Showing papers by "David Broido published in 2014"

Phonon thermal transport in strained and unstrained graphene from first principles

Abstract: A rigorous first principles Boltzmann-Peierls equation (BPE) for phonon transport approach is employed to examine the lattice thermal conductivity, ${\ensuremath{\kappa}}_{L}$, of strained and unstrained graphene. First principles calculations show that the out-of-plane, flexural acoustic phonons provide the dominant contribution to ${\ensuremath{\kappa}}_{L}$ of graphene for all strains, temperatures, and system sizes considered, supporting a previous prediction that used an optimized Tersoff empirical interatomic potential. For the range of finite system sizes considered, we show that the ${\ensuremath{\kappa}}_{L}$ of graphene is relatively insensitive to strain. This provides validation for use of the BPE approach to calculate ${\ensuremath{\kappa}}_{L}$ for unstrained graphene, which has recently been called into question. The temperature and system size dependence of the calculated ${\ensuremath{\kappa}}_{L}$ of graphene is in good agreement with experimental data. The enhancement of ${\ensuremath{\kappa}}_{L}$ with isotopic purification is found to be relatively small due to strong anharmonic phonon-phonon scattering. This work provides insight into the nature of phonon thermal transport in graphene, and it demonstrates the power of first principles thermal transport techniques.

Phonon thermal transport in Bi 2 Te 3 from first principles

Abstract: We present first-principles calculations of the thermal and thermal transport properties of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ that combine an ab initio molecular dynamics (AIMD) approach to calculate interatomic force constants (IFCs) along with a full iterative solution of the Peierls-Boltzmann transport equation for phonons. The newly developed AIMD approach allows determination of harmonic and anharmonic interatomic forces at each temperature, which is particularly appropriate for highly anharmonic materials such as ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. The calculated phonon dispersions, heat capacity, and thermal expansion coefficient are found to be in good agreement with measured data. The lattice thermal conductivity, ${\ensuremath{\kappa}}_{l}$, calculated using the AIMD approach nicely matches measured values, showing better agreement than the ${\ensuremath{\kappa}}_{l}$ obtained using temperature-independent IFCs. A significant contribution to ${\ensuremath{\kappa}}_{l}$ from optic phonon modes is found. Already at room temperature, the phonon line shapes show a notable broadening and onset of satellite peaks reflecting the underlying strong anharmonicity.

The Seebeck coefficient and phonon drag in silicon

Abstract: We present a theory of the phonon-drag Seebeck coefficient in nondegenerate semiconductors, and apply it to silicon for temperatures 30 < T < 300 K. Our calculation uses only parameters from the literature, and previous calculations of the phonon lifetime. We find excellent agreement with the measurements of Geballe and Hull [Phys. Rev. 98, 940 (1955)]. The phonon-drag term dominates at low temperature, and shows an important dependence on the dimensions of the experimental sample.

Ab Initio Thermal Transport

Abstract: Ab initio (or first principles) approaches are able to predict materials properties without the use of any adjustable parameters. This chapter presents some of our recently developed techniques for the ab initio evaluation of the lattice thermal conductivity of crystalline bulk materials and alloys, and nanoscale materials including embedded nanoparticle composites.

First principles determination of ultra-high thermal conductivity fo Boron Arsenide: A competitor for diamond?

Abstract: We have calculated the thermal conductivities (κ) of cubic III-V boron compounds using a predictive first principles approach. Boron arsenide is found to have a remarkable room temperature κ over 2000 W m(-1) K(-1); this is comparable to those in diamond and graphite, which are the highest bulk values known. We trace this behavior in boron arsenide to an interplay of certain basic vibrational properties that lie outside of the conventional guidelines in searching for high κ materials, and to relatively weak phonon-isotope scattering. We also find that cubic boron nitride and boron antimonide will have high κ with isotopic purification. This work provides new insight into the nature of thermal transport at a quantitative level and predicts a new ultrahigh κ material of potential interest for passive cooling applications.

High thermal conductivity materials for thermal management applications

Abstract: High thermal conductivity materials and methods of their use for thermal management applications are provided. In some embodiments, a device comprises a heat generating unit (304) and a thermally conductive unit (306, 308, 310) in thermal communication with the heat generating unit (304) for conducting heat generated by the heat generating unit (304) away from the heat generating unit (304), the thermally conductive unit (306, 308, 310) comprising a thermally conductive compound, alloy or composite thereof. The thermally conductive compound may include Boron Arsenide, Boron Antimonide, Germanium Carbide and Beryllium Selenide.