Preparation of high-T c superconducting film in the system of Bi-Sr-Ca-Cu-O from sol-gel process and its properties

High-T _c superconducting Bi₂Sr₂CaCu₂O_x films withT _c ^off =80 K were prepared by the dipping method of sol-gel processing using inorganic salts. The influence of the preparation conditions on the superconducting properties of the derived material is reviewed. Bi, Sr, Ca and Cu nitrates were used as raw materials. Glycerol was used as solvent. The thickness of films made by the dip method was about 0.5 m. The films were crystallized by heat-treatment at 830°C for 10 min.T _c ^off of films was 80 K andJ _c at 77 K was more than 8 kA cm^–2. Synthesis of high-T _c superconducting films was very easy and the crystallization of films was possible with a relatively low heat-treatment temperature.     

High-Resolution Thermal Expansion of MgB2

The thermal expansion of polycrystalline MgB₂ from 5–300 K is studied using high-resolution capacitance dilatometry. The thermal expansivity exhibits a small jump of –5.8×10^–8 K^–1 at T _c (in accord with expectations from the Ehrenfest relationship and published specific heat and pressure data) and a negative peak-like feature close to 5 K. No indications of any structural instabilities are observed.     

Influence of Ar ion etching on Tc of liquid-quenched Bi1.6Pb0.4Sr2Ca2Cu3Ox superconductor by sheet plasma source

The influence of argon-ion etching is investigated for liquid-quenched high-T_c Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_x. The argon-ion irradiation has no effect on the T_c value for a dose of 4.05 × 10¹⁷ (ions mm^–2). However, doses in excess of this level greatly decreases the T_c value. Therefore, a critical irradiation does value (D_c) to maintain a T_c value above 100 K is defined and determined. The D_c is about 4.94 × 10¹⁷ (ions mm^–2) for argon-ion irradiation at the low acceleration energy of 1 keV.     

Variable—Range hopping in Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-K<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> glasses

The dc conductivity of the glasses in the Fe₂O₃-Bi₂O₃-K₂B₄O₇ system was studied at temperatures between 223 and 393 K. At temperatures from 300 to 223 K, T^–1/4 (T is temperature) dependence of the conductivity was found, however, both Mott variable-range hopping and Greaves intermediate range hopping models are found to be applicable. Mott and Greaves parameters analysis gave the density of states at Fermi level N (E_F) = 3.13 × 10²⁰–21.01 × 10²⁰ and 1.93 × 10²¹–16.39 × 10²¹ cm^–3eV^–1 at 240 K, respectively. The variable-range hopping conduction occurred in the temperature range T = 300–223 K, since W_D was found to be large (W_D = 0.08–0.14 eV for these glasses) and dominated the conduction at T < 300 K.     

The preparation of high transition temperature superconducting Pb-Bi-Ge alloy filaments using the method of glass-coated melt spinning

The melt spinning of Pb-Bi-Ge alloys with Pyrex glass was investigated as a means of producing a superconducting long filament with highT _c. Continuous filaments with maximumT _c of more than 10 K of Pb_100-x-y Bi _x Ge _y (15x37 and 725) were obtained from the molten state at 1500 K with a winding speed of 0.95 m sec^–1. The Pb₄₉Bi₃₃Ge₁₈ filament, which was 34×10^–6 m diameter and a ductile material with a tensile strength of 20 MPa and elongation of 2.7%, exhibited superconductivity at the highestT _c of 14.3 K. These filaments were found to be polycrystalline with a grain size of more than 5000×10^–10 m and had a mixed structure of germanium (diamond) (h c p) and bismuth phases. The germanium element distributed homogeneously in the filament.     

Synthesis of melt-quenched Bi1.7V0.3Sr2Ca2Cu3O10+y superconducting glass-ceramics

A glass in the BSCCO system with Bi₁₇V_0.3Sr₂Ca₂Cu₃O_10+y nominal composition was prepared by the melt-quenching (glass) method. The suitability of the glass ceramic method has been assessed in terms of physical and electrical properties. Using an analysis developed for non-isothermal crystallization studies, information on some aspects of crystallization has been obtained. The result obtained indicated that substitution of vanadium for bismuth increased the activation energy compared to the unsubstituted BSCCO system but did not enhance the superconducting phase formation. The activation energy for crystallization of glass has been found, E _a=355 kJ mol^–1. The crystal structure was found to differ from that in the unsubstituted BSCCO system. Most importantly, the HT _c phase was formed by reaction between the constituent phases at lower temperatures and not directly from the glass material. The best electrical properties were obtained at T ₀=75 K and J _c=12 × 10³A cm^–2 at 4.2 K.     

Ion-channeling studies of lattice anomalies well aboveT c in YBa2CU3O7−y

Backscattering yields in the <001> axial channeling mode using D⁺ ions have been measured for YBa₂Cu₃O_7–y single crystals with T_c=59K. 68K and 91K at temperatures between 40K and 295K. In 60 K-class YBa₂Cu₃O_7–y with significant anomalies associated with the spin gap, it is found that the channeling anomalies are observed at 130–140 K(T_pa) in addition to anomalies at T_c. The channeling anomalies at T_c follow to the shift of T_c, suggesting the phonon anomalies induced by the superconducting-gap opening. On the other hand, T_pa is found to be almost unchanged for varying T_c, although T_pa appears near the temperature where the spin gap is opened. This leads a question whether the channeling anomalies at T_pa is directly related to the phonon anomalies induced by the spin-gap opening. The existence of lattice instability is suggested as one of possible explanations for the anomalies at T_pa.     

Thermal expansion of niobium in the range 1500–2700 K by a transient interferometric technique

The linear thermal expansion of niobium has been measured in the temperature range 1500–2700 K by means of a transient (subsecond) interferometric technique. The basic method involves rapidly heating the specimen from room temperature up to and through the temperature range of interest in less than 1 s by passing an electrical current pulse through it and simultaneously measuring the specimen temperature by means of a high-speed photoelectric pyrometer and the shift in the fringe pattern produced by a Michelson-type interferometer. The linear thermal expansion is determined from the cumulative shift corresponding to each measured temperature. The results for niobium may be expressed by the relation (l-l ₀)/l ₀=5.4424×10^–3–8.8553×10^–6 T+1.2993×10^–8 T ² –4.4002×10^–12 T ³+6.3476×10^–16T⁴ where T is in K and l ₀ is the specimen length at 20°C. The maximum error in the reported values of thermal expansion is estimated to be about 1% at 2000 K and not more than 2% at 2700 K.Paper presented at the Ninth International Thermal Expansion Symposium, December 8–10, 1986, Pittsburgh, Pennsylvania, U.S.A.     

Preparation and characterization of superconducting (YBa2Cu3O x -Ag) composites obtained by sol-gel method

For the first time a simple sol-gel method has been developed for the preparation of [YBa₂Cu₃O _x -Ag] composites using nitrates of Y, Ba, Cu, and Ag. For the two composites (with 10 and 20 wt% Ag), the electrical resistivities and magnetic susceptibilities (both alternating and direct current) show critical temperatureT _c (zero) around 90–91 K. The X-ray diffraction studies indicate no major change of lattice parameters for the orthorhombic phase. The critical current density,J _c, is found to be around 200 A cm^–2 at zero field and 80 K. The degradation ofT _c of the composites in presence of water is also very low, similar to the previous observations. This sol-gel method could be used for the large-scale production of superconducting composite powders necessary for drawing wires/tapes or targets for sputtering.     

Surface Superconducting State of Heavy-Fermion Superconductor CeIrIn5 Probed by CeIrIn5-Ag Junctions

The CeIrIn₅-Ag junctions of about 2×10^–9 cm^–2 area have been made using microfabrication techniques, and the surface superconducting state of CeIrIn₅, which has two characteristic temperatures T ₀ and T _c, has been investigated, where T ₀ and T _c are the transition temperature to zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. The temperature, below which superconducting anomalies are observed, varies from junction to junction, and yet it is always well above T _c=0.4 K. This result, together with no indication of transition at T _c, suggests that at least the surface of CeIrIn₅ is in the superconducting state above T _c. The data on the critical current I _c in superconducting anomalies point to the possibility to define a local transition temperature for each junction.     

P+ implantation and annealing effects on theT c in BiSrCaCuO films

TheT _c changes related to the microstructure as a function of annealing temperature for the BiSrCaCuO (BSCCO) film implanted with 170 keV P⁺ at two different doses were studied. The BSCCO films were prepared by d.c. sputtering on MgO substrates. For the film implanted at a dose of 5×10¹⁵ cm^–2 post-implantation annealing at 600–800°C enabled theT _cs of the film to be completely recovered. For the film implanted at a dose of 1.0×10¹⁷cm^–2 theT _cs were only partly recovered after 600°C annealing. On further annealing at 700°C the superconductivity of the film disappeared. TEM examination showed that significant amount of CaP, Ca₃P₂, and some unknown phases were formed. It is considered that the significant amounts of these phases formed during post-implantation annealing renders the recovery of the superconductivity of the P⁺-implanted BSCCO film difficult.     

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15−δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr₂Ba₄Cu₇O_15−δ (Pr247) showing a superconducting transition at T_c=10–16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T² dependence up to approximately 150 K. A clear specific heat anomaly was observed at T_c for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature T_N of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μ_B. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^α dependence (α=0.08–0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.     

The critical constants of long-chain normal paraffins

Because of the recent availability of the critical constants of normal alkanes up to octadecane, some modifications in the estimation procedures for the critical constants have become necessary. It has been shown that the equation of Ambrose for the critical temperature of normal alkanes leads to the result that as n , the limiting value for the critical temperature is equal to the limiting value for the normal boiling point and the limiting value for the critical pressure is 1 atm. Currently, the CH₂ increment for the critical volume is considered constant. The recent data of Teja have shown that the CH₂ increment increases indefinitely in a homologous series until the critical volume reaches its limiting value. This has made the current procedure for estimating the critical volume obsolete. Taking into account the new measurements of Teja, we have now developed new equations for estimating the critical constants. The limiting values for an infinitely long alkyl chain for T _b, T _c, P _c, and V _c have been found to be 1021 K, 1021 K, 1.01325 bar, and 18618 cm³ · mol^–1, respectively. These new concepts have been applied to the estimation of various properties other than the critical constants.Nomenclature M Molar mass, kg·mol ^–1 - V _c Critical volume, cm³·mol^–1 - V ₁ Saturated liquid volume, cm³·mol^–1 - P _c Critical Pressure, bar - T _c Critical temperature, K - T _b Normal boiling point, K - T _B Boyle temperature, K - T _A Temperature at which the third virial coefficient is zero, K - V _c Limiting value of critical volume = 18,618 cm³ · mol^–1 - P _c Limiting value of critical pressure=1.01325 bar - T _c Limiting value of critical temperature = 1021 K - T _b Limiting value of normal boiling point = 1021 K - P _b Pressure at the normal boiling point, 1 atm - Z _c Critical compressibility factor - Z _c Limiting value for the critical compressibility factor = 0.22222 - R Gas constant, 83.1448×10^–6m³ · bar · K^–1 · mol^–1 - Acentric factor - X (T _c–T _b)/T _c - X ₁ (T _c–T)/T _c - X ₂ 1–(T _B/T)^5/4 - X ₃ 1–(T _A/T)^5/2 - Y P _c/RT _c - Surface tension, mN · m^–1 - B Second virial coefficient, cm³ · mol^–1 - B Limiting value for the second virial coefficient = –30,463 cm³ · mol^–1 - C Third virial coefficient, cm⁶ · mol^–2 - C _b Third virial coefficient at the normal boiling point, cm⁶ · mol^–2 - C _c Third virial coefficient at the critical temperature, cm⁶ · mol^–2 - C _B Third virial coefficient at the Boyle temperature, cm⁶ · mol^–2 - H _vb Enthalpy of vaporization at the normal boiling point, kJ · mol^–1 - n Number of carbon atoms in a homologous series - p Platt number, number of C-C-C-C structural elements - a, b, c, d, e, etc Constants associated with the specific equation - T _c ^* , T _b ^* , P _c ^* , V _c ^* , etc. Dimensionless variables     

Measurement of the heat capacity of molybdenum (Standard Reference Material) in the range 1500–2800 K

Measurement of the heat capacity of molybdenum (Standard Reference Material 781 of the National Bureau of Standards) in the temperature range 1500–2800 K by a subsecond-duration, pulse-heating technique is described. The results of the measurements on three specimens are in agreement within 0.6%. The heat capacity of molybdenum in the temperature range 1500–2800 K based on the present results is expressed by the following function (standard deviation =0.5%): C _p =–3.0429+4.7215×10^–2 T–2.3139×10^–5 T ²+4.7090× 10^–9 T ³ where T is in K and C _p is in J · mol^–1 · K^–1. The inaccuracy of the reported results is estimated to be not more than 3%.     

In-situ deposition of superconducting YBCO thin films by magnetron sputtering from powder target

We have used stoichiometric Y₁Ba₂Cu₃O_7–x powder as magnetron sputtering target to deposit high-quality high-T _c superconducting thin films on MgO, SrTiO₃, and ZrO₂ substrates. The zero-resistance temperatures are 86–88 K, and the 77 K zero-field critical current density is 8 × 10⁵ A/cm². The films are highlyc-axis oriented. Films deposited on 10 × 10 × 1 mm³ ZrO₂ substrates have surface resistances below 25 m at 77 K and 94 GHz. Using powder targets instead of bulk targets has the following advantages: simple and low-cost target preparation, simple target replacement, and versatility for large-area deposition.     

Long multifilament Bi-2223 Ag-sheathed superconducting tapes and solenoids

Multifilament Ag-sheathed BiPbSrCaCuO (2223) superconducting tapes containing 49 filaments were fabricated by the powder-in-tube route and the roll-anneal process. The transport critical current densityJ _c was 1.3×10⁴ A cm^–2 at 77 K and 7×10⁴ A cm^–2 at 4.2 K in self-field. A 12-m-long tape was used to construct superconducting solenoids (50, 28, and 14 mm internal diameters) generating dc fields 380–1070 G at 4.2 K. Measurements of the variation ofJ _c with field (0–1.6 T) and bend strain (0–5%) are used to explain the performance of the solenoids. The critical bend strain of tapes was about 1.5%.     

The high-temperature thermal expansion of BiPbSrCaCuO superconductor and the oxide components (Bi2O3, PbO,CaO, CuO)

The thermal expansion of superconducting Bi_1.6Pb_0.4Sr₂Ca₂Cu₃Ox (BiPbSrCaCuO) and its oxide components Bi₂O₃, PbO, CaO and CuO have been studied by high-temperature dilatometric measurements (30–800°C). The thermal expansion coefficient for the BiPbSrCaCuO superconductor in the range 150–830°C is =6.4×10^–6K^–1. The temperature dependences of L/L of pressed Bi₂O₃ reveals sharp changes of length on heating (T ₁=712°C), and on cooling (T ₂=637°C and T ₃=577°C), caused by the phase transition monoclinic-cubic (T ₁) and by reverse transitions via a metastable phase (T ₂ and T ₃). By thermal expansion measurements of melted Bi₂O₃ it is shown that hysteresis in the forward and the reverse phase transitions may be partly caused by grain boundary effect in pressed Bi₂O₃. The thermal expansion of red PbO reveals a sharp decrease in L/L, on heating (T ₁=490°C), related with the phase transition of tetragonal (red, a=0.3962 nm, c=0.5025 nm)-orthorhombic (yellow, a=0.5489 nm, b=0.4756 nm, c=0.5895 nm). The possible causes of irreversibility of the phase transition in PbO are discussed. In the range 50–740°C the coefficient of thermal expansion of pressed Bi₂O₃ (_m=3.6 × 10^–6 and _c=16.6×10^–6K^–1 for monoclinic and cubic Bi₂O₃ respectively), the melted Bi₂O₃ (_m=7.6×10^–6 and _c=11.5×10^–6K^–1), PbO (_t=9.4×10⁶ and _or=3.3×10^–6K^–1 for tetragonal and orthorhombic PbO respectively), CaO (=6.1×10^–6K^–1) and CuO (=4.3×10^–6K^–1) are presented.     

A melt process to superconductive fibres of Bi2Sr2CaCu2 oxide

A particulate precursor of superconductive Bi₂Sr₂CaCu₂ oxide dispersed in an aqueous poly(vinyl alcohol) solution, was shaped into fibres. The fibres fired at a T _max of 850 °C were very porous. Although they exhibited a Meissner on-set transition at 85 K, electrical resistance at 0.02 Acm^–2 at 77 K was still not zero. Exposing the porous fibres to a temperature slightly above their melting point densified them. The treated fibres had a sharp Meissner transition and could carry critical current (J _c) as high as 331 A cm^–2 at 77 K. They were also more resistant to the effect of a magnetic field than the fibre fired at 850 °C. Eighty percent of the treated fibre was found to be the superconductive 2 2 1 2 phase.     

The mechanical and structural properties of Y1Ba2Cu3O7−x superconductors

Mechanical losses spectroscopy of two superconducting Y₁Ba₂Cu₃O_7–x samples (T _c=93 K) measured at 5, 10, 20 and 30 MHz revealed four peaks. As the frequency of measurement was increased, the peaks shifted to higher temperatures. The relaxation process was found to have an activation enthalpy of 0.188, 0.092, 0.209 and 0.314 eV with an attempt frequency,f _o, equal to 2.46 × 10¹⁵, 5.95 × 10¹⁰, 1.64 × 10¹⁴, and 7.82 × 10¹³ Hz for P₁, P₂, P₃ and P₄ peaks, respectively. The mechanism responsible for these relaxation peaks is discussed. The infrared spectra of some of these compounds provide evidence for oxygen rearrangement on going from tetragonal to orthorhombic phase. X-ray diffraction analysis shows that these compounds correspond to a single phase which is an orthorhombic perovskite structure.