Barium Cuprate Research Articles

The NQR picture of electronic phase separation in the lanthanum–barium cuprates

We report the results of Cu and La NQR investigations of the spin and charge separation in La 2− x Ba x CuO 4 compounds. For Ba doping x= 1 8 inducing the low-temperature tetragonal phase our Cu–NQR measurements revealed three inequivalent Cu sites in CuO 2 planes. In terms of the stripe-phase picture we assign them to the charged stripe, to its neighborhood and to the AF correlated regions free of holes correspondingly. The last site corresponds to the ordered Cu magnetic moment of 0.29 μ B.

Sources of flicker noise and the technology of superconducting microstripes based on yttrium barium cuprate films

It has been shown by annealing simulations of YBa2Cu3O7 epitaxial films that the predominant sources of flicker noise in superconducting microstripes in operating ranges of frequency and temperature are related to oxygen transitions in the vicinity of boundaries of the small-angle blocks. To make superconducting microstripes with a dimension of ∼1 µm the techniques of magnetron sputtering, as well as chemical and ion beam etching, were applied.

Effect of mixing three RE elements (Nd, Eu, Gd) on the REBa 2Cu 3O 7− x phase in RE–cuprate high- Tc superconductors

The relationship between average rare earth (RE) radius and the peritectic decomposition temperature was investigated through the differential thermal analysis (DTA) of mixed rare earth (Nd, Eu, Gd) barium cuprates. The peritectic decomposition temperature ( T p) of compositions within the `ternary' (Nd x Eu y Gd z )Ba 2Cu 3O 7− δ ( x+ y+ z=1), whilst maintaining a constant average RE cationic radius of 1.076 Å, was observed to be uniform. Analysis of the processed materials using powder X-ray diffraction showed the lattice parameters to be uniform also with such compositional changes. These results indicate uniform mixing of the RE elements in the RE123 phase.

Raman investigation of the metastable barium cuprate BaCu 3O 4 in thin films

Metastable BaCu 3O 4 has been synthesised by MOCVD on perovskite substrates. Strain effects lead to an epitaxial island-growth of micrometer-sized crystallites with two orientations relative to the substrate surface. They were used for a polarized single crystal analysis by micro-Raman spectrometry. The results can be used to detect BaCu 3O 4 as an impurity phase in RBa 2Cu 3O 7 thin films with a mole fraction level of less than 1%, a sensitivity which exceeds that of XRD by at least one order of magnitude.

Dependence of the microwave surface resistance on the structure and thickness of superconducting cuprate films

An investigation is made of the microwave surface resistance Rs measured at 10 GHz and 77 K, as a function of the thickness of superconducting epitaxial films of yttrium barium cuprate grown on lanthanum aluminate substrates by magnetron sputtering. Films between 200 and 1800 nm thick have low values Rs (0.3–0.5) mΩ and do not show any deterioration. The level of Rs achieved in films of comparatively large thickness is mainly attributable to the low rate of film growth (0.8 nm/min).

Cracking of rare earth barium cuprate films prepared by liquid phase epitaxy

A high growth rate of REBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (RE=rare earth elements) superconducting films is achieved with liquid phase epitaxy. The occurrence of cracks is one of the major problems for applications of these thick films. To achieve crack free films, it is necessary to understand the formation of cracks in REBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films. Investigations were carried out on YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and (Y,Pr)Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// films on NdGaO/sub 3/ substrates and on new SrPrGaO/sub 4/ substrates with tetragonal K/sub 2/NiF/sub 4/ structure (space group I4/mmm). A thermodynamic model for understanding the crack spacing together with mechanical properties determined in melt textured YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// bulk samples was applied.

Synthesis of polycrystalline BaZrO 3 coatings

Barium zirconate was prepared by liquid phase sintering at low temperatures and used as a substrate for a YBCO melt process. The liquid phase was provided by barium cuprate melt, barium peroxide melt or barium zirconate liquid (in case of plasma spraying). The infiltration resistance against the barium cuprate melt could be improved, but is not yet sufficient for application.

Phase formation and melt processing of Yb-123

The formation of Yb-123 has been studied at different temperatures in air and in reduced oxygen partial pressure. It is found that the stability and/or the formation kinetics of Yb-123 phase is a major hurdle in manufacturing phase-pure Yb-123 in air. However, under reduced oxygen partial pressure, Yb-123 forms rapidly and more than 90% phase-pure Yb-123 is achieved within three sintering steps. Rods made from this powder were melt-processed in air and showed a Tc of 90 K. Kinetic studies performed by interrupting the growth during the directional solidification of these rods revealed a growth mechanism similar to that of Y-123 and a maximum growth rate of 7.2 mm/h for a stable planar interface. EPMA of the interface showed the liquid to be rich in barium cuprates with a Ba:Cu ratio of 1:3.

Preparation of Tl-HTS-Films for Microwave Application

This paper presents the preparation of Tl2Ba2Ca2Cu3Ox thin films on 2″ LaAlO3-substrates for microwave applications and some aspects of phase formation. We found barium cuprate but no calcium cuprate as an intermediate phase before Tl2O is incorporated into the film. Steps necessary for the formation of Tl2Ba2Ca2Cu3Ox-films with c-axis orientation are outlined. Our highest quality film exhibits a surface resistance of Rs = 6.5 mΩ at 87 GHz and 77K .

Microstructural aspects of joining superconductive components using solder

Investigations of superconductive joints were performed using an infiltration technique. Gaps between two domains of were prepared mechanically and subsequently filled with powder. Barium cuprate liquid prepared by the peritectic decomposition of was infiltrated into the gap between the two domains. Microstructural analysis indicates a local epitaxy of the solder.

Effect of mixing Nd, Eu, and Gd rare earth elements on RE-cuprate high-Tc superconductors

The extent of the relationship between rare earth radius, peritectic decomposition temperature (Tp), and lattice dimensions within mixed rare earth (Nd, Eu, Gd) barium cuprate 123 superconductors has been investigated. The peritectic decomposition temperature of compositions within the “ternary” (NdxEuyGdz)Ba2Cu3O7 − δ (x + y + z = 1) material, while maintaining a constant average rare earth cationic radius of 1.076 A, was observed to be uniform. Powder X-ray diffraction analysis of processed materials also showed the lattice parameters to be uniform with these compositional changes. These results indicate a uniform mixing of the RE elements in the RE 123 phase. In contrast, the mixing of such RE elements leads to the formation of an inhomogeneous distribution of both Nd422 and RE211 second-phase particles in the matrix subsequent to melt processing.

Synthesis of Nd123 superconducting powder via oxalate coprecipitation

The preparation of pure and fine Nd123 powder was investigated in five ways. Precursor powders were generated by the oxalate coprecipitation. Some of the powders were heated in vacuum (770°C for 20 h at 10–15 Pa), followed by calcination at different temperatures and oxygen partial pressures. The vacuum step was omitted with the remaining powders. Calcination under pure oxygen requires excessively high temperatures ( T>940°C). Under these conditions the powder is badly fused, with grain sizes in the 7–12 μm range. In addition, the orthorhombicity of this powder is low due to the formation of Nd123 solid solution — implying the presence of residual BaCuO 2. The optimal route to produce the desired powder is in a low partial pressure of 350–1000 ppm oxygen in nitrogen, without a vacuum preheating step. Similar powders can be obtained in 1% O 2 with a vacuum preheating step. At 60 ppm O 2 the reaction between Nd 1+ x Ba 2− x Cu 3O 6.5+ x/2+δ and BaCuO 2 is incomplete, with about 5% of barium cuprate remaining. Finally, the T c of the powder produced at the optimal conditions was found by DC magnetization to be ∼ 99 K.

The surface chemistry of Hg-containing superconductors by HREM

The surface chemistry of Hg-containing superconductors has been investigated using high resolution electron Microscopy. It was found that the crystal surface of these materials usually decomposed in air by losing Hg, forming a thin amorphous layer containing Ba and Cu. This amorphous layer can be recrystallised into some crystallites of barium cuprate under the electron beam annealing. Such recrystallisation did not take place on the surface of Re-doped Hg-1201. Crystals decomposed heavily when they were ground in solvents. A surface dependence of the low temperature low field magnetic susceptibility of Hg-1201 was also observed.

Liquid phase epitaxy of crystalline REBa 2Cu 3O 6+δ films on reactive substrates using the BaF 2BaOCuO flux

Phase relations concerning the crystal formation of YBa 2Cu 3O 6+δ and NdBa 2Cu 3O 6+δ phases from the BaF 2BaOCuO flux have been investigated by the liquid phase epitaxial technique on various substrates. As the amount of BaF 2 in the solution increases, the temperature of REBa 2Cu 3O 6+δ (RE = Y, Nd) formation decreases down to about 920°C not only in the Y system but also in the Nd system. The decrease of the growth temperature allows us to form liquid phase epitaxial thick films on the reactive substrates such as yttria-stabilized zirconia and SrTiO 3, which rapidly react with REBa 2Cu 3O 6+δ and the melt of barium cuprate flux at high temperatures. Our studies have also revealed that the seed layer pre-deposited on the substrates plays an important role in epitaxial growth of REBa 2Cu 3O 6+δ to form thick films on these substrates.

Effect of barium cuprate on high temperature superconductors

BaCuO2 has a pronounced effect on the magnetization data of high-Tc superconductors. By subtracting the estimated magnetic contribution of BaCuO2 present in the superconducting samples, the susceptibility is seen to be purely diamagnetic.

Melt-textured YBa 2Cu 3O 7 − x wires having plate-like grains

YBCO wires which consist of well oriented plate-like fine grains are fabricated using a moving furnace to achieve higher mechanical strength. Melt-texturing experiments have been undertaken on YBCO wires with two different compositions: YBa 1.5Cu 2.9O 7− x , and YBa 1.8Cu 3.0O 7− x . Wires are extruded from a mixture of precursor powders (formed by a coprecipitation process) then textured by firing in a moving furnace. Size of secondary phases such as barium cuprate and copper oxide, and overall composition of the sample affect the orientation of the fine grains. At zero magnetic field, the YBa 1.5Cu 2.9O 7− x wire shows the highest critical current density of 1450 A cm −2 and 8770 A cm −2 at 77 K and 4.2 K, respectively. At 1 T, critical current densities of 30 A cm −2 and 200 A cm −2, respectively, are obtained at 77 K and 4.2 K. Magnetisation curves are also obtained for one sample to evaluate critical current density using the Bean model. Analysis of the microstructure indicates that the starting composition of the green body significantly affects the achievement of grain alignment via melt-texturing processes.

Polymer-metal chelate precursor reduced its firing temperature and time for preparing yttrium barium cuprate powder

Bulk YBa2Cu3Ox was prepared by a polymer chelate precursor method using poly[(N,Ndicarboxymethyl)allylamine] as a chelating polymer of which molecular weights were 3 × 104 (PDAA-L) and 3 × 105 (PDAA-H), respectively. X-ray diffraction (XRD) analysis of the precursor from PDAA-H shows that YBa2Cu3Ox(Y123) phase appeared after being calcined at 750 °C for 5 h and the mixture was completely converted to tetragonal Y123 phase after being calcined at 800 °C for 5 h. The phase evolution of the precursor from PDAA-H during isothermal experiment at 800 °C showed that pure tetragonal Y123 was produced even after the polymer chelate precursor was heated for 2 h in air, although a very small amount of BaCO3 was recognized. This BaCO3 phase was hardly recognized after 4 h calcination. The precursor prepared from PDAA-L was fully converted to pure tetragonal Y123 after 3 h calcining at 800 °C. On the other hand, the sample prepared from metal nitrate solution without PDAA was not fully transferred to Y123 phase after heating at 800 °C for 10 h. Large amounts of Y2O3, BaCO3 and CuO were observed. These results indicated that the greater homogeneity in the polymer chelate precursor leads to reduced firing times and temperature compared with the metal nitrate precursor.

ChemInform Abstract: Crystallization of Superconducting Yttrium Barium Cuprates from Peritectic Melts.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Top-seeded solution growth of YBa2Cu3O6 + d superconductive crystals

YBa2Cu3O6 + d superconductive crystals are formed by top-seeded solution growth under a temperature gradient in which the nutrient is a Y2BaCuO5 phase on the bottom and the solvent is molten barium cuprate with the cation ratio of Ba:Cu = 3:5. The crystal misorientation (< 0.2°) was determined by neutron diffraction. The superconducting transition temperature was about 90 K after appropriate oxygenation. Praseodymium-substituted crystals, (PrxY1 ± x)Ba2Cu3O6 + d, were also grown by adding PrBaO3 + BaCuO2 · 2CuO into the solution. We observed different types of growth steps on crystallographically different surfaces. The growth steps on the (001) face were formed by one lattice constant of c-axis height, while those on the (100) face were composed of macrosteps 60 nm high and microsteps about 1 nm high.

Introduction of lanthanide on the mercury site of the HgBa 2CuO 4 superconductor

This investigation of the substitution of lanthanides for mercury in the HgBa 2CuO 4+δ structure has shown that “1201” cuprates Hg 1−xLn xBa 2CuO 4+δ can be synthesized for Ln = Ce, Pr, Nd with x ≈ 0.15; whereas for Ln = Sm, Eu, Gd, lanthanide does not enter into the matrix. The XRD and HREM studies confirm the “1201”-type structure and show the statistical distribution of mercury and lanthanides within the mixed [Hg 1−xLn xO δ]- layer. In contrast to the homologous strontium phases Hg 1−xLn xSr 2CuO 4+δ, these barium cuprates exhibit superconducting properties, with T c ranging from 55 K to 83 K when as-synthesized. Annealing in a reducing atmosphere increases T c significantly, showing that the as-synthesized samples are all overdoped. The most important feature deals with the fact that the critical temperature and the narrow character of the transition for the annealed samples are improved as the size of the lanthanide cation increases, the best characteristics being obtained for the cerium phase that exhibits a T c of 90 K.