Papers by Author: Z. Balogh

Abstract: In a set of recent papers we have shown that the diffusion asymmetry in diffusion couples (the diffusion coefficient is orders of magnitude larger in one of the parent materials) leads to interesting phenomena: i) sharp interface remains sharp and shifts with non Fickian (anomalous) kinetics [1-5], ii) originally diffuse interface sharpens even in ideal (completely miscible) systems [6,7], iii) an initially existing thin AB phase in A/AB/B diffusion couple can be dissolved [8], iv) there exists a crossover thickness (typically between few nanometers and 1m) above which the interface shift turns back to the Fickian behaviour [9], v) the growth rate of a product of solid state reaction can be linear even if there is no any extra potential barrier present (which is the classical interpretation of the “interface reaction control” for linear kinetics) [10]. These latter results will be summarized and reformulated according to the usual expression for linear-parabolic law containing the interdiffusion coefficient, D, and interface transfer coefficient, K. Relation between the activation energies of D and K will be analyzed and compared with available experimental data.

Abstract: Diffusion controlled processes play a crucial role in the degradation of technical materials. At low temperatures the most significant of them is the diffusion along grain boundaries. In thin film geometry one of the best methods for determining the grain boundary (GB) diffusion coefficient of an impurity element is the Hwang-Balluffi method, in which a surface sensitive technique is used to follow the surface accumulation kinetics. Results of grain boundary diffusion measurements, carried out in our laboratory by this method in three different materials systems (Ag/Pd, Ag/Cu and Au/Ni) are reviewed. In case of Ag diffusion along Pd GBs the surface accumulation was followed by AES method. The data points can be well fitted by an Arrhenius function with an activation energy Q=0.99eV

Abstract: Depending on the thermodynamic, structural and diffusion properties of the system, a thin deposit dissolves into a substrate by different mechanisms. In this communication these different behaviours, investigated by surface analytical techniques (AES, XPS, STM, UPS, etc) [ - ], are reviewed. The experiments were also supported by computer simulations. The obtained results are compared and it is summarized how different parameters influence the dissolution of a thin film in a substrate. Furthermore, it is show that i) the volume dissolution kinetics is different on the atomic-/nano-scale than on the microscopic scale due to the diffusion asymmetry ii) the volume and GB diffusion in one measurement can be separated and iii) pure (C-kinetic) GB diffusivities can be determined from thin film kinetics measurements performed under adequate conditions.

Abstract: X-ray standing wave technique has been used to measure the kinetics of CoSi intermetallic phase growth in a-Si/Co/a-Si sandwich structure. The a-Si/Co/a-Si arrangement were placed into a waveguide structure formed by two Ta films. X-ray fluorescence and extended X-ray absorption fine structure analysis has been used in a combination with X-ray standing wave technique for depth profiling with sub-nanometer resolution of specimens annealed at 493K for different annealing time. The position and the thickness of the growing CoSi intermetallic phase have been monitored.

Abstract: Solid state reactions between amorpous Si and crystalline Co have been investigated by synchrotron radiation at Bessy (Berlin, Germany). The multilayered samples (with 10 periods of a-Si(15 nm)/Co(15 nm) layers) were produced by magnetron sputtering and isothermally heat treated at temperatures between 523 and 593 K. From the time evolution of the XRD spectra first the growth rate of the CoSi phase as well as the decay rate of the Co layer we determined (at 523 and 543 K). The kinetics were described by a power law; tk, and for the growth of CoSi k=0.65 while for the loss of the Co the k=0.77 was obtained, respectively. At higher temperatures (at 573 and 593 K) the formation and growth of the Co2Si layer, at the expense of the Co and already existing CoSi layers, was observed with exponents of about 1 for all the above kinetics. These results, together with the results of resistance kinetics measurements, in similar multilayered as well as bi-layered samples at similar temperatures, providing similar exponents will be presented. Possibility of the interface reaction control and/or the effect of the diffusion asymmetry (which was recently published for the interpretation of solid state reactions with non-parabolic kinetics on the nanoscale) will be discussed.

Abstract: Solid state reactions between amorphous Si and crystalline Co have been investigated by 4W electrical resistance and TEM. Multilayered (with 10 periods of 5nm a-Si/5nm Co and 10 nma- Si/10nm Co layers) as well as tri-layered samples (20nm a-Si/3nmCoSi/6nm Co) were produced by magnetron sputtering and isothermally heat treated at different temperatures between 473 and 523K. From the time evolution of the normalized resistance the kinetics of the process were determined by fitting a power law, tk, and k was between 0.8 and 1. Possibility of the interface reaction control and/or the effect of the diffusion asymmetry (which was recently published for the non-parabolic interface shifts on the nanoscale) will be discussed.

Abstract: Temperature dependence of different parameters (the position of the inflexion point and the saturation value on the root main square, RMS, values versus exciting field curves) of the Barkhausen noise is measured in structural steel (S 235 JRG1). It is shown that while the position of the inflexion point remained constant, the RMS value at the inflexion point and saturation value increased with the increasing temperature, T. Most interestingly the field required for saturation decreased with decreasing temperature and had a breakpoint at about 200K. Breakpoints at the same temperature on the critical exponents versus temperature functions (i.e. on the β(T) and α(T) curves, where β and α are the exponents of the probability distributions of peak heights and durations, respectively) were also observed. This temperature can be identified as the ductile-brittle transition temperature.

Abstract: Barkhausen noise properties of amorphous and nanocrystallized FINEMET type soft magnetic materials are investigated. It is obtained that the amorphous-nanocrystalline tranformation has a well observable effect on both the root main square, RMS, values and the critical exponents of the probability distributions of peak heights and durations.