Chromium Doping Research Articles

Influence of mild Cr3+doping on the structural, optical, photochromic, and thermochromic reversibility of nano-titania systems

We report on the effect of chromium doping on the band gap lowering of nano-titania (TiO2) and subsequent implications as regards coloration characteristics mediated via photochromism and thermochromism processes. As can be found in the X-ray diffractograms, the sol-gel derived, Cr3+-doped nano-TiO2systems have exhibited an anatase phase with the evidence of peak shifting towards a lower diffraction angle. The average crystallite size decreases, whereas lattice unit cell parameters and, consequently, cell volume, tend to increase with the inclusion of Cr3+into the titania host. To be specific, 1% Cr-doped titania system showed nearly 5.8% cell expansion as compared to its un-doped counterpart. As revealed from the optical absorption spectroscopy, a narrowing of band gap is observed for the Cr doped nano-titania system: 3.18 eV for the un-doped system, and 2.61 and 2.41 eV for 0.3% and 1% Cr doping cases, respectively, considering direct band-to-band transitions. Moreover, doping led noticeable lowering of the exponent (n value), from its normal value, which suggests inclusion of adequate non-parabolicity feature to the energy band scheme. The photochromic feature, for a given incident radiation, demonstrates a lowered transmission response with increasing Cr content. A reversible thermochromism response has also been demonstrated for doped nano-titania systems subjected to heating with temperature varying between 0–55 °C. The Cr3+doped nano-titania and similar systems would find scope in smart windows, display components, photocatalysis, etc., when a select coloration is desired.

ARTIFICIAL NEURAL NETWORK IN THE MODELLING OF THE EFFECT OF CHROMIUM DOPANTS ON THE MECHANICAL PROPERTIES OF AL-4%CU ALLOY

Artificial Neural Network (ANN) was used to model the effect of Chromium dopants on the mechanical properties duralumin (Al-4 %Cu). The results showed that the hardness, yield strength, and ultimate tensile strength increased, while the energy absorbed and percentage elongation decreased, with increasing %wt of Chromium dopants. Simulation results of ANN show strong agreement with experimental values, having satisfactory R-values of Mean Square Error. ANN can suitably be used to predict the mechanical properties of Al-4%Cu doped with Chromium.

(Invited) Structure-Function Studies of the Effects of F and Cr Doping on the Performance of Lithium-Rich Li1+ XMO2 (M= Ni, Mn, Co) Positive Electrodes

This work will present the results of studies aimed at understanding the chemistry− and structure−function relation of the lithium-rich layered metal oxide material Li1.2Ni0.23Co0.12Mn0.5O2 doped with F and Cr [1]. The electrode material is found to consist of a mixture of R m and C2/m space-group symmetry components in a weight ration of approximately 4:1. Doping with ~ 2% Cr and 5% F was successful in the main layered phase with R m space-group, with Cr residing at the 3a transition-metal site and F residing at the 6c oxygen site, as supported by neutron and X-ray powder diffraction as well as electron microscopy. The electrochemical performance of the doped and undoped materials were compared in coin cells against lithium metal, allowing the comparison of capacity and rate performance, as well as lithium ion diffusion constants within the material as derived from cyclic voltammetry. These differences in measured electrochemical performance were reconciled with the results of structural studies of the electrode powders, as well as differences in the change of these structures that occurred during operation in full 18650 cells against Li4Ti5O12, as measured using in operando neutron powder diffraction. [1] W.K. Pang, H.F. Lin, V.K. Peterson, C.Z. Lu, C.E. Liu, S.C. Liao, J.M. Chen “Effects of fluorine and chromium doping on the performance of lithium-rich Li1+ xMO2 (M= Ni, Mn, Co) positive electrodes” Chem. Mater. 2017.

Microstructure Characterization of Ni-75 at.% Al Raney Type Alloy as a Result of Cooling Rate and Chromium Doping

The effect of cooling rate on the phase composition of gas atomized Raney type catalysts was studied using the Ni-75 at.% Al composition. The resulting particles were sieved into 3 standard size fractions and analysed using XRD with Rietveld refinement: as expected the three phases, Al3Ni2, Al3Ni, and Al-Al3Ni eutectic were identified. Differing phase compositions in the 3 size ranges were identified offering a possible explanation for varying catalytic activity with cooling rate, the higher cooling rates experienced by the smaller droplets allow less time for the peritectic conversion of Al3Ni2 to Al3Ni to proceed. This in turn results in a more Al-rich residual liquid, increasing the volume fraction of eutectic. This was further confirmed when analysing the microstructure using SEM backscatter imaging. Al3Ni2 was found to be encased in a shell of Al3Ni characteristic of peritectic reactions. The remainder of the alloy was found to consist of Al-Al3Ni eutectic. The SEM backscatter imaging also indicated that the larger particles displayed and a more globular structure than smaller particles. Similar Raney type Ni-75 at.% Al doped with 1.5 at.% Cr were synthesised using the same method and sieved into the same 3 standard size fractions. It was found that the Cr doped alloys exhibited a more dendritic character than the undoped samples in the corresponding size fraction, although the material still displayed an increasingly dendritic character with increasing cooling rate. The phase composition found by Rietveld refinement also followed a similar trend to the undoped samples with decreasing amounts of Al3Ni formed at the higher cooling rates. However, significant amounts of an additional phase, Al13Cr2, were also observed. Rietveld refinement found that a larger amount of Al13Cr2 was present than could be accounted for by the addition of 1.5 at.% Cr . This can be explained by the substitution of Ni onto the Cr lattice, as confirmed by Rietveld refinement. Al13Cr2 was found to be located mostly at the boundary of the Al3Ni and Al-Al3Ni eutectic phases during elemental mapping and quantitative image analysis of backscattered electron micrographs. This indicates that precipitation of Al13Cr2 is towards the end of the solidification process. The relatively large amounts of the Al-rich Al13Cr2 may explain the enhanced catalytic activity observed following leaching of Cr-doped Raney catalysts.

Electrochromic performance of chromium-doped Co3O4 nanocrystalline thin films prepared by nebulizer spray technique

Pristine and chromium (Cr)-doped tricobalt tetraoxide (Co3O4) thin films were grown on glass and FTO substrates by nebulized spray pyrolysis technique. The structural, optical and electrical properties of the pristine and Cr-doped Co3O4 films were characterized by X-ray diffraction, scanning electron microscopy, Raman, and four-probe method. The chemical environment of the elements present in the films was identified from energy dispersive X-ray analysis and X-ray photoelectron spectroscopy. The effect of chromium doping on optical constants is discussed in detail. The electrochromic studies reveal that better optical modulation (ΔT = 38.3%) and high coloration efficiency (CE = 47 cm2/C) were obtained for 6 at. % of Cr-doped Co3O4 film. The present work suggests that incorporation of Cr in Co3O4 will enhance its electrochromic performance.

Structure evolution of chromium-doped boron clusters: toward the formation of endohedral boron cages.

The electron-deficient nature of boron endows isolated boron clusters with a variety of interesting structural and bonding properties that can be further enriched through metal doping. In the current work, we report the structural and electronic properties of a series of chromium-doped boron clusters. The global minimum structures for CrBn clusters with an even number of n ranging from 8 to 22 are proposed through extensive first-principles swarm-intelligence structure searches. Half-sandwich structures are found to be preferred for CrB8, CrB10, CrB12 and CrB14 clusters and to transform to a drum-like structure at CrB16 cluster. Endohedral cage structures with the Cr atom located at the center are energetically most favorable for CrB20 and CrB22 clusters. Notably, the endohedral CrB20 cage has a high symmetry of D2d and a large HOMO–LUMO gap of 4.38 eV, whose stability is attributed to geometric fit and formation of an 18-electron closed-shell configuration. The current results advance our understanding of the structure and bonding of metal-doped boron clusters.

Blistering and Helium Retention of Tungsten and 5% Chromium Doped Tungsten Exposed to 60 keV Helium Ions Irradiation☆☆Supported by the National Natural Science Foundation of China under Grant Nos 51471015 and 11675010, and the National Magnetic Confinement Fusion Program under Grant No 2015GB109003.

Pure tungsten (W) and chromium doped W (W-5%Cr) are prepared by powder metallurgy. The microstructure, blistering and helium retention are investigated by x-ray diffraction, scanning electron microscopy, transmission electron microscopy and thermal desorption spectroscopy (TDS). These results show that the average size and density of helium blisters on the surface of pure W are much larger than those on the W-5%Cr alloy. Vacancy-impurity pairs can reduce the migration coefficients of vacancy and vacancy-helium complexes, and Cr may play a role of such an impurity. Moreover, the TDS result shows that the highest desorption peak moves to higher temperature, which is attributed to the HemCrkVn complexes in the W-Cr alloy. In addition, the helium retention is found to be higher in W than in W-5%Cr.

Effect of Cr-doping on the physicochemical properties of blue TiO2 and its application in dye-sensitized solar cells via low-temperature fabrication process

Cr-doped blue TiO2 (Cr-BTiO2) nanoparticles were fabricated at room temperature using lithium-ethylenediamine (Li-EDA) as reducing agent. The addition of Li-EDA promotes the selective reduction of the rutile phase of TiO2 into the amorphous phase keeping anatase phase unaltered. Hence, the phase-selective reduction of TiO2 leads to the formation of blue TiO2 nanoparticles. Synthesized samples were characterized by equipment fitted with modern technology. The shifting of (101) peak to a lower angle (2θ) in Cr-BTiO2 in X-ray diffraction (XRD) pattern suggests the successful doping of chromium into TiO2 lattices. In Raman spectra, the shifting of the active Eg peak of Cr-BTiO2 nanoparticles to higher wavenumber also suggests the successful substitution of Ti by Cr. The blue TiO2 and Cr-BTiO2 show increased absorption of light in the visible region compared to TiO2 (P25). The modified TiO2 samples have improved electron-hole separation tendency as predicted by the photoluminescence spectra (PL). Also, doping of Cr- into TiO2 lattice results the formation of oxygen vacancy as detected by X-ray photoelectron spectroscopy (XPS). Among all samples, Cr-BTiO2 demonstrated improvement in Jsc and overall incident photon to current conversion efficiency. Therefore, the synthetic effect is thus responsible for the enhancement in efficiency of Cr-BTiO2 towards the dye-sensitized solar cell (DSSC) by 2.5 and 1.5 times higher than the P25 and blue TiO2, respectively.

The effect of holding temperature on the strength of the diffusion bond of Ti-alloy and stainless steel through the ultrafine-grained interlayers of Ni and Ni-2%Cr alloy

The manuscript considers the effect of diffusion bonding pressure, as well as holding at different temperatures, on the strength of the joints between titanium alloy and stainless steel, formed by diffusion bonding through ultrafine-grained interlayers of Ni and the Ni-2%Cr alloy. In samples with an interlayer of nickel after holding at 25 °C, the maximum strength of 380 MPa is achieved after diffusion bonding in the pressure range of 4-8 MPa. In samples with an interlayer of Ni-2%Cr, after holding at 25 °C, the highest strength of 490 MPa is achieved after bonding at a pressure of 12 MPa at 700 °C for 20 minutes. In the case of Ni-2%Cr interlayer, the strength of the joint falls more slowly than in the case of Ni interlayer. The mechanical properties of the joints are discussed in terms of the effect of chromium doping of nickel interlayer on the temperature range of transformation of austenite to martensite of the formed TiNi intermetallic compound.

Structural and Magnetic Property of Cr3+ Substituted Cobalt Ferrite Nanomaterials Prepared by the Sol-Gel Method.

Cobalt-chromium ferrite, CoCrxFe2−xO4 (x = 0–1.2), has been synthesized by the sol-gel auto-combustion method. X-ray diffraction (XRD) indicates that samples calcined at 800 °C for 3 h were a single-cubic phase. The lattice parameter decreased with increasing Cr concentration. Scanning electron microscopy (SEM) confirmed that the sample powders were nanoparticles. It was confirmed from the room temperature Mössbauer spectra that transition from the ferrimagnetic state to the superparamagnetic state occurred with the doping of chromium. Both the saturation magnetization and the coercivity decreased with the chromium doping. With a higher annealing temperature, the saturation magnetization increased and the coercivity increased initially and then decreased for CoCr0.2Fe1.8O4.

Phase Transition Dominated High-Rate Performances of the High Voltage LiNi0.5Mn1.5O4 Cathode: Improvement on Structure Evolution and Ionic Diffusivity by Chromium Doping

Phase transition can profoundly influence the electrochemical performances of cathode materials for lithium ion batteries. The intricate phase transitions upon electrochemical cycling constrain the...

Cation distribution, magnetic and structural properties of CoCrxFe2-xO4: Effect of calcination temperature and chromium substitution

Abstract The influence of chromium doping and the calcination temperature on the structural, morphological, cation distribution and magnetic properties of CoCrxFe2−xO4 where x = 0.0, 0.25, 0.5, 0.75 and 1 were investigated in detail. The X-ray diffraction (XRD) analysis using the Rietveld refinement technique proved the nanocrystalline single-phase, chromium doped cobalt ferrite samples. It is observed that the lattice parameter 'a' decreases with increasing the chromium concentrations, while lattice parameter gradually enhances with increasing of calcination temperature. The cation distribution was inferred from the analysis of intensity of XRD as well magnetic properties. Magnetic measurements informed that both of the coercivity (HC) and saturation magnetization (MS) overall decrease with increasing chromium substitution. Also, with rising of calcination temperature from 350 to 1000 °C, for un-doped cobalt ferrite sample, MS is increased from 31 to78 emu/gr, respectively. These magnetic behaviors could be explained by the cation distribution and crystallite size enhancement. The HC enhancement for the heat-treated samples until 700 °C can be attributed to the crystal growth within the single domain region, while decreasing HC by further increasing calcination temperature to 1000 °C, is due to the transformation of single domain nanoparticles to multi-domain particles. The reduced hysteresis loops due to decreasing MS and HC with increase in chromium concentration, which in turn lowered the coercive field and softened the material, favoring its application in high-frequency transformers.

Tailoring sub-bandgap of CuGaS2 thin film via chromium doping by facile chemical spray pyrolysis technique

Tailoring sub-bandgaps using intermediate states or bands with wide bandgap semiconductors is a promising and novel technique for the application in high efficiency solar cells. Pure and chromium (Cr) doped chalcopyrite CuGaS2 (CGS) thin films were prepared by facile chemical spray pyrolysis technique and annealed in vacuum, nitrogen and argon atmospheres. Structural characterization confirmed that the prepared films are in tetragonal chalcopyrite structure with polycrystalline nature. No secondary phases were present in both pure and Cr doped CGS thin films. Presence of Cr ions was confirmed by X-ray photoelectron spectroscopy and energy dispersive analysis of X-rays analyses. The optical direct and sub band gap of pristine and Cr doped CGS thin films were measured from UV absorption data. It was revealed that the pure CGS film has a band gap of 2.40 eV sub-band gap values were observed at 2.25 and 2.15 eV for 1 and 2 wt% of Cr doping, respectively. These gaps can be ascribed to the formation of intermediate bands due to hybridization of Cr d-states into the host electronic structure. Meanwhile, photoconductivity study demonstrated the photo-electric activity of the intermediate bands in the Cr doped thin films.

Enhanced thermoelectric performance of Mg2Si1−xSnx codoped with Bi and Cr

Magnesium silicides are favorable thermoelectric materials considering resource abundance and cost. Chromium (Cr) doping in magnesium silicides has not yet been explored. Using first-principles calculations, we have studied the stability of ${\mathrm{Mg}}_{2}\mathrm{Si}$ with chromium (1.85, 3.7, 5.55, and 6.25% Cr) and tin (12.5 and 50% Sn). Three ${\mathrm{Mg}}_{2}\mathrm{Si}$ compounds doped with Sn, (Sn + Bi), and (Sn + Bi + Cr) are used to explain doping effects on thermoelectric performance. Notably, Cr behaves nonmagnetically for $\ensuremath{\le}2%\phantom{\rule{0.28em}{0ex}}\mathrm{Cr}$, after which ferromagnetic ordering is favored ($\ensuremath{\le}12.96%$ Cr), despite its elemental antiferromagnetic state. With alloying of Sn (70.4%), ${\mathrm{Mg}}_{2}\mathrm{Si}$ remains an indirect-band-gap semiconductor, but adding small amounts of Bi (3.7%) increases the carrier concentration such that electrons occupy conduction bands, making it a degenerate semiconductor. ${\mathrm{Mg}}_{2}{\mathrm{Si}}_{0.296}{\mathrm{Sn}}_{0.666}{\mathrm{Bi}}_{0.037}$ is found to give the highest thermoelectric figure of merit (ZT) and power factor (PF) at 700 K, i.e., 1.75 and 7.04 $\text{mW}\phantom{\rule{4.pt}{0ex}}{\text{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\text{K}}^{\ensuremath{-}2}$, respectively. Adding small %Cr decreases ZT and PF to 0.78 and 4.33 $\text{mW}\phantom{\rule{4.pt}{0ex}}{\text{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\text{K}}^{\ensuremath{-}2}$, respectively. Such a degradation in thermoelectric (TE) performance is attributed to two factors: (i) uniform doping acting as an electron acceptor, decreasing conduction, and (ii) the loss of low-lying conduction band degeneracy with doping, decreasing the Seebeck coefficients. A study of configurations of Cr doping suggests that Cr has a tendency to form clusters inside the lattice, which play a crucial role in tuning the magnetic and TE performance of doped ${\mathrm{Mg}}_{2}\mathrm{Si}$ compounds.

Computer Simulation of Plastic Deformation in Tial Alloys in the Presence of Chromium

Quantum-mechanical calculations were used to investigate shear rupture in intermetallic titanium aluminide (TiAl) alloys in the presence of vacancy or chromium dopant. The substitution of both Ti and Al atoms by Cr atoms in the γ-TiAl crystal lattice was considered. The simulation of shear was carried out in the (111) slip plane along two directions, namely the [110] and [11-2]. The decrease in the shear resistance of the defects present in the γ-TiAl lattice was estimated. It was shown that when chromium occupies a titanium vacancy, it can compensate for this defect by increasing the shear modulus for the {111} <110> slip system.

Qualitative and Quantitative Phase-Analysis of Undoped Titanium Dioxide and Chromium Doped Titanium Dioxide from Powder X-Ray Diffraction Data

Undoped titanium dioxide (TiO2) and a series of chromium(III) doped TiO2 (Cr-doped TiOfig. 12) with various %wt Cr atom were prepared by a reflux technique. The undoped TiO2 and Cr-doped TiO2 of 1.1, 3.9, 4.4 %wt Cr atom have been successfully analyzed both qualitative and quantitative analysis of powder X-ray diffraction (XRD) data. The qualitative analysis was carried out with the identification of phases in all samples by comparison with Crystallography Open Database (COD) and International Centre for Diffraction Data (ICDD), while the quantitative phase analysis was calculated by reference intensity ratio (RIR) and whole-pattern fitting (Rietveld analysis) methods. The undoped TiO2 consist of three phases: anatase, rutile, and brookite. In the 1.1 %wt Cr-doped TiO2 are detected presenting two phases: anatase (major) and rutile (minor). In the 3.9 %wt Cr-doped TiO2 andin the 4.4 %wt Cr-doped TiO2 consist of anatase as major phase, while CrO2 and TiO2-II phases can be detected as minor phases. The undoped TiO2 was refined in the phase, crystal system and space group of anatase (tetragonal, I41/amd), rutile (tetragonal, P42/mnm) and brookite (orthorhombic, Pbca), while the 1.1 %wt Cr-doped TiO2 was refined based on anatase (tetragonal, I41/amd), rutile (tetragonal, P42/mnm). Finally, in the 3.9 %wt Cr-doped TiO2 and 4.4 %wt Cr-doped TiO2,respectively were refined in the crystal system and space group of anatase (tetragonal, I41/amd).

Effect of chromium doping on the structure and band gap of Bi3.15Nd0.85Ti3O12 thin films

The Bi3.15Nd0.85Ti3O12 (BNdT) and Cr-doped BNdT films on quartz substrates were prepared by a sol-gel process and the effects of Cr doping on the structure and band gap of BNdT were examined. Cr-doping does not change the three-layered perovskite structure of BNdT but transforms the grain morphology from equiaxed grains to rectangular grains. The utilization of visible light is increased by Cr doping and the large band gap reduction (∼1.1 eV) was obtained, which is analyzed from the electronegativity and the distortion caused by substitution. The present work provides an available way to make ferroelectric (FE) BNdT getting more extensively applied in the new photovoltaic cells and other novel optoelectronic devices.

Enhanced performance and anchoring polysulfide mechanism of carbon aerogel/sulfur material with Cr doping and pore tuning for Li-S batteries

The ultrafine Cr2O3 modified carbon aerogel (Cr-CA) composites were successfully synthesized through in situ and sol-gel method as cathode material for lithium sulfur battery. The chromium doping remarkably controls structure, decreases carbon sphere size and promotes network interconnect. The nano Cr-CA composites with large pore volume and high specific surface present stronger adsorption ability and effectively immobilize polysulfides. The Cr-CA/S composite cathodes display excellent electrochemical performance with high specific capacity and long cycling stability. Especially, the Cr2-CA/S cathode with 65.9 wt% of sulfur delivers initial specific capacity of 1343 and 987 mAh g−1 at the rate of 0.2 C and 2 C, respectively. It exhibits the outstanding long-term cyclic stability with the discharge capacity of 873 mAh g−1 at 0.5 C for the 300th cycle. The excellent performance is mainly attributed to special anchoring role of Cr2O3 to polysulfides and its catalytic effect for redox reaction process. Furthermore, the adsorb energy between polysulfides and Cr2O3 and theoretic analysis by DFT definitely reveals and supports immobilizing and catalytic mechanism.

Improving rate capability and decelerating voltage decay of Li-rich layered oxide cathodes by chromium doping

In order to improve the rate capability and mitigate the voltage decay of lithium rich layered oxides, chromium doped cathode materials Li1.2[Mn0.54Ni0.13Co0.13]1−xCrxO2 (x = 0,0.003,0.005,0.007) is synthesized via co-precipitation method followed by calcination. As a result, Cr-doped samples present shortened voltage platform at 4.5 V, indicating that the escape of lattice oxygen is suppressed by Cr doping. Among all samples, Li1.2[Mn0.54Ni0.13Co0.13]1−xCrxO2 (x = 0.005) sample shows the greatest rate capability (119.3 mAh g−1 at 10 C) and the minimum voltage drop (0.6167 V) after 200 cycles at 1.0 C. The reasons for the improved electrochemical performance are the enhanced structural stability, the stronger CrO bond than MnO band and the decreased metal–oxygen covalency induced by Cr doping.

Influence of Chromium Doping on Electrical and Magnetic Behavior of Nd0.5Sr0.5MnO3 System

With a view to understand the influence of chromium doping at the Mn site on the electrical and magnetic behavior of the Nd0.5Sr0.5MnO3 manganite system, a series of samples were prepared by the citrate sol–gel route method. The samples were characterized structurally by XRD. A systematic investigation of electrical resistivity over a temperature range 5–300 K was carried out mainly to understand the magneto-transport behavior in these materials. Studies on the variation of magnetization with temperature over a temperature range 80–330 K were undertaken. Investigation of magnetization at different magnetic fields at two different temperatures, viz. 80 and 300 K, was also carried out. The results show that chromium doping gave typical electrical and magnetic properties. It has been concluded that the coexistence of charge ordered and ferromagnetic phases induced by chromium doping plays an important role in the low-temperature behavior of the system.