Bismuth-based Superconductor Preparation Process And The Doping Effect

In order to prepare high-Tc Bi-superconductor , we researched the impaction of the sintering temperature and sintering time to formate superconductor c in the preparation technology, and systemically researched the structure, morphology and the critical temperature of the sample by XRD,SEM,R-T. At the same time, in order to find out the impact of Pb to the superconductivity, we changed the content of Pb and observed the XRD and Tc change of the sample. The experiment result shows that proper sintering temperature and sintering time have great impact on the formation of superconductivity material and appropriate content of Pb can help accelerate the formation of the superconductor.On the basis of the preparation of good superconductivity material, Mn and Zn doping at Cu, we analyzed the phase composition and morphology, symtemically measured the resistance transition characteristic the critical temperature Tc, and analyzed the relation the impression of dopant content. First time we researched Bi-system superconductor doped Mn and Zn by Raman and Infrared spectrum technology, and we made theoretical analysis and interpretation of the observing result, which provides new experiment basis for deeply discussing the high-temperature superconductor mechanism. We get the following conclusions:(1) To prepare high-Tc Bi-system superconductor , the sintering temperature is very important. First, a series of samples was prepared. Their temperature range is from 820°C-850°C and their sintering time is 8h. Then the curves of XRD,SEM,T-R of the samples were measured. We found: along with the increase of the sintering time, the content of the 2201 and Ca₂PbO₄ phase of the precursor powder dropped off and the 2212 and 2223 increased. Exceeding 840 degree, 2223 phase begins to form; below 840 degree, the major phases are 2212 and 2201 phase. According to multiple experiments we got: the best sintering time is 840-850 degree. Under the best sintering time, extending the holding time, we could find the content of the Bi-2223 phase increased and the Bi-2212 and other phases decreased from the XRD. When the sintering time is 90h, the content of the 2223 phase reached 88 percent and the Tc is 105K. Therefore, the sintering time and sintering temperature are interdependent. Only at proper sintering time, exceeding the sintering time can help the formation of high Tc phase.(2) The samples with different Pb contents have been studied using T-R and XRD methods. It is shown that the content of Ca2PbO4-phase in powder increased as the content of Pb increased. Pb in the powder only have two forms, i.e. (Bi, Pb) -2212 and Ca2PbO4. Increase of the initial powder Pb content, will naturally increase Ca2PbO4 content in the powder, thus accelerating the conversion from 2,212-phase to 2,223-phase. With the increase of doping content, the Tc will be increased. But the Ca2PbO4 phase in the sample intensity increased when x = 0.5, and then Tc declined. It was concluded that Pb content of excessive result in miscellaneous phases happen. We find that the Pb content also affect the superconductivity, in the sintering temperature, time and other elements ratio, and the Pb content of 0.4, it will promote high-Tc superconducting materials produced.(3) We have studied the replacement of Zn for Bi-superconductor. The results reveal that Zn doped Cu lead the Bi-2223 phase become unstable, and Tc is reducing,.When doping content is 0.05, the Tc decreased significantly to 98.8K, and the more doping quantity, the lower critical temperature. The tolerance coefficient t of ABO₃ which is of Perovskite structure become smaller, thus, Bi-2223 phase becomes unstable and decomposes easily for miscellaneous other material which result in the Tc decline rapidly by replacing Zn for Cu.(4) The replacement of Mn for Bi series samples was studied. It is shown that the miscellaneous in the XRD spectra is less obvious and the Tc is more than 100K, when doping is less than 0.15; When x is greater than 0.15, the Ca2PbO4 and Bi-2212 phase content in samples is gradually increasing, the Tc dropped to 90 K. The cation radius reduce after Mn substituted for Cu, which will cause the mutual exclusive increase between O atoms. But the Coulomb attraction between positive charge and negative charge can be enhanced by positive charge of the ions increase, and strengthen stability of crystal; we consider the effects of the energy, the valence of Mn³⁺⁽⁴⁺⁾ is higher than the Cu²⁺, and all of the energy coefficients are lager than Cu²⁺. The system energy is reduced after substitution, and the lattice energy increase, thus formation of Crystal is more stable. Therefore, the inhibition superconductivity by Mn substitutes for Cu is not stronger than Zn.(5) By analyzing the Infrared spectrum, the impact of the different element doping and the dopant content to the superconductivity of the samples was observed. By analyzing the shift of the 609 cm^-1 peak, it was proved that Mn and Zn surely doping at Cu on the CuO₂ plane. Either Mn or Zn doping, along with the increase of the doping content, the intension of the 609 cm^-1 peak reduced and Tc lowered, which shows that 609cm^-1 and superconductivity have some contact.(6) Basis on the assignment to the peak in the Raman spectrogram of undoping samples, the superconductor Raman spectrum of Zn,Mn doping was analyzed, which shows that the outside stretching vibration of oxygen atom in the GuO₂ plane. From the analysis to this spectrum, the Zn ion doping has great impact on the structure of the Bi-system superconductor. When the doping content is 0.3, the main phase of the conductor has become into 2212 phase., but to the Bi-system superconductor of Mn ion doping, when the doping content is 0.4, the characteristic peak of the main phase which was assigned 2212 phase still didn't appear. Therefore, among the Bi(Pb) superconductors, the Zn ion doping has more impact on the superconductivity than Mn ion doping.