Carbon solubility and superconductivity in MgB2

Successful replacement of B by C in the series MgB_2−xC_x for values of x upto 0.3 is reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x=0.30 into the MgB₂ lattice. The superconducting transition temperature, T_c measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x=0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch–Gruneisen formula for all the carbon compositions studied. The Debye temperature, θ_D, extracted from the fit, is seen to decrease with carbon content from 900 to 525 K, whereas the electron–phonon interaction parameter, λ, obtained from the McMillan equation using the measured T_c and θ_D, is seen to increase monotonically from 0.8 in MgB₂ to 0.9 in the x=0.50 sample. The ratio of the resistivities between 300 and 40 K versus T_c is seen to follow the Testardi correlation for the C substituted samples. The decrease in T_c is argued to mainly arise due to large decrease in θ_D with C concentration and a decrease in the hole density of states at N(E_F).     

Substrate dependent growth studies of MgB2 films by electrocrystallization

The comparative study of MgB₂ film growth on Al₂O₃ and glass substrate by electrocrystallization technique is discussed. The precursor magnesium films were deposited by vacuum evaporation method. These magnesium films were then used as electrode for the growth of MgB₂ films by electrocrystallization. The structural, morphological and superconducting properties of the electrocrystallized MgB₂ films on Al₂O₃ substrate were examined by using XRD, SEM and electrical resistivity measurement techniques and compared with that of MgB₂ films grown on glass substrate. The films deposited on Al₂O₃ substrates showed enhanced crystallinity and relatively higher T_c value compared to films deposited on glass substrates.     

关于MgB2/Al2O3超导薄膜电阻转变和各向异性的研究

利用电子束蒸发方法将MgB₂超导薄膜沉积到Al₂O₃(001)衬底上.采用标准的四引线法研究了磁场平行和垂直超导薄膜ab平面下的电阻转变.一个激活能模型 U(T,H)=U₀(1-T/(T_c+δ))ⁿ(1-H/H 关键词： 2/Al₂O₃')" href="#">MgB₂/Al₂O₃ 超导体 电阻转变 各向异性     

MgB2超导薄膜的变温拉曼光谱研究

MgB₂作为迄今为止超导转变温度最高的合金超导体,由于其具有结构简单、相干长度长、晶界间不存在弱连接、上临界场很高、电-声散射时间短等特点,MgB₂超导薄膜在电子学领域有着广阔的的应用前景。拉曼光谱是研究电-声子相互作用和超导能带的一种有效方法,且已广泛用于分析MgB₂材料的电子、声子特征以及超导体能带结构,研究表明,样品质量、晶粒尺寸以及测试条件对MgB₂拉曼峰的峰位和峰形影响很大,其中拉曼光谱随温度的变化也是一个研究重点,但目前关于MgB₂变温拉曼光谱的研究,测试的温度范围相对较小,局限在83 K到室温区域或是转变温度附近。研究了大范围温度区间内MgB₂薄膜的拉曼光谱变化,采用混合物理化学沉积法在(0001)SiC衬底上制备了MgB₂多晶薄膜,薄膜的晶粒尺寸约为300 nm,超导转变温度为39.3 K,对其在10～293 K之间的拉曼光谱进行了测试,测量的波数范围为20～1 200 cm-1。变温拉曼光谱的测试结果显示,在高频620 cm-1附近以及低频80和110 cm-1附近存在MgB₂的拉曼峰。经分析,低频区域出现的两个拉曼峰的频率与超导能隙宽度相对应,表明MgB₂的双能隙特性。考虑到MgB₂中四种声子模式的拉曼活性,高频620 cm-1附近的拉曼峰应是由E_2g振动模所贡献的,且随着测试温度的降低,该拉曼峰的峰位未发生明显的偏移,但半高宽显著变小,从293 K时的380.7 cm-1减小到10 K时的155 .7 cm-1,分析表明E_2g声子与电子系统的非线性耦合所引起的非简谐效应可能是拉曼峰半高宽线性变小的主要原因。     

Study of the sintering behaviour of MgB2 superconductor during hot-pressing

The densification behaviour of MgB₂ was studied when commercial powder was hot-pressed in the temperature range 1070–1190 °C. The mechanisms active during sintering were investigated by continuously recording the shrinkage vs. time and elaborating the data on the basis of Kingery’s model for liquid phase sintering. XRD and SEM analysis on final dense bodies were used to evaluate secondary phases formation during sintering and the effect of magnesium sublimation. AC magnetic susceptibility measurements were also performed to correlate the microstructural and morphological modifications induced by hot-pressing and the superconducting properties.     

千米量级多芯MgB2/Nb/Cu超导线材的制备及性能研究

以无磁性的Nb作为中心增强体和阻隔层材料,无氧铜作为稳定体包套材料,采用原位法粉末装管工艺(in-situ PIT)制备了千米量级7芯导体结构的MgB2/Nb/Cu超导线带材,由于Nb/Cu包套材料具有良好的塑形加工性能,整个加工过程中未进行中间退火热处理,复合多芯线材最终加工到Φ1.4mm;在真空热处理炉中680℃保温2小时进行成相热处理;对烧结后的线材进行了微观结构、超导电性、纵向电流分布均匀性及常温力学性能等分析检测.线带材的工程临界电流密度在20K,1T磁场条件下达到2.5×104 A/cm2.结果表明该工艺能够制备实用化高性能的MgB2线带材.     

Effect of pressure on the superconducting transition temperature of doped and neutron-damaged MgB2

Measurements of the superconducting transition temperatures for Al-doped, C-doped and neutron-damaged–annealed MgB₂ samples under pressure up to 8 kbar are presented. The dT_c/dP values change systematically with the decrease of the ambient pressure T_c in a regular fashion. The evolution of the pressure derivatives can be understood assuming that the change in phonon spectrum is a dominant contribution to dT_c/dP.     

葡萄糖掺杂对MgB2钉扎机制的影响

本文通过磁性测量获得了葡萄糖掺杂MgB2块材样品的临界电流密度Jc,并利用集体钉扎模型分析了掺杂对其磁通钉扎机制的影响．研究发现,在纯样中δTc钉扎为主导钉扎机制;而在掺杂样品中,随着掺杂量的增加础钉扎所占比重逐渐增大到90％．剩余电阻率的计算结果也验证了掺杂样品中的钉扎机制变化．我们推断础钉扎主要是由碳对硼位掺杂引起...     

Pinning enhancement in MgB2-magnetic particles composites

The temperature dependence of microwave absorption hysteresis is measured in superconducting composites, prepared from MgB₂ and powders of magnetic Ni, Co, and non-magnetic Ti. In MgB₂–Ni and MgB₂–Ti composites the increase of microwave absorption hysteresis, by 150% and 25%, respectively, is found. In both cases this is connected with pinning enhancement. In MgB₂–Co composites a decrease of the superconducting transition temperature and worsening of pinning properties are observed.     

新型超导体MgB2和MgCNi3热、电输运性质研究

报道了两种新型超导体MgB₂,MgCNi₃和氧化物高温超导体Bi_{2< /sub>Sr2Ca0.9Ce0.1Cu2O8+y 的热导率-温度关系和电阻率-温度关系.实验发现氧化物高温超导 体在进入超导态后热导有所上升，出现极大值后再下降，而MgB2和MgCNi3 则单调下降. 由Wiedemann-Fra 关键词： 热导率 超导体 2}')" href="#">MgB₂ 3')" href="#">MgCNi₃     

Scaling behavior of mixed-state hall effect in MgB2 thin films

The Hall resistivity (ρ_xy) and the longitudinal resistivity (ρ_xx) in c-axis-oriented superconducting MgB₂ thin films have been investigated in extended fields up to 18 T. We have observed a scaling behavior between the Hall resistivity and the longitudinal resistivity, , where the exponent (β) is observed to be independent of the temperatures and the magnetic fields. For a wide magnetic field region from 1 to 18 T and a wide temperature region from 10 to 28 K, a universal power law with β = 2.0 ± 0.1 was observed in c-axis-oriented MgB₂ thin films. These results can be well interpreted by using recent models.     

C掺杂MgB2超导线材的失超特性研究

研究了不同芯数(单芯、6芯和12芯)的C掺杂MgB2超导线材的失超特性.在制冷机环境(20K)下测量了C掺杂MgB2超导线材样品在自场下的最小触发能及失超传播速度.实验结果表明,失超传播速度随着工作电流的增大而线性增加.失超传播速度的大小跟测量位置有关,离触发区越近的传播速度越大.单芯样品的最小触发能最小,6芯和12芯线的最小触发能相差不大.最小触发能随工作电流的增加成指数形式衰减.     

混合物理化学气相沉积法制备MgB$lt;sub$gt;2$lt;/sub$gt;单晶纳米晶片的研究

利用混合物理化学气相沉积法在石墨衬底上制备出了晶形为六角结构、厚度不同、径向尺寸不一的MgB₂单晶纳米晶片. 利用纳米定向转移技术将此晶片转移到了碳支持膜铜网上, 以便对其精细结构等物性进行表征. 电输运测量和磁性测量结果都表明晶片具有超导电性: T_conset=38 K, T_c(0)=33 K. 扫描电子显微镜图像表明, 晶片表面平整、厚度分布在几个纳米到200 nm之间, 宽度从几微米到上百微米; 高分辨透射电镜图像显示出晶片具有周期性晶格条纹. 选区电子衍射数据与MgB₂已有的单晶衍射数据相符. 这些测量结果证实了其确为高质量单晶MgB₂超导纳米晶片. 本文不仅提出了一种全新的制备单晶MgB₂的方法, 也观察到了纳米尺度MgB₂单晶的零电阻现象, 为后续的磁通钉扎、纳米力学性能等领域的深入研究提供了合适的素材. 关键词： 2')" href="#">单晶MgB₂ 超导纳米晶片 零电阻 混合物理化学气相沉积法     

First-principles calculations for structure and equation of state of MgB2 at high pressure

We investigate the structure and the equation of state of compound MgB₂ at high pressure using the full-potential linearized muffin-tin orbital scheme within the generalized gradient approximation correction in the frame of density functional theory. Through the quasi-harmonic Debye model, in which the phononic effects are considered, we have obtained successfully the bulk modulus and the thermal expansion of MgB₂.     

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.     

A 3 kW average power tunable TEA CO2 laser

A high average power line-tunable TEA CO₂ laser is described. Average output power up to 3 kW is achieved at repetition rate of 180 Hz. The maximum output power is almost equal among the four spectrum bands of CO₂ laser (the P and R branches of 00°1–10°0 and 00°1–02°0 transition bands). Several special technologies including rotating spark gap switching, PCB (printed circuit board) preionization and zeroth-order grating coupling are employed.     

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     

STRUCTURE ANALYSIS OF NEW SUPERCONDUCTOR MgB2

The superconductivity of a sintered pellet of the newly discovered MgB₂ superconductor has been studied with magnetic measurements and its crystal structure was analysed using the Rietveld method of powder X-ray patterns. It has hexagonal symmetry (S.G. P6/mmm) with unit cell lattice parameters a=0.308136(14)nm and c=0.351782(17)nm.     

Iron doping effect on superconducting properties of MgB2

Iron-doped MgB₂ bulks are prepared by hybridized diffusion method using nano-powder and macro-powder of pure iron as iron source. The doping effect on superconductivity transition temperature, T_c, and critical current J_c have been investigated. It is found that both T_c and J_c of MgB₂ show quite different features depending on the particle size of the dopant powders. It is demonstrated that different from iron bulk or large size powders, iron nano-powders are active dopant for MgB₂ which suppresses both T_c and J_c of MgB₂.     

The microwave response of MgB2 /Al2O3 superconducting thin films

Microwave characteristics of MgB2/Al2O3 superconducting thin films were investigated by coplanar resonator technique. The thin films studied have different grain sizes resulting from different growth techniques. The experimental results can be described very well by a grain-size model which combines coplanar resonator theory and Josephson junction network model. It was found that the penetration depth and surface resistance of thin films with smaller grain sizes are larger than those of thin films with larger grain sizes.