thermochemical, and solar thermophotovoltaics. There exist a range of solutions with high absorptivity for low and intermediate temperatures. [1][2][3] However, for many applications, high operating temperatures (>700 K) are advantageous to achieve higher system effi ciencies. Conventional absorbers are unsuitable at these high operating temperatures since there are more considerations to be taken into account. [ 4 ] Firstly, the materials and structures need to be thermally stable and to maintain their optical properties at these high temperatures. Refractory metals are most advantageous due to their high melting point and low vapor pressure. Secondly, it is crucial that the absorber exhibits spectrally selective absorptance; namely high absorptivity in the shorter wavelength range to absorb most of the solar spectrum and low absorptivity (i.e., emissivity) in the longer wavelength range to minimize losses due to re-emission. Furthermore, this selectivity, i.e., the spectral range of high and low absorptivity, has to be tailored for the specifi c system operating conditions to achieve maximum system effi ciency.It is therefore advantageous to use PhCs which offer the possibility to tailor the spectral absorptance [ 5,6 ] and thus optimize system effi ciency. Several absorbers based on 1D multilayer stacks, [ 7,8 ] 2.5D structures such as pyramids, [9][10][11] 3D PhCs in refractory metals, [ 12,13 ] as well as metamaterials [14][15][16] have been proposed. Here, we demonstrate the suitability of a 2D PhC comprising a square lattice of cylindrical cavities etched into a Ta substrate as a highly effi cient and selective absorber at high temperatures, i.e., above 1000 K. While all of the above approaches achieve good spectral selectivity, the 2D PhC design is a compact and thermally robust structure, minimizing the number of interfaces as compared to multilayer or 3D PhC approaches which is crucial for high temperature stability. At the same time, fabrication is simple and scalable and can be achieved by standard semiconductor processes. In this 2D PhC design, the absorptivity of the material is selectively enhanced by the introduction of cavity modes and the spectral range of enhancement, i.e., high absorptivity, can be tuned A high-temperature stable solar absorber based on a metallic 2D photonic crystal (PhC) with high and tunable spectral selectivity is demonstrated and optimized for a range of operating temperatures and irradiances. In particular, a PhC absorber with solar absorptance α α = = 0.86 and thermal emittance ε ε = 0.26 at 1000 K, using high-temperature material properties, is achieved resulting in a thermal transfer effi ciency more than 50% higher than that of a blackbody absorber. Furthermore, an integrated double-sided 2D PhC absorber/ emitter pair is demonstrated for a high-performance solar thermophotovoltaic (STPV) system. The 2D PhC absorber/emitter is fabricated on a double-side polished tantalum substrate, characterized, and tested in an experimental STPV setup along with a fl at Ta absorber...

We present the results of extensive characterization of selective emitters at high temperatures, including thermal emission measurements and thermal stability testing at 1000 °C for 1h and 900 °C for up to 144 h. The selective emitters were fabricated as 2D photonic crystals (PhCs) on polycrystalline tantalum (Ta), targeting large-area applications in solid-state heat-to-electricity conversion. We characterized spectral emission as a function of temperature, observing very good selectivity of the emission as compared to flat Ta, with the emission of the PhC approaching the blackbody limit below the target cut-off wavelength of 2 μm, and a steep cut-off to low emission at longer wavelengths. In addition, we study the use of a thin, conformal layer (20 nm) of HfO(2) deposited by atomic layer deposition (ALD) as a surface protective coating, and confirm experimentally that it acts as a diffusion inhibitor and thermal barrier coating, and prevents the formation of Ta carbide on the surface. Furthermore, we tested the thermal stability of the nanostructured emitters and their optical properties before and after annealing, observing no degradation even after 144 h (6 days) at 900 °C, which demonstrates the suitability of these selective emitters for high-temperature applications.

After decades of intense studies focused on cryogenic and room temperature nanophotonics, scientific interest is also growing in high-temperature nanophotonics aimed at solid-state energy conversion. These latest extensive research efforts are spurred by a renewed interest in high temperature thermal-to-electrical energy conversion schemes including thermophotovoltaics (TPV), solar-thermophotovoltaics, solar-thermal, and solar-thermochemical energy conversion systems. This field is profiting tremendously from the outstanding degree of control over the thermal emission properties that can be achieved with nanoscale photonic materials. The key to obtaining high efficiency in this class of high temperature energy conversion is the spectral and angular matching of the radiation properties of an emitter to those of an absorber. Together with the achievements in the field of highperformance narrow bandgap photovoltaic cells, the ability to tailor the radiation properties of thermal emitters and absorbers using nanophotonics facilitates a route to achieving the impressive efficiencies predicted by theoretical studies. In this review, we will discuss the possibilities of emission tailoring by nanophotonics in the light of high temperature thermal-toelectrical energy conversion applications, and give a brief introduction to the field of TPV. We will show how a class of large area 2D metallic photonic crystals can be designed and employed to efficiently control and tailor the spectral and angular emission properties, paving the way towards new and highly efficient thermophotovoltaic systems and enabling other energy conversion schemes based on high-performance high-temperature nanoscale photonic materials.

The challenging problem of ultra-high-energy-density, high-efficiency, and small-scale portable power generation is addressed here using a distinctive thermophotovoltaic energy conversion mechanism and chip-based system design, which we name the microthermophotovoltaic (μTPV) generator. The approach is predicted to be capable of up to 32% efficient heat-to-electricity conversion within a millimeter-scale form factor. Although considerable technological barriers need to be overcome to reach full performance, we have performed a robust experimental demonstration that validates the theoretical framework and the key system components. Even with a much-simplified μTPV system design with theoretical efficiency prediction of 2.7%, we experimentally demonstrate 2.5% efficiency. The μTPV experimental system that was built and tested comprises a silicon propane microcombustor, an integrated high-temperature photonic crystal selective thermal emitter, four 0.55-eV GaInAsSb thermophotovoltaic diodes, and an ultrahigh-efficiency maximum power-point tracking power electronics converter. The system was demonstrated to operate up to 800°C (silicon microcombustor temperature) with an input thermal power of 13.7 W, generating 344 mW of electric power over a 1-cm 2 area.catalytic combustion | micro generator | thermal radiation W ith the recent proliferation of power-hungry mobile devices, significant research efforts have been focused on developing clean, quiet, and portable high-energy-density, compact power sources. Although batteries offer a well-known solution, limits on the chemistry developed to date constrain the energy density to ∼0.2 kWh/kg, whereas many hydrocarbon fuels have energy densities closer to 12 kWh/kg. The fundamental question is, How efficiently and robustly can these widely available chemical fuels be converted into electricity in a millimeter-scale system? Indeed, it is difficult to tap the full potential of hydrocarbon fuels on a small scale. However, their high energy density allows even relatively inefficient generators to be competitive with batteries. To this end, researchers have explored different energy conversion routes, such as mechanical heat engines (1), fuel cells (2, 3), thermoelectrics (4, 5), and thermophotovoltaics (TPVs) (6, 7).TPVs present an extremely appealing approach for small-scale power sources due to the combination of high power density limited ultimately by Planck blackbody emission, multifuel operation due to the ease of generating heat, and a fully static conversion process. Small-scale TPVs have yet to be demonstrated and are particularly challenging because of the need to develop strong synergistic interactions between chemical, thermal, optical, and electrical domains, which in turn give rise to requirements for extreme materials performance and subsystems synchronization.In this work, we present a proof of concept microthermovoltaic (μTPV) system, shown in Fig. 1, that validates the theoretical foundation and paves the way toward a new breed of ultra-highenergy-density, hi...

Articles you may be interested inFabrication of transferrable, fully suspended silicon photonic crystal nanomembranes exhibiting vivid structural color and high-Q guided resonanceThe authors present highly selective emitters based on two-dimensional tantalum (Ta) photonic crystals, fabricated on 2 in. polycrystalline Ta substrates, for high-temperature applications, e.g., thermophotovoltaic energy conversion. In this study, a fabrication route facilitating large-area photonic crystal fabrication with high fabrication uniformity and accuracy, based on interference lithography and reactive ion etching is discussed. A deep reactive ion etch process for Ta was developed using an SF 6 =C 4 F 8 based Bosch process, which enabled us to achieve $8:5 lm deep cavities with an aspect ratio of $8, with very steep and smooth sidewalls. The thermal emitters fabricated by this method show excellent spectral selectivity, enhancement of the emissivity below cut-off approaching unity, and a sharp cut-off between the high emissivity region and the low emissivity region, while maintaining the low intrinsic emissivity of bare Ta above the cut-off wavelength. The experimental results show excellent agreement with numerical simulations.

The use of molecular layers to modify the surface and interfaces of solid-state materials while retaining their bulk properties offers great potential. Despite the widespread interest, little work has been undertaken to characterize the growth and surface chemistry of the short-chain alkoxy silane molecular layers. Variable angle spectroscopic ellipsometry, contact angle goniometry, and X-ray photoelectron spectroscopy are used to undertake the work in the present study. Results indicate that 3-mercaptopropyltrimethoxysilane and 2-(trimethoxysilylethyl)pyridine are both unique among the short-chain alkoxy silanes and grow multilayer films in a toluene solution on hydroxylated SiO2 surfaces. In particular, the mercaptan molecular layers show evidence of a changing surface chemistry as a function of growth time. Further, added surface moisture on mercaptan molecular layers yields thicker films of a higher density with more reduced surface sulfur when subsequent growth is resumed as compared to a control sample. Further, the pyridine molecular layers possess negative optical birefringence much like the parylene polymers, polyimides, and phthalocyanine Langmuir-Blodgett films undertaken by previous researchers. In previous cases, the presence of a phenyl group with a large anisotropic molecular polarizability caused the large in-plane polarizability. Further, the pyridine molecular layers exhibited a high index of refraction of 1.567 ( 0.005 explaining its superior properties as a metallic diffusion barrier at dielectric/metal interfaces from previous research.

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The residual stress in thin films is a major limiting factor for obtaining high quality films. We present a strategy for stress reduction in sputter deposited films by using a nanostructured compliant layer obtained by the oblique angle deposition technique, sandwiched between the film and the substrate. The technique is all in situ, does not require any lithography steps, and the nanostructured layer is made from the same material as the deposited thin film. By using this approach we were able to reduce stress values by approximately one order of magnitude in sputter deposited tungsten films. These lower stress thin films also exhibit stronger adhesion to the substrate, which retards delamination buckling. This technique allows the growth of much thicker films and has enhanced structural stability. A model is developed to explain the stress relief mechanism and the stronger adhesion associated with the presence of the nanostructured compliant layer.