Sol-gel and architecture I: anti-reflective coatings

The use of light building envelopes, made of materials such as steel and glass, has progressively substituted the traditional concrete and brick structures. This change poses new design challenges in order to control parameters such as the solar factor, the thermal transmittance, visual transmittance, reflection index, among others [1]. The thermal and energetic efficiency of these enclosures, depicted by the 2002/91/CE EU Directive [2], require new large-scale solutions that prevent thermal losses.

One of the most promising lines is the deposition of thin films into architectural glasses. The presence of these coatings in glass industry is constantly increasing, for example, almost the 70% of the glass globally produced in 2000, included some kind of coating (anti-glare, solar control, decorative…) [3].

It has been proved that the application of spectrally selective coatings to glass building envelopes leads to large energy savings. A coating that reflects a high percentage of the infrared radiation while allowing visible light to pass through reduces considerably the air-conditioning consumption. Soutar and Nee reported in 2002 [4] a multi-layer sol-gel coating that selectively reflect radiation in the near infrared region while allowing high visible light transmission:

 

transmisión

Silver based low-E coating vs five layer sol-gel spectrally selective coating [4].

 

In order to minimize the reflectance, the thickness and refractive index of the coating must be carefully controlled. Reflectance can be reduced to almost zero if the length of the optical path in the layer is equal to one half of the light wavelength:

λ= 4nchc

where λ0 is the interference wavelength nc is the coating refractive index and hc is the coating thickness. It must be also satisfied that the amplitudes of reflected light from the coating and the medium-substrate are equal:

nc=(nons)1/2

where nc, no and ns are refractive indexes of the coating, incident medium and substrate, respectively. The refractive index is related to its density, an can be therefore reduced by increasing its porosity. This relationship between refractive index and porosity is described by:

npc=[(1−P/100)(ndc2−1)+1]1/2

where npc, ndc and P are the refractive indexes of porous and densified coatings, and the porosity percentage [5].

[1] Cannavale, A., Fiorito, F., Manca, M., Tortorici, G., Cingolani, R., Gigli, G., Multifunctional bioinspired sol-gel coatings for architectural glasses, Building and Environment, Volume 45, Issue 5, 2010, Pages 1233-1243

[2] Directive 2002/91/CE of the European Parliament and of the Council of 16 December 2002 on the energy performance of buildings, 2002.

[3] L. Martinu, D. Poitras, Plasma deposition of optical films and coatings: a review J Vac Sci Technol A, 18 (2000), pp. 2619-2645

[4] A. Soutar, T.S. Nee: Sol-gel spectrally selective coatings, SIMTech technical report, Singapore Institute of Manufacturing Technology (2002)

[5]  Prado, R., Beobide, G., Marcaide, A., Goikoetxea, J., Aranzabe, A., Development of multifunctional sol–gel coatings: Anti-reflection coatings with enhanced self-cleaning capacity, Solar Energy Materials and Solar Cells, Volume 94, Issue 6, 2010, Pages 1081-1088,

© Nadetech Innovations 2017

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