Shekhar Chandra, Giri Rajiv, Malik S K, Srivastav O N
Department of Physics, Banaras Hindu University, Varanasi 221005, India.
J Nanosci Nanotechnol. 2007 Jun;7(6):1804-9. doi: 10.1166/jnn.2007.720.
In the present study, we report a systematic study of doping/admixing of carbon nanotubes (CNTs) in different concentrations in MgB2. The composite material corresponding to MgB2-x at.% CNTs (35 at.% > or = x > or = 0 at.%) have been prepared by solid-state reaction at ambient pressure. All the samples in the present investigation have been subjected to structural/microstructural characterization employing XRD, Scanning electron microscopic (SEM), and Transmission electron microscopic (TEM) techniques. The magnetization measurements were performed by Physical property measurement system (PPMS) and electrical transport measurements have been done by the four-probe technique. The microstructural investigations reveal the formation of MgB2-carbon nanotube composites. A CNT connecting the MgB2 grains may enhance critical current density due to its size (approximately 5-20 nm diameter) compatible with coherence length of MgB2 (approximately 5-6 nm) and ballistic transport current carrying capability along the tube axis. The transport critical current density (Jct) of MgB2 samples with varying CNTs concentration have been found to vary significantly e.g., Jct of the MgB2 sample with 10 at.% CNT addition is approximately 2.3 x 10(3) A/cm2 and its value for MgB2 sample without CNT addition is approximately 7.2 x 102 A/cm2 at 20 K. In order to study the flux pinning effect of CNTs doping/ admixing in MgB2, the evaluation of intragrain critical current density (JJ) has been carried out through magnetic measurements on the fine powdered version of the as synthesized samples. The optimum result on Jc is obtained for 10 at.% CNTs admixed MgB2 sample at 5 K, the Jc reaches approximately 5.2 x 10(6) A/cm2 in self field, -1.6 x 10(6) A/cm2 at 1 T, approximately 2.9 x 10(5) A/cm2 at 2.6 T, and approximately 3.9 x 10(4) A/cm2 at 4 T. The high value of intragrain Jc in 10 at.% CNTs admixed MgB2 superconductor has been attributed to the incorporation of CNTs into the crystal matrix of MgB2, which are capable of providing effective flux pinning centres. A feasible correlation between microstructural features and superconducting properties has been put forward.
在本研究中，我们报告了对不同浓度碳纳米管（CNTs）掺杂/混入MgB₂的系统研究。通过常压下的固态反应制备了对应于MgB₂₋ₓat.% CNTs（35 at.%≥x≥0 at.%）的复合材料。本研究中的所有样品均采用X射线衍射（XRD）、扫描电子显微镜（SEM）和透射电子显微镜（TEM）技术进行了结构/微观结构表征。磁化测量由物理性能测量系统（PPMS）进行，电输运测量通过四探针技术完成。微观结构研究揭示了MgB₂ - 碳纳米管复合材料的形成。连接MgB₂晶粒的碳纳米管因其尺寸（直径约5 - 20 nm）与MgB₂的相干长度（约5 - 6 nm）相匹配以及沿管轴的弹道输运载流能力，可能会提高临界电流密度。已发现不同CNTs浓度的MgB₂样品的输运临界电流密度（Jct）有显著变化，例如，添加10 at.% CNTs的MgB₂样品在20 K时的Jct约为2.3×10³ A/cm²，而未添加CNTs的MgB₂样品在20 K时的值约为7.2×10² A/cm²。为了研究CNTs掺杂/混入MgB₂中的磁通钉扎效应，通过对合成样品的细粉版本进行磁性测量来评估晶粒内临界电流密度（JJ）。对于添加10 at.% CNTs的MgB₂样品在5 K时获得了关于Jc的最佳结果，在自场中Jc达到约5.2×10⁶ A/cm²，在1 T时为 - 1.6×10⁶ A/cm²，在2.6 T时约为2.9×10⁵ A/cm²，在4 T时约为3.9×10⁴ A/cm²。添加10 at.% CNTs的MgB₂超导体中晶粒内Jc的高值归因于碳纳米管掺入MgB₂的晶体基质中，这能够提供有效的磁通钉扎中心。提出了微观结构特征与超导性能之间的可行关联。
