近化学计量比铌酸锂晶体周期极化畴反转特性研究

采用K2O作助熔剂直接拉晶法和气相输运平衡技术制备出了高质量近化学计量比铌酸锂晶体,研究了铌酸锂晶体中的[Li][Nb]比含量对其畴反转结构和极化电场的影响.实验结果表明:随着晶体中[Li][Nb]比的提高,畴极化反转电场呈明显下降趋势,使用近化学计量比铌酸锂晶体,在4.0±0.5kVmm大小极化电场条件下,成功地实现了1.0mm厚度的周期极化畴反转.并用铌酸锂晶体的Li空位缺陷模型对上述实验结果给出了合理的解释. 关键词： 近化学计量比铌酸锂晶体 周期极化 畴反转     

Non-Arrhenius relaxation of the Heisenberg model with dipolar and anisotropic interactions

The dynamical properties of a 2D Heisenberg model with dipolar interactions and perpendicular anisotropy are studied using Monte Carlo simulations in two different ordered regions of the equilibrium phase diagram. We find a temperature defining a dynamical transition below which the relaxation suddenly slows down and the system apart from the typical Arrhenius relaxation to a Vogel-Fulcher-Tamann law. This anomalous behavior is observed in the scaling of the magnetic relaxation and may eventually lead to a freezing of the system. Through the analysis of the domain structures we explain this behavior in terms of the domains dynamics. Moreover, we calculate the energy barriers distribution obtained from the data of the magnetic viscosity. Its shape supports our comprehension of both, the Vogel-Fulcher-Tamann dynamical slowing down and the freezing mechanism.     

Experimental investigation and modeling of corona characteristics under lightning impulses

An experimental study is conducted on the corona characteristics under lightning impulses. The charge-voltage characteristics, i. e. the Q-U curves, are measured in a corona cage. The impulse waveshapes are varied to investigate their influence on the Q-U curves. Meanwhile, a corona model is proposed to compute the Q-U curves. The ionized zone is simplified as a cylindrosymmetric geometry and a one-dimensional subdivision is made in the radial direction. The Q-U curves can be given by performing an efficient computational procedure. The computed results are compared with the measured ones to check the validity of the model.     

Influence of the corona-wire diameter and length on corona discharge characteristics of a cylindrical tri-axial charger

In this paper, the corona discharge characterization in terms of current–voltage relationships of a unipolar cylindrical tri-axial charger on the effects of the corona wire diameter and length have been experimentally studied and discussed. A commercial computational fluid dynamics software package, COMSOL Multiphysics™, was used to predict the electric field distribution in the ion generation and charging zones of the charger and the ion penetration through the perforated screen opening on the inner electrode of the charger. It was found from experimental results that both positive and negative charging currents in the charging zone of the charger increased with increasing corona and ion-driving voltages. At the same corona and ion-driving voltages, both positive and negative coronas were decreased with increasing diameter of the corona-wire. Compared with the corona-wire of 22 mm in length, the magnitude of both positive and negative charging currents were markedly higher for corona-wire of 11 mm in length at the same corona voltage. It was found that the charging currents for negative coronas were about 1.2 times higher than those positive coronas at the same corona and ion-driving voltages. Numerical results of the electric field distribution and the ion and charged particles migrations in the discharge and charging zones of the charger is correlated to have the same direction with the experimental results of the current–voltage relationships. Also, this can be used to guidance in describing the electric field distribution and the behavior of ion and charged particle trajectories that cannot be seen from experiments in order to improve the applicably design and refinement of a unipolar cylindrical tri-axial charger.     

Effect of dipolar and exchange interactions on magnetic blocking of maghemite nanoparticles

Magnetic interparticle interactions compete with the magnetic blocking of ultrafine magnetic nanoparticles. We have prepared maghemite (γ-Fe₂O₃) nanoparticles by microwave plasma synthesis as a loose powder and in compacted form. In ZFC/FC measurements, blocking temperature of the compacted sample C is larger than that of the powder sample P. The frequency dependence of AC susceptibility of the sample C shows a large shift of blocking temperature with increasing frequency. Vogel-Fulcher law gives a large value of T₀ for the sample C. To get evidence of a possible spin-glass freezing in both samples, scaling law fitting is applied to the AC susceptibility data. The value of the exponent (zv) of the critical slowing down dynamics fits to the spin-glass regime for both samples. For the sample P, spin-glass freezing occurs on the surface of individual nanoparticles, while in the sample C surface spin-glass freezing is concomitant with a superspin-glass formation as a consequence of coupling between particles. The sample C also shows an enhancement of coercivity due to dipolar interactions among the nanoparticles. Exchange interactions are attributed only to touching nanoparticles across their interfaces. All these measurements indicate the presence of strong interparticle dipolar interactions in the compacted sample C.     

Explaining the impact of conductor surface type on wet weather HVDC corona characteristics

Corona of HVDC overhead-lines is important in the planning of transmission corridors. Effects that need to be considered include corona losses (CL), audible-noise (AN), radio-interference (RI) and ground level ion currents. Water on conductor surfaces influences these quantities. It was previously shown that certain conductor surfaces lead to considerable reductions in wet weather CL. This publication studies the causes of these differences using optical methods. A key finding is that the size of water drops is decisive for differences in CL. Furthermore, different discharge mechanisms are shown to be the reason for differences in AN during and after rain showers.     

Modulation effect produced by adjacent AC voltage on the characteristics of negative corona current pulses in a wire-cylinder electrode configuration

In this paper, a wire-cylinder electrode system was designed to study the negative DC corona pulses due to the influence of AC voltage applied to an adjacent conductor. The corona onset voltage and distribution pattern and the parameters of the corona current pulses such as the rise time, pulse duration and the average pulse amplitude are systematically analyzed. The AC voltage is found to have an interesting modulation effect on the time interval of the current pulses. At last, based on a simplified ion cloud model, a full explanation is given to reveal the mechanism behind the influence of the AC voltage on negative DC corona.     

生长方向对量子点应变与应变弛豫的影响

由于材料弹性的各向异性与表面能的各向异性, 不同的生长方向或生长面, 量子点有不同的力学性能与行为. 本文基于各向异性弹性理论的有限元方法, 以金字塔型自组织InAs/GaAs半导体量子点为研究对象, 分别在7个常见的生长方向或生长面上, 对其应变能和应变弛豫能、自由能等进行了分析计算, 得到了这些能量随生长方向的变化规律. 结果表明(211)量子点应变弛豫能最大, 而(100)量子点应变弛豫能最小. 这些结果可为可控制备量子点提供理论参考. 关键词： 量子点 生长方向 平衡形态 应变弛豫     

Effect of pollution severity class and service year on corona characteristics and electromagnetic environment degradation of aged conductors

Corona discharge and electromagnetic environment on AC long-term operated conductors are important issues for extra-high-voltage transmission lines. To investigate the relation between conductor age and electromagnetic environment, including audible noise and radio noise, several groups of long-term operated conductors in China are selected for corona cage experiment. According to corona tests, corona discharge becomes more serious as the applied voltage improves. Difference of electromagnetic environment between conductors of different operated year increases. Corona discharge and electromagnetic noise become more serious as the service year lengthens. When the operated years are close, the general environment is important in electromagnetic environment degradation.     

Laboratory ignition of hydrogen and carbon disulphide in the atmospheric air by positive corona discharge

Experimental attempts of ignition of sensitive explosive atmospheres by continuous positive corona discharges in coaxial electrode system were conducted in laboratory conditions. Sixty five explosions of hydrogen (H₂) and forty of carbon disulphide (CS₂) were forced. Both atmospheres were ignited by the minimum power 1–2 W, the minimum ignition current 100–130 μA at the ignition voltages 12–30 kV (for CS₂) and 16.5–25 kV (H₂). To prevent the energetic sparks, the high voltage resistor 1 MΩ was introduced in series with corona wire.     

Experimental and modelling study of the dependence of corona discharge on electrode geometry and ambient electric field

The results of experimental observations and mathematical modeling of corona formation on the tips of grounded rods are presented as a function of their tip height, curvature radius, the magnitude and polarity of the applied electric field producing corona. The investigations demonstrate that corona current depends on the active volume of zone in which electric field strength exceeds the breakdown criteria for air. The mathematical model was verified with the experimental data, enabling dependence of corona current on rod tip height, tip radius and applied electric field strength to be quantified with the need for a plethora of experiments.     

Simultaneous extraction of multiple orientational constraints of membrane proteins by 13C-detected N-H dipolar couplings under magic angle spinning

A 13C-detected N-H dipolar coupling technique is introduced for uniaxially mobile membrane proteins for orientation determination using unoriented samples. For proteins undergoing rigid-body uniaxial rotation in the lipid bilayer, the intrinsic equality between the dipolar coupling constants measured in unoriented samples and the anisotropic coupling measured in static oriented samples has been shown recently. Here, we demonstrate that the orientation-sensitive backbone N-H dipolar couplings can be measured with 13C detection using 2D and 3D MAS correlation experiments, so that maximal site resolution can be achieved and multiple orientational constraints can be extracted from each experiment. We demonstrate this technique on the M2 transmembrane peptide of the influenza A virus, where the N-H dipolar couplings of various residues fit to a dipolar wave for a helical tilt angle of 37 degrees , in excellent agreement with data obtained from singly 15N-labeled samples.     

Investigations of polymer dynamics in nanoporous media by field cycling NMR relaxometry and the dipolar correlation effect

The chain dynamics of short-chain perfluoropolyether melts confined in Vycor nanoporous media has been characterized by field cycling nuclear magnetic resonance relaxometry and the dipolar correlation effect. The slowdown of motions under confinement, leading to larger residual dipolar couplings, has been probed by looking at the quotient of stimulated and primary echoes. Using field cycling relaxometry, it has been shown that there is strong evidence of reptation-like motion, even for such short-chain polymers as shown by the frequency and molecular weight dependences of the spin-lattice relaxation time.     

电子散射的钠原子受激态取向参数研究

原子碰撞中的取向(orientation)参数的研究为原子碰撞动力学、原子受激态结构提供了丰富的信息.散射靶原子集合可用态多极(state multipole)描述，用它可以表征受激态原子的取向参数.通过取向参数与受激原子态退激的偶极辐射光子的Stokes参数之间的关系，可以进行实验与理论的比较.本文主要研究钠原子受电子散射S→P跃迁中取向参数，根据散射理论的扭曲波近似展开计算得到的散射振幅，带入态多极，然后计算钠原子3P态取向与散射角的关系及不同入射能下受激态取向参数变化特点，并与电子-光子符合散射实验所测数据进行比较.考虑到符合散射实验的测量困难，在误差范围内，理论分析与实验结果符合得比较好. 关键词： 取向参数 态多极 Stokes参数 电子-光子符合测量     

Influence of corona on strike probability of grounded electrodes by high voltage discharges

This paper presents the results of an investigation into the influence of corona formation processes at the tips of grounded rod electrodes on the probability of those rods being “struck” by high-voltage discharges. Experiments simulating the final stage of the attachment process were carried out with a composite voltage comprising a simultaneously applied impulse and DC potential of different levels to grounded rod electrodes simulating lightning rods, featuring either a spherical or pointed (conical) tip. The experiments show that corona does not influence the probability of the electrode being struck until a critical electric field (EF) strength is reached.     

Determination of structural and dynamic characteristics of biodegradable polymers by the NMR relaxation method

A theory of the NMR spectra of heterogeneous polymer systems is proposed that makes it possible to simulate signals observed over a wide temperature range with account of spectral diffusion. Based on this theory, a technique of rapid analysis of molecular structure parameters, including the degree of crystallinity and NMR line shape, is developed. The degree of crystallinity is demonstrated to be a linear function of the area under the NMR spectrum. It is shown that this dependence, universal for a given substance, makes it possible to reliably determine the degree of crystallinity over a wide temperature range (or the fraction of low-molecular additives in composites). The NMR spectra of chitosan is simulated and compared to experimental data. The universal dependence of the degree of crystallinity on the area under the spectrum of chitosan is calculated. A comparison with experimental signals allowed determining the degree of crystallinity.     

Lamellar orientation on the surface of a polymer determined by ToF-SIMS and AFM

The fold and lateral surfaces of chain-folded lamellae of poly(bisphenol-A-co-etheroctane), containing both aliphatic CH₂ and aromatic segments, were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). At low and high crystallization temperatures, the surfaces of the polymer films were shown to consist mainly of edge-on and flat-on lamellae, respectively. Surfaces with a mixture of edge-on and flat-on lamellae were produced at intermediate temperatures. The edge-on and flat-on lamellae were identified by using ions that recognize the flexible and rigid segments of the polymer. Ion images produced using selected ions that are related to the edge-on or flat-on orientation can be used to identify the location of these lamellae on the polymer surface. Our results indicate that ToF-SIMS can be used to detect different lamellar orientations at the surfaces of semi-crystalline polymers.     

Effects of corona discharge on the surface structure, morphology and properties of multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were modified by corona discharge and then heat treated in the air. The influences of corona discharge parameters such as treatment time and processing power were investigated. The results of energy dispersive X-ray analysis (EDX) and thermogravimetric analysis (TGA) indicated the introduction of oxygen-containing functional groups onto the surface of the MWCNTs after heat treatment. The water contact angle tests showed that the hydrophobicity of the MWCNTs was decreased to some extent. The static water contact angle was reduced from 146° to 122° when the time of the corona discharge treatment reached 3 min at the processing power of 200 W. The enhanced thermomechanical and mechanical properties of epoxy nanocomposites filled with the corona discharge treated MWCNTs were observed. The modified MWCNTs conferred better properties on the composites than the pristine MWCNTs because of the improved dispersion of MWCNTs in matrix and the enhanced interfacial interaction between the treated MWCNTs and epoxy.