7 - Lotus ceramics for counteracting urban heat island effects

Abstract

Porous mullite ceramics with unidirectionally oriented pores, designated as lotus ceramics, were prepared by an extrusion method using rayon fibers with various diameters as the pore formers to investigate their capillary rise properties, which is an important consideration when developing materials for the purpose of counteracting urban heat island effects. Rayon fibers 8.1–37.6μm in diameter and 800μm long were used as the pore formers. Those chopped fibers were kneaded with alumina powder, kaolin clay, and China earthen clay, adding 40 mass% of water to form pastes. The resulting pastes were extruded into cylindrical tubes (typically, outer diameter 50mm, inner diameter 30mm, and length 2100mm). The resulting green body was dried and fired at 1500°C for 2h. The chemical composition of the porous ceramics was set as Al2O3=57.7, SiO2=37.1, and Fe2O3=5.2 mass%, which is close to the composition of mullite. The pore size distributions showed a sharp peak arising from the burnt-out rayon fibers. The average bending strength of the tubular samples was 25.5MPa. The maximum capillary rise height obtained was 1780mm and was nearly five times higher than previously reported. Such excellent capillary rise ability was very effective at suppressing surface temperature increases from the heat of the sun and effective at counteracting urban heat island effects. We confirmed this effectiveness by developing the passive cooling wall (PCW) system using lotus ceramics.