Nanosized Alumina Powder Research Articles

Oxidation of nano-sized aluminum powders

Oxidation of aluminum nanopowders obtained by electro-exploded wires is studied. Particle size distributions are obtained from transmission electron microscopy (TEM) images. Thermo-gravimetric (TG) experiments are complemented by TEM and XRD studies of partially oxidized particles. Qualitatively, oxidation follows the mechanism developed for coarser aluminum powder and resulting in formation of hollow oxide shells. Sintering of particles is also observed. The TG results are processed to account explicitly for the particle size distribution and spherical shapes, so that oxidation of particles of different sizes is characterized. The apparent activation energy is obtained as a function of the reaction progress using model-free isoconversion processing of experimental data. A complete phenomenological oxidation model is then proposed assuming a spherically symmetric geometry. The oxidation kinetics of aluminum powder is shown to be unaffected by particle sizes reduced down to tens of nm. The apparent activation energy describing growth of amorphous alumina is increasing at the very early stages of oxidation. The higher activation energy is likely associated with an increasing homogeneity in the growing amorphous oxide layer, initially containing multiple defects and imperfections. The trends describing changes in both activation energy and pre-exponent of the growing amorphous oxide are useful for predicting ignition delays of aluminum particles. The kinetic trends describing activation energies and pre-exponents in a broader range of the oxide thicknesses are useful for prediction of aging behavior of aluminum powders.

Transformation kinetics of θ- to α-phase of alumina powders prepared from different alumina salts by chemical processing

The aim of this study was to produce nanosized alumina powders from two different aluminum salts (AlCl3 and AlNO3.9H2O) by precipitation method and to investigate transformation kinetics of α-Al2O3 crystallization obtained from quantitative DTA curves via Kissinger–Akahira–Sunose (KAS) method. Precipitates are dried at 80 °C for 24 h and then calcinated at 1200 and 1300 °C for 1 h. Dried powders have bayerite [Al(OH)3] and calcinated powders have theta- (θ) and alpha (α)-alumina phases which confirmed by XRD analysis, and the crystallite size of powders determined by Scherrer equation is about 30 nm for dried powders and ranged from approximately 15–35 nm for calcinated powders. Scanning electron microscopy studies revealed that the morphology and size of powders precipitated from two different salts are similar to each other, and powders contain mainly sub-micrometer-sized particles and close to spherical form. Activation energy values determined by KAS equation range from 717 to 411 kJ mol−1 for powders produced from aluminum nitrate salt and from 844 to 476 kJ mol−1 for powders produced from aluminum chloride salt depending on the transformation rate.

Invited Paper: Photoignited Aluminum Nanopowder Combustion in Air

Metal combustion has recently received renewed interest as a result of the ability to manufacture and characterize nanoparticles. These nanoparticles are known to exhibit desirable traits, mainly their high specific surface area. The objective of this work was to investigate experimentally the oxidation behavior of nanosized aluminum powders in air under white light photo-ignition conditions. Powder samples with three different nominal size particles of 20, 50, and 70 nm were characterized with HRSEM and TEM. The results showed that the reactivity of photo-ignited powder decreased with increasing size whereas the energy threshold for ignition had the opposite trend. The presence of a natural oxide coating and changes in the optical properties of the powder are proposed as the cause of this behavior.

Influence of the content of Mn2+ on the phase composition of the precursor of MnO x -Al2O3 catalyst and the catalytic activity in the oxidation reaction of methane

The influence of the concentration of Mn2+ in a solution for the hydrolysis of nanosized aluminum powder on the phase composition of the precursor and the content of manganese in samples of aluminum hydroxide is studied. Stages of the formation of catalysts from the precursors are studied by means of synchronic thermogravimetric (TG)-differential scanning calorimetry (DSC) analysis and X-ray phase analysis (XPA). The influence of the number of manganese ions in aluminum hydroxide on the phase composition and catalytic properties of MnOx-Al2O3 systems in the reaction of the deep oxidation of methane is also studied. It is shown that an increase in the concentration of manganese ions in the solution for hydrolysis raises the content of unreacted aluminum and X-ray amorphous hydroxides in the precursors of the catalysts. It is established that the phase composition of the catalyst and the catalytic activity in the reaction of the deep oxidation of methane depend on the content of manganese in the precursor.

Two-step preparation of AlON transparent ceramics with powder synthesized by aluminothermic reduction and nitridation method

Abstract

Effect of alumina nanofiller on the viscosity and electrical conductivity of glass-based seals for solid oxide fuel cells

Use of alumina nanofiller to strengthen the structure of the sealing glass used for solid oxide fuel cells (SOFCs) has been investigated by measurement of fracture strength, viscosity, and electrical conductivity at elevated temperatures. The electrical conductivity and viscosity of two glass-based seals for SOFCs were compared. Measurement of electrical conductivity and viscosity of the glass-based seals over the temperature range 400–600 °C showed that addition of 10 % nano-sized alumina powder to the sealing glass increased both the high-temperature electrical conductivity and viscosity of the sintered glass matrix. The higher conductivity and viscosity of glass composites containing alumina nanofiller were attributed to formation of bonds with bridging oxygen by partial dissolution of the alumina nanofiller in the sealing glass. Infrared spectroscopy and X-ray diffraction analysis were conducted to confirm this conclusion. Strengthening of the sealing glass structure to improve long-term durability may be possible by using glass composites containing alumina nanofiller.

Novel fabrication route for porous silicon carbide ceramics through the combination of in situ polymerization and reaction bonding techniques

ABSTRACTFor the first time, an in situ polymerization technique was applied to produce mullite‐bonded porous SiC ceramics via a reaction bonding technique. In this study, SiC microsized particles and alumina nanopowders were successfully coated by polyethylene (PE), which was synthesized from the particle surface in a slurry phase reactor with a Ziegler–Natta catalyst system. The thermal studies of the resulting samples were performed with differential scanning calorimetry and thermogravimetric analysis. The morphology analysis obtained by transmission electron microscopy and scanning electron microscopy (SEM) confirmed that PE was successfully grafted onto the particle surface. Furthermore, the obtained porous ceramics were characterized in terms of their morphologies, phase composition, open porosity, pore size distribution, and mechanical strength. SEM observations and mercury porosimtery analysis revealed that the quality of the dispersion of nanosized alumina powder into the microsized SiC particles was strongly enhanced when the particles were coated by polymers with in situ polymerization. This resulted in a higher strength and porosity of the formed ceramic porous materials with respect to the traditional process. In addition, the X‐ray diffraction results reveal that the amount of mullite as the binder increased significantly for the samples fabricated by this novel method. The effects of the sintering temperature, forming pressure, and polymer content on the physical and mechanical properties of the final porous ceramic were also evaluated in this study. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40425.

Fabrication of mullite-bonded porous SiC ceramics via a sol–gel assisted in situ reaction bonding

Abstract In the present work, mullite-bonded porous SiC ceramics were fabricated using reaction bonding techniques. The morphologies, phase composition, open porosity, pore size distribution and mechanical strength of porous ceramics were examined as a function of alumina sources (calcined nano-sized alumina powder and alumina sol prepared from hydrolysis of aluminum isopropoxide) and contents. It was found that the addition of alumina in powder form effectively enhanced the strength and decreased the porosity. In contrast, when alumina was added in sol form, a reverse effect was observed. Moreover, it was revealed that when a portion of calcined alumina was replaced by alumina sol, the mechanical properties improved significantly (more than 30%) as well as porosity compared to the traditional method. Pore size distribution analysis showed that the dispersion of nanosize alumina powder and SiC micro-particles in alumina sol is strongly improved compared to mixing in ethanol.

Effect of Al<SUB>2</SUB>O<SUB>3</SUB> Nano-Filler on Properties of Glass-Based Seals for Solid Oxide Fuel Cells

This study compares the viscosity and strength of three glass-based seals prepared with or without nano or micron-sized alumina powder used as filler material. Measurements of the viscosity and bending strength of the glass-based seals showed that addition of the nano-sized alumina powder to the glass increased both the high-temperature viscosity and the strength of the sintered glass matrix. Strength tests and observations of the microstructure of the fracture surface of the seal samples confirmed the strengthening of the glass network structure. Conversion of non-bridging oxygen to bridging oxygen is presumed to occur upon the addition of alumina to the glass sample. The strengthening of the alumina-glass composite seal was attributed to the alumina nano-filler and prolonged heat treatment at elevated temperatures.

Mechanochemical synthesis of alumina nanoparticles: Formation mechanism and phase transformation

Abstract Nanosized alumina powders were synthesized by mechanochemical treatment of stoichiometric mixture of anhydrous AlCl3 and CaO. X-ray powder diffraction (XRD), differential thermal and thermogravimetric analysis (DSC-TG), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to understand the formation mechanism and the exact phase transformation sequence in this system. Milling of the initial raw materials resulted in the formation of crystalline CaO and partially amorphous AlCl3, with no sign of chemical reaction between the constituents. Heating the as-milled powder led to the formation of amorphous aluminum hydroxide (Al(OH)3) and calcium chloride (CaCl2). Based on the results obtained in this study, AlCl3 hydrolyses in the heating stage and CaO adsorbs the produced HCl forming CaClOH and CaCl2 crystalline phases. Heat treatment of the amorphous Al(OH)3 resulted in the formation of amorphous alumina (Al2O3). Amorphous alumina transformed into η-, κ-, and α‐Al2O3 when calcined at higher temperatures. Based on Rietveld refinement, it was concluded that η-Al2O3 and κ-Al2O3 phases are stable up to an average critical crystallite size of around 13 nm and 39 nm respectively.

Synthesis of nanosized alumina powders by pulsed wire discharge in air flow atmosphere

Pulsed wire discharge (PWD) is a method to form nanosized powders which is based on evaporation of thin metal wires by applied large electric current. Nanosized oxide powders can be synthesized by PWD in oxygen atmosphere. Since only small amount of powders can be synthesized by single discharge to a piece of wire, the number of discharges has to be increased for mass production of nanosized powders. Then, PWD equipments with wire feeding and gas flow atmosphere systems were developed for mass production. However, effects of gas flow on formed powders have never been investigated. In this study, PWD in air flow atmospheres were tried to synthesize alumina powders from industrial viewpoints. The effects of air flow rate on the oxidation phenomena and the synthesized powders have been investigated. PWD of pure Al wires with diameters of ϕ 0.3–0.5mm were examined in air flow rates of 15L/min and 33L/min as well as 0L/min (closed atmosphere condition). The synthesized powders at all experimental conditions were dominantly composed of γ-Al2O3 and δ-Al2O3 phases. The δ/γ ratios of the Al2O3 phases increased with increasing air flow rate and wire diameter. Besides them, small amount of metallic Al phase was remained in the powders synthesized by PWD in the air flow atmosphere of 33L/min. It is concluded that appropriate air flow rate should be adopted for mass production of alumina powders in order to achieve enough collection of particles and avoid mingling of metallic Al phase.

Production of nanosized aluminum powders and their application as fillers for composite materials based on ultrahigh molecular weight polyethylene (UHMWPE)

A modified levitation-in-flow Gen-Miller technique is developed to produce nanodisperse aluminum particles 40–200 nm in size with a barrier coating of controlled thickness made of aluminum oxides and (oxy)nitrides for the use as a dielectric filler in composite materials. The polymerization filling technique (in situ polymerization) is used to synthesize new highly filled nanocomposite materials based on UHMWPE and nanodisperse aluminum with the barrier coating combining dielectric, heat-conducting, and plastic properties, which can replace brittle ceramics in a number of fields. The samples of such nanocomposites with a content of nanoaluminum ranging from 20 to 80 wt % were synthesized and characterized by electron microscopy and X-ray diffraction analysis.

Preparation and Light Transmission Properties of AlON Ceramics by the Two-Step Method with Nanosized Al2O3 and AlN

The single-phase aluminum oxynitride (AlON) powder used in this experiment was obtained at relatively low temperature by the solid-state-reaction method with nanosized AlN powder and nanosized alumina powder. The obtained powder was ground by ball milling and doped with Y2O3; then, it was shaped into pellets. Transparent ceramics sintering was carried out at 2153 K (1880 °C) for 5, 10, and 20 hours. Obtained samples for three duration times are all are transparent, and the light transmittance of samples increases with increasing the holding time at the sintering temperature. The transmittance of the sample with 20 hours holding time is 55 pct at near 5 μm. X-ray diffraction (XRD) measurement shows that all samples were single-phase AlON. The fracture surfaces of samples from scanning electron microscopy (SEM) analysis shows that with increasing the holding time, the grain size grows slightly, while the pore size and porosity decrease apparently, which bring about the transmittance increase. The pore size and the porosity are the main factors affecting the light transmitting properties of samples.

Explosion characteristics of nano-aluminum powder–air mixtures in 20 L spherical vessels

Using 20 L spherical explosion vessel, the explosion characteristics of nano-aluminum powders with different sizes were investigated. Compared with micro-scale aluminum powders explosion, nano-powders explosion processes and mechanisms were analyzed based on the auxiliary analysis of scanning electron microscopy (SEM) and X-Ray diffraction (XRD). The results show that the maximum explosion pressure and maximum rate of pressure rise mainly depended on the dust concentrations. With the increasing of dust concentration, the maximum explosion pressure increases gradually to the maximum when the dust concentrations below 1000 g/m 3 and then the maximum explosion pressure decreased especially for the dust concentrations higher about 1250 g/m 3. At the same time, the trends of maximum rate of pressure rise performed the similar rules with the dust concentrations. For the selected nano-powders, particle size change seems no obvious explosion differences. However, for micro-sized aluminum powders, explosion characteristic presents decreased change rules with the particle size increase. At the same time, the lower explosion concentration limits of aluminum powders explosion were measured and presented. Research result may have important implications for nano-sized aluminum powders utilization and safety operation.

Adsorption of Nickel Ions from Aqueous Solutions by Nano Alumina: Kinetic, Mass Transfer, and Equilibrium Studies

The presence of nickel in aquatic systems due to discharge of industrial effluents is of concern because of its toxic and nonbiodegradable nature. Its removal from water and wastewater is mandatory. Though activated carbon has been effective in removing metallic species including nickel from water and effluents, its high cost limits its large scale application to developing nations. The present paper addresses the removal of nickel from aqueous solutions by alumina nano particles. The adsorbent, nano alumina powder, was synthesized in the laboratory and was characterized by X-ray diffractometry (XRD), Fourier transformation infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). The BET surface area and porosity of nanosized alumina powder were found to be 78.79 m2 g−1 and 0.51, respectively. The initial concentration of nickel, agitation speed, and contact time were found to affect the removal of nickel from aqueous solutions. Kinetic studies were performed and pseudofirst order, second...

Excess energy in the electroexplosive nanopowders

The electroexplosive route for nanopowders production is a rapidly occurring nonequilibrium process under the action of high power density energy. The electrical explosion of conductor (EEC) powder particles have a spherical shape with protective oxide–hydroxide layer which is an adsorbed layer of working gas on the surface of particles. The formation of EEC end products (nanopowder) occurs in conditions of coexistence of metal in different states: the vapor form (cluster), plasma and superheated liquid. In this work, the formation of particles and excess energy stored during EEC were studied. Themal analysis is suggested as a method for the determination of excess energy value. According to the results of the thermal analysis, the excess energy value for nanosized aluminum powder with various storage time after passivation are 61.99–11.89 kJ/mole, respectively.

Effect of phase inversion on microporous structure development of Al 2O 3/poly(vinylidene fluoride-hexafluoropropylene)-based ceramic composite separators for lithium-ion batteries

To improve the thermal shrinkage of the separators that are essential to securing the electrical isolation between electrodes in lithium-ion batteries, we develop a new separator based on a ceramic composite membrane. Introduction of microporous, ceramic coating layers onto both sides of a polyethylene (PE) separator allows such a progress. The ceramic coating layers consist of nano-sized alumina (Al 2O 3) powders and polymeric binders (PVdF-HFP). The microporous structure of the ceramic coating layers is observed to be crucial to governing the thermal shrinkage as well as the ionic transport of the ceramic composite separators. This microporous structure is determined by controlling the phase inversion, more specifically, nonsolvent (water) contents in the coating solutions. To provide a theoretical basis for this approach, a pre-investigation on the phase diagram for a ternary mixture comprising PVdF-HFP, acetone, and water is conducted. On the basis of this observation, the effect of phase inversion on the morphology and air permeability (i.e. Gurley value) of ceramic coating layers is systematically discussed. In addition, to explore the application of ceramic composite separators to lithium-ion batteries, the influence of the structural change in the coating layers on the thermal shrinkage and electrochemical performance of the separators is quantitatively identified.

Computational fluid dynamic modeling of a chemical vapor synthesis process for aluminum nanopowder as a hydrogen storage precursor☆

A chemical vapor synthesis (CVS) process for synthesizing nano-sized aluminum powder as a precursor for various hydrogen storage materials was simulated by the use of computational fluid dynamic modeling. The fluid flow, heat transfer and chemical reaction phenomena taking place inside the reactor were analyzed together with particle formation and growth in the CVS process. The temperature, velocity and particle size distribution fields inside the reactor were computed. Chemical reaction rate and population balance model were used to calculate the particle formation and growth. The particle size computed by the program was compared with the experimental data, and the calculated average size of the final product particles was consistent with those obtained in the experimental work.

PHASE INVERSION PREPARATION AND MORPHOLOGICAL STUDY OF POLYVINYLIDENE FLUORIDE ULTRAFILTRATION MEMBRANE MODIFIED BY NANO-SIZED ALUMINA

In this study, polyvinylidene fluoride (PVDF) membranes with various structures ranging from dense to highly asymmetric morphologies were obtained by changing the effective parameters in the phase inversion process. The effect of some important processing parameters such as solution concentration, harshness of the precipitation bath, exposure time before immersion in coagulation bath and bath temperature was studied and the mentioned parameters were optimized. The membranes were then modified with nano-sized alumina powder in order to improve the hydrophilicity of the PVDF membranes. The surface morphology, surface and cross-sectional structures of the membranes were examined by scanning electron microscope (SEM) and atomic force microscope (AFM). The specific surface area of the membranes was determined using the Brunauer–Emmett–Teller (BET) method. The contact angles between water and the membranes' surfaces were measured in order to study the hydrophilicity changes of the membrane surface. The results indicated that the addition of nano-sized alumina to the casting dope increased the hydrophilicity of the PVDF membrane surface.

The effect of the particle size and the morphology of alumina powders on the processing of green bodies by electrophoretic deposition

The electrophoretic deposition (EPD) method was investigated for the preparation of compacts from nanosized alumina powders. Ethanol suspensions of commercially available nanosized transition alumina and α-alumina powders were used. In addition, submicron powders were also considered for comparison. Besides the influence of operational pH change, the influence of the addition of three different surfactants – two branched polyethylene imines with molecular masses 1800 and 10,000, and citric acid—on the zeta-potential of the particles in ethanol and afterwards on the EPD was investigated. The packing density of the green parts prepared by the EPD of nanosized α-alumina powders at 30 V/cm was low (the green density was 27 vol. % of the theoretical density—TD), and a bimodal pore size distribution was observed. In contrast, the packing density in deposits made with the transition alumina nanopowder was much higher (the green density was 42 vol. % of the TD) and a monomodal distribution of pores was observed. In general, for all the deposits made from nanosized alumina powders, the particle-packing density was found to be lower than for those made from the submicron alumina powders (a green density equal to 59 vol. % of the TD).