Cs. Cserháti

Investigation of the interplay of nickel dissolution and copper segregation in Ni/Cu(111) system

Abstract The effect of segregation on the dissolution of a thin Ni layer (3–14 equivalent-monolayers (eq-ML)) into a semi-infinite Cu substrate has been investigated at different temperatures (600–730 K) by Auger electron spectroscopy. Computer simulations have also been carried out to understand the details of the dissolution and a sophisticated method is proposed to analyse the experimental kinetics. The simulations indicated a step-wise character of dissolution and an interesting interference between segregation and dissolution. Because of the strong concentration dependence (there is a fast diffusion in Cu and there is practically no diffusion into Ni), the interface remains sharp and shifts proportionally with time until it does not reach the next-to-the-last Ni layer. Then the dissolution continues by the saturation of Cu in the topmost layer, according to the segregation isotherm. After the complete coverage of the surface by Cu, the final homogenisation takes place by the total dissolution of the next-to-the-last layer. According to these results, it was possible to separate the dissolution from the segregation and to determine the average speed of the interface shift, which is proportional to the intrinsic diffusion coefficient.

Segregation inhibited grain coarsening in nanocrystalline alloys

It will be shown, in the framework of a general statistical (atomistic) treatment, that segregation inhibited grain coarsening in binary nanocrystalline alloys can be described by analytical expressions, if the effect can be attained with highly saturated grain boundaries. Relatively simple relations describing the temperature dependence of the stabilized grain-boundary fraction will be derived. The validity conditions of the relation proposed very recently by R. Kirchheim [Acta Mater. 50, 413 (2002)] are also analyzed. Our conclusions are compared with experimental results and the reliability of the few experimental data, known at present, will be discussed as well.

Kinetics of surface segregation in alloys

Abstract The kinetics of surface segregation in ordering alloys are studied with the help of a multilayer model in slabs of different thicknesses. The time evolution of the concentrations of atomic layers perpendicular to the (100) planes of an AB-type BCC ordering alloy (CoFe) are calculated. As a result of the competition between the surface segregation of A atoms and ordering in the bulk, a metastable configuration with two anti-phase boundaries (APB) inside the slab, with odd numbers of layers, was obtained if we started from an ordered initial state with B atoms on the free surfaces (B-termination). The effect of the temperature was also investigated for a slab with 41 layers and from the calculation of the free energies of the metastable and stable states it was shown that the metastable state can exist below T 1 = 0.95 T c , where T c is the critical temperature of the order-disorder transition in bulk. It was obtained that T 1 decreases with decreasing thickness of the slab, while the difference of free energies of the metastable and stable states slightly increases. The effect of the slab thickness is a typical size effect: the time necessary to reach the steady state decreases with decreasing number of layers.

Size effects in surface segregation

The effect of the slab thickness (d) on the segregation behavior has been investigated for a phase separating system in a slab consisting of atomic planes parallel to the free surfaces. The calculations were carried out using the Fowler–Guggenheim approximation in the monolayer limit. It is shown, that by varying the surface fraction ξ, a surface phase transition can occur, similar to a semi-infinite system where a change in temperature can lead to a phase transition (S-shaped segregation isotherm). Furthermore, the existence of a minimum on the free energy curve versus d was examined. It was demonstrated that such a minimum can exist in the nanocrystalline region.

Nanoscale investigations of shift of individual interfaces in temperature induced processes of Ni–Si system by secondary neutral mass spectrometry

We describe a method for measurement of nanoscale shift of interfaces in layered systems by a combination of secondary neutral mass spectrometry and profilometer. We demonstrate it by the example of the investigation of interface shifts during the solid state reaction in Ni/amorphous-Si system. The kinetics of the shrinkage of the initial nanocrystalline Ni film and the amorphous Si layer as well as the average growth kinetics of the product phases were determined at 503 K. The results show that nanoscale resolution can be reached and the method is promising for following solid state reactions in different thin film systems.

Direct surface relief formation in As-S(Se) layers

The process of holographic recording based on a direct formation of periodic surface relief in AsxSe1-x (0 ≤ x ≤ 0.5) and As2S3 layers was investigated by in situ AFM depth profiling and compared with data on diffraction efficiency η of the similar relief holographic gratings, measured in a reflection mode. It is established, that the time (exposure) dependence of η has at least two components, which are connected with different components of the surface deformation Δd and relief formation up to the giant, Δd/d >10% changes in the best As0.2Se0.8 or As2S3 compositions. Correlation is found between light and e-beam induced surface deformations during recording in similar compositions. Applications for prototyping phase-modulated optoelectronic elements are considered.

Segregation and phase separation in nanocrystalline materials

Abstract Surface segregation has been calculated in a slab of thickness d. In ordering AB systems there is a competition between the surface segregation of A atoms and bulk ordering, and either a surface disturbed region or an antiphase boundary can be formed inside the slab. With decreasing d the average order can decrease well below the bulk value. In phase separating systems with strong segregation a phase separation from the surfaces develops and two layers of the A-rich phase cover the A-poor phase in the slab (lamellar structure) and there is an enrichment of A atoms in the near surface layers of the upper phase. For weakly segregating systems if T is below a certain temperature, Th, A-rich and A-poor phases can be in equilibrium in a columnar structure. With decreasing d there is an increased as well as a decreased solubility in A-poor and in A-rich phases, respectively. It was shown that a segregation stabilisation of the nanostructure is possible and with increasing d there is a sharp surface phase transition from a large to a small coverage.

Cathodoluminescent Features and Raman Spectroscopy of Miocene Hydrothermal Bio-mineralization Embedded in Cryptocrystalline Silica Varieties, Central Europe, Hungary

Variable cryptocrystalline silica varieties occur en masse in the fractures and cavities of the andesite host rock, showing different genetic and morphological features of variable microbial activity. Four types of CL features characterized silica phases. Raman spectroscopy supported variable amorphous carbon phases in inhomogenous distribution.

Segregation and phase separation in thin films

Abstract Surface segregation and phase separation in binary AB systems have been calculated in a slab consisting of atomic planes perpendicular to the free surfaces. For strongly segregating systems, if the average concentration lies in the two phase region of the binary phase diagram, a phase separation from the surfaces develops and two layers of the A-rich phase cover the A-poor central phase in the slab (lamellar structure). Furthermore, there is an enrichment (segregation) of A atoms in the near surface layers of the upper phase. For weakly segregating systems there exists a critical concentration C h . If T > T h (T h is the temperature on the miscibility gap at C h ), a similar phase separation and segregation has been observed as above. However, for T h , two different phases can be in equilibrium in a columnar structure. The concentrations in their central layers, if the slab is thick enough, correspond to the compositions of the miscibility gap at the given temperature. At the same time, there is a strong segregation of A atoms in the A-poor phase, while, due to the stronger separation tendency, the minority B atoms will ‘separate’ at the surface, leading to a desegregation of A atoms in the A-rich phase.