原位法MgB_2-B_4C复相超导体合成及相含量调控研究

以B4C和Mg为原料合成的MgB2-B4C复相超导体具有高的临界电流密度(Jc)和高的超导转变温度(Tc),是一种有潜力的实用MgB2超导材料,其成相机理对复相MgB2超导体的相含量调控和磁通钉扎研究具有重要意义。结合经典烧结理论,研究了B4C-Mg真空固相烧结制备MgB2-B4C复相超导体的超导相形成和晶粒生长过程,给出了B4C-Mg的金斯特林格扩散模型和MgB2晶粒生长过程。通过选择B4C原料粒径,MgB2-B4C复相超导体超导相体积相含量在18%-88%范围可控。相含量88%的MgB2-B4C复相超导体临界转变温度达33.5K,转变宽度1.5K。10 K环境6T外场下电流密度可以达到1×104A/cm2,表明MgB2-B4C复相超导体具有良好的磁通钉扎行为。     

热处理温度对MgB2超导材料的超导电性及微观结构的影响

采用粉末烧结方法在不同温度(650℃,700℃,750℃,800℃,850℃,900℃)制备了MgB2超导块材.采用X射线衍射(XRD)和扫描电镜(SEM)研究了热处理温度对MgB2超导材料的成相及微结构的影响.采用磁化法测定不同温度制备MgB2超导材料的超导电性.结果显示热处理温度对MgB2超导材料的晶粒尺寸、形状和超导电性有明显影响,700℃制备的MgB2超导体具有最高的临界电流密度和最好的磁通钉扎特性,细小的晶粒尺寸是样品磁通钉扎特性改善的原因.     

先位粉末套管法制备MgB2/Fe线材的超导性能研究

采用先位法将MgB2粉装入纯铁管中,制备出MgB2/Fe超导线材.用X射线衍射仪和扫描电子显微镜分析了样品的物相组成和显微结构;用SQUID测量了样品的超导转变温度;用标准四引线法测量了短样的临界电流.结果显示,超导样品的临界转变温度约为38.3 K,在4.2 K/4 T下MgB2/Fe线材的临界电流密度为～104A/cm2.研究结果表明,高温退火有效地减少了冷加工过程中产生的应力,改善了晶粒连接性,提高了芯材的致密度,可以显著地提高先位法制备线材的临界电流密度.     

热处理温度对MgB2超导材料的超导电性及微观结构的影响

采用粉末烧结方法在不同温度（650℃，700℃，750℃，800℃，850℃，900℃）制备了MgB2超导块材．采用X射线衍射（XRD）和扫描电镜（SEM）研究了热处理温度对MgB2超导材料的成相及微结构的影响．采用磁化法测定不同温度制备MgB2超导材料的超导电性．结果显示热处理温度对MgB2超导材料的晶粒尺寸、形状和超导电性有明显影响，700℃制备的MgB2超导体具有最高的临界电流密度和最好的磁通钉扎特性，细小的晶粒尺寸是样品磁通钉扎特性改善的原因．     

BiSrCaCuO体系中的临界电流特性

我们在低磁场下测量了 BiSrCaCuO 样品的临界电流和磁化曲线,发现 BiSrCaCuO 多相样品的临界电流比 YBaCuO 小一至二个数量级.从磁化曲线估计的晶粒区域的临界电流为10~3A/cm~2(77K),说明 BiSrCaCuO 体系中钉扎力相当小,有些样品的磁化曲线接近理想的第二类超导体.     

纳米C和SiC掺杂对MgB2带材超导性能的影响

采用X射线衍射仪,扫描电镜,超导量子干涉仪等仪器对纳米C和SiC掺杂的MgB2带材进行了表征,并采用标准四引线法对样品的临界电流进行了测试. 实验表明,C和SiC掺杂在提高MgB2带材高场下的临界电流密度方面具有显著效果. 在温度为4.2 K、磁场大于9 T条件下,C和SiC掺杂样品的临界电流密度与未掺杂样品相比均提高一个数量级以上. 掺杂样品高磁场下良好的临界电流性能主要归因于C对B的替代所产生的晶格畸变、位错等缺陷和局部成分变化而导致的有效晶内钉扎作用. 实验结果表明,SiC掺杂的MgB2带材之所以具有非常好的高场电流特性,和C掺杂的样品一样, C对B的替代起到十分关键的作用.     

强磁场诱导生长MgB_2超导体的磁各向异性研究

通过磁场诱导技术,我们制备出具有织构化取向的MgB2超导体.样品X衍射图显示晶格参数发生变化,随着诱导磁场的增加晶格参数a逐渐递减;当诱导磁场达到10T将会导致晶格变形,影响晶粒尺度大小;分别对诱导磁场为2T、4T、6T、10T样品进行磁测量,可发现临界电流得到明显提高,并且不可逆场也有所提高,样品表现出磁各向异性.     

颗粒超导体临界电流密度的尺寸效应

假设颗粒超导体内某点临界电流密度是该处磁场强度平均值的某种函数,计算了其实测临界电流密度与样品尺寸的关系.发现对目前实验中所用的烧结样品,实验测得的 YBaCuo超导体的临界电流密度有显著的尺寸效应.     

纳米ZrO2掺杂对烧结DyBaCuO块材微观结构及其超导电性的影响

研究了纳米ZrO2掺杂及其掺杂浓度对烧结DyBa2Cu3Oy超导体的转变温度Tc和临界电流Ic在零磁场和磁场中的行为,在磁场中发现掺杂样品的临界电流密度明显高于纯DyBaCuO样品.这种磁场行为可能与掺杂样品粒子分布更加均匀,富纳米粒子区明显增加,从而形成有效钉扎中心有关.     

利用电泳法在金属基底上制备MgB2超导厚膜

利用电泳技术在高熔点金属基底Ta,Mo和W上制备MgB2 超导厚膜 .厚膜中的MgB2 晶粒结合紧密 ,粒度小于1μm ,呈随机取向生长 .电阻测量表明沉积在Ta ,Mo,W上的MgB2 厚膜的超导起始转变温度分别为 3 6.5K ,3 4.8K ,3 3 .4K ,对应的转变宽度为 0 .3K ,1.5K和 2 .0K .三种基底上制备的MgB2 厚膜的临界电流密度在不同温度下随外磁场的变化情况基本相同 ,MgB2 Mo厚膜的临界电流密度在 5K ,1T时可达 1.2× 10 5A cm2 .本工艺可以随意控制膜的厚度、形状、大小 ,也可进行双面沉积 ,且制备出的MgB2 厚膜的超导性能优良 ,为MgB2 超导体的实用化提供了可能     

Pair-Breaking Critical Current Density of Two-Band Superconductor MgB2

Temperature dependence of tbe pair-breaking critical current density of MgB2, jd（T）, is studied using a two-band Ginzburg-Landau theory. The results are shown to be in good agreement with experimental data for the superconducting magnesium diboride MgB2.     

MgB2超导膜的厚度与其Jc(5K,0T)的关系

通过混合物理化学气相沉积法 (hybrid physical-chemical vapor deposition, HPCVD), 在(000l) SiC 衬底上制得一系列从10 nm到8 μm的MgB₂超导膜样品, 并对它们的形貌、超导转变温度T_c 和临界电流密度J_c与膜厚度的关系进行了研究. 观察到T_c随膜厚度增加上升到最大值后, 尽管膜继续增厚, 但T_c值保持近乎平稳, 而J_c则先随膜厚度增加上升到最高值后, 继而则随膜的厚度的增加而下降. MgB₂膜的T_c(0)和T_c(onset)值与膜厚的关系基本一致, T_c(0)在膜厚为230 nm处达到最大值T_c(0)=41.4 K, 而J_c(5K,0T)在膜厚为100 nm时达到最大值, J_c (5 K, 0 T)=2.3×10⁸A·cm^-2, 这也说明了我们能用HPCVD方法制备出高质量干净MgB₂超导膜. 本文研究的超导膜厚度变化跨度非常大, 从10 nm级的超薄膜到100 nm级的薄膜, 再到几微米的厚膜, 如此T_c和J_c对膜厚度变化的依赖就有了较完整、成体系的研究. 并且本文的工作对MgB₂超导薄膜制备的厚度选取具有实际应用意义. 关键词： 2超导膜')" href="#">MgB₂超导膜 混合物理化学气相沉积法 厚度 临界电流密度     

Use of the ac inductive method to assess the quality of bulk melt textured superconductors

Through a measurement of flux profiles, the inductive method of Campbell has not only been used for determining the bulk critical current density of large sized samples but has also been extended to study the coupling strength of the domains in melt textured NdBa₂Cu₃O_7−δ superconductors. An analysis of the profiles measured for the slow (1°C/h) and fast cooled (40°C/h) samples through peritectic temperature, T_p clearly showed that the domain boundaries in the fast cooled samples are decoupled in the presence of external dc fields. The observed features of flux profiles are correlated to the microstructural features of the samples. It is proposed that this technique can be used routinely to assess the quality of large size melt textured samples for high current applications like magnetic bearings or quasi permanent magnets.     

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。     

Zr掺杂形式对钨电子发射材料组织性能的影响

采用中频感应加热烧结方法制备了W-1.5%La₂O₃-0.1%Y₂O₃-0.1%ZrO₂和W-1.5%La₂O₃-0.1%Y₂O₃-0.08%ZrH₂电子发射材料,烧结样品的致密度约为95.5%。热电子发射测试结果表明,添加氢化锆的热电子发射材料样品的零场发射电流密度大于添加氧化锆的样品,分析认为是添加的氢化锆在烧结时,发生分解,生成活性的Zr可以捕获钨晶界中的杂质氧,净化晶界,从而提高了电子发射;维氏显微硬度表明添加氢化锆样品的硬度高于添加氧化锆的样品,分析表明是氢化锆的添加有效改善了钨晶粒之间的结合性,提升了钨电子发射材料的硬度。利用SEM,EDS,XRD、金相显微镜等表面分析设备对样品进行了表征,样品结构显示添加氢化锆与添加氧化锆相比,不仅钨晶粒尺寸由13.63 μm降至11.63 μm,而且稀土相尺寸由1.87 μm降至1.66 μm,这种组织结构的变化有利于电子发射。     

SQUID法和Campbell法测量超导材料的研究

本文对比研究了超导材料磁测量中的SQUID法和Campbell法; 并用高压PIT法制备的超导材料MgB₂作为测量样品,用两种方法,测量了超导样品的临界电流密度, 分别得到了样品的J_c-B关系曲线; SQUID法测量样品的外磁场可以达到6 T, 此时材料已经处于失超状态,此方法测得的结果是样品各个小区域结果的平均值, SQUID还可以用来进一步标度材料的钉扎力行为,研究材料磁特性. Campbell法测量只能测量到外磁场强度为0.4 T,外磁场的交流部分的频率可以达到800 Hz, 用这种测量方法得到的是整块样品的电流,由于测量计及材料内部微观结构缺陷等影响电流传输因素, 所测结果小于直流磁化法,但更切近材料实际电流,能用来深入研究材料内部结构差别对材料电性能的影响.     

Effect of Cu doping in MgB2 superconductor at various processing temperatures

The effects of Cu doping in MgB₂ superconductor has been studied at different processing temperatures. The polycrystalline samples of Mg_1−xCu_xB₂ with x = 0.05 were synthesized through the in-situ solid sate reaction method in argon atmosphere at different temperature range between 800–900 °C. The samples were characterized through X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and low temperature R–T measurement techniques for the phase verification, microstructure and superconducting transition temperature, respectively. The XRD patterns of Mg_1−xCu_xB₂ (x = 0.05) do not exhibit any impurity traces of MgB₄ or MgB₆ and they show the sharp transition in the samples prepared at 850 °C. The onset transition temperature of the prepared samples is around 39 K, which is almost the same as that for the pure MgB₂. This indicates that Cu doping in MgB₂ does not affect the transition temperature. The SEM micrograph of Mg_0.95Cu_0.05B₂ has shown that the sample is dense with grain size smaller than 1 μm.     

Improvement of sintering,nonlinear electrical,and dielectric properties of ZnO-based varistors doped with TiO_2

The effects of TiO_2 on sintering and nonlinear electrical properties of(98.5-x)ZnO–0.5MnO_2–0.5Co_2O_3-0.5Bi_2O_(3–x)TiO_2(x = 0.3,0.5,0.7,0.9 mol%) ceramic varistors prepared by the ceramic technique are investigated in this work.The optimum sintering temperature of the prepared samples is deduced by determining the firing shrinkage and water absorption percentages.The optimum sintering temperature is found to be 1200℃,at which each of the samples shows a maximum firing shrinkage and minimum water absorption.Also minimum water absorption appears in a sample of x = 0.9 mol%.Higher sintering temperature and longer sintering time give rise to a reduction in bulk density due to the increased amount of porosity between the large grains of ZnO resulting from the rapid grain growth induced by the liquid phase sintering.The crystal size of ZnO decreases with increasing TiO_2 doping.The addition of TiO_2 improves the nonlinear coefficient and attains its maximum value at x = 0.7 mol% of TiO_2,further addition negatively affects it.A decrease in capacitance consequently in the dielectric constant is recorded with increasing the frequency in a range of 30 kHz–200 kHz.The temperature and composition dependences of the dielectric constant and AC conductivity are also studied.The increase of temperature raises the dielectric constant because it increases ionic response to the field at any particular frequency.     

Critical current density of polycrystalline YBa2Cu3O7−δ consisting of tin-coated grains

Tin and its oxides have been introduced into the intergrain areas of polycrystalline YBa₂Cu₃O_7−δ by coating the crystalline grains with a thin layer of tin and sintering the ceramics in flowing oxygen and argon. The transport critical current density J_c at 77 K in a magnetic field of 0−1.5 T is enhanced as a result of the coating. A probable improvement of the intergrain weak links is suggested.     

Influence of the Tl- and Hg-content on magnetic and transport properties of the Pb, Sr-doped Tl-1223 and Hg-1223 superconductors

The effects of the thallium and mercury content x on the as-sintered and post annealed samples of M_xPb_0.4Sr_1.6Ba_0.4Ca₂Cu₃O_8+δ {M: Tl (0.32≤x≤0.74) or Hg (0.18≤x≤0.68)} have been studied by magnetization and transport measurements. For Tl-1223 we have found the optimum Tl doping level to be x=0.53 regarding the grain properties, the content of superconducting phase, the first penetration field H_pl^wl, the transport (J_c^tr), magnetic intergrain (J_c^M) and intragrain (J_c^g) critical current densities. The critical temperature T_c of the as-sintered Tl-1223 sample decreased with increasing Tl content. Post-annealing in oxygen improved the T_c for Tl contents of x≥0.53 and had generally positive effects on the critical current densities. The intergrain properties of the Hg-1223 samples were much worse than those of the Tl-based superconductors.