Ferromagnetic resonance in nanomagnetic metal core and noble metal shell systems

The change in the line widths in the ferromagnetic resonance (FMR) spectra of Co and Ni nanoparticles upon shell formation with noble metals like gold or silver are described. The Ni(core)Ag(shell), Co(core)Ag(shell), and CO(core)Au(shell) nanoparticles were prepared by a simple transmetallation reaction between the Co and Ni nanoparticles and the Ag+ or AuCl4- ions. It is revealed that the FMR line width decreases upon Ag shell formation whereas it increases upon core-shell composite formation with Au. Several probable explanations such as the differences in size distributions before and after the reaction or the changes occurring in shape anisotropy of the particles due to the shell formation or the different extents of electronic interaction between the core and shell materials have been offered for this observation.     

Spontaneous phase and morphology transformations of anodized titania nanotubes induced by water at room temperature

We report a spontaneous phase transformation of titania nanotubes induced by water at room temperature, which enables the as-anodized amorphous nanotubes to be crystallized into anatase mesoporous nanowires without any other post-treatments. These mesoporous TiO(2) nanomaterials have a markedly improved surface area, about 5.5 times than that of the as-anodized TiO(2) nanotubes, resulting in a pronounced enhanced photocatalytic activity. The present approach not only allows a flexible control over the morphology of TiO(2) nanostructures but can fundamentally eliminate the need for high temperature operations for crystallizing amorphous TiO(2).     

Cruciform fillet welded joint fatigue strength improvements by weld metal phase transformations

Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation‐Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non‐load‐carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.     

Compatibility and accommodation in displacive phase transformations

The role of disconnections and lattice-invariant deformation in displacive phase transformations is reviewed, particularly the defect structure of equilibrium habit planes, the mechanism of transformation, and the deformation accompanying growth. This is extended to the 3-D topological modeling of a product embedded in its parent phase. Stress concentrations arise where the interface orientation deviates from the equilibrium habit, such as plate tips and edges. Various mechanisms are discussed for the amelioration and accommodation of these stresses. These include the well established mechanism of self-accommodating assemblies of variants. New mechanisms are proposed relating to the defect structure within individual plates and its interaction with crystal dislocations. Some supporting experimental observations are presented.     

Microstructure and phase transformations in duplex 316 manual metal arc weld metals: the role of carbon

The nature and kinetics of the phase transformation of metastable -ferrite in two duplex stainless steel manual metal arc weld metals has been studied at two carbon levels. 0.042 and 0.076 w t%. The lean deposit exhibited a rapid transformation to intermetallic phases on post-weld heat treatment, whilst the higher carbon deposit retained sable -ferrite after similar heat treatment. These results appear to be consistent with previously published transformation models for similar materials.     

Near-field photochemical imaging of noble metal nanostructures

The sub-diffraction imaging of the optical near-field in nanostructures, based on a photochemical technique, is reported. A photosensitive azobenzene-dye polymer is spin coated onto lithographic structures and is subsequently irradiated with laser light. Photoinduced mass transport creates topographic modifications at the polymer film surface that are then measured with atomic force microscopy (AFM). The AFM images correlate with rigorous theoretical calculations of the near-field intensities for a range of different nanostructures and illumination polarizations. This approach is a first step toward additional methods for resolving confined optical near fields, which can augment scanning probe methodologies for high spatial resolution of optical near fields.     

Low-temperature hydrothermal reduction of metal hydroxides to metal powders

Hydroxides of copper, cobalt, nickel, bismuth, silver, palladium and osmium could be reduced to the metal powder below 250°C under hydrothermal conditions in formic acid medium. Particles in the resulting metal powder have single crystal morphology. Such powders show faster shrinkage kinetics. Powders of zirconia dispersed cobalt and nickel could be directly obtained by this method.     

Electrochemical reduction of noble metal compounds in ethylene glycol

The effect of temperature on both the electrochemical oxidation of pure ethylene glycol and the reduction of AuCl₄⁻ in ethylene glycol at a rotating disk glassy carbon electrode has been investigated using linear sweep voltammetry. As the temperature is increased from 25°C up to 60°C, ethylene glycol begins to oxidize at lower potentials, whereas the reduction potential of AuCl₄⁻ is independent of temperature. Reduction current densities, however, increase as temperature increases. Room temperature reduction of several noble metal species in ethylene glycol was also investigated. Metal reduction potentials at both a platinum and a glassy carbon electrodes follow the sequence: AuC1₄⁻>Ag⁺>PtC1₆²⁻>Pd(NH₃)₄²⁺. The oxidation potential of ethylene glycol at both electrodes was found to be more positive than the reduction potential of the gold, silver, platinum and palladium precursors. These results predict that the spontaneous formation of noble metal particles by chemical reduction with ethylene glycol is thermodynamically unfavorable at 25°C. Gold and silver particles, however, are easily prepared at room temperature using the polyol process, which is a redox based process for the preparation of finely divided metals by chemical reduction of the corresponding metal precursors with ethylene glycol. Since measured potentials are the sum of a thermodynamic and a kinetic contribution (the overpotential), metal reduction in the polyol process seems to be aided by the overpotential. Therefore, measured potentials have been correlated to the chemical conditions at which noble metal particles are synthesized in the polyol process. It was found that as the potential difference between ethylene glycol oxidation and metal reduction increases, both the reaction temperature and time needed for metal synthesis increases. These electrochemical results may contribute to have a better understanding of the fundamentals of the polyol process, and for optimizing such reaction parameters as temperature, time and solution chemistry.     

Crystal phase transformations in nylon 11

Three sample preparations of nylon 11, m-cresol cast, trifluoroacetic acid cast and sodium hydroxide-treated melt-crystallized, are examined by infrared analysis. Digital subtraction spectroscopy is used to distinguish between structural differences in different chemical treatments. M-cresol cast films of nylon 11 appear to be intermediate in conformational structure between melt-crystallized and trifluoroacetic acid cast films. Various bands of the infrared spectrum support this result. The basis for the doublet observed in the amide II band, which results from conformational changes rather than crystal field splitting, is discussed in this paper. Sodium hydroxide treatment promotes the α to γ conversion in melt-crystallized films of nylon 11.     

Synthesis and characterization of a binary noble metal nitride

There has been considerable interest in the synthesis of new nitrides because of their technological and fundamental importance. Although numerous metals react with nitrogen there are no known binary nitrides of the noble metals. We report the discovery and characterization of platinum nitride (PtN), the first binary nitride of the noble metals group. This compound can be formed above 45-50 GPa and temperatures exceeding 2,000 K, and is stable after quenching to room pressure and temperature. It is characterized by a very high Raman-scattering cross-section with easily observed second- and third-order Raman bands. Synchrotron X-ray diffraction shows that the new phase is cubic with a remarkably high bulk modulus of 372(+/-5) GPa.     

Study of interaction of ethylene glycol/PVP phase on noble metal powders prepared by polyol process

Noble metal powders (Au, Ag, Pt, Pd and Ru) have been synthesized by the polyol process in both the nanometer and submicron scales (sans Pd, Pt and Ru). They have been characterized by both microscopic (TEM and SEM) as well as spectroscopic techniques (FT-IR and XPS). Infrared spectroscopy was employed to study the colloid particles in the presence of ethylene glycol and PVP and the results show that the interaction between the organic phase and the metal particles vary according to the particle size. The role of the solvent, ethylene glycol, during the reduction process was also investigated and we observe formation of >C=O vibration band after the reduction process implying that the solvent reduces the metal ions thereby getting oxidized. XPS measurements carried out on the colloidal sols have shown the presence of the organic phase adsorbed onto the metal particles.     

Phase transformations and phase stability in nanocrystalline materials

The current state of the researches on diffusionless phase transformations, including allotropic, polymorphous and martensitic transformations, and phase stability are reviewed. The behaviors of phase transformation and phase stability in nanocrystalline materials are markedly affected by the non-equilibrium conditions involved in their preparation, as a result, in this review an ideal demonstrating method of critical size for the stability of a high-temperature phase at low-temperatures is suggested, and the intrinsic conditions of the phase stability are clarified. Our recent experiments exhibit that the reversal transformation temperatures of low-temperature phases in nanocrystalline Co bulk metal and Fe–30Ni wt% alloy are significantly raised up over 800 °C when their grain sizes are smaller than about 15 nm, while in the reported experiments of nanocrystalline particles or films the reversal transformation temperature lowers with decreasing grain size or is independent of grain size. Therefore, the author suggests that more experiments and theories for phase stability in reversal transformation should be performed. The study of grain growth kinetics of nanocrystalline materials, as a basic of investigating phase stability, is another attention aspect.     

Atomic structure and phase transformations in Pu alloys

Plutonium and plutonium-based alloys containing Al or Ga exhibit numerous phases with crystal structures ranging from simple monoclinic to face-centered cubic. Only recently, however, has there been increased convergence in the actinides community on the details of the equilibrium form of the phase diagrams. Practically speaking, while the phase diagrams that represent the stability of the fcc δ-phase field at room temperature are generally applicable, it is also recognized that Pu and its alloys are never truly in thermodynamic equilibrium because of self-irradiation effects, primarily from the alpha decay of Pu isotopes. This article covers past and current research on several properties of Pu and Pu-(Al or Ga) alloys and their connections to the crystal structure and the microstructure. We review the consequences of radioactive decay, the recent advances in understanding the electronic structure, the current research on phase transformations and their relations to phase diagrams and phase stability, the nature of the isothermal martensitic δ → α′ transformation, and the pressure-induced transformations in the δ-phase alloys. New data are also presented on the structures and phase transformations observed in these materials following the application of pressure, including the formation of transition phases.     

工作用贵金属热电偶校准能力验证

JJG 141——2013《工作用贵金属热电偶检定规程》实施后,为了解我国校准贵金属热电偶的能力,中国合格评定国家认可委员会(CNAS)组织工作用贵金属热电偶校准能力验证,全国28个省、市、自治区的88家实验室参加。该研究主要介绍工作用贵金属热电偶校准能力验证计划的试验背景和实施过程,包括方案设计、样品制备、稳定性试验、结果统计等,并对能力验证结果进行分析。结果显示:校准工作用贵金属结果满意率为98.86%,参加能力验证的绝大多数实验室可以按照修订后的新规程准确校准工作用贵金属热电偶,我国校准工作用贵金属热电偶实验室具有较高的水平。