Sugar Coating of Boron Powder for Efficient Carbon Doping of MgB2 with Enhanced Current‐Carrying Performance
Sugar Coating of Boron Powder for Efficient Carbon Doping of MgB2 with Enhanced Current‐Carrying Performance
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DOI：
10.1002/adma.200601659
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发表时间：
2007-05
影响因子：
29.4
通讯作者：
Sihai Zhou;A. Pan;D. Wexler;S. Dou
中科院分区：
文献类型：
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作者：
Sihai Zhou;A. Pan;D. Wexler;S. Dou
在过去的二十年中，超导电性的第二次繁荣是由于发现了转变温度(TC)为39K[1]的硼化镁[1]以及强大的市场应用潜力。对这种超导体发展的最大影响是发现了由纳米碳化硅掺杂引起的临界电流密度(JC)随外加磁场(BA)的显著提高。[2]从那时起，为了实现或改进所获得的增强，人们尝试了大量的掺杂剂。在这些掺杂工作中，绝大多数纳米添加剂是通过固相反应引入的；然而，一个重要的挑战是在少量的纳米添加剂和基质材料之间实现均质，因为任何干混合都会造成纳米颗粒团聚的主要问题。在本次通信中，我们选择了糖(C6H12O6)作为掺杂剂。这是最容易获得的碳水化合物，可以使液体均匀混合。我们已经证明，糖掺杂导致了硼化镁中碳的有效替代，因此在整个外场范围内，JC性能的显著提高是很容易实现的。该方法适用于各种碳基化合物和复合材料的制备。MgB_2超导体具有高的T_c、强的颗粒间连接性和低的各向异性，具有很强的应用潜力。一个主要的问题是纯材料中相对较弱的钉扎，这导致JC随着Ba值的变化而相当快地降解。用不同的方法，如辐照和化学掺杂，来提高MgB_2的钉扎强度。。化学掺杂是在超导体中引入钉扎位的一种理想的、相对容易且廉价的方法。纳米碳化硅被认为是增强JC最有效的掺杂剂。[2，5，6，15，20-22]碳基材料对MgB_2中的JC(Ba)性能的增强作用最强，这是因为碳可以通过取代硼进入到MgB_2的晶格中。这种取代导致了在该材料中发现的两个能带中的C取代位和镁空位上发生的电荷载流子散射增强。这种散射已被证明是导致上临界场显著增加的原因。一般来说，纳米颗粒是确保均匀掺杂过程所必需的，在我们的情况下，这应该会提供强大的钉扎力增强。然而，无论混合、研磨、球磨或超声波分散进行得多么好，掺杂过程通常都会因形成大的颗粒团聚而受阻。此外，前驱体材料通常处于钝化状态，这会进一步降低反应性和掺杂质量。可持续工业应用的另一个问题是纳米粉末价格昂贵。在这里，我们介绍了一种简单分子混合的新方法，该方法确保了i)均一，ii)具有最大反应表面的强反应性，iii)原子级掺杂层(C)和基质组分(B和Mg)之间新鲜和干净的界面，以及iv)排除纳米级添加剂的必要性。这是通过将糖溶液与硼粉“湿”混合并连续干燥浆料以实现均匀掺杂来实现的，从而产生可与碳化硅纳米级掺杂相媲美的JC(BA)增强。扫描电子显微镜(SEM)图像显示，原始的和糖包覆的硼粉没有差别。这两种粉末都有球状颗粒，尺寸小到几纳米。与热处理形成的纯硼化镁不同，添加糖的粉末首先经历糖的分解，最有可能的是在100°C以上分解成水和新鲜的高活性碳：C6H12O6→6H2O+6C。同时，还可能发生以下过程：C6H12O6→6CO+6H2。然而，第一种途径更有可能发生，从所获得的X射线衍射(X射线衍射)结果可以得出结论。对样品的X射线衍射谱分析表明，糖对样品的物相组成影响不大。图1a显示了不同C6H12O6含量的湿式预混镁样品的X射线衍射图。即使在不加糖的情况下(碳取代度x=0)，样品也是以MgB_2相为主，以少量的MgO相为主要杂质。这可能是加氢处理和/或糖中存在水的残余影响。从添加5%的糖开始(x=0.08)，C6H12O6掺杂样品出现两个峰，属于Mg2C3。对于x=0.03掺杂水平的样品，由于添加的糖量较少，只观察到一个很小的Mg2C3峰。令人惊讶的是，C O M M U N IC A IO N
The second boom in superconductivity during the last two decades has been powered up by the discovery of MgB2 having the transition temperature (Tc) of 39 K [1] as well as a strong market potential for applications. The strongest impact on the development of this superconductor has been made by the discovery of the significant enhancement of the critical current density (Jc) as a function of the applied magnetic field (Ba) that results from SiC nanodoping. [2] Since then, a large variety of dopants have been attempted in order to achieve or improve on the attained enhancement. In the vast majority of these doping works the nanodopants have been introduced via a solid-state reaction; however, an important challenge is achieving homogeneity between a small amount of nanoadditives and matrix materials, because any dry mixing poses the major problem of nanoparticle agglomeration. In this Communication, we have chosen sugar (C6H12O6) as the dopant. This is the most readily available carbohydrate that enables liquid homogeneous mixing. We have demonstrated that sugar doping resulted in an effective substitution of carbon for boron in MgB2, so that a significant enhancement of the Jc performance over the entire applied field range is readily achievable. This method is applicable to the fabrication of a wide range of carbon-based compounds and composites. The MgB2 superconductor has a strong potential for various applications because of its high Tc, strong connectivity between the grains, and low anisotropy. One major problem is posed by the relatively weak pinning in the pure material, which leads to rather rapid degradation of Jc as a function of Ba. Different approaches, such as irradiation and chemical doping, have been used to enhance the pinning strength in MgB2. . Chemical doping is a desirable, relatively easy, and cheap method to introduce pinning sites into the superconductor. Nanometer-size SiC has been found to be the most effective dopant for Jc enhancement. [2,5,6,15,20–22] The carbon-based materials introduce the strongest enhancement of Jc(Ba) performance in MgB2, owing to the fact that carbon can be incorporated into the MgB2 crystal lattice by replacing boron. This substitution results in the enhancement of chargecarrier scattering occurring on C-substituted sites and Mgvacancies in two energy bands discovered in this material. This scattering has been shown to be responsible for the considerable upper critical field increase. Generally, nanometer-sized particles are necessary to ensure a homogeneous doping procedure, which in our case should provide a strong enhancement of the pinning force. However, regardless of how well mixing, grinding, milling, or ultrasonic dispersion is carried out, the doping process is usually impeded by the formation of large agglomerates of particles. In addition, the precursor materials are commonly in their passivated state, which can further degrade the reactivity and doping quality. Another problem for sustainable industrial applications is that nanometer-sized powders are expensive. Herein, we introduce a new approach to simple molecular mixing that ensures i) homogeneity, ii) strong reactivity with a maximum reaction surface, iii) fresh and clean interfaces between the atomic-scale dopant layer (C) and matrix components (B and Mg), and iv) rules out the necessity of nanometer-scale additives. This is achieved by “wet” mixing of a sugar solution with a boron powder and successive drying of the slurry to achieve homogeneous doping, resulting in Jc(Ba) enhancements comparable to the best results achieved by SiC nanoscale doping. Scanning electron microscopy (SEM) images of the raw and the sugar-coated boron powders show no difference. Both powders have ball-shaped particles down to a few nanometers in size. In contrast with pure MgB2 formation upon heat treatment, the sugar-added powder first undergoes the decomposition of sugar to, most likely, water and fresh highly reactive carbon above 100 °C: C6H12O6 → 6H2O + 6C. Concurrently, the following process may also occur: C6H12O6 → 6CO + 6H2. However, the first route is more likely to occur, as can be concluded from the obtained X-ray diffraction (XRD) results. XRD pattern analysis of the MgB2 samples indicates that the sugar slightly affects the phase constituents. Figure 1a shows XRD patterns of wet premixed MgB2 samples with different levels of C6H12O6 content. The samples mainly consist of an MgB2 phase, with some MgO phase as the main impurity even for the sample with no sugar addition (carbon substitution level (x) equal to 0). This is probably a remnant effect of hydrotreatment and/or the presence of water in the sugar. Starting from 5 % sugar addition (x = 0.08), the C6H12O6-doped samples show two peaks that belong to Mg2C3. For the x = 0.03 doping level sample only one small Mg2C3 peak is observed, as a result of the low amount of sugar added. Surprisingly, the C O M M U N IC A IO N