The flexural strength and microhardness of YBa2Cu3O6+δ

The flexural strengths of rectangular YBa₂Cu₃O₆₊ bars, prepared from mixed oxides and carbonates or spray-dried precursors, have been measured at room temperature and at 77 K. Strengths ranged from 17.8 to 57.6 M Pa at room temperature, depending on processing history, and were 20% greater when measured at 77 K. Corrosion of YBa₂Cu₃O₆₊ in humid air at 38° C created two layers of corrosion products, but did not weaken the uncorroded core when failure loads were corrected for the decreased sample dimensions. The Knoop hardness of polycrystalline YBa₂Cu₃O₆₊ ranged from 436 to 447 KHN while the hardness of individual grains of YBa₂Cu₃O₆₊ was 498 KHN. Variations in flexural strength with microstructure were observed and are discussed.     

63Cu NMR in the normal state of HgBa2Ca2Cu3O8+δ

We present a⁶³Cu NMR study of HgBa₂Ca₂Cu₃O₈₊ underdoped single crystals with T_c 115 K. While the uniform spin susceptibility decreases below T_o 370 K, relaxation rate measurements demonstrate the opening of a spingap at Q = (, ) below T* 230 K, the highest temperature reported so far. The characteristic energy of spin fluctuations is shown to be higher than in underdoped YBa₂Cu₃O_7–, and the analysis of the quadrupole and hyperfine couplings suggests that the in-plane Cu-O hybridization is also stronger. The T-dependence of T₁ is the same in the three CuO₂ planes which seems hardly compatible with the pure interlayer spin-pairing picture.     

Ultrafast Optical Response of Tl2Ba2CuO6+δ

We present the first measurements of the ultrafast optical response of Tl ₂ Ba ₂CuO ₆₊ and show that the optical response is clearly sensitive to the electronic groundstate. In the superconducting state its decay can be interpreted as following the reformation of superconducting pairs and the dynamics of quasiparticles near the nodes in a d-wave gap.     

Current switching in semiconducting LaMn1−xMgxO3+δ

The semiconducting LaMn_1−xMg_xO_3+δ (LMMO) exhibits highly nonlinear, voltage-limiting, conduction (current switching) in the current–voltage (J–E) relationship. The nonlinearity is observed at field strengths less than 50 V/cm and below the magnetic transition temperature (T_c). The current density in the conducting region is very high, reaching tens of A/cm². The threshold electric field (E_TH) shifts to lower values in presence of an external magnetic field. No current switching is observed in ferromagnetic metallic, La_0.67Ca_0.33MnO_3+δ (LCMO) specimens. The nonlinearity observed in LMMO cannot be explained on the basis of antiferromagnetic/paramagnetic insulating phases co-existing with ferromagnetic metallic phases below T_c, because such giant clusters are observed in LCMO. Substitution of Mg increases the Mn⁴⁺ concentration, however, does not induce a transition to metallic state below T_c even at high hole concentration; instead, displays the behaviour of localisation. Localisation of carriers is due to the increased hole hopping distance (Mn³⁺–O–Mg²⁺–O–Mn⁴⁺) and by the spin canting. Below T_c, in the canted ferromagnetic state, the carriers can be delocalised by applying an electric field. The electric field enhances the mobility of the carriers and increases the hopping probability. Above E_TH, large number of carriers are delocalised and set in for conduction, yielding high current density.     

Thermal Conductivity Properties of Glass-Ceramic (Bi2-δ-γGaδTlγ)Sr2Ca2Cu3O10+x High-Tc Superconductors

Glass-ceramic BSCCO superconductors with nominal (Bi _2-- Ga Tl )Sr ₂ Ca ₂ Cu ₃ O _10+x compositions, where =0.20, 0.40, 0.60 and =0.10, 0.30, 0.40, have been prepared by the melt quenching method. The thermal conductivity (T) measurements were performed between 30K and 285K and results obtained found to be between 3.1 and 9.9 mW/cm.K depending on the substitution levels and temperature. Results obtained showed that the phonon-defect scattering dominates above the T _c for all samples but below the T _c a strong coupling mechanism occurs. The best electrical performance was obtained to be 113K and 107K for T _c and T _zero respectively.     

Phase equilibria in the reciprocal system 3Tl2S + Bi2Te3 ↔ 3Tl2Te + Bi2S3

Phase equilibria in the reciprocal system 3Tl₂S + Bi₂Te₃ ? 3Tl₂Te + Bi₂S₃ (A) have been studied by differential thermal analysis, X-ray diffraction, microhardness tests, and emf measurements using thallium concentration cells, and a number of vertical sections, the 300-K isothermal section of its phase diagram, and the liquidus projection have been mapped out. The system is shown to be reversibly reciprocal, with broad solid solution regions in the Tl₂S-Tl₂Te-Tl₉BiTe₆ subsystem and along the TlBiS₂-TlBiTe₂ join. We have determined the homogeneity ranges and primary crystallization fields of the phases and identified the nature and coordinates of invariant equilibria in system A.     

Metaln+-Metalδ+pair sites steer C-C coupling for selective CO2 photoreduction to C2 hydrocarbons

The major obstacle for selective CO2 photoreduction to C2 hydrocarbons lies in the difficulty of C-C coupling,which is usually restrained by the repulsive dipol...     

Bi2O3-Y2O3体系中纳米δ-Bi2O3相的生成规律研究

以分析纯的Bi(NO3)3·5H2O和Y (NO3)3*6H2O作为原料,经反向滴定法化学共沉淀、干燥和430℃焙烧后获得(75mol%Bi2O3+25mol%Y2O3)纳米复合粉体,粉体平均粒度30nm以下;通过常压反应烧结工艺制备了纳米Bi2O3-Y2O3快离子导体,对烧结过程中高导电相(纳米δ-Bi2O3)的形成规律研究结果表明烧结初期以固溶反应为主,后期以晶粒长大为主,晶粒生长规律符合(D-D0)2=K·t抛物线方程;用模式识别技术对δ-Bi2O3相生成的工艺条件进行了优化.     

Flux Flow Resistance and Transmittivity in Overdoped Tl2Ba2CuO6+δ

We have measured the surface resistance and transmittivity of overdoped Tl ₂ Ba ₂ CuO ₆₊ in magnetic field up to 7 T in order to investigate the unusual dc resistive transition. Introduction of columnar defects largely shifts the irreversibility line to higher field, showing that the resistive transition is caused by the melting of the vortex lattice. It is found that flux flow resistance of this compound is unusual as compared with that expected from the dc resistivity.     

Isotope Studies of the CMR Compounds La1−xCaxMnO3+δ

Oxygen isotope effect studies of the ferromagnetic Curie temperature T _c of La_1?x Ca _x MnO₃ are presented. The isotope exponent α₀=?ΔlnT _c/Δlnm ₀ changes from 0.4 to 0.14 in the range 0.2<x<0.43. The isotope exponent decreases strongly with increasing tolerance factor, or decreasing lattice distortion. Above Tc the conductivity is characteristic of small polarons. Raman scattering shows a prominent peak at 230 cm?1. The peak width could be related to site-dependent Jahn–Teller distortions above T _c, becoming significantly smaller at and below T _c. IR reflectivity data show a much larger zero frequency IR conductivity than dc conductivity. The IR peaks are independent of temperature between 150 and 295 K.     

Anomalous Scattering Rate and Microwave Absorption in Bi2Sr2CaCu2O8+δ and YBa2Cu3O7?δ

We determine the scattering rate from microwave measurements for an optimally doped Bi-2212 single crystal, using a simple two-fluid model with a d-wave symmetry order parameter. In the superconducting state, the calculated scattering rate is three orders of magnitude smaller than that determined from ARPES experiments. A similar anomalously large decrease in the scattering rate is also required to explain the data within a gap-quasiparticle scenario for other HTS, such as YBa₂Cu₃O_7–. The results suggest that the assumption of normal excitations vanishing at low T is invalid and an additional charge mode is responsible for the microwave absorption.     

超导YBa_2Cu_3O_(7+δ)/硅橡胶复合材料的压敏与介电特性

采用YBa_2Cu_3O_(+δ)(简称YBCO)多晶陶瓷超导粉末与硅橡胶(110型)按不同质量比进行配料, 经过特殊的制备工艺, 合成不同含量的超导YBCO/硅橡胶高分子复合材料, 分别测量样品的压敏效应和介电特性. 结果表明, 在不同应力作用下, 样品电阻值的变化范围在1～4个数量级. 样品电阻值随测量温度的降低(300~50 K)呈下降趋势, 测量温度降到90 K时, 样品电阻值发生突变, 但在90~50 K没有观察到超导零电阻现象. 室温下, 样品的介电常数随频率的增加(1 kHz~5 MHz)而减小, 介电损耗随测量频率的增大先增大后减小. 随着YBCO含量的增加, 形成的超电容网络微观结构也就越多, 样品的电阻逐渐减小电流加大, 导致超电容中电解质的极化强度有所增加, 两者共同作用的结果导致样品的介电常数、介电损耗均随着YBCO含量的增加而增大.     

Domain growth in partially melted bulk Nd1+xBa2−x Cu3O7−δ

Domain growth of bulk partially melted Nd_1+xBa_2–xCu₃O_7– in air was found to be severely limited by liquid-phase segregation and losses. The use of a reduced oxygen pressure greatly improved the stoichiometry control, leading to the formation of well-developed domains and enhanced superconducting properties. Direct nucleation of Nd_1+xBa_2–xCu₃O_7– from the liquid phase by dissolution of Nd₂BaCuO₅ and a spiral growth mechanism in three perpendicular directions of the platelets stacked in the domains, are suggested.     

c-axis Superfluid Response and Quasiparticle Conductivity in Bi2Sr2CaCu2O8+δ and Bi2Sr2CuO6+δ

Josephson plasma resonance for underdoped Bi ₂ Sr ₂ CaCu ₂ O ₈₊ and Bi ₂ Sr ₂ CuO ₆₊ have been measured by sweeping the microwave, frequency continuously. The resonance enables us to determine the superfluid density and quasiparticle conductivity in the c-axis accurately. We show that the. superfluid response and the low energy excitations out of the condensate in the c-axis of these materials are very different from those in the ordinary Josephson multilayer tunnel junctions. The Josephson coupling energy in single layer Bi ₂ Sr ₂ CuO ₆₊ is more than 5000 times smaller than is predicted in interlayer tunneling model.     

Eu3+和Sm3+共掺杂TiO2光催化剂的协同效应

采用溶胶-凝胶法制备了纯TiO2和Eu3+/Sm3+共掺杂TiO2复合粉体,采用XRD,DRS和SEM等技术进行表征,以亚甲基蓝(methylene blue,MB)的光催化降解为目标反应,评价了其光催化活性,探讨了Eu3+/Sm3+共掺杂对TiO2粉体光催化的影响机制.结果表明,Eu3+/Sm3+共掺杂可以显著提高TiO2粉体光催化活性,Eu3+/Sm3+共掺杂在TiO2粉体中产生协同作用,可以抑制TiO2由锐钛矿相向金红石相转变,使TiO2的粒径减小.Eu3+/Sm3+共掺杂增大了TiO2粉体的晶格畸变,使TiO2粉体吸收带边蓝移.当Eu3+/Sm3+的掺杂量分别为0.05%和0.1%,TiO2光催化活性最高,光催化降解率达到84.8%.     

Analysis of deformed YBa2Cu3O7−δ

In this study, polycrystalline YBa₂Cu₃O_7– (123) was deformed under controlled conditions with a confining pressure of 1.0GPa, temperatures of 25, 500 and 800° C, and a strain rate of 10^–4 sec^–1 in order to ascertain the micromechanisms of deformation that give rise to the macroscopic plastic behaviour. The deformed material was analysed using optical microscopy, transmission electron microscopy (TEM), and a SQUID magnetometer to study the effects of deformation on the microstructure of YBa₂Cu₃O_7– and how changes in the microstructure affected the superconducting properties. The results of these preliminary experiments suggest that the 123 material will be very difficult to deform plastically as slip occurs only on the (001) plane. The lack of multiple slip systems implies that this material will show some brittle behaviour up to a very high homologous temperature. Even when plastic behaviour can be sustained for high strains it may require high annealing temperatures to remove lattice imperfections which impede the superconducting currents. Densification by high pressure deformation may make reoxygenation difficult due to the reduced diffusion rates between the grains. These factors combined suggest that traditional fabrication techniques are not applicable to the 123 material. More work needs to be carried out to determine how annealing affects the microstructures of deformed materials and how these changes in microstructure affect the superconducting properties of these materials.     

Physical Properties of NaNbO3–A2+TiO3and NaNbO3–A2+Nb2O6Solid Solutions

Solid solutions of compounds with the ilmenite, columbite, or potassium tungsten bronze structure in the perovskite compound NaNbO₃were studied. The observed property–structure–composition relations were used to choose promising compositions for designing new piezoelectric ceramics.     

H3BO3对燃烧法制备SrAl2O4:Eu2+、Dy3+、Pr3+发光性能的影响

以硝酸盐和尿素为基质,采用燃烧法在较低炉温(600～620℃)下制备了SrAl2O4:Eu2+、Dy3+、Pr3+长余辉发光粉体.研究了H3BO3的加入对粉体的相组成、晶体结构、发光性能与长余辉特性的影响.通过对材料的XRD、激发和发射光谱、余辉衰减以及微观形貌分析.结果表明,H3BO3作为一种助溶剂,当其掺杂摩尔百分...     

Pr3+掺杂对SrAl2O4:Eu2+、Dy3+磷光体发光性能的影响

用燃烧法制备Pr~(3+)掺杂的SrAl_2O_4:Eu~(2+)、Dy~(3+)长余辉发光粉体,研究了Pr~(3+)掺杂对其发光性能的影响。结果表明,合成的单相SrAl_2O_4样品具有单斜晶系结构,荧光发射光谱是连续宽带谱,峰位于515 nm,激发光谱是峰值在320 nm和360 nm的连续宽带谱。掺杂Pr~(3+)对形成晶粒尺寸均匀的固溶体有一定的促进作用,使其余辉初始亮度为不掺杂时的3倍。     

(Ca1-xSrx)[(Li1/3Nb2/3)0.95Zr0.15]3+δ陶瓷微波介电特性

研究了A位Sr2 取代对(Ca1-xSrx)[(Li1/3Nb2/3)0.95Zr0.15]3 δ(0.0≤x≤0.2,CSLNZ)陶瓷的微观结构及微波介电特性的影响.当0≤x≤20%(摩尔分数)时,体系为单一钙钛矿相,随Sr2 含量的增加,介电常数和谐振频率温度系数增大,而品质因素先增大,x=0.05时,开始下降.用键价理论分析了谐振频率温度系数随B位键价的变化关系.当Sr2 含量为5%(摩尔分数)时,陶瓷微波介电性能最佳:εr=32.5,Qf=13500GHz,τf=-2.4×10-6/℃.