Chemistry of electron doped Ln2−xCexCuO4 superconductors

The electron doped Ln_2−xCe_xCuO₄ (Ln=lanthanide) oxides have intergrowth structures consisting of superconductively active CuO₂ sheets alternating with inactive (Ln, Ce)₂O₂ fluorite layers along the c-axis. Stabilization of such intergrowth structures requires bond length matching across the intergrowth interface. The bond length matching criterion causes a monotonic decrease in the Ce solubility limit from x=0.24 to x=0.15 as the size of Ln³⁺ decreased from Ln=La_0.5Nd_0.5 to Ln=Gd. Annealing in N₂ atm of Ln_2−xCe_xCuO₄ at temperatures above 900°C creates oxygen vacancies and the number of vacancies decreases with increasing Ce content. The value of x at which a semiconductor to superconductor transition occurs in Ln_2−xCe_xCuO₄ increases with decreasing size of Ln³⁺ due to an increasing Madelung energy caused by a decreasing Cu−O bond length.     

Phase relations and homogeneity region of the hightemperature superconducting phase (Bi,Pb)2Sr2Ca2Cu3O10+d

For the nominal composition of Bi_2.27−xPb_xSr₂Ca₂Cu₃O_10+d, the lead content was varied from x < 0.05 to 0.45. The compositions were examined between 800 and 890‡C which is supposed to be the temperatue range over which the so-called 2223 phase (Bi₂Sr₂Ca₂Cu₃O_10+d) is stable. Only compositions between x < 0.18 to 0.36 could be synthezised in a single phase state. For x <0.36, a lead-containing phase with a stoichiometry of Pb₄(Sr,Ca)₅CuO_d with a small solubiliy of Bi is formed, for x > 0.18 mainly Bi₂Sr₂CaCu₂O_8+d and cuprates are the equilibrium phases. The temperature range for the 2223 phase was found to be 800 to 890‡C but the 2223 phase has extremely varying cation ratios over this temperaure range. Former single phase 2223 samples turn to multiphase samples when annealed at slightly higher or lower temperaures. A decrease in the Pb solubility with increasing as well as decreasing temperature with a maximum at about 850‡C was found for the 2223 phase.     

Enhanced superconductivity in Nd1+xBa2-xCu3O7+δby low oxygen partial pressure annealing

Previous work has shown that unlike YBa₂ Cu₃ O_7-δ (Y123), the Nd-Ba-Cu-O system exhibits a solid solution Nd_1+xBa_2-x Cu₃O_7+δ (Nd123ss) for 0.04≤ × ≤0.6.^1–3) An earlier paper showed that although the superconducting properties decrease nonlinearly for increasing x, T_c can be varied by increasing the annealing temperature without changing the low temperature oxygen soak.² The changes in microstructure and T_c with increasing x are analogous with Y123 with increasing δ except that the total hole concentration remained constant. T_c was modeled in terms of oxygen disorder resulting from Nd³⁺ atoms on the Ba sites relocating chain oxygens to anti-chain sites. The variability in T_c as a function of x and processing conditions can be explained by the number of fourfold coordinated coppers on the chain sites. In this paper, the model has been further substantiated by processing in 1% O₂. The annealing in a reduced oxygen partial pressure followed by a 450°C oxygen soak resulted in a marked increase in T_c compared to the 100% PO₂ anneal. The low PO₂ anneal favors pairing of Nd³⁺ substituting for Ba²⁺ to conserve oxygen ions, resulting in fewer disrupted fourfold-coordinated coppers thus increasing charge transfer from the planes to the chains.     

Kinetic Studies of the Interfacial Reaction of the Ba2YCu3O6+x Superconductor with a CeO2 Buffer

Interfacial reactions between the Ba₂YCu₃O_6+x superconductor and the CeO₂ buffer layers employed in coated conductors have been modeled experimentally by investigating the kinetics of the reaction between Ba₂YCu₃O_6+x films and CeO₂ substrates. At 810°C, the Ba₂YCu₃O_6+x -CeO₂ join within the BaO-Y₂O₃-CeO₂-CuO _x quaternary system is nonbinary, thereby establishing the phase diagram topology that governs the Ba₂YCu₃O_6+x /CeO₂ reaction. At a mole ratio of Ba₂YCu₃O_6+x :CeO₂ of 40:60, a phase boundary was found to separate two four-phase regions. On the Ba₂YCu₃O_6+x -rich side of the join, the four-phase region consists of Ba₂YCu₃O_{6 +x} , Ba(Ce_1−z Y _z )O_3−x , BaY₂CuO₅, and CuO _x ; on the CeO₂ rich side, the four phases were determined to be Ba(Ce_1−z Y _z ) O_3−x , BaY₂CuO₅, CuO _x and CeO₂. The Ba₂YCu₃O_6+x /CeO₂ reaction is limited by solid-state diffusion, and the reaction kinetics obey the parabolic rule, x = Kt ^1/2, where x = thickness of the reaction layer, t = time, and K = a constant related to the rate constant; K was determined to be 1.6 × 10⁻³ μm/s^1/2 at 790°C and 4.7 × 10⁻³ μm/s^1/2 at 830°C. The activation energy for the reaction was determined to be E _act = 2.67 × 10⁵ J/mol using the Arrhenius equation.     

Internal friction of superconducting YBa2Cu3O6+x wires at 200 kHz under in situ heat treatment

Using piezoelectric LiNbO₃ crystal oscillator method at 200 kHz, we have measured internal friction and modulus of YBa₂Cu₃O_6+x short wires with 1 mm-diameter and about 5mm length under in situ cyclic heating-cooling treatment at temperatures of 300–1020K. Polycrystalline YBa₂Cu₃O_6+x wires show anelastic behavior as well as orthorhombic-to-tetragonal (O-T) phase transition within present temperature range. There appears a strong internal friction peak below the O-T transition temperature, corresponding to the atomic site relaxation between oxygen and a vacancy in the Cu-O plane of oxygen defective YBa₂Cu₃O_6+x. A novel peak is observed after the cyclic heat treatments at around 700K. This relaxation is attributed to the hopping movement of oxygen in the defective local structure of YBa₂Cu₃O_6+x.     

The phase transformation of Bi-Pb-Sr-Ca-Cu-O bulk materials are rapidly melted and solidified by a CO2 laser

The phase transformation of Bi_1.7Pb_0.4Sr_1.6Ca_2.4Cu_3.6O_y bulk materials rapidly melted and solidified by a CO₂ laser with the scanning speed of 40 mm/s were investigated. Results of x-ray diffraction pattern, scanning electron microscopy and energy-dispersive x-ray analysis showed the decomposition of the high-T_c phase in the laser irradiated region. Nonsuperconducting phases such as CaO and (Sr_1−xCa_x)CuO_y were found to be in the melting zone. On the other hand, (Sr_1−xCa_x)CuO_y and 2212 phase were also found in the heat-affected zone. When the irradiated samples were treated with 835‡C for 72 h in air, the laser treated region changed into the high-T_c as a major phase, in addition to the low-T_c phase and nonsuperconducting phase. However, the high-T_c phases are piled up randomly. The transport critical-current density of the laser treated samples after annealing is lower than that of the original sintered one, i.e. at 77K and zero magnetic field.     

Powder processing of high temperature ceramic superconductor Tl2Ba2Ca2Cu3O10

We have developed a technique to produce high quality Tl₂Ba₂Ca₂Cu₃O₁₀ powders used for making superconducting wire, tape, lead, shield, and other large scale bulk applications. Starting with T1₂O₃, BaO₂, CaO, and CuO, we mix and grind these chemicals with a machine ball mill and then press the ground mixture into pellets. The pellets are sintered at about 895‡C for at least 30 h in an oxygen atmosphere. The sintered material is mainly the Tl₂Ba₂Ca₂Cu₃O₁₀ compound. To get more homogeneous superconductor powders, we pulverize the sintered material and use a magnetic superconducting material selector to separate and grade the material. Finally, the top grade material has a phase purity of <98% and a T_c(r < 0) of 123–126K.     

YBa2Cu3-xZnxO7薄膜红外探测器的研究

在对红外探测器进行理论分析的基础上,设计并研制了液氮温度下的Yba2Cu3-xZnxO7薄膜红外探测器,系统地测试了器件的特征参数.最好的结果为:对于波长为10μm,调制频率为f=500Hz,带宽为Δf=1Hz的红外输入辐射Rv(500,10,1)=3587V/W,NEP(500,10,1)=6.5×10-12W/Hz1/2,D*(500,10,1)=7.2×1012cmHz1/2/W,τ(500,10,1)=1.2ms.     

Nanoscale analysis of YBa2Cu3O7−x/La0.67Ca0.33MnO3 interfaces

The structure of interfaces in superconducting/ferromagnetic YBa₂Cu₃O_7−x/La_0.67Ca_0.33MnO₃ superlattices has been analyzed by scanning transmission electron microscopy and high spatial resolution electron energy loss spectroscopy. Individual layers are flat over long lateral distances. The interfaces are coherent, free of defects, exhibiting no roughness, and are located at the BaO plane of the superconductor. Concerning chemical disorder, EELS measurements show the absence of measurable chemical interdiffusion within experimental error bars.     

Large Thermoelectric Power of La2−x Sr x CuO4 Possessing Two-Dimensional Electronic Structure

In order to gain deep insight into the origin of the large thermoelectric power and metallic electrical conduction observed for La_2−x Sr _x CuO₄ possessing a two-dimensional layered structure and the consequently obtained two-dimensional electronic structure, we investigate the energy–momentum dispersion and the lifetime of quasiparticles by using high-resolution angle-resolved photoemission spectroscopy. By calculating the thermoelectric power using the Bloch–Boltzmann theory and the experimentally obtained information about the electronic structure and electron scatterings, we found that the large thermoelectric power of La_2−x Sr _x CuO₄ is mainly brought about by the anisotropic electron scattering caused by the strong electron correlation, and that the two-dimensional electronic structure makes almost no important contribution to the large S(T) except for rather minor effect of the van Hove singularity near the Fermi level.     

Superconducting Properties of (M x /YBa2Cu3O7−δy )N Multilayer Films with Variable Layer Thickness x

The superconducting properties of (M _x /YBa₂Cu₃O_7−δy )_N multilayer films were studied for varying layer thickness x. Different M phases were examined including green-phase Y₂BaCuO₅ (211), Y₂O₃, BaZrO₃, CeO₂, SmBa₂Cu₃O_7−δ (Sm123), brown-phase La₂BaCuO₅ (La211), and MgO. Multilayer (M _x /YBa₂ Cu₃O_7−δy )_N structures were grown by pulsed laser deposition onto SrTiO₃ or LaAlO₃ single-crystal substrates by alternate ablation of separate YBa₂Cu₃O_7−δ (123) and M targets, at temperatures of 750°C to 790°C. The x layer thickness was varied from 0.1 nm to 4.5 nm, and the y 123 layer thickness was kept constant within a given range of 10 to 25 nm. Different M phase and x layer thicknesses caused large variations of the microstructural and superconducting properties, including superconducting transition (T _c), critical current density as a function of applied magnetic field J _c(H), self-field J _c(77 K), and nanoparticle layer coverage. Strong flux-pinning enhancement up to 1 to 3x was observed to occur for M additions of 211 and BaZrO₃ at 65 to 77 K, Y₂O₃ at 65 K, and CeO₂ for H < 0.5 T. BaZrO₃ had a noticeably different epitaxy forming smaller size nanoparticles ∼8 nm with 3 to 4x higher areal surface particle densities than other M phases, reaching 5 × 10¹¹ nanoparticles cm⁻². To optimize flux pinning and J _c (65 to 77 K, H = 2 to 3 T), the M layer thickness had to be reduced below a critical value that correlated with a nanoparticle surface coverage <15% by area. Unusual effects were observed for poor pinning materials including Sm123 and La211, where properties such as self-field J _c unexpectedly increased with increasing x layer thickness.     

Microstructural study of the effect of an excess of Y2BaCuO5 and BaSnO3 doping on the texturing process of YBa2Cu3O7−x bulk superconductors

Melt texture process of YBCO leads to a YBa₂Cu₃O_7−x matrix where Y₂BaCuO₅ particles are observed. The Y₂BaCuO₅ inclusions size and distribution depend upon several parameters: YBa₂Cu₃O_7-x grain size in the presintered, heating rate, dopants. The influence of an excess of Y₂BaCuO₅ and/or BaSnO₃ on these Y₂BaCuO₅ particles are observed in the liquid phase and in the texture domain. According to the dopant used, two kinds of coarsening of Y₂BaCuO₅ can be observed: an isotropic and an anisotropic. The control of the distribution of Y₂BaCuO₅ particle size is of primary interest to improve the efficiency of the MTG process. In particular large cooling rate (5°C/h) during the texture formation could be used by adding Y₂BaCuO₅ + BaSnO₃ to YBa₂Cu₃O_7−x composition.     

Effect of Hf and Hf-Ca substitution on the superconductivity of GdBa2Cu3O7 − δ

The structural and superconducting properties of (Gd_1−x−yCa_yHf_x)Ba₂Cu₃O_z samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen content measurements. The effect of increasing Hf concentration in (Gd_1−xHf_x)Ba₂Cu₃O_z lowers the oxygen content and decreases T_c which is attributed to hole filling by Hf. The substitution of Ca for Gd in (Gd_0.85−yCa_yHf_0.15)Ba₂Cu₃O_z provides proper matching between the ionic radius and valence of Gd³⁺ (0.94 Å) and the average ionic radius and valence of Hf⁴⁺ (0.78 Å) and Ca²⁺ (0.99 Å). As the Ca content increases, the T_c increases from 81 K (y = 0.05) to 86.5 K (y = 0.20, compensated oxide), closer to the value of 91 K for pure GdBa₂Cu₃O_7−δ due to the balance between the hole filling by Hf and hole doping by Ca. A comparative study of Hf doped samples of (R_1−xHf_x)Ba₂Cu₃O_z (R = Y, Er, Gd) indicates that the magnetic moment carried by R-ion plays an important role in the suppression of superconductivity and T_c.     

Structural and Spectroscopic Characterization of Nominal Yb3+:Ca8La2(PO4)6O2 Oxyapatite Single Crystal Fibers Grown by the Micro‐Pulling‐Down Method

Yb‐doped Ca₈La₂(PO₄)₆O₂ (CLPA) single crystals with the apatite‐type structure and having <0001> orientation were grown by the micro‐pulling‐down (μ‐PD) method. The apatite structure is represented by the monophased field of Ca₈(La_2–xYb_x)‐(PO₄)₆O₂ (CLYPA) where it is assumed that 2 Ca²⁺ sites are substituted by La³⁺ and Yb³⁺ cations. Its monophased range was found to be from x = 0.0 to 0.2. The segregation of Yb³⁺ in CLPA single crystals and the maximum Yb³⁺ concentration are discussed. The crystallinity was studied using X‐ray rocking curve analysis. Absorption, emission and fluorescence decay studies of Yb³⁺ ions in CLPA were also carried out both at low temperature and room temperature. Spectroscopic data reveal Yb³⁺ ion occupation within different crystallographic sites of the apatite‐type structure. The potential for a diode‐pumped Yb³⁺ laser is evaluated.     

Processing of large YBa2 Cu3Ox domains for levitation applications by a Nd1+x Ba2−x Cu3 Oy-Seeded melt-growth technique

YBa₂Cu₃O_x domains for levitation applications have been produced by a seeding technology that includes Nd_1+x Ba_2−x Cu₃O_y seeds and melt-processing technologies such as conventional melt-textured growth, melt-texturing with PtO₂ and Y₂BaCuO₅ additions, and the new solid-liquid-melt-growth technology. Large domains (∼20 mm) with high levitation forces (F₁ up to 8.2 N) have been produced. The reproducibility of the results is good, and the capability of producing a large number of pellets in a single batch indicates good potential for the production of large amounts of this material.     

Studies of the La1+xBa2−xCu3Oz prepared from highly homogeneous precursors

Single phase materials of La_1+xBa_2−xCu₃O_z with x=0.0,…,0.98, were synthesized from highly homogeneous precursors, prepared by spray-drying or the polymerized complex method. These samples were characterized using X-ray diffraction (XRD), Raman spectroscopy and AC susceptibility measurements. The effects of composition, processing time, temperature and oxygen partial pressure, on the formation and decomposition of La_1+xBa_2−xCu₃O_z prepared by different methods, were investigated. We found that: T_c gradually decreased with the increase of La content; orthorhombic distortions appear at x<0.25 and x>0.7; and single phase stoichiometric La-123 ceramic samples, obtained at low oxygen partial pressure and quenched, decomposed into La-123ss (x=0.2) and BaCuO₂ after annealing at a rather low temperature, below 500°C.     

Properties and use of ln0.5(AlxGa1-x)0.5P and AlxGa1-x as native oxides in heterostructure lasers

Data are presented demonstrating the formation of native oxides from high Al composition In_0.5(Al_xGa_1-x)_0.5P (x≳ 0.9) by simple annealing in a “wet” ambient. The oxidation occurs by reaction of the high Al composition crystal with H₂O vapor (in a N₂ carrier gas) at elevated temperatures (≥500° C) and results in stable transparent oxides. Secondary ion mass spectrometry (SIMS) as well as scanning and transmission electron microscopy (SEM and TEM) are employed to evaluate the oxide properties, composition, and oxide-semiconductor interface. The properties of native oxides of the In_0.5(Al_xGa_1-x)_0.5P system are compared to those of the Al_xGa_1-xAs system. Possible reaction mechanisms and oxidation kinetics are considered. The In_0.5(Al_xGa_1-x)_0.5P native oxide is shown to be of sufficient quality to be employed in the fabrication of stripe-geometry In_0.5(Al_xGa_1-x)_0.5P visible-spectrum laser diodes.     

Passivation of Hg1-xCdxTe by photochemical native oxidation: Quantitative analysis of oxide composition

The composition of photochemically grown native oxides on Hg_1-xCd_xTe (x = 0.3) has been analyzed and depth profiled using x-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy. The oxide films were grown in either N₂O or O₂ ambients, and differences in the oxidation process were examined by varying the time and temperature of oxide growth. Under all growth conditions, oxides grown in an O₂ ambient exhibited a higher Hg concentration in the bulk oxide region when compared to N₂O grown oxides. The Hg/Te ratio of all the oxides was found to be less than the starting Hg_1-xCd_xTe substrates and, in some cases, this may be leading to an accumulation of Hg in the oxide/Hg_1-xCd_xTe interface region. For growths at higher temperatures (∼75°C), the excess Hg was seen to move from the oxide/Hg_1-xCd_xTe interface region to the oxide surface. In O₂ ambients, the Hg accumulated at the surface of the oxide whereas for growths in N₂O, it was lost to the ambient. Previous results on photochemical oxidation of Hg_1-xCd_xTe show an inverse relationship between oxide growth rate and temperature. Evidence obtained in this study from oxide compositions, depth profiles and annealing at higher temperatures, suggest that this relationship between oxide growth rate and temperature is primarily due to temperature induced differences in the oxidizing ambient, and not the result of a change in the film growth mechanism due to changing diffusion characteristics with temperature.     

Precipitation and pinning in Ca and Sr-Rich High-Tc superconducting “Bi2Sr2CaCu2O8” ceramics

From a consideration of the phase equilibrium diagram of the system Bi₂O₃-SrO-CaO-CuO, a simple annealing procedure was developed to precipitate Bi_2+xSr_2+xCuO_6+d, Sr₁₄Cu₂₄O_41−x, and Bi₂Sr₃O₆ in high-temperature superconducting Sr-rich “Bi₂Sr₂CaCu₂O₈” ceramics and Ca₂CuO₃ and a liquid in Ca-rich “Bi₂Sr₂CaCu₂O₈” ceramics. The transformation results in an increase of the critical current density of which is believed to express improved pinning properties of the superconducting crystals, in particular an increased pinning energy, which reduces the probability for thermally activated depinning. Possible pinning centers which were introduced during precipitation of the second phases are the surface of the precipitates.     

Phase relations in Cu- RO1.5- O (R < Ho,Er, Yb) and gibbs energy of formation of Cu2R2O5 (R < Ho,Er, Yb) between 1000 and 1325K

Phase relations in Cu-RO_1.5-O(R < Ho,Er,Yb) ternary systems at 1273K have been established by isothermal equilibration of samples containing different ratios of Cu:R(R < Ho,Er,Yb) in flowing air or high purity argon atmosphere for four days. The samples were then rapidly cooled to ambient temperature and the coexisting phases were identified by powder x-ray diffraction analysis. Only one ternary oxide, Cu₂R₂O₅(R < Ho,Er,Yb) was found to be stable. The chemical potential of oxygen for the coexistence of the three phase assemblage, Cu₂O + R₂O₃ + Cu₂R₂O₅(R < Ho,Er,Yb) has been measured by employing the solid-state galvanic cells,< (−) Pt, Cu₂O + Ho₂O₃+ Cu₂Ho₂O₅//CSZ//Air (Po₂< 2.12 × 10⁴ Pa), Pt (+) (−) Pt, Cu₂O + Er₂O₃+ Cu₂Er₂O//CSZ//Air (Po₂< 2.12 × 10⁴ Pa), Pt (+) (−) Pt, Cu₂O + Yb₂O₃ + Cu₂Yb₂O₅//CSZ//Air (Po₂ < 2.12 × 10⁴ Pa), Pt (+) in the temperature range of 1000 to 1325K. Combining the measured emf of the above cells with the chemical potential of oxygen at the reference electrode, using the Nernst relationship, gives for the reactions, 2Cu₂O(s) + 2Ho₂O₃(s) + O₂(g) → 2Cu₂Ho₂O₅(s) (1) 2Cu₂O(s) + 2Er₂O₃(s) + O₂(g) → 2Cu₂Er₂O₅(s) (2) and 2Cu₂O(s) + 2Yb₂O₃(s) + O₂(g) → 2Cu₂Yb₂O₅(s) (3) δΜo₂ = −219,741.3 + 145.671 T (±100) Jmol⁻¹ (4) δΜo₂ = −222,959.8 + 147.98 T(±100) Jmol⁻¹ (5) and δΜo₂ = −231,225.2 + 151.847 T(±100) Jmol⁻¹ (6) respectively. Combining the chemical potential of oxygen for the coexistence of Cu₂O + R₂O₃ + Cu₂R₂O₅(R− Ho,Er,Yb) obtained in this study with the oxygen potential for Cu₂O + CuO equilibrium gives for the reactions, 2 CuO(s) + Ho₂O₃(s) → Cu₂Ho₂O₅(s) (7) 2 CuO(s) + Er₂O₃(s) → Cu₂Er₂O₅(s) (8) and 2 CuO(s) + Yb₂O₃(s) → Cu₂Yb₂O₅(s) (9) δG‡ < 22,870.3 − 23.160 T (±100) Jmol⁻¹ (10) δG‡ < 21,261.1 − 22.002 T (±100) Jmol⁻¹ (11) and δG‡ < 17,128.4 - 20.072 T (±100) Jmol^-1 (12) It can be clearly seen that the formation of Cu₂R₂O₅R < Ho,Er,Yb) from the component oxides is endothermic. Further, Cu₂R₂O₅(R < Ho,Er,Yb) are an entropy stabilized phases. Based on the results obtained in this study, the oxygen potential diagram for Cu-R-O(R < Ho,Er,Yb) ternary system at 1273K has been composed.