Zr-doped Sample Research Articles

Oriented Gradient Doping of Zirconium in Ni-Rich Cathode to Achieve Ultrahigh Stability and Rate Capability.

Ni-rich layered oxide materials exhibit great prospects for practical applications in lithium-ion batteries due to their high specific capacity. However, the poor cycling performance and suboptimal rate performance have caused obstacles for their widespread application. Herein, we developed a gradient Zr element doping method based on the bulk gradient concentration of Ni-rich layered oxide material to reinforce the cycle stability and rate performance of the cathode. In particular, the orientations of the gradient Zr doping were achieved via coprecipitation in a positive or negative correlation between the concentrations of Zr and Ni, and it was revealed that the material behaves better when the Zr content is abundant in the core. The gradient doping of Zr decreases the content of Ni2+ and mitigates the mixing degree of Li+ and Ni2+, implying the superior performance of doped cathode material. Compared with the bare sample (70.7%, 121.4 mAh g-1), the Zr-doped sample delivered a higher capacity retention of 85.6% after 300 cycles at 1C (1C = 180 mA g-1) and exhibited a considerable rate performance of 122.5 mAh g-1 at 20C. In particular, the Zr-doped cathodes performed dramatically on high rate cycling at 10C, with an initial capacity of 143.6 and 103.9 mAh g-1 after 300 cycles. Furthermore, the Zr-doped cathode delivered significant stability at a high potential of 4.5 V with a capacity retention of 72.1% after 300 cycles, while that of the bare sample was only 37.4%. The concept of gradient doping strategies during coprecipitation offers new insight into the design of advanced cathodes with excellent cycling stability and rate capability.

Low-temperature specific heat and heat transport of Tb2Ti2−xZrxO7 single crystals

We report a study on the specific heat and heat transport of Tb$_2$Ti$_{2-x}$Zr$_x$O$_7$ ($x =$ 0, 0.02, 0.1, 0.2, and 0.4) single crystals at low temperatures and in high magnetic fields. The magnetic specific heat can be described by the Schottky contribution from the crystal-electric-field (CEF) levels of Tb$^{3+}$, with introducing Gaussian distributions of the energy split of the ground-state doublet and the gap between the ground state and first excited level. These crystals has an extremely low phonon thermal conductivity in a broad temperature range that can be attributed to the scattering by the magnetic excitations, which are mainly associated with the CEF levels. There is strong magnetic field dependence of thermal conductivity, which is more likely related to the field-induced changes of phonon scattering by the CEF levels than magnetic transitions or spin excitations. For magnetic field along the [111] direction, there is large thermal Hall conductivity at low temperatures which displays a broad peak around 8 T. At high fields up to 14 T, the thermal Hall conductivity decreases to zero, which supports its origin from either the spinon transport or the phonon skew scattering by CEF levels. The thermal Hall effect is rather robust with Zr doping up to 0.2 but is strongly weakened in higher Zr-doped sample.

Unravelling the role of zirconium-substitution on the magnetic properties of nickel ferrite processed from nanocrystalline powders

Herein, the effect of substituting tetravalent zirconium (Zr4+) for trivalent iron (Fe3+) on the magnetic properties of nickel ferrite, derived from nanocrystalline powders, is probed. Via a tartrate-gel method, NiFe2O4 and zirconium-doped nickel ferrite (NiFe1.8Zr0.2O4) nanopowders have been synthesized and are well-characterized using XRD and TEM techniques. Subsequently, as-synthesized nanopowders were compacted and sintered at high temperature (1200 °C for 2 h) to attain high density ceramic products. Using XRD, SEM, and VSM techniques, further characterizations were made on the sintered products to assess their phase stability/purity, morphology, structural aspects, and magnetic parameters. Analysis of the powder patterns indicated that NiFe2O4 is phase pure. However, a small fraction of impurity phase, identified as zirconia (ZrO2), has been observed in Zr-doped sample, due to charge and size mismatch between and Zr and Fe. After incorporating Zr4+ into the lattice structure, MS decreased and HC increased. Considerable changes in the microstructural parameters and octahedral site preference of Zr4+ are responsible for the observed change in the magnetic parameters of Zr-substituted sample.

Screening of optimal dopants on cobalt-based ceramics for high-temperature thermochemical energy storage

To date, numerous dopants have been investigated to promote the initial heat storage performance or sintering resistance of Co3O4 for thermochemical energy storage. Owing to the different synthesis methods and heat storage test conditions, the performances of these materials are not comparable. In this work, nine dopants, including four unreported dopants, were systematically compared and screened under the same conditions. Results demonstrated that for five redox cycles, the Si- and Mg- doped sample had adverse effects on the conversion rate, and the others maintained the conversion rate above 0.95. Cu and Zr doping exhibited the highest re-oxidation rate values, and those of the others were lower than that of pure cobalt oxide. Moreover, except for Cu and Zr, all dopants adversely affected energy density. According to aforementioned heat storage properties, Cu-, Zr-doped samples, and pure cobalt oxide were subjected to 100 cycles for further comparison. The Cu-doped sample exhibited a higher re-oxidation rate and energy density, and the Zr-doped sample exhibited more stable cyclability. Finally, by doping Cu and Zr, the micromorphology evolution was more stable than that of pure oxide for multiple cycles, and X-ray photoelectron spectroscopy results revealed that the higher surface oxygen was the primary reason for the superior performance facilitated by Cu and Zr. The Cu- and Zr-modified cobalt oxides were the potential candidates for thermochemical energy storage.

Enhanced magnetic, dielectric and photoconductive properties of Zr doped BiFeO3 nanostructures

Lead-free ferroelectric BiFe1-xZrxO3 (x = 0.0, 0.015, and 0.025) thin films with varying concentrations of Zr4+ ions were fabricated using nanoparticles synthesized by the hydrothermal method. The X-ray diffraction analysis reveals that the as-synthesized pure and Zr-doped nanoparticles possess distorted rhombohedral structures. The diffraction peak at 32° shifts to the lower angle with increase in Zr content. For higher concentration of Zr4+ ions (2.5% Zr-doped BFO), the impure phase Bi2Fe4O9 evolved and increased the leakage current. Both the SEM and TEM images of pure and doped samples exhibit irregularly shaped clusters of agglomerated nanocrystallites. The SAED pattern of the samples confirms that the samples are highly crystalline and the diffracted rings matched with the lattice planes of single-phase pristine bismuth ferrite (BFO). The Raman E modes of 1.5% Zr-doped BFO nanoparticles are relatively higher in intensity as compared to other samples and exhibited a very weak dependence of dielectric constant on frequency. On comparing, a 1.5% Zr-doped BFO sample showed lowest dielectric constant (∼79) at 1 kHz due to the filling of oxygen vacancies by Zr4+ ions and hence displayed a significant photocurrent of ∼3.10 μA at 5 V. However, for 2.5% Zr dopant concentration, the dielectric constant is quite high at 1 kHz due to the reduction of Fe3+ ion and increased oxygen vacancies; and consequently showed very feeble photocurrent of ∼9.00 nA at 5 V. Furthermore, higher ferromagnetic ordering between Fe3+ and Zr4+ in 2.5% Zr-doped BFO nanostructure has resulted in larger remnant magnetization of 0.585 emu/g. The present study confirms that the optimum concentration of Zr ions substituted at the Fe-site of BiFeO3 reduces the leakage current and improves its photoconductive properties. Besides, the aging problem of BFO nanostructures caused by oxygen vacancies due to the reorientation of dipoles can be significantly reduced through Zr doping. Hence, the enhanced temperature-dependent photo sensing properties of doped BFO nanostructures are more suitable to serve as an active layer in self-powered photodetector systems.

Influence of dopant valence on the thermochromic properties of VO2 nanoparticles

The effect of valence of dopants on thermochromic properties of VO2 nanoparticles are investigated. In the present work, VO2 nanoparticles were synthesized by hydrothermal method and subsequently spin coated on a glass substrate. Dopants with various valences can break the structural symmetry of the V4+-V4+ chains, which results in various degrees of lattice distortion and changes in the crystal structure of VO2. Therefore, the transition temperature (Tc) can be associated with the valence of transition metal dopants, and it decreases with the increase of elements’ valence. Meanwhile, various valence results in the changeable optical band gap (Eg). The doping of lower valence elements results in narrowing of Eg, thus leading to enhancements in both luminous transmittance (Tlum) and solar energy modulation ability (ΔTsol). The phase transition temperature is significantly reduced by 20.2 °C for 1% doping of W. The Zr-doped film exhibits the highest Tlum and ΔTsol of 61.4% and 10.3%, respectively. The Mg-doped film has shown Tlum and ΔTsol of 59.4% and 9.5%, respectively, which is slightly inferior to Zr-doped sample. It can be deduced that W/Zr and W/Mg co-doped VO2 films can improve Tc, Tlum, and ΔTsol, simultaneously, which can provide a potential solution for manufacturing smart windows.

Defect states and their electric field-enhanced electron thermal emission in heavily Zr-doped β -Ga2O3 crystals

Performing deep level transient spectroscopy (DLTS) on Schottky diodes, we investigated defect levels below the conduction band minima (Ec) in Czochralski-grown unintentionally doped (UID) and vertical gradient freeze-grown Zr-doped β-Ga2O3 crystals. In UID crystals with an electron concentration of 1017 cm−3, we observe levels at 0.18 eV and 0.46 eV in addition to the previously reported 0.86 (E2) and 1.03 eV (E3) levels. For 1018 cm−3 Zr-doped Ga2O3, signatures at 0.30 eV (E15) and 0.71 eV (E16) are present. For the highest Zr doping of 5 × 1018 cm−3, we observe only one signature at 0.59 eV. Electric field-enhanced emission rates are demonstrated via increasing the reverse bias during measurement. The 0.86 eV signature in the UID sample displays phonon-assisted tunneling enhanced thermal emission and is consistent with the widely reported E2 (FeGa) defect. The 0.71 eV (E16) signature in the lower-Zr-doped crystal also exhibits phonon-assisted tunneling emission enhancement. Taking into account that the high doping in the Zr-doped diodes also increases the electric field, we propose that the 0.59 eV signature in the highest Zr-doped sample likely corresponds to the 0.71 eV signature in lower-doped samples. Our analysis highlights the importance of testing for and reporting on field-enhanced emission, especially the electric field present during DLTS and other characterization experiments on β-Ga2O3 along with the standard emission energy, cross section, and lambda-corrected trap density. This is important because of the intended use of β-Ga2O3 in high-field devices and the many orders of magnitude of possible doping.

Magnetization reversal behavior and first-order reversal curve diagrams in high-coercivity Zr-doped α-Fe/Nd2Fe14B nanocomposite alloys

In the present work, the magnetization reversal behavior for the melt spinning (Nd0.8Ce0.2)2Fe12Co2−xZrxB (x = 0, 0.5) permanent alloys with high coercivity was investigated by analyzing the hysteresis curves and the recoil loops. Compared to the Zr-free alloy, the Zr-doped sample obtains higher magnetic properties: coercivity of Hcj = 650.5 kA·m−1, squareness of Hk/Hcj = 0.76 and maximum energy product of (BH)max = 131.0 kJ·m−3. The first-order reversal curves (FORCs) analysis was taken to identify optimal conditions of exchange coupling for the Zr-free and Zr-doped alloys. The coercivity mechanism of the α-Fe/Nd2Fe14B nanocomposite alloys was analyzed by the angular dependence of the coercive field as measured for the Zr-doped sample. The results show that the magnetic reverse process of the Zr-doped sample can be explained by the pinning model.

Effect of Zr doping and aging on optical and photocatalytic properties of ZnS nanopowder

Abstract A facile single step chemical method is reported for the synthesis of Zr doped ZnS nanopowder and its photocatalytic performance has been studied. The average crystallite size of the synthesized powder estimated from X-ray diffraction (XRD) is nearly 1.1 nm. Transmission electron microscopy (TEM) study shows that nanoparticles are agglomerated having a particle size of nearly 10 nm. The optical band gap of ZnS and Zr-doped ZnS, estimated with the help of UV–visible spectroscopy, is 3.7 and 2.9 eV, respectively. Photoluminescence (PL) spectroscopy indicated the role of lattice defects (vacancy, interstitial defects) on optical emission characteristics of doped ZnS samples. The photocatalytic activity of synthesized samples has also been investigated by degrading methylene blue (MB) dye under UV–visible illumination. The degradation quenching has been observed beyond 0.4 mol% doping of Zr in ZnS with maximum efficiency i.e. 80% in 4 h. Thereafter, the effect of aging is monitored for 1.5 months in which the modified emission properties of defect sites modulated the photocatalytic performance of Zr-doped ZnS sample. Further, with the help of various spectroscopic techniques (absorbance, PL and XPS), a mechanism for the photodegradation of MB dye by using Zr-doped ZnS nanopowder has been proposed.

Effect of Zr Doping on the Magnetic and Phase Transition Properties of VO2 Powder

In this work, V1−xZrxO2 powder (x = 0, 0.01, 0.02, 0.04) was synthesized by two step hydrothermal method. The micro-topography, magnetic and phase transition properties have been investigated using various measurement techniques. All prepared V1−xZrxO2 powder samples exhibit monoclinic structure at room temperature. With the Zr4+ ions doping concentration increased, the shapes of VO2 particles change from spherical to rectangular slice. Besides, the saturation magnetic moment of the samples decrease with the increase of doped Zr4+ ions concentration, while their phase transition temperature increase gradually with Zr ions doping at a rate of around 2 °C/at% on average. We investigated the Zr doping effects on V-V dimers and confirmed the role of V-V dimers in phase transition. We speculate that more V-V dimers form with Zr doping by magnetic measurements, which result in the monoclinic phase of Zr-doped VO2 sample is more stable than rutile phase. Therefore the phase transition temperature is elevated by Zr doping in our experiment. We further consider that the VO2 phase transition should be ascribed to Peierls transition caused by the changing of V-V dimers.

Effect of Zr doping on the magnetic properties and FORC diagrams of (Nd0.8Ce0.2)1.6Fe12Co2B alloys

The microstructure and room temperature magnetic properties for (Nd0.8Ce0.2)1.6Fe12Co2BZrx (x = 0-5) nanocomposite alloys prepared by melt-spinning method and subsequent thermal annealing have been investigated. Zr addition has proved to result in relevant improvements in the microstructure and magnetic properties. The enhanced coercivity Hcj of 6.02 kOe and maximum energy products (BH)max of 15.2 MGOe have been obtained at the optimum temperature Ta with the Zr content at x = 2 and x = 3, respectively. The results of X-ray diffraction (XRD) and transmission electron microscope (TEM) indicate that a small amount of Zr addition can refine the grain size and cause a uniform distribution of soft magnetic α-Fe phase. A first-order-reversal curve (FORC) analysis is performed for the Zr-free (x = 0) sample and the Zr-doped (x = 3) sample. The FORC diagrams suggest the Zr-doped sample with a higher maximum ρ of the major peak and the flat surface is responsible for the better magnetic performance. The results of Kronmüller plot show that nucleation model is the dominating mechanism for the magnetization reversal in the Zr-free and Zr-doped alloys and give the evidence for the increase in microstructural parameter αex resulting in a drastic increase of coercivity in the Zr-doped alloy.

Effect of Zr doping and Li2O-2B2O3 layer on the structural electrochemical properties of LiNi0.5Co0.2Mn0.3O2 cathode material: experiments and first-principle calculations

In order to enhance the electrochemical performance of LiNi0.5Co0.2Mn0.3O2 (NCM), an in situ doping with zirconium (Zr) by wet grind-solid state method then coating with Li2O-2B2O3 (LBO) by crystal phase selection method is successfully developed. At the same time, based on the density functional theory (DFT) with the generalized gradient approximation (GGA), use of the first-principle calculation method theoretically proved the experimental results. The XRD results of experimental and the calculated both revealed the lattice parameters become larger after Zr doped. The thickness of the LBO layer uniformly deposited on the surface of the Zr-doped sample is 5 nm, which can scavenge and neutralize HF erosion. Particularly, the Zr-doped and LBO-coated NCM sample exhibited excellent cycling performance with 90.5% capacity retention after 100 cycles at 1 C, which are apparently higher than 76.97% of NCM. The initial discharge capacity of double optimized sample was 170.4 mAh g−1 at 1 C which is higher than 157.1 mAh g−1 of NCM. Zr and LBO layers can effectively reduce the energy of the system and the band gap. Obtained results were consistent with the experimental results. Therefore, co-modification method plays an import role to optimize the material from the structure to the surface and hence greatly improved the battery performance.

Reactivity of Ni, Cr and Zr doped ceria in CO2 splitting for CO production via two-step thermochemical cycle

This work focuses on modification and screening of ceria-based oxides for solar H2O/CO2 splitting via two-step thermochemical cycle. Ce1-xMxO2-δ (M = Zr, Ni, Cr; x = 0, 0.05, 0.10, 0.15, 0.20) were synthesized via sol-gel method and tested for CO2-splitting via two-step thermochemical cycles. Reduction was conducted at 1500 °C through a ramp rate of 10 °C/min and oxidation was performed at 1000 °C isothermally. Both Ni and Cr showed low solubility in ceria and no or very limited promoting effect on CO productivity. Cr could be reduced in the first reduction step but cannot be oxidized by CO2 in the following oxidation step. Zr doped sample showed advantages in both CO productivity and lattice stability. 15% Zr doped exhibited the best performance with the CO productivity of 315.40 μmol/g. However, the oxidation rate of Zr doped samples was much lower than that of pure ceria. Compromise between fuel productivity and fast kinetics should be made in practical application.

Synthesis and Enhanced Photocatalytic Activity of Black Porous Zr-doped TiO2 Monoliths

Black Porous Zr-doped TiO2 monoliths were successfully synthesized by a combined route including sol–gel method with polystyrene spheres as template followed by calcination at high temperature in the presence of oxygen. As-synthesized samples were then treated in hydrogen atmosphere at 500[Formula: see text]C for 6[Formula: see text]h to obtain black Porous Zr-doped TiO2 monoliths. This as-synthesized black porous Zr-doped TiO2 composite showed relatively narrow bandgap compared to the pure anatase phase of TiO2, which may have its origin from the contributions from the Zr impurity and oxygen vacancies. The photocatalytic activity of the black porous Zr-doped TiO2 monolith was examined by carrying out the dye degradation on Rhodamine B under visible irradiation. Our experimental results indicate that black porous 0.9% Zr-doped TiO2 sample exhibits high photocatalytic activity with the photodecomposition of 95.25% under visible irradiation for 120[Formula: see text]min.

Preparation of Zr-doped CaTiO3 with enhanced charge separation efficiency and photocatalytic activity

A series of Zr-doped CaTiO3 powders were prepared with the mild co-precipitation method and calcined at 850 °C for 3 h. The as-prepared Zr-doped CaTiO3 samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–Vis diffuse reflectance spectra (DRS) and X-ray photoelectron spectra (XPS). XRD result revealed the presence of single perovskite phase of CaTiO3. UV–Vis diffusive reflection spectra of Zr-doped CaTiO3 indicated that the absorbance obviously increased in the visible light irradiation. XPS analysis showed that two types of oxygen existed on the photocatalyst surface, including lattice oxygen and absorbed oxygen. Their photocatalytic activity in the case of the degradation of methyl orange in water and photoelectrochemical activity were also tested. The 5%Zr-doped (mole fraction) CaTiO3 sample showed the highest photocatalytic activity. The enhanced photocatalytic activity was ascribed to the change of the lattice structure, existence of oxygen vacancies and increase of the photogenerated charge separation efficiency.

Effect of doping ions on the structural defect and the electrical behavior of CaCu3Ti4O12 ceramics

The microstructure, dielectric and electrical properties of CaCu3Ti4−xRxO12 (R=Y, Zr, Ta; x=0 and 0.005) ceramics were investigated by XRD, Raman spectra, SEM and dielectric spectrum measurements. Positron annihilation measurements have been performed to investigate the influence of doping on the defects. The results show that all samples form a single crystalline phase. Y and Ta doping cause different defect types and increase the defect size and concentration, which influence the mobility of grain boundary and result in the different grain size. Y doping increases the dielectric constant and decreases the nonlinear property while Ta doping lead to an inverse result. Zr-doped sample has nearly the defect type, grain morphology and dielectric properties as pure CaCu3Ti4O12. The effects of microstructure including the grain morphology and the vacancy defects on the mechanism of the dielectric and electric properties by doping are discussed.

Two-step thermochemical cycles for CO2 splitting on Zr-doped cobalt ferrite supported on silica

Silica-supported Zr-doped cobalt ferrite complex oxides, Co1−2xZrxFe2O4/SiO2 (0⩽x⩽0.15) were prepared via a simple sol–gel auto-combustion method using aqueous silica colloid as the silica source. The products were used as reactive materials in two-step thermochemical cycles for CO2 splitting. Compared to undoped cobalt ferrite, the Zr-doped ones can significantly enhance the rates of the reduction and oxidation steps as well as the yields of CO. The Zr-doped sample also showed greater recyclability than the undoped one. When the sample Co0.8Zr0.1Fe2O4/SiO2 was used as reactive material, the average production of 24.4mL/gferrite for O2 and 40.2mL/gferrite for CO were obtained after 20 cycles at 1400 and 1100°C respectively. A kinetic analysis indicated the apparent activation energy in the CO2 splitting step over Co0.8Zr0.1Fe2O4/SiO2 is 118.0±1.2kJ/mol, while it is 133.0±2.1kJ/mol for CoFe2O4/SiO2.

Influence of Zr doping on the structural, magnetic and magnetoelastic properties of cobalt-ferrites

Cobalt-ferrite (CoFe2O4) has long been identified as a potential material for the magnetostrictive component of the torque sensor − for power-steering systems in automotive industries, owing to its superior strain sensitivity (dλ/dH). In this work we investigate the influence of Zr doping (in Fe site) on the structural, magnetic, and magnetoelastic properties of CoFe2O4. Zr-doped cobalt-ferrites, having nominal compositions Co1+xZrxFe2−2xO4 (0⩽x⩽0.2), were synthesized by conventional solid-state method. The X-ray spectral analysis indicated the presence of only cubic-spinel phase and the lattice parameter (a) increases from ∼8.3769Å (x=0) to ∼8.3921Å (x=0.2). Saturation magnetization (MS), at ∼4000kA/m, and coercivity (HC) increases from ∼413kA/m and ∼4.456kA/m respectively for x=0 to ∼540kA/m, ∼7.96kA/m respectively for x=0.2. A remarkable 300% increase in dλ/dH was observed in Zr-doped cobalt-ferrite sample (x=0.2) (∼3.32×10−9A−1m) compared to that of pure cobalt-ferrite (∼0.8×10−9A−1m).

Evaluation of undoped and M-doped TiO2, where M = Sn, Fe, Ni/Nb, Zr, V, and Mn, for lithium-ion battery applications prepared by the molten-salt method

Bare and Fe, Zr, Sn, Mn, V and Ni/Nb doped TiO2 prepared by the molten salt method, amongst these the Zr-doped sample exhibited a stable reversible capacity.

Magnetic properties, microstructure and corrosion behavior of Nd10Y1Fe85−xNb3.5Ti0.5Bx (x = 14–22) and Nd10Y1Fe69Nb3.5M0.5B16 (M = Ti, Zr, Cr, Mo) bulk nanocrystalline magnets

The bulk nanocrystalline magnets of Nd10Y1Fe85−xNb3.5Ti0.5Bx (x=14–22) and Nd10Y1Fe69Nb3.5M0.5B16 (M=Ti, Zr, Cr, Mo) in rod form with a diameter of 1.5mm were prepared by copper mold injection casting. The magnetic properties, microstructure and corrosion behavior of bulk magnets have been studied. It is shown that the glass forming ability of alloy is obviously improved with increasing the B content. For the Nd10Y1Fe85−xNb3.5Ti0.5Bx (x=14–22) alloys, the optimal magnetic properties of Br=6.0kG, iHc=7.8kOe and (BH)max=6.7MGOe can be achieved when x=16, although the sample of x=22 exhibits better corrosion resistance than the sample of x=16. For the Nd10Y1Fe69Nb3.5M0.5B16 (M=Ti, Zr, Cr, Mo) alloys, the sample of M=Zr exhibits the best magnetic energy product of (BH)max=7.0MGOe, with Br=6.3kG and iHc=8.2kOe. While, the highest intrinsic coercivity of iHc=8.8kOe, with Br=5.6kG and (BH)max=5.8MGOe is obtained in the sample of M=Mo. The electrochemistry analysis shows that the Zr-doped sample has the best corrosion resistance among the M-doped alloys. The thermogravimetric analysis reveals that the oxidation rates of bulk magnets are significantly enhanced when the temperature is above 350°C, and the Ti-doped sample has the best antioxidant ability among the M-doped alloys.