J.J. Yang

Unstable kinetic roughening during the island coalescence stage of sputtered tantalum films

Kinetic roughening of tantalum films during the initial growth stages has been studied by atomic force microscopy, scanning electron microscopy, and dynamic scaling theory. Different from the time-independent scaling behavior for continuous film growth, an intriguing unstable kinetic roughening occurs during island coalescence. In such case, roughness exponent α increases with growth time, accompanied by lower growth exponent β and higher coarsening exponent η. Detailed analysis of film surface morphology and simple phenomenological models suggests that this unstable behavior is related to the pronounced lateral growth of surface islands, which arises from the combined effect of the formation of grain boundary and the covering of heterogeneous substrate surface.

Structural instabilities and mechanical properties of U2Mo from first principles calculations

We perform detailed first principles calculations of the structural parameters at zero pressure and high pressure, the elastic properties, phonon dispersion relation, and ideal strengths of U2Mo with the C11b structure. In contrast to previous theoretical studies, we show that the I4/mmm structure is indeed a mechanically and dynamically unstable phase, which is confirmed by the negative elastic constant C66 as well as the imaginary phonon modes observed along the Σ1-N-P line. The calculations of ideal strengths for U2Mo are performed along the [100], [001], and [110] directions for tension and on (001)[010] and (010)[100] slip systems for shear load. The ideal shear strength is about 8.1 GPa, much smaller than a tension of 18-28 GPa, which indicates that the ductile U2Mo alloy will fail by shear rather than by tension.

Elastic anisotropy, vibrational, and thermodynamic properties of U2Ti intermetallic compound with AlB2-type structure under high pressure up to 100 GPa

Structural, elastic anisotropy, dynamical, and thermodynamic properties of U2Ti have been studied by employing density functional theory and density functional perturbative theory. The optimized lattice parameters a, c, unit volume V, bulk modulus B, and bond lengths dU-U, dU-Ti of U2Ti are in favorable agreement with the available experimental data and other theoretical values. The elastic constants under pressure were obtained using “energy-strain” method. The polycrystalline modulus, Poissons ratio, brittle/ductile characteristics, Debye temperature and the integration of elastic wave velocities over different directions, and hardness under pressure are also evaluated successfully. The anisotropy of the directional bulk modulus and the Youngs modulus is systematically predicted for the first time. It turns out that U2Ti should be stabilized mechanically up to 100 GPa, this compound just possesses slightly elastic anisotropy at zero pressure; however, the anisotropy becomes more and more significant w...

Suppression of surface roughening kinetics of homogenously multilayered W films

Using multi-step deposition mode, we developed an innovational strategy of homogenously multilayered (HM) structure to tailor the roughening kinetics of sputtered W films. Dynamic scaling analysis showed that all sublayers of HM W films exhibited the same anomalous roughening behavior, which originated from the limited surface diffusion. Intriguingly, different from its single-layered counterpart, the HM W films exhibited a suppression effect of kinetic roughening, which could be well manipulated by film modulation period. Detailed experimental characterization and phenomenological model analysis suggested that this roughening suppression arises from the multi-interruption of the continuous film roughening process, forming an interlayer interface after every interruption and restarting the sublayer roughening on the pre-sublayer surface.