Thin films consisting of transition metal nanoparticles in an insulating oxide exhibit a high solar absorptance together
with a low thermal emittance and are used as coatings on solar collector panels. In order to optimise the nanocomposites
for this application a more detailed understanding of their optical properties is needed. Here we use a highly efficient
recently developed numerical method to extract the spectral density function of nickel-aluminum oxide (Ni-Al2O3)
composites from experimental data on the dielectric permittivity in the visible and near-infrared wavelength ranges. Thin
layers of Ni-Al2O3 were produced by a sol-gel technique. Reflectance and transmittance spectra were measured by
spectrophotometry in the wavelength range 300 to 2500 nm for films with thicknesses in the range 50 to 100 nm.
Transmission electron microscopy showed crystalline Ni particles with sizes in the 3 to 10 nm range. The spectral
density function shows a multi-peak structure with three or four peaks clearly visible. The peak positions are influenced
by particle shape, local volume fraction distributions and particle-particle interactions giving rise to structural resonances
in the response of the composite to an electromagnetic field.
A spectrally selective reflector (SSR) can be fabricated by depositing a transparent conducting oxide on a reflective
substrate. SSRs can be incorporated in concentrating solar cells in order to minimize the heating. We deposited
TiO2:Nb thin films on glass by DC magnetron sputtering, extracted optical constants, and used those to model an
optimized SSR. Corresponding films were then successfully produced. The best wavelength-integrated reflectance
values were 79 % and 31 % in the ranges 300 < λ < 1100 nm and 1100 < λ < 2500 nm, respectively. These data are
better than those previously achieved using SnO2:F.
We discuss a number of aspects of a novel flexible electrochromic foil capable of varying its optical transmittance. The
foil includes thin films of tungsten oxide and nickel oxide laminated together by a polymer electrolyte. Starting with
scientific issues, we discuss the dominating defects in amorphous tungsten oxide and how they may yield a consistent
picture of the optical properties of tungsten oxide films versus nonstoichiometry and ion intercalation. We also present a
detailed model for the coloration/bleaching due to proton extraction/insertion in thin surface sheaths of nanocrystallites
of nickel oxide. Next we consider aspects of technology and treat options to enhance the bleached-state transmittance by
mixing the nickel oxide with another oxide having a wide band gap. We also cover pre-assembly charge insertion/extraction by facile gas treatments of the films as well as practical device manufacturing. The final part of the paper deals
with a number of applications, with emphasis on architectural "smart windows" which can improve indoor comfort at
the same time as they accomplish significant energy savings due to lowered requirements for air cooling. Applications
concerning electrochromics-based eyewear are introduced; these may be approaching market introduction.
Nanoparticle films of PdxWO3, with x being 0.01 or 0.12, were made by dual-beam gas deposition. Resistance noise as well as dc resistance were measured during exposure to ethanol and hydrogen gas. For ethanol concentrations exceeding 50 ppm, changes in the resistance noise gave 300 times larger detection sensitivity than changes in the dc resistance. This sensitivity reached a maximum at 250 °C and was very reproducible for ethanol sensing.
Thin films were made by spinning a dispersion of tin-doped indium-oxide particles, having an average diameter of 14 nm, onto glass substrates. As-deposited thin films displayed a resistivity (rho) of 0.3 (Omega) m and some optical absorption. Annealing in vacuum at 200 to 400 degree(s)C for 2 h, and subsequently in air at 500 degree(s)C for 2 h, produced films with (rho) equals10--3 (Omega) m and a visible transmittance exceeding 90 %. Leaving out the vacuum treatment yielded higher resistivity.
Sputtered thin films of amorphous tungsten oxide were deposited onto glass substrates coated by conductive indium- tin oxide. The films were sputtered in Ar/O2 with different oxygen contents. Elastic Recoil Detection Analysis determined the density and the stoichiometry while x-ray diffraction gave evidence of the amorphous character. The films were electrochemically intercalated with lithium ions. At several intercalation levels the optical reflectance and transmittance was measured in the wavelength range 0.3 micrometers to 2.5 micrometers . We study the effect of different sputtering conditions on the coloration efficiency of the films. Films deposited at pressures of 80 mTorr showed a distinctly different absorption spectrum after intercalation, as compared to films sputtered at lower pressure. Sputtered substoichiometric tungsten oxide exhibits an absorption peak similar to the case of lithium intercalation. Substoichiometric films can be made transparent by deintercalation.
In this study we investigate the structure, composition, diffusion coefficient, and electrochromic properties of nickel-vanadium oxide films as a function of deposition conditions. Polycrystalline films have been deposited by DC magnetron sputtering from a nonmagnetic target of Ni0.93V0.07 in an atmosphere of O2/Ar and Ar/O2/H2, with the gas flow ratios varied systematically to cover the range from nearly-metallic to overoxidized films. The results contradict the usual view that films deposited in O2/Ar are dark brown in their as-deposited state. While such films can easily be deposited, the optimum electrochromic properties have been observed at O2/Ar ratios giving nearly transparent films. Addition of hydrogen to the sputtering atmosphere improved cycling stability of the films. The diffusion coefficient has been determined by the Galvanostatic Intermittent Titration Technique (GITT).
Thin films of amorphous and crystalline tungsten trioxide were deposited onto glass covered with indium tin oxide, by reactive dc magnetron sputtering. The samples were intercalated with Li ions in 1M LiClO4 in propylene carbonate. The infrared reflectance was measured ex-situ at wavelengths between 2 and 50 micrometers for LixWO3 of different thickness, d, and at different Li/W ratios, x. The absorption bands (i.e. the dips in the reflectance spectra) of a thick amorphous film (d equals 1450 nm) get stronger up to the threshold value x equals 0.17. Above this value, the amplitude of the oscillations in the reflectance spectra decreases, which resembles the behavior of a thick crystalline film (d equals 1270 nm). The latter seems to have a lower threshold value, around x equals 0.08 and the spectra at large x are flatter than the ones for the amorphous film.
An optical measurement system composed of an optical microscope (Olympus BX60) and an optical multichannel analyzer (EG&G OMA 1460) has been assembled and tested. The optical microscope allows the user to make measurements on a small and well defined area of the sample. The light source, a 100 W halogen lamp, and the diode array detector, result in high sensitivity in the wavelength region of 450-750 nm. The spectral resolution of the instrument is listed as 0.59 nm/channel. The full width at half maximum (FWHM) of the strongest peaks in calibration measurements on a mercury lamp is 5 channels corresponding to 3 nm. Quasifractal clusters of gold particles have been produced with electron beam lithography. The clusters consists of different numbers of particles, giving a cluster size variation from 1.6 (mu) m to 50 (mu) m. The individual gold particles are 50 nm in diameter each. The measurement system has been used to measure both absolute transmittance and the relative transmittance using the uncoated substrate as a reference.
The temperature of a nanosatellite in orbit varies strongly as it goes into earth shadow or solar radiation. A variable emittance panel built with an electrochromic material, could improve the temperature control by providing an adaptive thermal control. The active function is due to an electrochromic layer, WO3, deposited by sputtering. Intercalation of Li+ leads to a change in the electron configuration, which modulates the radiation properties. Thin WO3 films were deposited onto Indium Tin Oxide (ITO) coated glass by reactive dc sputtering. IR measurements were done in a Perkin-Elmer 983 spectrophotometer. The IR emittance modulation, (Delta) (epsilon) of WO3 films, deposited on ITO coated glass, has been investigated. The emittance, (epsilon) , is computed from the reflectance, at 25 degrees C. Initial results show emittances in the range from 0.2 to 0.5 crystalline film has (Delta) (epsilon) equals 0.12, and the amorphous film has (Delta) (epsilon) equals 0.3. IR properties of WO3 need to be further investigated as well as the construction of a whole device.
Thermal control is an important issue in small-scale satellite design, and thin film coatings suit the limited mass and volume constraints. Group IVB transition metal nitride films meet the criteria that the satellite surface must be mechanically and chemically stable, and electrically conducting. Thin film TixAlyNz coatings have been investigated and tailored for temperature control. The films were deposited by reactive sputtering on aluminum substrates in N2/Ar-atmosphere. The solar absorptance, (alpha) , and thermal emittance, (epsilon) , were calculated from spectral reflectance measurements. It was found that an optimization of film composition leads to a reduced equilibrium temperature. The composition temperature. The composition Ti0.16Al0.41N0.43 has a flatter reflectance curve than TiN, and was found to be close to optimal. By varying the film thickness, interference effects could further reduce the equilibrium temperature. A 650 nm Ti0.16Al0.41N0.43 film showed a reflectance interference minimum positioned at the maximum of the blackbody spectrum, resulting in an increased emittance. Neglecting internal heat contributions, the lowest calculated equilibrium temperature was 34.6 degrees C for this film.
A light scattering model, based on scalar perturbation diffraction theory, has been used to derive surface roughness information from measurements on transparent thin film samples. The method utilizes the spectral behavior of the diffusely scattered reflectance (transmittance) as compared to the total reflectance (transmittance). By studying interference effects within the film, i.e. correlated and uncorrelated interface roughness contributions, it is possible to separate the origin of the scattering and extract statistical data of the boundaries. In this study, sputtered tin oxide films deposited onto glass substrate has been investigated. Optical characterization was made with a spectroscopic total integrating sphere (TIS) instrument in the wavelength range 0.4 less than lambda less than 1.0 micrometer. Surface roughness data from the light scattering model was compared with atomic force microscope (AFM) measurements through the use of power spectral density (PSD) functions. The AFM measurements made it possible to determine surface roughness scaling properties of sputtered tin oxide thin films with respect to film thickness and scan length.
This paper reports on the development of pigmented polyethylene foils for radiative cooling. The optical properties of the foils were optimized for applications in day-time radiative cooling and water condensation. We first study highly scattering foils used as convection shields. These cover foils combine a high solar reflectance and a high transmittance in the atmospheric window region in the infrared. Different pigment materials were studied and ZnS was the only one that could prevent heating of an underlying blackbody at noon, with the sun in its zenith. A 400 micrometers thick ZnS pigmented polyethylene foil with a pigment volume fraction of 0.15 was tested in Tanzania. At noon the observed temperature of the covered blackbody was only 1.5 K above the ambient. Secondly, we study the potential for condensation of water in an arid region. Pigmented foils for this purpose should combine a high solar reflectance and a high infrared emittance, in order to promote condensation by the radiative cooling effect. Titanium dioxide is a fairly good infrared emitter, but the emittance can be improved by using a mixture of TiO2 and BaSO4 pigments or only employing a composite SiO2/TiO2. Field tests with a 390 micrometers thick polyethylene foil with TiO2 and BaSO4 pigments gave encouraging results.
Thin films were deposited at an oblique angle of incidence by electron-beam evaporation as well as reactive and non-reactive sputtering. We have produced evaporated Cr films and sputtered metallic and partially oxidized films of Al, Cr and Ti. The optical transmittance of the films was measured as a function of the angle of incidence, and pronounced angular selectivity was found in many cases. The evaporated films show the highest angular selectivity in the luminous transmittance, but also some of the sputtered ones display promising optical properties. The optical properties of the sputtered films could be modeled with effective medium theories.
Selective absorbers utilize in many cases the enhanced solar absorptance of small metal particles embedded in a layered ceramic medium. A crucial factor in determining the performance of such an absorber is the degradation rate of this layer. The oxidation of ultrafine gas evaporated metal particles is examined here. Several metal develop a stable oxide film around the surface of small particles. It is shown that there is a linear dependence between the size of the metal core and the particle radius for Al, Mg, Be and Sn in room temperature and for Si at high temperatures. Charge quantization is proposed as a possible mechanism responsible for this phenomenon. Thermogravimetric measurements on Ni particles show a rapid oxidation for intermediate temperatures.
Plastic foils containing nonabsorbing pigments can display a high reflectance of solar radiation combined with a high transmittance in the atmospheric window region in the thermal infrared. Such foils can be applied as selective covers enabling radiative cooling of an underlying material during the night and avoiding heating in direct sunlight during the day. The foils could be used for condensing water or cooling food, buildings, etc. We have prepared ZnS pigmented polyethylene foils with various thicknesses and volume fractions of pigments. The optical properties of the foils were compared with theory, and good agreement was achieved for pigment volume fractions up to 0.1. The optimum solar reflectance of the foil is 0.825 for the available pigment powder; but should be 0.88 if heating were to be avoided at noon with the sun in its zenith. The cooling power for a radiator covered with the best sample is 52 Wm-2 at night, and the equilibrium temperature of a radiator is 12 K below the ambient temperature. Heating of the radiator is avoided 18 hours per day, and the radiator temperature at noon is 8 K above the ambience.
Electrochromic tungsten oxyfluoride thin films were made by reactive magnetron sputtering of W in Ar+O2+CF4. A moderate CF4 fraction led to deposition at enhanced rates and films with fairly neutral color in their dark state. Transparent vanadium oxyfluoride films were made analogously; they were annealing post-treated in Ar+O2+CF4. By fluorination it was possible to avoid the short-wavelength absorption that otherwise limits the usefulness of V2O5 in electrochromic smart windows. A transparent, adhesive, viscoelastic polymer electrolyte laminate was developed. It has good voltammetric cycling capability (106 times), good optical transmittance, and high conductivity, ranging from 10-6 S/cm (-10 degree(s)C) to 10-3 S/cm (80 degree(s)C).
We have studied the degradation of cermet selectivity solar absorbing coatings produced by magnetron sputtering. The samples consisted of a reflector layer of molybdenum, a graded Mo-Al2O3 cermet layer, and an antireflection layer of aluminum oxide. The coatings were exposed to temperatures between 673 K and 1123 K in an atmosphere where the pressure was kept below 10-4 Torr. The degradation of the solar absorptance was determined as a function of time and temperature. A remarkable stability at high temperatures in vacuum is reported. The optical properties could be interpreted within a multilayer optical model employing the Bruggeman effective medium theory for the cermet layer. It was found that the metal content of the cermet layer continuously decreased as the degradation proceeded. We assume that oxygen ions move through the amorphous aluminum oxide in microchannels and subsequently oxidize the molybdenum particles in the cermet layer. Theories for oxidation by anion movement are reviewed and used in order to interpret the experimental degradation kinetics.
Plastic foils containing nonabsorbing pigments can display a high reflectance of solar radiation combined with a high transmittance in the atmospheric window region in the thermal infrared. Such foils can be applied as selective covers enabling radiative cooling of an underlying material even in direct sunlight. Extensive calculations were performed of the optical properties of nonabsorbing foils pigmented with various oxides and sulphides. The calculations were carried out by the four flux theory using input calculations for single spheres by the Lorenz-Mie theory. The optical properties of the foils were optimized for radiative cooling applications with respect to particle radius, pigment volume fraction, thickness of the foil, and refractive index of the particles. In particular, ZnS is a suitable pigment material because of its high refractive index and low infrared absorption. It should be feasible to achieve a solar reflectance of 0.9 in combination with an infrared transmittance of 0.8 to 0.85 in the atmospheric window region by foils made of a transparent matrix material pigmented with ZnS. Initial experimental studies have been performed on pigmented polyethylene foils.
We introduce a model of the optical properties of nickel pigmented aluminium oxide selective solar absorbers that can
explain the degradation at elevated temperatures. The properties of the Ni-A1203 cermet layer are described by the Bruggeman
effective medium theory. On top of this layer we assume one homogeneous and one graded aluminium oxide layer. Our
optical model can qualitatively explain the change in opticai properties of the coatings upon heat treatments in air. The
degradation mechanism appears to be oxidation of the nickel particles throughout the cermet layer. We also propose a novel
theoretical model that can be used to account for optical degradation of solar collector coatings. The degradation is described in
terms of an activation energy and a kinetic exponent. These quantities are determined for the Ni-A1203 coatings and are
compared to data on the oxidation kinetics of nickel.
We have studied the optical properties, as well as the dielectric permittivity in the frequency range 1O-1O' Hz, for
electrochromic hydrated nickel oxide films. The films were produced by dc sputtering and were subsequently elecirochemically
treated in order to achieve different visible absorption. Dielectric and optical measurements were carried out on
clear and dark films as well as films of intermediate absorption. The dielectric spectra showed a dc conductivity at low
frequencies crossing over to a percolation behaviour at higher frequencies. For films in the clear state we also observed a
broad relaxation peak, superimposed on a background similar to the percolation behaviour. The optical spectra were
dominated by a broad absorption in the visible region, peaked at about 2.3 eV with a shoulder around 1 .5eV. We believe that
these peaks are due to charge transfer excitations. No evidence of polaron effects was found.
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