Micrometric Powders Research Articles

Microwave Ignited Combustion Synthesis as a Joining Technique for Dissimilar Materials

Microwave energy has been exploited to ignite combustion synthesis (CS) reactions of properly designed powders mixtures, in order to rapidly reach the joining between different kinds of materials, including metals (Titanium and Inconel) and ceramics (SiC). Beside the great advantage offered by CS itself, i.e., rapid and highly localized heat generation, the microwaves selectivity in being absorbed by micrometric metallic powders and not by bulk metallic components represents a further intriguing aspect in advanced materials joining applications, namely the possibility to avoid the exposition to high temperatures of the entire substrates to be joined. Moreover, in case of microwaves absorbing substrates, the competitive microwaves absorption by both substrates and powdered joining material, leads to the possibility of adhesion, interdiffusion and chemical bonding enhancements. In this study, both experimental and numerical simulation results are used to highlight the great potentialities of microwave ignited CS in the joining of advanced materials.

Distinctive Toxicity of TiO2 Rutile/Anatase Mixed Phase Nanoparticles on Caco-2 Cells

Titanium dioxide has a long-standing use as a food additive. Micrometric powders are, e.g., applied as whiteners in confectionary or dairy products. Possible hazards of ingested nanometric TiO(2) particles for humans and the potential influence of varying specific surface area (SSA) are currently under discussion. Five TiO(2)-samples were analyzed for purity, crystallinity, primary particle size, SSA, ζ potential, and aggregation/agglomeration. Their potential to induce cytotoxicity, oxidative stress, and DNA damage was evaluated in human intestinal Caco-2 cells. Only anatase-rutile containing samples, in contrast to the pure anatase samples, induced significant LDH leakage or mild DNA damage (Fpg-comet assay). Evaluation of the metabolic competence of the cells (WST-1 assay) revealed a highly significant correlation between the SSA of the anatase samples and cytotoxicity. The anatase/rutile samples showed higher toxicity per unit surface area than the pure anatase powders. However, none of the samples affected cellular markers of oxidative stress. Our findings suggest that both SSA and crystallinity are critical determinants of TiO(2)-toxicity toward intestinal cells.

Activity of Antimicrobial Silver Polystyrene Nanocomposites

A simple technique based on doping polymers with in situ generated silver nanoparticles (Ag/PS films) has been developed. In particular, an antiseptic material has been prepared by dissolving silver 1,5‐cyclooctadiene‐hexafluoroacetylacetonate in amorphous polystyrene, and the obtained solid solution has been heated for ca. 10 s at a convenient temperature (180°C). Under such conditions the metal precursor decomposes producing silver atoms that diffuse into the polymer and clusterize. The antimicrobial characteristics of the resulting polystyrene‐based material have been accurately evaluated toward Escherichia coli (E. coli) comparing the cytotoxicity effect of 10 wt.% and 30 wt.% (drastic and mild annealing) silver‐doped polystyrene to the corresponding pure micrometric silver powder. Two different bacterial viability assays were performed in order to demonstrate the cytotoxic effect of Ag/PS films on cultured E. coli: (1) turbidimetric determination of optical density; (2) BacLight fluorescence‐based test. Both methods have shown that silver‐doped polystyrene (30 wt.%) provides higher antibacterial activity than pure Ag powder, under similar concentration and incubation conditions.

STRUCTURAL AND MORPHOLOGICAL EVALUATION OF NANO-SIZED MoSi2 POWDER PRODUCED BY MECHANICAL MILLING

Nano-sized intermetallic powders have received great attention owing to their property advantages over conventional micro-sized counterparts. In the present study nano-sized MoSi 2 powder has been produced successfully from commercially available MoSi 2 (3 μm) by a mechanical milling process carried out for a period of 100 hours. The effects of milling time on size and morphology of the powders were studied by SEM and TEM and image analyzing system. The results indicate that the as-received micrometric powder with a wide size distribution of irregular shaped morphology changes to a narrow size distribution of nearly equiaxed particles with the progress of attrition milling up to 100 h, reaching an average particle size of 71 nm. Structural evolution of milled samples was characterized by XRD to determine the crystallite size and lattice microstrain using Williamson-Hall method. According to the results, the crystallite size of the powders decreases continuously down to 23 nm with increasing milling time up to 100 h and this size refinement is more rapid at the early stages of the milling process. On the other hand, the lattice strain increases considerably with milling up to 65 h and further milling causes no significant changes of lattice strain.

The study on the microwave sintering of tungsten at relatively low temperature

Tungsten and its alloys have to be of high density and good performance in the application for plasma facing materials in fusion reactors and for target materials in spallation neutron sources. In this paper the relatively high density tungsten was fabricated by microwave sintering at low temperatures (1673–1773K) in a short time (30min) using the ball milled micrometer and nanometer powders without any sintering-assistant additives. The density of the microwave-sintered samples increases with sintering temperature. The grain size can be controlled to be smaller than 1μm and the thermal conductivity can reach 140W/mK at room temperature when the WC ball milled nanometer tungsten powders were used. However, due to the introduction of some impurity phases during ball milling process, the thermal conductivity of the W samples microwave-sintered from the steel-ball milled powders is only about 50W/mK at room temperature, even though these samples have higher relative density.

One-Step Hydrothermal Synthesis and Characterization of ZnO Nanopowders

In the paper, ZnO nanopowders were synthesized by one-step hydrothermal synthesis using zinc acetate and sodium hydroxide as raw materials. The influences of molar concentration of NaOH and synthesis temperature on the properties of ZnO nanopowders were investigated. XRD and FSEM were used to characterize ZnO nanopowders. The results showed that; when the molar concentration of NaOH was 0.05 mol/L, 1mol/L and 2mol/L, ZnO micrometer powders were obtained. When the molar concentration of NaOH was 4mol/L, ZnO nanorods or nanosheets were obtained with different reaction temperature. When the temperature was 220°C, ZnO nanorods with the length of 500nm and diameter of 100nm, were synthesised. Pure ZnO nanopowders can be obtained at lower temperature of 100°C by using one-step hydrothermal synthesis. When the synthesis temperature was 100°C and the molar concentration of NaOH was 4mol/L, ZnO nanosheets were produced. The length, width and thickness of ZnO nanosheets were about 800 nm, 500nm and 80nm, respectively.

Sintering Behavior of Nano‐ and Micro‐Sized ZnO Powder Targets for rf Magnetron Sputtering Applications

In this work, the nonisothermal sintering behavior of as‐received commercial high purity ZnO micrometric (m_ZnO), submicrometric (sm_ZnO) and nanometric (n_ZnO) powders was studied. The sintering behavior for sputtering target production was evaluated by changing the green density of samples from 62% of theoretical density (TD) to 35%. We observed that for n_ZnO powder, the maximum shrinkage rate (MSR) temperature (TMSR) was not affected by the green density, and that it was reached at lower temperatures (~710°C) compared with m_ZnO and sm_ZnO powders. For these powders, the temperature of MSR increased from 803°C to 934°C and from 719°C to 803°C as TD changed from 62% to 35% TD, respectively. Small grain size (~0.560 μm) and high density targets were obtained for n_ZnO when sintered at temperatures below the TMSR. Heating rate from 1°C to 15°C/min led to lower activation energy for n_ZnO (~201 ± 3 kJ/mol) than for the submicrometric (sm_ZnO) (~332 ± 20 kJ/mol) and micrometric (m_ZnO) (~273 ± 9 kJ/mol) powders. Using the model proposed by Bannister and Woolfrey, an n value of 0.75 was found, which was correlated with a combination of viscous flow and volume diffusion mechanisms that should control the initial stage of n_ZnO sintering. No significant differences were observed for n_ZnO powder in terms of density when the size of targets (scale‐up effect) was increased, while in the case of m_ZnO and sm_ZnO, a delay in the densification was observed, which was related to the higher sinterability of n_ZnO powder. Two inches ZnO ceramic targets with different particle sizes and final densities were used in an rf magnetron sputtering system to produce ZnO films under the same deposition conditions. Films with thickness around 100 nm and good uniformity were produced using those targets, and no variation was observed in the optical and morphological properties. However, low electrical resistivity (1.4 Ω·cm) films were obtained with n_ZnO targets, which could be explained in terms of a nonstoichiometric Zn:O composition of the started powders.

Magnetic removal of surfactants from wastewater using micrometric iron oxide powders

The aim of this paper is the study of a sustainable process for the treatment of urban wastewater able to reduce surfactant concentrations close to the back-ground levels or, at least, lower than the values allowed by law for a reuse in agriculture. The considered process is based on the adsorption of surfactants (water diluted) on commercial iron oxide powders and their removal in a magnetic filtration system. The powders of hematite and magnetite used have a diameter of 0.5, 1 and 5 μm, respectively; they are non-toxic for humans and the environment and they have a relatively low cost. The removal of surfactants on a laboratory scale at concentrations in the wastewater range (0.2–4.2 mg/l) was studied applying the treatment on pure surfactants, mixtures of pure surfactants and detergents. With regard to the adsorption on magnetite, despite the large quantity of powder required (17–51 g/l), the tests led to positive results for cationic surfactants (up to 90% of removal) and relatively good for the anionic (up to 20%) and non-ionic ones (up to 40%). Adsorption on hematite has shown encouraging results with regard to all surfactants (from 50% to higher than 90% of removal) with a much lower amount of powder required (1–17 g/l). In all cases the adsorption took 10 min and the magnetic separation of the iron oxides was fully achieved after 10 min of filtration.

Experiments on Heat Transfer in Fe Micrometer and Nanometer Powders

This paper investigates characteristics of heat transfer in Fe micrometer and nanometer powder. The understanding for thermal properties of the powder is advantageous to the advancement of the processing technologies such as laser cladding, laser sintering, powder metallurgy and its other applications. Nanosized particles possess characteristic physical and chemical properties different from those of bulk materials due to the confinement of electrons, excitons, and photons into small volumes. Therefore it is valuable to discuss the thermal behaviours of powders constituted by nanometer-sized particles. The powder is wrapped up in the slender tube. One end of the slender tube filled with powder is connected to the low constant-temperature reservoir and the other end is kept at room temperature. The temperature histories at the 1cm location of the slender tube from the low constant-temperature reservoir are recorded using thermalcouples. Powders of particles with the diameter sizes 20nm and 5000nm are employed in this experiment. The results show that the thermal diffusion in the 20nm Fe powder is faster than that in the 5000nm Fe powder.

Effect of nano-ZrO2 on microstructure and thermal shock behaviour of Al2O3/SiC composite ceramics used in solar thermal power

The Al2O3-ZrO2(3Y)-SiC composite ceramics used in solar thermal power were prepared by micrometric Al2O3, nano-ZrO2 and SiC powders under the condition of pressureless sintering. The bulk density and bending strength of samples with 10vol% nano-ZrO2 sintered at 1480 °C were 3.222 g/cm3 and 160.4 MPa, respectively. The bending strength of samples after 7 times thermal shock tests (quenching from 1000 °C to 25 °C in air medium) is 132.0 MPa, loss rate of bending strength is only 17%. The effect of nano-ZrO2 content on the microstructure and performance of Al2O3-ZrO2(3Y)-SiC composite ceramic was investigated. The experimental results show that the bending strength of samples with above 10vol% nano-ZrO2 content has decreased, because the volume expansion resulting from t-ZrO2 to m-ZrO2 phase transformation is excessive; Adding proper nano-ZrO2 would be contributed to improve the thermal shock resistance of the composite ceramics. The Al2O3-ZrO2(3Y)-SiC composite ceramic has promising potential application in solar thermal power.

New route to densify tantalum carbide at 1400 °C by spark plasma sintering

Tantalum carbide with a TaC 0.9 composition was sintered to 96% relative density by spark plasma sintering at 1400 °C for 5 min, using commercial micrometric Ta and TaC powders as the starting materials. This achievement was dependent on the delicate rearrangement of the heating process which limits evaporation–condensation and promotes the mass transportation in association with 0.9TaC + Ta = TaC 0.9 reaction. The dense TaC 0.9 ceramic had a uniform microstructure composed of equiaxed grains, and showed flexure strength up to ∼498 MPa.

A comparative study of TiN and TiC: Oxidation resistance and retention of xenon at high temperature and under degraded vacuum

Dense TiN and TiC samples were prepared by hot pressing using micrometric powders. Xenon species (simulating rare gas fission products) were then implanted into the ceramics. The samples were annealed for 1 h at 1500 °C under several degraded vacuums with PO2 varying from 10−6 to 2×10−4 mbars. The oxidation resistance of the samples and their retention properties with respect to preimplanted xenon species were analyzed using scanning electron microscopy, grazing incidence x-ray diffraction, Rutherford backscattering spectrometry, and nuclear backscattering spectrometry. Results indicate that TiC is resistant to oxidation and does not release xenon for PO2≤6×10−6 mbars. When PO2 increases, geometric oxide crystallites appear at the surface depending on the orientation and size of TiC grains. These oxide phases are Ti2O3, Ti3O5, and TiO2. Apparition of oxide crystallites is associated with the beginning of xenon release. TiC surface is completely covered by the oxide phases at PO2=2×10−4 mbars up to a depth of 3 μm and the xenon is then completely released. For TiN samples, the results show a progressive apparition of oxide crystallites (Ti3O5 mainly) at the surface when PO2 increases. The presence of the oxide crystallites is also directly correlated with xenon release, the more oxide crystallites are growing the more xenon is released. TiN surface is completely covered by an oxide layer at PO2=2×10−4 mbars up to 1 μm. A correlation between the initial fine microstructure of TiN and the properties of the growing layer is suggested.

Mechanical Properties of Chitosan-Starch Composite Filled Hydroxyapatite Micro- and Nanopowders

Hydroxyapatite is a biocompatible ceramic and reinforcing material for bone implantations. In this study, Starch-chitosan hydrogel was produced using the oxidation of starch solution and subsequently cross-linked with chitosan via reductive alkylation method (weight ratio (starch/chitosan): 0.38). The hydroxyapatite micropowders and nanopowders synthesized by sol-gel method (10, 20, 30, 40 %W) were composited to hydrogels and were investigated by mechanical analysis. The results of SEM images and Zetasizer experiments for synthesized nanopowders showed an average size of 100 nm. The nanoparticles distributed as uniform in the chitosan-starch film. The tensile modulus increased for composites containing hydroxyapatite nano-(size particle: 100 nanometer) powders than composites containing micro-(size particle: 100 micrometer) powders. The swelling percentage decreased for samples containing hydroxyapatite nanopowder than the micropowders. These nanocomposites could be applied for hard-tissue engineering.

Influence of Plasma Intensity on Wear and Erosion Resistance of Conventional and Nanometric WC-Co Coatings Deposited by APS

The effects of plasma intensity and powder particle size on wear and erosion resistance have been evaluated for WC-12 wt.%Co coatings deposited by Air Plasma Spraying. Coatings were deposited from micrometric and nanostructured powders. SEM and XRD characterization showed the presence of WC, W2C, W, and an amorphous Co-rich matrix. The performance of the different coatings was compared in sliding wear tests (ball-on-disk), under dry friction conditions. Wear debris and tracks were analyzed by SEM. The debris generated during the test was found to have a great influence on the sliding properties. Wear follows a “three-body abrasive mechanism” and is dominated by coating spallation because of sub-surface cracking. In order to evaluate erosion behavior, solid particle erosion tests were conducted. Eroded coatings were analyzed by SEM, and erosion mainly occurs by a “cracking and chipping mechanism.” The study shows that wear and erosion behavior is strongly affected by plasma arc intensity.

Miniaturization of micrometric SiC from a detonation process of highly energetic material

Submicron- and, in a low quantity nano-sized carbide particles were successfully prepared through a dynamic process of energetic material detonation. For that, a commercial micrometric SiC powder, homogeneously coated by a polymer layer, was dispersed in an energetic 2,4,6-trinitrotoluene (TNT) matrix and then detonated. After detonation, the resulting carbide material was characterized by several techniques such as X-ray diffraction, infrared spectroscopy, electron microscopies, and nitrogen adsorption. The results showed a decrease in the initial micrometric SiC size with particle sizes reaching, in some cases, less than 20 nm. The specific surface area was also considerably increased since an approximate value of 7.8 m 2 g − 1 was obtained for a detonated SiC sample as opposed to 0.12 m 2 g − 1 for the initial micrometric SiC powder.

An energetic approach to the wear behaviour of plasma-sprayed alumina–13% titania coatings

The wear behaviour of plasma-sprayed Al2O3–13% TiO2 coatings was determined by pin-on-disc tests that were performed at different normal loads. Conventional coatings, deposited from micrometric powders, and modified coatings, deposited using agglomerates of nanoparticles as feeding powders, were studied. The wear rates of conventional and modified coatings were compared, and the influence of the microstructure was analysed by using an energetic approach to the wear. The enhanced properties of the modified coating were justified in terms of its microstructure. It is likely that the partially melted zones in this coating were responsible for the improvement in the wear resistance via their reinforcement of the ceramic matrix composite.

Microstructure and in-vitro behaviour of a novel High Velocity Suspension Flame Sprayed (HVSFS) bioactive glass coating

Bioactive glass coatings based on a novel SiO 2–P 2O 5–CaO–K 2O system (“Bio-K”) were deposited by the High-Velocity Suspension Flame Spraying (HVSFS) technique. Attrition-milled micrometric powder particles, dispersed in water + isopropanol, were sprayed onto Ti plates. The coatings remained entirely glassy, but the glass underwent some structural alterations during processing. After soaking in simulated body fluid (SBF), a rather uniform hydroxyapatite layer developed on the coatings, indicating that they may have the potential to faster osseointegration of prosthetic implants.

Formation of Carbon Nanofibers and Thin Films Catalyzed by Palladium in Ethylene−Hydrogen Mixtures

The nature of observed growth of solid carbon on palladium from ethylene−hydrogen mixtures is consistent with the supposition that the primary source of carbon for growth is homogeneously generated radicals. Evidence includes the lack of growth in the absence of a reacting mixture, sharp maxima as a function of temperature, and dramatic differences in temperature of growth as a function of mixture composition. The finding that the structure of the support strongly influenced the morphology of the solid carbon, and the temperature regime for deposition, is also consistent with this model. Carbon nanofibers were found to form on sputtered palladium films and palladium nanopowder (ca. 700 °C), whereas planar carbon structures deposited on palladium micrometer powder and foil (ca. 600 °C). A radical species growth mechanism is consistent not only with observations made herein but also with data presented in earlier studies.

Wear behaviour of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings produced using micron- and nano-sized powder suspensions

Three Al2O3-based suspensions were prepared using two nano-sized Al2O3 powders (having analogous size distribution and chemical composition but different agglomeration behaviour) and a micron-sized one. High velocity suspension flame sprayed (HVSFS) coatings were produced using these suspensions as feedstock and adopting two different sets of deposition parameters. The coatings produced by the micrometric powder suspension are denser and more wear resistant (as revealed by ball-on-disk testing) than those deposited using the two nanopowder suspensions, irrespective of the deposition parameters. The tribological behaviour of all coatings is dictated by the formation and subsequent removal of surface tribofilms: these tribofilms are more stable in the coatings obtained by the micrometric powder suspension, although they exhibit the largest tensile residual stress (as determined by X-ray diffraction and hole drilling techniques). Even though the as-deposited coatings are quite smooth (Ra ∼ 1.3–2 μm), polishing to Ra ∼ 0.1 μm further improves the wear resistance in all cases, as it delays the onset of tribofilm delamination phenomena.

Influence of particle size distributions on magnetorheological fluid performances

In this paper we investigate the influence that size distributions of the magnetic particles might have on the magnetorheological fluid performances. In our study, several size distributions have been tailored first by sieving a micrometric Fe powder in order to obtain narrow distribution powders and then by recomposing the new size distributions (different from Gaussian). We used spherical Fe particles (mesh -325) commercially available. The powder was sieved by means of a sieve shaker using a series of sieves with the following mesh size: 20, 32, 40, 50, 63, 80 micrometers. All magnetic powders were characterized through Vibrating Sample Magnetometer (VSM) measurements, particle size analysis and also Scanning Electron Microscope (SEM) images were taken. Magnetorheological (MR) fluids based on the resulted magnetic powders were prepared and studied by means of a rheometer with a magnetorheological module. The MR fluids were measured in magnetic field and in zero magnetic field as well. As we noticed in our previous experiments particles size distribution can also influence the MR fluids performances.