First principles study and comparison of vibrational and thermodynamic properties of XBi(X=In,Ga,B,Al)

In the present work, vibrational and thermodynamic properties of XBi（X = B, Al, Ga, In） compounds are compared and investigated. The calculation is carried out using density functional theory（DFT） within the generalized gradient approximation（GGA） in a plane wave basis, with ultrasoft pseudopotentials. The lattice dynamical properties are calculated using density functional perturbation theory（DFPT） as implemented in Quantum ESPRESSO（QE） code. Thermodynamic properties involving phonon density of states（DOS） and specific heat at constant volume are investigated using quasiharmonic approximation（QHA） package within QE. The phonon dispersion diagrams for InBi, GaBi, BBi, and AlBi indicate that there is no imaginary phonon frequency in the entire Brillouin zone, which proves the dynamical stability of these materials. BBi has the highest thermal conductivity and InBi has the lowest thermal conductivity. AlBi has the largest and GaBi has the smallest reststrahlen band which somehow suggests the polar property of XBi materials. The phonon gaps for InBi, GaBi, BBi and AlBi are about 160 cm^-1, 150 cm^-1, 300 cm^-1, and 150 cm^-1, respectively. For all compounds,the three acoustic modes near the gamma point have a linear behavior. C_V is a function of T-3 at low temperatures while for higher temperatures it asymptotically tends to a constant as expected.

Thermal Stability and Thermal Aging of Poly(vinyl chloride)/MgAl Layered Double Hydroxides Composites

MgAl-LDH（layered double hydroxides） were prepared with CO（NH2）2, NH4 Cl and NH3·H2O by the coprecipitation method, respectively. Corresponding composite membranes were prepared by the coating method. LDHs were characterized by WAXS, CO2-TPD and SEM. The morphology of the PVC/LDHs composite membranes were characterized by means of SEM. The thermal stability of the membranes was analyzed by air aging box and TGA-FTIR. The SEM results show that nano-particles can be compatible with poly（vinyl chloride）（PVC） matrix homogeneously by the stirring-ultrasound blend method with two steps. Furthermore, the air aging box results proved that MgAl-CO（NH2）2-LDH has the best effect on thermal stability of PVC. TGA-FTIR results show that MgAl-CO（NH2）2-LDH could adsorb more HCl that resulted from the degradation of PVC and improve the pyrolysis temperature of the first degradation stage by 15 K compared with PVC.

A New Approach for Refining Carbide Dimensions in M42 Super Hard High-speed Steel

Obtaining small carbides is crucial but difficult for high-speed steels.A new approach for refining carbide dimensions in M42 super hard high-speed steel by increasing cooling rate and spheroidizing treatment was proposed.The morphologies and properties of eutectic carbides formed at different cooling rates were investigated by means of scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,X-ray diffraction（XRD）,transmission electron microscopy（TEM）,electron back-scattered diffraction（EBSD）and differential scanning calorimeter（DSC）.The results show that eutectic carbides change from a lamellar shape into a curved-rod shape as cooling rate increases.Despite different morphologies,the two carbides are both of M2C type with a hexagonal close-packed structure and display a single crystal orientation in one eutectic colony.The morphology of M2C mainly depends on the growing process of eutectic carbides,which is strongly influenced by cooling rate.Compared with lamellar carbides,M2C carbides with curved-rod shapes are less stable,and decompose into M6 C and MC at lower temperatures.They are more inclined to spheroidize during heating,which ultimately and distinguishably refines the carbide dimensions.As small carbides are much easier to dissolve into matrices during austenization,the process described herein improves the supersaturation of alloying elements in martensite,which leads to an increment of hardness in M42 steel.