脉冲磁场处理对碳纳米管掺杂MgB2线材临界电流密度的影响

对碳纳米管(CNT)掺杂MgB2超导体磁场处理后的行为进行了研究. 结果表明,CNT掺杂MgB2超导体经5T脉冲磁场处理后临界电流密度Jc(H)在低磁场下提高了2-3倍,高场下提高一个数量级以上,扫描电镜结果显示CNT沿着处理磁场方向规则排列并且成为MgB2基体的形核中心和高效的磁通钉扎中心.     

MgB2 超薄膜的制备和性质研究

利用混合物理化学气相沉积法在6H-SiC(001)衬底上制备干净的MgB₂超导超薄膜.在本底气体压强、载气氢气流量等条件一定的情况下,改变B₂H₆流量及沉积时间,制备得到不同厚度的系列MgB₂超薄膜样品,并研究了超导转变温度T_c、剩余电阻率ρ_(42K)、上临界磁场H_c2等与膜厚的关系.该系列超薄膜沿c轴外延生长,随膜厚度的变小,T_c(0)降低,ρ_(42K)升高.膜在衬底上的生长遵循Volmer-Weber岛状生长模式.对于厚度为7.5 nm的MgB₂超薄膜,T_c(0) =32.8 K,ρ_(42K) =118 μΩcm,是迄今为止所观测到的厚度为7.5 nm的MgB₂超薄膜最高的T_c值;对于厚度为10 nm的MgB₂膜,T_c(0)=35.5 K,ρ_(42K)=17.7 μΩcm,上临界磁场μ₀H_c2估算为12 T左右,零磁场、4 K时的临界电流密度J_c=1.0×10⁷ A/cm²,是迄今为止10 nm厚MgB₂超薄膜的最高J_c值,且其表面连接性良好,均方根粗糙度为0.731 nm.这预示MgB₂超薄膜在超导纳米器件上具有广阔的应用前景. 关键词： 2超薄膜')" href="#">MgB₂超薄膜 薄膜生长 氢气流量 混合物理化学气相沉积     

纳米C和SiC掺杂对MgB2带材超导性能的影响

采用X射线衍射仪，扫描电镜，超导量子干涉仪等仪器对纳米C和SiC掺杂的MgB₂带材进行了表征，并采用标准四引线法对样品的临界电流进行了测试. 实验表明，C和SiC掺杂在提高MgB₂带材高场下的临界电流密度方面具有显著效果. 在温度为4.2 K、磁场大于9 T条件下，C和SiC掺杂样品的临界电流密度与未掺杂样品相比均提高一个数量级以上. 掺杂样品高磁场下良好的临界电流性能主要归因于C对B的替代所产生的晶格畸变、位错等缺陷和局部成分变化而导致的有效晶内钉扎作用. 实验结果表明，SiC掺杂的MgB2带材之所以具有非常好的高场电流特性，和C掺杂的样品一样， C对B的替代起到十分关键的作用. 关键词： 2带材')" href="#">MgB₂带材 C掺杂 SiC掺杂 临界电流性能     

利用电泳法在金属基底上制备MgB2超导厚膜

利用电泳技术在高熔点金属基底Ta，Mo和W上制备MgB₂超导厚膜.厚膜中的MgB2晶粒结合紧密，粒度小于1μm，呈随机取向生长.电阻测量表明沉积在Ta，Mo，W上的MgB₂厚膜的超导起始转变温度分别为36.5K，34.8K，33.4K，对应的转变宽度为0.3K，1.5K和2.0K.三种基底上制备的MgB₂厚膜的临界电流密度在不同温度下随外磁场的变化情况 基本相同，MgB₂/Mo厚膜的临界电流密 关键词： 2超导厚膜')" href="#">MgB₂超导厚膜 电泳 金属基底     

MgO(111)衬底MgB2 超薄膜的制备和性质研究

利用混合物理化学气相沉积法(HPCVD)在MgO(111)衬底上制备了干净的MgB₂超导超薄膜. 在背景气体压强, 载气氢气流量以及沉积时间一定的情况下, 改变B₂H₆的流量, 制备得到不同厚度系列的MgB₂超导薄膜样品, 并测量了其超导转变温度 T_c, 临界电流密度J_c等临界参量. 该系列超导薄膜沿c轴外延生长, 表面具有良好的连接性, 且有很高的超导转变温度T_c(0) ≈ 35-38 K和很小的剩余电阻率ρ(42 K) ≈ 1.8-20.3 μΩ·cm^-1. 随着膜厚的减小而减小, 临界温度变低, 而剩余电阻率变大. 其中20 nm的样品在零磁场, 5K时的临界电流密度J_c ≈ 2.3×10⁷ A/cm². 表明了利用HPCVD在MgO(111)衬底上制备的MgB₂超薄膜有很好的性能, 预示了其在超导电子器件中广阔的应用前景. 关键词： MgO(111)衬底 2超薄膜')" href="#">MgB₂超薄膜 混合物理化学气相沉积     

类MgB2硼化物晶体电子结构比较研究

应用准确的第一原理方法，对8种类MgB₂超导体结构的二硼化物进行了电子结构 比较研究，发现其中的超导体具有特殊的能带属性.对其在高压下态密度的变化情况做了对照. 关键词： 硼化物 态密度 能带 高压     

MgB2和Mg0.93Li0.07B2的电阻率与霍尔效应研究

测量了MgB₂和Mg_0.93Li_0.07B₂的电阻率ρ(T)与霍尔系数R_H(T)的温度依赖关系.电阻率的测量结果表明,MgB₂和Mg_0.93Li_0.07B₂的正常态电阻率与温度有平方的依赖关系.MgB₂和Mg_0.93Li_0.07B_{2关键词： 电阻率 霍尔效应}     

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     

铁基非晶的低频脉冲磁场处理效应

对非晶合金Fe₇₈ Si₉ B₁₃进行了低频脉冲磁场处理，用穆斯堡尔谱学、透射电 镜等方法观察处理试样的微观结构变化.研究发现，当脉冲频率20—25Hz,磁场16—32kA/m,作用时间≤2min，合金发生了纳米晶化，纳米相岐睩e(Si)晶粒尺寸为10nm. 而且，在低频脉冲磁场处理过程中，非晶试样的温升≤20℃. 关键词： 非晶态合金 脉冲磁场处理 纳米晶化     

Mg/B多层膜退火法中不同制备条件对MgB2超导薄膜性质的影响

用Mg/B多层膜退火的方法制备了一系列MgB₂超导薄膜,研究了退火温度、退火时间和薄膜厚度对于MgB₂薄膜性质的影响.厚度为250 nm的Mg/B多层膜经400 ℃低温退火后已经生成超导相,此厚度薄膜750 ℃下退火20—30 min实现最佳超导转变温度（T_c）.前驱膜分层厚度相同时,随着薄膜厚度减小MgB₂薄膜T_c明显降低,而且较薄的膜T_{c关键词： 2}超导薄膜')" href="#">MgB₂超导薄膜 电子束蒸发 超导成相     

分层掺B和吸附H2O碳纳米管的结构稳定性及电子场发射性能

运用基于第一性原理的密度泛函理论,系统研究了处于外电场中分层掺B并吸附不同数目H₂O碳纳米管体系的结构稳定性和电子场发射性能. 研究表明:第3层掺B并吸附5个H₂O的B₃CNT+5H₂O体系结构最稳定,管帽处Mulliken电荷最密集,尤其与单独掺B的B₃CNT和单独吸附H₂O的B₃CNT+5H₂O相比,其Fermi能级处态密度分别     

MgB2混合态热导率的反常增强

测量了MgB₂多晶样品的混合态热导率，磁场强度为0—7 T，温度范围为5—45 K .实验结果显示MgB₂热导率在低场下迅速上升，高场下趋于饱和，这与MgB₂的 二能隙电子结构有关.对实验结果的分析指出，低温强场下MgB₂多晶样品热导率的显著增强无法完全 用电子热导来解释，并对此进行了讨论. 关键词： 2')" href="#">MgB₂ 热导率 混合态     

Diffusion of CO2/CH4 confined in narrow carbon nanotube bundles

ABSTRACT

The diffusion of a CO₂/CH₄ mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO₂ and CH₄ are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO₂ and CH₄ molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO₂ in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO₂ and CH₄ in the pores. For CO₂, the interaction energies (CO₂–CO₂ and CO₂–CNT) are larger than that of CH₄ in all cases. But only a very small difference in the diffusion coefficient was observed between CO₂ and CH₄. Temperature has a negligible effect on the difference between diffusion coefficients of CO₂ and CH₄ in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO₂/CH₄ in CNT bundle membranes.     

Quasiparticle density of states of 2H-NbSe2 single crystals revealed by low-temperature specific heat measurements according to a two-component model

Low-temperature specific heat in a dichalcogenide superconductor 2H-NbSe2 is measured in various magnetic fields. It is found that the specific heat can be described very well by a simple model concerning two components corresponding to vortex normal core and ambient superconducting region, separately. For calculating the specific heat outside the vortex core region, we use the Bardeen-Cooper Schrieffer （BCS） formalism under the assumption of a narrow distribution of the superconducting gaps. The field-dependent vortex core size in the mixed state of 2H-NbSe2, determined by using this model, can explain the nonlinear field dependence of specific heat coefficient γ（H）, which is in good agreement with the previous experimental results and more formal calculations. With the high-temperature specific heat data, we can find that, in the multi-band superconductor 2H-NbSe2, the recovered density of states （or Fermi surface） below Tc under a magnetic field seems not to be gapped again by the charge density wave （CDW） gap, which suggests that the superconducting gap and the CDW gap may open on different Fermi surface sheets.     

Magnetocaloric effect in Gd6Co1.67Si3 compound with a second-order phase transition

The magnetic properties and the magnetic entropy change AS have been investigated for Gd6Co1.67Si3 compounds with a second-order phase transition. The saturation moment at 5 K and the Curie temperature TC are 38.1μB and 298 K, respectively. The AS originates from a reversible second-order magnetic transition around TC and its value reaches 5.2 J/kg.K for a magnetic field change from 0 to 5T. The refrigerant capacity （RC） of Gd6Co1.67Si3 are calculated by using the methods given in Refs.[12] and [21], respectively, for a field change of 0 5T and its values are 310 and 440 J/kg, which is larger than those of some magnetocaloric materials with a first-order phase transition.     

稀磁半导体Hg0.89Mn0.11Te磁化强度及磁化率的研究

利用MPMS-7(magnetic property measurement system)型超导量子磁强计对垂直布里奇曼法生长的Hg_0.89Mn_0.11Te晶片磁化强度变化规律进行了测量.试验采用了两种不同的外场和冷却条件.首先在5 K恒温下，-5200到5200 kA/m范围内改变磁场强度进行了测定.然后维持800 kA/m恒定磁场，分别在有场冷却和无场冷却条件下，从5到300 K范围内改变温度，研究了变温条件下的磁化特性.并采用分子场近似模型，用类布里渊函数，最小二乘法对磁化强度随磁场强度变化的实验结果进行拟合和分析，结果表明，Mn²⁺离子之间存在反铁磁相互作用.磁化率和温度关系分析表明：在测试范围内Hg_0.89Mn_0.11Te是单一的顺磁相，在高温区磁化率和温度服从居里-万斯定律，呈线性关系，低于40 K时，磁化率和温度的关系偏离居里-万斯定律，表现出顺磁增强现象. 关键词： 0.89Mn_0.11Te')" href="#">Hg_0.89Mn_0.11Te 磁化强度 磁化率 类布里渊函数     

Sr2RuO4正常态的c方向的磁阻的研究

研究了典型的层状钙钛矿结构超导单晶Sr₂RuO₄在c方向的磁阻(Δρ/ρ₀)(H∥ab,J∥c)的变化.实验发现,磁阻表现出强烈的各向异性,并且随着温度T的降低,磁阻效应越明显;当在平面ab内旋转磁场H的方向时,磁阻成周期性变化;实验表明,磁场沿(110)方向时,出现磁阻的极大值.分别从Sr₂RuO₄的费米面的各向异性、载流子散射率、c方向能带色散的各向异性等方面来解释这些输运性质. 关键词： 2RuO₄')" href="#">Sr₂RuO₄ 磁阻     

Effect of high magnetic field annealing on the microstructure and magnetic properties of Co-Fe layered double hydroxide

The effects of high magnetic field (10 T) on the products obtained by calcination of Co-Fe LDH precursors at different temperatures were investigated. The XRD results indicated that Fe^III substituted for Co^III in Co₃O₄ to yield Co^IICo^IIIFe^IIIO₄ under the calcination of Co-Fe LDH precursors at 400 °C. The products obtained by magnetic field annealing at 400 °C had a porous plate-like morphology, whereas the products without magnetic field annealing were composed of nanoparticles. It was seen that CoFe₂O₄ phase could be formed at low temperature (about 500 °C) under the magnetic field annealing. The grain size of products obtained by magnetic field annealing at 800 °C was larger than that of zero magnetic field. It was found that the saturation magnetization was significantly enhanced after magnetic field annealing, especially at lower temperature (≤600 °C). The possible reason for the effects on the microstructure and magnetic properties of products obtained by magnetic field annealing was discussed.     

Magnetoresistances and magnetic entropy changes associated with negative lattice expansions in NaZn13-type compounds LaFeCoSi

Magnetoresistances and magnetic entropy changes in NaZn13-type compounds La（Fel-xCox）11.9Si1.1 （x=0.04, 0.06, and 0.08） with Curie temperatures of 243 K, 274 K, and 301 K, respectively, are studied. The ferromagnetic ordering is accompanied by a negative lattice expansion. Large magnetic entropy changes in a wide temperature range from ～230 K to ～320 K are achieved. Raising Co content increases the Curie temperature but weakens the magnetovolume effect, thereby causing a decrease in magnetic entropy change. These materials exhibit a metallic character below Tc, whereas the electrical resistance decreases abruptly and then recovers the metal-like behaviour above Tc. Application of a magnetic field retains the transitions via increasing the ferromagnetic ordering temperature. An isothermal increase in magnetic field leads to an increase in electrical resistance at temperatures near but above Tc, which is a consequence of the field-induced metamagnetic transition from a paramagnetic state to a ferromagnetic state.     

Effects of applied magnetic field and pressures on the magnetic properties of nanocrystalline CoFe2O4 ferrite

Nanocrystalline CoFe₂O₄ ferrite with crystallite sizes of 30 nm have been successfully prepared by an emulsion method. X-ray diffractometer (XRD) shows that nanocrystalline CoFe₂O₄ ferrite possesses face center cubic structure. Crystal structure of the CoFe₂O₄ nanocrystals will not be changed by the applied magnetic field and pressures. The obtained CoFe₂O₄ nanocrystalline powders were pressed into thin columns with different pressures. Meanwhile, the dependences of the applied pressures and the direction of applied magnetic field on the magnetic properties of the CoFe₂O₄ nanocrystals were investigated in detail using vibrating sample magnetometer (VSM). The pressed CoFe₂O₄ nanocrystal gains the most excellent magnetisms in a parallel applied magnetic field.