Characterization techniques

The normal reflectance of prepared and aged samples was measured in the wavelength interval 0.3 to 20 pm. A Perkin-Elmer Lambda 900 spectrophotometer equipped with an integrating sphere of diameter 150 mm, circular beam entrance and sample ports of 19 and 25 mm, respectively was used in the wavelength interval 0.3 to 2.5 pm. The infrared wavelength interval, 2.5 to 20 pm, was covered with a Bomen Michelson 110 FTIR spectrophotometer with an integrating sphere of diameter 4 inches (102.4 mm) and a circular beam entrance and sample port of 11/8 inches (28.8 mm). An evaporated gold mirror was used as a reference mirror for the measurements done with the infrared spectrophotometer. The measurements were combined to create one spectrum and the normal asol and stherm values were calculated using the equations below [8]. Normal solar absorptance, asol, is theoretically defined as a weighted fraction between absorbed radiation and incoming solar radiation. The solar spectrum, Isoi, used here is defined according to the ISO standard 9845-1 (1992) with an air mass of 1.5. Normal thermal emittance, stherm, is as well a weighted fraction but between emitted radiation and the Planck black body distribution, Ip, at 100°C.

4.1

J Isol (A)( 1 — R(A))dA

J03_______________________________

4.1

J Iso, W dA

0.3

20

J Ip (A)(1 — R(X))dA

_ 2.5_______________________________

20

JIp (A)dA

2.5

Results

Absorbing layer

Samples heat treated with 5, 30 and 60 0Cmin-1 up to 580°C were tested for 150 hours. Two samples of each type were subjected to the condensation test. Tests showed that the higher temperature increase rate a sample had been subjected to the better did it perform in the accelerated ageing test. Samples were tested for 150 hours and the samples made with the lowest temperature increase rate showed strong absorption in the infrared wavelength range after the condensation test and consequently failed the performance criterion. The specific bands have not been analyzed but the locations of them indicate that hydroxide compounds are involved. It is most probable that surface alumina had reacted with water and formed some sort of aluminum hydroxide or oxo-hydroxide. The reflectance curve of samples treated with a higher temperature increase rate was not as largely affected. The surface appearance became slightly rough causing the impinging light to scatter more and hence the absorption in the visible wavelength range increased. The transition from low to high reflectance was also shifted towards shorter wavelengths. Samples made with a higher temperature increase rate than 30 0min-1 all passed the PC limit of 0.05.

The absorbing base layer typically attains a normal solar absorptance of 0.80 and a normal thermal emittance of 0.03.

Figure 1a and b. Comparison of absorber samples without an AR layer, before and after 150 hours of an accelerated ageing test. (a) heated with 5 °min1 up to 580°C (b) heated with 60 min1 up to 580oC.

Anti reflection layer

A base layer made of 65 volume percent nickel, heat treated with 50 °min’1 up to 550°C, has been coated onto all samples before the AR coating was applied. The only exception is the base layer for the alumina coated sample which has a 70 % nickel base layer heat treated with 30 °min’1 up to 580°C. The parameters stherm, asoiand PC for the aged samples can be found in table 1. Two samples of each type of coating were subjected to the condensation test. The five different AR materials were: A = alumina, S = silica, HS = hybrid silica (80 mol% TEOS and 20 mol% MTES), ST73 = silica-titania (70 mol% TEOS and 30 mol% TBOT).

The reflectance curve of the alumina coated absorber, sample 1, tested for 80 hours show strong absorption bands in the infrared, see Figure 2a, and consequently the normal thermal emittance value drastically increased. Since the sample already after 80 hours of testing exceeded the limit of the performance criterion, no further testing was needed.

All other AR materials proved to be very resilient, minor or no changes at all to the optical performance were seen, even after 600 hours of testing, see the figures below and Table 1. No difference in reflectance after 300 and 600 hours of testing for the S and HS samples heated to 350°C was observed. Consequently it was concluded that it was enough to test the remaining samples for 300 hours in order to see any trend if there was one.

Samples coated with silica and heated to 350°C showed a small decrease in normal solar absorptance and a small increase in normal thermal emittance. Silica and hybrid silica coated samples heated to 550°C showed an increase in thermal emittance. In general both S and HS samples proved to be very durable and the difference in performance between samples heated to 350 or 550°C was small. However one evident trend was that hybrid silica samples were more resilient than the corresponding silica samples. Samples treated with ST73

revealed as slight increase in normal thermal emittance while the normal solar absorptance value remained constant.

The best selective properties were obtained for samples coated with alumina or silica-titania 70/30 molar %. The normal solar absorptance and the normal thermal emittance values for ST73 were typically 0.91 and 0.03. Alumina coated samples attain the same values but are not that interesting since this material did not withstand the condensation test.

Figure 2a and b. Comparison of samples before and after an accelerated ageing test. (a) coated with alumina, heated to 580°C and tested for 80 hours (b) coated with silica, heated to 350°C and tested for 600 hours.

Figure 3a and b. Comparison of samples before and after an accelerated ageing test. (a) coated with silica, heated to 550°C and tested for 300 hours (b) coated with hybrid silica, heated to 350°C and tested for 600 hours.

Figure 4a and b. Comparison of samples before and after an accelerated ageing test. (a) coated with hybrid silica, heated to 550°C and tested for 300 hours (b) coated with silica(70%)-titania(30%), heated to 500°C and tested for 300 hours.

Discussion

The durability of the absorbing base layer revealed to be very dependent upon the temperature increase rate at which the samples were treated with. It seems like the absorbing layer becomes more durable when it was treated with a high temperature increase rate. Antireflection treated samples coated with silica, hybrid silica, or silica-titania proved to be very resilient. The absolute best test results were found for samples coated with hybrid silica. Hybrid silica seems to be more flexible and less prone to crack, in accordance with the expectations, which make it an excellent protecting layer. The only two layer absorbers which did not pass the condensation test were samples coated with alumina working as the AR layer.

The solution-chemical method investigated has proved to produce coatings with good selective optical properties. The study has however shown that it seems virtually impossible to achieve a durable two layer absorber with more than 0.91 in normal solar absorptance. To be able to compete with commercially available selective absorbers which have a as0i value of about 0.95, a third layer is most probably needed.

Other important factors for the creation of a successful solar selective coating are scratch resistance and adhesion. All samples produced in this study had, after the heat-treatment, excellent adhesion properties and were reasonably tolerable towards scratching. The adhesion ability of the coating solution on the aluminum substrate is very important for the quality of the film. The coating solution will not homogenously stick to a greasy aluminum surface, instead small droplets are formed on the surface and the resulting heat-treated film will appear stained. The pretreatments of substrates in this work were fully adequate. In conclusion the process is simple, utilizes readily available chemicals and does not demand sophisticated equipment, which makes it accessible for not only the industrialized world but also developing countries.

Spin-coating processes are very easy to handle but there is one considerable disadvantage. This technique cannot handle large surfaces. Instead two other wet coating methods could be of practical interest for industrial use, spray — or dip-coating. Spray-coating techniques are quick, easily adaptable to different coating solutions, complex shapes can be coated, suitable for the establishment of an in-line process and there is a minimum of material waste. One shortcoming is that one nozzle can coat only one surface at a time. Dip-coating processes are simpler and coat two sides at the same time but they are slow and the material waste is larger. The number of advantages with a spray-coating method suggests that this is the technique to prefer when up scaling the process.

Acknowledgements

First of all I would like to thank my supervisor Dr. Eva Wackelgard, who came up with the idea that led to this study, for her invaluable support and excellent counseling.

Dr. Gunnar Westin deserves a lot of credit for his guidance and for letting me use his technique even though it has not been patented yet. Further I would like to express my gratitude to Dr. Annika Pohl and Dr. Asa Ekstrand for providing me with help and their never — ending patience. Finally I would like to thank all members of the Solid State Physics group for their backing and encouragement.

The work has been carried out under the auspices of The Energy Systems Program which is financed by the Swedish Energy Agency.

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