Large Thermal Diffusivity Database Reveals a New Mechanism for Heat Transport in Geomaterials: Diffusion of IR-Polaritons Substantially Augments Phonon-Phonon Scattering
Abstract
Over the course of several years, we have measured heat transport to high temperatures for a large number (ca. 200) of minerals, rocks, glasses and melts using laser flash analysis which eliminates systematic errors (contact losses and boundary-to-boundary radiative transfer gains) that limit utility of conventional, contact techniques. The database is large enough to elucidate patterns. For most samples and particularly for our >60 non-metallic, large single-crystals, >30 glasses and >12 polycrystals, we show that thermal diffusivity is consistently represented by D(T) =F/T ^G + HT, permitting confident extrapolation from conditions in the laboratory to those in the mantle. The two distinct temperature terms describing D(T) suggest that two microscopic mechanisms of conduction exist in the electrical insulators explored. We propose that phonon scattering (the F/T^G term) sums with radiative diffusion of infrared (IR) light in the form of polaritons (the HT term). Speeds near that of sound over unit cell scale lengths exist for the polariton mechanism due to phonon-photon coupling, thereby distinguishing this proposed mechanism from high frequency diffusive radiative transfer which travels near the speed of light, and only is important following transient heating. For 63 single-crystals and many glasses unaffected by disordering or reconstructive phase transitions, G ranges from 0.3 to 2, depending on structure, and H is ~0.0001/ K, and so HT crosses F/T^G by ~1300 K (for most oxides), meaning that radiative diffusion of IR light is more important than phonon scattering inside the Earth. Importantly, the increase in heat transport due to elevated temperature is augmented by the increase due to high P inside planets, providing stability against convection. The popular view of a vigorously convecting interior needs revisiting, given known feedback in the temperature equation and the large size of the HT term. To understand the microscopic basis of HT term, we re-derive formula for the effective radiative thermal conductivity for an internally heated medium. Previous derivations violated the 2nd law of thermodynamics and in doing so incorrectly incorporated the index of refraction squared into formulae currently used. Hence, our findings not only affect geosciences but also astronomy, planetary, and materials sciences.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V13A2581B
- Keywords:
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- 3999 MINERAL PHYSICS General or miscellaneous;
- 3621 MINERALOGY AND PETROLOGY Mantle processes;
- 5460 PLANETARY SCIENCES: SOLID SURFACE PLANETS Physical properties of materials;
- 8130 TECTONOPHYSICS Heat generation and transport