Bulk Mg2Si crystals were grown using the vertical Bridgman melt growth method. The n-type and p-type dopants, bismuth (Bi) and silver (Ag), respectively, were incorporated during the growth. X-ray powder diffraction analysis revealed clear peaks of Mg2Si with no peaks associated with the metallic Mg and Si phases. Residual impurities and process induced contaminants were investigated by using glow discharge mass spectrometry (GDMS). A comparison between the results of GDMS and Hall effect measurements indicated that electrical activation of the Bi doping in the Mg2Si was sufficient, while activation of the Ag doping was relatively smaller. It was shown that an undoped n-type specimen contained a certain amount of aluminum (Al), which was due either to residual impurities in the Mg source or the incorporation of process-induced impurities. Thermoelectric properties such as the Seebeck coefficient and the electrical and thermal conductivities were measured as a function of temperature up to 850 K. The dimensionless figures of merit for Bi-doped and Ag-doped samples were 0.65 at 840 K and 0.1 at 566 K, respectively. Temperature dependence of the observed Seebeck coefficient was fitted well by the two-carrier model. The first-principles calculations were carried out by using the all-electron band-structure calculation package (ABCAP) in which the full-potential linearized augmented-plane-wave method was employed. The ABCAP calculation adequately presents characteristics of the Seebeck coefficients for the undoped and heavily Bi-doped samples over the whole measured temperature range from room temperature to 850 K. The agreement between the theory and the experiment is poorer for the Ag-doped p-type samples.

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The work described herein deals with efforts to make a persuasive correlation between structural characteristics and electrochemical lithium storage for a silicon oxycarbide prepared from poly(methylhydrogensiloxane) and divinylbenzene. Structural characterization reveals that the silicon oxycarbide includes excess free carbon in an amorphous network. The reversibility of lithiation and delithiation in the silicon oxycarbide reaches 74% between 0.005 and 3 V relative to lithium at the first cycle but falls to only ca. 30% between 0.4 and 3 V. We found two resonances at 0 and 2.4 ppm in the (7)Li magic angle spinning nuclear magnetic resonance spectrum of the silicon oxycarbide lithiated to 0.4 V, whose contributions are 67 and 33%, respectively, and thus are in good agreement with the reversibility observed between 0.4 and 3 V. The fully lithiated silicon oxycarbide shows a single resonance at ca. 3-9 ppm, which tends to broaden at lower temperatures to -120 °C, whereas the fully delithiated silicon oxycarbide has a single resonance at 0 ppm. These results indicate that both reversible and irreversible lithium species have ionic natures. The Li K edge in electron energy loss spectroscopy does not show clearly any identified near-edge fine structures in the inner part of the silicon oxycarbide after delithiation. Near the surface, on the other hand, LiF and oxygen- and phosphorus-containing compounds were found to be the major constituents of a solid electrolyte interface (SEI) layer. Over repeated lithiation and delithiation, the SEI layer appears to become thick, which should in part trigger capacity fading.