Molecular dynamics simulation of phase transition in AgNO3

Structural phase transition in AgNO₃ at high temperature is simulated by molecular dynamics. The simulations are based on the potentials calculated from the Gordon-Kim modified electron-gas formalism extended to molecular ionic crystals. AgNO₃ transforms into rhombohedral structure at high temperature and the phase transition is associated with the rotations of the NO₃ ions and displacements of the NO₃ and Ag ions.     

System Dynamics Modelling for Residential Energy Efficiency Analysis and Management

A System Dynamics model to simulate the substitution of installed household appliances by more efficient ones is presented. The model allows the construction of scenarios and also the analyses of several other issues such as: alternatives for technology penetration, electricity consumption growth, gas consumption growth and effects of pricing policies on various energy demands. The proposed methodology has been applied to assist the decision process in relation to gas penetration policies. The model also supports policy making on energy efficiency and it allows the calculation of total energy savings under different scenarios. Furthermore, government underpricing policies on tariffs and appliance acquisition (longer loan terms, lower interest rates and grace periods) may also be analysed.     

Dimension-Splitting for Simplifying Diffusion in Lattice-Gas Models

We introduce a simplified technique for incorporating diffusive phenomena into lattice-gas molecular dynamics models. In this method, spatial interactions take place one dimension at a time, with a separate fractional timestep devoted to each dimension, and with all dimensions treated identically. We show that the model resulting from this technique is equivalent to the macroscopic diffusion equation in the appropriate limit. This technique saves computational resources and reduces the complexity of model design, programming, debugging, simulation and analysis. For example, a reaction-diffusion simulation can be designed and tested as a one-dimensional system, and then directly extended to two or more dimensions. We illustrate the use of this approach in constructing a microscopically reversible model of diffusion-limited aggregation as well as in a model of growth of biological films.     

Study of spontaneous fission with EUROGAM

Prompt γ radiation following spontaneous fission has been measured using the EUROGAM 2 array. Using these data and newly developed analysis techniques a number of subjects were studied, which are briefly discussed. One of these subjects, shape transition in A≈100 Sr and Zr nuclei is discussed in more detail. Quadrupole moments were determined for rotational bands in the N=58,59,60,62 and 64 Sr and Zr isotopes. Deformation parameters, which increase from β₂ ≈ 0.1 at N=56 to β₂ ≈ 0.4 at N=64, suggest that in strontium and zirconium isotopes shape change happens gradually between N=56 and N=62 and is probably due to an occupancy of three or more deformation-driving orbitals of h_11/2 parentage.     

Air traverse time in grain bins

To maintain the quality of cereal grains during storage, it is necessary to keep the grain cool and free from insects, and typical methods for dealing with these problems are considered in this paper. In particular the insect population is controlled by fumigating the grain bed with carbon dioxide gas and the grain is cooled by forcing ambient air through the bed. In both problems, the equations which describe the physical processes contain a mixture of advection and diffusion or conduction terms. This paper explores the relationship between traverse time and heat and mass transfer and gains an insight into the grain storage processes that are controlled by forced convection. When heat and mass transport is dominated by the advection terms, the equations are simplified by changing variables from the (x,y) space coordinates to (ψ,τ), where ψ is the stream function for the problem and the traverse time τ at a point in the storage bin is the time taken for the air to travel to the point from the inlet duct. The conditions are described for the equations to be independent of ψ, with the main condition being that the derivatives of the metrics g₁₁, g₁₂ and g₂₂ with respect to ψ are small enough. If the equations are independent of ψ then the dependent variable (concentration or temperature) will be constant on lines of constant traverse time τ. This relationship between traverse time and the cooling or fumigation pattern can be used in the design of storage bins since it implies that the best outlet surface is a line of constant τ.     

Encapsulation of organotin compounds in metal acetate glasses

Triorganotin halides, oxides and sulphides can be dissolved in molten, mixed-metal acetates at ca 140–160°C without decomposition; quenching provides glasses into which are encapsulated the organotin species. Halide/acetate and oxide/acetate, but not sulphide/acetate, exchanges occur in the melt. Only partial exchange was found for hindered trineophyl tin chloride [(PhCMe₂CH₂)₃SnCl], in contrast to the complete exchanges observed for the butyl (Bu), phenyl (Ph) and cyclohexyl (Cy) analogues. Complete oxide/acetate exchange was found for (Bu₃Sn)₂O, partial exchange occurred for (Cy₃Sn)₂O, whilst no exchange resulted with bis(trineophyltin) oxide or (Ph₃Sn)₂O. Tin–tin bonds (e.g. as in Ph₃SnSnPh₃) and carbon–tin bonds (even the allyl–Sn bond in Bu₃SnCH₂CH?CH₂) are not affected. The acetate glasses dissolve in aqueous media with release of the organotin species and they have potential as slow-release systems which is currently being investigated.     

Diffusion competition between solvent and nonsolvent during the coagulation process of cellulose / ammonia / ammonium thiocynate fiber spinning system

An investigation of the diffusion competition between solvent and nonsolvent in a coagulation bath is presented for the formation of a new cellulosic fiber by wet-spinning. The system consisted of the spinnable cellulose solution with a mixture of liquid ammonia/ammonia thiocynate as the solvent and low-molecular-weight alcohols as the nonsolvents. The diffusion competition between solvent and nonsolvent was quantitatively characterized in terms of their mass transfer rate differences. The measurements of this rate difference were performed on the model filament shaped from gelled cellulose solutions. Results revealed that an increase in molecular size of coagulant, bath temperature, and coagulant concentration in the bath enhanced preferential diffusion of solvent from cellulose solution. Fiber spinning experiments showed that a higher value of the initial modulus of the fiber was attained with a coagulation condition providing a lower value of mass transfer rate difference. The importance of mass transfer rate difference was also shown in the influence of the fiber cross-sectional shapes.