Solution Combustion Synthesis And Properties Of High Near-infrared Reflective Pigments

Solar energy is a kind of renewable energy source,which brings many benefits to human beings.However,it has brought about some serious troubles.In particular,the roofs and exterior walls of buildings capture a large amount of solar radiation,and generating the temperature in the city center is 3-5°C higher than that in the suburbs and accelerating the formation of"urban heat island effect",which seriously increases the energy consumption of air conditioning.The application of high near-infrared reflective pigments on the roofs and exterior walls is one of the most effective strategies to alleviate the"urban heat island effect",but the existing near-infrared reflective pigments have poor color,low near-infrared reflectance and toxic elements.Consequently,the preparation of bright-colored and eco-friendly pigments with high near-infrared reflectance is imminent.In this thesis,Al₃Fe_5-xAl_xO₁₂（x=0,0.25,0.50,0.75,1.00）,La₂W_2-xFe_xO_9-σ（x=0,0.05,0.10,0.15,0.20）and Zn_1-xA_xWO₄（A=Co,Mn,Fe）high near-infrared reflective pigments were prepared by solution combustion method.The effects of doping ratio,sintering temperature,holding time and fuel ratio on the phase,morphology,color,particle size distributions and near-infrared reflectance were investigated.Moreover,the prepared pigments were made into heat-insulating coatings,and their thermal insulation tests and energy consumption simulations were carried out.Experimental results show that:（1）The Al₃Fe_5-xAl_xO₁₂（x=0,0.25,0.50,0.75,1.00）pigments with rice-like 200 nm particles not only have stable garnet-type structure,but also possess excellent high temperature resistance and acid/alkali resistance（△E*<5）.The Al₃Fe₄Al O₁₂ pigment maintains a bright color（L*=78.72,a*=-4.05,b*=40.04）and high near-infrared reflectance of 87.66%at the same time.By enhancing the calcination temperature and extending calcination time,the near-infrared reflectance was improved,but the color became darker.After the Al₃Fe₄Al O₁₂ pigment was prepared into a coating and placed under an infrared light for 5 minutes,the surface temperature was 7.6°C lower than that of a commercial pigmented coating with similar color,and the result demonstrated that the thermal insulation effect was extremely obvious.（2）As the doping ratio of Fe in La₂W_2-xFe_xO_9-σpigment increased,the transition fromαphase toβphase was occurred,and the band gap of La₂W_1.85Fe_0.15O_9-σand La₂W_1.8Fe_0.2O_9-σpigment was reduced from the undoped 3.71 e V to 3.07 e V and 2.96 e V,respectively.The representative La₂W_1.85Fe_0.15O_9-σpigment has the highest near-infrared reflectance of89.25%,excellent color（L*=74.20,a*=22.69,b*=22.42）and gorgeous chemical stability for its use in architectural coatings.The energy consumption simulation results show that applying this kind of heat-insulating coating on the roof and exterior walls of the 223.17 m² building can save 7.32 dollars per month,and the energy-saving effect is prospective.（3）All the prepared Zn_1-xA_xWO₄（A=Co,Mn,Fe）pigments with blue,yellow,brown and pale green colors have high near-infrared reflectance above 85%.The initial wolframite structure of the Zn WO₄ matrix has not changed after the incorporation of transition metals.The absorption of Zn WO₄ in the UV region is mainly due to a ligand-to-metal charge transfer from O 2p nonbonding orbits to W 5d antibonding orbits.By incorporating Co²⁺（3d⁷）,Mn²⁺（3d⁵）,Fe³⁺（3d⁵）transition metal ions to replace Zn²⁺,d-d transitions and metal-to-metal charge transfer from the occupied 3d orbits of A²⁺to unoccupied W 5d orbits will occur and generate bright colors.Significantly,these pigments have excellent thermal stability and chemical stability（△E*<5）,which are conducive to complex application environment.In summary,these high near-infrared reflective pigments have excellent heat-insulating properties,and own great research value and significance in the field of high near-infrared reflectance pigments used in heat-insulating coatings.