Oxidation Of Aluminum Powders Research Articles

Particle size control of alumina nanopowders in microwave plasma-chemical process

The effect of operating parameters of the oxidation of aluminum powder in a stream of air microwave plasma on the particle size of the obtained Al2O3 has been studied. The possibility of improving the particle size of the powder by pretreatment the starting aluminum with chemicals activating particle combustion in a reactor has been investigated. Ways of controlling the particle size of produced aluminum oxide nanopowders in the range of 20–80 nm have been found.

Oxidation kinetics of micron-sized aluminum powder in high-temperature boiling water

A new efficient method of hydrogen, heat and aluminum oxide/hydroxide co-production is proposed. Only micron-sized aluminum powder (without any chemical activation) and usual water are used as initial reagents. For aluminum to be effectively oxidized, water is converted into the high-temperature boiling state that creates high pressure inside oxidation reactor. Paper describes the oxidation kinetics of aluminum micron powder in high-temperature boiling water depending on powder size and reactor temperature (pressure). Kinetic experiments were carried out using four types of aluminum powders with different dispersity. Due to kinetic experiments it was established that micron-sized aluminum powder intensively oxidizes in boiling water at temperatures above 230 °C. Aluminum powders with average particle size of 4.1 μm, 7.2 μm and 22.5 μm were fully oxidized at 296 °C, 308 °C and 350 °C respectively. Aluminum powder with average particle size of 77.5 μm after 10 min staying under about 364 °C was 95% oxidized. Reaction times for 4.1 μm, 7.2 μm and 22.5 μm aluminum powders decreased from 870 s at 237 °C to 33 s at 359 °C, from 800 s at 273 °C to 41 s at 355 °C and from 780 s at 286 °C to 66 s at 350 °C respectively. Aluminum oxidation product X-ray analysis showed that aluminum oxidized to aluminum hydroxide AlOOH – boehmite. Microstructure analysis showed that micron-sized aluminum powder oxidation product represents nano-sized aluminum hydroxide.

Oxidation of aluminum powders at high heating rates

A kinetic model of oxidation of aluminum in oxygen has been previously established based on thermal analysis data using heating rates in the 1–40 K/min range. Ignition, on the other hand, involves heating rates that are many orders of magnitude higher, which requires extrapolation of the kinetic predictions. In this study, we use thermogravimetry at heating rates up to 500 K/min (8.3 K/s) to validate and refine the kinetic model of aluminum oxidation. Experiments are conducted in argon/oxygen mixtures. The stepwise aluminum oxidation process reported for lower heating rates is also observed for the higher heating rates addressed in this study. Activation energies for individual oxidation steps are generally consistent with previous data obtained from low heating rate measurements. Additional oxidation process parameters are quantified from the higher heating rate measurements. The overall oxidation model proposed earlier and involving growth of various alumina polymorphs and transformations between these polymorphs is validated in new experiments. The refined oxidation model is successfully used to interpret experiments on aluminum particle ignition in a laser beam, in which the particles are heated at more than 10 6 K/s.

Aluminum Powder Oxidation in CO2 and Mixed CO2/O2 Environments

An experimental study of oxidation of micrometer-sized aluminum powder in CO2 and CO2/O2 mixed oxidizers is described. The oxidation is studied using thermogravimetric (TG) measurements. Partially oxidized samples are recovered and their compositions are analyzed using X-ray diffraction. Oxidation of aluminum powders in CO2 occurs in several stages, similar to that in oxygen. Aluminum oxidation rates are effectively the same for O2 and CO2 oxidizers for the low-temperature oxidation processes controlled by the growth of the initial amorphous oxide, its transformation to the γ-Al2O3 polymorph, and initial growth of γ-Al2O3. At higher temperatures, the growth of transition alumina in oxygen occurs faster than it does in CO2. The thermal stability of the transition alumina polymorphs, primarily for θ-Al2O3, is extended to higher temperatures in the presence of CO2 so that the denser α-Al2O3 polymorph forms later than that in oxygen. Because of the lower diffusion resistance of transition alumina polymorphs a...

Kinetics of aluminum powder oxidation by water at 100°C

The kinetics of aluminum powder oxidation by water at 100°C has been investigated using three types of aluminum powders. A “shrinking sphere” model was used for description of the experimental data at the initial reaction stage, whereas a “parabolic” diffusion equation was used at the second stage. Variation of the specific reactivity is discussed.

Physicochemical Principles of the Synthesis of Porous Composite Materials through the Hydrothermal Oxidation of Aluminum Powder

The main versions of the synthesis of a new class of porous cermet materials such as Al2O3/Al, MOx/Al2O3/Al, and M1/MOx/Al2O3/Al and ceramic composites on their basis were analyzed. These ceramic composites were prepared through the stage of the hydrothermal oxidation of aluminum powder and were designed for catalytic and adsorption processes. Equations that express the dependence of the apparent density of the resulting composite on the density of the initial powder mixture, on the concentration of the powdered active component, and on the conversion of aluminum are given. It was found that the formal kinetics of aluminum oxidation with water at 100°C can be described by the Kolmogorov-Erofeev equation. The results were compared with data obtained in an autoclave at higher temperatures and steam pressures. The synthesis parameters that affect the total pore volume and the specific surface area of aluminum oxide obtained from aluminum powder were determined. For the case of the transfer of soluble components from an autoclave to a press mold, the molar coefficients of this process were calculated. The texture peculiarities of composites were analyzed. The texture exhibited a polymodal character with developed micropore, mesopore, and ultramacropore structures, which are responsible for the high permeability of granulated composites. Factors affecting the mechanical properties of metal ceramics were studied. The catalysts and products of composite materials were exemplified.

Ignition of Aluminum Powders Under Different Experimental Conditions

This paper presents a brief review of experimental measurements of aluminum ignition temperature. Models used to describe aluminum ignition are also reviewed. It is shown that the current models cannot describe ignition of aluminum powders of different sizes and ignited under various experimental conditions. The paper further discusses properties of and phase changes occurring in the alumina scale existing on the surface of aluminum particles at different temperatures. Results of recent thermal analysis studies of aluminum powder oxidation are presented and it is shown that the stepwise oxidation of aluminum is caused by the sequence of polymorphic phase transitions occurring in the growing oxide film. Finally, the qualitative connection between the processes governing the oxidation of aluminum powders and the ignition of aluminum particles is made and an explanation for the experimentally observed wide range of aluminum ignition temperatures is suggested.

Effect of polymorphic phase transformations in Al 2O 3 film on oxidation kinetics of aluminum powders

Thermogravimetry was used to study the oxidation of aluminum powders at elevated temperatures. Aluminum powders of various particle sizes and surface morphologies were heated in oxygen up to 1500 °C at different heating rates. Partially oxidized samples were recovered from selected intermediate temperatures and the oxide phases present were analyzed by X-ray diffraction. The experimental data were related to current information on stabilities and phase changes of Al 2O 3 polymorphs. Aluminum powders were observed to oxidize in four distinct stages in the temperature range from 300 to 1500 °C. During stage I, from 300 to about 550 °C, the thickness of the natural amorphous alumina layer on the particle surface increases. The rate of this process is controlled by the outward diffusion of Al cations. At about 550 °C, when the oxide layer thickness exceeds the critical thickness of amorphous alumina of about 4 nm, the oxide transforms into γ-Al 2O 3. The specific volume of γ-Al 2O 3 is less than that of amorphous alumina; therefore, the newly formed γ-Al 2O 3 only partially covers the aluminum surface. The oxidation rate increases rapidly at the onset of stage II, but it decreases when the γ-Al 2O 3 layer becomes continuous. During stage III oxidation, the γ-Al 2O 3 layer grows and partially transforms into the structurally similar θ-Al 2O 3 polymorph. Finally, oxidation stage IV is observed after the transition to stable α-Al 2O 3 results in an abrupt reduction of oxidation rate. Qualitative analysis of the rates of oxidation at the different stages enables one to understand the wide range of aluminum ignition temperatures observed for particles of different sizes.

Porous Al2O3/Al Metal Ceramics Prepared by the Oxidation of Aluminum Powder under Hydrothermal Conditions Followed by Thermal Dehydration: IV. Effect of Oxide Additives on the Composition and Texture Characteristics of MOx/Al2O3/Al Composites

The synthesis, structure, and texture of metal ceramics based on Al2O3/Al with powdered oxide additives (CaO, MgO, Al2O3, La2O3, and TiO2) were studied. Analytic expressions were derived to relate the main macroscopic characteristics of the composites.

Porous Al2O3/Al metal ceramics prepared by the oxidation of aluminum powder under hydrothermal conditions followed by thermal dehydration: III. The reactivity of aluminum, the reaction mechanism of its oxidation with water vapor, and the microtexture of cermets

The macrokinetics of aluminum oxidation under hydrothermal conditions at 150–250°C and water vapor pressures of ∼0.5–4.5 MPa was studied. The apparent kinetic characteristics of the process were determined. The diffusion of water through the layer of a hydrated product was found to be a rate-limiting step. It was found that diffusion coefficients for various aluminum samples differed by almost two orders of magnitude; the main reaction steps were revealed. Changes in the texture of aluminum oxide in cermets in the course of the hydrothermal oxidation reaction were analyzed. This texture was found to depend on the ratio between the specific rate of the oxidation reaction and the rate of aging of the oxidation products under hydrothermal conditions.

The Reaction‐Bonded Aluminum Oxide Process: I, The Effect of Attrition Milling on the Solid‐State Oxidation of Aluminum Powder

The effect of attrition milling on the solid‐state oxidation of aluminum powder is important for the reaction‐bonded aluminum oxide process. Attrition milling increased the surface area to 14.4 and 20.2 m2/g versus 1.2 m2/g for unmilled powder and smeared the Al particles, and the surface was hydrolyzed to form bayerite and boehmite. Upon heating the hydroxides decompose to form an 11–13 nm thick amorphous plus γ‐Al2O3 layer which subsequently retards oxidation kinetics. The oxidation per unit area decreases for the higher surface area powders at temperatures below the critical temperature but the total oxidation of the milled powder is ∼70% versus ∼9% for the as‐received powder because of the higher surface area. The critical temperature depends on Al particle surface characteristics and is defined as the transition temperature above which the initial rate of oxidation is linear, not parabolic. Above the critical temperature the oxidation per unit area decreases significantly. In addition, linear oxidation occurs faster than parabolic oxidation and thus the initial fast oxidation kinetics (i.e., linear) can cause thermal runaway during oxidation. Therefore, oxidation below the critical temperature is essential to maximize solid‐state oxidation and to prevent thermal runaway. The critical temperatures for the as‐received (1.24 m2/g), the 6 h (14.4 m2/g), and 8 h (20.2 m2/g) attrition‐milled Al powders were 500°, 475°, and 500°C, respectively. A model for oxidation during the parabolic and linear oxidation stages is presented.

Porous Al2O3/Al metal ceramics prepared by the oxidation of aluminum powder under hydrothermal conditions followed by thermal dehydration: I. Composition and macrocharacteristics of composites

The macrotexture and mechanical properties of porous Al2O3/Al metal ceramics prepared by the hydrothermal treatment of aluminum powder in a closed space are studied using gravimetry, pycnometry, mercury porosimetry, and scanning electron microscopy. Analytical expressions that relate the porosity, density, and mechanical strength of parent materials and final products to the composite synthesis conditions are derived.

Production of nitride containing mixture in oxidation of aluminum powder in air

The authors investigate the composition of the combustion products of mixtures of freely poured aluminum powders: ADS-1 industrial powder and ultradisperse aluminum powder produced by electric detonation of conductors in argon. It is demonstrated that the end products containing ∼ 50% AIN are easily destroyed sinters and can be used as the main component of the mixture for production of the nitride-containing ceramics.

Mechanism of oxidation of powdered metals during heating in air

It is shown that the oxidation of aluminum powders and rare-earth elements as the temperature rises is accompanied by cracking of a layer of interaction products at the particle surface. The intensity of cracking is reflected in the change of the specific powder surface during heating, and it depends on the phase formation in the barrier layer and its physicochemical properties. The real oxidation rate of disperse metals is higher than the diffusion rate in oxides, which indicates the determining role of cracking in the interaction mechanism.

Low/high temperature relationships in dinitramide salts by DEA/DSC and study of oxidation of aluminum powders by DSC/TG

The dinitramide salts of ammonia (ADN), hexamethylenetetramine (HDN), potassium (KDN), and sodium (NaDN) showed a linear relationship between the DSC rate of decomposition at the peak maximum and the DEA tanδ value at the low temperature transition peak. As the cation basicity increased in the series ADN<HDN<KDN<NaDN, there was an increase in the low temperature transition peak, the energy barrier for relaxation, and the decomposition peak temperature, and a decrease in the tanδ value at the low temperature transition peak, specific heat capacity, and the rate and enthalpy of decomposition. The more basic salts were more thermally stable (i.e., higher decomposition temperature) and less energetic (i.e., lower enthalpy of decomposition). The more internal free volume (disorder) present in these salts, the higher the rates of relaxation and decomposition. Five aluminum powders of different surface areas were analyzed by DSC in platinum sample pans, and it was found that the enthalpy and rate of oxidation increased as the particle size of Al decreased while the enthalpy of the Al melt decreased. TG showed a two-step weight gain in the oxidation of Al with plateaus in the 650 and 1130°C regions and the percent weight gain increased as the particle size of Al decreased. Variable DSC and TG heating rate studies showed that the activation energies for the first step in the oxidation process increased as the particle size of Al increased.

Effect of sodium and potassium polyvanadates on aluminum-powder oxidation

The effect of polyvanadates and vanadium oxide bronzes of alkali metals on aluminum-powder oxidation has been studied by thermogravimetry and x-ray phase analysis. The macromechanism of promotion of aluminum oxidation in which the catalytically active component is meltedNa2AlxV12O31 has been established. It is shown that the intermediate product of vanadium-containing slag processing can be used as an activator.

Effect of the size factor and alloying on oxidation of aluminum powders

The effect of particle size and alloying with rare-earth elements on oxidation of aluminum-based powders was studied by methods of derivative differential thermal analysis of the change in the specific surface and x-ray diffraction analysis. It is shown that the character of oxidation is determined by processes occurring in the barrier layer of interaction products on the particle surface.

Effect of the properties of the surface oxide layers on the oxidation of aluminum powders

The oxidation of ASD-4 and AV-000 aluminum powders in the temperature range 293–1773°K has been studied by the thermogravimetry (under conditions of programmed heating), thermal argon desorption, and x-ray phase analysis methods. The stresses generated during heating in the outer oxide films on aluminum particles lead to the formation of cracks in the former, which promotes oxidation of the aluminum. Marked increases have been observed in the oxidation rate and specific surface of the ASD-4 powder at temperatures corresponding to the temperature range of the γ → α-Al2O3 transition. The finding appears to be linked with an increase in the intensity of crack formation during the polymorphic transformation and with enhanced diffusional mobility in the oxide layers.