Improvement in Fabrication of Hg-1223 Superconductor by Te-Doping
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Improvement in Fabrication of Hg-1223 Superconductor by Te-Doping K.K. Yau, C.C. Lam, L.J. Shen1 and S.H. Li Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China. 1 Department of Physics, Nanjing University of Chemical Technology, Nanjing 210009, People’s Republic of China. ABSTRACT The sealed quartz tube method is one of the usual methods used for sintering mercury-based copper oxide superconductors. By this method, precursor pellets are inserted into the quartz tube together with certain mass ratio to the reagent sample pellets for controlling the mercury vapour pressure. A certain amount of excess HgO is required to ensure the fabricated sample to have the desired composition. In our research, it was found that if a small amount of TeO2 is added and mixed with the pulverized precursor after first calcination, Hg-1223 superconductors can be fabricated without using the precursor pellet and no excess HgO is required. Thus, the fabrication process can be simplified. As mercury and its oxide are toxic, if no excess Hg is involved, it is safe and good for the laboratory-environment. In this research, some amounts of 0.10 and 0.18 wt% (weigh percentage) of TeO2 were added into the precursor, it is shown that the critical temperature of the fabricated samples is comparable to that of the undoped Hg-1223 samples. For 0.10 wt% of TeO2, the Tc of the as-synthesized sample is about 116.7 K whereas the annealed Te-doped Hg-1223 is about 134.2 K. For 0.18 wt% of TeO2, the Tc of the assynthesized and annealed samples are 115.8 K and 134.6 K, respectively. The phase structure of samples fabricated in this research has been analyzed by the X-ray diffraction technique. It shows that the Hg-1223 phase is dominated. INTRODUCTION In 1993, Putilin et al. discovered a mercury-based copper oxide superconductor, HgBa2CuO4+δ, which is the first member of a homologous series of HgBa2Can-1CunO2n+2+δ [1]. In the same year, other members of the series, with n = 2, 3 and 4, were synthesized subsequently [2-4]. The critical temperature achieved by the third member, HgBa2Ca2Cu3O8+δ (Hg-1223), is the highest Tc (~ 134 K) under ambient pressure [3] recorded up to now. Its Tc can be increased to 164 K under a high pressure of about 30 GPa [5]. Apart from the stated sealed quartz tube method, there are some methods used for fabricating the Hg-based copper oxide superconductors [5-8], for examples high pressure method [6] and sol-gel method [9,10]. For the case of sealed quartz tube method, it can still be divided into Hg vapour diffusion process [5-7] and cationexchange process [8]. In this investigation, the sealed quartz tube method with Hg vapour diffusion process is used, but some modification process is adopted. In this investigation we used Te-doping. By this method the fabrication is simplified and the excess amount of toxic HgO is not required. The effects of Te-doping on Tc, phase structure and thermal stability were studied.
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EXPERIMENT 1. F
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