【作者】 杨烨；

【导师】 赵勇；

【作者基本信息】 西南交通大学 ， 材料学， 2009， 博士

【摘要】 全文主要包括以下方面内容：第一章简要介绍了MgB₂超导材料的研究背景,分析了目前所存在的主要问题,提出了本文的研究目标和研究内容。第二章回顾了超导电性的基本内容和MgB₂超导体的发现过程,着重介绍了MgB₂超导体钉扎性能的研究情况以及其它相关物性,并详细回顾了MgB₂化学掺杂的研究现状。第三章介绍了本文中用于制备MgB₂超导材料的三种制备方法:固相反应法、扩散法和化学溶液法,并简单介绍测量相关的表征手段。第四章研究了单质Al和碳纳米管单独掺杂对MgB₂超导电性和钉扎行为的影响。在单独掺Al的研究中发现,Al原子可以替代Mg进入到MgB₂的晶格当中,使c轴方向的晶胞参数减小。当Al掺杂量大于10%时,MgB₂晶格有相分离的情况出现,可能在材料中同时有两种合金相存在。另一方面,Al掺杂明显的抑制了MgB₂的超导电性,随着Al杂量的增加,超导转变温度线性下降。同时Al掺杂样品的H_c2、J_c等性能也受到了明显的抑制。然而,从钉扎力曲线的分析结果发现,掺杂并没有改变体系的钉扎机制,晶界钉扎仍是体系中主要的钉扎作用。分析单独CNTs掺杂对MgB₂的晶体结构和超导电性的影响时,发现碳纳米管掺杂可以在一定程度上提高MgB₂超导体的临界电流密度,但是当掺杂量过高时,掺杂样品在低场下的载流性能被明显的抑制。另一方面,用固相反应法制备的样品孔洞较多,实际密度只为理论密度的50%左右,因而其性能仍有较大的提升空间,用扩散法制备是解决这一问题的可能途径。在一体系中,还详细的研究了随着碳纳米管掺杂量的变化,系列样品中δT_c和δl钉扎的行为特性。发现在我们的MgB₂样品中,δT_c和δl两种钉扎行为同时存在,并且随着碳纳米管掺杂量的增加δl钉扎的比重逐渐增加,得到的实验结果和理论分析有很好的拟合,结果也说明了碳纳米管掺杂不但可以往样品中引入点钉扎的作用和,另外由于C掺杂引起的δl钉扎的增强,也可以极大的提高样品的钉扎作用。但在高温的情况下,由于温度等因素的影响,即使是高掺杂的样品在接近转变温度的区域,δT_c钉扎仍表现出较大的作用。在扩散法制备碳纳米管掺杂的MgB₂超导材料研究中,制备得到了较高致密性的超导块材,并且扩散法可以有效减少样品中MgO杂相的存在,同时极大的提高样品在低场下的临界电流密度。但是过于紧密的晶界连接反而削弱了MgB₂超导体晶界钉扎的作用从而导致高场下钉扎力较快衰退,因而有必要优化制备条件引入更有效的钉扎中心提高其高场下的性能。碳纳米管的掺杂在两组样品中都表现出了提高高场下钉扎力的能力,特别是0.5%掺杂量的样品,在整个磁场范围都有较好的表现。第五章研究了Al/C和Ti/C同时掺杂对MgB₂超导电性和钉扎性能的影响。研究MgB₂样品中同时掺杂Al和C时发现,Al替代Mg和C替代B这两种作用可以同时发生,并且当电子掺杂作用相当时,Al掺杂对超导转变温度（T_c）的抑制作用要更加明显,而另一方面Al掺杂却可以减缓C掺杂对T_c的影响。在对超导临界电流密度的影响方面,少量的Al掺杂可以提高MgB₂超导体在低场下的临界电流密度,C掺杂则可以显著改善高场下的性能。当样品中同时掺入1%Al和1%C时对J_c的改善达到最优化的效果,在0-7T范围内都表现出优于纯MgB₂样品的的临界电流密度性能。钉扎力性能的分析也印证了这一点,在低场下钉扎力达到最大值以前Al掺杂样品和共掺杂样品的钉扎力强度都要大于纯的样品,但当磁场增加以后共掺杂的样品则表现出了最好的性能。最后一节利用测量交流磁化率和直流磁化强度的方法研究了Ti,C共掺对MgB₂磁通钉扎行为的影响,发现Ti,C共掺显著提高了MgB₂超导体的临界电流密度,并且在整个外场下的综合性能要优于单掺Ti和单掺C的结果。通过交流磁化率的测量结果,我们分析了掺杂对钉扎势的影响。其中MgB₂和Ti,C共掺的样品分别得到了如下的关系式U（T,B,j）=U₀（?）和U（T,B,j）=U₀（?）,这一结果也同样表明Ti,C掺杂明显提高了MgB₂材料的钉扎性能。第六章首先研究了柠檬酸掺杂对MgB₂超导电性及临界电流密度的影响。研究发现,柠檬酸掺杂可以有效的提高MgB₂超导体在高场下的临界电流密度（J_c）,但同一般的C掺杂类似,柠檬酸掺杂会导致超导转变温度（T_c）下降,所以在高温下柠檬酸掺杂对J_c的提高并不是非常明显。临界电流密度和钉扎力性能的分析表明,柠檬酸掺杂可以有效的提高MgB₂在高场下的性能。在10K,4T条件下15%掺杂的样品得到了最优的结果,其临界电流密度值达到了1.2×10⁴A/cm^-2,要远高于没有掺杂的样品。但是当掺杂量进一步增加到20%时临界电流密度反而出现了下降,因而柠檬酸的最优掺杂可能在15.20%之间。其次对于山梨酸的结果,也进行了类似的分析。相对于柠檬酸,山梨酸含碳量相对较高,在相同掺杂量的情况下有更多的C被引入到MgB₂体系中,从而表现出较高的载流性能。为了更好的分析掺杂对MgB₂钉扎性能的影响,利用传统钉扎力理论的标度公f_p∝h^p（1-h）^q对两组不同掺杂样品的约化钉扎力曲线进行了拟合,发现掺杂使得拟合的结果向着晶界钉扎的结果靠近。我们推测在MgB₂体系中,临界电流密度除受到钉扎力的影响外可能还受到晶粒的各向异性等其它因素的影响,而山梨酸和柠檬酸掺杂所引入的C掺杂则可能会减少这类影响。而在磁测量的结果中,这一影响并不能很好的利用渗流模型来加以解释,为了更好的分析实验结果,在渗流模型的基础上进行了修正和补充,钉扎力标度的拟合公式和实验本质更为接近。另一方面,综合柠檬酸和山梨酸掺杂的结果,用化学溶液法进行掺杂与传统的固相反应法相比较有其自身的优点。因为纳米颗粒有团聚的性能,当用固相反应法进行掺杂时,纳米颗粒的团聚作用有可能使掺杂物无法很好的在体系中均匀分散,从而导致局部过掺杂的情况。用化学溶液法进行掺杂则可以有效的解决这一问题,当化学溶液挥发以后,析出的掺杂物会在原料B粉的表面形成一层包裹膜,这样就有利于掺杂反应的进行。因而在柠檬酸掺杂的样品中,我们得到均匀性较好的掺杂样品,适度掺杂的情况下,所有样品的超导转变都较窄,从而进一步反映出样品具有较好的超导性能。第七章研究了铁纳米线,纳米铁粉和二茂铁三种磁性物质对MgB₂掺杂的影响。发现铁掺杂对MgB₂超导电性的抑制主要是由于活性较强的Fe和B反应造成的,只有纳米铁粉对T_c表现出了较强的抑制作用,而纳米铁粉和二茂铁的作用则相对较小。铁纳米线掺杂提高了MgB₂超导体的临界电流密度（J_c）,其原因可能是是由于掺杂的铁纳米线可以在中高磁下场提供额外的钉扎力,增加了样品在高场下的的钉扎性能。二茂铁掺杂对J_c也具有提高的作用,但在二茂铁掺杂的样品中,钉扎力约化曲线的峰值向h=0.17方向偏移,并且在高场下（h=0.75）有弱小的第二个峰存在,这有可能是由于二茂铁掺杂的磁性钉扎所引起的。但这一观点仍需要进一步的实验来验证。更多还原

【Abstract】 The main contents of thesis are organized as follows:In Chapter 1, the background of MgB₂ is introduced, where the problems of the recent work are revealed. The aims as well as the main contents of the present dissertation are also explained. The development of investigation on MgB₂ has been reviewed in Chapter2 and the main contents are focus on the superconductivity and other physical properties of MgB₂, including crystal and electronic structure, critical current density and up critical field. Afterwords, the current research states of chemical doping on MgB₂ have been introduced. Chapter 3 has discussed the experimental details as well as the characterization approaches concerned in this project.In Chapter 4, the doping effect of Al on superconductivity of MgB₂ is studied as well as CNTs doping. In the section of Al doping, it is found that c-axis cell parameter is reduced by substitution of Al for Mg in the lattice. The phase separation is observed in samples doped at level higher than 10%. On the other hand the superconductivity of MgB₂ is suppressed severly by Al doping, while the mechanism of flux pinning in doped samples is not affected.The critical current, J_c, is ehanced by CNTs doping at low level, while depressed at high doping level. The density of samples prepared by solid state reaction is only 50% of theoretical values. It may be the key problem to improve the J_c in MgB₂ furtherly. Theδl andδT_c pinning properties of MgB₂ doped with CNTs is investigated and it is observed that the main pinning mechanism transforme fromδT_c toδl pinning with increasing CNTs doping.The density of MgB₂ is increased for samples prepared by diffusion methed. Less MgO impurity phase and better grain connection are also observed in these samples; however the behaviour of J_c is suppressed in high field range due to the weakening of grain boundaries.In Chapter 5, the cooperation doping effect of Al/C and Ti/C on superconductivity of MgB₂ is investigated. The observed decrease of T_c of samples could be attributed to the band filling effect and interband scattering. The similar behavior of H_irr versus contension carbon was observed in the both series of samples. It was found that transition temperature is suppressed more severely doping with aluminium than with carbon. The doping effect of carbon is depressed by aluminium doping when both elements are doped simultaneity.In the last section of Chapter 5, flux pinning behavior of carbon and titanium concurrently doped MgB₂ alloys has been studied by ac susceptibilityand dc magnetization measurements. It is found that critical current density and irreversibilityfield of MgB₂ have been significantly improved by doping C and Ti concurrently, sharply contrasted to thesituation of C-only-doped or Ti-only-doped MgB₂ samples. AC susceptibility measurement reveals thatthe dependence of the pinning potential on the dc applied field of Mg_0.95Ti_0.05B_1.95C_0.05 has been determinedto be U(B_dc)∝B_dc^-1 compared to that of MgB₂ U(B_dc)∝B_dc^-1.5. As to the U（J） behavior, a relationshipof U（J）∝J^-0.17 is found fitting well for Mg_0.95Ti_0.05B_1.95C_0.05 with respect to U（J）∝J^-0.21 for MgB₂. All theresults reveal a strong enhancement of the high field pinning potential in C and Ti co-doped MgB₂.The doping effect of citric acid on MgB₂ is studied in the first segment of Chapter6. It was observed that the J_c of MgB₂ is significantly enhanced by doping while the critical temperature, T_c, is suppressed as well as carbon doping. The best values of J_c is obtained in 15% doped sample at 10K, 4T, which is 1.2×10⁴ A/cm². The analogous results are obtained in samples doped with sorbic acid. The fluxpinning behaviors is fitted with expression of f_p∝h^p（1-h）^q which is often usedin conventional flux pinning theory, and it is found that the values of f_p decreasemuch rapidly than the values of surface pinning theory in the high field range. However, the experimental data could not be explained by the percolation theory which is proposed by Eisterer because our values of J_c are obtained from magnetichysteresis loop. In order to explain the result, a new method of fitting f_p isproposed. It is found that the behavior of f_p in high field range is mainly influenceby the anisotropy of irreversibility field of MgB₂.In Chapter 7, Mg_1-xFe_xB₂ superconductor was prepared by in situ solid state reaction to study the effects of nano-iron doping on the superconductivity of MgB₂. Two kinds of nano-iron sources have been used: nano-iron particles and nano-iron wires. It was found that crystal structure was not affected by nano-iron doping for either form of dopant source, while T_c, J_c, and H_m were severely suppressed by iron nano-particles doping. In contrast, with iron nano-wires doping, T_c of the MgB₂ superconductors were not changed remarkedly, while their J_c and H_irr were slightly enhanced at all the temperature ranges investigated in this work. Under 4 T field, the sample doped with 1% wt iron nano-wires reached the highest J_c of 1.1×10⁴ A/cm² at 10 K and 2.2×10² A/cm² at 20 K, respectively. It is argued that the Fe nano-wires may be introduced into MgB₂ as external pinning centers in high field region.In addition, ferrocene is also used to dope in the MgB₂. In this situation, the results may be consider as the codoping of Fe and C, because ferrocene is decomposed into iron and carbon at high temperature. It is deteced that the J_c of doped samples is increased and this may be due to the doping of carbon in MgB₂.更多还原