共查询到19条相似文献,搜索用时 203 毫秒
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Monte Carlo方法模拟了在非均质基底表面上团簇的生长过程.非均质基底由规则分布的A,B两类不同性质的物质(不同材料、不同温度等)组成.沉积粒子的初动能为E0,在不同相基底上单位长度扩散的能耗为EA,EB,粒子从A经过相界扩散到B的能耗为EAB.模拟中粒子的初动能E0.远大于EA,EB,粒子从B过相界到A能耗EBA取为0.结果表明,沉积在此类基底表面的粒子凝聚成具有分形结构的团簇.当EAB远大于EA,EB时,团簇的数目、团簇的回旋半径、团簇的分形维数均随E0/EAB呈周期性变化.
关键词:
分形团簇
非均质基底
Monte Carlo模拟
周期性 相似文献
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在无格点基底表面建立了存在排斥相互作用的原子团簇凝聚及扩散模型,对沉积在均匀带电硅油基底表面的银原子凝聚过程进行了研究.结果表明:当沉积原子凝聚成稳定的原子团簇后,由于带同种电荷,团簇之间存在库仑排斥作用,团簇数密度随时间呈指数形式衰减,衰减时间常数为Oaa;两团簇相互离散的相对平均速率V与它们之间的相对距离L在 统计意义上成正比,即V=HL.停止沉积后初期,H≈Oaa,然后随着扩散时间的增 加,H逐渐趋于零;随着液体基底黏滞系数增加,摩擦力增大,H逐渐减小,摩
关键词:
排斥相互作用
团簇
扩散 相似文献
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采用分子动力学方法对Ti3Al合金的形核机理进行了模拟研究,采用团簇类型指数法(CTIM),对凝固过程不同尺度的原子团簇结构进行了识别和表征,深入研究了临界晶核的形成和长大过程.结果表明,凝固过程体系包含了数万种不同类型的原子团簇结构,但其中22种团簇结构类型对结晶形核过程起关键性作用.在晶核的形成和长大过程,类二十面体(ICO)原子团簇、类BCC原子团簇和缺陷FCC及缺陷HCP原子团簇在3个特征温度点T1 (1110 K), T2 (1085 K)和T3 (1010 K)时达到数量上的饱和,并根据数量和空间分布随温度的变化,得到了它们在形核和长大过程相互竞争的关系.跟踪平行孪生晶粒形成和长大的过程发现,临界晶核是由FCC原子构成的单相结构,并未观察到亚稳BCC相优先形核的过程;平行孪生结构是由FCC单相晶核在沿密排面逐层生长过程中形成的.结果还表明, CTIM相比于其他微观结构表示方法,能更为准确地揭示凝固过程微观结构的转变特征. 相似文献
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采用离子注入技术将Zn离子注入Si(001)基片,并在大气环境下加热氧化制备了ZnO纳米团簇.利用电子探针、薄膜X射线衍射仪、原子力显微镜和透射电子显微镜,对注入和热氧化后的薄膜成分、表面形貌和微观结构进行表征,探讨了热氧化温度以及注入剂量对纳米ZnO团簇的成核过程及生长行为的影响.结果表明,Zn离子注入到Si基片表面后形成了Zn纳米团簇,热氧化过程中Zn离子向表面扩散,在表面SiO2非晶层和Si基片多晶区的界面处形成纳米团簇.热氧化温度是影响ZnO纳米团簇结晶质量的一个重要参数.随着热氧化温度的升高,金属Zn的衍射峰强度逐渐变弱并消失,而ZnO的(101)衍射峰强度逐渐增强.当热氧化温度高于800 ℃以后,ZnO与SiO2之间开始发生化学反应形成Zn2SiO4.
关键词:
ZnO纳米团簇
离子注入
微观结构
形貌分析 相似文献
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Paul Meakin 《Physics letters. A》1985,107(6):269-272
Cluster-cluster aggregation has been simulated in dimensions two to six using both linear and brownian cluster trajectories. Relatively efficient off lattice algorithms have allowed large clusters to be generated and values for the fractal dimensionalities of the aggregates have been obtained without finite concentration effects. The values for the fractal dimensionality are in good aggreement with lattice model simulations for euclidean dimensionalities 2–4. The effective dimensionality (Dβ) obtained from the dependence of the radius of gyration on cluster size increases with increasing cluster size for all of our models (particularly for d ≥ 4). For clusters in the accessible size range (up to 103-104) Dβ is slightly larger for cluster-cluster aggregation via linear trajectories than for brownian trajectories. For cluster-cluster aggregation via brownian trajectories, the limiting (large cluster size) fractal dimensionality is estimated to be 1.46 ± 0.04 for d=2,1.82 ± 0.10 for d = 3, 2.10 ± 0.15 for d = 4, 2.35 ± 0.15 for d = 4, 2.65 ± 0.25 for d = 6. For cluster- cluster aggregation via linear trajectories, the limiting fractal dimensionality is estimated to be 1.55 ± 0.04 for d = 2, 1.91 ± 0.10 for d = 3≥ 2.5 ± 0.06 for d = 5 and ≥2.64 ± 0.05 for d = 6. 相似文献
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Fractal growth of thin films at low temperature (50-175 K) is simulated by Monte Carlo method. It is shown that the thin film growth is quite different from the diffusion-limited aggregation (DLA) model when the coverage is larger than 0.1 ML. The average branch width of clusters increases with increasing temperature and it usually larger than the branch width (1.9 atom) in the classic DLA model. The average fractal dimension of clusters increases also with increasing coverage while the fractal dimension of DLA model remains constant. This difference comes from the weak screening effect during the late stage of thin film growth. The relationship between the saturation island number ns and deposition interval Δt is described in a power law: ns∝Δtγ, where γ=-0.332 is very close to the theoretical value -1/3 of rate equations from nucleation theory. 相似文献
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We perform molecular dynamics simulations to study the homogeneous nucleation in the freezing of molten potassium bromide clusters. The nucleation rates tend to decrease with increasing cluster size and temperature. The solid-liquid interfacial free energy σsl of 42.4-52.3 mJ/m2 is close to the values predicted by Turnbull's relation and comparable to the experimental observation by Buckle and Ubbelohde. It is interesting to find that there is no cluster size effect on the critical nucleus size. Critical nucleus sizes inferred from classical nucleation theory are of 6.5-20.7 K+Br− ionic pairs in the temperature range of 400-600 K. The critical nucleus size at bulk MD freezing temperature obtained by extrapolation is about 45 K+Br− ionic pairs, which is comparable to the experimental value of NaCl. 相似文献
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SIMULATION OF MULTIPLE FRACTAL AND COMPACT GROWTH OF ULTRA-THIN FILMS ON HEXAGONAL SUBSTRATE 总被引:2,自引:0,他引:2
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The multiple cluster growth of ultra-thin films with different deposition rate and different substrate temperature has been studied by kinetic Monte-Carlo simulation. With increasing diffusion rate along cluster edges (corresponding to an increasing substrate temperature), pattern structures change smoothly from fractal islands, compact islands with random shapes, to regular islands, and the average branch width of clusters increases continuously up to some constant value in the compact island limit. The formation of the multiple fractal and compact clusters can be described quantitatively by multifractal. The results of multifractal analysis show that with pattern change from fractal to compact islands, the Hausdorff dimension D0, the information dimension D1, and the correlation dimension D2 decrease, while the width and height of the multifractal spectra increase. 相似文献
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Cluster assembled selenium oxide (SeO2) thin films, as a function of oxygen flow pressure (OFP) have been synthesized by a low energy cluster beam deposition (LECBD) technique. The OFP dependent surface morphology leading to well separated nanoclusters (size ranging from 50 to 200 nm) and fractal features are confirmed from transmission electron microscopic (TEM) measurements. A diffusion limited aggregation (DLA) mediated fractal growth with dimension as 1.71 ± 0.01 has been observed for high OFP (60 mbar). Structural analysis by glancing angle X-ray diffraction (GXRD) and selected area diffraction (SAD) studies identify the presence of tetragonal phase SeO2 in the deposit. Micro-Raman studies indicate the shifts in bending and stretching vibrational phonon modes in cluster assembled SeO2 as compared to their bulk counter part due to the phonon confinement effect. 相似文献
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The growth of submonolayer Pt on Ru(0 0 0 1) has been studied with scanning tunneling microscopy. We focus on the island evolution depending on Pt coverage θPt, growth temperature TG and post-growth annealing temperature TA. Dendritic trigonal Pt islands with atomically rough borders are observed at room temperature and moderate deposition rates of about 5 × 10−4 ML/s. Two types of orientation, rotated by 180° and strongly influenced by minute amounts of oxygen are observed which is ascribed to nucleation starting at either hcp or fcc hollow sites. The preference for fcc sites changes to hcp in the presence of about one percent of oxygen. At lower growth temperatures Pt islands show a more fractal shape. Generally, atomically rough island borders smooth down at elevated growth temperatures higher than 300 K, or equivalent annealing temperatures. Dendritic Pt islands, for example, transform into compact, almost hexagonal islands, indicating similar step energies of A- and B-type of steps. Depending on the Pt coverage the thermal evolution differs somewhat: While regular islands on Ru(0 0 0 1) are formed at low coverages, vacancy islands are observed close to completion of the Pt layer. 相似文献
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V. D. Borman A. V. Zenkevich V. N. Nevolin M. A. Pushkin V. N. Tronin V. I. Troyan 《Journal of Experimental and Theoretical Physics》2006,103(6):850-868
The results of an experimental study of the formation of nanometer-size Au clusters on NaCl(100) and HOPG(0001) surfaces under pulsed laser deposition are presented. No clusters of small sizes (d ≤ 1 nm) have been found in the cluster size distribution. The distribution itself at d < 5 nm has the form of a percolation distribution. It has been established that the perimeter of clusters with sizes d < 5 nm has a fractal structure. The fractal dimension of clusters is different for NaCl(100) and HOPG(0001) surfaces with different symmetries; it decreases with increasing cluster size from D f ≈ 1.2–1.4 at d ≈ 1.5 nm to D f ≈ 1 at d ≈ 5 nm. A physical mechanism of nanocluster formation is suggested. Under pulsed laser deposition, the attainable densities of adatoms are close to the percolation threshold in the region of thermodynamically unstable states and many-particle correlation regions are formed in a spatially inhomogeneous adsorbate. Clusters are formed on the surface from many-particle correlation regions in several diffusion jumps. The suggested mechanism allows the fractal dimension of the clusters forming on surfaces with different symmetries, its dependence on cluster size, and the cluster size distribution functions to be calculated. 相似文献
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Viscous fingering (VF) in random Sierpinski carpet is investigated by means of successive over-relaxation technique and under the assumption that bond radii are of Rayleigh distribution. In the random Sierpinski network, the VF pattern of porous media in the limit M→∞ (M is the viscosity ratio and equals to η2/η1 where η1 and η2 are the viscosities of the injected and displaced fluids, respectively) is found to be similar to the diffusion-limited aggregation (DLA) pattern. The interior of the cluster of the displacing fluid is compact on long length scales when M=1, and the pores in the interior of the cluster have been completely swept by the displacing fluid. For finite values of M such as M≥10, the pores in the interior of the cluster have been only partly swept by the displacing fluid on short length scales. But for values of M in 1f(α) sites have velocites scaling as L-α; and the scaling function f(α) is measured and its variation with M is found. 相似文献
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The growth of magnetite (Fe3O4) on a polycrystalline iron substrate has been continuously followed in situ in the S.E.M. at 500°C in 13.3 N m?2 CO2. Medium resolution (250 Å) micrographs of the same area have been taken at time intervals over periods of oxidation in excess of 200 hr. Successive layers of oxide crystallites grow according to the same logarithmic rate law, but the overall thickening rate of the oxide film conforms to the parabolic rate law. A theoretical analysis of this situation suggests that the dominant factor determining the overall growth kinetics is the frequency of oxide nucleation steps. It can be shown that the basic logarithmic growth of individual oxide crystallites could lead to a variety of overall growth processes, depending upon the conditions required for nucleation. 相似文献