High-pressure shock modification and synthesis of superconducting ceramic oxides

Abstract

Our shock-compression efforts of high-temperature superconducting ceramic oxides are reviewed. These consist of: (i) shock-induced chemical synthesis on various mixtures of metal oxides of composition similar to those which yield known high-temperature superconductor oxides; and (ii) shock-modification studies on high quality YBa 2Cu 3O 6.9 (Y-123), Bi 2CaSr 2O 8 (Bi-2122) and Tl 2Ca 2Ba 2Cu 3O 10 (Tl-2223) powder compacts. Fundamentals of the Sandia shock-compression studies are reviewed here so as to present a more comprehensive view on processes influencing shock modification and/or synthesis. Various mixtures of metal oxides of compositions known to yield high-temperature superconducting oxides under conventional ceramic processing have been subjected to a range of shock-loading conditions to determine parameters leading to their shock-induced chemical synthesis. For La 1.85−Sr 0.15CuO 4 recovered samples show X-ray diffraction lines consistent with the formation of the K 2NiF 4 structure type crystallographic phase in high yields. Studies on the YBaCu system yielded diffraction lines consistent with those for a few percent yield of Y-123. In addition, studies on BaO and Y 2O 3 were carried out, since these oxides appeared to influence the reaction forming Y-123. Superconducting powders of Y-123 have been shock-modified at pressures of 20 and 27 GPa in the Sandia “Bear” gas-tight copper recovery capsules and subsequently characterized by static magnetization, electrical transport and X-ray diffraction. Magnetization data show considerable degradation in the superconducting properties of the shock-modified material which is attributed to a loss of oxygen during the shock process. This is confirmed by the X-ray studies which show partial conversion to the non-superconducting tetragonal phase. Subsequent oxygen annealing restores the magnetic response, and a field loop at 77 K demonstrates strongly endhanced flux pinning, presumably related to the presence of point, line and structural defects and/or impurities (ion substitution) introduced by the shock-modification process. No improvement in transport critical current is observed compared with our standard prepared materials, presumably because of the weak-link nature of the intergranular connections. Superconducting powders of Bi-2122 and Tl-2223 have been shock modified at pressures of 7.5 and 27 GPa, respectively. The Bi-2122 material contained a small amount (≈0.1%) of the so-called 100 K phase (Bi-2223), detectable only by its Meissner signature at such low concentration levels, which is not present following shock loading. The Tl-2223 sample showed a broadened transition and a decrease in expulsion, but the onset of superconductivity increased from 117 to 123 K. Although the potential for direct shock synthesis of superconductors has not been demonstrated, these studies suggest that superconducting material systems which are robust with respect to stoichiometry, particularly oxygen, may benefit from shock-modification processes which introduce defects serving as flux pinning sites in the superconductors. More detailed studies on flux motion in shock-modified materials remain to be carried out.