Nanocrystalline Fe - 1 wt. %C compacts obtained by high pressure torsion of mechanically alloyed powder

Powders of a nanocrystalline Fe - 1 wt.% C alloy produced by mechanical alloying were consolidated to form a bulk nanocrystalline sample by means of high pressure torsion at 540°C. The density of the compacts reached 96 % of the theoretical density. The fracture stress of the material was 1 GPa and the microhardness 7.5 GPa, respectively. The grain size was approximately 70 nm and remained stable up to 700°C. The material contained fine particles of oxides and carbides.     

Nanoscale precipitation in a maraging steel studied by APFIM.

This article summarizes findings from our previous investigations and recent studies concerning precipitation in a maraging steel of type 13Cr-9Ni-2Mo-2Cu (at.%) with small additions of Ti (1 at.%) and Al (0.7 at.%). The material was investigated after aging at 475 degrees C up to 400 h using both conventional and three-dimensional atom-probe analyses. The process of phase decomposition in the steel proved to be complicated. It consisted of precipitation of several phases with different chemistry. A Cu-rich phase was first to precipitate and Mo was last in the precipitation sequence. The influence of the complex precipitation path on the material properties is discussed. The investigation clearly demonstrated the usefulness of the applied techniques for investigation of nanoscale precipitation. It is also shown that, complementary methods (such as TEM and EFTEM) giving structural and chemical information on a larger scale must be applied to explain the good properties of the steel after prolonged aging.     

Application of focused ion beam to atom probe tomography specimen preparation from mechanically alloyed powders.

Focused ion-beam milling has been applied to prepare needle-shaped atom probe tomography specimens from mechanically alloyed powders without the use of embedding media. The lift-out technique known from transmission electron microscopy specimen preparation was modified to cut micron-sized square cross-sectional blanks out of single powder particles. A sequence of rectangular cuts and annular milling showed the highest efficiency for sharpening the blanks to tips. First atom probe results on a Fe95Cu5 powder mechanically alloyed in a high-energy planetary ball mill for 20 h have been obtained. Concentration profiles taken from this powder sample showed that the Cu distribution is inhomogeneous on a nanoscale and that the mechanical alloying process has not been completed yet. In addition, small clusters of oxygen, stemming from the ball milling process, have been detected. Annular milling with 30 keV Ga ions and beam currents >or=50 pA was found to cause the formation of an amorphous surface layer, whereas no structural changes could be observed for beam currents 相似文献   

表面光电压测量中的最佳相位

对基于锁相放大技术的表面光电压测量中的“最佳相位"的物理意义及影响因素进行了系统的理论分析及实验验证.结果表明,对于通常所采用的金属-绝缘体-半导体（MIS）“三明治"测量结构,其等效阻抗会很大程度地影响到最佳相位.对于体相材料,“最佳相位"仅与测量条件有关;对于纳米材料等体系,光照可以改变MIS结构的等效电阻(Rins)、等效电容(Cins),从而将引起最佳相位角的显著变化,使“最佳相位”与所研究材料的特性直接关联起来.因此,该手段有可能成为一种新的纳米材料光电检测方法.     

Synthesis and morphological characterization of nanocrystalline powders obtained by a gas condensation method

The preparation of nanocrystalline powders of Fe and Fe50Ni50 has been performed by a gas-condensation method under pure helium atmosphere. The characterization of the prepared materials which was carried out by means of Transmission Electron Microscopy, X-rays diffraction and Mössbauer Spectrometry, evidences for the presence of oxide phases. Fe and FeNi based ultrafine particles are observed with a size comprised within the range 10–70 nm and they occur as clusters or chains.     

Unusual catalytic properties of nanostructured nickel films obtained by laser electrodispersion

Nanostructured nickel films deposited by laser electrodispersion onto a silicon (semiconducting) or thermally oxidized silicon (insulating) substrate show a remarkably high catalytic activity (of the order of 10³–10⁴ (mol product) (mol Ni)^?1 h^?1) in the isomerization of chlorinated hydrocarbons and olefin hydrogenation. The special properties of the laser-deposited films are likely due to the small size (2.5 nm), developed surface, and amorphism of the nickel particles, as well as to highly active, charged particles appearing on the insulating substrate. The latter result from thermal fluctuations of electrons between closely spaced particles. In a film deposited on silicon covered with a natural oxide layer, a significant role is also played by charge redistribution between the substrate and metal particles.     

Synthesis and properties of magnetic FeNi3 alloyed microchains obtained by hydrothermal reduction

In this paper, stoichiometric FeNi₃ alloyed microchains were successfully synthesized via a simple hydrothermal method without surfactants or external magnetic fields. The effect of synthetic parameters, including citrate concentration, the reactant molar ratio and reaction time, on the formation and morphologies of the FeNi₃ assemblies were investigated. The as-prepared chain-like structures were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Furthermore, magnetic measurements by superconducting quantum interference device were also carried out to explore their magnetic properties, which confirmed that the as-prepared samples had the symmetric hysteresis loop behavior of ferromagnetic materials.     

Systematic investigation of mechanically alloyed Ti-Mg-Ni used as negative electrode in Ni-MH battery

In this study, a series of Ti-Mg-Ni alloys have been synthesized by mechanical alloying using a planetary high-energy ball mill. The effect of varied composition on the microstructure and electrochemical hydrogenation properties of Ti-Mg-Ni system have been investigated through the compared analysis of Ti_1-xMgNi_x and TiMg_1-xNi_x alloys. The structural transformation and discharge capacities were characterized by XRD and electrochemical measurements at galvanostatic conditions, respectively. Additionally, a separate milling process has been employed for improving the electrochemical properties of Ti-Mg-Ni alloys. It is shown that the separated milling process can extremely enhance the discharge capacities of Ti-Mg-Ni composite materials.     

Neutron powder diffraction study of strain and crystallite size in mechanically alloyed PbTe

We report on an analysis of strain and crystallite size effects in mechanically alloyed PbTe. The evolution of the microstructure was monitored by Rietveld refinements of the neutron powder diffraction data collected at room temperature. For milling times shorter than , the synthesis is not completed and the samples are clearly multi-phase with high concentrations of unreacted starting constituents. For longer milling times, the diffraction patterns are consistent with a single-phase PbTe. Within the range of reaction times studied, the crystallite size decreases with an exponential decay law and saturates to a value of . However, the strain parameter does not show such a monotonic behavior. Indeed, it first increases and reaches a maximum when the synthesis is achieved and then drops for longer milling time as a result of the thermal activated annealing induced by additional mechanical shocks.     

The oxidation kinetic study of mechanically milled ultrafine iron powders by thermogravimetric analysis

Journal of Thermal Analysis and Calorimetry - The effect of mechanical milling on the oxidation kinetics of ultrafine iron powders was investigated by thermogravimetric (TG) analysis. The initial...     

Designing of nanostructured hollow TiO2 spheres obtained by ultrasonic spray pyrolysis

Theoretical model describing the mechanism of droplet formation and structure of hollow TiO2 spheres prepared by the process of ultrasonic spray pyrolysis, using colloidal solution consisting of the 2.5-nm TiO2 nanoparticles as a precursor, is developed. The proposed model quantitatively defines each line in the size distribution spectrum. The mechanism of droplet formation and/or particle genesis is fully determined by harmonization between the physical fields inherent to the system as the consequence of its physical characteristics: external, e.g., ultrasound, and internal. Agreement between theoretically obtained basic structural parameters (size distribution and geometry) and experimentally determined values was found.     

Rapid classification of alloyed steel by means of optical emission spectroscopy and a supervised learning procedure

A supervised learning procedure for classification of steel samples analyzed by means of optical emission spectroscopy was developed. The method should be applicable in simple portable spectrometers, for various groups of materials and working relatively fast. Data vectors extracted from digital spectra of unknown samples are compared with average vectors evaluated from the data vectors of repeated measurements of reference samples. The classification is carried out on the basis of the multivariate distance between the data vector of the unknown sample and the nearest average reference vector and its deviation. The supervised learning procedure was tested by 10 steel samples which could be successfully classified.     

Anticariogenic properties of ent-pimarane diterpenes obtained by microbial transformation

In the present work, the anticariogenic activities of three pimarane-type diterpenes obtained by fungal biotransformation were investigated. Among these metabolites, ent-8(14),15-pimaradien-19-ol was the most active compound, displaying very promising MIC values (ranging from 1.5 to 4.0 μg mL(-1)) against the main microorganisms responsible for dental caries: Streptococcus salivarius, S. sobrinus, S. mutans, S. mitis, S. sanguinis, and Lactobacillus casei. Time kill assays performed with ent-8(14),15-pimaradien-19-ol against the primary causative agent S. mutans revealed that this compound only avoids growth of the inoculum in the first 12 h (bacteriostatic effect). However, its bactericidal effect is clearly noted thereafter (between 12 and 24 h). The curve profile obtained by combining ent-8(14),15-pimaradien-19-ol and chlorhexidine revealed a significant reduction in the time necessary for killing S. mutans compared with each of these two chemicals alone. However, no synergistic effect was observed using the same combination in the checkerboard assays against this microorganism. In conclusion, our results point out that ent-8(14),15-pimaradien-19-ol is an important metabolite in the search for new effective anticariogenic agents.     

Investigations of the solid state reaction process in mechanically alloyed Zr-Al-Cu-Ni bulk metallic glasses by analytical transmission electron microscopy

Starting with elemental powders, the progress of amorphization in mechanically alloyed Zr₆₅Al_7.5Cu_17.5Ni₁₀ has been investigated by x-ray diffractometry and transmission electron microscopy (TEM). Detailed investigations of the microstructural evolution during milling indicate that the amorphization proceeds by solid state reaction, similar to other well known binary or multicomponent systems.     

ZrO2 phase structure in coating films and powders obtained by sol-gel process

Zirconia coating film and powder obtained by the sol-gel route using zirconium n-propoxide as starting material and acid catalyst were investigated by the Perturbed Angular Correlations method, X-Ray Diffraction and Differential Scanning Calorimetry and Differential Thermal Analyses. The hyperfine interaction, measured after annealing the samples at increasing temperatures up to 1100°C, allowed to distinguish five different zirconium neighborhoods. Two of them describe rather disordered material which, on heating, crystallizes to the tetragonal phase and end finally in monoclinic zirconia. As compared with the powder, the film exhibits a minor fraction of an unidentified ordered form and a higher and more stable fraction of tetragonal phase. In addition, the tetragonal to monoclinic conversion takes place at higher temperatures and with a larger activation energy.Researcher for CONICET-ArgentinaResearcher for CICPBA-Argentina     

Understanding the high activity of a nanostructured catalyst obtained by a deposit of Pd on Ni: first principle calculations

The catalytic activity of a 4 monolayer deposit of Pd on a Ni(110) surface toward the hydrogenation of ethylene is investigated by using gradient-corrected periodic density functional calculations. The Pd/Ni(110) surface is strongly nanostructured, due to the anisotropic stress induced by the Ni(110) substrate on the Pd layer. A kinetic analysis, based on the investigation of the optimal reaction pathway for the hydrogenation of ethylene to ethane, is presented, allowing a comparison between Pd/Ni(110) and pure Pd(110) surfaces. The calculated activation energies allow one to reproduce the experimental result, which shows that the Pd/Ni(110) surface is about 30 times more active than the pure Pd(110) surface. This marked increase of the catalytic activity is a consequence of the specific nanostructure of the Pd/Ni(110) surface. By examining the structure of the adsorbed species and of the transition states and by analyzing the electronic properties, we show that this rate increase can be associated to the fact that the ethylene adsorption energy in the first hydrogenation step and the ethyl-hydrogen coadsorption energy in the second step are both much lower on Pd/Ni(110) than on pure Pd(110).     

Structural and dielectric characterization of LiNbO3 nano-size powders obtained by Pechini method

Lithium niobate is an artificially synthesized material with wide technological applications, due to its numerous physical properties, such as: ferroelectricity, birefringence and large pyroelectric, piezoelectric, non-linear, acousto-optic, electro-optic and photo-elastic coefficients. It also exhibits very strong bulk photovoltaic and photo refractive effects. In the present work, LiNbO₃ powders were prepared by the Pechini (sol?Cgel) method. The heat-treatment of the base amorphous powders, at 450?°C promotes the formation of LiNbO₃ crystalline phase. The electrical characterization, in function of the heat-treatment time, shows that the dc conductivity depends on the quantity of crystalline phase, which increases with the increase of heat-treatment time. The sample treated for 96?h shows a dielectric constant value of ~35, at low frequencies. A dielectric relaxation phenomenon independent of the temperature of measurement was detected in the samples treated at 48 and 96?h, shifting to higher frequencies with the increase of the heat-treatment time.