Electrical Conductivity of Other Nano Particles

6.2.6. Electrical Conductivity of Other Nano Particles

In the recent years, the nano materials have been extensively studied due to the expectation that the new physics would enlighten us on the extra-ordinary properties exhibited by such systems of mate- rials [31-34]. One of the important aspects of these nano-crystalline materials is their high surface-to- volume ratio and hence they contain larger volume fraction of grain-boundaries. The thin-films based on these nano materials have been exploited for the study of the structure and dynamics of the thin-films in confined geometries [35]. Recently, there has been a surge of activity in the domain of molecular dy- namics simulations, and such type of work on silicon grain boundaries show that there is an existence of an ‘amorphous equilibrium structure’, as found out by an extensive work by the Phillpot group [36, 37]. Das et al [24] were able to create a metal core-metal oxide ‘shell structure’ with nano-scale dimensions within a silica gel matrix. The percolative configuration of the composite nano particle generates a large ‘interface’ in the disordered matrix [38]. A drastic change in electrical conductivity of the resultant structure as compared to that of the parent gel indicated that there is a marked influence of the ‘intefaces’ on the tranbsport properties of such nano systems [24].

The target gel composition is chosen such that the copper phase on reduction formed a percolative chain of nano-sized copper particles. The gel composition in mole% was 60CuO - 40SiO 2 , with CuCl 2 .2H 2 O and Si(OC 2 H 5 ) 4 used as precursors. The composites of nanometer-sized copper core-cop- per oxide shell with sizes in the range of 6.1 nm to 7.3 nm, which are dispersed in this silica gel, were synyhthesized by a technique consisting of reduction followed by oxidation of a suitably chosen precur- sor gel. The hot-pressed gel powders mixed with nano particles of copper dispersed in a silica gel showed electrical resistivity several orders of magnitude lower than that of the precursor gel. The elec- trical resistivity of different composites was measured over the temperature range of 30 to 300°C [24].

The activation energies for the core-shell nano-structured composites were found to be a fraction of that of the precursor gel. Such dramatic changes are attributed to the presence of an ‘interfacial’ amorphous phase. For these composites, the variation of resistivity as a function of temperature was analysed on the basis of Mott’s small polaron hopping conduction model, as also done in case of basalt glass containing smaller nano particles of magnetite, as described in the previous section [23]. The effective dielectric constant of the interfacial phase as extracted from the analysis of the data was found to be much higher than that of the precursor gel. This has been explained by these workers as arising

251 from the generation of a very high pressure at the interface due to the oxidation step to which the nano

ELECTRICAL PROPERTIES

particles of copper are subjected [24]. In a related investigation, the conducting films consisting of silver particles of diameters ranging

from 4 nm to 12 nm have been grown in glass-ceramic by subjecting the latter to a Li + to or from Ag + ion-exchange, which was followed by a suitable reduction treatment. The DC electrical resistance of these films has been measured over the temperature range 80-300°K. The resistivity data have been analysed in terms of the Ziman theory of electron-phonon scattering. The effective Debye temperature (θ P ) has been estimated by fitting the experimental data to Ziman’s equation. The θ P is seen to vary from

98 to 192°K for silver particle sizes ranging from 4.3 nm to 11.0 nm. The silver particle aggregates in this system have a fractal microstructure with fractal dimensions of around 1.6 and 1.9 respectively [39].