Designing high temperature superconductor magnets

Quasi-permanent magnets made of melt textured yttrium barium copper oxide (YBCO) superconductor can now trap multi-Tesla fields and retain that field semi-indefinitely. Because the fields so trapped achieve near zero creepage decay, they can be used as replacements of permanent magnets. This paper investigates the fields expected from these magnets, their prediction, and design expectations when they are incorporated into large scale arrays     

Reliability of high temperature superconductor metal contacts

A thermodynamic criterion has been applied to determine the reliability of contact metals such as silver, aluminium, gold, chromium, and titanium on bismuth and yttrium based high temperature superconductors (HTSC). The chemical stability is presumed for positive heat of reactions, i.e., ΔH_reaction > 0, when a metal reacts with all the possible oxides present in the superconducting material. Based on these calculations, metals for reliable contacts to HTSCs are proposed.     

Low frequency voltage noise in high temperature superconductor Josephson junctions

The origin of 1/f voltage noise in different types of Josephson junctions fabricated from the high temperature superconductors (HTS) have been traced back to the trapping and release of charge carriers in trapping centers in an insulating barrier giving rise to correlated fluctuations of the junction critical current I_c and normal state resistance R_n. For the normalized fluctuations S_I and S_R a linear scaling with R_n has been observed which suggests an almost constant density of trapping centers for all investigated HTS Josephson junctions. Using this linear scaling we have made an approximate calculation of the density of the trapping centers.     

Hybrid high temperature superconductor/GaAs 10 GHz microwaveoscillator: temperature and bias effects

Hybrid YBa₂Cu₃O_7-x superconductor/GaAs microwave oscillators have been designed, fabricated and characterized. The planar oscillators were built on a single 10 mm×10 mm LaAlO₃ substrate. The active elements in the hybrid oscillators were GaAs MESFETs. A ring resonator was used to select and stabilize the frequency. A superconducting ring resonator had a loaded Q at 77 Kg which was 8 times larger than the loaded Q of a ring resonator fabricated out of copper. S-parameters of the GaAs FET were measured at cryogenic temperatures and used to design the oscillator which had a reflection mode configuration. The transmission lines, RF chokes and bias lines were all fabricated from YBa₂Cu₃ O_7-x superconducting thin films. The performance of the oscillators was measured as a function of temperature. The rate of change of the frequency as a function of temperature was smaller for an oscillator patterned from a pulsed laser deposited film than for an oscillator patterned from a sputtered film. As a function of bias at 77 K, the best circuit had an output power of 11.5 dBm and a maximum efficiency of 11.7% The power of the second harmonic was 25 dB to 35 dB below that of the fundamental, for every circuit. At 77 K, the best phase noise of the superconducting oscillators was 68 dBc/Hz at an offset frequency of 10 kHz and less than -93 dBc/Hz at an offset frequency of 100 kHz. At an offset frequency of 10 kHz, the superconducting oscillator had 12 dB less phase noise than the copper oscillator at 77 K     

新型高性能微晶玻璃折叠腔线偏振He-Ne激光器

介绍了一种新型微晶玻璃折叠腔线偏振He-Ne激光器的基本结构与制作工艺。该激光器的腔体材料采用一块外形尺寸为159 mm×80 mm×45 mm的极低膨胀系数微晶玻璃,采用W形折叠光路形式,总腔长为636 mm,增益长度为454 mm。新型激光器在结构设计上结合了全内腔与半外腔He-Ne激光器的双重优点,所有反射镜均采用石英镜片光胶连接,电极与布儒斯特窗片采用无应力超高真空铟封形式,激光器工作于低电压、小电流、低功耗的工作模式,谐振腔稳定性好。实验测试了工作电流与输出功率之间的关系,测试结果表明,该激光器的高阶模最大输出功率为14.9 mW,TEM00模最大输出功率为9.65 mW。     

Possible high-frequency cavity and waveguide applications of high temperature superconductors

Conventional superconducting materials used at liquid helium temperatures have been employed to greatly reduce ohmic power loss in microwave cavities. If suitable high temperature superconductor surfaces can be developed, it could be possible to alleviate constraints due to relatively low energy gaps and limits due to critical fields, and operate at more convenient temperatures with larger thermal margins. These features could be used to improve performance of present superconducting microwave cavity devices. They may also facilitate new applications and device designs at millimeter-wave and far infrared frequencies. Possible cavity and waveguide applications are described. Sensitivity of RF surface resistance to boundaries and nonaligned grains in the material and to small amounts of nonsuperconducting material may be an obstacle to development. Moreover, critical RF fields can be limiting for high power applications. An additional problem is the adverse effect of D.C. magnetic fields.     

Powder processing of high temperature ceramic superconductor Tl2Ba2Ca2Cu3O10

We have developed a technique to produce high quality Tl₂Ba₂Ca₂Cu₃O₁₀ powders used for making superconducting wire, tape, lead, shield, and other large scale bulk applications. Starting with T1₂O₃, BaO₂, CaO, and CuO, we mix and grind these chemicals with a machine ball mill and then press the ground mixture into pellets. The pellets are sintered at about 895‡C for at least 30 h in an oxygen atmosphere. The sintered material is mainly the Tl₂Ba₂Ca₂Cu₃O₁₀ compound. To get more homogeneous superconductor powders, we pulverize the sintered material and use a magnetic superconducting material selector to separate and grade the material. Finally, the top grade material has a phase purity of <98% and a T_c(r < 0) of 123–126K.     

光控高温超导衰减器及光斑偏移对其光响应特性的影响

研制了工作于液氮温度的高温超导系统使用的新型光控高温超导微波可变衰减器.该衰减器的实质是利用钇钡铜氧(YBCO)高温超导薄膜极低的微波表面电阻和卓越的激光响应特性,达到了优良的衰减性能.主要结果包括:可独立使用的光控高温超导微波可变衰减器样品尺寸为12 mm×8 mm×0.5 mm,与高温超导系统集成时无需外壳,体积与重量将大幅减小;高温超导衰减器的插入损耗小于0.2 dB,比常规衰减器低1个数量级;高温超导衰减器的可变衰减精度小于0.01 dB,比常规衰减器至少低1～2个数量级;在本实验条件下,当激光光斑中心偏移微带线中心0.3、0.5和0.7 mm时,其衰减器插损将由0.08 dB变为0.05、0.03和0.01 dB,说明激光光斑偏移量是影响YBCO高温超导衰减器性能的重要因素.     

Laterally resolved microwave surface-resistance measurement ofhigh-Tc superconductor samples by cavity substitutiontechnique

Microwave surface impedance measurement of a high-temperature superconductor (HTS) is a sensitive probe to test its quality, particularly if a microwave device is to be fabricated. Most microwave characterization employs resonance techniques in which the components of the surface impedance are extracted from the measured Q value and the shift in the resonance frequency. In this paper, we present a modification of the widely used complete end-plate substitution technique to measure the surface resistance of samples having dimensions smaller than the dimension of the end plate at 20 GHz, as well as to facilitate the laterally resolved surface resistance measurement of large-area HTS samples. From the knowledge of the electromagnetic-field configuration in a TE₀₁₁-mode cylindrical cavity, the loss contribution from the HTS sample is analyzed theoretically and measured experimentally in the temperature range of 20-100 K. The design of the cavity is discussed to optimize the sensitivity of the measurement by the placement of the sample and to maximize the difference in the measured Q value     

Microwave cavity perturbation technique: Part III: Applications

The resonant cavity perturbation method as described in the preceding two parts of this series is applied to study the electrodynamical properties of different materials in the microwave and millimeter wave spectral range. We briefly discuss the relevant uncertainties which are asociated with the different measurement techniques and we find that employing the amplitude technique it is possible to measure both the width and frequency to nearly the same precision. We then demonstrate the broad range of applicability of this technique by showing results obtained on several different materials, ranging from an insulator to a superconductor. The performance limitations of this technique are discussed in detail.     

Microwave cavity perturbation technique: Part I: Principles

This report reviews the analysis used to extract the complex conductivity of a compound from a microwave cavity perturbation measurement. We intend to present a generalized treatment valid for any spheroidally shaped sample of arbitrary conductivity which is placed at either the electric or magnetic field antinode of the cavity. To begin with, we establish the relationship between the measured parameters and the conductivity for a spherical sample. Next, we extend these results to the case of spheroids; and for the first time, we cover all different configurations that one can possibly use to study an arbitrary conducting sample inside a cavity: in particular, all possible orientations of the sample with respect to the applied field are solved.     

Microwave cavity perturbation technique: Part II: Experimental scheme

In this paper, the second in a three part series, we describe an experimental scheme used to measure the electrodynamical response of a material in the millimeter wave range of frequency. In particular, with this technique we can directly evaluate the complex conductivity from a measurement ofboth the bandwidth and characteristic frequency of a resonator containing the specimen. We will describe in detail all the technical improvements achieved which provide the required accuracy.     

超导温度下微波混合集成电路工艺可靠性初探

为研制可靠的高温超导微波组件设计了可靠性试验,研究了高温超导微波组件常用的微波混合集成电路工艺,如锡铅钎焊、共晶焊接和金丝焊接等在超导温度下的可靠性。结果表明:在经历温度循环、机械冲击、变频振动和恒定加速度试验后,所研究的工艺能满足GJB548B—2005的相关要求,其中锡铅钎焊的试样可以通过1×10–9Pa.m3/s的泄漏率要求,设计的环境试验对80Au-20Sn共晶焊接性能无显著影响。     

Calculations of the intrinsically higher temperature stability of electronic devices made from quaternary alloys

It is suggested that electronic devices made from quaternary alloys in which the carrier velocity is strongly influenced by alloy scattering will have an intrinsically higher temperature stability. This is illustrated by Monte Carlo computer simulations of the steady-state velocity-field characteristic of n-type Ga0.24In0.76As0.5P0.5 around the velocity peak.     

Pulsed and CW high temperature operation of InGaAs/GaAs strained layer vertical cavity surface emitting lasers

The performance of strained layer InGaAs/GaAs vertical cavity surface emitting lasers defined by ion implantation over a approximately 75 degrees C temperature range is reported on. Maximum CW output levels for the temperature extremes of 10 and 86 degrees C are 7.5 MW and 200 mu W, respectively for 20*20 mu m/sup 2 /devices. The temperature dependence of the CW threshold current exhibits exponential behaviour to 80 degrees C.<>     

The use of nanophase powders for high Jc superconductor tape production

Advances in processing of superconductors has demonstrated that ceramic powders with exact composition, super-homogeneity, and extremely fine particle size can be prepared by chemical processing to achieve high density and uniform microstructure. The chemical processing routes have great advantage in preparing powder with high reactivity thus promoting solid state reactions required to produce ‘purer’ superconducting phase. We have studied extensively the coprecipitation technique for the preparation of superconducting ceramic powders. Chemical solution methods have been used in order to model several superconducting systems and to predict optimum conditions for obtaining exact composition. Several improvements on the control of precipitation reaction have been implemented in order to obtain powders of few nanometers in size. This has been done by carrying out the reaction in the bulk of a solution or in microemlusion systems. The corecipitated precursor powder has been used for production of 20− 30 meter long Ag-clad tapes. These tapes require considerably reduced processing times. By controlled heat treatment conditions, these tapes have been processed to carry transport critical current density, J_c <4 × 10⁴ A/cm² at 77K in zero applied field.     

Far-infrared Fabry Perot filter of high transmission with high-Tc superconductor reflectors

We report on a far-infrared Fabry Perot filter with high-T _c superconductor reflectors that has a high peak-transmission. The filter consists of two plane parallel MgO plates, with the adjacent surfaces coated with YBa₂Cu₃O₇-gd thin films. The resonance frequencies are mainly determined by the distance between the films. We present data for a filter that has for the first order resonance at a frequency of 80 cm^?1a quality factor of 55 and a peak-transmissivity of 0.16. The shape of the transmission curve is determined by standing waves between the films and within the plates.     

Fabrication of nanosized superconductor structures with a critical temperature exceeding 100 K

The technology of preparation of layers (up to 100 nm thick) of a superconductor of composition (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ with a superconducting transition temperature above 100 K is considered. It is shown that, upon preparation of thin layers, the critical condition reduces to the maintenance of a stoichiometric composition on the surface of the substrate in the process of magnetron-sputtering deposition and the formation of the crystal lattice of the superconductor at a high temperature.     

Microwave self-modulation of a diode laser coupled to an external cavity

An external cavity coupled to a conventional Fabry-Perot GaAs diode laser operating continuously has been found to cause modulation of the light output at a frequency within the range 0.5 to several GHz. The modulation depth is close to 100 percent and the linewidth can be made as narrow as 180 kHz. The modulation is thought to be stimulated by the intensity noise fluctuations, which peak at the well-known spiking frequency fr. The oscillations are strongly enhanced by a frequency locking action of the external cavity, being efficient when the external cavity round-trip time2L/c, or a multiple thereof, corresponds to the inverse of the spiking frequency. Since the latter is dependent on both pump current and temperature, the system can simply be tuned by adjusting the pump current. For a fixed resonator length, the narrow-band oscillations occur in a small current range, in which an increase in frequency with increasing current at a rate of 400 kHz/mA is observed. A small-signal analysis based on simple rate equations shows the influence of the external cavity on the intrinsic resonance frequency fr. It demonstrates that self-modulation can only occur for small values of the coupling coefficient ε between the laser diode and the external cavity.     

基于流体填充微腔结构的柔性温度传感器件

通过在聚二甲基硅烷(PDMS)基底中嵌入填充GaIn液态合金的微型腔体结构,设计并制备了一种柔性多点温度传感器件。结合红外热成像表征结果和基于热扩散系数的理论分析,讨论了填充GaIn合金的腔体在嵌入PDMS基底后对器件中热量传导行为的影响。实验表征和理论分析表明,由于GaIn合金的热扩散系数远高于PDMS,腔体结构的引入可有效缩短器件的热响应时间。此外通过对器件各测温单元的电阻-温度曲线表征和弯曲对比实验,也展示了器件良好的线性响应特性、空间分辨率和测温位置稳定性。