Surface Review and Letters

Various rare earth oxide nanostructures were synthesized for the first time by dual-double pulsed gas feeding/pulsed laser deposition. The optically transparent and insulating nanostructures do not exhibit the standard columnar configuration of rare earth oxide thin films but rather dense structures and a significant chemical stoichiometry. More precisely, they exhibit single crystallographic low temperature phases with preferential textures, generally similar to that of the bulk used powder targets. For the cubic fluorine type CeO₂ and Ho₂O₃ films, an epitaxial growth is observed with a special feature noticed in the case of the Eu₂O₃ nanostructure. For this latter, localized and very large oriented crystallites embedded in disordered packed pyramidal crystallites are observed.

The electronic and optical properties of Mn-doped 3C-SiC films are investigated by the first-principles calculation. The structure of Mn-doped 3C-SiC is modeled by substituting Mn atom for C or Si atom in 3C-SiC lattice. The results suggest that Mn-C and Mn-Si bonds can exist in the Mn-doped 3C-SiC. Mn location in 3C-SiC lattice significantly affects the crystal structure of Mn-doped 3C-SiC, and the Mn atom substitution for C or Si sites of 3C-SiC lattice can induce to the difference of indirect or direct band structure. The calculated results also show that some new impurity energy levels occur in the band gap of Mn-doped 3C-SiC, and the imaginary part of dielectric function of Mn-doped 3C-SiC shifts toward the infrared region in comparison with the primitive 3C-SiC. The adsorption spectrum of Mn-doping 3C-SiC, due to the transition of electrons between Mn 3d states, presents some new prominent peaks at low frequency. These results can further confirm Mn-doped 3C-SiC to act as a potential material for optical applications.

Li-W co-doped ZnO (LWZO) thin films were deposited on quartz glass substrates by RF magnetron sputtering technology. The properties of LWZO films deposited with varied substrate temperatures were investigated. When the substrate temperature was lower than 120°C — according to X-ray diffraction (XRD) patterns, films keep hexagonal wurtzite structure with the (002) plane as preferred orientation — the optical transmittance was higher than 85%. When the substrate temperature was higher than 120°C, the results of XPS and XRD show that W⁶⁺ will work as donors, and the (101) peak appeared; the optical transmittance decreased slightly but still higher than 82%. Scanning electron microscope (SEM) and its two-dimensional Fourier transform images showed that films had smooth surface and columnar particles structure when the substrate temperature was lower than 120°C. The film surface became rougher and flaky-shaped particles structure could be observed when the substrate temperature was higher than 120°C. In addition, the lowest electrical resistivity of sample was 3.6 × 10^-3 Ω ⋅ cm which was obtained at substrate temperature 240°C.

This paper is based on the dry sliding wear of Stellite SF12-B₄C-TiN-Mo composite coating deposited on a pure Ti using a laser cladding technique, the parameters of which provide almost crack-free composites with low porosity. To the best of our knowledge, it is the first time that Stellite SF12-B₄C-TiN-Mo mixed powders are deposited as the hard composites by a laser cladding technique. Scanning electron microscope images indicate that the nanoscale particles are produced in such coating. The fact that due to the sufficiently rapid heating and cooling rates of the laser cladding technique, the ceramics, such as TiC or TiB₂ did not have enough time to grow up, resulting in the formation of the nanoscale particles. Compared with a pure Ti substrate, the increments of the micro-hardness and wear resistance are obtained for such composite coating.

Periodic modulations that appear in the low-temperature transient reflectivity signal of a GaAsP/AlGaAs single quantum well is studied. Similar anomalous oscillations are also reported in layered manganite [K. Kouyama et al., J. Phys. Soc. Jpn.76 (2007) 123702(1–3).] We show that such periodic modulations are caused by changes in the linear reflectivity of the sample during transient reflectivity measurements. Studies carried out on reflectivity of different materials under identical conditions show that these modulations on the true transient reflectivity signal are caused by condensation of residual gases on the surface of quantum well. Methods to obtain reliable transient reflectivity data are also described.

In order to improve the tribological performance of AISI 316 stainless steel (316 SS) under grease lubrication, electrochemical processing was conducted on it to obtain a rough (surface texturing-like) surface by making use of the high sensitivity of austenitic stainless steel to pitting corrosion in Cl^--rich environment. Numerous corrosion pits or micro-ditches acted as micro-reservoirs on the obtained surface. While the grease could offer consistent lubrication, and then improve the tribological performance of 316 SS. Tribological behaviors of raw 316 SS and the treated sample were measured using a reciprocating type tribometer sliding against GCr15 steel counterpart under dry and grease lubrication conditions. The results showed that the mass losses of the two samples were in the same order of magnitude, and the raw sample exhibited lower friction coefficient in dry sliding. When the tests were conducted under grease lubrication condition, the friction coefficients and mass losses of the treated sample were far lower than those of the raw 316 SS. The tribological performance of 316 SS under grease lubrication was drastically improved after electrochemical processing.

A thin strip of a Zr-based alloy with a composition of Zr₆₀Cu₂₅Fe₅Al₁₀ (in atom percent) was used as a raw material, and the composite coatings containing Zr-based amorphous phase and crystallites on Ti substrate were fabricated by a one-step laser cladding method without protection. The microstructure, phase constitution, microhardness and wear properties of the coatings were investigated. The results indicate that the microstructure of the coatings is strongly dependent on the laser scanning speed under the conditions of the laser power of 1300 W and laser beam diameter of 6 mm, and the composite coating mainly containing amorphous phase with a small amount of the crystallites can be obtained at the laser scanning speed of 10 mm/s. The composite coating exhibits much higher microhardness than the pure Ti substrate, and thus it behaves superior wear resistance in comparison with the substrate.

This work is based on the dry sliding wear of the MgAl₂C₂-TiB₂-FeSi composite coating deposited on a pure Ti using a laser cladding technique. Scanning electron microscope images indicate that the nanocrystals and amorphous phases are produced in such coating. X-ray diffraction result indicated that such coating mainly consists of MgAl₂C₂, Ti-B, Ti-Si, Fe-Al, Ti₃SiC₂, TiC and amorphous phases. The high resolution transmission electron microscope image indicated that the TiB nanorods were produced in the coating, which were surrounded by other fine precipitates, favoring the formation of a fine microstructure. With increase of the laser power from 0.85 kW to 1.00 kW, the micro-hardness decreased from 1350 ~ 1450 HV_0.2 to 1200 ~ 1300 HV_0.2. The wear volume loss of the laser clad coating was 1/7 of pure Ti.

Non-cyanide alkaline bath was used to prepare copper thin films. Influences of various temperatures on deposition rates, surface morphologies and microstructures of films were investigated. Copper thin films prepared from non-cyanide alkaline bath show typical nodular structures. Copper films fabricated at higher temperature possess rough surface due to hydrolysis of complexing agents. According to the XRD patterns, all deposited films were crystalline and showed Cu (111), Cu (200) and Cu (220) peaks. The intensity of peak (200) increases gradually with the rise on bath temperatures. Films with maximum thickness (7.5 μm) could be obtained at the temperature of 40°C. From the cyclic voltammetry curve, it was found that the cathodic polarization decreased slightly with increase of bath temperatures. In addition, when the bath temperature was equal to 50°C, current efficiency could reach to 96.95%.

SF₆–N₂ mixtures are becoming the best alternative for SF₆ in electrical systems. Plasmachemical modeling and the results of a positive surface streamer are reported in this paper. The model exhibits excellent streamer transition from the gas gap to the insulator surface. The surface charge accumulations on the dielectric insulators are compared and discussed. Results showed that the density of negative ions is much larger than that of electrons in the surface streamer channel. The dominant and less pronounced constitute the main negative ions in the electronegative surface streamer because of the strong electron attachments to SF₆ molecules.

Molecular hydrogen adsorption on MOF-210 was evaluated at the density functional theory level. The most stable H₂ adsorption occurs near the acetenyls in the organic linker, but its binding energy (0.113 eV) is not sufficient to satisfy the minimum value (0.24 eV) required for practical applications. Meanwhile, Li cation-decorated MOF-210 has the average hydrogen adsorption energies of 0.28 eV, and its saturated hydrogen storage capacity reaches 5.35 wt.%.

Melt grown ZnO single crystal was irradiated with varying fluence of 2 MeV proton beam in the range of 1 × 10¹⁶H⁺/cm² to 5 × 10¹⁷H⁺/cm². The irradiated spots exhibited varying degree of discoloration due to the irradiation indicating surface and volume defects. The surface roughness of the irradiated spots was measured using atomic force microscopy indicating surface modification of the sample. The extent of the damage on the ZnO single crystal from radiation was assessed using Raman spectroscopy. The emergence of the A₁(LO) band at 579 cm^-1 pointed to the significant surface alteration in the ZnO crystal due to proton bombardment.

The nano-crystalline iron nitride films with a mixture of γ-Fe₄N, εFe₃N and αFe₂N phases were synthesized on copper substrate by sol–gel technology. The structure, morphology and magnetic properties of the samples were characterized using X-ray diffraction, scanning electron microscopy, and vibrating sample magnetometer. The films are ferromagnetic at room temperature. Magnetic properties such as coercive forces and saturation magnetization were found to be 398 Oestered and 32.92 emu/cm³, respectively.

In this paper, effect of heat treatment on the microstructures and wear properties of laser alloying (LA) composites is investigated. LA of the T-Co50/FeSi/TiC/TiN/CeO₂ mixed powders on substrate of 45 steel can form the hard composites, which increased the wear resistance of substrate greatly. Such LA composites were investigated by means of a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The tempering promoted the growth of the block-shape hard phases, favoring an enhancement of the integrity of block-shape hard phases; and tempering also improved greatly the formation mechanism, guarantying the composites to have enough ability of intensity transfer. This research provided essential experiment and theoretical basis to promote the application of the laser and heat treatment technologies in the field of surface modification.

An accurate calculation of the fractal dimension of a fracture surface is of prime significance for the quantitative evaluation of mechanical behavior. A laser profilometer is employed to measure a surface roughness of a large sandstone sample. Based on the definition of Minkowski dimension, a new method, referred to as the volume covering method, is proposed to estimate fractal dimension by accurately calculating the number of virtual cell needed to cover prism-like volume. It is indicated that the method can fully use laser scanning-based elevation data of the surface and make an accurate estimation of fractal dimension of a fracture surface. Furthermore, another method related to the opposite corner is adopted to estimate the same rough surface with little error of the fractal dimension. A comparison analysis indicates that the volume covering method has the advantage of high accuracy and good applicability for a real fracture surface.

In order to improve the bonding strength of hydroxyapatite coating on carbon/carbon composites, a surface hydrophilic modification was performed on carbon/carbon composites using a combination of H₂O₂ and FeSO₄ ⋅ 7H₂O under ultraviolet irradiation. The hydroxyapatite coating was prepared by an ultrasound-assisted electrochemical deposition method. The results showed that the surface hydrophilic modification introduced a large number of oxygen-containing functional groups (C=O, C–O and COOH group) onto the surface of carbon/carbon composites. The surface hydrophily of the modified carbon/carbon composites was significantly improved with the water contact angle decreasing from 85.7 ± 4.1° to 11.9 ± 1.5°. The modified carbon/carbon composites had strong induction ability for the deposition of hydroxyapatite crystals. The hydroxyapatite showed a wavy-shaped crystal at initial deposition process and subsequently formed a compact and uniform coating. The bonding strength of hydroxyapatite coating on modified carbon/carbon composites (9.58 ± 0.40 MPa) was significantly higher than that of the coating on unmodified carbon/carbon composites (3.04 ± 0.20 MPa).

LaPO₄ inverse opal photonic crystals with different photonic band gaps were fabricated by template-assisted method. The Yb³⁺/Er³⁺ co-doped NaYF₄ nanoparticles were deposited on the surfaces of the inverse opals, and their up-conversion emission properties were investigated. The upconversion emissions of Yb³⁺/Er³⁺ co-doped NaYF₄ nanoparticles on the inverse opal surfaces have been enhanced when the upconversion emission bands of the nanoparticles are in the range of photonic band gaps of the inverse opals, which is attributed to an efficient and selective reflection of photonic band gaps.

Density functional theory calculations are performed to provide a molecular-level understanding of the mechanism of mercury adsorption on sulfuric acid-impregnated carbonaceous surface. The carbonaceous surface is modeled by a nine-fused benzene ring in which its edge carbon atoms on the upper side are unsaturated to simulate the active sites for reaction. SO₄ clusters with and without charge are examined to act as the representative species to model the sulfuric acid absorbed on the carbonaceous surface. All of the possible approaches of SO₄ clusters with and without charge on the carbonaceous surface are conduced to study their effects on mercury adsorption. The results suggest that sulfuric acid effect on the mercury adsorption capacity of the carbonaceous surface is very complicated, and it depends on a combination of concentration and charge of SO₄ cluster. SO₄ cluster presents a positive effect on mercury adsorption on the carbonaceous surface, but higher concentration of SO₄ cluster decreases the adsorption capacity of the carbonaceous surface for mercury removal because there is considerable competition for active sites between Hg and SO₄ cluster. Since all of the possible approaches of mercury on the carbonaceous surface with cluster, excluding one that mercury is adsorbed at bridge active site, can lead to the decrease in the adsorption energies of mercury on the carbonaceous surface, cluster presents a negative effect on the capacity of the carbonaceous surface for mercury adsorption regardless of the concentration of cluster. The results also indicate that SO₂ cluster and surface oxygen complex can be formed from SO₄ cluster with or without charge if mercury is adsorbed at bridge active site, which facilitates the mercury removal for the carbonaceous surface.

The decomposition of goethite and goethite/siderite concentrates into hematite with thermal modification was studied through the measurements of X-ray diffraction (XRD), scanning electronic microscope (SEM) and thermal gravimetric analysis (TGA). The experimental results showed that goethite decomposed into hematite directly at around 300°C without any intermediate phase and the decomposition of siderite completed at 500°C. Nanoscale granular structures were observed in the thermally treated products. It deserves highlighting that the decomposition processes proceeded from surfaces into bulks.