Combustion Synthesis of PrBa2Cu3O7−δ and YBa2Cu3O7−δ cuprate materials

PrBaCuO and YBaCuO cuprate materials were prepared from cooper, barium peroxide, and yttrium/praseodymium oxide by SHS and standard solid-state synthesis. SHS reactions were carried out using relatively large cooper particles (< 63 im) to obtain small product samples (13 mm in diameter). High ambient temperature was used to stabilize a combustion front in the ignited pellets. Explored was the effect of cooper particles size, starting density, and ambient temperature on phase evolutions in synthesized materials.     

New versatile synthesis for low dimension superparamagnetic YBa2Cu3O7−x nanoparticles

This paper describes the synthesis and characterization of YBa₂Cu₃O_7−x (YBCO) nanoparticles obtained through an environmentally friendly chemistry approach. Y-, Cu- acetates and Ba trifluoroacetate were used for the synthesis of the precursor gel. Moreover, sucrose and pectin reagents were added as chelating agents inducing the formation of small size oxide nanoparticles. The thermal decomposition process of the precursor powder was investigated by thermal analysis correlated with mass spectrometry. The chemical nature, structure and morphology of the particles were investigated by X-Ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Fourier Transform Infrared Spectroscopy. According to XRD analysis the nanoparticles have an orthorhombic structure and the average diameter between 18–30 nm, additionally confirmed by TEM measurements. The superparamagnetic behavior at room temperature of the YBCO nanoparticles has been clearly evidenced by magnetization measurements. Furthermore, the effect of the annealing atmosphere on the magnetic properties has been studied.     

Preparation of Y1−x Yb x Ba2Cu3O7−y superconducting films by chemical vapor deposition

High Tc Y_1−x Yb _x Ba₂Cu₃O_7−y films were prepared on SrTiO₃(100) substrates by chemical vapor deposition method. Yb₁Ba₂Cu₃O_7−y films were obtained at higher oxygen partial pressure compared with Y₁Ba₂Cu₃O_7−y films at the same deposition temperature. Tc,o (R=0) decreased about 1.5 K when Y was fully substituted with Yb. The caxis lattice parameter of Y_1−x Yb _x Ba₂Cu₃O_7−y films also decreased as the amount of Yb(x) increased.     

Flux pinning mechanisms of (YBa2Cu3Oy-d)1−x/(Dy2O3)x superconductors (x=0.1 and 0.5 wt%)

Polycrystalline (YBa₂Cu₃O_y-d)_1?x/(Dy₂O₃)_x superconductor samples were produced through the solid-state reaction route and by adding amounts of x = 0.1 and 0.5 wt% of Dy₂O₃ nanoparticles (NP-Dy₂O₃ with a size of about 10 nm) during the second stage of heat treatment. The structural, microstructural, critical current density and pinning properties were investigated using x-ray diffraction, scanning electron microscope, and DC magnetization at various temperatures ranging from 77 down to 10 K and under an applied magnetic field varying between ?6 and +6 Tesla. Both samples crystallize in the orthorhombic structure. It was found that the Dy₂O₃ nanoparticles reside in the grain boundaries. Although the sample with 0.5 wt% NP-Dy₂O₃ is characterized by a low zero resistive temperature of about 78 K that is close to the useful temperature for technological applications, it showed the highest J_c values and the excellent flux pinning capacity. The addition of an appropriate amount of NP-Dy₂O₃ up to 0.5 wt% extends the single bundle pinning regime and retards the liquid vortex regime.     

Intergranular properties of polycrystalline YBa2Cu3O7−δ superconductor added with nanoparticles of WO3 and BaTiO3 as artificial pinning centers

Pure (x = 0.0 wt%) superconducting YBa₂Cu₃O_7−δ (YBCO) sample and added YBCO sample with 0.1 wt% artificial barium titanate (BTO) and 0.1 wt% tungsten trioxide (WO₃) nanoparticles were prepared using the solid-state reaction route. Phase purity was analyzed via the X-rays diffraction technique. Scanning electron microscopy showed a high density of isolated W-rich secondary phases embedded within the YBCO added sample. Furthermore, both WO₃ and BTO nanoparticles tend to reside at the grain boundaries and play the role of bridges connecting the YBCO superconducting granules. Quantitative analysis performed on the areas where nanosized entities induced by BTO and WO₃ phases was evidenced by EDX analysis equipped with SEM instrument. The values of H_c2 increased from 1.6 T for pristine to 3.4 T for BTO/WO₃ added samples. The superconducting parameters determined by AC susceptibility measurements also showed an improvement with WO₃ and BTO nanoparticles co-addition. The value of J_cinter(0) increases from 1.18 kA/cm² for the pristine sample to 5.10 kA/cm² for BTO/WO₃ co-added sample. Hence, the incorporation of artificial BTO and WO₃ nanoparticles into the YBCO superconducting phase could be a useful way to make such compounds available in practical applications.     

Fabrication and characterization of La2Zr2O7 films on different buffer architectures for YBa2Cu3O7?δ coated conductors by RF magnetron sputtering

La₂Zr₂O₇ (LZO) films were grown on different buffer architectures by radio frequency magnetron sputtering for the large-scale application of YBa₂Cu₃O_7−x (YBCO)-coated conductors. The three different buffer architectures were cerium oxide (CeO₂), yttria-stabilized zirconia (YSZ)/CeO₂, and CeO₂/YSZ/CeO₂. The microstructure and surface morphology of the LZO film were studied by X-ray diffraction, optical microscopy, field emission scanning electron microscopy, and atomic force microscopy. The LZO films prepared on the CeO₂, YSZ/CeO₂, and CeO₂/YSZ/CeO₂ buffer architectures were preferentially c-axis-oriented and highly textured. The in-plane texture of LZO film on CeO₂ single-buffer architecture was ∆ φ = 5.5° and the out-of-plane texture was ∆ ω = 3.4°. All the LZO films had very smooth surfaces, but LZO films grown on YSZ/CeO₂ and CeO₂/YSZ/CeO₂ buffer architectures had cracks. The highly textured LZO film grown on CeO₂-seed buffered NiW tape was suitable for the epitaxial growth of YBCO film with high currents.     

CaO-MgO-Al2O3-SiO2 corrosion behavior of air-plasma-sprayed (LaxYb1−x)2Zr2O7

The CaO-MgO-Al₂O₃-SiO₂ (CMAS) corrosion of thermal barrier coatings (TBCs) is a crucial problem for the lifetime of blades and vanes of jet engine and gas turbine at high operating temperature. Although many new alternative materials for TBCs have been developed in recent years, their application is limited by the CMAS corrosion. On the other hand, the composition difference of CMAS between regions makes solving this problem very difficult. Therefore, in this study, the yearly composition changes of sand-dust around Beijing area were investigated. The high-temperature corrosion behavior of air-plasma-sprayed 8YSZ and newly developed (La_xYb_1−x)₂Zr₂O₇ TBCs by the representative sand-dust of Beijing was investigated. In comparison, a universally used CaO-riched composition of simulated silicate deposit was also adopted for the TBCs corrosion test. It is found that the (La_xYb_1−x)₂Zr₂O₇ TBCs performs much better anti-corrosion behavior than that of 8YSZ, both by Beijing sand-dust and simulated one. Particularly, Yb₂Zr₂O₇ TBCs appear to be the optimum material against silicate deposits attack. The mechanism of silicate deposits corrosion has also been discussed.     

Enhanced superconducting properties of YBa2Cu3O7−δ thin film with magnetic nanolayer additions

Vertically aligned nanocomposite (VAN) (La_0.7Sr_0.3MnO₃)_0.5(CeO₂)_0.5 and pure La_0.7Sr_0.3MnO₃ layers were incorporated into YBa₂Cu₃O_7−δ (YBCO) thin films as bilayer stacks for magnetic flux pinning enhancement. The films show high epitaxial quality, suggested by XRD and TEM study. The critical temperature T_c of the bilayers is about 90 K, which is close to that of pure YBCO films, while both the self-field J_c^sf and in-field critical current density J_c^in-field are largely enhanced. Among all samples, the film with VAN cap layer shows the highest J_c values in all field ranges. This study demonstrates an effective way towards the tunable pinning effect for YBCO coated conductors by both defect and magnetic pinning.     

Electrical,structural and thermal properties of nanoceramic (Bi2O3)1−x−y(Ho2O3)x(Tm2O3)y ternary system

Ho₂O₃ and Tm₂O₃ doped Bi₂O₃ composite electrolyte type materials for solid oxide fuel cells (SOFCs) operating at intermediate-temperature were investigated. The bismuth-based ceramic powders were produced by using conventional solid-state synthesis techniques. The products were characterized by means of scanning electron microscopy (SEM), X-ray powder diffraction (XRD), differential thermal analysis/thermal gravimetry (DTA/TG), and the four-point probe technique (4PPT). XRD and DTA/TG measurements indicate that all of the samples have the stable fluorite type face centered cubic (fcc) δ-phase. 4PPT measurements were performed in the temperature range 150–1000 °C in air and these measurements showed that the electrical conductivity of the samples decrease with increasing amount of Tm₂O₃. This increase in the electrical conductivity of the samples could be attributed to the increase in the numbers of highly polarizable cations and oxide ion vacancies. The highest conductivity value was found as 5.31×10^?1 Ω cm^?1 for the (Bi₂O₃)_1?x?y(Ho₂O₃)_x(Tm₂O₃)_y ternary system (for x=20 and y=5 mol%) at 1000 °C. The activation energies of the samples were calculated from log σ graphics versus 1000/T. These calculated results showed that the translation motion of the charge carriers, oxygen vacancies, and space charge polarizations are responsible for the change in activation energy as a function of temperature.     

Varistor and electrical properties of MgO.(Fe2O3)1−x(Bi2O3)x ceramics

In this study, MgO.(Fe₂O₃)_1−x(Bi₂O₃)_x (x = 0.01, 0.02, 0.04, 0.08) samples were prepared by the conventional ceramic process. Microstructure studies revealed that the samples contain MgFe₂O₄ grains surrounded by insulating Bi₂O₃-rich phases. DC electrical resistivity of the samples was increased by Bi₂O₃ content up to 1.1 MΩ.cm for the sample with x = 0.08. Current density- electric field investigations suggested that the samples with x = 0.01, 0.02 and 0.04 exhibited varistor properties. The sample with x = 0.01 showed excellent varistor properties with a non-linear coefficient of 45 and a threshold electric field value of 160 V/cm. Variation of D.C electrical conductivity versus temperature indicated that the activation energy values for the conduction were increased by Bi₂O₃ content from 0.334(5) eV to 0.857(5) eV. A.C electrical conductivity spectra of the samples obey the universal power law and the charge transport mechanism is based on electron hopping via sudden translational motion between the ferric and ferrous ions.     

Improvement of epitaxial growth and flux pinning of MOD-derived YBa2Cu3O7−δ nanocomposites films by self-seeding and multi-element doping strategies

YBa₂Cu₃O_7?δ (YBCO) nanocomposite films were fabricated by low-fluorine metal-organic deposition (LF-MOD) route, combining self-seeding and multi-doping strategies with elements of Zr, Hf, Sn and Mn. Under the same growth conditions, due to the self-seeding layer as a template for the c-axis growth of doped films, all the multi-doped films with 200 nm self-seeding layers exhibit good c-axis orientation, even for the heavy doping of 16 mol%. EDS confirmed the four doping elements in the multi-doped YBCO films appear independently, leading to higher density of nano-scale heterogeneous phases than the case of Zr single doping. The pinning force density (F_p) and critical current density (J_c) are improved significantly, which indicates that the optimal doping content is achievable as high as 12 mol% in the case of multi-doping. The multi-12% YBCO films have the highest Fp,max of 6.13 GN/m3 at 77 K, which is about 3.3 times higher than that of the undoped films, and the Jc can reach 4.3 MA/cm2 at 30 K, compared to 3.1 MA/cm2 for the undoped films.     

Fabrication of GdBa2Cu3O7−ẟ fine patterns by “chemically modified self-photosensitive” photolithography

In this study, GdBa₂Cu₃O_7−ẟ (GdBCO) sol with ultraviolet (UV) sensitivity was prepared using Gd(OAC)₃, Ba(OAC)₂, and Cu(OAC)₂ as the starting materials, methanol as the solvent, and 2,2′-bipyridine as the chemical modifier. Furthermore, chelation mechanism of the chemical modifier and metal ions in the sol was analyzed by infrared and UV absorption spectroscopy. Cu²⁺ in the sol was found to chelate with the chemical modifier to form a photosensitive chelate, rendering UV sensitivity to the sol. Next, the photosensitive chelate was decomposed by exposing it to UV light, and the decomposition product was barely soluble in organic solvents. By exploiting the self-sensitivity of GdBCO, fine patterns of its films were obtained (without photoresist throughout) with micron scale resolution. XRD and superconductivity performance tests revealed that compared to the GdBCO film without fine patterns, fine-patterned GdBCO fabricated herein does not exhibit significant degradation in terms of its crystal structure and superconductivity. This result indicates that fine-pattern technology developed for chemical film formation is simple in terms of process and equipment, and it does not affect the performance of the GdBCO film. Therefore, “chemically modified self-photosensitive” photolithography can be applied for the preparation of the GdBCO-based inorganic microstructure, which exhibits specific significance for the research and application of high-temperature superconductor electronic devices. In addition, during heat treatment, the technology of first introducing carbon dioxide and then water vapor also was adopted in this paper, which reduced the surface roughness of the GdBCO film.     

Fabrication of tunable bandgap epitaxial β-(AlxGa1−x)2O3 films using a spin-coating method

β-(Al_xGa_1−x)₂O₃ films have several critical properties of interest to the research community, including a wide bandgap that may be used in the development of new electronic, optoelectronic, and photonic devices. Here we demonstrate the first time fabricated metal-alkoxide-based spin-coated single-phase epitaxial β-(Al_xGa_1−x)₂O₃ films on c-sapphire substrates with orientation and good crystallinity that is comparable to the films fabricated using other film deposition techniques, such as molecular beam epitaxy and chemical vapor deposition. Using this technique, we generated films with broad Al compositions (x) of 0.3, 0.5, and 0.7 with bandgap energies of 5.15, 5.56, and 6.16 eV, respectively, estimated from the X-ray photoelectron spectroscopy inelastic energy-loss spectra. Photoluminescence emission spectra in the ultraviolet and visible (blue) wavelength range highlighted several intrinsic defects in the film structure that functioned as luminescence centers, including self-trapped exciton and recombining donor-to-acceptor band. Detailed analysis of the structural and optical properties of β-(Al_xGa_1−x)₂O₃ epitaxial films revealed that this low-cost and scalable solution-deposition approach coupled with a spin-coating technique could be used to fabricate β-(Al_xGa_1−x)₂O₃ films with tunable properties.     

Low temperature Mössbauer spectroscopic study of CaO(Fe2O3)1−x(Al2O3)x system

Low temperature ⁵⁷Fe Mössbauer spectroscopic studies on CaO(Fe₂O₃)_1−x(Al₂O₃)_x, written as CF_1−xA_x, have been carried out. In view of the earlier findings based on room temperature data, the spectra have been analysed by assuming three Fe sites: two belonging to CF phase and the third site for the Fe atoms distributed in the CA phase. The Fe sites pertaining to the CF phase experience hyperfine field of about 49T and 47T, respectively, at 20K; whereas the third site is paramagnetic at this temperature.     

Preparation and thermoelectric properties of Bi2SexTe3−x bulk materials

Bi₂Te₃ and Bi₂Se₃ nanoplates were synthesized by a microwave-assisted wet chemical method, and Bi₂Se_xTe_3−x (x=1, 2, 3) bulk nanocomposites were then prepared by hot pressing the Bi₂Te₃ and Bi₂Se₃ nanoplates at 80 MPa and 723 K in vacuum. The phase composition and microstructures of the bulk samples were characterized by powder X-ray diffraction and field-emission scanning electron microscopy, respectively. The electrical conductivity of the Bi₂Se_xTe_3−x bulk nanocomposites increases with increasing Se content, and the Seebeck coefficient value is negative, showing n-type conduction. The absolute Seebeck coefficient value decreases with increasing Se content. A highest power factor, 24.5 µWcm⁻¹ K⁻², is achieved from the sample of x=1 at 369 K among the studied samples.     

Fabrication and phase transition of La2−xLuxZr2O7 transparent ceramics

A series of transparent ceramics with the composition of La_2−xLu_xZr₂O₇ (x = 0−2.0) were prepared by solid-state reactive sintering in vacuum. With the increase of Lu content (x), phase transition from pyrochlore to defective fluorite occurred and a two-phase region existed in the range of x = 0.6−1.2. Grain sizes of the pyrochlore phase dominated samples (x < 0.5) were 11−14 μm, and that of the defective fluorite phase dominated samples were larger than 60 μm. However, grain sizes of the samples in the two-phase region were smaller than 3 μm. The La_0.8Lu_1.2Zr₂O₇ ceramic with the smallest grain size (∼2.5 μm) reached a highest in-line transmittance of 72.4% at 1100 nm among all the samples.     

The influence of inter-granular coupling on the magnetic susceptibility and critical current density of YBa2(Cu1−xAgx)3O7−δ superconductors prepared by ethylene glycol assisted solvothermal approach

Series of YBa₂(Cu_1−xAg_x)₃O_7−δ (0≤x≤0.09) powders were prepared by ethylene glycol assisted solvothermal approach. X-ray diffraction verified the formation of single phase crystal structure for all the investigated samples, implying the complete solubility of the silver ions into YBa₂Cu₃O₇. The partial substitution of the Ag atoms into the Cu sites caused a decrease of the lattice constant (c) and an increase of the lattice constants, a, and, b, to preserve the orthorhombic crystal structure of YBa₂Cu₃O₇. The scanning electron microscopy depicted that the incorporation of silver ions into copper sites adjusts the random orientations of the YBCO grains and suppresses the formation of grain boundary. The AC magnetic susceptibility measurement revealed that the superconducting performance of the YBa₂Cu₃O₇ is preserved at all Ag concentrations. The critical current density, J_c, measurement revealed that the addition of the Ag into the YBCO caused a strong pinning force is established due to the strong coupling among the grains instead of the weak Josephson junctions and in turn a remarkable increase of the zero field J_c from 2.7 MA/cm² to 9.1 MA/cm² was achieved.     

Microwave dielectric properties of (1−x)ZnTa2O6−xMgNb2O6 ceramics

Single phase MgNb₂O₆ and ZnTa₂O₆ powders were synthesized by solid-state method, and the high quality factor composite ceramics of (1?x)ZnTa₂O₆?xMgNb₂O₆ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25 and 1.0) were prepared using the as-synthesized powders. The microwave dielectric properties, microstructure, phase transition and sintering behavior of the composite ceramics were investigated. The X-ray diffraction analysis revealed that solid solution between ZnTa₂O₆ and MgNb₂O₆ phases appeared in the composite ceramic. SEM results show that the grain sizes of the composite ceramics increased with increasing x values. The temperature coefficient of resonant frequency of (1?x)ZnTa₂O₆?xMgNb₂O₆ composite ceramics reaches near-zero of 1.02 ppm/°C with ε_r=35.58 and a high quality factor of 65500 GHz when x=0.20 and sintered at 1350 °C for 2 h.