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Selected AbstractsTransport properties in n-type AlGaN/AlN/GaN-superlatticesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008J. Hertkorn Abstract In order to improve the lateral conductivity in optoelectronic devices, we have investigated Si-doped AlGaN/AlN/GaN-superlattices. As a first step we performed calculations of the band structure of Al-GaN/AlN/GaN modulation doped multi heterostructures. Based on these results we worked on optimizing the growth of low Al content (xAl, 20%) superlattices by MOVPE. Several tens of abrupt and graded AlGaN/AlN/GaN-layer pairs could be grown crack-free on 2 ,m thick n-GaN layers deposited on sapphire substrates with AlN nucleation. By Van-der-Pauw Hall measurements, we determined that the lateral conductivity of a 1.5 ,m thick superlattice structure is a factor of four higher than in highly n-doped bulk material with comparable thickness without compromising too much the vertical conductivity as confirmed by two step TLM-measurements. At 4K we could demonstrate an extremely high effective mobility of 18760 cm2/Vs at n=2×1014 cm,2 (R=1.6,/®), a clear verification of our excellent crystal quality. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Female reproductive biology of Platygaster diplosisae (Hymenoptera: Platygastridae) and Aprostocetus procerae (Hymenoptera: Eulophidae), two parasitoids associated with the African Rice Gall Midge, Orseolia oryzivora (Diptera: Cecidomyiidae)ENTOMOLOGICAL SCIENCE, Issue 2 2008Souleymane NACRO Abstract We investigated the female reproductive system of Platygaster diplosisae (Hymenoptera: Platygastridae) and Aprostocetus procerae (= Tetrastichus pachydiplosisae) (Hymenoptera: Eulophidae), two parasitoids associated with the African rice gall midge, Orseolia oryzivora (Diptera: Cecidomyiidae). Both optical and electron microscopy were used. The female reproductive system of P. diplosisae includes two large ovaries of the meristic polytrophic-type, each composed of several tens of ovarioles. The system includes also a venomous gland that extends to a common oviduct. This gland had a filiform secretory portion, in which the epithelium was thin and surrounded a common evacuation canal. The secretory cells secrete into a large reservoir. Parasitism due to P. diplosisae is gregarious. The female reproductive system of A. procerae includes two ovaries of the meristic polytrophic-type, and each ovary has a few ovarioles. Each ovariole includes one or two oocytes, which can be seen in the vitellarium. Two accessory glands, which extend to the oviduct, are also visible. The secretory epithelium of the accessory gland is made up of a dense network of secretory cells surrounded by muscle fibers. Females of A. procerae pierce the tissues of the gall and probably deposit one egg on or close to the pupa of the midge. Aprostocetus procerae is a solitary parasitoid of the midge. The two parasitoids exploit the same host at different developmental stages. These findings improve our knowledge of the reproductive biology of these two parasitoids associated with the African rice gall midge, an important pest in Africa. [source] Multifocal structure of the T cell , dendritic cell synapseEUROPEAN JOURNAL OF IMMUNOLOGY, Issue 6 2005Cédric Brossard Abstract The structure of immunological synapses formed between murine naive T cells and mature dendritic cells has been subjected to a quantitative analysis. Immunofluorescence images of synapses formed in the absence of antigen show a diffuse synaptic accumulation of CD3 and LFA-1. In electron microscopy, these antigen-free synapses present a number of tight appositions (cleft size ,15,nm), all along the synapse. These tight appositions cover a significantly larger surface fraction of antigen-dependent synapses. In immunofluorescence, antigen-dependent synapses show multiple patches of CD3 and LFA-1 with a variable overlap. A similar distribution is observed for PKC, and talin. A concentric organization characteristic of prototypical synapses is rarely observed, even when dendritic cells are paralyzed by cytoskeletal poisons. In T,DC synapses, the interaction surface is composed of several tens of submicronic contact spots, with no large-scale segregation of CD3 and LFA-1. As a comparison, in T,B synapses, a central cluster of CD3 is frequently observed by immunofluorescence, and electron microscopy reveals a central tight apposition. Our data show that it is inappropriate to consider the concentric structure as a "mature synapse" and multifocal structures as immature. [source] Controlled Synthesis of CdSe Nanowires by Solution,Liquid,Solid MethodADVANCED FUNCTIONAL MATERIALS, Issue 22 2009Zhen Li Abstract Semiconductor nanowires prepared by wet chemical methods are a relatively new field of 1D electronic systems, where the dimensions can be controlled by changing the reaction parameters using solution chemistry. Here, the solution,liquid,solid approach where the nanowire growth is governed by low-melting-point catalyst particles, such as Bi nanocrystals, is presented. In particular, the focus is on the preparation and characterization of CdSe nanowires, a material which serves a prototype structure for many kinds of low dimensional semiconductor systems. To investigate the influence of different reaction parameters on the structural and optical properties of the nanowires, a comprehensive synthetic study is presented, and the results are compared with those reported in literature. How the interplay between different reaction parameters affects the diameter, length, crystal structure, and the optical properties of the resultant nanowires are demonstrated. The structural properties are mainly determined by competing reaction pathways, such as the growth of Bi nanocatalysts, the formation and catalytic growth of nanowires, and the formation and uncatalytic growth of quantum dots. Systematic variation of the reaction parameters (e.g., molecular precursors, concentration and concentration ratios, organic ligands, or reaction time, and temperature) enables control of the nanowire diameter from 6 to 33,nm, while their length can be adjusted between several tens of nanometers and tens of micrometers. The obtained CdSe nanowires exhibit an admixture of wurtzite (W) and zinc blende (ZB) structure, which is investigated by X-ray diffraction. The diameter-dependent band gaps of these nanowires can be varied between 650 and 700,nm while their fluorescence intensities are mainly governed by the Cd/Se precursor ratio and the ligands used. [source] Magnetic and Structural Investigation of ZnSe Semiconductor Nanoparticles Doped With Isolated and Core-Concentrated Mn2+ IonsADVANCED FUNCTIONAL MATERIALS, Issue 15 2009Christina Graf X-Ray magnetic circular dichroism (XMCD) experiments on diluted magnetic semiconductor nanocrystals (2,7,nm) are reported in order to study their local electronic structure and magnetic properties. ZnSe nanoparticles containing either single manganese ions (Mn2+) distributed in the lattice of the entire particle or a MnSe core in the center are prepared using high temperature approaches. The Mn2+ concentration is varied between less than one to several tens of manganese ions per nanocrystal. For all samples it is shown that the Mn2+ is exclusively present in the bulk of ZnSe nanoparticles with no evidence for oxidation to higher Mn-oxidation states. The magnetic ions are highly polarized inside the nanocrystals reaching about 80% of the theoretical value of a pure d5 state under identical conditions for the case of isolated manganese ions. Nanocrystals with a MnSe core ZnSe shell structure reach <50% of this value. Thus, their polarization is significantly more hindered, which is due to the significantly enhanced Mn,Mn interactions and a more distorted crystalline lattice. In contrast, no coupling between the manganese centers is observed in the nanoparticles doped samples with low concentrations of Mn2+, indicating that these ions are isolated in the bulk of the nanoparticles. [source] Network-magnetotelluric method and its first results in central and eastern Hokkaido, NE JapanGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2001Makoto Uyeshima Summary A new field observation technique based on the magnetotelluric (MT) method has been developed to determine deep and large-scale 3-D electrical conductivity distributions in the Earth. The method is named ,Network-MT', and employs a commercial telephone network, to measure voltage differences with long dipole lengths ranging from 10 to several tens of kilometres. This observation configuration enables us to obtain the telluric field distribution with nearly continuous coverage over a target region. Response functions are estimated between the respective voltage differences and the horizontal magnetic fields at a reference point. Owing to the long electrode spacing, the observed responses are relatively free from the effects of small-scale near-surface heterogeneity with a scalelength shorter than the typical electrode spacing. Therefore, physically meaningful direct comparison between the observations and model responses is feasible even if the fine-scale features of near-surface heterogeneity are ignored. This extensively reduces the difficulty, especially in 3-D MT interpretation. The first Network-MT experiment was performed in central and eastern Hokkaido, NE Japan, in 1989. It took about five months to complete all of the measurements, and used 209 dipoles to cover the target area of 200(EW) × 200(NS) km2. The long electrode spacing enabled us to obtain the voltage differences with a high signal-to-noise ratio. For 175 dipoles, the squared multiple coherency between the voltage difference and the horizontal magnetic field at Memambetsu Geomagnetic Observatory was determined to be more than 0.9 in the period from 102 to 104 s. 193 MT impedances were computed in tensor form by linear combination of the response functions. The estimated impedances generally possessed smooth period dependence throughout the period range. No drastic spatial change was observed in the characteristics of the tensors for neighbouring sites, and some regional trend could be detected in the spatial distribution. Thus, we confirmed the merit of the Network-MT method, that its responses are little affected by small-scale near-surface structures. The regional feature of the response implied a significant influence of the coast effect, and was well correlated with the regional geological setting in Hokkaido. Conventional Groom,Bailey tensor decomposition analysis revealed that the target region is not regionally one- or two-dimensional. Therefore, we developed a 3-D forward modelling scheme specially designed for the Network-MT experiment, and tried to reproduce the Network-MT responses directly. In the 3-D model, a realistic land,sea distribution was considered. The resistivity of sea water was fixed to be 0.25 , m and, as a first trial of 3-D modelling, the resistivity of the land was assumed to be uniform and its value was determined to be 200 , m by a simple one-parameter inversion. Overall agreements between the observations and the best-fit model responses indicated the importance of the 3-D coast effect in the target region. However, there remained significant discrepancies, especially in the phase of the responses, which provide a clue to determining a regional deep 3-D structure. [source] Electrochemical Synthesis of High-Quality AgTCNQ Nanowires Using Carbon Nanotube ElectrodesADVANCED MATERIALS, Issue 46 2009Liang Ren High-quality charge-transfer complex AgTCNQ nanowires have been successfully synthesized by a facile nanoelectrochemical approach utilizing carbon nanotube (CNT) electrodes. These nanowires have diameters of 30,80,nm and lengths of up to several tens of micrometers, and exhibit remarkable electrical switching bistability. The uniform and high aspect ratio nature of nanowires originates from the one-dimensional structural feature and ultrasmall surface area of the CNTs. [source] Tapered Carbon Nanotubes from Activated Carbon Powders,ADVANCED MATERIALS, Issue 2 2006Q. Hu Tapered carbon nanotubes (TCNTs) are produced from activated carbon powders via simple thermal catalytic reactions in which the carbon nanotubes grow though a Sn-catalyzed vapor,liquid,solid growth process. The tapered nanotubes are well aligned and have a length of several tens of micrometers (see Figure). [source] Porous Tin Oxides Prepared Using an Anodic Oxidation Process,ADVANCED MATERIALS, Issue 3 2004H.-C. Shin Open, porous structures of tin oxides, fabricated by an anodic oxidation process, are characterized by irregular channels with diameters of several tens of nanometers, as shown in the Figure. The as-prepared amorphous tin oxides can be transformed by annealing to a crystalline stannic structure with very few micro-structural changes. The materials may find applications in batteries and chemical sensors. [source] Ab-initio simulations of materials using VASP: Density-functional theory and beyondJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2008Jürgen Hafner Abstract During the past decade, computer simulations based on a quantum-mechanical description of the interactions between electrons and between electrons and atomic nuclei have developed an increasingly important impact on solid-state physics and chemistry and on materials science,promoting not only a deeper understanding, but also the possibility to contribute significantly to materials design for future technologies. This development is based on two important columns: (i) The improved description of electronic many-body effects within density-functional theory (DFT) and the upcoming post-DFT methods. (ii) The implementation of the new functionals and many-body techniques within highly efficient, stable, and versatile computer codes, which allow to exploit the potential of modern computer architectures. In this review, I discuss the implementation of various DFT functionals [local-density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, hybrid functional mixing DFT, and exact (Hartree-Fock) exchange] and post-DFT approaches [DFT + U for strong electronic correlations in narrow bands, many-body perturbation theory (GW) for quasiparticle spectra, dynamical correlation effects via the adiabatic-connection fluctuation-dissipation theorem (AC-FDT)] in the Vienna ab initio simulation package VASP. VASP is a plane-wave all-electron code using the projector-augmented wave method to describe the electron-core interaction. The code uses fast iterative techniques for the diagonalization of the DFT Hamiltonian and allows to perform total-energy calculations and structural optimizations for systems with thousands of atoms and ab initio molecular dynamics simulations for ensembles with a few hundred atoms extending over several tens of ps. Applications in many different areas (structure and phase stability, mechanical and dynamical properties, liquids, glasses and quasicrystals, magnetism and magnetic nanostructures, semiconductors and insulators, surfaces, interfaces and thin films, chemical reactions, and catalysis) are reviewed. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Three-bed PVSA process for high-purity O2 generation from ambient airAICHE JOURNAL, Issue 11 2005Jeong-Geun Jee Abstract A three-bed PVSA (pressure vacuum swing adsorption) process, combining equilibrium separation with kinetic separation, was developed to overcome the 94% O2 purity restriction inherent to air separation in the adsorption process. To produce 97+% and/or 99+% purity O2 directly from air, the PVSA process with two zeolite 10X beds and one CMS bed was executed at 33.44,45.60 to 253.31 kPa. In addition, the effluent gas from the CMS bed to be used for O2 purification was backfilled to the zeolite 10X bed to improve its purity, recovery, and productivity in bulk separation of the air. PVSA I, which made use of a single blowdown/backfill step, produced an O2 product with a purity of 95.4,97.4% and a recovery of 43.4,84.8%, whereas PVSA II, which used two consecutive blowdown/backfill steps, produced O2 with a purity of 98.2,99.2% and a recovery of 47.2,63.6%. Because the primary impurity in the O2 product was Ar, the amounts of N2 contained in the product were in the range of 4000,5000 ppm at PVSA I and several tens of ppm at PVSA II. A nonisothermal dynamic model incorporating mass, energy, and momentum balances was applied to predict the process dynamics. Using the linear driving force (LDF) model with constant diffusivity for the equilibrium separation bed and a modified LDF model with concentration dependency of the diffusion rate for the kinetic separation bed, the dynamic model was able to accurately predict the results of the experiment. © 2005 American Institute of Chemical Engineers AIChE J, 2005 [source] Synthesis of poly[N -isopropylacrylamide- g -poly(ethylene glycol)] with a reactive group at the poly(ethylene glycol) end and its thermosensitive self-assembling characterJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2006Michihiro Iijima Abstract Poly[N -isopropylacrylamide- g -poly(ethylene glycol)]s with a reactive group at the poly(ethylene glycol) (PEG) end were synthesized by the radical copolymerization of N -isopropylacrylamide with a PEG macromonomer having an acetal group at one end and a methacryloyl group at the other chain end. The temperature dependence of the aqueous solutions of the obtained graft copolymers was estimated by light scattering measurements. The intensity of the light scattering from aqueous polymer solutions increased with increasing temperature. In particular, at temperatures above 40°C, the intensity abruptly increased, indicating a phase separation of the graft copolymer due to the lower critical solution temperature (LCST) of the poly(N -isopropylacrylamide) segment. No turbidity was observed even above the LCST, and this suggested a nanoscale self-assembling structure of the graft copolymer. The dynamic light scattering measurements confirmed that the size of the aggregate was in the range of several tens of nanometers. The acetal group at the end of the PEG graft chain was easily converted to the aldehyde group by an acid treatment, which was analyzed by 1H NMR. Such a temperature-induced nanosphere possessing reactive PEG tethered chains on the surface is promising for new nanobased biomedical materials. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1457,1469, 2006 [source] Synthesis and characterization of carbon nanotube/polypyrrole core,shell nanocomposites via in situ inverse microemulsionJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2005Yijun Yu Abstract We demonstrate here a feasible approach to the preparation of multiwalled carbon nanotube (MWNT)/polypyrrole (PPy) core,shell nanowires by in situ inverse microemulsion. Transmission electron microscopy and scanning electron microscopy showed that the carbon nanotubes were uniformly coated with a PPy layer with a thickness of several to several tens of nanometers, depending on the MWNT content. Fourier transform infrared spectra suggested that there was strong interaction between the ,-bonded surface of the carbon nanotubes and the conjugated structure of the PPy shell layer. The thermal stability and electrical conductivity of the MWNT/PPy composites were examined with thermogravimetric analysis and a conventional four-probe method. In comparison with pure PPy, the decomposition temperature of the MWNT/PPy (1 wt % MWNT) composites increased from 305 to 335 °C, and the electrical conductivity of the MWNT/PPy (1 wt % MWNT) composites increased by 1 order of magnitude. The current,voltage curves of the MWNT/PPy nanocomposites followed Ohm's law, reflecting the metallic character of the MWNT/PPy nanocomposites. The cyclic voltammetry measurements revealed that PPy/MWNT composites showed an enhancement in the specific charge capacity with respect to that of pure PPy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6105,6115, 2005 [source] Fabrication of a Porous Bioactive Glass,Ceramic Using Room-Temperature Freeze CastingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2006Ju-Ha Song The room-temperature freeze-casting method was used to fabricate porous bioactive glass,ceramics. In this method, a glass/camphene slurry prepared at 60°C was cast into a mold at 20°C, resulting in the production of a rigid green body that was comprised of three-dimensional dendritic camphene networks surrounded by highly concentrated glass powder walls. After the sublimation of camphene, the samples were sintered for 3 h at elevated temperatures ranging from 700° to 1100°C. As the sintering temperature was increased to 1000°C, the densification of the glass,ceramic wall was remarkably enhanced, while its highly porous structure was preserved. The sample sintered at 1000°C showed a high porosity of 53% and pore channels with a size of several tens of micrometers, as well as dense glass,ceramic walls. In addition, the fabricated samples effectively induced the deposition of apatite on their surfaces when immersed in simulated body fluid, implying that they are very bioactive. [source] Growing SiC Nanowires on Tyranno-SA SiC FibersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2004Wen Yang A new in situ process for growing SiC nanowires on Tyranno-SA SiC fibers (2-D, plain-woven) was developed using the thermal decomposition of methyltrichlorosilane in hydrogen. The process was performed using a chemical vapor infiltration system. ,-SiC nanowires ,100-nm thick and several tens of micrometers long were successfully synthesized on the fibers. The growing of the SiC nanowires suggests a conditions-dependent process. [source] Preparation of Highly Dispersed Ultrafine Barium Titanate Powder by Using Microbial-Derived SurfactantJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2003Hidehiro Kamiya To uniformly disperse ultrafine BaTiO3 particles with a stoichiometric composition and several tens of nanometers in diameter to primary particles during the sol,gel synthesis process, a new aqueous surfactant with a high hydrophilic group density and special cis-structure was prepared from a microbial product and added to solution before the sol,gel synthesis reaction. Because of the rapid formation of large and porous aggregates which were 30,50 ,m in diameter in suspension without addition of this unique structural surfactant, the prepared ultrafine BaTiO3 particles caused rapid sedimentation in suspension. The addition of the surfactant in the range of 7.1 wt% for the synthesized BaTiO3 particles made it possible to decrease the size of the aggregates in suspension as well as the sedimentation velocity while maintaining the stoichiometric composition. The optimum additive content to obtain the minimum aggregate size of about 100,200 nm in diameter and the highest dispersion stability in suspension while maintaining the stoichiometric composition of prepared ultrafine BaTiO3 particles without other phases was determined at about 7.1 wt%. Because the excess addition of this surfactant at more than 8.5 wt% inhibited the uniform synthesis of BaTiO3 particles, an amorphous phase with a highly specific surface area and a BaCO3 phase formed in the synthesized particles. [source] Photon echo in ruby doped only by 53Cr isotope ionsLASER PHYSICS LETTERS, Issue 8 2008V.V. Samartsev Abstract The signals of photon echo (PE) are investigated firstly in a ruby crystal doped only by the 53Cr isotope ions in a concentration of 0.03 , 0,05 wt%. The optical experiments were performed in backward regime at the wavelength of 693.4 nm both with a low magnetic field (200 G) and without it. Since the 53Cr isotope ions have hyperfine structure of levels the special attention was paid to the study of the stimulated photon echo and primarily to the investigation of its decay kinetics. It is established that this decay has a form which is typical to the signals of longlived PE. But in contrast to the long-lived PE the decay time in our case is less than the lifetime of the excited 2E () state. The signals of primary photon echo and stimulated photon echo at a low longitudinal magnetic field and their decay curves are investigated. We observed the beats of temporal shape of these signals with a period of several tens of nanoseconds. Theoretical analysis shows that they are due to the hyperfine interaction of valence electrons of 53Cr isotope ions with their own nuclei. The obtained decay curves allow us to estimate the phase relaxation time at the presence of a magnetic field. It proves to be equal to 98 ns. The spectrum of stimulated photon echo signal in the doped ruby exposed to a magnetic field is measured. (© 2008 by Astro Ltd., Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Fabrication of Regularly Patterned Microporous Films by Self-Organization of an Amphiphilic Liquid-Crystalline Diblock Copolymer in a Dry EnvironmentMACROMOLECULAR MATERIALS & ENGINEERING, Issue 1 2010Dong Chen Abstract An amphiphilic LCBC PEO -b- PAz consisting of flexible PEO as a hydrophilic block and poly(methacrylic acid) containing an azobenzene moiety in side chain as a hydrophobic LC segment was synthesized and used to fabricated microporous films by spin-coating method under a dry environment. With the help of a small amount of water, well-arranged ellipsoidal micropores embedded in a LC matrix were obtained and the pore size is in the range of several tens µm of water. The influence of water content and rotational speed was studied in detail. It was found that regularly patterned microporous films can be prepared with certain water content, and the pore size can be easily tailored through changing the rotational speed. The obtained microporous structures showed good thermal and photo stability. [source] Corrosion behaviour of aircraft coating systems in acetate- and formate-based de-icing chemicalsMATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 3 2009E. Huttunen-Saarivirta Abstract Corrosion behaviour of four coating systems, which are commonly used in aircraft components; namely cadmium-coated and subsequently chromate-treated steel AISI 4340, aluminium-coated steel AISI 4340, anodised aluminium alloy 7075-T6 and chromate-treated aluminium alloy 7075-T6, were investigated in six commercial acetate- and formate-based de-icing chemicals. The results show that the aluminium-coated steel specimens experience least corrosion among the four coating systems; no corrosion is detected in potassium acetate-based de-icing chemicals (Safeway KA HOT, Safegrip and Safegrip+), although some corrosion is observed in the other three chemicals (Safeway SD, Safeway SF and Meltium). In contrast, the coatings in the other three coating systems are damaged in all six de-icing chemicals. The thickness of the coating does not necessarily predict how well the coating will protect the substrate; the thickest coating, several tens of micrometres, is apparent on the cadmium- and chromate-coated steel specimens, while only a few micrometres thin aluminium coating on the steel substrate is the only coating system that can survive the corrosion tests in some of the de-icing chemicals. Comparison of the results from the two test methods used in this research, polarisation measurements and chemical exposure tests, shows that, for each coating system, the extent of corrosion, as evaluated on the basis of weight changes calculated from corrosion current density values and those experienced during chemical exposure tests, is different: the weight changes that materialise during exposure tests are, at least, from one to two orders of magnitude higher than those predicted by calculations. In this paper, these observations are discussed and explanations for them are presented. [source] X-ray imaging of various biological samples using a phase-contrast hard X-ray microscopeMICROSCOPY RESEARCH AND TECHNIQUE, Issue 9 2008Guk Bae Kim Abstract In this study, we visualized the internal structures of various bio-samples and found the optimum conditions of test samples for the 7 keV hard X-ray microscope of the Pohang light source. From the captured X-ray images, we could observe the intercellular and intracellular structures of dehydrated human cells and mouse tumor tissues without using any staining materials in a spatial resolution better than 100 nm. The metastasized lung tissue, which was several tens of micrometers in thickness, was found to be very well suited to this hard X-ray microscope system, because it is nearly impossible to observe such a nontransparent and thick sample with a high spatial resolution better than 100 nm using any microscopes such as a soft X-ray microscope, an optical microscope, or an electron microscope. Microsc. Res. Tech., 2008. © 2008 Wiley-Liss, Inc. [source] Wide-range length metrology by dual-imaging-unit atomic force microscope based on porous aluminaMICROSCOPY RESEARCH AND TECHNIQUE, Issue 3 2004Dongxian Zhang Abstract A new dual-imaging-unit atomic force microscope (DIU-AFM) was developed for wide-range length metrology. In the DIU-AFM, two AFM units were combined, one as a reference unit, and the other a test one. Their probes with Z piezo elements and tips were horizontally set in parallel at the same height to reduce errors due to geometric asymmetry. An XY scanner was attached to an XY block that was able to move in the X direction with a step of about 500 nm. A standard porous alumina film was employed as the reference sample. Both reference sample and test sample were installed at the center of the XY scanner on the same surface and were simultaneously imaged. The two images had the same lateral size, and thus the length of the test sample image could be accurately measured by counting the number of periodic features of the reference one. The XY block together with the XY scanner were next moved in the X direction for about 1.5 ,m and a second pair of reference and test images were obtained by activating the scanner. In this way, a series of pairs of images were acquired and could be spliced into two wide-range reference and test images, respectively. Again, the two spliced images were of the same size and the length of test image was measured based on the reference one. This article presents a discussion about the structure and control of the DIU-AFM system. Some experiments were carried out on the system to demonstrate the method of length calculation and measurement. Experiments show a satisfactory result of wide-range length metrology based on the hexagonal features of the porous alumina with a periodic length of several tens of nanometers. Using this method the DIU-AFM is capable of realizing nanometer-order accuracy length metrology when covering a wide range from micron to several hundreds of microns, or even up to millimeter order. Microsc. Res. Tech. 64:223,227, 2004. © 2004 Wiley-Liss, Inc. [source] A smoothed particle hydrodynamics simulation of the collapse of the interstellar mediumMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001Y. Golanski A smoothed particle hydrodynamic (SPH) code is used to model a collapsing region of the interstellar medium (ISM). A region of the ISM is seeded with coolant material ejected from a supernova. The resulting pressure imbalance between the cooling region and the surrounding ISM induces collapse. The collapse first forms dense low-temperature regions (protoclouds) each containing several tens of solar masses of material and in quasi-equilibrium with the surrounding ISM. Turbulence is generated within the collapsing regions as they form. Collisions between protoclouds leads to regions of further increased mass and density, the final outcome being a dense cold cloud with mean density 10,18 kg m,3, mean temperature about 20 K and total mass a few hundred solar masses. The final cloud is sufficiently turbulent for star formation to occur within it. A new form of boundary condition was used in the SPH simulation to solve the problem inherent in modelling a continuum. [source] Limits on the location of planetesimal formation in self-gravitating protostellar discsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2009C. J. Clarke ABSTRACT In this Letter, we show that if planetesimals form in spiral features in self-gravitating discs, as previously suggested by the idealized simulations of Rice et al., then in realistic protostellar discs, this process will be restricted to the outer regions of the disc (i.e. at radii in excess of several tens of au). This restriction relates to the requirement that dust has to be concentrated in spiral features on a time-scale that is less than the (roughly dynamical) lifetime of such features, and that such rapid accumulation requires spiral features whose fractional amplitude is not much less than unity. This in turn requires that the cooling time-scale of the gas is relatively short, which restricts the process to the outer disc. We point out that the efficient conversion of a large fraction of the primordial dust in the disc into planetesimals could rescue this material from the well-known problem of rapid inward migration at an approximate metre-size scale and that in principle the collisional evolution of these objects could help to resupply small dust to the protostellar disc. We also point out the possible implications of this scenario for the location of planetesimal belts inferred in debris discs around main sequence stars, but stress that further dynamical studies are required in order to establish whether the disc retains a memory of the initial site of planetesimal creation. [source] Palaeoclimate reconstruction on Big Lyakhovsky Island, north Siberia,hydrogen and oxygen isotopes in ice wedgesPERMAFROST AND PERIGLACIAL PROCESSES, Issue 2 2002Hanno Meyer Abstract Late Quaternary permafrost deposits on Big Lyakhovsky Island (New Siberian Islands, Russian Arctic) were studied with the aim of reconstructing the palaeoclimatic and palaeoenvironmental conditions of northern Siberia. Hydrogen and oxygen stable isotope analyses are presented for six different generations of ice wedges as well as for recent ice wedges and precipitation. An age of about 200 ka BP was determined for an autochtonous peat layer in ice-rich deposits by U/Th method, containing the oldest ice wedges ever analysed for hydrogen and oxygen isotopes. The palaeoclimatic reconstruction revealed a period of severe winter temperatures at that time. After a gap in the sedimentation history of several tens of thousands of years, ice-wedge growth was re-initiated around 50 ka BP by a short period of extremely cold winters and rapid sedimentation leading to ice-wedge burial and characteristic ice-soil wedges (,polosatics'). This corresponds to the initial stage for the Late Weichselian Ice Complex, a peculiar cryolithogenic periglacial formation typical of the lowlands of northern Siberia. The Ice Complex ice wedges reflect cold winters and similar climatic conditions as around 200 ka BP. With a sharp rise in ,18O of 6, and ,D of 40,, the warming trend between Pleistocene and Holocene ice wedges is documented. Stable isotope data of recent ice wedges show that Big Lyakhovsky Island has never been as warm in winter as today. Copyright © 2002 John Wiley & Sons, Ltd. [source] Low-temperature MBE-grown GaBiAs layers for terahertz optoelectronic applicationsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2009Vaidas Pa, ebutas Abstract Gallium bismide arsenide epitaxial layers were grown by molecular-beam-epitaxy at low substrate temperatures and investigated for their suitability in terahertz optoelectronic applications. Optical pump-terahertz probe measurements on these layers have shown that carrier dynamics can be described using two characteristic times. The faster decay component has characteristic times shorter than 1 ps, whereas the slower component decays in several tens of picoseconds. Fitting the electron lifetimes dependence on optical excitation level the electron trapping cross-section and trap density were determined. The possible mechanism of carrier recombination was discussed. The photoconductive terahertz emitters and detectors made from GaBiAs layers have been manufactured and used in time-domain spectroscopy system with a signal bandwidth larger than 4.5 THz. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Nanocrystalline alloys studied by positron annihilation lifetime spectroscopyPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007Martin Petriska Abstract Nanocrystalline alloys present a new class of materials that exhibit interesting magnetic properties. These stem primarily from the presence of crystalline grains with the size of several tens of nanometers embedded in a residual amorphous matrix. In this paper, we employ positron annihilation spectrometry to investigate the relation between positron lifetime and microstructure of the NANOPERM-type metallic Fe76Mo8Cu1B15 alloy. The behaviour of positron lifetime with respect to the annealing temperature is discussed for samples, which were prepared by one hour thermal treatment in a vacuum at temperatures ranging from 330 up to 700 °C. Samples in amorphous (as-quenched) state were analysed, too. The results of this work confirm that the optimum annealing temperature of 550 °C correlates with the minimum of free volume and coincide with the temperature at which superior magnetic properties are observed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Dry etching of N-face GaN using two high-density plasma etch techniquesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2007F. Rizzi Abstract This paper describes processing of GaN on the on the (000) N-face surface, using two different high-density plasma etch techniques, inductively coupled plasma (ICP) etch, and electron cyclotron resonance (ECR) etching. ICP experiments used several different conditions employing Cl2,Ar,BCl3 or Cl2,Ar plasmas. The resulting maximum etch rates of 370,390 nm/min are approximately twice as high as etch rates for Ga-face (0001) GaN with the same recipes. ECR etching employed a Cl2,CH4,Ar recipe, which produced an average etch rate of 55 nm/min in a 20-minute etch process on N-face GaN. Both etch techniques increased the roughness of N-face GaN, but could produce surfaces with average roughness values below 3 nm. Selection of conditions with a dominant chemical etch contribution is important to maintain smooth surfaces. The use of both ICP and ECR etching in sequence is advantageous in situations where a GaN substrate several tens of microns in thickness must be thinned from the backside, stopping the etch in a suitable marker layer. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] New implementation of a SNOM suitable to study topographical features over wide areasPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2005A. Ustione Abstract A new implementation of a SNOM is described, aiming at the topographical study of large areas and exploiting the advantages of the shear-force detection system. This technique finds very interesting application in the study of archeological or artistic samples, where it often occurs that an area to be examined at high resolution (optically, topographically or chemically) is enclosed in a wider one. The implemented system allows to obtain scans of wide areas by using dc-motors to move the sample under the probe tip, and thus it can face rough samples with height differences of several tens of micrometers. It allows the user to choose whether to use the SNOM to study the optical and topographical properties of a small part of the sample (up to tens of square micrometers), or to use the motor-driven scan technique, to study the topographical characteristic of a large area of the sample (up to some square millimeters) and even over rough surfaces. We show results detecting worn relieves over a coin. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Preparation and mechanical properties of well-aligned and well-oriented poly(vinyl alcohol) nanoribbonPOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 3 2009Mai Iwashita Abstract We report a novel, economical method of preparing well-aligned and well-oriented nanoribbons made of poly(vinyl alcohol) (PVA). The method requires no special equipment and easily produces nanoribbon. The structure of uniaxially drawn nanoribbon was investigated by polarized microscopy, polarized FT-IR, and X-ray diffraction, which showed high molecular and crystallite orientations. The well-aligned PVA nanoribbon, measuring several tens of micrometer width and around 400,nm thickness, had a Young's modulus of 9,GPa and a tensile strength of 350,MPa. Copyright © 2008 John Wiley & Sons, Ltd. [source] Preparation and mechanical properties of poly(chitosan- g - DL -lactic acid) fibrous mesh scaffoldsPOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 2 2008Ying Wan Abstract DL -lactic acid was grafted onto chitosan to produce poly(chitosan- g - DL -lactic acid)(PCLA) without using a catalyst. These PCLAs were then spun into filaments and further fabricated into fibrous mesh scaffolds using an improved wet-spinning technique. The diameter of filaments in different scaffolds could vary from a few micrometers to several tens of micrometers. The scaffolds exhibited various pore sizes ranging from about 20,µm to more than 200,µm and different porosities up to 80%. The several main processing conditions were optimized for obtaining the desired scaffolds with well-controlled structures. The tensile and compressive mechanical properties of the mesh scaffolds in both dry and hydrated states were mainly examined. Significantly improved tensile strength and modulus, enhanced compressive modulus, and stress as well as the dimensional stability for these mesh scaffolds in their hydrated state were observed. Copyright © 2007 John Wiley & Sons, Ltd. [source] | |||||||||||||||||||||||||||||||||||||