Yuri S. Kaganovsky

Diffusion of Silver in Silicate Glass and Clustering in Hydrogen Atmosphere

Annealing in a hydrogen atmosphere of silicate glass plates doped by Ag+ ions leads to the reduction of silver to a metallic state (Ag0) and to the formation of silver nanoclusters. The kinetics of clustering during hydrogen diffusion into the glass and diffusion of Ag0 atoms in the glass matrix have been studied in a temperature ranging from 160 to 200oC by SEM, AFM and optical spectrometry. The absorption spectra have a peak near 410 nm corresponding to the surface plasmon resonance in Ag clusters. The position of the peak moves as the clusters grow. A theoretical analysis of the absorption spectra allowed us to estimate the cluster size as a function of time, as well as the thickness of the layer filled by clusters, which also changes with time. From AFM data we could measure the kinetics of cluster growth on the surface. We have theoretically analyzed the kinetics of cluster growth during reactive hydrogen diffusion, the kinetics of bulk cluster growth, surface cluster growth, and thickening of the layer filled by clusters.

Lateral Diffusion Spreading of Two Competitive Intermetallic Phases over Free Surface

The kinetics of growth and lateral spreading of intermetallic layers during surface interdiffusion in Cu – Sn system has been studied in a temperature range 160 – 200oC by the methods of optical microscopy, SEM provided with X-ray microprobe, and AFM. Lateral phase spreading over the surface is characterized by competition between two phases: Cu6Sn5 and Cu3Sn. A steady state solution for concentration distribution on the surface of growing intermetallic phases, as well as kinetic equations of lateral spreading of growing phase layers have been obtained. By comparison of experimental data on intermetallic growth kinetics with the proposed theory, the dynamic surface diffusion coefficients have been calculated.

Size-Dependent Interdiffusion in Nanomaterials

The phenomenon of low-temperature homogenization (LTH) during interdiffusion is studied under condition a t Dv £ 2 / 1 ) ( (Dv is the bulk diffusion coefficient, a is the lattice parameter) using nano-objects of binary Cu-Ni and Cr-Ni systems compacted from nano-powders and produced by mechanical alloying. Two stages of LTH were detected: at the first stage (t £ 103 s) the volume fraction of solution rapidly grows; at the second stage (t > 103 s) the volume fraction of solutions grows slowly with practically constant average solution concentration. The first stage of LTH correlates with active grain growth caused by small size (l) of structural element and nonequilibrium structure of nano-objects. Obtained results are analyzed theoretically in terms of interdiffusion along migrating GBs due to grain growth at the first stage and DIGM mechanism at the second stage. It is shown that the GB concentration distribution during interdiffusion along migrating GBs and the kinetics of LTH depend on a parameter l/l where 2 / 1 ) / ( b b V sD d l= is the characteristic diffusion length. The mechanisms and criteria of LTH are proposed.

Grain Boundary Induced Lateral Propagation of Intermetallic Phases in Nano-Grained Cu-Sn Thin Film Couples

The kinetics of lateral Cu6Sn5 and Cu3Sn phase propagation induced by grain boundary (GB) interdiffuson in thin-film diffusion couples Cu-Sn were studied in a temperature range 160-180oC by optical microscopy, AFM, SEM, and energy-dispersive X-ray spectroscopy (EDS). Nano-grained Cu and Sn films were sequentially deposited on glass substrates with 5 – 20 µm overlap. To prevent surface diffusion and thus separate GB-diffusion contribution into kinetics of phase propagation, the surfaces of diffusion couples were covered by a thin (20 – 40 nm) carbon layer. It was found that the rates of lateral Cu6Sn5 and Cu3Sn phase spreading in thin-film couples exceed several times the spreading rates of the same phases over the surface of coarse-grained samples and 50 – 70 times exceed the rates in the bulk of massive samples. Kinetics of lateral phase spreading both in thin-film and in massive diffusion couples obeys parabolic law. Similarly to A and B regimes for GB tracer diffusion, A and B regimes of GB reactive diffusion were found in the spreading Cu3Sn phase. The kinetics of the phase propagation turned out independent of the film thickness (in the range 40 – 200 nm) if the films possessed similar grain size, whereas the kinetics was rather sensitive to the grain size and GB structure. Theoretical analysis of the phase propagation kinetics accelerated by GB diffusion has been done and the phase propagation rates have been calculated. By comparison experimentally measured phase propagation rates with the calculated ones we determined the GB diffusion coefficients of Sn in both growing phases.

Calculation of Surface Self-Diffusion Coefficients from AES Data on Decay of Thin Metal Films

We propose a simple method for calculation of surface self-diffusion coefficients using kinetic data on the decay of thin films – void growth and transformation of the island shape to the equilibrium. Calculations are made taking into account equilibrium wetting angle of the film on a substrate. The kinetic data on the decay of Pd thin films on sapphire and silica substrates were obtained using Auger electron spectroscopy. By in situ monitoring the intensity of the Auger signal from the film, three different stages of the decay could be distinguished. The surface self-diffusion coefficients were calculated for the temperature range 583 – 823 K. The values of the surface diffusion coefficients and the activation energies are discussed compared to those obtained by other methods.

Intermetallic Growth: Structure-Sensitive Effects

The kinetics of intermetallic growth in the bulk, along the surface and grain boundaries (GBs) was studied in the Cd-Ni, Cd-Cu and Cu-Sn systems. Bulk dynamic diffusion coefficients exceed by a few orders of magnitudes the tracer self-diffusion coefficients in homogeneous phases. The reasons for this difference are discussed in terms of departure of growing intermetallics from stoichiometry due to the simultaneous existence of two processes: interdiffusion through the growing phase layer and chemical reactions at interfaces. Accelerating contribution of GBs and free surfaces, as fast diffusion paths, into diffusion penetrability of growing intermetallics has been investigated. It was found that the rates of lateral phase spreading along free surfaces and GBs exceed several times the rate of phase growth in the bulk. Accelerating GB contribution depends on the grain size. Nano-dispersed thin films demonstrate maximal phase propagation rates, which several times exceed even the rates of lateral phase spreading along free surfaces in coarse-grained polycrystals.

Stress Relaxation Mechanisms during Cd21Ni5 Intermetallic Growth under High Hydrostatic Pressure

Stress relaxation processes accompanying intermetallic growth during reactive diffusion between Cd and Ni have been studied by the methods of optical and scanning electron microscopy, provided with X-ray microanalysis. The experiments were carried out with the two-layer Cd-Ni samples at 250 and 280oC under hydrostatic pressures 350-900 MPa. The observed processes have been compared with those occurred at low pressures to demonstrate that the mechanisms of stress relaxation and thus the kinetics of intermetallic growth essentially depend on applied hydrostatic pressure. New mechanisms of stress relaxation were found, such as Cd extrusion and Cd whisker growth, which accompanied formation of Cd21Ni5 compound. It is shown, that the whisker growth is more probable at lower temperatures when the grain size is smaller, and the stress gradient, which support a driving force for whisker formation, is higher. An atomic mechanism for whisker growth, based on diffusion climbing of dislocation loops produced by Bardeen-Herring source for building new atomic layers, has been discussed.

Spatially Periodic Formation of Nanoparticles in Metal-Doped Glasses

In the course of reactive diffusion of hydrogen in metal-doped glasses, at some conditions, metallic nanoparticles grow forming quasi-periodic layered structure. We have developed a model defining conditions necessary for the formation of the layered structures. The model indicates relatively narrow range of parameters providing the quasi-periodic growth of the nanoparticles. The layered structure arises at relatively low over-saturation by neutral metal in the diffusion zone, due to the competition of two processes: enrichment of the glass by neutral metal atoms via reducing of metal ions by diffusing hydrogen and depletion of the glass by the metal atoms caused their diffusion to the nanoparticles. The model can be also applied to other situations where reactive diffusion inducing the formation and growth of nanoparticles occurs.