共查询到20条相似文献,搜索用时 62 毫秒
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Mg粉、B粉按1.03:2的比例混合、压制成型后,在流通高纯氩气的条件下分别于660、700、750、800、850和900℃烧结1h,制备出MgB2块材.利用X射线衍射仪、扫描电子显微镜和阿基米德方法等,分别研究了样品的物相组成、显微结构和密度等.结果表明,随着烧结温度的升高,样品的非超导相Mg和MgBx(x=4、6或12)的含量、体积膨胀系数、质量损失率、密度及MgB2的晶粒尺寸都发生变化.750~850℃烧结时,可得到由MgB2和少量MgO组成、晶粒较小、密度较高的MgB2超导体. 相似文献
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尽管MgB2超导线材的临界转变温度相对较低,但MgB2没有晶界弱连接,因而具有低的加工成本,相对于BSCCO和YBCO表现出明显的应用优势.目前,粉末装管技术(PIT)广泛用于制备MgB2带材和线材.利用高能球磨工艺的超细粉末制备的MgB2线材临界电流密度达到3.0×105 A/cm2(20 K,自场).为了了解MgB2线材的实际使用条件和提高使用条件下的性能,很有必要对其进行机械性能方面的研究. 相似文献
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由德国德累斯顿金属材料研究所的约根·意科特领导的研究小组采用一种机械合金化工艺制备了具有高临界电流密度的大块MgB2超导体,这种超导体在20K,1T磁场下临界电流密度为10000A/cm2,并具有较高的不可逆场.他们认为,由于在这种材料中含有直径大约为40nm~100nm的球形晶粒,并存在大量的晶界,因此改善了磁通钉扎特性,提高了临界电流密度. 相似文献
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SiC掺杂对MgB2/Fe超导线材临界电流密度和显微结构的影响 总被引:1,自引:0,他引:1
利用原位粉末套管法制备出SiC微粉掺杂的MgB2-x(SiC)x2/Fe(z=0.00、0.05、0.10、0.20)超导线材。在750℃,流通高纯氩气的条件下热处理1h后,大部分SiC没有参与取代B位的反应。随着x的增大,线材中非超导相SiC和Mg的含量增加,MgB2的平均晶粒尺寸变小,从而使可作为磁通钉扎中心的晶界的面积相应增加。在外加磁场中,MgB2超导线材的临界电流密度(Jc)随x增大逐步升高,至x=0.10时Jc性能最好,其在6K,5T时的Jc达到了8480A/cm^2,比未掺杂线材的Jc高出约70%。但是,当x=0.20时,Jc却有所下降。Jc的这种变化规律与SiC掺杂引起的MgB2晶粒变小,以及非超导相物质含量之间的相互平衡有关,其中MgB2晶粒变小是Jc提高的主要原因。 相似文献
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REBCO(RE为稀土)块材超导体比传统的铁基永磁体能产生更大的磁场.因为块材超导体产生的磁矩与流过样品的回路电流和电流流过的面积成正比,因此,近年来许多工作集中在提高样品的临界电流密度和增大样品的尺寸. 相似文献
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A method is proposed for estimating the parameters of a mixture of exponential and Weibull distributions for which the accuracy of preliminary estimates obtained by graphical analysis is refined in accordance with the criterion of maximum likelihood. The efficiency of the proposed method is supported by the results of statistical modeling. 相似文献
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Liebert A Wabnitz H Grosenick D Möller M Macdonald R Rinneberg H 《Applied optics》2003,42(28):5785-5792
A novel method for the determination of the optical properties of tissue from time-domain measurements is presented. The data analysis is based on the evaluation of the first moment and the second centralized moment, i.e., the mean time of flight and the variance of the measured distribution of times of flight (DTOF) of photons injected by short (picosecond) laser pulses. Analytical expressions are derived for calculation of absorption and of reduced scattering coefficients from these moments by application of diffusion theory for infinite and semi-infinite homogeneous media. The proposed method was tested on experimental data obtained with phantoms, and results for absorption and reduced scattering coefficients obtained by the proposed method are compared with those obtained by fitting of the same data with analytical solutions of the diffusion equation. Furthermore, the accuracy of the moment analysis was investigated for a range of integration limits of the DTOF. The moment analysis may serve as a comparatively fast method for evaluating optical properties with sufficient accuracy and can be used, e.g., for on-line monitoring of optical properties of biological tissue. 相似文献
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O'Brien RS 《Radiation protection dosimetry》2003,105(1-4):95-100
Australia has several uranium mines and a large number of mineral sand mines, with associated processing facilities. Exposures resulting from these mining and processing operations usually involve intakes of mixtures of radionuclides. This work describes the development of a suite of first order, linear compartment models, based on the ICRP Publication 66 respiratory tract model, and an analytical solution to the decay equations, for assessing the consequences of such intakes. The computer programs based on these models directly compute excretion, organ retention and organ and whole-body doses for intakes of either single radionuclides or any mixture of radionuclides belonging to the same radioactive decay chain. The intake can be via inhalation, ingestion or injection, and can be acute, chronic or of limited duration. The starting concentration and degree of secular (dis)equilibrium can be specified for each radionuclide. No assumptions need to be made about the relative magnitudes of the radioactive half-lives of the different nuclides. 相似文献
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Conventional methods for summation of direct measurement errors are considered. Conditions under which these methods provide consistent results are stated. It is proposed to utilize for characteristic of the total error, its interval estimator or the half-sum of the value obtained by adding up its arithmetic and geometric random errors.Translated from Izmeritel'naya Tekhnika, No. 4, pp. 10–12, April, 1995. 相似文献
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由于空压机进气侧轴承箱蜗壳温度探头密封效果不好,使空压机及其后续管道受到油污染。后将温度探头的O形圈密封改为骨架油封,杜绝了轴承箱漏油问题。文章简介空压机的结构和技术参数,详细介绍了骨架油封的制作和优点。 相似文献
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V. T. Kornienko 《Strength of Materials》1991,23(7):760-765
The article suggests a method of evaluating the microstructural inhomogeneity of deformation of metals by modeling. The method includes the measurement of deformations of metal according to meshes of a dividing grid with small base and the calculation of the distribution parameters of microdeformations in model representation. It is shown that the modeling method makes it possible objectively to evaluate the microinhomogeneity of deformation of metals independently of the chosen mesh size of the dividing grid. New structural characteristics are introduced: the base and wave of variation expressing the degree of crumbling or amalgamation of metal grains by inhomogeneity of deformation.Translated from Problemy Prochnosti, No. 7, pp. 40–44, July, 1991. 相似文献
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M. M. Gadenin 《Inorganic Materials》2013,49(15):1352-1356
It is shown that conditions of attainment of marginal states in the material of structural components are formulated on the basis of analysis of force and deformation criteria of formation and development of destruction. The parameters of these criteria are data on types of structural materials, standard and special characteristics of their mechanical properties, structural forms of considered elements of the equipment, and their operating stress loading. In this case, the construction of a system of state equations for describing interrelation of current damage, external actions, and responses to them is based on the concept of calculating variations in properties of materials at all stages of the design life cycle. 相似文献
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V. Ya. Fateev 《Measurement Techniques》2011,54(7):774-780
Different constructions of the sensing elements of radio-frequency level sensors based on sections of long lines are numerically
investigated by means of the Matlab program for the purpose of minimizing the measurement error caused by the nonlinearity
of the output characteristic. It is shown that the nonlinearity is lowest for U-shaped designs. The nonlinearity factor KN
may be regulated by varying the capacitance of a correcting capacitor connected to the input of the sensing element and (or)
the length of one of the sections of the long line. In selecting an optimal capacitance of the correcting capacitor KN ≤ 0.15%
for control media with dielectric permittivity in the range 2.2–30. 相似文献