H.C. Wang

Nanocrystallization of Zr-Ti-Cu-Ni-Be bulk metallic glass

Abstract The nanocrystallization kinetics of Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 (at.%) bulk metallic glass was investigated by differential scanning calorimetry (DSC) in the mode of continuous heating and isothermal annealing. In the case of continuous heating, three exothermic crystallization peaks can be observed, and the peak temperatures display a strong dependence on the heating rates, which can be fitted by a first order decay equation. The activation energies for crystallization are estimated to be E p1 =166.83±8.85 kJ/mol, E p2 =256.15±9.34 kJ/mol, E p3 =174.36±12.56 kJ/mol, which correspond to peak temperatures of T p1 , T p2 , and T p3 , respectively, indicating the formation of different crystallization phases at different stages. In the case of isothermal annealing, the crystallization products under isothermal annealing were observed and determined by transmission electron microscopy (TEM). The precipitation phases of the sample heated at higher temperatures were identified by X-ray diffraction (XRD), showing that the body-centered tetragonal (bct) Zr 2 Cu and hexagonal ZrBe 2 are the primary phases, in spite of the presence of other phases. These results are consistent with the complexity of the DSC curves obtained during the continuous heating and isothermal annealing.

Thermoelectric Properties of Dy-Doped SrTiO3 Ceramics

Sr1−xDyxTiO3 (xxa0=xa00.02, 0.05, 0.10) ceramics were prepared by the reduced solid-state reaction method, and their thermoelectric properties were investigated from room temperature to 973xa0K. The resistivity increases with temperature, showing metallic behavior. The Seebeck coefficients tend to saturate at high temperatures, presenting narrow-band behavior, as proved by abxa0initio calculations of the electronic structure. The magnitudes of the Seebeck coefficient and the electrical resistivity decrease with increasing Dy content. At the same time, the thermal conductivity decreases because the lattice thermal conductivity is reduced by Dy substitution. The maximum value of the figure of merit reaches 0.25 at 973xa0K for the Sr0.9Dy0.1TiO3 sample.

Vibrational and thermal properties of small diameter silicon nanowires

We present the results of vibrational and thermal properties for small diameter silicon nanowires (Si-NWs) from first principles calculations. Phonon spectrums of the Si-NWs are obtained based on the density functional perturbation theory. We found that heat-carrying acoustic branches exhibit “bending,” which results from the strong interaction between acoustic and no-zero-frequency flexural modes. The bending of acoustic branches implies that the phonon group velocity (V=dω/dq) of Si-NWs is less than that of corresponding bulk silicon. Therefore, a lower lattice thermal conductivity of Si-NWs can be caused by the bending of acoustic phonon. In comparison with bulk silicon, optical branches of Si-NWs exhibit “blueshift,” which is due to the high frequency vibration of silicon atoms at the edge of Si-NWs. From the obtained phonon spectrums, specific heat is calculated. The specific heat of Si-NWs is also lower than that of bulk silicon crystal. The reduction in the specific heat is due to the small magnitu...

Crystallization of amorphous Al84.2Ni10La2.1Ce2.8Pr0.3Nd0.6 alloy

The crystallization of amorphous Al84.2Ni10La2.1Ce2.8Pr0.3Nd0.6 alloy was investigated by using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) techniques. The amorphous Al84.2Ni10La2.1Ce2.8Pr0.3Nd0.6 alloy crystallizes through the primary precipitation of fcc-Al phase from the amorphous matrix in the range of 490–550K, followed by the precipitation of Al3Ni and Al11(La, Ce)3 phases in the temperature range of 590–680 K. Comparing amorphous Al84.2Ni10La2.1Ce2.8Pr0.3Nd0.6 and Al84Ni10Ce6 alloys, the addition of La can improve the precipitation of primary fcc-Al phase upon reheating the samples. On the basis of the Kissenger equation, the activation energy for crystallization of Al3Ni phase is higher than that of the other two phases, fcc-Al and Al11(La, Ce)3 phases, implying that the thermal stability of the Al3Ni phase is higher than that of the other two phases.

Segregation growth of large Bi2Sr2CaCu2Oy 90° (100) bicrystal by the self-flux method

The growth of large Bi2Sr2CaCu2Oy bicrystals up to 8 x 4 x 1 mm3 by a self-flux method is reported for the first time. Bicrystals with single 2212 phase were grown by segregating 2201 phase and extra Sr and Ca in flux during crystal growth. The optical morphology and SEM indicated that the as-grown bicrystals have symmetrical 90° (i.e., a-b)(110) twin boundary. A characteristic layer growth morphology suggests that the twin formation occurred layer by layer during crystal growth.

Synthesis of a new series of rare-earth vanadates RBa2V3O11 (R = Y, La, Pr, Nd, Eu, Gd, Dy, Ho, Er, Tm)

Abstract A new series of rare-earth vanadates RBa 2 V 3 O 11 (R = Y, La, Pr, Nd, Eu, Gd, Dy, Ho, Er, Tm) are first synthesized by the conventional solid-state reaction. The single phase can be obtained at a sintering temperature of 1010°C for most samples. The X-ray powder diffraction analysis has shown that the new compounds have an orthorhombic structure with cell parameters a = 7.60 to 7.88 A , b = 7.58 to 7.85 A and c=23.60 to 24.35 A for R = Y, Pr, Tm, Er, Dy, Ho compounds or a tetragonal structure with a = 7.85 to 7.94 A and c = 23.95 to 24.27 A for R = La, Nd, Gd, Eu compounds. These cell parameters depend upon the rare-earth ionic radius. The RBa 2 V 3 O 11 can be decomposed to RVO 4 , Ba 3 (VO 2 ) 2 and V 2 O 5 by sintering temperature higher than 1010°C. The as-sintered samples are insulators with a resistivity of 10 12 Ω cm. Furthermore, if the as-sintered samples are annealed under H 2 atmosphere, the RBa 2 V 3 O 11 phase decomposes into RVO 3 and BaVO 4 despite the lower annealing temperature of 950°C.