We present here our analysis of thin film materials crucial for the development of thermophotovoltaic (TPV) selective emitters and thermal barrier coatings (TBCs), emphasizing their high refractive index, excellent thermal stability, and transparency to infrared radiation. Utilizing spectroscopic ellipsometer measurements across the wavelength range of 210 to 2500 nm and temperatures ranging from room temperature to 1000°C, we examine the impact of temperature variations on the electronic band structures of these materials. Our study begins with the characterization of magnesium oxide and strontium titanate substrates, followed by the analysis of individual composite layers consisting of cerium oxide and barium zirconate deposited onto these substrates. Subsequently, we extend our analysis to multi-layer samples comprising combinations of these materials, aiming to project their potential performance in TPV and TBC applications.
Characterizing thin-film materials with high refractive index, high thermal stability and infrared transparency is important for designing Thermophotovoltaic (TPV) selective emitters and Thermal Barrier Coatings (TBCs). Here, we report spectroscopic ellipsometer measurements of several thin films in the wavelength range 210 nm to 2500 nm from room temperature to 1000 deg C. Our findings provide insights into the potential impacts of temperature change on the aforementioned applications, induced by the underlying changes in their electronic band structures. In the first step, we present the properties of magnesium oxide and strontium titanate substrates. Next, we consider layers of cerium oxide and barium zirconate deposited on top of these substrates. Finally, we apply these initial characterizations to understand data obtained from multilayer samples comprised of a combination of layers from all these materials, and project the potential performance for TPV and TBC applications.
High frequency, and high power, radar and communications systems rely on high power amplifiers (HPAs) to increase signal strength immediately before it reaches the antenna. The heat generated in these devices quickly outpaces the heat removal capabilities of air-cooled architectures. Therefore, a variety of novel cooling approaches are needed to meet the needs for such high-power electronics. In this talk, we consider two methods - radiative cooling and two-phase cooling - with the potential to provide additional cooling power needed to achieve such goals, and also provide specific modeling and experimental results to quantify the performance improvements associated with this strategy.
Transition metal di-chalcogenides (TMDCs) have strong potential for ultra -thin electronic and photonic applications because of their range of electronic and optical properties, 2D layered structure, and tunability of properties by dopants and hybrid alloys. TMDCs have high atomic masses compared to commonly used semiconductors, which makes them resistant to damage by high energy particles in space. We have studied the fundamental electronic and optical properties of various tungsten-based TMDCs by Density Functional Theory (DFT) calculations. We then developed a solar cell model composed of heavier TMDCs with photon management features to design high-performing photovoltaic devices which are ultra-thin, lightweight, with significantly enhanced resistance to radiation-induced damage. Here, we model electro-optic properties and photovoltaic performance of various combinations of tungsten-based TMDCs containing sulfur, selenium and tellurium. Device simulations conducted using the AM0 space solar spectrum yield high efficiencies above 17% for the tungsten-based devices. The non-ionizing energy loss (NIEL) due to high energy protons for tungsten-based TMDCs are much lower than common photovoltaic semiconductors, such as silicon, resulting in significantly reduced displacement damage doses (DDD) from space radiation. Our results show that TMDCs have great potential for implementation in radiation-resistant electronic and photonic technologies in the space environment.
Thermophotovoltaics and thermal barrier coatings both require resilient materials capable of surviving extremely high temperatures associated with their applications. To obtain higher performance, it is also important that they have the ability to retain nanostructural integrity through large temperature swings. In this work, we present several concepts for achieving this goal, along with our initial characterization data demonstrating the value of this effort.
Converting high-temperature heat to electricity can theoretically provide new sources of energy, but there are several practical barriers to realization. Foremost among these challenges is the lack of data on visibly transparent materials with a suitable set of properties up to high temperatures. In this study, we examine candidate material options and report experimental findings of their key properties across a range of optical wavelengths and temperatures. We then perform a simulation to confirm that we have a correct and consistent understanding of their properties, and use that to design more complex structures for future thermophotovoltaic selective emitters, which can efficiently radiate heat that can be converted into electricity through a device similar to a solar cell.
Probabilistic computing with p-bits is a powerful, unique paradigm alternative to classical computing and holds experimental advantages over certain forms of quantum computing. Stochastic nanodevices have been experimentally demonstrated to act as artificial neurons in solving certain problems through probabilistic computing. Still, many open questions about the breadth and size of soluble problems remain. We demonstrate the capability of probabilistic computing made of a stochastic nanodevice network in solving likely NP (non-deterministic polynomial time)-complete number theory problems associated with combinatorial optimization, which can be implemented using a network of optical parametric oscillators. These simulation results show robustness across all problems tested, with great potential to scale to solve substantially larger problems.
Thermophotovoltaics (TPV) allow one to harvest excess heat as electricity. While higher temperature heat allows TPV to operate more efficiently, the materials used in the emitter are often limited in their operating temperatures and atmospheres. In this study, we specifically focus on designing a multilayer system consisting of compatible oxides, capable of reaching high temperatures. We then provide materials characterization data to support this approach and predict the final performance of a TPV system using this approach.
There is a great demand for high performance materials and technology in space photovoltaics (PV) to meet the power needs of satellites. Transition Metal Di-Chalcogenides (TMDCs) are strong candidates for such applications, as they are very lightweight and resilient to high energy radiation, compared to most PV semiconductors. We have modeled an ultra-thin photovoltaic system based on tungsten disulfide (WS2) and demonstrated performance enhancement by addition of light trapping and anti-reflection coating. Our photovoltaic model consists of a 100 nm thick WS2-based heterojunction solar cell, similar to the Hetero Junction Intrinsic Thin Layer (HIT) solar cell structure. A 1-D grating light trapping structure has been implemented using silver as the reflector material, with the grating period and thickness optimized for highest absorption enhancement. An antireflection coating layer was added to further enhance absorption, with the thickness optimized to minimize surface reflection. We have simulated our model under AM0 solar spectrum over the temperature range of geostationary satellite orbits (313-343K). The baseline photovoltaic model design was calculated to have an efficiency around 12%. The absorption enhancement from light trapping increased the short-circuit current (JSC) by 25%, which gave an efficiency around 16%. The additional absorption due to anti-reflection coating increased the JSC by a further 15%, leading to efficiency around 19%. In addition, TMDC-based solar cells have lower temperature coefficient for efficiency degradation compared to low bandgap semiconductor solar cells. These results show that our TMDC-based photovoltaic system with light trapping and anti-reflection coating is a strong candidate for space photovoltaic applications.
Solar thermal absorbers lose an increasing amount of heat at higher operating temperatures, limiting their efficiencies. An infrared mirror characterized by a high transmittance in the visible region of the spectrum while having high reflectance in the mid-IR could be crucial for a partial recovery of heat radiated by absorbers operating at high temperatures. Thanks to the cold-side external photon recycling mechanism, an IR mirror applied to an evacuated collector could lead to increase in efficiency of up to 60%. Here, we demonstrate the feasibility of such a mirror, which in principle is developed for high-vacuum flat solar thermal collectors, but is easily adaptable to other applications. The mirror presented has a very simple design, based on a discrete rugate filter scheme, and does not require multi-cathode deposition tools, which further simplifies the manufacturing process.
We propose a Nanodiamond based Biological Physical Unclonable Function for securing in-vivo and in-vitro biosensing devices connected to Internet-of Things healthcare systems for remote patient monitoring . We take advantage of nontoxic biocompatibility and antibacterial activity of nanodiamond films and cavities that can produce strong pseudo-random electromagnetic responses upon electrical or laser triggering due to the fluorescence coming from multiple color centres coupled to such cavities. Such quantum-photonic randomness provides robustness against modelling and side-channel attacks employing machine learning by remote-hackers and can be used to generate a framework of securing biomedical Internet of Things devices and architecture.
We present a silicon-based Chaotic-Cavity Optical Physically Unclonable Function(CCO-PUF) embedded inside smart-sensor based devices utilizing nonlinear-optical properties of bi-axial crystals present inside a chaotic cavity. The photonic circuit produces statistically diverse yet reproducible multimodal photonic outputs that provides a large statistical degree of freedom for generating randomness in the device responses upon electrical as well as photonic triggering that can be employed for producing authentication keys for users seeking on-demand access to IoT devices . Such PUFs can be very robust against side-channel attacks and machine-learning attacks for their wide operability range, complex nano-photonic operation and strong degree of pseudo-randomness.
We propose a dual-spin-bit Physically Unclonable Function for securing Internet-of-Things(IoT) from geometry variations of magnetic thin-films embedded inside IoT devices. Such structure-dependent device-specific keys serve for authentication,identification and privacy preservation of users against Identity theft, side-channel attacks and remote hacking in IoT networks. In our design,electrically tunable magnetic domain wall motion of the thin-films produces device-geometry sensitive spin-transfer-torque response, providing path for device-specific memory-based Spintronic Physically Unclonable key generations.An array of such magnetic films with each film containing memristive spin-bit act as the building block of spin-torque driven cryptographic logic circuits for the IoT devices.
On-demand single photon sources are a crucial element of quantum key distribution and quantum computing. Such devices face major challenges in achieving high performance, which include realized necessary environmental isolation, preservation of coherence over time, and reliable triggering of multi-photon emission. Nanodiamonds exhibit uniquely good characteristics at room temperature, but emission rates are still limited. Here, we consider how nitrogen and Xenon color centers with electronic triggering can increase emission rates, by carefully modeling the interaction between rapid voltage pulses and the resulting photonic emission. If realized experimentally, this work will pave the way for short-wavelength infrared quantum communications.
Physical and side channel attacks on Internet of Things (IoT) devices employing cryptographic software are an increasing threat to the security of the Internet. Preventing these attacks may require new, hardware-based approaches to encryption. Here, we present a possible solution, consisting of a multimodal metal-insulator-metal (MIM) plasmonic ring resonator-based Physically Unclonable Function (PUF). Device-specific field distribution patterns with strong sub-wavelength confinement act as a device-specific cryptographic identifier to ensure private communications. This is possible because our MIM structures are ultra-responsive to fabrication variations, such as sidewall roughness, metal/insulator thicknesses, coupling lengths, ring diameters, and material impurities.
Welcome to the conference on New Concepts in Solar and Thermal Radiation Conversion III. Here, we aim to explore a range of concepts and experiments relating to the effective conversion of radiation into alternative forms of energy and power. These include sessions on high-efficiency thermophotovoltaics, selective thermal transport, solar photovoltaics, reliability aspects of photovoltaics, artificial photosynthesis, solar fuels, and the stability of perovskite solar cells. A range of video and poster presentations will be available in this virtual forum, and participants are encouraged to follow-up with individual presenters with any specific questions or comments.
Solar thermal absorbers experience heat losses at high temperatures, thus limiting their efficiencies. In this work we present an InfraRed (IR) window coating capable of recycling thermal radiation back to the absorber to reduce losses and improve efficiencies. This mechanism is what we call cold-side external photon recycling. Several designs have been investigated in a preliminary study, ranging from a simple alternation of two different optical materials, as in the case of a Distributed Bragg Reflector (DBR) design, to more complex structures like the rugate filters. The ideal design of a rugate filter makes use of a huge number of different optical materials and layers, useful to reach a continuous variation of the refractive index profile which guarantee a minimization of the undesired reflections. Here, we show that some of these advantages can still be obtained by using a limited number of materials coupled with a proper design, and we extend prior theoretical work to include a realistic set of materials for ease of fabrication and validation. The resulting IR mirror, based on a rugate-multilayer structure, presents favorable properties: the high transmissivity in the visible region of the spectrum allows the Solar radiation to reach the absorber, without altering its performance greatly; instead, a reflection window width of almost 1 µm in the mid-IR region allows for partial recovery of heat radiated by the absorber that otherwise would be lost, and the reduced number of materials can simplify fabrication. Finally, the design has been developed for the particular case of a high-vacuum flat solar thermal panel, but it could be applied to any solar thermal device with an emissive surface without altering the absorber performance greatly.
Thermal management is extremely important for renewable energy systems such as photovoltaics (PV), thermophotovoltaics (TPV) and concentrating photovoltaics (CPV). Elevated operating temperatures not only reduce the efficiency of PV modules, but also substantially reduce their lifetimes. This is an even more critical issue for higher heat load systems, such as TPV and CPV, where low-bandgap solar cells are commonly used, making the system more sensitive to temperature increases. Radiative cooling can reject significantly more waste heat than convection and conduction at high temperatures by sending it directly into space. Therefore, in this study, we examine an outdoor radiative cooling system that consumes no power while realizing a substantial reduction in operating temperature, which may be suitable for substantially improving the performance and lifetime concentrating photovoltaics.
Radiative cooling is a uniquely compact and passive cooling mechanism. Significant applications can be found in energy generation, particularly concentrating photovoltaics (CPV) and thermophotovoltaics (TPV). Both rely on low-bandgap PV cells that experience significant reductions in performance and lifetime when operating at elevated temperatures. This issue creates a significant barrier to widespread adoption. To address this challenge, we demonstrate enhanced radiative cooling for low-bandgap PV cells under concentrated sunlight for the first time. A composite material stack is used as the radiative cooler. Enhanced radiative cooling reduces operating temperatures by 10 degrees C, translating into a relative increase of 5.7% in open-circuit voltage and an estimated increase of 40% in lifetime at 13 suns. Using a model, we also estimate the same setup could achieve an improvement of 34% in open-circuit voltage for 35 suns, which could reduce levelized costs of energy up to 33% for high activation energy failure modes. The radiative cooling enhancement demonstrated here is a simple and straightforward approach, which can be generalized to other optoelectronic systems.
Metasurfaces have emerged as elegant engineered interfaces capable of controlling optical phases and amplitudes within ultra-flat form factors. Recently, there has been an increasing effort to achieve reconfigurable metasurfaces incorporating various tuning mechanisms, including electrical, optical, mechanical or thermal driving forces. In particular, electronic tuning has previously been shown to provide potential control over virtually a full range of optical phases. However, practical implementation is limited by the maximum doping that can be achieved by applying bias, and by the inherent losses of the constituent materials. In this work, we apply electrically-tuned reconfigurable metasurfaces to achieve dynamically-controlled thermal sources. Kirchhoff’s law of thermal radiation suggests possible active control of spectral and angular properties of radiated heat in carefully designed metasurfaces. This goal can be achieved by coupling optical resonances that imply spectral and angular selectivity, to 2D plasmonic resonances in active structured 2D surfaces. We discuss the potential of different 2D materials, such as graphene, black phosphorus and transition metals dichalcogenides, with respect to their respective optical properties, bandgaps and inherent losses. The ultimate goal is to achieve maximal absorption in a dynamically selected direction at a given wavelength, by exciting surface-confined modes. Enabling active beam steering of coherent thermal sources may provide low-cost alternatives to existing infrared sources for applications such as sensing and thermal management.
Radiative cooling, a unique and uncommon passive cooling method for devices operating outdoors, has recently been demonstrated to be effective for photovoltaic thermal management. In this work, we investigate the effect of radiative cooling as a complement to existing passive cooling methods like convective cooling in a related system with much higher heat loads: a high-concentration photovoltaic (HCPV) system. A feasible radiative cooler design addressing the thermal management challenges here is proposed. It consists of low-iron soda-lime glass with a porous layer on top as an antireflection coating and a diamond layer as heat spreader. It is found that the proposed structure has strong mid-IR emittance as well as high solar transmission, allowing radiative cooling under direct sunlight and low loss in the concentrated solar irradiance. A systematic simulation with realistic considerations is then performed. Compared with a conventional copper cooler, the lowest temperature reached by the proposed radiative cooler is 14 K lower. Furthermore, less area of the proposed cooler is needed to reach a standard target temperature (333.15 K) for steady-state operation under high concentrations for the crystalline silicon PV module. In order to compare the coolers quantitatively, a figure of merit – cooling power per weight - is introduced. At the target temperature, the proposed cooler is determined to have a cooling power per weight of 75 W/kg, around 3.7 times higher than that of the conventional copper cooler.
Simultaneously controlling both the spectral and angular emission of thermal photons can qualitatively change the nature of thermal radiation, and offers a great potential to improve a broad range of applications, including infrared light sources and thermophotovoltaic (TPV) conversion of waste heat to electricity. For TPV in particular, frequency-selective emission is necessary for spectral matching with a photovoltaic converter, while directional emission is needed to maximize the fraction of emission reaching the receiver at large separation distances. This can allow the photovoltaics to be moved outside vacuum encapsulation. In this work, we demonstrate both directionally and spectrally-selective thermal emission for p-polarization, using a combination of an epsilon-near-zero (ENZ) thin film backed by a metal reflector, a high contrast grating, and an omnidirectional mirror. Gallium-doped zinc oxide is selected as an ENZ material, with cross-over frequency in the near-infrared. The proposed structure relies on coupling guided modes (instead of plasmonic modes) to the ENZ thin film using the high contrast grating. The angular width is thus controlled by the choice of grating period. Other off-directional modes are then filtered out using the omnidirectional mirror, thus enhancing frequency selectivity. Our emitter design maintains both a high view factor and high frequency selectivity, leading to a factor of 8.85 enhancement over a typical blackbody emitter, through a combination of a 22.26% increase in view factor and a 6.88x enhancement in frequency selectivity. This calculation assumes a PV converter five widths away from the same width emitter in 2D at 1573 K.
Radiative cooling has recently garnered a great deal of attention for its potential as an alternative method for photovoltaic thermal management. Here, we will consider the limits of radiative cooling for thermal management of electronics broadly, as well as a specific application to thermal power generation. We show that radiative cooling power can increase rapidly with temperature, and is particularly beneficial in systems lacking standard convective cooling. This finding indicates that systems previously operating at elevated temperatures (e.g., 80°C) can be passively cooled close to ambient under appropriate conditions with a reasonable cooling area. To examine these general principles for a previously unexplored application, we consider the problem of thermophotovoltaic (TPV) conversion of heat to electricity via thermal radiation illuminating a photovoltaic diode. Since TPV systems generally operate in vacuum, convective cooling is sharply limited, but radiative cooling can be implemented with proper choice of materials and structures. In this work, realistic simulations of system performance are performed using the rigorous coupled wave analysis (RCWA) techniques to capture thermal emitter radiation, PV diode absorption, and radiative cooling. We subsequently optimize the structural geometry within realistic design constraints to find the best configurations to minimize operating temperature. It is found that low-iron soda-lime glass can potentially cool the PV diode by a substantial amount, even to below ambient temperatures. The cooling effect can be further improved by adding 2D-periodic photonic crystal structures. We find that the improvement of efficiency can be as much as an 18% relative increase, relative to the non-radiatively cooled baseline, as well as a potentially significant improvement in PV diode lifetime.
Thermal emission from blackbodies and flat metallic surfaces is non-directional, following the Lambert cosine law. However, highly directional thermal emission could be useful for improving the efficiency of a broad range of different applications, including thermophotovoltaics, spectroscopy and infra-red light sources. This is particularly true if strong symmetry breaking could ensure emission only in one particular direction. In this work, we investigate the possibility of tailoring asymmetric thermal emission using structured metasurfaces. These are built from surface grating unit elements that support asymmetric localization of thermal surface plasmon polaritons. The angular dependence of emissivity is studied using a rigorous coupled wave analysis (RCWA) of absorption, plus Kirchhoff’s law of thermal radiation. It is further validated using a direct thermal simulation of emission originating from the metal. Asymmetric angular selectivity with near-blackbody emissivity is demonstrated for different shallow blazed grating structures. We study the effect of changing the period, depth and shape of the grating unit cell on the direction angle, angular spread, and magnitude of coupled radiation mode. In particular, a periodic sawtooth structure with a period of 1.5λ and angle of 8°was shown to create significant asymmetry of at least a factor of 3. Such structures can be considered arbitrary directional sources that can be carefully patterned on metallic surfaces to yield thermal lenses with designed focal lengths, targeted to particular concentration ratios. The benefit of this approach is that it can enhance the view factor between thermal emitters and receivers, without restricting the area ratio or separation distance.
Converting blackbody thermal radiation to electricity via thermophotovoltaics (TPV) is inherently inefficient. Photon recycling using cold-side filters offers potentially improved performance but requires extremely close spacing between the thermal emitter and the receiver, namely a high view factor. Here, we propose an alternative approach for thermal energy conversion, the use of an integrated photonic crystal selective emitter (IPSE), which combines two-dimensional photonic crystal selective emitters and filters into a single device. Finite difference time domain and current transport simulations show that IPSEs can significantly suppress sub-bandgap photons. This increases heat-to-electricity conversion for photonic crystal based emitters from 35.2 up to 41.8% at 1573 K for a GaSb photovoltaic (PV) diode with matched bandgaps of 0.7 eV. The physical basis of this enhancement is a shift from a perturbative to a nonperturbative regime, which maximized photon recycling. Furthermore, combining IPSEs with nonconductive optical waveguides eliminates a key difficulty associated with TPV: the need for precise alignment between the hot selective emitter and cool PV diode. The physical effects of both the IPSE and waveguide can be quantified in terms of an extension of the concept of an effective view factor.
Major sources of performance degradation and failure in glass-encapsulated PV modules include moisture-induced gridline corrosion, potential-induced degradation (PID) of the cell, and stress-induced busbar delamination. Recent studies have shown that PV modules operating in damp heat at -600 V are vulnerable to large amounts of degradation, potentially up to 90% of the original power output within 200 hours. To improve module reliability and restore power production in the presence of PID and other failure mechanisms, a fundamental rethinking of accelerated testing is needed. This in turn will require an improved understanding of technology choices made early in development that impact failures later.
In this work, we present an integrated approach of modeling, characterization, and validation to address these problems. A hierarchical modeling framework will allows us to clarify the mechanisms of corrosion, PID, and delamination. We will employ a physics-based compact model of the cell, topology of the electrode interconnection, geometry of the packaging stack, and environmental operating conditions to predict the current, voltage, temperature, and stress distributions in PV modules correlated with the acceleration of specific degradation modes. A self-consistent solution will capture the essential complexity of the technology-specific acceleration of PID and other degradation mechanisms as a function of illumination, ambient temperature, and relative humidity. Initial results from our model include specific lifetime predictions suitable for direct comparison with indoor and outdoor experiments, which are qualitatively validated by prior work. This approach could play a significant role in developing novel accelerated lifetime tests.
It has recently been proposed that designing selective emitters with photonic crystals (PhCs) or plasmonic metamaterials can suppress low-energy photon emission, while enhancing higher-energy photon emission. Here, we will consider multiple approaches to designing and fabricating nanophotonic structures concentrating infrared thermal radiation at energies above a critical threshold. These are based on quality factor matching, in which one creates resonant cavities that couple light out at the same rate that the underlying materials emit it. When this quality-factor matching is done properly, emissivities can approach those of a blackbody, but only within a selected range of thermal photon energies. One potential application is for improving the conversion of heat to electricity via a thermophotovoltaic (TPV) system, by using thermal radiation to illuminate a photovoltaic (PV) diode. In this study, realistic simulations of system efficiencies are performed using finite-difference time domain (FDTD) and rigorous coupled wave analysis (RCWA) to capture both thermal radiation and PV diode absorption. We first consider a previously studied 2D molybdenum photonic crystal with a commercially-available silicon PV diode, which can yield TPV efficiencies up to 26.2%. Second, a 1D-periodic samarium-doped glass emitter with a gallium antimonide (GaSb) PV diode is presented, which can yield efficiencies up to 38.5%. Finally, a 2D tungsten photonic crystal with a 1D integrated, chirped filter and the GaSb PV diode can yield efficiencies up to 38.2%; however, the fabrication procedure is expected to be more challenging. The advantages and disadvantages of each strategy will be discussed.
Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling
discontinuous dielectric materials, due to the inhererent discretization ("pixellization"). We show
that accuracy can be significantly improved by using a sub-pixel smoothing of the dielectric function,
but only if the smoothing scheme is properly designed. We develop such a scheme based on a
simple criterion taken from perturbation theory, and compare it to other published FDTD smoothing
methods. In addition to consistently achieving the smallest errors, our scheme is the only one
that attains quadratic convergence with resolution for arbitrarily sloped interfaces. Finally, we
discuss additional difficulties that arise for sharp dielectric corners.
A new on-chip silicon-based Bragg cladding waveguide with full CMOS compatibility is developed. This novel optical waveguide has a low refractive index core (SiO2) surrounded by a 1D photonic crystal cladding. The cladding consists of several dielectric bilayers, where each bilayer consists of a high index-contrast pair of layers of Si and Si3N4. This new waveguide guides light based on omnidirectional reflection, reflecting light at any angle or polarization back into the core. Its fabrication is fully compatible with current microelectronics processes. In principle, a core of any low-index material can be realized with our novel structure, including air. Potential applications include tight turning radii, high power transmission, nonlinear properties engineering and biomaterials sensors on silicon chip.
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