Ultrafine Aluminum Research Articles

Combustion of Mixtures of Commercial Aluminum Powders and Ultrafine Aluminum Powders and Aluminum Oxide in Air

The paper studies the combustion of mixtures of commercial aluminum powders (ASD-1 and ASD-4) and ultrafine powders of Al and γ-Al2O3 in air. It is shown that the combustion of coarsely dispersed commercial powders is accompanied by binding of air nitrogen with formation of AlN and AlON. The combustion of mixtures proceeds in two stages with the possible formation of intermediate gaseous and liquid products. The processes of sintering and incomplete combustion play an important role in the combustion of mixtures of commercial powders and ultrafine powders of aluminum.

Properties of Ultrafine Aluminum Powder Stabilized by Aluminum Diboride

The paper studies properties of an ultrafine aluminum powder produced by electric explosions of conductors, whose particles are stabilized by coating with aluminum diboride immediately during the synthesis of the powder. The ultrafine aluminum powder stabilized in such a manner has special properties: narrow particle size distribution, increased dispersity, and higher resistance to oxidation upon heating.

High Explosives Containing Ultrafine Aluminum ALEX

The sensitivity and performance of mixtures of HMX (octogen) and BTNNA (bis(2,2,2-trinitroethyl)nitramine) with 10–20% of ultrafine aluminum, ALEX, and other types of aluminum are studied. While the addition of aluminum results in only a small reduction of the detonation velocity, it decreases the Gurney energy of HMX much more than that of BTNNA, which is attributed to its positive oxygen balance. Formulations based on HMX and BTNNA reach top values of the Gurney velocity surpassing 3.0 mm/μs.

Air pollution and retained particles in the lung.

Epidemiologic evidence associates particulate air pollution with cardiopulmonary morbidity and mortality. The biological mechanisms underlying these associations and the relationship between ambient levels and retained particles in the lung remain uncertain. We examined the parenchymal particle content of 11 autopsy lungs from never-smoking female residents of Mexico City, a region with high ambient particle levels [3-year mean PM(10) (particulate matter < or = 10 microm in aerodynamic diameter)= 66 microg/m(3)], and 11 control residents of Vancouver, British Columbia, Canada, a region with relatively low levels (3-year mean PM(10) = 14 microg/m(3). Autopsy lungs were dissolved in bleach and particles were identified and counted by analytical electron microscopy. Total particle concentrations in the Mexico City lungs were significantly higher [geometric mean = 2,055 (geometric SD = 3.9) x 10(6) particles/g dry lung vs. 279 (1.8) x 10(6) particles/g dry lung] than in lungs from Vancouver residents. Lungs from Mexico City contained numerous chain-aggregated masses of ultrafine carbonaceous spheres, some of which contained sulfur, and aggregates of ultrafine aluminum silicate. These aggregates made up an average of 25% of the total particles by count in the lungs from Mexico City, but were only rarely seen in lungs from Vancouver. These observations indicate for the first time that residence in a region with high levels of ambient particles results in pulmonary retention of large quantities of fine and ultrafine particle aggregates, some of which appear to be combustion products.

Ultra-fine powder by wire explosion method

Development of new technologies for the production of various ultra-fine powder has become nowadays one of the most important topics. The pulse energy technology has a number of principal advantages for ultra-fine powder production: In the pulsed mode it is possible to provide high density of energy to a substance with a required dosage which can be controlled precisely. The energy supplied in the pulsed mode is used with higher efficiency. The process proceeds extremely fast. All the process can be regarded as adiabatic. Particle formation with designed characteristics would be possible. In the High Voltage Research Institute (HVRI, Tomsk, Russia) ‘wire electric explosion (WEE) method’ has been developed for the production of ultra-fine powder [1‐5]. In this study some theoretical backgrounds and experimental results for ultra-fine aluminium powder production are described.

Combustion of Ultrafine Aluminum in Air

The paper reports results of studying the combustion of ultrafine aluminum (surface average diameter of particles is ≈ 0.1 μm) in a sealed bomb at an initial air pressure of 1 atm. The combustion proceeds in two stages, similarly to combustion in air. It is shown that during the two‐stage combustion of ultrafine aluminum powder in the bomb, the mass concentration of chemically bound nitrogen in the final products increases by ≈ 20% in terms of aluminum nitride. An increase in nitrogen content in confined combustion validates the previously proposed mechanism of binding air nitrogen with participation of the gas phase during aluminum combustion.

Reactivity of Aluminum Powders

The reactivity of aluminum powders is determined using the following parameters: the temperature of the onset of oxidation, maximum oxidation rate, degree of conversion (degree of oxidation) of aluminum, reduced (conditional) ratio of the thermal effect to the weight increment. These parameters for estimating the activity of aluminum powders were chosen from the results of nonisothermal oxidation of powders of various particle sizes under conditions of programmed heating (oxidizer–air). In accordance with the testing method proposed, the most reactive powder studied was STPA‐4 ultrafine aluminum powder produced by electrical explosion of conductors.

Effect of process parameters on synthesis of aluminum nitride powder prepared by chemical vapor deposition

Ultrafine aluminum nitride (AlN) powders were obtained by the chemical vapor deposition (CVD) process via the AlCl3-N2-NH3 system operated at various temperatures and different mixing modes of AlCl3 and NH3 gases. X-ray diffraction (XRD), Fourier transform infrared spectroscope (FTIR), transmission electron microscope (TEM) and electron diffraction (ED) were utilized to study the effect of process parameters on the synthesis and characterization of the AlN powders. It had been shown that all of the obtained powders were exclusively identified to be the single phase AlN and were indifferent to the different mixing modes AlCl3 and NH3 gases. The yield of synthesized AlN powder was affected by the entries mixing position of the reacting gases of AlCl3 and NH3.The yield increased from 13% to 82% where the mixed position was shifted from the front edge to middle point of the quartz tube. On the other hand, the yield increased from 5% to 82% as the reaction temperature increased from 600°C to 1050°C. The morphology of the AlN powders was affected by the diameter of a feeding nozzle and mixing mode of AlCl3 and NH3 gases.

Sub-micron sized Al2TiO5 powders prepared by high-energy ball milling

High energy ball milling to obtain ultrafine aluminium titanate particles has been investigated. Tempered steel has been selected as material for the containers and balls because the desirable properties of aluminium titanate are not degraded by small amounts of Fe2O3. The starting powders have been milled during different periods (1–60 h) and the evolution of the morphology and crystallinity of the treated powders as well as the extent of contamination from the milling media have been characterised. Different experimental techniques, X-ray diffraction, BET-analysis, chemical analysis, scanning electron microscopy and low and high resolution transmission electron microscopy have been used. High energy ball milling has been proved to be an efficient route to obtain submicron sized (50–100 nm) aluminium titanate powders, but further milling of the powders is accompanied by contamination from the milling media and the formation of hard agglomerates.

Chemical Analysis of Aluminum as a Propellant Ingredient and Determination of Aluminum and Aluminum Nitride in Condensed Combustion Products

A brief survey of typical problems in the analysis of aluminum powder in aluminized solid propellants and in analysis of condensed combustion products of these propellants was carried out. Recommendations for applying the versions developed by the authors of the known methods are given. The permanganatometric variant of the titrimetric method was found suitable for most tasks concerning the measuring of the metallic/unburned aluminum. The determination of aluminum nitride in combustion products using the combination of chemical and X-ray diffraction methods was described and illustrated by results obtained from condensed combustion products of propellant formulations containing highly active ultrafine aluminum powder. Even for this formulation the content of aluminum nitride in the final condensed combustion products was found to be negligibly small independently of the nature of the gas (argon or nitrogen) used for bomb pressurization.

Specific features of the reaction between ultrafine aluminum and water in a combustion regime

The combustion of mixtures of an ultrafine electroexplosive aluminum powder with water thickened by a 3% polyacrylamide additive is investigated. The reaction in a combustion regime is accompanied by the formation of a superheated foamy layer in gel-like water. The incompleteness of aluminum burnout in a stoichiometric mixture, which is explained by boiling-out of water from the reaction zone, is shown. The maximum combustion temperatures are determined in various conditions by means of thermocouple measurements and combustion-product composition calculations. The possibility of producing ultrafine or monolithic corundum as a reaction product is shown.

Effect of heat treatment on phase transformation of aluminum nitride ultrafine powder prepared by chemical vapor deposition

Ultrafine aluminum nitride (AlN) powders were obtained by chemical vapor deposition via AlCl 3–NH 3–N 2 system operated at various temperatures and at a same 200 cm 3/min flow rate of NH 3 and N 2, respectively. It has been shown that when the reaction temperature of AlCl 3 and NH 3 was above 600°C, then crystalline AlN powder can be formed; whereas, amorphous AlN was obtained with NH 4Cl if reacted in a lower-temperature zone of the reaction chamber. The amorphous AlN powder was heat treated at 1400°C under NH 3 and N 2 atmosphere for 2 h, then the crystalline phases of the obtained powder belong to a single phase of AlN; a mixture of AlN and Al 2O 3 and only AlON, respectively. On the other hand, if crystalline AlN powder is heat treated at 1400°C under N 2 atmosphere for 2 h, the crystalline phases were composed of the major phase of AlN and a minor phase of Al 2O 3. The morphology, particle size and agglomerate size of the AlN powder were strongly dependent on the heat-treatment temperature. The particle size of AlN powder increases from 28.1 to 45.0 nm, as the heat treatment temperature increases from 800 to 1400°C.

Combustion of mixtures of ultrafine powders of aluminum and boron in air

Results of investigation of the combustion of mixtures of ultrafine aluminum and boron powders (the oxidizer is air) are presented. It is shown that the combustion proceeds in two steps, which differ in temperature. The addition of boron influences the concentrations of AlN, residual Al, and α-Al2O3 in the end products of combustion of mixtures of ultrafine powders of Al and B in air. For a fixed sample weight of 4 g, the maximum AlN content is observed in the combustion of an Al+20% B mixture of ultrafine powders, and the combustion temperature is also maximum in this case. When the sample weight is smaller than a certain critical value, the combustion proceeds in one step. Increasing the sample weight of the starting mixture of ultrafine powders of Al and B leads to an increase in the AlN content in the combustion products with simultaneous rise in the combustion temperature. A considerable part of the combustion products stabilizes as acicular polycrystals of micron and submicron sizes formed with participation of a gas phase during combustion.

Evaluation of Stored Energy in Ultrafine Aluminum Powder Produced by Plasma Explosion

Ultrae ne aluminum powder produced by plasma explosion (ALEX) exhibits burn behavior unlike that of ordinary aluminum powders. Others have previously suggested that the source of this unique behavior might be stored internal energy. The objective of this study is to evaluate theoretically the feasibility of energy being stored in ALEX as a result of work performed by means of the compression of the liquid portion of an ALEX particle by its shrinking solid shell during rapid solidie cation of the particle. A theoretical model of this process of energy storage was developed. This model was then formulated and numerically solved on a computer for a variety of conditions. Results show that the postulated mechanism results in measurable stored energy for only unrealistically high cooling rates. It is concluded that, although the postulated mechanism could store energy, the amount of energy stored is realistically negligible. This theoretical e nding is in agreement with recent experiments that show no observable stored energy in ALEX particles. Nomenclature A = surface area, m 2 B = bulk modulus, N/m 2 Cp = specie c heat, J/kg ¢K D = diameter, m Est = thermal energy stored during solidie cation, J H f = latent heat of fusion, J/kg h = heat transfer coefe cient, W/m 2 ¢K k = thermal conductivity, W/m ¢K m = mass, kg N = surface node P = pressure, Pa r = radius,m T = temperature, K Ti = initial temperature, K V = volume, m 3 W = work, J ¯ = thermal coefe cient of expansion, 1/K 1t = time-step increment, s Ω = density, kg/m 3

Comparison of Thermal Behavior of Regular and Ultra-fine Aluminum Powders (Alex) Made from Plasma Explosion Process

Alex powder (an ultra-fine aluminum powder produced by the plasma-explosion process) has been shown to be a very effective burning rate enhancer for solid propellants and fuels. The objective of this research is to investigate the mechanisms responsible for the beneficial effect of Alex. A TGA and DTA were employed to examine the thermal bahavior of Alex and regular aluminum particles in different gases. An environmental scanning electron microscope (ESEM) was utilized to study the particle surface bahavior when samples were heated in air and nitrogen. A transmission electron microscope (TEM) was employed for examining the microstructure of Alex particles. Alex powder was demonstrated in TGA and DTA experiments to behave very differently from the regular aluminum in air, oxygen and nitrogen. Rapid weight gain of Alex particles in air and nitrogen environment occurred around 548°C and 700°C, respectively. The low-temperature reaction in air corresponds to an oxidation reaction. Comparing DTA traces of Alex and regular aluminum in air, it was found that oxidation of Alex particles occurs at a lower temperature with a higher degree of reaction. This is believed to be mainly due to the higher reactivity associated with greater surface area of the Alex particles. In the comparison of DTA traces of Alex and regular aluminum in nitrogen, the rapid nitridation reaction of Alex particles occurs at temperature around 680°C and proceeds to near completion.

High resolution in situ SEM observations of early stages of fatigue crack growth in IN 9052 aluminium alloy

A study of the early stages of fatigue crack growth in vacuum from a notch in an ultrafine grain size aluminium alloy (IN 9052) has been carried out. Specimens were tested under cyclic loading at afrequency of 0.2 Hz in the vacuum chamber of a field emission gun scanning electron microscope (SEM) which provides a high spatial resolution of 25 nm. The notch, ∼20 μmlong by 1 μm wide and 2 μm deep, proved to be effective in initiating fatigue cracks in this alloy. The notch was formed by melting the material and ejecting it as vapour. At the bottom of the notch, defects resulting from the rapid solidification of the molten material were found. The combination of these defects and the geometrical stress concentration resulting from the presence of the notch ensure crack formation in association with the notch. Direct in situ SEM observations suggest that the transition from initial mixed mode (stage I) crack growth to mode I (stage II) crack growth in vacuum is discontinuous and intermittent. In addition, the transition can occur when the crack is very small (≤ 500 nm). The observations also suggest that interactions with an aggressive environment are not essential for stage I fatigue crack growth in this material. It was found that fatigue crack rewelding, blunting, and propagation are associated with the fatigue crack growth process. A sequential development of the crack tip during fatigue crack growth in vacuumfor the ultrafine grain size aluminium alloy (IN 9052) is suggested. The implications of these observations for the underlying mechanisms of fatigue crack growth in vacuum for such alloys are discussed.

Synthesis of Sub-Micron Alumina in a Thermal Plasma Jet

Ultrafine aluminium oxide powder has been synthesized in a thermal plasma reactor by oxidation of aluminium metal powder. The reactor consists of a dc plasma torch mounted on to a reaction chamber made of stainless steel with provision for injecting any reactive gas into the plasma jet. A mixture of argon and nitrogen is used as the plasma gas and aluminium metal powder is fed into the plasma using argon as the carrier gas. The metal particles are vaporized in the plasma stream and the vapours of aluminium metal are oxidized to aluminium oxide by compressed air injected downstream in the reactor. The synthesized powder has been characterized by X-ray diffraction, which showed that it consisted of γ-alumina. Scanning electron microscopy of the synthesized particles showed agglomerates of fine particles.

Study on the mechanism of aluminum nitride synthesis by chemical vapor deposition

Ultrafine aluminum nitride particles were synthesized by chemical vapor deposition at 973–1273 K. Effects of reaction temperature, gas total flow rate, aluminum chloride concentration on aluminum nitride particle morphology and particle size distribution were studied. The mechanism of the reaction between aluminum chloride and ammonia and the mechanisms for homogeneous formation of aluminum nitride particles and growth of aluminum nitride film by surface reactions were investigated.

INSTANTANEOUS REGRESSION BEHAVIOR OF HTPB SOLID FUELS BURNING WITH GOX IN A SIMULATED HYBRID ROCKET MOTOR

An experimental investigation using a high−pressure hybrid motor analog has been conducted to provide detailed solid fuel regression rate data at realistic operating conditions for both model validation and correlation development. The 2−D motor operated at a maximum pressure of 9 MPa (1300 psi) with maximum injected gaseous oxygen mass fluxes of 420 kg/m2·s (0.6 lbm/in2·s). Hydroxyl−Terminated Polybutadiene (HTPB, R−45 M) cured with Isonate 2143L was used as the baseline solid fuel. Either carbon black powder or ultra−fine aluminum (UFAL) powder was added to the HTPB fuel during the processing stage for some tests. Both ultrasonic pulse−echo and real−time X−ray radiography techniques were used to determine the instantaneous regression rate. An array of fine−wire thermocouples was used to determine fuel surface temperatures and subsurface temperature profiles. The deduced instantaneous regression rates displayed a complex dependency on axial location and flow conditions. Close to the sample leading edge, the regression rate, governed by the mixing motion of the entering GOX flow, did not vary with time. Following this region, the regression rates exhibited strong dependency on both axial location and flow structure. When the GOX mass fluxes were below 140 kg/m2·s (0.2 lbm/in2·s), gas−phase radiative heat transfer to the fuel surface was found to play a more important role in the regression of solid fuel. Solid fuel regression rates were found to correlate well with the injected oxidizer mass flux, chamber pressure, axial location, and port height. The addition of carbon black had no effect on regression behavior. Addition of 20% of UFAL powder, however, was found to increase the mass burning rate by 70%.

Characterization of plasma-synthesized alumina

Ultrafine aluminium oxide powder has been synthesized in a thermal plasma reactor by oxidation of aluminium metal powder. The reactor consists of a d.c. plasma torch mounted on to a reaction chamber made of stainless steel with provision for injecting any reactive gas into the plasma jet. A mixture of argon and nitrogen is used as the plasma gas and aluminium metal powder is fed into the plasma using argon as the carrier gas. The metal particles are vaporized in the plasma stream and the vapours of aluminium metal are oxidized to aluminium oxide by compressed air injected downstream in the reactor. The synthesized powder has been characterized by X-ray diffraction, which showed that it consisted of γ-alumina. Scanning electron microscopy of the synthesized particles showed agglomerates of line particles. The individual particles, ranging from a few nanometres in size to a few tens of nanometres could be resolved using transmission electron microscopy.