S15_NME220_Lecture21_Heat_Nanophotonics

S15_NME220_Lecture21_Heat_Nanophotonics - Lecture 21 Heat...

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Lecture 21: Heat cont'd and Nanophotonics Prof. James M. Carothers June 1, 2015 S15 NME 220
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Nanoscale heat transfer Goal: Introduce nanoscale heat transfer phenomena Temperature Boltzmann's constant Conduction Problem Thermal conductivity and mean free path Problem Convection Radiation
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Convection Convection: Collective movement of molecules Movement can occur through diffusion (Brownian motion) Movement can occur through advection (motion of larger currents) Newton's law of cooling describes rate of convective heat transfer: Q convect = h t A ( T 1 - T 2 ) Where, A is the cross-sectional area, and h t is the heat transfer coefficient ( h t has units W/m K and is analogous to k c, except that h t is not a material property and depends on the fluid flow – can be difficult to calculate at the nanoscale) Problem: True or False: Convection can readily occur in solids.
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Convection Convective heat transfer occurs through diffusion and advection in liquids and gases but cannot occur in solids. Convection: Collective movement of molecules Movement can occur through diffusion (Brownian motion) Movement can occur through advection (motion of larger currents) Newton's law of cooling describes rate of convective heat transfer: Q convect = h t A ( T 1 - T 2 ) Where, A is the cross-sectional area, and h t is the heat transfer coefficient ( h t has units W/m K and is analogous to k c, except that h t is not a material property and depends on the fluid flow – can be difficult to calculate at the nanoscale) Problem: True or False: Convection can readily occur in solids. Answer: False, neither diffusion or advection can occur in solids, therefore there is no convective heat transfer.
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Radiation Thermal radiation is transferred by photons and thus does not require a medium (unlike convection and conduction). Thermal Radiation: Recall: Planck's law describes the spectrum of blackbody radiation, which depends only on temperature. Wien's law gives the most likely frequency of the emitted radiation. Radiation emitted
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Radiation Thermal radiation is transferred by photons and thus does not require a medium (unlike convection and conduction). Thermal Radiation: Photons (electromagnetic radiation) are generated by the vibrational motion of particles of matter. Photons can travel through a vacuum When Phonons or electrons oscillate at frequencies similar to passing photons, resonant modes can occur For small (10 μ m) separations at room temp, resonant modes lead to energy transfer through photon tunneling (photon tunneling occurs when the separation is smaller than the wavelength of the dominant photon) Recall: Planck's law describes the spectrum of blackbody radiation, which depends only on temperature. Wien's law gives the most likely frequency of the emitted radiation.
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Nanophotonics Goal: Introduce concepts that dictate the interactions of light with materials at length scales smaller than λ .
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