Spherical Alumina Research Articles

Combustion behaviors and residues characteristics in hydrogen/aluminum dust hybrid explosions

Hybrid explosion experiments of hydrogen/aluminum dust in open space were performed. Aluminum dust with a median diameter of 56.18μm was mixed with hydrogen at different volume concentrations (0 %, 5 % and 10 %). Flame propagation was recorded by a high-speed camera. The explosion residues were observed by a scanning electron microscope, and their compositions were analyzed by X-ray photoelectron spectroscopy. The flame propagation velocities and structures, explosion residues and the combustion reaction mechanisms of hydrogen/aluminum dust mixtures were elucidated. The results show that the addition of hydrogen can increase the flame brightness and improve the continuity of the flame front. In the flame propagation process of a hydrogen/aluminum dust hybrid explosion, a micro-diffusion flame and asymmetric flame appeared simultaneously. Compared with pure aluminum dust combustion in air, when 5 % hydrogen-air mixtures were used to disperse the dust, the flame propagation velocities decreased by 0.11-0.15m/s. Attributable to a variety of intermediate products competing for oxygen and absorbing heat, the hybrid explosion residues cooled faster, porous oxide layers and incompletely oxidized aluminum spheres with small particle sizes were formed. The XPS showed that Al2O3, Al(OH)3, AlO(OH) and other complex products appeared in the combustion reactions. On this basis, a combustion model of hydrogen/aluminum dust hybrid explosion was established.

The Indirect Excitons Contribution to the Polarizability of a Dielectric Nanoparticle

The optical properties of dielectric nanostructures strongly depend on dielectric nanoparticles (NP) polarizability which can take the high values even interacting with low-intensity radiation. In this article, we are trying to find out if this is possible for electron transits to indirect exciton (IX) states at the NP surface. Because the exciton energy spectrum depends on NP shape and size, we research these factors influence on NP polarizability within the dipole approximation framework. For a quantitative assessment, we use a nanosystem containing a spherical dielectric alumina nanoparticle in a dielectric medium with a lower permittivity. We estimated the impact of NP size effect on the NP energy spectrum and the NP polarizability in case of exciton transitions to spatially separated electron and hole condition at the interface of dielectric nanoparticle and dielectric medium. Thus, using the variational method, we found that NP polarizability can achieve many orders of magnitude higher values than for atomic and molecular electron transitions case. It’s take place for infrared, visible and ultraviolet radiation ranges. Also was found that size-dependence of IE energy spectrum weakly affects on NP polarizability. The obtained results can be used for creating the nano- and heterostructures for advanced nanophotonic applications that operate in conditions of weak optical fields.

The catalytic oxidation of toluene at low temperature over palladium nanoparticles supported on Alumina sphere catalysts: effects of Palladium precursors and preparation method

Abstract In this study, 1 wt.% Pd/Al2O3 sphere catalysts were prepared using the wet-impregnation (WI) and deposition-precipitation (DP) method using palladium chloride and tetraamminepalladium (II) nitrate as salt precursors. All catalysts were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier-transform infrared (FTIR) spectroscopy. The catalytic activity in toluene oxidation under gas-phase conditions was measured. The obtained results showed that metal dispersion and catalytic activity were strongly dependent on the salt precursor and method of catalyst preparation. The use of tetraamminepalladium (II) nitrate as the precursor presented smaller particle size, an enhanced dispersion and higher specific surface area. Moreover, the catalyst prepared with this precursor also showed higher catalytic activity than that prepared with palladium chloride. At 1 wt.% Pd loading, complete oxidation of toluene was achieved at 250°C. However, there was only approximately 80–90% efficient at the same temperature when the catalyst was prepared with palladium chloride as the precursor.

Characterization of spherical cell porous aluminum alloy-polyurethane interpenetrating phase composites at different temperatures

The characterization, including deformation process, damping mechanisms within the plastic plateau region, damping and stiffness properties, of spherical cell porous aluminum alloy-polyurethane interpenetrating phase composites (SCPAA/PU composites) at different temperatures were investigated. Compressive tests show that seven distinct regions for the composites under the given loading pattern are exhibited. Then, the damping mechanisms for the composites within the plastic plateau region are found to be attributed to the following aspects, including the damping of the matrix and filler phase, the interfacial friction damping between the matrix phase and filler phase, and the macroscopic interface dislocation friction damping along macro-interface in the debonded region. Furthermore, no clear regularity between the loss factor and temperature is observed, while the secant stiffness decreases with the increment of temperature. Both loss factor and secant stiffness declines rapidly in the first few cycles, and reduces slowly until a stable state is achieved. At last, the variation of loss factor/secant stiffness with the number of cycles can be described by a power function model. The proposed model works well with the testing data.

Shielding performance of debris shield with separated rear wall

Generally, changing the properties of bumper or the stuffed wall in a debris shield is the traditional method to improve shielding performance. A new way to improve shielding performance by separating the rear wall of debris shield was explored in this paper. Both hypervelocity impact tests and simulations were performed to evaluate the performance of the shield with a separated rear wall. The projectiles were aluminum spheres with a diameter of 6.0 mm at the impact velocity of about 8.3 km/s, and a diameter of 4.0 mm at the impact velocity of about 4.7 km/s. The targets were the separated rear wall shield (SRW) and Whipple shield with the same areal density. The results show that the shielding performance of a SRW shield is improved and better than that of a Whipple shield with the same areal density, because the separated surface in SRW shield can attenuate the propagation of the impact energy wave, decrease the free surface velocity on the back of the rear wall, and then reduce the damage on the rear wall.

A novel hollow alumina sphere-based ceramic bonded by in situ mullite whisker framework

In this study, a novel hollow alumina sphere-based ceramic enhanced by in situ mullite whisker framework was recommended. Hollow alumina spheres selected as the main pore-forming agent were bonded together by an active mullite powder at high temperature. To improve the mechanical properties of the hollow alumina sphere-based ceramic, AlF3 powder was added into the active mullite powder to fabricate the mullite whisker framework during the sintering process. The mechanical properties of as-received hollow alumina spheres ceramic can be tailored by the sintering procedure. The results indicated that the sample sintered at 1400 °C exhibited a low bulk density of 1.005 g/cm3 and a low thermal conductivity of 0.656 W/mK. The high aspect ratio of mullite whisker interacting with each other formed a unique framework structure, which enhanced the flexure strength (reaching 5.05 MPa) of the sample.

Micrometeoroid impact-induced damage of GLAss fiber REinforced aluminum fiber-metal laminates

Through several decades of development, engineers have made the GLAss fiber REinforced aluminum material fit for aviation structures, e.g., the fuselage of Airbus A380. A request like ‘GLARE + impact’ in a web search engine gives hundreds if not thousands of scientific articles address the high impact resistance of GLAss fiber REinforced aluminum. GLAss fiber REinforced aluminum can be a suitable material for shielding systems to protect manned and unmanned spacecraft against micrometeoroids and orbital debris. However, it is hard to comprehend a rational reason why a thorough search in the relevant literatures yielded only a couple of articles focused on the ballistic impact response of GLAss fiber REinforced aluminum. Handful of studies have interrogated the damage of GLAss fiber REinforced aluminum using analytical, numerical and experimental methods. No physical model, yet has been proposed and validated to capture the damage of GLAss fiber REinforced aluminum upon collision with micrometeoroids. This study, therefore, introduced a new numerical model based on the smoothed particle hydrodynamics and the finite element method. The model was able to predict the exorbitant strain-rate of GLAss fiber REinforced aluminum. Besides, the model could approximate the cataclysmic amount of energy dissipated in the shockwave-induced collapse of GLAss fiber REinforced aluminum. The model assumed that the behavior of the S2-glass/FM94-epoxy composite material was orthotropic elastic prior to the onset of damage. After the damage initiation, the energy-based orthotropic softening governed the damage accumulation in the composite material. Looking at the outputs of the model, an impact of a 2 mm 2024-T3 aluminum sphere on a GLAss fiber REinforced aluminum 5–6/5–0.4 plate led to petals in the front aluminum layer, spallation of the rear aluminum layer and buckling of the inner aluminum layers. By contrast, the S2-glass/FM94-epoxy composite laminates conserved the imparted energy through membrane stretching before had been pierced. A test campaign, with the aid of a two-stage light-gas gun facility, was pursued to assess the model accuracy. It was found that the model predicted many of the experimental observations with a high degree of fidelity.

Development of thermal barrier coating systems from Al microparticles. Part I: Influence of processing conditions on the mechanisms of formation

This work presents the mechanisms of formation on pure nickel of full thermal barrier coating systems (aluminide coating, thermally grown oxide and thermal barrier top coating) from micro-sized Al particles dispersed in a slurry annealed in different atmospheres (Ar, synthetic air, water vapour and mixtures thereof). The simultaneous formation of nickel aluminides and of a thermal barrier made of sintered hollow alumina spheres involved self-propagating high-temperature synthesis in all conditions. However, the microstructures and adherence of the top coatings changed markedly with either a diffusion step (700°C-2 h) or a complete heat treatment (700°C-2 h + 1100°C-2 h) depending on the atmosphere. Whereas fast consumption of Al occurred in Ar to form the nickel aluminides, synthetic air and water vapour fostered the peripheral oxidation of Al micro-sized particles that impeded the release of Al and its diffusion towards the substrate. This resulted in heterogeneous diffusion layers but thicker top coatings with better sintering and thicker alumina shells.

Cyclic response of spherical cell porous aluminum alloy-polyurethane composites

This work presents the experimental characterization of open-cell spherical cell porous aluminum alloy-polyurethane composites (SCPAA/PU composites) under a single full unloading/reloading cycle and seven complete repeated cycles, respectively. Three batches of SCPAA/PU composites are prepared for each loading pattern. The present paper mainly focuses on the three characteristic parameters, including unloading modulus, plastic strain and stress degradation ratio. The test results show that cyclic stress-strain curves of the composites under the two loading patterns exhibit the post-peak strain softening. Moreover, unloading modulus of the composites increases firstly and then reduces rapidly. The effect of the repeated cycles on the unloading modulus is closely associated with the applied strain. Initially, plastic strain ascends linearly with the increment of the applied strain, whereas, after a threshold strain, the growth rate slows down gradually. And the plastic strain is independent of the repeated cycles. Stress degradation ratio remains almost stable within a threshold strain and then reduces swiftly. The effect of the repeated cycles on the stress degradation ratio is also observed to be related to the applied strain.

Efficient and environmental-friendly perovskite solar cells via embedding plasmonic nanoparticles: an optical simulation study on realistic device architectures.

Solution-processed, lead halide-based perovskite solar cells have recently overcome important challenges, offering low-cost and high solar power conversion efficiencies. However, they still undergo unoptimized light collection due mainly to the thin (∼350 nm) polycrystalline absorber layers. Moreover, their high toxicity (due to the presence of lead in perovskite crystalline structures) makes it necessary that the thickness of the absorber layers to be further reduced. Here we address these issues via embedding spherical plasmonic nanoparticles of various sizes, composition, concentrations, and vertical positions, in realistic halide-based perovskite solar cells. We theoretically show that plasmon-enhanced near-field effects and scattering leads to a device photocurrent enhancement up to ∼7.3% when silver spheres are embedded inside the perovskite layer. An even further enhancement, up to ∼12%, is achieved with the combination of silver spheres in perovskite and aluminum spheres inside the hole transporting layer (PEDOT:PSS). The proper involvement of nanoparticles allows the employment of much thinner perovskite layers (up to 150 nm), reducing thus significantly the toxicity. Providing the requirements related to the design parameters of nanoparticles, our study establishes guidelines for a future development of highly-efficient, environmentally friendly and low-cost plasmonic perovskite solar cells.

Detection technique of the optical and thermoradiative characteristics with compensation effect of reflection and transmittance indicatrices for the semitransparent materials with high subsurface scattering

The experimental set-up is developed for detection of reflection and transparence coefficients of semitransparent materials within visible and IR radiation diapasons. The peculiarity of the set-up is using the spectral photometric integrating spheres for effect compensation of reflection indicatrices of tested materials with fixed wavelengths 0.63 µm 1.15 µm (near IR), 2.79 µm and 3.39 µm (middle infrared) of probing laser radiation. In the first case the integrating sphere is a photometric ball which made from porous fluoroplastic with high reflectivity up to 97-99%. For the second range an aluminum sphere (sand blasting) was used. The application of the photometric sphere reduced the measurement error, connected with different sensitivity of photosensors to beam slope angle of detected radiation reflected (transmitted) for flat material samples with different indicatrices of reflection (transmission). The proposed method of experimental measurements of spectral photometric characteristics of reflection and transparent improve the accuracy of theoretical estimations of the optical (thermoradiation) parameters (absorption and scattering indexes), as well as the functions of a thermal radiation heat source for the calculation of temperature fields in semitransparent materials. For the first time, a computational and experimental estimation of scattering and absorption indexes (with an accuracy not less than 1%) were carried out for semitransparent materials with high scattering (porous ceramics based on stabilized zirconium dioxide) in the middle infrared range of wavelengths which is most problematic for optical measurements.

Fuel-rich aluminum–nickel fluoride reactive composites

Composite powders comprising aluminum and nickel fluoride are prepared by arrested reactive milling. All compositions are fuel-rich, with 50–90 wt% aluminum. Adequate mixing between components and enhanced reactivity is achieved for up to 70 wt% Al. Materials were characterized using electron microscopy, thermal analysis, as well as custom ignition and combustion experiments. Ignition temperatures were lower for powders with 50 and 70 wt% of Al compared to pure Al. Correlation of thermal analysis and ignition tests suggests that exothermic fluorination of Al with an activation energy of ca. 80 kJ/mol likely governs ignition at high heating rates. In air, the composites with 50 and 70 wt% of Al burn faster than spherical aluminum with comparable particle sizes. The flame temperature for 50Al·NiF2 composite does not exceed the Al boiling point suggesting a purely heterogeneous reaction. For 70Al·NiF2 composites, higher flame temperatures are measured suggesting the presence of vapor-phase reactions. In constant volume explosion experiments, combustion of 70Al·NiF2 occurred with shortest ignition delays and resulted in the pressures matching or exceeding those of pure Al, while also essentially matching the pressures predicted for this composite by equilibrium calculations. The burn times observed for the composite powders injected into an air–acetylene flame are similar to those of pure aluminum; it is hypothesized that side reactions of NiF2 with CO, CO2 and H2O diminish its effectiveness as an oxidizer for Al in these environments.

Constructing a “pea-pod-like” alumina-graphene binary architecture for enhancing thermal conductivity of epoxy composite

Constructing a three-dimensional thermal transport framework is an effective strategy for enhancing thermal conductivity of polymer composites. Vacuum-assisted filtration has been utilized as an effective method to prepare heat spreader films, which always exhibit poor heat dissipation performance in the axial direction. Herein, with the incorporation of spherical alumina, a pea-pod-like binary alumina-graphene architecture has been fabricated via vacuum filtration and impregnated with epoxy to obtain alumina-graphene/epoxy composite. The thermal conductivity of epoxy composite in axial direction has risen to 13.3 W m−1 K−1 with the assistance of pea-pod-like binary alumina-graphene foam with 12.1 wt% graphene and 42.4 wt% alumina loading. Thermal conductivity of the composite in radial direction reached 33.4 W m−1 K−1, which exhibits about 166 times enhancement than that of pure epoxy. The test result of the thermal conductivity of the composite under various conditions and heat transport applications confirmed the excellent thermal transport performance of the composite. Our work provides a new idea to significantly enhance the thermal conductivity of thermal management materials.

Size scaling of hypervelocity-impact ejecta mass and momentum enhancement: Experiments and a nonlocal-shear-band-motivated strain-rate-dependent failure model

Impact tests were performed from 4 to 5.77 km/s of 3.0-cm-diameter aluminum spheres impacting 2024-T351 aluminum targets. This data was compared to previously published data for smaller scale impactors (0.1588 cm to 1.27 cm). It was seen that there are no size-scaling effects for the crater depth. The slight size-scaling effect in the crater diameter directly correlated with the nonlinear size-scaling effect in the ejecta mass. There is a qualitative change in the ejecta mass somewhere in the vicinity of 1.27-cm-diameter impactors, in that impactors with diameters less than this have ejecta mass showing a strong diameter dependence while for impactors larger than this diameter there is little size dependence and the ejecta mass scales only with impact velocity squared. A strain-rate-dependent failure model is developed based on a mechanical model of shear-band slip over a characteristic finite length. The underlying mechanical model is nonlocal, but, through certain assumptions, leads to a failure strain expression that depends on the equivalent plastic strain rate to the 2/3rds power. This failure model was implemented in the hydrocode CTH and showed excellent agreement with experimental ejecta mass data. It can explain the origin of the size scale dependence in the ejecta mass as well as the observed saturation. The saturation in the ejecta mass is due to the static failure properties of the material. The new data also shows that the 0.45 power dependence on the projectile diameter for the momentum enhancement β continues up through the 3.0-cm-diameter sphere impactors. This size scaling behavior of the momentum enhancement is surprising in that there is not a change in the momentum enhancement scaling behavior even though there is a qualitative and quantitative change in the ejecta mass scaling behavior. The new damage model does not predict the experimentally-observed momentum enhancement size-scaling behavior, even though it does predict the ejecta mass behavior.

Obtainment of the alumina polyamide-coated spherical particles

The indirect selective laser sintering (SLS) process consists of a powder-based additive manufacturing (AM) technique. Recently, an attempt was carried out using the indirect-SLS method on ceramics to obtain complex geometric parts without the need to fabricate tools or molds. In this context, the ceramic particles employed in indirect-SLS must be coated with a polymer layer and retain spherical format. Currently, several processes of encapsulation and balling up of ceramic particles are available, among which thermally induced phase separation (TIPS) is noteworthy. The present study describes the synthesis, via TIPS, of spherical micrometric alumina particles coated with polyamide 12 to be employed in indirect-SLS. Dimethyl sulfoxide (DMSO) was used as a solvent to obtain these particles. After encapsulation, the agglomerates were characterized according to granulometry, microstructure, thermal behavior, and flowability. The obtained results delineated the ideal conditions to synthesize agglomerates with adequate flowability, particle size profile, and similar thermal behavior to that verified for PA-12, a currently used material in SLS printing. Based on the results of the present study, it is possible to confirm the effectiveness of the TIPS technique in obtaining ceramic polymer-coated spherical particles that are suitable for use in AM processes via indirect-SLS.

Impulse Generation and Stabilization Performance of a Doughnut–Spherical Laser Launch System

A conceptual laser launch system of a spherical vehicle propelled by a doughnut-shaped laser beam is expected to accelerate the spherical vehicle continuously to the orbit velocity along a laser beam. Once lateral offset between the axis of the laser beam and the center of the sphere is generated, lateral force is generated to recede the offset. In this study, a pulsed Neodymium-doped Yttrium Aluminum Garnet laser beam of a doughnut-shaped cross section is focused on a 10-mm-diameter aluminum sphere. The impulse components, parallel and lateral to the laser beam, are first investigated experimentally using a pendulum method. As a result, the ratio of the doughnut beam radius to the target radius is found to have a major impact on the impulse. For a relatively small value of at 0.56, at atmospheric pressure, the impulse is enhanced due to the focusing of the toroidal blast wave, whereas the shock focusing effect is negligible for a larger value of at 0.86. On the other hand, must be larger than 0.83 to generate the restoring lateral force.

Novel design of high-surface-area poly-hollow alumina spheres via a facile ball dropping method based on particle-stabilized emulsions

Herein, the preparation of millimeter-sized poly-hollow alumina spheres (PHASs) with high-surface-area is proposed via an innovative and facile ball dropping method based on particle-stabilized emulsions. The PHASs fabricated by this method possess high specific surface area because of their hierarchical pore structures and high porosity, and exhibit remarkable superiority over the commercially available active alumina materials with specific surface areas of 200–300 m2g-1. The resultant millimeter-sized PHASs have diameters in range of 2–6 mm, average mesoporous sizes of 4–8 nm, and excellent specific surface areas of 400–500 m2 g-1, which are achieved with oil-water volume ratio of 3:1, rotation speed of 900 rpm, and sintering temperature of 500 °C. These high-performance PHASs with ultra-high specific areas and hierarchical pore structures can potentially be useful in various applications.

Effects of internals on phase holdup and backmixing in a slightly-expanded-bed reactor with gas–liquid concurrent upflow

Five different internals were designed, and their effects on phase holdup and backmixing were investigated in a gas–liquid concurrent upflow reactor where the spherical alumina packing particles of three diameters (3.0, 4.5 and 6.0 mm) were slightly expanded under the conditions of varied superficial gas velocities (6.77 × 10−2 - 3.61 × 10−1 m·s−1) and superficial liquid velocities (9.47 × 10−4 - 2.17 × 10−3 m·s−1). The experimental results show that the gas holdup increases with the superficial gas velocity and particle size, opposite to the variational trend of liquid holdup. When an internal component is installed amid the upflow reactor, a higher gas holdup, a less liquid holdup and a larger Peclet number characterizing the weaker backmixing are obtained compared to those in the bed without internals under the same operating conditions. Additionally, the minimal backmixing is observed in the reactor equipped with the internals with a novel multi-step design. Finally, empirical correlations were proposed for estimating gas holdup, liquid holdup and Peclet number with the relative deviations within 11%, 12% and 25%, respectively.

Three-dimensional imaging of crack growth in L chondrites after high-velocity impact experiments

Small asteroids such as Itokawa are covered with an unconsolidated regolith layer of centimeter-sized or smaller particles. There are two plausible formation mechanisms for regolith layers on a sub-kilometer-sized asteroid: (i) fragments produced by thermal fatigue by day-night temperature cycles on the asteroid surface and (ii) impact fragment. Previous studies suggest that thermal fatigue induces crack growth along the boundary surface of the mineral grain while impact phenomena may induce crack growth regardless of the boundary surface of the mineral grain. Therefore, it is possible that the crack growth within a mineral grain (and/or a chondrule) differs depending on the crack formation mechanism, be it thermal fatigue or an impact.In order to investigate how mineral grains and chondrules are affected by impact-induced crack growth, we fired spherical alumina projectiles (diameter ~1 mm) into 9 mm side length cubic targets of L chondrites at a nominal impact velocity of 2.0 km/s. Before and after the six successful impact experiments, the cracks within mineral grains and chondrules in the respective targets are examined using X-ray microtomography at a resolution with the voxel size of 9.0 μm. The results show that most cracks within chondrules and troilite (FeS) grow regardless of the boundary surfaces of the grains while most cracks within ductile Fe-Ni metal grow along the boundary surfaces of the grains. This may indicate that crack growth is largely affected by the strength of mineral grains (and/or chondrules). From the experimental results and the fact that the shapes of polymineralic and monomineralic particles from Itokawa are similar, we conclude that the Itokawa particles have not been produced by thermal fatigue but instead are likely to be impact fragments, as described in previous papers (Tsuchiyama et al., 2011, 2014; Michikami et al., 2018).

A three dimensional modeling method for spherical open cell aluminum foams based on spherical core stratification algorithm

Because of the complexity and irregularity of the internal structure of aluminum foam, it is difficult to establish a three-dimensional model that can accurately reflect this structure. In this study, an algorithm, named spherical core stratification algorithm, for three-dimensional modeling of spherical aluminum foam was proposed, and by using this algorithm, a three-dimensional model for sphere aluminum foam with random pores has been successfully constructed. The constructed model not only has a high similarity with the real structure of spherical open cell aluminum foam, but also can match its pore size and thickness by adjusting the size and number of holes in the random pore. In order to verify the feasibility of the modeling method, firstly, the three-dimensional model of the cylindrical spherical aluminum foam with a size of ∅35 mm × 20 mm and pore diameter of 5 mm has been generated by using the new algorithm. Secondly, taking the influence of relative density and shape function on the compressive properties of spherical open cell aluminum foams into consideration, a quasi-static constitutive model suitable for the material has been established based on the Sherwood-Frost classical compression constitutive model, which provides material parameters for quasi-static compression simulation. The comparison results show that the established constitutive equation has a good fit with the experiment, with a fitting correlation coefficient of above 0.99. Finally, the quasi-static compression simulation was carried out by ABAQUS, and the simulated nominal stress-strain curve was obtained. The simulation results indicate that the simulated stress-strain curve had the same trend with the one obtained by the quasi-static compression experiment with a small deviation.