Current dependent reorganization in superconducting Y<Subscript>1</Subscript>Ba<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-δ</Subscript>

We present strong non-linear dynamic responses developing due to magnitude and type of driving current in bulk polycrystalline superconducting Y₁Ba₂Cu₃O_7-δ sample at zero magnetic field. Several novel types of dynamic changes induced by the transport current were observed via the time evolution of the voltage (V - t curves). The physical observations appearing in V - t curves were interpreted mainly with the reorganization of driving current in a multiply connected network of weak-link structure. It was found that such a dynamic process could cause an enhancement or suppression in superconducting order parameter due to the magnitude of the driving current and coupling strength of weak link structure together with the chemical and anisotropic states of the sample. It was shown that the general behavior of decays evolving in V - t curves is consistent with an exponential relation which is analogous to the glassy state relaxation.     

Current-induced voltage oscillations in superconducting Y<Subscript>1</Subscript>Ba<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-δ</Subscript>

We investigated influence of bidirectional square wave current with long periods and dc current on the evolution of the voltage-time (V - t) curves in superconducting polycrystalline bulk Y₁Ba₂Cu₃O_7-δ (YBCO) material at the temperatures near the critical temperature. In a well-defined range of amplitudes and periods of driving current, and temperatures, novel type of non-linear dynamic responses was observed by means of the V-t curves. It was seen that such a non-linear response to bidirectional square wave current sometimes reflects itself as regular sinusoidal-like voltage oscillations. The sinusoidal-like and non-sinusoidal oscillations were discussed mainly in terms of the dynamic competition between pinning and depinning and significant relaxation effects which appear in this competing process. The density fluctuations associated with the current induced self-magnetic flux (SMF) lines and semi-elastic coupling of SMF lines with the pinning centers were also considered as possible physical mechanisms in the interpretation of the experimental results.     

Promising performances for a La<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>Co<Subscript>0.8</Subscript>Fe<Subscript>0.2</Subscript>O<Subscript>3-δ</Subscript> cathode with a dense interfacial layer at the electrode-electrolyte interface

As for the commonly studied La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (6428), here, a very low area-specific resistance (ASR) was measured for La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (6482) cathode deposited on a Ce_0.9Gd_0.1O_2-δ (GDC) electrolyte with addition of a thin (1 μm) dense LSCF film deposited by spin coating at the interface between the GDC electrolyte and a 40-μm-thick screen-printed electrode. The ASR ranged from 1 Ω.cm² at 500 °C, 0.11 Ω.cm² at 625 °C and value as low as 0.03 Ω.cm² at 700 °C. Impedance spectra collected in between 500 and 700 °C were carefully studied. They could all be modelled with two R//CPE in series which are likely associated to the oxygen reduction reaction itself (dissociation/adsorption/ionization) at low frequency and to the oxide ion transfer at the electrode/electrolyte interface at high frequency.     

Preparation of dense La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>Ga<Subscript>0.8</Subscript>Mg<Subscript>0.2</Subscript>O<Subscript>3-δ</Subscript> with high ionic conductivity by solid-state synthesis

The results of a systematic investigation on the effects of processing steps, via solid-state reactions, on structural phase characteristics and ionic conductivity of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ solid electrolyte are reported. The main purpose of this work is to establish an optimized route for obtaining good densification and high ionic conductivity of this solid electrolyte. Processing routes with three successive calcinations at 1250 °C followed by attrition milling (R1), and with two sequences of calcination at 1350 °C with intermediary attrition milling (R2) give rise to near full density at 1450 °C sintering temperature. The rate of grain growth is fast when the relative density reaches 95%. Elemental mapping reveals uniform distribution of the constituents in the matrix along with La₄Ga₂O₉, LaSrGa₃O₇ and sub-micrometer MgO grains at grain boundaries. The ionic conductivity of grains remains unchanged with the processing route and sintering profile. The blocking effect of charge carriers at grain boundaries decreases with increasing the dwell temperature.     

Surface of underdoped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-</Subscript><Subscript>δ</Subscript> as revealed by STM/STS

We performed scanning tunneling microscopy and spectroscopy on untwinned crystals of underdoped YBa₂Cu₃O_{7- δ} at δ = 0.4. A comprehensive statistical analysis of our topographic data indicates a doping dependent cleaving behavior of this material. We find in particular that at δ = 0.4 the material primarily cleaves in multiples of one unit cell along the c-axis with a high corrugation of the topmost layer. Our data suggest that the low temperature cleaving mainly results in a disruption of the CuO chain layers involving a redistribution of the layer atoms onto the two cleaving planes. In a few instances, fractional step heights (in terms of the c-axis lattice constant) are observed as well. Scanning tunneling spectroscopy reveals that such fractional steps connect surfaces which differ significantly in their tunneling conductance.     

XRD,impedance, and Mössbauer spectroscopy study of the Li<Subscript>3</Subscript>Fe<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> + Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> composite for Li ion batteries

The synthesis procedure of the Li₃Fe₂(PO₄)₃?+?Fe₂O₃ composite is presented. The monoclinic (A type) and hematite phases were detected by X-ray diffraction after the synthesis of the composite. The structural α–β (at a temperature of 460 K) and β–γ (at a temperature of 523 K) phase transitions in the composite were indicated by the anomalies of the electrical conductivity, dielectric permittivity, and changes of activation energies of conductivity. Two phase transitions have been detected in the Li₃Fe₂(PO₄)₃?+?Fe₂O₃ composite by ⁵⁷Fe Mössbauer spectroscopy: the phase transition in Li₃Fe₂(PO₄)₃ from the paramagnetic to antiferromagnetic phase at temperature T _N?=?29.5 K and the Morin phase transition in Fe₂O₃ at temperature T _M?=?235 K.     

Preparation,structure and properties of La<Subscript>0.67</Subscript>Pb<Subscript>0.33</Subscript>(Mn<Subscript>1-x</Subscript>Co<Subscript>x</Subscript>)O<Subscript>3-δ</Subscript>

La_0.67Pb_0.33(Mn_1-xCo_x)O_3-δ ceramics with x=0, 0.01, 0.03, 0.06, 0.1 and 0.15 have been prepared in a two-step procedure. Precursor gels were made by the wet chemical malic acid method. The gels were calcined and then converted into ceramics by heat treatment at 950 °C and 1000 °C in air. X-ray diffraction showed that the compounds were phase pure. The crystal structure symmetry of the compounds was confirmed to be rhombohedral (space group R3̄c) for the whole investigated range of x. All compounds undergo a paramagnetic–ferromagnetic phase transition between 335 K and 225 K. The basic magnetic characteristics such as the Curie temperature , the paramagnetic Curie temperature θ, the effective magnetic moment and the saturated magnetization decrease with increasing Co doping. The ferromagnetic transition is accompanied by an anomaly in the electrical resistance for all compounds. The high-temperature insulator–metal transitions () do not coincide with the relevant . A large magnetoresistance peak of about 15% was observed for all compounds at . PACS 72.80.Ga; 75.47.Lx; 75.60.Ej     

Large electrostrictive strain in lead-free Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–BaTiO<Subscript>3</Subscript>–KNbO<Subscript>3</Subscript> ceramics

In the present work, (1−x)(0.935Bi_0.5Na_0.5TiO₃–0.065BaTiO₃)–xKNbO₃ (BNT–BT–KN, BNT–BT–100xKN) ceramics with x ranging from 0 to 0.1 were prepared by the conventional ceramic fabrication process. A large electrostrictive coefficient of ∼10⁻² m⁴ C⁻² is obtained with the composition x ranging from 0.02 to 0.1, which is close to the well-known electrostrictive material Pb(Mg_1/3Nb_2/3)O₃. Under an electric field of 4 kV/mm, the electrostrictive strain can reach as high as 0.08%. Besides, the electric field induced strain behavior indicates a temperature independent behavior within the temperature range of 20 to 150°C. The large electrostrictive strain is suggested to be ascribed to the formation of non-polar (NP) phase developed by the KNbO₃ substitution, and the high electrostrictive coefficient of BNT–BT–KN ceramics makes them great candidates to be applied in the new solid-state actuators.     

Mössbauer study of 1% <Superscript>57</Superscript>Fe doped ferromagnetic insulator La<Subscript>0.825</Subscript>Ca<Subscript>0.175</Subscript>MnO<Subscript>3</Subscript>

We have employed magnetization measurements, M?ssbauer and ESR spectroscopic techniques, in order to study the ferromagnetic insulating (FMI) compound La_1-xCa_xMnO₃ (x=0.175) doped with 1% ⁵⁷Fe. We have used two samples; one prepared in air which has cation vacancies and a second in inert atmosphere, which is stoichiometric. An abrupt change of the experimental results is obtained, by all techniques, in the ferromagnetic insulating regime, in the temperature region of T_O/O//≈60 K, where an orbital rearrangement is suggested to occur. An analysis of these findings points to an orbital rearrangement transformation. Ferromagnetic resonance reveals considerable differences between stoichiometric and cation deficient samples, indicating anisotropy of the exchange interactions in the former sample with significant temperature dependence, most pronounced in the vicinity of T_O/O//     

Magnetic and magnetoresistive properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Sr<Subscript>2</Subscript>FeMoO<Subscript>6–δ</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoheterostructures

A method has been developed for fabricating nanoporous matrices based on anodic aluminum oxide for the deposition of ferromagnetic nanoparticles in them. The modes of deposition of strontium ferromolybdate thin films prepared by the ion-plasma method have been worked out, and the magnetic and magnetoresistive properties, structure, and composition of the films have been investigated. It has been revealed that the microstructure and properties of the strontium ferromolybdate films deposited by ionplasma sputtering depend on the deposition rate and the temperature of the substrate. Based on the measurement of the electrical resistivity of nanoheterostructures in a magnetic field, it has been found that the magnetoresistance reaches 14% at T = 15 K and B = 8 T, which is due to the manifestation of tunneling magnetoresistance.     

Cluster spin glass and superparamagnetism in RuSr<Subscript>2</Subscript>Eu<Subscript>1.5</Subscript>Ce<Subscript>0.5</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10-</Subscript><Subscript>δ</Subscript>

We investigate in detail the dc magnetization and nonlinear ac susceptibility behavior of the superconducting ferromagnet RuSr₂Eu_1.5Ce_0.5Cu₂O_{10- δ} (Ru1222) to develop a comprehensive understanding of the spin glass and superparamagnetism in this material. The structural properties of the system result in the formation of magnetic (ferromagnetic) clusters of different sizes, shapes and properties. The magnetic clustering of the system leads to observation of various features in dc magnetization and ac susceptibility consistent with superparamagnetism and cluster spin glass states, which can coexist or stand alone, depending on the temperature range considered. Experimental results of magnetic measurements in combination with their analysis have enabled us to explain and distinguish these phenomena, as well as to propose a temperature dependent scenario of the system behavior.     

Electrical switching in the MoO<Subscript>3</Subscript>–WO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> and MoO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

High field electrical switching on blown films of MoO₃(60%)–P₂O₅(40%), MoO₃(50%)–WO₃(10%)–P₂O₅(40%), and MoO₃(45%)–WO₃(15%)–P₂O₅(40%) having different thicknesses was studied and compared. Switching was observed using two terminal samples. S-type current–voltage characteristic (current-controlled negative resistance—CCNR) with memory was observed in molybdenum–phosphate glasses, but N-type characteristic (voltage-controlled negative resistance—VCNR) with threshold in tungsten–molybdenum–phosphate glasses was observed. The important observation was that with the addition of WO₃ to binary MoO₃–P₂O5 led to a change of I–V characteristic from CCNR with memory to VCNR with threshold. The measurements of density and molar volume showed linear relation between MoO₃ content and density which decreased with the increase of MoO₃ content. The samples’ thickness had no significant effect on threshold voltage. The attained results also indicated that the electrode material had no effect on switching property of devices. The switching behavior of the devices did not show any dependence on the polarity of the applied voltage. In terms of the effect of heat on the switching behavior of molybdenum–phosphate glasses, it was found that threshold voltage decreases with increasing of temperature. Finally, the switching phenomenon was explained by thermal (formation of crystalline filaments) and electronic models.     

Structural properties of composite cathode material LiFePO<Subscript>4</Subscript>–Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Composite cathode material LiFePO₄–Li₃V₂(PO₄)₃ is synthesized through a chemical reduction and lithiation using FeVO₄·xH₂O as both iron and vanadium sources. The structural properties of LiFePO₄–Li₃V₂(PO₄)₃ are investigated. X-ray diffraction results show the composite material containing olivine type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ phases. High-resolution transmission electron microscopy and energy-dispersive X-ray spectrometry results indicate that mutual doping effects take place between the LiFePO₄ and Li₃V₂(PO₄)₃ particles with V³⁺ doping the LiFePO₄ while Fe²⁺ dopes the Li₃V₂(PO₄)₃. LiFePO₄–Li₃V₂(PO₄)₃ nanocomposites are formed in the carbon webs. There is no structural compatibility between monoclinic (Li₃V₂(PO₄)₃) and olivine (LiFePO₄) domains in composite material LiFePO₄–Li₃V₂(PO₄)₃.     

Calculation of refractive indices for the (NH<Subscript>2</Subscript>CH<Subscript>2</Subscript>COOH)<Subscript>2</Subscript> · HNO<Subscript>3</Subscript> crystal

The refraction R of the diglycine nitrate (DGN) crystal, (NH₂CH₂COOH)₂ · HNO₃, in the para-and ferroelectric phases has been calculated in the model of noninteracting diatomic chemical bonds of the elementary unit cell of the crystal on the basis of the longitudinal and transversal polarizabilities of these bonds. The calculated magnitudes of the principal refractive indices n _p , n _m , and n _g and the orientations of the optical indicatrix of the crystal agree satisfactorily with experimentally observed values. Introducing the coefficient of Lorenz-Lorentz interaction x into the corresponding formula permits better agreement of the calculated and experimental refractive indices of DGN crystal to be obtained. The temperature changes of these x coefficients upon the ferroelectric phase transition in the DGN crystal have been analyzed.     

The effect of strontium doping on densification and electrical properties of Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>2−<Emphasis Type="Italic">δ</Emphasis></Subscript> electrolyte for IT-SOFC application

The effect of strontium doping on densification and ionic conductivity of gadolinium-doped ceria (GDC) was investigated. Doped (Sr-GDC) and un-doped GDC green specimens were subjected to dilatometric measurements to evaluate their sintering behavior and to identify the sintering temperature regimes. XRD spectra show the crystal structure of the sintered samples to be cubic. Strontium doping has exhibited a relatively larger grain size as is evident by the microstructural characterization. AC impedance analysis exhibited a threefold increase in ionic conductivity for Sr-GDC (0.072 S/cm) in comparison to GDC (0.028 S/cm) samples which can be attributed to improved density and increased grain size, resulting in enhancement of total conductivity. Additionally, strontium doping to GDC lattice not only increases the oxygen vacancies but also decreases the lattice binding energy, leading to increase in oxygen ion mobility which is also confirmed by the lower activation energy exhibited by the Sr-GDC formulation. Our experimental results established that co-doping is very effective in identifying new materials with remarkably high ionic conductivity with substantial reduction in the cost for solid oxide fuel cell application.     

Synthesis and characterization of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>·LiMn<Subscript>0.33</Subscript>Fe<Subscript>0.67</Subscript>PO<Subscript>4</Subscript>/C cathode materials

A new polyanionic cathode material, Li₃V₂(PO₄)₃·LiMn_0.33Fe_0.67PO₄/C for lithium-ion batteries, was synthesized using a sol-gel method and with N,N-dimethyl formamide as a dispersion agent. The analysis of electron transmission spectroscopy and X-ray diffraction revealed that the composite contained two phases. The material has high crystallinity with a grain size of 20–50 nm. The valence states of Mn, V, and Fe in the composite were analyzed by X-ray photoelectron spectroscopy. The electrochemical kinetics in Li₃V₂(PO₄)₃ is effectively enhanced by the incorporation of LiMnPO₄ and LiFePO₄, via structure modification and reduced Li diffusion length. The Li₃V₂(PO₄)₃·LiMn_0.33Fe_0.67PO₄/C materials displayed high rate capacity and steady cycle performance with discharge capacity remained 148 mAh g^?1 after 50 cycles at the rate of 0.2C. In particular, the composite exhibited excellent reversible capacities, with the values of 157, 134, 120, 102, and 94 mAh g^?1 at charge/discharge 0.2, 0.5, 1, 2, and 5C rates, respectively.     

Temperature dependence of the critical current of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> films

The temperature dependence of the critical current of YBa₂Cu₃O_7?δ films is studied experimentally. The performed analysis allows separating two components of the critical current owing to pinning of vortices both on defects in the volume of the superconductor and on oxygen vacancies in the CuO₂ planes. The established temperature dependences of components make it possible to correctly describe the behavior of the total critical current in the studied temperature range from 4.2 K to the irreversibility temperature.     

Meissner effect in the underdoped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6+<Emphasis Type="Italic">δ</Emphasis></Subscript> HTSC phase

The dependence of the superconducting (Meissner) phase volume V _m on the YBa₂Cu₃O_6+δ doping level was studied. It was found that V _m monotonically decreases as the doping level is lowered and vanishes at the same value of δ ～ 0.3 as T_c does. It was concluded that the T_c decrease and the increase in the pseudogap formation temperature T* as the doping level is lowered are caused by a decrease in the average size of superconducting clusters. This conclusion suggests an extraordinary superconductivity mechanism in HTSC.     

A PEG-assisted rheological phase reaction synthesis of 5LiFePO<Subscript>4</Subscript>⋅Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C as cathode material for lithium ion cells

5LiFePO₄⋅Li₃V₂(PO₄)₃/C composite cathode material is synthesized by a polyethylene glycol (PEG)-assisted rheological phase method. As a surfactant and dispersing agent, PEG can effectively inhabit the aggregation of colloidal particles during the formation of the gel. Meanwhile, PEG will coat on the particles to play the role of carbon source during the sintering. The samples are characterized by X-ray diffraction (XRD), scanning electron microscopy, and electrochemical methods. XRD results indicate that the 5LiFePO₄⋅Li₃V₂(PO₄)₃/C composites are well crystallized and contain olivine-type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ phases. The composite synthesized at 650 °C exhibits the initial discharge capacities of 134.8 and 129.9 mAh g⁻¹ and the capacity retentions of 96.2 and 97.1 % after 50 cycles at 1C and 2C rates, respectively.