High temperature broadband reflectors

Introduction

The heat radiation contribution to the total heat transfer in gas turbine becomes significant at high temperature. The issue is raised by the transparency of some ceramics used in the thermal barrier coating (TBC) in the infra-red (IR) region. Especially for the widely used TBC material: yittria-stabilized zirconia (YSZ), is fairly transparent in the wavelength region of 1-6mm, which corresponds to the region of maximum blackbody emission for operating temperature of present gas turbine engines [1]. Thus, the combination of high IR radiation and low absorption suggests that the radiation at high temperature may compete with the solid conduction for the heat transport across the coating.

The recent progress in photonic crystals (PhCs) allows for the efficient controls of electromagnetic waves. PhCs are structures where the periodicity in dielectric prevents electromagnetic wave from propagation through the structure due to Bragg reflection. If the PhCs combine with the low thermal conductivity ceramic materials, the resulting high temperature photonic crystals could be attractive and promising for control of total heat transport at temperature > 1000°C. Thus the concept of 3D PhCs for high temperature heat radiation reflector is proposed. The application can also be extended to the spectral filter for thermo-photovoltaic (TPV).

Goals

The high temperature photonic crystal research in the institute for Optical and Electronic Materials is concentrated on designing a broadband IR radiation reflector that can operate in high temperature environment. The goal is to realize such a high temperature photonic structure with following properties:

Broadband reflection over the wavelength region of 1-6 mm.
Based solely on low thermal conductivity and high thermal stability ceramic materials.
Highly reflective for all incidence angles and polarizations.
Ability to resist high temperature during operation without having a significant alteration of its photonic properties.

Methods

Simulation methods:

Finite Element Method (MWS CST) [2]
Plane Wave Expansion method (MPB) [3]

Fabrication methods

Self-assembly of colloidal micro-particles
Atomic layer deposition (ALD)
Sol-gel infiltration
Template removal via calcinations / HF etching
Sol-gel co-assembly

Characterization methods:

Structural characterization with Scanning electron microscope (SEM) & optical microscopy
Transmission and reflectance measurement with UV-Vis and FTIR spectrometers.
Diffuse reflectance measurement with integrating sphere.

Results

The optical transmission and reflection spectra of the direct opal and inverse opal structures have been simulated for different lattice constants. The results show a 10 layers YSZ inverse opal offers a broadband stop gap (Figure 1a) with good extinction (> -16dB) in the IR region. The stopgap’s location and gap width can be tailored according to the spectral distribution of IR radiation with the pore size, filling fraction and refractive index of the infiltrated material. By stacking several inverse opal structures with varying lattice constants, the closely positioned stop gaps are merged in the reflection/transmission spectrum, and consequently the bandwidth is effectively enhanced (Figure 1b). In addition, the design improves the wide-angle feature of the structure (high reflectance for any incidence angle and polarizations) [4].

Figure 1 a) Calculated transmission and reflectance spectra of a YSZ inverse opal with the pore size of 930 nm. Inset shows the schematic illustration of the simulation model.

b) Angle resolved transmittance of a single stack- (upper right) and a multi-stack inverse opal (lower right). The blue region at wavelength 2000 nm represents the stop band of the structure, where the propagation of radiation is prohibited.

Preliminary PhC structures have been fabricated with titanium dioxide (TiO₂) and YSZ. Starting with the self-assembly of polystyrene spheres, followed by atomic layer deposition (ALD) with TiO₂ or YSZ, and finally the removal of the polymer template by calcination. The fabricated structures show distinctive reflectance peak in the spectrum.

Figure 2 a) Hemispherical reflectance of a single-(dashed line) and a double-layer titanium dioxide (TiO₂) PhC structure (solid line) obtained in experiment. The double-layer structure exhibit high reflectance over a broader wavelength range, which evidently

b) The cross-sectional SEM of the double-layer structure is shown. Top layer A and bottom layer B consist of PhC with the pore size of 608 nm and 756 nm, respectively.

List of publications

Lee, H. S. et al. Thermal radiation transmission and reflection properties of ceramic 3D photonic crystals, J. Opt. Soc. Am. B 29, 450, (2012).

Kubrin, R. et al. Stacking of Ceramic Inverse Opals with Different Lattice Constants, J. Am. Ceram. Soc 95, 2226–2235, (2012).

Responsible

Hooi Sing Lee

Collaborations

Prof. Dr. Gerold Schneider

TUHH, Keramische Hochleistungswerkstoffe

Prof. Dr. Kornelius Nielsch

Uni Hamburg, Angewandte Physik

References

1. Shklover, V., Braginsky, L., Witz, G., Mishrikey, M. & Hafner, C. High-Temperature Photonic Structures. Thermal Barrier Coatings, Infrared Sources and Other Applications, J. Comput. Theor. Nanos. 5, 862–893, (2008).

2. Available at www.cst.com ,

3. Avaialbe at ab-initio.mit.edu/wiki/index.php/MIT_Photonic_Bands ,

4. Lee, H. S. et al. Thermal radiation transmission and reflection properties of ceramic 3D photonic crystals, J. Opt. Soc. Am. B 29, 450, (2012).

5. Kubrin, R. et al. Stacking of Ceramic Inverse Opals with Different Lattice Constants, J. Am. Ceram. Soc 95, 2226–2235, (2012).