Tetrahydrofurfuryl alcohol (H-OTHF) was successfully reacted with a series of aluminum alkyls (AlR{sub 3}) to yield compounds of the general formula [R{sub 2}Al({mu}-OTHF)]{sub 2} where R = CH{sub 3} (1), CH{sub 2}CH{sub 3} (2), and CH{sub 2}CH(CH{sub 3}){sub 2} (3). Further, reactivity studies showed that the alkyls for 1 were easily exchanged, forming compounds of the general formula [Me(OR)Al({mu}-OTHF)]{sub 2} where OR = OC{sub 6}H{sub 3}(Me){sub 2}-2,6 (4), OC{sub 6}H{sub 3}(CMe{sub 3}){sub 2}-2,6 (5a), and OSi(C{sub 6}H5){sub 3} (6). For 5a, reflux temperatures were required to get the full exchange; otherwise the asymmetric derivative [Me(OR)Al({mu}-OTHF){sub 2}AlMe{sub 2}] (5b) was isolated. The bulk powders of 1-6 were found to be in agreement with the crystal structures on the basis of elemental analyses and multinuclear solid state NMR studies. Multinuclear solution state NMR studies indicate that the alkyl OTHF derivatives have cis/trans isomers due to the chiral proton on the OTHF ligand.
The synthesis and characterization of a family of alternative precursors for the production of CdE nanoparticles (E = S, Se, and Te) is reported. The reaction of Cd(NR{sub 2}){sub 2} where NR{sub 2} = N(SiMe{sub 3}){sub 2} with n HOR led to the isolation of the following: n = 1 [Cd({mu}-OCH{sub 2}CMe{sub 3})(NR{sub 2})(py)]{sub 2} (1, py = pyridine), Cd[({mu}-OC{sub 6}H{sub 3}(Me){sub 2}-2,6){sub 2}Cd(NR{sub 2})(py)]{sub 2} (2), [Cd({mu}-OC{sub 6}H{sub 3}(CHMe{sub 2}){sub 2}-2,6)(NR{sub 2})(py)]{sub 2} (3), [Cd({mu}-OC{sub 6}H{sub 3}(CMe{sub 3}){sub 2}-2,6)(NR{sub 2})(py)]{sub 2} (4), [Cd({mu}-OC{sub 6}H{sub 2}(NH{sub 2}){sub 3}-2,4,6)(NR{sub 2})(py)]{sub 2} (5), and n = 2 [Cd({mu}-OC{sub 6}H{sub 3}(Me){sub 2}-2,6)(OC{sub 6}H{sub 3}(Me){sub 2}-2,6)(py){sub 2}]{sub 2} (6), and [Cd({mu}-OC{sub 6}H{sub 3}(CMe{sub 3}){sub 2}-2,6)(OC{sub 6}H{sub 3}(CMe{sub 3}){sub 2}-2,6)(THF)]{sub 2} (7). For all but 2, the X-ray crystal structures were solved as discrete dinuclear units bridged by alkoxide ligands and either terminal -NR{sub 2} or -OR ligands depending on the stoichiometry of the initial reaction. For 2, a trinuclear species was isolated using four {mu}-OR and two terminal -NR{sub 2} ligands. The coordination of the Cd metal center varied from 3 to 5 where the higher coordination numbers were achieved by binding Lewis basic solvents for the less sterically demanding ligands. These complexes were further characterized in solution by {sup 1}H, {sup 13}C, and {sup 113}Cd NMR along with solid-state {sup 113}Cd NMR spectroscopy. The utility of these complexes as 'alternative precursors' for the controlled preparation of nanocrystalline CdS, CdSe, and CdTe was explored. To synthesize CdE nanocrystals, select species from this family of compounds were individually heated in a coordinating solvent (trioctylphosphine oxide) and then injected with the appropriate chalcogenide stock solution. Transmission electron spectroscopy and UV-vis spectroscopy were used to characterize the resultant particles.
This paper presents a three-dimensional generalization of the bridging scale concurrent method, a finite temperature multiple scale method that couples molecular dynamics (MD) to finite elements (FE). The generalizations include the numerical calculation of the boundary condition acting upon the reduced MD region, as such boundary conditions are analytically intractable for realistic three-dimensional crystal structures. The formulation retains key advantages emphasized in previous papers, particularly the compact size of the resulting time history kernel matrix. The coupled FE and reduced MD equations of motion are used to analyze dynamic fracture in a three-dimensional FCC lattice, where interesting physical phenomena such as crack branching are seen. The multiple scale results are further compared to benchmark MD simulations for verification purposes.
The junction temperature of AlGaN/GaN ultraviolet (UV) Light-Emitting Diodes (LEDs) emitting at 295 nm is measured by using the temperature coefficients of the diode forward volt-age and emission peak energy. The high-energy slope of the spectrum is explored to measure the carrier temperature. A linear relation between junction temperature and current is found. Analysis of the experimental methods reveals that the diode-forward voltage is the most accurate method (,,b 3 ,,aC). A theoretical model for the dependence of the diode junction voltage (Vj) on junction temperature (T) is developed that takes into account the temperature dependence of the energy gap. A thermal resistance of 87.6 K/W is obtained with the AlGaN/GaN LED sample mounted with thermal paste on a heat sink.
Thermal stratification has the potential to reduce pressure-rise rates and allow increased power output for HCCI engines. This paper systematically examines how the amount of thermal stratification of the core of the charge has to be adjusted to avoid excessive knock as the engine speed and fueling rate are increased. This is accomplished by a combination of multi-zone chemical-kinetics modeling and engine experiments, using iso-octane as the fuel. The experiments show that, for a low-residual engine configuration, the pressure traces are self-similar during changes to the engine speed when CA50 is maintained by adjusting the intake temperature. Consequently, the absolute pressure-rise rate measured as bar/ms increases proportionally with the engine speed. As a result, the knocking (ringing) intensity increases drastically with engine speed, unless counteracted by some means. This paper describes how adjustments of the thermal width of the in-cylinder charge can be used to limit the ringing intensity to 5 MW/m2 as both engine speed and fueling are increased. If the thermal width can be tailored without constraints, this enables smooth operation even for combinations of high speed, high load, and combustion phasing close to TDC. Since large alterations of the thermal width of the charge are not always possible, combustion retard is considered to reduce the requirement on the thermal stratification. The results show that combustion retard carries significant potential since it amplifies the benefit of a fixed thermal width. Therefore, the thermal stratification required for operation with an acceptable knocking intensity can be decreased substantially by the use of combustion retard. This enables combinations of high engine speed and high fueling rate even for operation with the naturally occurring thermal stratification. However, very precise control of the combustion phasing will likely be required for such operation.
The solution of the governing steady transport equations for momentum, heat and mass transfer in fluids undergoing non-equilibrium chemical reactions can be extremely challenging. The difficulties arise from both the complexity of the nonlinear solution behavior as well as the nonlinear, coupled, non-symmetric nature of the system of algebraic equations that results from spatial discretization of the PDEs. In this paper, we briefly review progress on developing a stabilized finite element ( FE) capability for numerical solution of these challenging problems. The discussion considers the stabilized FE formulation for the low Mach number Navier-Stokes equations with heat and mass transport with non-equilibrium chemical reactions, and the solution methods necessary for detailed analysis of these complex systems. The solution algorithms include robust nonlinear and linear solution schemes, parameter continuation methods, and linear stability analysis techniques. Our discussion considers computational efficiency, scalability, and some implementation issues of the solution methods. Computational results are presented for a CFD benchmark problem as well as for a number of large-scale, 2D and 3D, engineering transport/reaction applications.
This study investigates algebraic multilevel domain decomposition preconditioners of the Schwarz type for solving linear systems associated with Newton-Krylov methods. The key component of the preconditioner is a coarse approximation based on algebraic multigrid ideas to approximate the global behavior of the linear system. The algebraic multilevel preconditioner is based on an aggressive coarsening graph partitioning of the non-zero block structure of the Jacobian matrix. The scalability of the preconditioner is presented as well as comparisons with a two-level Schwarz preconditioner using a geometric coarse grid operator. These comparisons are obtained on large-scale distributed-memory parallel machines for systems arising from incompressible flow and transport using a stabilized finite element formulation. The results demonstrate the influence of the smoothers and coarse level solvers for a set of 3D example problems. For preconditioners with more than one level, careful attention needs to be given to the balance of robustness and convergence rate for the smoothers and the cost of applying these methods. For properly chosen parameters, the two- and three-level preconditioners are demonstrated to be scalable to 1024 processors.
Least-squares finite-element methods for Darcy flow offer several advantages relative to the mixed-Galerkin method: the avoidance of stability conditions between finite-element spaces, the efficiency of solving symmetric and positive definite systems, and the convenience of using standard, continuous nodal elements for all variables. However, conventional C{sup o} implementations conserve mass only approximately and for this reason they have found limited acceptance in applications where locally conservative velocity fields are of primary interest. In this paper, we show that a properly formulated compatible least-squares method offers the same level of local conservation as a mixed method. The price paid for gaining favourable conservation properties is that one has to give up what is arguably the least important advantage attributed to least-squares finite-element methods: one can no longer use continuous nodal elements for all variables. As an added benefit, compatible least-squares methods inherit the best computational properties of both Galerkin and mixed-Galerkin methods and, in some cases, yield identical results, while offering the advantages of not having to deal with stability conditions and yielding positive definite discrete problems. Numerical results that illustrate our findings are provided.
The effect of block copolymers on the cocontinuous morphology of 50/50 (w/w) polystyrene (PS)/high density polyethylene (HDPE) blends was investigated using symmetric polystyrene-polyethylene block copolymers (PS-PE) with molecular weights varying from 6 to 200 kg/mol. The coarsening rate during annealing was compared to the Doi-Ohta theory. An intermediate molecular weight PS-PE, 40 kg/mol, showed remarkable results in reducing the phase size and stabilizing the blend morphology during annealing. Mixing small amounts of 6, 100 or 200 kg/mol PS-PE in the blend did not reduce the phase size significantly, but did decrease the coarsening rate during annealing. In stabilizing the morphology, 6 kg/mol PS-PE was inferior to 100 and 200 kg/mol. The existence of an optimal molecular weight block copolymer is due to a balance between the ability of the block copolymer to reach the interface and its relative stabilization effect at the interface.