Cyclotron He++ implantations effect in superconductors YBa2Cu3O7-δ

Full text: Introduction In our radiation damage research using cyclotron simulations, we are accomplishing a study of high Tc superconductor under '' particles' implantation at lEN's CV-28 cyclotron. Using a special system for energy degrading, it was possible homogeneous He implantation with fluency of 7.2 to 6.0x1016''.cm2. The preliminary results showed decreasing values for Tc(onset) and T(offset) with increasing fluency. The magnetic permeability curves showed bump structures with relative position to Tc(onset) varying with the fluency, suggesting phase transition as consequence of He implanted. The samples annealing in oxygen atmosphere showed evidence of restoring the superconductivity property. Some results available in the literature are conflicting to some high Tc superconducting ceramics: Voronova, irradiated Nb3Sn with '' particles getting monotonically decreasing of Tc(onset) with increasing fluency; Chrisey, irradiated films of YBaCuO with 65 MeV '' particles and fluency lower than 1014''.cm-2, and showed no changes on Tc(onset); Maish, irradiated YBaCuO with 1014p.cm-2 protons, and observed no evidence for phase transition, despite of contradicting Voronova's results. Some results of this study also differ from some authors above mentioned. However, some of them are in agreement with others authors such: Bourgault, which investigating samples of YBaCuO, using 3.5 GeV Xe ions, fluency of 1012''.cm-2, verified decreasing of the Tc(onset) with fluency, and a more strong decreasing of T(offset), also in agreement with present work. As a next step, is scheduled to perform positron lifetime measurements as function of temperature, to study the behavior of Tc and its changes as consequence of Cyclotron He++ implantation in this ceramic. Sample Preparation and Characterization - The samples were prepared from Y2O3, Ba(NO3)2 e CuO, following the conventional procedure. The mixture, in parts of 1:2:3 in Y, Ba, and Cu was dry heated in an alumina crucible, with oxygen flux of 5.6 ml.s-1 a 1223K during 20 h. After pressing, we got the samples in pellets with 350 μm thickness and density 5.8 gcm-3. The sintering was done in the same condition of the preparing for 18 hours, when the temperature was reduced (7.7 K.min-1) to 673 K and brought to room temperature. In that stage we took it out pallets, with mass around 5.86 mg, and size compatible with the probe for magnetic permeability measurements (2x1x1 mm). Irradiation Procedure - The samples were fixed, with metallic indium, in a water-cooled target holders made of copper. The indium was used for proper heat dissipation and as temperature controller during irradiation. In earlier works, X-ray diffraction and magnetic permeability measurements showing no changes due to indium adsorption or absorption. The irradiations were done at lEN's CV-28 Cyclotron. The '' particle beam with 28 MeV maximum energy was periodically degraded with a rotating disk with aluminum foils of different thickness. This way, it was possible a homogeneous ion implantation into the sample thickness. The typical currents were 500 nA, corresponding to a flux of 1.64x1012''.cm-2s-1. Results - The magnetic permeability measurements were performed using a conventional system for zero detection ('Lock-in amplifier' 124 PAR). The temperature range was from 66 to 94 K. After irradiations it was observed structure's bump like, in the permeability curve. This bumps also showed changes in relative position in the curve, followed by changes in Tc(onset). This behavior suggests that changes in Tc(onset) could be attributed to a defined amount of implanted ions. This hypothesis is justified considering that each bump is the register of a starting phase transformation, as consequence of the helium concentration. Although Maish had also observed similar structure in resistivity measurements as function of temperature, in YBaCuO irradiated with 63 MeV protons' 1014pcm-2, it was discarded phase transformation by X ray diffraction. Nevertheless, considering the different experimental situations, it should not be excluded the possibility of changes on the crystal structure due to presence of implanted He, giving rise to oxygen ion disorder followed by phase transformation. Conclusion - In spite of some divergences and diversity of experimental situations, all of them have in common the study of the behavior of the superconductors under radiation. Other questions are being considered, and the positron annihilation technique is being applied, trying to clarify the referred questions. Despite these questions, it is possible to make some conclusions. One of the most important is that the superconducting changed by ion implantation can be partially recovered by a proper annealing. It is expected that positron technique results could be able to clarify or better describe the behavior of Tc(onset), T(offset), bumps structures and Tc. (author)