The urea combustion method in the preparation of precursors for high-TC single phase HgBa2Ca2Cu3O8+δ superconductors

An alternative sol–gel route for synthesizing precursors of mercury based superconductors is presented. This route is based on urea as a combustion agent and has been applied in the synthesis of HgBa₂Ca₂Cu₃O_8+δ (Hg-1223). Moreover, the effects of the precursor preparation on the synthesis of Hg-1223 were studied. This method is further compared with other sol–gel routes used in the synthesis of this family of compounds.     

The study of novel multi-doped spinel Li1.15Mn1.96Co0.03Gd0.01O4 + δ as cathode material for Li-ion rechargeable batteries

Novel spinel Li_1.15Mn_1.96Co_0.03Gd_0.01O_4 + δ was synthesized by high temperature solid-state reaction method. The product was identified as well-defined spinel phase by X-ray diffraction (XRD); the SEM images illustrated that the particle distribution was well-proportioned. The initial special capacity was 126.5 and 128.1 mAh g^? 1 at 25 and 50 °C. The fading rate was 0.017% and 0.098% per cycle under 0.5 °C at 25 and 50 °C, respectively. The results showed that Li_1.15Mn_1.96Co_0.03Gd_0.01O_4 + δ displayed excellent capacity and cycleability.     

Magnetic properties of ball-milled Mn nanoparticles

Nanoparticles of Mn of sizes  < 500 Å were prepared by the ball-milling technique. The temperature dependence of the magnetic susceptibility χ showed systematic variation with particle size. Peaks observed in χ were attributed to the magnetic ordering of the oxides Mn₃O₄and MnO. Peaks found in ∂(χT) / ∂T were associated with the Neel temperature ofα -Mn. We estimated that our samples contain about 0.4% of Mn₃O₄. This low concentration of Mn₃O₄was not detected by X-ray diffraction experiments but contributed significantly to the magnetization measurements.     

Synthesis and characterisation of La1−xMnO3±δ nanopowders prepared by acrylamide polymerisation

La_1−xMnO_3±δ (x=−0.02 to 0.35) nanocrystalline powders were prepared by a new sol–gel method. It is used the acrylamide gelification to form an organic 3D tangled network where a solution of the respective cations is soaked. This method was adapted to cover a broad range of high impact electro–ceramic oxides, which a particular example is the CMR nanopowders reported in this work. The acrylamide sol–gel process is a fast, cheaper and easy to scale-up method for obtaining fine powders of complex oxides. This synthesis method allows performing 100 g of highly pure nanopowders in one run with simple laboratory scale. The sponge like powder obtained consists of thin sheets composed of nanocrystallites whose size varies from 66 nm to 30 nm, depending on composition. The oxygen content of the manganite powder is shown to decrease with vacancy-doping on lanthanum site. Such a evolution can be explained for La/Mn<0.9 by considering a demixtion of the powder into La_0.9MnO₃ and Mn₃O₄ phases, while for La/Mn>0.9, the high oxygen excess leads to consider vacancies on both lanthanum and manganese sites. Both hypotheses are supported by magnetic measurements, which show a constant Curie temperature of 295 K for La/Mn<0.9, while for La/Mn>0.9, the occurrence of vacancies on manganese sites progressively impedes the ferromagnetic interactions, leading to a cluster–glass behaviour in the case of the highly manganese-deficient La_0.94Mn_0.92O₃ compound.     

A-deficit LSCF for intermediate temperature solid oxide fuel cells

A-deficit La_0.54Sr_0.44Co_0.2Fe_0.8O_3 ? δ cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) was synthesized by a citrate complexation (Pechini) route. Using La_0.54Sr_0.44Co_0.2Fe_0.8O_3 ? δ as cathode material, a superior cell performance with the maximum power density of 309, 470 and 855 mW cm^? 2 at 600, 650 and 700 °C was achieved, in contrast with the maximum power density of 266, 354 and 589 mW cm^? 2 using conventional La_0.6Sr_0.4Co_0.2Fe_0.8O_3 ? δ as cathode material at the same temperatures. The reason of this improvement was analyzed on the basis of defect chemistry. Thermal shrinkage experiment testified that the oxygen vacancies in La_0.54Sr_0.44Co_0.2Fe_0.8O_3 ? δ are more mobile than in La_0.6Sr_0.4Co_0.2Fe_0.8O_3 ? δ. Furthermore, theoretical calculation in terms of their composition and the shift of peak position in XRD pattern showed that the concentration of oxygen vacancies of La_0.54Sr_0.44Co_0.2Fe_0.8O_3 ? δ is higher than that of La_0.6Sr_0.4Co_0.2Fe_0.8O_3 ? δ. Therefore, the oxygen ion conductivity via vacancies transfer mechanism is enhanced, which induces the polarization resistance of La_0.54Sr_0.44Co_0.2Fe_0.8O_3 ? δ being decreased with a result of cell performance improved.     

Effect of the synthesis route on the structural properties and shape of the indium oxide (In2O3) nano-particles

Nano-crystalline indium oxide (In₂O₃) particles have been synthesized by sol–gel and hydro-thermal techniques. A simple hydro-alcoholic solution consisting indium nitrate hydrate and citric acid (in sol–gel method) and 1, 4-butandiol (in hydro-thermal method) have been utilized. The structural properties of indium oxide nano-powders annealed at 450 °C (for both methods) have been characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and specific surface area (SSA) analysis. Structural analysis of the samples shows cubic phase in sol–gel and cubic-hexagonal phase mixture in hydro-thermally prepared particles. The nano-particles prepared by sol–gel method have nearly spherical shape, whereas hydro-thermally-made ones display wire- and needle-like shape in addition to the spherical shape. The obtained In₂O₃ nano-particles surface areas were 23.2 and 55.3 in sol–gel and hydro-thermal methods, respectively. The optical direct band gap of In₂O₃ nano-particles were determined to be ∼4.32 and ∼4.24 eV for sol–gel and hydro-thermal methods, respectively. These values exhibit ∼0.5 eV blue shift from that the bulk In₂O₃ (3.75 eV), which is related to the particle size reduction and approaching the quantum confinement limit of nano-particles.     

Oxygen nonstoichiometry and defect structure of the double perovskite GdBaCo2O6 − δ

The results of oxygen nonstoichiometry, δ, measured by means of coulometric technique as a function of oxygen partial pressure, p_O2, in temperature range 900 ≤ T °C ≤ 1050 are presented for GdBaCo₂O_6 − δ with double perovskite structure. Partial molar enthalpy and entropy of oxygen in GdBaCo₂O_6 − δ structure were calculated. Both thermodynamic properties were shown to increase dramatically in the vicinity of the oxygen nonstoichiometry value equal to 1. The p_O2 dependences of oxygen nonstoichiometry and the δ dependences of the partial molar properties were found to have inflections when the oxygen content of GdBaCo₂O_6 − δ is equal to 5.0 exactly. The modeling of the defect structure of the double perovskite GdBaCo₂O_6 − δ was carried out by considering different reference states. Only the model based on the cubic perovskite GdCoO₃ as a reference state was shown to fit the experimental data on oxygen nonstoichiometry of GdBaCo₂O_6 − δ good enough. Equilibrium constants of the appropriate defects reactions were, therefore, determined. Concentrations of all defect species defined within the framework of this model were calculated as functions of temperature and oxygen nonstoichiometry. Oxygen vacancies were shown to be formed during p_O2 diminution in gas environment in the layers of GdBaCo₂O_6 − δ crystal lattice where they are ordered until oxygen nonstoichiometry of the oxide becomes equal to unity afterwards oxygen vacancies are formed randomly in oxygen polyhedrons.     

Spectroscopic properties of Co2+: ZnAl2O4 nanocrystals in sol–gel derived glass–ceramics

Transparent glass–ceramics containing zinc–aluminum spinel (ZnAl₂O₄) nanocrystals doped with tetrahedrally coordinated Co²⁺ ions were obtained by the sol–gel method for the first time. The gels of composition SiO₂–Al₂O₃–ZnO–CoO were prepared at room temperature and heat-treated at temperature ranging 800–950 °C. When the gel samples were heated up to 900 °C, ZnAl₂O₄ nanocrystals were precipitated. Co²⁺ ions were located in tetrahedral sites in ZnAl₂O₄ nanocrystals. X-ray diffraction analysis shows that the crystallite sizes of ZnAl₂O₄ crystal become large with the heat-treatment temperature and time, and the crystallite diameter is in the range of 10–15 nm. The dependence of the absorption and emission spectra of the samples on heat-treatment temperature were presented. The difference in the luminescence between Co²⁺ doped glass–ceramic and Co²⁺ doped bulk crystal was analysed. The crystal field parameter D_q of 423 cm⁻¹ and the Racah parameters B of 773 cm⁻¹ and C of 3478.5 cm⁻¹ were calculated for tetrahedral Co²⁺ ions.     

Defect equilibria and partial molar properties of (La,Sr)(Co,Fe)O3−δ

The oxygen nonstoichiometry δ of La_1−xSr_xCo_1−yFe_yO_3−δ (x = 0.6 and y = 0.2, 0.4) was investigated by thermogravimetry in the range 703 ≤ T/°C ≤ 903 and 1E−5 < pO₂/atm < 1. The oxygen deficit increases with increasing T and decreasing pO₂. Electronic conductivities σ were measured as a function of pO₂ in the range 1E−5 < pO₂/atm < 1 at 700 ≤ T/°C ≤ 900. At constant T, a p-type pO₂-dependence of σ is observed. Oxygen nonstoichiometry data are analyzed with regard to the enthalpy and entropy of oxidation ΔH_ox^θ and ΔS_ox^θ, as well as to the partial molar enthalpy and entropy of oxygen with respect to the standard state of oxygen (pO₂^θ = 1 atm), (h_O − H_O^θ) and (s_O − S_O^θ), respectively. For 2.67 ≤ (3 − δ) ≤ 2.79, (h_O − H_O^θ) decreases with increasing δ, while (s_O − S_O^θ) is constant within the limits of error. Defect chemical modelling was performed by an ideal solution model under consideration of three different valence states for B-site ions (Co or Fe). The dependence of σ on δ is modelled, using calculated defect concentrations as functions of δ. Deviations from the ideal behaviour suggest an immobilization of n-type charge carriers by oxygen vacancies.     

Antiferromagnetic phase transition in garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12

Ferrimagnetism has been extensively studied in garnets, whereas it is rare to find the antiferromagnet. Present work will demonstrate antiferromagnetism in the two Mn–V-garnets. Antiferromagnetic phase transition in AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ has been found, where the magnetic Mn²⁺ ions locate only on octahedral A site. The heat capacity shows sharp peak due to antiferromagnetic order with the Néel temperature T_N=23.8 K for AgCa₂Mn₂V₃O₁₂ and T_N=14.2 K for NaPb₂Mn₂V₃O₁₂. The magnetic entropy change over a temperature range 0–50 K is 13.9 J K^?1 mol-Mn²⁺-ions^?1 for AgCa₂Mn₂V₃O₁₂ and 13.6 J K^?1 mol-Mn²⁺-ions^?1 for NaPb₂Mn₂V₃O₁₂, which are in good agreement with calculated value of Mn²⁺ ion with spin S=5/2. The magnetic susceptibility shows the Curie–Weiss behavior over the range 29–350 K. The effective magnetic moment μ_eff and the Weiss constant θ are μ_eff=6.20 μ_B Mn²⁺-ion^?1 and θ=?34.1 K (antiferromagnetic sign) for AgCa₂Mn₂V₃O₁₂ and μ_eff=6.02 μ_B Mn²⁺-ion^?1 and θ=?20.8 K for NaPb₂Mn₂V₃O₁₂.     

Effect of Eu dopant concentration in SrCe1 − xEuxO3 − δ on ambipolar conductivity

The europium dopant concentration in strontium cerate was studied to achieve maximum hydrogen permeation. In order to determine high ambipolar conductivity, total conductivity and open circuit potential measurements were performed. Among the three different compositions of Eu-doped SrCe_1 ? xEu_xO_3 ? δ (x = 0.1, 0.15 and 0.2) studied, SrCe_0.9Eu_0.1O_3 ? δ showed highest total conductivity between 600 °C and 900 °C. However, transference number measurements showed increasing electronic conductivity with increasing dopant concentration and a stronger temperature dependence for electronic conduction. Therefore, the highest ambipolar conductivity was obtained over the compositional range from SrCe_0.85Eu_0.15O_3 ? δ to SrCe_0.8Eu_0.2O_3 ? δ depending on temperature. Finally, the hydrogen permeation flux was calculated based on the ambipolar conductivity and compared with experimental results.     

Particle size effects on magnetic properties of yttrium iron garnets prepared by a sol–gel method

We present detailed measurements of field—and temperature—dependence of magnetization in nanocrystalline YIG (Y₃Fe₅O₁₂) particles. The fine powders were prepared using sol–gel method. Samples with particle sizes ranging from 45 to 450 nm were obtained. We observe that the saturation magnetization decreases as the particle size is reduced due to enhancement of the surface spin effects. Below a critical diameter D_s≅190 nm, the particles become single domains and the coercive forces reaches a maximum at diameters close to the critical value. As the particle size decreases the coercivity diminishes and at D_p≃35 nm diameters the upper limit of superparamagnetic behavior is reached. A quantitative comparison of temperature and particle size dependence of coercivity shows a satisfactory agreement that is expected for an assembly of randomly oriented particles.     

Electrical conductivity of fully densified nano CaZr0.90In0.10O3 − δ ceramics prepared by a water-based gel precipitation method

Water-based gel precipitation method was first applied to synthesize high purity nano CaZr_0.90In_0.10O_3 ? δ powders suitable for fabricating dense ceramics at lower temperature. By using CaCO₃, Zr(NO₃)₄ and In(NO₃)₃ as raw materials, PEG as dispersant, CaZr_0.90In_0.10O_3 ? δ with an average particle size of about 40 nm was obtained at 850 °C, which was nearly 350 °C lower than that of traditional solid-state reaction method. Fully densified ceramics with an average grain size of 200–300 nm were obtained at 1350 °C, a temperature about 250 °C lower than that of traditional sintering techniques. Experimental results showed that the flexure stress, total, bulk and grain boundary protonic conductivities of the ceramics were more favorable than those of the ceramics fabricated at 1500 °C and 1600 °C from the powders synthesized by solid-state reaction method.     

Investigation on the microscopic features of layered oxide Li[Ni1 / 3Co1 / 3Mn1 / 3]O2 and their influences on the cathode properties

Three kinds of samples of Li[Ni_1 / 3Co_1 / 3Mn_1 / 3]O₂ were prepared respectively from direct solid-state reaction method, combustion method and co-precipitation route and their microscopic structural features have been investigated using Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), magnetic susceptibility measurement and X-ray photoelectron spectroscopy (XPS). The microscopic features such as uniform distribution of transition metal ions at 3b-site and the site-exchange ratio between lithium and nickel were found to be significantly dependent on the synthetic routes. The electrochemical properties of three samples were monitored using 2016 coin-cell by galvanostatic charge–discharge cycling test and cyclic voltammetry, which showed that the microscopic structural features are deeply related with the electrochemical performance. The obtained results also suggested that the combustion method may become a much simple alternative synthetic route to the complicate co-precipitation method.     

Performance of perovskite-related oxide cathodes in contact with lanthanum silicate electrolyte

The cathodic performance of selected mixed-conducting electrodes, including perovskite-type SrMn_0.6Nb_0.4O_3 ? δ, Sr_0.7Ce_0.3Mn_0.9Cr_0.1O_3 ? δ and Gd_0.6Ca_0.4Mn_0.9Ni_0.1O_3 ? δ, and Ruddlesden–Popper La₂Ni_0.5Cu_0.5O_4 + δ, LaSr₂Mn_1.6Ni_0.4O_7 ? δ, La₄Ni_3 ? xCu_xO_10 ? δ (x = 0–0.1) and La_3.95Sr_0.05Ni₂CoO_10 ? δ, was evaluated in contact with apatite-type La₁₀Si₅AlO_26.5 solid electrolyte at 873–1073 K and atmospheric oxygen pressure. The electrochemical activity of porous nickelate-based layers was found to correlate with the concentration of mobile ionic charge carriers and bulk oxygen transport, thus lowering in the series La₄Ni_2.9Cu_0.1O_10 ? δ > La₄Ni₃O_10 ? δ > La_3.95Sr_0.05Ni₂CoO_10 ? δ and decreasing on copper doping in K₂NiF₄-type La₂Ni_1 ? xCu_xO_4 ? δ. The relatively high overpotentials of nickelate-based cathodes, varying in the range ? 240 to ? 370 mV at 1073 K and current density of ? 200 mA/cm², are primarily associated with surface diffusion of silica from La₁₀Si₅AlO_26.5, which partially blocks the electrochemical reaction zone. As compared to the intergrowth nickelate materials, the manganite-based electrodes exhibit substantially worse electrochemical properties, in correlation with the level of oxygen-ionic and electronic conduction in Mn-containing phases. The effects of cation interdiffusion between the cell components as a performance-deteriorating factor are briefly discussed.     

Structure,optical, and magnetic properties of rutile Sn1−xMnxO2 thin films

Sn_1?xMn_xO₂ (x ≤ 0.11) thin films were fabricated by sol–gel and spin-coated method on Si (1 1 1) substrate. X-ray diffraction revealed that single-phase rutile polycrystalline structure was obtained for x up to about 0.078. Evolution of the lattice parameters and X-ray photoelectron spectroscopy studies confirmed the incorporation of Mn³⁺ cations into rutile SnO₂ lattice. Optical transmission studies show that the band gap energy (E_g) broadens with the increasing of Mn content. Magnetic measurements revealed that all samples exhibit room temperature ferromagnetism (RTFM), which is identified as an intrinsic characteristic. Interestingly, the magnetic moment per Mn atom decreases with the increasing Mn content. The origin of RTFM can be interpreted in terms of the bound magnetic polaron model.     

Influence of simultaneous doping of Sb and Pb on phase formation,superconducting and microstructural characteristics of HgBa2Ca2Cu3O8+δ

We report systematic studies of structural, microstructural and transport properties of (Hg_0.80Sb_0.2−xPb_x)Ba₂Ca₂Cu₃O_8+δ (where x = 0.0, 0.05, 0.1, 0.15, 0.2) compounds. Bulk polycrystalline samples have been prepared by two-step solid-state reaction route at ambient pressure. It has been observed that simultaneous substitution of Sb and Pb at Hg site in oxygen deficient HgO_δ layer of HgBa₂Ca₂Cu₃O_8+δ cuprate high-T_c superconductor leads to the formation of Hg-1223 as the dominant phase. Microstructural investigations of the as grown samples employing scanning electron microscopy reveal single crystal like large grains embodying spiral like features. Superconducting properties particularly transport current density (J_ct) have been found to be sensitive to these microstructural features. As for example (Hg_0.80Sb_0.05Pb_0.15)Ba₂Ca₂Cu₃O_8+δ compound which exhibits single crystal like large grains (∼50 μm) and appears to result through spiral growth mechanism, shows highest J_ct (∼1.85 × 10³ A/cm²) at 77 K. A possible mechanism for the generation of spiral like features and correlation between microstructural features and superconducting properties have been put forward.     

Thermal expansion behavior and chemical compatibility of BaxSr1−xCo1−yFeyO3−δ with 8YSZ and 20GDC

Perovskite oxides of the composition Ba_xSr_1−xCo_1−yFe_yO_3−δ(BSCF) were synthesized via a modified Pechini method and characterized by X-ray diffraction, dilatometry and thermogravimetry. Investigations revealed that single-phase perovskites with cubic structure can be obtained for x ≤ 0.6 and 0.2 ≤ y ≤ 1.0. The as-synthesized BSCF powders can be sintered in several hours to nearly full density at temperatures of over 1180 °C. Thermal expansion curves of dense BSCF samples show nonlinear behavior with sudden increase in thermal expansion rate between about 500 °C and 650 °C, due mainly to the loss of lattice oxygen caused by the reduction of Co⁴⁺ and Fe⁴⁺ to lower valence states. Thermal expansion coefficients (TECs) of BSCF were measured to be 19.2–22.9 × 10^− 6 K^− 1 between 25 °C and 850 °C. Investigations showed further that Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ is chemically compatible with 8YSZ and 20GDC for temperatures up to 800 °C, above which severe reactions were detected. After being heat-treated with 8YSZ or 20GDC for 5 h above 1000 °C, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was completely converted to phases like SrCoO_3−δ, BaCeO₃, BaZrO₃, etc.     

Synthesis and characterization of (Pr0.75Sr0.25)1 − xCr0.5Mn0.5O3 − δ as anode for SOFCs

The complex perovskite (Pr_0.75Sr_0.25)_1 − xCr_0.5Mn_0.5O_3 − δ (PSCM) has been prepared and studied as possible anode material for high-temperature solid oxide fuel cells (SOFCs). PSCM exhibits GdFeO₃-type structure and is both physically and chemically compatible with the conventional YSZ electrolyte. The reduction of PSCM resulted in structural change from orthorhombic Pbnm to cubic Pm-3m. Selected area electron diffraction (SAED) analysis on the reduced phases indicated the presence of a √2 × √2 × 2 superlattice. The total conductivity values of ∼ 75% dense Pr_0.75Sr_0.25Cr_0.5Mn_0.5O_3 − δ at 900 °C in air and 5% H₂/Ar are 9.6 and 0.14 S cm^− 1 respectively. The conductivity of PSCM drops with decreasing Po₂ and is a p-type conductor at all studied Po₂. The average TEC of Pr_0.75Sr_0.25Cr_0.5Mn_0.5O_3 − δ is 9.3 × 10^− 6 K^− 1, in the temperature range of 100–900 °C and is close to that of YSZ electrolyte. The anode polarization resistance of PSCM in wet 5%H₂ is 1.31 Ω cm² at 910 °C and in wet CH₄ at 930 °C; the polarization resistance is 1.29 Ω cm². PSCM was unstable at 900 °C in unhumidified hydrogen. Cell performance measurements carried out using graded PSCM and La_0.8Sr_0.2MnO₃ as anode and cathode respectively yielded a maximum power density of 0.18 W cm^− 2 in wet 5%H₂/Ar at 910 °C and the corresponding current density was 0.44 A cm^− 2 at 0.4 V. The activation energy for the electrochemical cell operating in wet (3% H₂O) 5%H₂/Ar fuel is 85 kJ mol^− 1.     

Effect of Fe3+ doping on structural,optical and electrical properties δ-Bi2O3 thin films

Fe³⁺ doped δ-Bi₂O₃ thin films were prepared by sol–gel method on quartz glass substrate at room temperature and annealed at 800 °C. The thin films were then characterized for structural, surface morphological, optical and electrical properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption measurements and d.c. two-probe, respectively. The XRD analyses revealed the formation δ-Bi₂O₃ followed by a mixture of Bi₂₅FeO₄₀ and Bi₂Fe₄O₉. SEM images showed reduction in grain sizes after doping and the optical studies showed a direct band gap which reduced from 2.39 eV for pure δ-Bi₂O₃ to 1.9 eV for 10% Fe³⁺ doped δ-Bi₂O₃ thin film. The electrical conductivity measurement showed the films are semiconductors.