Nanostructured Al2O3 Research Articles

High frequency induction heated sintering of nanostructured Al2O3–ZrB2 composite produced by MASHS technique

Nanostructured Al2O3–ZrB2 composite was synthesized using an alternative route called Mechanically Activated Self-propagating High-temperature Synthesis (MASHS). It was demonstrated that Al2O3–ZrB2 composite with nanometric structure could be produced via a very fast combustion front in contrast with the SHS process. Ball milling for 5h with ball to powder weight ratio of 5:1 was found to be the optimum condition for activation of the reactants. Using the high frequency induction heated sintering method, the composite was consolidated within 3min. The relative density of the sintered sample was 99% of the theoretical density at an applied pressure of 20MPa and temperature of 1800°C. The average hardness and fracture toughness were 19.5GPa and 4.1±0.3MPam1/2, respectively.

Process Parameter Optimization of Plasma Sprayed Nanostructured Al2O3-13 %TiO2 Coating Based on Genetic Algorithm

Process Parameter Optimization of Plasma Sprayed Nanostructured Al2O3-13 %TiO2 Coating Based on Genetic Algorithm

Towards highly electrically conductive and thermally insulating graphene nanocomposites: Al2O3–graphene

Highly electrically conductive materials with low heat transfer rates are of very high importance for high temperature fuel cell technologies and the refractory material industry. We aim to develop such materials with high electrical conductivities/high thermal resistivities by creating composite materials of graphene and Al2O3. Here we describe a novel and facile method for the synthesis of Al2O3–graphene composites. Graphite oxide, which was prepared by the Hofmann method, was reduced by active hydrogen generated by the reaction of aluminum with a solution of sodium hydroxide. This reaction led to the formation of a nanocrystalline composite of graphene and aluminum hydroxide. The Al(OH)3–graphene composite was then calcined and pressed into pellets. Sintering of the pellets yielded a nanostructured Al2O3–graphene composite. We characterized the properties of the Al(OH)3–graphene and Al2O3–graphene composite materials in all steps to get an understanding of the process of the nanocomposite formation. The materials were analyzed by XRD, high resolution XPS, Raman spectroscopy, SEM, SEM-EDS, STEM, STA and AFM. The resistivity and thermal conductivity of the final Al2O3–graphene composite were measured. The Al2O3–graphene nanocomposite is a promising conductive material for high-temperature applications.

Microstructural characterization of plasma sprayed conventional and nanostructured coatings with nitrogen as primary plasma gas

Air plasma sprayed conventional and nanostructured Al2O3–13wt.%TiO2, WC–12–17wt.%Co, and ZrO2–7–8wt.%Y2O3 coatings were deposited using nitrogen as the primary plasma gas. In nanostructured coatings, as critical plasma spray parameter (CPSP) – which is defined as the ratio of gun or arc power to the primary gas flow rate – controls the volume fraction of unmelted/partially melted regions, which in turn control the properties of coatings, coatings for all systems considered in this work were obtained as a function of CPSP. Coating properties, e.g. porosity, microhardness and percentage of partially melted/unmelted regions, for nanostructured coatings, were seen to be monotonic functions of CPSP. However, the effect of CPSP on percentage of partially melted/unmelted regions was significantly smaller for nitrogen as compared to argon as the primary plasma gas, particularly for nanostructured Al2O3–13wt.%TiO2 coatings.

Preparing of nanostructured Al2O3–TiO2–ZrO2 composite powders and plasma spraying nanostructured composite coating

Nanostructured Al2O3–TiO2–ZrO2 composite powders for plasma spraying were prepared by spray drying granulation technology. The effects of processing parameters on the microstructure and properties of composite powders were investigated. The results show that with increasing the slurry solid content, the particle size of powders increases, and the bulk density of powders increases, and the flowability of powders increases firstly and then decreases. With increasing the binder content, the particle size of powders increases, and the bulk density of powders increases, and the flowability of powders increases firstly and then decreases. With increasing the spray drying temperature, the particle size of powders increases, and the bulk density and flowability of powders increases firstly and then decreases. The most appropriate spray drying parameters are the slurry solid content of 40 wt.%, the binder content of 2.0 wt.% and the spray drying temperature of 250 °C. The nanostructured composite coating was successfully prepared by using the as-prepared nanostructured Al2O3–TiO2–ZrO2 composite powders as feedstocks. The nanostructured coating possessed higher hardness and toughness compared with the conventional microstructured one, which was attributed to the use of the nanostructured composite powders feedstocks.

Friction and wear behaviors of Al2O3–13 wt%TiO2 coatings

Nanostructured and conventional Al2O3–13 wt%TiO2 coatings were manufactured by air plasma spray. Friction and wear behaviors of coatings were investigated at room and elevated temperatures using an SRV wear test machine. The nanostructured coating has “two regions” microstructure, while the conventional coating has typical layered microstructure with obvious interfaces among splats. The coefficient of friction decreases with rising of temperature because of the formation of tribo-layer at elevated temperatures. The wear resistance of the nanostructured coatings is higher than that of the conventional coating, and the wear threshold of applied load is 30 N for conventional coating and 40 N for nanostructure coating. The wear resistance difference is related to the “two regions” microstructure of nanostructure coating, which could blunt or branch the cracks propagation. In our test ranges, the wear rates rising are more sensitive with the applied wear load rising than with the temperature rising. Extensive tribo-layer area emerged from the wear scar of nanostructure coating obtained at 500 °C. The tribo-layer area is compact and smooth and plays an important role to reduce the coefficient of friction. The elevated temperature enhanced the formation of tribo-layer, which lead to the decreasement of friction coefficient with rising of temperature.

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr3+ centers in submicron and nanostructured Al2O3 crystals of different phase compositions at temperatures of 300 and 7.5K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of 2E, 4T2, and 4T1 excited states of Cr3+ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al2O3 nanoscale crystals is formed by intracenter transitions (2.5–5.5eV region), by charge transfer band (6.9eV) and by effective formation of impurity-bound excitons (9.0eV region). Such impurity-bound excitons correspond to O2p→Al3s electron transition in surroundings of an impurity Cr3+ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al2O3 crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al2O3 at an excitation energy above 19eV (more than 2Eg).

Tribological behaviour of nanostructured Al2O3 coatings

Nanostructured alumina coatings have been deposited on SS 304 substrate using atmospheric plasma spray process. In the present work, nanostructured feed powder is obtained by manual granulation of nano-Al2O3 powder particles followed by exhaustive sieving, which is helpful in their proper consolidation. The tribological performance of the nano-alumina coatings, such as dry sliding, slurry erosion and cavitation erosion, has been investigated and compared with conventional coatings. The superior wear and erosion resistance of nanostructured coatings as compared to conventional coatings are due to its higher hardness and effective hindrance to crack propagation. Wear mechanism was explained based on their microstructure and worn surface morphologies. The microhardness and porosity of the two coatings were also experimentally investigated.

Nanostructure, Excitations, and Thermoelectric Properties of Bi2Te3-Based Nanomaterials

The effect of dimensionality and nanostructure on thermoelectric properties in Bi2Te3-based nanomaterials is summarized. Stoichiometric, single-crystalline Bi2Te3 nanowires were prepared by potential-pulsed electrochemical deposition in a nanostructured Al2O3 matrix, yielding transport in the basal plane. Polycrystalline, textured Sb2Te3 and Bi2Te3 thin films were grown at room temperature using molecular beam epitaxy and subsequently annealed at 250°C. Sb2Te3 films revealed low charge carrier density of 2.6 × 1019 cm−3, large thermopower of 130 μV K−1, and large charge carrier mobility of 402 cm2 V−1 s−1. Bi2(Te0.91Se0.09)3 and (Bi0.26Sb0.74)2Te3 nanostructured bulk samples were prepared from as-cast materials by ball milling and subsequent spark plasma sintering, yielding grain sizes of 50 nm and thermal diffusivities reduced by 60%. Structure, chemical composition, as well as electronic and phononic excitations were investigated by x-ray and electron diffraction, nuclear resonance scattering, and analytical energy-filtered transmission electron microscopy. Ab initio calculations yielded point defect energies, excitation spectra, and band structure. Mechanisms limiting the thermoelectric figure of merit ZT for Bi2Te3 nanomaterials are discussed.

Micro- and nanostructured Al2O3 surfaces for controlled vascular endothelial and smooth muscle cell adhesion and proliferation

The effect of the micro- and nanotopography on vascular cell-surface interaction is investigated using nano- and microstructured Al2O3 as model substrate. Two different nanostructured Al2O3 surfaces composed of low density (LD) and high density (HD) nanowires (NWs) were synthesized by chemical vapour deposition (CVD) and commercially available microstructured Al2O3 plates were used for comparison. A clear diverging response of human umbilical vein endothelial cells (HUVEC) and human umbilical vein smooth muscle cells (HUVSMC) was observed on these nano- and microstructured surfaces. LD Al2O3 NWs seem to enhance the proliferation of HUVECs selectively. This selective control of the cell-surface interaction by topography may represent a key issue for the future stent material design.

Single-Crystalline, Stoichiometric Bi2Te3 Nanowires for Transport in the Basal Plane

Bi2Te3 nanowires were grown in a nanostructured Al2O3 matrix by potential-pulsed electrochemical deposition. The wires had diameters of 50 nm to 80 nm and length of about 56 μm. Cross-sections prepared for scanning electron microscopy revealed uniform growth with deviations in length of less than 10%. The wires were deposited with reduction potentials between −150 mV and −250 mV. A reduction potential of −200 mV yielded an average Te content of 63.2 at.% as determined by calibrated, high-accuracy energy-dispersive x-ray spectrometry in transmission electron microscopy (TEM). The chemical composition was uniform along the length of the nanowires but depended on their diameter. The Te content increased by about 1 at.% when the diameter of the nanowires decreased from 80 nm to 50 nm. Selected-area electron diffraction combined with dark-field TEM imaging revealed single-crystalline wires; no grain boundaries were detected. The nanowires grew along the [110] and [210] directions; the c axis was perpendicular to the wire axis, allowing basal plane transport unaffected by grain boundaries.

Wear of Plasma Sprayed Conventional and Nanostructured Al2O3 and Cr2O3, Based Coatings

An ever increasing demand for high-performance ceramic coatings has made it inevitable for developing techniques with precise control over the process parameters to enable the fabrication of coatings with the desired microstructure and improved structural properties. The literature on plasma sprayed nanostructured ceramic coatings such as of Al2O3, Cr2O3, and their composites obtained using reconstituted nano sized ceramic powders has been reviewed in this study. Ceramic coatings due to their enhanced properties are on the verge of replacing conventional ceramic coatings used for various applications like automotive systems, boiler components, power generation equipment, chemical process equipment, aircraft engines, pulp and paper processing equipment, land-based and marine engine components, turbine blades etc. In such cases, the advantage is greater longevity and reliability for realizing the improved performance of ceramic coatings. It has been observed that the plasma sprayed nanostructured ceramic coatings show improvement in resistance to wear, erosion, corrosion, and mechanical properties as compared to their conventional counterparts. This article reviews various aspects concerning the plasma sprayed ceramic coatings such as (i) the present understanding of formation of plasma-spray coatings and factors affecting them, (ii) wear performance of nanostructured Al2O3, Cr2O3 and their composite ceramic coatings in comparison to their conventional counterparts, and (iii) mechanisms of wear observed for these coatings under various conditions of testing.

Stoichiometry Controlled, Single‐Crystalline Bi2Te3 Nanowires for Transport in the Basal Plane

AbstractThermoelectric Bi2Te3 based bulk materials are widely used for solid‐state refrigeration and power‐generation at room temperature. For low‐dimensional and nanostructured thermoelectric materials an increase of the thermoelectric figure of merit ZT is predicted due to quantum confinement and phonon scattering at interfaces. Therefore, the fabrication of Bi2Te3 nanowires, thin films, and nanostructured bulk materials has become an important and active field of research. Stoichiometric Bi2Te3 nanowires with diameters of 50–80 nm and a length of 56 μm are grown by a potential‐pulsed electrochemical deposition in a nanostructured Al2O3 matrix. By transmission electron microscopy (TEM), dark‐field images together with electron diffraction reveal single‐crystalline wires, no grain boundaries can be detected. The stoichiometry control of the wires by high‐accuracy, quantitative enegy‐dispersive X‐ray spectroscopy (EDX) in the TEM instrument is of paramount importance for successfully implementing the growth technology. Combined electron diffraction and EDX spectroscopy in the TEM unambiguously prove the correct crystal structure and stoichiometry of the Bi2Te3 nanowires. X‐ray and electron diffraction reveal growth along the [110] and [210] directions and the c axis of the Bi2Te3 structure lies perpendicular to the wire axis. For the first time single crystalline, stoichiometric Bi2Te3 nanowires are grown that allow transport in the basal plane without being affected by grain boundaries.

Sliding wear behavior of plasma sprayed nanoceramic coatings for biomedical applications

This paper reports on the sliding wear performance of the nanostructured Al2O3–13TiO2, ZrO2 and the bilayered (ZrO2/Al2O3–13TiO2) coated biomedical Ti–13Nb–13Zr alloy in simulated body fluid condition. The nanopowders were sprayed using an atmospheric plasma spray technique and the reciprocatory sliding wear behavior of all the above coatings was evaluated using wear testing machine. The bilayered (ZrO2/Al2O3–13TiO2) coating which has not been reported hitherto exhibited two hundred and five hundred fold increase in the wear resistances when compared with that of the nanostructured Al2O3–13TiO2 (AT) and ZrO2 (YSZ) coatings. This substantial improvement in the wear resistance of the bilayered coating is attributed to its lower porosity and higher adhesion strength when compared to the AT and YSZ coatings. This study suggests that this new type of bilayered coating may be a preferred approach to be tried for obtaining much higher

Microstructure, Corrosion, and Fatigue Properties of Alumina-Titania Nanostructured Coatings

Air Plasma spray process was used to deposit a conventional and nanostructured Al2O3-13 wt% TiO2 coatings on a stainless steel substrates. Morphology of the powder particles, microstructure and phase composition of the coatings were characterized by XRD and SEM. Potentiodynamic polarization tests and Electrochemical Impedance Spectro- scopy (EIS) were used to analyze the corrosion of the coated substrate in 3.5% NaCl solutions to determine the opti-mum conditions for corrosion protection. The fatigue strength and hardness of the coatings were investigated. The experimental data indicated that the nanostructured coated samples exhibited higher hardness and fatigue strength compared to the conventional coated samples. On the other hand, the conventional coatings showed a better localized corrosion resistance than the nanostructured coatings.

Performance evaluation of laser surface alloyed hard nanostructured Al2O3–TiB2–TiN composite coatings with in-situ and ex-situ reinforcements

Hard and wear resistant Al2O3–TiB2–TiN composite coatings have been developed on low carbon steel (AISI 1025) substrate by following two different routes involving laser surface treatment. In the first (termed ‘in-situ’ process), reinforcing phases TiB2 and TiN, as well as the matrix Al2O3 of the composite are synthesized in-situ by laser-triggered self-propagating high temperature synthesis (SHS) from a mixture of Al, TiO2 and h-BN and coated onto the substrate surface by laser surface alloying (LSA). In the second (termed ‘ex-situ’ process), the constituents Al2O3, TiB2 and TiN of the coating are provided directly as a pre-placed precursor powder mix and laser surface alloyed onto the substrate. Of these two laser assisted manufacturing procedures, it is of interest to determine the one that is more appropriate for the development of a hard, wear resistant coating. In the present work, investigation of the comparative merits and demerits of Al2O3–TiB2–TiN coatings produced by in-situ and ex-situ processes is attempted through analysis of microstructure and evaluation of mechanical and tribological properties.

Study of radiation induced effects in the luminescence of nanostructured Al2O3: Yb, Er crystals

Alumina crystals doped with Yb and Er were obtained by sol gel process and their morphologies and luminescence properties were discussed. Nanocrystals formations composed by Er2O3, Yb2O3 e Yb3Al5O12 were observed by TEM images, EDS, electron beam diffraction and XRD, at the surface of the alumina grains. The size of the nanocrystals were of about (36±2) nm and (182±8) nm for the samples calcinated at 1200oC and 1600oC, respectively. The sample codoped with 1mol% of Er and 2 mol% of Yb supplied the best results for Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL). The growth intensity of dosimetric TL peak at 205oC was linear with gamma radiation doses and the same behavior was observed in OSL results.

UV Thermoluminescence and Phosphorescence Properties of Mg2+ and Nd3+ Doped Nanostructured Al2O3

Mg2+ and Nd3+ doped aluminium oxide samples were produced by polymer calcination method. Mg2+ doped samples did not exhibited significant fluorescence emission, using IR (LED, emission centered at 862nm) or green (Xe-lamp plus optical filter, emission centered at 520 nm) sources. Nonetheless, high thermostimulated luminescence was detected, with high emission peak at 190°C. A nanoscopic layer (about 50 nm width) of magnesium spinel was observed by Transmission Electronic Microscopy (TEM) for 2.61mol% doped sample; this layer can be the responsible for TL enhancement. Nd3+ doped sample exhibited low phosphorescence emission in the UV (Schott U-340) using IR source. TL peaks were detected at 185 and 265°C; the intermediary peak showed the highest emission. Occurrence of NdAl and NdAl2 structures were detected in 5 mol% doped sample and NdAl2 and NdAl4 structures in 10 mol% doped sample.

Adhesion of fibroblasts on micro- and nanostructured surfaces prepared by chemical vapor deposition and pulsed laser treatment

The development of micro- and nanostructured surfaces which improve the cell–substrate interaction is of great interest in today's implant applications. In this regard, Al/Al2O3 bi-phasic nanowires were synthesized by chemical vapor deposition of the molecular precursor (tBuOAlH2)2. Heat treatment of such bi-phasic nanowires with short laser pulses leads to micro- and nanostructured Al2O3 surfaces. Such surfaces were characterized by scanning electron microscopy (SEM), electron dispersive spectroscopy and x-ray photoelectron spectroscopy. Following the detailed material characterization, the prepared surfaces were tested for their cell compatibility using normal human dermal fibroblasts. While the cells cultivated on Al/Al2O3 bi-phasic nanowires showed an unusual morphology, cells cultivated on nanowires treated with one and two laser pulses exhibited morphologies similar to those observed on the control substrate. The highest cell density was observed on surfaces treated with one laser pulse. The interaction of the cells with the nano- and microstructures was investigated by SEM analysis in detail. Laser treatment of Al/Al2O3 bi-phasic nanowires is a fast and easy method to fabricate nano- and microstructured Al2O3-surfaces for studying cell–surface interactions. It is our goal to develop a biocompatible Al2O3-surface which could be used as a coating material for medical implants exhibiting a cell selective response because of its specific physical landscape and especially because it promotes the adhesion of osteoblasts while minimizing the adhesion of fibroblasts.

Microstructure and Thermal Properties of Nanostructured 4 wt.%Al2O3-YSZ Coatings Produced by Atmospheric Plasma Spraying

Microstructure and phase composition of the nanostructured Al2O3 doped YSZ coatings by atmospheric plasma spraying method have been characterized with XRD, TEM and SEM. The nanostructured 4AlYSZ coatings consist mainly of t-ZrO2, crystalline Al2O3 phase is absent in the coatings and the grain size of the 4AlYSZ coating is about 65 nm. The APS 4AlYSZ coating is characterized by nanozones, dense area and voids. After doping, the coefficient of thermal expansion of YSZ is decreased to 10.928 × 10−6/K. The addition of Al2O3 has a great influence on decreasing the thermal conductivity of nano-YSZ, which is mainly caused by the point defect scattering and grain-boundary scattering. The lifetime of nanostructured 4AlYSZ coating is about 1000 cycles at 1100 °C.