Preparation And Performance Of Solar-reflectance Ceramics

In the areas that are characteristic of hot summer and warm winter,it is important to reduce the sunlight absorption of the buildings and savethe energy consumption of indoor refrigerations.Covering high-reflectance coatings on exterior walls and roofs is one of the most effective methods to deal with the building energy efficiency issue.However,organic membrane-forming materials are inlack of durability and not resistant to dirt.In order to maintain beautiful surface and keep the high-reflectance performance,such walls and roofs have to be repainted after a few years.Ceramic exterior wall tiles are widely used in our country,and they have obvious advantages in weather resistance and mechanical strength.Therefore,it is of great importance to study the characteristics and mechanism of reflectivity performance and developethe ceramic tilesfor the buildingenergy-saving in thehot summer and warm winter areas.Based on the preivous research,the author studied the influence of chemical composition on solar reflectance by orthogonal experiment,and found that the solar reflectance of ceramic could be improved by generating titanite crystal in glaze.By developing thermodynamic calculations and experiments,the most possible way to generate titanite was found.First,CaCO₃ was decomposed.Then CaTiSiO₅ was synthesized by the Ca O,TiO₂ and SiO₂.Besides,small amount of CaTiO₃ was formed from CaCO₃&TiO₂,and then synthesized CaTiSiO₅ with SiO₂.In order to improve the homogeneity of the crystal in glaze,the raw materials were melted and became the frit glaze.The frit glaze could directly generate titanite crystalfrom glass phase at 820°C.According to the calculation based on Kissinger model,the crystallization activation energy of titanite in frit glaze was Ea=450.92 kJ/mol.Moreover,the Avrami index of titanite in frit glaze was 2.11-3.06,which meant the crystallization of titanite was three-dimensional diffusion-controlled growth.The reflectance of frit glaze was higher than that of raw glaze because the titanite crystal in frit glaze was better-distributed and it covered much more areas of the glaze.Crystal content,glaze layer thickness,and sintering process are main factorsto influence reflecting property of glaze.In this research,the influence of TiO₂ content on titanite content in glaze was investigated.Results showed that only titanite phase appeared only when the mole weight of TiO₂ was lower than that of CaO;otherwise,titanite phase and rutile phase could be detected.The reflectance value accorded with the standard of high reflectance materials when the raw materials contained 10.02 wt%TiO₂.By controlling the solid content of the slurry and the dipping time,we could obtain different thickness of glaze layer.Reflectance increased significantly with the layer thickness increasing when the layer was not thick enough,and reflectance would not increase a lot if the layer thickness was above 300?m.Furthermore,results showed that the glaze which was fired at 1150°C reflected more light than samples fired at other temperatures.The crystal would be fused in glaze if the soakingperiod was too long.When ZnO,BaO,&MgO was respectively added into the frit glaze,the crystallization behavior of titanium-glaze changed dramatically,and the reflectance of glaze enhanced significantly.Added ZnO stopped the crystallization of titanite and promoted the crystallization of rutile.The refract index of rutile is higher than titanite.As a result,the reflectance of glaze was improved.Added BaO inhibited crystal growth in glaze,which was also a good way to improve the reflecting property.The size of titanite crystal in such glaze was around 200-300 nm when 2 wt%BaO was added.The reflectance was improved because of the high coverage of the crystal.The influence of added MgO on crystallization behavior of glaze was similar to the influence of added Zn O.However,the transition of titanite to rutile caused by MgO was less intense.Consequently,the ratio between titanite and rutile was easier to be controlled by added suitable amount of MgO.Thus,a higher visible reflectance and near-infrared reflectance glaze could be prepared.The experimental results showed that the glaze with added MgO performed better in reflecting property than the glaze with added ZnO/BaO.The highest reflectance of the glaze added with MgO was 97.21%.By adding iron-zircon red pigment and zirconia-praseodymium yellow pigment into the glaze,we found that the reflectance of glaze performed better when the pigment contained high raito of high-refract-index crystal.The titanite pigments with different colors could be synthesized by doping with some subgroup elements.The glaze which was added with such pigments could meet standard requirements of colorized heat-reflecting materials.It was found that covering a colorized zirconia glaze layer or a colorized transparent glaze layer on the titanium glaze layer was a feasible method to develop high-reflectance and aesthetic ceramic tiles.