Dislocation motion-induced strain in nanocrystalline materials: Overlooked considerations

The amount of plastic strain caused by the motion of a single dislocation across an individual nanosize grain is drastically higher than the amount recorded for larger grain sizes. As a result, in nanocrystalline materials, only a small number of dislocations would need to move within each individual grain in order to accommodate the plastic strain on the entire sample. This observation leads to a quantitative criterion for determining if observed dislocation activity is sufficient to accommodate realistic applied plastic strains. This new criterion is directly applicable to the interpretation of in situ TEM experiments and computational molecular dynamics simulations.     

Dislocation self-interaction in TiAl: Evolution of super-dislocation dipoles revealed by atomistic simulations

As one of the fundamental outcomes of dislocation self-interaction,dislocation dipoles have an important influence on the plastic deformation of materials,especially on fatigue and creep.In this work,superdislocation dipoles in γ-TiAl and α_2-Ti_3 Al were systematically investigated by atomistic simulations,with a variety of dipole heights,orientations and annealing tempe ratures.The results indicate that non-screw super-dipoles transform into locally stable dipolar or reconstructed cores at low temperature,while into isolated or interconnected point defect clusters and stacking fault tetrahedra at high temperature via short-range diffu sion.Non-screw super-dipoles in γ-TiAl and α_2-Ti_3 Al exhibit similar features as fcc and hcp metals,respectively.Generally,over long-term annealing where diffusion is significant,60° superdipoles in γ-TiAl are stable,whereas the stability of super-dipoles in α_2-Ti_3 Al increases with dipole height and orientation angle.The influence on mechanical properties can be well evaluated by integrating these results into mesoscale or constitutive models.     

Piezoresistivity in films of nanocrystalline manganites

Rare earth manganites having perovskite structure are susceptible to lattice strain. So far most investigations have been done with hydrostatic pressure or biaxial strain. We have observed that hole doped rare-earth manganites, which are known to display colossal magnetoresistance (CMR) also show change in its resistance under the influence of uniaxial strain. We report the direct measurement of piezoresistive response of La0.67Ca0.33MnO3 (LCMO) and La0.67Sr0.33MnO3 (LSMO) of this manganite family. The measurements were carried out on nanostructured polycrystalline films of LCMO and LSMO grown on oxidized Si(100) substrates. The piezoresistance was measured by bending the Si cantilevers (on which the film is grown) in flexural mode both with compressive and tensile strain. At room temperature the gauge factor approximately 10-20 and it increases to a large value near metal-insulator transition temperature (Tp) where the resistivity shows a peak.     

Misfit dislocation dipoles in nanodimensional films with periodically modulated composition

A model is suggested that describes the misfit defects of a new type—dislocation dipoles—in nanodimensional films with periodically modulated chemical composition. The critical thicknesses of such inhomogeneous films are determined, above which the formation of misfit dislocations or their dipoles becomes energetically favorable. It is shown that a critical thickness for the dislocation dipole nucleation can be smaller than that for the ordinary misfit dislocations.     

Dislocation processes in the deformation of nanocrystalline aluminium by molecular-dynamics simulation

The mechanical behaviour of nanocrystalline materials (that is, polycrystals with a grain size of less than 100 nm) remains controversial. Although it is commonly accepted that the intrinsic deformation behaviour of these materials arises from the interplay between dislocation and grain-boundary processes, little is known about the specific deformation mechanisms. Here we use large-scale molecular-dynamics simulations to elucidate this intricate interplay during room-temperature plastic deformation of model nanocrystalline Al microstructures. We demonstrate that, in contrast to coarse-grained Al, mechanical twinning may play an important role in the deformation behaviour of nanocrystalline Al. Our results illustrate that this type of simulation has now advanced to a level where it provides a powerful new tool for elucidating and quantifying--in a degree of detail not possible experimentally--the atomic-level mechanisms controlling the complex dislocation and grain-boundary processes in heavily deformed materials with a submicrometre grain size.     

掺硼nc-Si:H薄膜中纳米硅晶粒的择优生长

利用等离子体增强化学气相沉积(PECVD)生长的系列掺硼氢化纳米硅(nc-Si:H)薄膜中纳米硅晶粒(nanocrystallinesilicon,简称nc-Si)有择优生长的趋势。用HRTEM、XRD、Raman等方法研究掺硼nc-Si:H薄膜的微观结构时发现:掺硼nc-Si:H薄膜的XRD只有一个峰,峰位在2θ≈47o,晶面指数为(220),属于金刚石结构。用自由能密度与序参量的关系结合实验参数分析掺硼nc-Si:H薄膜择优生长的原因是:适当的电场作用引起序参量改变,导致薄膜在适当的自由能范围内nc-Si的晶面择优生长。随着掺硼浓度的增加,nc-Si:H薄膜的晶态率降低并逐步非晶化。nc-Si随硅烷的稀释比增加而长大,但晶态率降低。nc-Si随衬底温度升高而长大,晶态率提高。nc-Si随射频功率密度的增大而长大,晶态率增大的趋势平缓。但未发现掺硼浓度、稀释比、衬底温度、射频功率密度的变化引起薄膜中nc-Si晶面的择优生长。     

纳米晶FeCuNbSiB多层膜的巨磁阻抗效应研究

研究了纳米晶态下Fe73.5Cu1Nb3Si13.5 B9多层膜的巨磁阻抗(GMI)效应。研究结果表明纵向巨磁阻抗(LMI)效应在3MHz时取得最大值为44％,横向巨磁阻抗(TMI)效应在6MHz时取得最大值为46％。LMI与TMI随外磁场有不同的变化行为,TMI曲线具有阁值行为,超过阈值磁场后出现明显的磁阻抗效应。晶化后出现最大值阻抗效应所对应的频率下降,由非晶态下的13MHz下降为晶化后的3MHz。薄膜样品的磁阻抗效应与样品中磁矩的空间分布密切相关．磁矩垂直面向分布时。磁阻抗效应下降为5％     

Surface plasmon resonance in nanocrystalline gold-copper alloy films

Nanocrystalline Au(x)Cu(1-x) films were synthesized by depositing Cu/Au/Cu multilayer in nanocrystalline thin film form with requisite thickness of individual layers onto fused silica substrates by high pressure sputtering technique. The absorbance spectra showed only one surface plasmon peak for all the compositions with the exception that the peak position did not indicate gradual shift as gold concentration was increased. Peak position for the two compositions corresponding to the two superlattice structures, AuCu3 and AuCu, deviated significantly from linear variation. The experimental results have been discussed in light of the existing Mie theory and the Core-shell model.     

纳米硅薄膜及其太阳电池研究进展

纳米硅薄膜具有新颖的结构特征和一系列独特的物理性质,可望应用于新型光电子器件、量子功能器件、集成电路等领域.本文综述了纳米硅薄膜的研究现状及其优良的光电性能和纳米硅薄膜太阳电池的研究进展,指出在生产制备与性能方面纳米硅薄膜太阳电池所具有的优势,具有良好的发展前景.     

Dislocation wall formation during interdiffusion in thin bimetallic films

Transmission electron microscopy observations are described of the diffusion-induced behaviour of misfit dislocations originally present in the interface of thin bimetallic films. Experiments were carried out with specimens consisting of a layer of approximately 500 Å Cu vapour deposited onto an electropolished Ni substrate approximately 1000 Å thick. Diffusion anneals were performed in situ in the electron microscope at annealing temperatures in the range 450–600°C. The dislocation behaviour in Cu/Ni bicrystals with originally a (100) interface was photographed and video-recorded. A cross-grid of misfit dislocations parallel to 〈110〉 directions was present in the original interface. The Burgers vectors were of type $\text{1}\text{2}$a〈110〉 lying in the interface. During diffusion the misfit dislocations became distributed in the diffusion zone. When $\text{2Dt}$ (where D is the diffusion coefficient and t is the annealing time at a given temperature) exceeded a value of 40–50 Å, the dislocations started to align vertically forming dislocation walls along 〈110〉 directions parallel to the original interface. This resulted in a dislocation cell structure. Lengthwise the dislocation walls grew with shocks. The elastic strain energy of a finite edge dislocation array was estimated. Using this result an energy criterion for the formation of dislocation walls was derived. From this criterion it followed that dislocation wall formation may start to occur when $\text{2Dt}$≈ 45 Å, in good correspondence with the experimental results. Some additional observations of recrystallization phenomena during interdiffusion are reported.     

Optical properties of nanocrystalline gallium nitride films

Nanocrystalline GaN films with different crystallite sizes were deposited onto quartz and NaCl substrates by magnetron sputtering of a GaN target in argon plasma. All the films showed predominant hexagonal phase. The band gap values were always found to be higher than that of the bulk. This blue shift in band gap could be attributed to the quantum confinement effect. The optical absorption in these films could be explained by the combined effects of phonon and inhomogeneity broadening along with optical loss due to light scattering at the nanocrystallites. Band edge luminescence is absent in these GaN nanocrystalline films. The line shapes of the photoluminescence (PL) spectra are asymmetric and broad. The film deposited at lower substrate temperature showed broader PL peak. It may be observed that no significant energy shift in the peak positions was observed with reduction in crystallite size but the intensity of the peak decreased for films with the reduction in crystallite size. Below band gap emission observed in this study may also originate due to the presence of polarization-induced electric field present in wurtzite GaN deposited here.     

Sputtered rutile stoichiometric TiO2 nanocrystalline films

Titanium dioxide (TiO₂) films were prepared at room temperature on silicon and glass substrates using DC reactive magnetron sputtering at the rate of 0.1 nms⁻¹ from a pure metal Ti target. X-ray diffraction (XRD) studies on freshly prepared samples showed a purely rutile phase. It is found from AFM that annealing of fresh TiO₂ films at 550°C for 30 min produced an increase in grain size by a factor of at least 1.5. X-ray photoelectron spectroscopies (XPS) gives correct ratio for purely TiO₂ stoichiometry. Optical band gap was estimated to be 3.2 eV from UV–vis transmission spectra.     

Microstructure of nanocrystalline PbS powders and films

Nanocrystalline PbS powders and nanostructured PbS films have been produced by different deposition procedures. According to scanning and transmission electron microscopy results, the average size of PbS particles in the nanopowders is 8 to 20 nm, and the average grain size in the nanofilms is 40 to 80 nm. As shown by electron diffraction, the nanocrystalline PbS powders have the B1 structure (sp. gr. Fm[`3]mFmbar 3m).     

Electrodeposition and characterization of nanocrystalline CoNiFe films

Nanocrystalline Co₄₅Ni₁₀Fe₂₄ films have been fabricated using cyclic voltammetry technique from the solutions containing sulfate, then characterized by scanning electron microscopy, X-ray diffraction and vibrating sample magnetometer. Meanwhile, Electrochemical Impedance Spectroscopy technique has been employed to probe into the nucleation/growth behavior of Co₄₅Ni₁₀Fe₂₄ films. The results show that, the obtained Co₄₅Ni₁₀Fe₂₄ film possesses low coercivity of 973.3 A/m and high saturation magnetic flux density of 1.59 × 10⁵ A/m. Under the experimental conditions, the nucleation/growth process of Co₄₅Ni₁₀Fe₂₄ films is mainly under activation control. With the increase of the applied cathodic potential bias, the charge transfer resistance for CoNiFe deposition decreases exponentially.     

Magnetic force microscopy on nanocrystalline Co films

Pioneer works in ultrathin magnetic films have shown perpendicular magnetic domains in the demagnetized state. The source of this perpendicular anisotropy is the interface anisotropy developed at the interface. Similar domains could be observed in tetragonally distorted ultrathin films due to the magnetoelastic anisotropy. On the other hand, single-crystalline hexagonal close packed (hcp) Co films when grown epitaxially with the c-axis oriented perpendicular to the film plane may show perpendicular stripe magnetic domains even up to a thickness of about 500 nm. In that case the source of perpendicular anisotropy was the magnetocrystalline anisotropy of bulk Co, which favors the c-axis. In this work, we have grown by radio frequency magnetron sputtering Co films in the thickness range 15-4500 nm. We have used various substrates, such as Corning glass, silicon and Al-foil. The substrate temperature was about 350 K. The films have been found by X-ray diffraction experiments to present various structures and textures depending on the preparation conditions, mainly the Ar-pressure and deposition rate. Stripe- and labyrinth-like domain configurations are observed in films textured along the c-axis, and in films with a mixture of hcp and fcc grains, repectively. Films which show mainly fcc or amorphous structure do not form perpendicular domains. The results are discussed with respect to magnetization loops.     

Grain boundary structure dependent fracture in nanocrystalline Au films

Nanocrystalline Au films were in situ strained in a high resolution transmission electron microscope, which demonstrated that the diffusion-assisted intergranular fracture was the dominant failure mode. Grain orientation with respect to grain boundaries (GBs) imposes important effect on the crack propagation and blunting. The low surface energy and high diffusion mobility of {111} planes lead to a notch-like crack. The stress concentration at the tip may help breaking {111} planes layer by layer and thus advance the crack. Cracks can be diverted from the preset path by GBs and grow into the grain interior, which has never been revealed by other experiments and molecular dynamics simulations.     

Synthesis of nanocrystalline CdS thin films in PVA matrix

Nanocrystalline thin films of CdS are deposited on glass substrates by chemical bath deposition technique using polyvinyl alcohol (PVA) matrix solution. Crystallite sizes of the nanocrystalline films are determined from broadening of X-ray diffraction lines and are found to vary from 5·4–10·2 nm. The band gap of the nanocrystalline material is determined from the UV spectrograph. The absorption edge is shifted towards the lower wave length side (i.e. blue shift) and are found to be within the range from 2·48–2·8 eV as grain sizes decrease from 10·2–5·4 nm. This is also supported by the spectral response curves. An increase of molarity decreases the grain size which in turn increases the band gap.