János L. Lábár

Electron Diffraction Based Analysis of Phase Fractions and Texture in Nanocrystalline Thin Films, Part I: Principles

A method for phase analysis, similar to the Rietveld method in X-ray diffraction, was not developed for electron diffraction (ED) in the transmission electron microscope (TEM), mainly due to the dynamic nature of ED. Nowadays, TEM laboratories encounter many thin samples with grain size in the 1–30 nm range, not too far from the kinematic ED conditions. This article describes a method that performs (semi)quantitative phase analysis for nanocrystalline samples from selected area electron diffraction (SAED) patterns. Fractions of the different nanocrystalline components are determined from rotationally symmetric ring patters. Both randomly oriented nanopowders and textured nanopowders, observed from the direction of the texture axis produce such SAED patterns. The textured fraction is determined as a separate component by fitting the spectral components, calculated for the previously identified phases with a priori known structures, to the measured distribution. The Blackman correction is applied to the set of kinematic diffraction lines to take into account dynamic effects for medium grain size. Parameters of the peak shapes and the other experimental parameters are refined by exploring the parameter space with the help of the Downhill-SIMPLEX. Part I presents the principles, while future publication of Parts II and III will elaborate on current implementation and will demonstrate its usage by examples, respectively.

Revealing the grain structure of graphene grown by chemical vapor deposition

The physical processes occurring in the presence of disorder: point defects, grain boundaries, etc. may have detrimental effects on the electronic properties of graphene. Here we present an approach to reveal the grain structure of graphene by the selective oxidation of defects and subsequent atomic force microscopy analysis. This technique offers a quick and easy alternative to different electron microscopy and diffraction methods and may be used to give quick feedback on the quality of graphene samples grown by chemical vapor deposition.

Simultaneous growth of Ni5Ge3 and NiGe by reaction of Ni film with Ge

The reaction between nanometric Ni films and Ge is analyzed using isothermal x-ray diffraction measurements and transmission electron microscopy. It is found that NiGe is formed during deposition at room temperature. The metal rich phase that grows during heat treatment has been clearly identified to be Ni5Ge3. The simultaneous growths of Ni5Ge3 and NiGe have been observed on amorphous and polycrystalline germanium. This is in contrast with the usual sequential growth reported in thin films.

Electron Diffraction Based Analysis of Phase Fractions and Texture in Nanocrystalline Thin Films, Part II: Implementation

This series of articles describes a method that performs (semi)quantitative phase analysis for nanocrystalline transmission electron microscope samples from selected area electron diffraction (SAED) patterns. Volume fractions of phases and their textures are obtained separately in the method. First, the two-dimensional SAED pattern is converted into an X-ray diffraction-like one-dimensional distribution. Volume fractions of the nanocrystalline components are determined by fitting the spectral components, calculated for the previously identified phases with a priori known structures. Blackman correction is also applied to take into account dynamic effects for medium grain sizes. Peak shapes and experimental parameters (camera length, etc.) are refined during the fitting iterations. Parameter space is explored with the help of the Downhill-SIMPLEX algorithm. Part I presented the principles, while Part II now elaborates current implementation, and Part III will demonstrate its usage by examples. The method is implemented in a computer program that runs under the Windows operating system on IBM PC compatible machines.

Partial amorphization of a Cu-Zr-Ti alloy by high pressure torsion

High pressure torsion was applied to produce disk-shape specimen of Cu60Zr20Ti20 composition. Radial dependence of the microstructure was monitored by x-ray diffraction, scanning, and transmission electron microscopies. The disk consists of a top surface layer, homogeneous on a micrometer scale with an average thickness of 10–20μm, and an inhomogeneous bulk region of 200μm thickness. Calorimetric studies revealed that the disk contains detectable amount of amorphous phase. Characteristics of this amorphous content were compared to a fully amorphous melt-quenched Cu60Zr20Ti20 ribbon.

High pressure torsion of amorphous Cu60Zr30Ti10 alloy

High pressure torsion was applied to produce a disk-shaped specimen of Cu60Zr30Ti10 composition. The dependence of the morphology, microstructure, and thermal behavior on the applied shear strain was monitored by scanning and transmission electron microscopies, synchrotron x-ray diffraction, and calorimetry. The disk consists of a gradient microstructure ranging from large homogeneous blocks (about 20 μm) to finely dispersed nanocrystals (about 20 nm) of two stable hexagonal phases and continuously decreasing amorphous content with increasing strain. The evolution of such microstructure was interpreted by using a model based on heat conduction generated by the extensive shear deformation.

Kirkendall voids and the formation of amorphous phase in the Al‐Pt thin‐film system prepared by high‐temperature successive deposition

We report on the amorphization processes taking place in the Al‐Pt thin‐film system prepared by high‐temperature successive deposition. The formation of Kirkendall voids in the Al film was directly proved and intensively studied in relation to the percentage of Al and Pt components. An estimation of the composition of the amorphous phase is given, and the model proposed for the amorphization process is discussed. The films were analyzed by transmission electron microscopy, cross‐sectional transmission electron microscopy, selected area electron diffraction, and energy dispersive x‐ray microanalysis.

Initial formation and growth of an amorphous phase in Al-Pt thin films and multilayers: Role of diffusion

Despite their technological importance for nanoscale technologies, the initial stages of reaction at interfaces (and their dependence on experimental conditions) are still poorly understood. In this article we analyze the initial stages of solid state reaction (SSR) in Al/Pt multilayers (period: 54 nm, overall composition: Al4Pt) and compare the results to those recently obtained by high temperature sequential deposition (HTSD). These two methods differ in several aspects, the most important being the state of reacting Pt (solid in SSR, vapor in HTSD) and its flux (limited in HTSD by the deposition rate). The chemical driving force for the Al/Pt reaction are thus different. We have shown that: (i) The first reaction product observed during SSR or HTSD is the same: namely, a metastable amorphous Al2Pt phase (a-Al2Pt): (ii) This amorphous phase grows in a layer by layer mode. Its growth is diffusion controlled during SSR and stops when all the available Pt is consumed. In the case of HTSD the layer growth i...

Microstructure and Mechanical Behavior of Ultrafine-Grained Titanium

Ultrafine-grained titanium was processed by severe plastic deformation (SPD). The SPD was carried out by equal channel angular pressing (ECAP) at high temperature. The ECAPprocessed sample was further deformed by conventional techniques such as radial forging and drawing. The microstructure was characterized quantitatively by X-ray diffraction line profile analysis and transmission electron microscopy after each step of deformation. The effect of procesing routes on the mechanical behavior was also studied. It was found that the conventional deformation processes after ECAP result in further increment in dislocation density and strength at the expense of ductility.

Lattice and grain-boundary diffusion of As in Ni2Si

The diffusion coefficient of As in 260 nm thick polycrystalline Ni2Si layers has been measured both in grains and in grain boundaries (GBs). As was implanted in Ni2Si layers prepared via the reaction between a Si layer and a Ni layer deposited by magnetron sputtering on a (100) Si substrate covered with a SiO2 film. The As concentration profiles in the samples were measured using secondary ion mass spectroscopy before and after annealing (400–700 °C). The diffusion coefficients in the grains and the GBs have been determined using two-dimensional finite element simulations based on the Fisher model geometry. For short time annealing (1 h) and temperatures lower than 600 °C, lattice diffusion has not been observed. However, GB diffusion was evidenced for temperatures as low as 400 °C. For higher thermal budgets, As diffuses simultaneously in the volume of the grains and in the GBs. Lattice diffusion is characterized by a pre-exponential factor D0v∼1.5×10−1 cm2 s−1 and an activation energy Qv∼2.72±0.10 eV. I...