October 15, 2008

FONTIERS IN MATERIALS SCIENCE
VINSE COLLOQUIUM SERIES 

Dr. David Norris
University of Minnesota
"Tailoring the Glow of a Heated Metal:  Photonic Crystals and Thermophotovoltaics"

Abstract.  Photonic crystals are materials that are patterned on an optical length scale.  When these structures are heated, their “glow” (or thermal emission) can be modified due to optical interference.  This may allow the elimination of unwanted heat from thermal emission sources, such as the tungsten filament in a conventional light bulb.  Furthermore, this effect may lead to efficient thermophotovoltaic devices.  The generation of electricity from sunlight is hindered by the ener­geti­cally broad distribution of solar photons.  Because standard photovoltaic de­vices trans­form these pho­tons into isoenergetic electrons, en­ergy mismatches lead to waste.  While many approaches aim to improv­e this direct photon-to-electron conver­sion, a less-studied alternative is to trans­form sunlight first into low-en­ergy pho­tons.  A mate­rial, heated by the sun, can ther­mally emit infrared light that is matched to a photo­voltaic cell.  In principle, this thermo­photo­vol­taic process can be effi­cient, but has been limited by the emission spectra of real mate­rials.  Because of their ability to tailor thermal emission, metallic photonic crys­tals can pro­vide an interesting new class of thermophotovoltaic emitters.  In this talk, we will discuss the fabrication of me­tal structures and the measurement of their thermal emission, in particular for thermophotovoltaics.  We will demonstrate molybdenum photonic crystals that, when heated to 650°C, should generate over ten times more power than solid emit­ters while having an optical-to-elec­trical con­ver­sion effi­ciency above 32%.  At such relatively low tempera­tures, these emitters have promise not only in solar energy but also in har­nessing geothermal and in­dustrial waste heat.