YBa_2Cu_(3-x)Fe_xO_(7-δ)陶瓷样品的超声特性研究

本文研究了YBa_2Cu_(3-x)Fe_xO_(7-δ)(0≤x≤0.4)高温氧化物多晶超导体系的电阻,超声声速和超声衰减随温度的变化特性。对于超导样品,声速-温度曲线出现三次明显异常,首先是在超导转变温度附近声速呈现一极小值,表明样品中出现声子软化现象,其次分别在170K和240K附近声速发生两次大的突变,同时超声衰减曲线也相应地出现两个峰,而不超导的样品(x=0.40)只有170K和240K两处出现超声异常,而低温下无明显的声子软化现象,这说明,高温超导相变的发生会导致样品的弹性性质发生变化,而240K处体系发生的相变同高温超导电性无必然的联系,文中还初步讨论了170K超声异常的起因。     

Y2BaCuO5相含量对大尺寸YBCO织构样品磁性的影响

用ＰＭＰ（粉末熔化法）方法制备出大尺寸织构ＹＢＣＤ样品，名义成分为磁测量结果表明，当ｘ＝１５时．在７７Ｋ下达到较好水平；通过观察样品的组织形貌说明颗粒细小及分布均匀的Ｙ２ｌｌ相能起到较好的钉扎作用。     

超导研究及其中的化学

1987年十大新闻之一:今年是超导研究取得飞跃的一年……。本文将略述超导研究的历史,超导的物理和化学原理,介绍化学在超导研究中的作用以及超导技术的未来用途。     

掺氟汞系1223相的结构与超导电性

合成了掺氟汞基铜酸盐超导体 .研究证明氟进入了 1 2 2 3相 ,占据元胞中O( 3)的晶格位置 .掺氟烧结而不退火样品的超导临界温度Tc 为 1 33.8K ,比不掺氟烧结标准样品的Tc 提高约 2 2K ,显示氟掺杂能够使样品中载流子浓度nh 不经退火一步接近最佳值 .F代O( 3)收缩了CuO2 面与电荷储存层间距离及氟的电负性比氧大是提高Tc 的主要原因 .     

样品晶粒尺寸及高度偏差对XRD的影响

样品晶粒尺寸及样品高度偏差对XRD测试结果存在极大影响。实验结果表明,晶粒尺寸过大会导致异于标准卡片的衍射峰出现,且衍射谱图不具有重现性;样品高度偏差则会导致衍射峰位移动,正高度偏差使衍射峰向高角度方向移动,负高度偏差使衍射峰向低角度方向移动,峰位移动量与高度偏差呈正比:△2θ=360cosθ/πR△h。样品晶粒和高度偏差过大以致结果异常的情况应视为制样错误,测试过程中应予重视。     

Bi系2212相超导材料的制备及其单晶生长与表征——推荐一个无机固体化学的综合实验

介绍用无机固态反应制备Bi：Sr：CaCu：0。（Bi-2212）超导陶瓷材料,助熔剂法生长Bi-2212超导单晶,x射线衍射法鉴定晶体结构,差热分析确定晶体生长温区,碘量法测定样品的氧含量以及标准四引线法测量固体材料与单晶的超导转变温度的全过程实验方法．并对实验结果进行了详细讨论。     

三维有序In2O3纳米线阵列的合成及纳米结构有序度对气敏性能的影响

采用SBA-15硬模板复制技术合成出三维有序纳米线阵列结构的In₂O₃, 通过离心分离获得了2个尺寸不同的块状颗粒样品. 利用XRD、 SEM和紫外-可见光谱对样品的晶体结构、 晶粒尺寸、 形貌及带隙宽度进行了分析. 结果表明, 2个样品具有相同的纳米微结构, 均为由晶粒尺寸约12 nm的近球形In₂O₃晶粒规则有序排列生长而成的三维纳米线阵列结构, 纳米线的直径约为12 nm, 间距约为2 nm, 且块状尺寸较大样品的带隙宽度略大. In₂O₃样品对乙醇气体的气敏性能测试结果显示, 当乙醇气体在空气中的体积分数为100×10^-6, 温度为320 ℃时, A样品的灵敏度达到50. 6, 优于尺寸较小的B样品. 通过对比研究发现该纳米结构样品的气敏性能明显优于同级别纳米颗粒和介孔结构材料, 纳米材料的气敏性能随纳米结构形态有序度的增加而提高.     

低温合成超导体Ba1-xKxBiO3

先以BaCO3和Bi2O3为原料由传统的固相反应制备纯BaBiO3,然后在低温下将用KOH-KF熔盐处理这样的拓扑反应合成了Ba1-xKxBiO3超导体.所有样品均进行了粉末X射线衍射(XRD)和磁性表征.XRD结果表明,所得Ba1-xKxBiO3样品均为纯相,且均可用赝立方晶胞指标化.磁性测量表明所有样品具有超导电性,最高超导转变温度(Tc)为30.6 K.讨论了反应时间、前驱体与熔盐质量比对超导转变温度的影响.最佳的反应条件为:反应温度450℃,反应时间4h,BaBiO3∶KOH∶KF质量比1∶5∶2.5.     

低温合成超导体Ba1-xKxBiO3

先以BaCO3和Bi2O3为原料由传统的固相反应制备纯BaBiO3,然后在低温下将用KOH-KF熔盐处理这样的拓扑反应合成了Ba1-xKxBiO3超导体.所有样品均进行了粉末X射线衍射(XRD)和磁性表征.XRD结果表明,所得Ba1-xKxBiO3样品均为纯相,且均可用赝立方晶胞指标化.磁性测量表明所有样品具有超导电性,最高超导转变温度(Tc)为30.6 K.讨论了反应时间、前驱体与熔盐质量比对超导转变温度的影响.最佳的反应条件为:反应温度450°C,反应时间4 h,BaBiO3:KOH:KF质量比1:5:2.5.     

YBa2Cu3O7-x高Tc超导体的热转变

自朱经武等最先报道液氮温区Y—Ba—Cu—O体系超导体以来，世界上涌现第二次高Tc超导体研究热潮。材料的烧结与退火均在高温下进行，尤以退火条件对超导性能好坏影响为大。因此，高温热转变的研究对于优良材料的获得及超导机制的认识都至为重要。迄今     

强磁场辅助TFA-MOD法制备YBCO厚膜的理论分析

为了实现YBa2Cu3O7-x(YBCO)涂层导体在电力工业中的实际应用，必须有效增加YBCO超导层的厚度。提出一个强磁场辅助TFAMOD法制备YBCO厚膜的新工艺，并从磁热力学和磁性物理出发理论分析其可行性。沿垂直基底平面方向施加强磁场时，磁场将促进c轴取向YBCO晶核的形成，诱导厚膜中非c轴取向晶核转动到c轴平行磁场取向的位置。另外，取向晶粒间磁性相互作用力使晶粒沿磁场方向定向聚合，有利于获得致密的厚膜。因此，强磁场辅助TFA—MOD法有望制备高临界电流密度的YBCO织构厚膜。     

Fabrication of YBCO/CeO2/YBCO multilayer films through an all chemical solution deposition approach

In this paper, the growth characteristics and superconducting properties of YBCO/CeO₂/YBCO multilayer films derived from an all chemical solution deposition process layer by layer are investigated. X-ray diffraction results show that the c-axis epitaxial growth feature of the bottom YBa₂Cu₃O_7-δ (YBCO) film was perfectly inherited. Alternating epitaxial growth of YBCO/CeO₂/YBCO multilayer films was confirmed by high resolution transmission electron microscopy in combination with selected area electron diffraction, which will lay the groundwork for preparation of SIS-type Josephson junctions. In addition, nanosized BaCeO₃ particle rather than BaCeO₃ transition layer was observed in the interface between YBCO and CeO₂. The superconductivity measurement results show that the proposal method gives better superconducting properties only for high magnetic field and at low magnetic field it deteriorates the superconducting properties.     

Chelating agent effects in the synthesis of mesoscopic-size superconducting particles

Several routes and procedures have been used in these last years as an effort to achieve single-phase mesoscopic-size superconducting samples. In this paper, the effects of using citric acid (CA), tartaric acid (TA) and ethylenediaminetetraacetic acid (EDTA) as chelating agents and ethylene-glycol (EG) as polyhydroxy alcohol were studied in order to establish conditions to avoid the occurrence of BaCO₃ undesirable secondary phase in YBa₂Cu₃O_7−δ (YBCO). Thermal evolution of intermediate compounds formed during the calcinations process by the use of different chelating agents was traced using thermogravimetric and spectroscopic methods. The obtained results indicated that the polymer breakdown of samples prepared using EDTA occurs at higher temperatures than others chelating agents and also reduces the occurrence of BaCO₃ secondary phase as studied by X-ray diffraction measurements. Furthermore, the magnetic response of the mesoscopic-size YBCO specimens obtained was verified showing that samples present different superconducting response.     

Analytical Techniques Applied in Optimization of Electrophoretic Deposition of Thin Film YBCO Superconductors

 YBa₂Cu₃O _7−x (x = 0.1–0.2) compounds (YBCO) were produced by the oxalate coprecipitation and the solid state reaction methods. The powders obtained were used for the production of YBCO superconducting coatings on Pt/Si wafers, by the electrophoretic deposition technique. The optimum process conditions for the production of both powders and coatings were found by using a combination of modern analytical techniques. The thermal treatment of the samples was followed by thermogravimetry (TG) and differential scanning calorimetry (DSC). The optimization and characterization of the superconducting properties of the powders and coatings were achieved by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), optical microscopy, magnetic susceptibility and electrical resistivity measurements.     

Enhancement of critical current density in superconducting YBa2Cu3O7−x films by nanostructure development of substrate surface using sol–gel method

In this paper, a simple sol–gel route has been adopted in developing substrate surface for the first time. Only by adjusting the concentration of precursor solution, LaAlO₃ (LAO) nanodot arrays were directly obtained on LAO single crystal substrate, without incorporating additional processing steps. Subsequently, YBa₂Cu₃O_7?x (YBCO) superconducting films were prepared on the nanodot arrays using a low-fluorine solution process. In contrast, J _c of YBCO films grown on the substrate developed with nanodot arrays is almost three times as large as that of YBCO films grown on undeveloped substrate in an applied magnetic field with the strength of 3 Tesla. The pinning force density (F _p ) of YBCO film on the nanodot arrays developed substrate is 2.7 GN/m³ which exceeds that (1.8 GN/m³) of YBCO films on undeveloped substrate. The results of cross-section transmission electron microscope (TEM) images show that the extended defects along the a–b planes of YBCO, induced by the developed substrate with nanodot arrays, should be the origin of the J _c enhancement.     

Scanning Auger analysis for the clarification of oxidation and interdiffusion processes during laser ablation of YBa2Cu3O7-x on alloy/oxide substrates

Scanning Auger investigations are reported on layer systems consisting of a metallic substrate, an oxide buffer layer and the YBa₂Cu₃O_7?x (YBCO) high temperature superconductor. By bending the samples under UHV conditions the internal interfaces have been made accessible for the Auger analysis. The examination of the interfaces have shown that an oxidation of the substrate alloy had taken place during the deposition of the YBCO in spite of the buffer layer already being present. Additionally, segregated sulphur has been found on the substrate/buffer interface, which can lead to a loss of adhesion of the layer system. By line scan investigations on samples prepared by ball cratering a carbon enriched zone of approximately 50 nm thickness has been detected at the interface of buffer and YBCO.     

The influence of interparticle surface forces on the coagulation of weakly magnetic mineral ultrafines in a magnetic field

In this paper, it is shown that the coagulation of dispersions of weakly magnetic mineral ultrafines (such as hematite and chromite) in an external magnetic field can be described theoretically by invoking interparticle forces. Essentially, coagulation occurs when the short-range London—van der Waals interactions and the long-range magnetic forces outweigh the stabilizing electric double layer repulsion. From classical colloid chemistry theory, we have calculated the various components of the potential energy for different-sized particles at a series of ionic strengths and magnetic field intensifies. Principles governing the stability of the suspensions were derived and the computations lead to the establishment of criteria which can be used to predict the stability of the suspensions of weakly magnetic oxide mineral ultrafines in a “wet magnetic separation process”.

Experimentally, the magnetic-field induced coagulation of ultrafines of natural hematite and chromite in aqueous suspensions at moderate ionic strength was investigated using a laboratory-scale electromagnetic solenoid. The experimental results relate the coagulation process (as determined by magnetosedimentation analysis) to particle size, slurry pH and the external magnetic field. In the magnetic fields, maximum coagulation occurred near the pH of the point of zero charge (pH_PZC) of the minerals (where the electrostatic double layer repulsion was reduced to a minimum) enabling the particles to enter the “primary minimum” energy sink. In contrast, in cases where the electrostatic repulsion was not suppressed, the long-range magnetic forces enabled coagulation to occur in the “secondary minimum”. This caused the formation of chains which appeared to be relatively stable at enhanced rates of settling. The experimental results could be interpreted from a theoretical analysis of the interparticle forces controlling the process.     

单分散磁性P(St/BA/MAA)微球的制备

在共沉淀法合成超细磁流体的基础上 ,以苯乙烯 (St)、丙烯酸丁酯 (BA)和甲基丙烯酸 (MAA)为共聚单体 ,在不同的介质体系中采用无皂乳液聚合法制备了单分散 ,粒径范围为 80～ 2 30nm的磁性P(St BA MAA)微球 .详细探讨了介质极性、磁流体中表面活性剂含量对磁性高分子微球粒径和单分散性的影响 .实验结果表明 ,在一定范围内随介质极性降低 ,磁性高分子微球的单分散性提高 ,随表面活性剂用量增加 ,单分散性变差 .总体来看 ,磁性高分子微球的单分散性与其表面静电斥力密切相关 ,过大或过小的静电斥力均会导致磁性高分子微球单分散性的降低 .     

Scanning Auger analysis for the clarification of oxidation and interdiffusion processes during laser ablation of YBa2Cu3O7-x on alloy/oxide substrates

Scanning Auger investigations are reported on layer systems consisting of a metallic substrate, an oxide buffer layer and the YBa₂Cu₃O_7–x (YBCO) high temperature superconductor. By bending the samples under UHV conditions the internal interfaces have been made accessible for the Auger analysis. The examination of the interfaces have shown that an oxidation of the substrate alloy had taken place during the deposition of the YBCO in spite of the buffer layer already being present. Additionally, segregated sulphur has been found on the substrate/buffer interface, which can lead to a loss of adhesion of the layer system. By line scan investigations on samples prepared by ball cratering a carbon enriched zone of approximately 50 nm thickness has been detected at the interface of buffer and YBCO.     

Molecular dynamics of a liquid-crystalline polymer with laterally fixed mesogenic side groups

Abstract

The long range molecular dynamical behaviour of liquid-crystalline side chain polymers with the mesogenic groups linked laterally to the backbone have been studied by using dielectric relaxation spectroscopy over a broad temperature and frequency range. The samples were oriented homeotropically and homogenously by electric and magnetic fields and the relaxations were recorded during alignment and with the fully aligned samples. By fitting the data to theoretical relaxation curves, accurate relaxation parameters could be determined, allowing us to perform a comparison with end-fixed liquid-crystalline side chain polymers on the one hand and with low molecular weight liquid crystals on the other. The relaxation in homeotropic alignment for the laterally fixed compound has more analogies in some aspects, for example, the relaxation time distribution, with low molecular weight liquid crystals than with the corresponding end-fixed compounds, though the activation energy is very large (241 kJ/mol). We relate this to the length of the rigid mesogenic unit and the resulting stronger repulsion by the neighbouring side chains during reorientation. In homogeneous alignment the relaxation is very broad and also has a large activation energy. Different molecular processes are related to this relaxation regime. The relationship between the different relaxation processes and the molecular structure is discussed.