Chromium Impurities Research Articles

Analysis of impurity effects on the coloration of corundum by laser-induced breakdown spectroscopy (LIBS).

Corundum was analyzed using laser-induced breakdown spectroscopy (LIBS) for impurity effects in their multiple colorings. Qualitative measurements were attained for impurities of chromium, magnesium, iron, and titanium in red, yellow, and blue samples. Moreover, treatment with a beryllium diffusion, which can modify corundum to obtain an attractive color, was tested in the yellow sample. In this work, most of the measurements were acquired using a laser pulse energy of 5 mJ and impurity emissions were appreciable. The signal-to-noise ratios were 11, 6.5, 10, and 4 for the Cr 425.44 nm, Fe 404.58 nm, Be 313.04 nm, and Mg 285.21 nm lines, respectively, for five laser shots. The amount of damage to the corundum samples was also monitored by measuring the craters after laser analysis. It was found that the crater size was about 30 μm after 10 laser shots. As such, the damage to corundum sample is almost imperceptible after the LIBS analysis.

⁵²Mn production for PET/MRI tracking of human stem cells expressing divalent metal transporter 1 (DMT1).

There is a growing demand for long-term in vivo stem cell imaging for assessing cell therapy techniques and guiding therapeutic decisions. This work develops the production of 52Mn and establishes proof of concept for the use of divalent metal transporter 1 (DMT1) as a positron emission tomography (PET) and magnetic resonance imaging (MRI) reporter gene for stem cell tracking in the rat brain. 52Mn was produced via proton irradiation of a natural chromium target. In a comparison of two 52Mn separation methods, solvent-solvent extraction was preferred over ion exchange chromatography because of reduced chromium impurities and higher 52Mn recovery. In vitro uptake of Mn-based PET and MRI contrast agents (52Mn2+ and Mn2+, respectively) was enhanced in DMT1 over-expressing human neural progenitor cells (hNPC-DMT1) compared to wild-type control cells (hNPC-WT). After cell transplantation in the rat striatum, increased uptake of Mn-based contrast agents in grafted hNPC-DMT1 was detected in in vivo manganese-enhanced MRI (MEMRI) and ex vivo PET and autoradiography. These initial studies indicate that this approach holds promise for dual-modality PET/MR tracking of transplanted stem cells in the central nervous system and prompt further investigation into the clinical applicability of this technique.

The fate of chromium impurities during acid sulphate digestion of ilmenite concentrates

Ilmenite concentrates from the Murray Basin region of southeastern Australia are contaminated with chromium impurities that must be removed for the ilmenite to become a satisfactory feedstock for the sulphate route to titania pigment production. The chromia is present primarily as discrete, compositionally variable, chrome-rich spinel grains with a smaller amount as intra-grain chromia distributed as coatings in fractures and pores of weathered ilmenite grains. Characterisation of chromia deportment through a simulated acid sulphate digestion process showed a small but non-negligible solubility of the spinels. Most spinels were resistant to dissolution with the exception of those containing high Fe(Al,Cr)2O4 and magnetite (Fe3O4) components. Intra grain chromia was highly soluble. Processing to achieve low bulk chromia using a magnetising roast procedure must ensure that well crystallised rutile is not produced because of its insolubility in the sulphate process. It is also important that the roast conditions do not substantially increase the magnetite content of the spinels making them more susceptible to dissolution. This work highlights the importance of characterising all spinel composition types within ilmenite concentrates in addition to the level of intra-grain chromia associated with the ilmenite when considering the suitability of Murray Basin primary ilmenites as a feedstock to sulphate route titania pigment plants.

Influence of the chromium and ytterbium co-doping on the photoluminescence of zinc selenide crystals

The luminescent properties of ZnSe, ZnSe:Cr (0.05 at.% Cr), ZnSe:Yb (0.03 at.% Yb) and ZnSe:Cr:Yb (0.05 at.% Cr, 0.05 at.% Yb) crystals, doped during the growth process by the chemical vapor transport method, were studied within the temperature interval of 6–300 K. At the 6 K temperature in the visible spectral range 2 bands were observed: a band in the excitonic spectral region and a band of self-activated luminescence. It was shown that co-doping of zinc selenide crystals with the chromium and ytterbium led to the combination of the impurities influence on the photoluminescent properties. At the liquid helium temperature in the middle infrared range of the spectra of the ytterbium and chromium co-doped crystal a band with the maximum localized at 1.7 μm was observed, which was overlapped with a complex band in the middle-IR spectral range, characteristic for the chromium doped ZnSe crystals. On the basis of obtained data an interaction mechanism of the chromium and ytterbium co-doping impurities was proposed. Guided by the existent model of the ytterbium ion incorporation in the selenide sublattice of the ZnSe crystals, an assumption about stabilization of single charged chromium ions in the zinc sublattice crystal nodes, by means of formation of the local charge compensating clusters, was made. It was assumed that the resonant energy transfer from one chromium ion to another, which led to the concentration quenching of the IR emission in the ZnSe:Cr PL spectra, would lead to the broadening of the IR emission in the spectra of ytterbium and chromium co-doped zinc selenide crystals.

Optical and structural properties of chromium impurities in niobium–gallium oxide

In this work, the optical properties of the gallium–niobium oxide doped with chromium ions were investigated. The samples were obtained by wet chemical and solid-state methods, investigated by X-ray powder diffraction and the data were refined by Rietveld methods. The morphology was determined by scanning electron microscope (SEM) images. The photoluminescence, excitation and photoacoustic measurements showed broad and intense bands in the red region of the spectrum, associated with energy transitions of chromium ions in octahedral sites. From the optical spectra and using the crystal field theory, the energy parameters were obtained.

Influence of the Magnetic Field on the Solubility of Crystals

Influence of the magnetic treatment of KDP and ADP crystals and solutions on their equilibrium temperature is revealed and investigated. The maximum change of the temperature is observed right after magnetization. Then the effect monotonically decreases and in 5 h the temperature approaches its equilibrium value. It is established that the observed decrease in the solubility depends on the impurity content and is maximum for magnetically treated KDP crystals with chromium impurity; it further decreases in the series: KDP crystal, KDP–ADP crystals in KDP–ADP solution, KDP crystal in KDP–ADP solution, and KDP solution. Relaxation times are determined for all five examined systems. It is revealed that the effect fast increases and then is saturated at B = 0.02 T.

Impurity-induced enhancement of perpendicular magnetic anisotropy in Fe/MgO tunnel junctions

Using first-principles calculations, we investigated the impact of chromium (Cr) and vanadium (V) impurities on the magnetic anisotropy and spin polarization in Fe/MgO magnetic tunnel junctions. It is demonstrated, using layer resolved anisotropy calculation technique, that while the impurity near the interface has a drastic effect in decreasing the perpendicular magnetic anisotropy (PMA), its position within the bulk allows maintaining high interfacial PMA while reducing the bulk magnetization and correlatively the easy-plane demagnetizing energy. As a result, the effective magnetic anisotropy tends to increase as a function of the Cr or V concentration resulting in an increase in the critical magnetic thickness at which the crossover from out-of-plane to in-plane anisotropy takes place. At the same time, the interfacial spin polarization is not affected by the magnetic layer bulk doping by Cr or V impurities and even enhanced in most situations thus favoring an increase of tunnel magnetoresistance (TMR) amplitude.

Theoretical unification of hybrid-DFT andDFT + Umethods for the treatment of localized orbitals

We formulate the on-site occupation dependent exchange correlation energy and effective potential of hybrid functionals for localized states and connect them to the on-site correction term of the DFT+U method. Our derivation provides a theoretical justification for adding a DFT+U-like onsite potential in hybrid DFT calculations to resolve issues caused by overscreening of localized states. The resulting scheme, hybrid- DFT+Vw, is tested for chromium impurity in wurtzite AlN and vanadium impurity in 4H-SiC, which are paradigm examples of systems with different degree of localization between host and impurity orbitals.

Thermo- and photostimulated luminescence and stimulated emission of magnesium oxide crystals irradiated in a nuclear reactor

The luminescence of nominally pure magnesium oxide crystals irradiated in a nuclear reactor is studied. It is shown that, in the entire fluence range (1015–1018 n/cm2), the active luminescence centers are chromium impurity ions, which are initially ( 1017 n/cm2), after the radiation-induced transformation Cr2+ → Cr3+, are in the form of Cr3+ ions. The role played by F centers is also shown. Pulsed stimulated emission is observed in crystals irradiated with a fluence of 1018 n/cm2.

Investigation of biomass depolymerization by surface techniques

The application of time‐of‐flight (ToF)‐SIMS to cellulose valorization is described. Cellulose samples subjected to ball milling or sulfuric acid impregnation, or combinations thereof, were subjected to hydrolysis. The material, which was impregnated in the last treatment step, no matter whether previously milled or not, exhibited the highest hydrolysis activity because of the highest accessibility of surface sulfonic groups. When milling followed impregnation, the activity was decreased because of possible encapsulation of sulfonic groups in the bulk. The ToF‐SIMS analysis revealed that both the ball mill and the stainless steel reactor may be a source of chromium and iron impurities, which can decrease the hydrolysis yield. Copyright © 2014 John Wiley & Sons, Ltd.

Influence of chromium interaction with native and impurity defects on optical and luminescence properties of ZnSe:Cr crystals

Luminescent properties of the chromium doped zinc selenide crystals with various concentrations of the doping impurity in 0.03–0.30 at. % range within the temperature interval from 6 K to 300 K were studied. Chromium doping was performed in various technological processes: during the growth by chemical vapor transport method; during the growth from melt; by thermal diffusion from the Bi + Cr melt or by thermal diffusion from gas. Possibility of chromium-bound excitons formation was shown, as well as chromium impurity participation in formation of emission bands in IR spectral region. A mechanism of chromium doping impurity interaction with uncontrolled background impurities and zinc selenide structural defects was proposed based on experimental data. An assumption about resonant energy transfer between double charged chromium ions and complexes based on crystals vacancy defects was made.

EPR study of the effect of ionizing radiation on chromium centers in Mg2SiO4: Cr,Li laser crystals

Forsterite single crystals doped with chromium and lithium and exposed to ionizing radiation have been studied using multifrequency electron paramagnetic resonance (EPR) spectroscopy. It has been found that ionizing irradiation up to a dose of 108 rad does not lead to a significant change in the concentration of single chromium impurity centers. At the same time, γ-ray irradiation of the crystal leads to a decrease in the concentration of active laser centers, which form an associate of trivalent chromium and monovalent lithium in the crystallographic positions M2 and M1, respectively, and to the formation of new centers of divalent chromium. The structure and magnetic properties of the new centers have been discussed.

EPR study of the effect of partial pressure of oxygen in the growth atmosphere on concentration of chromium centers in synthetic forsterite

Samples of synthetic forsterite doped with chromium and grown in an argon atmosphere with different oxygen concentrations (at a partial pressure of oxygen in the range from 0.03 to 0.78 kPa) have been studied using multifrequency electron paramagnetic resonance (EPR) spectroscopy. It has been demonstrated how the oxidizing properties of the atmosphere during the growth of crystals affect the relative concentration of di-, tri-, and tetravalent chromium ions in the samples. The structure of trivalent chromium impurity centers has been discussed.

Improved performance of amorphous Si thin-film solar cells on 430 stainless steel substrate by an electrochemical mechanical polishing process

This study reports the electrochemical mechanical polishing (ECMP) technique for 430 stainless steel (SS) substrate to improve the conversion efficiency of hydrogenated amorphous silicon (a-Si:H) thin-film solar cells. The surface roughness of the untreated 430 SS substrate was ∼38nm and decreased to ∼23nm after ECMP processing of the 430 SS substrate. The average diffuse reflection rate decreased from 4.2% of the untreated 430 SS substrate to 1.1% for the ECMP processed 430 SS substrate. The analysis of chemical composition of passive layer on the ECMP processed 430 SS substrate was carried out by the X-ray photoelectron spectroscopic. The passive layer of ECMP processed 430 SS substrate is composed of Cr2O3 and Fe2O3. The SIMS result indicated that the diffusion of iron (Fe) and chromium (Cr) impurities from the 430 SS substrate into the a-Si:H solar cell can be suppressed by the passive layer which was formed by the ECMP process. In particular, the Cr impurity was effectively blocked from diffusing into a-Si:H layer of the ECMP processed 430 SS substrate. The decreased series resistance and increased shunt resistance for the a-Si:H solar cells on the ECMP processed 430 SS substrate, due to a reduction in the Cr and Fe deep-level defect density, resulted in the cell conversion efficiency increasing to 5.4%. The ECMP process provides an available surface treatment technique for achieving high conversion efficiency of a-Si:H thin-film solar cells on a 430 SS substrate.

Effect of magnetic treatment of KDP and ADP crystals and solutions on their equilibrium temperature

The effect of magnetic treatment of KDP and ADP crystals and solutions on their equilibrium temperature has been revealed and studied. The change in the temperature reaches a maximum immediately after the magnetization. Then, the effect decreases monotonically, and the equilibrium temperature approaches the equilibrium value for 5 h. It has been found that the observed decrease in the solubility depends on the impurity concentration, and it is maximum in magnetically treated KDP crystals with a chromium impurity; then, it decreases in the series: KDP crystal, KDP-ADP crystals in KDP-ADP solution, KDP crystal in KDP-ADP solution, and KDP solution. For all five systems under study, the relaxation times have been determined. It has been revealed that the effect rapidly increases and reaches saturation at B = 0.02 T. The energy effects in the crystals and solutions have been estimated.

Spin-dependent transport in carbon nanotubes with a chromium impurity

Nature of the spin-dependent electron transport in carbon nanotubes with chromium atoms adsorbed at the surface has been clarified. It has been shown that the magnitude of the spin-dependent transport is determined by the relative displacement (under the effect of an external magnetic field) of the energy levels of electrons (Coulomb gaps arising in the vicinity of the Fermi level) for different spin projections.

IGSCC of Non-Sensitised Austenitic Stainless Steels in BWR Environment – Effect of Deformation Mode on Grain Boundary Susceptibility

Intergranular Stress Corrosion Cracking (IGSCC) of austenitic Stainless Steels (SS) in Boiling Water Reactor (BWR) environment is generic in nature in both the sensitised and the non-sensitized conditions. IGSCC in non-sensitized austenitic SS in the strain hardened condition has been reported without any grain boundary chromium depletion or impurity segregation. The present study ascertains the reason for IGSCC in BWR environment in non-sensitized condition and investigates the effect of nitrogen content in SS on the susceptibility to IGSCC. Two heats of type 304LN stainless steel with 0.08 and 0.16 wt. % nitrogen were used. Strain hardening was done by cross rolling at 200 °C to 20 % thickness reduction (warm rolling) to simulate the weld induced strain in constrained welds. Subsequently, Transmission Electron Microscopic (TEM) examination was carried out on the rolled SS. The deformation mode observed due to warm rolling was predominantly elongated twin and shear band (SB) formation in both the SS, terminating at the grain boundary regions. This resulted in higher stresses and strains making grain boundary susceptible to IGSCC. Presence of more dislocations at grain boundaries is a key feature for such enhancement in the susceptibility of non-sensitized SS to IGSCC. Formation of twins and SB occurred to a greater extent in the SS with higher nitrogen content indicating greater susceptibility to IGSCC in BWR environment. Crack growth studies done in simulated BWR environment at different Dissolved Oxygen (DO) levels showed higher crack growth rates in the SS with higher nitrogen content in the non-sensitised and strain hardened condition, confirming the higher susceptibility of SS with a higher level of nitrogen.

Interaction of impurity and intrinsic defects in forsterite and its role in the formation of spectral and luminescence properties

This paper reports on the results of investigations of the coefficients of chromium distribution between a crystal and a melt of forsterite, the absorption and luminescence spectra, and the electron paramagnetic resonance spectra of chromium centers in Mg2SiO4: Cr, Mg2SiO4: Cr: Sc, and Mg2SiO4: Cr: Li crystals. It has been established that the concentration dependences of these properties vary upon changing over from the range of trace concentrations of chromium, scandium, or lithium impurities in the melt to the range of higher concentrations of these impurities. The observed phenomenon is explained by the interaction of impurities with intrinsic defects of the crystal, which is called as the microimpurity trapping effect. According to the performed estimations, the concentration of predominant intrinsic defects (magnesium Frenkel defects) in the forsterite crystals grown from the melt is equal to (7.5 ± 0.3) × 10−6 atomic fractions. The energy of the formation of magnesium Frenkel defects can be estimated as 4.2 ± 0.2 eV.

Neptunium(III) application in extraction chromatography

This paper describes a novel strategy for actinide separation by extraction chromatography with Np(III) valence adjustment. Neptunium(IV) was reduced to Np(III) using Cr(II) and then selectively separated from uranium (IV) on a TEVA resin. After elution, Np(III) was retained on a DGA resin in order to remove any detrimental chromium impurities. Neptunium(III) formation was demonstrated by the complete and selective elution of Np from TEVA resin (99±7%) in less than 12mL of 9M HCl from U(IV) (0.7±0.7%). It was determined by UV–visible and kinetic studies that Cr(II) was the only species responsible for the elution of Np(IV) as Np(III) and that the Cr(II) solution could be prepared from 2 to 30min before its use without the need of complex degassing systems to prevent the oxidation of Np(III) by oxygen. The methodology proposed here with TEVA/DGA resins provides removal of Cr(III) impurities produced at high decontamination factors (2.8×103 and 7.3×104 respectively).

Effect of extended phosphorus diffusion gettering on chromium impurity in HEM multicrystalline silicon

We have investigated the extended phosphorus diffusion gettering (PDG) effect on chromium impurities (Cr) in p-type multicrystalline silicon (mc-Si) grown by Heat Exchanger Method (HEM). The study was made after phosphorous diffusion and according to different extended annealing temperatures. The secondary ion mass spectrometry (SIMS) analysis revealed a significant accumulation of 52Cr in heavily phosphorus doped (HPD) region. Using quasi-steady state photoconductance (QSSPC) technique, the apparent lifetime dependent minority carrier density curves have been obtained. The results showed an increment of the bulk minority carrier lifetime for specific annealing temperatures. Appropriate calculations based on QSSPC results allowed us to determine the lifetime curves associated to gettered impurities. Their fitting by Shockley-Read-Hall (SRH) model reveal that the origin of the lifetime increment is the reduction of interstitial chromium (Cri) density in the bulk. Furthermore, the estimation of electron to hole capture cross-section ratio (k=σn/σp) through the modelling of apparent lifetime curves using Hornbeck–Haynes model, confirmed the effectiveness of Cri gettering and identified the nature of dominant recombination centres after gettering process.