Bruce L. Davis

Nanophononic metamaterial: thermal conductivity reduction by local resonance.

We present the concept of a locally resonant nanophononic metamaterial for thermoelectric energy conversion. Our configuration, which is based on a silicon thin film with a periodic array of pillars erected on one or two of the free surfaces, qualitatively alters the base thin-film phonon spectrum due to a hybridization mechanism between the pillar local resonances and the underlying atomic lattice dispersion. Using an experimentally fitted lattice-dynamics-based model, we conservatively predict the metamaterial thermal conductivity to be as low as 50% of the corresponding uniform thin-film value despite the fact that the pillars add more phonon modes to the spectrum.

Thermal characterization of nanoscale phononic crystals using supercell lattice dynamics

The concept of a phononic crystal can in principle be realized at the nanoscale whenever the conditions for coherent phonon transport exist. Under such conditions, the dispersion characteristics of both the constitutive material lattice (defined by a primitive cell) and the phononic crystal lattice (defined by a supercell) contribute to the value of the thermal conductivity. It is therefore necessary in this emerging class of phononic materials to treat the lattice dynamics at both periodicity levels. Here we demonstrate the utility of using supercell lattice dynamics to investigate the thermal transport behavior of three-dimensional nanoscale phononic crystals formed from silicon and cubic voids of vacuum. The periodicity of the voids follows a simple cubic arrangement with a lattice constant that is around an order of magnitude larger than that of the bulk crystalline silicon primitive cell. We consider an atomic-scale supercell which incorporates all the details of the silicon atomic locations and the void geometry. For this supercell, we compute the phonon band structure and subsequently predict the thermal conductivity following the Callaway-Holland model. Our findings dictate that for an analysis based on supercell lattice dynamics to be representative of the properties of the underlying lattice model, a minimum supercell size is needed along with a minimum wave vector sampling resolution. Below these minimum values, a thermal conductivity prediction of a bulk material based on a supercell will not adequately recover the value obtained based on a primitive cell. Furthermore, our results show that for the relatively small voids and void spacings we consider (where boundary scattering is dominant), dispersion at the phononic crystal unit cell level plays a noticeable role in determining the thermal conductivity.

Thermal conductivity prediction of nanoscale phononic crystal slabs using a hybrid lattice dynamics-continuum mechanics technique

Recent work has demonstrated that nanostructuring of a semiconductor material to form a phononic crystal (PnC) can significantly reduce its thermal conductivity. In this paper, we present a classical method that combines atomic-level information with the application of Bloch theory at the continuum level for the prediction of the thermal conductivity of finite-thickness PnCs with unit cells sized in the micron scale. Lattice dynamics calculations are done at the bulk material level, and the plane-wave expansion method is implemented at the macrosale PnC unit cell level. The combination of the lattice dynamics-based and continuum mechanics-based dispersion information is then used in the Callaway-Holland model to calculate the thermal transport properties of the PnC. We demonstrate that this hybrid approach provides both accurate and efficient predictions of the thermal conductivity.

Analysis of Periodicity Termination in Phononic Crystals

While resonant propagation modes are non-existent within band gaps in infinite periodic structures, it is possible for anomalous band-gap resonances to appear in finite periodic structures. We establish two criteria for the characterization of band-gap resonances and propose approaches for their elimination. By considering flexural periodic beams, we show that as the number of unit-cells is increased the vibration response corresponding to band-gap resonances (1) does not shift in frequency, and (2) drops in amplitude. Both these outcomes are not exhibited by regular pass-band resonances, nor by resonances in finite homogenous beams when the length is changed. Our conclusions stem from predictions based on Timoshenko beam theory coupled with matching experimental observations.Copyright

A Three-Dimensional Lumped Parameter Model of Nanoscale Phononic Crystals

This work focuses on modeling nanoscale phononic crystals by setting up the appropriate Lagrangian equations of motion. The atomic structure and force constants are accounted for by means of a lumped parameter mass-spring model. In particular we focus on a simple cubic lattice with one mass per primitive unit cell. We use the model to predict the wave propagation frequency spectrum. We then use the model to conduct a series of studies on the influence of defects intentionally introduced to the lattice at a supercell level. One area of interest is the effect of such alterations on the size and location of band gaps.Copyright

Comparative Analysis of Satellite Aerodynamics and Its Application to Space-Object Identification

The accuracy of atmospheric density measurements inferred from satellite drag is limited by errors in drag coefficient estimates. In this work, we use a unique opportunity in which the Drag and Atmospheric Neutral Density Explorer satellite and three Polar Orbiting Passive Atmospheric Calibration Spheres are deployed from a common launch vehicle. Each object flies through similar atmospheric conditions but has a different area-to-mass ratio. This allows aerodynamic analysis that is independent of atmospheric density via comparisons of measured and modeled ballistic coefficient ratios. A test particle method combined with a satellite energy accommodation model is used to model the aerodynamics of these objects. Fitted ballistic coefficients computed as a result of special-perturbations orbit analysis are then compared to the model results. The drag coefficient model and observations agree at the 1–2% level when coefficient ratios are compared. Comparisons of an additional shape with model predictions are m...

Silicon Thin-Film Lattice Dynamics and Thermal Transport Properties

Thin films composed of dielectric materials are attracting growing interest in the solid state physics and nanoscale heat transfer communities. This is primarily due to their unique thermal and electronic properties and their extensive use as components in optoelectronic, and potentially in thermoelectric, devices. In this paper, an elaborate study is presented on silicon thin films ranging from a few nanometers in thickness to very thick bulk-like thicknesses. Full lattice dynamics calculations are performed incorporating the entire film cross section and the relaxation of the free surfaces. The phonon properties emerging from these calculations are then incorporated into Holland-Callaway models to predict the thermal conductivity and other phonon transport properties. A rigorous curve fitting process to a limited set of available experimental data is carried out to obtain the scattering lifetimes. Our results demonstrate the importance of proper consideration of the full thin-film dispersion description and provide insights into the relationship between thermal conductivity, film thickness and temperature.Copyright

NEMS Components Design Using Intentionally Defected Dispersive Building Blocks

We propose a hierarchical approach for the design of NEMS components with favorable dynamical characteristics. The approach consists of two steps: (i) design of several defect-engineered crystalline materials through intentional introduction of uniformly distributed defects, and in doing so altering precisely the frequency band structure of these materials, and (ii) allocation of patches of these designed materials to various regions in the component. Through this multiscale dispersive design approach, NEMS components can be designed to act as filters forbidding the transmission of vibrations at certain frequencies or waveguides confining the flow of energy to predetermined paths. Case studies are presented for 1D and 2D nanostructures.Copyright