晶粒尺度对热致变色材料辐射特性的影响

研究了原材料在不同粒径、不同烧结温度条件下所生成钙钛矿锰氧化物热致变色材料的晶粒尺度变化,进而分析了晶粒尺度对材料辐射特性的影响。结果表明:不同粒径原材料在同一烧结温度下烧结的样品晶粒尺度近乎相等,发射率变化幅度相当,相变温度保持不变。同粒径原料在不同温度下烧结,材料晶粒尺度随烧结温度升高而增大;经比较不同晶粒尺度材料的辐射特性,可知大晶粒尺度材料在高温时发射率较高,低温发射率较低,使发射率整体变化幅度提高,而对材料相变温度则几乎没有影响。     

相变材料热物理性质的分子动力学模拟

为从微观尺度探寻相变材料的热物性变化机理, 本文采用分子动力学的方法, 构建了由正二十二烷组成的无定形结构的相变材料体系, 采用周期性边界条件以及COMPASS力场对相变材料的比热以及导热系数进行了模拟, 并对纯正二十二烷进行了DSC测试. 结果表明, 模拟所得的相变材料热容与文献实验值的偏差是6.5%, 熔点与DSC实验值的偏差是0.98%. 当温度为288–318 K时, 相变材料的导热系数在0.1–0.4 W·m^-1·K^-1 范围内波动, 且随着压力增大略呈下降趋势. 关键词： 扩散系数 比热 导热系数 分子动力学     

相变材料模拟金属构件热特性的数值研究

通过理论分析和数值计算论证了用相变材料模拟多种常用金属构件热特性的可行性.利用相变材料模拟了厚度分别为20 mm,30 mm,50 mm和100 mm的金属板材,发现在模拟较厚的金属板材时,相变材料的低导热系数对模拟效果产生了不利影响.在此基础上,利用导热系数增强的相变材料对厚度大于100 mm金属板材进行模拟,取得了较好的模拟效果.并利用相变材料模拟了金属构件轮和管的热特性.     

基于六水氯化钙的单相变材料热二极管的实验研究

基于相反传热方向上相变程度不同引起的传热形式和系数差异设计的相变材料热二极管被认为是有潜力的热管理器件.然而多种材料的使用或仅依靠数值模拟的研究使其结构复杂或理想化,降低了其实际应用的可能性.因此,本文结合材料固液相变和自然对流过程的传热形式和传热系数变化,提出了一个仅含有CaCl₂·6H₂O单相变材料的简单热二极管结构,并制备了相应的器件,搭建了稳态热通量测试系统用于实验研究,其测量结果与文献记载值相近,具有良好的准确度,实验研究了冷热端温差和正反传热方向对热二极管热整流效果的影响规律.结果表明:热二极管的热通量随冷热源温差的减小而降低,正向和反向分别沿逆重力和重力方向时,热整流比最高可达1.58,最佳冷源温度范围为20—25℃,接近室温,所提出的相变材料热二极管结构在建筑节能和热管理等方面具有一定的应用潜力.     

电触发二氧化钒纳米线发生金属-绝缘体转变的机理

二氧化钒(VO_2)是一种强关联相变材料,在341 K下发生金属-绝缘体转变.尽管对于VO_2相变的物理机理进行了大量研究,但科学家仍未形成统一认识.与热致VO_2相变相比,电触发VO_2相变应用前景更为广阔,但其机理也更为复杂.本文利用原位通电杆和超快相机技术,在透射电镜下原位观察了单晶VO_2纳米线通电时的相转变过程,记录了相变过程中对应的电压-电流值,并在毫秒尺度下捕捉到了VO_2的过渡相态.发现VO_2电致相变并非由焦耳热引起,推断其机理是载流子注入.同时观察到电子结构相变和晶体结构相变存在解耦现象,进一步支持了上述推断.将VO_2纳米线两端施加非接触式电场,观察到VO_2纳米线在电场中的极化偏移,而未观察到相变发生,该现象同样支持相变的载流子注入机理.研究表明VO_2的金属-绝缘体转变遵循电子-电子关联机理,即根据电子关联的Mott转变进行.     

低熔点合金相变热控数值模拟与实验研究

本文主要对47合金与58合金两种低熔点合金相变材料的相变热控性能与相变过程传热现象进行研究。首先通过将相变热控实验与数值模拟相结合的方法验证了焓–多孔介质模型与Boussinesq假设的可靠性。之后通过数值模拟的方法对比分析了两种低熔点合金相变材料与熔点相近似的传统有机类相变材料热控性能。结果表明,47和58合金低熔点合金相变材料的控温时间明显更长,较传统有机相变材料热控性能更好。另外,自然对流对低熔点合金相变热控装置的温度分布及固液相界面几乎无影响,低熔点合金熔化过程中自然对流效应可忽略。     

介孔尺度及结构对混合硝酸盐热输运特性的影响

米用分子动力学方法模拟介孔尺度和结构对混合硝酸盐热输运特性的影啊.便用Material Studio软件分别建立不同尺度、两种结构的混合硝酸盐模型以及达到共晶状态的不同比例的NaNO₃-KNO₃模型,通过对模型进行运算并整理计算结果,对纳米尺度下混合硝酸盐热输运特性的微观机理进行分析.结果表明:太阳盐的相变温度随着纳米孔尺度的增大呈现先增加后减小的趋势,最终与宏观尺度下的熔点一致;阳离子的比例对混合硝酸盐的相变温度有很大的影响,且纳米线结构也会改变硝酸盐的相变温度.硝酸盐的体热膨胀系数随着介孔尺度的增大而减小,随着NaNO₃含量的增加而增大,随着介孔结构的改变而改变.离子之间的相互作用的增强会使导热率增大,但对定压比热容没有太大影响.     

纳米石墨烯片-正十八烷复合相变材料制备及热物性研究

本文分别制备了纳米石墨烯片质量分数为0%, 0.5%, 1%, 1.5%, 2%的纳米石墨烯片-正十八烷复合相变材料,并通过扫描电镜测试、红外光谱分析、差示扫描量热实验及导热分析等实验对其形貌结构及热物性进行表征和研究.实验表明本文制备的纳米石墨烯-正十八烷复合相变材料具有很好的相变稳定性;当纳米石墨烯片的质量分数达到2%时,复合相变材料的导热系数相对于纯十八烷高出了89.4%.     

相变蓄冷材料的研究进展

变蓄冷材料具有较高的储能密度,在能源高效利用和节能等方面有着广阔的应用前景。本文对固液相变点在20℃以下的相变蓄冷材料进行了分类,并详细地总结了目前研究较多的和已经商用的各类相变材料及其热物性参数,比较了不同种类相变材料的热物性和化学性质。最后,本文对相变蓄冷材料的研究和应用前景进行了展望。     

基于LBM的多孔陶瓷有效热导率和相变传热分析

多孔氮化铝陶瓷具有高导热、耐熔盐腐蚀等优点在相变材料封装方面具有较大应用前景。本文采用微米计算机断层扫描(CT)得多孔氮化铝的灰度图像,并精准重构三维孔隙结构。并采用四参数随机生长、堆积颗粒、维诺结构等数值方法建立结构模型。基于介观尺度的格子玻尔兹曼方法 (LBM),计算了多孔陶瓷的有效热导率,结果表明维诺结构与CT真实重构的热导率接近。采用三维焓法LBM模拟了多孔骨架内的相变传热过程,获得了孔隙尺度对流相变的传热机理,并分析了不同孔隙率时的储热密度和复合材料的有效热导率。基于CT扫描和LBM的相变传热模拟有助于快速分析骨架的热特性,为多孔骨架的设计提供理论基础。     

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.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

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.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

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.