铋系超导体制备工艺和掺杂效应研究

【作者】 汪静；

【导师】 李平林； 张逸民；

【作者基本信息】 郑州大学 ， 凝聚态物理， 2007， 硕士

【摘要】 为了制备出具有高Tc的Bi系铜氧超导体，在制备工艺方面我们研究了烧结温度和烧结时间对超导相形成的影响，用XRD、SEM测试手段对样品的结构、形貌做了系统的研究；为了解Pb含量对超导电性的影响，改变Pb的含量，观察了样品的XRD和Tc变化。实验结果表明，适当的烧结温度和时间对超导体的形成有很大的影响，且合适的Pb含量有助于加速超导体的形成。在制备出具有良好超导电性材料的基础上，用Mn和Zn对Cu位进行替代，利用XRD、SEM分析了超导体的相组成和形貌，系统地测量了临界温度Tc，并分析了它们与掺杂量之间的关系。首次用拉曼及红外光谱技术研究了Mn和Zn掺杂的Bi系超导体，并对观测结果进行了理论分析和解释，为深入探讨高温超导机制提供了新的实验依据。从研究中我们得出了结论如下：（1）合适烧结温度对制备高Tc的Bi系超导体是十分重要的。首先在温度范围：820-850℃，样品烧结时间8h，制备了系列样品。测得样品的XRD、SEM、T-R的曲线，发现温度低于840℃时，2212和2201相较多。经过多次实验探索发现：最佳烧结温度是840-850℃。在最佳烧结温度下，延长恒温时间，XRD显示Bi-2223相的含量增加，Bi-2212和杂相含量减少。烧结温度为850℃保温90小时条件下，2223相的含量达到了88％，其Tc为105K。因此，烧结时间和烧结温度对样品质量都有很大影响。只有在合适的烧结温度下，延长烧结时间才有利与高Tc相的形成。（2）通过对不同铅含量的样品的T-R和XRD研究表明，随着Pb含量的增加其Tc相应增加。但当x=0.5时样品内Ca₂PbO₄相强度增加，Tc反而下降。这说明Pb含量过多时容易形成杂相。从实验结果发现，Pb含量为0.4时可以制备出最佳样品。（3）对Zn替代的Bi系列样品研究结果表明，Cu位Zn掺杂会使Bi-2223相变得不稳定，其Tc降低，当掺杂量为0.05时，Tc下降为98.8K，随着掺杂量的增加，临界转变温度更低。Zn取代Cu后钙钦矿型结构ABO₃中的容忍系数t变小，因此，Bi-2223相会变得不稳定，易分解为其他物质而形成较多杂相，导致Tc下降较快。（4）对Mn替代的Bi系列样品的研究结果表明，当掺杂量少于等于0.10时，XRD结果显示未出现杂相，Tc在100K以上；当x大于0.15时，样品中Bi-2212相和Ca₂PbO₄相的含量逐渐增多，Tc降到93K以下。Mn替代Cu位后阳离子半径减少会引起O^2-间的排斥力增大，但是替代离子正电荷的增加可以增强正负离子的库仑引力，增强晶体稳定性。因此Mn替代Cu位对超导电性的抑制没有Zn强烈。（5）通过测试Bi系掺杂超导样品红外吸收谱，观测到不同元素掺杂及不同掺杂量的样品超导性能的差异。通过分析609cm^-1峰位的移动，证明了Mn和Zn确实替代了Cu位。而且，无论是Mn还是Zn掺杂，随着掺杂量的增加609cm^-1峰位的强度都减弱，Tc降低，我们认为这是由于掺杂影响了铜氧键的振动特征，进而影响了样品的超导电性。（6）我们先对未掺杂样品拉曼光谱图中的峰位进行指证，在此基础上对Zn、Mn离子掺杂的超导体拉曼光谱进行分析，确定453cm^-1为CuO₂面上氧原子的面外伸缩振动引起的。从图谱分析得出，Zn离子掺杂对Bi系超导体的结构有很大的影响，当掺杂量为0.3时，超导体的主相已成为2212相，而Mn离子掺杂的Bi系超导体，掺杂量为0.4时，仍未出现2212相的特征峰。因此，Bi系超导体中，Zn离子掺杂比Mn更显著影响了样品的超导电性，其原因可能主要是CuO₂面上氧原子的面外伸振动引起的。更多还原

【Abstract】 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 Mn^3+（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.更多还原