Structures and stabilities of the donor–acceptor complexes HXPY (X=Al,B; Y=H,F, OH)

The optimized geometries, complexation energies, etc. of HXPY (X?=?Al, B; Y?=?H, F, OH) donor–acceptor complexes have been investigated at the B3LYP/6-311+G(d,p), MP2/6-311+G(d,p) and/or CCSD(T)/6-311+G(d,p) levels. The results show that HBPY (Y?=?H, F, OH) is more stable than the corresponding HAlPY (Y?=?H, F, OH), F (or OH) substitution on phosphorus results in decreasing complex stability, and the stronger the electron-attracting nature of the substitution atom, the more stable the complex. Moreover, the thermodynamic and kinetic properties of the formation reaction of these donor–acceptor complexes were also examined within the temperature range 200–800?K using the general statistical thermodynamics and Eyring transition state theory with Wigner correction. It is concluded that the formation of HBPY is thermodynamically favoured over that of the corresponding HAlPY, especially at low temperature, and is kinetically favoured over that of the relevant HAlPY (Y?=?H, F, OH), especially at high temperature.     

A generalised model of hydrogen diffusion in metals with multiple trap types

Continuum modelling of hydrogen diffusion in metals, which accounts for both trapping and an imposed force field, is revisited. A generalised model of hydrogen diffusion and trapping is developed as a continuous interpretation of the discrete random-walk theory. A system of nonlinear equations describing the phenomenon of diffusion with multiple types of traps is derived without the assumption of a local equilibrium among hydrogen populations in dissimilar positions. Lattice-trap interchange kinetics can degenerate into local equilibrium as a limit case. Moreover, certain terms in general equations may be negligible in specific situations. By removing these terms, known particularised models of hydrogen diffusion and trapping are recovered. Determining the terms, which are disregarded in reduced models, enables a straightforward assessment of the applicability of these models. The advantages and limitations of particularised models applied to hydrogen embrittlement analyses are discussed.     

Heterogeneous fixation of N2: Investigation of a novel mechanism for formation of NO

Formation of NO initiated by heterogeneous fixation of N₂ during pyrolysis is investigated experimentally and theoretically. The experiments were conducted with beech wood as well as with the pure biomass components cellulose, xylan, and lignin. The NO formation during char oxidation was recorded as function of pyrolysis atmosphere (N₂ or Ar), pyrolysis temperature (700–1050 °C), and oxidizing atmosphere (O₂ in N₂ or Ar). The results confirm earlier reports that biomass char may be enriched in N during pyrolysis at 900 °C and above. The N-uptake involves re-capture of N-volatiles as well as uptake of N₂. During char oxidation, the captured N is partly oxidized to NO, resulting in increased NO formation. The NO yield from oxidation of beech wood char made in N₂ increases with pyrolysis temperature, and is about a factor of two higher at 1050 °C than the corresponding yield from chars made in Ar. The experiments with pure materials show that the lignin char has the strongest ability to form NO from uptake of N₂, while xylan char forms only small amounts of NO from N₂. Density Functional Theory (DFT) calculations on model chars have revealed a number of chemisorption sites for N₂, many of which are weakly bound and therefore expected to have a short half-life at the higher pyrolysis temperatures. However, the chemisorption of N₂ across a single ring of the armchair surface was found to have an activation energy of 344 ± 30 kJ mol⁻¹ and form a stable, exothermic product with cyano groups. This demonstrates that at least one channel exists for the high-temperature incorporation of N₂ into a char which could give rise to the observed increase in NO release in subsequent char oxidation.     

Controlled Space Radiation concept for mesh-free semi-analytical technique to model wave fields in complex geometries

Numerical modelling of the ultrasonic wave propagation is important for Structural Heath Monitoring and System Prognosis problems. In order to develop intelligent and adaptive structures with embedded damage detector and classifier mechanisms, detailed understanding of scattered wave fields due to anomaly in the structure is inevitably required. A detailed understanding of the problem demands a good modelling of the wave propagation in the problem geometry in virtual form. Therefore, efficient analytical, semi-analytical or numerical modelling techniques are required. In recent years a semi-analytical mesh-free technique called Distributed Point Source Method (DPSM) is being used for modelling various ultrasonic, electrostatic and electromagnetic wave field problems. In the conventional DPSM approach point sources are placed along the transducer faces, problem boundaries and interfaces to model incident and scattered fields. Every point source emits energy in all directions uniformly. Source strengths of these 360° radiation sources are obtained by satisfying interface and boundary conditions of the problem. In conventional DPSM modelling approach it is assumed that the shadow zone does not require any special consideration. 360° Radiation point sources should be capable of properly modelling shadow zones because all boundary and interface conditions are satisfied. In this paper it is investigated how good this assumption is by introducing the ‘shadow zone’ concept at the point source level and comparing the results generated by the conventional DPSM and by this modified approach where the conventional 360° radiation point sources are replaced by the Controlled Space Radiation (CSR) sources.     

Structure, oxygen stoichiometry and electrical conductivity in the system Sr-Ce-Co-O

Mixed oxides in the system S-Ce-Co-O were prepared by solid state reaction and by freeze-drying of precursor compounds followed by thermal treatment. Two types of perovskite oxides exist in the system: Solid solutions of the type Sr_{1 − y}Ce_yCoO_{3 − x} and mixed oxides of the type (1 − y)SrCeO₃ − ySrCoO_{3 − x}. Microstructures and phase compositions were determined by electron microscopy and X-ray diffraction. SrCoO_{3 − x} forms a solid solution of ceria on the A-site in the strontium cobaltite lattice up to 0.15 mol Ce. This solid solution corresponds to the high-temperature structure of pure SrCoO_{3 − x} and is characterized by high oxygen exchange and electrical conductivity. The oxygen deficiency x was measured by solid electrolyte coulometry. The oxygen deficiency of solid solutions Sr_{1 − y}Ce_yCoO_{3 − x} increases with temperature and decreases with pO₂ in the ambient atmosphere and with increasing Ce dopant concentration. The pO₂-T-x diagram of the solid solution was determined. The T, pO₂ and dopant concentration dependencies of electrical conductivity were measured by a four-point d.c. technique. By Ce doping strontium cobaltite becomes a stabilized high-conductive material (maximum conductivity: 500 S cm⁻¹ at 400 °C, E_a = 0.025 eV, p-type). Above this temperature the T-coefficient of the conductivity changes from positive (semiconducting) to negative values.     

SCDA for 3D lattice gases with repulsive interaction

The selfconsistent diagram approximation (SCDA) is generalized for three-dimensional lattice gases with nearest neighbor repulsive interactions. The free energy is represented in a closed form through elementary functions. Thermodynamical (phase diagrams, chemical potential and mean square fluctuations), structural (order parameter, distribution functions) as well as diffusional characteristics are investigated. The calculation results are compared with the Monte Carlo simulation data to demonstrate high precision of the SCDA in reproducing the equilibrium lattice gas characteristics. It is shown that similarly to two-dimensional systems the specific statistical memory effects strongly influence the lattice gas diffusion in the ordered states. Received 7 August 2002 / Received in final form 22 January 2003 Published online 24 April 2003     

Integer linear programming formulation of the material requirements planning problem

Lot sizing procedures for discrete and dynamic demand form a distinct class of inventory control problems, usually referred to asmaterial requirements planning. A general integer programming formulation is presented, covering an extensive range of problems: single-item, multi-item, and multi-level optimization; conditions on lot sizes and time phasing; conditions on storage and production capacities; and changes in production and storage costs per unit. The formulation serves as a uniform framework for presenting a problem and a starting point for developing and evaluating heuristic and tailor-made optimum-seeking techniques.     

山姜素与人血清白蛋白相互作用的荧光光谱法研究

利用荧光光谱法和紫外-可见光谱法研究了山姜素与人血清白蛋白(HSA)之间的相互作用。证实了山姜素对HSA的荧光猝灭为动态猝灭过程,并测定了不同温度下的猝灭常数; 根据Fōrster非辐射能量转移理论,计算出山姜素在蛋白质中的结合位置与色氨酸残基间的距离为4.05 nm; 由求得的热力学参数,推断了山姜素与HSA之间主要靠疏水作用力结合;用三维荧光光谱及同步荧光光谱技术探讨了山姜素对HSA构象的影响。