Radiation Transport

Energy transport via electromagnetic waves is an exciting and challenging research area with a broad range of applications. One area of interest is thermophotovoltaic (TPV) energy conversion. The principle of TPV conversion is similar to solar energy conversion using photovoltaic (PV) cells. The PV layer is common to both technology (of course, the types of PV cells differ). The source in the case of TPV conversion is a terrestrial thermal emitter at 1500 - 2000 K. Compared to the sun, most of the energy from such terrestrial emitters would be more concentrated in lower energy (or longer wavelength) regions. The PV cell converts the portion of radiation with energy higher than its band gap into useful electrical power. Improving the efficiency as well as power density of TPV converters will make this technology more competitive.

We are carrying out studies involving far-field spectral control and near-field enhancement of thermal radiation transfer, and thermal transport by surface phonon-polaritons. We have also studied electromagnetic metamaterials and photonic crystals.

Selective Radiation Emitters

The performance of TPV energy conversion systems is greatly affected by the radiation characteristics of the thermal emitter. Only photons with energy larger than the band gap of the photovoltaic material can be converted to electrical power whereas those below the band gap add to the energy loss. Ideally, one would want a selective emitter with high emissivity above the band gap and low emissivity below the band gap. Various approaches have been proposed to fabricate effective selective emitters with 2D or 3D photonic crystals (PCs), which involve considerable intricate microfabrication.1,2 Instead we have proposed a simpler method to fabricate 1D structure which exhibits many of the features of its 2D and 3D counterparts.3

Figure 1: Click to enlarge

The key has been to use ultra thin metallic films arranged as a 1D photonic crystal or an alternating periodic multilayer stack with a suitable dielectric material. Figure 1 shows the numerical computation of total hemispherical emissivity of two such structures as a function of wavelength. The structures have 10 unit cells, each unit cell consisting of 10 nm film of tungsten and 60 nm of alumina. For comparison, the emissivity of bare tungsten and the 3D PC are provided.

Figure 2: Click to enlarge

In addition to improving the selective emission of thermal radiators, enhancement in energy transfer can increase the power density of thermal- to-electric energy conversion devices. Though near-field enhancement of radiative energy transfer has been investigated for a long time, certain aspects are only being investigated now. Electromagnetic surface waves, like surface phonon polaritons or surface plasmon polaritons can increase the energy transfer by two or three orders of magnitude compared to the near-field enhancement between materials that do not support such surface waves (Fig. 2). Our work has shown that such enhancements in thermal radiative transfer can increase not only the power density of TPV devices4 but also contribute to the improvement of thermoelectric devices.5

Surface Phonon-Polaritons

Surface polaritons have recently received much renewed interest due to applications in fields such as nano-optics, Surface-Enhanced Raman Spectroscopy (SERS), extraordinary sub-wavelength transmission, and lasers. Of particular interest to our group is the application of surface polaritons to increasing energy transfer at the nanoscale.

Figure 3: Click to enlarge

Polaritons are a hybrid interaction of the transverse electromagnetic field and a resonant oscillation of a material. A surface polariton (Figure 3) is a mode which is trapped on the surface of an appropriately active material. The most commonly encountered surface polaritons are surface plasmons and surface phonon-polaritons, where the material resonances are the electron plasma oscillation or phonon vibrations, respectively.

Figure 4: Click to enlarge

For a thin film, there will be surface polaritons on both the top and bottom surfaces. When the film is thin enough to allow these two modes to interact, they split into an anti-symmetric mode and a symmetric mode. We model the in-plane energy transfer due to these surface polaritons as a diffusive process in an absorbing medium. Due to the well-known very long propagation length of the anti-symmetric mode, we find that the surface polaritons can make a significant contribution to the effective thermal conductivity along thin films. In particular, for a 40 nm thick film of amorphous silicon dioxide, we calculate a total thermal conductivity of 4 W m-1 K-1 at 500 K, which is an increase of ~100% over the intrinsic phonon thermal conductivity.

Photonic Structures and Nanogaps

Figure 5: Click to enlarge

We have proposed to increase power density of TPV converters by using structured devices such as the one shown on the right. The interpenetrating fingers act like radiative fins. Due to the extended surface area of the structure, an increase in radiative energy transfer can be realized. We are also exploring photonic bandgap effects in such structures.

In addition, we are investigating the possibility of using nanometer-scale gaps to couple surface waves through tunneling. The energy flux carried by the surface waves are near monochromatic and under proper conditions, can exceed that of free space blackbody radiation. This phenomenon can be exploited for developing high power density and high efficiency thermophotovoltaic devices.

References

  1. S. Y. Lin, J. Moreno, and J. G. Fleming. (2003). "Three-dimensional photonic crystal for thermal photovoltaic power generation." Appl. Phys. Lett. 83, 380-382.
  2. A. Heinzel, V. Boerner, A. Gombert, B. Blasi, V. Wittwer, and J. Luther. "Radiation filters and emitters for the NIR based on periodically structures metal surfaces." J. of Mod. Opt. 47, 2399 (2000).
  3. A. Narayanaswamy and G. Chen, "Thermal emission control with one-dimensional metallodielectric photonic crystals," Phys. Rev. B 70, 125101 (2004)
  4. A. Narayanaswamy and G. Chen, "Surface modes for near-field thermophotovoltaics," Appl. Phys. Lett. 82, 3544-3546 (2003).
  5. R. Yang, A. Narayanaswamy, and G. Chen, submitted for publication (2005).
  6. Chen, D.-Z. A., Narayanaswamy, A., and Chen, G., "Surface phonon-polariton mediated thermal conductivity enhancement of amorphous thin films", Physical Review B, Volume 72, Issue 15, 155435, October 2005.
  7. Chen, D.-Z. A., Narayanaswamy, A., and Chen, G., "Enhancement of In-Plane Thermal Conductivity of Thin Films via Surface Phonon-Polaritons", Proceedings of the ASME International Mechanical Engineering Congress and Exposition, IMECE2005-83051, Orlando, FL, November 5-11, 2005.