Dezső L. Beke

Determination of diffusion coefficients of Zn, Co and Ni in aluminium by a resistometric method

Abstract The impurity diffusion parameters of Zn, Co and Ni have been determined in aluminium by measurements of electrical resistance. It is shown experimentally that in the case of Co and Ni, surface layers with constant concentrations were formed during the isothermal diffusion treatments and so the resistance measurements made possible the determination of the temperature dependence of the saturation concentrations too. It is also shown that the determination of the diffusion activation energy is possible from resistance measurements when the initial and boundary conditions are not exactly known (but when they do not vary with temperature). The measurements gave for the pre-exponential factors (D 0) and the energies of activation (Q) : D 0 Zn = (0.20 ± 0.08) cm2/s, QZn = (1.25 ± 0.02) eV; D 0co = (141 ± 137)cm2/s, Q Co = (1.75 ± 0.07) eV; D 0 Ni = (4.4 ± 3.l) cm2/s, Q N1 = (1.51 ± 0.06) eV, respectively. The diffusion parameters of Zn and Co are in good agreement with previous results in the literatur...

Volume diffusion of iron in Fe3Al: Influence of ordering

Abstract The lattice self diffusion coefficient of 59 Fe in Fe 3 Al has been determined in the temperature range 847–1503 K using the classical radio-tracer method. Below 1014 K the Arrhenius plot has a downwards curvature resulting from the B2 type of atomic ordering. The temperature dependence of the self-diffusion coefficient can be described by: D v = ( 3.2 1.5 + 2.8 × 10 − 5 ) × exp ( − ( 204 ± 6.1 kJ / mol ) × ( 1 + ( 0.1 ± 0.03 ) × S 2 ) R T ) ( m 2 / s ) where S is the long-range order parameter.

Interdiffusion in amorphous Si/Ge multilayers by Auger depth profiling technique

It has been shown by the Auger depth profiling technique that the concentration profile at the initially sharp Si/Ge interface in amorphous Si/Ge multilayers shifted but remained still sharp after a heat treatment at 680 K for 100 h. At the same time the fast diffusion of Si resulted in the formation of an almost homogeneous Ge(Si) amorphous solid solution, while there was practically no diffusion of Ge into the Si layer. This is direct evidence on the strong concentration dependence of the interdiffusion coefficient in amorphous Si/Ge system, and it is in accordance with the previous indirect result obtained from the measurements of the decay of the small angle Bragg peaks, as well as with finite difference simulations.

Laser-induced optical changes in amorphous multilayers

It is shown that the well-known blueshift of the fundamental absorption edge in as-deposited compositionally modulated amorphous Si/Ge and As6Se94/Se80Te20 multilayers (with periods of 4–8 nm) is further enhanced due to the thermal or laser-induced intermixing of adjacent layers. The laser-induced intermixing process, as supported by experiments and model calculations, can be attributed to both the local heating and photo-effects in As6Se94/Se80Te20 multilayers, while only the thermal effects were observed for Si/Ge multilayers. Structural transformations, based on this enhanced interdiffusion, provide good capability for spatially patterning optoelectronic devices and digital information recording.

Grain-boundary diffusion: Analysis of the c kinetic regime

Abstract The possibility of measuring the ‘true’ grain boundary (gb) diffusion coefficient in the so-called C kinetic regime is demonstrated for self-diffusion of silver. Although the criteria for the C-type diffusion have sometimes been experimentally established, we point out the existence of a transitional region in which the classical analysis of gb diffusion profiles (assuming either B- or C-type conditions) is wrong. In this region an alternative method, initially proposed by one of us, is tested. It allows the determination of both the true gb double products and the true gb diffusion coefficients from the experimental profiles. The ratio of these values gives the thickness δ of the gb, and the values obtained justify the usual assumption δ=0·5 nm.

Diffusion-induced bending of thin sheet couples: Theory and experiments in Ti-Zr system

Numerical and analytical calculations of concentration and stress distributions of thin-sheet diffusion couples have been carried out as well as the time dependence of the Kirkendall shift, xk, and the curvature has also been determined. It is shown that the concentration distribution is not sensitive to the boundary conditions (bent and planar, constrained, samples) and is influenced mainly by the feeding back effects of stresses (described by the stress term in the genealized diffusion potential) only. The stress distributions obviously are different for bent and planar samples and the effect of cutting off, caused by the dislocation glide, is also illustrated. It is found that the Kirkendall shift follows the parabolic law only in high creep rate limit. For intermediate creep rates, as a function of the time, t, a change of the slope of the xk(t) function is expected due to the stress development and relaxation. It is shown that the curvature of samples, caused by the diffusion stresses, is proportional to the annealing time and the difference of the intrinsic diffusion coefficients in a wide range of input parameters. By the example of experiments on Ti-Zr thin-sheet diffusion couples it was illustrated that the theoretical results are in good agreement with the measurements.

Magnetic properties of ball milled nanocrystalilne Ni and Fe

Abstract Magnetic properties of pure nanocrystalline nickel and iron produced by ball milling were investigated by low field magnetization measurements and by Barkhausen-noise technique at room temperature. The coercivity He followed the predictions of the random anisotropy model, shows a peak in the nanocrystalline region. The saturation magnetization is practically independent of the grain size in iron and similar behaviour can be deduced for nickel if the effect of iron contamination (caused by ball milling) is extracted from the magnetization curve.

Comparative Microstructural Study of the Diffusion Zone between NiCr Alloy and Different Dental Ceramics

Our knowledge on the bonding mechanisms between the metal and ceramic parts of dental systems is very limited. This work tested the hypothesis that the details of the interface processes can be described in the framework of a chemical diffusion model. The development of interfacial phases was investigated by cross-sectional analytical transmission electron microscopy between a NiCr (Wiron 99) alloy and three different dental ceramics (Carat, Vita VMK 95, and Vision). All systems were investigated at normal firing conditions (suggested by the manufacturer) and at increased firing times as well. The conclusions are based on the results that the formation of a nanocrystalline Cr2O3 layer and amorphous silicon oxide inclusions were detected in the early stage of the firing process in all investigated systems, and that, in the case of Carat and Vision ceramics, formation of complex NiCr and NiCrTi oxides was also observed at longer annealing times. It is shown by transmission electron microscopy that, in the reaction processes taking place at the NiCr alloy/dental ceramics interface, nanocrystalline Cr2O3 first forms and amorphous silicon oxide inclusions appear, then, at longer firing times, complex NiCr and NiCrTi oxides form.

Production and magnetic properties of nanocrystalline Fe and Ni

Abstract Magnetic properties of pure nanocrystalline iron produced by ball-milling and heavy cold deformation are investigated, using low field magnetization measurements at low temperatures and Barkhausen-noise technique at room temperature. It was observed that the saturation magnetization was practically constant up to the grain size of D = 7 nm. On the other hand, the coercive force, H, determined from the Barkhausen-noise curves, was very sensitive to the grain size. Furthermore, the Barkhausen-noise values showed a systematic, but different behavior with the grain size for the ball-milled powders and for the cold-worked wires. Similar measurements were carried out for cold-worked nickel wires.

Inversion of the direction of photo-induced mass transport in As20Se80 films: Experiment and theory

Diffusion mass transfer in thin chalcogenide films under illumination by a focused Gaussian beam have been studied both experimentally and theoretically. It is demonstrated that depending on the light intensity, waist of the beam, and the film thickness, one can obtain formation of either hillocks or dips in the illuminated regions. By comparison of the kinetics of hillock or dip formation on a surface of As20Se80 glass films with the results of our theoretical analysis, we have estimated the photo-induced diffusion coefficients, D, at various light intensities, I, and found D to be proportional to I (D = βI), with β ≈ 1.5 × 10−18 m4/J.