Zoltán Balogh

Interface sharpening in miscible Ni/Cu multilayers studied by atom probe tomography

Interfaces of Ni/Cu multilayers were studied by atom probe tomography. To this aim, specimens with sharp or artificially smeared interfaces were prepared and investigated before and after annealing at 773u2009K. Owing to three-dimensional subnanometer resolution of the atom probe, local chemical analysis of layer interfaces becomes possible without interferences of grain boundaries or geometric roughness. In contrast to the classical expectation for a miscible system, but in agreement with more recent theoretical considerations, diffusion reduces the chemical width of the interfaces by up to 50%.

Atom probe tomography of lithium-doped network glasses.

Li-doped silicate and borate glasses are electronically insulating, but provide considerable ionic conductivity. Under measurement conditions of laser-assisted atom probe tomography, mobile Li ions are redistributed in response to high electric fields. In consequence, the direct interpretation of measured composition profiles is prevented. It is demonstrated that composition profiles are nevertheless well understood by a complex model taking into account the electronic structure of dielectric materials, ionic mobility and field screening. Quantitative data on band bending and field penetration during measurement are derived which are important in understanding laser-assisted atom probe tomography of dielectric materials.

Triple line diffusion in nanocrystalline Fe/Cr and its impact on thermal stability.

The thermal reaction of iron-chromium multilayers is analyzed by atom-probe tomography. Samples were prepared using ion-beam sputter deposition and cutting by focused ion beams. Isothermal and isochronal annealing sequences were carried out in a vacuum furnace. Effects of atomic transport are observed at temperatures above 773 K. Segregation along line-shaped zones is noticed to very high concentrations. These zones, with a diameter of 1.5 nm, are identified as triple lines of the grain structure. While these defects could not be resolved by TEM, the outstanding potential of a 3D analysis provided by APT allowed their detailed investigation. Evaluating the dependence of the segregation amplitude on time and temperature, the segregation enthalpy and diffusivity of the triple lines are quantified. The segregation enthalpy is determined to be 0.076 eV, which indicates the considerable excess volume at the triple line.

Investigation of Interfaces by Atom Probe Tomography

We investigated the thermodynamic and transport properties of buried interfaces with atom probe tomography. Owing to the 3D subnanometer resolution and single atom sensitivity of the method, it is possible to obtain composition profiles with high accuracy both along or normal to the interfaces. We have shown that the width of the chemical interface between the Fe and Cr system follows the Cahn–Hilliard relation with a gradient energy coefficient of 1.86xa0×xa010−22xa0Jxa0nm2. Sharpening of the Ni/Cu interface as a result of kinetic control was directly observed. We investigated the grain boundary and triple junction transport in Fe/Cr and Ni/Cu. Cr segregation enthalpy into Fe triple junctions was found to be 0.076xa0eV, which falls in between the surface (0.159xa0eV) and grain boundary (0.03xa0eV) segregation enthalpies. In the investigated 563xa0K to 643xa0K (290xa0°C to 370xa0°C) range, Ni transport is 200 to 300 times faster in the triple junctions of Cu than in the grain boundaries. The diffusion activation enthalpy in the triple junctions is two-thirds that of the grain boundaries (0.86 and 1.24xa0eV, respectively). These investigations have shown that triple junctions are defects in their own right with characteristic segregation and diffusion properties: They are preferred segregation sites and can be considered as a diffusion shortcut in the grain boundary network.

Diffusion in Metals and Alloys

Transport in materials via random individual atomic migration steps (jumps) is called ‘diffusion’. General understanding and detailed theory of diffusional transport represent a classical field of material science. While in liquids and gases, diffusion rate ranges up to millimeters or even centimeters per second, transport in solid materials is rather slow. In densely packed metals near to the melting point, one can expect about a micrometer per second and this rate drops down to about a nanometer per second at half the melting temperature. At room temperature atomic migration is practically frozen, except for few exceptional cases of small interstitial impurities. Measurement of diffusion in solids needs therefore sensitive techniques and well-developed microscopy. Not surprising that a scientific proof of diffusion in solid metals came rather late in history of science (Roberts-Austen, 1896).

Trapping Effect on the Kinetic Critical Radius in Nucleation and Growth Processes

The critical nucleus size—above which nuclei grow, below dissolve—during diffusion controlled nucleation in binary solid-solid phase transformation process is calculated using kinetic Monte Carlo (KMC). If atomic jumps are slower in an A-rich nucleus than in the embedding B-rich matrix, the nucleus traps the A atoms approaching its surface. It doesn’t have enough time to eject A atoms before new ones arrive, even if it would be favourable thermodynamically. In this case the critical nucleus size can be even by an order of magnitude smaller than expected from equilibrium thermodynamics or without trapping. These results were published in [Z. Erdélyi et al., Acta Mater. 58 (2010) 5639]. In a recent paper M. Leitner [M. Leitner, Acta Mater. 60 (2012) 6709] has questioned our results based on the arguments that his simulations led to different results, but he could not point out the reason for the difference. In this paper we summarize our original results and on the basis of recent KMC and kinetic mean field (KMF) simulations we show that Leitner’s conclusions are not valid and we confirm again our original results.

Concentration Dependence of the Diffusion in the Ni/Cu System

Deviations from the Fickian-laws of diffusion in the case of concentration dependent diffusion coefficients and high composition gradients gain more and more acceptance nowadays. The cause of this phenomenon is the finite permeability of the atomic layers, or in other words “interface control”. The consequences are wide-spreading e.g. linear diffusion kinetics, deviations in the nucleation behavior of reaction products and kinetically determined interface shape in miscible alloys. Furthermore, if the original chemical interface is broader than the optimum width, even a sharpening of the interface by diffusion can be observed. Previous experiments proving these effects used more or less ideal specimens (e.g. single crystalline or amorphous samples with very flat interfaces) and some doubts can be raised whether these effects can be observed in a realistic specimen with a more complex grain structure. In this talk we will present the results of atom probe measurements on sputter deposited Ni/Cu multilayers (containing surface roughness, lattice defects, etc.). Samples with sharp and smeared Ni/Cu interfaces were produced and later annealed. We found an asymmetry on the interface width in the as-prepared specimens depending on the stacking order. After annealing this asymmetry vanished and remarkably the Cu/Ni interface sharpened by diffusion. After short diffusion time, the interface width became independent on the sample origin (sharp or smeared interface) proving the kinetic control of the interface. Atom probe tomography also allows the direct, local investigation of the grain boundary diffusion in any single grain boundaries. Surprisingly the best description of the shortcut transport can be achieved by assuming a concentration-independent grain boundary diffusion coefficient.

Interfacial reaction and phase growth for various metal/amorphous silicon system

The practical importance of the reactions between a semiconductor and a metal system cannot be overstated. Fundamental physics itself offers also a wide variety of interesting phenomena. Unlike the “clean” metal-metal reactions nucleation or interface control is observed for some metal-silicon reactions.

Physics on the Top of the Tip: Atomic Transport and Reaction in Nano-Structured Materials

Nanoscale systems show a wide variety of physical properties that cannot be observed in the bulk. Using atom probe tomography, it is possible to study nanostructured materials with almost atomic resolution in all three dimensions. In this article, we will present a short review of the latest atom-probe measurements carried out at University of Münster with particular focus on diffusion and segregation measurements in triple junctions and interface analysis.