泡沫Al阻尼性能研究

通过测定不同频率、温度和应变振幅下的内耗，研究了泡沫Al的阻尼性能.泡沫Al组织中孔洞对阻尼的贡献是使背景内耗特别是高温背景内耗增加.泡沫Al阻尼的另一个特点是较为显著的振幅效应，这一效应随孔径减小而变得更加突出，由于泡沫Al所特有的非均质性孔洞组态和高密度缺陷，应力应变分布极不均匀，加上外部应力的干扰，造成缺陷密度增加，因此缺陷阻尼是主要的阻尼机制. 关键词：     

泡沫铝的低频内耗特征研究

采用粉末冶金发泡法制备的泡沫铝试样，研究了室温下低频范围（0.04—4Hz）泡沫铝的内耗与孔结构（孔隙率和孔径）、频率、振幅之间的关系．结果表明泡沫铝的内耗具有以下特征：孔径一定时，内耗随孔隙率的增加而增大；孔隙率一定时，内耗随孔径的减小而增大；在测量频率范围内，内耗与频率近似无关；小应变振幅时内耗随振幅的增大而增大，具有正常振幅效应．提出了泡沫铝的内耗产生机制，根据等效夹杂方法和钉扎位错内耗特点进行了理论分析，较理想地解释了实验结果 关键词：     

铁锰合金在正、反马氏体型相变温度范围内出现的位错内耗峰

用扭摆法测量了含锰18.5％的铁锰合金的内耗,在正和反马氏体型相变温度范围内都观察到一个稳定的内耗峯。在降温时,正马氏体型相变区域内的内耗峯出现在100℃左右;在重新加热时,反马氏体型相变区域内的内耗峯出现在200℃左右。对于这两个变为稳定以后的内耗峯进行了系统的研究,它们的共同特征是:1.内耗峯的位置与测量内耗所用的振动频率无关;2.稳定内耗峯的高度,随着测量内耗所用的应变振幅的增大而显著地升高;3.随着对于试样的加热和冷却循环次数的增加,内耗峯的位置离相变开始点的温度越远,而且内耗峯的高度也下降。从以上的特点可以看出,内耗峯的位置与频率无关表示产生内耗的基本过程不是受热激活所控制的。此外,内耗与振幅有关这一现象进一步指出,产生内耗的机构不是一种滞弹性效应。电子衍衬金相观测已经指出,作为扩展位错的组成部分的堆垜层错可以作为马氏体型相变的核,因此,这种内耗现象可能是由于扩展位错的应力感生加宽和收缩所引起的。这种模型可以统一地解释在铁锰合金中所观察到的各种内耗现象。     

铝-0.1%镁合金中表现反常振幅效应的低频位错内耗峰

用低频扭摆测量了经过轻度冷加工的Al-0.l％Mg合金在室温时效过程中的内耗变化,观察到表现反常振幅效应的时效内耗峯。用经过充分时效而内耗已达到稳定值的试样进行升温和降温内耗测量,也在30—55℃的温度范围内观察到表现反常振幅效应的温度内耗峯。这些现象在高纯铝试样和经过完全退火的Al-0.l％Mg合金试样中都不出现。在Al-0.l％Mg合金中也能观察到以前在Al-0.5％Cu合金中所观察到的反常内耗现象,这就排斥了任何用在点阵中所发生的一般脱溶沉淀过程来解释这种反常内耗现象的可能性,困为在含0.1％Mg的铝合金中,在实验测量所涉及的温度范围内,是不能够在点阵中发生一般的脱溶沉淀过程的。为了解释铝镁合金中所呈现的反常内耗现象的特点,可以假设在铝镁合金中能够形成两种位错气团:一种是“镁原子-空位”对与位错所组成的气团,这种气团能够在应变时效过程中形成;另一种气团是镁原子与位错所组成的气团,当经过范性形变的试样的温度升高到100℃左右时,镁原子与空位脱离,在退火过程中就单独与位错形成气团。前一种气团对于位错的钉札较第二种为强,因而它的振幅内耗峯出现在较高的振幅值。     

钼在千周范围内的位错内耗

用Marx方法(频率为86kc/s及102kc/s)测量了区熔提纯钼单晶及工业纯钼多晶的内耗振幅依赖关系。钼单晶在2000℃氢气炉中退火后可测得Granato-Lücke型的内耗-振幅曲线。微量冷加工后内耗增大,并使内耗振幅曲线上出现转折平台(或称极大值),与铝在低温低振幅下测得的结果相似。根据位错蚀坑的观察结果,初步认为此转折平台的出现是与冷加工在晶体中产生的未被钉扎的新位错有关,并求得转折平台处内耗的增值与新位错密度成正比的关系。此外还做了温度和时效(200—300℃)对内耗振幅曲线的影响,随着温度的降低和时效时间的增长,内耗明显下降。以上的结果我们用推广的GL理论进行了讨论。测量了冷加工对工业纯钼多晶的内耗振幅曲线的影响,基本符合GL理论。     

Al-0.5%Cu合金中表现反常振幅效应的低频位错内耗

用低频扭摆的方法,系统地研究了能够较有把握地观测到在Al-0.5％Cu合金中所出现的反常位错内耗的实验程序。结果指出,把试样在拉力试验机上预先拉伸到某一适当的形变量后,立即测量它在某一适当温度下时效的过程中的内耗,可以在内耗-时效时间曲线上观察到一个表现反常振幅效应(即内耗-振幅曲线上出现一个峯值)的时效内耗峯。用经过充分时效而内耗已经达到稳定值的试样,逐渐增加振幅,测量内耗,也可以观察到一个应变振幅内耗峯(当内耗表示为应变振幅的函数时)。此外,用一定的应变振幅在较高温度下,测量经过充分时效的试样的内耗时,也可以观察到一个表现反常振幅效应的温度内耗峯(当内耗表示为温度的函数时)。这些实验结果肯定地指出,在一定测量温度下的振幅内耗峯和表现反常振幅效应的温度内耗峯是存在的。为了找出以前在Al-0.5％Cu中所观察到的反常内耗现象重复性不好的原因,系统地研究了测量内耗所用的应变振幅对于出现振幅内耗峯的影响以及预形变量对于出现时效内耗峯和温度内耗峯的影响。对于出现反常内耗现象的实验条件进行了分析。所观察到的关于反常内耗现象的实验结果,都可用位错拖着溶质原子气团运动的模型来作定性的解释。具体的模型和理论分析将另行报导。     

锰基合金的反铁磁转变与fct马氏体相变内耗

孪晶型阻尼材料已被实际应用,(011)孪晶通过fcc-fct马氏体相变形成,而γMn基合金中,马氏体相变又与合金的反铁磁转变密切相关.因此研究γMn基孪晶型阻尼材料,无疑必须探讨反铁磁转变与一级马氏体相变的之间关系,反铁磁转变和马氏体转变对孪晶形成的作用.本文通对富锰的γMn基合金(Mn-Cu,Mn-Fe,Mn-Ni)的内耗和模量的测量,研究这二类相变在不同材料,不同成分合金中的耦合的机制,以及反铁磁转变和马氏体相变对孪晶形成的作用.结果显示,马氏体相变和反铁磁转变耦合或马氏体相变与孪晶阻尼峰耦合都可以获得材料的高阻尼性能.当锰含量较高时,反铁磁转变和马氏体相变发生耦合,或马氏体相变内耗与孪晶内耗叠加,在室温附近形成高内耗阻尼;当锰含量较低时,马氏体相变温度降到室温以下,反铁磁转变形成的微孪晶亦能产生内耗阻尼峰.     

铝一镁合金中的反常位错内耗峰

Al-0.03at％ Mg合金在倒扭摆上-65℃在原位进行拉伸,形变量为0.5％,然后在升温过程中测量内耗,于-30℃及60℃附近观测到内耗峰,随后从80℃的降温过程中仍在相同温度观测到内耗峰。研究了不同温度下的内耗-振幅曲线,在降温过程中出现内耗峰的温度范围内观测到了振幅内耗峰,并由此计算得到等效激活能分别为0.32及0.22eV。初步认为,当低温冷加产生的位错弯结在外力作用下作沿边往复运动时,“镁原子-空位”对和镁原子被拖曳运动是分别引起-30℃峰和60℃峰的可能的原因。 关键词：     

石墨颗粒/CuAlMn形状记忆合金复合材料中的位错内耗峰

利用渗流技术制备出了以石墨颗粒为阻尼增强相、以Cu-11.9Al-2.5Mn(wt%)形状记忆合金为基体的复合材料，对该合金的内耗行为进行了研究. 在淬火态样品的内耗-温度曲线上观察到两个内耗峰，分别位于240 ℃和370 ℃附近. 对其中低温峰的变化规律和机理进行了研究. 实验发现，低温峰仅在复合材料中出现，峰位与频率无关，峰高随频率升高而上升；随升温速率增加，峰高增加，峰位移向高温；随石墨颗粒体积分数增加，峰高增加；经多次热循环后该内耗峰消失. 由以上特征和微观观察，可以证明该峰起因于外加交变应力与位错的相互作用.     

Mn-Cu系高阻尼合金的凝固组织控制及阻尼特性的改善

Mn-Cu系阻尼合金兼有较高的阻尼特性和良好的力学性质,因而具有较大的实用前景.这类合金通常经过铸造,塑性加工和热处理来获得要求的性能和组织.加工后的Mn-Cu合金存在一个特定温度,在此温度以上合金的阻尼性能将会消失,因而影响了合金的在较高温度下的使用.该温度和合金的Mn含量有关,然而提高合金的Mn含量有会降低合金的加工性及力学性能.一般Mn-Cu合金的热处理都是利用400℃附近的时效来获得相分解后的局部高Mn组织.但是目前的时效处理后的Mn-Cu阻尼合金的最高使用温度只在80℃以下.为解决Mn-Cu阻尼合金使用温度的局限性,本研究选用凝固过程控制的方法在铸造组织中来获得较大幅度的Mn含量分布,从而在Mn-Cu合金得到较高的高阻尼特性温度.本工作利用铸型温度控制的方法,将M2052(Mn-20Cu-5Ni-2Fe)合金在250～0.1 K/s冷却速度范围内控制凝固.随凝固冷却速度的降低在合金的铸态组织中观察到二次枝晶间距和晶粒尺寸的明显增大.同时还发现缓冷凝固的合金的成分比快冷凝固有较大的分布幅度.铸态下的合金阻尼性能评价也证实了凝固冷却速度对合金的凝固组织有很大的影响.尽管铸态组织的合金的高温阻尼性能并没有很大的改善,然而通过对铸态组织实施时效处理后发现缓冷凝固合金的高温阻尼性能有很大的改善.凝固冷却速度对时效处理后合金的阻尼性能有明显的影响.实验结果表明0.1 K/s的冷却速度下缓慢凝固的合金在时效处理后高阻尼特性可持续高达120℃.     

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.     

Hybrid Wavelength and Code Division Multiple Access in Optical Networks

A review of schemes for multiple access in fiber optic networks shows that a hybrid of wavelength and code division multiple access (WCDMA) combines the best features of both. In particular, the hybrid scheme retains the large information carrying capacity of wavelength division multiple access (WDMA) and flexibility of code division multiple access (CDMA). In this paper WDMA, optical CDMA (OCDMA), and WCDMA networks are discussed. In OCDMA networks, concept of incoherent and coherent coding including inverse decoding and matched filter is introduced. The delay performance of both WDMA and WCDMA networks, under the simple suboptimum access protocols based on cyclic search, is computed. It has been shown quantitatively that tuning delay significantly affects the delay performance of both WDMA and WCDMA networks. Futhermore, delay performance of WCDMA networks is always better than the WDMA networks for the same tuning delay, load, and number of users.     

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.     

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.     

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.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.