Some thermodynamic aspects of highT c superconductors

Thermochemical and thermodynamical properties of HTSC phases are reviewed for the Y-Ba-Cu-O system and also presented for the newly calculated Bi-Sr-Cu-O system stressing out stoichiometric and phenomenological viewpoints. Simulated data are listed for (H ₂₉₈ ^o -H _o ^o , phase transformation temperatures, standard entropies, standard enthalpies of formation, heat capacities in crystalline phase, etc. Pseudobinary phase diagrams are treated showing the effect of oxygen partial pressure particularly illustrated on the (Sr, Bi, Ba)-Cu-O system.The work was carried out under the project No. A 2010532 supported by the Grant Agency of Academy of Sciences of the Czech Republic and the grant No. 104/97/0589 financed by the Grant Agency of the Czech Republic.     

Thermal characterization by photodeflection method

The photodeflection technique is useful not only for thermal diffusivity measurements but also to supply a thermal imaging system. The experimental setup and the basic theoretical aspects for determining the temperature profile are discussed together with the experimental results on a semiconductor laser diode.     

Thermoanalytical studies of the processing of bulk and thin film BSCCO highT c superconductors by the Edita-Gel route

The decomposition of EDTA gel precursors for BSCCO superconductor manufacture has been studied using STA (TG/DTA) and dilatometry in conjunction with FTIR. Ther thermoanalytical data are discussed in relation to the sequence of phase formation necessary for the production of the 2223 BSCCO superconducting phase. Thin film preparation of Bi-based highT _c superconductors have been carried out on MgO (100). Grain orientation of oxide thin films has been investigated. Well orientated 2212 grains have been achieved, with the (001) planes parallel to the substrate. The EDTA-gel method has been modified by the addition of glycerol to achieve the appropriate viscosity for spin coating. STA has been used to study the decomposition of these modified gels for the formation of thin films.     

Synthesis and characterization of high temperature superconductors

The high temperature superconductors of the system Bi-Pb-Sr-Ca-Cu-O show promising properties and large scale applications can be considered. The (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀₊ (Bi,Pb(2223)) phase is attractive because of its high superconducting temperature (110 K) and high current transport capabilities.Simultaneous DTA/TG has proved an excellent tool for synthesizing, characterizing these high temperature superconductors, confirming phase diagrams and finding the optimal annealing temperature for Bi,Pb(2223)/Ag tapes in order to obtain high critical current densities around 30 kA cm^–2 at 77 K and 0T. The identification of the main DTA peaks of measurement of different phase mixtures made it possible to observe the formation of an intermediate phase Bi,Pb(2212) during the formation of Bi,Pb(2223).     

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.     

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 barium complexes used for depositing thin films of YBa2Cu3O7 superconductors

The thermal properties of several barium containing complexes are compared by using thermogravimetric analysis and solids probe mass spectrometry. The properties of complexes containing flourinated and non-fluorinated β-diketonates with and without the presence of a polyether to occupy additional barium coordination sites are compared. Barium complex thermal properties are heavily dependent on the structure of the β-diketones and polyether comprising the complex. Barium complexes containing fluorinated ligands are better suited for chemical vapour deposition of barium than complexes containing non-fluorinated ligands because they efficiently evolve vapours containing barium at lower temperatures.     

Sol-gel route for obtaining bismuth high T c superconducting oxide compounds

Bismuth high T _c superconducting oxide compounds were obtained by sol-gel method using acetate precursors. A study of the decomposition of quasi-amorphous acetates has been carried out to establish the conditions of superconducting phase formation. A comparison of the acetate sol-gel route with the oxalate coprecipitation method (previously used by the authors) reveals comparable results. Both methods require much shorter periods of thermal treatment for the formation of superconducting phases, as compared with the solid state reactions. The sol-gel route leads to better results from this point of view.     

Analytical TEM investigations of high-temperature superconductors

Electron diffraction and energy dispersive X-ray spectroscopy as analytical transmission electron microscopic methods have been applied to Y-Ba-Cu-O superconductors. The evaluation of diffraction patterns by means of the MS-WINDOWS program ELDISCA is demonstrated. The course of X-ray linescan signals in the transition range between YBa₂Cu₃O_7– matrix and Y₂BaCuO₅ inclusions is explained by calculations based on a mathematical model which is described. Errors of quantiative X-ray spectroscopic results and their reasons are shown.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Type H superconductors and the vortex lattice.

Superconducting material is used, for example, in magnetic resonance imaging for medical examinations and particle accelerators in physics. Knowledge about superfluid liquids can give us deeper insight into the ways in which matter behaves in its lowest and most ordered state. Work by the author on superconduction in liquid helium established the existence of type II superconductors and proved that vortex lattices exist in superfluid helium, in the presence of magnetic fields. He showed that the Ginzburg-Landau theory could be extended to include this "new" type of superconductors, which today are in common use. His work on phase transitions of these superconductors under the influence of magnetic fields was groundbreaking, although he has worked in many other areas since then. He was awarded the Nobel prize in 2003 "for pioneering contributions to the theory of superconductors and superfluids" with V L. Ginzburg and A. J. Leggett.     

The application of microbalance technique in study of the high temperature superconductors

Many physical-chemical properties of oxide superconductors depend on oxygen content and its distribution in the sample. Investigations of the thermal decomposition of the oxide superconductors in dynamic vacuum are valuable, particularly in view of the high oxygen diffusion coefficient in these cuprates. Dynamic vacuum protects against the readsorption of oxygen from the environment. Samples of the high-temperature superconductors were synthesised from stoichiometric mixtures of high-purity oxides and carbonates.The lattice parameters of all preparations were controlled, in both initial and final experiments using a Stadi P (Stoe) diffractometer with a positron-sensitive detector (CuK radiation). The oxygen content was measured by iodometric titration with a reproducibility of at least 0.02.DC four probe resistivity measurements were performed on pellets to which copper contacts were attached by using silver paint. Thermogravimetric analyses (TG) were performed by using a Cahn RG ultramicrobalance system.The purpose of the present paper is to show the application of microbalance technique in study of high-temperature superconductors.This revised version was published online in November 2005 with corrections to the Cover Date.     

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.     

Determination of total heterogeneity and fractal dimensions of high-temperature superconductors

Using thermo-analytical and sorptometric methods physicochemical properties and especially surface heterogeneity of HgBa₂Ca₂Cu₃O₈₊, (Hg-1223) was investigated. The desorption energy distribution was derived from mass loss Q-TG and differential mass loss Q-DTG curves of thermodesorption in quasi-isothermal conditions of pre-adsorbed n-octane and water vapour. It is shown that the superconducting Hg-1223 phase is highly sensitive to water vapours. The mechanism of water adsorption depends largely on the activation time. By water vapour saturation in a period of 90 min, physisorption takes place. Prolonged periods result in a chemical decomposition. From nitrogen ad- and desorption isotherms the fractal dimension of superconductors were calculated. A new approach is proposed to calculate fractal dimension from Q-TG curves.This revised version was published online in November 2005 with corrections to the Cover Date.     

On the chronicle of high-T c oxide superconductors

After briefly noting some development records in phase diagrams, the stoichiometry is dealt with in more details showing the structural layer ordering in the superconducting cuprates and mentioning some correlation between thermodynamics and BSC theory of superconductivity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Laser production of clusters as precursors of ceramic materials, semi- and superconductors

In this paper, a short review of the processes induced by laser ablation of ceramic materials and precursors of semi- and superconductors is presented. After a short introduction on methods such as physical and chemical vapor deposition, the formation and properties of cluster intermediates of the laser vaporization technique are discussed. Some examples of the reactions taking place in the plume produced by the interaction of laser light and ceramic materials are also given.     

Thermal conductivity analysis of polymeric crystal by mirage effect

Photothermal Techniques are based on the conversion of the modulated light energy into heat within the sample. Using the Photothermal Probe Beam Technique, where the analysis of a laser beam deflected by the mirage effect near the sample leads to the thermal properties of this sample, we have determined the three components of the thermal conductivity tensor of an orthorhombic polydiacetylene single crystal. A numerical simulation of the probe beam deflection is also presented and compared to the experimental data.     

Thermal analysis in the study of the new oxide-based superconductors

Thermoanalytical methods offer a convenient means for testing the starting materials used in the synthesis of the ceramic superconductors and for establishing the reaction and annealing temperatures as well as the stoichiometry of the end product. In addition, the stability and other thermal properties of the superconducting materials can be investigated including phase transformations during thermal cycling and possible reactions with substrates and the environment. The applications of TG, DSC/DTA and other thermal techniques for the study of the YBa₂Cu₃O_7–x superconductor are briefly reviewed using results from own experiments and selected literature data to illustrate the examples given.

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Zusammenfassung Thermoanalytische Methoden eignen sich zum Erproben von Ausgangsmaterialien zur Synthese von Keramiksupraleitern und zur Ermittlung der Reaktions- und Glühtemperaturen sowie der stöchiometrischen Zusammensetzung der Endprodukte. Außerdem können die Stabilität und andere thermische Eigenschaften der supraleitenden Materialien, einschließlich deren Phasenübergängen beim Wärmecycling sowie möglicher Reaktionen mit Substraten oder mit der Umwelt untersucht werden. Es wird kurz die Anwendung von TG-, DSC/DTA- und anderen thermischen Verfahren bei der Untersuchung des Supraleiters YBa₂Cu₃O_7–x beschrieben sowie Ergebnisse aus eigenen Experimenten un zur Illustration der aufgeführten Beispiele ausgewählte Literaturangaben gegeben.

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, , , . , , . , / YBa₂Cu₃O_7–x, , .

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This work has been supported by the Technology Development Centre (TEKES) through grant 4126/1988. The stimulating discussions and cooperation with Professor T. Katila and his group as well as Mr. K. Härkönen, M.Sc. are gratefully acknowledged. Mr. I. Danielsson from Oy Philips Ab is thanked for the permission to publish the high-temperature XRD pattern.     

Synthesis and properties of superconductors prepared from ethylenediaminetetraacetic acid (EDTA)-ethylenediamine (ED) polyamide YBC chelate

Single-phase Y₁Ba₂Cu₃O_7-x has been synthesized by a polymer-metal chelate precursor technique. Ethylenediaminetetraacetic acid (EDTA)-ethylenediamine (ED) polyamide was used as chelating polymer for Y, Ba, and Cu. The titration curves of the polymer in the absence and presence of each metal ion showed the chelate formations. The polymer was dissolved in water and the metal nitrates of Y, Ba, and Cu (1:2:3 in mole ratio) were stoichiometrically added to form a homogeneous molecular complex. The clear polymermetal chealate film was obtained by removal of the solvent without phase separation. This precursor was dried, calcined, and sintered to develop the superconducting (1,2,3) phase. The 1,2,3 phase produced was found to be pure according to x-ray diffraction pattern and it possessed good superconducting properties, i.e., the onset of superconductivity was seen at 90.0 K and the resistance rapidly decreased with decreasing temperature, reaching substantially zero at 88.6 K. Scanning electron microscope (SEM) and face and point analyses by electron probe microanalysis (EPMA) showed that Y, Ba, and Cu elements were uniformly dispersed at least micron order. © 1994 John Wiley & Sons, Inc.