First-principles calculation of dehydrogenating properties of MgH2-V systems

Based on experimental results in which VH_0.81/MgH₂ interface was found during the process of mechanically milling MgH₂+5at%V nanocomposite, a VH/MgH₂ interface is designed and constituted in this work. A first-principles plane-wave pseudopotential method based on Density Functional Theory (DFT) has been used to investigate the vanadium alloying effects on the dehydrogenating properties of magnesium hydride, i.e., MgH₂. A low absolute value of the negative heat of formation of VH/MgH₂ interface compared with that of MgH₂ indicates that vanadium hydrides befit to improve the dehydrogenating properties of MgH₂. Based on the analysis of the density of states (DOS) and the total valence electron density distribution of MgH₂ before and after V alloying, it was found that the improvement of the dehydrogenating properties of MgH₂ caused by V alloying originates from the increasing of the valence electrons at Fermi level (E _F) and the decreasing of the HOMO-LUMO gap (ΔE _H-L) after V alloying. The catalysis effect of V on dehydrogenating kinetics of MgH₂ may attribute to a stronger bonding between V and H atoms than that between Mg and H atoms, which leads to nucleation of the α-Mg at the VH/MgH₂ interface in the MgH₂-V systems easier than that in pure MgH₂ phase.     

First-principles calculation of dehydrogenating properties of MgH_2-V systems

1 Introduction As potential energy storage materials, magnesium-based hydrogen storage alloys have been extensively researched due to their high hydrogen storage capacity, light weight and low cost[1―3]. However, a slow hydriding and dehydrogenating kinetics and high disso- ciation temperature limit its practical application for hydrogen storage. The mechanical alloying of MgH2 and transition metal elements has been experimentally confirmed to be an efficient method. For example, Ni, V, Ti,…     

Interface structure of Ag (111)/SnO<Subscript>2</Subscript> (200) composite material studied by density functional theory

Electric contact material of Ag/SnO₂ was successfully synthesized by in situ process method. The interface structure was characterized by high-resolution transmission electron microscopy (HTEM) and simulated at atomic scale on computer. The mean-square displacements of atoms near the interface were calculated, and the results showed that near the interface both Ag side and SnO₂ were mismatched and this effect decays rapidly far from the interface. By inspecting the calculated density of states (DOS), we found that the electric-conductivity of this composite material was decreased because of the localized 4d and 2p electrons of Ag and O near the Fermi surface, respectively. Electron density changed acutely across the interface, so that there was no extra compound precipitated. A micro-electric field also formed in the whole material due to the interface structure, and this may affect the electron conduction and the related electric-conductivity of the composite. It is found that the interface cohesive energy of Ag (111)/SnO₂ (200) was −3.50 J/m², which is higher than the experimental results. Supported by the National Natural Science Foundation of China (Grant No. 2008CB617609), the Natural Science Foundation of Yunnan Province (Grant No. 2006E003Z) and Science Innovation Foundation of Kunming University of Science and Technology     

Structure and migration characteristic of heterointerfaces during the phase transformation from L1<Subscript>2</Subscript> to DO<Subscript>22</Subscript> phase

Based on the microscopic phase-field model, the structure and migration characteristic of ordered domain interfaces formed between DO₂₂ and L1₂ phase are investigated, and the atomistic mechanism of phase transformation from L1₂ (Ni₃Al) to DO₂₂ (Ni₃V) in Ni₇₅Al _x V_25−x alloys are explored, using the simulated microstructure evolution pictures and the occupation probability evolution of alloy elements at the interface. The results show that five kinds of heterointerfaces are formed between DO₂₂ and L1₂ phase and four of them can migrate during the phase transformation from L1₂ to DO₂₂ except the interface (002)_D//(001)_L. The structure of interface (100)_D//(200)_L and interface (100)_D//(200)_L·1/2[001] remain the same before and after migration, while the interface (002)_D//(002)_L is formed after the migration of interface (002)_D//(002)_L·1/2[100] and vice versa. These two kinds of interface appear alternatively. The jump and substitute of atoms selects the optimization way to induce the migration of interface during the phase transformation, and the number of atoms needing to jump during the migration is the least among all of the possible atom jump modes.     

Study on hydrogen atom adsorption and diffusion properties on Mg (0001) surface

Hydrogen atom adsorption and diffusion properties on clean and vacancy defective Mg (0001) surface have been investigated systematically by using a first-principles calculations method based on the density functional theory. The calculation results of adsorption energy and diffusion energy barrier show that hydrogen atom is apt to be adsorbed at fcc and hcp sites on clean Mg (0001) surface, and fcc adsorption site is found to be more preferred. The highest diffusion energy barrier is estimated as 0.6784 eV for the diffusion of hydrogen from clean Mg (0001) surface into its bulk. Surface effects, which affect hydrogen diffusion obviously, results in a slow diffusion velocity of hydrogen from surface to subsurface, while a fast one from subsurface to bulk, indicating the range of surface effects is only restricted within two topmost layers of Mg (0001) surface. Comparatively, Mg atom vacancy on Mg (0001) surface not only enhances the chemisorption interaction between H and Mg surface, but also benefits H atom diffusion in Mg bulk with relatively more diffusion paths compared with that of clean surface. Besides, hydrogen atom is found to occupy mostly the tetrahedral interstice when it diffuses into the Mg bulk. Further analysis of the density of states (DOS) shows that the system for hydrogen atom to be adsorbed at fcc site has a lower DOS value (N (E _F)) at Fermi level and more bonding electrons at the energy range blow the Fermi level of H/Mg (0001) system as compared with that at hcp site. On the other hand, the enhanced chemisorption interaction between hydrogen and defective surface should be attributed to the fact that the electronic structures of Mg (0001) surface are modified by an Mg vacancy, and the bonding electrons of the topmost layer Mg atoms are transferred from low energy range to Fermi level, which is in favor of improving the surface activity of Mg (0001) surface. Supported by the PhD Programs Foundation of Ministry of Education of China (Grant No. 200805321032), the Science and Technology Program Project of Hunan Province (Grant No. 2008GK3083) and the Program for Changjiang Scholars and the Innovative Research Team in university (Grant No. 531105050037)     

Crystal structure and hydrogen storage behaviors of Mg/MoS<Subscript>2</Subscript> composites from ball milling

The Mg/MoS₂ composites were prepared by ball milling under argon atmosphere, and the effect of MoS₂ on the crystal structure and hydrogen storage properties of Mg was investigated. It is found that 10 wt% of MoS₂ is sufficient to prevent particle aggregation and cold welding during the milling process. The crystallite size of Mg will remain constant at slightly less than 38.8 nm with the milling process due to the size confinement effect of MoS₂. The dehydrogenation temperature of MgH₂ is reduced to 390.4-429.4 °C due to the crystallite size reduction. Through fitting by Johnson-Mehl-Avrami model, it is found that Mg crystal grows by three dimension controlled by interface transformation during the process of MgH₂ decomposition. MoS₂ has a weak catalyst effect on the decomposition of MgH₂ and activation energy of 148.9 kJ/mol is needed for the dehydrogenation process calculated by the Arrhenius equation.     

Chemial Bond and Stability of Adsorption of [Au（ AsS3 ）]^2- on the Surface of Kaolinite

Density function theory and discrete variation method (DFT-DVM) were used to study the adsorption of [Au( AsS3 )]^2- on the surface of kaolinite. The correlation among structure, chemical bond and stability was discussed. Several models were selected with [ Au( AsS3)]^2- in different directions and sites. The results show that the models with gold on the edge of kaolinite basal layer contain pincerlike bond among gold and several oxygen atoms and form strong Au - 0 covalent bond, so these models are more stable than those with gold above or under the layer. The models with gold near to [ AlO2(OH)4] octahedra are more stable than those with gold near to the vacancy withont aluminium. These two stable tendencies in kaolinite- [ Au( AsS3)]^2- are stronger than that in kaolinite-Au systems. The interaction between [ Au( AsS3 )]^2- and kaolinite is stronger than that between gold and kaolinite, and this interaction is strong enough to form the surface complexes.     

Ferroelectric properties of Bi<Subscript>4</Subscript>Zr<Subscript>0.5</Subscript>Ti<Subscript>2.5</Subscript>O<Subscript>12</Subscript> thin films prepared on LaNiO<Subscript>3</Subscript> bottom electrode by sol-gel method

The Bi4Zr0.5Ti2.5O12 (BZT) thin films were fabricated on the LaNiO3 bottom electrode using sol-gel method. The structure and morphology of the films were characterized using X-ray diffraction, AFM and SEM. The results show that the films have a perovskite phase and dense microstructure. The 2Pr and 2Vc of the Pt/BZT/LaNiO3 capacitor are 28.2 μC/cm2 and 14.7 V respectively at an applied voltage of 25 V. After the switching of 1×1010 cycles, the Pr value decreases to 87% of its pre-fatigue values. The dielectric constant (ε) and the dissipation factor (tanδ) of the BZT thin films are about 204 and 0.029 at 1 kHz, respectively. The films show good insulating behavior according to the test of leakage current. The clockwise C-V hysteresis curve observed shows that the Pt/BZT/LaNiO3 structure has a memory effect because of the BZT film's ferroelectric polarization.     

Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> thin films prepared by sulfurizing different multilayer metal precursors

Cu₂ZnSnS₄ (CZTS) thin films were successfully fabricated on glass substrates by sulfurizing Cu-Sn-Zn multilayer precursors, which were deposited by ion beam sputtering and RF magnetron sputtering, respectively. The structural, electrical and optical properties of the prepared films under various processing conditions were investigated in detail. Results showed that the as-deposited CZTS thin films with the precursors by both ion beam sputtering and RF magnetron sputtering have a composition near stoichiometric. The crystallization of the samples, however, has a strong dependence on the atomic percent of constituents of the prepared CZTS films. A single phase stannite-type structure CZTS with a large absorption coefficient of 10₄/cm in the visible range could be obtained after sulfurization at 520°C for 2 h. The samples relative to the RF magnetron sputtering showed a low resistivity of 0.073 Ωcm and band gap energy of about 1.53 eV. The samples relative to the ion beam sputtering exhibited a resistivity of 0.36 Ωcm and the band gap energy is about 1.51 eV. Supported by the National Natural Science Foundation of China (Grant No. 10574106), the Planned Science and Technology Project of Guangdong Province (Grant No.2003C05005) and the Natural Science Fund of Zhanjiang Normal University (Grant No.200801)     

Thermal Wave Effect and Sintering Activation Energy at the Initial Stage of Field Assisted Sintering Process for Non-conductive Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Powders

The effect of thermal wave at the initial stage for non-conductive Al₂O₃ powders compact in field assisted sintering technique (FAST) was investigated. The Lord and Shulman type generalized thermoselastic theory was introduced to describe the influence of thermal-mechanical interaction, as well as the heat transport and thermal focusing caused by thermal wave propagation. The expression of vacancy concentration difference of the particles was deduced by considering transient thermal stress. Subsequently, the relationship between activation energy and vacancy concentration difference was obtained. The mechanism of surface diffusion, volume diffusion, simultaneous surface and volume diffusion was analyzed. The numerical simulations indicate that low sintering temperature can obtain high local temperature by the superposition effect of thermal wave. Vacancy concentration differences were improved during FAST compared with hot-pressure and pressureless sintering, thereby decreasing the sintering time. By contrast, the activation energy declined with the decrease of vacancy concentration difference in the neck growth process.     

Fabrication and magnetic properties of Sm3(Fe,Ti)29Nx/α-Fe dual-phase nanocomposite permanent magnetic material

Sm3(Fe,Ti)29Nx/α-Fe dual-phase nanometer magnetic material was fabricated through rapid solidification, crystallization and nitridation of Sm-Fe (Ti) alloy. The effect of combination of rapid solidification and Ti alloy addition on the phase formation and microstructure of the Sm-Fe alloy is investigated in this paper. The microstructure of amorphous phase and dual-phase nano-grain crystals before and after crystallization annealing were observed using a high-resolution transmission electron microscope (HREM). The dual-phase nano-grains after annealing were compacted together with a clear interface with the direct exchange-coupling mechanism. Different annealing processes were used to examine the melt-spun alloy. Comparison of the images of SEM showed that annealing at 750℃ for 10 min was most suitable to get homogeneous and nano-grains. No obvious kink was detected in the second quadrant of the hysteresis loop like a single hard magnet, and strong exchange coupling was found between hard magnets and soft magnets.     

Sensing characterization of Sn/In/Ti nanocomplex oxides for CO,CH<Subscript>4</Subscript> and NO<Subscript>2</Subscript>

The nanocomplex oxides of Sn-In and Sn-In-Ti were prepared by controlled co-precipitation method as sensing materials of semiconductor gas sensors for detection of CO, CH4 and NO2. Through manipulating the Sn/In cation ratio, metal salt total concentration, precipitation pH value and aging time, the nanocrystalline powders were successfully derived with chemical homogeneity and superior thermal stability, compared with the single component oxides. The particle size and morphology, surface area, and thermal and phase stabilities were characterized using TEM, TG-DTA, BET and XRD. The sensing tests showed that the Sn-In composites exhibit high sensitivity and selectivity for CO and NO2. The introduction of TiO2 enhanced CH4 sensitivity and selectivity, particularly, additives of Pd and Al2O3 as a dopant and surface modification greatly enhanced the sensing properties. The sensitivity depended on the composition of composites, calcination temperature and operating temperature. The optimal values were (25%In2O3- 75%SnO2)-20%TiO2 for ternary composite, 600 and 300℃, respectively. Temperature-programmed desorption (TPD) studies were employed to explain the gas adsorption behavior displayed by the surface of nanocomposites and X-ray photoelectron spectroscopic (XPS) analysis was used to confirm the electronic interactions existing between oxide components. The sensing mechanism of the nanocomposites was attributed to chemical and electronic synergistic effects.     

Valence electron structure and properties of the ZrO<Subscript>2</Subscript>

To reveal the properties of ZrO₂ at the atom and electron levels, the valence electron structures of three ZrO₂ phases were analyzed on the basis of the empirical electron theory of solids and molecules. The results showed that the hybridization levels of Zr and O atoms in the m-ZrO₂ were the same as those in the t-ZrO₂, while those in the c-ZrO₂ rose markedly. The electron numbers and bond energies on the strongest covalent bonds in the m-ZrO₂ phase were the greatest, the values were 0.901106 and 157.5933 kJ/mol, respectively. Those in the t-ZrO₂ phase took second place, which were 0.722182 and 123.9304 kJ/mol, and those in the c-ZrO₂ phase were the smallest, which were 0.469323 and 79.0289 kJ/mol. According to the product of the bond energy on the strongest covalent bond and equivalent bond number (this value reflected the crystal cohesive energy), the order from the greatness to smallness was the c-ZrO₂> t-ZrO₂ > m-ZrO₂. This showed that the m-phase bonds were the tightest, their energy was the smallest, the crystal cohesive energy of the m-phase was the largest, and the m-phase existed most stably at room temperature. So it must need energy or higher temperature to take apart the stronger covalent bonds to form a new phase. Supported by the Major Project of the National Natural Science Foundation of China (Grant No. 90505015)     

First-principles study on adsorption of Au atom on hydroxylated SiO<Subscript>2</Subscript> surface

Adsorption of single gold (Au) atom at three kinds of sites (hollow, bridge and top) on the hydroxylated β-cristobalite SiO2 (1 1 1) surface was studied using the first-principles calculations with general gradient approximation (GGA). The results of adsorption energies and density of electronic states (DOS) suggest that the hollow and bridge sites have the basically equal capability of binding Au, while the ability of the Top site is weaker. Two new energy levels emerge after the adsorption at all sites; i...     

Synthesis and characterization of (PEO)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>−(V<Subscript>0.85</Subscript>Mo<Subscript>0.15</Subscript>)<Subscript>2</Subscript>O<Subscript>5</Subscript> nanocomposite films

(PEO) _x −(V_0.85Mo_0.15)₂O₅(x=0,0.5,1.0) nanocomposite films were prepared by a modified sol-gel method. The structure of the films was analyzed by XRD, and the DC electrical conductivity. Cyclic voltammogram and optical spectral transmittance were investigated. The results show that the (V_0.85Mo_0.15)₂O₅ xerogel has a layered structure and its interlayer space increased from 1.3181 nm at x=0 to 1.7897 nm at x=1.0. The introduction of MoO₃ improved the DC electrical conductivities of the films due to the generation of V₄₊ to maintain the electrical neutrality of the oxides. PEO intercalated in the interlayer of (V_0.85Mo_0.15)₂O₅ oxides has interaction with the oxides, enhancing the amount of Li⁺ ions inserted into the interlayer of the oxides. Moreover, the intercalation of PEO into the interlayer of (V_0.85Mo_0.15)₂O₅ oxides improved the cathodic electrochromic property in near ultraviolet region and anodic electrochromic property in visible range. JIANG Cong-sheng: Born in 1963 Supported by the Science Foundation of Hubei Province (Grant No. 2001ABB083)     

Dielectric tunable properties of BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript>-doped Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> microwave composite ceramics

BaTi₄O₉-doped Ba_0.6Sr_0.4TiO₃ (BST) composite ceramics were prepared by the conventional solid-state reaction and their structure, dielectric nonlinear characteristics and microwave dielectric properties were investigated. The secondary phase of the orthorhombic structure Ba₄Ti₁₃O₃₀ is formed among BST composite ceramics with the increase of BaTi₄O₉. At the same time, a duplex or bimodal grains size distribution shows fine grains in a coarse grain matrix. The degree of frequency dispersion of dielectric permittivity below T _m is increased initially and then decreased with respect to BaTi₄O₉. As the BaTi₄O₉ content increases, the tunability of composite ceramics decreases, while the Q value increases. Interestingly, 70 wt% BaTi₄O₉-doped BST has a tunability ∼4.0% (under 30 kV/cm biasing) versus a permittivity ∼68 and quality factor ∼134.1 (at ∼3.2 GHz). Supported by the Ministry of Science and Technology of China through 973-project (Grant No. 2009CB623302), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (Grant No.707024), Shanghai Committee of Science and Technology (Grant No. 07DZ22302), and Shanghai Foundation Project under 06JC14070     

The theoretical study of O<Subscript>2</Subscript> adsorption on NiTi (100) and (110) surfaces

The discrete variational Xa method (DV-Xα) within the framework of density-functional theory was applied to study O₂ molecule adsorption on NiTi (100) and (110) surfaces. The bond order and charge distribution between Ti and O atoms for two possible O₂ molecule adsorption ways on NiTi (100) and (110) surfaces were calculated. It is found that the adsorption way for O−O bond perpendicular to NiTi surface is preferred to that for O−O bond parallel to NiTi surface, and O₂ molecule only interacted with one nearest surface titanium atom during the adsorption process. Mulliken population and the partial density of state analysis show that the interaction between Ti and O atoms is mainly donated by O 2p and Ti 4s electrons on NiTi(110) surface, O 2p and Ti 4s, 4p electrons on NiTi(100) surface, respectively. The total density of state analysis shows that NiTi(100) surface is more favorable for O₂ molecule adsorption. HUA Ying-jie: Born in 1966. Funded by the National Natural Science Foundation of China (No. 50081001)     

Conformation of Organic Chain in Phase Transition of Hybrid （Cl2H25NH3）2MnCl4

The structural change in phase transition of hybrid （Cl2H25NH3）2MnCl4 was investigated. The temperature and the structures of the phase transition is investigated by thermal gravimetry （TG） and differential scanning calorimetry （DSC）, infrared spectrum （IR） and X-ray diffraction （XRD）. The results suggest that the phase transition is reversible and the structural change arises from the conformation change of the organic chain. The interlayer distance increases when the hybrid transforms from low temperature phase to high temperature phase. This is explained by the diffusion of gauche-bond along the organic chains and they move away from each other when the phase transition occurs. Combining the experimental data with theoretical calculation, we propose that organic chain of the hybrid in high temperature phase is the conformation of gauche-bond alternating with trans bond （noted as GTG＇TGTG＇TGTG＇T）.     

High-temperature oxidation behavior of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiAl matrix composite in air

The oxidation behavior of Al₂O₃/TiAl in situ composites fabricated by hot-pressing technology was investigated at 900° in static air. The results indicate that the mass gains of the composites samples decrease gradually with increasing Nb₂O₅ content and the inert Al₂O₃ dispersoids effectively increase the oxidation resistance of the composites. The higher the Al₂O₃ dispersoids content, the more pronounced the effect. The primary oxidation precesses obey approximately the linear laws, and the cyclic oxidation precesses follow the parabolic laws. The oxidized sample containing Ti₂AlN and TiAl phases in the scales exhibits excellent oxidation resistance. The oxide scale formed after exposure at 900°C for 120 h is multiple-layered, consisting mainly of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner TiO₂+Al₂O₃ mixed layer. From the outer layer to the inner layer, TiO₂+Al₂O₃ mixed layer presents the transit of Al-rich oxide to Ti-rich oxide mixed layer. Near the substrate, cross-section micrograph shows a relatively loose layer, and micro- and macro-pores remain on this layer, which is a transition layer and transferres from Al₂O₃+TiO₂ scale to substrate. The thickness of oxide layer is about 20 μm. It is also found that continuous protective alumina scales can not be observed on the surface of oxidation scales. Ti ions diffuse outwardly to form the outer TiO₂ layer, while oxygen ions transport inwardly to form the inner TiO₂+Al₂O₃ mixed layer. Under long-time intensive oxidation exposure, the internal Al₂O₃ scale has a good adhesiveness with the outer TiO₂ scale. No obvious spallation of the oxide scales occurs. The increased oxidation resistance by the presence of in situ Al₂O₃ particulates is attributed to the enhanced alumina-forming tendency and thin and dense scale formation. Al₂O₃ particulates enhance the potential barrier of Ti ions from M/MO interface to O/MO interface, thereby the TiO₂ growth rate decreases, which is also beneficial to improve the oxidation resistance. Moreover, the multi-structure of the TiO₂+Al₂O₃ mixed layer decreases the indiffusion of oxygen ions and also avails to improve the high temperature oxidation resistance of the as-sintered composites. Supported by the Special Program for Education Bureau of Shaanxi Province, China (Grant No. 08JK240) and Scientific Research Startup Program for Introduced Talents of Shaanxi University of Technology, China (Grant No. SLGQD0751)     

Thermal-dynamical properties and pressure dependence of phonon spectrum of ordered Si<Subscript>50</Subscript>Ge<Subscript>50</Subscript> alloy based on <Emphasis Type="Italic">ab initio</Emphasis> methods

The phonon spectrum of ordered zincblende Si₅₀Ge₅₀ alloy is calculated by ab initio method. The energy band structure at zero pressure and the pressure dependence of phonon dispersion curves are shown up to 20 GPa. The calculation finds a pressure-induced softening of the transverse acoustic phonon mode and the mode frequency reaching zero at about 14 GPa, which indicate breaking of the symmetry and formation of a new phase under high pressure. Supported by the National Natural Science Foundation of China (Grant No. 50771090), the State Key Program for Basic Research of China (Grant No. 2005CB724404) and the Program for Changjiang Scholars and Innovative Team (Grant No. IRT0650)