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.
When a beam of light is incident on a translucent sample, a significant fraction of the light is scattered at high angles. Some of this scattered light may be trapped inside the substrate through multiple reflections and total internal reflection, similar to light coupling into an optical fiber. The trapping depends on factors such as the surface roughness of the external surfaces and/or the size and distribution of scattering particles inside the sample. The scattered light may thus escape out of the sample at a shifted position relative to the incident beam. This leads to port losses in an integrating sphere. The detected signal from the light entering the sphere then underestimates the hemispherical transmittance. In this paper the signal versus lateral position has been measured in an attempt to estimate the error and to find an extrapolation procedure for the correct transmittance value. The lateral measurements were carried out by moving a detector behind the sample, a procedure carried out at several angles of incidence. Different illumination methods have also been studied both theoretically and experimentally to further investigate what effect light trapping can have when characterising scattering samples.
The angular dependent optical properties of low-e and solar control glazings have been investigated in a European project, ADOPT, within the Standards Measurements and Testing programme. Two new predictive algorithms have been proposed and tested. One algorithm is based on an empirical formula and the other is based on a hybrid equivalent model, which approximates the actual coated glass. The validation procedure has included measurements as well as Fresnel calculations. The algorithms allow the user to calculate the solar heat gain coefficient versus angle of incidence for any transparent window combination without having to perform time-consuming measurements. For the tested window configurations the precision is mostly within one percentage point as compared to measured results.
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.
Antireflection (AR) coatings can be incorporated into highly transmitting glazings that, depending upon their cost, performance, and durability of optical properties, can be economically viable in solar collectors, agricultural greenhouses, and PV systems. A number of AR-coated glazings have been prepared under the auspices of the International Energy Agency Working Group on Durability of Materials for Solar Thermal Collectors. The AR coatings are of two types, including (1) various sol-gels applied to glass and (2) an embossed treatment of sheet acrylic. Typically, for unweathered glazings, a 4 - 5% increase in solar-weighted transmittance has been achieved. For AR-coated glass, reflectance values as low as 0.5% - 0.7% at selected wavelengths (680 - 720 nm) were obtained. To determine the durability of the hemispherical transmittance, several collaborating countries are testing these materials both outdoors and in accelerated weathering chambers. All materials exposed outdoors are affixed to mini-collector boxes to simulate flat-plate collector conditions. Results for candidate AR coatings weathered at geographically disperse outdoor test sites exhibit changes in spectral transmittance primarily in the high visible range (600 - 700 nm). Accelerated testing at measured levels of simulated solar irradiance, and at different constant levels of temperature and relative humidity have been performed in different countries. Parallel testing with different levels of laboratory-controlled relevant stress factors permits the time-dependent performance of these materials to be compared with measured results from in-service outdoor exposure conditions. Coating adhesion and performance loss resulting from dirt and dust retention are also discussed.
Commercially available anodized aluminum, vacuum evaporated aluminum foil, and roofing aluminum have been optically characterized in the wavelength range 300 - 2500 nm in a single beam Edwards sphere at oblique angle of incidence (60 degrees). The first two materials are long-term stable, which makes them promising to use as external reflectors. The third one is of interest because of its low cost. A model is proposed for the correction factors for the single beam sphere. A comparison between the single beam sphere and a Beckman 5240 standard spectrophotometer equipped with an integrating sphere was made. The agreement between the measurements was excellent. Furthermore, angle resolved measurements were made with a scatterometer at 633 nm at the incidence angle 60 degrees for all three samples. These data were used in a simulation model to calculate the intensity in the collector plane, assuming a semi-infinite collector and dito external reflector. A calculation of a two-dimensional ideal Lambertian sample was also made. It is shown that optically dull materials, such as roofing aluminum, which has rolling grooves that scatter light, perform equally well as optically smooth materials, provided the scatter is confined to low angles. It is also concluded that integrating sphere measurements of the specular component of diffuse sample in standard spectrophotometers are misleading. The low-angle scattering of diffuse samples also contributes to the intensity in the collector plane and should therefore also be included in the measurement. For Lambertian samples the scatter is too high and such a surface is not a feasible choice of material.
The three transition metal nitrides TiN, ZrN and HfN have remarkably high stability due to their bonding: a mixture of covalent and ionic contributions. The optical properties of these nitride compounds are free-electron like to a surprisingly large extent, in particular in comparison with the corresponding carbides. It is argued that the interband optical excitations of the d-electrons are restricted by selection rules, resulting in a Drude like behavior of these d-electron compounds. Hitherto, one of the main optical applications has been as opaque, wear-resistant replacements for gold-coatings. This review includes the efforts to study, understand and enhance the optical selectivity of group IVB transition metal nitrides as the selective surface in high temperature thermal solar absorbers, as the metal layer in LE-coatings on energy-efficient or solar control windows and as a Langmuir probe coating. The reflectance edge is not in the optimum position for absorber applications and attempts to shift it with alloying were unsuccessful. Recent development of inhomogeneous, cermet- type nitrides hold some promise. The LE-coatings will not reach as high selectivity as the current noble metal based multilayers, but many find use in aggressive environments because of their excellent stability.
Spectral diffuse reflectance and transmittance measurements have been performed on single films of tin oxide on Corning glass substrates. The measurements have been performed on a spectrophotometer equipped with an integrating sphere in the wavelength range 0.3 - 1.0 micrometers . The front surfaces of the films have been characterized by means of spectroscopic light scattering and optical profiler measurements after the deposition of a thin aluminum film. These measurements have given different results, which can be explained by the different bandwidths of the methods. The front surfaces have thus been found to have a short correlation length, indicating roughness with short spatial wavelengths. The front surface data obtained from the light scattering measurements have been used in a model to characterized both film interfaces. The calculated diffuse spectra agreed with experimental spectra both for reflectance and transmittance. The difference between the highest and the lowest scattering levels was more than an order of magnitude. The model calculations made it possible to establish an upper limit for the substrate-film interface.
The Drude-like behavior of the group IVB metal nitrides: TiN, ZrN and HfN furnishes the physical basis for the use of these hard, inert materials as replacement for noble metals in optically selective multilayers. A low value of the refractive index, n, in the visible region and rapidly increasing extinction coefficient, k, when the wavelength increases into the infrared, is characteristic for these nitrides, although to a lesser extent than for the noble metals. A screened Drude model can be fitted to the experimental dielectric function over the near infrared and at least part of the visible spectrum to determine the parameters: plasma resonance energy hvp and relaxation time (tau) . Systematic studies of TiN and ZrN films show that n increases with decreasing film thickness below 60 nm when the film transmits. This increase can be modelled with a increasing Drude parameter (tau) and has previously been explained as an extrinsic effect from defects etc. It is argued that most of this change can be understood as an effect of diffuse scattering against the back surface of the film and is therefore not cured by improvements in deposition technology.
Optical properties of some aerogel samples were investigated. The samples were provided by Japan, Norway and Sweden as part of a collaborative program of research on behalf of the International Energy Agency. For each sample the total near-normal hemispherical spectral transmittance and reflectance as well as the diffuse near-normal hemispherical spectral transmittance and reflectance were measured for the solar spectral range using a Beckman integrating sphere. The ratio of diffuse to total transmittance was calculated at each wavelength. Solar optical parameters as well as chromaticity coordinates were determined for both the total and the diffuse components. The influence of bulk and surface scattering is discussed and the effect of sample thickness and incident beam size in relation to sample area in respect of measured optical properties was also investigated and significant differences were observed which are relevant for good practice aspects of the measurement technique. No significant specular reflectance component was detected for any sample. The investigated samples vary considerably in density, but it was found that the visible scattering was nearly the same for all samples. Much stronger absorption is evident in the near infrared spectral range for the Norwegian sample and this is attributed to enhanced water content and higher density.
Optical characterization of windows with advanced glazing materials is discussed. Measurements at oblique incidence introduce errors, which make it difficult to properly estimate the solar energy performance of a window unit with several panes. The influence of experimental errors of the individual glazing products on the total performance of a window is discussed. A new formula for the evaluation of the annual energy balance of a window is presented. The total heat flow through the window during the heating season, or the total cooling load during the summer can be estimated with this formula. The method is useful for the comparison of different window configurations and for the evaluation of their annual energy performance.
Opaque and semi-transmitting film of ZrN have been reactively magnetron sputtered and their optical performance for solar control coating applications evaluated. The optical constants of the opaque films were determined from near normal reflectance measurements and Kramers- Kronig calculations over the interval 0.23 - 3.0 micrometers . The thin film optical constants were determined with combined R- and T-measurements. Systematic variations with growth conditions were observed. Thin films were observed to have higher n-values in the visible, a change that could be partly compensated by substrate heating. For the triple layers: glass / ZrO2 / ZrN / Zro2 it was noted that the crystallinity of the first oxide did not influence the refractive index of this oxide, but improved significantly the selectivity of the nitride growing upon it. Triple layers for solar control applications with 57% luminous transmittance and close to 1/3 each of solar transmittance, reflectance and absorption have been realized.
Triple layer structures of TiO2/TiN/TiO2 and quadruple layer structures of TiO2/Al/TiN/TiO2 have been sputtered on glass substrates at temperatures ranging from room temperature to 300 degree(s)C. The reflectance and transmittance were measured in the visible and the near infrared wavelength regions. Accelerated degradation tests with respect to high temperature and acid exposure have been performed with these laboratory samples of low-e coatings and the degradation has been compared with that of commercial silver based window coatings. As expected the durability of the nitride based coatings is far superior to the stability of those based on noble metals. Furthermore, the nitride coatings with an aluminum sacrificial layer have been found to resist aging at elevated temperatures (as high as 350 - 400 degree(s)C) far better than similar coatings without the aluminum. It has also been shown that the aluminum layer protects the nitride film during deposition of the top oxide layer. The effects of high temperature annealing have been modeled with optical multilayer calculations. Comparison of two degradation mechanisms demonstrates that the silver layers fail by agglomeration while the nitride suffers successive oxidation. This explains the effectiveness of the aluminum layer which forms a dense oxide during the initial stages of TiO2- deposition.
Several new materials are available for glazing applications, many of which require careful optical characterization, especially with regards to light scattering. Measuring scattering requires special equipment and is inherently difficult. An integrating sphere can be used for the total and diffuse components but great care must be taken in interpreting the instrument readings. Angular resolved scattering measurements are necessary for a complete characterization, and this is difficult for low levels of scattering. In this paper, measurements on electrically switchable NCAP materials and thick panes of aerogel are reported. The NCAP films switch reversibly from a translucent, scattering state to a transparent, clear state with the application of an ac-voltage. Airglass has a porous SiO2 structure with a refractive index n equals 1.04 and a very low heat transfer coefficient. Integrated scattering measurements were performed in the wavelength range 300 to 2500 nm on a Beckman 5240 spectrophotometer equipped with a 198851 integrating sphere. In this instrument we can measure the total and diffuse components of the reflectance or transmittance separately. The angular distribution of the scattered light was measured in a scatterometer, which can perform scattering measurements in the wavelength range 400-1100 nm in both transmittance and reflectance mode with variable angle of incidence.
There are several different kinds of coatings, both active and passive, for the control of the radiation flux through a window system. These entail coatings for the reduction of thermal heat losses in cold climates, and coatings for the reduction of not only the thermal part of the solar spectrum but also the visible part. Considerable effort is devoted to the study of optically switching films, for which the optical properties depend on certain conditions. When these coatings and films are to be used in the windows of buildings, scattering is usually unwanted, while in some applications strong scattering is a characteristic feature of the film. In either case it is important to be able to measure the scattered transmittance accurately. In this paper it is demonstrated that large errors can occur when diffuse transmittance is measured with an ordinary integrating sphere. A model is presented for the calculation of the true transmittance value from the measured signal. The separation into the diffuse and specular components is always instrument dependent. These components can vary considerably depending on the size and geometry of the sphere ports and on how the measurement is performed. By using two different modes of operation this difficulty can be reduced.
The chemical and mechanical stability of tin oxide coated aluminium reflectors have been investigated.
The tin oxide has been applied with the pyrolytic spray technique which results in a hard crystalline
coating. Tin oxide is a wide bandgap semiconductor and is transparent in the visible part of the
spectrum. The visual appearance of tin oxide coated aluminium is therefore similar to the uncoated
metal. Interference effects can give a slightly coloured appearance, and owing to the higher refractive
index and extinction coefficient of the tin oxide the reflectance is lower than for an anodized aluminium
surface. The tin oxide was applied both on electropolished aluminium and on evaporated aluminium
films on glass. In both cases an improved resistance both to alcaline and acid solutions was noted. It
was found that the chemical stability was better than for an anodized surface. The mechanical stability of
the evaporated film was considerably improved. The electropolished samples were also characterized
with respect to film hardness and resistance to erosive and abrasive wear. The hardness of the tin oxide
was found to be higher than that of the anodized layer while the opposite relation applied for the erosive
wear resistance. The abrasive wear resistance was about equal for tin oxide coated and anodized