在Ag基带上用PLD法沉积YBCO超导薄膜的研究

Ag是目前唯一不需要隔离层的高温超导基带材料,而且研究表明,在YBCO中掺入适量的Ag,将有利于超导性能的改善和表面电阻的降低.本研究采用准分子激光法(PLD),在经不同表面处理的{110}<110>取向的双轴织构银基带上直接沉积了YBCO薄膜,得到了强双轴织构的超导膜,系统地研究了银基带中的孪晶、晶界等缺陷对YBCO超导微观组织的影响.     

高温超导Ni—W基带的织构研究

涂层超导是目前国际超导界的研究热点和重点，所采用的韧性金属基带多为Ni和NiW合金．本文采用粉末冶金方法制备了Ni-3 at．％W和Ni-5 at．％W合金．利用轧制辅助双轴织构基带（咖TS）技术制备了Ni-W合金基带，系统研究了Ni-W合金形变和再结晶织构转变规律．研究表明：通过大变形量的冷轧和高温再结晶退火，可以得到强立方织构的Ni-W合金基带．     

改善YBCO高温超导银基带的织构

采用部分再结晶的方法对形成{110}<110>多晶Ag织构的"领先再结晶取向"进行了研究.通过控制银的纯度及冷轧和再结晶退火过程,得到了很强{110}<110> 织构的银基带,并对织构形成过程进行了探索.     

短时高温热处理工艺对YBCO超导薄膜结构和性能的影响

通过无氟高分子辅助金属有机物沉积法(PA-MOD)制备了YBCO超导薄膜,研究了785～845℃的不同短时高温热处理对YBCO薄膜双轴织构、表面形貌及超导性能的影响.X射线衍射(XRD)和扫描电镜(SEM)的结果表明,经800℃短时高温处理的YBCO薄膜具有良好的双轴织构和平整致密的表面形貌.物性测量(Quantum-DesignSQUID)的结果表明,该薄膜超导转变温度达到90K,77K自场下的临界电流密度(Jc)为2MA/cm2.     

电子束蒸发制备YBCO超导薄膜研究

采用电子束沉积制备YBCO超导薄膜,研究了760℃—840℃的不同退火温度下高温热处理对YBCO薄膜双轴织构、表面形貌及超导性能的影响。超导临界电流密度测试、X射线衍射(XRD)和扫描电镜(SEM)的结果表明,退火温度在在800℃时,YBCO薄膜具有良好的织构和平整致密的表面形貌,在77K自场下的临界电流密度J可达4.2×106/cm2。     

喷雾沉积热分解法在SrTiO3和织构的Ag基底上制备YBa2Cu3O7-x涂层超导膜

本文研究了用喷雾沉积热分解法在(110)Ag单晶、{110}<011>Ag带、(100)SrTiO3三种不同的基底上制备YBa2Cu3O7-x涂层超导薄膜.通过对溶液成份,载气流量,沉积温度,退火时间等条件的系统研究,我们发现上述参数与薄膜的成份,表面形貌,织构以及超导性能有密切的关系.在Ag单晶基底上所获的YBCO薄膜具有很强的平面双轴织构,起始转变温度达到了90K,Jc值超过了104A/cm2(77K,0T);改进试验装置后,以织构Ag为基底,制备出了15cm长的YBCO超导带材,Jc值达到了104A/cm2(77K,0T).同时,我们发现温度梯度在热分解区间内对薄膜的形成起到了重要的作用.随后我们又在喷雾加热系统中附加一个金属加热圆筒,以改善加热区间的温度梯度,薄膜的性能得到了明显的提高.在(100)SrTiO3上的YBa2Cu3O7-x薄膜,(102)极图的FWHM值达到了0.70°,薄膜的起始超导转变温度为91K(ΔTc=1.5K),Jc值接近105A/cm2.     

高温超导Ni-W基带的织构研究

涂层超导是目前国际超导界的研究热点和重点,所采用的韧性金属基带多为Ni和Ni-W合金.本文采用粉末冶金方法制备了Ni-3 at.%W和Ni-5 at.%W合金.利用轧制辅助双轴织构基带(RABiTS)技术制备了Ni-W合金基带,系统研究了Ni-W合金形变和再结晶织构转变规律.研究表明:通过大变形量的冷轧和高温再结晶退火,可以得到强立方织构的Ni-W合金基带.     

银基带再结晶织构的EBSD分析

本文采用EBSD（电子背散射衍射）等分析手段对{110}〈110〉取向的银基带的织构形成机理进行了深入研究，发现了冷轧织构与不同退火温度下再结晶织构间的取向转变关系，研究发现，{110}面附近且非{110}〈211〉取向的冷轧织构组分，900℃退火可以转变为强的{110}〈110〉织构．织构转变中有孪晶形成，{110}〈110〉和{110}〈114〉取向互为孪晶关系．     

低氟MOD法制备YBCO及其银掺杂效应

金属有机溶剂法(MOD)是制备高质量YBCO涂层导体的主要方法之一.银掺杂可以有效降低晶界的弱连接改善薄膜的表面形貌,提高涂层导体的超导性能.目前,主要以物理方法和块材为主。本文采用低氟MOD方法进行银掺杂,在哈氏合金缓冲层上制备了YBCO-Ag,首先研究了银对YBCO织构的影响,然后研究了银对YBCO成相温区的影响。实验结果表明银掺杂可以抑制a轴成核,YBCO-Ag在较宽的成相温区具有良好c轴织构,样品表面平整致密无裂纹.     

YBCO涂层导体用Ni5W基带的制备和立方织构的发展

为了制作能满足YBCO涂层导体（coated conductor）所需要的高强度、低磁性的立方织构基带，本工作用粉末冶金方法制作了Ni-5at％W合金基带．为评估基带中立方织构的发展，用March-Dollase函数对各种热处理样品的择优取向度进行了研究，结果与用X射线极图法和电子背散射衍射法得到的结果基本一致．研究结果表明，在实验中所用的工艺参数范围内，随总加工率和热处理温度的提高，基带中立方织构百分数明显增高．提高总加工率实际增加了冷加工样品中立方织构晶粒或立方核心的数量．实验中得到了较好的和实用的工艺制度，用这种工艺可以制作出具有99％～100％立方织构百分数，并具有很好一致取向度的Ni5W基带．     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.