A systematic approach for calibrating the direct simulation Monte Carlo (DSMC) collision model parameters to achieve consistency in the transport processes is presented. The DSMC collision cross section model parameters are calibrated for high temperature atmospheric conditions by matching the collision integrals from DSMC against ab initio based collision integrals that are currently employed in the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) and Data Parallel Line Relaxation (DPLR) high temperature computational fluid dynamics solvers. The DSMC parameter values are computed for the widely used Variable Hard Sphere (VHS) and the Variable Soft Sphere (VSS) models using the collision-specific pairing approach. The recommended best-fit VHS/VSS parameter values are provided over a temperature range of 1000-20 000 K for a thirteen-species ionized air mixture. Use of the VSS model is necessary to achieve consistency in transport processes of ionized gases. The agreement of the VSS model transport properties with the transport properties as determined by the ab initio collision integral fits was found to be within 6% in the entire temperature range, regardless of the composition of the mixture. The recommended model parameter values can be readily applied to any gas mixture involving binary collisional interactions between the chemical species presented for the specified temperature range.
Vibrational energy transport in disordered media is of fundamental importance to several fields spanning from sustainable energy to biomedicine to thermal management. In this work, we investigate hybrid ordered/disordered nanocomposites that consist of crystalline membranes decorated by regularly patterned disordered regions formed by ion beam irradiation. The presence of the disordered regions results in reduced thermal conductivity, rendering these systems of interest for use as nanostructured thermoelectrics and thermal device components, yet their vibrational properties are not well understood. Here we establish in detail the mechanism of vibrational transport and the reason underlying the observed reduction. The hybrid systems are found to exhibit glass-crystal duality in vibrational transport. Lattice dynamics reveals substantial hybridization between the localized and delocalized modes, which induces avoided crossings and harmonic broadening in the dispersion. Allen/Feldman theory shows that the hybridization and avoided crossings are the dominant drivers of the reduction. Anharmonic scattering is also enhanced in the patterned nanocomposites, further contributing to the reduction. The systems exhibit features reminiscent of both nanophononic materials and locally resonant nanophononic metamaterials, but operate in a manner distinct to both. These findings indicate that such "patterned disorder" can be a promising strategy to tailor vibrational transport through hybrid nanostructures.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))
Ion beam irradiation has recently emerged as a versatile approach to functional materials design.We show in this work that patterned defective regions generated by ion beam irradiation of silicon can create a phonon glass electron crystal (PGEC), a longstanding goal of thermoelectrics. By controlling the effective diameter of and spacing between the defective regions, molecular dynamics simulations suggest a reduction of the thermal conductivity by a factor of ∼20 is achievable.Boltzmann theory shows that the thermoelectric power factor remains largely intact in the damaged material. To facilitate the Boltzmann theory, we derive an analytical model for electron scattering with cylindrical defective regions based on partial wave analysis. Together we predict a figure of merit of ZT ≈ 0.5 or more at room temperature for optimally patterned geometries of these silicon metamaterials. These findings indicate that nanostructuring of patterned defective regions in crystalline materials is a viable approach to realize a PGEC, and ion beam irradiation could be a promising fabrication strategy.
A general approach for constructing finite rate surface chemistry models using time-of-flight (TOF) distribution data acquired from pulsed hyperthermal beam experiments is presented. First, a detailed study is performed with direct simulation Monte Carlo (DSMC) to analyze the TOF distributions corresponding to several types of reaction mechanisms occurring over a wide temperature range. This information is used to identify and isolate the products formed through different reaction mechanisms from TOF and angular distributions. Next, a procedure to accurately calculate the product fluxes from the TOF and angular distributions is outlined. Finally, in order to derive the rate constant of the reactions within the system, the inherent transient characteristic of the experimental pulsed beam set up must be considered. An analysis of the steady-state approximation commonly used for deriving the rate constants reveals significant differences in terms of the total product composition. To overcome this issue, we present a general methodology to derive the reaction rate constants, which takes into account the pulsed setup of the beam. Within this methodology, a systematic search is performed through the rate constant parameter space to obtain the values that provide the best agreement with experimentally observed product compositions. This procedure also quantifies the surface coverage that corresponds to the rates of product formation. This approach is applied to a sample system: oxidation reaction on vitreous carbon surfaces to develop a finite-rate surface chemistry model. Excellent agreement is observed between the developed model and the experimental data, thus showcasing the validity of the proposed methodologies.
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