R. de Coss

Phase-pure TiO2 nanoparticles: anatase, brookite and rutile

We report on the synthesis of phase-pure TiO(2) nanoparticles in anatase, rutile and brookite structures, using amorphous titania as a common starting material. Phase formation was achieved by hydrothermal treatment at elevated temperatures with the appropriate reactants. Anatase nanoparticles were obtained using acetic acid, while phase-pure rutile and brookite nanoparticles were obtained with hydrochloric acid at a different concentration. The nanomaterials were characterized using x-ray diffraction, UV-visible reflectance spectroscopy, dynamic light scattering, and transmission electron microscopy. We propose that anatase formation is dominated by surface energy effects, and that rutile and brookite formation follows a dissolution-precipitation mechanism, where chains of sixfold-coordinated titanium complexes arrange into different crystal structures depending on the reactant chemistry. The particle growth kinetics under hydrothermal conditions are determined by coarsening and aggregation-recrystallization processes, allowing control over the average nanoparticle size.

Electrodeposition and characterization of nanostructured black nickel selective absorber coatings for solar–thermal energy conversion

Selective coatings consisting of a bright nickel interlayer and black nickel overlayer for solar-to-thermal energy conversion have been electrodeposited onto stainless steel substrates from a nickel(II) chloride bath. During electrodeposition of black nickel, multiple reduction processes combined with oxidative processes result in a complex film composition, containing α-Ni(OH)2, NiOOH, Ni2O3, NiO, water and metallic Ni, as indicated by a detailed X-ray photoelectron spectroscopy study. Reflectance measurements confirm that the selective coatings have a high absorbance in the wavelength range of the solar spectrum, and that the emissivity in the near to mid infrared is low. Electron microscopy showed that the films consist of nanostructured flakes, oriented perpendicular to the surface. The best coatings were obtained by using a two-pulse galvanostatic deposition method.

Electronic structure of FCC carbon

We report first-principles calculations of the electronic structure for carbon in the fcc structure with the experimentally observed lattice parameter. The calculated orbital population shows that the chemical bond in fcc carbon is close to the s2p2 bonding with a small s-p hybridization. We find that, in contrast to graphite and diamond, fcc carbon exhibits metallic behavior with an electronic density of states at the Fermi level of 0.5 states/(eV atom).

Polaronic Signatures in Phonon Isotopic Shifts

The effect of O(16) by O(18) isotopic substitution in the excitation spectrum of a model electron-phonon Hamiltonian, previously used to describe the dynamics of the O(4)-Cu(1)-O(4) cluster in YBa{sub 2}Cu{sub 3}O{sub 7}, is presented. This model includes electronic correlations and electron-phonon interactions, exhibiting the presence of polaron tunneling. The calculated isotopic shifts of phonon excitations differ from those found using harmonic or anharmonic potentials, and are consistent results of optical measurements of c-axis phonons. The isotopic substitution changes the dynamics of polaron tunneling and produces a change in the local structure.

First-Principles Calculations of Electronic Structure and Structural Properties for MoV, MoNb, and MoTa

First-principles total-energy electronic structure calculations based on the full-potential linearized augmented plane wave (LAPW) method have been used to study the electronic and elastic properties of MoV, MoNb, and MoTa with the B2 (CsCl) estructure. From the calculated values for the bulk modulus we have determined the melting temperature using an empirical correlation. The chemical bond and the electronic structure around the Fermi level are analyzed. In particular, we found that MoTa which have the experimental determined highest melting point of the studied materials, present the largest bulk modulus and the highest degree of covalence bonding of these intermetallic compounds.