CeO_2掺杂MgB_2超导材料的特性研究

Characteristics of MgB_2 superconductor doped with CeO_2

以Mg粉、B粉、Ce O2粉末为原料,用真空固相反应法制备了系列按1Mg B2+x Ce O2摩尔量比Ce O2掺杂的样品,其中x=0,0.03,0.06,0.09,0.13,0.135,0.14。通过X-射线和扫描电子显微镜,分析了样品的成相和织构。除Mg B2和少量Mg O相外,在Ce O2掺杂的样品中还检测到了Ce B6和少量残留的Ce O2相。XRD的计算表明,随着Ce O2掺杂量的增加,Mg B2的平均晶粒尺寸受到了明显的压制,且Ce B6相含量增加。电阻-温度特性测量表明,掺杂量在x=0.06以内时,Mg B2的超导电输运特性无明显影响,正常态电阻率低,超导转变温度(Tc)超过38K,转变宽度($Tc)小于1K;掺杂量大于0.09以后,观察到正常态电阻率升高,超导电性快速退化;超过0.14后,尽管存在电阻转变,但在测量温度范围内已观测不到零电阻效应。实验除展示Ce O2掺杂抑制Mg B2的成相和晶粒生长外,同时表明晶粒尺度的大小和分布对Mg B2的超导特性影响显著。

Using magnesium （ Mg）, boron （B） and cerium oxide （ CeO2 ） powder as the raw materials and performing solid phase reactions in a vacuum furnace, a series of bulk samples of MgB2 doped with CeO2 by mole ratios of 1 （ MgB2 ） ： x （ CeO~ ） were prepared, where x =0, 0.03, 0.06, 0.09, 0.13, 0. 135, 0.14. X - ray and scanning electron microscopy （SEM） meas- urements revealed the phase constitution and microscopic texture of the samples. The results showed that CeB6 and little unreact- ed CeO2 were detected in the doping samples except MgB2 and a small amout of MgO. Through the calculation of the XRD, we found an obvious suppression in the average grain size of MgB2 and an increasing of CeB6 fraction in the samples with the increas- ing doping level. From the temperature dependence of resistance measurement, we could conclude that a lower doping level at x 〈0.06 did not affect significantly the electrical transport properties of MgB2, in which samples a lower normal state resistivity and a higher normal - superconducting transition temperatures （ T ） than 38K with a transition temperature width （ AT, ） lower than 1K were observed. With the mole doping content of CeO2 increasing up greater than O. 09, an obvious rising in the normal state resistivity and a rapid degradation in the superconductivity of MgB2 were noticed. At a doping level to x = 0.14, a resistance transition was found but no zero resistance temperature was detected in the experiments. These results showed the suppression in the phase formation and the grain growth of MgB2 by CeO2 doping and suggested that grain size and distribution in MgB2 polycrys- talIine sample would impact on its superconductivity.