Substitutional solution of silicon in cementite: A first-principles study

Cementite precipitation from austenite in steels can be suppressed by alloying with silicon. There are, however, no validated thermodynamic data to enable phase equilibria to be estimated when silicon is present in cementite. The formation energies of Fe₃C, and have therefore been estimated using first-principles calculations based on the total energy all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation to density functional theory. The ground state properties such as lattice constants and bulk moduli have also been calculated. The calculations show that and have about 52.06 kJ mol⁻¹ and 37.17 kJ mol⁻¹ greater formation energy, respectively, than Fe₃C. The formation energy for hypothetical cementite Si₃C has also been calculated to be about 256 kJ mol⁻¹. Silicon substitution significantly reduces the magnetic moments at the Fe(4c) site for both and , irrespective of the Si substitution sites. The calculated electronic structures indicate that the magnetic moment reduction at the Fe(4c) site by the Si substitution at 4c site is indirect through the neighboring carbon atom, whereas at the 8d site it is direct.     

Diffusion and formation energies of adatoms and vacancies on magnesium surfaces

This paper reports classical molecular statics calculations of magnesium {0 0 0 1}, , , and surfaces, specifically formation energies of defects (adatoms and surface vacancies) and flat surfaces and diffusion energy barriers of the defects. The formation energies show that the surface is thermodynamically more favorable than , and surfaces; in contrast, literature reports have often ignored the surface. The diffusion energy barriers of both adatoms and surface vacancies show strong diffusion anisotropy on , , and surfaces. Based on this anisotropy, the ratio of diffusion distances (of either adatoms or surface vacancies) along two orthogonal directions on is 37–55 at room temperature. Using the results of formation energies and diffusion energy barriers we develop a more complete understanding of surface orientations in Mg nanoblades synthesized by physical vapor deposition [F. Tang, T. Parker, H.-F. Li, G.-C. Wang, T.-M. Lu, J. Nanosci. Nanotechnol. 7 (2007) 3239]. In contrast to previous reports, we postulate that the side surfaces of Mg nanoblades are because (a) they have the second lowest surface formation energy and (b) the ratio of diffusion distances on them agrees with the experimental value of approximately 50.     

Scintillation fiber array detector for measurement of neutron beam profile

We built and tested a detector to measure the profile of fast-neutron beams delivered by the MC50 cyclotron at the Korea Institute of Radiological and Medical Science (KIRAMS). The core component of the detector is a 2×46 array of scintillation fibers. The light output of the scintillation fibers is transformed into a current signal by a 46-channel silicon photodiode and digitized by a current-mode signal processor. This scanning device was designed to cover a neutron beam area of . The detector was tested in a neutron beam delivered by the MC50 cyclotron at KIRAMS. We demonstrate that the detector can successfully measure the neutron beam profile at various beam currents from 10 to . The proposed neutron beam profile detector will be useful, for example, in radiotherapy applications with neutron intensities above .     

Measurements of high-energy -rays with detectors

The full-energy peak efficiency calibration and the energy resolution measurements of the LaBr₃ γ-ray detector are presented for γ-ray energies in the 700 keV–17.6 MeV range. Measurements were done using a combination of proton-capture nuclear reactions on , , , and for high-energy γ-rays, and radioactive sources such as and for the lowest energies. At high energies, two γ-rays in a cascade from proton resonance capture were employed using Al, Na₂WO₄, K₂SO₄ and LiBO₂ targets. The obtained results were compared to the simulations performed using a GEANT4 code.     

Development of thick-foil and fine-pitch GEMs with a laser etching technique

We have produced thick-foil and fine-pitch gas electron multipliers (GEMs) using a laser etching technique. To improve production yield we have employed a new material, liquid crystal polymer, instead of polyimide as an insulator layer. The effective gain of the thick-foil GEM with a hole pitch of , a hole diameter of , and a thickness of reached a value of 10⁴ at an applied voltage of 720 V. The measured effective gain of the thick-foil and fine-pitch GEM ( pitch, diameter, and thick) was similar to that of the thick-foil GEM. The gain stability was measured for the thick-foil and fine-pitch GEM, showing no significant increase or decrease as a function of elapsed time from applying the high voltage. The gain stability over 3 h of operation was about 0.5%. Gain mapping across the GEM showed a good uniformity with a standard deviation of about 4%. The distribution of hole diameters across the GEM was homogeneous with a standard deviation of about 3%. There was no clear correlation between the gain and hole diameter maps.     

New bounds on the reliability of the consecutive k-out-of-r-from-n:F system

The main goal of this article is to show that the lower and upper bounds on the failing probability resp. reliability of consecutive k-out-of-r-from-n:F reliability systems developed by Sfakianakis, Kounias and Hillaris [M. Sfakianakis, S. Kounias, A. Hillaris, IEEE Trans. Reliability 1992;R-41:442–447] can be improved by applying fourth order Boole–Bonferroni bounds. Further we propose the application of the Hunter–Worsley bound in the framework of reliability system analysis, too. Numerical results of the formerly published test examples and harder problems are given. The computer code was written in and is available on request from the authors.     

Molybdate inhibition of environmental fatigue crack propagation in Al–Zn–Mg–Cu

effectively inhibits environmentally assisted fatigue crack propagation in 7075-T651 stressed during full immersion in low-chloride solution, as understood by hydrogen environment embrittlement and film rupture where -enhanced passivity reduces H production and uptake due to reduced crack hydrolysis, buffered pH, and a diffusion-barrier film. Inhibition is governed by the balance between crack tip strain rate and repassivation kinetics which establish the stability of the passive film. Inhibition is promoted by reduced loading frequency, reduced stress intensity range, increased crack tip concentration, and potentials at or anodic to free corrosion. The inhibiting effect of parallels that of , but molybdate effectiveness is shifted to a lower frequency regime suggesting the Al_xMo_yO_z passive film is less stable against crack tip deformation. For high R loading at sufficiently low frequencies fully inhibits EFCP, quantified by reduced crack growth rate to that typical of ultra-high vacuum, reduction in crack surface facets typical of hydrogen embrittlement, and crack arrest. Chromate did not produce such complete inhibition. Methods exist to incorporate molybdate or Mo in self-healing coating systems, but the complex effects of mechanical and electrochemical variables must be understood for reliable-quantitative fatigue performance enhancement.     

Depth of interaction detection for -ray imaging

A novel design for an inexpensive depth of interaction capable detector for γ-ray imaging has been developed. The design takes advantage of the strong correlation between the width of the scintillation light distribution in monolithic crystals and the interaction depth of γ-rays. We present in this work an inexpensive modification of the commonly used charge dividing circuits which enables the instantaneous and simultaneous computation of the second order moment of light distribution. This measure provides a good estimate for the depth of interaction and does not affect the determination of the position centroids and the energy release of γ-ray impact. The method has been tested with a detector consisting of a monolithic LSO block sized and a position-sensitive photomultiplier tube H8500 from Hamamatsu. The mean spatial resolution of the detector was found to be for the position centroids and for the DOI. The best spatial resolutions were observed at the center of the detector and yielded for the position centroids and for the DOI.     

Selective adsorption of cations on single-walled carbon nanotubes: A density functional theory study

The selective adsorption of cation on single-walled carbon nanotubes (SWNTs) is systemically studied by using density functional theory calculations. It is found that the adsorption energy of cations on SWNTs depends on the concentration of cations and the diameter and the electronic structure of SWNTs. The binding strength of on each SWNT increases monotonically as the concentration of decreases, undergoing a change from endothermic to exothermic reaction. Generally speaking, the binding of on SWNTs becomes weaker as the diameter increases. In the medium-diameter region (9 < d < 11 Å), prefers to interact with metallic SWNTs (m-SWNTs) rather than semiconducting SWNTs (s-SWNTs) at the same concentration of . In the small-diameter region (d < 9 Å), the binding of is nearly independent of metallicity, but it is stronger than that of on the medium-diameter s-SWNTs. In the large-diameter region (d > 11 Å), the dependence of adsorption on the electronic structure is complicated, but the binding of is weaker than that on the medium-diameter s-SWNTs. Our results are in agreement with the experimental report that the small-diameter m- and s-SWNTs and the medium-diameter m-SWNTs are etched away by while the medium-diameter s-SWNTs and the large-diameter m- and s-SWNTs are intact.     

Recrystallization mechanism of as-cast AZ91 magnesium alloy during hot compressive deformation

Dynamic recrystallization (DRX) behavior of as-cast AZ91 magnesium alloy during hot compression at 300 °C and the strain rate of 0.2 s⁻¹ was systematically investigated by electron backscattering diffraction (EBSD) analysis. Twin DRX and continuous DRX (CDRX) are observed in grains and near grain boundaries, respectively. Original coarse grains are firstly divided by primary {} tensile twins and {} compression twins, and then {}–{} double twins are rapidly propagated within these primary compression twins with increasing compressive strain. Some twin-walled grains are formed by the mutual crossing of twins or by the formation of the {}–{} double twins and furthermore, subgrains divided by low-grain boundaries in the double twins are also formed. Finally, DRXed grains are formed by the in situ evolution of the subgrains with the growth of low-angle boundaries to high-angle grain boundaries in twins. CDRX around the eutectic Mg₁₇Al₁₂ phases at grain boundaries occurs together with the precipitation of discontinuous Mg₁₇Al₁₂ phase and the fragmentation of the precipitates during compression. The discontinuous fragmented precipitates distribute at the newly formed CDRXed grain boundaries and have remarkable pinning effect on the CDRXed grain growth, resulting in the average grain size of about 1.5 μm.     

A device to measure the effects of strong magnetic fields on the image resolution of PET scanners

Very high resolution images can be achieved in small animal PET systems utilizing solid state silicon pad detectors. As these systems approach sub-millimeter resolutions, the range of the positron is becoming the dominant contribution to image blur. The size of the positron range effect depends on the initial positron energy and hence the radioactive tracer used. For higher energy positron emitters, such as and , which are gaining importance in small animal studies, the width of the annihilation point distribution dominates the spatial resolution. This positron range effect can be reduced by embedding the field of view of the PET scanner in a strong magnetic field. In order to confirm this effect experimentally, we developed a high resolution PET instrument based on silicon pad detectors that can operate in a 7 T magnetic field. In this paper, we describe the instrument and present initial results of a study of the effects of magnetic fields up to 7 T on PET image resolution for and point sources.     

Microstructured semiconductor neutron detectors

Perforated semiconductor neutron detectors are compact diode detectors that operate at low power and can be fashioned to have high thermal neutron detection efficiency. Fabricated from high-purity Si wafers, the perforations are etched into the diode surface with ICP-RIE and backfilled with ⁶LiF neutron reactive material. The intrinsic thermal neutron detection efficiency depends upon many factors, including the perforation geometry, size, and depth. Devices were fabricated from high resistivity  cm n-type Si with conformal p-type shallow junction diffusions into the perforations, which demonstrate improved neutron detection performance over previous selectively diffused designs. A comparison was made to previous selectively diffused designs, and pulse height spectra show improved signal-to-noise ratio, higher neutron counting efficiency, and excellent gamma-ray discrimination. Devices with wide deep sinusoidal trenches yielded intrinsic thermal neutron detection efficiencies of 11.94±0.078%.     

Impurity-defect induced noncentrosymmetricity in nonlinear optical processes

Noncentrosymmetric nanosize-material processes in cadmium iodide are formed by doping it with the impurity copper. The noncentrosymmetricity in the processes are probed by the observation of the second-order optical susceptibility . The value of is found to depend fashionably on the impurity content of the nanomaterials. The results also show that a significant enhancement in the noncentrosymmetric response is achieved in nanomaterials with reduced sizes and at low temperatures.     

First-principles study on electronic structure and absorption spectra for the CaWO4 crystal with oxygen vacancy

The electronic structure and absorption spectra for the perfect CaWO₄ and the CaWO₄ containing oxygen vacancy have been calculated using density functional theory code CASTEP with the lattice structure being optimized. The results indicate that the CaWO₄ crystal containing , an additional absorption spectrum in the region of visible light. And the absorption spectrum can be fit into two Gaussian-shape absorption spectra with peaks at 360 and 415 nm. These peaks are located at the experimentally observed position. It is predicted that the 340 and the 420 nm absorption spectra are related to the existence of in the CaWO₄ crystal.     

Semi-empirical atomistic study of point defect properties in BCC transition metals

We have constructed a set of embedded atom method (EAM) potentials for Fe, Ta, W and V and used them in order to study point defect properties. The parametrizations of the potentials ensure that the third order elastic constants are continuous and they have been fitted to the cohesive energies, the lattice constants, the unrelaxed vacancy formation energies and the second order elastic constants. Formation energies for different self-interstitials reveal that the split dumbbell is the most stable configuration for Fe while for Ta, W and V we find that the split dumbbell is preferred. Self-interstitial migration energies are simulated using the nudged elastic band method and for Fe and W the migration energies are found to be in good agreement with experimental and ab initio data. Migration energies for Ta and V self-interstitials are found to be quite low. The calculated formation, activation and migration energies for monovacancies are in good agreement with experimental data. Formation energies for divacancies reveal that the second nearest neighbor divacancy is more energetically favorable than nearest neighbor divacancies and the migration energies indicate that nearest neighbor migration paths are more likely to occur than second nearest neighbor migration jumps. For Fe, we have also studied the influence of the pair potential behavior between the second and third nearest neighbor on the stability of the split dumbbell, which revealed that the higher the energy level of the pair potential is in that region, the more stable the split dumbbell becomes.     

Metallization and metallicity: Universal conductivity limits

The Mott–Ioffe–Regel minimum metallic conductivity based on a minimum mean free path of inter-atomic spacing has been used as a robust criterion for signalling Fermi liquid behaviour. Instead we examine the conductivity of a system in universal terms of the excitation energy, E_exc, of a charge carrier bound to its hole. The expression for conductivity depends simply on probability of charge transfer expressed as E_exc/. The minimum conductivity for Fermi liquid state is obtained as  S cm⁻¹ (C = 1;  eV is the maximum excitonic binding energy for a mobile exciton). From simple considerations in the t–J model of the Larmor precession time, , due to an internal exchange magnetic field and the residence time, τ_W, for a charge carrier with energy E_W, we express condition for Fermi liquid () and non-Fermi liquid behaviour (). For such NFL liquids, we find that one requires C_NFL = (1/π²π^2/3)  1/21.2 to account for the reduced probability of charge transfer with conservation of spin in NFL systems. The maximum value of the conductivity, σ^±, at which the temperature coefficient of resistance (TCR) changes sign at an insulator metal transition is given by  S cm⁻¹. This value is close to that observed in several systems. We discuss these universal values of the conductivity in the context of the Herzfeld criterion, Mooij criterion, exciton transfer rates, chemical reaction rate theory, universal sheet resistance at insulator–superconductor transition, as well as the changes in resistivity during the metallization of molecular hydrogen, oxygen and iodine.     

Response of the CALICE Si-W electromagnetic calorimeter physics prototype to electrons

A prototype silicon–tungsten electromagnetic calorimeter (ECAL) for an international linear collider (ILC) detector was installed and tested during summer and autumn 2006 at CERN. The detector had 6480 silicon pads of dimension . Data were collected with electron beams in the energy range 6–45 GeV. The analysis described in this paper focuses on electromagnetic shower reconstruction and characterises the ECAL response to electrons in terms of energy resolution and linearity. The detector is linear to within approximately the 1% level and has a relative energy resolution of . The spatial uniformity and the time stability of the ECAL are also addressed.     

Arguments for a “US Kamioka”: SNOLab and its implications for North American underground science planning

We argue for a cost-effective, long-term North American underground science strategy based on partnership with Canada, initial construction of a modest US Stage I laboratory designed to complement SNOLab, and follow-up stages to create clean horizontal access to greater depths. We show, by reviewing the requirements of detectors now in the R&D phase, that SNOLab and a properly designed US Stage I facility would be capable of meeting most needs of North America's next wave of underground experiments.One opportunity for creating such a laboratory is the Pioneer tunnel in Washington State, a site that could be developed to provide dedicated, clean, horizontal access. This unused tunnel, part of the deepest (1040 m) tunnel system in the US, would allow the US to establish, at low risk and modest cost, a laboratory at a depth (.w.e., or kilometers of water equivalent) quite similar to that of the Japanese laboratory Kamioka (.w.e.). The site's infrastructure includes highway and rail access to the portal, a gravity drainage system, redundant power, proximity to a major metropolitan area, and a system of crosscuts connecting to the parallel Great Cascade tunnel and its ventilation system. We describe studies of cosmic ray attenuation important to properly locating such a laboratory, and the tunnel improvements that would be required to produce an optimal Stage I facility.We describe the unique role this location could play in formulating an international plan for high-energy accelerator physics that includes, as one component, a neutrino factory. The site has a “doubly magic” baseline—a separation from both KEK and CERN—as well as an appropriate baseline for CP violation studies, should FermiLab host the neutrino factory.We also describe how new space at greater depth could be added in response to the needs of future experiments, building on the experience gained in Stage I. We discuss possible designs for Stage II (.w.e.) and Stage III (.w.e.) developments at the Pioneer tunnel, should future North American needs for deep space exceed those available at SNOLab. This staging could be planned to avoid duplication of SNOLab's capabilities while minimizing construction and operations costs. We describe the existing geotechnical record important to future stages, including past tunneling histories, borehole studies and analyses, and recent examinations of the Pioneer tunnel. We also describe the significant broader impacts of this project in improving the efficiency, safety, and security of one of the nation's key transportation corridors.     

The optics beamline at the Swiss Light Source

We present the successful installation and the performance of a new beamline for optics and instrumentation research and developments at the Swiss Light Source. The beamline covers the photon energy range from 5.5 to 22.5 keV at a bending magnet with a cryogenically cooled Si(1 1 1) channel cut monochromator followed by a bendable toroidal mirror with 1:1 focusing. Monochromator and focusing mirror can be retracted independently to allow monochromatic and pink beam mode with and without focusing. In focused monochromatic mode we measured a usable photon flux of at 11 keV within a focus of (FWHM v×h). The higher order contamination has been determined with absorption foils. We measured 0.025% of second order light at 9 keV and 17% of third order contamination at 6 keV. In pink beam mode we measured with a thermopile sensor a radiation power of 10.6 W for 1 mrad acceptance.     

Development of front-end electronics for the luminosity detector at ILC

The design and measurements of the prototype front-end electronics for the luminosity detector (LumiCal) at the International Linear Collider (ILC) are presented. The challenges of the LumiCal front-end electronics are discussed and the proposed architecture comprising switched-gain preamplifier, pole-zero cancellation circuit (PZC) and switched-gain shaper is described. The preamplifier works for a wide range of input capacitance of up to 100 pF. The input charge dynamic range extends from <0.4 fC to and covers more than four orders of magnitude. To conform with the timing requirements a peaking time (T_peak) of about 70 ns was chosen. The prototype ASIC comprising eight channels was designed and fabricated in CMOS technology. The results of measurements on gain, noise, high count rate performance and crosstalk are presented.