Improving thermal efficiency of building is a concern when it comes to reducing the overall cooling cost. Cool roofing has been adopted to increase the solar reflectance in the visible as well as near infra-red wavelengths and thermal emittance of roofs in urban areas. This has reduced thermal radiation absorption and consequently cooling power demand. Cool roofs are also important in mitigating summer heat islands, therefore, increasing human comfort.
A valuable cool roofing solution implements functional engobes with a high albedo, and can be implemented to ceramic tiles used for roofing. However, while albedo coating is valuable, ceramic tiles may warm to environmental temperature through conduction while in contact with the air, and may transfer this heat into the building. Therefore, to address this problem, the use of porous substrate is being considered as a promising candidate. Above all, a combination with a reflective glaze will represent a promising approach for obtaining cool tiles with superior thermal management attributes.
The porous substrates need to be tailored to have low thermal conductivity and sufficient mechanical strength. Chemical composition of the glaze and substrate layers should be similar in order to match their thermal expansion coefficients.
Researchers led by professor Enrico Bernardo at the University of Padova in Italy considered glass compositions that would be suitable for the synthesis of porous and dense sintered glass-ceramics. Dense tiles can be suitable for paving while porous tiles can be good for cladding. Their work is now published in Journal of the European Ceramic Society.
The authors selected the commercial Neoparies® glass ceramics (from Nippon Electic Glass) as reference materials. They produced a glass with chemical composition mimicking that of Neoparies using raw materials and a small amount of pure chemicals. The raw materials were dried and melted in refractory crucibles. The molted glass did not corrode the crucibles, therefore, its chemical composition was unchanged. After complete melting, the authors poured the melt into water producing a glass frit. This rapid quenching produced several fragments that were then dried, ball milled and sieved to get particles sized below 90µm.
For dense samples preparation, the authors used pure frit, while for porous samples they applied foaming additives and cold pressed the mixture at a pressure of 40 MPa. The samples were then polished before testing in a bid to eliminate surface flaws.
The authors obtained dense glass ceramics through sintering a frit powder with a controlled precipitation of hardystonite and wollastonite crystals in a bid to realize a high reflective color with little absorption of the sunlight radiation. Foaming the starting frit, the authors were to obtain induced porosity that reduced thermal conductivity from 1.38-0.61W (mK)-1. This high reflectance together with low thermal conductivity indicated the developed porous glass-ceramic would be suitable for heat barrier application therefore providing a good thermal insulation for buildings.
Using engineered amounts of silicon nitride and gypsum as foaming elements would be helpful in tuning the amount as well as the distribution of porosity. The two ingredients displayed a synergistic property in defining oxidation/reduction couple and key benefit of not degrading the white color of the obtained glass-ceramic.
Glaze application was as well important in improving the material’s reflectance. It prevented dust and water incorporation, therefore, favoring longer durability. In view of the lightweight and high albedo, the porous tile could be implemented as cladding materials for enhanced building insulation. Above all, in view of their high strength, these materials could be used for paving.
M. Marangoni, B. Nait-Ali, D.S. Smith, M. Binhussain, P. Colombo, E. Bernardo. White sintered glass-ceramic tiles with improved thermal insulation properties for building applications. Journal of the European Ceramic Society, volume 37 (2017), pages 1117–1125.Go To Journal of the European Ceramic Society