Glassy carbon - A promising substrate material for pulsed laser deposition of thin Li1+xMn2O4−δ electrodes

The spinel LiMn₂O₄ is a promising candidate for future battery applications. If used as a positive electrode in a battery, the charging capacity of such a battery element is limited by the formation of a solid electrolyte interphase like layer between the electrolyte and the spinel. To study the electrolyte-electrode interaction during electrochemical cycling, spinel thin films are deposited as model electrodes on glassy carbon substrates by pulsed laser ablation. The obtained polycrystalline oxide thin films show a well defined surface morphology and are electrochemical active. Adhesion of these thin films on glassy carbon is in general poor, but can be improved considerably by a surface pretreatment or adding a thin metallic coating to the substrate prior deposition. The best adhesion is obtained for films deposited on argon plasma pretreated as well as Pt coated glassy carbon substrates. During the electrochemical characterization of Li_1.06Mn₂O_3.8 thin film electrodes, no additional reactions of the substrate are observed independent of the used electrolyte. The best cycle stability is achieved for films on Pt coated glassy carbon substrates.     

Role of growth temperature and oxygen partial pressure on the structural and electrical properties of pulsed laser deposited La1−xSrxnO3−δ thin films

The paper presents the fabrication and characterization of La_0.65Sr_0.35MnO_3−δ (LSMO) polycrystalline thin films deposited directly on Si (1 0 0) substrates using pulsed laser deposition technique. Various deposition parameters like substrate temperature and oxygen partial pressure have been varied systematically to obtain stoichiometric, crack-free films with smooth surface morphology having nearly monodisperse grain size distribution. The substrate temperature variation from 600 to 800 °C had profound effects on the microstructure and topography of the deposited film, with optimum result being obtained at 700 °C. The variation of partial pressure of oxygen controls the deposition kinetics as well as the stoichiometry of the film in terms of oxygen vacancy, which influences the magnetic and electrical transport properties of the manganate films. The microstructure and crystallinity of the deposited films have been studied using X-ray diffraction, scanning electron microscopy and atomic force microscopy. A correlation between the oxygen stoichiometry and micro-structural and transport properties of the deposited films has been obtained.     

Electrical conductivity and structural stability of SrCo1−xFexO3−δ

Perovskite compounds in the system of SrCo_1−xFe_xO_3−δ (x=0.2, 0.4 and 0.6) were synthesized by solid state reaction. SrCo_1−xFe_xO_3−δ shows the p-type small polaron conduction behavior. Electrical conductivity and oxygen vacancy content decrease with increase in Fe content. The incorporation of Fe increases the structural stability of SrCo_1−xFe_xO_3−δ at low temperatures, while decreasing the structural stability at high temperatures. Oxygen partial pressure has a strong influence on electrical conductivity. At low oxygen partial pressure, SrCo_0.8Fe_0.2O_3−δ will transform from cubic to orthorhombic structure. This structure can remain in 5%H₂/Ar only for a short time and then dissociates into Sr₃Fe₂O_6.64 and Co due to the reduction of B-site elements.     

Co-L3 X-ray absorption near-edge structure analysis of Pr1−xCaxCoO3−δ and Pr1−xSrxCoO3−δ

The valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ has been investigated by an analysis of the Co-L₃ X-ray absorption near-edge structure (XANES) profile. The observed intensity distributions of Co-L₃ XANES change continuously with increasing concentration of alkaline-earth ions. To investigate the origin of this change in the XANES profile, charge transfer multiplet calculations were carried out, which could successfully explain the change in the spectral profile; they also suggest that the valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ is between 3+ and 4+ and increases gradually with the concentration of alkaline-earth ions.     

High temperature transport and thermoelectric properties of Ca3−xErxCo4O9+δ

Ca_3−xEr_xCo₄O_9+δ (x=0, 0.05, 0.15, 0.3 and 0.5) samples were prepared using a sol-gel method followed by hot-pressing sintering technique. Powder X-ray diffraction analysis showed the single-phases of Ca_3−xEr_xCo₄O_9+δ were obtained up to x=0.3 and the crystallinity of the samples decreased with increasing Er-doping amount. The high-temperature (323-1073 K) thermoelectric properties of the samples were investigated. The substitution of Er³⁺ for Ca²⁺ resulted in the decrease of electrical conductivity, thermal conductivity and the increase of Seebeck coefficient for all the samples except the x=0.05 one. The dimensionless figure of merit ZT reached 0.28 at 1073 K for Ca_2.7Er_0.3Co₄O_9+δ, indicating the thermoelectric properties of CaCo₄O_9+δ can be enhanced by doping Er in the system.     

Enhanced flux pinning in pulsed laser deposited Y Ba2Cu3O7−δ : BaTiO3 nanocomposite thin films

The effect of incorporation of BaTiO₃(BTO) nanoparticles on the flux pinning properties of pulsed laser deposited YBCO:BTO thin films was studied. Substantial increase in the critical current density (J_C) and the pinning force density (F_p) of the nanocomposite thin films was observed. At 77 K, and zero applied magnetic field, the value of J_C for YBCO and YBCO:BTO (2%) thin films were 2.93 MA/cm² and 6.43 MA/cm², respectively. At the same temperature and an applied magnetic field of 4 T, the value of J_C increases from 3.6×10⁴ A/cm² for YBCO thin film to 2.7×10⁵ A/cm² for YBCO:BTO (2%) nanocomposite thin film. The study of temperature and field dependence of of YBCO and YBCO:BTO thin films indicates similar type of pinning. The lattice mismatch between YBCO and BTO seems to introduce more defects resulting in the improvement of flux pinning properties.     

The effect of La-substitution on magnetic properties of nanosized Sr1−xLaxTi0.05Zn0.2(Fe)11.75−δ(Fe)δO19 powders

A series of La-substituted M-type Sr hexaferrite powders Sr_1−xLa_xTi_0.05Zn_0.2Fe³⁺_11.75O₁₉, wherein x ranges from 0.1 to 0.5 with a step of 0.1, have been prepared by the conventional ceramic method and were then milled in a high energy mill to prepare nanosized powders. XRD investigation of the calcined and the milled powders shows that single phase hexaferrite structure has been formed after calcining and has not changed after milling. The lattice parameters and the mean crystallite sizes of the samples have been determined from the XRD data and Scherrer's formula. The results show that the lattice parameters (“а” and “c”) decrease with increase in La-substitution and the mean crystallite size of the milled powders is about 17 nm. Coercivities and magnetizations of the samples in a magnetic field of 16 kOe have been determined from the room temperature hysteresis loops. It was found that both parameters increase with La substitutions up to 0.3 and then decrease for higher substitutions. These variations were attributed to the enhancement of hyperfine field and spin-canting magnetic structure when La content increases. In addition, the magnetizations were smaller for the nanosized samples in comparison with those of bulk ones, which were discussed according to the core-shell model. Also the results show that annealing of the nanosized samples up to 500 °C can enhance coercivity and magnetization of the samples, which is discussed based on crystallite size growth.     

Studies on sintering of solid state synthesized Ba1−xKxBiO3−δ superconducting phase

Superconducting Ba_1−xK_xBiO_3−δ pellets were synthesized by solid state reaction followed by sintering. Thermo-gravimetric and differential thermo-gravimetric analysis (TG-DTA) of the mixture of nitrates was carried out to study the reactions during the phase formation. The effect of different sintering temperatures on the phase formation was studied. The X-ray diffraction data confirms the formation of superconducting Ba_0.6K_0.4BiO_2.23 phase at 700 °C. The surface morphological studies as a function of sintering temperature were studied by SEM. It is observed that the pellets prepared by solid state reaction followed its sintering at 700°C shows the superconducting transition at 26.8 K.     

Structure and magnetism in Ho1−xSrxCoO3−δ

We have magnetically and structurally characterized the Ho_1−xSr_xCoO_3−δ family of materials where 0.67≤x≤0.95. The solid solution range and evolution of the structure as a function of x is established and correlated with the broad range of magnetic behavior observed. The structure is shown to be tetragonal I4/mmm although is possibly cubic when x=0.95. For 0.67≤x≤0.9 the material shows antiferromagnetic long range order and ferromagnetic clusters. At x=0.95 the magnetic transition is at 120 K and the imaginary susceptibility becomes non-zero and the temperature of the cusp in the ac susceptibility shows a frequency dependence indicative of glassiness.     

Investigation on cation distribution and magnetic properties in self-doped manganites La0.6−xSr0.4MnO3−δ

Self-doped manganites with nominal composition La_0.6−xSr_0.4MnO_3−δ (0≤x≤0.175) have been prepared by the sol–gel method. The X-ray diffraction (XRD) patterns and magnetic measurements indicate that the samples have two phases with the ABO₃ perovskite structure being the dominant phase and Mn₃O₄ being the minor phase when the doping level x≥0.05. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions in the perovskite phases were estimated, in which there are Mn²⁺ ions and no vacancies at A sites. The ion contents have been corrected by Rietveld fitting of the powder samples' X-ray diffraction data. The change tendency of the Curie temperature T_C vs. the Mn⁴⁺ ion content ratio at the B sites of ABO₃ perovskite phase is in accord with the experimental result of the samples La_1−xSr_xMnO₃.     

Study on the structure and optical property of Zn1−xCuxO sol–gel thin films on quartz substrate

Zn_1−xCu_xO thin films (x=0, 1.0, 3.0, 5.0%) are prepared on quartz substrate by sol–gel method. The structure and morphology of the samples are investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The results show that Cu ions were effectively penetrated into the ZnO crystal lattices with substitutional and interstitial impurities to form stable solid solutions without changing the polycrystalline wurtzite structure. Two peaks at 420 nm (2.95 eV, violet), 485 nm (2.56 eV, blue) have been observed from the photoluminescence (PL) spectra of the samples. It is concluded that the violet peak may correspond to the exciton emission; the blue emission corresponds to the electron transition from the bottom of the conduction band to the acceptor level of zinc vacancy. The optical test shows that the optical band gap E_g is decreased with the increase amount of Cu doping in ZnO. The band gap decrease from 3.40 eV to 3.25 eV gradually. It is also found that the transmission rate is increased rapidly with the increase of Cu ions concentration.     

Effect of Ar+ irradiation on the electrical conductivity of BaCe0.9Y0.1O3−δ

The effects of 10 keV Ar⁺ ion irradiation on the electrical characteristics of BaCe_0.9Y_0.1O_2.95 subject to fluences of 0, 1.0 × 10¹⁷, 5.0 × 10¹⁷ and 1.0 × 10¹⁸ ions/cm² at room temperature, has been investigated using elastic recoil detection analysis (ERDA), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and alternating current (AC) impedance measurements. It was confirmed from the ERDA results that the hydrogen concentration near the surface increased with increase of Ar⁺ ion fluence. This increase may be associated with the increasing quantities of hydrogen generated by interaction between oxygen vacancies, formed by irradiation, and H₂O from exposure to air. SEM images showed clearly that the number of surface defects due to modification increased with increasing fluence. In addition, the size of the defects showed a tendency to increase with increasing fluence. From the results of XPS analyses, providing information on the electronic states on the surface, it was evident that with increase in the Ar⁺ ion fluence, the quantity of excess oxygen, such as hydroxide, increased in the oxygen 1s XPS spectrum. In addition, it was indirectly found, from decomposition of the Ce 3d, spectrum that the concentration of oxygen vacancies increased with fluence, since the percentage of Ce³⁺ also increased. Accordingly, the surface modification led to the formation of more oxygen vacancies and a greater hydrogen concentration on the surface, since the H₂O interacted with some of them. From the results of the DC conductivity and AC impedance measurements, the proton conductivity was shown to predominate over the temperature range from 473 K to 823 K. It was concluded that the increase in these protons and vacancies generated from surface modification contributed to the increase of proton conductivity.     

Structural and magnetic properties in the self-doped perovskite manganites with nominal composition La0.7Sr0.3−xMnO3−δ

Perovskite manganites with nominal composition La_0.7Sr_0.3−xMnO_3−δ (0.00≤x≤0.20) have been prepared by the sol-gel method with the highest heat treatment temperature being 1073 K. The XRD patterns indicate that when the doping level is x≤0.10 the samples have only a single phase, with the R3?c perovskite structure, while for x>0.10, the samples have two phases with the R3?c perovskite being the dominant phase and Mn₃O₄ being the second phase. A quantitative analysis and Rietveld fitting of the X-ray powder diffraction data indicate that on the basis of the thermal equilibrium theory of crystal defects there are Mn²⁺ ions at the A sites and Mn³⁺ plus Mn⁴⁺ ions at the B sites in the ABO₃ perovskite phase. The curves of magnetization versus applied magnetic field at 10 K showed that the magnetic moments of the Mn²⁺ ions at the A sites are antiparallel to those of the Mn³⁺ and Mn⁴⁺ ions at the B sites.     

Manifestation of vortex depinning transition in nonlinear current-voltage characteristics of polycrystalline superconductor Y1−xPrxBa2Cu3O7−δ

We present our recent results on the temperature dependence of current-voltage characteristics for polycrystalline Y_1−xPr_xBa₂Cu₃O_7−δ superconductors with x=0.0, 0.1 and 0.3. The experimental results are found to be reasonably well fitted for all samples by a power like law of the form V=R(I−Ic)^a(T). Here, we assume that a(T)=1+Φ₀IC(T)/2πkBT and IC(T)=IC(0)(1−T/TC)^3/2 for the temperature dependences of the power exponent and critical current, respectively. According to the theoretical interpretation of the obtained results, nonlinear deviation of our current-voltage characteristics curves from Ohmic behavior (with a(TC)=1) below TC is attributed to the manifestation of dissipation processes. They have a characteristic temperature Tp defined via the power exponent as a(Tp)=2 and are related to the current induced depinning of Abrikosov vortices. Both TC(x) and Tp(x) are found to decrease with an increase of Pr concentration x reflecting deterioration of the superconducting properties of the doped samples.     

Low-field magnetic properties of LaMnO3+δ with 0.065≤δ≤0.154

In a weak magnetic field LaMnO_3+δ exhibits at δ=0.065 below the paramagnetic-to-ferromagnetic (FM) Curie temperature, T_C, a mixed (spin-glass and FM) phase followed by a frustrated FM phase at δ between 0.100 and 0.154. The same behavior is observed in La_1−xCa_xMnO₃ with x between 0 and 0.3. This can be understood by the similar variation of the Mn⁴⁺ concentration, c between ≈0.13 and 0.34, in both materials when x or δ is increased. On the other hand, considerable differences are found between these compounds in the values of the magnetic irreversibility, in the dependencies of T_C(c) and the magnetic susceptibility, χ(c), as well as in the critical behavior of χ(T) near T_C. These differences can be explained by distortions of the cubic perovskite structure, by the reduced lattice disorder and by the more homogeneous hole distribution in LaMnO_3+δ than in La_1−xCa_xMnO₃.     

Effect of Mg doping and substrate temperature on the properties of pulsed laser deposited epitaxial Zn1?xMgxO thin films

In this paper, we report on the pulsed laser deposition of epitaxial (0002) oriented Zn_1−x Mg _x O thin films onto (0001) sapphire substrate in O₂ ambient at different deposition temperatures. Pulsed laser deposited Zn_1−x Mg _x O films showed (0002) oriented hexagonal wurtzite structure up to 34% of Mg concentration. The bandgap of Zn_1−x Mg _x O thin films is successfully tuned from 3.3 to 4.2 eV by adjusting the Mg concentration x=0.0 to x=0.34. Pulsed laser deposited Zn_1−x Mg _x O thin films were characterized by XRD, AFM, SEM, PL and UV–VIS spectrometer. We have also studied the effect of deposition temperature on to the structure, surface morphology and optical properties of Zn_1−x Mg _x O thin films.     

Effect of granularity and inhomogeneity in excess conductivity of YBa2Cu3O7−δ+xBaTiO3 superconductor

Synthesis of polycrystalline YBCO+xBaTiO₃ (x=1.0, 2.5, 5.0) superconductor has been done and the effects of granularity and inhomogeneities due to inclusions of nano-BaTiO₃ in excess conductivity are reported in this work. The phase formation, texture and grain alignments were analyzed through XRD and SEM techniques. SEM results reveal that the grain size is reduced and morphology is improved with the incorporation of nano-BaTiO₃ particles. Superconducting order parameter fluctuation (SCOPF) studies on the electrical conductivity were investigated from the resistivity vs. temperature data in the experimental domain relatively above T_c. Log(Δσ) vs. log(ε) plots show that the 2D to 3D crossover temperature (T_LD) that demarcates dimensional nature of fluctuation inside the grains is influenced by BaTiO₃ incorporation in YBCO matrix. An upward shift of T_LD in the mean field region has been observed as a consequent dominance of 3D region with increase in 1 wt% BaTiO₃ in the composites as compared to higher inclusions. It has been analyzed that microscopic inhomogeneities produced as a result of diffusion of a fraction of Ti ions into the grains affect fluctuations in the excess conductive region. The interplay of microscopic inhomogeneities produced inside the grains and mesoscopic inhomogeneities in the grain boundaries on the excess conductivity has been explained in terms of thermal fluctuations for the composites.     

Epitaxial growth of YSZ buffer layer for YBa2Cu3O7−δ coated conductor by reel-to-reel pulsed laser deposition

Yttria-stabilized zirconia (YSZ) buffer layers were deposited on CeO₂ buffered biaxially textured Ni-W substrate by reel-to-reel pulsed laser deposition (PLD) for the application of YBa₂Cu₃O_7−δ (YBCO) coated conductor and the influence of substrate temperature and laser energy on their crystallinity and microstructure were studied. YSZ thin films were prepared with substrate temperature ranging from 600 to 800 °C and laser energy ranging from 120 to 350 mJ. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to investigate how thin film structure and surface morphology depend on these parameters. It was found that the YSZ films grown at substrate temperature below 600 °C or laser energy above 300 mJ showed amorphous phase, the (0 0 1) preferred orientation and the crystallinity of the YSZ films were improved with increasing the temperature, but the surface roughness increased simultaneously, the SEM images of YSZ films on CeO₂/NiW tapes showed surface morphologies without micro-cracks. Based on these results, we developed the epitaxial PLD-YSZ buffer layer process at the tape transfer speed of 3-4 m/h by the reel-to-reel system for 100 m class long YBCO tapes.     

Synthesis and magnetic properties of Co1−xZnxFe2O4+γ nanoparticles as materials for magnetic fluid hyperthermia

Nanoparticles of the single spinel phase Co_1−xZn_xFe₂O_4+γ of mean size 3-23 nm, as determined by X-ray diffraction analysis, were synthesized by the co-precipitation method followed by a temperature treatment. Magnetic studies carried out in the range of 4.5-550 K revealed gradual transition from ferrimagnetic to superparamagnetic to paramagnetic behaviour depending on the composition and particle size. The observed behaviour indicates a broad distribution of volume sizes of the nanoparticles. Particular importance can be ascribed to the composition of x=0.6 where the observed transition temperature to the paramagnetic state at 310-334 K suggests applicability of this material for magnetic fluid hyperthermia in a self-controlled regime.