P. Vasek

Academy of Sciences of the Czech Republic, Praha, Praha, Czech Republic

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Publications (26)17.02 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Series of the samples with nominal compositions Bi3.2Pb0.8Sr4Ca5Cu7Ox and Bi3.2Pb0.8Sr4Ca5Cu7Ox/Ag (10 wt%) were prepared by the standard solid state reaction at the same calcination and sintering temperature and time. Three starting mixtures were used, carbonates, oxides and silver i) with nanoparticles, ii) with micrometer size of particles and iii) powder obtained by modified sol-gel process. The main phase in all samples was Bi-2223 and silver formed the separate phase. The sol-gel process was found to be effective way in term of porosity, bulk density and connectivity of the grains. The effect of the microstructure on the transport properties were described by the temperature dependence of resistivity and critical current densities.
    International Journal of Modern Physics B - IJMPB. 01/2007; 21:3246-3249.
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    ABSTRACT: In the first part of the work two routes of preparation of undoped superconductor Bi-2223 are compared. The influence of addition of extra PbO is studied as well. The route that leads to higher content of the phase 2223 is then chosen as the appropriate way for preparation of fluorine doped samples of general stoichiometry Bi1.6Pb0.4Sr2Ca2.5Cu3.5Ox-z Fz (z = 0, 0.3, 0.6 and 0.9). These samples are characterized by measuring of their temperature dependence of electrical resistance and critical current density. Their phase composition is determined by XRD. While greater amount of fluorine widenes the superconducting transition and worsenes transport properties, the less doped sample shows increasement of critical current density.
    physica status solidi (c) 01/2006; 3(9):3077-3080.
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    ABSTRACT: Samples with the nominal composition Bi1.8−xAxPb0.26Sr2Ca2Cu3Oy (x=0–0.5; A=B, Al, Ga, In) were prepared. Measurements of resistivity, AC susceptibility, critical current density and magnetization have been used for characterization of the samples. All boron-substituted samples had a 2223 phase and narrow transition with the critical temperature Tc≈106–108K and the critical current density Jc (77K)≈105A/m2. The presence of aluminium caused decrease of Tc. The 2212 phase became evident and Jc (77K) sharply decreased at x⩾0.3. The resistivity curves of all the samples with Ga were steeper. The Tc values ranging from 103 to 60K were considerably influenced by the amount of dopant. The volume of 2212 phase increased rapidly at x=0.2 and Jc decreased to zero below 77K. Introducing indium into the samples had a rather positive effect on Tc since the resistivity–temperature curves were always shifted towards higher temperatures with the narrow transition at ∼107K although the XRD patterns showed that the samples also contained the 2212 phase. The samples with x=0.05–0.4 had Jc (77K)≈105A/m2. The highest content of In worsened the quality of the samples and decreased the Jc (77K) to zero.
    Physica B Condensed Matter 01/2002; 321(1):292-294. · 1.28 Impact Factor
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    ABSTRACT: Formation of the high temperature Bi-2223 phase has been studied for samples with nominal composition Bi3,2-xPb0.8MxSr4Ca5Cu7Oy. M represented V, Nb, Ta, their contents was altered from 0 to 0.6 mol %. The effect of the additives on the growth of the phase 2223, lattice parameters and the superconducting properties of the samples was detected. It was found that the additives enhanced the volume of the 2223 phase, improved the intergrain connection resp. the critical current density. The additives did not affect the lattice parameters and Tc significantly.
    International Journal of Modern Physics B 01/1999; 13:3738-3740. · 0.46 Impact Factor
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    ABSTRACT: The effect of the sintering temperature, the cooling rate and the additive on the critical parameters was studied on samples with nominal composition Bi1.8(Pb0.26)Sr2Ca2Cu3O8+x both undoped and with addition of 5, 9, 12.7 mol % of boron (as B2O3), respectively. The sample with 5 mol % of boron cooled slowly had the critical current density five times higher than undoped sample. The rapid cooling of the samples caused enhancement of the volume of the 2223 phase.
    International Journal of Modern Physics B 01/1999; 13:3729-3731. · 0.46 Impact Factor
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    ABSTRACT: The influence of dopants on the properties of BiaPbbMcSr4Ca5Cu7Ox (M = V, Mn, Sb) samples was studied by the measurement of the temperature dependence of the electrical resistivity, magnetic susceptibility and XRD analysis. In general the partial replacement of V or Mn for Bi (c = 0.2 - 0.6) with the constant content of Pb (b = 0.8) led to samples with high content of the 2223 phase and Tc about 110 K and 105 K respectively. The presence of Sb suppressed the quality of intergrain connection.
    Physica C Superconductivity 01/1997; 282:851-852. · 1.11 Impact Factor
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    ABSTRACT: The effect of the stochiometric replacement of Pb and Bi by V in the Bi - Sr - Ca - Cu - O system has been studied. The content of Bi, Pb and V in the 0953-2048/9/9/009/img7 was altered according to the formula: a + b + c = 4 (a = 2.6 - 3.6, b = 0 - 0.8, c = 0 - 0.6). The influence of the dopant on the properties of the samples was detected by XRD analysis, by the measurement of the electrical resistivity, critical current density and magnetic susceptibility. The increasing amount of V on account of Pb led to the decreasing content of the 2223 phase and to the deterioration of the superconducting properties. On the other hand, the samples with V substituting Bi in material with a constant concentration of Pb (b = 0.8) reveal improved intergrain boundaries and a large content of the 2223 phase. The XRD analysis confirmed a decrease of the lattice parameter c of the 2223 phase from 37.173 nm to 37.079 nm for increasing amount of V in these samples. An unknown phase detected on x-ray diffraction patterns (Cu 0953-2048/9/9/009/img8) was attributed to the presence of V.
    Superconductor Science and Technology 01/1996; 9(9):761-765. · 2.80 Impact Factor
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    ABSTRACT: A series of Bi2+xSr2-yCuO6+delta single crystals in the compositional range 0.11 < x < 0.4 and x = y was investigated by means of resistivity, thermopower and Hall effect measurements. In order to elucidate the influence of the oxygen stoichiometry the studies were carried out successively in the as-grown, oxygen and argon post-annealed crystals. The superconductivity appears in an extremely narrow composition range, 0.11 < x < 0.15. This fact is discussed in the context of the doping level and charge carrier localization ascribed to bismuth ions which occupy the Sr sites partially.
    Physica C Superconductivity 01/1996; 264:220-226. · 1.11 Impact Factor
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    ABSTRACT: The influence of Pb and Sb dopants on the properties of Bi-based high-temperature superconductors was systematically investigated. The samples with nominal composition BiaPbbSbcSr4Ca5Cu7O (a+b+c=3.6-4.4; b=0.2-0.8; c=0.1-0.6) were prepared and characterized by DTA, XRD analysis, resistivity and susceptibility measurements. Our results showed that the addition of a small amount of Sb for a constant or decreasing concentration of Pb has little influence on Tc but suppresses T0. The important characteristic of this system is the presence of a new phase for higher concentrations of Sb with strong reflections near 2 Theta =35 degrees (Cc K alpha ) in the XRD pattern.
    Superconductor Science and Technology 04/1995; 8(4):219. · 2.80 Impact Factor
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    ABSTRACT: The anisotropy of the vortex motion has been studied on textured samples of Bi based superconductors. For some orientation of the magnetic field and transport current with respect to the preferential orientation of grains a Hall (transverse) voltage even in a magnetic field has been observed in the mixed state in the low-temperature range. The observed dependences have been discussed as a result of the guided motion of vortices due to intrinsic pinning due to the layered structure of the material under study. The increase of the temperature suppresses the anisotropy of the pinning and even the voltage disappears.
    Physica C Superconductivity 01/1995; 247(3):381-384. · 1.11 Impact Factor
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    ABSTRACT: Electric transport properties of the R-2201 phase were studied on Bi2-xSr2-yCu1O6+delta (x=y/3) single-phase samples annealed under different oxygen activities. It was found that only the composition of y=0.2, treated in N2 at 700°C, is superconducting while an additional incorporation of oxygen into the structure deteriorates this property. An increase of the bismuth concentration has a similar effect. The observed behaviour is ascribed to an occupancy of the originally vacant O(2) sites in -(SrO)- causing local deformation in the former case, and to the substitution of bismuth for strontium ions which reduce the hole concentration and act as trapping centers in the latter case.
    Physica C Superconductivity 01/1994; 232:104-110. · 1.11 Impact Factor
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    ABSTRACT: The Bi2+xSr2+xCuO6+delta single crystals with 0.11
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/1994; 235:1389-1390.
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    ABSTRACT: The Hall (transverse) voltage has been measured on textured samples of Bi based superconductors. For samples with preferential orientation of grains with c axes parallel to the surface of the samples transverse voltage even in the magnetic field has been observed in the mixed state in the low temperature range. The observed results has been discussed in the frame of the model of the guided motion of vortices due to the layered structure of the material under study.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/1994; 235:3145-3146.
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    ABSTRACT: The X-ray study of YBa2Cu3-yCoyO7 solid solutions showed that in addition to the orthorhombic-tetragonal transition existing in the compositional range of 0.0
    Physica C Superconductivity 01/1992; 197:371-377. · 1.11 Impact Factor
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    ABSTRACT: A comparative study of the influence of mechanical and heat treatments on the mechanism of Bi2Sr2Ca2Cu3O10+gamma phase formation was carried out on samples of the nominal composition Bi1.6Pb0.4Sr2Ca2Cu3O10+gamma. Uniaxial cold pressing apparently creates lattice defects which play a substantial role as centers for phase formation and exaggerated grain growth which in turn promotes c-axis alignment.
    Physica C Superconductivity 01/1992; 203:59-67. · 1.11 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/1992; 23(35):304-304.
  • Physica Status Solidi B-basic Solid State Physics - PHYS STATUS SOLIDI B-BASIC SO. 01/1992; 171(2):469-475.
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    ABSTRACT: The suppression of superconductivity is studied in the system of Pr0.5R0.5Ba2Cu3O7-y (R=rare-earth element). X-ray structural investigation reveals the different changes of cell volume with increasing ion radius in these compounds in comparison to X0.5R0.5Ba2Cu3O7-y (X=Ho, Sm). Results are discussed in terms of Pr-O hybridization. It is suggested that there exists a critical average ion radius above which superconductivity in the RPr systems does not persist.
    Physica C Superconductivity 01/1992; 196:90-94. · 1.11 Impact Factor
  • physica status solidi (a) 01/1991; 126(1). · 1.21 Impact Factor
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    ABSTRACT: The synthesis and superconduting properties of various samples of the BiSrCaCuO system with different nominal composition are investigated. Resistivity and magnetic susceptibility measurements demonstrate superconducting properties in the n = 2 and n = 3 phases with Tcm ≈ 78 K and Tcm ≈ 108 K, respectively. The observed transition temperature and the volume fraction of the high-Tc phase (Tcm ≈ 108 K) sensitively depend upon sample processing, in particular the annealing temperature and cooling rates. The superconducting properties and the stability of the different phases are strongly influenced by the substitution of Pb for Bi. Improved superconducting properties due to a large volume fraction of the high Tc-phase are found in Pb doped BiSrCaCuO ceramics. From R(T) dependences in high magnetic fields the critical parameter Bc2 is determined and the role of weak links in the superconducting properties are discussed for undoped and Pb-doped samples of the Bi–based ceramics.Synthese und Supraleitungseigenschaften verschiedener Proben des BiSrCaCuO–Systems mit unterschiedlicher nomineller Zusammensetzung werden untersucht. Widerstands- und Suszeptibilitätsmessungen zeigen Supraleitungseigenschaften in den n = 2 und n = 3-Phasen mit Tcm ≈ 78 K bzw. Tcm ≈ 108 K. Die beobachtete Übergangstemperatur und der Volumenanteil der Hoch-Tc-Phase (Tcm ≈ 108 K) hängen empfindlich von der Probenbehandlung, insbesondere der Temperungstemperatur und Abkühlrate ab. Die Supraleitungseigenschaften und die Stabilität der verschiedenen Phasen werden stark beeinflußt durch die Substitution von Pb für Bi. Verbesserte Supraleitungseigenschaften durch einen großen Volumenanteil der Hoch-Tc-Phase werden in Pb-dotierten Bi-Sr-Ca-Cu-O-Keramiken gefunden. Aus den R(T)-Abhängigkeiten in hohen Magnetfeldern wird der kritische Parameter Bc2 bestimmt, und es wird die Rolle schwacher Verbindungen in den Supraleitungseigenschaften für undotierte und Pb-dotierte Proben der Keramik auf Bi-Basis diskutiert.
    physica status solidi (a) 07/1990; 120(1):185-197. · 1.21 Impact Factor