Michael G. Hutchins

Angular-dependent optical properties of low-e and solar control windows—: Simulations versus measurements

Abstract 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. The object of the project has been to identify reliable ways of predicting the angular dependency without having to perform measurements or detailed calculations. Two new predictive algorithms have been developed and validated. For the investigated coatings the accuracy of these predictive algorithms is mostly within 1% of the value obtained by measurements or Fresnel calculations. This is an improvement over previously used algorithms, which have failed to distinguish between different types of coatings. The improved accuracy is of importance in energy simulation of buildings and makes improved product specification possible.

Thin film angular selective glazing

Angular selective glazing based on special obliquely deposited thin films, has considerable potential for optimizing the overall performance of a window in temperate to hot climates. In contrast to most currently used glazings for these environments it has the ability to control solar heat gain and glare while providing a clear view through the window and considerable but not excessive light gain. Various deposition techniques and film materials will be discussed and the spectral and integrated optical performance of single and double-layered systems presented for various directions of incidence. Full optical characterisation requires measurements for incidence directions over half a hemisphere. Some of this is presented but most data is confined to planes of incidence where angular selectivity is greatest as determined by deposition geometry. Two layer systems are designed to provide a low emittance and better solar control than the single layer systems while retaining good visible angular selectivity. The single layers are primarily to provide angular selective control of the visual radiation since they have moderately high transmittance in the near infrared and a high emittance. Theoretical models used to extract structural information and model the complex optical properties are introduced and their predictions assessed. Preliminary results on simulation of the performance of full size angular selective windows in comparison to other types of solar control glazing such as low e heavily tinted windows are discussed in terms of their relative impact on cooling load, lighting contribution, glare control and total energy savings.

The electrochromic properties of hydrated nickel oxide films formed by colloidal and anodic deposition

Abstract Hydrated nickel oxide NiOxHy films were deposited onto indium tin oxide (ITO) coated glass by two methods (i) colloidal precipitation and (ii) anodic electrodeposition. The electrochromic properties of hydrated nickel oxide films were studied by transmittance measurements (UV/VIS/NIR), and Fourier transform infrared reflectance spectroscopy as a function of the key deposition parameters. The solar transmittance was calculated for films switched in both bleached and coloured states. The best results were achieved for films produced by anodic electrodeposition from stable solutions with solar transmittance Ts(bleached) = 0.82 and Ts(coloured) = 0.22. Corresponding optimum values for the films produced by colloidal precipitation were solar transmittance Ts(bleached = 0.82 and Ts(coloured) = 0.47. Fourier transform spectrophotometry was used for elucidating changes in hydration, hydroxylation and for the characterization of structural characteristics in the bleached and coloured states. It was found that free OH stretching vibration at 3647 cm−1 corresponds to Ni(OH)2 for both anodic and colloidal deposited films in the reduced (bleached) state. In the oxidised state hydrogen bonded OH at 3360 cm−1 is observed.

Electrical conduction mechanisms in thermally evaporated tungsten trioxide (WO3) thin films

Thin films of amorphous tungsten trioxide, a-WO3, have been thermally evaporated onto glass substrate held at 350K. Annealing at 723K caused the formation of polycrystalline tungsten trioxide, c-WO3, with a monoclinic structure. The dark DC electrical conductivity of both a-WO 3 and c-WO3 was studied over a temperature range from 298 to 625K in two environmental conditions (air and vacuum). A simple Arrhenius law, a polaron model and a variable range hopping model have been used to explain the conduction mechanism for a-WO3 films. Using the variable range hopping model, the density of localized states at the Fermi level, N(EF), was found to be 1.08 × 1019eV -1cm-3. The mechanism of electrical conduction in c-WO3 films is explained by means of the Seto model. The Seto model parameters were determined as the energy barrier (Eb ≤ 0.15eV), the energy of trapping states with respect to the Fermi level (Et ≤ 0.9eV) and the impurity concentration (ND ≤ 4.05 × 1015eV-1cm-3). The thickness dependence of resistivity of c-WO3 films has been found to decrease markedly with increasing film thickness, which is explained on the basis of the effective mean free path model. Using this model, the mean free path of electrons in c-WO 3 films was evaluated. The temperature dependence of the thermoelectric power for a-WO3 films reveals that our samples are n-type semiconductors.

Infrared reflectance modulation in tungsten oxide based electrochromic devices

Abstract Tungsten oxide thin films prepared by reactive magnetron sputtering are investigated for their use in infrared reflectance modulation electrochromic devices. The temperature dependence of the charge capacity and associated reflectance modulation are studied for W oxide films deposited in the temperature range 100–350°C. Films were cycled in either 1 M lithium triflate in propylene carbonate or 1 M H 2 SO 4 electrolytes. The charge capacity of films of thickness ∼200 nm is observed to decrease with increasing deposition temperature from ∼50 to less than 25 mC cm −2 for films prepared at the highest temperatures. Infrared spectral optical properties were measured and the optical constants, n λ and k λ , then derived using an optical model. For all films prepared at 200°C and above the optical constants are increased by ion insertion. The dependence of n λ and k λ on the quantity of charge inserted for films prepared at different substrate temperatures is established. The data enabled the design of variable reflectance devices utilising either the charge dependence of n λ or of k λ . Examples of such devices prepared experimentally are presented.

Measurement and prediction of angle-dependent optical properties of coated glass products: results of an inter-laboratory comparison of spectral transmittance and reflectance

The European Union ADOPT project has been concerned with the accurate determination of the optical properties of coated glass products, describing reliable procedures for angle-dependent measurements and the development of predictive algorithms to enable accurate angle-dependent performance characterisation based upon near-normal values only. An essential element of the ADOPT project was the inter-laboratory comparison of transmittance and reflectance measurements for commercially available glazings. Thirteen European and three USA laboratories participated. The selected samples represent major categories of coated glass products used in buildings. Each participant received a set of samples and measured the angle-dependent spectral transmittance and reflectance under appropriate polarisation conditions for incident wavelengths within the solar spectral distribution. Calibrated mirrors were also provided for reference purposes. This paper presents the results of the inter-comparison which show that near-normal solar, visible and spectral transmittance and reflectance can be measured with an uncertainty of better than ±0.02. The inter-comparison illustrates the difficulties still experienced in making accurate oblique incidence measurements. The predictive algorithms, which require near-normal input data only, are capable of predicting the angle-dependent transmittance, reflectance and total solar energy transmittance for double and triple glazed windows to an accuracy which enables reliable calculation of the annual solar heat gain through windows.

FTIR spectroscopic investigations and the thermal stability of thickness sensitive spectrally selective (TSSS) paint coatings

Abstract Various black paints suitable for coil coating applications with different PVC ratios (27%–39%) were made from phenoxy resin and FeMnCuOx pigment. The influence of the paint formulation on the spectral selectivity, surface properties and thermal response was established. Optimisation ( a s > 0.92, e T ) of the TSSS paint coating was obtained by addition of fumed silica and a silane coupling agent at a PVC ratio of about 35%. Thermal degradation of the paint coating was investigated by measuring the infrared spectra of coatings during a 60 hour heat treatment. Comparison of the corresponding FTIR spectra revealed that oxidazation of the phenoxy resin took place. The paint is stable up to 135°C, while at 220°C outgassing is already beginning.

Preparation and Properties of Electrochemically Deposited Tungsten Oxide Films

Tungsten oxide films of thicknesses 1095 nm have been cathodically deposited onto ITO coated glass substrates kept at room temperature, at a rate of 13 nm/min. The electrolyte solution has been prepared by adding 0.025 M tungsten (dissolved in 30% H 2 O 2 ) to 0.25 M H 2 SO 4 . XRD observations showed that all films are amorphous. The film resistivity at room temperature was found to be 9 × 10 6 Ω cm, and from the resistivity-temperature dependence, the thermal activation energy was obtained as 0.29 eV. The refractive index, n, and the extinction coefficient, k, have been computed from the corrected transmittance and reflectance over the spectral range 400 to 800 nm. Absorption versus photon energy curves give an indirect transition with optical energy gap of 3.25 and 3.36 eV for the as-deposited and coloured states, respectively. The electrochromic properties of the prepared films have been investigated in situ and both solar transmittance and optical density have been evaluated in the coloured and bleached states. The electrochemical coloration and bleaching were performed in an electrolyte of 0.4 M H 2 SO 4 solution. The coloration efficiency has been evaluated as 50 cm 2 /C at λ = 600 nm, which is comparable with the results obtained from films prepared by other methods.

A comparison of the electrochemical properties of lithium intercalated amorphous and crystalline tungsten oxide

This work investigated the effect of cycling tungsten oxide at low potentials (below 2 V versus Li/Li+). Crystalline and amorphous tungsten oxide thin films were deposited by reactive sputtering in an Ar–O2 plasma onto various substrates (ITO glass, K-glass, glassy carbon, stainless steel). Crystalline films were obtained by heating the samples at a temperature above 400°C during the deposition or by annealing the samples at 400°C after deposition. The electrochemical behaviour of the films was studied using slow scan cyclic voltammetry and ac impedance spectroscopy. The electrolyte used was a 0.5 M solution of lithium triflate in PC. Normal conditions for the electrochemical study of tungsten oxide films were defined by a potential range of about 2 V versus Li/Li+ to 4.5 V versus Li/Li+. The samples were taken to lower potentials (down to 1.6 V versus Li/Li+). Amorphous tungsten oxide electrodes showed a loss of current and charge during cycling down to 1.6 V versus Li/Li+. Crystalline samples showed a good reproducibility during cycling. No evidence of a damage at the surface of the electrode was shown by the scanning electron microscopy. Spectrophotometric titration of the electrolyte showed no evidence of dissolution of tungsten. Impedance spectroscopy of the amorphous samples showed an increase of the interfacial resistance when the cell was left at 1.6 V versus Li/Li+. This was consistent with a loss of adhesion at the interface between the tungsten oxide and the underlying conductive layer.

Optimisation of Preparative and Performance Parameters on Electrochromic Properties of Electrochemically Deposited Tungsten Oxide Films

Tungsten oxide films of 240–1080 nm thickness were deposited on indium tin oxide (ITO) coated glass substrates using an electrochemical deposition technique. All films were amorphous, as proved by X-ray diffraction (XRD), and had an electrical resistivity of 106 Ωcm and spectral transmittance exceeding 75% in the visible region. The electrochromic (EC) properties were measured in situ during colouration and bleaching cycles. The EC parameters, Tsolb, Tsolc, ΔTsol and Δ(OD)sol and the solar colouration efficiency ηsol were evaluated at different preparation and performance parameters. The results showed that at small film thickness, the solar colouration efficiency changes linearly and tends toward saturation at larger thickness. At colouration potentials ≥2 V, the solar colouration efficiency is almost constant whereas the active sites are transformed to colour centres. In contrast, the efficiency has an exponential dependence on electrolyte concentration. The optimum values are: film thickness = 1080 nm, colouration potential - 2 V and electrolyte concentration = 0.4 M. The corresponding EC parameters are: ΔTsol =0.458, Δ(OD)sol=0.632 and ηsol=34 cm2/C.