SEARCH · Search NASA
Results for “CHROMIUM IONS”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Spectroscopic and mechanical studies of lithium aluminoborate glasses doped with chromium ions
Aluminoborate glasses, doped with chromium ions, were prepared using melt quenching technique. Spectroscopic and mechanical tools were utilized to study the impact of B{sub 2}O{sub 3} substitution with Al{sub 2}O{sub 3} on the structure of the glass system. Infrared spectra uncovered that substitution of B{sub 2}O{sub 3} with Al{sub 2}O{sub 3} causes concealment of non-bridging oxygen NBOs and creation of inflexible AlO{sub 4} tetrahedral. The UV–Vis absorption spectra demonstrated a blue-shift of the optical absorption edge with increasing Al{sub 2}O{sub 3} content. The optical band gap was found to increase, while the refractive index diminished with addition of Al{sub 2}O{sub 3} content. The calculated crystalline field Dq, Racah parameters (B and C) and Dq/B indicated the diminishment in inter-electron repulsion with increasing the covalency. In particular, a value of Dq/B larger than 2.3 assures that chromium ions are in strong ligand field sites. The elastic moduli increase with increasing Al{sub 2}O{sub 3} content, due to the reduction NBOs and, consequently, the increased average bond connectivity.
Magnetic properties of manganites doped with gallium, iron, and chromium ions
The magnetization and the crystal structure of the La{sub 0.7}Sr{sub 0.3}Mn{sub 1−x}M{sub x}O{sub 3} (M = Ga, Fe, Cr; x ≤ 0.3) systems are studied. The substitution of gallium and chromium is shown to cause phase separation into antiferromagnetic and ferromagnetic phases, whereas the substitution of iron for manganese stabilizes a spinglass state. The ferromagnetic phase in the chromium-substituted compositions is much more stable than that in the case of substitution by iron ions or diamagnetic gallium ions. The magnetic properties are explained in terms of the model of superexchange interactions and the localization of most e{sub g} electrons of manganese. The stabilization of ferromagnetism in the chromium-substituted compositions can be caused by the fact that the positive and negative contributions to the superexchange interaction between Mn{sup 3+} and Cr{sup 3+} ions are close to each other but the antiferromagnetic part of the exchange is predominant. Moreover, some chromium ions are in the tetravalent state, which maintains the optimum doping conditions.
Coherent control and high-fidelity readout of chromium ions in commercial silicon carbide
Abstract Transition metal ions provide a rich set of optically active defect spins in wide bandgap semiconductors. Chromium (Cr 4+ ) in silicon-carbide (SiC) produces a spin-1 ground state with a narrow, spectrally isolated, spin-selective, near-telecom optical interface. However, previous studies were hindered by material quality resulting in limited coherent control. In this work, we implant Cr into commercial 4H-SiC and show optimal defect activation after annealing above 1600 °C. We measure an ensemble optical hole linewidth of 31 MHz, an order of magnitude improvement compared to as-grown samples. An in-depth exploration of optical and spin dynamics reveals efficient spin polarization, coherent control, and readout with high fidelity (79%). We report T 1 times greater than 1 s at cryogenic temperatures (15 K) with a T 2 * = 317 ns and a T 2 = 81 μs, where spin dephasing times are currently limited by spin–spin interactions within the defect ensemble. Our results demonstrate the potential of Cr 4+ in SiC as an extrinsic, optically active spin qubit.
Radiolytic Production of Radical Species in Gibbsite Doped with Iron and Chromium Ions
Gibbsite (aluminum hydroxide, Al(OH) 3 ) nanoparticles, synthesized from aluminum chloride or aluminum nitrate, were doped with metal ions, Cr(III) or Fe(III), and then irradiated with γ rays to determine the effect of the dopants on radiolytic hydrogen (H 2 ) production and radical generation. The addition of Cr(III) and Fe(III) ions at concentrations of 0.5% or 5% decreased the concentration of stable oxygen-centered radicals, with the strongest suppression in Cr(III) doped samples. A decrease in H 2 yields was observed with increasing Cr(III) or Fe(III) concentrations, with the greatest effect observed for the Fe(III)-doped samples. Reduction of the Cr(III) to Cr(II) and Fe(III) to Fe(II) was also observed, probably due to scavenging of radiolytically produced electrons. However, further processes differ for Fe(III)- and Cr(III)-doped systems. Both ions are reduced by the free electrons, leading to a decrease in H 2 production, but they react differently with the oxygen radicals. Cr(III) can be oxidized by oxygen radicals, whereas Fe(III) cannot. Fe(II) can interact with peroxides, possible products of intermediate oxygen oxidation, converting back to Fe(III) and leaving oxygen radicals behind. These oxidation reactions lead to a difference in the observed relative effects on H 2 yields and oxygen radical production between Cr(III)- and Fe(III)-doped gibbsite. In conclusion, the connection between electron scavenging and H 2 production indicates that radiolytically produced electrons are precursors to H 2 .
Selective detection of heavy metal ions by self assembled chemical field effect transistors
Multiple layer-by-layer sensor material modifications were designed and implemented to achieve selectivity of semiconductor based chemical field effect transistors (ChemFETs) to particular heavy metal ions. The ChemFET sensors were fabricated and modified in three ways, with the intent to initially target first mercury and lead ions and then chromium ions, respectively. Sensor characterization was performed with the gate regions of the sensor elements exposed to different concentrations of target heavy metal ion solutions. A minimum detection level in the range of 0.1 ppm and a 10%–90% response time of less than 10 s were demonstrated. By combining layer-by-layer gold nanoparticles and lead ionophores, a sensor is produced that is sensitive and selective not only to chromium but also to Cr{sup 3+} and Cr{sup 6+}. This result supports the claim that high selectivity can be achieved by designing self-assembled bonding for lead, arsenic, chromium, cesium, mercury, and cadmium.
Electron Paramagnetic Resonance of Cr{sup 3+} Ions in Single Crystals of Yttrium Aluminum Borate YAl{sub 3}(BO{sub 3}){sub 4}
Single crystal of yttrium aluminum borate YAl{sub 3}(BO{sub 3}){sub 4} doped with chromium ions (1 at %) was studied using electron paramagnetic resonance spectroscopy. It is shown that chromium ions introduced into the sample occupy yttrium ion sites in the crystal structure. The parameters of the spin Hamiltonian of Cr{sup 3+} ions in the YAl{sub 3}(BO{sub 3}){sub 4} matrix are determined at different temperatures. The sign of the fine structure parameter D allows the conclusion that the chromium ions in YA1{sub 3}(BO{sub 3}){sub 4} single crystals have an easy-plane anisotropy.
Highly Selective Detection of Cr{sup 3+} Ion with Colorimetric & Fluorescent Response Via Chemodosimetric Approach in Aqueous Medium
So far, very few numbers of chemosensors for Cr{sup 3+} ion have been reported. However, the main drawback of reported receptors are the lack of selectivity and other trivalent cations such as Fe{sup 3+}, Al{sup 3+} and anions like F{sup −} and {sup −}OAc frequently interfere with such assays. This paper present the synthesis, characterization & sensor studies of Schiff base containing naphthalene moiety which selectively detect Cr{sup 3+} ion by chemodosimetric approach. Using FT-IR, {sup 1}H NMR, {sup 13}C NMR and ESI mass spectroscopic techniques the probe was characterized. This receptor exhibit more selectivity and sensitivity towards Cr{sup 3+} than other divalent and trivalent cations like Mn{sup 2+}, Zn{sup 2+}, Co{sup 2+}, Ni{sup 2+}, Cd{sup 2+}, Cu{sup 2+}, Hg{sup 2+}, Fe{sup 3+}, and Al{sup 3+} ions. After the addition of chromium ion the receptor get change from yellow to colorless in aqueous medium. But no color change was observed on the addition of other metal ions. Using UV-Vis and PL studies, it was confirmed that the selective hydrolysis of imine group of receptor by Cr{sup 3+} ions takes place with high fluorescence enhancement that is corresponding to 1-naphthylamine. Receptor acts as selective chemodosimeter for Cr{sup 3+} ions with 2:1 stoichiometry and micro molar detection limit. This chemodosimetric approach was applied successfully for bio-imaging of HeLa cells.
High temperature gamma radiation-induced chromium redox chemistry via in situ spectroscopic measurements
Chromium ions can make their way into the primary coolant of nuclear power reactors from the corrosion of stainless-steel reactor components, decreasing the material's corrosion resistance and resulting in increased transport of further corrosion products. Despite these potential effects, the radiation-induced redox speciation of chromium ions in aqueous solution is not well understood, especially at the elevated temperatures experienced by reactor coolants. Here, in the present work, we report new experimental results demonstrating that in aerated aqueous solution, the radiolytic oxidation of Cr(III) to Cr(VI) occurs at pH 4, while the reduction of Cr(VI) to Cr(III) occurs at pH 2. The oxidation of Cr(III) is primarily attributed to the reaction of the hydroxyl radical (˙OH) with the Cr(OH) 2+ species, while the reduction of Cr(VI) is attributed to reactions involving the hydrated electron (e aq - ) and hydrogen atom (H˙). Additionally, the steady-state equilibrium yield of Cr(VI) from the gamma irradiation of pH 4 Cr(III) solutions decreased with increasing temperature (over a range of 37–195 °C). This observation indicates that the activation energy of the Cr(VI) reduction reactions is higher than that for the Cr(III) oxidation reactions, such that it becomes relatively more favorable at higher temperatures. Overall, these data are important for the development of complementary multiscale models for the prediction of metal ion speciation in high temperature radiation environments.
Spectroscopic studies of Cr{sup 3+} ions doped in poly(vinylalcohol) complexed polyethylene glycol polymer films
Polymer films of Poly(vinylalcohol) (PVA) complexed with Polyethylene glycol (PEG) with different dopant concentrations of Cr{sup 3+} ions are prepared by solution cast technique. Electron paramagnetic resonance (EPR), Optical absorption and FT-IR studies have been carried out on the polymer films. The EPR spectra of the entire samples exhibit resonance signal at g ≈1.97 which is attributed to the isolated Cr{sup 3+} pairs. The temperature variation EPR studies show that the population of spin-levels participating in the resonance decreases with an increase in temperature, which is in accordance with the Boltzmann Law. The paramagnetic susceptibilities (X) have been calculated from the EPR data at different temperatures. The linewidth of the g ≈1.97 resonance signal has been found to be decreasing with an increase in temperature, which confirms the pairing mechanism between Cr{sup 3+} ions. The Optical absorption spectrum of chromium ions in (PVA+PEG) polymer films exhibits three bands, corresponding to the d-d transitions {sup 4}A{sub 2g}(F)→{sup 4}T{sub 1g}(F), {sup 4}A{sub 2g}(F)→{sup 4}T{sub 2g}(F) and {sup 4}A{sub 2g}(F)→{sup 2}T{sub 1g}(G), in the order of decreasing energy. The crystal field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. From the ultraviolet absorption edges, Optical band gap (E{sub opt}) and Urbach (ΔE) energies are evaluated. FT-IR spectrum exhibits few bands which are attributed to O-H, CH, C=C and C=O groups of stretching and bending vibrations.
Metal-organic-framework and walnut shell biochar composites for lead and hexavalent chromium removal from aqueous environments
Extensive research in recent years has explored the realm of porous carbon composites for various applications, including electrochemistry, structural materials, environmental remediation, and more. In particular, the fabrication of porous carbon composites using a metal-organic framework (MOF) and biochar (BC) for aqueous remediation is a fairly new avenue of research. In this study, a MOF-BC composite was synthesized with unmodified and chemically modified BCs using solvothermal synthesis. The composites were used as adsorbents to remediate heavy metals, such as lead (II) and chromium (VI), from aqueous environments. Here, it was verified that the MOF was homogeneously deposited onto the BC's surface using various material characterization techniques. Lead and chromium adsorption studies revealed a high adsorption capacity with greater than 99% removal for lead and ∼65% for chromium, respectively. Impressively, for lead, the highest observed experimental adsorption capacity of the MOF-chemically modified BC composite was 535 mg/g, compared to 240 mg/g for pristine BC. Meanwhile, the adsorption capacity of the same MOF-BC composite for chromium ions was low at 18 mg/g, compared to 80 mg/g for the chemically modified BC. The MOF-BC had a rapid adsorption rate, achieving equilibrium at only 150 min of reaction time for lead ions. MOF-BCs have higher adsorption for cationic lead through physisorption and ion-exchange mechanisms, whereas, for anionic chromium, removal is dominated only by physisorption mechanisms. The outcomes and methodological developments attained in this study offer a novel and compelling approach for synthesizing MOF-BC composites for aqueous remediation applications.
Magnetic order in the structurally disordered helicoidal magnet Cr{sub 1/3}NbS{sub 2}: NMR at {sup 53}Cr nuclei
The Cr{sub 1/3}NbS{sub 2} magnet is studied by nuclear magnetic resonance (NMR) at {sup 53}Cr nuclei in a zero applied magnetic field. The following two frequency ranges are distinguished in the {sup 53}Cr NMR spectrum at T = 4.2 K: ν{sub 1} = 64–68 MHz and ν{sub 2} = 49–51 MHz. They can be related to two valence states of chromium ions, namely, Cr{sup 4+} and Cr{sup 3+}. The components of the electric field gradient, the hyperfine fields, and the magnetic moment at chromium atoms are determined. The NMR data demonstrate that the magnetic moments of chromium lie in plane ab and form a magnetic structure consisting of regions with a helicoidal magnetic order and regions where this order is broken.
Recovery of Chromium(III) from Tannery Wastewater by Nanoparticles and Whiskers of Chitosan
Chitosan, its nanoparticles and whiskers present an excellent capacity to complex chromium ions. However, this phenomenon is influenced by different parameters. In our search, we determined the appropriate range of pH to form chitosan–Cr(III), nanoparticles Cr(III) and whiskers–Cr(III) complex. We studied also the influence of chromium concentration and nature of chitosan-based materials on complexation process. Our main aim is approximate the optimal conditions to remove chromium(III) from tanning bath, recuperated from tannery wastewater of Marrakech in Morocco. However, the results of adsorption kinetic in tannery wastewater revealed that chitosan, its nanoparticles, whiskers and biocomposites are good sorbent of chromium as well, even if the adsorbed quantity is less compared to chromium solution. Although, according to ICP-OES analysis in this real effluent, nanoparticles are the best complexing ligand, after 24 h of contact nanoparticles can remove 70% of chromium from this tannery wastewater.
Paramagnetic Chromium Centers in Y{sub 2}SiO{sub 5} and Sc{sub 2}SiO{sub 5} Crystals
The EPR spectrum of yttrium and scandium silicate single crystals doped with chromium has been studied at various frequencies. Y{sub 2}SiO{sub 5} has one triclinic Cr{sup 3+} center, while the Sc{sub 2}SiO{sub 5} crystals demonstrate two nonequivalent centers. The spin Hamiltonian parameters of these centers have been determined. These centers are found to be tricharged chromium ions localized in the yttrium and scandium positions.
Enhanced removal of aqueous hexavalent chromium (Cr(VI)) as a nuclear industry water contaminant by a polymer-modified activated carbon - 18354
Cr (VI) is produced through industrial processes including nuclear activities. It is one of the highly toxic pollutants defined by US EPA, classified as a know human carcinogen. Some of the health effects caused by Cr (VI) exposure are: severe skin and eye damage, nose, throat and lung irritations, damage to the mucous membranes results in ulcers. At Hanford and Savannah River nuclear sites in the United States, the chromium concentration vary from 20 - 30,000 μg/L in water. The maximum level of Cr(VI) for drinking water has been regulated by California Environmental Protection Agency (Cal/EPA) at 0.01 mg/L. Activated carbon (AC) is a widely-used adsorbent for water treatment. However, AC has a limited selectivity for chromium species. Thiol compounds (R-S-H) have also been reported to reduce Cr(VI) to Cr(III) which is harmless. To enhance the removal efficiency of chromium, polysulfide rubber polymer containing thiol was synthesized and coated on Activated Carbon (AC-PSR). Polymer coating on activated carbon enhances its sorption capacity by increasing the chromium binding sites. Conclusions: Polysulfide rubber polymer successfully coated on granular activated carbon. AC-PSR provided greater Cr(VI) removal capacity, due to the reduction of Cr(VI) to Cr(III) by thiol compounds in PSR, as well as physical adsorption onto AC. Future Work: Investigating intra-particle diffusion mechanism of of Cr(VI) in AC-PSR. Evaluating reusability of AC-PSR. Investigating the competition of mercury and chromium ions to be adsorbed onto AC-PSR.
Formation of a regular domain structure in TGS–TGS + Cr crystals with a profile impurity distribution
A complex investigation of TGS–TGS + Cr crystals with a profile impurity distribution of chromium ions Cr{sup 3+} has been carried out at the macrolevel (measurement of dielectric properties by the method of nematic liquid crystals) and microlevel (domain structure according to atomic force microscopy data). It is established that periodic doped layers are formed only in individual growth pyramids in the regions where the polarization vector has a nonzero component along the normal to the growth faces rather than throughout the entire crystal volume. The domain configuration at the boundary of growth layers with different impurity compositions has been studied by piezoelectric force microscopy. The static unipolarity of layers with and without chromium impurity is approximately identical, whereas the domain-wall density in doped regions is higher than that in undoped ones by a factor of about 7.
Characterizing Radiation-Induced Chromium Redox Chemistry
The redox behavior of Cr(III) and Cr(VI) in nuclear environments were studied by analyzing aqueous irradiated chromium ions using UV-Visible spectroscopy. The formation of different chromium oxidation states can occur through gamma radiolysis of chromium solutions. Characterization of radiation-induced chromium is not well understood, and yet important as chromium can make its way into the primary coolant of reactors because of the corrosion of stainless steel reactor components. Furthermore, Cr(VI) is toxic and not suitable for environmental release, therefore Cr speciation is important. Fricke dosimetry was performed to collect doses rates, specific extinction coefficients were obtained from various analytical methods for the two oxidation states of chromium. Chromium samples--conditioned at various pH's and concentration to understand the effects--were then irradiated using a Coblat-60 gamma irradiator. Three optical spectroscopy analytical methods were used , including direct oxidation state measurements and complexation by either EDTA or DPC. Complexation by EDTA was found to not be a reliable method. Radiolytic oxidation of Cr(III) to Cr(VI), and reduction of Cr(VI) to Cr(III) were clearly seen, except for Cr(III) samples were a pH less than or equal to 2, where no change occurred.