Chromium Impurities Research Articles

Influence of mono- and triple (Cr–La–Ba) doping on the mechanical, optical and gas diffusion properties of corundum

The effect of triple doping with chromium, lanthanum and barium on the mechanical, degassing and gas diffusion properties of fine-crystalline corundum synthesized in a supercritical water fluid was studied. The influence of chromium monodoping on the mechanical and optical properties of fine-crystalline corundum was also investigated. It was found that even small amounts of dopants of lanthanum and chromium during triple and monodoping of corundum significantly improve its mechanical properties (increase the abrasive ability) and reduce it several times the content of volatile gas-liquid impurities, which contributes to increase transparency of the resulting ceramics. The role of the interaction of a chromium impurity with oxygen vacancies in corundum on the increase in the static strength of crystals was suggested. The diffusion coefficients of water in doped corundum were determined. Comparison of the degassing and diffusion properties of corundum doped with La, Cr, Ba with undoped corundum and corundum doped with lanthanum was carried out. The results obtained contribute to the creation of new materials based on doped corundum with high abrasive, luminescent, and gas diffusion characteristics.

INVESTIGATION OF TRAPS IN beta-Ga2O3:0.05%Cr AND beta-Ga1,9In0,1O3:0,05%Cr CRYSTALS BY FRACTIONAL THERMAL-GLOW TECHNIQUE

The research results of the luminescence spectra and thermally stimulated luminescence (TSL) in β-Ga 2 O 3 and β-Ga 1,9 In 0,1 O 3 single crystals doped with chromium ions, grown by the optical melting zone method, have been presented. The Cr 3+ impurity concentration was 0.05%. Luminescence was excited by light with a wavelength of 440 nm from the region of the broad absorption band 4 A 2 → 4 T 1 of Cr 3+ ions. Two sharp R-lines ( 2 E → 4 A 2 transitions) are dominated in the luminescence spectrum of investigated samples at the temperature of liquid nitrogen. A number of weak narrow lines associated with the phonon-induced sidebands of the R -lines are observed in the wavelength range of 700-740 nm. At room temperature, a broad luminescence band extending from 650 to 900 nm is dominated and corresponds to electron-vibrational 4 T 2 → 4 A 2 transitions in chromium ions. The R -lines are also observed on the background of this broad luminescence band in the luminescence spectrum of chromium impurity in gallium oxide at room temperature. The luminescence of chromium impurity in solid solutions is concentrated only in a broad luminescence band with a maximum near 775 nm ( 4 T 2 → 4 A 2 transitions). Doping of β-Ga 2 O 3 with a chromium impurity leads to the appearance of a thermoluminescence maximum at 285 K, which corresponds to traps with an energy depth of 0.59 eV. In the crystals of the solid solution, there is a shift of the TSL maximum to the range of low temperatures up to 225 K and an increase in its half-width. The fractional thermal-glow technique was used to analyze the TSL complex maximum at 225 K. It was established that this maximum consists of at least 4 elementary maxima, which are caused by the release of electrons from traps with activation energies in the ranges of 0.4-0.42 and 0.48-0.495 eV. The method of the initial increase in the intensity of the TSL glow was used to calculate the activation energy. Keywords : β-Ga 2 O 3 , solid solutions, thermally stimulated luminescence, activation energy.

Paramagnetic centers in chromium-doped Na-=SUB=-5-=/SUB=-AlF-=SUB=-2-=/SUB=-(PO-=SUB=-4-=/SUB=-)-=SUB=-2-=/SUB=- single crystal

The paramagnetic resonance of Na5AlF2(PO4)2 single crystals with chromium impurity was studied. The Cr3+ centers have been found to replace the triclinic Al3+ positions. In the local coordinate system, the spectra of these centers were described by the spin Hamiltonian of rhombic symmetry. In addition, vanadium centers and unidentified triclinic symmetry centers have been observed. Keywords: impurity ions, aluminum fluoride phosphate, paramagnetic resonance.

A novel process to prepare high-purity vanadyl sulfate electrolyte from leach liquor of sodium-roasted vanadium slag

The leach liquor of sodium-roasted vanadium slag in vanadium industry is an alkaline solution containing vanadium, sodium, chromium, and other impurities, and preparing high-purity vanadyl sulfate electrolyte (used in vanadium redox flow battery) directly from this solution is challenging but cost-effective. A novel process consisting of selective reduction–precipitation of chromium and solvent extraction of vanadium was proposed and optimized in this study. Approximately 90% of Cr(VI) in the solution was reduced to Cr(III) and precipitated after adding sodium sulfite at a pH of 5.77 and 25 °C, while V(V) remained in this process and needed a further reduction to V(IV) prior to extraction. More than 99.9% of V(IV) remained in the solution was extracted by two-stage extraction using the “P507 + TBP + kerosene” extractant system at a pH of 2.00 and an organic-to-aqueous phase ratio of 1.5:1. Sodium entrained in the organic phase was washed off through three-stage scrubbing. A high-purity vanadyl sulfate solution of 2.1 mol L−1 was prepared by two-stage stripping using 5.5 mol L−1 sulfuric acid. According to the slope analysis and infrared spectra, the extraction of V(IV) by P507 follow the cation exchange mechanism. This study provides a feasible way to remove chromium and recover vanadium in the form of vanadyl sulfate from the industrial leach liquor of sodium-roasted vanadium slag.

High efficiency CIGS solar cells on flexible stainless steel substrate with SiO2 diffusion barrier layer

The copper indium gallium selenium (CIGS) thin film is recognized as the most potential material for photovoltaics applications, and the thin film solar cell with flexible substrate makes the application scenarios more extensive and the product advantages more obvious. In this contribution, we prepared high efficiency CIGS thin film solar cells on flexible stainless steel substrate by three-stage coevaporation method. In this study, silicon dioxide film was proposed as the diffusion barrier layer of iron, chromium and other impurity elements in stainless steel flexible substrate to the CIGS absorber, which were found to reduce the performance of the device. The thickness of the SiO2 barrier layers varying from 0 to 1000 nm was studied. The structure, crystal morphology and electrical properties of each sample were studied. The results showed that the conversion efficiency increased with the increase of thickness of sputtered silicon dioxide barrier until the thickness is greater than 200 nm.

Analytical survey of tattoo inks-A chemical and legal perspective with focus on sensitizing substances.

Tattoo inks have been reported to elicit allergic contact dermatitis. To investigate the labels and the contents of metals and pigments in tattoo inks, considering restrictions within the European Union. Seventy-three tattoo inks currently available on the market, either bought or donated (already used), were investigated for trace metals and pigments by inductively coupled plasma mass spectrometry and by matrix-assisted laser desorption/ionization time of flight tandem mass spectrometry. Ninety-three percent of the bought tattoo inks violated European, legal requirements on labeling. Fifty percent of the tattoo inks declared at least one pigment ingredient incorrectly. Sixty-one percent of the inks contained pigments of concern, especially red inks. Iron, aluminium, titanium, and copper (most in green/blue inks) were the main metals detected in the inks. The level of metal impurities exceeded current restriction limits in only a few cases. Total chromium (0.35-139 μg/g) and nickel (0.1-41 μg/g) were found in almost all samples. The levels of iron, chromium, manganese, cobalt, nickel, zinc, lead, and arsenic were found to covary significantly. To prevent contact allergy and toxic reactions among users it is important for tattoo ink manufacturers to follow the regulations and decrease nickel and chromium impurities.

Diffusion of chromium in zinc chalcogenides during hot isostatic pressing

The diffusion of chromium impurities in zinc chalcogenides (ZnS, ZnSe) during hot isostatic pressing (HIP) in the temperature range T = 1000–1330 °C and pressures P = 100–190 MPa has been investigated. The chromium concentration and its distribution over the sample volume with a step from 5 to 50 μm was determined by FTIR spectroscopy in combination with an IR microscope. The diffusion coefficients of Cr in ZnS and ZnSe at various temperatures and pressures were determined. The diffusion coefficients of Cr at 1300 °C were (7.0 ± 0.3) × 10−8 cm2/s in ZnSe and (3.4 ± 0.4) × 10−8 cm2/s in ZnS. It is shown that the temperature dependences are described by the Arrhenius equation, and the isostatic pressure does not affect the value of the diffusion coefficients.

Correlation between electrical conductivity and luminescence properties in β-Ga2O3:Cr3+ and β-Ga2O3:Cr,Mg single crystals

The photoluminescence, excitation, and absorption spectra as well as the electrical conductivity of β-Ga2O3:Cr and β-Ga2O3:Cr,Mg single crystals were studied. The as-grown β-Ga2O3:Cr crystals had a green color, the conductivity at about 10−2–10−3 Ω−1 cm−1, and a low yield of Cr3+ impurity luminescence. Annealing in oxygen atmosphere led to a strong increase in Cr3+ red luminescence yield, increase in the resistivity, and changes in the absorption and excitation spectra. Similarly, increases in the Cr3+ luminescence yield and resistivity were observed after codoping of β-Ga2O3:Cr crystals with magnesium (Mg2+). The registered changes in the Cr3+ luminescence yield, electrical conductivity, and in the absorption and excitation spectra are considered to be due to the shift in the Fermi level. In the as-grown β-Ga2O3:Cr crystals, the Fermi level is located near the bottom of the conduction band, and most chromium ions are in the Cr2+ charge state. Annealing in an oxygen atmosphere as well as codoping of the crystals with chromium and magnesium impurities moves the Fermi level toward the middle of the bandgap and recharges the chromium ions to the Cr3+ state.

Formation of nanosized relief on the CdTe single crystals surface by bromine-emerging etchants

The chemical-dynamic and chemical–mechanical polishing of the CdTe single crystals surfaces by bromine-emerging etching compositions based on aqueous solutions of K2Cr2O7–HBr–solvent has been investigated. The dependences “solution concentration—polishing rate” have been established and the concentration limits of polishing etching compositions have been determined. The effect of the organic component on the rate and character of their dissolution, the quality of the treatment of the single crystals surfaces has been determined. The dependences of the chemical–mechanical polishing rate on the dilution of the base polishing solution for ethylene glycol and glycerol were found. The surface quality of single crystals after etching in the developed solutions has been investigated using methods of metallographic and profilometric analyzes atomic force and scanning electron microscopy. Using X-ray microanalysis, it was found that after treatment of the semiconductor surfaces by the polishing solutions [Cd]/[Te] ratio was stoichiometric and chromium impurities were absent. The polishing etchants composition and conditions for forming of nanosized relief (Ra < 1 nm) on the CdTe single crystals surface have been optimized.

Парамагнитные центры в легированном хромом монокристалле Na-=SUB=-5-=/SUB=-AlF-=SUB=-2-=/SUB=-(PO-=SUB=-4-=/SUB=-)-=SUB=-2-=/SUB=-

The paramagnetic resonance of Na5AlF2(PO4)2 single crystals with chromium impurity was studied. The Cr3+ centers have been found to replace the triclinic Al3+ positions. In the local coordinate system, the spectra of these centers were described by the spin Hamiltonian of rhombic symmetry. In addition, vanadium centers and unidentified triclinic symmetry centers have been observed.

Comparison of chromium poisoning between lanthanum strontium manganite and lanthanum strontium ferrite composite cathodes in solid oxide fuel cells

Degradation mechanisms of chromium poisoning have been extensively investigated for last two decades on various cathode materials for solid oxide fuel cells. While most researchers used half-cell experiments to investigate the effects of chromium impurities, this paper compares the degradation phenomena of LSM-YSZ and LSF-GDC cathodes in full cells under different operating conditions, namely polarized current density and humidity over the cathode.Under cathodic load, performance degradation of LSF-GDC cathode is significantly less than that of LSM-YSZ cathode. Most interestingly, under humidified air condition, LSF-based cell hardly experiences performance degradation while LSM-based cell shows a catastrophic degradation. Cr deposition in LSM-based cells occurs mainly near the electrolyte under both dry and humidified air conditions. In contrast, Cr deposition in LSF-based cell concentrates at the cathode surface under dry air condition, but concentrates both at cathode surface and near electrolyte under humidified air condition. Microstructure analysis reveals Mn-associated Cr deposits in LSM-based cathode, and Sr- and Fe-associated Cr deposits in LSF-based cathode.With the help of distribution of relaxation time analysis, the different electrochemical behaviors of LSM- and LSF-based cells are carefully interpreted. The effects of electrochemical deposition, Sr surface segregation, and humidity on LSM- and LSF-based cathodes are discussed.

Fundamental problems and prospects for the use of titanium raw materials in Russia

The article provides a detailed analysis of research results on the use of various types of titanium raw materials from Russian deposits: primary (titanomagnetite, ilmenite-titanomagnetite), metamorphosed (buried leucoxene and ilmenite-leucoxene sandstones) and complex zircon-rutile-ilmenite placers. All titanium deposits in Russia are characterized by low ore quality, which does not meet technological requirements for the production of pigment TiO 2 and metallic titanium. Main deposits are based on titanomagnetites containing from 3 to 17 % of TiO 2 . Ilmenite is in a subordinate position. Leucoxene sandstones of the Yarega deposit have high titanium content (about 10 %o TiO 2 ). But when they are dressed by known methods, low-quality high-silicon leucoxene concentrates are obtained containing 40 - 50 %o TiO 2 with significant titanium losses. Ilmenite-leucoxene sandstones of the Pizhemskoe deposit contain 3 - 10 %o of TiO 2 . The main titanium-containing phases are pseudorutyl Fe 2 O 3 3TiO 2 and leucoxene. The cementing binder of grains in sandstones is siderite with magnetic properties, which greatly reduces the degree of minerals disclosure during crushing and worsens the conditions for ore dressing in general. Ilmenite-bearing placers consist of fine-grained disseminated ilmenite and are difficult to concentrate. Complex zircon-rutil-ilmenite placers are infected with chromium and other undesirable impurities, which do not allow existing dressing methods to obtain a conditioned ilmenite concentrate. The main problems of using hard-to-concentrate titanium ores of all the above types of deposits and scientifically based ways to solve them are discussed. It is aimed at obtaining high-quality raw materials for the production of titanium metal and pigment TiO 2 with the simultaneous extraction of other valuable components. Particular attention is devoted to studies on the use of mass complex raw materials - titanomagne-tites, which are allocated in three phased directions, lasting for about 200 years - from the beginning of the XIX century to the present.

Features of Formation of Cr3+ Paramagnetic Centers in Strontium Titanate (SrTiO3) Implanted with Chromium Ions

We present results of structural (XPS) and magnetic resonance (EPR) studies of single-crystal plates of strontium titanate (SrTiO3) heavily doped with chromium utilizing ion implantation technique. It was shown that the temperature of the oxide matrix during ion implantation (300 or 900 K) significantly affects the valence state of the implanted chromium impurity (Cr0 or Cr3+) and the depth profiles of its distribution in the SrTiO3 matrix. Using the EPR method, it was established that as a result of implantation of chromium impurity at an elevated substrate temperature during irradiation, the dominating color centers in the surface implanted oxide layer appear in the form of trivalent chromium ions in a cubic environment with a g-factor g = 1.976 and the value of the hyperfine coupling constant with the magnetic nucleus of 53Cr isotope, A = 16.2 × 10–4 cm–1. From a comparison of these data with parameters for the cubic Cr3+ impurity center in the SrTiO3 single crystal, it was concluded that this center is localized in the structural positions of titanium. Upon implantation of a chromium impurity in SrTiO3 at room temperature of the substrate, in addition to cubic Cr3+ centers, new signals were detected in the EPR spectra, apparently associated with the formation of oxygen vacancies in the implanted oxide. However, the number of these additional centers does not change significantly even after high-temperature heat treatment of samples in air at temperatures up to 900 K. It is noted that signals of this type also appear in the spectra of SrTiO3 samples obtained as a result of hot implantation and then subjected to high-temperature thermal annealing.

Explanation of the Colour Change in Alexandrites

Alexandrites are remarkable and rare gemstones. They display an extraordinary colour change according to the ambient lighting, from emerald green in daylight to ruby red in incandescent light from tungsten lamps or candles. While this colour change has been correctly attributed to chromium impurities and their absorption band in the yellow region of the visible light spectrum, no adequate explanation of the mechanism has been given. Here, the alexandrite effect is fully explained by considering the von Kries model of the human colour constancy mechanism. This implies that our colour constancy mechanism is real (objective) and primarily attuned to correct for the colour temperature of black-body illuminants.

Effect of Uncontrollable Impurities on the Absorption Spectrum of a NaGd(WO4)2 Laser Crystal

Three NaGd(WO4)2 single crystals were grown by the Czochralski method using charges of different chemical purity. The impurity compositions of the grown crystals were analyzed by inductively coupled plasma mass spectrometry. The optical absorption spectra of the crystals were measured, and the effect of uncontrollable impurities on their optical properties and color was analyzed. The green color encountered in NaGd(WO4)2 crystals in some cases was shown to be due to the uncontrollable impurities of d elements, among which the chromium impurity had the most significant effect.

Modeling Iron Pentacarbonyl Ultrapurification in a Vertical Distillation Apparatus

This paper presents a mathematical model for the ultrapurification of substances via distillation in a closed vaporization–condensation system, where vapor condenses on a flowing down liquid film. We jointly analyze the mechanisms behind vaporization, vapor transport, condensation, condensate motion, and impurity diffusion in vaporizing liquid. Examining the removal of cobalt impurities from iron pentacarbonyl as an example, we assess the degree of purification as a function of vaporization and condensation temperatures, vaporization area, the fraction of liquid vaporized, and the radius and height of the condensation tube. Using experimentally determined temperature-dependent effective separation coefficients and the mathematical model, we find diffusion coefficients and equilibrium separation coefficients of cobalt, tungsten, and chromium impurities in iron pentacarbonyl.

Peculiarities of the Dielectric Spectra of Triglycine Sulfate Crystals with a Profile Distribution of Chromium

The temperature evolution of the dielectric spectra of TGS–TGS + Cr crystals (TGS stands for triglycine sulfate) with a periodic distribution of chromium impurity (Cr3+) has been studied in comparison with nominally pure TGS crystals in the frequency range from 10 to 107 Hz upon heating and cooling. It is shown that impurity crystals are characterized by the absence of dielectric dispersion in both measurement modes. An approach is proposed to explain the peculiarities of the dielectric dispersion of crystals with a periodic distribution of chromium impurity from the standpoint of mobile activity of certain elements of the TGS crystal structure.

Laser alloying of nanocrystalline coatings

A new method for obtaining high-entropy alloys in the near-surface layer of a metal substrate is proposed. As a metal base, steel containing impurities of chromium, nickel and titanium is used. An ion-plasma method is applied to this substrate with an AlFe043 alloy 15 microns thick. Then the resulting coating was irradiated with a laser. As a source of laser radiation, a neodymium-doped aluminum-garnet laser was used. The duration of the flash of the laser pumping lamps operating in the free-running mode was 2×10-3 s. The energy of the laser pulse was 1 J, the repetition rate of the laser pulses was controlled from 0.1 to 35 Hz. Elemental analysis showed the formation of high-entropic coating with high performance characteristics.

The electronic, half-metallic, and magnetic properties of Ca1-xCrxS ternary alloys: Insights from the first-principle calculations

The electronic, structural, and magnetic characteristics of Cr atom substituting Ca atom in rocksalt CaS have been investigated within the formalism of (GGA + PBE) and PBE with Hubbard correction (GGA + U). Our findings point out that the ternary alloys are dynamically stable depending on the obtained results of elastic constants. For structural properties, it is clear that the lattice constants decrease and bulk modulus increases with increasing concentration of chromium impurity. Interestingly, the perceived total magnetic moments increase with the Cr concentration and reaches the maximum for Ca0.25Cr0.75S, which is mainly composed of Cr atoms. Besides, it is found from PBE and PBE + U calculations that the Cr-substituted CaS gives half-metallic ferromagnetism (HMF). Finally, the deduced results of minority-spin bands demonstrate a half-metallic ferromagnetic gap and half-metallic (HM) gap. The predicted results confirmed that Ca1-xCrxS could be considered as a promising candidate material for spintronics applications.

Electron Transport in Carbon Nanotubes with Adsorbed Chromium Impurities.

We employ Green’s function method for describing multiband models with magnetic impurities and apply the formalism to the problem of chromium impurities adsorbed onto a carbon nanotube. Density functional theory is used to determine the bandstructure, which is then fit to a tight-binding model to allow for the subsequent Green’s function description. Electron–electron interactions, electron–phonon coupling, and disorder scattering are all taken into account (perturbatively) with a theory that involves a cluster extension of the coherent potential approximation. We show how increasing the cluster size produces more accurate results and how the final calculations converge as a function of the cluster size. We examine the spin-polarized electrical current on the nanotube generated by the magnetic impurities adsorbed onto the nanotube surface. The spin polarization increases with both increasing concentration of chromium impurities and with increasing magnetic field. Its origin arises from the strong electron correlations generated by the Cr impurities.