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1.
1984年,D.Shechtman等人报道了在急冷的Al-Mn合金的约 2 μm大小的晶粒中,用电子衍射发现了一种除显示出二次和三次对称性图象外,还出现了五次对称的图象.该图象斑点明锐,具有与晶格平移不符的点对称群m35,他们称之为二十面体相[1].此后,立即引起了巨大的反响,国内外的报刊杂志纷纷报道和评论了这个惊人的发现[2-5]。 经典晶体学不允许有五次对称.五次对称破坏了点阵的平移对称性,因而不能不留空隙地用五边形铺满平面,也不能用具有五次对称轴的二十面体不留空隙地填满三维空间.因此二十面体相不是一个服从传统晶体学规律的晶体,而是一种组…  相似文献   
2.
张留琬  蔡培新  陈廷国 《物理学报》1995,44(7):1148-1157
利用等温原位电阻测量和热重分析等方法,深入研究Bi_(1.6)Pb_(0.4)Sr_2Ca_2Cu_3O_(10+5)超导材料在不同温度下氧气和空气中的退火行为,发现电阻率和重量均随退火时间单调增加,最后趋于平衡.同时观察到类Ca_2PbO_4杂相析出,调制结构由Pb型(波矢q=β_1b)逐渐转变为Bi型(q=β_2b+c),适量的杂相析出使T_c(0)有所增高.根据电阻率变化的规律判断,类Ca_2PbO_4杂相析出受2223母相Bi_2-O_2层中Pb~(+2)离子的二维扩散控制,求得Pb~(+2)离子 关键词:  相似文献   
3.
Bi1.6Pb0.4Sr2Ca2Cu3Ox超导体在65O℃O2中等温退火,调制结构由纯Pb型,经由Bi,Pb混合型,最后变为纯Bi型;反之,经Q2处理后,又在650℃流动N2中等温退火,调制结构又由纯Bi型,过渡到Bi,Pb混合型,最终将恢复到纯Pb型,整个过程基本可逆.调制结构的变化与Pb离子运动密切相关,P 关键词:  相似文献   
4.
谢晓明  陈廷国 《物理学报》1992,41(11):1830-1836
研究了YBa2Cu3O7-δ正交相和四方相中氧在Cu(1)-O基平面上跳动引起的两低频内耗峰;研究了在正交-四方相变过程中它们随氧含量的变化规律,并据此分析了YBCO中正交-四方相变的级次,认为在~200℃该相变实际上可能是一个一级相变。 关键词:  相似文献   
5.
Six Q-1 peaks in freshly-worked PbNb2O6 ceramics are observed by low-frequency internal friction measurement so far as we know for the first time. P1, P2, and P4 appear at the first heating process with remarkable shear modulus anomalies. But they are not observed during the cooling process or in the well-annealed samples. Those peaks are considered to be related with the working processes. P3 is a widened Debye relaxation peak in nature. The activation energy and pre-exponential factor are about 1.01 eV and 2.5×10-12 s, respectively. This peak is attributed to the interaction of domain walls and oxygen vacancies. P6 is corresponded with the paraelectric and ferroelectric phase transition. P5, appearing near Tc, is thought to be caused by the viscous movement of the domain walls.  相似文献   
6.
张留碗  陈廷国 《物理学报》1998,47(11):1906-1911
研究了高温热处理对BPSCCO-2223超导体高温电阻率的影响,观察到电阻率的奇异变化.实验结果表明,高温氮气退火,除Bi-O层外,Cu-O层也发生脱氧,同时伴随金属离子的运动.氧含量和氧重布及金属离子的运动引起了BPSCCO-2223相微结构的多样性. 关键词:  相似文献   
7.
Low-frequency internal friction (IF) of SrBi2Ta2O9(SBT) ceramics has been measured at temperatures ranging from 293 to 623 K. Two IF peaks, with their locations around 393 K (P1) and 569 K (P2) (f≈1.58 Hz), respectively, are observed. P1 is found to be associated with a relaxation process with activation energy of 0.97 eV and pre-exponential factor of 3.58×10-14 s. Peak shape analyses reveal that P1 can be well described by a broadened Debye peak with the width parameter β from 2.70 to 2.92. The mechanism responsible for P1 is proposed to be relaxation of oxygen vacancies near domain walls(DWs). P2 is considered to arise from viscous motion of DWs since it shows characteristics of static hysteresis type. Comparisons for the IF behavior between SBT and Pb(Zr, Ti)O3(PZT) suggest that in SBT oxygen vacancies are much less localized near DWs than that in PZT. This result provides a possible explanation for the weak DW pinning due to oxygen vacancies in SBT.  相似文献   
8.
对Y-系(123)相超导材料的微波焊接进行了初步研究,结果表明微波快速焊接后的试样,经960℃空气中退火15h后炉冷,其Tc可恢复至89.7K,比焊接前试样的Tc低1.6K。利用电子探针对焊接前后样品的显微结构进行了比较,发现焊区组织致密,但在后处理过程中超导相发生了再结晶,导致焊缝变宽,气孔因聚集而变大。同时用电子微探针分析仪发现,焊接后焊区普遍存在Y2BaCuOx相、Ba2-yCuy 关键词:  相似文献   
9.
Bi系超导材料的微波焊接及其显微结构研究   总被引:2,自引:0,他引:2       下载免费PDF全文
微波焊接技术近年来发展较快,它有下列优点:1)能耗低;2)升温速度快;3)接头质量高等。本文研究了Bi系超导材料微波焊接的可行性。结果表明,经855℃60h热处理后,焊接试样的Tc可达107K,与焊接前试样的Tc一致,焊区强度已经高于基体。利用电子探针对焊接前后的显微结构进行了比较,发现焊区组织致密,但在后处理过程中发生再结晶,导致焊区晶粒较大,焊缝变宽且焊区内存在较多杂相。 关键词:  相似文献   
10.
The influences of oxygen content (by quenching from different temperatures in air or oxygen) and its re-distribution (by annealing in nitrogen at 200℃) on Tc and room temperature resistivity were studied. It was found that Tc as a function of oxygen content or charge carrier concentration exhibits a maximum. Upon oxygen re-distribution, the room temperature resistivity would always inclose but Tc would change in a complicated way. Tc increases for the oxygen-rich state and decreases for the oxygen-deficient state Internal friction measurements showed that there are two kinds of oxygen defects in the Bi2-O2 layers. One is the isolated oxygen interstitials and the other is oxygen vacancies on the excess oxygen chains. It is proposed that the isolated interstitial oxygen will produce a set of impurity states near the Fermi level, and will create holes on the Cu3d-O2p band through charge transfer from the Cu3d-O2p band to the impurity states. The content of the isolated oxygen interstitial is believed to be the major factor for determining the charge carrier concentration. Variation of Tc and room temperature resistivity with oxygen re-distribution is sat-isfactorily explained as to be induced by combination of oxygen interstitials with oxygen vacancies on the excess oxygen-chains.  相似文献   
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