Polycrystalline materials, interfacial

The interfacial structure of a solid electrode depends on various factors. The interatomic distance varies with the exposed crystallographic face and with the interaction energy between the crystallites in a polycrystalline material there are breaks in the structure and onedimensional and two-dimensional defects, such as screw dislocations, etc. Adsorption of species can be facilitated or made more difficult, and at the macroscopic level we observe the average behaviour. 

Before we do so, let us mention a particular difference between solid and liquid systems which is very important in this context, namely the occurrence of a network of internal boundaries in multiphase or polycrystalline materials. The proportion of boundary regions can be so high that the overall conductivity can be interfacially dominated. 

Before dealing with the superposition in polycrystalline materials, let us first consider the conductance of a thin film with an accumulation148 effect at the interface to the (identical) neighboring phases for different thicknesses and consider Fig. 36. In the top figure the film is so thick that the interfacial effect is not seen the carrier... 

The polycrystalline microstructure is the microstructure we observe after sintering a powder compact to full density. In homogeneous polycrystalline materials, this microstructure is determined, in general, by the interfacial tension. Figure 3.1 illustrates an equilibrium state between three interfacial tensions. For this geometry, the sine law is satisfied so that... 

Nanocarbon hybrids have recently been introduced as a new class of multifunctional composite materials [18]. In these hybrids, the nanocarbon is coated by a polymer or by the inorganic material in the form of a thin amorphous, polycrystalline or single-crystalline film. The close proximity and similar size domain/volume fraction of the two phases within a nanocarbon hybrid introduce the interface as a powerful new parameter. Interfacial processes such as charge and energy transfer create synergistic effects that improve the properties of the individual components and even create new properties [19]. We recently developed a simple dry wrapping method to fabricate a special class of nanocarbon hybrid, W03 /carbon nanotube (CNT) coaxial cable structure (Fig. 17.2), in which W03 layers act as an electrochromic component while aligned... 

Interfacial electron transfer across a solid-liquid junction can be driven by photoexcitation of doped semiconductors as single crystals, as polycrystalline masses, as powders, or as colloids. The band structure in semiconductors (281) makes them useful in photoelectrochemical cells. The principles involved in rendering such materials effective redox catalysts have been discussed extensively (282), and will be treated here only briefly. 

Polycrystalline Films/Coatings Materials Specific Multilayer Stacks Extensive Interfaces Interfacial Stability... 

Extensive interface research is crucially essential for developing long-life, cost-effective, multilayer, polycrystalline, thin-film stacks for SECS. Microchemical analysis and other interface measuring techniques must be employed to solve the interfacial stability problems in the stacks. Important topical areas in solar materials interface science include thin films grain, phase, and interfacial boundaries corrosion and oxidation adhesion chemisorption, catalysis, and surface processes abrasion and erosion photon-assisted surface reactions and photoelectrochemistry and interface characterization methods. 

Typical values of transfer coefficients a and ji thus obtained are listed in Table 4 for single crystal and polycrystalline thin-film electrodes [69] and for a HTHP diamond single crystal [77], We see for Ce3+/ 41 system (as well as for Fe(CN)63 /4 and quinone/hydroquinone systems [104]), that, on the whole, the transfer coefficients are small and their sum is less than 1. We recall that an ideal semiconductor electrode must demonstrate a rectification effect in particular, a reaction proceeding via the valence band has transfer coefficients a = 0, / =l a + / = 1 [6], Actually, the ideal behavior is rarely the case even with single crystal semiconductor materials fabricated by advanced technologies. Departure from the ideal semiconductor behavior is likely because the interfacial potential drop is located in part in the Helmholtz layer (due e.g. to a high density of surface states), or because the surface states participate in the reaction. As a result, the transfer coefficients a and ji take values intermediate between those characteristic of a semiconductor (0 or 1) and a metal ( 0.5). 

Such efficient minority-carrier injectors have been proposed as emitters for high-power transistors (Kroemer, 1957) and demonstrated using an n-type oxygen-rich polycrystalline Si emitter on a p-type c-Si base (Oh-uchi et al., 1979). The main problem expected to occur at heterojunctions between dissimilar materials is that associated with interfacial states that may either pin the Fermi level or act as generation-recombination centers. However, in the case of a-Si H the abundance of atomic hydrogen should help eliminate the interfacial states. 

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