Microbalance and Related Studies of High-temperature Superconductors

Many properties of oxide superconductors depend on the oxygen concentration and its distribution in the samples. The microgravimetric method is very useful to study of oxide superconductors, as it allows investigations in vacuum and controlled environments in classical applications: thermogravimetric analysis for the study of solid-state reactions, determinations of oxygen contents in redox reactions and the combined measurement of mass and additional parameters, e.g. evolved gas analysis etc. Single-phase samples of high-temperature superconductors were synthesized from stoichiometric mixtures of high-purity oxides and carbonates. Appropriate amounts of the precursor powders were homogenized manually or by a mechanical ball mill and subsequently calcined at temperatures in the 800 to 950°C range with intermediate grinding to ensure homogenous reaction. The lattice parameters of all preparations were controlled, in both initial and final experiments, by the X-ray powder method (CuKα radiation), using a Stadi P (Stoe) diffractometer with a position-sensitive detector. It is well known that temperature and mass can be measured with an accuracy higher by orders of magnitude than it is still possible for the thermoanalyst to determine the transformation temperatures or the mass changes due to overlapping partial reactions. Applications of Cahn microbalance to study of high-temperature superconductors are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermodynamic modelling of CVD of high-T c superconductors

Thermodynamic modelling of CVD of the superconducting phases in the Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O systems is reviewed. Both MOCVD and halide CVD are considered. The results are discussed in terms of calculated CVD stability diagrams. The influence of deposition parameters, including selection of precursors, on the existence and extension on those stability regions, where the superconducting phases can be deposited, is described. The results of the thermodynamic modelling are compared with experimental investigations. The agreement between thermodynamics and experiments is satisfactory, indicating that thermodynamic modelling can be used as a guide to predict the optimum deposition conditions.Financial support from the Swedish National Board for Industrial and Technical Development is gratefully acknowledged.     

Thermal properties of high-T c superconductors

The termal stabilities of several families of high-T _c superconductors (HTSC), as well as the dependence of phase transitions on temperature and stoichiometry, have been studied by X-ray diffraction, DTA, TG and DSC. Experimental results are discussed in the context of decomposition models.     

Microgravimetric Investigations of High-Temperature Superconductors

It has been confirmed that, in oxide superconductors, the superconducting transition temperature T_c depends strongly on the oxygen content and oxygen ordering. The microgravimetric method is very useful in the analysis of oxide superconductors, because it allows investigations in vacuum and controlled environments in classical applications: thermogravimetric analysis, kinetics and thermodynamics of reaction, determination of oxygen contents in redox reactions, investigations of the correlation of the deviation from stoichiometry and carrier concentration and also the combined measurement of mass and additional parameters, e.g. evolved gas analysis, etc. Selected papers are reviewed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Far infrared reflectivity of narrow band materials

The application of a Fourier spectrometer to the study of the electronic structure in the far infrared of some intermediate valence or heavy fermion materials or of highT _c superconductors showed that FT-reflectivity is a powerful tool of investigation at very low energies. A very careful control of the experimental conditions and of the reference measurements is necessary to eliminate spurious reflectivities.The results show optical transitions at very low energy and permit to evaluate the electronic structures and the effective mass of the carriers in the vicinity of the Fermi level.     

Time and temperature-based study for the production of high T C phase by sol-gel technique in Pb-bscco system

The high T_C superconducting phase Bi₂Sr₂Ca₂Cu₃Ox (2223) in the Pb-BSCCO system has been produced by EDTA-gel processing using nitrate solutions. The precursor has heated in two stages, at 300 and 800°C each for 2 h, to avoid the burning of the important species involved in the final product. The effects of time (6 to 48 h) and temperature (845 and 855°C) on the formation of the 2223 phase have been studied by sintering the samples in air. Thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and a vibrating sample magnetometer (VSM) have been employed to investigate the powder produced at different stages of decomposition, oxidation and formation of sintered materials from the powders. The volume-fraction of the 2223 phase at 845°C increases with time, the maximum value of the 2223 phase was obtained at 120 h. It has been observed that the formation of the high T_C phase is remarkably enhanced at the temperature of the endothermic peak of the DTA curve. The best result has been obtained in the sample sintered for 24 h at the temperature 855°C (endothermic peak). This also indicated that at 855°C, the large volume-fraction of 2223 phase with T_C 113 K grew in short time and as the sintering time increased, it decomposed into the Bi₂Sr₂CaCu₂Ox (2212) phase and other phases.     

Sodium and silver phosphate glasses doped with chlorides of Fe,Mn and Zn

A number of samples of sodium and silver phosphate glasses doped with various compositions of some transition metals viz. iron, manganese and zinc chlorides alongwith undoped samples of sodium and silver phosphate glasses were synthesized and characterized by X-ray diffraction, IR spectral, electrical conductivity and differential scanning calorimetry (DSC). The glass transition temperature (T _g) and crystallization temperature (T _c) values obtained from DSC curves were found to increase with increasing concentration of the dopant Fe/Mn/Zn chlorides in both sodium and silver phosphate glasses and the following sequence is observed: T _g(–FeCl₃)>T _g(–MnCl₂)>T _g(–ZnCl₂) T _c(–FeCl₃)>T _c(–MnCl₂)>T _c(–ZnCl₂) The increase in T _g and T _c values indicate enhanced chemical durability of the doped glasses. The electrical conductivity values and the results of FTIR spectral studies have been correlated with the structural changes in the glass matrix by the addition of different transition metal cations as dopants.     

Vibronic pairwise charge transfer in copper-oxide sheets: A possible approach to high temperature superconductivity theory

Summary To accommodate the number of holes and fractional number of atoms in doped highT _c superconductors, and to produce a periodic structure with given symmetry, we postulate a quadruple cell with four copper atoms on the CuO₂ layer. The quadruple cell structure hasD _2h symmetry which can be distorted toC _2h geometry underB _1g vibration. Such a structure allows the interconversion of different spin angular momenta into paired spins similar to Cooper pairs. It also provides vibronic interactions that lower the energy of the ground state. For electron (hole) pairing, we construct the running wave Bloch sums consisting of linear combination of bonding/antibonding geminals (instead of one-electron atomic orbitals) in these quadruple cells. For bond movement we construct the Bloch sums consisting of linear combination of Covalon waves in quadruple cells related to the movement of conjugate (alternating) bonds. In both cases the pair-wise charge (hole/electron) transfer is coupled with antisymmetric vibrations under a double-well potential related to Peierls distortion. The vibronic mixing of different running bonds with different antisymmetric vibrations at various distances, accounts for the different long-range order of charge transfer. Our formulations represent an alternative view of BCS theory, Bisoliton theory and Resonanting Valence Bond theory by using a quantum chemical, position-space approach to a more tight binding situation.     

The formation of superconducting oxide phases in the (Bi,Pb)−Sr−Ca−Cu−O System

The crystallization mechanism of superconducting phases in the (Bi,Pb)?Sr?Ca?Cu?O system was determined on the basis of the results of DTA, DTG and TG studies, supplemented by X-ray examination of ceramic powders obtained by the sol-gel method. It has been demonstrated that the factor determining the formation of superconducting phases: Bi₂Sr₂CaCu₂O_x (low-T _c ) and Bi₂Sr₂Ca₂Cu₃O_x (high-T _c ) is the kinetics of reaction of calcium and strontium carbonates with molten CuBi₂O₄. As a result of the reaction of the bimetallic compound CuBi₂O₄ with SrCO₃ in the liquid phase the compound Bi₂Sr₂CuO₆ is formed. This compound, reacting with calcium and copper oxides, yields superconducting phases: the low-T _c and the high-T _c phase. It has been also observed that an increase in the volume fraction of high-T _c phase in powder subjected to thermal treatment takes place probably due to the repeated disproportionation of low-T _c phase and its repeated synthesis from Bi₂Sr₂CuO₆, CuO and CaO.     

GaPO4 high temperature crystal microbalance demonstration up to 720°C

Quartz-homeotypic gallium (ortho-) phosphate, GaPO₄, is of special interest for resonator applications asking for temperature compensated cuts with higher electro-mechanical coupling than quartz and operational temperatures up to 970°C. The crystal microbalance technique, well known for quartz (QCM) which can be used only at moderate temperatures, can now be extended to much higher temperatures using GaPO₄ crystals, benefiting from all three advantages mentioned above. Two different experiments were done to demonstrate the advantages of a crystal microbalance based on GaPO₄. First, the GaPO₄ resonator was used for film thickness determination and compared with a commercial QCM. This experiment demonstrated that the measuring range can be extended by using GaPO₄ resonators instead of quartz. The second experiment demonstrates the possibility for thermogravimetric analysis up to 720°C by using a new concept for resonator mounting.This revised version was published online in November 2005 with corrections to the Cover Date.     

Atomistic lattice simulation of high Tc oxides

Summary A survey is outlined of some of the normal-state crystal properties of high T_c oxides that have been calculated recently by atomistic lattice simulations.     

Hole concentration in the three-CuO2-plane copper-oxide superconductor Cu-1223

Strongly overdoped samples of the three-CuO₂-plane copper-oxide superconductor, CuBa₂Ca₂Cu₃O_8+z or Cu-1223, were obtained through high-pressure synthesis and post-annealed to various hole-doping levels so as to have the value of T_c range from 65 to 118 K. A concomitant decrease in the average valence of copper from ∼2.20 to ∼2.05 was evidenced by means of wet-chemical and thermogravimetric analyses and Cu L-edge X-ray absorption near-edge structure (XANES) spectroscopy. The valence value as low as ∼2.05 that corresponds to the highest T_c (=118 K) may be understood by taking into account multiple ways for holes to be distributed among the different Cu-O layers. In terms of actual chemical composition of the Cu-1223 phase, both Cu L-edge and O K-edge XANES results suggest that some portion of charge-reservoir copper atoms may have been replaced by CO, i.e., (Cu_1−xC_x)Ba₂Ca₂Cu₃O_8+x+z. The variation range of excess oxygen was estimated at Δz≈0.3.     

Approximation to the laser floating zone preparation of high temperature BSCCO superconductors by DSC

Differential scanning calorimetry (DSC) has been used to simulate laser fusion of ceramic precursors to prepare BSCCO high temperature superconducting materials. At fast heating rates, typical of the laser floating zone, the energy required for complete melting of the precursor increases with the Bi to Sr+Ca ratio, in agreement with the critical current results obtained. At low heating rates, solid-state reactions involving ionic rearrangements take place before incongruent melting. These changes can be divided into three processes and can be quantified from calorimetric measurements. Lower heating rates and isothermal experiments allow detailed visualisation of the ionic arrangements taking place. The results prove that the mechanism of the final phase formation is dependent on the fabrication procedure.     

The dual role of cytochrome c2 in the facultative phototrophic bacteriumRhodopseudomonas capsulata

Succinate oxidation is stimulated by addition of cytochrome c₂ in cytochrome c₂-deficient spheroplasts from the M6-mutant and from the wild type strain ofRhodopseudomonas capsulata. Inhibition of the alternative oxidase in the wild type by CO facilitates this observation. The finding confirms a dual role of cytochrome c₂, in photosynthetic and in respiratory electron transport.

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Die doppelte Rolle von Cytochrome c₂ in Rhodopseudomonas capsulata

Zusammenfassung Die Oxidation von Succinat wird durch Zugabe von Cytochrom c₂ zu Cytochrom c₂-defizienten Sphäroplasten der Mutante M6 und des Wildtyps vonRhodopseudomonas capsulata stimuliert. Die Hemmung der alternativen Oxidase durch CO im Wildtyp erleichtert diese Beobachtung. Der Befund bestätigt die Doppelrolle von Cytochrom c₂ im photosynthetischen und respiratorischen Elektronentransport.

     

TheT g displacement measurements: A way to foresee the physical behaviour and the use of glassy polymers

The effect of ageing temperature (belowT _g) on physical ageing of glassy PET has been investigated by differential scanning calorimetry. The position of the glass transition endotherm was found to be strongly dependent on the annealing conditions (time, annealing temperature). A single relaxation time model is proposed for the analysis of physical ageing.The variations of the equilibrium relaxation time _iso with annealing temperature allow the determination of the parameters of the model. Finally, using these constants, one can determine the thermal past and foresee the thermal future of unknown samples which are of prime importance in processes using temperatures around the glass transition temperature.

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Zusammenfassung Mittels DSC wurde der Einfluß der Alterungstemperatur (unterhalbT _g) auf das physikalische Altern von glasartigen PET untersucht. Die Lage der Glasumwandlungsendothermen hängt stark von den Temperungsbedingungen (Dauer, Temperungstemperatur) ab. Es wird ein Relaxationszeitmodell für die Analyse physikalischen Alterns vorgeschlagen.Die Parameter dieses Modelles knnen aus der Änderung der Gleichgewichtsrelaxationszeit _iso mit der Temperungstemperatur ermittelt werden. Unter Benutzung dieser Konstanten kann die thermische Vergangenheit unbekannter Proben bestimmt und die thermische Zukunft derselben vorhergesagt werden, was für Vorgänge bei Temperaturen um den Glasumwandlungspunkt von primärer Wichtigkeit ist.

     

Analysis of thermo-magnetic fluctuations above the glass-transition temperature in Bi1.6Pb0.5Sr2−xEuxCa1.1Cu2.1O8+δ (0.000 ≤ x ≤ 0.180) system

The resistivity of Bi_1.6Pb_0.5Sr_2−xEu_xCa_1.1Cu_2.1O_8+δ (0.000 ≤ x ≤ 0.180) superconductor has been measured as a function of temperature and magnetic field. The resistivity shows a glassy behavior even at higher temperatures and magnetic fields for the Eu-doped samples as compared with the Eu free sample. The values of glass-transition temperature [T_g], magnetic field dependent activation energy [U₀(B)] and the temperature and magnetic field dependent activation energy [U₀(B,T)] are found to be maximum for optimal doping levels (x = 0.135) which shows that the flux lines are effectively pinned in this sample. Also for temperatures below the superconducting transition temperature (T_C), a scaling of measured resistivity curves in magnetic field (B = 0.4 and 0.8 T) is obtained and this scaling is quite useful for better understanding of the behavior of the flux vortices in high temperature superconductors.     

Kleppa type calorimeter for the study of high temperature processes

A Kleppa type microcalorimeter was designed and constructed for the application to measure the thermal change in a reaction tube at T>1300 K. The details of new calorimeter are described. In order to verify the performance of the new calorimeter, the standard enthalpies of formation for NiSi and Ni₃Al were measured by direct synthesis in the calorimeter at T=1323 K. The enthalpies of formation obtained were −(79.3 ± 1.8) and −(189.3 ± 6.3) kJ · mol⁻¹ for NiSi and Ni₃Al, respectively. A comparison of results in the present work with available reported values has been made.     

A ductile-brittle zone in polyimides depending on the reaction temperature

The mechanical and optical properties of polyimides were studied in this paper and the influence of the variation of the reaction temperature on the physico-chemical properties of the polymers was evaluated. From this the dependence of the stress-strain diagrams on the reaction temperature, as well as the stresses and strains at fracture were experimentally determined. Moreover, the elastic moduli and Poisson's ratios, as well as the refractive index of the polymers were evaluated for different temperatures of imidization. In order to define also the behaviour of the polymers as thin membranes at fracture, simple tension tests with edge-cracked thin strips were executed up to fracture. The method of caustics was used, with the specimens loaded inK _I mode of deformation at different stress-levels to evaluate the stress intensity factors of the materials in the non-linear zone of loading. TheK _I -factor was evaluated by applying the simple Dugdale-Barenblatt model for the ductile materials, whereas for brittle samples the elastic theory was used. Interesting results concerning the physico-mechanical properties of the polyimides were derived.     

Electron kinetics in a collision-dominated SiH4 rf plasma including self-consistent rf field strength calculation

The electron kinetics of a collision-dominated rf plasma in silane has been studied by solving the nonstationary electron Boltzmann equation. Ionization and attachment processes and the spatially averaged electron loss to the plasma wall by ambipolar diffusion have been included in the kinetic approach. This makes it possible to calculate, in addition to the time-resolved energy distribution, the self-consistent rf field amplitude which is necessary for the maintenance of the steady-state rf discharge. The impact of the rf field frequency, of the density ratio of negative ions to electrons, and of superelastic (second kind) collisions with excited silane molecules was studied. In particular, large, rf field amplitudes of about 100 V cm^–1 Torr^–1 result, connected with large modulations of the energy distribution for field frequencies in the megahertz region.     

Electronic structures of superconductors YBa2Cu3−x M x O y (M=Sn,Ni)

Based on the EHMO approach, the energy band structures for superconductors YBa₂Cu_3–x Sn _x O _y (y>7) and YBa₂Cu_3–x Ni _x O_y (y<7) were calulated in the present paper. The influence of the cation doping at the Cu site in the unit cell and the oxygen content on their electronic structures was studied. The results showed that the cation doping at the Cu site resulted in the great decreases in the bandwidths of the broad anisotropic Cu-O bands and the densities of states. In YBa₂Cu_3–x Sn _x O _y , however, these decreases are compensated by the increase in the oxygen content caused by the Sn-doping, which results in a small change in the total densities of states. For YBa₂Cu_3–x Ni _x O _y , the effect of the doping on its electronic structures in dominant. The Ni-doping, therefore, results in a great change in the electronic structures. In addition, the study on the projected densities of states of the Ni-doped system revealed that the 2D Cu-O planes in the Y-Ba-Cu-O system played a dominant role in superconductivity.