Abstract: Transition metal oxides form a series of compounds with a uniquely wide range of electronic properties. Some have been known since antiquity, whereas other properties, such thermoelectricity (TE) have been discovered rather recently. In developing such material systems, Mn, Cu, Co or Ni oxides and their binary combinations were considered for thermoelectric applications over forty years ago at the Westinghouse Research Laboratory [1,2]. Complex quaternary compositions can potentially deliver more flexibility in terms for structural variations and their transport mechanisms, anticipating better performance for thermoelectric properties . Over a wide range of compositions, containing Mn, Cu, Ni, and Co, the crystal structure basically takes on the spinel (AB2O4) configuration, where oxygen tetrahedrally coordinates A-sites and octahedrally coordinates the B-sites; however, the unit cell contains 56 atoms with 8 A-site atoms, 16 B-site atoms, and 32 oxygen atoms. Electrical conduction in similar oxide compounds has been shown to originate from a charge hopping mechanism: either variable range hopping or small-polaron hopping [4,5]. A small-polaron is a charge that resides on a cation but has a wave function extending beyond that of a normal valence electron. The potentially delocalized nature of this charge combined with the strain field generated by the neighboring atoms has two defining characteristics of a polaron [6, 7]. The consensus is that hopping occurs between the Mnoccupied B-sites of the unit cell, and these sites lie along the <110> directions. The hopping between adjacent B-B sites provides the shortest inter-site gaps, as compared to the A-B or A-A inter-site distances. Current thermoelectric materials are suited to room temperature applications, yet it remains highly desirable to identify new materials that function efficiently at elevated temperatures. Oxides are a natural choice due to their high temperature stability.
In an attempt to develop complex multi-component oxide systems having specific properties for thermoelectric applications, we describe here our strategy in finding optimum compositions through “combinatorial material search (CMS)”. Once the desired compositions are selected, the materials are then fabricated by low temperature synthesis, stabilizing thermodynamically metastable valence states of the ions. The present study was aimed at finding material’s compositions having unique thermoelectric properties; however, the strategy described here can be used, in general, to search new material systems for specific applications of interest.
The application of CMS, originally developed by Xiang and Schultz , promises to increase an ability to identify and optimize the material compositions and their properties in a very efficient way. We have recently demonstrated an effective use of CMS on a Ni-Cu-Mn based oxide system. Experimentally, three target materials (NiO, CuO and Mn2O3) were sequentially deposited in thin film forms using a pulsed laser deposition (PLD) technique, in which thickness of the each layer was tapered over the substrate. After sequentially depositing three material layers, the process was repeated for multiple rounds until desired thickness is achieved on the substrate. By proper annealing, three materials were mutually diffused to form continuously graded ternary compositions on the substrate. In Fig. 1, the CMS process is schematically illustrated.
Abstract: This paper describes the metallurgical interfacial reactions at elevated temperatures between reactive zirconium metal and stable oxide ceramics, specifically beryllia, yttria, and magnesia- zirconia composite ceramic. The ceramic/metal systems were preheated at 600°C, and then heated to peak temperatures of 1800°C or above, depending of the system, in ultra pure Argon atmosphere. After a short stay at the peak temperature, each system was cooled to room. The interaction was monitored during heating by a video camera and the interfaces were microscopically examined after the thermal cycle. The microstructure and chemical changes at the interface were evaluated via SEM and EDS. During heating of the beryllia/Zr system, the ceramic was initially reduced and Be alloyed the Zr metal in solid solution, causing Zr to melt locally at the interface at about 1600°C instead of 1855°C. The alloy Zr-Be liquid is what later dissolved the beryllia and infiltrated partially into the ceramic substrate. It seems that there was no solid state reaction between the Zr metal and yttria since Zr melted at its melting temperature of 1855°C; it is evident, however, that the liquid Zr partially dissolved yttria at the interface; yttrium and oxygen segregated to the grain boundaries. The solidified metal tightly bonded to the ceramic substrate as the system cooled to room temperature. In the Zr-MgO/ZrO2 system, Zr melted at 1855°C and it reduced the magnesia, but at the same time the magnesium was volatilized.
Abstract: In the near twenty years, with the rapid increase of Chinese economy, Chinese welding technology developed very fast. In 2003, the annual output of steel in China has reached 230 megatons and above 40 percent was manufactured by welding process. The main welding engineering projects include: the Three Gorges power station, bridges over the Changjiang River, high pressure heavy-duty chemical container, pipeline project of transferring natural gas from West to East and manned spacecraft etc. Meanwhile, Chinese annual output of welding materials exceeds 1,200 kilotons, welding equipments exceed 200,000 sets and the welding automation rate reached 40 percent.
The State Key Laboratory of Advanced Welding Production Technology in Harbin Institute of Technology is the only welding laboratory of national level in China. It is the base of applied fundamental research and high quality welding technicians training. It has completed many projects of welding technology development and some significant applied engineering tasks. Every year, about 15-20 Doctors, 40-50 Masters and 70-100 Bachelors graduate from the Laboratory. Also, it is opened to the whole world.
Abstract: The paper gives a brief overview of our recent work on atomistic computer modeling of ordered intermetallic compounds of the Ni-Al and Ti-Al systems. Atomic interactions in these systems are modeled by semi-empirical potentials fit to experimental and first-principles data. The methodology includes a large variety of techniques ranging from harmonic lattice dynamics to molecular dynamics and Monte Carlo simulations. The properties studied include lattice characteristics (elastic constants, phonons, thermal expansion), point-defect properties, atomic diffusion, generalized stacking faults, dislocations, surfaces, grain boundaries, interphase boundaries, and phase diagrams. We especially emphasize the recent progress in the understanding of diffusion mechanisms in NiAl and TiAl, calculation of stacking fault energies in Ni3Al in relation to dislocation behavior, and calculations
of / 0 interface boundaries in Ni-Al alloys.
Abstract: Ab initio pseudopotential calculations of Cu/Al2O3 and Au/TiO2 interfaces have revealed strong effects of interface stoichiometry. About the Cu/Al2O3 system used for coatings and electronic devices, the interfacial bond of the O-terminated (O-rich) Cu/Al2O3(0001) interface is very strong with ionic and covalent Cu-O interactions, although that of the Al-terminated (stoichiometric) one is rather weak with electrostatic and Cu-Al hybridization interactions. About the Au/TiO2 system with unique catalytic activity, the adhesive energy between non-stoichiometric (Ti-rich or O-rich) TiO2(110) surface and a Au layer is very large, and there occur substantial charge transfer and orbital hybridization, which should have close relations to the catalytic activity.
Abstract: Feature sizes of useful electronic devices are becoming smaller and reaching nanometer ranges. There is increasing demand to perform dynamic simulations of such nano-devices with realistic sizes. To date, various kinds of simulation methods have been used for materials and devices including the density-functional theory (DFT) and the molecular dynamics (MD) for atomistic mechanics and the finite element method for continuum mechanics. We review recent progresses in our multiscale, hybrid simulation schemes that combine those methods. The coarse-grained particles (CG) method originally proposed by Rudd and Broughton [Phys. Rev. B58 (1998), p. R5893] has features suitable to such hybridization. We improve the CG method so that it is applicable to realistic nanostructured materials with large deformations. A novel hybridization scheme that couples the DFT method with the MD method is presented, which is applicable to virtually any selection of the DFT region in a wide range of materials. Hybrid DFT-MD simulations of the H2O reaction with nanostructured Si and alumina systems under stresses are performed, to demonstrate significant effects of stress on the chemical reaction.
Abstract: The interface of fiber and matrix strongly influences the performance and strength of fiber-reinforced composite materials. Due to the limitations of continuum mechanics at the nanometer length scale, atomistic level computer simulation has started to play an important role in the understanding of such interfacial systems. Our study focuses on a typical crosslinked interfacial system of glass-epoxy composite with the presence of silanes. To explore the mechanical properties of the interfacial network system, Coarse-grained Molecular Dynamics is used. Currently it is not possible to study mechanical properties of interfacial systems purely through ab initio molecular dynamics simulations because of the huge computational resources required. Although pure atomistic classical molecular dynamics simulations have been used to study systems comprising billions of atoms, classical MD simulation do not take into account the effects of crosslinking of molecular chains. A new force field, which combines the Lennard-Jones potential and a finiteextensible nonlinear elastic attractive potential, is proposed and incorporated in a bead-spring model to simulate glass/epoxy interfacial system with the crosslinked structure of silanes. The finite-extensible nonlinear elastic attractive potential is included to control the motion and breakage of polymer chains. Interfacial adhesion and mechanical properties were studied through the simulation of mechanically separating the interfacial system.
Abstract: The behaviors of a material are nonlinear in the large deformed region. The hyper elastic models can describe such non linear materials. If the hyper elastic material is applied to the hydrostatic tensile load, the void begins to grow when the load exceed the critical value. It is important to study the coalescence of the void growth in order to consider the destruction of the material. In this paper, the void growth simulations in the hyper-elastic material with multiple seeds are studied. The unit rectangular cell with small voids is subjected to the hydrostatic tensile load. This problem can be analyzed by FEM. However, the simulation with the larger number of the voids is not possible. Thus, the CA (Cellular Automaton) is used to describe the behaviors of the void coalescence and the possibility of CA is discussed.
Abstract: The structural and electronic properties of isolated neutral ZnmCdn clusters for m+n £ 3 have been investigated by performing density functional theory calculations at B3LYP level. The optimum geometries, vibrational frequencies, electronic structures, and the possible dissosiation channels of the clusters considered have been obtained. An empirical many-body potential energy function (PEF), which comprices two- and three-body atomic interactions, has been developed to investigate the structural features and energetics of ZnmCdn (m+n=3,4) microclusters. The most stable structures were found to be triangular for the three-atom clusters and tetrahedral for the four-atom clusters. On the other hand, the structural features and energetics of Znn-mCdm (n=7,8) microclusters, and Zn50, Cd50, Zn25Cd25, Zn12Cd38, and Zn38Cd12 nanoparticles have been investigated by performing molecular-dynamics computer simulations using the developed PEF. The most stable structures were found to be compact and three-dimensional for all elemental and mixed clusters. An interesting structural feature of the mixed clusters is that Zn and Cd atoms do not mix in mixed clusters, they come together almost without mixing. Surface and bulk properties of Zn, Cd, and ZnCd systems have been investigated too by performing molecular-dynamics simulations using the developed PEF. Surface reconstruction and multilayer relaxation on clean surfaces, adatom on surface, substitutional atom on surface and bulk materials, and vacancy on surface and bulk materials have been studied extensively.
Abstract: An analytic embedded-atom potentials was developed. It was applied to calculating mono-vacancy formation energy, divacancy binding energy, elastic constants, energy difference of different structures, the surface energy, and the phonon spectra of iron and europium. The formation enthalpies of Fe-Eu binary alloy were also calculated. The calculated physical properties are in agreement with the experiments available or other theoretical results. The formation enthalpies are in good agreement with the results obtained by Miedema’s theory.