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12.1 GOING TO ATOMIC DIMENSIONS New instrumentation and devices for gaining insight into the structure of matter on the atomic level has opened the door to this fascinating nanoworld. Research and development of nanoscience and nanotechnology in electrochemistry follow several main streams. One of them is the investigation of the surface with atomic resolution,which has been described throughout this book. A number of reviews exist about this surface nanoelectrochemistry.1 Another area of study has been the preparation of atomic-level surface topographies assembled from clusters of nanostructures like nanodots, nanowires, nanoarrays, and nanoelectrodes, etc.2 Additionally, supramolecular carbon nanostructures have been made from nanoballs, -rods, and-tubes.3 New materials can be formed with the different nanosized structures but preparation of their materials and properties is somewhat different than for traditional materials. For example, nanocrystalline metals are much harder.4 These materials and unusual properties combine to create composites, for example, from the co-deposition of particles with metals in a plating process or incompositionally modulated multi-layers. Also of interest are core-shell particles, which consist of an inorganic core and a conducting polymer shell, and the traditional composites with larger subunits. Ways leading to the preparation of such nanocomposites is an interesting topic and will be discussed in this chapter. 12.2 CO-DEPOSITION Co-deposition describes the embedding of particles in a metal matrixduring a plating process. The ideal structure of such materials is shown in Figure 12.1. Spherical particles of different sizes are embedded in a homogeneous metal matrix. Another form of particles is flakes. Co-deposition of mica flakes with Zn2 is shown as an example in Figure 12.2. The principles have been known since the 1960s but the mechanism of co-deposition and the conditions for optimizedparticle incorporation are still unclear. In Table 12.1 some examples of systems mentioned in the literature are given.
Figure 12.1 Idealized picture of a composite layer formed by metal plating.
Figure 12.2 Co-deposition of mica flakes in a zinc matrix.5
Table 12.1 Some of the particle–metal matrix combinations andapplications that are known so far Metal matrix Ni Ni Ni Ni Ni Co CoNi Ag Au Zn Particles SiC PTFE Al2O3 Micro capsules NiFeS2 Cr2C3 Cr2C3 Al2O3, nanodiamond Al2O3, nanodiamond SiO2 Application Wear resistant Friction control Dull nickel Different applications Catalytic properties Temperature resistant Temperature resistant Wear resistant Wear resistant Corrosion resistance Industrial branch Carindustry
Air industry Air industry Electronic industry Electronic industry
For co-deposition the particles must be dispersed in the plating bath. Either the dispersion can be made in a separate bath that is mixed later on with the plating bath or the particles can be directly added to the plating bath. In both cases, in the formation of a stable dispersion theremust be no separation of the particles and little or no agglomeration. The theoretical background for a stable particle solution is provided by the DLVO theory (Derjaguin– Landau–Verwey–Overbeek theory6). In a stable dispersion electrostatic repulsion forces must compensate van der Waals and other attraction forces. Electrostatic forces are dependent upon the charge on the particle surface anddominate at larger distances between the particles, which creates a larger dilution. Van der Waals forces dominate at a low particle charge and smaller particle distance. For sufficiently large electrostatic repulsion forces, the sum of both charges has a maximum, which is shown in Figure 12.3. The maximum is an activation barrier that prevents or reduces agglomeration. For a stable dispersion the...
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