区域熔炼法制备YBa_2Cu_3O_(6+x)的非对数磁弛豫

考虑到有效钉扎势U(J)与临界电流密度的非线性关系,将U(J)展开为三级泰勒级数,最后推导出一个普遍适用于描述高温超导体磁弛豫的等温衰减方程,并用这一方程研究了具有高J_c,强钉扎的区域熔炼法制备YBa_2Cu_3O_(6+x)的磁弛豫,而且对目前被普遍采用的几个钉扎模型进行了讨论. 关键词：     

YBa_2Cu_3O_7中Cu、O离子价态的研究

采用不同的原子集团模型研究了 YBa_2Cu_3O_7高 Tc 超导材料中 Cu、O 离子的价态,用局域密度泛函理论自洽求解单电子方程得到系统的电子结构及离子价态.详细讨论了原子集团周围环境离子的电子分布对原子团内 Cu、O 离子价态的影响.结果表明,Cu 离子均为+2价,空穴分布在氧上,这与实验结果一致.在四种氧位中,空穴主要分布在 Ba-O 层内的O(4)上,空穴的分布易受原子团周围离子的电子分布变化的影响.     

水份对YBa_2Cu_3O_7超导电性的影响

在高 T_c 超导体的应用中,首先遇到和必需解决的是稳定性问题,特别是在潮湿空气和浸水条件下,对样品超导电性的影响.但在至今所进行的相应的研究中,都不能定量地测定样品中的水含量.本文采用热中子透射法测定了样品的水含量,从而定量地研究了水含量对超导样品性能的影响.     

YBa_2Cu_3O_(7-x)的高温氧呼吸特性

采用热分析仪和高温静态吸附等实验研究了 YBCO材料的氧的呼吸特性。实验表明 ,该材料对氧有强烈的选择吸附性 ,并且呼吸过程可以很好的重复。提出了把 YBCO做为除氧分子筛的设想。     

YBa_2Cu_3O_(7-δ)超导体的微波特性

本文对YBa_2Cu-3O_(7-δ)材料的表面阻抗的系列研究工作进行了归纳.与经典超导体相比,该类氧化物超导体的微波表面电阻在低温区所呈现的反常,当源于材料的内在特性.我们认为它是材料中存在一定量的未配对载流子所致;而其中 CuO 链的影响是应当考虑的.基于YBa_2Cu_3O_(7-δ)的格胞排序组构,借鉴 Allender 模型,我们提出了修正电导率σ.∝ln(Δ/(?)ω)·[c_1e~(-Δ/K_BT)+(1-c_1)((K_BT)/Δ)]来表征材料的传导电性.基于该设想的微波表面阻抗理论值能很好地吻合实验数据;进而,据此可估计出 YBa_2Cu_3O_(7-δ)的序参数值.如 H_c_1H_c_2,ξ,Δ(0),l.     

YBa_2Cu_3O_(7-y)单晶体的研制

用助熔剂缓冷法成功地生长出了YBa_2Cu_3O_(7-y)单晶体。晶体的最大尺寸可达2×1×0.5mm~3。单个晶粒的粉末X射线衍射确认该晶粒为正交的YBa_2Cu_3O_(7-y)相。劳厄照相证明此晶粒为单晶体。对上述单晶体的交流磁化率测量表明,该晶体的超导转变温度为90.5K。对生长中观察到的一些现象也进行了简单讨论。     

YBa_2Cu_3O_7固相生成反应的历程

以碳酸盐,氧化物为原料,生成 YBa_2Cu_3O_7的固相反应历程,在升温过程(750℃→950℃)及等温过程(956℃)中是不同的.在升温过程为反应历程(Ⅰ):BaCO_3+CuO→BaCuO_2+CO_2↑,BaCuO_2+Y_2O_3→Y_2BaCuO_5,Y_2BaCuO_5+3BaCuO_2+2CuO+(1/2)O_2→2YBa_2Cu_3O_7在等温过程,混合存在着两个反应历程,即如上之(Ⅰ)及(Ⅱ):BaCO_3+Y_2O_3→Y_2BaO_4+CO_2↑,Y_2BaO_4+CuO→Y_2BaCuO_5,Y_2BaCuO_5+3BaCuO_2+2CuO+(1/2)Q_2→2YBa_2Cu_3O_7     

融熔织构YBa_2Cu_3O_7的电阻展宽

测量了融熔织构 YBa_2Cu_3O_7在磁场中电阻转变的展宽效应.发现 R(T,H)=R_0exp[-U_0/k_BT],仍满足 Arrhenius 关系,钉扎能 U_0(H)仅为 eV 量级.讨论了影响 J_c 和磁通蠕动的因素.     

YBa_2Cu_3O_（7－x）和GdBa_2Cu_3O_（7－x）薄膜中涡旋

本文给出了具有高密度孪晶界ｃ取向ＹＢａ２Ｃｕ３Ｏ７－ｘ外延薄膜存在涡旋玻璃转变及很好的标度行为的实验证据．在有α方向晶粒或４５°晶界的ＧｄＢａ２Ｃｕ３Ｏ７－ｘ外延薄膜中存在涡旋玻璃转变但在转变温度附近观察到磁通蠕动和热激活磁通流动的某些特征．这些结果可以在涡旋玻璃理论，Ａｎｄｅｒｓｏｎ－Ｋｉｍ磁通蠕动模型以及热激活磁通流动模型框架内给于很好的解释．     

YBa_2Cu_3O_7_薄膜热激发磁通蠕动研究

本文选择两块零电阻温度分别为82K 和71K 超导性能不同的簿膜徉品, 同步进行了电阻在外滋场中展宽的测量, 并用Palstra 所提出的Arrhenius 定律研究两样品热激活能的差别。实验发现样品热激活能随磁场增加线性下降. 由测量结果推出的不可逆线仍有B * -(l -T*/T_c_)**3/2形式. 关键词：     

Low temperature heat capacity of YBa2Cu3O7

We have measured the heat capacity of superconducting, single phase YBa₂Cu₃O₇ in the temperature range 2 to 18 K. An extrapolation of the data between 4 and 9 K gives aC/T (T → 0) of ∼ 25 mJ/mole K². The Debye temperature obtained from the high temperature linear portion ofC/T vsT ² plot is 325 K.     

YBa2Cu3O7−y composites for improved fabricability

Composites of YBa₂Cu₃O_7−y with silver powder have been investigated for their fabricability and superconductivity. Silver does not react with the oxide powder. The composites can be easily rolled into thin strips. There is no appreciable degradation inT _c either during sintering or rolling.     

Field and temperature dependence of intergranular hysteresis in YBa2Cu3O7

We have studied the variation of low field magnetization hysteresis in YBa₂Cu₃O₇ as a function of the maximum magnetic field applied during a hysteresis cycle (1 G<H _max<7.3 G) and also as a function of temperature (77 K<T<95 K). The remnant magnetization is studied as a function ofH _max andT and the measured dependences are explained using the extended critical state model. The potential of this technique as a contactless method of probing the temperature dependence ofJ _c is discussed.     

Enhancement of thermopower in the highT c superconductor YBa2Cu3O7 and related compounds

The absolute thermopower of single phase YBa₂Cu₃O₇ and Y_0.8Er_0.2Ba₂Cu₃O₇ has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show a large enhancement of thermopower in the range 150 K down toT _c. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms.     

Structure and oxygen content in the high-temperature superconductor YBa2Cu3O7−δ

The effects of oxygen deficiency on the processes of ordering the oxygen vacancies and of the formation of superstructures are discussed in the layered perovskite structure of YBa₂Cu₃O_7−δ. Samples subjected to different heat and gas treatments are studied with X-ray diffraction techniques to get this information.     

Microstructural investigations of melt grown YBa2Cu3O7

The results of microstructural investigations on bulk YBa₂Cu₃O₇ prepared by melt growth process are reported.     

Stages of thermally induced gas-exchange in the high-temperature superconductor YBa2Cu3O7−δ

The oxygen stoichiometry in the YBa₂Cu₃O_7−δ high temperature superconductors is studied, using DSC/DTA techniques on samples subjected to different heat and gas treatments. Characteristic temperature regions of high temperature superconductivity exchange with oxygen in gas phase are identified and the resulting structural changes are discussed.     

Anomalous change in the photoacoustic phase near superconducting transition in YBa2Cu3O7

Photoacoustic amplitude and phase measurements have been carried out near the superconducting transition temperature in the highT _csuperconductor YBa₂Cu₃O₇. In both the samples investigated, the photoacoustic phase undergoes an anomalous step-like decrease aroundT _c≈ 96 K and increases at lower temperatures. A clear temperature hysteresis for the phase change has been noticed. No anomalous variation has been observed in the amplitude values. Neither phase nor amplitude shows any anomaly in a non-superconducting sample of the same composition.     

The scaling of the current voltage characteristics of the YBa2Cu3O7:Y2BaCuO5 composites

We have investigated features in the current voltage (I—V) characteristics of YBa₂Cu₃O₇:Y₂BaCuO₅ composite sinters which indicate the occurrence of a second-order phase transition. For each sinter composition similar families of characteristics are found for a wide range of magnetic fields. The relation between V and I is seen to be of the form V = f(I, ζ(B, T)) where a single parameter ξ contains all of the field and temperature dependence. The locus of points in the B, T plane for which ζ(B, T) is a constant have been examined, and a low field and a high field régime have been identified. Samples of different composition show families of I—V characteristics which scale onto one another in a simple way, hence we are led to suppose the V = f(I, ζ(B, T, p)) where p is the dilution fraction. Data taken at temperatures below the phase transition agrees well with a simple scaling picture of dissipation close to a second order phase transition, with exponents which are independent of magnetic field and composition. Above the transition, however, the behaviour is more complex.     

高压下超导体YBa2Cu3O7弹性性质的第一性原理研究

通过从头算密度泛函理论研究了YBa2Cu3O7的结构和弹性性质。发现c轴最容易压缩。另外,我们还计算了各种弹性性质与压强的关系,例如弹性模量,剪切模量,杨氏模量,泊松比。最后,我们获得了不同压强下的德拜温度和声速。