【作者】 赵倩；

【导师】 刘永长；

【作者基本信息】 天津大学 ， 材料学， 2010， 博士

【摘要】 随着人们对掺杂MgB2超导体研究的不断深入,掺杂元素对MgB2的成相过程及超导性能的影响具有越来越重要的指导意义。本文首先采用传统的固相反应法对纯MgB2进行烧结并确定最佳的工艺参数。在此基础上,选取不同粒度的金属Ni作为掺杂对象,结合显微组织观察、差热分析技术、粉末烧结理论,热力学和动力学分析手段,系统研究了Ni的颗粒大小对MgB2相的形成、微观形貌以及超导电性的影响。另外,以金属Ni纳米颗粒作为催化剂,在B基体上沉积单质碳并掺入MgB2超导体,试图通过纳米相的引入来增加磁通钉扎中心,以提高MgB2的临界电流密度值。上述研究包含的主要内容及获得的结论有:利用高精度差热分析仪对Mg和B的混合粉末进行烧结处理,并分析不同升温速率(5、10、20和30 K/min)对MgB2相的形成过程产生的影响。临界电流密度测试结果表明,以5 K/min的加热速率烧结可以促进MgB2在低温下的反应过程,进而提高MgB2晶粒间的连接性,改善高磁场下MgB2的临界电流密度值。然而,高的升温速率(30K/min)容易导致孔隙率的增加和MgB2晶粒间连接性的恶化,从而使得MgB2样品的超导性降低。同样采用热分析手段,进一步研究了Ni掺杂对MgB2体系成相过程的影响,结果表明:在Ni掺杂的MgB2体系烧结过程中,Mg和Ni在506℃下形成共晶液相,大大提高了Mg和B原子的扩散速率,使大部分MgB2颗粒在固相阶段形成。Mg-Ni-B体系的反应模型表明,MgNi2.5B2相的析出是从MgB2晶粒边界处转至晶粒内部。实验选取不同粒度的金属镍粉对MgB2超导体进行掺杂,通过研究粒度对MgB2的成相过程以及化学成分的影响,进一步比较不同条件下超导性能的变化,以研究第二相颗粒大小在其中所起作用。结果表明:小尺寸镍颗粒可以促进第二相MgNi2.5B2在更低的温度下形成。在镍粒度为10μm的掺杂样品中,MgNi2.5B2相颗粒呈螺旋状沿MgB2晶粒内部析出,进一步揭示了MgB2晶体的生长符合螺旋位错的生长机制。临界电流密度测试表明,临界电流密度大小与第二相颗粒的大小相关,较大的MgNi2.5B2相颗粒更容易导致临界电流密度值的降低。利用还原法在B基体上成功制备出金属Ni纳米颗粒,通过与Mg粉的混合烧结制得了掺杂Ni纳米颗粒的MgB2超导体。对前驱体粉末和MgB2样品进行透射电镜观察、X-射线衍射以及超导性能测试后发现:在B基体上形成了平均晶粒直径为5纳米的Ni纳米颗粒,与大颗粒Ni掺杂相比,更能够改善MgB2晶粒间的连接性,提高低磁场下MgB2超导体的临界电流密度值。以Ni纳米颗粒作为催化剂,利用化学气相沉积法在500℃下将甲烷分解并在B基体上沉积生成碳洋葱及碳纳米管。经过与Mg粉混合烧结得到碳掺杂的MgB2超导体,并获得了具有较高临界电流密度的超导试样,结果表明:B基体中沉积的碳含量随着通甲烷时间的延长逐渐增多,但是在1.5小时达到一个饱和值,此后的碳含量维持不变。由于碳原子层的包裹,使得Ni无法与Mg/B反应生成第二相MgNi2.5B2。但是提高的临界电流密度表明,碳包覆Ni纳米颗粒可以作为有效的磁通钉扎中心。更多还原

【Abstract】 The effect of doping elements on the formation mechanism and the superconducting properties of MgB2 sample have become more and more important for guiding the preparation of high quality MgB2 superconductors. In the present paper, the appropriate experimental parameters were firstly defined by traditional solid-state sintering method. Based on the analysis, the in-situ sintering mechanism, microstructure and superconducting properties of Ni-doped MgB2 samples were systematically explored by means of metallographic analysis, differential thermal analysis, powders sintering theory, thermodynamic and kinetics analysis. Additionally, the MgB2 samples doped with carbon were successfully obtained by deposition of carbon on B matrix, which was expected to enhance the critical current density ( J c) by inducing Ni nanoparticles as pinning centers.A high-resolution Differential Thermal Analysis apparatus was employed to sinter the mixture of Mg and B powder and analyze the kinetics of MgB2 phase formation with the different heating rates (5, 10、20 and 30 K/min).The measured results of the critical current density suggested that the 5 K/min sample had the best performance in high-field because of the better connectivity in MgB2 grain formed at low temperature. However, more vacancy in the sample and the worse grain connectivity will be generated by the fast heating rate (30 K/min), which is the main reason for the degeneration of superconducting properties in magnetic field.The effect of Ni doping on the formation of MgB2 was investigated by the thermal analysis. It shows that the low-melting eutectic liquid firstly formed by Mg-Ni at 506°C in the Ni-doped MgB2 sample, which could provide a favorable area for the further diffusion between Mg and B atoms and promote the reaction between Mg and B at solid state. The sintering model of Mg-Ni-B system indicated that the precipitation of MgNi2.5B2 phase vary from grain boundaries to MgB2 intra-grains as the sintering temperature increases.The different grain sizes of Ni were adopted to detect the influence of the grain size on the formation process of MgB2 phase and the variety of the superconducting properties. It is found that the smaller grain size of Ni will promote the formation of the secondary MgNi2.5B2 phase at low temperature. The microstructure shows that the precipitation of the MgNi2.5B2 phase is from the MgB2 intra-grains, the morphology of secondary phase further reveal the screw dislocation mechanism of the formation of MgB2 crystalline. The value of J c have the strong correlation with the secondary phase, the large grain size may lead to the degeneration of J cas impurities.Ni nanoparticles were successfully introduced for preparing polycrystalline MgB2 samples by a novel reduction method. The microstructure and magnetization measurements of the precursor powders and MgB2 samples were explored. It was found that the obtained Ni nanoparticles with average grain size of 5 nm are distributed in the B matrix without agglomeration. The sample doped with Ni nanoparticles has better critical current density ( J c) with respect to the commercial Ni-doped sample because of the improvement of grain connectivity.The carbon onions and carbon nanotubes were prepared on the B matrix at 500°C using Chemical vapor deposition (CVD) method by decomposition of methane. The carbon doped MgB2 sample with high critical current density ( J c) was obtained by sintering the mixture of Mg and precursor powders.The content of the carbon on the B matrix will increase with the reaction time, but cease when the reaction time exceeding 1.5 hours. The Ni could not react with Mg/B forming the secondary MgNi2.5B2 phase because of the carbon wrap. However, the Ni nanoparticles covered by carbon can act as effective pinning center to enhance the J cat magnetic field.更多还原