Polycrystalline Alumina Research Articles

Ce supported Al2O3 nanoparticles synthesized by coprecipitation method; morphology and optoelectronic study

In this research, ceria (Ce)- doped alumina (Al2O3) nanoparticles (NPs) were fabricated in different level of cerium using coprecipitation method. To characterize the morphology, crystal structure and optical properties, the FESEM, XRD, TEM, UV-DRS and FTIR analyses were performed. The structure analysis of undoped samples showed the multiple phases of γ, δ and θ–alumina, and the structure remained unchanged when cerium atom was doped into the alumina. The morphology and structure studies were carried out to investigate the effect of Ce doping on the polycrystalline alumina nanoparticles. The optical analysis was also done to study of the bandgap energy of the Al2O3 NPs. The novelty of this work is the synthesis of Ce-doped Al2O3 NPs with new precursors and improving their optical properties.

Effect of loading path on grain misorientation and geometrically necessary dislocation density in polycrystalline aluminum under reciprocating shear

Solid phase processing (SPP) is a promising alloy fabrication technique to produce fine and homogeneous grain structures for high-performance alloys. However, there is very limited modeling capability to understand and predict the grain refinement during SPP. In this work, the crystal plasticity theory was used to study elastic–plastic deformation in polycrystalline aluminums under large shear deformation. Two approaches, kernel averaged misorientation (KAM) and grain reference orientation deviation (GROD), were used to assess the grain misorientations. The geometrically necessary dislocation (GND) density was computed with the plastic strain rate. The deformation simulations were carried out under two loading conditions to investigate the effect of loading paths on the evolutions of grain misorientation and GND density. The results show that the regions with high misorientation and GND density first appear near grain boundaries. These regions then extend toward interior grains. The loading path affects plastic deformation accumulation and recovery, hence dislocation evolution and misorientation. Both two- and three-dimensional simulations showed that the spatial and temporal evolutions of GROD, KAM, and GND density are closely correlated, which indicates they all can be used as criteria of grain refinement or recrystallization, the essential information for grain refinement simulations.

Orienting dilute thin films of non-planar spin-1/2 vanadyl-phthalocyanine complexes.

The molecular orientation as well as the electronic and magnetic properties of vanadyl–phthalocyanine (VOPc) diluted into titanyl–phthalocyanine (TiOPc) thin films on Si(100) and polycrystalline aluminum substrates have been investigated by soft X-ray absorption spectroscopy (XAS), X-ray linear dichroism (XLD) and X-ray magnetic circular dichroism (XMCD). On the bare substrates the films grow with a standing-up geometry. By contrast, on template layers of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA), they assume a lying-down orientation. Moreover, a theoretical model based on the normalized intensity of the nitrogen K-edge XLD is established in order to extract the molecular orientation angle quantitatively without the need for crystallinity and with the sub-monolayer sensitivity of soft-XAS. XMCD reveals that the vanadium magnetic properties are preserved in both non-diluted and diluted films. The results pave the way toward the use of VOPc as nanometer-sized spin quantum bits.

Exploration of the best reference material on anelastic measurement by cyclic loading under high pressure

ABSTRACT Anelastic measurement by cyclic loading under high pressure has been developed by means of in situ X-ray observation at a synchrotron facility. In this method, the reference material is a key factor to precisely determine attenuation and moduli of unknown materials. We compared the performance of three types of reference materials (dense polycrystalline alumina, alumina single-crystal parallel to c-axis, and flexible graphite) under the pressure of 3 GPa and the temperature range between 1173 and 1373 K. The phase lags of strain between reference materials and samples show that the flexible graphite is less attenuated than dense polycrystalline alumina and alumina single crystal in various periods. The strain ratios show that the flexible graphite is much softer and can produce the measurable strain in the limited displacement. The flexible graphite, as reference material, is more excellent to detect the relatively lower energy dispersion of mantle minerals at high pressure.

Influence of synthesis parameters and thermal annealing on grain size of polycrystalline aluminum thin film

The thin polycrystalline aluminium films were synthesized on monocrystalline silicon substrates by ion-plasma sputtering. The synthesis was carried out at temperatures of 80 and 160°C and deposition rate of 10 and 110 nm/min. As-deposited films were annealed for 15 h at 550°C. The morphology of aluminium films before and after annealing was obtained using SEM images. The surfaces of as-deposited Al thin films, synthesized at high temperature, were uneven, while for low temperature films they were smooth enough with Al hillocks on the top of the film. After thermal annealing, morphology of the films was changed slightly. XRD patterns were obtained to calculate the average Al grain size of as-deposited and annealed films. The XRD analysis showed that an increase in the synthesis temperature leads to an increase in the average grain size from 50 to 84 nm and that increase in the rate of Al film synthesis leads to an increase in the average grain size from 50 to 63 nm. As the result of annealing, the average grain size increased for all samples and the final meaning was from 78 to 140 nm.

Investigation on Plastic Flow Behaviors of FCC Polycrystalline Aluminum under Pre-Cyclic Tension-Compression Loading: Experiments and Crystal Plasticity Modeling.

The plastic flow behaviors of FCC polycrystalline aluminum after pre-cyclic tension-compression deformation are mainly investigated in tension–torsion stress space by the physically based crystal plasticity model introducing a back-stress. A global finite element model (GFEM) constructed of sufficient grains was established to simulate the same-size thin-walled tube specimen constrained and loaded as the experiments of yield surfaces. The computational results showed that the shape of subsequent yield surfaces and the plastic flow directions directly depended on the given offset strain levels and the applied re-loading paths under different pre-cyclic deformations. The angle deviation between the plastic flow direction and the theoretical orthogonal direction further indicated that there was a large difference between them in the inverse pre-straining direction, but the difference was negligible in the pre-straining direction. From the influence of the anisotropic evolution of the subsequent yield surfaces on plastic flow, we found that the plastic normality rule followed the smooth yield locus; conversely, the significant non-associated flow was attributed to the distorted yield locus. Furthermore, it was also demonstrated that the anisotropic evolution and the plastic flow trend of the subsequent yield surfaces obtained by experiments can be better reproduced by the crystal plasticity model.

Ab-initio study of paramagnetic defects in Mn and Cr doped transparent polycrystalline Al2O3 ceramics

Birefringence is a major source of difficulty in sintering of transparent polycrystalline alumina ceramics, especially as the grain size exceeds a few hundred nanometers, which ultimately leads to complete opacity, mainly due to scattering of light. Recent studies have made it clear that by application of a strong magnetic field, alumina grains can be aligned along a particular crystallographic orientation, which minimizes scattering due to birefringence, and enhances transparency. Defects that cause spin delocalization are known to induce a paramagnetic behavior in alumina ceramics. Therefore, such defects have become a focal point of research for magnetic field assisted sintering of transparent polycrystalline alumina, in order to reduce the necessary magnetic field strength during production process. In light of recent studies on paramagnetic potentials of transition metal doped alumina, we have applied Spin Polarized Density Functional Theory (SP-DFT) calculations on manganese and chromium doped and co-doped alumina to calculate the magnetic moments, density of states and defect formation energies, which should be expected from this system of dopants, along with their interactions with oxygen vacancies. The results clearly indicate that formation of a point defect comprised of chromium and manganese positioned substitutionally at adjacent aluminum sites, in vicinity of an oxygen vacancy can induce a magnetic moment equivalent to 5 Bohr magnetons (μβ), outperforming previously reported defects. Based on this study we find it likely that chromium and manganese co-doping in alumina can further reduce the required magnetic field strength for production of transparent polycrystalline alumina.

Specific Features of Electroplastic Effect in Mono- and Polycrystalline Aluminum

Specific Features of Electroplastic Effect in Mono- and Polycrystalline Aluminum

Evolution of Mesoscopic Deformation-Induced Surface Roughness and Local Strains in Tensile Polycrystalline Aluminum

Evolution of Mesoscopic Deformation-Induced Surface Roughness and Local Strains in Tensile Polycrystalline Aluminum

Unraveling the implications of finite specimen size on the interpretation of dynamic experiments for polycrystalline aluminum through direct numerical simulations

Normal and Pressure-shear plate impact (NPI and PSPI) experiments are popular experimental techniques for studying the mean-field macroscopic behavior of polycrystalline metals under high-rate dynamic loading. However, since both configurations rely upon geometry for subjecting the specimen to high strain rates, these experiments often involve a limited specimen size. Moreover, because of the inherent heterogeneities present within polycrystalline metals, it is difficult to ascertain if the size of the specimen and/or regions where measurements are made are sufficiently large for making representative inferences about the mean-field macroscopic properties from single-point velocity measurements. In the present study, we quantify the expected measurement variability on observable point measurements in NPI and PSPI experiments by carrying out direct numerical simulations (DNS) of statistically representative polycrystalline microstructures subjected to dynamic compression and compression-shear loading. In particular, we consider the role of specific material heterogeneities (e.g. the grain-to-grain difference in size, crystallographic orientation) on dispersion in the normal and transverse particle velocity records and on local fluctuations in key state variables (e.g. velocity, accumulated plastic strain) by incorporating these effects directly into a representative synthetic microstructure geometry and crystalline description of pure polycrystalline aluminum. The form of the present study is a large parametric investigation, consisting of ten ensembles of one hundred simulations. Each of the thousand simulations reflects a randomly realized synthetic microstructure in one of five cases of decreasing average grain size for the two loading configurations. Our analysis of the DNS results demonstrates that for both of these experimental configurations, the grain size directly correlates with the coefficient of variation (CV) in simulated point measurements, showing a convergent decrease in CV to zero (i.e. particle velocity record approaches the mean-field value) with decreasing grain size. Remarkably, the magnitude of variations in the particle velocity record is shown to be largest where the deviatoric stresses are most significant. In the case of NPI, this occurs at the elastic and plastic wavefront, whereas, in the case of PSPI, the magnitude of fluctuations are approximately constant throughout the experimental window time. The reasoning for the scatter in particle velocity due to the heterogeneous microstructure is demonstrated to be dependent on the mechanisms for accommodating deformation and on the interaction of reflection waves generated at sites of heterogeneities occurring at the scale of grains. Lastly, we develop a power-law description for the magnitude of scattering versus characteristic length, which provides a statistical framework for assessing the required number of grains per characteristic specimen dimension for minimizing scatter within these two experimental configurations (NPI, PSPI).

Estimation via Laser Ultrasonics of the Ultrasonic Attenuation in a Polycrystalline Aluminum Thin Plate Using Complex Wavenumber Recovery in the Vicinity of a Zero-Group-Velocity Lamb Mode

In this paper, we present a method to recover the complex wavenumber dispersion relations using spatial Laplace transform from experimental spatiotemporal signals measured by laser ultrasonic technique. The proposed method was applied on zero-group-velocity Lamb modes in order to extract the ultrasonic attenuation in a polycrystalline aluminum plate of about 70 μm thickness. The difference between the experimental and theoretical Laplace Fourier transforms was minimized in the least square sense to extract the complex amplitudes and complex wavenumbers of the modes at about 40 MHz. The experimental results were compared to values reported in the literature that were measured by other means and those estimated by using the quality factor extracted from a single temporal signal.

The interfacial structure underpinning the Al-Ga liquid metal embrittlement: disorder vs. order gradients

Embrittlement of polycrystalline aluminum by liquid gallium represents a classic example of liquid metal embrittlement, whereas the interfacial atomic structure of Ga penetration front remains elusive despite decades of investigation. We performed aberration-corrected scanning transmission electron microscopy characterization to uncover the atomistic structure of Ga-enriched complexion within Al general grain boundaries. Multiple-layer adsorption of Ga with ordering gradients is revealed, consisting of disordered Ga layers at the GB core that are sandwiched by two more ordered adsorbate layers connecting to both aluminum grains. The ordering gradients within the interfacial complexion are further verified by the hybrid Monte Carlo and molecular dynamic simulations. This study advances our fundamental understandings of complexion structures within general grain boundaries.

Preparation of silicon and aluminum nanoparticles in an aqueous solution

The interest in nanomaterials is due to a significant change in their physical properties compared to massive materials. Changes in the physical and chemical properties of particles containing several hundred or thousands of atoms are determined by changes in the density of electronic states.Aqueous solutions of aluminum and silicon nanoparticles were obtained by laser ablation. Pulsed laser treatment was performed in the mode of double pulses in distilled water. The invested energy was: for aluminum 1000–1500 J, the exposure time is 10–25 min., for silicon-50–1000 J with exposure times from 1 to 10 min.The features of the morphology of the obtained particles, as well as the surface of polycrystalline aluminum and silicon craters, were determined by scanning electron microscopy. It is shown that an ensemble of particles of different sizes from 20 nm to 2.0 microns is formed. Some aluminum particles are cut, which allows us to talk about their cristallinity. It is shown that the structure of the target surface after laser exposure consists of grains separated from each other by thermal etching grooves, since the evaporation of the material occurs primarily along the grain boundaries.

Identification of various orthodontic materials as foreign bodies via panoramic radiography, cone beam computed tomography, magnetic resonance imaging, and ultrasonography: an in vitro study.

We evaluated the in vitro detection sensitivity of orthodontic materials (serving as foreign bodies) using panoramic radiography, cone beam computed tomography (CBCT), magnetic resonance imaging (MRI), and ultrasonography. Five different orthodontic materials served as foreign bodies: titanium-molybdenum alloy wire (TMA; ORMCO, Orange, CA, USA; 0.017 × 0.025 in in cross-sectional dimensions and 1cm long); stainless steel bracket tooth #34 (American Orthodontics, Sheboygan, WI, USA); a monocrystalline, sapphire ceramic bracket tooth #34 (Skyortho Dental Supplies Medical, China); a polycrystalline alumina clear bracket, Damon clear bracket tooth #34 (ORMCO); and a 1 × 1 × 0.1cm polyurethane-based thermoplastic material, Invisalign clear aligner (Align Technology, San Jose, CA, USA). Panoramic radiography, CBCT, MRI, and ultrasonography were used, and four observers scored all findings independently. The TMA and stainless steel bracket were visualised in all fields by panoramic radiography and CBCT. The sapphire and Damon brackets were very clear on CBCT. The Invisalign in air was evident only on CBCT. MRI was unable to identify any material in muscle. Ultrasonography detected the TMA, sapphire bracket, and the Invisalign in muscle but only the TMA on bone. Panoramic radiography does not reveal nonmetallic orthodontic equipment in air and reveals them only poorly in muscle. CBCT was the optimal imaging modality for all materials in all fields except for the Invisalign in muscle and bone. CBCT was the only method that revealed the Invisalign in air. MRI and ultrasonography should be used to detect orthodontic materials in muscle.

Rapid synthesis of AlON powders by nitriding combustion synthesis precursor

Polycrystalline aluminum oxynitride (AlON) powders have been prepared by the carbothermal reduction nitridation of amorphous precursor obtained by solution combustion synthesis (SCS). Firstly, a mixed precursor of amorphous transition alumina and incompletely combusted organic matter has been combustion synthesized of a mixed solution of aluminum nitrate, urea and glucose. Then the pure phase AlON powders with D50 = 0.846 μm can be prepared by calcining the SCS precursor at 1700 °C for only 10 min. The influences of different urea/aluminum nitrate nonahydrate ratio, calcining temperature and holding time on the properties of products have been discussed in detail. The research can provide a valuable reference for the efficient synthesis of AlON powders.

Influence of technological regimes of plasma chemical deposition on texture formation in aluminum nitride films

Textured polycrystalline aluminum nitride films were grown by magnetron sputtering of an aluminum target in a mixture of argon and nitrogen, followed by the formation of a film on sitall substrates. The influence of the pressure of the working gases, the power dissipated on the target, and the ratio of the Ar / N2 gas fluxes on the texture of the aluminum nitride films are investigated. The phase composition and degree of crystallinity of the obtained films were determined using scanning electron microscopy and X-ray diffraction. The results of studies of the phase composition and parameters of the crystal structure of aluminum nitride films are presented: the size of the coherent scattering regions, the predominant orientation of the crystallites in the growing film, and the degree of crystallinity. It is noted that the films are predominantly oriented along the (111) and (002) planes. The degree of crystallinity of the A1N films varied from 10 to 30% depending on the conditions of the deposition process. The relative microdeformation of the lattice (Δd/d), depending on the parameters, ranged from 1.7·10−3 to 6.9·10−3. The size of the crystallites is affected by the microdeformation of the lattice, with its increase by 4 times, the size of the crystallites decreased by 3.6 times from 29 to 8 nm.

The influence of carbon on the microstructure and wear resistance of alumina

AbstractThe influence of carbon as a dopant on grain growth and wear resistance of polycrystalline alumina was evaluated. Carbon was introduced into alumina by sintering in a carbon‐rich environment (graphite furnace under flowing He), and/or by residual carbon from organic binders used during the green body consolidation process. Samples were sintered at 1600°C for 2 h. Doping alumina with carbon resulted in a reduced grain size after sintering, correlated to solute‐drag, and graphite particle‐drag for high concentrations of carbon (~3 wt.%). The material response to abrasive wear was quantified by measuring the sample area cut for a defined time using a diamond wafering saw, as a function of grain size and carbon content. Sintering alumina with carbon resulted in a significant increase in wear resistance, as a result of the reduced grain size.

High-frequency K[formula omitted] internal friction peaks in polycrystalline aluminum and magnesium near the melting points at longitudinal and torsional resonance

By using a modified piezoelectric ultrasonic composite oscillator technique (M-PUCOT), the temperature dependence of Young’s modulus, shear modulus and related internal frictions of 4 N polycrystalline aluminum and magnesium were measured from room temperature to 900 K at longitudinal and torsional resonance under several tens of kilohertz. The grain boundary internal friction peaks (Keˆ peaks) accompanied by large moduli relaxations were observed near the melting point (~0.86 Tm and 0.78 Tm for Al and Mg respectively) in both vibration modes. The activation energy of grain boundary relaxation is almost the same for the two vibration modes, which is 31.2 kcal/mol for aluminum and 36 kcal/mol for magnesium. The peak values of grain boundary internal frictions are significantly dependent on the vibration mode, and for both materials, the torsional peak is about 1.5 times higher than that of longitudinal peak. Finally, the experimental results were evaluated by using existing grain boundary slipping models (including 3D and 2D Zener’s model, Budiansky and O’Connell’s model and Ghahremani’s model). Results show that the periodic hexagonal grains sliding model (Ghahremani’s model) can reproduce the experiments best.

Adiabatic shear bands formation in polycrystalline aluminium

The microstructure and texture evolutions during dynamic (or high strain rate) deformation of commercially pure AA1050 alloy have been studied to elucidate the mechanisms of shear band (SB) formation and its propagation across the flattened grains arranged in layers with special attention to texture evolution. The hat-shaped samples were compressed using drop hammer at a strain rate of 5.3 × 102 s−1. Detailed microstructure and texture characterizations were carried out using SEM equipped with EBSD facility. The shear displacement moves the upper part of the hat-shaped sample towards the bottom one leading to a strain localization in the form of SB. It was found that the strain localization occurs via deflection of flattened grains within narrow areas leading to kink-type bands that are considered to be the precursors of SB. The strain-induced crystal lattice rotation led to the formation of new texture components, different from those identified inside the undeformed matrix. It also facilitates slip propagation across the grain boundaries. The mechanism of macro-SB formation is essentially crystallographic in nature since in all grains of the sheared zone, the crystal lattice rotated in such a way that one of the {111} slip planes became nearly parallel to the shear plane and the <011> direction became parallel to the direction of maximum shear.

Ultrasonic experimental study on the elasticity of aluminum to 4.1 GPa in multi-anvil apparatus

Ultrasonic experiments have been performed to measure compressional and shear wave velocities of polycrystalline aluminum under hydrostatic pressure up to 4.1 GPa at room temperature in a multi-anvil apparatus. The sample pressure was determined by the new Z-cut quartz calibrant. Two types of data processing methods, three-order finite strain method and the Anderson's method, were utilized to calculate the elasticity of aluminum. The results from this work are in good agreement with previously reports and so it demonstrates the accuracy and convenience of our experimental methods, including the validity of the Z-cut quartz calibrant for pressure determination. We believe it's valuable for the measurements of elasticity of other material in multi-anvil apparatus, especially those experiments using compressible specimens which have small elastic moduli, and lacking of X-ray source.