Phase evolution and gas-phase particle size distributions during spray pyrolysis of (Bi,Pb)SrCaCuO and Ag(Bi,Pb)SrCaCuO powders

Phase evolution, gas-phase particle size distributions and lead loss were studied during formation of (Bi,Pb)SrCaCuO powders and their composites with silver by spray pyrolysis starting from nitrate solutions. The 10 wt% Ag/90 wt% Bi_1.8Pb_0.44Sr₂Ca_2.2Cu₃O_x composite powders made at 700°C consisted of 20–60 nm grains of silver and mixed-oxide phases with a fine dispersion of Ag grains within the particles. At 700°C, the primary phases present in (Bi,Pb)SrCaCuO powders were (Bi,Pb)₂Sr₂CuO_x (2201), Ca₂PbO₄ (plumbate), (Bi,Pb)₂Sr₂CaCu₂O_x(2212), and (Bi,Pb)₃Sr₂Ca₂Cu₁O_x(3221). For T≥800°C, the powders were considerably depleted in lead, and the plumbate and 3221 phases were absent. For T = 900°C, a large number of ultrafine particles (<30 nm) were formed, probably from the PbO vapor released from the reactor walls. Using spray pyrolysis, it is easy to control stoichiometry and limit the phase segregation at the nanometer-scale so that homogeneous and phase-pure materials can be obtained rapidly during subsequent processing.     

Evidence for two superconducting components in oxygen-annealed single-phase YBaCuO

The complex susceptibility of a sintered YBaCuO superconductor is strongly dependent on a.c. field amplitude, h. Very small values of h must be used for the real part of susceptibility, χ′, to reach a value corresponding to bulk diamagnetism just below the critical temperature, T_c. The imaginary part, χ″, represents hysteresis loss in the sample. Thus, χ″ versus temperature becomes positive when h exceeds the lower critical field, H_cl of the superconductor.Annealing the material in oxygen gives rise to two distinct components, a relatively high-T_c, high-H_cl superconductor (denoted as ‘G’ or ‘good’) and a relatively low-T_c, low-H_cl superconductor (denoted as ‘B’ or ‘bad’). Curves of susceptibility versus increasing temperature reflect the dual nature of the annealed sample: χ′ has an inflection point at T_c of the B component and approaches zero at T_c of the G component, while χ″ has a peak at each T_c. Both critical temperatures decrease linearly with increasing h, though at very different rates. H_cl of the G component is considerably greater than H_cl of the B component. The lower critical fields are linearly decreasing functions of temperature.Two models might explain the susceptibility data. In the grain model, the G component consists of superconducting grains and the B component is either intergranular material, unfavourably orientated anisotropic grains, or oxygen-depleted grain boundaries. In the surface model, the G component is in the interior of the sample and the B component is at the sample's surface. This condition could arise if there was oxygen depletion at the surface subsequent to total enrichment during annealing.     

Influence of preparation parameters on the tunnelling characteristics of PbBiIn/oxide/PbBiIn junctions

The quasiparticle millimetre wave mixer requires high quality tunnel junctions with a large gap voltage, a normal resistance R_n of several tens of ohms and areas A of only a few square micrometres or less. We show that water vapour at the upper surface of the PbBiIn alloy junction can increase the product R_nA by more than two orders of magnitude. However, indium at the upper surface can drastically decrease R_nA but does so mostly at the expense of the quality of the junction. Appropriate annealing allows the normal resistance to be increased or decreased to close to the desired value.