La0.67Ca0.33MnO3/YBa2Cu4O8/La0.67Ca0.33MnO3三层膜的时间效应研究

报导了La0.67Ca0.33MnO3/YBa2Cu4O8/La0.67Ca0.33MnO3薄膜中超导特性的时间效应.样品在空气中存放11个月后,超导转变温度都降低,而且每个系列样品的临界厚度增加.对用La0.67Ca0.33MnO3作保护层的YBa2Cu4O8薄膜进行时间效应测量,结果表明当La0.67Ca0.33MnO3层的厚度大于40nm时可以防止水对YBa2Cu4O8薄膜的影响.     

Ca-La2O3-Ni体系的热力学研究

计算了用Ca还原La2O3生成纯金属La(3Ca La2O3=2La 3CaO),La与Ni反应生成LaNi5合金（La 5Ni=LaNi5)的标准吉布斯自由能和吉布斯自由能变化,以及反应平衡时的钙分压,在1223,1273,1323,1373K温度下制得了LaNi5合金,XRD鉴定了反应产物的物相,热力学计算说明在La-NI系化合物中生成物只能是L aNi5。     

溶胶-凝胶法制备YBa2Cu3O7-x超导薄膜的研究

传统的TFA-MOD法制备YBa2Cu3O7-x薄膜,采用Y、Ba、Cu 3种金属的三氟醋酸盐(TFA)为先驱体,在热分解时,会产生大量的HF气体,即使通过将近20h的缓慢升温过程来对薄膜进行热分解,也难以获得较为光洁的表面,从而无法实现厚膜的制备.本文提出了一种新的含氟溶胶-凝胶工艺,通过减少溶胶中F的含量,并利用二乙醇胺做修饰剂,缩短了热分解时间,提高了薄膜的表面光洁度.利用该方法在(010)LaAlO3衬底上制备了具有良好c轴取向,临界转变温度为89K的YBCO超导薄膜.     

a轴取向YBa2Cu3O7-x薄膜的制备及其Raman光谱研究

采用直流磁控溅射法在SrTiO3(100)衬底上制备a轴取向的YBa2Cu3O7-x薄膜,用四引线法测量R-T曲线Tc0=86K.对于YBa2Cu3O7-x薄膜进行喇曼光谱测量,发现0(4)振动的声子峰(～500cm-1)强度远远大于0(2)-0(3)反相振动的声子峰(～340cm-1)强度,应用群论分析证明薄膜主要是沿a轴生长的.在较低的衬底温度下(<780℃),YBa2Cu3O7-x薄膜沿a轴生长,随着衬底温度的升高,薄膜中沿c轴生长的组分逐渐增加.     

BaFe12O19/A12O3-SiO2-K2O微晶玻璃陶瓷的柠檬酸SOl-Gel合成及其微波性能的研究

采用柠檬酸sol-gel工艺合成了BaFe12O19/A12O3-SiO2-K2O微晶玻璃陶瓷,并对其介电常数及其磁导率在1MH2～6GHz下的变化规律进行了研究.结果表明,BaFe12O19/Al2O3-SiO2-K2O微晶玻璃陶瓷的合成与体系中Fe/Ba、烧结温度密切相关;其介电常数、磁导率基本都随测试频率的增加而下降;介电损耗值最大可达到0.30磁损耗值较小.     

脉冲激光蒸发熔融织构靶制备的YBa2Cu3O7-δ超导薄膜

用脉冲激光蒸发熔融织构靶材在SrTiO3(001)单晶基片上制备了YBa2Cu3O7-δ超导薄膜。用X射线衍射仪、透射电镜和原子力显微镜分别对薄膜的微观结构和表面形貌进行了分析。结果表明，薄膜呈c轴取向，薄膜中有一定数量的颗粒状Y2O3杂相，且具有择优取向，而靶材中所含的Y2BaCuO8相却没有在薄膜中形成。沉积温度升高，有利于[001]取向，但不利于[111]取向的Y2O3形成。与传统的粉末烧结靶相比，用熔融织构靶制备YBa2Cu3O7-δ薄膜可以明显抑制薄膜表面颗粒的形成。     

Cu2O薄膜磁控溅射制备及特性分析

采用反应射频磁控溅射法，在氩-氧氛围制备Cu-O体系薄膜。在溅射功率、氩气流量一定的条件下，通过改变氧分压，得到不同戍份的Cu-O体系薄膜。在氧分压较小条件下，薄膜中主要以金属铜为主，存在少量的Cu2O，随着氧分压的增加，Cu2O成分增加，在氧分压1．42×10^-2pa时得到单相Cu2O薄膜。随着氧分压继续增加，Cu2O逐渐转变成CuO。薄膜成份通过X射线分析，薄膜电阻率采用四探针仪测量，薄膜光学特性采用紫外-可见光谱分析，并计算出本次实验所得单相Cu2O光学带隙为2．51eV。     

Fe和Fe3O4硫化制备的FeS2薄膜的性能

用溅射Fe和电沉积Fe3O4先驱体硫化制备出FeS2薄膜,研究了不同先驱体对硫化过程和FeS2薄膜性能的影响．结果表明,两种先驱体结晶成的FeS2能够在一定程度上保留先驱体形貌特征．Fe生成FeS2的热力学驱动力比较高,虽然可能生成FeS的过渡相;Fe硫化生成的薄膜平整致密,晶粒生长比较充分,尺寸较大,其禁带宽度接近理论值．Fe3O4硫化生成FeS2的热力学驱动力较低,生成的薄膜表面疏松多孔,晶粒细小;薄膜的晶界等面缺陷比例较大和几何连续性较低使其电阻率较高、禁带宽度和载流子迁移率低于Fe膜硫化FeS2薄膜．     

Ba0.6 Sr0.4 TiO3-ZnNb2O6复相微波介质陶瓷的结构与介电性能研究

以Ba0.6Sr0.4TiO3为基体材料,采用传统陶瓷制备工艺,制备了Ba0.6 Sr0.4 TiO3-ZnNb2O6(简称BSTZ)复相微波介质陶瓷.结果表明,BSTZ复相陶瓷可在1200℃烧结成瓷,烧结温度明显低于Ba0.6Sr0.4TiO3陶瓷的烧结温度,并反应生成新相BaNb3.6O10.在室温低频下,随ZnNb2O6含量的增加,BSTZ复相陶瓷的介电常数下降;在1.6kV/mm的直流偏压下,各BSTZ复相陶瓷的可调性随ZnNb2O6添加量增加而减小.     

Ba置换改性(Ca1-xBax)[(Mg1/3Nb2/3)0.6Ti0.4]O3陶瓷结构与微波介电性能的研究

研究了Ba置换改性对Ca [(Mg1/3Nb2/3)0.6Ti0.4]O3陶瓷微观结构与介电性能的影响.通过XRD与SEM分析发现,当x=0.15,(Ca1-xBax) [(Mg1/3Nb2/3)0.6Ti0.4]O3形成了正交晶系钙钛矿结构的单相固溶体;当x=0.20～0.80时,改性陶瓷为正交与六方钙钛矿结构的两相复合固溶体;当x=0.85 时,所形成(Ca0.15Ba0.85) [(Mg1/3Nb2/3)0.6Ti0.4]O3陶瓷为六方钙钛矿结构的单相固溶体.(Ca1-xBax) [(Mg1/3Nb2/3)0.6Ti0.4]O3系陶瓷微波介电性能的变化与Ba在材料内部的分布状态密切相关,与基材CMNT陶瓷相比:当x=0.15时,陶瓷的介电常数提高,介电损耗降低,谐振频率温度系数向负方向移动:εr=55,Qf值=32000GHz(6.5GHz下),τf=-36.82ppm/℃;当x=0.20～0.80间变化时,(Ca1-xBax)[(Mg1/3Nb2/3)0.6Ti0.4]O3两相复合陶瓷的微波介电性能由于复合效应而表现出连续变化的规律:εr= 45～33 ,Qf值= 30500～40200GHz(6.3GHz～7.6GHz下),τf = -17.7～12.52ppm/℃;当x=0.85时,单相钙钛矿固溶体(Ca0.15Ba0.85) [(Mg1/3Nb2/3)0.6Ti0.4]O3获得良好的微波介电性能:εr=31,Qf值达到44000GHz(8.5GHz下),τf=10.81ppm/℃.     

Electroless deposition of Alpha-PbO2 and Tl2O3

A chemical method for electroless deposition of thin film ofa-PbO₂ and Tl₂O₃ has been developed. The deposition has been performed by ammonia, persulfate ion and metal ions at a higher temperature. The electrical resistance, mobility and carrier concentration have been measured with variation of thickness of the films. Optical absorption spectra reveal the band edges which are 1·7 eV and 1·95 eV ofa-PbO₂ and Tl₂O₃ respectively.     

Low-temperature structural studies of Tl2Ba2CaCu2O x

Low-temperature x-ray diffraction studies of superconducting Tl-2212 ceramics revealed anomalies in thec cell parameter andc-axis thermal expansion coefficient near the superconducting transition. Air and vacuum anneals in the range 400–550‡C demonstrate that these anomalies correlate with the superconducting properties of the material. The anomalous temperature variation of thermal expansion suggests that the superconducting transition is accompanied by significant changes in the phonon spectrum.     

Alteration of amorphous Bi4Ge3O12 by Tl2O and Na2O

Ternary thallium and sodium bismuth germanate glasses were prepared and their densities refractive indices, and infra-red spectra obtained. The effect of univalent cations on the stability of the arrangement of decoupled GeO₄ tetrahedra in amorphous Bi₄Ge₃O₁₂ was compared with the effect of divalent cations. The molar volumes of glasses with the nominal mol% composition 20 M₂O(MO).20 Bi₂O₃.60 GeO₂ are directly related to the size and charge type of M^z+. However, the molar volumes of such glasses are inversely related to the ionic potential (z/r) of M^z+. The infra-red spectra of these ternary glasses exhibitv _Ge?O shifts that reflect the presence of both isolated and small clusters of GeO₄ tetrahedra compared to amorphous Bi₄Ge₃O₁₂. This slight increase in the degree of polymerization appears to be directly related to the ionic potential of M^z+.     

Inelastic neutron scattering from Tl2CaBa2Cu2O8

Coherent inelastic neutron scattering measurements have been carried out on the high temperature superconductors Tl₂CaBa₂Cu₂O₈ (Tl-2122,T _c =107 K) and YBa₂Cu₃O₇ (Y-123,T _c =92 K), at the Dhruva reactor at Trombay. The density of phonon states in Tl-2122 is enhanced at 6–17 meV and reduced at 40–70meV compared to that in Y-123.     

Flux Dynamics in Tl2Ba2CaCu2O8 Thin Films

Investigations on flux dynamics of ring-shaped T1₂Ba₂CaCu₂O₈ superconducting thin films have been carried out by measuring the temperature dependent magnetization M(T) during field-cooling (FC) and zero-field-cooling (ZFC) processes. For a given magnetic field, from the magnetization behavior two distinct temperatures, T_kink and T_irr, can be defined. Below T_kink, a clear hysteretic behavior of M(T) is observed leading to a large irreversible signal ΔM = M_FC ? M_ZFC. Above T_kink, this irreversible signal, though being very small, is still non-zero until it eventually vanishes at a higher temperature T_irr. Above T_irr, both curves M_FC(T) and M_ZFC(T) merge together and become temperature independent. We attribute the first region ( T< T_kink) to a 3D vortex-glass phase, the second region (T_kink < T < T_irr) to a vortex line liquid state and the third region (T_irr < T < T_c) to a pancake liquid state.     

Preparation and properties of Tl2Ba2CaCu2O8 thin films

Anex situ process has been developed to produce thin superconducting Tl₂Ba₂CaCu₂O₈ films. The properties of films grown on different substrates using different annealing regimes were studied. Critical temperatures of 103–107 K were measured on films prepared in a broad range of annealing temperatures on SrTiO₃, LaAlO₃, and Y-ZrO₂ substrates. A critical current density,J _c, of 2×10⁶ A/cm² at 77 K was measured on LaAlO₃. Film morphology was studied by SEM, AFM, and STM.     

Thermoelectric power of single-phase samples of Tl2CaBa2Cu2O y and Ba2CaSr2Cu2O y

Single-phase 2122 samples of thallium and bismuth superconductors were made by the precursor matrix method. The thermopower of these samples was measured in the temperature range 250 K-T _c . The thermopower was positive and decreased linearly with increasing temperature aboveT _c (onset). The exponential enhancement of thermopower seen in the undoped and doped YBCO was not observed in these samples. The linear variation of thermopower can be explained on the basis of either a two-band model or a narrow band model.     

Quantum tunneling of vortices in single crystal Tl2CaBa2Cu2O8 superconductors

Data are presented on the temperature-dependent time-logarithmic magnetic relaxation rate S(T) = ¦dM/d_in t¦ of the high-T_c superconductor Tl₂CaBa₂ Cu₂O₈. It is found that at low temperatures the relaxation rate has the form S(T) = A(H) exp[(T/T^*)²], which does not extrapolate to zero at T = 0, thus excluding conventional thermally activated flux creep and providing evidence of quantum vortex tunneling. From a quantum flux tunneling theory, it is shown that S(T) I/_er _p ² . The measurements of the relaxation rate thus provide information about the effective viscosity _e of fluxons.     

Solid-state chemistry of new Tl2OCl2 and Tl4Cl6

Solid thallous carbonate reacts with chlorine gas with the formation of brown-coloured Tl₂OCl₂ as an intermediate compound. Further exposure to chlorine gas gives the yellow compound Tl₄Cl₆. Both the compounds contain Tl(I) and Tl(III) ions. The kinetic data obeyw ²=kt. The energy of activation was found to be 41.98 kcal mol^?1. Thermo-gravimetry (TG) indicates that Tl₂OCl₂ decomposes into TlCl, Tl₂O₃ and chlorine gas above 250° C. In contrast, Tl₄Cl₆ is stable up to 420° C.     

Flux Dynamics and Vortex Phase Diagram in Tl2Ba2CaCu2O8 Thin Films

By doing sensitive magnetic measurement, the current-voltage ( I-V or E-j ) characteristics were determined for Tl ₂ Ba ₂ CaCu ₂ O ₈ thin films at temperatures between 4.2 K and 70 K in magnetic fields up to 6T. We found a crossover of the dimensionality of flux dynamics from 3D at low fields to 2D at high fields. The vortex matter melts at a finite temperature at a low field ( 3D ) and at zero K at a high field ( 2D ). Finally we attempt to give a vortex phase diagram based on this analysis.