Han Xiaodong

Thermal stability of Mg2Si epitaxial film formed on Si（111） substrate by solid phase reaction

A single crystalline Mg2Si film was formed by solid phase reaction (SPR) of a Si(111) substrate with an Mg overlayer capped with an oxide layer(s), which was enhanced by post annealing from room temperature to 100 degrees C in a molecular beam epitaxy (MBE) system. The thermal stability of the Mg2Si film was then systematically investigated by post annealing in an oxygen-radical ambient at 300 degrees C, 450 degrees C and 650 degrees C, respectively. The Mg2Si film stayed stable until the annealing temperature reached 450 degrees C then it transformed into amorphous MgOx attributed to the decomposition of Mg2Si and the oxidization of dissociated Mg.

Cross-over of the Plasticity Mechanism in Nanocrystalline Cu *

Via in situ uniaxial tensile tests in a high-resolution transmission electron microscope, we directly observed a cross-over of plastic deformation mechanisms in a nanocrystalline (nc) Cu thin film containing nano-twin lamellae. For a certain twin lamellae length, the twin lamellae with a larger thickness emit dislocations inclined (Schmidt-factor dislocations, i.e., S-dislocations) toward the twin boundaries. Upon decreasing the twin lamellae thickness to a critical value, such as approximately 15 nm, the plasticity switches toward emission of twinning partial dislocations (T-dislocations) parallel to the twin planes that cause migration of the twin boundaries. The critical twin thickness value also depends on the length of the twin. These results provide direct evidence for the strengthening and softening mechanisms in nano-twinning structured metals.

The electronic structures, Born effective charge tensors, and phonon properties of cubic, tetragonal, orthorhombic, and rhombohedral K0.5Na0.5NbO3: A first-principles comparative study

The electronic structures, Born effective charges (BECs), and full phonon dispersions of cubic, tetragonal, orthorhombic, and rhombohedral K0.5Na0.5NbO3 are investigated by the first principles method based on density functional theory. The hybridized states of Nb 4d and O 2p states are observed in the valence band, showing the formation of a strong Nb—O covalent bond which should be responsible for the displacement of Nb and O atoms. The abnormally large BECs of Nb and O indicate the possibility of phase instability induced by the off-center displacement of Nb and O atoms. The phonon dispersions reveal that the ferroelectric instability of K0.5Na0.5NbO3 is dominated by Nb and O displacements with significant Na characteristics. In addition to the ferroelectric instability, there is also rotational instability coming from the oxygen octahedra rotation around one axis. Moreover, the Γ phonon properties of orthorhombic KNbO3, NaNbO3, and K0.5Na0.5NbO3 are also studied in detail.

Local thermal conductivity of polycrystalline AlN ceramics measured by scanning thermal microscopy and complementary scanning electron microscopy techniques

The local thermal conductivity of polycrystalline aluminum nitride (AlN) ceramics is measured and imaged by using a scanning thermal microscope (SThM) and complementary scanning electron microscope (SEM) based techniques at room temperature. The quantitative thermal conductivity for the AlN sample is gained by using a SThM with a spatial resolution of sub-micrometer scale through using the 3ω method. A thermal conductivity of 308 W/m·K within grains corresponding to that of high-purity single crystal AlN is obtained. The slight differences in thermal conduction between the adjacent grains are found to result from crystallographic misorientations, as demonstrated in the electron backscattered diffraction. A much lower thermal conductivity at the grain boundary is due to impurities and defects enriched in these sites, as indicated by energy dispersive X-ray spectroscopy.