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Proc Natl Acad Sci U S A ; 109(11): 4070-3, 2012 Mar 13.
Artigo em Inglês | MEDLINE | ID: mdl-22375035


Earth's magnetic field is sustained by magnetohydrodynamic convection within the metallic liquid core. In a thermally advecting core, the fraction of heat available to drive the geodynamo is reduced by heat conducted along the core geotherm, which depends sensitively on the thermal conductivity of liquid iron and its alloys with candidate light elements. The thermal conductivity for Earth's core is very poorly constrained, with current estimates based on a set of scaling relations that were not previously tested at high pressures. We perform first-principles electronic structure computations to determine the thermal conductivity and electrical resistivity for Fe, Fe-Si, and Fe-O liquid alloys. Computed resistivity agrees very well with existing shock compression measurements and shows strong dependence on light element concentration and type. Thermal conductivity at pressure and temperature conditions characteristic of Earth's core is higher than previous extrapolations. Conductive heat flux near the core-mantle boundary is comparable to estimates of the total heat flux from the core but decreases with depth, so that thermally driven flow would be constrained to greater depths in the absence of an inner core.

J Phys Condens Matter ; 24(5): 055401, 2012 Feb 08.
Artigo em Inglês | MEDLINE | ID: mdl-22251624


Due to its large pressure range of stability and inert nature, cubic boron nitride has been proposed as a potential pressure standard for high pressure experiments. It is extremely refractive upon compression, although its melting temperature is not known beyond 10 GPa. We apply first-principles molecular dynamics to evaluate the thermodynamics of zincblende structured (cubic) and liquid boron nitride at extreme temperatures and pressures, and compute the melting curve up to 1 TPa by integration of the Clapeyron equation. The resulting equations of state reveal that liquid boron nitride becomes denser than the solid phase at pressures of around 0.5 TPa. This is expressed as a turnover in the melting curve, which reaches a maximum at 510 GPa and 6550 ± 700 K. The origin of this density crossover is explained in terms of the underlying liquid structure, which diverges from that of the zincblende structured solid as the phases are compressed.

Proc Natl Acad Sci U S A ; 108(44): 17901-4, 2011 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-22021444


The amount of heat flowing from Earth's core critically determines the thermo-chemical evolution of both the core and the lower mantle. Consisting primarily of a polycrystalline aggregate of silicate perovskite and ferropericlase, the thermal boundary layer at the very base of Earth's lower mantle regulates the heat flow from the core, so that the thermal conductivity (k) of these mineral phases controls the amount of heat entering the lowermost mantle. Here we report measurements of the lattice thermal conductivity of pure, Al-, and Fe-bearing MgSiO(3) perovskite at 26 GPa up to 1,073 K, and of ferropericlase containing 0, 5, and 20% Fe, at 8 and 14 GPa up to 1,273 K. We find the incorporation of these elements in silicate perovskite and ferropericlase to result in a ∼50% decrease of lattice thermal conductivity relative to the end member compositions. A model of thermal conductivity constrained from our results indicates that a peridotitic mantle would have k = 9.1 ± 1.2 W/m K at the top of the thermal boundary layer and k = 8.4 ± 1.2 W/m K at its base. These values translate into a heat flux of 11.0 ± 1.4 terawatts (TW) from Earth's core, a range of values consistent with a variety of geophysical estimates.

Phys Rev Lett ; 103(12): 125902, 2009 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-19792447


A method is presented by which the lattice thermal conductivity can be computed from first principles using relatively small system sizes and simulation times. The method uses the relation for thermal conductivity of a kinetic gas, with phonon lifetimes and frequencies determined by combining equilibrium first principles molecular dynamics and first principles lattice dynamics. To illustrate the method, the lattice conductivity is computed for MgO periclase. For individual wave vectors and vibrational modes, phonon lifetimes in periclase are found to be inversely proportional to temperature, with optic modes shorter lived than acoustic modes, contributing only approximately 5% to the lattice conductivity. Computed thermal conductivity values show excellent agreement with experimental measurements, and suggest that the radiative contribution to thermal transport in periclase starts playing a role above approximately 1500 K.