Temperature-dependent thermal conductivity in nanoporous materials studied by the Boltzmann Transport Equation

Nanostructured materials exhibit low thermal conductivity because of the additional scattering due to phonon-boundary interactions. As these interactions are highly sensitive to the mean free path (MFP) of a given phonon mode, MFP distributions in nanostructures can be dramatically distorted relative to bulk. Here we calculate the MFP distribution in periodic nanoporous Si for different temperatures, using the recently developed MFP-dependent Boltzmann Transport Equation. After analyzing the relative contribution of each phonon branch to thermal transport in nanoporous Si, we find that at room temperature optical phonons contribute 18 % to heat transport, compared to 5% in bulk Si. Interestingly, we observe a steady thermal conductivity in the nanoporous materials over a temperature range 200 K < T < 300 K, which we attribute to the ballistic transport of acoustic phonons with long intrinsic MFP. These results, which are also consistent with a recent experimental study, shed light on the origin of the reduction of thermal conductivity in nanostructured materials, and could contribute to multiscale heat transport engineering, in which the bulk material and geometry are optimized concurrently

Physically founded phonon dispersions of few-layer materials, and the case of borophene

An increasing number of theoretical calculations on few-layer materials have been reporting a non-zero sound velocity for all three acoustic phonon modes. In contrast with these reports, here we show that the lowest phonon dispersion branch of atomistically described few-layer materials should be quadratic, and this can have dramatic consequencies on calculated properties, such as the thermal conductivity. By reformulating the interatomic force constants (IFC) in terms of internal coordinates, we find that a delicate balance between the IFCs is responsible for this quadraticity. This balance is hard to obtain in ab-initio calculations even if all the symmetries are numerically enforced a posteriori, but it arises naturally in our approach. We demonstrate the phenomenon in the case of borophene, where a very subtle correction to the ab-initio IFCs yields the physically correct quadratic dispersion, while leaving the rest of the spectrum virtually unmodified. Such quadraticity nevertheless has a major effect on the computed lattice thermal conductivity, which in the case of borophene changes by more than a factor 2, and reverses its anisotropy, when the subtle IFC correction is put in place

The role of lighter and heavier embedded nanoparticles on the thermal conductivity of SiGe alloys

We have used an atomistic {\it ab initio} approach with no adjustable parameters to compute the lattice thermal conductivity of Si$_{0.5}$Ge$_{0.5}$ with a low concentration of embedded Si or Ge nanoparticles of diameters up to 4.4 nm. Through exact Green's function calculation of the nanoparticle scattering rates, we find that embedding Ge nanoparticles in $\text{Si}_{0.5}\text{Ge}_{0.5}$ provides 20% lower thermal conductivities than embedding Si nanoparticles. This contrasts with the Born approximation which predicts an equal amount of reduction for the two cases, irrespective of the sign of the mass difference. Despite these differences, we find that the Born approximation still performs remarkably well, and it permits investigation of larger nanoparticle sizes, up to 60 nm in diameter, not feasible with the exact approach.Comment: 13 pages, 5 figures, Accepted for publication in Physical Review

Phonon transmission through defects in carbon nanotubes from first principles

The original published version of this article may be found on the Physical Review B website: http://dx.doi.org/10.1103/PhysRevB.77.033418 Copyright (2008) American Physical SocietyWe compute the effect of different isolated defects on the phonon transmission through carbon nanotubes, using an ab initio density functional approach. The problem of translational and rotational invariance fulfillment in the nonperiodic system is solved via a Lagrange-multiplier symmetrization technique. The need for an ab initio approach is illustrated for the case of phonon transmission through a nitrogen substitutional impurity, for which no reliable empirical interatomic potentials exist. This opens an avenue for the accurate parameterfree study of phonon transport through general systems with arbitrary composition and structure, without any need for semiempirical potential descriptions

Doublet structures in quantum well absorption spectra due to Fano-related interference

In this theoretical investigation we predict an unusual interaction between a discrete state and a continuum of states, which is closely related to the case of Fano-interference. It occurs in a GaAs/AlxGa1-xAs quantum well between the lowest light-hole exciton and the continuum of the second heavy-hole exciton. Unlike the typical case for Fano-resonance, the discrete state here is outside the continuum; we use uniaxial stress to tune its position with respect to the onset of the continuum. State-of-the art calculations of absorption spectra show that as the discrete state approaches the continuum, a doublet structure forms which reveals anticrossing behaviour. The minimum separation energy of the anticrossing depends characteristically on the well width and is unusually large for narrow wells. This offers striking evidence for the strong underlying valence-band mixing. Moreover, it proves that previous explanations of similar doublets in experimental data, employing simple two-state models, are incomplete.Comment: 21 pages, 5 figures and 5 equations. Accepted for publication in Physical Review

From "Sirups" to Biocarbons: A 30 Year Research Cooperation for Better Biomass Utilization with Michael J. Antal, Jr

The results of a 30 year U.S.-Hungarian research cooperation are surveyed. The head of the cooperating U.S. laboratory, Michael J. Antal, Jr., died on Oct 21, 2015. He was a leading person in biomass research. The collaboration started with pyrolysis studies. In this phase of the work, the aim was to clarify the factors that enhance the formation of the valuable volatile products ("sirups"). For this purpose, the kinetics and mechanism of the biomass pyrolysis were studied with a particular emphasis on the behavior of the cellulose component. Later, the interest of the cooperation gradually shifted to the solid products of the pyrolysis: chars, charcoals, and biocarbons. Hence, the formation, properties, and uses of these products were studied. The present paper illustrates the 3 decades of the common work by selected results. Such examples are shown that (i) are thought to be useful in the planning of future studies on pyrolysis and combustion of biomass materials (ii) and/or may help in the interpretation of the existing literature data. The presented results include the choice of the proper experimental conditions, the evaluation of experiments with linear and nonlinear temperature programs by the method of least squares, the assessment of complex mechanism schemes by a suitable series of experiments, and the kinetic modeling of the combustion of inhomogeneous chars in the kinetic regime

Diffusion and Transport Coefficients in Synthetic Opals

Opals are structures composed of the closed packing of spheres in the size range of nano-to-micro meter. They are sintered to create small necks at the points of contact. We have solved the diffusion problem in such structures. The relation between the diffusion coefficient and the termal and electrical conductivity makes possible to estimate the transport coefficients of opal structures. We estimate this changes as function of the neck size and the mean-free path of the carriers. The theory presented is also applicable to the diffusion problem in other periodic structures.Comment: Submitted to PR

Critical load and congestion instabilities in scale-free networks

We study the tolerance to congestion failures in communication networks with scale-free topology. The traffic load carried by each damaged element in the network must be partly or totally redistributed among the remaining elements. Overloaded elements might fail on their turn, triggering the occurrence of failure cascades able to isolate large parts of the network. We find a critical traffic load above which the probability of massive traffic congestions destroying the network communication capabilities is finite.Comment: 4 pages, 3 figure