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The present study investigates the alumina particle size effect on hydrogen production from ammonia decomposition by dielectric barrier discharge plasma (DBD). Two different alumina particle diameters of 1 mm and 2 mm were used as a catalyst activation material. A low NH3 concentration of 0.5% was fed into the catalytic reactor at flow rates range of 0.1-1 L/min and dielectric barrier discharge plasma (DBD) voltage range of 12–18 kV. The decomposed gas samples were collected and measured by gas chromatography (GC) for both experiments. It was found that the hydrogen concentration results of plasma-alumina particle size 1 mm were higher than that obtained from alumina catalyst particle size of 2 mm. also, the conversion rate was determined at different applied voltage and flow rates for both alumina catalytic systems. The maximum conversion rates were 83.19, 80.35% at Al2O3 particle diameter of 1 mm, and 2 mm, respectively. Also, the maximum energy efficiency results of NH3 decomposition using alumina particle size of 1 mm and 2 mm were 0.29 and 0.225% which obtained at NH3 flow rate of 1, and 0.5 L/min, respectively. It can be concluded that the activation surface area is influenced by the catalyst material particle size hence the H 2 production from the NH3 decomposition process.

Thin-walled 3D-customized ceramic components can be used, e.g. for inert and thermal resistant housings as well as special surface structured kiln furniture. For manufacturing such components, deep drawing of ceramic green tapes has been used. The technology has been almost not applied for processing of ceramic green tapes. A new approach was developed to realize a homogenous ceramic particle packing and hence uniform green density within a deep drawn green tape. After debinding and sintering, defect-free model structures with a dense microstructure (density > 99%) were achieved. Within this study, the deep drawing of ceramic green tapes by using a new approach was investigated. The developed approach brings several beneficial properties together, e.g. a reproducible form deviation and more importantly a homogeneous particle distribution allowing homogeneous and dense structures. With this results, the transformation of non-complex and cheap ceramic green tapes to 3D-customized and near-net-shape thin-walled mouldings becomes possible.

Aluminum hydroxide is an essential material for the industrial production of ceramics (especially insulators and refractories), desiccants, absorbents, flame retardants, filers for plastics and rubbers, catalysts, and various construction materials. The calcination process of Al(OH)3 first induces dehydration and, finally, results in α-Al2O3 formation. Nevertheless, this process contains various intermediary steps and has been proven to be complicated due to the development of numerous transitional alumina. Each step of the investigation is vital for the entire process because the final properties of materials based on aluminum trihydroxide are determined by their phase composition, morphology, porosity, etc. In this paper, five dried, milled, and size-classified aluminum hydroxide specimens were thermally treated at 260, 300, and 400 °C, then, they were studied in order to identify the effects of temperature on their properties, such as particle morphology, specific surface area, pore size, and pore distribution. The major oxide compounds identified in all samples were characteristic of bauxite—namely, Al2O3 * 3H2O, SiO2, Fe2O3, Na2O, and CaO. Particles with smaller sizes (&lt, 10 µm = 76.28%) presented the highest humidity content (~5 wt.%), while all samples registered a mass loss of ~25 wt.% on ignition at 400 °C. The identified particles had the shapes of hexagonal or quasi-hexagonal platelets and resulted in large spherulitic concretions. The obtained results suggest that ceramic powders calcined at 400 °C should be used for applications as adsorbents or catalysts due to their high specific area of about 200–240 m2/g and their small pore width (3–3.5 nm).

ThermoCatalytic Decomposition of methane (TCD) is studied as a method to convert natural gas into hydrogen and functional carbon. In these processes the carbon typically formed on top of a catalyst phase leading to particle growth. Therefore, the development of a particle growth model is necessary to understand the limitations of thermocatalytic decomposition of methane and to assess optimal parameters and process conditions. In this paper, a particle growth model is presented to describe the growth of functional carbon on the catalyst particle. This coupled model requires kinetic equations and information on deactivation rates which have been studied from literature. The morphology of the particle changes due to carbon formation, which leads to eventual deactivation. Therefore, these kinetic expressions are coupled to a particle growth model based on the analogy with the growth of particles in polyolefin production. To combine the effects of particle growth, kinetics, and internal heat and mass transfer, the Multi-Grain Model (MGM) was used. Results confirm that with the currently available catalysts the carbon yield is not affected by heat and mass transfer limitations, however, with the availability of more active catalysts these limitations will become important. Temperature, however, has a significant role in that it regulates the kinetic rate and thus growth rate, which in turn influences the catalyst deactivation. The optimum temperature for the production of nano-carbon, within a reasonable process time, therefore sensitively depends on the choice of catalyst.

Smelter grade alumina (SGA) plays multiple roles in the Hall&ndash, H&eacute, roult process for primary aluminum production. Given its very porous nature, one major role of SGA is to adsorb toxic hydrogen fluoride (HF) in the dry scrubber. However, also because of its porous nature, SGA inevitably adsorbs ambient moisture. This paper discusses the influence of alumina properties, including pore size distribution and specific surface area, on the physical adsorption of water vapor on SGA, as well as the adsorption kinetics. The result shows that the adsorption enthalpy of moisture on SGA is in the range of 4&ndash, 13 kJ/mol. The adsorption capacity increases significantly with the particle specific surface area and total pore volume. A higher adsorption temperature indicates a much faster adsorption rate but corresponds to a lower equilibrium adsorption capacity.

Due to the increased use of nanoparticles in everyday applications, there is a need for theoretical descriptions of particle transport and attachment in porous media. It should be possible to develop a one dimensional model to describe nanoparticle retention during capillary transport of liquid mixtures in porous media. Water-glycerol-nanoparticle mixtures were prepared and the penetration process in porous Al2O3 samples of varying pore size is measured using NMR imaging. The liquid and particle front can be measured by utilizing T2 relaxation effects from the paramagnetic nanoparticles. A good agreement between experimental data and the predicted particle retention by the developed theory is found. Using the model, the binding constant for Fe2O3 nanoparticles on sintered Al2O3 samples and the maximum surface coverage are determined. Furthermore, we show that the penetrating liquid front follows a square root of time behavior as predicted by Darcy's law. However, scaling with the liquid parameters is no longer sufficient to map different liquid mixtures onto a single master curve. The Darcy model should be extended to address the two formed domains (with and without particles) and their interaction, to give an accurate prediction for the penetrating liquid front.

Molecular dynamics (MD) simulations and thermodynamic analysis are conducted to investigate size effect on melting point of alumina nanocrystals. Different geometries including spherical and cubic particles, planar thin films, and spherical shells are considered. The atomic interactions in MD simulations are captured using Vashishta et al.’s potential function. Thermophysical properties of concern such as the bulk melting point, latent heat, density, and surface free energy are calculated using MD simulations and fed as inputs to the thermodynamic models. Predictions of MD simulations are compared with those of thermodynamic melting models. For all cases, heterogeneous melting is observed, where nucleation of the melt phase occurred at the free surface and the melting front propagated into the interior regions of the crystal. Results suggest that the melting point drops sharply below a threshold particle size, which is a function of the geometry. The threshold particle size of spherical particles is greater than that of planar films. Melting point predictions are quite sensitive to the choice of thermodynamic model and thermophysical property values. To ascertain the effects of curvature and core size on shell melting point, melting points of planar semi-infinite films and spherical shells are calculated and compared. For particle sizes of concern to practical applications (∼100 nm or greater), shell melting point is nearly independent of core size and the semi-infinite planar film approximation is reasonable to estimate the melting point of alumina shells. In this size regime, melting points of 2–4 nm thick spherical oxide shells vary roughly in the range of 1800–2350 K, which is relatively close to the bulk melting point. Results of the present study do not indicate an enormous depression in the melting point, as the previous works suggest. This implies that the melting point depression of the oxide layer cannot fully describe the scatter in the measured ignition temperature of aluminum particles., by Nikhil Joshi, Nilkumar Mathur, Tejas Mane and Dilip Sundaram

We implement the C-shaped sample test method and micro-cantilever beam testing to measure the local strength of microscopic, low-aspect-ratio ceramic particles, namely high purity vapor grown a-alumina Sumicorundum (R) particles 15-30 mu m in diameter, known to be attractive reinforcing particles for aluminum. Individual particles are shaped by focused ion beam micromachining so as to probe in tension a portion of the particle surface that is left unaffected by ion-milling. Mechanical testing of C-shaped specimens is done ex situ using a nanoindentation apparatus, and in the SEM using an in-situ nanomechanical testing system for micro-cantilever beams. The strength is evaluated for each individual specimen using bespoke finite element simulation. Results show that, provided the particle surface is free of readily observable defects such as pores, twins or grain boundaries and their associated grooves, the particles can achieve local strength values that approach those of high-perfection single-crystal alumina whiskers, on the order of 10 GPa, outperforming high-strength nanocrystalline alumina fibers and nano-thick alumina platelets used in bio-inspired composites. It is also shown that by far the most harmful defects are grain boundaries, leading to the general conclusion that alumina particles must be single-crystalline or alternatively nanocrystalline to fully develop their potential as a strong reinforcing phase in composite materials. (C) 2017 The Authors. Published by Elsevier Ltd.

Composites modified with nanoparticles are of interest to many researchers due to the large surface-area-to-volume ratio of nano-scale fillers. One challenge with nanoscale materials that has received significant attention is the dispersion of nanoparticles in a matrix material. A random distribution of particles often ensures good material properties, especially as it relates to the thermal and mechanical performance of composites. Typical methods to quantify particle dispersion in a matrix material include optical, scanning electron, and transmission electron microscopy. These utilize images and a variety of analysis methods to describe particle dispersion. This work describes how photoluminescent spectroscopy can serve as an additional technique capable of quickly and comprehensively quantifying particle dispersion of photoluminescent particles in a hybrid composite. High resolution 2D photoluminescent maps were conducted on the front and back surfaces of a hybrid carbon fiber reinforced polymer containing varying contents of alumina nanoparticles. The photoluminescent maps were analyzed for the intensity of the alumina R1 fluorescence peak, and therefore yielded alumina particle dispersion based on changes in intensity from the embedded nanoparticles. A method for quantifying particle sedimentation is also proposed that compares the photoluminescent data of the front and back surfaces of each hybrid composite and assigns a single numerical value to the degree of sedimentation in each specimen. The methods described in this work have the potential to aid in the manufacturing processes of hybrid composites by providing on-site quality control options, capable of quickly and noninvasively providing feedback on nanoparticle dispersion and sedimentation.

The process of advanced aluminum-alumina powders production for selective laser melting was studied. The economically effective method of obtaining aluminum&ndash, alumina powdery composites for further selective laser melting was comprehensively studied. The aluminum powders with 10&ndash, 20 wt. % alumina content were obtained by oxidation of aluminum in water. Aluminum oxidation was carried out at &le, 200 &deg, C. The oxidized powders were further dried at 120 &deg, C and calcined at 600 &deg, C. Four oxidation modes with different process temperatures (120&ndash, C) and pressures (0.15&ndash, 1.80 MPa) were investigated. Parameters of aluminum powders oxidation to obtain composites with 10.0, 14.5, 17.4, and 20.0 wt. % alumina have been determined. The alumina content, particle morphology, and particle size distribution for the obtained aluminum&ndash, alumina powdery composites were studied by XRD, SEM, laser diffraction, and volumetric methods. According to the obtained characteristics of aluminum&ndash, alumina powdery composites, they are suitable for the SLM process.

Simulation of fluidization of a bed of solid particles in a two-dimensional box is performed using molecular dynamics methods by assuming that superposition of buoyant, gravitational, viscous, collision force will make the particle to move in two dimensions. Two different fluid type is used in this simulation, i.e., gas and liquid fluid. By increasing the fluid velocity from small to high, then minimum fluidization condition reached. As a result, fluidization of alumina particle with liquid fluid has a minimum fluidization velocity lower than gas fluid.

We analyzed the formation of the aluminum (Al) nanoparticles (NPs) with triangular shape obtained by ablating Al bulk in liquid using pulses with different durations (5 ns, 200 ps, and 30 fs) and wavelengths (355 nm, 800 nm, and 1064 nm). We report three stages of synthesis and aging of Al NPs: Formation, transformation, and stable stage. The NPs prepared by different pulses are almost identical at the initial stage. The effects of duration and wavelength of the ablation pulses on the aging of NPs are revealed. Pulse duration is determined to be essential for morphological transformation of NPs, while pulse wavelength strongly influences particle sizes. NPs produced by ultra-short pulses have smaller sizes and narrow size distribution. We demonstrate that oxidation and hydrolysis of Al in water are the results of ablation for all pulse durations and wavelengths, which also strongly modify the preferable reaction path of NPs in water, thus affecting the composition and morphology of triangle NPs. The results of modeling of the NPs generation in water due to a 50 ps laser pulse interacting with a thick Al target are presented. Water-based effects in the formation of NPs, their evolution, and solidification are considered from the mechanical and thermophysical points of view. The detailed analysis of the modeling results allowed for determination of the main mechanism responsible for the ablation process followed by the NPs formation.

Edited by Jean-Pierre Bellot, Pascal Gardin, Frédéric Gruy; International audience; This paper presents a process and its variants to prepare alumina particle suspension in liquid tin alloy. Complication may arise due to the non-wettability of alumina by tin alloy. For this purpose, prior to introduce the powder into the liquid metal, the particles of alumina are covered by a thin layer of tin metal by using a controlled milling process. The operating conditions leading to an optimal coverage without particle damage and to a good dispersibility in the liquid metal have been searched.

Planetary ball mills are able to perform dry and wet grinding. Most experimental analyses and computer simulations in this field are mainly about dry grinding. In order to empirically evaluate the wet grinding process, test grindings benches were manufactured, calcined alumina powder was ground, and the particle distribution analyzed. Effects of the media geometry (size and shape) on particle distribution, morphology, and phase composition of ground alumina were investigated. Smaller media leads to smaller particle size and cylindrical media provides a poor grinding performance but its product has the narrower particle size distribution. Alumina showed no phase change in the grinding tests. The experiments presented in this study can be used as an aid in the proper choice of grinding media or even to validate emerging models capable of predicting breakage in wet grinding.

Micro-abrasion remains a test configuration hugely used, mainly for thin coatings. Several studies have been carried out investigating the parameters around this configuration. Recently, a new study was launched studying the behavior of different ball materials in abrasive particles’ dynamics in the contact area. This study intends to extend that study, investigating new ball materials never used so far in this test configuration. Thus, commercial balls of American Iron and Steel Institute (AISI) 52100 steel, Stainless Steel (SS) (AISI) 304 steel and Polytetrafluoroethylene (PTFE) were used under different test conditions and abrasive particles, using always the same coating for reference. Craters generated on the coated samples’ surface and tracks on the balls’ surface were carefully observed by Scanning Electron Microscopy (SEM) and 3D microscopy in order to understand the abrasive particles’ dynamics. As a softer material, more abrasive particles were entrapped on the PTFE ball’s surface, generating grooving wear on the samples. SS AISI 304 balls, being softer than the abrasive particles (diamond), also allowed particle entrapment, originating from grooving wear. AISI 52100 steel balls presented particle dynamics that are already known. Thus, this study extends the knowledge already existing, allowing to better select the ball material to be used in ball-cratering tests.

Alumina based particles were prepared from aluminium chloride hydroxide as starting material by sol-gel technique. One series of particles was doped with ferrous oxide. Both series of particles were calcinated at three different temperatures: 700 degrees C, 800 degrees C and 900 degrees C. Poly(methyl methacrylate), PMMA, was used as a matrix and two different types of alumina based particles were added into the matrix to form the composites. All composites consisted of 3 wt% of alumina based particles. The aim of this study was to examine whether and how the temperature of particle calcination affects the microhardness and mechanical properties of the composite. The particles were characterized by the X-ray diffraction (XRD) and physical absorption methods. The morphology of the composites was examined using a field emission scanning electron microscope (FESEM). The microhardness of composites was measured using a traditional Vickers hardness (HV) method. The mechanical characteristics of obtained composites were determined using tensile test and impact testing.

Previous studies on vapor-phase synthesis of titania (TiO2), silica (SiO2), and alumina (Al2O3) nanoparticles were reviewed. Interactions between physicochemical phenomena involved in the particle growth and the operating variables were investigated. Strategies to produce non-aggregated spherical particles of the metal oxides are discussed. Model predictions based on the sintering laws without any adjustments in the sintering parameters rarely agreed with experimental data. There remains more to be understood in reaction and nucleation kinetics, sintering, and fragmentation mechanisms until the technology is developed to the stage of designing reactors for mass production of non-aggregated spherical particles for titania, silica and alumina

The aim of this study was to evaluate the effect of an 11.6Li2O-16.8ZrO2-68.2SiO2-3.4Al2O3 (LZSA) glass ceramic on the grain growth of alumina obtained by pressureless sintering. Three contents of the LZSA glass ceramic (7, 15, and 21 vol%; mean particle size of 1.52 µm) and three types of alumina (mean particle sizes of 0.5, 1.7, and 2.8 µm) were used. The powder compositions were formed by cold pressing. The samples (5 mm × 5 mm × 13 mm) were sintered in an optical dilatometer. The grain size and morphology were evaluated by scanning electron microscopy. The results show that elongated alumina particles were obtained for the investigated composites compared with pure alumina because of the LZSA addition. The grain growth of alumina was less pronounced at higher LZSA contents. High relative densities were obtained for higher LZSA contents at lower temperatures and holding times compared with pure alumina.

In this study, the performance of an alumina nanofluid cooled double pipe gas cooler for trans-critical CO2 refrigeration cycle is theoretically compared to that of water cooled gas cooler. Equal pumping power comparison criterion is adopted besides conventional equal Reynolds number comparison base. Nanofluid is loaded with 0.5%, 1.5% and 2.5% of particle volume fraction under turbulent flow conditions. Drastic variation of thermal and transport properties of CO2 in the vicinity of pseudo critical temperature is taken care of by employing an appropriate discretization technique. Effect of gas cooler pressure, Reynolds number, pumping power and nanoparticle volume fraction on COP of refrigeration system, gas cooler overall conductance, effectiveness and its capacity has been studied. Results indicate that at equal Reynolds number comparison, performance for alumina nanofluid cooled system is better than that of water cooled system. On the other hand, at equal pumping power comparison basis, the performance of water cooled system is superior. Even at equal mass flow rate comparison criterion, the performance of nanofluid cooled system degrades with increase in particle volume fraction. This study is expected to help to assess the nanofluid as a coolant before expensive experimentation.

International audience; Aluminum (Al) powders with spherical and irregular particle shapes were mixed with two alumina (Al2O3) powders with either a spherical or an angular particle shape to achieve high-performance cold-sprayed coatings onto steel. Two effects of the aluminum particle shape were observed. First, coating microstructure observation showed impinging heterogeneity depending on particle shape. Second, particle jet differences depending on particle morphology were shown by velocity maps. From the latter, SEM and XRD, three effects of the alumina particle shape were also shown, i.e., higher in-flight velocity of angular particles, fragmentation of spherical hollow particles and embedding of alumina particles with aluminum. Numerical simulation of particle impacts was developed to study the densification of Al coating due to Al2O3 addition through elucidation of Al-Al2O3 interaction behavior at the scale of the coating. Al/Al and Al/Al2O3 interfaces were investigated using TEM to understand coating strengthening effects due to alumina addition at the scale of the particle. As a whole, Al and Al2O3 particle shape effects were claimed to explain coating mechanical properties, e.g., microhardness and coating–substrate bond strength. This study resulted in specifying criteria to help cold spray users in selecting powders for their applications, to meet economic and technical requirements.

The wetting of angular alumina particle preforms by Cu-Sn alloys is investigated by means of pressure infiltration experiments conducted at 1150 A degrees C, using a system enabling dynamic and precise measurements of the metal volume injected into the preform. Wetting is quantified in terms of drainage curves, which plot the volume fraction of molten metal in the packed powder preform (also called saturation) versus the applied pressure. The shape of the curves confirms earlier findings, namely (i) the initial stage of infiltration is dominated by percolation and obeys a universal scaling relation while the metal shape is fractal; and (ii) at higher saturation, incremental pore-filling is dictated by local pore geometrical characteristics, saturation curves obeying then the Brooks-Corey correlation. According to sessile drop data in the literature, the Cu-Sn system is characterized by relatively small changes in the contact angle with alloy composition (128A degrees-122A degrees), while the metal surface tension changes by more than a factor two over the whole composition range. One would, therefore, expect that the drainage curves be indifferently proportional to the molten metal surface tension or the work of immersion: the present data show that this is indeed verified.

In the present work on pseudo-eutectic alumina-yttria stabilized zirconia (YSZ) composite, the influence of spray parameters on amorphous phase formation during suspension plasma spray deposition is investigated. Several variables were evaluated as the most probable key factors influencing the amorphous phase formation. These variables include powder feed size, in-flight particle characteristics (temperature and velocity), spraying robot travel speed, preheating the substrate, number of deposition passes and the presence of bond coat. It was found that larger particle size, higher robot speed and substrate preheating lead to larger amounts of amorphous phase. Moreover, it is shown that particle velocity and temperature need to be reasonably low to get greater amorphous content while the presence of a bond coat has no significant influence on the amorphous phase formation. In contrast, increasing the number of deposition passes is detrimental to this phase. This work also discusses some correlations observed between the amorphous phase content and the in-flight particle characteristics and coating grain sizes. © 2012 Elsevier B.V.

The hydrodynamic study of nanofluids in microchannel is carried out. For this, three microchannels of hydraulic diameters of 130, 211 and 300 ?m are fabricated by photolithographic and wet etching processes on silicon wafers. Alumina nanofluids with concentrations 0.25. vol.%, 0.5. vol.% and 1.0. vol.% with particle sizes 45. nm and 150. nm are prepared, stabilized and characterized by standard methods like sonication, pH variation, Transmission Electron Microscope (TEM) and Dynamic Light Scattering (DLS) measurements. The base fluids used are water and Ethylene Glycol. The effect of volume fraction, channel size, particle size and base fluids are presented and analyzed. It is shown that there is an early transition to turbulence for 211 and 301 ?m channels. In addition, numerical modeling is carried out by using mixture rule and discrete phase modeling approach. This approach is found to be more suitable at higher Reynolds numbers whereas at lower Reynolds numbers the conventional mixture model can be used. Shear induced particle migration is identified to be an important phenomenon at higher Reynolds number and hence the reason for difference between these two models. � 2012 Elsevier Inc.

The composite solid electrolytes, NaSn2F5, dispersed with submicron size Al2O3 fillers of various concentrations and particle sizes have been synthesized through mechanochemical milling technique. X-ray diffraction and microstructure results indicate the biphasic nature of the composite materials. The transport properties of the present composite materials have been investigated by means of impedance spectroscopy, and the results show that the improvement in conductivity increases with decrease in the particle size of the filler. An enhancement in conductivity of two orders in magnitude is obtained for NaSn2F5 with Al2O3 dopant concentration of 10 mol%. The activation energy responsible for conductivity relaxation, calculated from the modulus spectra, is found to be almost the same as the value obtained from temperature variation of dc conductivity. The scaling result of the imaginary part of the modulus shows the temperature-independent relaxation behavior. � 2012 Springer-Verlag.

Particle-based simulations using the discrete element method are applied for the sintering of thin ceramic stripes that are constrained by a rigid substrate. The inhibition of lateral particle movement in vicinity of the substrate leads to a preferred shrinkage direction perpendicular to the substrate, which causes shape distortion as well as delamination of the stripe at the interface edges. Multiple processing parameters are varied in simulations to analyze their influence on the mentioned effects. The amount of particle rearrangement and the height to width aspect ratio of the cross-section are identified to be factors that determine the level of shape distortion and edge delamination. The simulation results are compared with measurements of stripes produced by soft micromolding in capillaries. Good accordance is observed regarding parameters like axial and lateral shrinkage or delamination angle.

Experimentos do fluxo da água usando um meio poroso não saturado compoto por diferentes sistemas de a- alumina molhada não-saturada indicam que a carga superficial da partícula diminuiu com o aumento da água movimentada. O movimento máximo de água ocorreu quando a carga superficial era zero. O movimento da água foi afetado pela espessura da dupla camada elétrica no pH em que as partículas têm carga líquida (a partir do IEPpH). Estes resultados sugerem estruturação da água, induzida por carga, adjacente a superfícies de partículas carregadas. Experiments of water flow using a non saturated porous medium constituted by different a- wet unsaturated alumina systems indicate that particle surface charge decreased the amount of water moved. Maximum water movement occurred when surface charge was zero. Water movement was affected by thickness of the electrical double layer at pH where particles have net charge (out of the IEPpH). These results suggest charge-induced structuring of water adjacent to charged particle surfaces. Fil: Torres Sanchez, Rosa Maria. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Tecnología de Recursos Minerales y Cerámica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Tecnología de Recursos Minerales y Cerámica; Argentina

Gold nanoparticles (Au NPs) were prepared by reducing HAuCl4 with NaBH4. Their average particle sizes could be tuned in the range of 1.7 and 8.2 nm, by adjusting the amount of NaBH4 used during synthesis. The obtained Au NPs (colloids) were then loaded onto a commercial Al2O3 support to prepare Au/Al2O3 catalysts with tunable Au particle sizes. An optimal pH value (5.9) of the Au colloid solution was found to be essential for loading Au NPs onto Al2O3 while avoiding the growth of Au NPs. Au NPs and Au/Al2O3 catalysts were tested in the reduction of p-nitrophenol with NaBH4. Interestingly, the catalytic activity depended on the size of Au NPs, being the highest when the average size was 3.4 nm. Relevant characterization by UV-Vis, TEM, and XRD was conducted.

The transmission of transparent colored ceramics based on Al(2)O(3) doped with light transition metals is measured in the visible and infrared range. To clarify the role of the dopands we perform ab initio calculations. We discuss the electronic structure and present optical spectra obtained in the independent particle approximation. We argue that the gross spectral features of Co- and Ni-doped Al(2)O(3) samples are described by our model, while the validity of the approach is limited for Cr-doped Al(2)O(3).

International audience; Plasma spraying involves the total or partial melting of micrometer sized particles, flattening of the particles in about one microsecond onto a substrate to build a coating by layering the resulting solidified splats. The coating adhesion is essential and depends mainly on the behaviour of first lamellae in contact with the substrate. But in the plasma spray process about 108 particles/s impact onto the substrate, and thus it is difficult to understand the role of the different spray parameters onto the coating adhesion. In order to get a better understanding of phenomena involved, it is necessary to study the formation of a single lamella. An experimental set-up has been designed to study this process and is composed of a fast (50 ns) two-colour pyrometer and an imaging system, comprising two fast (1-10 ls) CCD cameras triggered by the velocity signal of the particle in flight prior to its impact. This work is focused on alumina particles flattening onto stainless steel (304L) substrates preheated at different temperatures during different times.

Understanding the mechanism of cracking during the drying of aqueous colloidal dispersions is important to preventing film failure. While most of the reported work has dealt with stable aqueous dispersions, a few studies have focused on flocculated systems. The latter especially assumes importance because the role of particle packing in the mechanism of cracking is not completely understood. In this work, we study the cracking of colloidal films cast from flocculated aqueous dispersions of alpha-alumina. Here, the extent of flocculation is controlled by varying the pH of the dispersion and characterized in terms of the final packing volume fraction of the dried film. The influence of varying the close-packed volume fraction on the critical cracking thickness and critical cracking stress is measured. The measurements are compared with the model predictions based on Griffith's energy balance, and good agreement is found between theory and experiments, suggesting that the model is universal and applies equally well to stable as well as flocculated systems.

In this study, 2618 aluminum alloy metal matrix composites (MMCs) reinforced with two different sizes and weight fractions of SiCp particles upto 10% weight were fabricated by stir cast method and subsequent forging operation. The effects of SiCp particle content and size of the particles on the mechanical properties of the composites such as hardness, tensile strength, hot tensile strength (at 120 �C), and impact strength were investigated. The density measurements showed that the samples contained little porosity with increasing weight fraction. Optical microscopic observations of the microstructures revealed uniform distribution of particles and at some locations agglomeration of particles and porosity. The results show that hardness and tensile strength of the composites increased, with decreasing size and increasing weight fraction of the particles. The hardness and tensile strength of the forged composites were higher than those of the cast samples. � 2008 Springer Science+Business Media, LLC.

In the present study, nano aluminium particles were produced by wire explosion process (WEP) in nitrogen, argon and helium atmospheres. Thus produced nano particles were characterized through certain physico-chemical diagnostic studies using wide angle X-ray diffraction (WAXD) and by energy dispersive analysis by X-rays (EDAX). The size and shape of the powder were analysed by using transmission electron microscopic (TEM) studies. The particle size distribution studies were performed by adopting log-normal probability distribution. The relationship between size of the particle generated in the explosion process and the type of inert gas/pressure was analysed. The mechanisms of nano particle formation, the factors which can aid the process of formation of nano particle in the wire explosion process were analysed. It is realized that energy deposited to the conductor and duration of current flow have major impact on particles produced by this process. � Indian Academy of Sciences.

A uniform distribution of zirconia particles in an alumina matrix can be achieved by chemical processing methods. Three chemical processing routes were investigated to obtain better powders. The sintered density and the t-ZrO 2 content were correlated with powder size and morphology.

Rheology of powder suspensions is a key factor in many processing routes, and better understanding of the parameters that control the rheology improves technical progress by reducing the empirical factors in the formulation of powder slurries. The strict requirements for the production of transparent polycrystalline alumina demand a fundamental understanding of powder handling and processing steps. The use of dopants (Mg, Y, La) has proven useful, but produces noticeable effects on suspension rheology and influences the choice of the processing route. With the prospect of spray granulation of such slurries, the rheological behaviour of doped alumina suspensions was investigated from a fundamental approach taking into account slurry and particle characteristics, and computing the interparticle potentials. This information was then used in a yield stress model which successfully predicted the rheological behaviour of the doped alumina slurries. A strategy for improved slurry formulation is presented.

particle beds with high purityAl (99.99%). These materials feature 40–60 vol. pct reinforcement homogeneously distributed in a pore-freematrix. Their tensile behaviour is studied as a function of reinforcement size and shape. Internal damage, inthe form of particle fracture and matrix voiding, occurs from the onset of plastic straining. Its evolution withstrain is monitored through changes in (i) stiffness and (ii) peak stress after incremental plastic straining andannealing. The influence of damage on the flow curves of the composites can be accounted for using basicpostulates of continuum damage mechanics. Failure strains vary between 2 and 4%, and are a function ofthe rate of damage accumulation. An expression is derived to predict elongation to failure of damagingmaterials that fail by tensile instability, which gives good agreement with the experimental observations. 2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.Keywords: Composites; Dislocations; Aluminium; Stress–strain relationship measurements; Damage1. INTRODUCTION

It is commonly considered that, in ceramic particle reinforced metals, the volume fraction of ceramic should not exceed 30% if the composite is intended for structural applications: otherwise its toughness and ductility generally become unacceptably low. Particle reinforced metal matrix composites produced by infiltration provide a material with which this assumption can be tested. We first present a summary of recent results from our laboratory showing that aluminium matrix composites containing 50% or more ceramic particles can be tough, strong and relatively ductile, despite the high ceramic loadings. We then discuss toughening mechanisms that can explain the mechanical properties displayed by these composites, to highlight the importance of the particle strength distribution on the composite fracture toughness.

This study investigates focused electron beam induced etching for the removal of alumina particles on patterned extreme ultra violet (EUV) mask using nitrosyl chloride (NOCl) as assist gas. As potential contaminant, particles of aluminum oxide (alumina, Al2O3) have been successfully removed, leaving the underlying layers undamaged. Particles were applied onto an EUV mask, consisting of a multilayer Bragg mirror capped with a thin ruthenium layer and a structured tantalum nitride (TaN∕TaON) absorber/antireflective film. Alumina particles were selectively etched using the chlorine-based gas, NOCl. Neither the Ru nor the absorber was significantly etched during the process in spite of a square area scanned by the focused electron beam being larger than the particle. The process resolution is discussed based on Monte Carlo electron scattering simulations. Thermodynamic driving forces for the electron-induced reactions and its selectivity are discussed and a chemical rationale is proposed.

We investigate processing-microstructure relationships in the production of Al2O3 particle reinforced copper composites by solidification processing. We show that during production of the composites by gas-pressure infiltration of packed Al2O3 particle preforms with liquid Cu or with liquid Cu8wt%Al at either 1,150 or 1,300 A degrees C, capillarity-driven transport of alumina can cause rounding of the Al2O3 particles. We use quantitative metallography to show that the extent of particle rounding increases markedly with temperature and with the initial aluminum concentration in the melt. An analysis of the thermodynamics and kinetics governing the transport of alumina in contact with molten copper, considering both interfacial and volume diffusion, leads to propose two mechanisms for the rounding effect, namely (i) variations in the equilibrium concentration of oxygen in the melt as affected by the initial aluminum concentration, or (ii) segregation of aluminum to the interface with the ceramic.