Superconducting electrical motorsMoteurs électriques supraconducteurs

Superconductors have always been very promising materials because they offer the possibility of building lighter and more efficient electrical machines than by any other conventional solution. These characteristics make superconducting motors very attractive drives for embarked equipment with low weight requirements. The historical development of superconducting machines is quickly reviewed. The high critical temperature superconductors have renewed the works about superconducting machines. Different motor options are presented (fully superconducting machine and permanent magnet superconducting motor). For each, the choice of the superconductor (low or high critical temperature) is discussed, according to their performance and technological problems, especially about cryogenics. The different solutions are compared through a preliminary electromagnetic design for a 10 MW–100 rpm torque motor. We carry out a research and development program on the permanent magnet superconducting motors. After a first 15 kW–750 rpm successfully tested model a second 150 kW–400 rpm demonstrator is under development.     

Electrical characteristics of a dc superconducting cable

A dc superconducting cable is ideal for transmitting large blocks of electrical power over a long distance. However, it must be designed to operate reliably within the constraints of the electrical system. Therefore, system analysis must be performed for each application. The conductor losses caused by the harmonics on the dc must be within the design goals; a system fault should not drive the cable normal with eventual damage to the cable and interruption of power flow; and, the dielectric system of the cable must be designed to be compatible with the expected transient voltages by proper insulation coordination. Transient overvoltages are of concern to electrical power systems; these are especially critical to cryogenic cables because of the susceptibility of the cryogenic enclosures to these transients. This paper discusses the electrical system constraints which are particularly applicable to a dc superconducting cable and shows how such a cable can be designed to be compatible with the electrical system.This paper also summarizes the work on low temperature dielectrics performed at Los Alamos. It shows the variation of break down voltage of dielectric materials, in sheet form and cable configuration, with temperature and pressure under dc and impulse voltage. The surface flashover characteristics with large creepage distance as well as electrical conductivity of dielectric materials at cryogenic temperatures are discussed. These studies are essential for the design of high voltage apparatus operating in cryogenic environments.     

Development of Superconducting Coaxial Cables for Cryogenic Detectors

Fast readout with low noise is essential to apply superconducting detectors to such experiments as time-of-flight mass spectrometry or X-ray spectroscopy, where high counting efficiency is required. We have developed a thin seamless superconducting coaxial cable employing NbTi alloy in both of the outer and inner electrical conductors. The outer diameter is 1.60 mm, and both conductors are separated by dielectric material of PTFE. The coaxial cable revealed to become superconductive below about 10 K. The thermal conductance was measured between 0.4 and 6 K and consistent with literature. Performance at high frequency was also measured at 4.2 K. The attenuation was very small and less than 1 dB up to about 7 GHz. The effect of mechanical treatment to thermal and electrical properties of NbTi alloy seems to be small in the present forming process of coaxial cable.      

Cu—YBaCuO粉末复合材料组织与性能的研究

预期得到了一种具有较高强度而又不降低导电率的铜基粉末复合材料，选用超导陶瓷粉末ＹＢａＣｕＯ为强化相，以冷等静压，热挤压等粉末冶金方法研制了两种（Ｃｕ－５％ＹＢａＣｕＯ）和Ｃｕ－２０％ＹＢａＣｕＯ粉末复合材料，在常温和液氮温度下测试了这两种复合材料的力学性能与导电性能，研究表明，该复合材料的力学性能与导电性能，研究表明，该复合材料低温（７７Ｋ）时的力学性能优于室温，但导电率并没有改善，其原因在于所加     

液氮低温环境下电阻应变片测试性能的试验研究

分别采用低温电阻应变片及拉线式位移传感器的电测技术,该文开展了液氮浸泡下的悬臂梁结构在静载条件下的应变测量。对两种实验测试结果和理论分析结果进行了对比和精度分析,给出了相应的实验标定曲线,并探讨了测量过程中应变片粘接、电桥连接方式、液氮冷却和数据采集对实验结果精度的影响因素等。结果表明:在采用温度补偿和应变片的正确粘贴和良好固化情形下,低温应变片能够在液氮低温区给出较高精度的应变测量;拉线式位移传感器几乎不受低温的影响,测量简单易行。相关技术和结果将为中科院近代物理研究所自主研制的兰州潘宁离子阱7T超导磁体的低温下应变测量提供方法和指导。     

Evidence for quantum tunneling of vortices in superconductors

Flux creep in disordered superconductors may be governed by quantum tunneling of Abrikosov vortices rather than by thermal activation processes. The expectation is that in the quantum tunneling regime the creep rate would be temperature independent. This assumes that the parameters describing the pinning potential and other aspects of the superconducting films are temperature independent. In the case of extremely thin superconducting films the coherence length retains its temperature dependence well into the quantum tunneling regime, leading to an unusual temperature dependence of the electrical resistance in this regime. This has been observed in ultrathin superconducting films of Pb, Al, and Bi. In low magnetic fields, at low temperatures, sheet resistances vary with temperature as RR₀ exp(T/T₀), where T₀ and R₀ are constants.     

Numerical simulation of the electro-acoustical response of a transducer excited by a time-varying electrical circuit

Existing methods for the modeling of piezoelectric transducer response are generally frequency domain-based. The major disadvantage of this type of model is that they cannot take into account the electrical elements present in the emitting or receiving circuit whose values vary with respect to time. The need for a method that accounts for time-varying elements arises, for example, when the circuit comprises active electrical elements, such as diodes, or when the transducer is excited by capacitive discharge via a switch. Indeed, in this last example, it is known that the output impedance of the generator depends on the state of the switch: if it is off, its value is high; if it is on, its value is low. A time-domain-based method is presented to compute the electro-acoustical response of a piezoelectric transducer and its electrical circuit, taking into account the presence of time-varying elements. An application to a current example makes it possible to show the influence of these elements on waveforms and the capacity of our model to account for them     

Properties and stability of Cu-YBaCuO composites elaborated by extrusion of an elemental powder mixture

To produce mechanical reinforcement of a Cu matrix without any loss of conductivity for low temperature applications, it is proposed to replace classical alumina particles by superconducting particles. This paper describes a first attempt to synthesize copper composites containing 5 wt% YBaCuO and 20 wt% YBaCuO by using a powder processing route. The resulting mechanical and electrical properties are related comparatively to usual ODS Cu alloys. It is shown that a crucial difficulty remaining to be cleared up is the decomposition of the orthorhombic YBa₂Cu₃O_7–x superconducting phase occurring during the elaboration process. Addition of an in situ oxygen donor, such as Cu₂O oxides in the initial powder mixture, appears insufficient to stabilize the oxygen stoichiometry of the superconducting particles. Conclusions are drawn as a guideline for further process designing.     

Ferroelectric Exchange Bias Affects Interfacial Electronic States

In polar oxide interfaces phenomena such as superconductivity, magnetism, 1D conductivity, and quantum Hall states can emerge at the polar discontinuity. Combining controllable ferroelectricity at such interfaces can affect the superconducting properties and sheds light on the mutual effects between the polar oxide and the ferroelectric oxide. Here, the interface between the polar oxide LaAlO₃ and the ferroelectric Ca-doped SrTiO₃ is studied by means of electrical transport combined with local imaging of the current flow with the use of scanning a superconducting quantum interference device (SQUID). Anomalous behavior of the interface resistivity is observed at low temperatures. The scanning SQUID maps of the current flow suggest that this behavior originates from an intrinsic bias induced by the polar LaAlO₃ layer. Such intrinsic bias combined with ferroelectricity can constrain the possible structural domain tiling near the interface. The use of this intrinsic bias is recommended as a method of controlling and tuning the initial state of ferroelectric materials by the design of the polar structure. The hysteretic dependence of the normal and the superconducting state properties on gate voltage can be utilized in multifaceted controllable memory devices.     

Piezoceramics for high-frequency (20 to 100 MHz) single-elementimaging transducers

The performance of transducers operating at high frequencies is greatly influenced by the properties of the piezoelectric materials used in their fabrication. Selection of an appropriate material for a transducer is based on many factors, including material properties, transducer area, and operating frequency. The properties of a number of piezoceramic materials have been experimentally determined by measuring the electrical impedance of air-loaded resonators whose thickness corresponds to resonance frequencies from 10 to 100 MHz. Materials measured include commercially available compositions of lead zirconate titanate (PZT) with relatively high dielectric constants and a modified lead titanate (PT) composition with a much lower dielectric constant. In addition, materials which have been designed or modified to result in improved properties at high frequencies are studied. Conclusions concerning the influence of the microstructure and composition on the frequency dependence of the material properties are made from the calculated properties and microstructural analysis of each material. Issues which affect transducer performance are discussed in relation to the properties. For transducers larger than about 1 mm in diameter, the use of a lower dielectric constant material is shown to result in a better electrical match between the transducer and a standard 50 Ω termination. For transducers whose impedance is close to that of the connecting cables and electrical termination, equivalent circuit model simulations show improved performance without the need for electrical matching networks. Measurements of fabricated transducers show close agreement with the simulations, validating the measurements and showing the performance benefits of electrically matched transducers     

Superconductivity on Interfaces of Nonsuperconducting Granules La<Subscript>2</Subscript>CuO<Subscript>4</Subscript> and La<Subscript>1.56</Subscript>Sr<Subscript>0.44</Subscript>CuO<Subscript>4</Subscript>

Composite materials fabricated by annealing of nonsuperconducting ceramics La₂CuO₄ and La_1.56Sr_0.44CuO₄ at 910 °C during various time are investigated. Areas of superconducting La_1.85Sr_0.15CuO₄ phase arises at boundaries of contacting nonsuperconducting granules. The volume fraction of the superconducting phase increases with increasing annealing time. A model describing the magnetic and transport properties of the samples at low magnetic fields is constructed. The magnetotransport characteristics of obtained samples at low magnetic fields (～ 100 Oe) are defined by weak links network formed by superconducting areas. At high fields, behavior of the system is defined by a magnetization of the disconnected superconducting islands. The average size of the superconducting areas has been estimated from an extended critical state model.     

Ultrasonic imaging using a 5-MHz multilayer/single-layer hybrid array for increased signal-to-noise ratio

Conventional diagnostic ultrasound scanners are bulky and require significant amounts of electrical power during operation. Reducing the size, weight, and consumption of electrical power is made easier through the use of highly integrated compact transmit and receive electronics that may be incorporated in the transducer handle. This necessitates the use of low voltage transmitters and low power receive preamplifiers. Conventional scanners typically use approximately 100-V pulses during transmit; therefore, decreasing the transmit voltage to 15 V decreases the transmit sensitivity. Conventional receive electronics that are located at the scanner degrade the received signal-to-noise ratio (SNR) because the array element cannot efficiently drive the coaxial cable. Transmit sensitivity and received SNR can be radically improved using a multilayer/single-layer hybrid array making integration of electronics into the transducer handle more feasible. In this paper, we discuss the design, fabrication, and testing of a 5-MHz hybrid linear array. The hybrid array included 16 multilayer transmit elements (10 Omega impedance) and 24 single-layer receive elements at a half wavelength element pitch. Low voltage transmitters with an output resistance of 7 Omega and high impedance JFET preamplifiers using 15 V for biasing were located adjacent to the hybrid array in the transducer handle. The transmit sensitivity and received SNR of the hybrid array were compared with a conventional array using 50-Omega transmitters and receive preamplifiers at the scanner. The transmit sensitivity improved by 12.8 dB, and the received SNR improved by 7.8 dB, yielding an overall improvement of 20.6 dB, which compared well with predictions from the KLM model. Images of phantoms and in vivo images of the kidney obtained with the Siemens Model 1200 phased array system showed the increased SNR using the hybrid array. Estimates of penetration in tissue mimicking phantoms (alpha=0.5 dB/(cm MHz)) improved by 7 cm compared with the control.     

Investigation of High Voltage Discharges in Low Pressure Gases Through Large Ceramic Superconducting Electrodes

A device has been built and tested, in which a ceramic superconducting cathode and a copper anode cause electrical discharges in low pressure gases, at temperatures between 50 and 70 K. The electrodes are connected to a capacitors array charged up to 2000 kV; peak currents are of the order of 10⁴ A. The cathode has the diameter of 10 cm and is fabricated by OCMTG technology. In discharges at voltage above 500 kV two new phenomena were observed, probably related to each other. First, the discharge does not look like a spark, but is a flat, glowing discharge, which originates from the whole surface of the superconducting electrode. Furthermore, a radiation pulse is emitted at the discharge, which propagates orthogonally to the cathode, towards the anode and beyond it, in a collimated beam, apparently without any attenuation. The radiation pulse carries an energy of 10^–3 J at least. The features and the nature of this radiation have been investigated by several means, still it was not possible to identify it; we can only exclude that it is electromagnetic radiation or any other radiation with energy-momentum relationship E=cp.     

Noise analysis in air-coupled PVDF ultrasonic sensors

In this paper we analyze the noise generated in a piezo-polymer based sensor for low frequency ultrasound in air. The sensor includes two curved PVDF transducers for medium and short range applications. A lumped RLC equivalent circuit was derived from the measurement of the transducer's electrical admittance, in air, by taking into account both mechanical and dielectric losses, which we suppose are the major sources of noise in similar devices. The electrical model was used to study and optimize the noise performance of a 61 kHz transducer and to simulate the electrical behavior of the complete transmitter-receiver system. The validity of the overall electrical model with low noise was confirmed after verifying, with Pspice, agreement of the practical and theoretical results.     

Increased homogenous clusters in superconducting paths with diffusion of optimum Ni impurities into Bi-2223 crystal

This study deals with variations of electrical and superconducting features of Bi-2223 superconducting materials exposed to Ni impurity diffusion at different annealing temperatures (650 °C?≤?T?≤?850 °C) by temperature-dependent resistivity measurements. It is found that the characteristic properties improve with annealing temperature up to 700 °C as a result of enhancement in the truly-metallic characteristics, interaction quality, formation of Cooper-pairs and overlapping of Cu-3d and O-2p wave functions. Similarly, the optimum annealing temperature of 700 °C diminishes the omnipresent flaws and structural defects. Additionally, we design a strong theory (Percolation) to discuss the role of nickel impurities on fundamental aspects of material science and physical quantities as regards stabilization of superconductivity in the homogeneous regions and formation of superconducting clusters in the paths for the first time. Further, we develop an empirical relationship between the structural problems and transition temperatures to obtain a superconductor exhibiting the highest electrical and superconducting features.     

几种金属间化合物电学性能的研究进展与展望

金属间化合物具有高强轻质及电阻率较高的特点,是一种极具发展前途的功能材料。影响其电学性能的因素有添加元素、合金化元素的原子半径、电子层结构、金属键的数量及晶体结构的无序性。总结了不同类型金属间化合物的电学性能方面的研究成果,同时介绍了其作为超导材料、离子电池的负极材料、电热元件、半导体材料的应用,并展望了以后的研究和发展方向。     

Design and fabrication of a 40-MHz annular array transducer

This paper investigates the feasibility of fabricating a five-ring, focused annular array transducer operating at 40 MHz. The active piezoelectric material of the transducer was a 9-microm thick polyvinylidene fluoride (PVDF) film. One side of the PVDF was metallized with gold and forms the ground plane of the transducer. The array pattern of the transducer and electrical traces to each annulus were formed on a copper-clad polyimide film. The PVDF and polyimide were bonded with a thin layer of epoxy, pressed into a spherically curved shape, then back filled with epoxy. A five-ring transducer with equal area elements and 100-microm kerfs between annuli was fabricated and tested. The transducer had a total aperture of 6 mm and a geometric focus of 12 mm. The pulse/echo response from a quartz plate located at the geometric focus, two-way insertion loss (IL), complex impedance, electrical crosstalk, and lateral beamwidth all were measured for each annulus. The complex impedance data from each element were used to perform electrical matching, and the measurements were repeated. After impedance matching; fc approximately equal to 36 MHz and -6-dB bandwidths ranged from 31 to 39%. The ILs for the matched annuli ranged from -28 to -38 dB.     

A low-loss SAW-TV-IF filter with an extended impedance matching range

A novel low-loss SAW (surface acoustic wave) filter for an intermediate frequency (IF) circuit in a color TV receiver has been developed. It consists of an apodized bidirectional and an unapodized group-type unidirectional transducer. The unidirectional transducer is designed to use different numbers of finger pairs in sending and reflecting electrodes for extension of the impedance-matching range. A thin-film capacitor for use as a phase shifter is monolithically fabricated on a 128 degrees Y-X LiNbO(3) substrate. A low insertion loss (11.3 dB) and impedance matching without adjustment are achieved at the same time without increasing the device chip size or number of electrical parts.     

Ultrasonic Transducer Sensitivity and Model-Based Transducer Characterization

Abstract

It is shown that the role that an ultrasonic piezoelectric transducer plays in both generating and receiving ultrasound in an ultrasonic nondestructive evaluation (NDE) measurement system can be completely described in terms of the transducer's electrical impedance and open-circuit, blocked force receiving sensitivity. Furthermore, it is shown that both of these quantities can be obtained experimentally via a model-based approach and purely electrical measurements. The measurement of sensitivity uses a method originally developed for lower-frequency acoustic transducers. However, it is shown that at the higher frequencies found in ultrasonic NDE applications electrical cabling effects play an important role and must be compensated for in determining the transducer sensitivity. Examples of experimental measurement results using these new approaches are given.     

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15–δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr₂Ba₄Cu₇O_15?δ (Pr247) showing a superconducting transition at T_c=10–16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T² dependence up to approximately 150 K. A clear specific heat anomaly was observed at T_c for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature T_n of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μ_B. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^α dependence (α=0.08–0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.

PACS: 74.72.Jt; 74.25.Bt; 75.40.Cx; 74.25.Fy