Effect Of Ca Doping Research Articles

Co-doping effects of Ca and Ce on the Superconducting Properties in Y1−x Ca x (Ba1−y Ce y )2Cu3O7−δ

A series of Y1−x Ca x (Ba1−y Ce y )2Cu3O7−δ (0≤x≤0.3, 0≤y≤0.3) polycrystalline superconductor samples were prepared using the solid-state reaction technique. The phase identification, crystal structure, and superconducting transition temperature (T c ) were studied by means of X-ray diffraction (XRD) and resistivity measurements. The results indicted that the phase of samples changed from orthorhombic phase to tetragonal phase with increasing Ca concentration x and Ce concentration y, and Ce did not form the superconducting structure. The lattice constants had a little change. The a-axis and c-axis lattice parameters increased. The b-axis lattice parameter decreased. The T c and resistance had an obvious dropping tendency with increasing Ca and Ce concentrations. The transition width became sharper with the increase of x (=y). We drew a conclusion that the Ce-doping had an effect for strengthening the intergrain connectivity, and it counteracted the weakening effect of Ca-doping which introduced the hole causing a reduction in the interlayer coupling strength.

Structural evolution of one-dimensional spin-ladder compounds Sr14−xCaxCu24O41with Ca doping and related evidence of hole redistribution

Incommensurate crystal structures of spin ladder series Sr14-xCaxCu24O41 (x=3, 7, 11, 12.2) were characterized by powder neutron scattering method and refined using the superspace group Xmmm(00{\gamma})ss0 (equivalent to superspace group Fmmm(0,0,1+{\gamma})ss0); X stands for non-standard centering (0,0,0,0), (0,1/2,1/2,1/2), (1/2,1/2,0,0), (1/2,0,1/2,1/2)) with a modulated structure model. The Ca doping effects on the lattice parameters, atomic displacement, Cu-O distances, Cu-O bond angles and Cu bond valence sum were characterized. The refined results show that the CuO4 planar units in both chain and ladder sublattices become closer to square shape with an increase of Ca doping. The Cu bond valence sum calculation provided new evidence for the charge transfer from the chains to ladders (approximately 0.16 holes per Cu from x=0 to 12.2). The charge transfer was attributed to two different mechanisms: (a) the Cu-O bond distance shrinkage on the ladder; (b) increase of the interaction between two sublattices, resulting in Cu-O bonding between the chains and ladders. The low temperature structural refinement resulted in the similar conclusion, with a slight charge backflow to the chains.

Effect of Ca doping on the magnetic properties of Nd 0.5Sr 0.5MnO 3 studied by electron spin resonance

The magnetic properties of Ca-doped Nd 0.5Sr 0.5MnO 3 have been studied by electron spin resonance (ESR) and dc magnetization measurements. The antiferromagnetic order and charge order are found to occur separately at T N=200 K and T co=150 K, respectively. Compared to the undoped Nd 0.5Sr 0.5MnO 3, the ferromagnetic correlations are suppressed by doping of the small Ca 2+ ion. In addition, the antiferromagnetic transition temperature is enhanced to 200 K, which can be explained by an increase of superexchange interaction between Mn 3+ and Mn 4+ ions as their distance decreases.

Effect of Ca-doping on the ionic conductivity of LiSi2N3

Effect of Ca-doping on the ionic conductivity of LiSi2N3 was studied. The compositions of Li1-2xCaxSi2N3 (x=0-0.2) were synthesized by the reaction of Li3N, Si3N4, and Ca3N2 at temperature of 1873K-2073K. Ca was incorporated into the LiSi2N3 host lattice and formed the solid solution, Li1-2xCaxSi2N3. Activation energy for ionic conduction was decreased and ionic conductivity at room temperature was enhanced by Ca doping. At 298K, the ionic conductivity of densified Li1-2xCaxSi2N3 (x=0.075) ceramics achieved 1.6×10−5Sm−1, almost 4 orders of magnitude higher than that of densified Li1-2xCaxSi2N3(x=0) ceramics (3.1×10−9Sm−1). The LiSi2N3 framework change by Ca doping decreased the interaction between the ions and increased the defects of the structure. Thus, mobile Li+ ion could migrate easier. Moreover, incorporation of aliovalent substitutional Ca2+ ions in the LiSi2N3 lattice might be expected to create a Li+ vacancy (VLi) for charge compensation (Li1-2xCaxVLiSi2N3), thereby increasing the number of mobile Li+ ions.

Effect of Ca-doping on the structural and electrical properties of CuY 1− xCa xO 2 (0 ≤ x ≤0.10) ceramics

Delafossites CuY 1− x Ca x O 2 (0 ≤ x ≤0.10) ceramics have been prepared by solid state reaction using Cu 2O, Y 2O 3 and CaCO 3. Liquid phase sintering, which obviously accelerates the reaction speed of Cu 2O–Y 2O 3–CaCO 3 system and promotes the formation of CuYO 2 phase is evidenced for the Ca-doped samples. During the sintering process, CuO can react with CaO to form two intermediate compounds, CaCu 2O 3 and Ca 2CuO 3, which decompose into CaO and liquid phase during 1273–1323 K. In the dopant range of 0 ≤ x ≤0.10, both electrical conductivity and density of the samples are increased by Ca-doping. The room temperature conductivity of CuY 0.94Ca 0.06O 2 is more than four orders of magnitude higher than that of CuYO 2.

Effect of Ca Doping on the Thermoelectric Performance of Yb14MnSb11

Complex Zintl phases possess low thermal conductivity and can be easily doped to modify the transport properties. Therefore, these phases have the potential to be good thermoelectric materials by simply controlling carrier concentration. Yb_(14)MnSb_(11) is a Zintl phase that has shown promise as a p-type thermoelectric material for high-temperature power generation. A Sn-flux synthetic route was used to make the new phase, Yb_(13)CaMnSb_(11). The high-temperature thermoelectric properties were measured on polycrystalline hot-pressed pellets and compared with Yb_(14)MnSb_(11). Substitution of the lighter isovalent Ca for Yb should reduce the lattice thermal conductivity by mass disorder scattering, and a noticeable reduction is seen in thermal diffusivity measurements at high temperature. There may also be a carrier concentration effect by employing the more electropositive Ca.

Effect of Ca doping on the performance of CeO2–NiO catalysts for CH4 catalytic combustion

Methane catalytic combustion was carried out over the Ce0.9–xNi0.1CaxOδ (0 0.1), more carbonate species (mainly carbonate calcium) can form on the catalyst surface, which would severely debase the catalytic activity of Ce0.9–xNi0.1CaxOδ.

Effect of doping on metal doped semiconductor

The effect of doping on position of interface states for metal doped bathocuproine (BCP) was studied with density functional theory (DFT). The doping of Ca atoms with BCP induces the formation of interface states with shift in their relative positions from Fermi level and approximately no shift in HOMO position of BCP molecule. The shift in the position of interface states towards higher binding energy was believed to be due to the presence of doping excess electrons from Ca at the interface. The analysis of modification in intensity of LUMO or E F or interface states, suggests the formation of multiply charged anions in heavily doped film. It clearly gives the direct evidence for the origin of the doping interface states in organic molecules. The effects of Ca doping on electrical properties were discussed.

Effect of Ca doping on the catalytic performance of CuO–CeO2 catalysts for methane combustion

Methane catalytic combustion was carried out over a series of Ca doped Ce0.9 − xCu0.1CaxOδ (0 < x ≤ 0.3) catalysts prepared by the citric acid complexation–combustion method. The catalyst with x = 0.05 showed the optimum activity, with T50 = 478 °C, 60 °C lower than that of Ca undoped catalyst. The Ca doping promoted the formation of oxygen vacancies of the catalyst. With the increase in Ca amount, more carbonate species were formed on the catalyst surfaces. The carbonate species produced on the catalysts after long-term reaction, were proposed to be responsible for the activity loss of the catalysts.

Effects of Ca Doping on Nodule Formation of on ITO Target during DC Magnetron Sputtering

Effects of Ca Doping on Nodule Formation of on ITO Target during DC Magnetron Sputtering

The effect of Ca doping on the properties of melt-processed Gd-based bulk superconductors

The effect of Ca doping to melt-processed Gd-based bulk superconductors fabricated by the oxygen-controlled melt growth (OCMG) method was studied. First, we simply added CaCO 3 to the starting materials to dope Ca and observed an increase in the trapped field and the critical current density ( J c) up to, at least, 0.31 wt.% CaCO 3. However, an additional oxygen annealing resulted in a decrease of J c in magnetic fields and in a monotonous exponential-like field dependence of J c. This suggests that oxygen deficiency had caused the enhancement of J c, although the same annealing process as the non-doped sample was applied. We also prepared samples by adding not only CaCO 3 but also BaCO 3 and CuO in a molar ratio of Ca:Ba:Cu = 1:2:3, and observed a large peak effect of J c up to at least 0.31 wt.% CaCO 3. It is likely that this large peak effect is also caused by oxygen defects, although the superconducting transition temperature was significantly large. These results indicate that J c at 77 K can be significantly enhanced by introducing a proper amount of oxygen deficiency and compensating at the same time the decrease of carrier density by Ca doping.

Effect of Ca doping on the structure and scintillation properties of ZnWO4

AbstractThe future application of ZnWO4 scintillator in a cryogenic search for rare events is the motivation for optimization of this material. We present results on the effect of Ca doping on the structure and scintillation properties of ZnWO4. X‐ray diffraction analysis revealed that there is no mixing in the CaWO4–ZnWO4 pseudobinary system due to a significant mismatch of the crystal structures of CaWO4 and ZnWO4. The lattice parameters of Ca‐doped ZnWO4 samples obtained from X‐ray powder diffraction data confirmed this finding. It is also shown that ZnWO4 retains the monoclinic wolframite structure when cooling, at 12 K exhibiting the following lattice parameters: a = 4.6826(2) Å, b = 5.7088(2) Å, c = 4.9230(2) Å and β = 90.541(2)°. The scintillation light yield of the Zn1−x Cax WO4 was measured using the multi‐photon counting technique and it is found that small concentrations of Ca (x = 0.001 – 0.02) cause no deterioration of this parameter. Ca doping of ZnWO4 is expected to facilitate production of a single‐crystalline scintillator. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effects of Ca doping on the electrochemical properties of LiNi 0.8Co 0.2O 2 cathode material

LiNi 0.8Co 0.2O 2 and Ca-doped LiNi 0.8Co 0.2O 2 cathode materials were synthesized via a rheological phase reaction method. It is found that the Ca doping significantly improves reversible capacity, cycling performance, thermal stability and rate capability. The Ca-doped LiNi 0.8Co 0.2O 2 cathode material maintains nearly its initial discharge capacity up to 100 cycles at room temperature. It also delivers an initial discharge capacity of 183 mA h g − 1 and still keeps 131 mA h g − 1 even after 120 cycles at 60 °C. These results, together with the X-ray diffraction and electrochemical impedance spectroscopy analysis, reveal that Ca 2+ ions occupy Li + ion sites to form Ca Li defects and lithium vacancies ( V Li′), which reduce the resistance and increases conductivity of LiNi 0.8Co 0.2O 2.

Effect of Ca doping on the optical properties of La1.2Sr1.8Mn2O7

We present x-ray-diffraction, infrared reflectivity, and Raman-scattering measurements of La1.2(Sr1.8−xCax)Mn2O7 as a function of temperature and doping (x=0.0, 0.4, 0.6, and 0.8). At room temperature, x-ray-diffraction data show that the replacement of Sr ions with smaller Ca ions causes a shrinkage of the unit-cell volume. The far-infrared conductivity spectra of all doped samples are typical of an insulator, showing only phonons. When doped with Ca on Sr and a corresponding systematic reduction in Curie temperature (Tc), there is a suppression of the low-frequency optical spectral weight. For the x=0.0 compound, the Raman-active phonon associated with the internal stretching of the oxygen atoms in MnO6 octahedra shows a noticeable hardening below Tc. A polaron transport can be attributed to the observed frequency shifts. Interestingly, this hardening is reduced as the Ca content increases. These observations suggest that Ca doping disrupts the effects of the double-exchange interaction on the lattice degree of freedom, thereby accounting for the characteristic changes in the phase diagram.

Effects of Ca doping on the Curie temperature, structural, dielectric, and elastic properties of Ba0.4Sr0.6−xCaxTiO3 (0⩽x⩽0.3) perovskites

The effects of Ca doping on the Curie temperature, structural, dielectric, and elastic properties of Ba0.4Sr0.6−xCaxTiO3 (0⩽x⩽0.3) has been studied. Powder x-ray diffraction revealed that the cubic lattice constant a decreases linearly with increasing x from 0 to 0.15, while showing an anomalous expansion between x=0.15 and x=0.18. This, together with the anomalies in the dielectric constants, Curie temperature TC, and elastic constants observed for 0.15&amp;lt;x&amp;lt;0.18, suggests that a small amount of Ca ions substituted for Ti ions. Correlated with the evolution of a with x, TC increases linearly with increasing x from 0 to 0.15; while deviating from linear behavior for x&amp;gt;0.15, TC increases persistently up to x=0.25 and thereafter shows a decrease. These variations of TC with x have been interpreted in terms of Ca-doping-induced A-site cation size variance, a substitution of a small amount of Ca ions for Ti ions, and structural phase separation. Upon cooling the longitudinal elastic constant CL shows drastic softening near TC, arising from the electrostrictive coupling between the polarization fluctuations and the elastic strains. Moreover, it was found that Ca doping induces the hardening of CL just below TC, and the magnitude of the relative hardening of CL (i.e., ΔCL∕CL) increases with increasing x, implying that the tetragonal ferroelectric phase was increasingly stabilized by Ca doping.

Effect of Ca doping for Y on structural/microstructural and superconducting properties of YBa2Cu3O7−δ

We report systematic studies of structural, microstructural and transport properties of Y1−xCaxBa2Cu3O7−δ bulk samples with 0.20⩾x⩾0.0. The partial substitution of Ca2+ at Y3+ site in YBa2Cu3O7−δ leads towards slightly overdoped regime, which along with disorder in CuO2 planes is responsible for decrease in superconducting transition temperature (Tc) of the doped system. The microstructural variants being studied by transmission electron microscopy (TEM) technique in imaging and selected area diffraction modes revealed an increase in the density of twins with increase in Ca concentration despite the fact that there is slight decrease in the orthorhombicity. This is against the general conviction of decrease in twin density with decreasing orthorhombicity. An increase in twin density results in sharpening of twin boundaries with increasing Ca concentration. These sharpened twin boundaries may work as effective pinning centers. A possible correlation between microstructural features and superconducting properties has been put forward and discussed in the present investigation.

Ca-doping effects on N-type ferrimagnetism of NdVO 3

Ca 2+-doping effects were studied on the N-type ferrimagnet of NdVO 3. The chemical pressures by Ca 2+-doping induced lowering of ferrimagnetic transition temperature T c and compensation temperature θ c, resulting in the phase transition from N- to P-type ferrimagnetic phase. In the N-phase, spontaneous magnetization M sp becomes zero at finite temperature θ c and in the P-phase, M sp is positive in whole temperature range. It was revealed that NdVO 3 and Ca 0.1Nd 0.9VO 3 located in the N-phase and Ca 0.2Nd 0.8VO 3 in the P-phase. This N→P transition by the chemical pressure was discussed by the intra- and inter-sublattice exchange integrals estimated from the molecular field approximation.

The effects of space charge, dopants, and strain fields on surfaces and grain boundaries inYBCO compounds

Statistical thermodynamical and kinetically-limited models are applied to study the origin andevolution of space charges and band-bending effects at low-angle [001] tilt grain boundaries inYBa2Cu3O7 and the effects of Ca doping upon them. Atomistic simulations, using shell models ofinteratomic forces, are used to calculate the energetics of various relevant point defects. Theintrinsic space charge profiles at ideal surfaces are calculated for two limits of oxygen contents,i.e. YBa2Cu3O6 andYBa2Cu3O7. At one limit,O6, the system is aninsulator, while at O7 it is a metal. This is analogous to the intrinsic and doping cases of semiconductors. The siteselections for doping calcium and creating holes are also investigated by calculating theheat of solution. In a continuum treatment, the volume of formation of doping calcium atY-sites is computed. It is then applied to study the segregation of calcium ions to grainboundaries in the Y-123 compound. The influences of the segregation of calcium ions onspace charge profiles are finally studied to provide one guide for understanding theimprovement of transport properties by doping calcium at grain boundaries in the Y-123compound.

The structure and superconductivity of Y 1− xCa xBa 2Cu 3O 7− δ coatings on YSZ single crystal substrate

It is reported that Ca-doping can improve the weak links between grain boundaries and increase the critical current density J c of YBa 2Cu 3O 7− δ polycrystalline films [Phys. Rev. B 58 (1998) 9543; Nature 407 (2000) 162]. In this paper, we investigated the preparation and superconducting properties of YBa 2Cu 3O 7− δ (YBCO) and Y 1− x Ca x Ba 2Cu 3O 7− δ ( x = 0.05, 0.1, and 0.2) (YCBCO) coatings on yttrium stabilized zirconia (YSZ) single crystal substrates by powder melting growth process. The X-ray diffraction (XRD) indicated that: (1) the coatings were of YBa 2Cu 3O 7− δ (Y123) phase with c-orientation; (2) the Y 2BaCuO 5 (Y211) phase remained clearly in the YBCO coating, but much less in the YCBCO coatings. The effect of Ca-doping on the formation of the Y211 phase was analyzed with the help of the differential thermal analysis (DTA). The results of R– T and I– V relationship at 77 K demonstrated that the zero resistance temperature T c0 decreased with the Ca content, while the critical current density J c (77 K) was improved for x = 0.05.

The effect of Ca doping on the superconductivity of NdBa 2Cu 3O y oxides

Two series of single phase triple-perovskite superconducting samples with nominal compositions of (Nd 1− x Ca x )Ba 2Cu 3O y and Nd(Ba 2− x Ca x )Cu 3O y are prepared by the solid-state reaction method. It is found that both the T c and the orthorhombicity of the unit cell for all the samples decrease with increasing Ca content. T c is also correlated to the distance of Cu(2)–Cu(2) along the c-axis, which decreases linearly with increasing Ca doping.