In order to suppress the failure of preserving positivity of density or pressure, a positivity-preserving limiter technique coupled with h-adaptive Runge-Kutta discontinuous Galerkin (RKDG) method is developed in this paper. Such a method is implemented to simulate flows with the large Mach number, strong shock/obstacle interactions and shock diffractions. The Cartesian grid with ghost cell immersed boundary method for arbitrarily complex geometries is also presented. This approach directly uses the cell solution polynomial of DG finite element space as the interpolation formula. The method is validated by the well documented test examples involving unsteady compressible flows through complex bodies over a large Mach numbers. The numerical results demonstrate the robustness and the versatility of the proposed approach.

The polymerization mechanism of a modified Pechini process was investigated by 87Sr nuclear magnetic resonance spectroscopy for the mixed solution of strontium nitrate, citric acid, and ethylene glycol. The C-ratio (the ratio of citric acid to metal ions in the polymer complex) is suggested to have a strong influence on the quality of the derived film. An analysis of the chemical shift variation, as a function of C-ratio, indicates the presence in the solution of two species: solvated strontium ions and strontium ions bound to the polymer complex, with a stoichiometry of 1:7. A polymeric precursor model based on this stoichiometry is proposed. Through a relaxation rate study of the strontium sites, it was found that the polymerization mechanism is predominantly bimolecular within the concentration region being studied. The equilibrium rate constant for the polymerization was calculated to be 104 s−1. A kinetic study of the fast cation exchange between the two identified strontium sites indicates that the inhomogeneity of the polymeric network leads to film cracking during pyrolysis.

A modified Pechini process has been successfully developed for preparation of thin films of La1−zSrzCo1−yFeyO3−x (LSCF) on both dense and porous substrates. Results indicatc that the most important processing parameter is the ratio of the polymerization/complexation agent to metal ions. Ceramic films derived from solutions with a relatively low ratio of citric acid to metal ions are usually cracked, while films derived from solutions with a relatively high ratio are crack-free and uniform. The use of ethylenediamine as an additional chelating agent further improves film quality, especially the adhesion and uniformity of the films. A single coating of solution typically yields a ceramic film of thickness about 0.4 μm, and thicker films can be prepared by application of successive coatings. For deposition of thin-film membranes on a porous substrate, however, it is necessary to modify the surface of the porous substrate in order to prevent solution from infiltrating into the pores due to capillary force, and to prevent oxide films from cracking due to surface roughness. The application of an intermediate polymer film to the surfaces of porous substrates has effectively overcome the problems and has resulted in uniform, nonporous membranes of LSCF on porous substrates. Successful deposition of thin-film ceramic membranes on porous substrates is important to fabrication of various ionic and micro-ionic devices based on ceramic thin films.

The use of transient and steady-state 4-probe techniques for the evaluation of the limitations to performance in solid-state electrochemical devices is described. The application of sequential, bipolar square-wave current pulses to a solid polymer cell with two internal reference electrodes allows separation of the polarization behavior of the various components of the cell, and indicates the rate limiting processes in the cell. Careful design of the experimental cell and appropriate selection of current pulse width allows evaluation of fundamental kinetic and transport properties, as well as activation parameters. Steady state constant-current measurements using 4-probe cells allows in situ observation of interface stability and/or polarization as a function of time as the cell is cycling.

Potential and current distributions and local energy dissipation due to Joule heating in metal-superconductor junctions have been computed as a function of geometric parameters and interfacial resistance. The primary current distribution and power dissipation are highly nonuniform in the system. The secondary current distribution and power dissipation, however, become more uniform as the interfacial resistance increases. Analysis indicates that zero contact resistance is not a stable situation since the primary distribution leads to local current densities exceeding the critical current density of the superconducting phase near the corner of the junction. Local contact failure might then initiate. A finite contact resistance is necessary for a practical application, and the minimum value of the contact resistance can be estimated from the operating current density (javg) of the device and the critical current density (jcri) of the superconducting phase. To obtain an optimum value of the contact resistance, however, one further has to take into consideration the stability and reliability of the device performance, which is, in turn, directly related to the uniformity of the current distribution and power dissipation, to temperature fluctuation of the superconducting phases brought about by local power dissipation, and to the thermal management of the system. Furthermore, a nonuniform contact resistance layer of appropriate profile can redistribute the current more effectively and more uniformly and hence reduce the total power dissipation in the system for a given jmax/javg ratio obtained by a uniform resistance layer.