Nanosized Alumina Powder Research Articles

Low temperature phase transition mechanism of amorphous Al-oxalate to nano α-alumina

α-Alumina (α-Al2O3), widely used as a raw material or production in advanced manufacturing industry, has attracted much attention in terms of its preparation temperature. A simple dissolution-evaporation-precipitation process was employed to obtain the amorphous Al-oxalate (AAO) precursor, serving as the starting material for the low temperature preparation of nano α-Al2O3. The crystalline structure and thermal behavior of the AAO were compared with commercially available aluminum hydroxide (Al(OH)3) powder using the X-ray powder diffraction (XRD) instrument, the thermal gravity analysis and differential scanning calorimetry (TG-DSC). The phase transition mechanism of AAO was investigated by the fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), scanning electron microscope (SEM), and 27Al magic angle spinning nuclear magnetic resonance (27Al-MAS-NMR). The DSC results indicated that AAO underwent a phase transformation at 982 °C. The α-alumina diffraction peaks appeared in AAO at 1000 °C, while α-Al2O3 phase was observed for Al(OH)3 at 1200 °C. Furthermore, an absorption peak at 452 cm−1, corresponding to Al–O bond in α-alumina was detected at 1000 °C. The proposed low-temperature preparation mechanism of α-alumina from AAO is as follows: four coordination Al-oxalate species entities coexist within the AAO sample; with increasing temperature, organic acid functional groups oxidize and volatilize, resulting in the formation of tetra-coordinate [AlO4], penta-coordinate [AlO5], and hexa-coordinate [AlO6] atomic groups; these atomic clusters further polymerize to form a structurally dense α-alumina crystal at the nano-scale (∼50 nm) at around 1000 °C. The experimental findings present a novel approach and method for the low-temperature preparation of nano-sized alumina powder. Additionally, a feasible theoretical reference for the low-temperature phase transition mechanism of amorphous Al-oxalate was provided.

Improvement of the Hydrogen Storage Characteristics of MgH2 with Al Nano-Catalyst Produced by the Method of Electric Explosion of Wires.

A new composite with a core-shell structure based on magnesium hydride and finely dispersed aluminum powder with an aluminum oxide shell was mechanically synthesized. We used magnesium chips to produce magnesium hydride and aluminum wire after exploitation to produce nano-sized aluminum powder. The beginning of the hydrogen release from the composite occurred at the temperature of 117 °C. The maximum desorption temperature from the MgH2-EEWAl composite (10 wt.%) was 336 °C, compared to pure magnesium hydride-417 °C. The mass content of hydrogen in the composite was 5.5 wt.%. The positive effect of the aluminum powder produced by the electric explosion of wires method on reducing the activation energy of desorption was demonstrated. The composite's desorption activation energy was found to be 109 ± 1 kJ/mol, while pure magnesium hydride had an activation energy of 161 ± 2 kJ/mol. The results obtained make it possible to expand the possibility of using magnesium and aluminum waste for hydrogen energy.

Study on the oxidation mechanism of micron- and nano-sized aluminum powders influenced by alumina shell

Thermal behaviors for aluminum particles at oxidative atmosphere gradually heating system were discussed. Heating micron- and nano-sized aluminum powders from room temperature to 1400℃ using thermal analysis method. The results showed that the reaction process for micron-sized aluminum particles can be divided into four stages which include the phase change of oxidized shell, increasing thickness of alumina shell, main broken shell oxidizing process and the phase change of alumina shell in high temperature. Thermal analysis for micro aluminum particles was carried out with shell thickness adjustments via ambient manual means. It turned out that the oxide layer thickness would have full limitation effects on the slow oxidation for micro aluminum particles with shell structure when it was increased above twice of the original thickness. The limitation effects of oxide shell thickness on the heating processes in gradually heating system were proposed. Humidity environment can make the thickness of alumina increase in a certain range when the particles are in nano-scale. Complete oxidation processes were observed but an increase of the oxidation temperature was identified.

Effects of K2SiF6 on Ignition and Heat Release Characteristic of Nano-Sized Aluminum Powders

ABSTRACT This study investigates the effect of potassium fluosilicate (K2SiF6) on the ignition and heat release characteristics of n-Al50, n-Al100, and n-Al800. It is found that K2SiF6 can effectively improve the ignition and heat release characteristics of nano-aluminum powder (n-Al). This is because K2SiF6 effectively increases the reaction rate. The heat release characteristics of the n-Al/K2SiF6 mixture are improved as the particle size of aluminum powder decreases and the heating rate increases. However, when the particle size reaches a certain level, the enhancement effect will become smaller and smaller. The study revealed that the addition of K2SiF6 influences the ignition delay time, reaction, and combustion products of the n-Al/K2SiF6 mixture. A small amount of K2SiF6 can greatly reduce the ignition delay time of nano-aluminum powder. With the increase of K2SiF6 content, the ignition delay time of n-Al50 decreases first and then increases, while the ignition delay time of n-Al100 and n-Al800 decreases continuously but the extent of this reduction progressively diminishes. The morphology and composition of the combustion products were analyzed. This paper found that K2SiF6 fluorinates the oxidized shell of aluminum, thereby promoting its ignition and combustion. The influence mechanism of K2SiF6 on n-Al ignition in air is discussed.

Molecular dynamics study on the thermal decomposition of 1,3,5-trinitro-1,3,5-triazinane (RDX) catalyzed by aluminum nanoparticles with different contents

The mechanism of thermal decomposition of 1,3,5-trinitro-1,3,5-triazinane (RDX) catalyzed by nano-sized aluminum powder remains unclarified. In the present study, reactive molecular dynamics simulations using the parameterized reactive force field with low gradient correction (ReaxFF-lg) were conducted to study the microscale process of the RDX thermal decomposition catalyzed by different Al contents, and the density functional theory calculations were used to analyze the initial decomposition pathways of RDX on the Al surface. The thermal disintegration and the released energy of RDX are significantly facilitated by an increase in Al concentration at a lower level and the optimal value is around 35 wt%, while a higher concentration of Al will lead to an opposite effect. The dissociation of nitro group is the most probable initial reaction pathway for RDX decomposition on the Al surface. In addition, the evolutions of Al-containing clusters, key intermediates, and final products were analyzed, providing more information about this reaction. The findings suggest that the addition of Al nanoparticles improves the properties of RDX-based explosives, but the optimal amount of Al is critical for achieving the desired effects. The detailed analysis of the reaction mechanism and intermediate products provides valuable insights into the underlying mechanisms of the Al/RDX system.

Laser Ignition of Ammonium Perchlorate/Aluminum Composition Confined into PMMA Capsule

AbstractA two‐component composition of ammonium perchlorate and nanosized aluminum powder can be ignited by millisecond Nd laser radiation. However, the mixture exposed to laser radiation pulse through polymethyl methacrylate (PMMA) capsule demonstrates less sensitivity when compared to ignition of the uncovered sample. Such behavior is uncommon to gasified energetic materials. It is shown that the sensitivity of uncovered samples does not change if the surrounding atmosphere is changed from air to argon. It was found that the cover of PMMA decreases the sensitivity of the mixture due to heat dissipation from the ignition zone, thereby reducing its reactivity.

Fabrication and Mechanical Properties of Mo-Al2O3 Cermets by Using Ultrafine Molybdenum and Nano-sized Alumina Powders

Fabrication and Mechanical Properties of Mo-Al2O3 Cermets by Using Ultrafine Molybdenum and Nano-sized Alumina Powders

Effects of Nano Aluminum Powder on the Mechanical Sensitivity of RDX-Based Explosives.

As nano-aluminum powder (NAP) can improve the detonation performance of aluminum-containing explosives, more and more explosives with NAP as the metal ingredient have been studied. It is believed that the mechanical sensitivity of explosives can be significantly enhanced by the added nano-sized aluminum powder. However, the mechanism for the enhancement has not been clarified. In order to illuminate the effects of NAP on the mechanical sensitivity of explosives, two RDX-based aluminum-containing explosives with the same weight ratio and preparation process were investigated despite the aluminum powders with different nano-size and micron-size. The morphology and surface atomic ratio of the two explosives were examined by scanning electron microscopy with energy dispersive spectroscopy tests. The contact angle and other microstructures properties of the explosives were calculated by Material Studio software. Results revealed that the impact and friction activity was determined by the aluminum particle sizes and explosive components. This paper clarified the mechanism for the increase in explosives sensitivity by the addition of NAP, which provide reference for the scientific and technical design of novel explosives.

Promotional effect of silica on the combustion of nano-sized aluminum powder in carbon dioxide

This paper presents how the combustion performance of nano-sized aluminum (nAl) powder in carbon dioxide are affected by silica. The ignition and combustion performance of nAl powder with silica addition were studied by a high-temperature tube furnace. An s-type thermocouple and a high-speed motion acquisition instrument were performed to evaluate the ignition temperature, maximum combustion temperature, maximum change of rate of temperature, and combustion propagation speed. The combustion efficiency and combustion products were measured and analyzed by a gas-volumetric method and an X-ray diffraction. The results show that silica added into nAl powder can enhance its maximum combustion temperature and maximum change of rate of temperature, while its ignition temperature increases slightly. The nAl powders with addition of 6.00 wt.% and 12.00 wt.% silica present high combustion propagation speeds, especially for the latter, it has high combustion efficiency. The effect mechanism of silica on the combustion of nAl powder in carbon dioxide was discussed.

Effect of ZrO2 Nanoparticles and Mechanical Milling on Microstructure and Mechanical Properties of Al–ZrO2 Nanocomposites

AbstractNano-aluminum powders and nano-ZrO2 reinforcement particles were mechanically milled and hot-pressed to produce Al–ZrO2 nanocomposites. Microstructure and mechanical properties of Al–ZrO2 nanocomposites were investigated using scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) analyses and by performing hardness and compression testing. Uniform particle distribution was obtained up to 3 wt% of nano-ZrO2 particles using nano-sized aluminum powders as matrix powders and by applying a mechanical milling process. As the nano-ZrO2 reinforcement particles were uniformly distributed in the matrix, the relative density of the Al–ZrO2 nanocomposites increased up to 3 wt% nano-ZrO2 particles with an increase in milling time; on the other hand, the relative density decreased and the porosity increased with high-weight fractions (>3 wt%) of nano-ZrO2 particles due to the negative combined effect of less densification and an increase in the number of particle clusters. The hardness and compressive strength of the Al–ZrO2 nanocomposites improved despite increased porosity. However, the compressive strength of Al–ZrO2 nanocomposites with a high amount (>3 wt%) of nano-ZrO2 particles began to decrease due to the negative combined effect of the less densification of the powder particles and the clustering of nano-ZrO2 reinforcement particles. The brittle-ductile fracture occurred in the Al–ZrO2 nanocomposites.

Enhancing the quartz-clay-feldspar system by nano-Al2O3 addition for electrical insulators: From laboratory to prototype scale

Nanotechnology applications have been increasing in the last years, offering innovative nanomaterials with enhanced characteristics and performance compared to conventional materials. In this work, a nanotechnology concept is proposed to improve the properties of a quartz-clay-feldspar ceramic system. The application target is in the electrical industry as an outdoor insulator. Experimental compositions were obtained by incorporating nano-alumina (nano-Al2O3) in siliceous porcelain to study their influence on the microstructure, physical, mechanical, and electrical properties. Industrial grade raw materials with a typical particle size distribution were used to prepare siliceous porcelain mixtures. Nano-sized alumina powder was added to the raw materials and then mixed to obtain good dispersion and high homogeneity. Specimens were prepared by uniaxial pressed and plastic extrusion processes for laboratory and prototype scale respectively and then were dried and fired, reaching a maximum temperature of 1250 °C. According to the SEM and XRD analysis, it was observed that alumina nanoparticles promote a mechanical strengthening mechanism in the ceramic body, mainly due to the increment of mullite. It was also found that dispersed nano-Al2O3 reinforces the glassy matrix providing a strong barrier that causes the crack deflection when loading. Nanostructured ceramic formulations exhibit higher flexural strength and superior breakdown voltage than conventional porcelain. The results include in this work strengthen the knowledge of nanotechnology concepts to develop advanced electrical porcelain systems.

Effects of nano-sized aluminum on detonation characteristics and metal acceleration for RDX-based aluminized explosive

Nano-sized aluminum (Nano-Al) powders hold promise in enhancing the total energy of explosives and the metal acceleration ability at the same time. However, the near-detonation zone effects of reaction between Nano-Al with detonation products remain unclear. In this study, the overall reaction process of 170 nm Al with RDX explosive and its effect on detonation characteristics, detonation reaction zone, and the metal acceleration ability were comprehensively investigated through a variety of experiments such as the detonation velocity test, detonation pressure test, explosive/window interface velocity test and confined plate push test using high-resolution laser interferometry. Lithium fluoride (LiF), which has an inert behavior during the explosion, was used as a control to compare the contribution of the reaction of aluminum. A thermochemical approach that took into account the reactivity of aluminum and ensuing detonation products was adopted to calculate the additional energy release by afterburn. Combining the numerical simulations based on the calculated afterburn energy and experimental results, the parameters in the detonation equation of state describing the Nano-Al reaction characteristics were calibrated. This study found that when the 170 nm Al content is from 0% to 15%, every 5% increase of aluminum resulted in about a 1.3% decrease in detonation velocity. Manganin pressure gauge measurement showed no significant enhancement in detonation pressure. The detonation reaction time and reaction zone length of RDX/Al/wax/80/15/5 explosive is 64 ns and 0.47 mm, which is respectively 14% and 8% higher than that of RDX/wax/95/5 explosive (57 ns and 0.39 mm). Explosive/window interface velocity curves show that 170 nm Al mainly reacted with the RDX detonation products after the detonation front. For the recording time of about 10 μs throughout the plate push test duration, the maximum plate velocity and plate acceleration time accelerated by RDX/Al/wax/80/15/5 explosive is 12% and 2.9 μs higher than that of RDX/LiF/wax/80/15/5, respectively, indicating that the aluminum reaction energy significantly increased the metal acceleration time and ability of the explosive. Numerical simulations with JWLM explosive equation of state show that when the detonation products expanded to 2 times the initial volume, over 80% of the aluminum had reacted, implying very high reactivity. These results are significant in attaining a clear understanding of the reaction mechanism of Nano-Al in the development of aluminized explosives.

Operational properties of chromium coatings obtained from self-regulating electrolyte with addition of nano-dimensional particles

The article presents the results of laboratory studies of microhardness and wear resistance of basic and nanocomposite electroplating chromium coatings. The most effective strengthening phase was chosen - nanosized alumina powder at a concentration of 3 g / l. The use of nano-sized aluminum oxide powder makes it possible to increase the microhardness and wear resistance of nanocomposite coatings by 1.5-1.8 times as compared to the base ones.

Electromagnetic-wave sintering of alumina ceramics from nano-sized particles: possible material for high-pressure cell for millimeter-wave electron spin resonance

Electromagnetic-wave sintering of alumina ceramics using 28 GHz gyrotron has been performed aiming for a high fracture toughness value in order for use as pistons for pressure cells of high-frequency electron spin resonance (ESR) measurements. We have tried to improve the fracture toughness by using nano-sized alumina powder (140 nm in average particle size) having smaller particle size than our previous work (400 nm in average particle size). Rapid densification was observed around the sintering temperature of 1200 °C. We obtained the relative density over 99 % above 1400 °C. It was found that alumina ceramics made in this work at sintering temperatures have smaller grain size and higher density simultaneously as compared to the previous work. These results suggest that alumina ceramics made from powder of smaller particle size sintered by electromagnetic-wave sintering possibly have high fracture toughness which can be used as materials for the pressure cell for ESR.

Spectrophotometrical analysis for fabrication of pH-independent nano-sized γ-alumina by dealumination of kaolin and precipitation in the presence of surfactant composites.

The challenge with γ-Al2O3 is its pH-dependent removal efficiency and relatively low adsorption capacity in recovery of wastewaters contaminated by cationic dyes. Therefore; the objective of present investigation was to fabricate a pH-independent nano-sized alumina powder by dealumination of kaolin for uptake of dye. The dealumination of meta-kaolin was carried out by nitric acid solution and the amorphous aluminum hydroxide was precipitated with ammonia in the presence of combined surfactants containing cetyltrimethylammonium bromide (CTAB), polyethylene glycol (PEG) and polyoxyethylene octyl phenyl ether (TX100). The nano-sized gamma alumina particles were fabricated after calcination at 800 °C and then were analyzed based on spectrophotometrical method. The rod-like nano-particles with average width of 5 nm and length of 20 nm were synthesized by precipitation in the presence of admixed surfactant, CTAB-TX100, which improves the removal efficiency due to efficient modification in the interaction of particles. TX100 also showed a significant affinity for increasing the specific surface area because of ability to prevent the particle aggregation which provides a potential for synthesis of alumina adsorbent without foaming, normally observed in the presence of CTAB. Both of mentioned powders indicated relatively pH-independent removal efficiency with higher adsorption capacity. Moreover, adsorption isotherm parameters were determined to verify the improvement of removal efficiency. The presented fabrication technique eventually overcame the pH-dependent recovery problem which is feasible for the development of adsorbents because the starting material is abundant, very low energy is needed, no hazardous waste is generated and applicable in the acidic environments.

Shock Compressibility of Mixtures of Micro- and Nano-Sized Nickel and Aluminum Powders

Shock compressibility of porous samples prepared from mixtures of micro- and nanosized nickel and aluminum powders is experimentally studied in the pressure range up to 60 GPa. Shock wave profiles in the samples are recorded, and Hugoniots are determined. The equation of state of the samples is derived within the framework of the Zel’dovich model. The shape of the shock wave profiles does not reveal any specific features that can be associated with a possible reaction between the components. The Hugoniots of the samples of two types of powder mixtures coincide within the experimental error despite significantly different sizes of powder particles, which implies that either there are no noticeable chemical transformations or, vice versa, they are completed within the shock loading time. The predicted Hugoniot with the reaction between the components being ignored passes in an immediate vicinity of the experimental data, which testifies to the absence of the reaction.

Accelerated ageing of micron- and nano-sized aluminum powders: Metal content, composition and non-isothermal oxidation reactivity

Nano-sized Al (nAl) is an attractive candidate for the performance enhancement of current solid propellant formulations, and its characteristics are relevant for the development of innovative solid fuels for hybrid rocket propulsion. Nano-sized Al powders feature a higher reactivity than their micron-sized counterpart. While desirable for effective energy conversion during the combustion, this characteristic yields possible powder storage issues. This paper investigates the ageing behavior of air-passivated nAl powders under two different relative humidity conditions (RH) of 80% and 10%. Fresh and aged powders are characterized by active metal content determination (CAl), electron microscopy, X-ray diffraction, and thermal analysis. The marked sensitivity of nAl (40 nm) to ageing is testified by a nearly full CAl loss in just 24 h (with RH = 80%). Under the same condition, the conventional 30 μm-counterpart loses only 13% of the original active metal content in a time of 336 h.

Thermal Reaction Characteristics of Nano/Micron-Sized Aluminum Mixtures in Carbon Dioxide

Thermal reaction characteristics of nano/micron-sized aluminum mixtures in a carbon dioxide atmosphere were investigated by thermogravimetry that aimed to examine the interactions between nanosized aluminum powder and micron-sized aluminum powder. Thermal reaction characteristics of nano/micron-sized aluminum mixtures at different ratios were studied. The synergistic effect mechanism was discussed by comparing experiment result and theoretical calculation. The morphologies and compositions of products were obtained by scanning electron microscopy and X-ray diffraction technologies. Results indicated that there were three weight gain stages for nano/micron-sized aluminum mixtures at different ratios. Although the temperature range of synergistic effect of nano/micron-sized aluminum mixtures with various ratios had somewhat differences, there was a significant synergistic effect from about 900 to 1040°C. The products analyses indicated that the products morphologies of nano/micron-sized aluminum particles showed molten and stuck together, and products contained aluminum and α-Al2O3.

Shock Compressibility of Mixtures of Microand Nano-Sized Nickel and Aluminum Powders

Shock Compressibility of Mixtures of Microand Nano-Sized Nickel and Aluminum Powders

Metal-rich aluminum–polytetrafluoroethylene reactive composite powders prepared by mechanical milling at different temperatures

Aluminum–polytetrafluoroethylene (Al·PTFE) composite materials with 90 wt% Al are prepared by mechanical milling at both room and cryogenic temperatures. Distribution of PTFE in the material is more homogeneous in the cryogenically milled materials. Thermal analysis in inert atmosphere shows at least three distinct exothermic reaction steps below Al melting as well as evolution of AlF3 at temperatures above 800 °C. The heat of low-temperature exothermic reactions first increases and then decreases as a function of the milling time. In an oxidizing environment, all materials oxidize qualitatively similar to pure Al, and a composite milled cryogenically for 6 h oxidizes as fast as or faster than nano-sized aluminum powder. When heated at several thousand degrees per second, all composites ignite around 730 °C. Only the powders prepared by cryogenic milling could be ignited by electrostatic discharge (ESD) with energies of 720 mJ, while materials milled at room temperatures could not be ignited with energies as high as 2 J. In the ESD ignition experiments, the optical emission pulse is delayed compared to the pressure pulse, suggesting that a gas-generating PTFE decomposition triggers the ignition. The material cryogenically milled for 6 h is the most attractive, based on the magnitude of both pressure and emission pulses generated upon its ESD ignition, and based on the rate and extent of its oxidation in thermo-analytical experiments.