Raman scattering spectroscopy has been used for the characterization of zinc oxide nanoparticles obtained by mechanical activation in a high-energy vibro-mill and planetary ball mill. Raman modes observed in spectra of nonactivated sample are assigned to Raman spectra of the ZnO monocrystal, while the spectra of mechanically activated samples point out to the structural and stoichiometric changes, depending on the milling time and the choice of equipment. Observed redshift and peak broadening of the E 2 high and E 1 (LO) first-order Raman modes are attributed to increased disorder induced by mechanical milling, followed by the effects of phonon confinement due to correlation length decrease. The additional modes identified in Raman spectra of activated ZnO samples are related to the surface optical phonon modes, due to the intrinsic surface defects and presence of ZrO 2 as extrinsic defects introduced by milling in zirconia vials.

The effects of adding small quantities of SnO 2 to the basic ZnO-Bi 2 O 3 varistor composition were studied in terms of phase reactions, microstructural development, and the formation of inversion boundaries. Scanning and transmission electron microscopy studies showed that the inversion boundaries, triggered by the addition of SnO 2 , cause anisotropic grain growth in the early stages of sintering. ZnO grains that include inversion boundaries grow exaggeratedly, at the expense of normal grains, until they dominate the microstructure. Higher additions of SnO 2 lead to an increase in number of grains with inversion boundaries and to a more fine-grained microstructure. The increasing amount of secondary phases is also related to a higher level of SnO 2 addition; however, the influence of these phases on ZnO grain growth is subordinate to the role of inversion boundaries.

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We investigated the high-temperature thermoelectric properties of Ga:ZnO bulk compounds, synthesized using a simple and scalable solid-state process. The effects of a low gallium content (x ≤ 0.04 in ZnGaO) on the structural features and electrical/thermal properties are reviewed. Transmission electron microscopy analyses showed that 2D, nonperiodic defects had formed from a doping content as low as x = 0.01 Ga. The structural description of these nanoscale interfaces is, for the first time, carefully investigated in such low-Ga-content samples by HAADF-STEM analyses combined with structural modeling. It was found that the formation of head-to-head inversion twin (h-IT) boundaries and tail-to tail inversion boundaries (t-IB) in the bulk compounds is responsible for strong phonon scattering, while maintaining relatively good electrical conductivity and thereby enhancing the thermoelectric properties. The absolute value of the Seebeck coefficient decreases abruptly from 475 μV/K for x = 0 down to 60 μV/K for x = 0.005 at 350 K. At the same time, the electrical resistivity drops from 1 ohm cm for x = 0 to 1.7 × 10 ohm cm for x = 0.005. For higher Ga additions (x > 0.01), the increase in electrical resistivity is likely linked to the formation of interface defects at a larger extent in the wurtzite structure. The thermal conductivity also drops sharply with the increase in the Ga content from ∼33 W/m K for x = 0 to ∼8 for x = 0.04 at 350 K. This study is progress toward the synthesis of other thermoelectric materials where nanoscale interfaces in bulk compounds provide tremendous opportunities for further enhancing both the phonon scattering and the overall figure of merit.

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