氧掺杂Y0.8Ca0.2Ba2 Cu3 Oy超导体正交-四方相变的正电子湮没实验研究

利用正电子湮没技术以及X射线衍射等实验手段对Y0.8Ca0.2Ba2Cu3Oy(y=6.32～6.84)体系进行了系统研究.结果表明,在氧含量y=(6.50±0.05)附近发生正交-四方相变.正电子寿命参数在正交-四方相变发生前后均有明显变化,说明样品内部微观电子结构在发生正交-四方相变附近区域发生了显著变化.表征样品内部电子结构整体平均效应的体寿命参数τb随y的降低而逐渐增大,表明由于氧含量降低导致了样品内部局域电子密度的降低,这和Tc随y降低而降低是一致的.认为该体系在y约为6.50附近正交-四方相变区域Tc出现平台式响应可能与相变过程中区域体系内部电子结构变化上的某种弛豫效应存在某种关联.且讨论了YBCO体系中正电子湮没机制以及它与正交-四方相变与和高温超导电性之间的关联.     

Coaxial magnetic brakes using single-domain YBCO

In coaxial magnetic brakes, the changing field produced by movement of a solenoidal magnet induces a current in the wall of a conductive tube. The interaction of the field and current leads to a repulsive force that slows the motion of the magnet. For brake applications that require high force density, melt-textured YBCO is a clear choice of material for the magnet because it can carry high currents at a given field and temperature, and is inherently capable of operating in persistent current mode. We present calculations of the performance of this type of brake as a function of magnet current density for catch tubes composed of aluminum and titanium. These results are validated with low speed (20 m/s) tests. Calculations indicate that melt-textured magnets can decelerate projectiles with a mass of 1 kg from 2000 m/s to rest in distances on the order of 10 m. This suggests that this type of brake is suitable for use in hypervelocity experiments, which sometimes requires nondestructive deceleration of projectiles for diagnostic purposes.     

Influence of crystal plane on the welding quality of YBCO bulk superconductor

The microstructure of welds parallel to the {1 0 0} and the {1 1 0} crystal planes of the single-grain YBa₂Cu₃O₇/Y₂BaCuO₅ (Y123/Y211) bulk superconductors welded by Ag foil has been studied. It is shown that a zone free of small Y211 particles is formed along the both sides of the welds. Formation of this zone is explained by higher Y content in the melt produced during heating and leaking out of the weld. The different morphology of the rest solidified liquid in the welds, which has a tendency to form films in the {1 0 0} welds and granules in the {1 1 0} welds, is associated with different surface energies of welded surfaces. Cracking suppression of the {1 1 0} welds was observed and is ascribed to higher fracture toughness for this plane. The quality of the weld is confirmed by the trapped field measurements.     

ICP-AES法测定塑料中的钙铅钡

本文叙述了 ICP-AES 法测定塑料中钙、铅、钡的过程,方法简便,RSD 为1.2～10%,回收率96～98%。     

Magnetothermal transition in YBCO melt textured materials

Conductors cut from large YBCO single domains textured by a combination of Melt Texturing Growth and Top Seeding techniques were studied for Fault Current Limiter applications. In self field, homogeneous transport critical current densities as large as 4 10⁴ and 9 10⁴ A/cm² were measured in shaped YBCO single domains up to 5 cm in length. Above J_c, experiments focused on magnetothermal transitions showed that the silver coating should have an adequate thickness so as to prevent the conductors from burning. Capability of textured materials to support high instantaneous power (40 GW/m³) and energy dissipation (190 MJ/m³) during the transition was demonstrated in a 4 T magnetic field.     

Magnetoacoustic measurements on pure and Zn doped YBCO crystals

Investigations on the temperature dependence in the range from 4 to 100 K of the ultrasonic attenuation (T, B) and the relative change Δv(T,B)/v of the sound velocity for various applied magnetic fields have been made. The ultrasonic attenuation was measured by the pulse-echo technique and the relative change of the sound velocity Δv/v by a phase-comparison method. The measurements serve for the examination of the elastic behaviour of the flux line lattice (FLL) of pure and Zn doped YBCO crystals and YBCO/Ag composites. These crystals are important for applications as permanent magnets. In the framework of the thermally assisted flux flow model (TAFF) the activation energies U(B) were deduced from the ultrasonic data.     

Grain orientation studies in YBCO superconductors by molten salt synthesis

The molten salt or the flux method is utilized in this study to fabricate a grainoriented YBa₂Cu₃O_7−x superconductor. The formation of YBa₂Cu₃O_7−x in the presence of molten salts of Na, K, Li belonging to chloride and iodide systems does not appear feasible due to the instability of the superconducting phase in these salt systems. Studies using the “green phase,” Y₂BaCuO₅, as seed crystals suggest a two-stage approach in the formation of YBa₂Cu₃O_7−x . The process uses Y₂BaCuO₅ formed by molten salt synthesis which has been observed to be stable in water and against most of the salts. The 211 crystals can be mixed with oxides and converted to 123. Analysis of X-ray data and SEM micrographs indicates a certain degree of grain orientation, which can be further enhanced by tape-casting or not-forging.     

Preparation and properties of melt-processed thick films of YBCO

A detailed study has been carried out of the successful fabrication of high current capacity, thick films of YBCO on yttria-stabilized zirconia substrates. This has involved the optimization of a number of processing parameters including material composition, processing temperatures and times, ink additives and so on. As a result of this work, thick films have been obtained with thicknesses of as little as 7μ, which are superconducting withT _c=91·5 K and have critical current densities at 77 K of almost 3×10³ Acm⁻². This current decreases rapidly with increasing magnetic field strength, reaching an almost constant value of 100–200 Acm⁻² at 0·4 T.     

Characterization of doped Nd-based superconductors

Y-, Dy-, and Gd-doped neodymium (Nd) superconductors (RE_0.1Nd_0.9Ba₂-Cu₃O_y) exhibited sharp resistivity transitions at 92 K. DC susceptibility measurements gave a common curve with diamagnetic onsets at 93 K and transitions complete at 50 K. Lattice constants for an assumed orthorhombic structure were calculated from X-ray diffraction measurements. These values were used to create a simulated diffraction pattern with 2 values and relative intensities that agreed well with observed values. Oxygen contents determined by instrumental neutron activation analysis, ranged from 14.7 to 15.1±0.1 wt%(6.59y in O _y 6.73). This range of oxygen content for YBa₂Cu₃O _y gives 50–60 K transitions in marked contrast to the 92 K (midpoint) values for the materials of this study.     

Sputtered YBCO bicrystal dc-SQUIDS

An analysis of YBa₂Cu₃O_{7 –} bicrystal dc-SQUIDs is reported. The devices have been fabricated by dc cylindrical magnetron sputtering from a single target. SQUIDs operating at the liquid nitrogen temperature have been obtained employing high-quality films fabricatedinsitu exhibiting a critical temperature as high as 89–90 K. Two different geometrical configurations for the loop inductance have been investigated. A good control of the transport parameters of the junctions, together with a reasonable correlation between the calculated inductance and the voltage response to an applied magnetic field, have been achieved. A study of the voltage response vs. the applied magnetic field as a function of both temperature and bias current has been carried out. SQUIDs working at temperatures as high as 87 K have been characterized.     

Fluctuation Magneto-Conductivity in La–Ba–Ca–Cu–O Superconductors

Temperature variation of magneto-resistivity measurements have been carried out at 1 T magnetic field on (La_1.5Gd_0.5)Ba₂CaCu₅O _z and, on pure and 5 wt.% Ag doped (La_1.6Y_0.4)Ba₂Ca_0.8Cu_4.8O _z superconductors. These data were analyzed in terms of fluctuation magneto-conductivity by using Aslamazov—Larkin and Maki—Thompson models for layered superconductors. These materials exhibit strong pair breaking effect and the typical phase breaking time at 100 K is found to be in the order of 10^?16 s. The typical values of coherence lengths along ab plane and along c-direction at 0 K are found to be 16.6 and 0.5 Å.     

液相烧结YBCO超导体材料的研究

将 YBCO 固相烧结坯料在高于1050℃温度下进行液相烧结,再在123相形成区域内缓冷,定向凝固,最后进行氧化处理。微观组织不同于传统的固相烧结样品,成分均匀的大尺寸的片状晶择优取向生长并以层状分布,定向生长的 YBCO 体材经氧化处理后孪晶密度明显提高,观察出大量的位错线。其性能在0场下大幅度提高。在77K、OT 下 J_c 大于2×10~4A/cm~2。在磁场下,下降幅度明显减少,H=1T,J_c≥6600A/cm~2;H=2T,J_c≥5800A/cm~2;H=5T,J_c≥3600A/cm~2。     

Ba源配比对MOCVD法制备YBCO薄膜性能的影响研究

采用单一液相混合源进液及闪蒸的MOCVD系统在LaAlO₃(001)单晶基片上制备YBa₂Cu₃O_7-x(YBCO)薄膜, 研究混合源中Ba含量对YBCO超导薄膜成分、结构及电流承载能力的影响。结果表明, 当Ba含量较小时, YBCO薄膜中易于形成尺寸较小的CuO颗粒; 随着Ba含量的增加, 薄膜中形成Ba₂CuO₃晶粒, 并且Ba₂CuO₃晶粒尺寸随Ba含量的增加而逐渐增大。杂质相的含量、尺寸以及与YBCO的晶格匹配程度对YBCO薄膜的双轴取向生长和电流承载能力具有重要影响。当原料摩尔配比Ba/Y=3.9时, 成功制备出了具有优异面内面外取向、结构致密的YBCO超导薄膜, 77 K下的300 nm厚度薄膜的临界电流密度达到4.0 MA/cm², 该研究结果对于第二代涂层导体的发展具有重要意义。     

白光LED用Ca2SiO3Cl2:Tb3+单一基质荧光粉的制备及性能研究

采用溶胶-凝胶法在较低温度下合成了不同浓度Tb3+掺杂的Ca2SiO3Cl2∶mTb3+单一基质白光荧光粉,并对其发光性质进行了研究。近紫外光激发下,发射光谱出现了明显的多色谱(415、440、460、486、544、595、619和700 nm)混合后发射白光。随着Tb3+浓度的增加,蓝光强度先增强后减弱,绿光不断增强,红光不断减弱,当m=0.003时荧光粉的色坐标为x=0.3174,y=0.3485,非常接近标准白光(x=0.33,y=0.33),样品呈现色温TC=6161K的正白色发光。Ca2SiO3Cl2∶Tb3+是一种具有良好白光发射的LED用单一基质荧光粉。     

Single-crystal neutron diffraction study of ferroelectric Ba0·92Ca0·08TiO3

Crystal structure of BaTiO₃ doped with 8% Ca²⁺ is refined using single-crystal neutron diffraction data and it is shown that the doped Ca²⁺ ion substitutes only at the Ba sites. The refined cell (P4 mm) parameters area=b=3·982(3) Å,c=4·003(3) Å with a finalR value of 0·02 (onF). Existence of multiple domains in the crystal is ruled out based on refinement with multidomain model.     

An equation to calculate oxygen content relating to copper oxidation level of doped cuprates

A new equation relating oxygen contenty to copper valence 2+p has been derived and proposed. Compared to the previous equation, there is a corrected function in the new equation. The corrected function depends on dopant contentx and copper oxidation levelp. As a metalM is substituted for the metal of the sample, the corrected function has a different form, and the function is discussed in detail. For each case, taking high-T _c cuprate for example, itsp is determined andy calculated by the new and previous equations. Changes in the corrected function are shown in the figures. All the discussions show that it is necessary to use the new equation to calculatey if we are to get accurate experiment results. The new equation can be widely applied to calculate the oxygen content of multicomponent doped or undoped cuprates, and the exact stoichiometries and superconducting or impurity phases of samples need not be known.     

Vibrating reed study of the flux line dissipation of ceramic and single-crystal (Ba, K)BiO3

Vibrating reed (VR) and dc and ac magnetic susceptibility measurements were performed on a nominal single crystal of composition (Ba_0.64K_0.36)BiO₃ in an applied magnetic fieldH. Field-cooling and field-sweep data revealed multiple peaks in the reed dissipation 1/Q located below the superconducting transition temperatureT _c (29.6 K forH=0). A shoulder or onset (with increasingT) of dissipation appears forT18 K, which may be a signature of a flux lattice melting transition. VR data for dense ceramic samples of composition (Ba_0.625K_0.375)BiO₃ (T _c=28.6K) exhibit a relatively narrow and smooth peak in 1/Q that corresponds well to a broad, intermediate-temperature peak in the crystal data. Resistivity experiments demonstrate that the single ceramic peak occurs well below the temperature at which the electrical resistanceR0, suggesting that the higher-temperature crystalline peaks are positioned close to the upper critical field line and may be strongly dependent upon grain size or surface properties. Both ac and dc susceptibility data show no clear evidence for multiple phases or gross compositional inhomogeneities in the crystalline sample. Our results demonstrate that the VR technique is an extremely sensitive method to probe sample inhomogeneities and their role in flux pinning phenomena.     

Calculation of the Critical Field and the Nernst Effect in Cuprate Superconductors

A recent experiment which measured the Nernst effect, the diamagnetic signal and the H _c2 field was interpreted as a support to the scenario which the pseudogap has the pair condensate without long-range phase coherence. We present here calculations which qualitatively reproduces the onset of the Nernst signal temperature T _ν (ρ) and T _c (ρ) as function of the doping level ρ. Together with our previous calculations to the magnetization and H _c2, we conclude that the phase separation scenario supports also the new Nernst effect experiments. This work has been partially supported by CAPES and CNPq.     

Microstructure of YBCO bulk superconductors with CeO2 addition

The refinement of Y₂BaCuO₅ (Y211) particles by CeO₂ in the single-grain YBa₂Cu₃O₇/Y₂BaCuO₅ (Y123/Y211) bulk superconductors of nominal composition Y_1.5Ba₂Cu₃O_x prepared by top-seeded melt-growth process with Y₂O₃ addition has been studied by polarised light microscopy. It is shown that cerium is active in the Y211 particle refinement when it is solved in the peritectic melt, L_p. Observed growth of Y211 particles during sintering stage is ascribed to higher amount of melt induced by CeO₂ reaction with Y123 phase producing barium cerate and CuO.     

Improved magnetic trapped field in thin-wall YBCO single-domain samples by high-pressure oxygen annealing

Oxygen high pressure (up to 16 MPa) has been introduced in the oxygen-annealing step necessary to make the YBa₂Cu₃O_x phase superconducting (change x from 6 to about 7). It enables a displacement in the equilibrium phase diagram towards higher temperatures, which means the possibility to achieve the same final oxygen content as the one at low temperature, but with the benefit of higher diffusion rates. Initial development made on thin bars (1.5–3 mm thick) has confirmed the interest of using a high pressure of oxygen. TEM observations have shown an increase of twin density associated with higher Jc. This is in agreement with other works claiming the possibility of higher Jc by twin engineering, and more precisely by twin refinement while annealing at high temperature. We report the successful application of this process without any adjustment to so-called thin-wall single-domain samples. These samples are obtained by growing a crystal on a pellet already shaped with an array of holes. The advantage is that, as far as diffusion processes are concerned, the typical length is not anymore the diameter of the sample, but the thickness of the walls between holes. The trapped field of 16 mm diameter Y123 thin-wall single-domain samples was doubled (0.6 T vs. 0.3 T at 77 K) in a rather short annealing time (about 3 days). Microstructures as well as magneto-optical observations of plain and thin-wall samples evidenced a reduction of cracks in the thin-wall samples. Improved performances were confirmed by further characterizations performed from 77 down to 20 K using the pulse-field facilities of the LNCMP at Toulouse.