Tl Ca Research Articles

Magneto-resistance and Excess Conductivity Studies of Tl(Ba2−y Mg y )Ca 2 Cu 3 O 10−δ Samples

The flux pinning characteristics of Tl(Ba2−yMgy)Ca2Cu3O10−δ(0.0≤y≤ 1.5) samples have been studied under the influence of various dc magnetic fields in the light of thermally activated flux flow model. It was observed form the magneto resistivity measurements that the activation energy U has been increased in the sample with Mg incorporation of y=0.5, however, it is decreased in the samples with higher Mg doping. This shows that lower Mg doping concentration promotes enhancement, whereas its higher concentration suppresses the flux pinning characteristics of Tl(Ba 2−yMg y)Ca 2Cu 3O10−δ (0.0 ≤y≤1.5) samples. The excess conductivity analysis of Tl(Ba 2−yMg y)Ca 2Cu 3O10−δ (0.0 ≤ y ≤ 1.5) samples has shown that the coherence length along the c-axis ξc(0), the interlayer coupling J, the phase relaxation time of the carriers τϕ, and the Fermi velocity vF of the carriers are suppressed with Mg doping. However, the values of Bc0(T), Bc1(T), and Jc(0) are enhanced which complement the magnetoresistivity analysis. These results showed that the flux pinning characteristics are enhanced for lower doping concentration of Mg at the charge reservoir layer sites.

Loss of circadian organization of sleep and wakefulness during hibernation.

We investigated circadian and homeostatic regulation of nonrapid eye movement (NREM) sleep in golden-mantled ground squirrels during euthermic intervals between torpor bouts. Slow-wave activity (SWA; 1-4 Hz) and sigma activity (10-15 Hz) represent the two dominant electroencephalographic (EEG) frequency components of NREM sleep. EEG sigma activity has a strong circadian component in addition to a sleep homeostatic component, whereas SWA mainly reflects sleep homeostasis [Dijk DJ and Czeisler CA. J Neurosci 15: 3526-3538, 1995; Dijk DJ, Shanahan TL, Duffy JF, Ronda JM, and Czeisler CA. J Physiol (Lond) 505: 851-858, 1997]. Animals maintained under constant conditions continued to display circadian rhythms in both sigma activity and brain temperature throughout euthermic intervals, whereas sleep and wakefulness showed no circadian organization. Instead, sleep and wakefulness were distributed according to a 6-h ultradian rhythm. SWA, NREM sleep bout length, and sigma activity responded homeostatically to the ultradian sleep-wake pattern. We suggest that the loss of sleep-wake consolidation in ground squirrels during the hibernation season may be related to the greatly decreased locomotor activity during the hibernation season and may be necessary for maintenance of multiday torpor bouts characteristic of hibernating species.

The microstructural evolution of a silver-containing spray deposited 1223 Tl–Ca–Ba–Cu oxide superconductor

This paper reports a study of the microstructural evolution of Ag-containing 1223 Tl-Ca-Ba-Cu oxide superconductors in spray-deposited films. The films were formed by spray depositing nitrates of Ca, Ba, Cu, and Ag onto a polycrystalline yttria-stabilized zirconia substrate. These deposits were then converted to a mixture of oxides (calcia, calcium-copper oxide, and barium cuprate) and metallic silver by heating in oxygen. When thallium oxide vapor was passed over the film, the thallium was incorporated into the film and the 1223 phase was formed. The evidence strongly suggests that the development of the 1223 superconductor involves the formation of a liquid phase. Our analysis suggests that the initial phase to form a liquid is CaO which contains thallium, barium, copper, and silver. Once this initial liquid is formed, it incorporates more thallium which, in turn, allows it to dissolve other types of oxides present in the film. In this way the liquid spreads across the surface. The equilibrium 1223 phase precipitates from it.

Structural studies on Tl–Ca–Ba–Cu−0 superconducting thin films

The microstructure of mixed phase, c-axis oriented Tl–Ca–Ba–Cu–O thin films, exhibiting excellent superconducting properties, has been examined by transmission electron microscopy and x-ray diffraction. Although the stoichiometry of the as-deposited films corresponds to the Tl2Ca2Ba2Cu3O10 phase (Tl-2223, c = 3.6 nm), x-ray diffraction showed these films contain up to 50% of the Tl2CaBa2Cu2O8 phase (Tl-2122, c ≍ 2.9 nm). The x-ray diffraction peaks of the 3.6 nm phase are noticeably broader than those of the 2.9 nm phase, suggesting a smaller coherent size along the c-axis for the 3.6 nm phase Lattice-fringe images of the 2.9 nm phase typically show only an occasional or no defect layer; however, such layer defects are more abundant in the 3.6 nm phase and, though random, occur with sufficiently wide separation and regularity to be consistent with an average coherency of ∼40 nm. The coherent sizes along the c-axis, as determined by x-ray line profile analysis, were 140 nm and 40 nm, respectively, for the 2.9 nm and 3.6 nm phases, in good agreement with those determined by electron microscopy. The estimated width of the layer defects observed in the lattice-fringe images is suggestive, for the most part, of the four Cu-layered phases (Tl-1324 or Tl-2324) and to a lesser degree the two Cu-layered phases (Tl-1122 or Tl-2122).

Phase stability and properties of near-equilibrium TlCaBaCuO superconductors

High-temperature phase stability limits of TlCaBaCuO superconductors were determined as a function of Tl 2O partial pressure at constant O 2 partial pressure (≈630 Torr), sample temperature (880°C), and sample Ca:Ba:Cu ratio (1:2:2, 2:2:3, and 3:2:4). Both phases with double CuO 2 sheets in the structure, 2122 and 1122, were stable over wide ranges of P(Tl 2O) for stoichiometric (1:2:2) samples. At relatively high P(Tl 2O), 2122 was also the stable superconducting phase even for the Ca-and Cu-rich stoichiometries. Energy-dispersive X-ray spectra and c-axis differences suggested variable cation stoichiometry in 1122 and 2122 prepared under such a range of conditions. However, the superconducting transition temperatures and a-axes of both 1122 and 2122 were nearly constant for all conditions of preparation, suggesting a nearly constant carrier density. No range of P(Tl 2O) was found where either 1223 or 2223 was stable as a single superconducting phase for the P(O 2) and temperature employed. Intergrown mixtures of 1223 and 2223 or Tl-free phases were found when P(Tl 2O) was below the 2122 formation boundary.

Superconducting electronics in the TlCaBaCuO System

The high‐Tc superconductor (HTS) Tl‐Ca‐Ba‐Cu‐O is an attractive material for applications because of its high transition temperature and structural properties. The bassic structural features, methods of film growth, device processing sequences, and some device and microwave circuit applications are discussed, with special attention being paid to active devices based both on Josephson junctions and on controlled flux flow in the non‐Josephson domain.

Processing of the Tl–Ca–Ba–Cu–O high-Tcphase via the sol–gel route

Tl2CaBa2Cu2O8 +x, and Tl2Ca2Ba2Cu3O10 +x superconductors have been produced via a sol–gel route. Thallium butoxide, calcium methoxyethoxide, barium methoxyethoxide, and copper(II) ethoxide were the alkoxide precursors for the synthesis. 2-Amino-2-methylpropanol was added to the copper(II) ethoxide in order to enhance its solubility in butan-1-ol, 2-methoxyethanol, methyl ethyl ketone, and toluene. Characterization of sols was done by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Samples were fabricated by heating the sols under vacuum, prefiring the resulting gels to 798 K, pelletizing, and firing at 1158 K in closed containers. DTA was performed to observe decomposition (432 K) and crystallization (691 K) reactions of the gels. The superconductors were characterized by X-ray diffraction, magnetic susceptibility, four-point probe resistivity, chemical analysis and scanning electron microscopy. The 2122 and 2223 superconducting phases were produced using 3 : 2 : 2 : 3 and 1 : 3 : 1 : 3 starting ratios, respectively. Tc(zero) values were 105 (2122) and 115 (2223) K with increases in these transitions occurring with firing times. Microstructural analysis revealed that a large degree of directional plate-like crystal growth occurred on the surface of the pellets.

Transport and pinning in high quality TlCaBaCuO films

TlCaBaCuO High temperature superconductors, especially Tl 2Ca 2Ba 2Cu 3O 10 with a T c of 120K, are of considerable interest for practical applications. Thin films in this system have demonstrated high T c′s, compositional tolerance, and a lack of weak links at the grain boundaries. Flux pinning in the system is appreciably weaker than the YBa 2Cu 3O 7 materials as evidenced by large magnetic relaxation and the decrease in critical current density at high magnetic field. Recent work has evaluated the effect of improving film quality on the pinning and explored methods of increasing the pinning. We have made substantial improvements in the film quality that resulted in “decreased” pinning; ion irradiation slightly enhances the pinning.

The synthesis of superconducting Tl–Ca–Ba–Cu–oxide films by the reaction of spray deposited Ca–Ba–Cu–oxide precursors with Tl2O vapor in a two-zone reactor

Superconducting Tl–Ca–Ba–Cu–oxide films have been prepared on yttria stabilized zirconia substrates via the reaction of Tl2O vapor with precursor Ca–Ba–Cu–oxide films prepared by the spray pyrolysis of a solution of the metal nitrates. The vapor reaction process, evaluated in both air and oxygen ambients, was carried out in a two-zone reactor which permitted the independent control of the temperatures of the sample and of a boat containing thallium oxide. Sample temperatures of 865–905 °C and boat temperatures of 775–870 °C were investigated. X-ray diffraction analysis of the best samples, which were prepared in an oxygen ambient at sample temperatures of 895–900 °C and boat temperatures of 805–810 °C, revealed the presence of a highly oriented Tl2Ca2Ba2Cu3O10+y phase with a trace of the Tl2CaBa2Cu2O8+y phase. FWHM's of ∼2°θ were obtained in x-ray rocking curve analyses of the oriented 2223 phase. Tc (0) values of 95–105 K were measured for the better samples, and a zero field critical current density of 28 800 amps/cm2 was measured at 77 K for the best sample. SEM micrographs taken of the highest Jc samples reveal a highly textured structure consisting of platelike crystals.

Annealing study on single crystal Tl-2223 superconductors

We have shown that crystals in the TlCaBaCuO system are robust to anneals (oxygen, vacuum or 10%H 2/90%N 2) to 500°C. Improvements in the superconducting properties, as determined from Meissner data, are observed for anneals above 300°C and suggest that as-grown crystals may not have fully equilibrated during growth despite an extended cool down cycle. There appears to be no systematic shift in T c with oxygen content in the TlO layer. Single crystal X-ray structure refinements have determined small atomic displacements which, in part, suggest the possibility of small amounts of Tl 1+ being present in the TlO layer of as-grown crystals which subsequently oxidizes during mild oxygen anneals to Tl 3+. Hence, changes in oxygen content may not result in altered carrier concentration in the CuO layers. Further, the nature of the oxygen position in the TlO layer, as determined by the usual least-squares refinement of X-ray data, appears to be related to the amount of oxygen present in the particular sample.

A study of the structural series in the TlCaBaCuO superconducting system

Three possible superconducting phases have been identified in the system TlCaBaCuO using a combination of energy dispersive X-ray emmission spectroscopy (EDS) and high resolution electron microscopy (HREM). HREM image contrast confirms that a structural series exists in this system, all the members of which are superconducting, and that these share a basic structure related to that of the superconducting phase in the BiSrCaCuO system.

Formation route of higher member phases from lower ones in the Tl–Ca–Ba–Cu–O system

An experiment was designed to follow the formation of higher member phases from lower member phases in the Tl2Can−1Ba2CunO2n+4 series. Coarse particles of the lower member phase were directly interfaced with various Ca–Cu–O sources and the pressed pellets were heat treated in air in a gold pocket at 850 °C for a period of 2–4 h. The reacted samples were polished and examined by scanning electron microscopy with energy dispersive x-ray spectrometry analysis (EDS). For the 2021 based reaction systems, the results suggest that the 2122 phase can be formed via reactions which involve the decomposition of the 2021 phase, the formation of a complex liquid phase, and finally the reaction between this liquid phase and a Ba/Ca compound. The formation of the 2223 phase from the 2122 phase is likely to follow a similar reaction route. The involvement of a liquid phase in the formation of the higher member phases may lead to a glassy phase, and hence to a barrier to the achievement of a high critical current density.

FORMATION REACTIONS OF THE Tl2Ca2Ba2Cu3O10 AND Tl2Ca1Ba2Cu2O8 PHASES

Solid state formation of the superconducting phases in the Tl–Ca–Ba–Cu–O system has been studied by DTA, TGA , and XRD analyses. It was found that the formation reaction of the high Tc Tl 2 Ca 2 Ba 2 Cu 3 O 10 and the low Tc Tl2Ca1Ba2Cu2O8 phases takes place at around 900°C and 840°C respectively, within a reaction time of 30 minutes. Activation energies of these reactions are found to be 50 kcal/mol and 35 kcal/mol, respectively. The use of precursor Ca 3 BaCu 3 O x yielded superconducting materials with superior characteristics as compared to those prepared from the direct mixture.

Synthesis of TlCaBaCuO superconductors

Starting compositions of Tl 2Ca 2Ba 2Cu 3O x, TlCa 2Ba 2Cu 3O x, and TlCa 3BaCu 3O x were used to synthesize TlCaBaCuO superconductors. The atmosphere was pure oxygen of pressure 10 5 to 6 × 10 5 Pa, and the temperature range was 890 to 940°C. Chemical analysis, X-ray diffraction, differential thermal analysis, scanning electron microscopy, and resistivity and magnetic susceptibility measurements revealed that the TlCa 3BaCu 3O x composition formed the most phase pure compound. The highest T c was 120 K.

The formation and role of a phase with glassy appearance in the Tl-Ca-Ba-Cu system

A phase with a glassy appearance has been detected in samples of the Tl–Ca–Ba–Cu system. The presence of such a phase is expected to have a negative effect on the critical current density in a bulk superconductor. Samples of 2-2–2-3 starting composition were prepared from Tl2O3, BaO2, and relatively large particles of CaO and CuO. These samples were then heat-treated at 867°C for various periods of time. The use of large particles of CaO and CuO facilitates the examination of the morphological changes associated with the formation of the superconducting phases. Using scanning electron microscopy, a layer of the 2122 phase was found to form on the surface of the large CaO particles whereas near the surface of the large CuO particles, large voids were observed. By varying some of the experimental conditions, a liquid phase with a composition close to 2-0–2-3 was inferred to form. The voids observed in the surroundings of the CuO particles may be due to the formation of such a melt. This liquid phase, after cooling, apparently leads to the “glassy” phase detected in the reacted samples. This glassy phase may play a role as an intermediate in the formation of the 2122 phase, which could result by a direct reaction between the liquid phase and CaO, A part may remain, however, among the superconducting grains, thus having a negative effect on the critical current density.

A sol-gel route to prepare Tl Ca Ba Cu O superconductor with Tc above 120 K

The preparation of Tl Ca Ba Cu O superconductor with Tc above 120 K from Ca Ba Cu O precursor mixed with thallium oxide was obtained. The Ca Ba Cu O precursor was prepared by sol-gel method from the proper complexation and polymerzation of the metal nitrates by citric acid and ethylene diamine. The excellent homogeneity and small particle size make the sol-gel precursor a potential candidate for Tl Ca Ba Cu O film preparation by screen-printing or spin-coating and thallium vapor diffusion.

Superconducting Tl Ca Ba Cu O thin films prepared by diffusion of Tl into an excimer laser ablated Ca 2 Ba 2 Cu 3 O x film

Abstract By laser ablation with an excimer laser (308 nm) cuprate films from a Ca 2 Ba 2 Cu 3 O x > sintered bulk target were deposited onto SrTiO 3 (100) single crystal substrates. Thallium was added by a diffusion process by simultaneously annealing of the cuprate films together with a sintered Tl 2 Ca 2 Ba 2 Cu 3 O 10 bulk sample both enclosed in a covered metal box; this film production method allowed to reduce the contamination by thallium. Scanning electron micrographs of the film surface show crystalline structures, i.e. a network of needle shaped crystallites laying in the film plane. For comparison, Nd:YAG laser produced films show rough surfaces consisting of clusters of grains.

Ultrasonic attenuation anomaly of Tl Ca Ba Cu O at superconducting transition

Abstract For longitudinal as well as transverse waves, a frequency independent ultrasonic attenuation anomaly was observed at temperature close to the superconducting transition temperature T c (103K) of a polycrystalline TlCaBaCuO. Temperature derivative of shear wave velocity at 10 MHz shows a discontinuity around T c . These variations of attenuation and velocity near T c may be intrinsic to high temperature superconductors.

Comparison of flux pinning in superconducting Tl Ca Ba Cu O crystals and thin films

We report direct observation of the flux pinning behavior of a Tl 1 Ca 2 Ba 2 Cu 3 O y highly oriented thin film, a randomly oriented polycrystalline thin film and a Tl 2 CaBa 2 Cu 2 O y single crystal. From the temperature dependence of the critical current densities J c at zero field, activation energies U 0 are estimated to be 110, 110 and 40 meV, respectively. Strong time-logarithmic magnetic relaxation M ( t , T ) was observed both in single crystals and in thin films. Within a thermally activated flux motion (TAFM) model, the distribution of activation energies ϱ ( U 0 ) is determined from data on the time and temperature dependence of M ( t , T ) for the highly oriented film. The presence of a high-energy tail in the distribution suggests that the TAFM model can not be described by a single activation energy U 0 ; the effective U 0 value increases rapidly with temperature.

Optical emission studies and preparation of high T c Tl Ca Ba Cu O thin films

Abstract Thin films of Tl Ca Ba Cu oxide were sputter-deposited from dual oxide targets using a symmetrical RF diode sputtering system. Spatially resolved optical emission spectra obtained using a scanning optical fiber for CuO, BaO-doped CuO, and Tl Ca Ba Cu O targets suggest no significant bombardments of the growing film by energetic particles in the plasma, resulting in a film composition close to that of the sputtering targets used. The superconducting properties of these films were found to depend strongly on the post-annealing conditions as a result of a substantial loss of thallium during annealing. Highly textured c-axis oriented films comprised mostly of Tl2Ca2Ba2Cu3Ox, with a zero resistance Tc of 122 K have been synthesized on MgO substrates by annealing in a sealed quartz tube of films wrapped in gold foil with Tl2Ca2Ba2Cu3Ox powders.