Various amorphous alloys of Fe with Y, Th or Zr were prepared either by melt spinning or by vapour deposition. Magnetic and electrical-transport properties were studied. The alloys are either Pauli paramagnetic or show a random antiferromagnetic behaviour. In several of them we observed a negative temperature coefficient of the electrical resistivity. The ⁵⁷Fe Mössbauer effect was utilized to study the nature of charge transfer in these alloys. Indications were obtained that charge transfer in the amorphous as well as in the crystalline materials comprises s and d electrons in amounts of comparable magnitude.     

Theory of the coercive field in amorphous ferromagnetic alloys

The coercive field of amorphous ferromagnetic alloys is determined by defect structures, surface irregularities, relaxation phenomena and intrinsic fluctuations of the material properties. Within the framework of micromagnetism these different contributions are determined quantitatively. It is shown that in magnetostrictive alloys defect structures are dominant whereas in non-magnetostrictive materials surface irregularities and relaxation effects become important.     

Some applications of NMR to the study of magnetically-ordered materials with emphasis on the short-range order in (Fe-B)-based crystalline and amorphous alloys

In this review, we summatize recent developments in nuclear magnetic resonance (NMR) studies on (Fe-B)-based crystalline and amorphous alloys, focusing on the application of NMR in identifying the existence of short-range order (SRO), determining the types of SRO, characterizing the behavior of the SRO and exploring the effect of the SRO on the magnetic properties for the Fe-B system. NMR experiments reveal that certain local environments surrounding the B atoms exist in both crystalline and amorphous Fe-B alloys. The type of SRO existing in this rapidly quenched system can be either o-Fe₃B or bct-Fe₃B, or a mixture, depending on the composition and processing factors, especially the carbon content and quenching speed. The SRO originates from a strong covalent bonding between the B and Fe atoms. As this interaction plays the same role in both crystalline and amorphous Fe-B alloys, the SRO which occurs in the amorphous Fe-B alloys is similar to the SRO which exists in their crystalline counterparts. NMR, in combination with magnetization measurements, provides evidence indicating that the SRO existing in the amorphous Fe-B alloys has a significant effect on their soft magnetic properties and that different types of SRO may act differently, thus providing an opportunity to improve the magnetic properties by changing the SRO. In connection with reviewing the achievements of NMR studies in recent years, brief comments concerning the advantages and potential of NMR experiments in the investigation of other magnetically-ordered materials will also be presented.