Nanometric Alumina Research Articles

Study of potential transfer of aluminum to the brain via the olfactory pathway

Many employees in the aluminum industry are exposed to a range of aluminum compounds by inhalation, and the presence of ultrafine particles in the workplace has become a concern to occupational health professionals. Some metal salts and metal oxides have been shown to enter the brain through the olfactory route, bypassing the blood-brain barrier, but few studies have examined whether aluminum compounds also use this pathway. In this context, we sought to determine whether aluminum was found in rat olfactory bulbs and whether its transfer depended on physicochemical characteristics such as solubility and granulometry. Aluminum salts (chloride and fluoride) and various nanometric aluminum oxides (13nm, 20nm and 40–50nm) were administered to rats by intranasal instillation through one nostril (10μg Al/30μL for 10days). Olfactory bulbs (ipsilateral and contralateral relative to instilled nostril) were harvested and the aluminum content was determined by graphite furnace atomic absorption spectrometry after tissue mineralization. Some transfer of aluminum salts to the central nervous system via the olfactory route was observed, with the more soluble aluminum chloride being transferred at higher levels than aluminum fluoride. No cerebral translocation of any of the aluminas studied was detected.

Microstructural behaviour of spark plasma sintered composites containing bimodal micro- and nano-sized Al2O3 particles

ABSTRACTSpark plasma sintering (SPS) has been recognised, in the recent past, as a very useful method to produce metal matrix composites with enhanced properties if compared with traditional powder metallurgy techniques. Obviously, the materials final properties are strongly related to the reinforcement types and percentages as well as to the processing parameters employed during synthesis. The present paper analyses the effect of microscopic and nanometric alumina particles, blended to pure aluminium in different combinations, on the final properties of metal matrix composites produced via SPS. A strong variation in the microstructural behaviour has been observed by varying the nano to micro alumina particles percentage blended with aluminium ones.

Microstructural and mechanical behavior of bimodal reinforced Al-based composites produced by spark plasma sintering and FSP

Spark plasma sintering (SPS) is a powder metallurgy technique where uniaxial force and pulsed direct current are employed to perform metallic or ceramic particle consolidation in very short times. The high heating and cooling rates allow to prevent excessive grain growth favoring densification. Spark plasma sintering has been recognized, in the recent past, as a very useful method to produce metal matrix composites characterized by enhanced mechanical and wear properties. Obviously, the materials final properties are strongly related to the reinforcement types and percentages as well as to the processing parameters employed during synthesis. The present paper analyses the effect of microscopic and nanometric alumina particles, blended to pure aluminum in different combinations, on the final properties of metal matrix composites produced via SPS. The SPSed composites were friction stir processed (FSPed), and the processing forces and the heat input in the materials were analyzed. The microstructural and mechanical behavior of the processed materials are shown to be strongly dependent on the starting material properties and on the processing parameters employed during FSP.

Influence of Al2O3 Particle Size on Microstructure, Mechanical Properties and Abrasive Wear Behavior of Flame-Sprayed and Remelted NiCrBSi Coatings

The influence of micrometric alumina (low surface area-to-volume ratio) and nanometric alumina (high surface area-to-volume ratio) on microstructure, hardness and abrasive wear of a NiCrBSi hardfacing alloy coating applied to an AISI 304 substrate using flame spraying (FS) combined with surface flame melting (SFM) is studied. Remelting after spraying improved the mechanical and tribological properties of the coatings. Microstructural characterization using XRD, SEM and EDS indicated that alumina additions produced similar phases (NiSi, Ni3B, CrC and Ni31Si12) regardless of the alumina size, but the phases differed in morphology, size distribution and relative proportions from one coating to another. The addition of 12 wt.% nanometric Al2O3 increased the phases concentration more than five- to sixfold and reduced the hard phases size about four-to threefold compared with NiCrBSi + 12 wt.% micrometric Al2O3. Nanoalumina led to reduced mass loss during abrasive wear compared to micrometric alumina and greater improvement in hardness.

Effects of Nano-Sized Al on the Combustion Performance of Fuel Rich Solid Rocket Propellants

Several industrial- and research – type fuel rich solid rocket propellants containing nano-metric aluminum metal particles, featuring the same nominal composition, were prepared and experimentally analyzed. The effects of nano-sized aluminum (nAl) on the rheological properties of metal/HTPB slurries and fuel rich solid propellant slurries were investigated. The energetic properties (heat of combustion and density) and the hazardous properties (impact sensitivity and friction sensitivity) of propellants prepared were analyzed and the properties mentioned above compared to those of a conventional aluminized (micro-Al, mAl) propellant. The strand burning rate and the associated combustion fl ame structure of propellants were also determined. The results show that nAl powder is nearly “round” or “ellipse” shaped, which is different from the tested micrometric Al used as a reference metal fuel. Two kinds of Al (nAl and mAl) powder can be dispersed in HTPB binder suffi ciently. The density of propellant decreases with increasing mass fraction of nAl powder; the measured heat of combustion, friction sensitivity, and impact sensitivity of propellants increase with increasing mass fraction of nAl powder in the formulation. The burning rates of fuel rich propellant increase with increasing pressure, and the burning rate of the propellant loaded with 20% mass fraction of nAl powder increases 77.2% at 1 MPa, the pressure exponent of propellant increase a little with increasing mass fraction of nAl powder in the explored pressure ranges.

Microstructure and mechanical properties of the PM precipitation-hardening alloy N07626: effect of HIP temperature and solution annealing

The microstructure and mechanical properties of the precipitation-hardening powder metallurgy Ni-base alloys UNS N07626 are investigated. The material was compacted by hot isostatic pressing and solution treated adopting distinct conditions to reveal the effect of processing temperature on material properties. The microstructure is analysed by scanning electron microscopy, optical microscopy and electron backscatter diffraction and energy-dispersive X-ray spectroscopy. The prior particle boundaries (PPBs) are related to the presence of thermally stable nanometric Nb-carbides and aluminium oxides. An increase of processing temperature conditions is associated to a slight decrease of mechanical strength and to an appreciable improvement of toughness, as suggested by Instrumented V-Notch Charpy experiments. This behaviour is attributed to the reduction of the density of PPBs precipitates acting as void-nucleating sites in a micro-ductility mechanism. The precipitates’ density is related to a limited grain growth and the resulting separation of PPBs from grain boundaries.

Comments on: “Combustion of nano-sized aluminum particles in steam: Numerical modeling”, by V.B. Storozhev and A.N. Yermakov

This comment points out the large discrepancy between the predictions of the recently published model by Storozhev and Yermakov (Storozhev and Yermakov, Combust. Flame 162 (2015) 4129–4137) and the large body of accumulated literature on aluminum combustion. Their model predicts burning times between 40-63 ms for a cloud of nanometric aluminum particles (20 or 50 nm) burning in steam at initial temperatures around 2300 K. These predictions are roughly two orders of magnitude larger than the measured combustion times of micron and submicron aluminum particles from various experiments, which are on the order or hundreds of microseconds to milliseconds. Experimentally measured flame speeds of aluminum flames in the products of hydrocarbon flames, and the burn rates of metal-water propellants, also point to much faster burning rates of aluminum particles in steam than predicted by this model. As such, the predictions of the model by Storozhev and Yermakov cannot be reconciled with the available experimental evidence.

Influence of stirring conditions on Ni/Al2O3 nanocomposite coatings

The influence of the stirring conditions in the electrolyte on the structure and properties of Ni/Al2O3 nanocomposite coatings was studied. The coatings were produced by electrochemical reduction on a copper substrate in a Watts bath modified by nickel grain inhibitor, cationic surfactant and low concentrated nanometric alumina. The process has been carried out with different types of agitation (mechanical, ultrasonic stirring and with a copper rotating disc electrode). The structures of the produced coatings were characterised by scanning and transmission electron microscopy as well as X-ray diffraction. The influence of Al2O3 particles on the microhardness of composite coatings was also analysed. The adhesion of the composite coatings to the substrate and coefficient of friction were determined by means of the scratch test. The studies have shown that the bath stirring method affects the content and distribution of the alumina, as well as the properties of the produced composite Ni/Al2O3 coatings.

Role of molecular structure of monosaccharides on the viscosity of aqueous nanometric alumina suspensions

The effect of molecular structure of d-galactose, d-mannose, d-glucose, 2-deoxy-d-glucose and 3-O-methyl-d-glucose on the viscosity of aqueous alumina nanoparticle suspensions was investigated by rheological measurements. The viscosity was found to be dependant on the molecular structure of these hexopyranoses. The research reveals that the orientation of primary and quaternary hydroxyl groups in a pyranose ring plays a crucial role in dispersing properties of monosaccharides. Moreover, the most important is the mutual orientation of these hydroxyl groups. Monosaccharides with axial and equatorial orientation of hydroxyl groups at first (C1) and fourth (C4) carbon atom in pyranose ring decrease the viscosity more than monosaccharides with two axial hydroxyl groups in these positions. Due to the differences in molecular structure of saccharides resulting in different shape of molecules and different compatibility to the water structure, the saccharides differ in ability to disturb water layers around alumina particles.

The size-effect of Al2O3 on the sinterability, microstructure and properties of glass-alumina composites

The glass-ceramic composites were prepared from low-softening glass CaO–B2O3–SiO2, micrometer and nanometer alumina. The influences of nano-Al2O3 on the sinterability, microstructure, dielectric property, mechanical strength, and thermal expansion of glass-alumina composites were investigated. The densification mechanism of nano-Al2O3 modified glass-alumina composites was revealed further. The results show that nano-Al2O3 effectively lowers the densification temperature of glass-alumina ≤900°C. The density gradually increases with increasing nano-Al2O3 to 15 wt %, and then decreases. Dielectric constant, dielectric loss, and flexural strength respectively versus nano-Al2O3 exhibit a similar change trend to the density. The sample with 15 wt % nano-Al2O3 sintered at 900°C could form the dense microstructure (~2.95 g/cm3) and achieve the high performance: low dielectric constant (6.13), low dielectric loss (5.2 × 10−4), low thermal expansion (4.21 × 10−6/°C), and high mechanical strength (178 MPa).

Modelling mesoporous alumina microstructure with 3D random models of platelets.

This work focuses on a mesoporous material made up of nanometric alumina 'platelets' of unknown shape. We develope a 3D random microstructure to model the porous material, based on 2D transmission electron microscopy (TEM) images, without prior knowledge on the spatial distribution of alumina inside the material. The TEM images, acquired on samples with thickness 300 nm, a scale much larger than the platelets's size, are too blurry and noisy to allow one to distinguish platelets or platelets aggregates individually. In a first step, the TEM images correlation function and integral range are estimated. The presence of long-range fluctuations, due to the TEM inhomogeneous detection, is detected and corrected by filtering. The corrected correlation function is used as a morphological descriptor for the model. After testing a Boolean model of platelets, a two-scale model of microstructure is introduced to replicate the statistical dispersion of platelets observed on TEM images. Accordingly, a set of two-scale Boolean models with varying physically admissible platelets shapes is proposed. Upon optimization, the model takes into account the dispersion of platelets in the microstructure as observed on TEM images. Comparing it to X-ray diffraction and nitrogen porosimetry data, the model is found to be in good agreement with the material in terms of specific surface area.

Functionalized Al2O3 particles as additives in proton-conducting polymer electrolyte membranes for fuel cell applications

This study reports on the synthesis and characterization of sulfated Al2O3 and of composite membranes, prepared by dispersing the functionalized oxide in Nafion, acting as electrolytes in proton-exchange membrane fuel cells.Different synthetic routes were explored to obtain nanometric alumina particles with sulfate groups. Structural and morphological characteristics of the inorganic compounds and the nature of the bond of the sulfates with the oxide were investigated by x-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, N2 adsorption and thermal gravimetric analysis. Key properties of the hybrid membranes were elucidated in terms of thermal characteristics, water uptake and ionic exchange capacity. Functionality of the nanocomposite membranes, compared with plain Nafion, was tested in hydrogen-fed fuel cells. Polarization and power density curves and in-situ electrochemical impedance spectroscopy were accomplished to evaluate the effect of temperature on the cell performance.It is shown that well-addressed variations in the synthetic routes are able to determine different morphologies and dimensions of the particles and different degrees of functionalization. The incorporation of alumina in Nafion changes the characteristics of the membrane, with special regard towards hydration. In-situ fuel cell electrochemical tests reveal improved electrode-composite membrane interface properties as the working temperature of the cell increases.

Biophysicochemical Interaction of a Clinical Pulmonary Surfactant with Nanoalumina.

We report on the interaction of pulmonary surfactant composed of phospholipids and proteins with nanometric alumina (Al2O3) in the context of lung exposure and nanotoxicity. We study the bulk properties of phospholipid/nanoparticle dispersions and determine the nature of their interactions. The clinical surfactant Curosurf, both native and extruded, and a protein-free surfactant are investigated. The phase behavior of mixed surfactant/particle dispersions was determined by optical and electron microscopy, light scattering, and zeta potential measurements. It exhibits broad similarities with that of strongly interacting nanosystems such as polymers, proteins or particles, and supports the hypothesis of electrostatic complexation. At a critical stoichiometry, micron-sized aggregates arising from the association between oppositely charged vesicles and nanoparticles are formed. Contrary to the models of lipoprotein corona or of particle wrapping, our work shows that vesicles maintain their structural integrity and trap the particles at their surfaces. The agglomeration of particles in surfactant phase is a phenomenon of importance that could change the interactions of the particles with lung cells.

Sol–gel route for the building up of superhydrophobic nanostructured hybrid-coatings on copper surfaces

A wet chemical route is herein presented with the aim of building up a superhydrophobic coating on copper (Cu). A thin film of flower-like alumina – obtained by sol–gel – was deposited and combined with fluoroalkylsilane moieties, resulting in a hybrid coating with excellent repellence to water (static contact angle of 179±1°) and self-cleaning properties (contact angle hysteresis of 5±1°). The wetting performances were strictly related to the peculiar morphology of the coating's inorganic component and to the chemistry of the outer organic layer. The combination of the nanometric alumina lamellas with the micrometric roughness of sandblasted Cu surface proved to be essential to the formation of the hierarchical scaled structure allowing superhydrophobicity. However, the surface extension of alumina layer and its functional effectiveness were threatened by Cu oxides occasionally formed during the annealing steps necessary to stabilize the coating. Field emission-scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) analyses of the surfaces, in fact, revealed the simultaneous presence of different chemical species and morphologies. Grains with cube-like aspect (attributable to Cu2O) were formed on coated surfaces thermally treated at 200–300°C, while microwires of CuO appeared at higher temperature. Once the thermal conditions are carefully tuned and the temperature kept not higher than 200°C, it is possible to limit the presence of Cu oxides which, in turn, means preserving a high level of performances, also avoiding brittleness phenomena and keeping unchanged the surface optical properties. The coating's stability and the maintenance of superhydrophobicity were preliminarily investigated following the water contact angle evolution after immersion of samples in ethanol in an ultrasonic bath.

Mechanism of dislocation kinetics under magnetoplastic effect

The plasticity of material is associated closely with the movement and proliferation of dislocation. Therefore, in the deformation and plasticity theory the dislocation kinetics is an important topic. In the case of no magnetic field, the conventional dislocation kinetics normally focuses on the dislocation microstructure, nucleation and mobility, and the inherent relationship between electron spin and plasticity is seldom concerned. As a matter of fact, the electron rotation is directionless and unordered in the absence of magnetic field, so the electron behavior will not take an apparent effect on the microstructure and properties of material. Nevertheless, in the presence of magnetic field the case is different. The magnetic field will influence the electron spin and, therefore, atomic rearrangement. The dislocation behavior and plasticity will also be affected by the magnetic field, which is called the magnetoplastic effect. In this paper, on the basis of magnetoplastic effect the dislocation kinetics involving dislocation stress, mobility and others is discussed both qualitatively and quantitatively. It has rarely reported currently in the literature. The pulsed magnetic field is first utilized to process solid nanometer alumina particulates reinforced aluminum matrix composites. The experimental results demonstrate that the dislocation density increases with magnetic induction intensity increasing from zero to 3 T. The phenomenon reveals the characteristic of plastic deformation in a processed sample. The further theoretical analysis displays that the generated magnetic force is not large enough to activate the dislocation movement. The fundamental reason lies in the magnetoplastic effect, that is, the magnetic field brings about the transition of electron spin in the radical pairs between paramagnetic dislocation cores and obstacles. The radical pairs tend to be conversed from the singlet state with high bonding energy to the triplet state with low bonding energy, therefore, the prerequisite energy for dislocation to surmount the obstacles will be lowed and the depinning tendency will be apparent. In a period of dislocation movement, the rate limiting consists in the dislocation stopping at the obstacle; on the contrary, the electron excitation and atomic arrangement governed by the magnetic field take negligible time. Hence, it can be seen that the performance of magnetic field is highly efficient. The critical magnetic induction intensity is calculated to be 3 T. That is, when the intensity is lower than 3 T, the magnetoplastic effect becomes strong with the increase of magnetic induction intensity and action time; when the intensity is higher than 3 T, the effect changes gently. Under this critical magnetic induction intensity, the dislocation velocity is deduced to be on the order of 10-3 m/s. Moreover, the dislocation length will be increased by two orders of magnitude. The displacement of dislocation is proportional to the square of magnetic induction intensity and action time of magnetic field. To sum up, the magnetic field treatment has been proved to be an efficient approach to improve the plasticity of material. The prospective research will focus on the mechanical properties of alloys or composites subjected to magnetic field, together with tensile stress so as to acquire the effect of magnetic field parameters of macro plasticity of materials.

EFFECTS OF NANO-METRIC ALUMINUM POWDER ON THE PROPERTIES OF COMPOSITE SOLID PROPELLANTS

Several industrial−and research−types of hydroxy-terminated polybutadiene (HTPB)-based composite solid propellants were experimentally analyzed. In general, they feature the same nominal composition, but different nano-metric aluminum particles (nAl) are investigated as metal fuels and contrasted to a conventional propulsion grade micrometric aluminum (5.6 µm average grain size), which are taken as reference. Strand burning rates and the associated combustion flame structures of propellants were studied. The mechanical sensitivity and microstructure surface of the formulations were compared to that of a conventional aluminized propellant already certified as steady. The results show that the common Al powders are of irregular shapes, while the tested nAl particles are of disperse and approximate "spherical shapes", and tend to pack or stick together easily into cold clots. The aluminum are insensitive for friction and impact stimuli (0%, >24.66 N · m) and all the propellant formulations containing different aluminum particles size were sensitive to impact and friction (36%, 100%) except the based propellant with common aluminum powder (32%, 96%). The nano-sized metal additives can affect the combustion behavior and change the burning rate effectively. The burning rate can be increased more than 55.0% for 5% of nAl replaced common Al powder in the formulations. The burning rate and pressure exponent values depend on the formulation of the fraction of nAl powder, and a change of part mass fraction of nAl particles may boost the burning rate higher than that of the common Al powder. Both the burning rate and the pressure exponent can be increased up to 90.0% (13 MPa), and from 0.38 to 0.54 (1−15 MPa), respectively, for 10% of nAl replaced common Al powder in the propellant formulations.

Thermal decomposition of d-metal nitrates supported on alumina

The thermal decomposition of cobalt, nickel, manganese, zinc, and copper nitrates supported on nanometric alumina was investigated and compared with decomposition of corresponding bulk nitrates. TG, DTA, and MS measurements in air were performed. The supported nitrates decompose in lower temperatures than the bulk ones and their decomposition proceeds in fewer stages which are better separated. Synthesized materials and bulk nitrates before degradation of nitrates groups undergo dehydration. For decomposition of manganese and copper nitrates, the last step of water vapour releasing is combined with degradation of nitrate groups thus formation of anhydrous metal nitrate during decomposition is not achievable. Thermal decomposition of bulk nitrates leads to oxides—Co3O4, NiO, MnO2, ZnO, and CuO—respectively, as the solid residue. The nickel, zinc, copper, and manganese nitrates while supported on alumina decompose to corresponding oxides (NiO, ZnO, CuO, MnO2) as well. For decomposition of cobalt nitrate while supported on Al2O3 as the solid residue CoAl2O4 were identified. The correlation between dehydration and degradation of nitrates groups temperatures for bulk and supported nitrates was analysed in terms of atomic properties of d-metals.

Synthesis of Alumina Using Aluminum Acetate

Alumina is a material used in a wide range of applications, including refractories, structural materials, sensors, catalysts, etc. Several synthesis methods are available for the production of submicrometric and nanometric alpha alumina. However, there is few studies on the use of acetate for the synthesis of alumina. Thus, the present work has as aim the synthesis of alumina powders using aluminum acetate. The precursors were obtained from the decomposition of aluminum acetate under temperatures of 650oC and 850oC, resulting in an amorphous precursors and gamma-alumina. These materials were submitted to dissolution and calcination. Based on the results, it is observed a reduced in the temperature formation of alpha phase, at around 900oC, when using the adopted methodology of synthesis. Amorphous precursor was more susceptible to the synthesis procedure used and generated alpha alumina in lower temperatures.

Research on the tribological performance of Cr2O3 filled with bronze‐based PTFE composites

ABSTRACTEnhancement of the wear resistance of bronze‐filled polytetrafluoroethylene (PTFE) composites has been achieved using various fillers, for example, chromic oxide (Cr2O3), molybdenum disulfide (MoS2), graphite, and nanometer aluminum oxide (n‐Al2O3), in the present study. The wear resistance was evaluated by a block‐on‐ring wear tester, and the effects of fillers on the wear resistance as well as the mechanism were investigated. The wear rate for the composite where the recipe containing 59% PTFE + 40% bronze + 1% Cr2O3 was 0.5 × 10−5 mm−3/N m and for the composite in the recipe containing 60% PTFE + 40% bronze was 4.2 × 10−5 mm−3/N m, which meant that that Cr2O3 increased the wear resistance by approximately 10 times. The differential scanning calorimetry measure analysis showed that Cr2O3 had a positive effect on the crystallization of PTFE; the crystallinity of PTFE composites increased from 45% to 52%, which exhibited improved wear resistance. Wear testing and scanning electron microscope analysis had shown that Cr2O3 had a positive effect on the formation of transfer film and keeping it stable to exhibit improved wear resistance. X‐ray photoelectron spectroscopy results also showed that Cr2O3 was effective in tribochemical reactions during sliding against stainless ring; these maybe responsible for forming transfer film and lowering wear rate of composite. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41117.

Softening behaviour of alumina reinforced copper processed by equal channel angular pressing

In this present work, the softening behaviour of Cu–1·1 wt-%Al2O3 alloy containing nanometric alumina particles was investigated. Single pass equal channel angular pressing was first applied on cylindrical specimens, and then, the deformed samples were subjected to annealing treatments in the temperature range of 700–1000°C. Afterwards, finite element analysis, microstructural observations, Vickers hardness and shear punch testing were employed to study strain distribution as well as the recrystallisation behaviour of the examined alloy. It was found that the alloy is softened at 800°C or above, which shows its high thermal stability. Furthermore, a very fine completely recrystallised microstructure was obtained via a continuous mechanism of recrystallisation at 1000°C. The activation energy of recrystallisation was measured to be ∼181 kJ mol−1, which offers a high thermal stability in high temperature applications of this alloy.