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Item type:Publication, Access status: with files , 2D photonic crystal emitter at 1,400°C for thermophotovoltaic energy harvesting(Elsevier, 2025-10-15); ; ; ; Spectrally selective emitters that endure extreme temperatures (exceeding 1,000°C) are vital for thermophotovoltaic energy harvesting. Here, we report a 2D photonic crystal emitter composed of yttria-stabilized zirconia particles on a tungsten mirror, fabricated through a simple and scalable self-assembly process. The emitter demonstrates spectral stability for 2 h and structural stability for 14 h at 1,400°C under high vacuum, with degradation attributed to zirconium nitridation, which is avoided under Ar or forming gas atmospheres. Long-term durability is demonstrated over 6 months and 200 thermal cycles at 1,050°C, effectively mitigating tungsten oxidation. The emitter achieves 52% spectral efficiency for a 0.72 eV photovoltaic band gap. This work offers perspectives on designing and implementing spectrally selective emitters that remain stable at high temperatures and resilient in harsh environments, representing a significant step forward in developing robust thermophotovoltaic energy-harvesting systems.Publicationtype: Journal Article21 22 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Iridium-based selective emitters for thermophotovoltaic applications(Elsevier, 2023-10-12); ; ; ; Raudsepp, RagleThe long-term operation of refractory-metal-based metamaterials is crucial for applications such as thermophotovoltaics. The metamaterials based on refractory metals like W, Mo, Ta, Nb, and Re fail primarily by oxidation. Here, the use of the noble metal Ir is proposed, which is stable to oxidation and has optical properties comparable to gold. The thermal endurance of Ir in a 3-layer-system, consisting of HfO2/Ir/HfO2, by performing annealing experiments up to 1240 °C in a pressure range from 2 × 10−6 mbar to 1 bar, is demonstrated. The Ir layer shows no oxidation in a vacuum and inert gas atmosphere. At temperatures above 1100 °C, the Ir layer starts to agglomerate due to the degradation of the confining HfO2 layers. An in situ X-ray diffraction experimental comparison between 1D multilayered Ir/HfO2 and W/HfO2 selective emitters annealed at 1000 °C, 2 × 10−6 mbar, over 100 h, confirms oxidation stability of Ir while W multilayers gradually disappear. The results of this work show that W-based metamaterials are not long-term stable even at 1000 °C. However, the oxidation resistance of Ir can be leveraged for refractory plasmonic metamaterials, such as selective emitters in thermophotovoltaic systems with strong suppression of long wavelength radiation.Publicationtype: Journal ArticleScopus© Citations 6 29 89 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Thermal stability of magnetron sputtered W/HfO₂ and Ir/HfO₂ thin films used as selective emitter in thermophotovoltaic applicationsHochtemperaturstabilität und Langlebigkeit eines selektiven Emitters sind die beiden entscheidenden Parameter, um hohe Strahlungsleistung und einen höheren Wirkungsgrad beim Einsatz in einem thermophotovoltaischen System (TPV) zu erzielen. Bei hohen Temperaturen sind thermisch-aktivierte Prozesse wie Diffusion, Kornwachstum, Phasentransformation, Erholung, chemische Reaktionen insbesondere Oxidation sowie Sublimation von Metalloxiden wirksam und somit stellt die thermische Stabilität eines Dünnschichtsystems eine besondere Herausforderung dar. In dieser Arbeit wurden die thermische Stabiliät und die Versagensmechanismen einer 1D-Metamaterial-Vielfachschicht als selektiver Emitter untersucht und analysiert. Zwei Dünnschichtsysteme: W/HfO2 und Ir/HfO2 standen aufgrund ihrer besonderen Materialeigenschaften (hoher Schmelzpunkt und niedriger Vakuumdampfdruck) im Fokus. Sie wurden mittels Magnetron sputtering hergestellt und durch Röntgenbeugung (XRD) untersucht. Die in-situ Hochtemperaturuntersuchungen (bis zu 2000 ⁰C) unter Verwendung von Heizbändern und die Abschätzung der Probentemperatur waren herausfordernd, dadurch wurde ein spezielles Wissen für die Hochtemperaturvalidierung unterschiedlicher Materialien aufgebaut. Der erste Teil der Arbeit zeigt die Heizexperimente an selektiven W/HfO2-Emitterschichten in verschiedenen Temperatur- (1000 ⁰C bis 1520 ⁰C) und Vakuumdruckbereichen (2 × 10-2 mbar bis 2 × 10-6 mbar). Eine selektive W/HfO2-Emitterschicht ist bis 6 h bei 1400 ⁰C und 2 × 10-5 mbar stabil, welches bisher einer der weltweit höchsten erreichten Werte ist. Darüber hinaus erfolgte eine detaillierte in-situ Röntgenanalyse am Schichtsystem HfO2/W/HfO2 zwischen 300 ⁰C und 1520 ⁰C im Hinblick auf die Mechanismen, die zu strukturellen und mikrostrukturellen Veränderungen wie Relaxation, Phasenumwandlung, Kornwachstum und schließlich zum Versagen (Degradation) führen. Der Hauptgrund des Versagens in einer selektiven W/HfO2-Emitterschicht ist auf die Oxidation des Wolframs gefolgt durch eine Sublimation des Suchtigen Wolframoxids zurückzuführen. Für diesen Prozess wurde eine Aktivierungsenergie von 1.2 eV abgeschätzt. Im Falle des HfO2 wurde eine polymorphe Phasentransformation beobachtet. Die strukturelle Umwandlung der Phasen verursacht einen Anstieg des Volumens der Einheitszelle, der wiederum zur Bildung von Leerstellen, Poren sowie der Entstehung von Transportkan alen entlang der Korngrenzen in der keramischen Schicht führt. Im zweiten Teil wird ein systematischer Ansatz verfolgt, um die thermische Stabilität des Metalls Iridium gegen Oxidation und die Lebensdauer der selektiven Ir/HfO2-Emitterschicht zu testen. Zunächst wurden in-situ XRD-Untersuchungen am Schichtsystem: HfO2/Ir/HfO2 zwischen 800 ⁰C - 1100 ⁰C bei 2 × 10-6 mbar durchgeführt. Iridium zeigte eine sehr gute thermische Stabilität bis zu 1000 ⁰C unterstützt durch HfO2 als hervorragende Deckschicht. Bei Temperaturen über 1000 ⁰C begann die Iridium-schicht zu agglomerieren, was bedeutet, dass die treibende Kraft eine Minimierung der Oberflächenenergie ist. Die thermische Stabilität wurde auch durch exsitu XRD-Untersuchungen bei unterschiedlichen Drücken von 1 bar bis zu 2 × 10-5 mbar im Schichtsystem HfO2/Ir/HfO2 verifiziert, welche bis zu 100 h bei 1000 ⁰C und 2 × 10-2 mbar stabil ist. Satellitenreflexe wurden nahe am primären (111) Ir-Peak beobachtet, die als wichtiger Hinweis für die thermische Stabilität dienen können. Ein finaler Vergleich erfolgte zwischen verschiedenen Metamaterialschichten bestehend aus W/HfO2 und Ir/HfO2 bei 1000 ⁰C und 2 × 10-2 mbar. Über 100 h weist Iridium eine exzellente thermische Stabilität im Vergleich zu Wolfram auf. Die in dieser Arbeit durchgeführten Hochtemperaturexperimente und - analysen bieten eine neue Perspektive auf die thermische Stabilität, die in Dünnschichtsystemen praktisch erreicht werden kann und ermöglichen die Entwicklung selektiver Emitter für TPV-Systeme der nächsten Generation.Publicationtype: Doctoral Thesis115 182 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Which factor determines the optical losses in refractory tungsten thin films at high temperatures?(Elsevier, 2022-03-04); ; ; ; Refractory tungsten (W) plays an important role in high temperature photonic/plasmonic applications. Previously room temperature bulk single/poly-crystalline optical constants were extensively used to calculate the optical properties of W nanostructures at high temperatures. This might lead to a significant deviation between the predicted and measured optical properties due to the exclusion of electron–phonon interaction, as well as grain-boundary and surface-scattering. Herein, we show experimentally, how film thickness and temperature affects the optical losses in W. A drastic increase in the effective electron collision frequency is observed with decreasing the film thickness down to 5 nm, due to the grain-boundary and surface-scattering mechanisms. At sufficiently high temperatures (greater than 200 °C for W), the electron–phonon interaction eventually becomes the dominant mechanism, linearly increasing collision frequency with temperature, and it is independent of the geometry of the thin film structure. The impact of thickness and temperature-dependent optical properties of W is showcased with a hyperbolic 1D metamaterial structure acting as a thermophotovoltaic emitter. This work opens new directions in accurate prediction of the optical properties of nanostructures and design of efficient devices in various applications, such as aerospace, energy-efficient lighting, radiative cooling and energy harvesting, by incorporating thickness and temperature-dependent optical constants.Publicationtype: Journal ArticleScopus© Citations 6 86 441 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Unprecedented thermal stability of plasmonic titanium nitride films up to 1400 °C(Wiley-VCH, 2021-05-29); ; ; ; Titanium nitride (TiN) has emerged as one of the most promising refractory materials for plasmonic and photonic applications at high temperatures due to its prominent optical properties along with mechanical and thermal stability. From a high temperature standpoint, TiN is a substitution for Au and Ag in the visible to near-infrared wavelength range, with potential applications including thermophotovoltaics, thermoplasmonics, hot-electron and high temperature reflective coatings. However, the optical properties and thermal stability of TiN films strongly depend on the growth conditions, such as temperature, partial pressure of the reactive ion gas, ion energy, and substrate orientation. In this work, epitaxial TiN films are grown at 835 °C on an Al2O3 substrate using a radio frequency sputtering method. The oxidization behavior of TiN is investigated at 1000 °C under a medium vacuum condition of 2 × 10–3 mbar, which is relevant for practical technical applications, and the thermal stability at 1400 °C under a high vacuum condition of 2 × 10–6 mbar. The TiN film structure shows an unprecedented structural stability at 1000 °C for a minimum duration of 2 h under a medium vacuum condition, and an exceptional thermal stability at 1400 °C, for 8 h under a high vacuum condition, without any protective coating layer. The work reveals, for the first time to the authors’ knowledge, that the TiN film structure with columnar grains exhibits remarkable thermal stability at 1400 °C due to low-index interfaces and twin boundaries. These findings unlock the fundamental understanding of the TiN material at extreme temperatures and demonstrate a key step towards fabricating thermally stable photonic/plasmonic devices for harsh environments.Publicationtype: Journal ArticleScopus© Citations 36 95 247 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Structural degradation of tungsten sandwiched in hafnia layers determined by in-situ XRD up to 1520 °C(Macmillan Publishers Limited, part of Springer Nature, 2021-02-08); ; ; ; The high-temperature stability of thermal emitters is one of the critical properties of thermophotovoltaic (TPV) systems to obtain high radiative power and conversion efficiencies. W and HfO are ideal due to their high melting points and low vapor pressures. At high temperatures and given vacuum conditions, W is prone to oxidation resulting in instantaneous sublimation of volatile W oxides. Herein, we present a detailed in-situ XRD analysis of the morphological changes of a 3-layer-system: HfO /W/HfO layers, in a high-temperature environment, up to 1520 °C. These samples were annealed between 300 °C and 1520 °C for 6 h, 20 h, and 40 h at a vacuum pressure below 3 × 10 mbar using an in-situ high-temperature X-ray diffractometer, which allows investigation of crucial alterations in HfO and W layers. HfO exhibits polymorphic behavior, phase transformations and anisotropy of thermal expansion leads to formation of voids above 800 °C. These voids serve as transport channels for the residual O present in the annealing chamber to access W, react with it and form volatile tungsten oxides. An activation energy of 1.2 eV is calculated. This study clarifies the limits for the operation of W-HfO spectrally selective emitters for TPV in high-temperature applications. 2 2 2 2 2 2 2 –6Publicationtype: Journal ArticleScopus© Citations 17 183 213 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Thermal stability of tungsten based metamaterial emitter under medium vacuum and inert gas conditions(Macmillan Publishers Limited, part of Springer Nature, 2020-02-27); ; ; ; Commercial deployment of thermophotovoltaics (TPV) is lacking behind the implementation of solar PV technology due to limited thermal stability of the selective emitter structures. Most of the TPV emitters demonstrated so far are designed to operate under high vacuum conditions (~10−6 mbar vacuum pressure), whereas under medium vacuum conditions (~10−2 mbar vacuum pressure), which are feasible in technical implementations of TPV, these emitters suffer from oxidation due to significant O2 partial pressure. In this work, the thermal stability of 1D refractory W-HfO2 based multilayered metamaterial emitter structure is investigated under different vacuum conditions. The impact of the O2 partial pressure on thermal stability of the emitters is experimentally quantified. We show that, under medium vacuum conditions, i.e. ~10−2 mbar vacuum pressure, the emitter shows unprecedented thermal stability up to 1300 °C when the residual O2 in the annealing chamber is minimized by encapsulating the annealing chamber with Ar atmosphere. This study presents a significant step in the experimental implementation of high temperature stable emitters under medium vacuum conditions, and their potential in construction of economically viable TPV systems. The high TPV efficiency, ~50% spectral efficiency for GaSb PV cell at 1300 °C, and high temperature stability make this platform well suited for technical application in next-generation TPV systems.Publicationtype: Journal ArticleScopus© Citations 35 284 317 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Metamaterial emitter for thermophotovoltaics stable up to 1400 °CHigh temperature stable selective emitters can significantly increase efficiency and radiative power in thermophotovoltaic (TPV) systems. However, optical properties of structured emitters reported so far degrade at temperatures approaching 1200 °C due to various degradation mechanisms. We have realized a 1D structured emitter based on a sputtered W-HfO 2 layered metamaterial and demonstrated desired band edge spectral properties at 1400 °C. To the best of our knowledge the temperature of 1400 °C is the highest reported for a structured emitter, so far. The spatial confinement and absence of edges stabilizes the W-HfO 2 multilayer system to temperatures unprecedented for other nanoscaled W-structures. Only when this confinement is broken W starts to show the well-known self-diffusion behavior transforming to spherical shaped W-islands. We further show that the oxidation of W by atmospheric oxygen could be prevented by reducing the vacuum pressure below 10 −5 mbar. When oxidation is mitigated we observe that the 20 nm spatially confined W films survive temperatures up to 1400 °C. The demonstrated thermal stability is limited by grain growth in HfO 2 , which leads to a rupture of the W-layers, thus, to a degradation of the multilayer system at 1450 °C.Publicationtype: Journal ArticleScopus© Citations 77 434 366