Wolfgang Graf

Mechanism of the gasochromic coloration of porous WO3 films

Films of WO3 were coated with platinum and coloured by exposure to diluted hydrogen gas. Bleaching was achieved with diluted oxygen. Because of its simple layer structure, this so-called gasochromic device is suitable for large-area window applications. By variation of several parameters, such as film thickness and gas concentration, and by considering the bleaching in vacuum and argon, we found a model for the mechanism of the reaction. It is strikingly different to the ideas of the double injection model, and there are indications that the gasochromic and electrochromic reactions are similar for porous films. According to our model, H2 is dissociated on the platinum, transferred into a pore or grain boundary of the WO3 and creates water and an oxygen vacancy. The O vacancy diffuses into the interior of the grain and the water slowly leaves the film. The bleaching in argon or vacuum is assumed to be the reverse reaction. The bleaching by oxygen is suggested to occur by by the dissociation of the O2 on the platinum, transfer into a pore and recombination with the oxygen vacancy.

The role of water in gasochromic WO3 films

Abstract Gasochromic films consist of an electrochromic layer such as WO 3 and a very thin coating of a catalyst. Hydrogen gas is dissociated on the catalyst into H atoms, which colour the electrochromic film. Bleaching is achieved with oxygen. In this paper the role of water for proton diffusion and the reaction process is discussed. Structural water is important to obtain a high proton mobility on the pore surface of the WO 3 . During coloration, oxygen vacancies and water are produced. For films with a high initial water content, the additional water produced in coloration leads to saturation of the pore surface, blocking further reactions. For films characterised by rapid proton diffusion, the water blocks the pore surface. This determines the first time constant of the coloration process. The relatively slow desorption of the water leads to a second time constant. Films with a low water content show a deep coloration, which can be explained with the framework of this model. The bleaching in Ar, (the reverse reaction of the coloration in H 2 ), is incomplete and slow. This is a consequence of the water blocking the reactive surface during the coloration process. The mechanism of the reaction is expressed in differential equations. Their solution agrees very well with the experimental data.

Development of large area sputtered solar absorber coatings

Abstract For effective solar collectors a simple black paint is inadequate. A solution is the use of solar selective coatings, which one of is known as black chromium, deposited electrochemically. The coatings we prepared were produced by reactive dc-magnetron sputtering. The coating consists of a graded mixture of chromium oxide and chromium nitride. Experimental design procedures (face centered cubic plans) were applied for the optimization of the production parameters. The maximum sample size was 0.6 m × 1.1 m in the lab and 2 m × 3 m in a following pilot production plant. The coating is deposited directly on copper sheets without the commonly used anti-corrosion nickel coating in between, on a metallic infrared reflecting coating on float glass or high temperature stable polymers. The durability of the absorbers was found to be sufficient for the application in ventilated flat plate collectors containing moisture according to the tests and requirements proposed by the Solar Heating and Cooling Program of the International Energy Agency.

Influence of incoherent superposition of light on ellipsometric coefficients

Reflections from the back surface of a transparent substrate influence the evaluation of optical constants of thin films from ellipsometric measurements. If the thickness of the substrate is large compared with the coherence length of the light, the relative phase between the p and s mode, which commonly is measured by ellipsometry, cannot be defined properly. We show how the reflections from the back surface of the substrate are taken into account in ellipsometric measurements by calculating the intensities of reflections for arbitrary angles of polarization. Applications of the new method, such as transmittance ellipsometry, ellipsometry at the back surface of the substrate, and the determination of the optical constants at the substrate-layer interface, are compared with measurements.

Optical properties of inhomogeneous media

Abstract Inhomogeneous media such as ceramic–metal composites are used in a wide range of applications as functional optical coatings e.g. in selective solar absorber coatings. Their optical properties can be calculated by the use of effective medium theories. These theories need the optical constants of the components as input parameters. Especially for the metallic component, these parameters depend strongly on the deposition conditions and on the particle sizes. The limitation of the mean free path of the conduction electrons can be explained by the well-known Drude theory. It was found that additional size effects occur due to a size- dependent modified band structure of the metal particles. The size dependence of the band structure and the resulting effects on the optical constants are discussed for the model system of sputtered small gold particles embedded in an amorphous carbon matrix produced simultaneously by hydrogenated, plasma- activated, chemical vapour deposition. Changes in the band structure were analysed by ultraviolet and X-ray photoelectron spectroscopy (UPS/XPS). The optical constants of the cermets were determined from reflectance and transmittance measurements as well as from spectral ellipsometry. Changes in the optical constants of the gold particles were found from comparison of these measurements with effective medium theories. The size of the metal particles and the surface structure of the cermets were determined from TEM images and atomic force microscopy. The size-dependent changes of the optical constants of the metal particles were correlated with the UPS spectra.

The gasochromic colouration of sputtered WO3 films with a low water content

Abstract Sputtered WO 3 films were coated with platinum and coloured by exposure to diluted hydrogen gas. Films with a high water content show a fast exponential increase of the optical density with time, whereas films with a low water content show a square root dependence, which is typical for slow diffusion. The combination of several experiments led us to a mechanism for the diffusion: the WO 3 layer consists of columns with surfaces, which are covered by water. The hydrogen gas is dissociated by the Pt, and reacts with the water to form H 3 O + and electrons, which reduce the WO 3 . The protons diffuse via the formation of H 3 O + along the water on the interior WO 3 surface.

Characterization of a-C:H/W and a-C:H/Cr solar selective absorber coatings

Tungsten and chromium containing hydrogenated amorphous carbon films were deposited in a process which combines plasma activated chemical vapor deposition of methane and r.f.- sputtering of a metallic target. The metal content of the deposits can be adjusted by the ratio of gas flow between argon and methane and was determined by X-ray Photoelectron Spectroscopy (XPS). For the a-C:H/W composites the XPS data are presented in detail and supply information about the chemical state of carbon and tungsten. The presence of W in carbidic state could be proven. Furthermore the optical constants n and k of a-C:H/W were obtained in the wavelength range between 0.4 and 2.6micrometers and for W concentrations of up to 33 at.%. The morphology of the deposits, determined by atomic force microscopy, ranges from very porous to compact and can be controlled by the substrate bias voltage. Accelerated ageing investigations were performed in air in order to characterize the deterioration mechanisms of a-C:H/W and a-C:H/Cr films deposited on different substrate materials. The significance of the morphology of the coating and the roughness of the substrate for the ageing mechanisms could be shown. Very promising results concerning the film stability were obtained for a-C:H/Cr.

The dependence of the chemical potential of WO3 films on hydrogen insertion

Knowledge of the chemical potential of electrochromic films coloured by hydrogen is important for matching the elements of an electrochromic device, for understanding the colouring mechanism and for obtaining information about the microscopic structure of the film. The dependence of the chemical potential on the hydrogen concentration was measured electrochemically for tungsten oxide films of different crystallinity and water content. A new method for determining the chemical potential by catalytic coloration by hydrogen gas is introduced. It revealed that the increase in electromotive force with increasing crystallinity is due only to different binding energies of the protons. We expect the protons to be located in the centres of hexagons, which are created by WO6 octahedra. According to our model, amorphous sputtered films show a hexagonal structure which is similar to that of evaporated films, but the hexagons are connected, leading to more hexagon centre sites, which increases the electromotive force.

Effects of roughness on TiNxOy-Cu selective absorbers

Smooth TiNxOy-Cu absorbers show low hemispherical emittancies (epsilon) < 0.05 at 250 degree(s)C and poor absorptances of < 0.08, when choosing optimal thickness and composition. Thin TiNxOy films were deposited on rough copper substrates in order to increase the absorptance. Specular and diffuse reflectance measurements of these coatings are compared with measurements on smooth films. An increase of 0.05 ... 0.1 in absorptance can be achieved. With moderate roughnesses (delta) 01 approximately equals 50 ... 60 nm, the absorptance can be increased to values above 0.85 without increasing the emittance significantly. Thin films of approximately equals 30 nm and moderate roughness (delta) 01 equals 55 nm should be favored against thick films because the emittance is lower at high temperature. A theoretical model is presented, explaining the experiments for not too rough interfaces. This model is suitable to be used for optimizations, provided the temperature dependence of optical constants is taken into account. Coatings on very rough substrates (delta) > 150 nm can achieve absorptances of more than 0.9 but accompanied with emittancies greater than 0.1. The interference peak at short wavelengths, typical for TiNxOy-Cu, cannot be suppressed completely; (alpha) is limited by this interference peak. For high temperature applications roughnesses greater than 60 to 80 nm are of no use.

Evaluation of the optical properties of molybdenum particles in metal/ceramic composites

The optical constants of thin molybdenum layers and molybdenum/tin oxide cermets, produced by a batch sputtering deposition system, were determined by spectroscopic ellipsometry. The geometrical structure of the layers and the embedded particles was analysed with an atomic force microscope. The results were used for the investigation of a suitable effective medium theory to describe the optical constants of cermets with relatively high filling factors.