Table of contents

Gives a critical review of idealised two-phase geometrical models. These treatments derive expressions for the resistivity and Hall coefficient of a composite material in terms of the properties of its constituents. The authors show that these models can be applied to the interpretation of transport measurements in polycrystalline films and powder layers. Important distinctions are made depending on whether the depletion layers extend completely or partially through the grains, whether the Debye length is greater or less than the grain size and whether the mean free path is greater or less than the grain size. The authors discuss the theoretical treatment of the Hall effect in percolative systems, as geometrical models neglect percolation. The modulation of Hall coefficient and conductivity by illumination and the adsorption and desorption of ambient gases are also considered.

The classical picture of the pre-solar nebula is that of a hot, well-mixed cloud of chemically and isotopically uniform composition. Recent measurements have shown this conception to be erroneous, however. Anomalies have been discovered in the isotopic composition of a number of elements which cannot be explained by processes known to be going on within the solar system at present. Rather, they appear to reflect primordial heterogeneities, testifying to variations in space and/or time of the isotopic composition of these elements within the proto-solar nebula. They contribute to our understanding of processes and of time scales in the early solar system, before and after its formation. Furthermore, they allow one to identify modes of nucleosynthesis and to derive relevant parameters of such production modes.