NMP 2008
 |
Prashant Nagpal |
ADVISOR:
David Norris |
Efficient Low Temperature Thermophotovoltaic Emitter
Prashant Nagpal,1 Sang Eon Han,1 Andreas Stein2 and David
J. Norris1
1Department of Chemical Engineering & Material Science,
2Department of Chemistry
University of Minnesota, Minneapolis, MN 55455
New
materials are sought to increase the efficiency of thermal-to-electrical
energy conversion. Metallic photonic
crystals - metal structures that are
three-dimensionally (3D) patterned on an optical length scale - provide one
possible solution. Due to their periodicity, the thermal emission spectrum
of these structures can be tailored to emit preferentially in a narrow band
of frequencies. The modified spectrum could then be efficiently converted
to electricity by a conventional semiconductor device. However, one challenge
is to fabricate three-dimensional photonic crystals from refractory metals
such as tungsten, which are necessary to achieve high temperatures. This
has limited the exploration of this approach. Here we utilize direct laser
writing (DLW) to obtain a specific type of photonic crystal known as the “woodpile
structure.” In general, DLW uses the beam of an infrared laser that
is tightly focused within a thin film of photoresist. Because multiphoton
polymerization occurs only within a small focal volume, any contiguous 3D
structure can be precisely defined by moving the focus. Consequently, DLW
is an extremely flexible and relatively quick method to obtain photonic crystals.
We create woodpile photonic crystals by combining DLW with a thermally stable,
methylsilsesquioxane-based photoresist previously developed in our laboratory.
This structure is then used as a template and coated with varying shell thicknesses
of refractory metals such as tungsten, molybdenum and tantalum using chemical
vapor deposition. According to theory the resulting structures should exhibit
modified thermal emission. To test this, we characterized their optical properties.
Large absorption for light with wavelengths between 1.5 and 1.9 micrometers
was observed. Therefore, Kirchhof’s law predicts that thermal emission
would occur that is well-matched to an InxGa1-xAs-based photovoltaic. Thermal
emission measurements at 893K will be presented, to assess the efficiency
of photonic emitter. More generally, the flexibility of this approach means
that a large variety of metallic photonic crystals can be fabricated and
studied for thermophotovoltaic applications.
Read Prashant Nagpal's current CV.