Heat Mirror Research Articles

Low-Temperature and Corrosion-Resistant Gas Diffusion Multibarrier with UV and Heat Rejection Capability-A Strategy to Ensure Reliability of Organic Electronics.

When placed in an outdoor environment, organic electronic devices (OEDs) can degrade on exposure to moisture, UV light, and heat, owing to the chemical sensitivity and decomposition of the organic materials. Therefore, to protect OEDs from outdoor environments, thin-film passivation, which can block harmful elements from reaching organic materials, is required. To meet the demands and trends in encapsulation technologies, in this study, we developed a low-temperature, simple, and effective gas diffusion multibarrier (GDM), which is UV and heat reflective as well as corrosion resistant. The designed UV- and heat-reflective GDM (UHGDM) has a multistacked structure in the form of a UV filter/Ag/gas diffusion barrier (GDB)/polymer based on a dielectric/metal/dielectric (DMD) configuration. First, the DMD structure was used as a heat mirror for infrared reflectance. Second, the bottom dielectric layer of the DMD structure was used as the UV filter, and it consisted of a ZnS/LiF multistacked structure with large differences in refractive indexes. Third, a nanolaminate-based GDB barrier with multi-interfacial and defect-decoupling systems, which achieved a water vapor transmission rate of 1.58 × 10-5 g/m2/day at a thickness of 60 nm, was used as the top dielectric layer of the DMD structure. Finally, an inorganic/organic hybrid polymer layer was coated on the DMD structure to provide corrosion-resistance and waterproofing properties. The fabricated UHGDM showed high transparency in the visible region and excellent reflectance in the UV and IR regions, resulting in excellent UV and heat rejection capability in practical UV and heat reflection tests. In addition to optical functionalities, the UHGDM maintained its functionality against harsh environmental conditions because of the GDB/polymer structure. Finally, the feasibility of the UHGDM was demonstrated using organic solar cells through water immersion and shelf lifetime tests.

Enhancement of plasmonic transmittance of porous gold thin films via gold/metal oxide bi-layers for solar energy-saving applications

Thin films of noble metals, such as gold, have high visible transparency and infrared reflectance. The former is due to the presence of surface plasmons, and the latter is due to the large density of free electrons. Gold thin films have a transmittance peak at 520 nm, which is closely matched to the maximum emission of solar radiation. These properties have been utilized in several applications, including energy-saving transparent heat mirrors that consist of a bi-layer of a thin gold film over-coated with a dielectric layer. These coatings transmit light and reflect heat. For such an application, the transmittance of the thin gold film needs to be enhanced; this is achieved by the dielectric layer that functions as an antireflective coating. In this work, the dielectrics were metal oxides that are transparent in the visible range. The selection criteria based on the band gap, and refractive index were first imposed to identify potential metal oxides. This resulted in the choice of ten metal oxides (CeO2, HfO2, MoO3, Nb2O5, NiO, SnO2, Ta2O5, TiO2, WO3, and ZrO2). The bi-layer performance was then optimized theoretically. The gold-metal oxide layers were subsequently fabricated, and their optical spectra were measured over the ultraviolet–visible-near infrared ranges. The performance of the heat mirrors was assessed in terms of their visible and solar transmittance and reflectance. A figure of merit was defined to evaluate the suitability of the oxides. The optical properties suggest that tungsten oxide (WO3) is the best oxide; however, hafnium oxide (HfO2) has the best performance for solar energy applications.

Applicability of Heat Mirrors in Reducing Thermal Losses in Concentrating Solar Collectors

Abstract In the present work, a novel parabolic trough receiver design has been proposed. The proposed design is similar to the conventional receiver design except for the envelope and the annulus part. Here, a certain portion of the conventional glass envelope is coated with Sn-In2O3 and also Sn-In2O3 coated glass baffles are provided in the annulus part to reduce the radiative losses. The optical properties of the coated glass are such that it allows most of the solar irradiance to pass through, but reflects the emitted long wavelength radiations back to the absorber tube. Sn-In2O3 coated glass is referred to as “transparent heat mirror.” Thus, effectively reducing the heat loss area and improving the thermal efficiency of the solar collector. A detailed one-dimensional steady-state heat transfer model has been developed to predict the performance of the proposed receiver design. It was observed that while maintaining the same external conditions (such as ambient/initial temperatures, wind speed, solar insolation, flow rate, and concentration ratio), the heat mirror-based parabolic trough receiver design has about 3–5% higher thermal efficiency as compared to the conventional receiver design. Furthermore, the heat transfer analysis reveals that depending on the spatial incident solar flux distribution, there is an optimum circumferential angle (θ = θoptimum, where θ is the heat mirror circumferential angle) up to which the glass envelope should be coated with Sn-In2O3. For angles higher than the optimum angle, the collector efficiency tends to decrease owing to increase in optical losses.

Enhanced optical absorption of graphene-based heat mirror with tunable spectral selectivity

Graphene based heat mirror (i.e., graphene/Cu/graphene multilayer coating) was designed and developed to achieve enhanced optical absorption with tunable spectral selectivity. The concept of a heat mirror and a high/low/high refractive index stack was used to develop the graphene/Cu/graphene absorber and the transition from high transmittance to high absorptance was realized on quartz substrates. The thickness of the Cu layer has an important role in creating the destructive interference, for achieving high absorptance and low emittance. The refractive index of the multilayer graphene transferred on Si substrate was studied using phase-modulated spectroscopic ellipsometry. An absorptance of 0.91 and emittance of 0.22 was achieved for the SiO2/graphene/Cu/graphene multilayer coating. For the first time, the feasibility of graphene/Cu/graphene multilayer coating for photothermal applications have been demonstrated.

Indium free electrode, highly flexible, transparent and conductive for optoelectronic devices

WO3/Ag/WO3 multilayer structures were used as ITO free transparent electrode, transparent heat mirrors and transparent heaters. WO3/Ag/WO3 stacked layers were deposited by sequential sublimation, evaporation under vacuum. After optimization of Ag thickness (16 nm), they exhibit low sheet resistance (8 Ω/sq), high transmittance in the visible (TMax = 91.5%, averaged T400-700 = 80.6%) and high reflection in the near infrared and infrared regions. These values are optimal when it is used as transparent electrode but, as transparent heat mirrors 18 nm are better due to higher reflection in the NIR and IR. All these properties made possible to use them in different devices. When used as transparent anode in organic photovoltaic cells, they allow achieving performance similar to those obtained with ITO. Their transmission and reflection spectra show that they can also be employed as transparent heat mirrors. Similarly, studies dedicated to heating properties of the WO3/Ag/WO3 multilayer structures show that their performance are comparable to those obtained with another possible substituent to ITO, silver nanowires thin films.

Solar Selective Volumetric Receivers for Harnessing Solar Thermal Energy

Given the largely untapped solar energy resource, there has been an ongoing international effort to engineer improved solar-harvesting technologies. Toward this, the possibility of engineering a solar selective volumetric receiver (SSVR) has been explored in the present study. Common heat transfer liquids (HTLs) typically have high transmissivity in the visible-near infrared (VIS-NIR) region and high emission in the midinfrared region, due to the presence of intramolecular vibration bands. This precludes them from being solar absorbers. In fact, they have nearly the opposite properties from selective surfaces such as cermet, TiNOX, and black chrome. However, liquid receivers which approach the radiative properties of selective surfaces can be realized through a combination of anisotropic geometries of metal nanoparticles (or broad band absorption multiwalled carbon nanotubes (MWCNTs)) and transparent heat mirrors. SSVRs represent a paradigm shift in the manner in which solar thermal energy is harnessed and promise higher thermal efficiencies (and lower material requirements) than their surface absorption-based counterparts. In the present work, the “effective” solar absorption to infrared emission ratio has been evaluated for a representative SSVR employing copper nanospheroids/MWCNTs and Sn-In2O3 based heat mirrors. It has been found that a solar selectivity comparable to (or even higher than) cermet-based Schott receiver is achievable through control of the cut-off solar selective wavelength. Theoretical calculations show that the thermal efficiency of Sn-In2O3 based SSVR is 6–7% higher than the cermet-based Schott receiver. Furthermore, stagnation temperature experiments have been conducted on a laboratory-scale SSVR to validate the theoretical results. It has been found that higher stagnation temperatures (and hence higher thermal efficiencies) compared to conventional surface absorption-based collectors are achievable through proper control of nanoparticle concentration.

Effect of annealing treatment on optical properties and microstructural variation of $$\hbox {WO}_{3}/\hbox {Ag}/\hbox {WO}_{3}$$ WO 3 / Ag / WO 3 multilayer nano-films

Structural and optical properties of \(\text {WO}_{3}/\text {Ag}/\text {WO}_{3}\) nano-multilayer composites were investigated for heat mirror applications. \(\text {WO}_{3}/\text {Ag}/\text {WO}_{3}\) thin films were fabricated through a physical vapour deposition method by using electron-beam evaporation at the vacuum chamber at 10\(^{-5}\) Torr. \(\text {WO}_{3}\) nano-layer was fabricated at 40 nm. Annealing treatment was carried out at 100, 200, 300 and 400\(^{\circ }\)C for 1 h after the deposition of first layer of \(\text {WO}_{3}\) on the glass. On \(\text {WO}_{3}\) film, Ag nano-layers with 10, 12 or 14 nm thickness were deposited. Individual layers morphology was investigated using atomic force microscopy (AFM) and deduced that a smoother layer can be achieved after the annealing at 300\(^{\circ }\)C. Ellipsometry analysis was executed to determine both layers, Ag film thickness and inter-diffusion between the \(\text {WO}_{3}\)–Ag–\(\text {WO}_{3}\) layers. It was inferred that there was almost no interfering among the \(\text {WO}_{3}\)–\(\text {WO}_{3 }\) layers in the samples with 12 and 14 nm Ag thickness; while silver was deposited on the annealed \(\text {WO}_{3}\) layer at 300\(^{\circ }\)C. UV–visible spectrophotometer showed that the annealing treatment of the first \(\text {WO}_{3}\) layer enhanced the transparency of films in the visible region. The innovations of the present study have been based on the annealing of the films and finding an optimum thickness for the Ag film at 12–14 nm. Heat mirrors efficiency was assessed according to the principle of their optical behaviour and optimum performance obtained for 14 nm of Ag film, deposited on annealed tungsten oxide at 300\(^{\circ }\)C.

Microstructure tuning facilitated photo-efficiency enhancement and environmental benign nature of HfO2/Mo/HfO2 multilayer films

Nano-layered, multilayered films consisting of HfO2(dielectric)/Mo(metal)/HfO2(dielectric), (D/M/D architecture) with controlled layer thickness, chemistry and microstructure were demonstrated for efficient utilization of solar energy with well-designated spectral control. The effect of Mo metal interlayer on the energy performance and efficiency of HfO2/Mo/HfO2 multilayer films made by electron-beam deposition onto silicon and glass substrates has been evaluated. The Mo interlayer thickness [Mo(t)] was varied in the range of 5–25 nm while the top/bottom HfO2 layer thickness was kept constant at ∼50 nm. Structural, morphological and optical characterization was performed to understand the effect of Mo(t) induced microstructure on mechanical durability and optical behavior of the D/M/D multilayer films. Structural studies revealed that D/M/D multilayer films with relatively low Mo(t) were amorphous, while higher Mo(t) induces the amorphous-to-nanocrystalline microstructure transformation, which also induces surface roughening effects. The D/M/D multilayer film with Mo(t) = 20 nm, where the nano-columnar morphology of the D-M-D layers are aligned perpendicular to the substrate surface and exhibits the optimum spectral selectivity. Such microstructure and morphological tailoring also facilitates the environmentally friendly (self-cleaning/stain repellent) nature of these multilayer films by the high contact angle (∼102°) achieved. Molybdenum interlayer facilitated higher contact angle leads to super-hydrophobic nature of the HfO2/Mo/HfO2 multilayered film surfaces. The low surface roughness values (0.2–0.8 nm), which are primarily due to Mo at the interface, of the HfO2/Mo/HfO2 multilayered films satisfy the general requirements of heat mirror applications. In addition, the microstructure of intermediate Mo layer and HfO2-Mo interfaces significantly control the mechanical properties of D/M/D multilayer films. Optimal mechanical properties (hardness = 30 GPa, elastic modulus = 312 GPa, wear resistance = 0.094, adhesion strength = 2050 µN) have been obtained for Mo(t) = 20 nm D/M/D multilayer film. The mechanical response strongly supports flexible and wear resistive nature of HfO2/Mo/HfO2 multilayer films.

Synthesis of CdO:SnO2 alloyed thin films for solar energy conversion and optoelectronic applications

CdO:SnO2, alloyed thin films were successfully deposited on glass substrates under the deposition condition of 60 °C of NaOH solution, using two solution based methods: successive ionic layer adsorption and reaction and solution growth technique. The crystallographic studies were done using X-ray diffractometer and scanning electron microscope. The deposited alloyed samples were annealed between 100 and 250 °C using Master Chef Annealing Machine. Rutherford backscattering Spectroscopy analysis confirmed the percentage of the elements of cadmium, tin, and oxygen in the alloyed thin films. The optical characterization was carried out using UV-1800 double beam spectrophotometer. The alloyed thin films of samples A14 and A16 of CdO:SnO2 show optical transmittance of 16–60% in the ultraviolet region, 61–59% in the visible, and 59–58% in the near-infrared regions of electromagnetic spectrum for sample A14 annealed at 100 °C. Sample A16 annealed at 150 °C has an optical transmittance of 27–70% in the ultraviolet region, 70–84% in the visible and 85–87% in the near infrared regions of electromagnetic spectrum. The two samples, have equal direct wide band gap of 3.70 ± 0.05 eV. From the spectral qualities, these compound alloyed thin films may be found useful in heat mirror applications, active layer in various types of solar cells and passive layer in heat and cold mirror application, galvanizing materials. It can also found interesting as semiconductor materials for electronic applications.

Self-Cleaning Transparent Heat Mirror with a Plasma Polymer Fluorocarbon Thin Film Fabricated by a Continuous Roll-to-Roll Sputtering Process.

This paper proposes a novel self-cleaning transparent heat mirror (SC-THM) produced by depositing a plasma polymer fluorocarbon thin film on a silver-and-SiN x multilayer structure fabricated by continuous roll-to-roll sputtering. The optimal structure and the thickness of each thin film of three-layer and five-layer SC-THMs were derived from optical simulation. In the five-layer SC-THM, the visible light transmittance was 60.67% at a wavelength of 406 nm and the infrared (IR) transmittance was 6.86% at a 1000 nm wavelength and 2.50% at a 1500 nm wavelength. The value of the performance parameter Tvis/ Tsol was 1.70. The SC-THM exhibited self-cleaning with a very good water repellency of more than 111°, achieved by applying a low-surface-energy fluorocarbon thin film to the top layer. This study successfully demonstrated the IR blocking properties of an SC-THM through IR reflection and IR irradiation experiments.

Study of Effect of Various Parameters on Desalination process

The fresh water shortage is current problem faced by many nations and fresh water resources are shrinking. The demand for fresh water will keep on increasing due to urbanization. Large amount of population is now staying along the sea coast line. The desalination could be the ideal solution for the generation of the distilled water. The desalination can be carried out by evaporation of saline water or it could be carried out with reverse osmosis. The evaporation of water to generate the fresh water is more common. The evaporation with solar has been useful for large scale systems. The different parameters on the solar desalination depends are solar insolation, Aperture diameter, receiver diameter, Wind speed, heat loss coefficient, mirror strip receiver bottom temperature, etc. This present paper discusses the effect of heat loss coefficient (overall) and mirror strip receiver temperature with the varying parameters like wind speed, aperture diameter, absorber diameter, material used for absorber. Wind speed (ranging from 1 m/s to 5 m/s) has significant effect on the receiver surface temperature and overall loss coefficient. It is observed that there is deviation of 7.90% in maximum receiver surface temperature at various wind speeds and different aperture and receiver diameter and maximum useful heat energy was obtained when the wind speed was 1 m/s in case of aluminium as receiver material. The effect of overall heat transfer was studied and it is observed that the maximum temperature of the surface is around 665.02 K.

The Preparation of Tin Oxide Film Heat Mirrors by A Sol Gel Process

The Preparation of Tin Oxide Film Heat Mirrors by A Sol Gel Process

Bi-layered energy efficient coatings as transparent heat mirrors based on vanadium oxide thin films

Energy-efficient coatings based on two-layer-structured transparent heat mirrors were fabricated by depositing a vanadium oxide thin film on a silver thin film by thermal evaporation. Transparent heat mirrors are energy efficient coatings designed to save energy in hot climates by allowing only the visible part of solar radiation to pass through them and reflecting the infrared heat. These transparent heat mirrors can be used as energy-saving windows in energy-efficient buildings and green house agriculture. First, the optical, structural, and chemical properties of individual vanadium oxide thin films were investigated related to their application in transparent heat mirrors. Then, in order to realize their potential as energy efficient coating, optical characterization was performed on a two-layer structure. The performance of these coatings was calculated through integrated spectral transmittance and reflectance in the range of visible and infrared radiations. In addition, chemical depth profiling was performed to probe the diffusion of elements along the deposited layers. The fabricated coatings were found to show the expected behavior as transparent heat mirrors of high visible transparency and high infrared reflection.

Investigations of optical and surface properties of Ag single thin film coating as semitransparent heat reflective mirror

Abstract The paper presents results of optical and surface morphology investigations of semitransparent silver single thin films deposited on glass substrate in relation to their heat radiation treatment. The thickness of 15 nm for the silver thin films was selected using computer designing of optical spectra and the films were deposited using electron beam evaporation process. Optical transmission and reflection were investigated for as deposited samples and after exposition to heat radiation from quartz-halogen lamp. The changes in the optical spectra were observed which suggested degradation of deposited heat mirrors. Structure and surface morphology studies performed using scanning electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy allowed us to conclude about formation of nanometric silver islands, regularly distributed over the surface of the glass substrate after exposure to heat treatment.

Indium-free transparent TiOx/Ag/WO3 stacked composite electrode with improved moisture resistance

Amorphous TiOx, WO3 and Ag mid-layer films were deposited on polyethersulfone (PES) and soda-lime glass via electron beam evaporation at room temperature. The crystallinity, microstructure and surface morphology of asymmetric TiOx/Ag/WO3 multilayers (TAW) and symmetric WO3/Ag/WO3 (WAW) multilayers were investigated. The polygrains oriented along the (111) planes in the inserted Ag thin films were adopted to supply carriers into both of the dielectric layers and reduce the sheet resistance of the multilayer. The TAW multilayer with a sufficiently large Ag thickness (>15 nm) exhibited low resistance, and highly visible transmittance, low infra-red transmittance, to serve as a transparent conductive electrode and a good heat mirror. The dependence of the Ag and TiOx thickness on the optical and electrical properties of TAW multilayers was also explored. A figure of merit (FOM) was used to find an optimal layer structure for a TAW multilayer with superior conductivity and highly visible transparency. A high FOM of 1.2 × 10−1 (Ω−1) was achieved at the visible wavelength of 550 nm for a asymmetric TAW stacked layer on PES with a 15-nm-thick Ag layer, a 40-nm-thick TiOx and a 40 nm-thick WO3 layer. The moisture resistances of the TAW stacking layer showed more robustness than that of the WAW sample after damp heat exposure.

Optical and EPR studies of Cu2+ doped SnO2 thin films by spray pyrolysis

The study and application of thin film technology is entirely entered in to almost all the branches of science and technology. Semiconductor oxide thin films are materials with numerous applications in electronic and optoelectronic devices as well as some other applications such as protective coatings, heat mirrors and catalysis. In the context of world energy demand, particularly energy conversion, transparent conductive oxides based semiconductor oxides plays a crucial role in the development of new-fashioned thin film solar cells. Present study describing the synthesis and study of optical and structural characteristics of copper ions doped tin oxide (SnO2) is really more interesting for researchers due to its vast applications. Cu2+-doped tin oxide thin films were prepared by chemical spray pyrolysis synthesis and characterised by powder XRD, optical and EPR studies. Powder XRD data revealed that the crystal structure belongs to tetragonal rutile phase and its lattice cell parameters, average crystallite size were evaluated. Optical and EPR data confirmed that the doped Cu2+ enter into host material as distorted octahedral symmetry.

Multi-nano layer ZnS/Ag/ZnS constructed as a heat mirror for energy saving

Multilayer ZnS and Ag thin films at nano-scale thickness and different substrate temperatures were investigated. Deposition carried out by PVD method using electron beam depositing mode on Corning glass substrate at room temperature, 60, 100 and 150°C. The optical performance of single and multi layers were characterised by XRD, ellipsometry analysis and scanning probe microscopy. XRD justifies that the film is crystallised in the hexagonal and cubic structure. Ellipsometry spectroscopic analyses reveal that the interlayer between Ag and ZnS layers contains a physically mixed layer and a compound semiconductor layer. Based on the characteristic matrix theory, the optimised multilayer system was speculated and tested. An optimised structure with good heat mirror performance was successfully prepared, including some new extremely asymmetric multilayer structures. Morphological characterisations carried out by scanning probe microscopy on the AFM mode to study the particle size, packing and roughness of the films.

Substrate temperature effect on the nanoscale multilayer ZnS/Ag/ZnS for heat mirror application

AbstractNanoscale multilayers of ZnS/Ag/ZnS were deposited on Corning glass substrates at different substrate temperatures. The depositions were carried out in high vacuum using electron beam deposition technique at 20, 60, 100 and 150 °C, respectively. The optical and electrical performance of each single layer and the accomplished ZnS/Ag/ZnS multilayer system were characterized using spectroscopic ellipsometry analysis, XRD and finally AFM. Based on these analyses and associated theories, such as the characteristic matrix theory, the optimized multilayer system was speculated and tested. Crystallographic structures of the films were studied by X-ray diffraction. In addition to X-ray diffraction, morphological characterizations were carried out by AFM in order to observe the deposited particle size, packing and roughness of the films. The optimum performance was achieved at the substrate temperature of 60 °C.

Experimental Results and Design of Mirror Antenna and MW Gyrotron for Control of High Intermittent Heat Flux in GAMMA 10 Tandem Mirror

Development of an electron cyclotron heating (ECH) mirror antenna and MW gyrotrons for power modulation experiments in GAMMA 10 has been started in order to generate and control the high heat flux and to make the edge-localized mode (ELM)-like intermittent heat load pattern for divertor simulation studies. A peak heat flux of more than 10 MW/m2 on the GAMMA 10 axis has been obtained during ECH injection within the available power of ECH. This value almost corresponds to the steady-state heat load of the divertor plate of ITER. However, we need a substantial upgrade of the heating power to approach the ITER level ELM energy density. We carry out a design study for a mirror antenna and higher power gyrotrons in order to generate higher intermittent heat flux in GAMMA 10 tandem mirror for future divertor simulation studies.

Solar heat harvesting and transparent insulation in textile architecture inspired by polar bear fur

Solar thermal technology is a promising key strategy for future renewable energy production. Various concepts exist that use solar collectors and heat mirrors, built from rigid materials, to gather thermal energy from solar radiation. A new approach is the utilization of textile materials to build solar thermal collector systems with flexible material properties, lightweight design and improved material-efficiency. A solar collector, based on a multi-layer arrangement of technical textiles and foil membranes, has been realized by the ITV Denkendorf (Institute of Textile Technology and Process Engineering Denkendorf). The proposed collector system allows transparent insulation in textile-based buildings while gathering thermal energy simultaneously. The system is inspired by the transparent insulation and heat harvesting strategies of polar bear fur and can inform textile-based envelopes of future transparent buildings. In this study, different material arrangements and the influence of different parameters on the temperature distribution along the collector were tested. Air temperatures up to 150°C (302°F) could be generated inside the collector system. Furthermore, a closer look at the polar bear fur and other related principles in nature delivered additional concepts for energetic optimization.