Volume 14, Issue 2
April 1991, pages 93-530
pp 93-94 April 1991 International Conference On Superconductivity—I
pp 95-104 April 1991 International Conference On Superconductivity—I
In this report we present the most important results of our recent analysis (Kupriyanov and Likharev 1990) of the Josephson effect in both the natural (intergrain) and artificial junctions using high-Tc superconductors (HTS). A comparison of the experimental data with the BCS-based theories of the Josephson effect in various tunnel-junction-type and weak-link-type structures has been carried out. The main conclusion is that the data presently available do not enable one to either confirm or reject the theories, and thus to reveal possible deviations of the real microscopic mechanism of the high-Tc superconductivity from the BCS mechanism. We suggest several experiments which would be more fruitful for this purpose, as well as for finding ways of reproducible fabrication of practically useful Josephson junctions.
pp 105-110 April 1991 International Conference On Superconductivity—I
The apparentTc of some high temperature superconductors such as BiSrCaCuO and TlBaCaCuO measured by low (10–20,000 Hz) a.c. inductance techniques is shifted downward by up to 30K in the presence of modest (400 Oe) d.c. magnetic fields. Dc magnetization (SQUID) measurements of the same samples show no such shifts in the same magnetic fields. Other materials like YBaCuO and TlPbSrCaCuO do not show such shifts ofTc in a.c. inductance measurements in d.c. magnetic fields. While BiSrCaCuO and TlBaCaCuO are known to exhibit flux creep/flux flow phenomena resulting in low irreversibility line behaviour as measured by both a.c. and SQUID techniques, the aboveTc shift is apparently a new phenomenon. Materials exhibiting this property contain two or more layers of metal ions between the CuO conductance layers (2212, 2223 structure). Materials with only one metal ion layer between the CuO layers (123, 1223 structure) do not show the apparentTc shift, suggesting that the phenomenon is a result of the properties of magnetically-isolated CuO layers. Implications of the latter will be discussed.
pp 111-116 April 1991 International Conference On Superconductivity—I
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 withTc=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.
pp 117-123 April 1991 International Conference On Superconductivity—I
We investigated the magnetic specific heat and entropy of a CuO single crystal in the wide temperature range from 1·5 K to 1000 K and determined the anisotropy of its magnetic susceptibility below 300 K. CuO behaves like an anisotropic low-dimensional antiferromagnet. The magnetization discontinuity is most clearly visible atTN2=214 K for fields in theb-c plane. The measured magnetic entropy at 1000 K,Sm(1000)=5·67 J/mol K, approaches the expected value ofSm=Rln2=5·76 J/mol K.
pp 125-130 April 1991 International Conference On Superconductivity—I
Hall constants (RH) at 300 K of three families of CuO-based superconductors are presented—the 123 YBaCuO with Fe substitution, the 2212 and 2223 Bi systems with Pb, Gd, Nd. Sm substitutions. In all these systems.RH is positive indicating hole conductivity.Tc increases initially as the hole density (p=1/RHe) increases and shows a maximum. Each family shows a maximum value ofTc but at different values ofp. These data along with the observation ofn-type conductivity in NdCeCuO are discussed in the light of certain models.
pp 131-134 April 1991 International Conference On Superconductivity—I
A generalized Gorkov formalism of superconductivity in a layered system with non-primitive lattice structure is given. This is used to justify theoretically a model of highTc oxide superconductors proposed recently by one of us (SSJ). A numerical estimation of the critical temperature as a function of the number of superconducting layers is mentioned.
pp 135-142 April 1991 International Conference On Superconductivity—I
The tetragonal (Tc=55 K) and orthorhombic (non-superconducting) Tl2Ba2CuO6 (Tl 2201) phases have been synthesized by aca 20 GPa planar shock wave in a microsecond time frame. Because of local rapid quench rates (up to 106 K/s) defects related to metastable phases are frozen in the sample. In the 2201 phase the predominant defect identified by high resolution lattice imaging corresponds to a Tl-Ba-Cu-Tl-Cu (Tl 1212) phase which is metastable but can be synthesized via a low temperature reaction. Defects of this type may account for the enhanced flux trapping observed in the material using field modulated microwave absorption. Attempts at shock synthesizing the more complex Tl2Ba2CaCu2O8 and Tl2Ba2Ca2Cu3O10 phases are also discussed. Shock processing pre-synthesized YBa2Cu3O7 powder in the radial geometry followed by an O2 anneal at 890 C produces near-theoretical density cylinders that sustain inter-grain critical currents at zero field of 1350 and 750 amps/cm2 at 60 and 77 K respectively. The magnetic field dependence of the critical current and the magnitude of flux-pinning in this material relative to sintered pellets are enhanced by a factor of 2–3. Preliminary investigations of the defect microstructure of this material are discussed.
pp 143-147 April 1991 International Conference On Superconductivity—I
The results are presented of the experimental studies on forming weak links in yttrium ceramics by scribing and high-voltage discharge. The energy resolution of SQUIDs and the magnetic field sensitivity of magnetometers produced according to these methods were 6×10−28 J/Hz and 10−28 J/Hz, 5×10−13 T/Hz1/2 and 2·5×10−13 T/Hz1/2, respectively. Different designs of HTSC interferometers sensitive to the external magnetic field variation are described. The factors affecting the sensitivity of r.f. HTSC SQUID-magnetometers are considered.
pp 149-156 April 1991 International Conference On Superconductivity—I
Ab initio unrestricted Hartree-Fock Cluster investigations have been carried out on the electronic structures of the YBa2Cu3O7 and YBa2Cu3O6 systems. The results of these investigations provide satisfactory explanations of available63Cu and17O nuclear quadrupole interaction data. The electron distributions obtained rule out the presence of Cu3+ ions and are supportive of the presence of Cu2+, Cu1+, O1− ions in the O7 system and Cu2+, Cu1+ and O2− in the O6 system with actual charges departing significantly from the formal charges, especially in the O7 system, indicating the importance of covalency effects. Suggestions will be made regarding possible sources that can bridge the remaining gap between theoretical and experimental results for the nuclear quadrupole interaction parameters.
pp 157-159 April 1991 International Conference On Superconductivity—I
pp 161-166 April 1991 International Conference On Superconductivity—I
Preparation, resistivity (ρ), field-cooled (FC) and zero field-cooled (ZFC) susceptibility (χ) of YSrCuO are reported. Zeroρ is observed at 72K. FCχ shows a superconducting transition at 68 K and also an antiferromagnetic transition at 15 K which is due to Y2Cu2O5 as deduced from electron diffraction and composition analyses. It is tentatively proposed that the Y-rich phase could be responsible for the observed superconductivity.
pp 167-171 April 1991 International Conference On Superconductivity—I
An improved method for hot pressing of high temperature superconducting (HTSC) powders prepared by conventional solid phase synthesis of the initial BaCO3, CuO and Ln2O3 allows one to obtain HTSC targets and magnetic shields possessing a high degree of homogeneity. By sputtering such targets HTSC films with critical current density of 3.3·106 A/cm2 have been deposited. Shields, prepared according to this method, show a shielding coefficient of 105 andHc of 79 Öe in the constant magnetic field and in the alternating magnetic field the amplitude is 90 Öe in the frequency range of 70–3000 Hz.
pp 173-177 April 1991 International Conference On Superconductivity—I
Fine powders of yttrium, barium and copper oxide were synthesized by two different chemical precipitation methods using a spray atomizer. The average particle size in the sintered ceramic was in the range 1–2.5 microns and the superconducting transition temperature of different powders was in the range 60–90 K. With few exceptions, it was found that recalcination and annealing in oxygen will not improve the superconducting transition temperature.
pp 179-183 April 1991 International Conference On Superconductivity—I
Effect of water vapour on the YBa2Cu3O7−x ceramics of different density has been studied using Auger electron and X-ray photoelectron spectroscopy. The initial period of decomposition of this compound when exposed to H2O is accompanied by Ba(OH)2 deposition on the ceramic grain surface and partial reduction of copper to Cu2O in the oxide. With this barium diffuses to free surfaces of ceramic grains. Degradation of HTSC ceramics proceeds by increase of thickness of the changed layer on free surfaces of grains. Violation of the initial composition may at first lead to some improvement of transitional characteristicsTc and ΔTc and with their subsequent degradation, the transition width ΔTc is in particular broadened. Dense ceramics possess higher resistance to a humid atmosphere.
pp 185-188 April 1991 International Conference On Superconductivity—I
We have developed a process for synthesizing orthorhombic YBa2Cu3Ox (123) superconducting powders by calcination of the precursor powder under reduced total oxygen pressure. Because a single calcination at 800°C for 4 h in flowing oxygen at a pressure of 2 mm Hg results in essentially phase-pure material, total calcination times have been drastically reduced. At liquid nitrogen temperature, sintered pellets made from this powder have critical current densities of ∼1000 A/cm2 in zero applied magnetic field.
pp 189-193 April 1991 International Conference On Superconductivity—I
High temperature superconducting LaBa2Cu3O7−δ has been prepared by ceramic and nitrate methods to understand the influence of preparation conditions on superconductivity. The characterization was carried out by X-ray diffraction and magnetic susceptibility measurements.Tc onset was observed at 88 K. Meissner effect has been observed above the liquid nitrogen temperature.
pp 195-197 April 1991 International Conference On Superconductivity—I
Samples of high temperature superconducting system ErBa2Cu3Oy prepared by the standard ceramic technique were found to have single phase. To vary the oxygen content in the samples, the mother compound was reduced by vacuum-annealing at different temperatures for a fixed period. The oxygen content was measured by gravimetric and iodometric titration procedures. The structural changes induced due to the variations in the oxygen content were studied by X-ray methods.
pp 199-206 April 1991 International Conference On Superconductivity—I
Influence of sintering time and quenching in Bi2 −xPbxCa2Sr2Cu3Oy(x=0.0, 0.1, 0.2, 0.25, 0.3 and 0.4) samples have been studied by resistance and XRD measurements. In samples sintered at 850°C for 4 days,Tc(0) increases with Pb concentration.Tc(0) increased from 81 K forx=0.0 to 109 K inx=0.30 sample and then decreased. Increasing the sintering time to 10 days decreased theTc Quenching further decreased theTc(0). From X-ray diffraction patterns, the intensity peaks of low and highTc phases have been measured. The addition of Pb promotes highTc-phase. Sintering time, slow cooling and rapid quenching studies show that there is an optimum sintering time and cooling rate to produce a highTc-phase.
pp 207-209 April 1991 International Conference On Superconductivity—I
Since the discovery of Bi-containing high-Tc superconductors, difficulties in obtaining monophase samples have been known. The study of phase formation in the system Bi2Sr(3−x)CaxCu2Oy, where 0.5<x<2.5 and conditions for raising HT phase (110 K) content by loading a superstoichiometric excess of Ca and Cu ions into the initial composition as well as by a substitution of lead for a part of bismuth is the object of the present paper.
pp 211-216 April 1991 International Conference On Superconductivity—I
The effect of sintering in air and annealing in oxygen flow at various temperatures and times on the superconducting transition of Bi2Sr2Ca1Cu2O8 ceramic was studied. Samples sintered at 1138 K for 24 h in air and slowly-cooled to room temperature are single-phase with the highest zero resistivity temperature of 88 K. XRD pattern showed that the single-phase was characteristic of tetragonal structure witha=0.5398 nm andc=3.0620 nm. Both a.c. susceptibility and resistivity measurements give similar values for the onset (100 K) of superconducting transition.
pp 217-222 April 1991 International Conference On Superconductivity—I
The role of preparation conditions and the effect of addition of Pb in Bi-Sr-Ca-Cu-O (BSCCO) superconductor, on theTc,Jc and grain orientation have been studied. Calcination at a temperature higher than the melting point of Bi2O3 after the prereaction at 800°C leads to formation ofc-axis oriented nearly single phase material. The presence of lead yields a nearly highTc phase (2223) exhibiting a maximum transition temperatureTc=110 K and ΔTc=2 K. The addition of lead and grain orientation together result in an increase inJc by nearly two orders of magnitude. The degradation behaviour was studied in terms of changes inTc,Jc and structure on exposure to atmosphere up to 250 days. Samples with 0.6 Pb were found to be superconducting withTc=97 K even after 250 days. The highTc phase was stable against degradation. Our studies indicate that the degradation of leaded BSCCO is a surface phenomenon rather than bulk phenomenon.
pp 223-226 April 1991 International Conference On Superconductivity—I
We report an elegant method for the synthesis of single-phase Bi-2223 superconductor from a stoichiometric composition Bi1.7Pb0.3Ca2Sr2Cu3Oy by a matrix reaction route. The superconducting transition temperatureTc (R=0) of this single-phase compound is 120 K. The effect of Pb-content and sintering temperature on the formation and stability of Bi-2223 phase is described.
pp 227-233 April 1991 International Conference On Superconductivity—I
Fabrication processes for increasing the critical current densityJc in BiSrCaCuO superconductor have been studied. Calcining mixtures of raw material in a reduced oxygen partial pressure or in vacuum after in-air calcination is found to be effective. Intermediate cold work (ICW) i.e. uniaxial pressing, cold flat rolling, etc. incorporated in the sintering Agsheathed tape is also effective for raisingJc. From a preliminary experiment on Ag-added tape sample, a combination of both processes appeared to be most effective. The highestJc obtained is 4.37×104 A/cm2 at 4.2 K and 12 Tesla. Effects of additional element Ag etc. onJc have also been described.
pp 235-239 April 1991 International Conference On Superconductivity—I
EnhancedTc in calcium-free Tl compounds of the series TlmBa2Can−1CunOx (2201) has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under optimized conditions (sintering temperature 970°C and duration 3–10 min) the highestTc(onset) ranges from 103 K to ∼ 115 K andTc (zero) ∼ 95 K was found. XRD studies showed the transformation of all the three nominal compositions into 2201 phase with differentTcs.
pp 241-246 April 1991 International Conference On Superconductivity—I
This paper describes the synthesis and microwave absorption studies of single phase Tl-2223 compound, withTc=(129 ± 1) K. The single phase compound has been synthesized using a precursor matrix method. It has been found that Tl-deficient starting composition gives the best results.
pp 247-249 April 1991 International Conference On Superconductivity—I
The superconducting transition with onset temperature of 95K has been confirmed in SmBa2Cu3O7−δ system by resistivity and AC susceptibility measurements. Pressure dependence ofTco andTcf of the title superconductor is described. The superconducting transition onset temperature and complete transition temperature increases with increase of pressure.
pp 251-255 April 1991 International Conference On Superconductivity—I
The effect of compositions, sintering temperatures and furnace atmospheres on the superconducting transition temperatures of several members of the series Pb2Sr2R1−xCaxCu3Oz(0.0⩽x⩽0.7, R=Y, Gd) has been studied. The effect of partial replacement of Pb, Ca and Y by In has also been studied. The shapes of the resistivity-temperature curves and the zero resistance temperatures are found to be extremely sensitive to the synthesis parameters employed. Superconductivity is not observed in samples treated in oxidizing atmosphere or heated at very high temperatures. Samples sintered in flowing nitrogen showed incomplete transition in resistivity starting at about 75 K and extending down to 15 K. A new and comparatively simple synthesis procedure involving treatment of the samples in vacuum has been employed to obtain nearly single-phase materials showing metallic behaviour. Using this procedure, samples of the above composition, but containing no Ca(i.e. Pb2Sr2RCu3Oz, R=Y and Dy), are also found to be superconducting with zero resistance up to 48 K.
pp 257-261 April 1991 International Conference On Superconductivity—I
V A Alekseev V M Baranovsky A A Vedenov A V Velichko L L Zaytzeva A S Kovalenko M I Konarev G F Leshenko T S Mylnikova A Ya Parshin Yu F Ryzhkov B S Skidan A I Sukhih N A Usov L I Feklina N V Shishkov
The influences of the host material on the superconducting properties of the composite material are studied in the following cases: (i) niobium powder—aluminium oxide, (ii) technetium—rare earth oxides and (iii) Nb3Sn powder—unsaturated polymer of fluorine derivative of ethylene. The critical temperatures, critical magnetic fields and theI-V characteristics of these systems are discussed.
pp 263-267 April 1991 International Conference On Superconductivity—I
The creation of local magnetic moments at the plane and chain Cu-sites in YBa2Cu3O7 due to incorporation of Ga3+ and Zn2+ has been studied with respect to the superconducting transition temperature and structural parameters of the compound. The result indicates that the local moments are created due to slowing down of spin and charge fluctuations in the planes and chains respectively due to doping.
pp 269-273 April 1991 International Conference On Superconductivity—I
The effect of the addition of 0–15 wt.% alumina powder on the grain size, morphology and superconducting properties of sintered YBa2Cu3O6+x was investigated as a function of alumina concentration in the composites. The results suggest that the grain size of YBa2Cu3O6+x decreases with an increase in the additive concentration. Some surface erosion of the YBa2Cu3O6+x grains was also noticeable in Al2O3/YBa2Cu3O6+x composites. The results also indicate that the additive tends to stabilize the nonsuperconducting tetragonal phase of the YBa2Cu3O6+x at the expense of the superconducting orthorhombic crystal phase.
pp 275-278 April 1991 International Conference On Superconductivity—I
We have synthesized CaBaLaCu3O7−δ and related compounds Ca0.9BaLaCu3O7−δ, Ca11 BaLaCu3O7−δ and Ca0.9Ce0.1 BaLaCu3O7−δ in order to investigate the effect of doping and non-stoichiometry at Ca-sites on the superconducting behaviour of tetragonal superconductor CaBaLaCu3O7−δ. The superconducting transition temperature (Tc) of Ca0.9 BaLaCu3O7−δ is the same as that of pure CaBaLaCu3O7−δ(67 ± 1 K). Calcium excess compound Ca1.1 BaLaCu3O7−δ showed a drop inTc to 72 K while the cerium-substituted compound Ca0.9 Ce0.1 BaLaCu3 O7−δ showed a drop inTc to 43 K. The results are explained on the basis of difference in valency between the host and the dopant ion.
pp 279-285 April 1991 International Conference On Superconductivity—I
Careful characterization of (Y0.95Sr0.05)(Ba1.5Sr0.5)Cu3O7−δ and Y(Ba1.5Sr0.5)Cu3O7−δ ceramic samples has confirmed the simultaneous substitution of Sr for both Y and Ba in the YBa2Cu3O7 structure. The substitution at the Y site has a distinct effect on the superconducting properties of the material. The inter-grain transport properties of both samples in an external d.c. field are enhanced with respect to undoped YBa2Cu3O7 ceramics.
pp 287-290 April 1991 International Conference On Superconductivity—I
The compounds Nd2−xCexCuO4−y forx=0 to 0.18 have been synthesized and characterized. Superconductivity is observed forx=0.14 to 0.16. The normal state resistivity and d.c. magnetic susceptibility of these compounds have been measured in the range 4.2 to 300 K. Both the superconducting and non-superconducting compounds exhibit a Curie-Weiss behaviour at high temperatures, giving an effective moment for the Nd ion close to the free-ion value.
pp 291-296 April 1991 International Conference On Superconductivity—I
The role of Al and Ag dopants in the nominal composition, 4336 of Bi-Sr-Ca-Cu oxide has been studied, using a new chemical phase analysis method known as differential dissolution besides XRD and resistivity and magnetic susceptibility measurements. Our investigation reveals that (a) it is relatively easy to realize high-Tc phase (2223) in both Al- and Ag-doped samples with the same nominal composition and heat treatment history (b) both samples consist of superconducting phases with more or less the same basic composition and equal amounts, (c) 2112 phase may be a superconductor and (d) superconducting properties with Al and Ag dopants are different.
pp 297-302 April 1991 International Conference On Superconductivity—I
The results of our investigation on the specimens Bi4Ca3Sr3Cu4−xTxOy (T=Fe, Co, Ni,x⩽0.5) synthesized in air are presented. Fe and Co substituents result in the formation of Bi2Sr2CuOy-type of phase, with considerable depression ofTc of the main phase. However, Ni is completely soluble with Cu in this concentration range without significant depression of superconducting transition temperature (Tc). This difference in the solubility behaviour of Fe and Co on the one hand and Ni on the other is explained taking into account ionic charge and coordination number mismatches.
pp 303-308 April 1991 International Conference On Superconductivity—I
Samples with the nominal composition Bi2Sr2Ca1−xNaxCu2Oy (x=0, 0·1, 0·2 and 0·3) were prepared by solid-state reaction of the individual compounds. X-ray diffraction patterns indicate that the samples have a majority 2212 phase with 2223 also being present. From the DC four-probe resistance data, we have observed that the furnace-cooled samples show metallic behaviour while the quenched samples show superconductivity up to 97 K.
pp 309-314 April 1991 International Conference On Superconductivity—I
Basic and substituted superconductors in the homologous series Bi2Sr2Can−1Cun O2n+4+y, Bi2−x/2Sr2−x/2LxCan−1CunO2n+4+y and Bi2Sr2−xLxCan−1CunO2n+4+y(L=Sn, Pb or Ag,x=0−0.4,n=2, 3 or 4) have been synthesized. All the prepared ceramic samples show superconducting behaviour with zero resistance atTc=70 to 85 K. The compounds withn=3 or 4 showed onset temperature around 115K indicating involvement of a disproportionate solid-state reaction and formation of a two-phase system. The phase involving tin or lead oxides showed similar superconducting properties. Final rapid quenching of samples contributed to preservation of the high temperature equilibria with higher solubility of tin oxide in the quaternary system Bi-Sr-Ca-Cu-O. Silver was not soluble but precipitated in a colloidal form at interfaces between the crystalline grains.
pp 315-320 April 1991 International Conference On Superconductivity—I
The layer-type compound, Ca0·85Sr0·15CuO2, which is the prototype of the cuprate family of highTc oxide superconductors is an electronic insulator, and has a low ordered moment at 300 K indicating that Cu2+ ions are highly correlated. Doping with lithium (up tox=0·15) decreases the room temperature resistivity drastically (from 103 ohm·cm to 0·6 ohm·cm) but metallic behaviour is not induced below 300 K. The magnetic moment on Cu2+ remains low but our studies did not show any antiferromagnetic ordering below 300 K in any of the Li-doped phases.
pp 321-325 April 1991 International Conference On Superconductivity—I
Superconductivity with a maximumTc, zero of 58 K forx > 0·75 has been observed in a nominal starting composition, ThxTl2Sr2Ca2Cu3Oy. X-ray data show that the 1212 phase gets stabilized due to part substitution of Th at the Tl-site forx⩾0·25; however,Tc is low forx<0·75.
pp 327-333 April 1991 International Conference On Superconductivity—I
Samples of the series Bi1·9−xPbxSb0·1Sr2Ca2Cu3Oy withx=0, 0·1, 0·2, 0·3 and 0·4 were prepared by the solid-state route. The X-ray and d.c. electrical resistivity data on furnace-cooled and quenched samples are presented. Though the starting composition is 2223, the end products were multiphase with 4334 as the major phase. A superconducting transition withTc=100K was observed in the pure 2223 sample after quenching. The furnace-cooled samples were metallic, while samples withx=0·1, 0·2 and 0·3 were superconducting after quenching. The amount of the 4334 phase decreases with increasing Pb content. Quenching seems to be favourable for the formation of the 4334 phase.
pp 335-338 April 1991 International Conference On Superconductivity—I
The effect of humid atmosphere (59 and 86%) on phase composition and properties of YBa2Cu3O7−y powder and ceramics prepared by hot pressing has been studied. It has been found that with increase of water content in the powder to 1·6 mass.% the superconducting properties of the corresponding ceramics gradually improve; on further increase of the water content the ceramics degrade resulting in a complete loss of superconductivity. The observed phenomenon can be explained by a plastifying effect of water on the powder by hot pressing on the one hand and by a solid-phase reaction of the products of decomposition (1-2-3) at a high temperature annealing, on the other.
pp 339-342 April 1991 International Conference On Superconductivity—I
Superconducting transition of minor-dispersed phase in modified YBa2Cu3O7−δ samples is found to occur at around 140K. The amount of this minor phase is enough to provide zero resistivity above 135 K. The measurements of the electrical resistivity indicated that the material is stable, thermally recyclable and reproducible. X-ray analysis of the sample with the highestTc shows a major phase with perovskite-like structure witha=3·820(1) Å;b=3·873(1) Å andc=11·659(2) Å along with several unidentified weak peaks. Magnetic measurements confirmed the mixed-phase nature with diamagnetic transition temperatures at 137,91 and 86 K. The minor phase responsible for superconductivity with zero resistivity above 135 K is about 0·4% of the bulk and its nature is still unidentified. The details of the preparation and chemical modification process and the results are presented.
pp 343-350 April 1991 International Conference On Superconductivity—I
Bi-Sr-Ca-Cu-O glasses and glass ceramics of various compositions were synthesised. The glass transition temperature varies from 396 to 422°C depending on the glass composition. The bulk glass ceramics of 4334, 4336, 2223 and 4246 compositions show superconductivity when the corresponding glass samples were heat-treated in air at 820°C for 3, 9, 12 and 24 h respectively. X-ray diffraction studies show that the superconducting phase present in all these compositions is Bi2Sr2Ca1Cu2Ox. The 4334 glass ceramic is almost a single-phase material with a preferred orientation such that thec axis is normal to the sample surface. The 2223 glass ceramic has a higherTc (onset) than the other three compositions indicating the presence of highTc phase (110K) also. ESR studies on the glass samples indicate the existence of Cu2+. The effect of heat treatment on ESR shows that the intensity of resonance decreases with increase in heat-treatment duration. This effect is more pronounced for the 4334 and 2223 compositions. The advantages of synthesizing superconducting materials by glass route are discussed in view of practical applications.
pp 351-357 April 1991 International Conference On Superconductivity—I
Superconducting compounds with nominal compositions Bi1.7Pb0.3Sr1.6Ca2 Cu3.4Ox and Bi1.9Sb0.1Sr2Ca2Cu3Oy have been synthesized by ceramic and glass routes and characterized by X-ray diffraction, electrical resistivity and dielectric constant measurements. The zero-electrical resistance temperatures are about 70K. The dielectric constant of the glasses at room temperature is around 30.
pp 359-365 April 1991 International Conference On Superconductivity—I
Bulk materials and thin films of pure and homogeneous YBa2Cu3O7−x and Bi2Sr2CaCu2O8+x compounds were prepared by a nanocomposite solution-sol-gel (SSG) method. The superconducting oxides of YBa2Cu3O7−x and Bi2Sr2CaCu2O8+x were prepared at very low temperatures i.e. 750°C and 850°C, respectively by SSG method. Pellets sintered from these nanophasic sol powders showed sharp resistivity drops atTc ∼ 90°K for YBa2Cu3O7−x andTc∼67°K for Bi2Sr2CaCu2O8+x. Thin films were prepared using triphasic sol of Y, Ba, Cu and tetraphasic sol of Bi, Sr, Ca and Cu on MgO and SrTiO3 substrates. The triphasic sol coated on SrTiO3 substrates and calcined at 800°C for 12h showed the formation of superconducting phase, YBa2Cu3O7−x with preferred orientation along theC-axis. X-ray diffraction patterns of the Bi2Sr2CaCu2O8+x films on MgO substrate showed the formation of the superconducting phase with preferential orientation along the C-axis and the microwave absorption data as a function of temperature of this film revealed the onset temperature to be 90°K.
pp 367-375 April 1991 International Conference On Superconductivity—I
Synthesis and thermal decomposition characteristics of acetate-modified citrate precursor have been investigated. The intermediates of thermal decomposition exhibit enhanced reactivity. Quantitative estimation of the intermediate phases above 1125 K has been carried out and a directed reaction between Y2Cu2O5 and BaCuO2 phases is postulated for the formation of pure 1–2–3 phase. Isothermal calcination of the precursor at 1175 K for 9 h is sufficient for the complete conversion of the intermediate phases to SmBa2Cu3O7−δ. The particles thus obtained are nearly spherical and monosized to give a BET surface area of 3 m2/g. The oxygenated powder showsTc onset at 97 K for magnetic susceptibility and a narrow hysteresis for magnetization withHc1 andJc values of 105Oe and 6×104 A/cm2 respectively at 77 K. EPR and EDAX techniques confirm the phase purity. Electron diffraction studies confirm XRD results. For oxygenated powder the excess charge (p+) on square planar copper site [Cu-O]p* is estimated to be 0.28.
pp 377-379 April 1991 International Conference On Superconductivity—I
We present an overview of our powder synthesis procedure and give some initial results concerning the superconducting properties of YBa2Cu3O7−x.
pp 381-385 April 1991 International Conference On Superconductivity—I
Local deviations of Y:Ba:Cu ratio from the stoichiometric value 16·7:33·3:50·0 in single-phase superconducting samples of Y1Ba2Cu3O(7−z) have been studied by EDXS in grains and grain boundaries. Two types of single-phase pellets, sintered from solution derived citrate complex and mechanically ground powders, are studied here.
pp 387-393 April 1991 International Conference On Superconductivity—I
Single crystals of YBa2Cu3O7−δ and Ba0.88K0.12BiO3 have been grown from the half-melt flux. Largest crystals with size up to 12×7×0.5 mm3 have been obtained from previously synthesized powders of YBa2Cu3O7−δ, BaCuO2 and copper oxide on MgO single crystal plates. The 3.5BaO-B2O3-BaF2-(1−x)CuO·xCu2O solvents is a promising system for controlled growth of YBa2Cu3O7−δ single crystals on seeds.
pp 395-402 April 1991 International Conference On Superconductivity—I
The microstructure of HTS YBa2Cu3O7−x single crystals withx⩾0.5 has been investigated by TEM and selected area diffraction. An inhomogeneous oxygen distribution over the crystal was observed. Large differences between the bulk crystal structure and the surface have been established. The bulk structure was presented by orthorhombic blocks (≈100×10×20 nm in size) embedded in tetragonal matrix withx≈1. The bulk structure was not of a superconductive type. The crystal surface was enriched by oxygen and had the usual orthorhombic twinned structure. The superconductive properties of the whole specimen were determined by surface layer about 5 µm thick.
pp 403-410 April 1991 International Conference On Superconductivity—I
We report here on the growth of large crystals of Gd-Ba-Cu-O superconductor by using the ‘K2CO3-method’ adopted earlier for preparing oriented ceramics and large crystals of Y-Ba-Cu-O superconductors. The temperature favourable for the growth of GdBaCuO crystals is found to be higher than that required for YBaCuO crystals. Also, the crystal perfection is better for these crystals and as a result, the oxygenation of the sample becomes more difficult. Resistivity measurements on these samples show a sharp drop in resistivity at 90 K which then tails off at lower temperatures and attains zero resistance around 35 K. Such behaviour is normally due to an inhomogeneous oxygen content in the sample. Chemical analysis shows that the deviation from ‘123’ composition in GdBaCuO-samples is much less as compared to YBaCuO-samples obtained by the same technique.
pp 411-414 April 1991 International Conference On Superconductivity—I
Films of Bi-Sr-Ca-Cu-O with lead doping using SrF2 substrates have been grown from KCl solution. The films were highly textured withC-axis perpendicular to the substrate surface. The onset of the resistive transition was at 81 K and zero resistance at 78 K. This technique is permising in view of the possibility to grow large films of both Bi2Sr2CaCu2Ox and Bi2Sr2Ca2Cu3Oy phases.
pp 415-422 April 1991 International Conference On Superconductivity—I
Crystallization of high-temperature superconductors was studied in La-Sr-Cu-O, Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O systems. Platelet crystals YBa2Cu3O6.5+x were obtained by spontaneous crystallization from homogeneous nonstoichiometric melts enriched in barium and copper oxide. La2−xSrxCuO4 was prepared by slow cooling of melts enriched in copper oxide. Bi2(Sr,Ca)n + 1CunOy, (n=1; 2) was obtained by melting zone travelling. The crystals show transition to superconducting state atT=93 K, ΔT 0.2–0.5 K (Y, Ba cuprate),T=87 K, ΔT 2K (Bi, Sr, Ca-cuprate). La, Sr-cuprate single crystals obtained by Czochralski method did not show transition to superconducting state. For flux-grown crystalsTc was 5–26 K depending on the composition, growth and heat treatment. The short characterization of some accessory phases (Ba3Y2Cu3PtO10, Ca1.75Sr1.5Cu0.75PtO6, BaCuO2, Ba41Cu44O84Cl2) is reported.
pp 423-427 April 1991 International Conference On Superconductivity—I
The deposition of yttria-stabilized zirconia (YSZ) as buffer layer on (100) silicon has been studied by rf sputtering with a view to subsequently preparing superconducting films of YBa2Cu3Ox on it. As-deposited films were found to be (100) oriented. The thermal mismatch and reaction between Si and YSZ at high temperatures were found to give rise to cracks in the films. Grain growth of buffer layer on annealing helped in the formation of superconducting phase.
pp 429-434 April 1991 International Conference On Superconductivity—I
Sequentially-deposited layers of Cu/BaF2/Y on buffered Si and SrTiO3 substrates have been synthesised to produce YBa2Cu3O7−δ thin film. An attempt has been made to see the effect of the ZrO2 and Al buffer layers on Si of the annealing of the layered structure. X-ray diffraction studies reveal that on ZrO2-buffered Si the annealing of the layered structure leads to formation of 211 phase while on Al-buffer layer 123 phase is obtained. Noticeable effect in annealed-layered structure of oblique deposition of BaF2 from a Knudson Cell is seen in X-ray diffraction.
pp 435-441 April 1991 International Conference On Superconductivity—I
Ion beam and thermally-induced interface reactions between highTc superconducting thin film of Y1Ba2Cu3O7−x and metal overlayer of Ag are studied with a view to control the interfacial property of contact resistance. The interface reaction is induced by 100 keV Ar+ ion beam with different ion dose values ranging from 5 × 1013 to 3×1014 ions/cm2. The YBaCuO film-metal interface is characterized by using the small angle XRD to study the structural properties of the interfacial phases. The electrical property of the interface, specifically contact resistance, has been investigated for different dose values and thermal treatments. Three-probe vs four-probe configuration has been adopted to measure the contact resistance.
pp 443-449 April 1991 International Conference On Superconductivity—I
Thin films of Y-Ba-Cu-O superconductor have been deposited on different substrates by pulsed excimer laser ablation from a superconducting pellet. The dependence of various process parameters such as substrate temperature, laser energy density, oxygen partial pressure, applied bias field and cooling rates on the quality of the films has been studied.
pp 451-455 April 1991 International Conference On Superconductivity—I
Films of Y-Ba-Cu-O system have been prepared by rf sputtering from composite Y-Ba-Cu-O targets in an axial magnetic field in 4:1 Ar/O2 mixture on 〈100〉 SrTiO3 and 〈100〉 MgO substrates at 270°C and 450°C. Although back-sputtering caused due to negatively-charged oxygen ions was found to drastically vary the film composition about the centre of the sputtering axis, by properly orienting the substrates with respect to the sputtering axis near-stoichiometric films have been obtained. Films annealed at 900°C in oxygen have shown superconducting onset at 88°K and zero resistance transition at 72°K.
pp 457-459 April 1991 International Conference On Superconductivity—I
Plasma-spraying is a potential technique for forming flexible tapes from the brittle highTc oxides. It is possible to obtain superconducting YBa2Cu3Ox coatings by a suitable heat-treatment after spraying. In an effort to improve the critical current densities of the coatings, the effect of additions of Ag2O, Bi2O3 and SnO2 to the YBa2Cu3Ox powder used for spraying was studied. The maximumJc value of 75 A/cm2 was obtained for the sample doped with 5% SnO2.
pp 461-467 April 1991 International Conference On Superconductivity—I
Thin films of Y-Ba-Cu-O material have been prepared by rf diode sputtering using a single oxide target on strontium titanate substrates kept at an ambient temperature. It was found difficult to attain stoichiometry of the films identical to that of the target due to resputtering of the films. The influence of sputtering parameters such as target-substrate distance, rf power, gas pressure and substrate temperature in attaining a particular stoichiometry of the film has been studied. The I–V characteristics of the bridges show Shapiro steps when the bridges are irradiated with microwave radiations. The thin film bridges have the dimensions larger than coherence length; thus all the Josephson effect features are understood to arise due to intergranular junctions.
pp 469-472 April 1991 International Conference On Superconductivity—I
Y-Ba-Cu-O films were prepared by low temperature codeposition of three components. The Y and Cu contents were evaporated from metallic sources, while Ba was vacuum-evaporated from Ba, BaO and BaF2 sources in separate codeposition experiments. The lowest temperature at which superconducting YBa2Cu3Ox thin films (about 0.5µm thick) preparedin situ was near 500°C. This process enables preparation of superconducting films on various substrates (SrTiO3, MgO, Al2O3, Si) without a buffer layer. Zero-resistance critical temperature was as high as 88 K and the critical current density was 104 A/cm2 at 4.2 K. The morphology of the films was granular with disordered grain orientation, the average grain size being typically 0.5µm.
pp 473-477 April 1991 International Conference On Superconductivity—I
We describe the technique ofin situ preparation of thin (d=0, 1-1µm) YBCO films by the RF magnetron sputtering of the ceramic target in the argon-oxygen atmosphere on the heated substrates, and report the preliminary results of investigation of the multilayer structure based on such films.
pp 479-483 April 1991 International Conference On Superconductivity—I
Thin films of highTc superconductor YBa2Cu3O7−x were obtained by magnetron sputtering. MgO, YSZ, YSH and Al2O3 single crystals were used as substrates. Epitaxial films with tetragonal structure havingTc 55–60 K grow at substrate temperaturesTs between 930 K and 980 K. Orientation of the films in thisTs range was (100) and (001) for (100) MgO substrate, (111) and (001) for (1012) Al2O3 and (111) YSH and (113) or (103) on (110) YSZ and (111) YSH. Single crystalline films with orthorhombic structure and (001) orientation were grown on all the substrates whenTs exceeded 980 K. They haveTc>80 K.
pp 485-491 April 1991 International Conference On Superconductivity—I
Thin-textured YBa2Cu3Oy(Ag) films were prepared by a new CVD process consisting of aerosol deposition and low temperature annealing, both operations not exceeding 600°C. MgO and SrTiO3 single crystals were used as substrates. The stoichiometry of the prepared films was not ideal 1–2–3, having the atomic quotient of copper always lower than 3. For an unoptimized preparation process, the onset temperatures were around 80 K and the zero resistanceTc values around 70K. Properties of samples obtained by low-temperature annealing are compared with those obtained by high temperature annealing. “Cleaning” effect of vacuum-annealing upon removing the nitrate and hydroxide groups from the prepared films is supposed.
pp 493-499 April 1991 International Conference On Superconductivity—I
High temperature superconducting films of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O have been deposited on different substrates using conventional techniques, like flash evaporation and spray pyrolysis. The microstructural investigation of the films by SEM technique reveals the partially oriented nature of the crystallites. In the case of spray-deposited Bi-Sr-Ca-Cu-O HTSC films it has been found that film/substrate mismatch is not the decisive factor for the superconducting transition temperatureTc.
pp 501-502 April 1991 International Conference On Superconductivity—I
This paper considers the several deposition techniques currently in use for deposition of thin film superconductors with reasonably good results.
pp 503-510 April 1991 International Conference On Superconductivity—I
HighTc films belonging to Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems have been fabricated by reactive sputtering of single targets in a planar magnetron and Ar + O2 sputtering atmosphere. Although it was possible to deposit films of correct composition, resputtering related composition variation was a problem. The key to obtaining correct chemistry was a proper control of the deposition parameters. The pressure and oxygen content of the sputtering gas were found to be the most critical parameters. Results of the variation of these parameters on the cation chemistry are presented in this paper. Results from Monte-Carlo simulation of the sputtering process are also presented showing that low pressure and oxygen content of the sputtering gas result in a higher yield of energetic reflected neutrals which can cause compositional variation in the film mainly due to preferential sputtering of the growing film. The effect was particularly noticeable directly underneath the target. The energetic particle bombardment can be controlled by using moderately high pressures and low oxygen concentration in the gas.
pp 511-515 April 1991 International Conference On Superconductivity—I
Bi(Pb)SrCaCuO films have been deposited on single (MgO) and polycrystalline (poly MgO, CuO) substrates by DC sputtering. All the films became superconducting after post-annealing them in air. The films on single crystal MgO showedTc onset at 120 K andTc at 92 K. However, the extrapolated zero resistivity is obtained at 106 K. The ac susceptibility showed an onset at 106 K and a sharp transition at 85 K. The films on poly MgO and poly CuO had aTc=72 K. The preparation, X-ray diffraction and morphology of these samples are presented.
pp 517-520 April 1991 International Conference On Superconductivity—I
As-deposited superconducting films of Y1Ba2Cu3O7−δ with zero resistance at a temperature of ⋍ 80 K have been successfully grown using a Nd:YAG laser. A substrate temperature in the range 450–550°C was found most appropriate. The use of a Nd:YAG laser instead of an excimer laser is likely to improve the cost effectiveness of the laser ablation technique.
pp 521-524 April 1991 International Conference On Superconductivity—I
Thick films of Bi-Sr-Ca-Cu-O were deposited on (100) MgO substrates by screen-printing technique with the starting composition 1112. To attain the superconducting state, the films were subjected to two-step heat-treatment. R-T and XRD have been studied for films annealed at different durations of the second step. InitiallyTc (R=0) increased from 77 to 103 K as the annealing duration was increased after whichTc decreased. Kinetics of the growth of highTc phase is discussed in the light of our results.
pp 525-530 April 1991 International Conference On Superconductivity—I
Superconducting Bi-Sr-Ca-Cu-O films have been prepared on Sr2Ca2Cu4Oy substrates by thermal evaporation of metallic Bi layer and heat-treatment at 830°C for a few minutes in air. The zero resistance temperature of Bi2Sr2Ca1Cu2Oy film of 1–µm in thickness is about 70 K. The surface diffusion process of Bi on the granular structure of Sr2Ca2Cu4Oy was observed by scanning electron microscope and thermogravimetry analysis. The stripe pattern of superconducting film, typically 2µm thick and 0·3 mm wide, is formed by using a mask.
Volume 42 | Issue 5
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