V. Ozolins

First-Principles Prediction of Thermodynamically Reversible Hydrogen Storage Reactions in the Li-Mg-Ca-B-H System

Introduction of economically viable hydrogen cars is hindered by the need to store large amounts of hydrogen. Metal borohydrides [LiBH(4), Mg(BH(4))(2), Ca(BH(4))(2)] are attractive candidates for onboard storage because they contain high densities of hydrogen by weight and by volume. Using a set of recently developed theoretical first-principles methods, we predict currently unknown crystal structures and hydrogen storage reactions in the Li-Mg-Ca-B-H system. Hydrogen release from LiBH(4) and Mg(BH(4))(2) is predicted to proceed via intermediate Li(2)B(12)H(12) and MgB(12)H(12) phases, while for Ca borohydride two competing reaction pathways (into CaB(6) and CaH(2), and into CaB(12)H(12) and CaH(2)) are found to have nearly equal free energies. We predict two new hydrogen storage reactions that are some of the most attractive among the presently known ones. They combine high gravimetric densities (8.4 and 7.7 wt % H(2)) with low enthalpies [approximately 25 kJ/(mol H(2))] and are thermodynamically reversible at low pressures due to low vibrational entropies of the product phases containing the [B(12)H(12)](2-) anion.

First-principles theory of competing order types, phase separation, and phonon spectra in thermoelectric AgPbmSbTem+2 alloys

Using a first-principles cluster expansion, we shed light on the solid-state phase diagram and structure of the recently discovered high-performance Pb-Ag-Sb-Te thermoelectrics. The calculated bulk thermodynamics favors the formation of coherent precipitates of ordered Ag m Sb n Te m+n phases immiscible with rocksalt PbTe, such as AgSbTe 2 . The solubility is high for Pb in AgSbTe 2 and low for (Ag,Sb) in PbTe (8% vs 0.6% at 850 K). The differences in the phonon spectra of PbTe and AgSbTe 2 suggest that these precipitates enhance the thermoelectric performance by lowering thermal conductivity.

Quantifying the relation between the morphology and performance of polymer solar cells using Monte Carlo simulations

Morphology is a crucially important factor determining the efficiency of photocurrent generation in bulk heterojunction polymer solar cells. Morphology, which depends on the characteristics of the polymers as well as on the conditions of phase separation, affects the performance of solar cells by influencing the rate of exciton dissociation and the efficiency of charge carrier transport. Using Monte Carlo simulations, we investigate the effects of annealing time on the morphology of phase separation and charge transfer behavior inside the active layers of polymer solar cells. We find that a suitably defined correlation distance is an effective parameter that quantitatively characterizes different morphologies and can be used to establish a direct link with transmission electron microscopy images of real polymer solar cells. Optimal morphologies have been investigated, showing results that are consistent with experimental data.

In situ measurements of stress evolution for nanotwin formation during pulse electrodeposition of copper

In situ stress measurements were performed during high frequency pulse electrodeposition of nanotwinned Cu thin films. Periodic stress changes during pulse-on and pulse-off periods were observed. The stress profile showed an abrupt increase in tensile stress to about 400 MPa during the pulse-on period and a stress relaxation during the pulse-off period. First-principles calculations predict that a complete relaxation of the tensile stress allows the formation of nanotwins separated by 28 nm or more. This is in good agreement with the results obtained from microstructural analysis of the Cu films fabricated during in situ stress measurements.

A technique for the measurement of surface diffusion coefficient and activation energy of Ge adatom on Si(001)

A method for determining the surface diffusion coefficient and the activation energy of Ge adatoms on Si(001) has been developed. Ge self-assembled quantum dots (SAQDs) grown on a relaxed SiGe buffer layer preferentially nucleate over a network of buried 60° dislocations. The surface sites over the buried dislocations act as sinks of Ge adatoms. When the average dislocation spacing becomes larger than the surface diffusion length of Ge adatoms, denuded zones free of Ge SAQDs appear on both sides of buried dislocations and separating the preferentially nucleated SAQDs from the randomly nucleated ones. The denuded zone width and the inter-dot spacing of randomly nucleated SAQDs are completely dependent on the surface diffusion coefficient. By varying the substrate temperature during the growth, the activation energy for the surface diffusion can be determined from the equilibrium inter-dot spacing of the randomly nucleated Ge SAQDs. Moreover, the pre-exponential term in the diffusion constant can be determi...

Ripening of L12 Ni3Ti precipitates in the framework of the trans-interface diffusion-controlled theory of particle coarsening

Abstract Data are presented on the kinetics of coarsening of γ′-type Ni3Ti precipitates (L12 crystal structure) in three binary Ni–Ti alloys containing 10.31, 11.84, and 13.72at.% Ti aged at 720°C for times up to 64 h. Data on the distributions of particle sizes (PSDs) are also presented. These data, as well as previously published data, are analyzed in light of a new theory of coarsening in which diffusion is controlled by transport through the non-sharp interface between the matrix and precipitate phases. The new theory, called the trans-interface diffusion-controlled (TIDC) theory of coarsening, predicts time (t)-dependent behavior of the type 〈r〉n ∝ t for the growth of precipitates of average radius 〈r〉 and XTi ∝ t−1/n for the depletion of the solute concentration of the matrix, XTi. The exponent n is intimately related to the width of the interface between the precipitate and matrix phases, δ, which is assumed to depend on the particle radius as δ ∝ m, where n = m + 2. The shape of the scaled distribution of particle sizes (PSD) depends on n and the thermo-physical kinetic constants are independent of volume fraction. The data on kinetics are evaluated and compared for n=2.375, determined from analyses of the PSDs, and for n=3, which is the traditionally accepted value. The agreement between the data on kinetics and predictions of the TIDC theory is acceptable, and the TIDC theory is the only one capable of explaining the experimentally observed absence of an effect of volume fraction on the kinetics at larger volume fractions, and the shapes of the PSDs.

Obtaining correct orbital ground states in f -electron systems using a nonspherical self-interaction-corrected LDA + U method

The electronic structure of lanthanide and actinide compounds is often characterized by orbital ordering of localized

First-principles study of phase stability of Gd-doped EuO and EuS

f

First-principles computational discovery of materials for hydrogen storage

electrons. Density-functional theory studies of such systems using the currently available local-density approximation

Trans-interface diffusion-controlled coarsening.

(text{LDA})+U