用于分析氧离子导体的氧表面交换行为的脉冲同位素交换方法（英文）

本文详细描述了一种用于测量氧化物离子导体的氧表面交换速率的新型脉冲~(18)O-~(16)O同位素交换(PIE)技术.该技术采用装有氧化物粉末的连续流动填充床微反应器实现.通过在线气相质谱法测量通过反应器的富含180的脉冲的等温响应.表观氧交换速率可以通过在给定的反应器停留时间和可用于交换的表面积下氧化物吸收~(18)O的数量来计算.相比于其他氧同位素交换的技术,PIE技术不需要快速加热/淬火步骤,具有快速、简单、非常适合筛选材料和系统研究反应机理的优点.此外,利用氧同位素~(18)O_2、~(16)O~(18)O和~(16)O_2在流出脉冲中的相对分布可以深入研究氧的表面交换反应机理.本文采用PIE技术分析YSZ、Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ)和La_2NiO_(4+δ)在350～900℃范围内表面氧交换性能.根据同位素交换反应的两步式反应模型分析,对于混合导体BSCF和La_2NiO_(4+δ),O_2分子在氧化物表面的解离吸附速率为速率控制步骤.对于YSZ,在800℃时,其表面氧交换的速率控制步骤由氧离子的晶格溶入变成O_2分子在氧化物表面的解离吸附步骤.     

双δ力与表面修正δ力和(s-d)壳层氧同位素核能谱

本文用双δ力和表面修正δ力作为有效相互作用, 用组态混合计算了(s-d)壳层氧同位素^17-22O核的能谱. 结果表明: 双δ力比修正表面δ力能更好地重现氧同位素的能谱; 表面δ相互作用的两体矩阵元对能量的贡献比体δ相互作用要大得多.     

周期数N不同的(Ce_(0.8)SmO_(2-δ))/YSZ)_N超晶格薄膜的阻抗性质

采用脉冲激光溅射技术,在MgO单晶片衬底上,以SrTiO_3作为缓冲层,交替沉积Ce_(0.8)Sm_(0.2)O_(2-δ)(SDC)和8 mol%Y_2O_3:ZrO_2(YSZ),制备了不同周期数的(SDC/YSZ)N超晶格电解质薄膜.利用扫描电子显微镜,X射线衍射和交流阻抗对其形貌、相结构和电学性能进行了表征.研究结果表明,薄膜具有优良的超晶格结构,层与层间的界面清晰.阻抗分析表明,周期数越多的样品显示出很小的活化能(约0.768 eV).该结果表明:周期数越多的SDC/YSZ超晶格是更为理想的低温固体氧化物燃料电池电解质.     

La1.85Sr0.15CuO4中的同位素效应

利用同位素间 Eliashberg 函数的关系及 Eliashberg 方程,我们计算了 La_(1.85)Sr_(0.15)CuO_4对应于~(16)O 及~(18)O 的 T_c,从而得到同位素指数α的值.计算结果与不同实验进行了比较.传统的电声机制可以解释这个同位素效应.     

Y_1Ba_2Cu_3O_(7-δ)的氧离子缺位数,氧空穴数变化与结构相变的关系

在空气、氮气、真空(10~(-3)乇)、氧气等气氛条件下,利用高温 X 射线动态跟踪法和 TG-DTA测定了 Y_1Ba_4Cu_3O_(7-δ)的结构和氧缺位量随温度的变化,注意到不同气氛对超导体中晶格氧和吸附氧的动态平衡的影响;测得氧缺位量δ与晶体结构转变之间的关系.根据文献[11]关于Cu-O 间电子转移的结论,计算出 Cu-O_2层中 O 的平均空穴数 n_h 与结构变化的对应关系.O≤δ≤0.25,0.25≥n_h≥0.125为正交超导相的稳定区,氧的空穴浓度为(1.44—0.72)x10~(21)/cm~30.25≤δ<0.5,0.125≥n_k>0为正交、四方相的转变过渡区;0.5≤δ≤1,n_h=0为四方非超导相的稳定区.因此,除了四方非超导结构区氧负离子取2价外,其它情况氧负离子也不总是取负2价.     

钙钛矿型氧载体SrCo_(0.8)Fe_(0.2)O_(3-δ)制氧性能的实验研究

钙钛矿型金属氧化物是优良的制备供富氧燃烧所需的O_2/CO_2的氧载体,本文采用柠檬酸法制备系列SrCo_(1-x)Fe_xO_(3-δ)(x=0.2,0.4,0.6,0.8)钙钛矿型氧化物,并采用X射线衍射分析来表征SrCo_(0.8)Fe_(0.2)O_(3-δ)(SCF182)反应前后的物相变化和晶体结构.同时在固定床上选择不同的运行条件对SCF182的释氧性能进行研究,包括吸附温度、吸附时间、脱附温度和循环特性.结果表明,SCF182的最佳吸附温度和脱附温度均为850℃,最佳吸附时间为1 h,循环性能良好,是可以为富氧燃烧提供稳定的O_2/CO_2循环气体的良好材料。     

CH_4在Cu/γ-Al_2O_3颗粒上催化燃烧分区及反应特性

采用固定床微分反应器,实验研究了甲烷在不同氧烷分压比(P_(O_2)/P_(CH_4))下Cu/γ-Al_2O_3催化剂上的反应速率及燃烧特性,确定了反应动力学分区,并探讨了甲烷催化燃烧反应的表面机理及动力学特性。结果表明,当P_(O_2)/P_(CH_4)2时,微分反应器中的反应速率与CH_4分压成正比而与O_2分压无关,催化剂表面呈饱和氧吸附状态;0.1P_(O_2)/P_(CH_4)2时,反应速率受CH_4和O_2分压的共同影响,吸附氧未完全覆盖活性位表面;当0P_(O_2)/P_(CH_4)0.1时,反应速率仅与O_2分压呈正比关系,与CH_4无关,金属表面裸露,表面吸附氧被快速吹离。动力学研究表明,3个反应动力学区间所对应的催化剂表面吸附氧覆盖率有明显不同,3个区间依次为(O~*)、(O~*+*)和(*),所对应的表观活化能依次下降146.3kJ/mol、99.8 kJ/mol、60.8 kJ/mol。     

La_(2-x)Sr_xCuO_4的高氧压处理与XPS研究

本文采用高氧压对 La_(2-x)Sr_xCuO_4进行处理.高氧压处理(2.3GPa,～800℃)不但对x=0组分及附近的样品提供过量的氧,使之成为超导体,而且可以使无氧缺位的状态保持到x=0.3.但电导率测量表明,高温超导相范围并没有扩展,因此排除了氧缺位的出现使超导相存在范围降低的看法.XPS 测量表明表面完全污染的 La_(2-x)Sr_xCuO_4样品与半导体 La_2CuO_4或 CuO 中 Cu~(2+) 谱一致,而未完全污染的样品表现出很宽的峰,明显包含局域态的 Cu~(2+)及LaCuO_3中的 Cu_(3+)成份.论文首次引入 La3d XPS 谱的伴线来研究超导体的电子态.对于“Clean”La_(1.85)Sr_(0.15)CuO_4及 La_2CuO_4,La3d XPS 谱具有完全不同的结构,前者伴线为 Shakedown,表明较低的费米能级位置,意味着强氧化态.后者为 Shake up,意味着较高的费米能级位置.在超导样品中,无法利用 O_(15)谱来区分污染样品表面还是处在深度氧化态的表面时,可用 La3d 谱来鉴别样品的表面.     

Ni与钇稳定的氧化锆(111)表面相互作用以及界面活性的第一性原理研究

采用基于密度泛函理论的第一性原理方法, 系统研究了Ni原子在钇稳定的氧化锆(YSZ)(111)和富氧的YSZ(YSZ+O)(111)表面不同位置的吸附, 以及CO和O₂分子在Ni₁(单个镍原子)/YSZ和Ni₁/YSZ+O表面吸附的几何与电子结构特征. 结果表明: 1) 单个Ni原子倾向于吸附在O原子周围, 几乎不吸附在Y原子周围, 且Ni原子在氧空位上吸附最稳定; 2)和YSZ相比, 单个Ni原子在YSZ+O表面易发生氧化现象, Ni原子失去1.06 e电子, 被氧化成了Ni⁺, 吸附能力更强; 3)被氧化的Ni催化活性大幅下降, 大大减弱了表面对O₂和CO等燃料气体的吸附作用.     

Y_1Ba_2Cu_3O_(7-δ)的氧离子缺位数,氧空穴数变化与结构相变的关系

在空气、氮气、真空(10^(-3)乇)、氧气等气氛条件下,利用高温 X 射线动态跟踪法和 TG-DTA测定了 Y_1Ba_4Cu_3O_(7-δ)的结构和氧缺位量随温度的变化,注意到不同气氛对超导体中晶格氧和吸附氧的动态平衡的影响;测得氧缺位量δ与晶体结构转变之间的关系.根据文献[11]关于Cu-O 间电子转移的结论,计算出 Cu-O_2层中 O 的平均空穴数 n_h 与结构变化的对应关系.O≤δ≤0.25,0.25≥n_h≥0.125为正交超导相的稳定区,氧的空穴浓度为(1.44—0.72)x10^(21)/cm^30.25≤δ<0.5,0.125≥n_k>0为正交、四方相的转变过渡区;0.5≤δ≤1,n_h=0为四方非超导相的稳定区.因此,除了四方非超导结构区氧负离子取2价外,其它情况氧负离子也不总是取负2价.     

Exchange magnetic moments

Exchange magnetic moments are investigated by adopting a phenomenological approach and a field theoretical model. Their contributions are estimated for nuclei with a particle or hole outside a closed shell. They are expressed in terms of corrections of gyromagnetic factors.     

图解热传导

在热传导过程中,高温物体放热,低温物体吸热,在不计热损失时,高温物体放出的热量等于低温物体吸收的热量,高温物体放热降温,低温物体吸热升温,当高温物体的温度和低温物体的温度相等时热传导停止.     

Exchange bias theory

Research on the exchange bias (EB) phenomenon has witnessed a flurry of activity during recent years, which stems from its use in magnetic sensors and as stabilizers in magnetic reading heads. EB was discovered in 1956 but it attracted only limited attention until these applications, closely related to giant magnetoresistance, were developed during the last decade. In this review, I initially give a short introduction, listing the most salient experimental results and what is required from an EB theory. Next, I indicate some of the obstacles in the road towards a satisfactory understanding of the phenomenon. The main body of the text reviews and critically discusses the activity that has flourished, mainly during the last 5 years, in the theoretical front. Finally, an evaluation of the progress made, and a critical assessment as to where we stand nowadays along the road to a satisfactory theory, is presented.     

Exchange Dynamical Quantum Groups

For any simple Lie algebra ? and any complex number q which is not zero or a nontrivial root of unity, %but may be equal to 1 we construct a dynamical quantum group (Hopf algebroid), whose representation theory is essentially the same as the representation theory of the quantum group U _q (?). This dynamical quantum group is obtained from the fusion and exchange relations between intertwining operators in representation theory of U _q (?), and is an algebraic structure standing behind these relations. Received: 24 March 1998 / Accepted: 14 February 1999     

Exchange bias in nanostructures

The phenomenology of exchange bias and related effects in nanostructures is reviewed. The types of systems discussed include: lithographically fabricated ferromagnetic (FM)—antiferromagnetic (AFM) nanostructures, chemically surface modified FM particles, FM particles embedded in an AFM matrix, controlled core–shell particles, nanoparticles with surface effects and coupled AFM–AFM systems. The main applications of exchange biased nanostructures are summarized. Finally, the implications of the nanometer dimensions on some of the existing exchange bias theories are briefly discussed.     

Exchange interactions in polynuclear complexes

We present detailed optical and magneto-optical data on Cs₃Cr₂Cl₉ as a single crystal containing only pairs of Cr³⁺ ions. Measurements have been carried out in a wide range of temperatures down to pumped helium and in the whole spectral region covering all singly and several doubly excited states. MCD helped considerably in resolving the absorption spectra into their numerous components, and proved especially useful in demonstrating the occurrence of a double excitation to ⁴ T ₂. It provides also parallel g factors for several excited terms.

The main conclusions of our work are: (i) all transitions are electric dipole allowed; (ii) transitions to singly excited doublet states are essentially promoted via a single ion mechanism; (iii) vibronic interaction must be invoked to understand the absorption data on sharp lines; (iv) the trigonal plus spin-orbit interaction competes favourably with exchange interaction to produce excited states splittings.

We present also a new non-phenomenological model to describe the splitting of the ⁴ A ₂Θ² E manifold under the combined effects of exchange, trigonal and spin-orbit interaction. This model includes no adjustable parameter. It leads to λ₀ ～ 180 cm^-1 and offers a good starting basis for a coherent interpretation of most of our experimental data.     

Exchange couplings in magnetic films

Recent advances in the study of exchange couplings in magnetic films are introduced.To provide a comprehensive understanding of exchange coupling,we have designed different bilayers,trilayers and multilayers,such as anisotropic hard/soft-magnetic multilayer films,ferromagnetic/antiferromagnetic/ferromagnetic trilayers,[Pt/Co]/NiFe/NiO heterostructures,Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide(AAO) template.The exchange-coupling interaction between soft-and hard-magnetic phases,interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer,ferromagnetic layer,and antiferromagnetic layer.Some particular physical phenomena have been observed and explained.     

Exchange reactions with Dick Dalitz

An account is given of the close collaboration of Dick Dalitz with the European K⁻ Collaboration over many decades in many aspects of hypernuclear physics. In particular, emphasis is given to the topics of double hypernuclei and the discovery and resolution of p-wave Λ strangeness exchange states. A brief review of early work on non-mesonic decays of hypernuclei is also given.