Development and properties of nanostructured thermal spray coatings

Nanostructured thermal spray coatings have been intensively studied because of their potential in a wide variety of industrial applications. In the present paper, current development status of nanostructured thermal spray coatings is presented, mainly based on the results of the authors. In the nanostructured WC–Co wear-resistant coatings, the influence of feedstock characteristics on the coating properties was discussed to suggest the desirable morphology of feedstock for thermal spraying. For the nanostructured Cr₂O₃ based solid-lubricant coatings, the advanced feedstock has been developed in order to solve the inhomogeneity problem of the conventional coatings. Various properties of the nanostructured coatings were evaluated and compared with those of the conventional counterparts. These results clearly demonstrate that the significant improvement in coating performance can be achieved by utilizing proper nanostructured coatings.     

Eye tissue structure and refraction alterations upon nondestructive laser action

A new approach to alterations in eye refraction upon nondestructive laser action on the sclera and cornea is studied. It is demonstrated in in vivo experiments on rabbit eyes that sequential laser irradiation of the sclera and cornea yields a significant alteration in the eye refraction. The collagen structure of the sclera and cornea is studied after the nondestructive laser action with noninvasive polarization-sensitive optical coherence tomography. It is demonstrated that collagen fibers that provide for the cornea tension and applanation partially survive in the zone of the local denaturation of sclera. An irradiation mode that corresponds to an increase in the cornea’s plasticity and does not cause visible structural changes is chosen. The simplest theoretical model for alterations in the eye refraction upon the nonablative laser action on sclera is analyzed. The alteration in the cornea curvature upon stretching resulting from the local sclera coagulation and the corresponding decrease in its volume is calculated. The model makes it possible to approximately estimate the laser irradiation modes that provide the desired alterations in eye refraction.     

Density of phonon states in nanostructured copper

Neutron spectroscopy analysis has been used to study the vibrational states in samples of coarse-grained and nanostructured copper. It has been revealed that the phonon spectrum for the nanostructured copper is characterized by a decrease in the local densities of states corresponding to the longitudinal (∼28 meV) and transverse (∼19 meV) vibrational modes. The total contribution to the molar heat capacity of the oscillators having the dispersion laws of coarse-grained copper decreases by about 18%. The experimentally observed increase in the energy (heat capacity) of the crystal in the nanostructured state is attributed to the spectrum transformation in the form of the transfer of the energy from the high-frequency phonons to the long-wavelength region, where the new acoustic branches of oscillations along the grain boundaries (the “second” ion sound effect) are developed. This means that the features of the scattering spectra of the slow neutrons in the samples are determined by the presence of two types of the scatterers.     

Depth-resolved monitoring of glucose diffusion in tissues by using optical coherence tomography

We demonstrate the capability of the optical coherence tomography (OCT) technique for depth-resolved monitoring and quantifying of glucose diffusion in fibrous tissues (sclera). The depth-resolved and average permeability coefficients of glucose were calculated. We found that the glucose diffusion rate is not uniform throughout the tissue and is increased from approximately 2.39+/-0.73 x 10(-6) cm/s at the epithelial side to 8.63+/-0.27 x 10(-6) cm/s close to the endothelial side of the sclera. Results demonstrated that the OCT technique is capable of depth-resolved monitoring and quantification of glucose diffusion in sclera with a resolution of approximately 40 mum.     

Simulation of transport and relaxation of hot electrons in the near-surface layers of nanostructured and crystalline silicon dioxides

Computer simulation has been performed to investigate the transport and energy relaxation of photoelectrons in the near-surface layers of nanostructured and crystalline silicon dioxides in the presence and absence of an electric field. Calculations have shown that nanostructured samples have a shorter hot-electron thermalization time and exhibit weaker influence of an electric field on the electron energy relaxation process than the bulk crystal. The size effect calculated in terms of electron thermalization time is most pronounced at particle sizes less than 5 nm.     

Homogenization of nanostructured media in magnetic field

Problem of homogenization of nanostructured media in magnetic field has been considered. Possibility of introduction of effective material parameters dielectric permittivity and magnetic permeability for three classes of media such as magnetic metal nanostructures, film metal–dielectric composite media and 3D-nanocomposites on the base of opal matrices has been investigated. It has been stated that the introduction of effective parameters far from magnetic resonance conditions is possible at millimeter waveband frequencies. Strict introduction of effective magnetic permeability of nanostructured media near magnetic resonance is not possible.     

铝纳米晶的低温导电特性研究

采用真空热压技术将电磁感应加热-自悬浮定向流法制备的铝纳米粉末压制成块体样品.通过X射线衍射、透射电子显微镜、扫描电子显微镜及X射线能谱分析了铝纳米晶的微观结构,并用四探针法测量了不同温度下(8—300 K)样品的电阻率,研究了铝纳米晶的电阻率(ρ)随温度的变化规律.结果表明:由于晶界(非晶氧化铝)对电子的散射以及晶界声子对电子的散射效应,低温(40 K)下,铝纳米晶的本征电阻率随温度变化关系明显不同于粗晶铝,不仅呈现出T~4变化,还表现出显著的T3变化规律.因晶界等缺陷和非晶氧化铝杂质对电子的散射,铝纳米晶残余电阻率比粗晶铝电阻率大5—6个数量级.     

Optical spectra of nanoceramics of yttrium-iron garnet Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> obtained by the method of intense plastic deformation

The features of the optical properties of nanostructured samples of iron-yttrium garnet Y₃Fe₅O₁₂ (YIG) in the range of 0.5 to 4.7 eV, which includes both the fundamental-absorption region and low-energy electron excitations, have been investigated by spectroscopic ellipsometry. The results are discussed in comparison with the measurement data of a YIG single crystal. The dispersion of the optical functions in nanostructured samples is significantly different from that for the single crystal: the spectral density is redistributed from the energy region above the fundamental absorption edge to the region below the edge. It has been shown that the energy positions of the main electronic transitions in nanostructured samples are on the whole the same as in the single crystal; at the same time, the intensity of low-energy transitions increases. The possible causes of this increase and the resolution of the fine absorption structure in the bandgap of nanostructured Y₃Fe₅O₁₂ are discussed.     

Effect of tunneling on the thermoelectric efficiency of bulk nanostructured materials

The possibility of increasing the thermoelectric figure of merit for bulk nanostructured materials has been investigated theoretically. The kinetic coefficients of the nanostructured material have been calculated and evaluated under the assumption that the dominant role in the transfer is played by the tunneling of electrons between nanoparticles. The limiting case of the absence of phonon thermal conductivity through barrier layers has been considered. It has been demonstrated using the estimates obtained for materials based on Bi₂Te₃ that the thermopower in the nanostructured material can be sufficiently high and that, despite the low electrical conductivity, the dimensionless thermoelectric figure of merit can be as large as 3–4 at room temperature.     

Optical and magneto-optical resonances in nanocorrugated ferromagnetic films

We report here on optical and magneto-optical spectra investigations of nanostructured Co and Ni films deposited on top of polymethyl methacrylate (PMMA) colloidal crystal. Resonant peculiarities have been observed in both spectra; their positions are scaled with the PMMA sphere diameter and depend on incident angle. Asymmetry of the resonance lineshapes as well as change of the direction of magneto-optical rotation has been observed. Both the surface plasmon resonance and the interference between reflections from the colloidal crystal and from the nanostructured film should be considered to explain the obtained results.     

SIZE AND GRAIN-DISTRIBUTION EFFECTS IN ELECTRONIC PROPERTIES OF TWO-DIMENSIONAL NANOCRYSTALLINE RECTANGLE SYSTEMS

According to the structural characters of real nanostructured systems, a model for two-dimensional nanostructured systems has been proposed. By developing a renormalization-group Green's function scheme, local electronic density of states at a type of crystallire sites is presented on the pure hopping model. It is found that the changes of all the grain-distribution rules, the average atomic number and the atomic interaction are connected with the spectral structures of nanostructured systems. However, bule shifts of the spectra of nanostruetured systems seem to be modulated mainly by the changes of the bond-parameters which are correlated with the lattice distortions of the nano-sized grains.     

Preparation and Dielectric Properties of Nanostructured ZnO Whiskers

By a novel controlled combustion synthesis method, a large number of nanostructured ZnO whiskers with different morphologies, such as tetra-needles, long-leg tetra-needles and multi-needles, are prepared without any additive in open air at high temperature. The morphologies and crystalline structures of the as-prepared ZnO nanostructured whiskers are investigated by SEM and XRD. The possible growth mechanism on the nanostructured ZnO whiskers is proposed. The experimental results indicate that the dielectric constants and losses of the nanostructured ZnO whiskers are very low, demonstrating that the nanostructured ZnO whiskers are low-loss materials for microwave absorption in X-band. However, obvious microwave absorption in nanostructured ZnO whiskers is observed. The quasi-microantenna model may be attributed to the microwave absorption of the ZnO whiskers.     

Controlling optical properties of biological tissues: II. Coherent optical methods for studying the tissue structure

Correlation and polarization properties of dynamic speckle fields formed upon scanning in vitro human sclera samples subjected to immersion agents with a focused laser beam were studied. The immersion agent diffusion into the tissue reduces the relative refractive index of structure-forming elements (collagen fibrils) with respect to the base sclera substance and thus stimulates the passage from multiple to single scattering of a probing beam. Experimental data are presented on the evolution of the autocorrelation function of intensity fluctuations and the intensity ratio between polarized and depolarized components of laser radiation passed through a sclera layer due to artificial clarification of the tissue samples analyzed. Possible applications of the developed method to the tissue structure diagnostics are discussed.     

Charge states of cobalt ions in nanostructured lithium cobaltite: X-ray absorption and photoelectron spectra

The charge states of ions in nanostructured lithium cobaltite prepared by severe plastic deformation under pressure have been determined using X-ray absorption spectroscopy and photoelectron spectroscopy, as well as calculations of the atomic multiplets with allowance for the charge transfer. It has been found that small deformations (pressures up to 5 GPa and angles of anvil rotation up to 30°) lead to the generation of lithium vacancies in the bulk of the nanostructured material and the formation of the Li₂O phase on the surface. The charge compensation occurs at the expense of holes in oxygen 2p states; the electronic configuration of cobalt ions is 3d ⁶ L, where L is a hole in oxygen 2p states. It has been shown that nanostructured lithium cobaltite belongs to the class of insulators with a negative charge transfer energy. An increase in the degree of deformation of lithium cobaltite (at a pressure up to 8 GPa) leads to the formation of Co²⁺ ions (with the electronic configuration 3d ⁷).     

Thermally stable coherent domain boundaries in complex-structured Cr2Nb intermetallics

The strength and hardness of nanostructured materials are significantly enhanced owing to the large amount of grain boundaries (GB) produced by a reduced grain size. The thermal stability of the GB is a key to maintaining the grain size and thus the strength/hardness in nanostructured materials at high temperatures. In this work, coherent domain boundaries (DB) were introduced by compressive processing to sub-divide a complex-structured intermetallic Cr₂Nb into nanograins of size down to 2 nm. These DB persisted after an annealing of 10 h at 1273 K. The coherent DB have been investigated by aberration-corrected high-resolution transmission electron microscopy and first-principles calculations. The high thermal stability is evidently a result of low formation energies of the DB.     

Luminescence,vibrational and XANES studies of AlN nanomaterials

The paper reports comparative studies on synthesized aluminium nitride nanotubes, nanoparticles and commercially available micron-sized AlN powder using different spectroscopic techniques: cathodoluminescence measurements (CL), X-ray absorption near edge spectroscopy (XANES) and Fourier-transform infrared spectroscopy (FTIR). Crucial distinctions in CL spectra are observed for nano- and microsized aluminium nitride powders; systematic shift of the IR absorption maximum has been detected for nanostructured aluminium nitride as compared to commercial samples. Through XANES experiments on Al K-edge structural differences between nano- and bulk AlN are revealed, intensity of features in absorption spectra has been found to be a function of wurtzite and zincblend phases amount in nanostructured samples.     

Formation of the structure and physicomechanical properties of a quasicrystalline Al–Cu–Fe alloy upon plasma spraying

Quasicrystalline coatings prepared under various thermal conditions of spraying have been studied. Initial quasicrystalline powders with dispersion of 10–50 μm were prepared in a low-pressure arc discharge plasma. The coatings have been sprayed on copper rings using a swinging plasmatron. It is found that the increase in the quenching rate of melt droplets increases the chemical homogeneity and leads to formation of nanostructured formations. The precipitation of nanostructured grains (d < 100 nm) in the sprayed alloy leads to an increase in the mechanical characteristics (hardness, deformation, and ductility) and can be considered as an additional factor of hardening of the material.     

Detection of internal cracks and ultrasound characterization of nanostructured Bi₂Te₃-based thermoelectrics via acoustic microscopy

The search for thermoelectric (TE) materials for highly efficient devices aims at improving the TE efficiency and broadening their areas of applications. We created nanostructured thermoelectric Bi-Sb-Te-family materials by high energy (ball milling) pre-treatment of the parent materials followed by high-pressure/high-temperature treatment. Bi_0.5Sb_1.5Te₃ compositions with the superfluous maintenance of tellurium was used for the synthesis of the samples with p-type electrical conductivity. Acoustic microscopy was used to study elastic properties and bulk irregularities and to detection of internal cracks both in the parent materials and in the created nanostructured samples. The data has been used for optimization of parameters of synthesis of nanostructured thermoelectrics.