The Disordered Surface Layer

The increase of rate capability of cathode materials (for use in hybrid electric vehicles for instance) could be achieved by decreasing as much as possible the size of the particles to improve the effective surface that is active for electrochemical reactions. In addition, smaller size means reduced path for the electrons and the Li ions inside the particles. Since the electronic and ionic conductivity are small [30], this reduction is expected to benefit the performance, especially at high C-rates. The experimental results, however, are not as simple as one might have expected, because the reduction in size impUes that surface effects become more important, and the surface layer does not necessarily have the same properties as the bulk, with important impact on the electrochemical properties. The bulk properties (i.e., physical and chemical properties big enough so that surface effects are negUgible) are now well understood. That is not the case, yet, for surface effects that are still under debate. 

Several experiments have evidenced the existence of a disordered layer (DSL) at the surface of particles of oxide, typically few nanometers that modified the intrinsic properties of electrode material for Li-ion batteries [77-83]. Yet attention must be 

The preparation and characterization of nanoparticles of LiFeP04 is then of primary importance, in as much as their structural properties may depend on their size. 

Fig 12.10 FTIR absorption spectra of molten-state samples at different stages of the grinding process (before carbon coating) roll mill, jet mill cyclone, and jet mill collector 

For nanoparticles, the fraction (1 — y) of iron ions in the surface layer is not negligible, and we found that this contribution is different from that of the bulk that was investigated by magnetic measurements [80]. The response of the magnetic moments of the iron ions is thus different for the ions in the bulk and the ions in the surface layer. Therefore, one has to add to the bulk contribution to the magnetic susceptibility the contribution coming from the iron ions inside the surface layer (Fig. 12.11). We found that this contribution satisfies the Curie law, so that/(T) takes the form  

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Fig. 12.7 Schematic representation of the simple core-shell model for description of the importance of the disordered surface layer in nanoparticle...

As an ex situ technique for structural information on surfaces, STM is an excellent complement to the standard electron and ion diffraction probes of surface order. The STM method can identify both short range order and long range periodicity, as well as disordered surface layers (e.g., images of sorbic acid on Highly Ordered Pyrolitic Graphite (HOPG), vida infra). In contrast,... 

Fig. 6. One-dimensionally disordered sulfur chains adsorbed on Ni(l 11) at 0 = 0.22. Solid circles represent adsorbed sulfur atoms open circles, nickel atoms in the adsorbent surface layer (Ref. 47).

In addition to the examples already mentioned we shall discuss the brominatlon of silver and Ag-Cd alloys. In all cases a parabolic rate law was observed above 200°C. Contrary to the previous example, the AgBr surface layer is an ionic conductor characterized by a Frenkel-type disorder, i.e., n g. n Ag we must write t t g B Hr 0> and consequently t becomes the rate determining factor in Bq. (27). According to Frenkel ... 

The above exposed copolymers were ultramicrotomed in cross section. The disordered surface exhibited a loosely bound 50-nm osmophilic layer,... 

The work of rupture is not transformed into heat quantitatively because the strata next to the rupture surfaces, as a rule, are in a more disordered state than the bulk of the two solid fragments see the above section on Rupture ork and Plastic Deformation. The energy residing in the disturbed surface layers received the name "cuticular". Thus, fracture work = heat + cuticular energy [2]. 

We find that we can similarly model the range in properties of the solitons formed by charge injection into the accumulation region of the MIS and MISFET structures. The accumulation layer in the MIS structure is very closely localised to the interface with the insulator. For silicon-based MISFET inversion layer structures, typical confinement distances are of order 2 nm [68] and we can expect values here to be no larger for the polyacetylene devices. We can therefore expect that the accumulation layer will be very sensitive to the local structure of the polymer at the interface with the insulator. We consider that in the case where this is silicon dioxide, the polymer surface layer is significantly more disordered than in the bulk of the polymer. In contrast, where the... 

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