Description of hard sphere crystals and crystal-fluid interfaces: a critical comparison between density functional approaches and a phase field crystal model

In materials science the phase field crystal approach has become popular to model crystallization processes. Phase field crystal models are in essence Landau-Ginzburg-type models, which should be derivable from the underlying microscopic description of the system in question. We present a study on classical density functional theory in three stages of approximation leading to a specific phase field crystal model, and we discuss the limits of applicability of the models that result from these approximations. As a test system we have chosen the three--dimensional suspension of monodisperse hard spheres. The levels of density functional theory that we discuss are fundamental measure theory, a second-order Taylor expansion thereof, and a minimal phase-field crystal model. We have computed coexistence densities, vacancy concentrations in the crystalline phase, interfacial tensions and interfacial order parameter profiles, and we compare these quantities to simulation results. We also suggest a procedure to fit the free parameters of the phase field crystal model.Comment: 21 page

Scale-bridging phase-field simulations of microstructure responses on nucleation in metals and colloids

In the present studies we investigate the connection between atomistic simulation methods, i.e. molecular dynamics (MD) and phase-field crystal (PFC), to the mesoscopic phase-field methods (PFM). While the first describes the evolution of a system on the basis of motion equations of particles the second uses a Cahn–Hilliard type equation to described an atomic density field and the third grounds on the evolution of continuous local order parameter field. The first aim is to point out the ability of the mesoscopic phase-field method to make predictions of growth velocity at the nanoscopic length scale. Therefore the isothermal growth of a spherical crystalline cluster embedded in a melt is considered. We also show simulation techniques that enable to computationally bridge from the atomistic up to the mesoscopic scale. We use a PFM to simulate symmetric thermal dendrites started at an early stage of solidification related to nucleation. These techniques allow to simulate three dimensional dendrites from the state of nuclei (≈50 Å) converted from MD up to a size of some μm where ternary side-arms start to grow

A comparative study of measurements from radiosondes, rocketsondes, and satellites

Direct comparisons of operational products derived from measurements of radiance by satellites to measurements from conventional in situ sensors are important for the evaluation of satellite systems. However, errors in the in situ measurements themselves complicate such comparisons. Atmospheric temporal and spatial variability are also influential. These issues are investigated by means of a special field program composed of flights of dual radiosondes and multiple radiosondes launched near the time of NOAA-6 overpasses. Satellite derived mean layer temperatures, geopotential heights, and winds are compared with the same quantities determined from the in situ sensors. Of particular interest is the impact of in situ errors on these comparisons. It is shown that the radiosonde provides a precise pressure height relationship and therefore precise data for synoptic type use. Radar tracking of the radiosondes reveals, however, an imprecise pressure measurement which causes large differences between the actual altitude of the radiosonde and the altitude at which it is calculated to be. Radiosondes should be radar tracked and pressures calculated if the data are to be used for purposes other than synoptic use. Evaluation of rocketsonde data reveals a temperature precision of 1 to 2 K below about 55 km. Above 55 km, the precision decreases rapidly; rms differences of up to 11 K are obtained