Metastable states of the 2223 phase in the Bi(Pb)SrCaCuO system

By the magnetically modulated microwave absorption method (MAMMA) we observed the modifications induced by different calcination temperatures (between 830°C and 870°C) on the 2223 phase formed in a system sintered at (855±5)°C with the starting composition Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O _y . The presence of the 2223 phase in almost distinct states (consequently, multiple 2223 phases) in the same sample was observed. As the calcination temperature was increased up to 850°C, the highest temperature state of the 2223 phase intensified. Higher calcination temperatures resulted in the enhancement of other lower-temperature states. The homogeneity of the 2223 phase was greatly improved by annealing the samples at, 800°C for 5 min in a flowing nitrogen atmosphere. We labeled as metastable the lower-temperature states having excess oxygen, which, by easily losing the supplementary oxygen under the above annealing procedure, were shifted to higher temperatures.     

Synthesis and neutron structural studies of (Pb/Cu)-1201 superconductor

A 1201-phase superconductor (Pb_0.5Cu_0.5)SrLaCuO_5± exhibiting a zero-resistance (R=0) state at 21 K has been synthesized. Neutron Rietveld refinement of the structure shows substantial oxygen disorder in the (Pb/Cu)-O(3) monolayer and excess occupancy of oxygen at the O(3) sites. The superconducting behavior of (Pb_0.5Cd_0.5)SrLaCuO_5± , prepared under identical conditions, yields a value ofT _c (R=0) equal to 24 K. The presence of extra oxygen appears to be responsible for trapping of holes within the rock-salt (Pb/Cu)-O(3).     

On structure and stability of (Hg, Mo)-1212 superconductors based on SAED and EDS analysis

A superconductor with nominal composition Hg_0.8Mo_0.2Sr₂Y_0.5Ca_0.5Cu₆₊ withT _c up to 96 K has been isolated and carefully examined by electron diffraction and X-ray energy dispersive spectrum analyses. Its structure can be attributed to space groupP4/mmm and lattice constantsa=3.82 å andc=11.91 å. Inhomogeneous distribution of the cations was observed, which is considered to be the key to the broadening ofT _c in theR-T curve. The effect of humid atmosphere on the stability of the sample has also been investigated.     

Effect of Cd Addition on Superconducting Fluctuations and Mechanical Properties of Bi1.82Pb0.36Sr2Ca2Cd x Cu3O y System

A study was made of the effect of Cd additions on the superconducting and mechanical properties of Bi_1.82Pb_0.36Sr₂Ca₂ Cd_xCu₃O_y (x = 0.0, 0.15, 0.25, 0.35 and 0.55). Characterization of the Cd-samples using XRD, DTA, and SEM techniques, has confirmed that remarkably formation of low-T _c phase (2212) by the addition of Cd up to 0.35. High-resolution electrical resistivity ρ (T) data on the composition of Cd = 0.35 have been taken for investigating critically the superconducting fluctuations. Using the Aslamazov and Larkin (AL) and Lawrence and Doniach (LD) models of excess conductivity. Excess conductivity analysis shows that this composition (Cd = 0.35) is 2D in the temperature range 137.8–163.7 K and a 3D one below 137.8 K. Thus, a crossover from 2D to 3D is observed at 137.8 K. Sample microhardness and density are greatly improved by Cd-additions (0.35). This trend is probably due to the intercalation of cadmium between superconducting grains in compositions may provide a plastic-flow region that allows relaxation of undesirable stresses resulting from the grain anisotropy of superconductors.     

The Influence of BaZrO3 on the Magnetic Response of (Bi,Pb) : 2223 High-Temperature Superconductors

DC magnetization and AC complex susceptibility measurements on (Bi,Pb) : 2223 high-temperature superconductors impurified with various amounts of BaZrO₃ are presented. The results are discussed in the frame of the critical state model, and the values of the inter- and intragranular critical current density as well as of the field for full penetration are estimated. The values of the intergranular critical current density are consistent with those obtained from transport measurements. The intragranular critical current density and the field for full penetration have similar values from both DC magnetization and AC susceptibility measurements. It was shown that, in the (Bi,Pb) : 2223 system, BaZrO₃ impurification changes only the properties of the intergrain matrix, while the superconducting properties of the grains are not modified.     

Structure and Superconducting Properties of Cd0.8Ba2(Y0.7Ca0.4)Cu3.5Oy

A new Cd-containing superconductor with nominal composition ofCd_0.8Ba₂(Y_0.7Ca_0.4)Cu_3.5O _y was synthesized and investigated. The obtained Cd and Ca-doped 123 phase exhibits an orthorhombic (T _c=80 K) or tetragonal (T _c=65 K) modification depending on the reaction atmosphere. It was shown that the combined Cd and Ca substitution facilitates the 123 phase formation. The results of the EDX analysis, as well as the comparison of the obtained lattice parameters with those of undoped, Cd-doped, and Ca-doped 123 have shown that both Cd and Ca enter the 123 phase and form a new Cd–Ba–Y–Ca–Cu–O superconducting compound.     

Microstructure controls and their effects on the properties of Bi2Sr2Ca1Cu2Ox superconductors

This paper reports our recent progresses in the development of Bi₂Sr₂Ca₁Cu₂O _x /Ag tape conductors for the applications of magnetic field generation in liquid helium or around 20 K, using a refrigerator. We have carried out extensive work to optimize the processing parameters, investigating the relationship between the microstructure and transportJ _c. We have found that the partial melting in oxygen atmosphere is effective to have large transportJ _c with good reproducibility. The pre-annealing and intermediate rolling (PAIR) process has been successfully applied to the multilayer conductors to improve the grain alignment and transportJ _c. TheJ _c of 5×10⁵A/cm² at 4·2 K and 10 T has been achieved, which is the highest value reported so far. Two magnets fabricated by using different types of Bi-2212/Ag conductors were tested. One is a magnet designed as an insert magnet for a 18 T-class large bore Nb-Ti/Nb₃Sn superconducting magnet. The conductor of this magnet was multifilamentary tape processed by powder-in-tube method. TheI _c was 98 A in the backup field of 18 T, which generated the self field of 1·79 T. A large pancake coil was fabricated with multilayer conductor and tested under the operation of cryocooler system. The coil was stably operated up to theJ _c of the coil at the temperatures below 30 K.     

Effect of Sr and Sr-Ca substitution on the superconductivity of (Er0.76Ca0.24)Ba2(Cu2.76Co0.24)Oz

The structural and superconducting properties of (Er_1-xCa_x)Ba_2-ySr_y(Cu_2.76Co_0.24)O_z are investigated using X-ray diffraction, resistivity, ac susceptibility, and oxygen content measurements. Increasing the Sr content in (Er_0.76Ca_0.24)Ba_2-ySr_y(Cu_2.76Co_0.24)O_z lowers the oxygen content and decreases theT _c, which is attributed to the rearrangement of oxygen atoms in the basal plane. This suppression ofT _c at_{y =0.6} cannot be improved by appropriate hole doping with Ca in (Er_1-xCa_x)Ba_1.4Sr_0.6(Cu_2.76Co_0.24)O_z forx = 0.24-0.48.     

Superconducting YBa2Cu3O7-δ thick films on Ba2RETaO6 (RE = Pr5 Nd, Eu, and Dy) substrates

Thick films of YBa₂Cu₃O_7-δ fabricated on polycrystalline Ba₂RETaO₆ (where RE = Pr, Nd. Eu, and Dy) substrates by dip-coating and partial melting techniques are textured and oaxis oriented, showing predominantly (00/) orientation. All the thick films show a superconducting zero resistance transition of 90 K. SEM studies clearly indicate platelike and needlelike grain growth over a wide area of the thick films. The values of the critical current density for these thick films are ∼10⁴ A/cm² at 77 K as determined by the nonresonant R.F. absorption method. Various processing conditions that affect the critical current density of thick films are also discussed.     

Study on the Physical and Electrical Properties of Bi2 ? δZnδSr2Ca2Cu3O10 + y Glass-Ceramic Superconductors

We have fabricated a series of glass-ceramic (Bi_{2 –} Zn) Sr₂Ca₂Cu₃O_{10 + y} , where = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, and investigated the effect of Zn ions on the glass formation, crystallization, thermal, electrical, and on the magnetic properties of the BSCCO-2223 superconductor system. The structural symmetry was found to be tetragonal in all the substitution levels. The best electrical performance was obtained from the = 0 sample, the T _c and T _zero was obtained at 110 K and 107 K, respectively. The J _c values of the samples were determined using the magnetization hysteresis and Bean's model. The crystallization kinetics were investigated using nonisothermal models of Augis–Bennett. The calculated activation energy, E _a, of the system was found to be in the range of 258–336 kJ/mol.     

Superconductivity and Ferromagnetism of the Ru-1232 Compounds in the (Ru1?xNbx)Sr2(GdCe1.8Sr0.2)Cu2Oz System

The Ru-1232 compounds have been synthesized in the (Ru_1–xNb _x )Sr₂(GdCe_1.8Sr_0.2)Cu₂O _z system, and effects of Nb substitution for Ru on superconductivity and ferromagnetism of the Ru-1232 compounds have been investigated. First, X-ray powder diffraction study shows that nearly the single 1232 phase samples can be obtained in the x composition range from 0.0 to 0.3. Then, from the electrical resistivity study, it is found that each of the samples shows resistivity dropping phenomenon at two temperatures of T _c ^l and T _c ^h, which originates from superconductivity of the Ru-1232 phase and the Ru-1222 one, respectively. Both of the starting temperatures are lowering with increasing Nb content x. Lastly, from the magnetic susceptibility study, it is found that superconducting transition temperature T _c is 20 K for the Ru-1232 sample with x = 0.0 and the ferromagnetic transition temperature T _m is about 90 K. This study also shows that both of the values of T _c and T _m become low with increasing x from 0.0 to 0.3.     

Superconducting transition temperatures in the Ru-1232 system, RuSr2(Gd1? x Ln x Ce1.8Sr0.2)Cu2O z (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperatureT _c in the Ru-1232 system, RuSr₂(Gd_1−x Ln _x Ce_1.8Sr_0.2)Cu₂O _z (Ln = Sm, Dy, and Ho). At first, in the case of the samples with Ln = Sm among almost the single 1232 phase samples, the values of the superconducting onset temperatureT _co are almost the same forx=0.00−0.15, and each of the lattice parametersa andc is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample withx=0.05 shows the maximum values for both the superconducting onset temperatureT _co and the zero resistivity temperatureT _cz. Especially for the sample with Ln = Dy, the values ofT _co andT _cz are 18.5 and 6.5 K, respectively. These are higher than those of the mother sample of RuSr₂(GdCe_1.8Sr_0.2)Cu₂O _z . Moreover, from variations ofT _co, lattice parameters ofa andc in the RuSr₂(Gd_1−x Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy contentx, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.     

SrCuO2 and related high-pressure phases

Treatment at high pressures and temperatures of 6 GPa and 1120 K, typically, have proved to be useful to find new layered complex cupric oxides. Among these, SrCuO₂ crystallizing in the infinite-layer structure may be considered to be the key material not only because of its simple composition and structure but also because of the fact that interesting homologous series of high-pressure phases like Sr _n+1Cu _n O_2n+1+ and Sr _n–1Cu _n+1O _2n converge to SrCuO₂ asn increases.     

Structural commonalities of high-temperature superconductors

It is shown that all of the presently known high-temperature superconductors belong to one structural type with a basic cell of approximate 4/mmm,D _4h symmetry in tetragonal compounds andmmm,D _2h symmetry in orthorhombic compounds. In the Ba(Pb1–xBi _x )O₃ and YBa₂Cu₃O_7– compounds the basic cell is the unit cell and in the (La_1–x Sr _x )₂CuO₄, BiSrCaCuO and TIBaCaCuO compounds it is half the unit cell. The features of more than one copper oxide layer per basic cell, oxygen-free cation layers (e.g., Ca, Y) between CuO₂ layers, and slightly puckered CuO₂ planes seem to favor higher transition temperatures. The feasibility of several proposed or potentially superconducting structures are analyzed by this scheme.     

A new high-Tc superconductor containing thallium and its crystal structure: The “1212” phase (Tl1-xBix)1.33Sr1.33Ca1.33Cu2O6.667+δ

Joining YBa₂Cu₃O_6.5+ (123 phase) and Bi₄Sr₄Ca₂Cu₄O₁₆₊ (4424 phase) as structurally characterized high-T _c , superconductors, the thallium-containing superconductor (Tl_.75Bi_.25)_1.33Sr_1.33Ca_1.33Cu₂O_6.667+ with the ideal stoichiometry (Tl,Bi)₁Sr₂Ca₁Cu₂O_6.5+ (1212 phase) is reported here. As prepared from the component oxides, 1212 has an initial deviation from resistance linearity at 120 K, a superconducting onset temperature of 92 K, and zero resistance at 75 K. The tetragonal unit cell (P4/mmm, a=3.800 Å;c=12.072 Å, deduced from powder data) contains double copper oxygen sheets (like 4424 and 123) that alternate withsingle thallium-bismuth oxygen sheets (in contrast to 4424, which containsdouble bismuth oxygen sheets), resulting in a total of three stacked perovskite-like cells (as in 123). The copper oxide sheets (with intersheet spacing 3.38 Å) are separated by Ca²⁺ and the Cu oxide sheets and (Tl,Bi) oxide sheets (with spacing 4.35 Å) are separated by Sr²⁺, Ca²⁺, and excess (Tl,Bi)³⁺. The 1212 cell constitutes the building block for the centered, more complex 4424 cell. The 1212 structure persists to Bi contents as low as 1% and can also be stabilized by Pb instead of Bi; Tl cuprates also form other superconductors with lowerT _c .     

A new family of superconductors containing carbonate group

Three new oxycarbonate superconductors recently obtained in our laboratory are described. (1) Sr₂CuO₂(CO₃)_1–x (BO₃) _x (T _c = 35 K) was synthesized under ambient pressure, providing a new method for carrier doping on the layered copper oxycarbonate system. (2) Ca _n (Ca, Sr)₂Cu _n+1(CO₃)O _y (n = 1, 2, 3) was synthesized under high pressure. Superconductivity was achieved by substituting (BO₃)^3– for a part of (CO₃)^2–. (3) A new Hg system HgBa₂Sr₂Cu₂(CO₃)O₆₊ was synthesized.T _c obtained from the magnetic susceptibility is about 66 K.     

Dependency of Flux Jump on Temperature and Field-Sweep Rate for Textured (Nd1/3Eu1/3Gd1/3)Ba2Cu3O7- δ Bulk Superconductor with Gd-211 Particles

Anisotropy flux jumps in the mixed state of a textured (Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_7-δ bulk superconductor with Gd-211 doping particles have been studied by means of magnetization measurements in the model of the magnetic field paralleling and perpendicular to the c axis. A typical anisotropy flux jump was observed at a temperature ranging from 2.0 to 3.0 K. Under the magnetic field perpendicular to c axis, no flux jump was found at whole temperature range until the sweep rate of 200 Oe/sec. For the magnetic field paralleling to the c axis, the number of flux jumps decreased with the increase of temperature, and the third quadrant of the M–H curve is the most flux-instability quadrant. The magnetic field sweep rate dependences of flux jumps were also studied and the influence of flux creep on flux jumps was also discussed.     

Effect of praseodymium substitution on the growth and properties of Y1−x Pr x Ba2Cu3O7−δ (o <x < 0·4) single crystals

In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y_1−x Pr _x Ba₂Cu₃O_7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm³ and 4 × 1 × 1 mm³, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y_0·8Pr_0·2Ba₂Cu₃O_7−δ and Y_0·6Pr_0·4Ba₂Cu₃O_7−δ materials are ≈ 2 × 3 × 0·1 mm³ and 5 × 1 × 1 mm³ and ≈ 1 × 1·5 × 0·1 mm³ and 7 × 0·2 × 0·1 mm³, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (T _c) of the oxygenated crystals were found to depend on the Pr-content. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

掺钾对PMN基陶瓷有序微区及介电和电致伸缩性能的影响

使用二次合成法制备了不同钾含量的0.80Pb(Mg1/3Nb2/3)O3-0.20PbTiO3陶瓷，首先研究了掺钾对PMN基预烧粉体和陶瓷试样相组成的影响，在此基础上详细研究了掺钾对PMN基陶瓷有序微区、介电和电致伸缩性能的影响规律。掺钾对PMN基预烧粉体和陶瓷的相组成影响很小；理论计算和Raman散射光谱分析都表明掺钾使PMN基陶瓷的有序微区的尺寸稍微减小；掺钾也小幅度地减小了PMN基陶瓷的介电和电致伸缩性能，最后分析了掺钾降低介电和电致伸缩性能的原因。