Surface Review and Letters

Positron annihilation spectroscopy (PAS) is one of the nuclear techniques used in material science. The present measurements are used to study the behavior of defect concentration in one of the most important materials aluminum alloys which is the 7075 alloy. It has been shown that positrons can become trapped at imperfect locations in solids and their mean lifetime can be influenced by changes in the concentration of such defects. No changes have been observed in the mean lifetime values after the saturation of defect concentration. The mean lifetime and trapping rates are studied for samples deformed up to 58.3%. The concentration of defect range vary from 10¹⁵ to 10¹⁸cm^-3 at the thickness reduction from 2.3 to 58.3%. The dislocation density varies from 10⁸ to 10¹¹cm/cm³.

In-situ synthesized TiB₂ particles reinforced Ni-base composite coatings with different compositions were fabricated on medium carbon steel using laser cladding. Cracking behavior in different coatings was studied by microanalysis. Results show that the coating quality is good with optimal laser parameters and coating compositions; whereas the cracks formed in the coating with high content of ceramic phases mainly include the following: the transcrystalline cracks in the bonding metal matrix, the high density cracks in the edge of the coating, micro-cracks on the interface of the matrix and the hardy ceramic phases and in the vicinity of the bonding interface in the heat-affected zone etc. Cracking behavior in the coating is mainly attributed to the thermal stress induced by the difference of the thermal expansion coefficient of the coating materials and the metal matrix, and the phase transformation stress plays an important role too.

The n-type conductivity nitrogen-doped amorphous carbon (n-C) films have been grown by surface wave microwave chemical vapor deposition (SWP-CVD) on plastic and quartz substrates. The bonding, electrical and photovoltaic properties have been studied. The Tauc plot of UV-Visible measurement have shown that n-C films deposited on plastic have smaller optical band gaps compared to the films grown on quartz substrate at the same parameters. Temperature dependence and photoresponse measurements also have shown that the properties of n-C thin films deposited on plastic have higher photoelectrical response. Our experimental results have shown that the flexible polytetrafluoroethene plastic substrate is a reliable candidate for future photovoltaic applications.

Phosphorus-doped amorphous carbon (n-C:P) films were grown by r. f.-power-assisted plasma-enhanced chemical vapor deposition at room temperature using a novel solid red phosphorus target. The influence of phosphorus doping on material properties of n-C:P based on the results of simultaneous characterization are reported. Moreover, the solar cell properties such as series resistance, short circuit current density, open circuit current voltage, fill factor and conversion efficiency along with the spectral response are reported for the fabricated carbon-based n-C:P/p-Si heterojunction solar cell that was measured by standard measurement technique. The cells performances have been given in the dark I–V rectifying curve and I–V working curve under illumination when exposed to AM 1.5 illumination condition (100mW/cm², 25°C). The maximum of open-circuit voltage (V_oc) and short-circuit current density (J_sc) for the cells are observed to be approximately 236 V and 7.34,mAcm² respectively for the n-C:P/p-Si cell grown at lower r. f. power of 100 W. The highest energy conversion efficiency (η) and fill factor (FF) were found to be approximately 0.84% and 49%, respectively. We have observed that the rectifying nature of the heterojunction structures is due to the nature of n-C:P films.

The effect of boron weight percentage in the camphoric carbon target of pulsed laser deposition on the preparation of boron-doped amorphous carbon (a-C:B) films has been studied using standard measurement techniques. XPS results showed the a-C:B films bonding properties almost unchanged at lower Bwt% up to 10 Bwt%, after which it changes with the increase of Bwt%, indicating increasing doping concentration with increase of Bwt% in the target. This phenomenon is further supported by FTIR and Visible-Raman spectroscopy analyses. The variation of bonding and structural properties are also correlated with the optical gap (E_g) and electrical resistivity (ρ) characteristics which are related to successful doping of B for low content of B in the amorphous carbon (a-C) films as the bonding, structural and E_g remain almost unchanged, and the ρ decreased untill the film deposited at 10 Bwt%. Since both the E_g and ρ decrease sharply with higher Bwt%, this phenomenon can be related to the graphitization.

Carbon films have been deposited on quartz and single-crystal silicon substrates by pulsed laser deposition technique. The soot for the target was obtained from burning camphor, a natural source. The effect of nitrogen (N) incorporation in camphoric carbon film is investigated. Optical gap for the undoped film is about 0.95 eV. The optical gap remains unchanged for low N content and decreases to about 0.7 eV. With higher N content, the optical gap increases. The resistivity of the carbon film increases with N content, initially and decreases with higher N content up till the film that is deposited at 30 mTorr. The results indicate successful doping for the film deposited at low nitrogen content. The J–V characteristics of N-incorporated carbon/silicon photovoltaic cells under illumination are observed to improve upon N-incorporation in the carbon layer.

The microstructure of dendritic titanium carbides fabricated by laser surface alloying of pure titanium with graphite powder is investigated using electron probe microanalyzer (EPMA) and high-resolution transmission electron microscope (HRTEM). The growth mechanism of TiC crystals is that crystal grows by the model of continuous growth at the beginning of solidification and then the crystal surfaces that are suited to lateral growth develop fully by the model of spiral growth. So, the final morphology of dendritic TiC looks like it is composed of tiny crystal grains in a linear arrangement.

In this paper, spectral properties of optical bandpass Fibonacci filter are studied using the transfer matrix approach. The dispersion relation, transmission coefficient of quasiperiodic optical filter structure arranged according to recurrent Fibonacci rule is demonstrated for normal, s-polarized and p-polarized modes of radiation. Numerical calculations for different generations of Fibonacci structure at various angles of incidence are presented. The idea is based on the famous Kroning–Penny model in the band theory of solids. An interesting method for the existence of forbidden photonic oblique bands with the help of dispersion and transmission diagram of filter is presented. A Fibonacci filter can act as a virtually ideal polarizer for the radiation incident on the surfaces inside a filter at angles of 30° and greater.

Langmuir-Blogdett (LB) monolayers of behenic acid (BA) and cadmium behenic were prepared on mica surfaces by the vertical precipitation method. Atomic force microscopy (AFM) and lateral force microscopy (LFM) were used to study the morphologies of the films before and after reorganization and friction image. The LB films of fatty acid were found to self-organize into multilayers in water simply by the addition or removal of one layer of molecules, suggesting that the reorganization is a common phenomenon for monolayer on substrates in water conditions. It is considered that the H₂O molecules in the first place interact with the BA amphiphilic molecules at the edge of the pinholes. The assembly of molecules at the edge of the pinholes changes with a little protuberance in the morphology. The reorganization extent was also relevant to the pH value of the water medium.

We theoretically investigate the spin-dependent transport properties of electrons in realistic magnetic-electric barrier nanostructures, which are produced by the deposition, on top of a heterostructure, of a metallic ferromagnetic stripe with an applied voltage. The degree of the electron-spin polarization is found to be closely associated with this voltage, although the use of applied voltage itself induces no spin polarization effect. As a positive voltage is applied to the stripe the electron-spin polarization shifts towards the low-energy region and increases; it shifts towards the high-energy direction and reduces for a negative applied voltage. These results shown in this work imply that the degree of electron-spin polarization can be tuned by means of an applied voltage on the stripes of system, which may result in a practical voltage-controlled spin filter.

ZrO₂-Al₂O₃ laminated ceramics of 3 layers were prepared by dry-pressing and pressureless sintering, and their mechanical properties and toughening mechanisms were studied. Furthermore, their tribological behaviors and wear mechanisms under dry friction and water lubrication conditions were also investigated, and compared with those of monolayer ZrO₂-Al₂O₃ ceramics under the same conditions. The results show that the bending strength, fracture toughness and hardness of the laminated ceramics are all better than those of the monolayer ceramics, moreover the friction factor and wear rate of the laminated ceramics are clearly lower than those of the monolayer ceramics. The excellent mechanical properties of the laminated ceramics, especially toughness, result mainly from effects of residual stress in the specially-designed layer, interface structure and phase transformation together. Good tribological properties of the laminated ceramics are supposed due to the high toughness and low shearing stress in the wear surface. The friction and wear of the composites are effectively decreased by water lubrication because of the transformation of the main wear mechanisms from abrasive and adhesive wear under dry friction to tribo-chemical and fatigue wear under water lubrication.

The effect on the vibrational properties of nanostructures at the interface between two thin films is presented. The model used consists of the coupling of two semi-infinite plans A and B having three and two atomic layers, respectively. Theoretical calculations of localized phonon modes and vibrational field were carried out using the matching procedure. Close to the interface, the transversal translational symmetry is broken down inducing Raleigh-like branches. The polarization degeneracy of the ordered surface Raleigh mode in the longitudinal direction is lifted. The presence of acoustic and optical-like phonons is discussed in terms of the elastic constant forces. The density of states and the spectral densities are presented for sample atomic sites. A hyperfine resonance structure is obtained. It allows the analysis of the dynamical evolution from thin film A to thin film B.

The procedure for the preparation of NiZn-ferrite powder with average particle size of 10 nm and saturation magnetization of 50 emu/g by using precipitation reaction between acidified sulphate precursor salts and NH₄OH as the precipitating agent in the water-in-oil domain of the microemulsion CTAB/1-hexanol/water at 45°C is presented herein. TEM measurements have revealed relatively uniform morphology of the particles, which are associated in coral-like agglomerates. EDS measurements have been used for qualitative analysis of the sample, whereby FAAS measurement has been performed in order to reveal the proportion of the cations in the sample — Ni : Zn : Fe = 0.17 : 0.18 : 2.64.

The electrical resistivity (ρ), resistive thermal coefficient (α_r), thermal expansion coefficient (α_t) and stress (σ) of Al and Au thin films deposited by thermal evaporation were measured while films were heated by Joule effect. Electrical resistivity measured by the four-probe technique was simultaneously measured with surface film temperature in real time. Au films show important variations in the α_r and ρ properties when thickness ranged from 0.1μm to 1μm; contrarily, Al films show insignificant variations in the 0.1μm to 2γm thickness range. The role of the surface aluminum oxide on these measurements and the intrinsic stress on films determined with grazing incidence X-rays are discussed.

To clarify the thermionic property of solids, the mean work function effective for positive ion from polycrystalline surfaces (ϕ⁺) is theoretically studied together with that for electron emission (ϕ^e). The theoretical values of ϕ⁺ and ϕ^e thus evaluated typically with W are in good agreement respectively with those experimental ones accepted today. The thermionic contrast (Δϕ* ≡ ϕ⁺ – ϕ^e) is determined to be 0.56–0.57eV for W. For other polycrystalline metals (Nb, Mo, Ta, Re, Ir and Pt), it ranges from about 0.4 to 0.8eV in contrast with monocrystalline ones having Δϕ* = 0. Consequently, the fact of Δϕ* > 0 should be taken into consideration with any polycrystalline surface, and ϕ⁺ should be adopted whenever we try to analyze those data on positive ion emission due to any of the processes such as thermal stimulation and ion bombardment at heterogeneous surfaces. This article concludes that such typical data on thermal- and secondary-positive ion emissions are analyzed reasonably by adoption of ϕ⁺ instead of ϕ^e.

For the evaporation of the pure liquid under the condition of constant temperature and constant external pressure, the phase equilibrium of the liquid vapor in the bubble and the liquid outside the bubble is always a kind of stable equilibrium whether there is air or not in the bubble. If there is no air in the bubble, the bubble and liquid cannot coexist in the mechanical equilibrium when the vapor pressure of the liquid in the bubble is less than or equal to the external pressure; the bubble and liquid can coexist in an unstable equilibrium of mechanics when the vapor pressure of the liquid is greater than the external pressure. If there is air in the bubble, the bubble and liquid can coexist in a stable equilibrium of mechanics when the vapor pressure of the liquid is less than or equal to the external pressure; the bubble and liquid can coexist in a stable and an unstable equilibrium of mechanics when the vapor pressure of the liquid is greater than the external pressure and less than a certain pressure p_m; the bubble and liquid cannot coexist in the mechanical equilibrium when the vapor pressure of the liquid is equal to or greater than p_m.

Recent developments of laser surface modification of titanium alloys for increasing their corrosion, wear and oxidation resistance are introduced. The effects of laser processing parameters on the resulting surface properties of titanium alloys are reviewed. The problems to be solved and the prospects in the field of laser modification of Ti alloys are discussed. Due to the intrinsic properties, a laser beam can be focused onto the metallic surface to produce a broad range of treatments depending on the input energy. Thus, composite strengthening coatings can be fabricated by the methods of laser alloying, cladding, pulse laser deposition (PLD), etc., which are promising techniques of producing a layer of new materials on the surface of titanium alloys.