铋系陶瓷超导体的制备和陶瓷超导体的可能机制

1 Bi系陶瓷超导体的试制与周期律当1—2—3(Y,Ba_2Cu_3Ox)陶瓷的超导性得到确认以后,为了找到T_c更高,稳定性更好的陶瓷超导物质,人们发现最现实最方便的途径,是根据元素的周期律,用性质比较接近的元素来对1—2—3陶瓷超导体的某些元素做置换实验,据文献报导,用各个稀土元素代替Y元素均可得到高T_c的超导性。     

Bi_(1.6)Pb_(0.4)Sr_2Ca_2Cu_3O_y超导体的DSC研究

新型高T_c氧化物超导体Bi—Sr—Ca—Cu—O系,在100K以上呈现超导性,存在高T_c(～110K)相和低T_c(～80K)相,其零电阻转变温度(T_(c0))总是低于85K.适当掺Pb可以增强高T_c相,抑制低T_c相,降低烧结温度,提高零电阻转变温度.用固相反应法制得组分为Bi_(1.6)Pb_(0.4)Sr_2Ca_2Cu_3O_y的超导体,零电阻温度为107K,稳定性较好。现以此样品作示差扫描量热(DSC)分析,并配合XRD分析,试图了解该体系的结构相变及其与超导电性的关系.     

Y-Ba-Cu-O系高临界温度超导陶瓷组成与性能关系的研究

研究了主组成及少量外加物对Y_xBa_(1-x)CuO_3系超导陶瓷导电性与超导电性的影响.研究表明,当配料组成Y∶Ba≈1∶2时最容易制取稳定的、零电阻为88K以上的高临界温度T_c的超导材料;采用Bi、Pb、Na、Co等对阳离子进行部分替位更换,仍能获得超导特性,但对T_c值没有改进;少量外加物对YBa_2Cu_3O_(7-δ)系超导陶瓷的T_c值有明显的影响,微量钴离子的掺入可使T_c提高到110K,但掺入量较多时反使T_c降低.     

Bi-Sr-Ca-Cu氧化物的高Tc超导电性

目前,不含稀土的两类(Bi系和Tl系)新型高Tc氧化物超导体正日渐引起国内外有关学者的高度关注。Tl系高于110K的超导相较易稳定,零电阻温度已达114K,然而Tl的昂贵(比稀土Y还贵几倍)及剧毒性(神经性毒物,致死量0.8g)不能不成为其致命的弱点。比较之下,Bi系材料无毒、价廉,更具前途,但迄今其零电阻温度却只有84～85K左右。显然,进一步提高Bi系超导体的零电阻温度必将成为人们研究的重要方     

高温超导体的正电子湮没研究(续)

本文在文献的基础上,进一步测量稀土系YBaCu_xO_y在T_c附近的正电子寿命谱,同时测量了成分相同非超导样品,实验结果证实超导寿命谱在T_c附近有跃变发生,伴随无规则振荡现象,至于非超导样品,τ-T曲线形式则无跃变发生.还测量了非稀土系高T_c超导体BiSrCaCu_xO_y在相变温区的正电子寿命谱,也发现在T_c附近有跃变、无规则振荡,但振荡区域比稀土系样品要宽.样品YBaCuO中,Y,Ba,Cu的氧化物按1:2:3配比,以不同的氧含量,得到T_c=90 K的超导样品Ⅰ和非超导样品Ⅱ.样品BiSrCaCu_xO_y中,按配比Bi:Sr:Ca:Cu=1:1:1:2,以适量氧含量,得到T_c=83 K的超导体Ⅲ,所有样品均为圆片,直径约为15mm,厚为1.5mm.放射源为强度8×10~5Bq的~(20)Na正电子源,衬底为Mylar膜.两片相同的样品夹住正电子源,置于样品架中.测量在空气中进行,测量所用的寿命谱仪为标准的块一块符合系统,对~(60)Co     

双钙钛矿型Ba2YSbO6的水热合成和结构表征

报道了温和条件下双钙钛矿型复合氧化物Ba2YSbO6的水热合成,并通过XRD、IR、ICP、XPS对产物物相、组成、结构、锑离子价态等进行了表征.Ba2YSbO6产物为立方钙钛矿结构,属于Fm3m空间群,晶胞参数为a=0.840955(7)nm,V=0.594727(8)nm3,Y和Sb呈岩盐型有序排列,锑为 5价.     

密度泛函理论的电负性均衡值与稀土氧化物超导性

提出确定稀土氧化物中元素电负性均衡值X_(cq)的计算公式,并用该式计算了200多种稀土氧化物超导体的电负性均衡值X_(cq),得出了稀土氧化物的超导性与稀土氧化物电负性均衡值之间的关系．结果表明,稀土氧化物超导体的电负性均衡值X_(cq)多数集中在1．654～2．650之间,而电负性均衡值在此范围之外的稀土氧化物一般不具有超导性．因此,电负性均衡值X_(cq)可作为稀土氧化物超导性的新的经验判据,这对进一步探讨稀土氧化物的超导机制有一定意义．     

组分简单环境友好的铜基高温超导材料:“铜系”

铜基高温超导材料是目前唯一能在常压环境实现液氮温区超导的物质,对其深入认识入选Science发布的125个重大科学问题之一,具有重要的科学价值和巨大的应用开发前景.为了合成铜基超导材料,通常需要引入重金属元素如稀土、Bi、Tl和Hg等昂贵、易挥发或有毒元素以稳定基本结构,这些体系相应称为"钇系"、"铋系"、"铊(汞)系"铜基超导材料.探索组分简单环境友好且在液氮温区超导性能优良的新体系,是超导新材料研发的前沿和重大挑战.本文基于铜基超导材料结构特点,通过介绍运用高压高温合成技术研制钙钛矿型功能材料的优势,重点讨论作者参与设计发现的Cu12(n–1)n(简称"铜系")铜基超导材料体系."铜系"只含铜和碱土氧化物,这是构成铜基超导材料最简单的成分,组分经济环境友好.如同人造金刚石一样,"铜系"块体材料虽然需高压高温合成,但在常压条件能够保持结构稳定,本文讨论的性能都是常压条件测量."铜系"具有独特的全钙钛矿型构型,呈现强的层间耦合,常压超导特性优良:(1)起始超导转变温度(T_c )可达120 K以上,高于"铋系"超导材料,仅次于常压T_c 最高的铜基Hg1223超导体;(2)在液氮温区临界电流密度特性优于"铋系"铜基超导材料,可媲美综合性能最好的"钇系"铜基超导材料,(3)成分只有碱土和铜氧化物,环境友好不含有毒元素,且在空气环境中可保持性能长期稳定.     

蓝色荧光粉Y2GeO5:Bi3+,Tm3+的制备及其发光性能研究

采用固相法制备蓝色荧光粉Y2GeO5:Bi3+,Tm3+,通过X衍射分析、扫描电镜观察、激光粒度分析和光致发光光谱分别对其物相、形貌、粒度、发光性能进行表征,并研究了Bi3+和Tm3+离子掺杂量对其发光性能的影响。结果表明:Bi3+和Tm3+分别作为敏化剂和激活剂进入到Y2GeO5的晶格中,其最佳掺杂量(摩尔分数)分别为1.5%和2%;Y2GeO5:Bi3+,Tm3+荧光粉的D50约为7.11μm。激发光谱由314 nm宽峰和355 nm锐利峰组成,前者是由于Bi3+离子和基质激发峰复合而成;在314 nm激发下,荧光粉发射365 nm宽峰和455 nm锐利峰;掺杂Bi3+离子可以使Y2GeO5Tm3+的发光强度提高4倍。蓝色荧光粉Y2GeO5:Bi3+,Tm3+可应用于UV-LED器件中。     

Zn元素在高TC氧化物超导体中的作用

元素替代效应是研究高TC氧化物超导体的电子结构、晶体结构变化及宏观物性间相互关联、相互制约的重要途径,是获得高TC超导氧化物的重要手段。本文就高TC氧化物超导体中的Zn元素替代效应,尤其对Y系、Bi系高TC氧化物超导体的CuO2平面上Cu(2)位部分取代,导致TC被强烈抑制的机理进行了分析研究,讨论了从中给出的有关高温氧化物超导电性微观机制的信息。     

Y-Ba-Cu-O系超导体的水解性研究

高转变温度T_c的Y—Ba—Cu—O系超导体是目前最引人注目的超导材料。这种超导体在液氮沸点温度以上有稳定的零电阻温度T_o。对于不同的Y、Ba和Cu组分比,只要采用相应的烧结条件,均能制备出T_c>90K的超导样品。我们在实验中发现,此种体系超导体无论组分如何,均有明显的水解特性,最终失去超导电性。水解的最终生成物由X射线衍射测定决定。另外,利用计算机对超导体完全水解的温度-时间关系进行了模拟计算。我们以组分比Y:Ba:Cu=1:2:3均匀混料,在850℃下预烧4小时(h),然后磨碎并过筛,压制成直径16mm、厚度1.5mm的圆片,在960℃下通氧烧结9小时,得到     

少量元素掺杂对Y-Ba-Cu-O强化的影响

近期文献报道了选择满足ABO_3化合物中B位要求条件下的过渡族元素以及其它元素如V,Cr,Bi等,掺入(YBa)CuO_3中,它们可能取代Cu的位置,可以作为“结构探针”引出与其周围O的2p电子,氧缺位以及Cu、稀土离子等相互作用信息的变化,这有助于对超导微观机理的了解;且从少量过渡族元素和其它元素掺杂所带来的变     

The structure of the high-energy spin excitations in a high-transition-temperature superconductor

In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity. The high transition temperatures (high-T(c)) of the copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials. The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at T(c). The most obvious feature in the magnetic excitations of high-T(c) superconductors such as YBa2Cu3O6+x is the so-called 'resonance'. Although the resonance may be strongly coupled to the superconductivity, it is unlikely to be the main cause, because it has not been found in the La2-x(Ba,Sr)(x)CuO4 family and is not universally present in Bi2Sr2CaCu2O8+delta (ref. 9). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa2Cu3O6.6. We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at T(c), and have spectral weight far exceeding that of the 'resonance'. The discovery of similar excitations in La2-xBa(x)CuO4 (ref. 10) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.     

Microscopic electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x

The parent compounds of the copper oxide high-transition-temperature (high-Tc) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are 'doped' away from stoichiometry. For the compound Bi2Sr2CaCu2O8+x, doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers ('holes') into the CuO2 planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high-Tc superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi2Sr2CaCu2O8+x, on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of approximately 14 A. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.     

不同烧结温度对Bi系2212相超导电性的影响

对采用不同温度烧结的Ｂｉ系２２１２相样品的Ｒ—Ｔ曲线测量表明，随着烧结温度的增高，Ｒ—Ｔ曲线发生由超导相向半导体相的转变，而且这种转变无法以ＣｕＯ２面的结构畸变来解释．认为是因为烧结温度的不同导致了样品中载流子浓度的变化而引起的样品超导电性的变化．烧结温度升高，样品中的载流子浓度减小，导致了样品由超导相向半导体相的转变，与铜氧化物超导体相图所显示的规律性是一致的．     

Study on Growth of GdBaSrCu_3O_(7-δ) Single Crystal

1 Results It is generally known that CuO2 planes of layered cuprate superconductors play a major role on the variation of critical temperature,Tc.In order to investigate their microscopic and electronic properties,preparation of such materials in single crystal form with highly structure orientation is very important.Crystal growth techniques for copper oxide materials have greatly improved since the discovery of high Tc superconductor materials[1].However,a strong reaction between CuO flux and crucible material has been a serious problem in crystal growth by using crucible,especially in a self-flux slow cooling method that needs long time.It is important to avoid the contamination from crucible material and also the ‘creeping out' problem of CuO flux during long time and high sintering temperature.In this study,we attempted to grew the GdBaSrCu3O7-δ single crystals via self-flux slow cooling technique.They were grown from CuO-rich nonstoichiometric solutions as similar as the YBCO case with approximately cation and powder ratio used as reported before[2-3].     

Electron-phonon coupling reflecting dynamic charge inhomogeneity in copper oxide superconductors

The attempt to understand copper oxide superconductors is complicated by the presence of multiple strong interactions in these systems. Many believe that antiferromagnetism is important for superconductivity, but there has been renewed interest in the possible role of electron-lattice coupling. The conventional superconductor MgB2 has a very strong electron-lattice coupling, involving a particular vibrational mode (phonon) that was predicted by standard theory and confirmed quantitatively by experiment. Here we present inelastic scattering measurements that show a similarly strong anomaly in the Cu-O bond-stretching phonon in the copper oxide superconductors La(2-x)Sr(x)CuO4 (with x = 0.07, 0.15). Conventional theory does not predict such behaviour. The anomaly is strongest in La(1.875)Ba(0.125)CuO4 and La(1.48)Nd(0.4)Sr(0.12)CuO4, compounds that exhibit spatially modulated charge and magnetic order, often called stripe order; it occurs at a wave vector corresponding to the charge order. These results suggest that this giant electron-phonon anomaly, which is absent in undoped and over-doped non-superconductors, is associated with charge inhomogeneity. It follows that electron-phonon coupling may be important to our understanding of superconductivity, although its contribution is likely to be indirect.     

氧化物超导体交流磁化率的测量

描述了用交流互感电桥测量高Ｔｃ氧化物超导材料的交流磁化率装置，采用锁相放大器作为指零仪，温度变化范围从液氮温区到室温，考虑了装置本身存在的误差，我们测量了钇系和铋系超导材料的磁化率随温度的变化关系，利用对比法定出材料的抗磁讯号的百分比，与文献［１］比较结果是满意的。     

An hour-glass magnetic spectrum in an insulating, hole-doped antiferromagnet

Superconductivity in layered copper oxide compounds emerges when charge carriers are added to antiferromagnetically ordered CuO(2) layers. The carriers destroy the antiferromagnetic order, but strong spin fluctuations persist throughout the superconducting phase and are intimately linked to superconductivity. Neutron scattering measurements of spin fluctuations in hole-doped copper oxides have revealed an unusual 'hour-glass' feature in the momentum-resolved magnetic spectrum that is present in a wide range of superconducting and non-superconducting materials. There is no widely accepted explanation for this feature. One possibility is that it derives from a pattern of alternating spin and charge stripes, and this idea is supported by measurements on stripe-ordered La(1.875)Ba(0.125)CuO(4) (ref. 15). Many copper oxides without stripe order, however, also exhibit an hour-glass spectrum. Here we report the observation of an hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the family of superconducting copper oxides. Our system has stripe correlations and is an insulator, which means that its magnetic dynamics can conclusively be ascribed to stripes. The results provide compelling evidence that the hour-glass spectrum in the copper oxide superconductors arises from fluctuating stripes.     

激光溅射产生团簇负离子C_nX~－（X＝N，P，As，Sb和Bi）

通过激光溅射固体样品，产生了ⅤＢ簇元素的杂碳团簇ＣｎＸ－（Ｘ＝Ｎ，Ｐ，Ａｓ，Ｓｂ和Ｂｉ）．随着从Ｎ到Ｂｉ的变化，ＣｎＸ－的奇偶效应也产生了由奇强偶弱变化到偶强奇弱变化．根据Ｐｉｔｚｅｒ等人提出的杂化轨道理论和Ｈｕｃｋｌｅ模型，结合ⅤＢ簇元素性质的变化规律，对实验现象给予了可能的解释：本文认为ＣｎＮ团簇取直链构型（∶Ｃ≡Ｃ—Ｃ≡Ｃ···Ｃ≡Ｎ∶），Ｃｎ与Ｎ之间形成共价键；对于ＣｎＢｉ团簇，Ｃｎ与Ｂｉ之间可能形成离子键（Ｃｎ仍为类似聚炔的直链共轭体系）；对于其它三种团簇可能存在Ｃｎ与Ｘ之间离子键与共价键共振，随着电负性由大到小，共价键成分逐渐减弱，而离子键成分逐渐增强，因此出现了奇偶效应的变化．