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1.
钨作为一种重要的核材料,在辐射环境下的微观演化行为与自间隙原子缺陷的扩散行为密切相关.研究不同构型自间隙原子的扩散行为有助于全面理解材料的微观演化过程.本文采用分子动力学方法重点考察了钨中具有不同构型的双自间隙原子随温度变化的扩散行为.结果表明:彼此互为最近邻的111双自间隙原子,随着温度的升高,从一维扩散演变成三维扩散,在111方向保持稳定的最近邻结构;次近邻111双自间隙原子在一定温度范围内沿111方向一维扩散,当温度高于600 K将解离成两个独立运动的自间隙原子;而三近邻结构在温度高于300 K就将解离.非平行结构的双自间隙原子在一定温度范围内形成固着性结构,几乎不移动,但在温度高于1000 K时将转化成移动性缺陷.通过将微动弹性带算法获得的自间隙原子迁移能与阿伦尼乌斯关系拟合的结果进行对比,表明了钨中单自间隙原子和双自间隙原子的扩散系数随温度的变化规律不适于用阿伦尼乌斯关系来描述,而线性关系则能合理地描述这一规律.  相似文献   

2.
王天民  王顺花  赖文生 《物理学报》1995,44(7):1091-1100
采用Finnis-Sinclair类型的多体势模型,用分子动力学方法对L1_2型有序金属间化合物Cu_3Au中的单、双空位进行了计算机模拟研究.选取了空位的三种可能的迁移方式,计算了单、双空位的迁移激活能.这三种迁移方式是:最近邻空位跳跃、一组由相关的六步最近邻跳跃构成的空位循环运动和次近邻空位跳跃.计算了单空位的形成能和双空位的结合能.讨论了对扩散起主要作用的微观机制.所得计算结果与实验值符合得很好. 关键词:  相似文献   

3.
采用格点自旋消约方法,将具有最近邻和次近邻耦合作用的镶嵌正方Ising晶格变换成等效的具有最近邻、次近邻和四体耦合作用的正方Ising晶格,得到系统近似解的临界点在K′C=0.4406868.结果表明:在相变点最近邻耦合作用K1和次近邻耦合作用K2之间满足一定关系.如果只计及镶嵌正方Ising晶格的最近邻耦合作用K1,则其严格解的临界点在K1C=0.7635.由此可以推断在正方格点间安放两个自旋的双镶嵌正方Ising晶格,在只计及最近邻耦合作用情况下,也是严格可解的.  相似文献   

4.
采用部分格点自旋消约变换,将镶嵌正方晶格上具有最近邻耦合作用K1和次近邻耦合作用K2的Ising模型变换成等效的具有最近邻、次近邻和四体耦合作用的正方Ising晶格.发现系统的临界点在(K1C,K2C)=(0.5125,0.2134),由此决定系统的临界温度,幷讨论了系统的普适性.  相似文献   

5.
基于高效的单团簇生长算法,采用蒙特卡罗方法模拟了考虑最近邻、次近邻,直至第五近邻格点的二维正方格子的键渗流.计算得到了二十余种格点模型高精度的键渗流阈值,并深入探讨了渗流阈值pc与格点结构之间的关联.通过引入参数ξ=∑i zir2~2/i(其中zi和ri分别为第i近邻格点的配位数及到中心格点的距离)来消除“简并”,研究发现pc随ξ的变化较好地满足幂律关系pc∝ξ,数值拟合得γ≈1.  相似文献   

6.
优化车流的交通流格子模型   总被引:5,自引:0,他引:5       下载免费PDF全文
薛郁 《物理学报》2004,53(1):25-30
在一维交通流格子模型的基础上,分别提出考虑最近邻车和次近邻车以及考虑前、后近邻车相互作用进行车流优化的一维交通流格子模型.应用线性稳定性理论和非线性理论进行分析,得出车流的稳定性条件,并导出了描述交通阻塞相变的mKdV方程.用数值模拟验证了mKdV方程的解,数值模拟结果表明考虑最近邻车和次近邻车的优化车流能够增强车流稳定性,而考虑前、后近邻车的优化车流将使稳定性减小. 关键词: 交通流 交通相变 稳定判据 mKdV方程  相似文献   

7.
Pt单原子在低温CO氧化反应中具有很高的催化活性.利用扫描隧道显微术与密度泛函理论,研究了Pt单原子在还原性TiO_2(110)表面的吸附行为及其与CO和O_2分子的相互作用.研究发现在80 K低温下,TiO_2表面的氧空位缺陷是Pt单原子的最优吸附位.将CO和O_2分子分别通入Pt单原子吸附后的TiO_2表面,研究相应的吸附构型.实验表明在低覆盖度下,单个Pt原子会俘获一个CO分子,CO分子同时与表面次近邻的五配位Ti原子(Ti_(5c))成键,进而形成非对称的Pt-CO复合物构型.将样品从80 K升温到100 K后,TiO_2表面的CO分子会迁移到Pt-CO处形成Pt-(CO)_2的复合结构.对于O_2分子,单个Pt原子同样会吸附一个O_2分子,O_2分子也会与最近邻或次近邻的Ti_(5c)原子成键形成两种Pt-0_2构型.这些结果在单分子尺度上揭示了CO和O_2与Pt单原子的相互作用,呈现了CO与O_2反应中的初始状态.  相似文献   

8.
奥氏体不锈钢由于具有良好的力学性能和耐腐蚀性,被广泛应用于核电工业。奥氏体不锈钢受辐照会产生多种点缺陷,进而影响材料力学性能。为了研究304NG奥氏体不锈钢中点缺陷的产生机理,以面心立方Fe-Cr-Ni合金为模型,利用基于密度泛函理论的第一性原理方法研究了多种点缺陷结构的晶格常数,形成能,结合能,扩散迁移及相互作用,并计算了γ-Fe中相应的的性质作为对照.研究结果表明:Fe空位和Fe-Fe <100>哑铃是Fe-Cr-Ni合金中容易形成的点缺陷.基于对这两种点缺陷的迁移能垒分析,发现Fe-Fe <100>哑铃要比空位更易迁移.在Fe-Cr-Ni合金中相邻Fe空位存在吸引作用,会促进孔洞的形成;Fe空位-Fe-Fe <100>哑铃及Fe空位-Fe-Cr <100>哑铃均存在较弱的排斥作用,此外,在第一近邻位置和第二近邻位置的八面体间隙Fe原子都会和空位结合,使体系恢复为完整晶胞.  相似文献   

9.
赵向荣  朱静 《物理学报》1991,40(1):161-168
本文应用Monte-Carlo统计物理计算方法,计算了L12结构有序度随温度的变化。计算中考虑了最近邻有序相互作用J1和次近邻有序相互作用J2=0,-0.3J1,0.3J1三种情况。计算结果显示存在有序无序一级相变,临界温度Tc与准化学方法等理论结果相符,同时发现次近邻有序相互作用J2敏感地影响到有序无序转变温度Tc,为间隙原子对L12结构金属间化合物塑性的作用提供了一种可能的解释。 关键词:  相似文献   

10.
虞家琪 《发光学报》1980,1(2):57-61
把一个杂质离子放在晶体中的不同位置上,杂质离子周围的最近邻离子和次近邻离子有各种几何构型和化学本质。例如Cu1+离子替位Zn2+离子,最近邻的四个S2-离子构成一个正四面体,如图1所示。  相似文献   

11.
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.  相似文献   

12.
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.  相似文献   

13.
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.  相似文献   

14.
15.
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.  相似文献   

16.
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.  相似文献   

17.
A transient suppression method for piezoelectric ceramic transducer is presented. By adding some special pedestal pulses to the original transmitted signal, the transient effects in the output of the transducer can be just precisely transmission by use of a narrowband transducer. canceled, thereby achieving the broadband The experimental results show that the transient responses of the sinusoid pulses and coded signals are suppressed successfully and the acoustic waveforms radiated into the surrounding water are improved significantly.  相似文献   

18.
We experimentally demonstrate amplitude squeezed soliton utilizing intensity-dependent self-phase modulation in an asymmetric Sagnac interferometer.The system.whose components are connected via ferrule connector/physical connection(FC/PC)fiber connectors,constitutes all-fiber configuration to generate squeezed soliton.Soliton amplitude reduction measured by homodyne detection is near 4.0 dB below the shot-noise level.Optimal squeezing fields in both simple and compact all-fiber configuration are obtained.  相似文献   

19.
The(2 1)-dimensional nonlinear Schr(o)dinger(NLS)equation with spatially inhomogeneous nonlinearities is investigated,which describes propagation of light in(2 1)-dimensional nonlinear optical media with inhomogeneous nonlinearities.New types of optical modes and nonlinear effects in optical media are presented numerically.The results reveal that the regular split of beam can be obtained in (2 1)-dimensional nonlinear optical media with inhomogeneous nonlinearities,by adjusting the guiding parameter.Furthermore,the stability of beam regular split is discussed numerically,and the results reveal that the beam regular split is stable to the finite initial perturbations.  相似文献   

20.
In the real-world application of face recognition system, owing to the difficulties of collecting samples or storage space of systems, only one sample image per person is stored in the system, which is so-called one sample per person problem. Moreover, pose and illumination have impact on recognition performance. We propose a novel pose and illumination robust algorithm for face recognition with a single training image per person to solve the above limitations. Experimental results show that the proposed algorithm is an efficient and practical approach for face recognition.  相似文献   

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