Predicting the domain structures and properties in both bulk single crystal and thin film ferroelectrics using the phase-field approach requires the knowledge of fundamental mechanical, electrical, and electromechanical coupling properties of a single-domain state. In this work, the elastic properties and structural parameters of cubic single crystals as well as tetragonal, orthorhombic, and rhombohedral BaTiO3 single domain states are obtained using first-principles calculations under the local density approximation. The calculated lattice constants, bulk modulus, and elastic constants are in good agreement with experiments for both the cubic paraelectric phase and the low-temperature ferroelectric phases. Spontaneous polarizations for all three ferroelectric phases and the electrostrictive coefficients of cubic BaTiO3 are also computed using the Berry’s phase approach, and the results agree well with existing experimentally measured values.

A hybrid of ammonia borane (AB) and a metal–organic framework (MOF), which contains unsaturated coordinated Tm3+, Tm­(BTC) (BTC = 1,3,5-benzenetricarboxylic), was synthesized through the solvent-based impregnation method (named as AB@Tm­(BTC)–CH3OH). Also two other materials AB@Tm­(BTC)-milled and AB@Tm2O3-milled were prepared by physical milling separately. TPD-MS results show that the H2-release peaks of the three materials shift to lower temperature, 77, 79, and 85 °C, respectively, compared with neat AB (114 and 150 °C). To avoid the undesirable volatile byproduct, only AB@Tm­(BTC)–CH3OH shows superior performance without any byproduct, especially ammonia. The three samples exhibit enhanced dehydrogenation kinetics compared to neat AB, but AB@Tm2O3-milled presents much slower than the other two materials. The dehydrogenation activation energies of AB@Tm­(BTC)–CH3OH, AB@Tm­(BTC)-milled, and AB@Tm2O3-milled are 98.1, 103.1, and 116.4 kJ·mol–1, respectively. The mechanisms of the AB@MOF thermal dehydrogenation system especially for the prevention of ammonia have been discussed. The interaction between AB and the unsaturated coordinated metal sites in MOFs plays a key role for inhibiting ammonia during AB thermolysis.

No abstract

The thermal conductivity of various carbon nanotubes with defects or intramolecular junctions was studied using nonequilibrium molecular dynamics approach. The results show that the thermal conductivity of both armchair and zigzag carbon nanotubes increased with the decrease of the radius of the tube. The thermal conductivity of armchair tube is higher than that of zigzag tube when the radii of the two tubes are kept almost same. Discontinuities appear on the temperature profile along the tube axial at the region of IMJ, resulting in the large temperature gradient and thus lower thermal conductivity of(n,n)/(m,0)tube with one IMJ and(m,0)/(n,n)/(m,0)tube with two IMJs. For the(m,0)/(n,n)/(m,0)tube with two IMJs, phonon mean free path of the middle(n,n)tube is much smaller than that of the isolate(n,n)tube.

Transition metal compounds usually have various stoichiometries and crystal structures due to the coexistence of metallic, covalent, and ionic bonds in them. This flexibility provides a lot of candidates for materials design. Taking the V-C binary system as an example, here we report the first-principles prediction of a new type of vanadium carbide, V5C3, which has an unprecedented stoichiometry in the V-C system, and is energetically and mechanically stable. The material is abnormally much harder than neighboring compounds in the V-C phase diagram, and can be further hardened by tuning the Fermi energy.