Effect Of Pb Substitution Research Articles

Enhanced irreversibility field and current stabilization by Pb substitution in aligned Tl-1223 high- Tc superconductor

Using magnetic methods, we have studied the effects of Pb substitution on the irreversibility line and other features of c-axis aligned Tl based cuprate superconductors with the “1223” structure. Specifically, the properties of (Tl 0.5Pb 0.5)Sr 2Ca 2Cu 3O 9 are compared with those of grain-aligned TlBa 2Ca 2Cu 3O 9 and Tl(BaSr) 2Ca 2Cu 3O 9 materials. The irreversibility line H irr( T) is notably higher for the Pb substituted material ( 1.7 T at 77 K with Hc) and markedly anisotropic for all of the samples. Also, results for the persistent current density are reported. Flux-creep studies of the current-decay rate show that the current is the most stable in the aligned (Tl 0.5Pb 0.5)Sr 2Ca 2Cu 3O 9, and that the decay rate of all three materials increases monotonically as the temperature increases.

X-Ray Diffraction Analysis of Strontium in Barites

AbstractA procedure for Sr-analysis of vein barites by X-ray powder diffraction is described. It is based on the measurement of barite d210 spacing change referenced to NaCl as the internal standard. This procedure covers a range of Sr isomorphic substitution of 0 to 12% SrSO4. Comparison with other values, derived from literature data, shows they are in good agreement with the experimental ones. Statistical analysis of the results yields a maximum absolute error of ±0.50 %SrSO4 in Sr determination. This measurement error proves to be acceptable for most studies of barite in mineral deposits. The effect of Pb-substitution is smaller, in most cases, than this error.The reported method has the advantage of being fast and simple. It can routinely handle large number of samples as well as small samples isolated from large crystals. Moreover, it allows the recovery of the original sample after the analysis by dissolving the NaCl in water. The main disadvantage is that fair amounts of quartz interfere with the measurement of the position of the barite 210 line.

Monitoring phase formation of Pb-substituted Bi-Sr-Ca-Cu-O superconducting samples at different preparative stages using Raman spectroscopy and X-ray diffraction

Raman spectroscopy and X-ray diffraction were used to follow structure and phase formation at various stages in order to study the effect of Pb substitution in (Bi1−xPb x Sr3Ca3Cu4O y samples. We found that major reactions involving Bi2O3 occurred at around 700°C and that reactions with Ca, Cu, and Pb started at a lower temperature. The amount of Ca2PbO4 formed increases as a function of lead concentration and annealing temperature (up to 800°C), but excess lead substitution (50%) destroys the superconductivity. The high-temperature superconducting phase (2223) is only observed in the 15% and 20% leaded samples. These two samples exhibit a higher amount of Ca2PbO4 during intermediate processing stages, which suggests that the presence of the Ca2PbO4 is important for the formation of the high-T c phase. It was found that theT c increases with lead concentration (up to 20% Pb).

Effect of Pb Substitution of Superconducting Transitions in Bi-Sr-Ca-Cu-O

Effect of Pb Substitution of Superconducting Transitions in Bi-Sr-Ca-Cu-O

EFFECT OF Pb-SUBSTITUTION ON THE THERMAL BEHAVIOR OF Bi-Sr-Ca-Cu-O SUPERCONDUCTOR

The effect of Pb-substitution on the Bi-Sr-Ca-Cu-O superconductor is studied by thermal gravimetric analysis and differential scanning calorimetry. When studied under different atmospheres (oxygen, nitrogen and air), the Bi-bascd compounds behave similarly to the YBa2Cu3O6+x, but the transition temperature is very different. The weight loss under an oxygen atmosphere is reversible, indicating the dynamic nature of "some" lattice oxygen. While when studied under a nitrogen atmosphere, an irreversible loss was observed during the cooling cycle. A unique transition at around 420°C was observed for the Bi-based compounds. Interestingly, the magnitude of this transition depends on the amount of Pb-substitution. The nature of this transition is believed to be due to the formation of an unstable phase probably associated with a Bi-O site or a mixed-oxide site, and is time dependent. This finding suggests the importance of careful sample handling to avoid possible inconsistent electrical and magnetic measurements as a result of the unstable phase formation.

Single-phase Bi2+x(Ca,Sr)n+1CunO2n+4+δ,n= 0, 1, 2, 3 AND ∞, and some substitutional DERIVATIVES

Abstract The preparation, structure and properties of five members of the homologous series Bi2+x(Ca,Sr) n+1 Cu n O2n+4+δ are discussed, namely for n = 0, 1, 2, 3 and ∞. The two end members are insulating phases with a fixed oxygen stoichiometry but the members n = 1, 2 and 3 are superconducting phases with Tc depending on the calcium to strontium ratio and on oxygen stoichiometry as determined by annealing temperature and oxygen partial pressure. Maximum zero resistance Tc 's obtained are n = 1: 76 K, n = 2: 91 K and n = 3: 106 K. The effects of Pb-substitution in n = 2 and n = 3 are discussed with particular reference to the stabilisation of the latter phase. Rare-earth substitution is shown to raise Tc for n = 2 to at least 101 K without the introduction of the n = 3 phase.

Single-phase Pb-substituted Bi2+ yCan−1 Sr2CunO2 n+4+ δ, n=2 and 3: Structure, T c and effects of oxygen stoichiometry

The effects of Pb-substitution for Bi in the homologous superconducting series Bi 2+ y Ca n−1 Sr 2 Cu n O 2 n+4+ δ are investigated for nearly single phase n = 2 and 3. Under our reaction conditions a solid solution exists for up to 16% substitution above which the n = 1 phase begins to appear and the composition Bi 1.9 Pb 0.35 Ca n−1 Sr 2 Cu n O 2 n+4 remains stable for a range of Pb-rich start ing compositions for both n=2 and n=3. The c-axis satisfies the rule c = 18.2+6.3 n A ̊ and two b-axis and c-axis superstructures are evident from electron diffraction which are identical in n = 2 and n = 3. The effects of T c of anneals at different temperatures and oxygen partial pressures are explored and the conditions for maximising T c at 93 K for n = 2 and 106 K for n = 3 are found.

Stabilisation of 110 K superconducting phase in BiSrCaCuO Pb substitution

The effect of Pb substitution for Bi in the Bi-Sr-Ca-Cu-O systems, together with heat treatment, on the superconducting transition was investigated. Up to 40% Pb substitution, the crystal structure remains essentially unchanged, but the sub-unit cell parameters decrease with an increase of Pb substitution; some extra peaks from the X-ray diffraction patterns could be indexed assuming c=37 A ̊ . The Pb substitution tends to stabilise the high- T c phase transition at 110 K and 95 K even for a wide range of heat treatment conditions due to the increase of Ca and Cu concentration in the superconducting phase. Ag addition sharpens all transitions due to the improvement of the connectivity between grains. Higher temperature treatment promotes the 95 K phase transition and raises the zero resistance temperature. Quenching treatment results in the enhancement of the 95 K phase transition for specimens without Pb substitutions; it also causes semiconductor behaviour for Pb-substituted specimens depending upon the optimisation of the levels of Cu 3+ concentration and oxygen deficiency.

NEW SUPERCONDUCTING OXIDES IN THE Bi-Sr-Ca-Cu-O SYSTEM : MAGNETIC MEASUREMENTS AND STRUCTURAL DETERMINATION

Detailed neutron diffraction structure analysis of Bi2SrzCaCu20s is reported. The effects of Pb-substitution on BizSrzCaCuzOs and Bi2Sr2CuOs are discussed, and magnetization measurement results on the 110 K phase in the Bi-Sr-Ca-Cu-0 system are given. Since the discovery of high Tc superconductiv- ity in the Bi-Sr-Ca-Cu-0 system (I), two supercon- ducting phases have been unambiguously identified, namely BizSrzCuOa (Tcw 10 K) and Bi2SrzCaCuzOs (T,x 80 K) , hereafter denoted as 2201 and 2212. The presence of transition temperatures around 110 K in mixed phase samples suggested the existence of a third phase, the composition of which could be 2223, based on an analogy with the later discovered thallium com- pounds (2). We report here a detailed structural anal- ysis of the 2212 phase and physical characterizations of the 110 K phase. Three alternative models for the structure of Bi2Sr2CaCuzOs were first proposed on the basis of X- ray single crystal diffraction data (3). Then, Bordet et al. 141 used powder neutron diffraction to show that the double Bi-0 layers do not adopt the usual Au- rivilius structure, but are arranged in an NaC1-type structure. The problem with this model, refined in the Fmmm space group, is that the in-plane Bi-0 dis- tances are too long to satisfy the normal ~i~+ coor- dination. In fact, these oxygens appear in this model. We show here a more precise analysis of our neutron data allowing a better determination of the Bi co-ordination. As the space group Amaa or its non-centric sub- group A2aa have been confirmed by convergent beam electron diffraction (5), the neutron data were refined in Amaasy permitting all atoms to move off the y-axis special position of the Bordet et al. structure taken as the starting model. Only the BiO plane oxygen 03 (which appeared disordered in Fmmm) was much displaced. In Amaa, the best refinement was obtained by splitting 03 between two sites either side of the center of the Bi-square. In an attem~t to remove the need for split atoms, we simply removed the center of symmetry (space group A2aa). 03 was then not con- strained in any way, and immediately moved to occupy

Electronic states of BaPb1-xBixO3 in the semiconducting phase investigated by optical measurements.

The semiconducting ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ (0.35lx\ensuremath{\le}1) is investigated in detail by optical measurements. The optical spectrum shows that ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ has a clear optical gap over the whole semiconducting compositional range. It is clearly shown by resonant Raman scattering measurements that the origin of this gap is a charge-density wave (CDW) accompanied by the breathing-mode distortion. No apparent effect of Pb substitution can be observed in the reflectivity spectra for the sample with xg0.7. For x\ensuremath{\le}0.7, the CDW gap decreases with x and simultaneously the absorption tail extends into the gap. This spectral behavior can be well interpreted in terms of the recently proposed theoretical model with emphasis on a local CDW and/or a substantial energy difference between Pb and Bi sites arising from the strong electron-phonon interaction. Thus it can be concluded that the strong electron-phonon interaction plays an essential role in the semiconducting phase of this material as well as in the metallic phase.