Molecular Dynamics Simulation of Methane Hydrate Decomposition in the Presence of Alcohol Additives.

The decomposition process of methane hydrate in pure water and methanol aqueous solution was studied by molecular dynamics simulation. The effects of temperature and pressure on hydrate structure and decomposition rate are discussed. The results show that decreasing pressure and increasing temperature can significantly enhance the decomposition rate of hydrate. After adding a small amount of methanol molecules, bubbles with a diameter of about 2 nm are formed, and the methanol molecules are mainly distributed at the gas-liquid interface, which greatly accelerates the decomposition rate and gas-liquid separation efficiency. The radial distribution function and sequence parameter analysis show that the water molecules of the undecomposed hydrate with ordered ice-like configuration at a temperature of 275 K evolve gradually into a long-range disordered liquid structure in the dynamic relaxation process. It was found that at temperatures above 280 K and pressures between 10 atm and 100 atm, the pressure has no significant effect on hydrate decomposition rate, but when the pressure is reduced to 1 atm, the decomposition rate increases sharply. These findings provided a theoretical insight for the industrial exploitation of hydrates.

Computational prediction of a high ZT of n-type Mg3Sb2-based compounds with isotropic thermoelectric conduction performance.

N-type Mg3Sb2-based Zintl compounds are proved to be high-performance thermoelectric materials with multiple degenerate valleys and low lattice thermal conductivity. Here, we investigate the electronic band structure and the thermoelectric properties of n-type Mg3Sb2 using first-principles density functional theory. A high ZT of 3.1 at 725 K is obtained when the minimum lattice thermal conductivity and the optimal carrier concentration are reached. The calculated thermoelectric performance demonstrates that Mg3Sb2 possesses an isotropic character in thermoelectric transport. Furthermore, the calculated lattice thermal conductivity κL reveals that the unusually low κL in Mg3Sb2 predominantly originates from the large Grüneisen parameter γ.

[Lattice vibration of Sr3TaGa3Si2O14 single crystal].

Based on the space group theory, the normal vibration modes of Sr3 TaGa3Si2O14 (STGS) single crystal were predicted, and the Raman scattering intensities of non-polar and polar modes were calculated. The Raman spectrum of STGS crystal was measured, and lattice vibration modes of STGS crystal were assigned. For symmetry species A1, six typical Raman-active optical modes have been recorded at 126, 245, 557, 604, 896 and 991 cm(-1), respectively. It is easy to assign the mode of 126 cm(-1) as the relative translation between SiO4, Sr and the TaO6. The mode 245 cm(-1) corresponds to the twisting vibration of SiO4 correlating with the Sr-TaO6-Sr stretching vibration. The mode 557 cm(-1) was assigned as the O-Ta-O stretching vibration, while the mode 604 cm(-1) as the O-Ga-O stretching vibration. The band at 896 cm(-1) in the Raman spectrum was assigned to be the O-Si-O stretching vibration of the two SiO4 tetrahedra in the primitive cell of STGS single crystal. Meanwhile, the band at 991 cm(-1) in the Raman spectrum was assigned to be the Si-O stretching vibration of the two SiO4 tetrahedra in the primitive cell of STGS single crystal. The layer structure of STGS crystal was identified by both theory study and Raman spectroscopy experiment results. The small anisotropy and piezoelectric modulus of STGS crystal were ascribed to the weak distortion of decahedral unit.

Theoretical study of CCl(4) adsorption and hydrogenation on a Pt (111) surface.

The adsorption and hydrogenation of carbon tetrachloride (CCl(4)) on a Pt (111) surface have been investigated using density functional theory (DFT). We have performed calculations on the adsorption energies and structures of CCl(4) on four different adsorption sites of a Pt (111) surface using the full adsorbate geometry optimization method. The results show that the adsorption energy of all of the potential sites is less than -17 kcal/mol, which indicates that CCl(4) is physiosorbed on a Pt (111) surface through van der Waals interactions. The dissociation and hydrogenation pathways were investigated by a transition state search. For the Pt(15), Pt(19), and Pt(25) cluster surfaces, the activation energies of dissociation obtained in this work are 15.69, 16.94, and 16.77 kcal/mol, respectively. The hydrogenation of CCl(3). was studied at the on-top site of the Pt(15) cluster, and the calculated activation energy is 5.06 kcal/mol. The small activation energies indicate that the Pt (111) surface has high catalytic activity for the CCl(4) hydrogenation reaction. In addition, the Hirshfeld population analysis reveals that the charge transfer from the Pt (111) surface to the adsorbates occurs in both the dissociation and hydrogenation pathways.

[Raman scattering spectra of Ca3NbGa3Si2O14 (CNGS) crystals].

The CNGS crystal belongs to the trigonal system, the 32 point group and the P321 space group, which is the same as the LGS crystal. Its lattice constants are a = 0.808 73 nm and c = 0.497 98 nm. The primitive cell of the CNGS crystal contains only one formula unit (23 atoms). The vibration modes were measured with Raman spectroscopy technique, assigned with ab initio molecular orbit calculation method. In order to model the crystal, two clusters Ca3NbGa2SiO12 and Ca3NbGaSi2O12 were designed, and calculations were carried out on them. Their configurations were optimized, and the vibration frequencies were calculated with UHF method. By investigating the force constants and activities of the assigned Raman spectra, the structure of CNGS crystal was analyzed, and its layer structure and piezoelectric properties were confirmed. It is concluded that CNGS crystals have more excellent anisotropy and piezoelectric properties than LGS crystals.

[Spectra analysis of [MnHg(SCN)4 (H2O)2 x 2C4H9NO (MMTWD) crystals].

The factor group symmetry analysis method was used to predict lattice vibrational modes of MMTWD single crystal, and it was found that there are no more than 297 and 148 theoretically observable Raman peaks and infrared reflection bands, respectively. The Raman and IR spectra were measured at room temperature. All the Raman and IR spectra peaks were assigned on the basis of the group theory and lattice dynamics theory. According to the Raman and IR spectra, the molecular structure of MMTWD crystal was discussed. From the results of the present theoretical and experimental works, the three-dimensional network structure of MMTWD crystal was identified. This structure provides the crystal with a large interaction force between molecules, which in turn induces high stability and macroscopic nonlinear optical properties.

[The analysis for Raman spectra of Nd:YVO4].

The factor group symmetry analysis method was used to analyse the Raman spectra of neodymium-doped yttrium orthoanadate Nd:YVO4 which were measured at room temperature with different geometrical configurations. All the Raman peaks were assigned on the basis of the group theory and lattice dynamics thoery. According to the peak numbers of the Raman spectra, we concluded that the primitive cell of Nd:YVO4 contains four formula units.

An experimental apparatus for simultaneously measuring Seebeck coefficient and electrical resistivity from 100 K to 600 K.

In this paper, we report a fully automated experimental apparatus for measuring Seebeck coefficient and electrical resistivity of a sample simultaneously in a temperature range of 100-600 K. The Seebeck coefficient is measured using a quasi-steady temperature differential method in which two ceramic heaters are employed to alternately heat the sample. The sample holder is designed to reduce temperature disturbance on its base during a measurement cycle. To demonstrate the accuracy and reliability of the experimental setup, we have performed tests on reference materials including constantan and platinum.