Galvanomagnetic size studies of metallic surface processes

This review gives an account of basic ideas concerning scattering of current carriers at metallic surfaces. It deals with the physical picture and theory of surface-sensitive galvanomagnetic size effects, i.e., transverse magnetic resistance, static skin effect, Sondheimer oscillations, and conduction electron focusing. These processes are employed to study the peculiarities of current carrier surface scattering with regard to the electron–hole transfers. Diffraction of conduction electrons at adsorbed submonolayer lattices is considered. An outlook is given of the application of these phenomena to the study of adsorption and ordering of adsorbed submonolayer films of various symmetries and chemical content, involving, in particular, precursor states. Influence of the diffusion of adsorbed atoms into the substrate on the kinetic size effects is considered in detail.     

Discrete resistance fluctuations in pressure-type point contacts

In high ohmic pressure-type metallic point contacts (resistance range 50 Ω to 3 k Ω) the point-contact resistance is observed to switch randomly between two or more discrete levels. This effect can be explained by the motion or reorientation of single defects, thereby changing their cross section for electron scattering. From the temperature- and voltage-dependence of the characteristic times of the fluctuations, electromigration parameters for a defect in silver are extracted.     

优化Rpa电压-电感耦合等离子体质谱法测定铁皮石斛中不同部位的14种金属元素

建立了优化Rpa电压-电感耦合等离子体质谱法测定铁皮石斛中不同部位的Mg、Al、K、Ca、Cr、Mn、Fe、Cu、Zn、Ni、As、Se、Cd和Pb 14种金属元素的方法。样品经微波消解后,采用动态反应池去除样品基体带来的质谱干扰,同时针对部分高含量元素的信号溢出问题,实验优化了Rpa电压,免去了样品的二次稀释。各元素的检出限为0.02~32μg/L,日内精密度(RSD,n=6)和日间精密度(RSD,n=12)分别在0.7%~4.6%和1.5%~5.8%之间;样品的加标回收率为83.9%~112%。采用该方法对柑橘叶标准样品(GBW10020)进行了验证,测定值均在参考值范围内。方法对大棚种植的铁皮石斛的不同部位样品进行了测定,发现重金属含量处于较低水平,其中叶和花的重金属含量要显著高于茎中的含量。     

Sensitive determination of 17β-estradiol in river water using a graphene based electrochemical sensor

In this study, a novel material for the electrochemical determination of 17β-estradiol using an electrode based on reduced graphene oxide and a metal complex porphyrin has been applied to environmental monitoring. The electrochemical profile of the proposed electrode was analyzed by differential pulse voltammetry, which showed a shift of the oxidation peak potential of 17β-estradiol to 150 mV in a less positive direction compared to the bare reduced graphene oxide electrode. DPV experiments were performed in PBS at pH 7.0 to determine 17β-estradiol without any previous step of extraction, cleanup, or derivatization, in the range of 0.1–1.0 μmol L⁻¹ with a detection limit archived at 5.3 nmol L⁻¹ (1.4 μg L⁻¹). The proposed sensor was successfully applied in the determination of 17β-estradiol in a river water sample without any purification step and was successfully analyzed under the standard addition method. All the obtained results were in agreement with those from the HPLC procedure.     

The influence of electron confinement,quantum size effects,and film morphology on the dispersion and the damping of plasmonic modes in Ag and Au thin films

Plasmons are collective longitudinal modes of charge fluctuation in metal samples excited by an external electric field. Surface plasmons (SPs) are waves that propagate along the surface of a conductor. SPs find applications in magneto-optic data storage, optics, microscopy, and catalysis.The investigation of SPs in silver and gold is relevant as these materials are extensively used in plasmonics. The theoretical approach for calculating plasmon modes in noble metals is complicated by the existence of localized d electrons near the Fermi level. Nevertheless, recent calculations based on linear response theory and time-dependent local density approximation adequately describe the dispersion and damping of SPs in noble metals.Furthermore, in thin films the electronic response is influenced by electron quantum confinement. Confined electrons modify the dynamical screening processes at the film/substrate interface by introducing novel properties with potential applications. The presence of quantum well states in the Ag and Au overlayer affects both the dispersion relation of SP frequency and the damping processes of the SP.Recent calculations indicate the emergence of acoustic surface plasmons (ASP) in Ag thin films exhibiting quantum well states. The slope of the dispersion of ASP decreases with film thickness.High-resolution electron energy loss spectroscopy (HREELS) is the main experimental technique for investigating collective electronic excitations, with adequate resolution in both the energy and momentum domains to investigate surface modes.Herein we review on recent progress of research on collective electronic excitations in Ag and Au films deposited on single-crystal substrates.     

Atomistic simulation of the structure and elastic properties of gold nanowires

We performed atomistic simulations to study the effect of free surfaces on the structure and elastic properties of gold nanowires aligned in the 〈100〉 and 〈111〉 crystallographic directions. Computationally, we formed a nanowire by assembling gold atoms into a long wire with free sides by putting them in their bulk fcc lattice positions. We then performed a static relaxation on the assemblage. The tensile surface stresses on the sides of the wire cause the wire to contract along the length with respect to the original fcc lattice, and we characterize this deformation in terms of an equilibrium strain versus the cross-sectional area. While the surface stress causes wires of both orientations and all sizes to increasingly contract with decreasing cross-sectional area, when the cross-sectional area of a 〈100〉 nanowire is less than , the wire undergoes a phase transformation from fcc to bct, and the equilibrium strain increases by an order of magnitude. We then applied a uniform uniaxial strain incrementally to 1.2% to the relaxed nanowires in a molecular statics framework. From the simulation results we computed the effective axial Young's modulus and Poisson's ratios of the nanowire as a function of cross-sectional area. We used two approaches to compute the effective elastic moduli, one based on a definition in terms of the strain derivative of the total energy and another in terms of the virial stress often used in atomistic simulations. Both give quantitatively similar results, showing an increase in Young's modulus with a decrease of cross-sectional area in the nanowires that do not undergo a phase transformation. Those that undergo a phase transformation experience an increase of about a factor of three of Young's modulus. The Poisson's ratio of the 〈100〉 wires that do not undergo a phase transformation show little change with the cross-sectional area. Those wires that undergo a phase transformation experience an increase of about 10% in Poisson's ratio. The 〈111〉 wires show, with a decrease of cross-sectional area, an increase in one of Poisson's ratios and small change in the other.     

Magnetocaloric and magnetoresistance properties in Co-based (Co0.402Fe0.201Ni0.067B0.227Si0.053Nb0.05)100−xCux (x=0–1) glassy alloys

The magnetocaloric and magnetoresistance properties of amorphous Co-based (Co_0.402Fe_0.201Ni_0.067B_0.227Si_0.053Nb_0.05)_100?xCu_x (x = 0, 0.5, 0.75 and 1) ribbons were investigated. Cu additions changed the crystallisation temperature (T_x) and the Curie temperature (T_C). The saturation magnetisation (M_s) and coercivity (H_c) for alloys were in the range of 65.51–38.49 emu/g and 1.99–6.84 A/m, respectively. Under an applied magnetic field change of 2.2 T, the (?ΔS_M)^max for (Co_0.402Fe_0.201Ni_0.067B_0.227Si_0.053Nb_0.05)_100?xCu_x with x = 0, 0.5, 0.75 and 1 are 0.77, 0.71, 0.89 and 0.67 Jkg^?1 K^?1, respectively. The values of refrigeration capacity (RC) for the as-spun glassy alloys are comparable with those of previously studied Fe-based metallic glasses such as Fe₈₀B₁₀Zr₉Cu₁, (Fe_0.76B_0.24)₉₆Nb₄ and Fe₈₂Ni₂Zr₆B₁₀. In addition, the maximum magnetoresistance (MR) values for (Co_0.402Fe_0.201Ni_0.067B_0.227Si_0.053Nb_0.05)_100?xCu_x with x = 0, 0.5, 0.75 and 1 are found to be 110, 38, 23 and 1% around the Curie temperatures under an applied magnetic field change of 1 T, respectively. With good RC, negligible hysteresis due to very low coercivity values and large magnetoresistance, these Co-based amorphous alloys can be used as the high temperature magnetic refrigerants and multifunctional applications working in the temperature range of 450–600 K.     

Bounds and estimates for the effect of strain gradients upon the effective plastic properties of an isotropic two-phase composite

Predictions are made for the size effect on strength of a random, isotropic two-phase composite. Each phase is treated as an isotropic, elastic-plastic solid, with a response described by a modified deformation theory version of the Fleck-Hutchinson strain gradient plasticity formulation (Fleck and Hutchinson, J. Mech. Phys. Solids 49 (2001) 2245). The essential feature of the new theory is that the plastic strain tensor is treated as a primary unknown on the same footing as the displacement. Minimum principles for the energy and for the complementary energy are stated for a composite, and these lead directly to elementary bounds analogous to those of Reuss and Voigt. For the case of a linear hardening solid, Hashin-Shtrikman bounds and self-consistent estimates are derived. A non-linear variational principle is constructed by generalising that of Ponte Castañeda (J. Mech. Phys. Solids 40 (1992) 1757). The minimum principle is used to derive an upper bound, a lower estimate and a self-consistent estimate for the overall plastic response of a statistically homogeneous and isotropic strain gradient composite. Sample numerical calculations are performed to explore the dependence of the macroscopic uniaxial response upon the size scale of the microstructure, and upon the relative volume fraction of the two phases.     

Thermal stability of magnesium-rich primers based on glycidyl carbamate resins

Coatings of outstanding thermal stability were obtained by the combination of two novel technologies, that of a magnesium-rich primer and a silane-modified glycidyl carbamate binder. While conducting a study to evaluate the new binder system with respect to properties of the magnesium-rich primer, during thermogravimetric analysis of samples, previously unobserved and unexpected properties were noted. The samples transformed into an intact solid residue, with the amount of the residual char ranging between 40 and 90% weight depending on the pigment volume concentration (PVC) of the magnesium particles in the composition. It appears that the hitherto unobserved property is essentially a function of the metallic pigment particles in the coating. The discovery of the exceptional thermal stability potentially increases the range of application for these primers and these can be further developed for use as a thermal barrier coating.     

Observation of new two-dimensional periodic structures in the confined regions of the reconstructed Pt(100) surface

New 2D periodic structures have been observed by STM in the regions of the reconstructed Pt(100) surface which are confined by domain boundaries or lattice steps. These structures can be seen only in a very narrow energy window near the Fermi level, and they are strongly correlated to the original atomic arrangements of the surface. These structures arise most probably from a modification in the distribution of the electronic density of states which is strongly coupled to the ion cores of the surface to produce a periodic shift in the atomic rearrangements in order to minimize the strain and free energy of the surface.