Effect of α-Al2O3 on the properties of cold sprayed Al/α-Al2O3 composite coatings on AZ91D magnesium alloy

Composite coatings using pure Al powder blended with α-Al₂O₃ as feedstock were deposited on AZ91D magnesium alloy substrates by cold spray (CS). The content of α-Al₂O₃ in the feedstock was 25 wt.% and 50 wt.%, respectively. The effects of α-Al₂O₃ on the porosity, microhardness, adhesion and tensile strength of the coatings were studied. Electrochemical tests were carried out in neutral 3.5 wt.% NaCl solution to evaluate the effect of α-Al₂O₃ on the corrosion behavior of the coatings. The results showed that the composite coatings possessed lower porosity, higher adhesion strength and tensile strength than cold sprayed pure Al coating. The corrosion current densities of the composite coatings were similar to that of the pure Al coating and much higher than that of bare AZ91D magnesium alloy.     

Preparation and spectroscopic characterization of γ-Al2O3 thin films

The electronic structure and surface properties of γ-Al₂O₃ thin films are studied. We have prepared the films by oxydizing Al foils under controlled conditions and we characterize the γ-Al₂O₃ samples by means of XPS, UPS, and TEM and found no charging. Pronounced effects in temperature-dependent changes of the work function are observed which result from changes in band bending and electron affinities by reorganisation and migration of defects. Thereby the ability of these systems for prototype studies in catalysis and analysis of defects is demonstrated.     

Specific features of luminescence of radiation-colored α-Al2O3 single crystals

Results of a comparative study on specific features of luminescence of thermochemical and radiation-colored crystals of aluminum oxide have been described. It is shown that crystals containing oxygen vacancies of different origins have identical main optical properties. A vacancy or vacancy-impurity origin of deep traps has been assumed. Some specific features of thermoluminescence (TL), which are common for both types of crystals, such as the dependence of the TL output on the heating rate and the occupancy of deep traps described by the model of interactive trap system, have been established.     

Growth of copper and palladium on α-Al2O3(0001)

Non-contact atomic force microscopy (NC-AFM) has been used to image the room-temperature growth of copper and palladium on the (1×1) and terminations of α-Al₂O₃(0001). Three-dimensional (3D) clusters of palladium are observed on both the (1×1) and the terminations, with 3D clusters of copper observed on the reconstructed surface. There is evidence of step-edge-dominated growth of palladium on the termination.     

Study of adsorption of oxygen on β-Al2O3 + Au and β-Al2O3 + Pt: Work function measurements—proposition of a model

To improve the understanding of the electrochemical effects observed on an original potentiometric gas sensor, interactions of oxygen with the device were investigated. This gas sensor is made of a solid electrolyte (treated Na-β-alumina) associated with two metallic electrodes (gold and platinum) located in the same gas mixture. Adsorption of charged oxygen species, considered responsible for the electrical response developed by the sensor, was investigated by work function measurements. Results showed that charged oxygen species only form on partially gold or platinum covered solid electrolyte. Comparison of these results with those obtained in a previous calorimetric study of interactions between oxygen and the same materials suggests the existence of at least two different oxygen species adsorbed on the surface of the sensitive element. The first one, located on the solid electrolyte surface, is neutral and characterized by an endothermal reaction of formation. The second one is charged and probably produced at the gas/solid electrolyte/metallic electrode interface. A mechanism based on the concept of “three phase boundary” and similar to the “reverse spillover” phenomenon is proposed to account for the adsorption of these oxygen species.     

Molecular dynamics analysis of impurity ions behavior in β″-Al2O3

The effects of foreign impurity ions in the conduction plane on the β″-Al₂O₃ lattice have been analyzed by molecular dynamics. As impurity ions, K and Ca ions were chosen and Na sites in the conduction plane were replaced with these ions. Results are summarized as follows: (1) The β″-Al₂O₃ lattice expands perpendicular to the conduction plane when K ions are doped; (2) Ca ions do not contribute to expansions of the β″-Al₂O₃ lattice; (3) both K and Ca ions reduce the mean square displacement of Na ions, which can be attributed to decreased Na ion diffusion.     

UV-luminescence and thermostimulated processes in nonlinear crystals LiB3O5

The results of study on luminescence and recombination processes in the large LiB₃O₅ single crystals are reported. It was shown that the luminescence at 295 nm is excited by photons with energy exceeding the band gap energy, by an electron beam, and also in recombination processes. Four peaks of thermoluminescence at 130, 180, 240 and 330 K are revealed, their parameters are fixed, and the origin of the trapping and recombination centers are determined. The most acceptable models of recombination processes and the origin of the LiB₃O₅ luminescence are discussed.     

Structural refractometry of β- and β″-Al2O3-type aluminates and gallates

The ionicity of oxide and β- and β″-Al₂O₃ crystals is identified via Lorentz–Lorenz molar refractivity. The investigation was extended to sodium gallates, the refractivity and average refractive index of which are estimated.     

Adsorption and diffusion of a single Pt atom on γ-Al2O3 surfaces

Motivated to better understand the interactions between Pt and γ-Al₂O₃ support, the adsorption and diffusion of a single Pt atom on γ-Al₂O₃ was studied using density functional theory. Two different surface models with atoms of various coordination (3–5) were used, one derived from a defected spinel structure, and another derived from the dehydration of boehmite (AlOOH). Adsorption energies are similar for the two surfaces, about −2 eV for the most stable sites, and involve Pt binding to surface O atoms. An unusually strong trapping geometry whereby Pt moves into the surface was identified over the boehmite-derived surface. In all cases the surface transfers 0.2–0.3 e⁻ to the Pt atom. The bonding is explained as being a combination of charge transfer between the surface and Pt atom, polarization of the metal atom, and some weak covalent bonding. The similarity of the two surfaces is attributed to the similar local environments of the surface atoms, as corroborated by geometry analysis, density of states, and Bader charge analysis. Calculated activation barriers (0.3–0.5 eV) for the defected spinel surface indicate fast diffusion and a kinetic Monte Carlo model incorporated these barriers to determine exact diffusion rates and behavior. The kinetic Monte Carlo results indicate that at low temperatures (<500 K) the Pt atom can become trapped at certain surface regions, which could explain why the sintering process is hindered at low temperature. Finally we modeled the adsorption of Pt on hydrated surfaces and found adsorption to be weaker due to steric repulsion and/or decreased electron-donating ability of the surface.     

Luminescence properties of α-Al2O3 dosimetric crystals exposed to a high-current electron beam

The spectra of pulsed cathodoluminescence (PCL) and thermoluminescence (TL) in TLD-500 detectors, which were exposed to a strong beam from a pulsed electron accelerator, have been studied. Additional bands in the PCL spectrum and new peaks in the TL curves, which are due to impurity ions, have been revealed. Luminescence bands of F- and F⁺-centers cannot be used in the dosimetry of strong electron beams using TLD-500 detectors because of the saturation of dose dependence and the decrease in the TL yield. It is shown that high doses from these beams can be measured by recording TL in the luminescence band of impurity titanium ions.     

Low temperature magnetocaloric effects in Dy1Ba2Cu3O7−δ and Y1Ba2Cu3O7−δ high-Tc single crystals

The heat release has been measured in high temperature superconductors during and after a change of the external magnetic field μ₀H_a ≤ 0.5 T applied parallel to the c-axis at 0.1 K ≤ T ≤ 0.45 K. Two Y123 and one Dy123 twinned single crystals were investigated. An analysis of the heat release based on the Bean model and on a model for thin superconductors in perpendicular geometry recently proposed by Brandt et al. has been carried out. During field sweep we measured ∝ H_a²dH_a/dt. This dependence can be understood within the Bean model for longitudinal geometry. The critical current densities obtained with this method are in good agreement with those from magnetization measurements. We also present measurements, and a first quantitative analysis, of dissipation due to the time relaxation of the flux line lattice after a field sweep. For the heat release a t⁻¹ dependence has been found which corresponds to a logarithmic time dependence of the magnetization. The normalized relaxation rate we obtained is in agreement with literature values.     

Thermogalvanic power and fast ion conduction in δ-Bi2O3 and δ-(Bi2O3)1−x(R2O3)x with R = Y, Tb---Lu

The thermogalvanic power (Seebeck coefficient) of O^2- conducting δ-Bi₂O₃ and δ-(Bi₂O₃)_1−x(Y₂O₃)_x has been measured directly as a function of temperature and partial oxygen pressure in N₂---O₂ mixtures. The of δ-(Bi₂O₃)_0.75(R₂O₃)_0.25 with R = Tb---Lu was indirectly determined using an isothermal concentration cell technique. Except for pure δ-Bi₂O₃, the heat of transport is much smaller than the activation energy for O^2- conduction for all materials. The vibrational freedom of O²⁻ ions in all δ-stabilized materials is reflected in their IR spectra at room temperature. Two prototypes of a thermogalvanic P_O2 meter were tested.     

Self-assembly of β-Ga2O3 nanobelts

Self-assembly of β-Ga₂O₃ (beta-gallium oxide) nanobelts with diameters of 50–100 nm and lengths of tens to hundreds of microns have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Under appropriate conditions such as nanobelts concentration, controlled solvent evaporation, β-Ga₂O₃ nanobelts assemble into a fan-like structure on the substrate. A tendency of these nanobelts to align parallel to each other was also observed. The mechanism behind the formation of self-assembly of β-Ga₂O₃ nanobelts has been proposed on the basis of lateral capillary forces.     

Polyacrylonitrile electrolytes: 1. A novel high-conductivity composite polymer electrolyte based on PAN, LiClO4 and α-Al2O3

In this work, a series of novel solid-type α-Al₂O₃-containing polyacrylonitrile (PAN)-based composite polymer electrolytes (CPE) with high conductivity and high mechanical property at room temperature has been prepared. The effect of the addition of α-Al₂O₃ on the properties of the PAN-based composite polymer electrolyte has been analyzed. The best conductivities obtained at room temperature is 5.7×10⁻⁴ S cm⁻¹ from the CPE with 7.5 wt.% α-Al₂O₃ and 0.6 LiClO₄ per PAN repeat unit. The stress–strain test result indicates that the membranes prepared possess high yield stress (73 kg cm⁻²) suitable for serving as separators in the solid-state lithium and lithium ion batteries and high yield elongation (225%) pliable to form good interface with electrodes. Also discussed are the effects of the addition of the ceramics on the interactions in the system and the possible conduction mechanism.     

Friction between α-Al2O3(0 0 0 1) surfaces and the effects of surface hydroxylation

Molecular dynamics simulations were performed to study the friction between hydroxylated α-Al₂O₃(0 0 0 1) surfaces at the temperature of 300 K. Effects of the degree of surface hydroxylation and sliding velocity have been discussed. Results indicate that the friction coefficient decreases with increased degrees of hydroxylation. For all degrees of surface hydroxylation, the friction law crosses over from thermal activation to viscous damping at sliding velocity of 80 m/s.     

Structural characterization of Ga-doped Mg0.1Zn0.9O layers grown on ZnO/α-Al2O3 templates by P-MBE

In this study, structural properties of epitaxial Ga-doped Mg_0.1Zn_0.9O layers grown on ZnO/α-Al₂O₃ templates by plasma-assisted molecular beam epitaxy have been investigated by high-resolution transmission electron microscopy (HRTEM), and high resolution X-ray diffraction (HRXRD). From analysis of the diffraction pattern, the monocrystallinity of the Mg_0.1Zn_0.9O layer with hexagonal structure is confirmed. The orientation relationship between Mg_0.1Zn_0.9O and the template is determined as (0 0 0 1)_Mg0.1Zn0.9O(0 0 0 1)_ZnO(0 0 0 1)_Al2O3 and [ [ ]_ZnO[ . The density of dislocations near the top surface layers measured by plan-view TEM is about 3.610¹⁰ cm⁻², one order of magnitude higher than the value obtained for ZnO layers on α-Al₂O₃ with a MgO buffer. Cross-sectional observation revealed that the majority of threading dislocations are in the [0 0 0 1] line direction, i.e. they lie along the surface normal and consist of edge, screw, and mixed dislocations. Cross- sectional TEM and X-ray rocking curve experiments reveal that most of dislocations are edge dislocations. The interface of Mg_0.1Zn_0.9O and ZnO layers and the effect of excess Ga-doping in these layers have been also studied.     

Thermoluminescence α-Al2O3:C detectors (TLD-500K): Some results of a long-term testing

Anion-defective α-Al₂O₃:C single crystals, commercially grown in Russia as TLD-500K detectors, have been increasingly used for environmental radiation measurements. These detectors have attracted great interest because of their high sensitivity and simple annealing procedure. The application of TLD for routine measurements in environmental monitoring, involving storage periods which may extend up to one year, imposes strict requirements on the fading, the change in sensitivity with long periods of time and other characteristics of the dosimeters. Many of these properties of TLD-500K have not yet been completely investigated and need to be further explored. This paper presents some results of long-term testing of the detectors. The variation in sensitivity within the detector group as well as changes of the sensitivity with time and number of read-outs were estimated. All tests were made by using low doses of irradiation, adequate for environmental dosimetry.     

Fabrication and characterization of β-Ga2O3 optical nanowires

Monoclinic gallium oxide (β-Ga₂O₃) nanowires with lengths of tens of micrometers and diameters ranging from 100 to 250 nm are synthesized using simple physical evaporation based on vapor–liquid–solid (VLS) mechanism. The as-synthesized straight β-Ga₂O₃ nanowires show excellent diameter uniformity and sidewall smoothness, making them suitable for optical wave-guiding. Light from a fiber taper is launched into the nanowire by means of evanescent coupling. Measured propagation loss of the nanowire at 633 nm wavelength is on the order of 10 dB/mm. Favorable mechanical strength of these nanowires for elastic bending is also observed. Our results suggest that β-Ga₂O₃ nanowires are promising building blocks for micro- and nanophotonic circuits and devices.     

Ion-implantation-induced structural modifications in Y1Ba2Cu3O7−δ superconductor

Ion-implantation-induced structural modifications in Y₁Ba₂Cu₃O_7−δ superconductor are examined by a grazing angle X- ray diffraction technique. By employing a range of grazing angles from 0.3° to 10° it is shown that 100 KeV Ar⁺ inplantation of the superconductor leads to amorphization as well as modification of grain size and orientation at dose values lower than 10¹⁶ ions/ cm². At the dose of 5 × 10¹⁶ ions/ cm² the X-ray diffraction intensity is a factor of 6 less as compared to the original pellet, though the lines themselves are sharp. This shows coexistence of perovskite grains and amorphous matrix.     

The insertion of magnesium into α-U3O8

The chemical and electrochemical insertion of magnesium into α-U₃O₈ was achieved at ambient and elevated temperatures with the topochemical formation of a product Mg_xU₃O₈ (0<x<0.65). Galvanometric discharges in both non-aqueous and aqueous cells show the existence of two single phase regions at 0.20≤x≤0.27 and x≥0.40. These regions shifted to higher x-values when the discharge was performed at 100°C with DMF as the electrolyte solvent. A fcc phase (a ≈ 5.42 Å) was identified in the cathodic products when cells were discharged beyond x ≈ 0.65. This could be related to the known high temperature fluorite phase Mg_xU_1−xO_2±y. Equilibrium potential measurements were made to determine the integral free energy of magnesium insertion. Kinetic measurements showed magnesium diffusivity in Mg_xU₃O₈ to be three to four orders of magnitude less than that for lithium in the compound Li_xU₃O₈.