Journal of Nano Research Vol. 7

Abstract: . In intermetallic compounds, random vacancy motion is not possible as it would disrupt the equilibrium ordered arrangement of atoms on lattice sites. In view of this limitation, various atomistic models have been proposed, which allow atom-vacancy exchanges to take place without concomitant long range disordering. For a L12 -type A3B structure, the major element A diffuses faster than the minor element B. The trend is attributed to the different diffusing paths; A atoms can diffuse through site exchanges with a neighbouring vacancy on its own sublattice, while the jump of a B atom to a neighbouring site always creates wrong bonds. For L10-type structures such as γ-TiAl, significant diffusion anisotropy is observed; Ti atoms diffuse on the Ti sublattice, while Al atoms also diffuse on the Ti sublattice. The formation of hollow metal oxide nanoparticles through the oxidation process has been studied by transmission electron microscopy for Cu, Zn, Al, Pb and Ni. The hollow structure is obtained as a result of vacancy aggregation, resulting from the rapid outward diffusion of metal ions through the oxide layer during the oxidation process. This suggests the occurrence of two different diffusion processes in the formation of hollow oxides.

Abstract: Results of kinetic Monte Carlo simulation of the formation of a hollow nanosphere by interdiffusion from a core-shell binary system are presented for the first time. The faster diffusing species is located in the core whilst the slower diffusing species form the shell. With its self-generated vacancy composition all stages of the hollow sphere formation process are observed in our model: interdiffusion, the supersaturation of the core of the nanosphere by vacancies, precipitation of pores and eventual void formation. Results of this simulation confirm the experimental conclusions that interdiffusion accompanied by the Kirkendall effect and Kirkendall porosity is one of the mechanisms responsible for the formation of hollow nano-objects.

Abstract: Recently, there has been a great deal of interest in the properties of hollow nanoparticles for use in advanced technologies. The diffusion phenomenon known as the Kirkendall effect features in one of the important experimental methods of synthesis of hollow binary nanoparticles. Diffusion naturally features prominently in shrinkage mechanisms of hollow nanoparticles. In this paper, we summarize the progress made so far in understanding the formation and shrinkage by diffusion processes of hollow nanoparticles and their apparent stability.

Abstract: The major problem of a powder material authority – the prediction of structural-phase composition in the production of new multicomponent materials by means of the mathematical description and making of the computer software for homogenization processes modeling in the binary metal-metal systems is solved. The objects of study are processes of nanopowders homogenization in solid-state metals nanosystems of the transition elements such as Fe-Mo, Fe-Cu, Fe-Ni, Fe-Cr. A complex theoretical and experimental study of homogeni¬zation processes in solid-state nanosystems is carried out. The analytical dependences of degree of homogenization on time and temperatures of the sintering process are established, the coefficients of mutual diffusion in systems are calculated, physical and chemical model of homogenization are offered, means of the computer nu¬merical analysis with visualization of the studied processes are created. The estimation of convergence of the numerical methods employed are carried out, the accuracy of calculations is determined, estimated calcula¬tions for considered systems are carried out. The obtained results will allow the reduction in the number of real experiments in the creation of new mate¬rials with required properties.

Abstract: It is shown that investigations of the isotope exchange kinetics in nanocrystalline oxides allows one not only to solve problems connected with determination of process parameters and characteristic scenarios, but also to substantially extend experimental opportunities in analysis of diffusion properties of oxides. A specific behavior of the oxygen isotope exchange was studied in nanocrystalline LaMnO3 and ZrO2:Y2O3 oxides. The former oxide is characterized by very small values of oxygen volume diffusion coefficients, while the latter one is, on the contrary, an ionic conductor. The study was carried out using powdered nanomaterials prepared from original ceramic materials by grinding in planetary mills or by laser evaporation. The study revealed future trends in the use of nanomaterials for analysis of the diffusion kinetics in oxides. It is demonstrated that in this case the sensitivity of traditional isotope methods increases considerably and fundamentally new opportunities are provided for analysis of processes on the "gas phase  solid state" interface as well as for the study of volume diffusion in polycrystals. Considering a strong dependence of the isotope exchange rate on the particle size, it is also topical to conduct studies dealing, along with determination of kinetic parameters of the process, with the use of the obtained data for certification of dimensional characteristics of oxide nanopowders.

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: Among various biocompatible materials, hydroxyapatite (HA) is widely used in medical applications. Hydroxyapatite can be used as temporary substitute material for the human bone. Despite of the risk of contamination during milling, the mechanochemical method shows higher reproducibility and low processing cost. In this investigation, the mechanochemical method has been carried out to produce nanocrystalline powders of hydroxyapatite using two experimental procedures (HA1: CaHPO4 + Ca (OH) 2; HA2: CaCO3 + CaHPO4) in polymeric and metallic vials at different milling time. The Effects of milling time, milling media and also chemical composition of initial materials on the crystallinity and morphological properties of obtained materials using X-ray diffraction (XRD) and transmission electron microscopy (TEM) were studied. Appropriate equation and graphs for determining crystallinity degree were used. The obtained results show that the crystallites sizes are within the nanometer range and also indicated that nanocrystalline hydroxyapatite with spherical morphology and high crystallinity degree can be produced much better in polymeric vials; therefore using polymeric vials with high wear resistance can have better performance during the mechanochemical process for the production of high quality nanocrystalline hydroxyapatite. Further work is needed to expand the idea for mass production.

Abstract: 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.

Abstract: Microscopic diatoms, unicellular algae that are abundant in the earth’s waters, have been interesting to researchers because of their unique outer shell structure known as frustules. The frustules are composed of amorphous silica, naturally nano-fabricated, while displaying unparalleled diversity in structure and morphology. The nanoporous structures of the frustules allow them to be used for specialized filtration procedures. These diatom frustules however haven’t been fully characterized for fluid/particle flow. Each diatom frustule contains two to three porous layers. The inner layer of Coscinodiscus sp. for instance has holes of around 1.2 µm and the outer layer comprises holes of around 300 nm diameter. This research presents diffusion studies to characterize the flow of particles through these frustule pores. In order to successfully carry out the diffusion studies, a single frustule of diatom was stuck onto the end of a hollow capillary fibre with internal diameter of 15.5 µm. The capillary was then inserted into 500 µm glass tube filled with fluorescent dye, Rhodamine B, and the experiment carried out with Nikon epifluorescence microscopy. We observed that diffusion through the inner layer was affected by its porosity resulting in the reduction of the diffusivity i.e. D = 1.1×10-10 m2/s. The diffusion through both the inner and outer layer however resulted in diffusion coefficient (D = 3.1×10-11 m2/s) that was influenced not only by the porosity but also the tortuosity. Thus we notice that the intricate three dimensional structure of the diatom imposes different boundary conditions to the flow of the Rhodamine B molecules resulting in different diffusion rate.