A joint use of experimental and theoretical techniques allows us to understand the key role of intermediate- and short-range defects in the structural and electronic properties of ZnO single crystals obtained by means of both conventional hydrothermal and microwave-hydrothermal synthesis methods. X-ray diffraction, Raman spectra, photoluminescence, scanning electronic and transmission electron microscopies were used to characterize the thermal properties, crystalline and optical features of the obtained nano and microwires ZnO structures. In addition, these properties were further investigated by means of two periodic models, crystalline and disordered ZnO wurtzite structure, and first principles calculations based on density functional theory at the B3LYP level. The theoretical results indicate that the key factor controlling the electronic behavior can be associated with a symmetry breaking process, creating localized electronic levels above the valence band.

The effect of different synthetic conditions (both template and chloride precursor concentration) on the morphologies of BaZrO 3 microcrystals was systematically investigated. Three different templates, polyethylene glycol (PEG), cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate (SDS), and two different chloride concentrations were used in the experiments. BaZrO 3 microcrystals were obtained using the microwave assisted hydrothermal method (MWHT) at 140 C for 40 min. BaZrO 3 microstructures were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), Fourier transform Raman spectroscopy (FT-Raman) and photoluminescence (PL) measurements. The growth mechanism for the formation of BaZrO 3 with decaoctahedron-shape was analyzed in detail, and we have found that the nature of the mechanism follows a non-classical growth process involving mesoscale self-assemblies of nanoparticles. Based on these findings, a relationship between the morphology and the photoluminescent behavior is proposed.

Mg2(Si,Sn)-based compounds have shown great promise for thermoelectric (TE) applications, as they are nontoxic and comprised abundantly available constituent elements. In this work, the crystal structures and TE properties of polycrystalline materials with nominal compositions Mg2Si0.35Sn0.65–x Bi x (x = 0, 0.015, 0.030, and 0.045) and Mg2Si y Sn0.97–y Bi0.03 (y = 0.30, 0.325, and 0.35) have been investigated. The electrical conductivity, Seebeck coefficient, and thermal conductivity are strongly affected by the presence of Bi. Undoped samples showed higher values of Seebeck coefficients (below 600 K), lower electrical conductivity, and lower thermal conductivity (above 600 K) in comparison to the Bi-doped samples. Furthermore, the signs of Seebeck coefficients are all negative, confirming that n-type conduction is dominant in these materials. Electrical conductivity was enhanced by increasing the Bi content up to 3% on the Si/Sn site because of the increasing amount of electron donors, and the absolute value of Seebeck coefficient decreased. When the Bi content is greater than 3%, lower zT values were obtained at 773 K. Thermal conductivity values might decrease with increasing Sn alloying for Mg2Si y Sn0.97–y Bi0.03, as mass and strain fluctuation caused by alloying can effectively scatter phonons. However, a different behavior was observed in higher Sn content material, possibly because of the absence of Mg atoms at the interstitial site [Mgi, on (1/2, 1/2, 1/2)] and vacancies of Mg atoms at the (1/4, 1/4, 1/4) site, as confirmed by Rietveld refinements. Outstanding figure of merit values in excess of unity were achieved with all samples, culminating in zT max = 1.35.

In this work, polycrystalline n-type Mg2Si0.30Sn0.67Bi0.03 dispersed with x wt % β-SiC nanoparticles (x = 0, 0.5, 1.0, 1.5, and 3.0) thermoelectric materials were fabricated by a solid-state reaction in a low-cost container, consolidated by hot-pressing. We obtained figure of merit values zT above 1.4 at 773 K along with enhanced mechanical properties by adding β-SiC into an Mg2Si0.30Sn0.67Bi0.03 matrix. Incorporation of SiC nanoparticles has thusly simultaneously increased toughness and, depending on the SiC content, thermoelectric performance. The peak figure of merit was improved from zT = 1.33 for Mg2Si0.30Sn0.67Bi0.03 to 1.45 for Mg2Si0.30Sn0.67Bi0.03 with 3 wt % at 773 K.

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