Study of nanocrystalline Fe－Al－N soft magnetic thin films

Fe－Al－N films were fabricated by reactive sputtering using a radio-frequency magnetron sputtering system. The effects of Al and N content and annealing temperature on microstructure and magnetic properties were investigated. The Fe－Al－N films, which have good soft magnetic properties, consist of nanocrystalline α-Fe grains and a small amount of other phases in the boundaries of α-Fe grains. The average α-Fe grain size is about 10－15nm. A slight amount of Fe－N and Al－N compounds precipitate in the boundaries of α-Fe grains and suppress their growth. Annealing improves the soft magnetic properties slightly by releasing the residual stress and reducing defects.     

Structure and morphology of chemical bath-deposited CdS films and clusters

Thin films of cadmium sulfide have been deposited on glass substrates and the structural properties of films have been investigated using scanning electron microscopy and X-ray diffraction techniques. The films consist of domains (groups of grains) and weakly bound grain clusters. The structural parameters of grains, domains and clusters and the effect of film thickness on these parameters are reported. From the measurement of lattice constants in CdS films and in free CdS clusters, it has become evident that the films on glass substrates have a tensile strain along their planes. The effect of thermal annealing on the partial relaxation of the strain is discussed. Received: 29 January 2001 / Accepted: 30 January 2001 / Published online: 3 May 2001     

A study on anomalous yield drop behaviour in modified beta titanium alloys

ABSTRACT

The yield drop phenomenon observed in the Ti–15V-3Al–3Sn-3Cr (Ti–15–3) beta-titanium alloy and its anomalous behaviour in the boron and carbon added Ti–15–3 alloys have been studied. While the base and the carbon containing alloys exhibit yield drop, the boron containing alloy with smaller grain size than base alloy does not appear to show this phenomenon. Tensile tests were interrupted at different stress levels followed by analyses of slip lines and sub-structural characteristics using scanning and transmission electron microscopes to understand this anomalous yield point phenomenon. Infrared thermal imaging technique was used to map the strain localisation and the spatiotemporal evolution of deformation along the gauge length of the specimens during the tensile tests. Deformation in these alloys initiates only in a few grains. Pile-up of dislocations in these grains subsequently triggers the formation of dislocations in other grains and their rapid multiplications. The spreading of deformation by the generation of dislocations from pile up dislocations in one grain to neighbouring un-deformed grains and their rapid multiplication to new regions influence the yield drop phenomenon and its characteristics. It is shown in this study that microscopic instability in the grain level is a necessary, but not the sufficient condition for the manifestation of macroscopic instability during tensile deformation in polycrystalline materials. The presence of boride particles at grain boundaries restricts the slip transfer across the grains as well as the spreading of deformation to new regions, which causes the suppression of yield drop in the boron containing alloy.     

Effect of microstructure on ~3He migration in TiT_(1.9) films

Two kinds of films were prepared to study the effect of microstructure on helium migration in Ti tritides. Both films showed different release behaviors and helium bubble distributions. In the film consisting of columnar grains, a twolayered structure was observed. Inclusions with a strip feature were found at the grain boundary, and no helium bubbles were distributed in these inclusions. However, helium preferred to migrate to the boundaries of these inclusions. Bubble linkage as a ribbon-like feature developed parallel to the film surface in the film consisting of columnar grains. More cracks were developed at the grain boundaries of the film consisting of columnar grains, although the helium content in the film consisting of columnar grains was less than that in the film consisting of equiaxed grains. A surface region with a small number of bubbles, or "depleted zone", was observed near the surface. The cracks extending to the film surface were the pathways of the critical helium released from the film. The helium migration was strongly influenced by the grain microstructure.     

Molecular Dynamics Study of the Formation of Solid Al–C Nanocomposites

Migration of graphene fragments along the aluminum matrix in the solid phase was studied by molecular dynamics. The structure of the Al–C nanocomposite grain was studied by statistical geometry. The distributions of the topological and metric characteristics of truncated polyhedra were calculated for the Al subsystem; the distributions for the polyhedra constructed at the centers of mass of the hypothetical geometrical neighbors were calculated for the carbon subsystem. The graphene fragments are concentrated at the structural grain boundaries. The nanocomposite grains are preferably separated by single-layer graphene rather than by the two-layer graphene membrane.     

Quantitative TEM analysis of Al/Cu multilayer systems prepared by pulsed laser deposition

Thin films composed of alternating Al/Cu/Al layers were deposited on a (111) Si substrate using pulsed laser deposition (PLD). The thicknesses of the film and the individual layers, and the detailed internal structure within the layers were characterized by means of transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy-filtered TEM (EFTEM). Each Al or Cu layer consists of a single layer of nano-sized grains of different orientations. EFTEM results revealed a layer of oxide about 2 nm thick on the surface of the Si substrate, which is considered to be the reason for the formation of the first layer of nano-sized Al grains. The results demonstrate that the PLD technique is a powerful tool to produce nano-scale multilayered metal films with controllable thickness and grain sizes.     

Preparation and Characterization of Silver Selenide Thin Film

Silver selenide, a phase-changing chalcogenide material, is prepared using electro deposition method for various molarities. X-ray diffraction studies show the cubic lattice of the material. The micro-structural properties such as grain size, strain, dislocation density, and texture coefficient are examined. The lattice constant is calculated using Nelson-Relay function. Morphological studies are done and uniform distributions of grains are observed. High purities of thin films are confirmed by energy dispersive X-ray analysis. The band gap is calculated using UV-vis spectroscopy and photoluminescence technique, and hence, the Stokes’s effect is observed in silver selenide thin films. It is the first time that the lattice constant and the Urbach energy for various molarities in the case of silver selenide thin films are reported.     

Optical and electrical properties of nanocrystal zinc oxide films prepared by dc magnetron sputtering at different sputtering pressures

The nanocrystal thin films of zinc oxide doped by Al (ZnO:Al) were deposited by dc reactive magnetron sputtering on the glass substrates, in the pressure range of 33-51 Pa. From the X-ray diffraction patterns, the nanocrystalline structure of ZnO:Al films and the grain size were determined. The optical transmission spectra depend from the sputtering pressure, but their average value was 90% in the range from 33 Pa to 47 Pa. Also, the sputtering pressure changes the optical band gap of ZnO:Al films, which is highest for films deposited at 37 Pa, 40 Pa and 47 Pa. The obtained films at room temperature have a sheet resistance of 190 Ω/cm² which increases with time, but the films annealed at temperature of 400 °C have constant resistance. The surface morphology of the films was studied by Scanning electron microscopy. XPS spectra showed that the peak of O1s of the as-deposited films is smaller than the peak of the annealed ZnO:Al films.     

体心立方多晶膜中应变能密度的各向异性分析

根据弹性理论和多晶膜的屈服强度公式，计算了附着在基体上体心立方多晶薄膜中不同取向晶粒中的应变能密度.结果表明：1）在屈服之前，对Fe和Ta两种薄膜，4个最小的应变能密度对应的晶粒取向依次为（100），（510），（410）和（511）；对Cr，Mo，Nb和V四种薄膜，4个最小的应变能密度对应的晶粒取向依次为（111），（332），（322）和（221）；对W膜，应变能密度与晶粒取向无关.2）在屈服的体心立方多晶膜中，4个最小的应变能密度对应的晶粒取向依次为（100），（111），（110）和（411）.从 关键词： 体心立方多晶薄膜 晶粒取向 应变能密度 织构     

H/Al共掺杂对ZnO基透明导电薄膜光电性质和晶体结构的影响

利用H在ZnO中作为浅施主杂质的特性,研究了H掺杂对ZnO：Al透明导电薄膜特性的影响。通过降低ZnO：Al中Al的含量并同时引入H掺杂,解决了透明导电薄膜中高导电性与高透过率之间的矛盾。H的掺杂可以显著降低ZnO基透明导电薄膜的电阻率,这是由于H一方面作为施主可以提供电子从而提高了自由载流子浓度;另一方面与ZnO晶界中的O^-结合降低了晶界势垒,提高了载流子迁移率。利用H掺杂,可以在Al掺杂量降低10倍的情况下,仍然能获得低电阻率（6.3×10^-4 Ω·cm）的透明导电薄膜,同时其近红外波段（1 200 nm）透光率从64%提高到90%。这种具有高导电性和高透光性的透明导电薄膜可以应用于各类薄膜太阳能电池中以提升器件效率。     

薄膜中异常晶粒生长理论及能量各向异性分析

针对柱状晶薄膜，建立了异常晶粒生长理论模型.指出薄膜中的晶粒生长，除像传统的整体材料中的晶粒生长一样考虑晶界能外，还应当考虑表面能、界面能和应变能.对能量的各向异性进行了回顾性分析.从表面能的最小化考虑，面心立方和体心立方薄膜的择优取向或织构应分别为（111）和（110）；而从应变能的最小化考虑，面心立方和体心立方薄膜的择优取向或织构应分别为（110）和（100）. 关键词： 薄膜 异常晶粒生长 模型 织构     

The grain size distribution in nanocomposite films

The grain size distributions and related mechanisms in nanocomposite films with nanostructures comprising a nanocrystalline (nc) phase surrounded by an amorphous (a) matrix under different amorphous phase amounts (V _a) have been analyzed by using a Monte Carlo grain growth model. The results show that with the V _a value increasing to a critical value of ～28%, the grain size distribution approaches lognormality, and it becomes off-lognormal when the V _a value is larger or smaller than ～28%. The simulated results are in a good agreement with the experiment. It is shown that the homogenous or inhomogeneous grain growth mode, determined by the energy exerted on the grain boundary, originates in lognormal or off-lognormal grain size distributions in nanocomposite films. Also, in a system with lognormal grain size distribution, the amorphous phase just covers all grain boundaries (GBs) and the length obtained by summing the boundary circumference of all nanograins is the longest. It is expected that this microstructure can result in exceptional properties of nanocomposite films.     

利用微带谐振技术研究MgB_2/Al_2O_3和YBa_2Cu_3O_(7-x)/LaAlO_3超导薄膜的微波性质(英文)

利用微带谐振技术研究了MgB2/A12O3和YBa2Cu3o7-X/LaAlO3(YBCO/LAO)超导薄膜的微波性质.一个颗粒尺寸模型被用来分析超导薄膜的穿透深度和表面电阻.结果表明,对于高质的YBCO/LAO,微波性质主要由颗粒决定.对于低质的MgB2/Al2O3,该薄膜的微波性质主要由颗粒边界决定.     

NiFe/FeMn双层膜交换偏置的形成及热稳定性研究

研究了在铁磁(NiFe)/反铁磁(FeMn)双层膜之间，交换偏置的形成过程和热稳定性，特别是NiFe/FeMn的交换偏置作用与FeMn层晶粒尺寸的关系.和以前作者不同的是，本文方法采用非磁性Ni-Fe-Cr合金作缓冲层材料，改变Cr的含量就可以获得不同晶粒尺寸的反铁磁FeMn层.实验表明，晶粒尺寸较小的FeMn产生较强的铁磁/反铁磁交换偏置场；但是，对于较大晶粒的FeMn层，出现交换偏置作用所要的临界厚度较小.这符合Mauri提出的理论模型.交换偏置场的热稳定性实验表明，具有较大晶粒尺寸的FeMn层给出较 关键词： 交换偏置 热稳定性 反铁磁 晶粒尺寸     

Synchrotron X-ray diffractometry and imaging of strains and defects in icosahedral quasicrystal grains

In order to better understand the long-range propagation of quasicrystalline order, as well as quasicrystal stability, it is important to know if defects are generated in the quasicrystal grains during their growth. Previously, we studied the degree of perfection of about ten icosahedral quasicrystal grains of various alloys (Al–Pd–Mn, Al–Cu–Fe, Zn–Mg–Y), as grown or annealed, and we disclosed that some of them were much more perfect than the others. In this work we have concentrated on another slice of such a grain of the Al–Pd–Mn alloy. Similarly, we have performed an extensive synchrotron X-ray topographic investigation of strain and defects in this grain, combined with phase-contrast radiography and high resolution X-ray diffraction examinations. Very few two-lobe contrasts associated with pores and no loop-shaped contrasts were observed on the X-ray topographs, but straight line segments and band contrasts have been identified. Line segments could be considered as the result of the climbing of polygonal dislocation loops, as observed by TEM by Caillard and coworkers. This would indicate that most strains and defects observed in quasicrystal grains, at room temperature, are the result of stresses (external and internal) acting after growth.     

Computational study of the formation of aluminum-graphene nanocrystallites

The molecular dynamics method is used to study the formation of the Al/graphene nanocomposite in the structural grains of different size under the action of internal stresses. The behavior of graphene sheets inside an individual structural grain as well as in the process of two Al grains containing graphene are joined is investigated. The motion of graphene films, starting from the middle of the aluminum matrix, ends with their location at the crystallite boundaries. Graphene moves in the Al matrix along closely packed planes. In this case, graphene sheets acquire curvature. An intergrowth of graphene sheets is also observed. A contact between two Al-C nanocrystallites through a graphene interlayer is created. The self-diffusion coefficients of atoms and the partial potential energies increased with decreasing nanocrystallite size. The angular distribution of the nearest geometric neighbors and the distribution of distances to the nearest neighbors are determined using the construction of Voronoi polyhedra.     

Formation of surface roughness on nanocrystalline aluminium samples under straining by molecular dynamics studies

The surface roughening of nanocrystalline aluminium samples was investigated by molecular dynamics simulations. Attention was focused on the fact that roughness increases with the grain size and the strain. The elastic–plastic transition was found at around 3.5% strain and a reverse Hall–Petch effect was observed under straining conditions. Then, different strain distributions in grains and grain boundaries at the sample surface were highlighted, yielding to the formation of local roughness. Finally, a linear relationship between the magnitude of roughness and the out-of-plane strain component was found.     

生长温度对磁控溅射ZnO薄膜的结晶特性和光学性能的影响

采用反应射频磁控溅射方法，在Si (100) 基片上制备了具有高c轴择优取向的ZnO薄膜.利用 原子力显微镜、透射电子显微镜、X射线衍射分析、拉曼光谱等表征技术，研究了沉积温度 对ZnO薄膜的表面形貌、晶粒尺度、应力状态等结晶性能的影响；通过沉积温度对透射光谱 和光致荧光光谱的影响，探讨了ZnO薄膜的结晶特性与光学性能之间的关系.研究结果显示， 在室温至500℃的范围内，ZnO薄膜的晶粒尺寸随沉积温度的增加而增加，在沉积温度为500 ℃时达到最大；当沉积温度为750℃时，ZnO薄膜的晶粒尺度有所减小；在室温至750℃的范 围内，薄膜中ZnO晶粒与Si基体之间均存在着相对固定的外延关系；在沉积温度低于500℃时 ，制备的ZnO薄膜处于压应变状态，而750℃时沉积的薄膜表现为张应变状态.沉积温度的不 同导致ZnO薄膜的折射率、消光系数、光学禁带宽度以及光致荧光特性的变化，沉积温度对 紫外光致荧光特性起着决定性的作用.此外，探讨了影响薄膜近紫外光致荧光发射的可能因 素. 关键词： ZnO薄膜 表面形貌 微观结构 光学常数     

Microstructures of grain boundaries in (001)-oriented strontium bismuth tantalate thin films grown by pulsed laser deposition

(001)-oriented strontium bismuth tantalate thin films have been grown on Pt/TiO₂/SiO₂/Si (100) substrates by pulsed laser deposition. The room-temperature current–electric field dependence of the films has been investigated, which revealed a space-charge-limited conduction mechanism. The microstructures of grain boundaries and structural defects in these films were also examined by transmission electron microscopy and high-resolution transmission electron microscopy, respectively. The grains of the films deposited at 550 °C exhibited polyhedral morphologies, and the average grain size was about 50 nm in length and 35 nm in width. At a small misorientation angle (8.2°) tilt boundary, a regular array of edge dislocations with about 3-nm periodic distance was observed, and localized strain contrast near the dislocation cores was also observed. The Burgers vector b of the edge dislocation was determined to be [110]. At a high misorientation angle (39.0°) tilt grain boundary lattice strain contrast associated with the distortion of lattice planes was observed, and the mismatching lattice images occurred at about 2 nm along the boundary. The relationship between microstructural defects at grain boundaries and leakage currents of these films is also discussed. Received: 8 September 2000 / Accepted: 18 December 2000 / Published online: 28 February 2001     

Effect of Al mole fraction on structural and electrical properties of AlxGa1−xN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy

The effect of Al mole fractions on the structural and electrical properties of Al_xGa_1−xN/GaN thin films grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si (1 1 1) substrates has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and current-voltage (I-V) measurements. X-ray results revealed that the AlGaN/GaN/AlN was epitaxially grown on Si substrate. By applying Vegard's law, the Al mole fractions of Al_xGa_1−xN samples were found to be 0.11, 0.24, 0.30 and 0.43, respectively. The structural and morphology results indicated that there is a relatively larger tensile strain for the sample with the smallest Al mole fraction; while a smaller compressive strain and larger grain size appear with Al mole fraction equal to 0.30. The strain gets relaxed with the highest Al mole fraction sample. Finally, the linear relationship between the barrier height and Al mole fraction was obtained.