RESUMO
We report a combined theoretical and experimental search for thermoelectric materials based on semiconducting zinc antimony. Influence of three new doping elements (sodium, potassium and boron) on the electronic properties was investigated as well as the carrier concentration and temperature dependence of the thermoelectric coefficients obtained through density-functional calculations and Boltzmann transport theory. Distortion of the electron arrangement caused by the doping elements is displayed as a deformation charge density around the atoms. Based on the band structures, the density of states, and the transport properties, we found that the presence of Na and K in the ZnSb matrix leads to a slightly improved p-type conductivity, whereas the B substitution leads to a n-type doping. Because of the stronger need for obtaining n-type ZnSb-based material, the B(0.01)Zn(0.99)Sb structure has been transferred to the laboratory to be synthesized by direct melting. The sample was investigated using x-ray diffraction and scanning electron microscopy.
RESUMO
Neoisostegane, the first naturally occurring steganin without a functional group at C-5, was isolated from two different collections of Steganotaenia araliacea. The structure of neoisostegane, which contained five aromatic methoxyl groups rather than the more usual three methoxyls and one methylenedioxy moiety, was elucidated by comparison of the pmr coupling constants with those of synthetic steganes and by selective decoupling experiments. The structure was confirmed by cmr data, by preparation of a derivative having five aromatic methoxyl moieties from stegane, which was compared spectroscopically to neoisostegane, and by thermal isomerization experiments.
