Rare-earth garnet selective emitter

Abstract

Thin film Ho-YAG and EroyAG emitters with a platinum substrate exhibit high spectral emitlance in the emission band ( _.'_0.75,4115/2 "4113/2, for Er -YAG and E_. _ 0.65, 517 -518 for Ho-YAG) at 1500K. In addition, low out-of-band spectral emittance, ¢_. < 0.2, suggest these materials would be excellent candidates for high efficiency selective emitters in thermophotovoltaic (TPV) systems operating at moderate temperatures (1200-1500K).Spectral emittance measurements of the thin films were made (1.2<_.<3.0p.m) and compared to the theoretical emittances calculated using measured values of the spectral extinction coefficient.In this paper we present the results for a new class of rare earth ion selective emitters. These emitters are thin sections ( < I mm) of yttrium aluminum garnet (YAG) single crystal with a rare earth substitutional impurity. Selective emitters in the near IR are of special interest for thermophotovoltaic (TPV) energy conversion 1,2. The most promising solid selective emitters for use in a TPV system are rare earth oxides. Early spectral emittance work 3 on rare earth oxides showed strong emission bands in the infrared (.9 -3 microns). However, the emittance outside the emission band was also significant and the efficiency of these emitters was low. Recent improvements in efficiency 4,5 have been made with emitters fabricated from fine (5-10_m) rare earth oxide fibers similar to the Welsbach mantle used in gas lanterns. However, the rare earth garnet emitters are more rugged than the mantle type emitters.A thin film selective emitter 6 on a low emissivity substrate such as gold, platinum etc., is rugged and easily adapted to a wide variety of thermal sources. The garnet structure and its many subgroups have been successfully used as hosts for rare earth ions, introduced as substitutional impurities, in the development of solid state laser crystals 7. Doping, dependent on the particular ion and crystal structure, may be as high as 100 at. % (complete substitution of yttrium ion with the rare earth ion). These materials have high melting points, 1940 C for YAG (Yttrium Aluminum Gamet), and low emissivity in the near infrared making them excellent candidates for a thin film selective emitter.As previously stated, the spectral emittance of a rare earth emitter is characterized by one or more well defined emission bands. Outside the emission band the emittance(absorptance) is much lower. Therefore, it is expected that emission outside the band for a thin film selective emitter will be dominated by the emitter substrate. For an efficient emitter (power in the emission band/total emitted power) the substrate must have low emittance, _s" Within the emission band the spectral emittance is governed by the index of refraction, nf, and the spectral extinction coefficient, _. (sum of the absorption coefficient, aX, and scattering coefficient, c_X),the emitter temperature, T E, and thickness, d. In reference 6 the emitter emittance and efficiency are derived as a function of the optical depth, K_,= _d...

Thermal emission properties of Al2O3/Er3Al5O12 eutectic ceramics

“…[1][2][3][4] TPV systems have no moving parts, high power density and a wide-ranging heat source. A specific feature of interest with the rare-earth oxides, for example, Er 2 O 3 , Yb 2 O 3 and Ho 2 O 3 is the strong band emission, which ranges from the visible to the nearinfrared wavelength region.…”

Heterogeneous metasurface for high temperature selective emission

“…(Other system considerations have reduced demonstrated efficiencies of combustion-TPV systems to $2.5%. 3 ) Thus, a significant amount of work over the past 30 years has focused on improving the optical-toelectrical conversion efficiency by recycling out of band photons, 4 using multiple bandgap cells, 5 modifying the emissivity of an object away from the typical blackbody, [6][7][8] or a combination of these techniques. 9 Numerous geometries for modifying the emission spectrum have been studied, including metal (such as tungsten) photonic crystals, 7,10 inverse opals, 11 and metal-dielectricmetal (MDM) metasurfaces.…”

Radiative performance of rare earth garnet thin film selective emitters

“…Sincethe Ho dopingconcentration, 1.5%,andhencethe emission at 2.0 _tm,is significantly lessthenthe Er dopingconcentration, 40%, efficiency resultsareexpected to besimilar to thatof the 40%Er YAG emitter in ref. 5. • , Several methods were used to calculate values for emitter efficiency using eq.1.…”