Microstructural characterization of γ-TiAl base alloy by electron probe x-ray microanalysis and electron backscatter diffraction

Electron backscatter diffraction (EBSD) and electron probe x-ray microanalysis (EPMA) in combination with x-ray diffraction (XRD) were applied for phase identification of the ternary precipitates and accompanying phases in Ti-49.6Al-1.9Fe alloy after heat treatment at 1400°C followed by furnace cooling. The heat treatment resulted in formation of the duplex structure consisting of equiaxed grains of the phase (AuCu type) and lamellae of the and ₂ (Ni₃Sn type). The ternary ₂ (Mn₂₃Th₆ type) phase, containing 21–22 at. % Fe, was revealed on the grain boundaries of the -matrix and lamellae, and is accompanied by ₂ precipitates. Different morphologies of the ₂ + ₂ colonies were found to differ in chemical composition, coarse particles being depleted in titanium, and the fine particles enriched in it. The combination of EPMA and EBSD in scanning electron microscopy proved to be very effective in local phase identification of specimens with fine multiphase structure.     

Continuous and multiple electron/positron injection: Can pulsars account for the electron/positron spectrum?

We investigate the observed spectrum of cosmic-ray electrons and positrons from astrophysical sources, especially pulsars, and the physical processes for making the spectrum spiky or smooth via continuous and multiple cosmic-ray injections. We find that (1) the average spectrum with the local birth rate of pulsars (including the off-axis ones) is relatively smooth, consistent with the PAMELA data, but requires an energetic source for the ATIC/PPB-BETS peak. Such a source should not occur repeatedly at the same rate. (2) A continuous injection produces a broad peak and a high energy tail above the peak, which can constrain the source duration ( 10⁵ years with the current data). We also discuss the H.E.S.S. data in the TeV range which constrain the total energy of young sources.     

Scanning electron microscope observations of polymorphic transitions occurring on a 12CaO · 7Al2O3 surface under electron beam action

The compound 12CaO · 7Al₂O₃ has been widely studied for a long time. There are many controversies concerning its polymorphism and other properties in the literature. The variable conditions of synthesis, i.e. Temperature, atmosphere, as well as the different cooling rate conditions allowed us to obtain an optically anisotropic polymorphic modification of the 12CaO · 7Al₂O₃ in vacuum. X-ray investigation of that anisotropic phase indicated that the d-values (in nm) of the diffraction maxima did not correspond to that of the isotropic cubic phase presented in the literature. The polymorphic transition temperature was determined and reported earlier. SEM observations shed new light on the 12CaO · 7Al₂O₃ polymorphism problem. A sample of CaO/Al₂O₃ of weight ratio 0.94, which has been synthesized and cooled in air, was examined under the scanning electron microscope. The sample was exposed to electron beam action for 20 min. In the dark spots formed during this operation, bulges appeared which then enlarged and cracked. On the surface a transverse crack was visible. This phenomenon could also be observed in other areas of the sample surface during exposure. This phenomenon is connected presumably with the polymorphic transition of the 12CaO · 7Al₂O₃ phase.     

Spinor–electron waveguided optics based on the Dirac equation

A preliminary quantum analysis, based on the Dirac equation, of the propagation of spinor–electron waves in electron waveguides is presented. The wave equations for spin-up (SU) and spin-down (SD) electron waves in electron guides are derived and their analogy with TE and TM light modes in dielectric guides is stressed. The spinor–electron waveguided propagation in a electron wave slab waveguide is solved exactly using the Dirac equation, that is, the exact amplitudes and dispersion equations of the spinor–electron waveguided modes are calculated. The main consequences related to the spinor modal structure are discussed: phase retardations (spin polarization), modal cutoff conditions and the non-relativistic limit.     

Control of electron transfer from lead-salt nanocrystals to TiO₂

The roles of solvent reorganization energy and electronic coupling strength on the transfer of photoexcited electrons from PbS nanocrystals to TiO(2) nanoparticles are investigated. We find that the electron transfer depends only weakly on the solvent, in contrast to the strong dependence in the nanocrystal-molecule system. This is ascribed to the larger size of the acceptor in this system, and is accounted for by Marcus theory. The electronic coupling of the PbS and TiO(2) is varied by changing the length, aliphatic and aromatic structure, and anchor groups of the linker molecules. Shorter linker molecules consistently lead to faster electron transfer. Surprisingly, linker molecules of the same length but distinct chemical structures yield similar electron transfer rates. In contrast, the electron transfer rate can vary dramatically with different anchor groups.     

Efficient bulk heterojunction solar cells based on D–A copolymers as electron donors and PC70BM as electron acceptor

Two low band gap conjugated polymers P1 (alternating phenylenevinylene containing thiophene and pyrrole rings) and P2 (alternating phenylenevinylene with dithenyl (thienothiadiazole) segments) having optical band gap 1.65 eV and 1.74 eV, respectively, were used as electron donor along with the PC₇₀BM as electron acceptor for the fabrication of bulk heterojunction solar cells. The power conversion efficiency (PCE) of BHJ devices based on P1:PC₇₀BM and P2:PC₇₀BM cast from THF solvent is about 2.84% and 2.34%, respectively, which is higher than the BHJ based on PCBM as electron acceptor. We have investigated the effect of mixed (1-chloronaphthalene (CN)/THF) solvent, modification of PEDOT:PSS layer and inserting of TiO₂ layer, on the photovoltaic performance of polymer solar cell. We have achieved power conversion efficiency of 5.07% for the polymer solar cells having structure ITO/PEDOT:PSS (modified)/P1:PC₇₀BM (CN/THF cast)/TiO₂/Al. The effect of solvent used for spin coating, modification of PEDOT:PSS layer and inclusion of TiO₂ layer has been discussed in detail.     

Structural studies on γ-MnO2 by transmission electron microscopy

Single-crystal electron diffraction patterns were obtained on five specimens of -MnO₂: one natural, two electrolytical (EMD) and two chemical (CMD) samples. EMDs are best described by the orthorhombic structure proposed by De Wolff which is derived from the ramsdellite structure. A CMD prepared from MnCO₃ fits the hexagonal cell of -MnO₂. Flaky grains from the natural sample and fibres from a CMD prepared from Mn(NO₃)₂ are hexagonal with a new cell:a 0.494,c 0.539 nm. No simple relation between chemical composition, morphology and structure could be found.     

An electron microscopic investigation of oxides related to ß-alumina

High resolution electron microscopic (HREM) investigation of potassium-alumina and the related gallate and ferrite has revealed that whereas the aluminate and gallate are highly disordered, consisting of random sequence of and units, the ferrite is more ordered. The aluminate and gallate are sensitive to electron beam irradiation exhibiting beam-induced damage similar to sodium-alumina. Significantly, the ferrite is beamstable, the difference in behaviour amongst these related oxides arising from the different mechanisms by which alkali metal nonstoichiometry is accommodated. Barium hexaaluminate and hexaferrite are both highly ordered; specimens prepared by the barium borate flux method exhibit a new 3a×3a superstructure of the hexagonal magnetoplumbite cell.Contribution No. 241 from the Solid State and Structural Chemistry Unit.     

Transmission electron microscopy study of Ni–Si nanocomposite films

Nickel induced crystallization of amorphous Si (a-Si) films is investigated using transmission electron microscopy. Metal-induced crystallization was achieved on layered films deposited onto thermally oxidized Si(3 1 1) substrates by electron beam evaporation of a-Si (400 nm) over Ni (50 nm). The multi-layer stack was subjected to post-deposition annealing at 200 and 600 °C for 1 h after the deposition. Microstructural studies reveal the formation of nanosized grains separated by dendritic channels of 5 nm width and 400 nm length. Electron diffraction on selected points within these nanostructured regions shows the presence of face centered cubic NiSi₂ and diamond cubic structured Si. Z-contrast scanning transmission electron microscopy images reveal that the crystallization of Si occurs at the interface between the grains of NiSi₂ and a-Si. X-ray absorption fine structure spectroscopy analysis has been carried out to understand the nature of Ni in the Ni–Si nanocomposite film. The results of the present study indicate that the metal induced crystallization is due to the diffusion of Ni into the a-Si matrix, which then reacts to form nickel silicide at temperatures of the order of 600 °C leading to crystallization of a-Si at the silicide–silicon interface.     

Crystal interface and high-resolution electron microscopy—the best partner

Several contributions of HRTEM on the interface science are reviewed in chronological order. The first contribution of HRTEM is the observation of gold (113)S°11 boundary, giving experimental proof of the CSL model. An observation of the asymmetric (112)S°3 boundary follows. A SiC grain boundary is effectively assessed not by the density of CSL point but the number of dangling bonds in the boundary. A ZnO/Pd interface provides an example that a misfit dislocation does not necessarily accommodate the lattice mismatch. Segregated interface shows characteristic HRTEM image contrast, suggesting change in atomic bonding. An atomic height step in the semiconductor hetero interface is observed by the Chemical Lattice Image technique. In the diamond grain boundary a dangling bond may not elevate the boundary energy, being contradictory of the least dangling bond rule. Super–high resolution of the HVHRTEM enable us to determine atomic species in the grain boundary. Combined use of HRTEM and EELSE allows us to discuss the correlation between atomic structure and nature of the corresponding interface. It is not exaggeration to say that modern interface science does not exist witout HRTEM. On the other hand, many complicated interfaces found by HRTEM remained as unaswered questions. An innovative structural model is requested to appear on the scene.     

High resolution transmission electron microscopic observation of β-Sn thin films

Thin films of β-Sn were studied by high resolution transmission electron microscopy (HRTEM). Diffraction patterns and experimental images agreed with those calculated on the basis of the known crystal structure of β-Sn. By matching the observed images to those calculated, the thicknesses of the films were estimated. Defects were observed as illustrated by a high resolution image of a [100] tilt grain boundary (Σ = 13, θ = 27°). The 20 nm thick films were found to be covered by a carbonaceous deposit. The presence of small clusters of β-Sn in the amorphous contamination layers covering the films was also revealed by HRTEM and by their conversion into SnO₂ particles in the electron beam.     

Mixed pronton–electron conducting properties of Yb doped strontium cerate

The electrical conductivity and hydrogen permeation properties of membranes were studied as a function of temperature and gradient. The bulk conductivity of was an order of magnitude higher than the grain boundary conductivity over the temperature range 100–250 °C in feed gas of 4% H₂/balance He (pH₂O = 0.03 atm). The significantly lower grain boundary conductivity indicates that larger-grained materials might be more suitable for proton transport. The hydrogen flux through the membranes is proportional to thickness down to 0.7 mm. The hydrogen permeation flux increases with an increase in gradient where the increase in hydrogen flux was explained by an increase in electron conduction as a function of temperature. The ambipolar conductivity calculated from hydrogen permeation fluxes shows the same and dependence as electron concentrations. The hydrogen and oxygen potential dependence of the ambipolar conductivity (, ) was understood from the defect structure. From this, it was confirmed that hydrogen permeation might be limited by electron transport at wet reducing atmosphere. From the temperature dependence of the electronic conductivity, the activation energy calculated at wet reducing conditions is 0.63 eV.     

Modification of UHMWPE by ion,electron and γ-ray irradiation

Due to good wear resistance Ultrahigh Molecular Weight Polyethylene (UHMWPE) is the material of choice for the load bearing surfaces of total joint implants. In order to improve its performance polymer parts are often modified by the use of ionizing radiation. Here we report on the use of electron and ion beams and γ-rays for the purpose. UHMWPE samples were irradiated with 600 keV and 1.5 MeV electron beam with doses ranging from 50 to 500 kGy and bombarded with 1–10 MeV He- and 9 MeV Cl-ions to fluences ranging from 10¹² to 5 × 10¹⁶ ions/cm². Co-bomb was used for γ-ray irradiation. Polymer radiolysis due to the irradiations was studied by means of nuclear reaction analysis (NRA) using the ¹H(¹⁵N, αγ)¹²C reaction. Hydrogen release increases with the applied dose and was correlated to the linear energy transfer (LET). Irradiated polymers oxidize rapidly when exposed to the air. Oxygen uptake profiles were determined using RBS. Correlation between radiolysis and oxidation has been revealed. Enriched in oxygen region extends to the depth at which radiation induced hydrogen release took place. Once started oxidation proceeds until the saturation concentration of about 10 at.% was attained.     

Nanosecond time resolved electron diffraction studies of the α→β in pure Ti thin films using the dynamic transmission electron microscope (DTEM)

The transient events of the α–β martensitic transformation in nanocrystalline Ti films were explored via single-shot electron diffraction patterns with 1.5 ns temporal resolution. The diffraction patterns were acquired with a newly constructed dynamic transmission electron microscope (DTEM), which combines nanosecond pulsed laser systems and pump-probe techniques with a conventional TEM. With the DTEM, the transient events of fundamental material processes can be captured in the form of electron diffraction patterns or images with nanosecond temporal resolution. The transient phenomena of the martensitic transformations in nanocrystalline Ti is ideally suited for study in the DTEM, with their rapid nucleation, characteristic interface velocities ∼1 km/s, and significant irreversible microstructural changes. Free-standing 40-nm-thick Ti films were laser-heated at a rate of ∼10¹⁰ K/s to a temperature above the 1155 K transition point, then probed at various time intervals with a 1.5-ns-long, intense electron pulse. Diffraction patterns show an almost complete transition to the β phase within 500 ns. Post-mortem analysis (after the sample is allowed to cool) shows a reversion to the α phase coupled with substantial grain growth, lath formation, and texture modification. The cooled material also shows a complete lack of apparent dislocations, suggesting the possible importance of a “massive” short-range diffusion transformation mechanism.

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Analytical electron microscopy of graphite-rich inclusions in sintered α-silicon carbide

Intragranular inclusions and multiphase regions at triple grain junctions in (B+C)-doped sintered -SiC were investigated using analytical and high-resolution electron microscopy. Both regions were two-phase, composed of graphite and amorphous material. The triple junctions also contained pores. The amorphous regions were principally carbon and oxygen. Graphite was formed by partial transformation of the amorphous regions. Only the triple-junction regions contain typical impurities from the starting -SiC powder, inferring that they are the main sinks for all grain-boundary/surface impurities in the material system.     

Features of the electron structure and magnetic susceptibility of δ-plutonium

Density of electron states, exchange-enhanced spin susceptibility, and orbital magnetic susceptibility of plutonium in the region of existence of a stable δ phase have been calculated within a generalized s(p)df model that takes into account both the band motion of electrons and their on-site and intersite interactions. It is shown that splitting of the electron spectrum by fluctuating intrinsic exchange fields leads to the formation of temperature-induced local magnetic moments, temperature-dependent changes in the occupancies of d, f bands, and spin-fluctuation-induced charge fluctuations.     

Nanofabrication by advanced electron microscopy using intense and focused beam∗

Abstract

The nanogrowth and nanofabrication of solid substances using an intense and focused electron beam are reviewed in terms of the application of scanning and transmission electron microscopy (SEM, TEM and STEM) to control the size, position and structure of nanomaterials. The first example discussed is the growth of freestanding nanotrees on insulator substrates by TEM. The growth process of the nanotrees was observed in situ and analyzed by high-resolution TEM (HRTEM) and was mainly controlled by the intensity of the electron beam. The second example is position- and size-controlled nanofabrication by STEM using a focused electron beam. The diameters of the nanostructures grown ranged from 4 to 20 nm depending on the size of the electron beam. Magnetic nanostructures were also obtained using an iron-containing precursor gas, Fe(CO)₅. The freestanding iron nanoantennas were examined by electron holography. The magnetic field was observed to leak from the nanostructure body which appeared to act as a ‘nanomagnet’. The third example described is the effect of a vacuum on the size and growth process of fabricated nanodots containing W in an ultrahigh-vacuum field-emission TEM (UHV-FE-TEM). The size of the dots can be controlled by changing the dose of electrons and the partial pressure of the precursor. The smallest particle size obtained was about 1.5 nm in diameter, which is the smallest size reported using this method. Finally, the importance of a smaller probe and a higher electron-beam current with atomic resolution is emphasized and an attempt to develop an ultrahigh-vacuum spherical aberration corrected STEM (Cs-corrected STEM) at NIMS is reported.     

Chemical thinning of III–V compound semiconductors for transmission electron microscopy

A vapour jet etching method and suitable equipment are tested in order to thin monocrystalline and polycrystalline material of GaAs, InP and InAs for transmission electron microscopy. The reactive vapour jet consists of air, bromine and methanol. The thinning procedure is monitored by an optical microscope. Large thin areas could be obtained using a bubbler temperature of 60° C, 20% bromine in methanol, a distance between nozzle and sample surface of 1.5 mm and a nozzle mouth diameter of 0.7 mm. During pre-etching the vapour flow rate was 0.71 min^–1, and 0.31 min^–1 was realized during final etching up to perforation. The etching times ranged between 1 to 2 min with an initial sample thickness of 350 to 365 m. Large thin areas have only been obtained using an excentric position of the nozzle with respect to the sample centre in combination with a manual rotation of the specimen holder.