Subsurface damage in alumina and alumina–silicon carbide nanocomposites

The subsurface plastic deformation below alumina (Al₂O₃) and Al₂O₃–silicon carbide (SiC) nanocomposite surfaces subjected to grinding, polishing and annealing has been measured by high-resolution grazing-incidence parallel-beam X-ray powder diffraction and transmission electron microscopy. The variation with angle in the full width at half-height maximum (FWHM) of the X-ray Bragg peaks was successfully modelled by a FWHM distribution that fell exponentially with increasing depth. Consistent parameters were extracted from data taken using both prism and pyramidal reflecting planes. Correlation was found between the depth at which the FWHM fell to 1/e of the surface value and the depth of damage observed by transmission electron microscopy. The associated surface strain in the nanocomposite was found to increase linearly with increasing diameter of the diamond polishing particles. In ground 5?vol.%?SiC nanocomposite, these random surface strains fell by a factor of 7 and the depth of damage increased by a factor of 3 after annealing at 1250°C for 2?h. No differences were observed in the Bragg peak FWHM as a function of angle for material polished with 1?µm diamond grit before and after annealing.     

Polarization properties of porous anodic alumina with Y-branched Cu nanowires

Porous anodic alumina(PAA)templates with branch structure are fabricated by the two-step anodic oxidation processes,and then the Y-branched Cu nanowires are synthesized in the templates using an alternating current(AC)deposition method.We observe the morphology image of the samples by scanning electron microscopy(SEM),and measure the transmission spectrum and the polarization spectrum of the samples by the spectrophotometer.The results show that PAA films with Y-branched Cu nanowires have better transmittance in the near infrared region.An extinction ratio of 15-18 dB and an insertion loss of 0.1-0.4 dB are obtained in this region.Therefore PAA with Y-branched Cu nanowires can be used as a near-infrared micropolarizer,and this kind of micropolarizer would have a promising future in the field of photoelectricity integration.     

The production of nickel–alumina composite coating via electroplating

A series of electroplating works have been conducted to investigate the best condition for the coelectrodeposition of nickel–alumina (Ni/α–Al₂O₃) composite coating. Co-electrodeposition was done onto mild steel as cathode at ambient temperature (27°C) with current density of 30 mA/cm² under α-Al₂O₃ concentration of 2 g/l and various agitation speeds of 50, 100, 150, 200, and 250 rpms. The cross-section of the composite coatings portrayed α-Al₂O₃ particles was co-deposited. Under field emission scanning electron microscopy analysis, the coating shows a coarse surface morphology, while cross-sectional microstructures shows a compact embedding of α-Al₂O₃ particle in the Ni matrix. Elemental analysis by EDX detected the presence of Ni and α-Al₂O₃. Vickers microhardness testing shows that the coating hardness increases almost 60% at the highest agitation speed, i.e., 250 rpm.     

Surface chemistry of Nextel-720, alumina and Nextel-720/alumina ceramic matrix composite (CMC) using XPS–A tool for nano-spectroscopy

Oxide-based ceramic matrix composites (CMCs) are prime candidates for high temperature turbine applications. Increasing demand of CMCs necessitates the development of quality monitoring procedures. Sol–gel derived Nextel-720 fiber/alumina matrix CMC is one of the potential candidate material for land-based gas turbine applications. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were utilized to investigate any surface/interface chemical alteration of the Nextel-720 fiber reinforcement and the alumina matrix during fabrication. The calculated XPS spectra of the composite were obtained by simply adding the spectra of the as-received Nextel-720 fiber and the alumina matrix. The calculated XPS spectra and the acquired XPS Al(2p), Si(2p³), and O(1s) spectra from the as-received materials were compared using a superimposition method to investigate any chemical alteration during composite fabrication for quality control measures. This paper is aimed to serve as a reference for future XPS studies of CMCs exposed to aggressive turbine environments.     

Structure and bonding properties of Y doped ∑37 grain boundary in alumina

The microscopic structures and the bonding properties of Y-doped and undoped(0118)/[0441]/180?(Σ37) grain boundaries in alumina are investigated by using ab initio method.The formation energy of grain boundary and the segregation energy of Y to grain boundary are acquired.Electronic structures,potential distributions,bond orders and effective charges of Y-doped and undoped Σ37 GB systems are calculated.Our results reveal that the higher strength Y-O bond than Al-O bond is ascribed to the hybridization of Y(4p,3d) with O(2s).Meanwhile,dopant Y also causes a change in potential distribution in the grain boundary region,thereby further aflecting the transport property of ceramic alumina.     

Preparation of alumina nanowires, nanorods, and nanowalls by chemical etching

Alumina (Al₂O₃) nanowires, nanorods, and nanowalls have been prepared from anodic aluminum oxide (AAO) templates by chemical etching in NaOH solution. Heating the template prior to etching is crucial to the morphology of subsequent prepared alumina nanostructures, which greatly depend on the phases of the AAO templates. It has been found that the templates with amorphous Al₂O₃, γ-Al₂O₃, and α+γ-Al₂O₃ phases will grow nanowires, nanorods, and nanowalls, respectively. A possible mechanism for forming different alumina nanostructures is proposed.     

Nanostructured layers of anion-defective gamma–alumina – New perspective TL and OSL materials for skin dosimetry. Preliminary results

Thin- layer material based on nanostructured Al₂O₃ of the surface density 5 mg/cm² was obtained. The material is characterized by high OSL and TL yields comparable with those for TLD-500 which is one of the leaders among the TL and OSL detectors. The dose response, fading and dependence of TL yield on heating rate was studied. It is established that high luminescence yield of the samples under study correlates with the content of anion vacancies and γ-phase of Al₂O₃. The data for time-resolved luminescent spectroscopy are presented, which evidence for possible correlation between high TL and OSL activity and the F-type centers. It is noted that the material needs to be modified for successful use in dosimetry. In addition further studies to decrease the contribution of unstable (at 300 К) components to OSL and TL yields are required.     

Thermal stability of alumina thin films containing γ-Al2O3 phase prepared by reactive magnetron sputtering

The paper reports on thermal stability of alumina thin films containing γ-Al₂O₃ phase and its conversion to a thermodynamically stable α-Al₂O₃ phase during a post-deposition equilibrium thermal annealing. The films were prepared by reactive magnetron sputtering and subsequently post-deposition annealing was carried out in air at temperatures ranging from 700 °C to 1150 °C and annealing times up to 5 h using a thermogravimetric system. The evolution of the structure was investigated by means of X-ray diffraction after cooling down of the films. It was found that (1) the nanocrystalline γ-Al₂O₃ phase in the films is thermally stable up to 1000 °C even after 5 h of annealing, (2) the nanocrystalline θ-Al₂O₃ phase was observed in a narrow time and temperature region at ≥1050 °C, and (3) annealing at 1100 °C for 2 h resulted in a dominance of the α-Al₂O₃ phase only in the films with a sufficient thickness.     

Extraction of some trapping parameters from experimental thermoluminescence (TL) signal of alumina (α-Al2O3) using analytical models

The exploitation of thermoluminescence (TL) signals constitutes the background of important and phenomenological methods used for estimating the kinetic parameters such as the activation energy (E), the frequency factor (s) and the general order of kinetics (b). These parameters are, in general, poorly known and all the used methods often diverge on their determined values. In the present paper, we investigate the suitability of the general phenomenological equations used to extract some trapping parameters from experimental TL signals of alumina (α-Al₂O₃). α-Al₂O₃ TL signal exhibits, under hard uv-irradiation (4.8 eV), two TL-main peaks located at 415 nm and 696 nm corresponding to F and Cr³⁺ centers respectively. To this aim, we numerically computed the analytical expression of TL which results from general order kinetic (b) taking into account the trapped charge concentration and the radiative/retrapping yield R. The computed curves for R = 1 fit perfectly the experimental data, allowing the extraction of some TL data as E, b and s from TL bands of alumina.     

Size-dependent oxidation of Pdn (n ? 13) on alumina/NiAl(110): Correlation with Pd core level binding energies

Small Pd clusters Pd_n (n = 1, 4, 7, 10, 13) deposited on alumina/NiAl(110) at room temperature were examined by X-ray photoelectron spectroscopy (XPS), as-deposited and after exposure to O₂ at temperatures ranging from 100 to 500 K. After O₂ exposure at 100 K, the Pd clusters showed XPS shifts indicative of oxidation. The exception was Pd₄, which did not oxidize under any conditions. The inertness of Pd₄/alumina/NiAl(110) appears to be correlated with a significantly higher-than-expected Pd 3d binding energy, which we attribute to a particularly stable valence shell. None of the clusters examined oxidized during O₂ exposures at 300 K or above, but He⁺ scattering showed that oxygen was bound on the cluster surfaces. Upon heating, all the oxygen associated with these small clusters appeared to spill over and react with the alumina/NiAl(110) support.