Distinct signature of local tetrahedral ordering in the scattering function of covalent liquids and glasses

Tetrahedral amorphous materials such as SiO2, GeO2, Si, Ge, C, and chalcogenides are extremely important in nature and technology. It is known that covalent bonding favors local tetrahedral order in these materials. However, how to extract information on this structural order from the scattering function has remained elusive. By analyzing the structure of simulated SiO2 and experimental data of various tetrahedral materials, we show that the lowest wave number peak, known as the first sharp diffraction peak (FSDP), and a few higher wave number ones in the scattering functions come from the characteristic density waves of a single tetrahedral unit. FSDP is thus a direct measure of the tetrahedrality. This finding opens the door for long-awaited experimental access to the characterization of disordered amorphous structures.

Modulation of the directions of the anisotropic axes of DyIII ions through utilizing two kinds of organic ligands or replacing DyIII ions by FeIII ions

Two complexes based on Dy4 or Fe2Dy2 tetrahedral unit have been obtained by employing mixed organic ligands. The directions of the easy magnetization for the DyIII in both complexes were successfully modulated.

Manufacture and prefabrication practice on a test model of a novel six-bar tetrahedral cylindrical lattice shell

A six-bar tetrahedral unit of a rhombic projection plane is a basic geometric invariable body with a simple configuration. A six-bar tetrahedral cylindrical lattice shell can be assembled by rhombic projection plane six-bar tetrahedral units with identical geometry sizes, which is conducive to standardized design, industrial production, and prefabricated construction. The popularization and application requirements of green architecture industry can also be met using such a shell. A model test of a six-bar tetrahedral cylindrical lattice shell was developed to simulate the practical application of the prefabricated structural system. A new type of joint for the prefabricated structure was proposed that is easy to assemble and is lightweight. A tridimensional fine-tuning bed jig was developed to ensure the manufacturing precision of the pre-made six-bar tetrahedral units. Stacking and transport of the units were practiced. The assembly process from units to the integral shell was conducted in an experimental hall; the result testified the advantages of easy stack, convenient transport, and highly efficient fabrication. The structural assembly errors of the shell were measured, and the measurement results verified the installation accuracy of the test model. This practical study lays the foundation for further engineering applications of six-bar tetrahedral cylindrical lattice shells.

Synthesis and Properties of P2O5-SnO2-ZnO Penetrated into Silica Aerogel Matrix by Sol-Gel Technique

This work demonstrated the synthesis of SiO2-P2O5-SnO2-ZnO quaternary porous aerogel by sol-gel followed supercritical carbon dioxide drying, starting from tetraethoxysilane (TEOS) as precursor for SiO2and triethyl phosphate, tin (iv) chloride pentahydrate and zinc nitrate hexahydrate as precursor for P2O5-SnO2-ZnO. It has been recorded that prepared P2O5-SnO2-ZnO sol was added to silica sol, then formed the quaternary aerogel by base catalyst. The microstructure, morphology and properties of the quaternary aerogel were studied by TG/DTA, FTIR, solid-state29Si NMR, BET, scanning electron microscopy (SEM) measurement and BJH nitrogen gas adsorption. The silica aerogels tetrahedral subunit structure and the influence of the loaded oxide have been observed. The results indicated that the quaternary aerogel exhibited average pore diameter of 14.15 nm, cumulative pore volume of 2.31 ml/g, the specific surface area as high as 796.29 m2/g and bulk density of 0.275g/cm3. There were four types of Si (O1/2)4tetrahedral unit structure in the quaternary aerogel. From this study, different chemical bond of P2O5-SnO2-ZnO penetrated into the silica network structure.

Pressure loss and heat transfer mechanisms in a lattice-frame structured heat exchanger

A novel heat exchanger medium, a high-porosity (0.938) lattice-frame material (LFM), has been introduced for possible use in mechanically and thermally loaded heat exchanger applications. The LFM is made up of circular cylinders, forming tetrahedral unit cells. This paper describes the results of experiments and numerical simulation leading to a detailed understanding of the flow structure, pressure loss and heat transfer mechanisms. It is shown that the circular LFM struts are responsible for approximately 85 per cent of the overall pressure losses in the unit cell by means of form drag at high Reynolds number. The LFM causes heat removal from the substrate by promoting flow mixing and also contributes to the overall heat transfer by convection from the strut surfaces. If a high thermal conductivity material is used, the strut and substrate contribute 57 and 43 per cent respectively of the total heat transfer. Steady numerical simulations show that a porosity of approximately 0.8 provides the best heat transfer performance for a fixed mass flowrate. However, the pressure loss monotonically increases as the porosity decreases within a range of porosity, 0.7 ≤ ε ≤ 0.938.

Comparison of several tetrahedra-based lattices

A comparison of the quantum Heisenberg anti-ferromagnetic model on the pyrochlore lattice, the checkerboard lattice, and the square lattice with crossing interactions is performed. The three lattices are constructed with the same tetrahedral unit cell and this property is used to describe the low-energy spectrum by means of an effective Hamiltonian restricted to the singlet sector. We analyze the structure of the effective Hamiltonian and solve it within a mean-field approximation for the three lattices. PACS No.: 75.10Jm

The Nonlinear Elastic Behavior of Open-Cell Foams

A constitutive model for the nonlinear elastic behavior of isotropic low-density opencell foams with three-dimensional structure is formulated in terms of a strain energy function. The theory is based on micromechanical analysis of an idealized tetrahedral unit cell of arbitrary orientation that contains four half-struts joining at equal angles. The force-displacement relations for each strut are expressed by compliances for bending and stretching that do not depend on the magnitude of applied force. Contributions to the strain energy from large deformation effects are assumed to depend on strut reorientation and stretching, and are determined by analyzing a pin-jointed structure. The analysis is considered to be valid for finite strains below the onset of yielding associated with strut buckling.

Chemical Bonding Effects in X-Ray Emission Spectra - A Molecular Orbital Model

Molecules or ions usually exist as discrete units, in crystals of chemical compounds. Intermolecular or interionic coValent interactions are slight so the bond structure of such, solids is very similar to the pattern of energy levels in each individual molecule or ion. Simple molecular orbital theory can therefore be used to generate a qualitative picture of the energy levels in a molecule or an ion; and this picture can then be used directly to interpret X-ray emission spectra. The application of molecular orbital theory, using group theory to simplify the calculations is described for a tetrahedral unit ML4. The origin of peak shifts and of low-energy satellite peaks are rationalised. A consideration of orbital amplitudes shows that the ‘cross-over' theory of O'Brien and Skinner cannot explain the observed intensities of low-energy satellite peaks. It is suggested that the use of the M. 0. model for the interpretation of X-ray emission spectra permits far greater analytical and structural use to be made of peak shift and satellite data. Ligands can be identified even when their own characteristic emissions are not detected (e.g. oxygen and fluorine). Relative peak intensities can be correlated with atomic orbital participation in bond formation. Such information is of great interest to chemists and can often be used to identify the bonding r61e of specific orbitals (e.g. the 3d orbitals of second row, main group, elements).