高温超导薄膜临界电流密度的无损测量研究

临界电流密度Jc是超导薄膜的一个重要参量,它可以衡量超导薄膜的功率承载能力。大面积高温超导薄膜制成后,其Jc需要被无损精确测量。文中提出了一种新的交流磁场下的高温超导薄膜临界态模型:(1)基于此模型对薄膜的临界电流密度进行了精确无损测量;(2)并将实验测量的三次谐波电压曲线进行拟合研究。首先,根据麦克斯韦方程和伦敦方程,计算外加直流磁场超导薄膜Meissner态下电流和磁场在薄膜内的分布;然后分析薄膜进入临界态后内部电流的变化,在考虑顶扎力作用的情况下,提出了临界态电流和磁场非均匀分布模型;最后根据其模型,推导出三次谐波电压的表达式。为了验证该理论,分别对四片超导薄膜在不同频率下进行了三次谐波和临界电流密度测量。实验结果表明:三次谐波电压的理论与实验曲线一致;与四点传输法的测量结果相比较,该方法测量超导薄膜临界电流密度的误差在5%左右,具有高精度、无损伤、方便快捷等优点。     

用于ITER的NbTi超导股线临界电流测试方法

介绍了一种用于ITER的NbTi超导股线在不同温度下临界电流测试的方法和装置。该装置包括一孔径为70mm、最高磁场高达16T的背景磁体以及变温杜瓦等系统。对NbTi超导线变温测试设计了二元电流引线。用该系统得到了其NbTi超导股线在不同磁场不同温度下的临界电流,用“Luca”定标律拟合了测试结果,并对测试结果进行了分析。     

Magnetic field dependence of critical current density in YBa2Cu3O7

We have measured the transport critical current densityJ _cof sintered YBa₂Cu₃O₇, in various applied fields up to 185 Oe at 77 K. We find a sharp decay ofJ _cwith magnetic field. We show that this sharp decay is consistent with the low field hysteresis results of Groveret al. We argue that the observed field dependence is not caused by intragranular weak links.     

Influence of thickness,width and temperature on critical current density of Nb thin film structures

The density of critical currents j_C in Nb thin films with thickness smaller than 15 nm and width between 100 nm and 10 μm has been measured in a wide temperature range. We have found that the temperature dependencies of j_C in sub-micrometer wide bridges at 0.7T_C < T < T_C are well described by the Ginzburg–Landau de-pairing critical current. In wider bridges already at T < 0.9T_C the j_C value is significantly reduced due to the penetration and de-pinning of magnetic vortices.     

晶粒细化对MgB_2超导临界电流密度的作用

在多晶系统的MgB2超导体中存在晶粒间较小的整体电流和晶粒内大的局域电流.用改变升温速率的方法制备了不同晶粒大小和晶粒连接性的MgB2样品,并对它们的晶粒大小进行了统计.采用一种测量超导临界电流密度的Campbell法,分别测量和计算得到了它们的整体电流和局域电流密度.研究表明:长时间的烧结造成晶粒变大,材料有较大的整体临界电流密度,而短时间烧结的样品则相反;同时发现晶粒细化只提高了样品的局域临界电流密度,而且样品内部缺陷、杂质及晶界等因素是影响MgB2超导体整体电流传输的主要因素.     

Effects of surface damage on critical current density in MgB2 thin films

We investigated the influence of surface damage on the critical current density (J_c) of MgB₂ thin films via 140-keV Co-ion irradiation. The J_c(H) of the surface-damaged MgB₂ films was remarkably improved in comparison with that of pristine films. The strong enhancement of J_c(H) caused by a surface damage in MgB₂ films can be ascribed to additional point defects along with an atomic lattice displacement introduced through low-energy Co-ion irradiation, which is consistent with the change in the pinning mechanism, from weak collective pinning to strong plastic pinning. The irreversible magnetic field (H_irr) at 5 K for surface-damaged MgB₂ films with a thickness of 850 and 1300 nm was increased by a factor of approximately 2 compared with that of a pristine film. These results show that the surface damage produced by low energy ion irradiation can serve as an effective pinning source to improve J_c(H) in a MgB₂ superconductor.     

The effect of interlayer coupling on critical current density in Bi2212 system

The dependences of critical current density J _c on the interlayer coupling strength and magnetic field in Bi2212 crystals were obtained by measuring the magnetic loop of the crystals with different interlayer coupling strengths. It was revealed that J _c decreases with the decrease in the interlayer coupling of the crystals. The relation of J _{c ∞ exp} (−H ^β) was also found in the crystals, and further analysis indicated that it was the result of Zeldov pinning potential model. __________ Translated from Chinese Journal of Low Temperature Physics, 2005, 27(1) (in Chinese)     

Crystal orientation dependence of the critical current density in a-axis- and c-axis-oriented MgB2 films

We have investigated the critical current density for MgB₂ films having various crystal orientations prepared by using a hybrid physical-chemical vapor deposition system. An enhancement of the critical current density is clearly presented in MgB₂ films with an a-axis or a b-axis orientation rather than a c-axis orientation. X-ray diffraction patterns reveal a suppression of c-axis orientations while a (100) orientation becomes dominant, and the surface morphology of the a-axis-oriented film shows that the orientation of the c-axis-oriented MgB₂ grains parallel to the plane of the substrate. As the a-axis orientation becomes more dominant in the MgB₂ films, the field performance of the critical current density clearly becomes better. These results suggest that the synthesis of MgB₂ with high ab-plane orientations is one of the keys to enhancing the critical current density in MgB₂.     

高温超导电机电枢线圈临界电流密度研究

高温超导线材的临界电流密度受磁场的影响,在设计计算时,通常假设在线圈截面上电流密度是均匀分布的。但是,实际上在线圈截面上,电压是一致的。由于线圈中的磁场不均匀分布,各处的临界电流密度是不同的。采用数值模拟的方法,分析了高温超导线圈截面上的临界电流分布。结果显示,临界电流密度的分布是不均匀的,总的临界电流可比按均匀分布时的计算结果提高25%以上。     

Angular dependences of critical current density in YBCO thin films with crossed columnar defects

We investigated the dependences of the critical current density J_c on the magnetic field angle θ in YBa₂Cu₃O_7−δ thin films with the crossed configurations of the columnar defects (CDs). To install the crossed CDs, the films were irradiated using the high energetic Xe ions at two angles relative to the c-axis. The additional peak around the c-axis appears in the J_c(θ) for all irradiated films. In lower magnetic fields, the height of the J_c(θ) peak caused by the crossed CDs with the crossing angles θ_i = ±10° was higher than that for the parallel CDs. It is considered that the correlation of the flux pinning by the crossed CDs along the c-axis occurs even in the case of θ_i = ±25°, which was also suggested by the kink behaviors of the scaling parameters of the current–voltage characteristics near 1/3 of the matching field. In higher magnetic fields, on the other hand, the height and width of the J_c(θ) peak for the crossed CD configurations rapidly reduce with increasing the magnetic field compared to the parallel ones. In the crossed CD configurations, the dispersion in the direction of CDs would prevent the correlation of flux pinning along the c-axis in high magnetic fields, which occurs in the parallel CD configurations due to the collective pinning of flux lines including the interstitial flux lines between the directly pinned flux lines by CDs.     

临界电流密度对场冷超导磁悬浮力的影响研究

基于Kim临界态模型,通过考虑超导块材内部屏蔽电流的穿透历史过程,讨论了场冷条件下临界电流密度对高温超导悬浮系统磁悬浮排斥力和吸引力的影响.结果显示:最大超导磁悬浮排斥力和吸引力均随临界电流密度的增加呈指数关系增加,并趋于饱和;场冷条件下的磁悬浮力回滞能量损耗远高于零场冷情况;存在一个磁悬浮力比率κ,κ值对于评价大电流...     

A new analytical model of high voltage silicon on insulator （SOI） thin film devices

A new analytical model of high voltage silicon on insulator (SOI) thin film devices is proposed, and a formula of silicon critical electric field is derived as a function of silicon film thickness by solving a 2D Poisson equation from an effective ionization rate, with a threshold energy taken into account for electron multiplying. Unlike a conventional silicon critical electric field that is constant and independent of silicon film thickness, the proposed silicon critical electric field increases sharply with silicon film thickness decreasing especially in the case of thin films, and can come to 141V/μm at a film thickness of 0.1μm which is much larger than the normal value of about 30V/μm. From the proposed formula of silicon critical electric field, the expressions of dielectric layer electric field and vertical breakdown voltage (V_B,V) are obtained. Based on the model, an ultra thin film can be used to enhance dielectric layer electric field and so increase vertical breakdown voltage for SOI devices because of its high silicon critical electric field, and with a dielectric layer thickness of 2μm the vertical breakdown voltages reach 852 and 300V for the silicon film thicknesses of 0.1 and 5μm, respectively. In addition, a relation between dielectric layer thickness and silicon film thickness is obtained, indicating a minimum vertical breakdown voltage that should be avoided when an SOI device is designed. 2D simulated results and some experimental results are in good agreement with analytical results.     

MgB2超导膜的厚度与其Jc(5K,0T)的关系

通过混合物理化学气相沉积法 (hybrid physical-chemical vapor deposition, HPCVD), 在(000l) SiC 衬底上制得一系列从10 nm到8 μm的MgB₂超导膜样品, 并对它们的形貌、超导转变温度T_c 和临界电流密度J_c与膜厚度的关系进行了研究. 观察到T_c随膜厚度增加上升到最大值后, 尽管膜继续增厚, 但T_c值保持近乎平稳, 而J_c则先随膜厚度增加上升到最高值后, 继而则随膜的厚度的增加而下降. MgB₂膜的T_c(0)和T_c(onset)值与膜厚的关系基本一致, T_c(0)在膜厚为230 nm处达到最大值T_c(0)=41.4 K, 而J_c(5K,0T)在膜厚为100 nm时达到最大值, J_c (5 K, 0 T)=2.3×10⁸A·cm^-2, 这也说明了我们能用HPCVD方法制备出高质量干净MgB₂超导膜. 本文研究的超导膜厚度变化跨度非常大, 从10 nm级的超薄膜到100 nm级的薄膜, 再到几微米的厚膜, 如此T_c和J_c对膜厚度变化的依赖就有了较完整、成体系的研究. 并且本文的工作对MgB₂超导薄膜制备的厚度选取具有实际应用意义. 关键词： 2超导膜')" href="#">MgB₂超导膜 混合物理化学气相沉积法 厚度 临界电流密度     

Shift of the superconducting critical parameters due to correlated disorder

In a nonuniform superconductor with randomly distributed local critical temperature both the macroscopic critical temperature and the upper critical magnetic field strongly depend on the characteristic correlation length ρ₀ of correlated disorder. The shift of the macroscopic critical parameters from those for non-correlated disorder, which does not exist for white noise, is obtained for small ρ₀ in the framework of the Ginzburg-Landau theory.     

The effects of C substitution and disorder on the field dependent critical current density in MgB2 with nano-SiC additions

In this work, nano sized SiC powders were mixed with Mg and B and reacted by either a one-step insitu or two-step method resulted in different level of C substitution. X-ray diffraction shows the presence of Mg₂Si signifying that the reaction between SiC and Mg occurred leading to the release of C in samples reacted in one-step method. Moreover, the much reduced value of a-axis indicates C substitution took place. Resistivity measurements showed higher intragrain scattering owing to a higher density of defects and/or impurities. These samples also show higher H_irr and H_c2 at 20 K in comparison to samples with mainly unreacted SiC (hence lower C substitution). More importantly, their J_c’s are more insensitive to high magnetic field (>4 T) at 6 K. However, at 20 K the effect of C content on J_c(H) is less pronounced. Finally, the order of magnitude of J_c(H) at both 6 K and 20 K is rather dominated by pinning.     

Enhancement of the critical current density in YBCO/Ag composites

YBa₂Cu₃O_7 + xAg?(x = 0.0, ?5.0, ?15.0 and 20 wt%) composite samples have been prepared by the solid state reaction method. The changes in structure are confirmed from the X-ray diffraction analysis and SEM measurements. The critical current density is calculated using Bean's formula from the magnetization measurement. We find that the addition of silver in YBCO enhances the critical current density (?J_C) by a factor of nearly six (for 15% Ag) in comparison to pure YBCO. Enhancement of the pinning force (F_P) by a factor of ten is also reported. The enhancement in ?J_C is investigated over a wide range of magnetic fields. These significant changes in ?J_C and F_P are attributed to the presence of Ag particles as efficient artifical pinning centers in YBCO.     

二极管伏安特性曲线测试电路的改进

指出了文献中所给出的伏安法测二级管特性曲线电路存在的问题，并给出了改进电路。     

3 MV感应电压叠加器的磁感应腔研制

根据脉冲驱动源和负载参数,提出了3 MV感应电压叠加器磁感应腔的设计指标为1.2 MV/70 ns。由感应腔的电流传输效率和真空同轴线绝缘要求,确定了磁芯的几何尺寸;研制了矩形比为0.5的预退火非晶磁环,明确磁环数量不少于6只;在30 T/s时,实验测量的最大脉冲相对磁导率与饱和波模型计算结果相当;估算磁感应腔的等效激磁电感约为7.3 μH,涡流损耗电阻约为139 Ω。根据临界击穿场强的经验公式,采用电场数值分析方法,确定了磁感应腔的电气结构;实验验证了磁感应腔设计有效性;建立了基于磁感应腔的3 MV感应电压叠加器全电路模型,阳极杆箍缩二极管电压、电流计算波形,与实验结果基本相符。     

测量条件对掺锡氧化铟薄膜电学测量结果的影响

用van der Pauw方法测量磁控溅射制备的掺锡氧化铟(ITO)薄膜的电学性能,研究了在室温和冰水混合条件下测量电流大小对电阻率测量结果的影响;在室温条件下测量电流和磁场的大小对载流子浓度和迁移率测量结果的影响.实验结果显示,测量电流为5 mA和测量磁场为0.5 T时,测量结果最佳.