Sample preparation

The absorber was produced out of two layers. The substrate was first coated with a solution containing nickel and aluminum ions [1] which after a heat treatment formed the absorbing layer. AR coatings made of varioius oxides was added on top. The substrate was optically smooth highly specularly reflecting aluminum with a surface roughness rms (root mean square) value of 0.02 pm [5]. Cutting aluminum plates into 55 x 55 mm squares produced suitably sized substrates. The substrates had to be cleaned before being coated, since the coating solution has poor adhesive abilities onto a contaminated aluminum surface.

The sol-gel route used to produce the silica and hybrid silica solutions originates from a paper by Tadanaga et al [3]. Tetraethoxysilane, TEOS, and methyltrietoxysilane, MTES, were used as starting material. If only TEOS is used the resulting film will consist of 100% silica. Mixing TEOS and MTES will generate a hybrid silica film, the higher the proportion of MTES in relation to TEOS is the more flexible will the resulting coating be. TEOS was mixed with ethanol before H2O containing 0.06 wt% HCl was added to the solution. The resulting mixture was stirred for one hour at room temperature in order to hydrolyze TEOS. A proper molar ratio of MTES was then poured into the solution. After being stirred for 24 hours in a closed container, to ensure full hydrolysis, the obtained solution was used for coating. The molar ratios of ethanol and H2O to the total alkoxide (TEOS + MTES) were 5 and 4, respectively.

The silica-titania mixtures were produced with a sol-gel technique originating from Dawnay et al [6]. TEOS, ethanol, H2O and HCl were mixed and stirred for 30 minutes. Ethanol was added to dilute the solution to a suitable concentration. Lastly acetylacetone, Acac, and tetrabutylorthotitanate, TBOT, were added and the resulting solution was stirred for 6 hours before being used. TBOT had to be added in drops in order to get a homogenous solution. The amounts of TEOS and TBOT were varied to get solutions with 70/30 and 50/50 Si/Ti molar ratios, respectively. The molar ratios of TEOS:EtOH:H2O:HCl were 1:1:2:0.1 and TBOT to Acac was 1:1.

There exist several different methods to coat a surface with a liquid medium. Some methods worth mentioning are spin-, flow-, spray — and dip-coating. The most suitable and used process for these laboratory experiments turned out to be spin-coating. Spin-coating methods utilize a spinner, where the number of revolutions per minute can be chosen. A spin coater of make Chemat Technology and model KW-4A was used. A syringe with approximately 0.35 ml of coating solution was employed to eject the liquid on top of the center of the substrate. In a fraction of a second the substrate was fully covered with coating solution and a completely homogenous and even film was acquired. Further evaporation of solvents and as a result an increase in coating stability was acquired by letting the spinning process continue for about 30 seconds after the solution was ejected. By changing the spin rate it was easy to vary the film thickness. Rates between 1200 to 6000 rpm were used. The total metal ion concentration of the solution is the second most important parameter that determines how thick the resulting film becomes. The higher the total metal ion concentration of the solution is, the thicker the film on the substrate becomes. The metal concentration can be increased through
evaporation of the solution solvents. A parameter that additionally can influence the film thickness and/or film properties is the substrate material.

After the substrate was coated it was heat treated inside a glass tube of 60 mm in diameter inserted in a split able oven of type ESTF 50/14-S and make Entech. Two parameters could be varied during the heat treatment process, the temperature increase rate and the final temperature, Ts. If the final temperature was set too low, residual organic groups would not be completely removed and a poor coating quality would be the result. The standard heat treatment for the absorbing layer was performed according to the following procedure; starting from room temperature the temperature was increased with varying speeds up to the final temperature at 550-580°C. The heat treatment for the AR layer varied in final temperature from 350 to 580°C but the temperature increase rate was always 50°/min"1.

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