HTS Joint Resistance for High-Field Magnets: Experiment and Temperature-Dependent Modeling

Journal of Superconductivity and Novel Magnetism - Benefiting from the high critical temperature and high critical current, the second-generation high-temperature superconducting (HTS) tape is...     

Temperature Stability and Flux Losses Over Time in Sintered Nd–Fe–B Permanent Magnets

Demagnetization occurring with time in a constant temperature is often assumed to be negligible, provided that the operating point of the magnet is above the knee of the $Bhbox{–}H$ curve. However, because it is important to ensure that no permanent flux losses occur in permanent magnets during their use in industrial applications, there is a clear need to quantify these time-dependent losses. We measured losses over time for four commercial sintered Nd–Fe–B magnet grades at five different temperatures (23$,^{circ}$ C–150$,^{circ}$ C). We tested samples with three different permeance coefficient (Pc) values for each material. The time-dependent losses measured fitted the logarithmic law of magnetic viscosity well. We demonstrated that the total flux loss in a lifetime of 30 years can be estimated according to the temperature, coercivity of the material, and the permeance coefficient of the magnet. With proper selection of the magnet material, in accordance with the designed Pc of the application, the total flux loss in 30 years can be minimized almost to zero even at 150$,^{circ}$ C.      

High Energy and High Coercivity Sintered NdFeB Magnets by Low Oxygen Process

1. IntroductionSince NdFeB magnets were discovered[ll, theirmagnetic performance has been improved by many researchers. The maximum energy product of sinteredNdFeB magnets has been increased from 240 kJ/m'to 430 kJ/m' by Kaneko et al.[2] in 1993. The maximtun energy product commercially available todayis close to 358 kJ/m'. Because of this high performance of NdFeB products, the applications have increased enormously in the last decade. For example,hard disk drivers (HDD), magnetic reso…     

基于双环永磁体结构的多稳态能量采集器建模及性能分析

宽频带能量采集已经成为当前国际上振动能量采集的研究热点之一.提出基于双环永磁体结构的非线性多稳态能量采集器.采用库伦模型建立双环永磁体磁力数学模型,分析外环永磁体结构参数对能量采集装置势能函数的影响规律,确定能量采集器在单稳态、双稳态到三稳态之间变化时外环永磁体结构参数范围.通过数值仿真分析能量采集器处于双稳态和三稳态...     

Measurement and Modeling of Rotational Core Losses of Soft Magnetic Materials Used in Electrical Machines: A Review

In many situations, for example, in the cores of a rotating electrical machine and the T-joints of multiphase transformers, the magnetic flux varies with time in terms of both magnitude and direction, i.e., the local flux density vector rotates with varying magnitude and varying speed. Therefore, it is important that the magnetic properties of the core materials under various rotational magnetizations be properly investigated, modeled, and applied in the design and analysis of electromagnetic devices with rotational flux. Drawing from the huge amount of papers published by various researchers in the past century, this paper presents an extensive survey on the measurement and modeling of rotational core losses of soft magnetic materials used in electrical machines, particularly from the view of practical engineering application. The paper aims to provide a broad picture of the historical development of measuring techniques, measuring apparatus, and practical models of rotational core losses.     

General Properties of Low-frequency Power Losses in. Fe-based Nanocrystalline Soft Magnetic Alloys

1. IotroductlonAs early as 1969, it was reported that the dynamicloss at 50 Hi in the high-grade grain-oriented 3%SiFereaches about 75% of the practical measured total lossP and is much larger than the so--called classical eddycurrent loss Pc,l']; It was also observed that the total loss per cycle p/f nonlinearly increases with increasing frequency f in the low frequency range notmore than 100 Hi[1]. The above mentioned nonlineardependences have early been observed in many crystalline and am…     

25th Anniversary Article: A Soft Future: From Robots and Sensor Skin to Energy Harvesters

Scientists are exploring elastic and soft forms of robots, electronic skin and energy harvesters, dreaming to mimic nature and to enable novel applications in wide fields, from consumer and mobile appliances to biomedical systems, sports and healthcare. All conceivable classes of materials with a wide range of mechanical, physical and chemical properties are employed, from liquids and gels to organic and inorganic solids. Functionalities never seen before are achieved. In this review we discuss soft robots which allow actuation with several degrees of freedom. We show that different actuation mechanisms lead to similar actuators, capable of complex and smooth movements in 3d space. We introduce latest research examples in sensor skin development and discuss ultraflexible electronic circuits, light emitting diodes and solar cells as examples. Additional functionalities of sensor skin, such as visual sensors inspired by animal eyes, camouflage, self‐cleaning and healing and on‐skin energy storage and generation are briefly reviewed. Finally, we discuss a paradigm change in energy harvesting, away from hard energy generators to soft ones based on dielectric elastomers. Such systems are shown to work with high energy of conversion, making them potentially interesting for harvesting mechanical energy from human gait, winds and ocean waves.     

Thermal Contact Resistance: Experiment Versus Theory

With a knowledge of the thermal resistance value associated with an asperity, a model for the thermal contact resistance of a given interface is obtained by considering that each asperity of surfaces in contact is a flux tube. Calculation of any of the thermal conductances depends on the contact radius of each asperity. This radius increases with load, as asperities are compressed. Contact area is calculated for a plastic load. Values of thermal contact conductance (or resistance) are compared with experimental results.     

聚酰亚胺微悬臂梁理论及实验

设计制作了一种电磁驱动式悬臂梁．为降低驱动电流,选择柔韧性较好的聚酰亚胺（PI）作为梁结构材料,其优良的绝缘性和耐热性保证了驱动线圈的功能实现．通过理论计算及有限元模拟仿真对悬臂梁动力响应及结构模态做了系统分析;根据分析结果,优化得到悬臂梁的结构参数．介绍了悬臂梁的制作工艺及流程,通过PI刻蚀、A7A903光刻胶光刻、电铸等工艺过程,制作出PI悬臂梁及其末端驱动线圈．     

Thermal Diffusion Through Multilayer Coatings: Theory and Experiment

Thermal diffusion through aluminum oxide/molybdenum multilayers (10, 20, and 40 bilayers on molybdenum substrates) was studied through thermal pulse experiments. In these experiments, a thermal pulse (<50 ns) was delivered by a laser to the front surface of the specimen and the resulting temperature transient on the rear surface, typically several degrees, was recorded. An integral solution for the equations governing heat transfer was used to relate experimental results to the thermal resistance of the interfaces. Experimental studies were conducted at temperatures from 950 to 1150°C, relevant to thermal barrier applications. Significant densification of the oxide layers was observed during the experiments at all but the lowest temperature. As a result of these experiments, an upper bound has been placed on the interface thermal resistance.     

Renewal Rate of Filament Lamps: Theory and Experiment

We calculate the renewal rate of fused lamps in an organization having large installation of filament lamps. This is achieved by using the standard mortality curve over the specified fractions of the half-life and setting up an algebraic relation for the renewed fraction at successive stages. The procedure for data analysis is described in detail for an actual experiment performed by us.     

Generalized Dispersion Equations of Plasma: Theory and Experiment

A correspondence is established between the classical experimental data of Looney and Brown and the theoretical results found within the framework of generalized Boltzmann physical kinetics.     

Light-induced Distributed Feedback in a Quasi-waveguide Thin-film Laser: Theory and Experiment

Abstract

The results of theoretical and experimental investigations of the quasi-waveguide laser system with light-induced distributed feedback (DFB) are given. The DFB theory in the system is developed. The threshold and spectral angular characteristics of operation are calculated. Frequency-tuneable operation of the system in single-mode and multimode regimes have been investigated.     

Evidence for Polaron Formation in High-Temperature Superconducting Cuprates: Experiment and Theory

We provide compelling support for the key role played by polaron formation to the physics of cuprate superconductors, which is evidenced above the pseudogap temperature T ^*, is the origin of the pseudogap phase itself and persists in the superconducting phase. Experimental and theoretical results are compared and show convincing agreement with each other.     

Quantum Griffiths Effects and Smeared Phase Transitions in Metals: Theory and Experiment

In this paper, we review theoretical and experimental research on rare region effects at quantum phase transitions in disordered itinerant electron systems. After summarizing a few basic concepts about phase transitions in the presence of quenched randomness, we introduce the idea of rare regions and discuss their importance. We then analyze in detail the different phenomena that can arise at magnetic quantum phase transitions in disordered metals, including quantum Griffiths singularities, smeared phase transitions, and cluster-glass formation. For each scenario, we discuss the resulting phase diagram and summarize the behavior of various observables. We then review several recent experiments that provide examples of these rare region phenomena. We conclude by discussing limitations of current approaches and open questions.     

Spin Down of Superfluid-Filled Vessels: Theory Versus Experiment

The spin up of helium II is studied by calculating the spin-down recovery of a superfluid-filled container after an impulsive acceleration and comparing with experiments. The calculation takes advantage of a recently published analytic solution for the spin up of a Hall-Vinen-Bekharevich-Khalatnikov superfluid that treats the back-reaction torque exerted by the viscous component self-consistently in arbitrary geometry for the first time. Excellent agreement at the 0.5% level is obtained for experiments at T=1.57 K, after correcting for the non-uniform rotation in the initial state, confirming that vortex tension and pinning (which are omitted from the theory) play a minimal role under certain conditions (small Rossby number, smooth walls). The dependence of the spin-down time on temperature and the mass fraction of the viscous component are also investigated. Closer to the lambda point, the predicted onset of turbulence invalidates the linear Ekman theory.     

Viscosity of Cobalt Melt: Experiment,Simulation, and Theory

The results of experimental measurements, molecular dynamics simulation, and theoretical calculations of the viscosity of a cobalt melt in a temperature range of 1400–2000 K at a pressure p = 1.5 bar corresponding to an overcooled melt at temperatures of 1400–1768 K and an equilibrium melt with temperatures from the range 1768–2000 K are presented. Theoretical expressions for the spectral density of the timedependent correlation function of the stress tensor \(\tilde S\)(ω) and kinematic viscosity ν determined from the frequency and thermodynamic parameters of the system are obtained. The temperature dependences of the kinematic viscosity for the cobalt melt are determined experimentally by the torsional oscillation method; numerically, based on molecular simulation data with the EAM potential via subsequent analysis of the time correlation functions of the transverse current in the framework of generalized hydrodynamics; and by the integral Kubo–Green relation; they were also determined theoretically with the Zwanzig–Mori memory functions formalism using a self-consistent approach. Good agreement was found between the results of theoretical calculations for the temperature dependence of the kinematic viscosity of the cobalt melt using experimental data and the molecular dynamics simulation results. From an analysis of the temperature dependence of the viscosity, we obtain an activation energy of E = (5.38 ± 0.02) ×10^–20 J.