Fine Al Particles Research Articles

Conjugate heat and mass transfer at heterogeneous burning surface of AP/HTPB/Al composite propellant at 2–10 MPa

Conjugate heat and mass transfer at heterogeneous burning surface has significant effect on combustion characteristics of AP/HTPB/Al composite propellant. In the present study, the conjugate heat and mass transfer characteristics of AP/HTPB/Al composite propellant are studied experimentally and numerically based on formulated combustion model and experimental facilities. With the increase of pressure and Al content, flame temperature increases significantly and leads to stronger heat transfer to the burning surface. With the content of Al varying from 18 wt% to 9 wt%, the total heat transfer coefficient decreases for all the sampled positions of the burning surface with a maximum decrease of 12% at the AP/HTPB interface. It is found that heat convection accounts for more than 97% of the total heat transfer coefficient. However, with pressure increasing from 2 MPa to 10 MPa, the total heat transfer coefficient and the respective contribution of heat convection and radiation vary slightly. Owing to the heterogeneous sandwich-like structure of the propellant, the temperature distribution at the burning surface of propellant is highly non-uniform. Therefore, heat transfer characteristics vary significantly at different positions of the burning surface. The heterogeneity of the burning surface is exacerbated by the non-uniform distribution of heat transfer coefficients at different positions of the burning surface. The burning rate of the propellant varies from 0.5 mm/s to 9.5 mm/s for different positions of the burning surface at 10 MPa. The results of micro-scale flame structure show that the mass transfer of fine Al particles in the combustion of AP/HTPB/Al composite propellant can be basically divided into four stages, i.e., migration, melting, agglomeration and oxidation/combustion. Coral reef-like aggregate structure is formed by some of the fine Al particles before oxidation and combustion. Increasing pressure is conducive to avoiding formation of large Al agglomerates and enhancing mass transfer at the heterogeneous burning surface.

Combustion characteristics of cross-linked fluorinated polymer supported aluminum/oxidizer microsphere in HTPB propellant

Combustion behaviors of propellant depend heavily on the architecture of the pocket bounded by coarse AP particles. In order to achieve a better control of the meso-scale architecture of the pocket, overcoming random mixing of the fine particles inside propellant, two kinds of cross-linked fluorinated polymer supported Aluminum/Oxidizer microspheres containing fine Al and oxidizer particles are fabricated by emulsion solvent evaporation and in-situ polymerization method (ESV-ISP), which are used to replace the fine Al and oxidizer particles of the Al/AP/HMX/HTPB propellant. The morphologies of the Al/Oxidizer microspheres and the corresponding propellants are studied, it shows that the fine Al and oxidizer particles are well dispersed and arranged in the microspheres. Besides, the average sizes of the microspheres are ranging from 113.9 ± 4.3 μm to 183.0 ± 5.9 μm, and equivalent to that of the Al-rich clusters located inside the pocket. The energy property and combustion performance of the propellants indicated that the combustion heat of the propellants containing Al/Oxidizer microsphere is increased by about 200 J/g, while the fraction of residual active aluminum is decreased by 77.8%, additionally, the D50 and D(4,3) of the combustion residue are both reduced significantly. Large Al aggregate nearby the burning surface is observed clearly in the blank propellant, while it could not be observed in the propellants containing Al/Oxidizer microsphere. Furthermore, compared to the pressure exponent (n), 0.34, of the B-Propellant, the n of the Al/AP-Propellant and the Al/HMX-Propellant significantly drop to 0.24 and 0.25, respectively, and the burning rate (r) could be adjusted and controlled by the Al/Oxidizer microsphere consisting of different fine oxidizer particles. It demonstrates that using the structurally ordered Al/Oxidizer microsphere could successfully achieve a better control of the meso-scale architecture of the pocket in typical Al/AP/HMX/HTPB propellant.

Influence of Force Chains Behaviors on Strength of Al/W/PTFE Granular Composite

Mesoscale simulation is conducted to investigate the effect of force chains between metal particles on the mechanical behavior of aluminum-tungsten-polytetrafluoroethylene (Al/W/PTFE) granular composite under a strain-controlled loading. A two-dimensional model followed the random distribution of particles is developed. Dynamic simulations are performed with variations in the size of Al particles to reveal the strength and fracture mechanisms of the composites. The results indicate that, force chains governed by the number and the size of metal particles significantly affects the global compressive response and macro-cracks propagation. The stability and reconstruction of mesoscale force chains explain the phenomenon that a higher strength is observed in the material with fine Al particles. Combined with the angle between particles, we examine the properties of force chains and the network as they evolve during the course of the deformation. Findings indicate that reactive composites tend to produce shorter chains, and straighter force chains with a smaller force angle result in a macroscopically stronger granular material.

Influence of poly(vinylidene fluoride) coating layer on exothermic reactivity and stability of fine aluminum particles

Simultaneous achievement of both reactivity and stability to oxidation is crucial in practical applications of reactive aluminum (Al) powders, which can be used for propellant and explosives. In this study, we introduce a poly(vinylidene fluoride) (PVDF) coating layer a few tens of nanometers thick onto the surface of fine Al particles instead of alumina (Al2O3). The PVDF-coated Al particles exhibited hydrophobic surface behavior and a characteristic interface where Al fluorides were spontaneously formed. The exothermic enthalpy of PVDF-coated Al particles was approximately two times higher than that of the uncoated particles owing to the synergetic effect of the active formation of Al-F and Al-O reactions. Furthermore, the accelerated oxidation test revealed that the enhanced reactivity was retained even under high humidity. Thus, the PVDF coating layer protects pure Al core material from explosively reacting with external oxygen, simultaneously increasing exothermic enthalpy.

Synergetic enhancement in the reactivity and stability of surface-oxide-free fine Al particles covered with a polytetrafluoroethylene nanolayer

Surface oxide (Al2O3) of reactive fine aluminum (Al) particles for solid fuels, propellants, and brazing materials often restricted oxidative performance, though the passivation film acts to protect Al particles from exploding. Here, we report fine Al particles fully covered with a polytetrafluoroethylene (PTFE) layer instead of an Al2O3 film on the surface. This advance is based on the introduction of strong Al–F bonds, known to be an alternative to the Al–O bonds of surface oxides. The DSC results on the PTFE-coated Al particles exhibit higher reactive-exothermic enthalpy energy (12.26 kJ g−1) than 4.85 kJ g−1 by uncoated Al particles. The artificial aging test of the PTFE layer on the Al particles show long-time stability to the external circumstance compared to those by Al2O3. The activation energy for oxidation was investigated from cyclic voltammetry assessment and the measured peak potentials of the anode curve for PTFE/Al (− 0.45 V) and uncoated Al (− 0.39 V) are achieved, respectively. This means that the PTFE layer is more stable against a sudden explosion of Al particles compared to Al2O3. These results are very useful given its capability to control both the reactivity and stability levels during the oxidation of Al particles for practical applications.

Self-Propagating Heat Synthetic Reactivity of Fine Aluminum Particles via Spontaneously Coated Nickel Layer

Aluminum powders are known to provide outstanding volumetric exothermic enthalpy energy during thermal oxidation. However, the amount of energy released tends to be limited by the dense surface oxide (Al2O3) layer of the powder. Hence, a prerequisite for improving the reactivity of passivated Al particles is to remove the Al2O3 film from the surface. Considering that the self-propagating high-temperature synthesis (SHS) reaction of Ni and Al can generate additional exothermic heat in Al powder, Ni can be considered as a promising alternative to the surface oxide layer. Here, we report oxide-layer-free fine Al particles with a characteristic Ni/Al interface, where a Ni layer replaces the Al2O3 film. The microstructure of the synthesized powder consists of a 200-nm-thick Ni layer homogeneously coated on the Al surface, which has nanosized craters caused by the geometrical removal of Al2O3. Thermal analysis and in-situ heating transmission electron microscopy (TEM) results clearly show that active interdiffusion of atoms through the Ni/Al interface results in the formation of intermetallic compounds to provide additional exothermic energy, compared to the result for simply mixing Ni and Al powders. Hence, these findings provide new routes for the design and application of reactive metallic particles using the SHS reaction.

Splat Morphology and Influence of Feeding Rate During Reactive Plasma Spray of Aluminum Powder

Fabrication of aluminum nitride (AlN) coatings using conventional plasma spraying processes directly has been deemed impossible. It is attributed to the thermal decomposition of the AlN feedstock particles during spraying without a stable melting phase. Using the reactivity of the plasma (reactive plasma spraying: RPS) showed a promising consideration for in situ formation of AlN thermally sprayed coatings. Several AlN-based coatings were fabricated through the RPS of aluminum powders in the N2/H2 plasma. The focus of this study is in discussing the morphology of splat deposition during the nitriding of Al particles. Furthermore, the influence of the feeding rate during the RPS and nitriding of Al powders will be investigated. The nitride content, as well as the unreacted molten Al phase, strongly influences splat deposition and morphology during the RPS of Al. The collected splats can be divided into reacted, partially reacted, and unreacted splats. The reacted splats tend to show a disk or egg-shell shape. The partially reacted mainly had outside nitride shells and an unreacted molten Al part in the center. The unreacted splats tended to show a splash shape. The main controlling factor is the time of the droplet impact on the substrate during the reaction sequence. The particle size and spray distance showed significant effects on the splat formation due to their effect on the nitriding conversion and the melting behavior of the particles during RPS nitriding. The powder feeding rate was investigated through increasing the injection rate and by using a low carrier gas flow rate. Increasing the powder feeding rate significantly improved the coating thickness. However, it suppressed the nitriding conversion of the large Al particles. Thus, with increasing the amount of the powder in the plasma, the Al molten particles are easily aggregated and agglomerate together upon colliding on the substrate with an AlN shell on the surface. This prevents the N2 from having access to all of the aggregated particles. Therefore, the fabricated coatings using large Al particles consist of surface AlN layers and the central parts of AlN and Al composite layers. On the other hand, it was possible to fabricate about 500-μm-thick AlN coatings using fine Al particles of 15 μm and increasing the feeding rate. Using the fine particles improved the nitriding reaction due to the improvement of the surface area (the reaction area). Moreover, the nitriding process of the Al particles with increasing the feeding rate was also investigated.

Effect of surface tension on the temperature of burning metal droplets

An effect of Laplace pressure resulting in elevated boiling temperatures for fine burning metal droplets is discussed. The effect is evaluated for both Al and Mg. An increase in the boiling temperature suggests that the temperature of fine Al particles can become equal to or even higher than the temperature of the vapor phase Al flame. The high particle surface temperature may be associated with accelerated surface reaction rates. For Al, such surface reactions may generate vapor-phase aluminum suboxides, resulting in a reduced heat effect and extended burn times for fine particles and nanoparticles. For Mg, the particle temperature remains below the vapor phase flame temperature despite an elevated boiling point. For fine particles, surface reaction producing solid MgO growing directly on the particle surface is possible.

Preparation of size-controlled fine Al particles for application to rear electrode of Si solar cells

Micron-sized aluminum (Al) particles are widely used in the fabrication of rear electrodes of Si solar cells. Moreover, the rear electrode of Si solar cells can be fabricated at relatively low electrode firing temperature using submicron Al particles whose sizes can be easily controlled, but there have not been any clear methods to obtain submicron Al particles yet. In this study, we first successfully prepared size-controlled Al submicron particles via a wet chemical process using dibutyl ether solvent and then to fabricate rear electrodes of Si solar cells; to our knowledge, this is the first such application of submicron Al particles. The geometric mean diameter ( D p) of the Al particles could be controlled from 139 to 614 nm by adjusting the reaction temperature, and the prepared Al particles showed geometric standard deviations ( σ g) of 1.25–1.30. The Al particle size was reduced to ∼35 nm by adding an organic surfactant to the precursor solution for Al particle preparation. Rear electrodes were fabricated by firing the screen printed Al paste films comprising Al particles with geometric mean diameters of ∼379 nm from 600 to 900° C. The electrode fired at 750 °C showed the minimum electrical sheet resistivity of 77.9 mΩ/□ (specific resistivity: 97.3 μΩ cm) and contained a BSF (back surface field) with a thickness of ∼3 μm. Our results indicate that the reported method can be used to minimize the thermal defects in the rear electrode of Si solar cells by lowering the electrode firing temperature.

Reactive Atmospheric Plasma Spraying of AlN Coatings: Influence of Aluminum Feedstock Particle Size

Feedstock powder characteristics (size distribution, morphology, shape, specific mass, and injection rate) are considered to be one of the key factors in controlling plasma-sprayed coatings microstructure and properties. The influence of feedstock powder characteristics to control the reaction and coatings microstructure in reactive plasma spraying process (RPS) is still unclear. This study, investigated the influence of feedstock particle size in RPS of aluminum nitride (AlN) coatings, through plasma nitriding of aluminum (Al) feedstock powders. It was possible to fabricate AlN-based coatings through plasma nitriding of all kinds of Al powders in atmospheric plasma spray (APS) process. The nitriding ratio was improved with decreasing the particle size of feedstock powder, due to improving the nitriding reaction during flight. However, decreasing the particle size of feedstock powder suppressed the coatings thickness. Due to the loss of the powder during the injection, the excessive vaporization of fine Al particles and the completing nitriding reaction of some fine Al particles during flight. The feedstock particle size directly affects on the nitriding, melting, flowability, and the vaporization behaviors of Al powders during spraying. It concluded that using smaller particle size powders is useful for improving the nitriding ratio and not suitable for fabrication thick AlN coatings in reactive plasma spray process. To fabricate thick AlN coatings through RPS, enhancing the nitriding reaction of Al powders with large particle size during spraying is required.

Effects of Fine Particle Content in Al Paste on Screen Printed Contact Formation and Solar Cell Performance

Al screen printed Si solar cells are fabricated in a belt system, and the impact of the fine Al particle content in the Al paste on the alloying process in relation to solar cell performance is investigated. Scanning electron microscopy and transmission electron microscopy analyses show that a lack of fine Al particles in the Al paste causes nonuniform back surface field (BSF), Al-rich extrusions at the Al–Si alloy/Si interface with faceting along the {111} planes of Al, and unmelted Si-rich rods in the Al–Si alloy layer after alloying results in lower , , and fill factor. The quality of the BSF and the Al–Si alloy layer could thus be improved by optimizing the fine Al particle content in the Al paste. Of the four pastes with different fine Al particle contents prepared for comparison, the content of 27 wt % renders the best contact quality, whereas an even higher content would cause a finer-grained polycrystalline Al–Si alloy to form and the fired Al layer to peel off from the Al–Si alloy layer.

Effect of the powder deposition rate on the mechanical properties and the structure of the APS-NiCr/Al coating

Effect of the powder deposition rate on the mechanical properties and the structure of the APS-NiCr/Al coating

Attenuation of pulsed CO2 laser radiation in an argon medium containing fine aluminum particles

The attenuation of 10.6-μm radiation by aluminum aerosol particles dispersed in argon is studied experimentally. An optimal method of forming the initiating beam is found. The size and concentration of fine Al particles in the medium are estimated.

Improvement of surface finish on SKD steel using electro-discharge machining with aluminum and surfactant added dielectric

Electrical discharge machining (EDM) process is widely used to process hard materials in the industry. Electrical discharge distribution effects can be achieved by the addition of Al powder in the dielectric. A fine surface roughness value of the workpiece is thus obtained. However, the electrostatic force among fine Al particles is found to agglomerate the Al powders in the dielectric. A surfactant can be adopted to separate the Al powder in the dielectric homogenously. A better surface even the mirror-like quality of the EDMed workpiece is thus desired. In the study, the effect of surfactant and Al powders added in the dielectric on the surface status of the workpiece after EDM is investigated. It is observed the best distribution effect is found when the concentrations of the Al powder and surfactant in the dielectric are 0.1 and 0.25 g/L, respectively. An optimal surface roughness (Ra) value of 0.172 μm is achieved under the following parameter—positive polarity, discharge current 0.3 A, pulse duration time 1.5 μs, open circuit potential 140 V, gap voltage 90 V and surfactant concentration 0.25 g/L. The surface roughness status of the workpiece has been improved up to 60% as compared to that EDMed under pure dielectric with high surface roughness Ra of 0.434 μm.

Preparation of CoAl intermetallic compound by combustion synthesis in self-propagating mode

The production of cobalt aluminide (CoAl) from elemental powder compacts was conducted by self-propagating high-temperature synthesis (SHS) in this study. Effects of the initial sample density, particle size of the reactants, and initial sample temperature on the combustion characteristics, as well as on the composition and morphology of final products were studied. Unlike most of the intermetallic reactions, the combustion on the CoAl powder compact is self-sustained upon initiation even without any preheating prior to ignition, implying that the reactions are highly exothermic. The combustion process was preceded by the fast propagation of the flame-front, followed by vigorous bulk combustion. The flame-front propagation velocity and combustion temperature were found to increase with increasing sample green density. The use of fine particles of Al (10 μm) also increased the flame-front velocity. In addition, the combustion products synthesized from the samples using fine Al particles were slightly shrunk or retained the original shape. However, when the coarse particles of Al (350 mesh) were used the sample exhibited a volume expansion after combustion, resulting in an increase in the sample porosity. According to the XRD analysis, combustion products obtained in this study were all made up of a single-phase composition of CoAl without the presence of any secondary phases or unreacted constituent elements. Based upon the measured data of this study, the activation energy with a value of 121.7 kJ/mol was deduced for the synthesis of CoAl by SHS.

High-performance Ni3Al synthesized from composite powders

Specimens of Ni3Al + B of high density (>99.3 Pct RD) and relatively large dimension have been synthesized from composite powders through processes of replacing plating and electroless Ni-B plating on Al powder, sintering, and thermal-mechanical treatment. The uniformly coated Ni layer over fine Al or Ni core particles constituting these coating/core composite powders has advantages such as better resistance to oxidation relative to pure Al powder, a greater green density as a compacted powder than prealloyed powder, the possibility of atomically added B to the material by careful choice of a suitable plating solution, and avoidance of the expensive powder metallurgy (PM) equipment such as a hot isostatic press (HIP), hot press (HP),etc. The final Ni3Al + B product is made from Ni-B-Al and Ni-B-Ni mixed composite powders by means of traditional PM processes such as compacting, sintering, rolling, and annealing, and therefore, the dimensions of the product are not constrained by the capacity of an HIP or HP. The properties of Ni3Al composite powder metallurgy (CPM) specimens tested at room temperature have been obtained, and comparison with previous reports is conducted. A tensile elongation of about 16 Pct at room temperature was attained.

メカノフュージョンプロセスにより作製したＮｉ‐Ａｌ複合粉とそのプラズマ溶射皮膜の性質

The Ni-55%Al and -20%Al composite powders were prepared by the mechanofusion process and were plasma sprayed on mild steel substrates in air. The properties of the powders and the sprayed coatings were examined, and were compared with those of several commercial powders and their coatings. The fine Al particles adhered well to the Ni core particles, but did not homogeneously coat them while mhanofusing. The mechanofused powders were found to differ from the commercial products regarding morphology and microstructure. The intermetallic compounds were not formed in the coatings in any case of air plasma spraying because of partial oxidation of the components in the powders and short reaction time for forming aluminides. The microstructure, hardness, adhesion strength and abrasive wear resistance of the coatings, which were plasma sprayed with the mechanofused powders, were almost the same as those of the coatings obtained by using the commercial powders.

Optical properties of very fine Al particles: Quantum size effect

The strong attenuation of the parallel-band absorption observed in very fine Al particles at 0.85 μm is interpreted in terms of a quantum mechanical size effect. Applying a quantum theoretical model, the dielectric constant for fine particles is calculated, and the effective dielectric constants for aggregated films are obtained classically from the Maxwell–Garnett theory. Calculated results of transmittance for Al blacks in the spectral range from 0.35 to 1.0 μm are found in very good agreement with experimental results.

Log-normal size distributions from magnetization measurements on small superconducting Al particles

Field-dependent magnetization of superconducting fine Al particles has been measured down to 0.5 K with a SQUID system. The results can be explained in considerable detail by the existence of a log-normal distribution of particle size.