A Trustworthy Proof Checker

Proof-carrying code (PCC) and other applications in computer security require machine-checkable proofs of properties of machine-language programs. The main advantage of the PCC approach is that the amount of code that must be explicitly trusted is very small: it consists of the logic in which predicates and proofs are expressed, the safety predicate, and the proof checker. We have built a minimal proof checker, and we explain its design principles and the representation issues of the logic, safety predicate, and safety proofs. We show that the trusted computing base (TCB) in such a system can indeed be very small. In our current system the TCB is less than 2,700 lines of code (an order of magnitude smaller even than other PCC systems), which adds to our confidence of its correctness.     

Mass transfer into simulated fermentation media

The rates of oxygen mass transfer into a simulated fermentation medium, made up of 16 kg of paper pulp per m³ of aqueous sodium sulphite solution with a cupric ion catalyst, were determined in vessels of 0·187, 0·291 and 0·451 m dia., using flat-bladed turbine impellers, and the effect of varying impeller dimensions and operating speed were investigated. Above a critical impeller tip speed the volumetric mass transfer coefficients obtained at the same speed with different power inputs (produced by variations in the impeller blade dimensions) could be represented by the sum of two terms, one depending on the impeller speed, the diameters of the impeller and vessel, and the height of pulp suspension in the vessel, and the other function of the power input per unit volume and the air velocity through the vessel. At each impeller speed the increase in the volumetric mass transfer coefficient with power input was found to be greater below a certain power input per unit volume, and a correlation for this power per unit volume was based on it corresponding to a change in the rate of air recirculation through the impeller. The expression for the mass transfer coefficient provides a more accurate basis for scale-up than the use of simple rules, such as constant power per unit volume.     

Numerical study of the thermoelectric power factor in ultra-thin Si nanowires

Low dimensional structures have demonstrated improved thermoelectric (TE) performance because of a drastic reduction in their thermal conductivity, ?? _l . This has been observed for a variety of materials, even for traditionally poor thermoelectrics such as silicon. Other than the reduction in ?? _l , further improvements in the TE figure of merit ZT could potentially originate from the thermoelectric power factor. In this work, we couple the ballistic (Landauer) and diffusive linearized Boltzmann electron transport theory to the atomistic sp³d⁵s*-spin-orbit-coupled tight-binding (TB) electronic structure model. We calculate the room temperature electrical conductivity, Seebeck coefficient, and power factor of narrow 1D Si nanowires (NWs). We describe the numerical formulation of coupling TB to those transport formalisms, the approximations involved, and explain the differences in the conclusions obtained from each model. We investigate the effects of cross section size, transport orientation and confinement orientation, and the influence of the different scattering mechanisms. We show that such methodology can provide robust results for structures including thousands of atoms in the simulation domain and extending to length scales beyond 10?nm, and point towards insightful design directions using the length scale and geometry as a design degree of freedom. We find that the effect of low dimensionality on the thermoelectric power factor of Si NWs can be observed at diameters below ??7?nm, and that quantum confinement and different transport orientations offer the possibility for power factor optimization.     

Atomistic calculations of the electronic, thermal, and thermoelectric properties of ultra-thin Si layers

Low-dimensional semiconductors are considered promising candidates for thermoelectric applications with enhanced performance because of a drastic reduction in their thermal conductivity, κ _l , and possibilities of enhanced power factors. This is also the case for traditionally poor thermoelectric materials such as silicon. This work presents atomistic simulations for the electronic, thermal, and thermoelectric properties of ultra-thin Si layers of thicknesses below 10 nm. The Linearized Boltzmann theory is coupled: (i) to the atomistic sp³d⁵s^? tight-binding (TB) model for the electronic properties of the thin layers, and (ii) to the modified valence-force-field method (MVFF) for the calculation of the thermal conductivity of the thin layers. We calculate the room temperature electrical conductivity, Seebeck coefficient, power factor, thermal conductivity, and ZT figure of merit of ultra-thin p-type Si layers (UTLs). We describe the numerical formulation of coupling TB and MVFF to the linearized Boltzmann transport formalism. The properties of UTLs are highly anisotropic, and optimized thermoelectric properties can be achieved by the choice of the appropriate transport and confinement orientations, as well as layer thickness.     

Lattice-based volumetric global illumination

We describe a novel volumetric global illumination framework based on the Face-Centered Cubic (FCC) lattice. An FCC lattice has important advantages over a Cartesian lattice. It has higher packing density in the frequency domain, which translates to better sampling efficiency. Furthermore, it has the maximal possible kissing number (equivalent to the number of nearest neighbors of each site), which provides optimal 3D angular discretization among all lattices. We employ a new two-pass (illumination and rendering) global illumination scheme on an FCC lattice. This scheme exploits the angular discretization to greatly simplify the computation in multiple scattering and to minimize illumination information storage. The GPU has been utilized to further accelerate the rendering stage. We demonstrate our new framework with participating media and volume rendering with multiple scattering, where both are significantly faster than traditional techniques with comparable quality.     

Non-equilibrium Green’s function (NEGF) simulation of metallic carbon nanotubes including vacancy defects

The electronic behavior of metallic carbon nanotubes under the influence of externally applied electric fields is investigated using the Non-Equilibrium Green’s function method self consistently coupled with three-dimensional (3D) electrostatics. A nearest neighbor tight binding model based on a single pz orbital for constructing the device Hamiltonian is used. The 3D Poisson equation is solved using the Finite Element Method. Carbon nanotubes exhibit a very weak metallic behavior, and external electric fields can alter the electrostatic potential of the tubes significantly. A single vacancy defect in the channel of a metallic carbon nanotube can decrease its conductance by a factor of two. More than one vacancy can further decrease the conductance.     

Calculation of Confined Phonon Spectrum in Narrow Silicon Nanowires Using the Valence Force Field Method

We study the effect of confinement on the phonon properties of ultra-narrow silicon nanowires of side sizes of 1 nm to 10 nm. We use the modified valence force field (MVFF) method to compute the phononic dispersion and extract the density of states, the transmission function, the sound velocity, the ballistic thermal conductance, and boundary-scattering-limited diffusive thermal conductivity. We find that the phononic dispersion and the ballistic thermal conductance are functions of the geometrical features of the structures, i.e., the transport orientation and confinement dimension. The phonon group velocity and thermal conductance can vary by a factor of two depending on the geometrical features of the channel. The 〈110#x232A; nanowire has the highest group velocity and thermal conductance, whereas the 〈111#x232A; has the lowest. The 〈111#x232A; channel is thus the most suitable orientation for thermoelectric devices based on Si nanowires since it also has a large power factor. Our findings could be useful in the thermal transport design of silicon-based devices for thermoelectric and thermal management applications.     

Study of thermal properties of graphene-based structures using the force constant method

The thermal properties of graphene-based materials are theoretically investigated. The fourth-nearest neighbor force constant method for phonon properties is used in conjunction with both the Landauer ballistic and the non-equilibrium Green’s function techniques for transport. Ballistic phonon transport is investigated for different structures including graphene, graphene antidot lattices, and graphene nanoribbons. We demonstrate that this particular methodology is suitable for robust and efficient investigation of phonon transport in graphene-based devices. This methodology is especially useful for investigations of thermoelectric and heat transport applications.     

Analysis of Thermoelectric Properties of Scaled Silicon Nanowires Using an Atomistic Tight-Binding Model

Low-dimensional materials provide the possibility of improved thermoelectric performance due to the additional length scale degree of freedom for engineering their electronic and thermal properties. As a result of suppressed phonon conduction, large improvements in the thermoelectric figure of merit, ZT, have recently been reported in nanostructures, compared to the raw materials. In addition, low dimensionality can improve a device’s power factor, offering an additional enhancement in ZT. In this work the atomistic sp³d⁵s* spin-orbit-coupled tight-binding model is used to calculate the electronic structure of silicon nanowires (NWs). The Landauer formalism is applied to calculate an upper limit for the electrical conductivity, the Seebeck coefficient, and the power factor. We examine n-type and p-type nanowires with diameters from 3 nm to 12 nm, in [100], [110], and [111] transport orientations, at different doping concentrations. Using experimental values for the lattice thermal conductivity in nanowires, an upper limit for ZT is computed. We find that at room temperature, scaling the diameter below 7 nm can at most double the power factor and enhance ZT. In some cases, however, scaling does not enhance the performance at all. Orientations, geometries, and subband engineering techniques for optimized designs are discussed.