Near infrared reflective pigments based on Bi₃YO₆ for heat insulation

Abstract

In this paper, a series of high performance near-infrared reflective pigments based on Bi₃YO₆ doped with Fe₂O₃ and Tb₄O₇ were prepared by high-temperature calcination method. The pigments were characterized by some analytic techniques, such as XRD, Particle size analyzer, SEM, EDS, UV–vis–NIR spectrophotometer, CIE L*a*b* (1976) color space and infrared thermal imaging. Experimental results showed that the Fe₂O₃ doping in Bi₃YO₆ could change the color from bright yellow to ochre red. The substitution of Tb (IV)-doped pigments changed the color to dark orange. The doping of Fe (III) and Tb (IV) contributed to enriching the color diversities of pigments. Among the synthesized pigments, Bi_3-xFe_xYO₆ and Bi₃Y_1-xTb_xO_6+δ displayed a disordered defective fluorite-type structure. The effect of doping Fe (III) and Tb (IV) reduced the near-infrared reflectance of the pigment. Excepting for Bi_3-xFe_xYO₆ (x = 0.8), the near infrared reflectance of some other pigments were higher than the China National standard (CNs). Chemical resistance tests showed that the pigments had excellent acid and alkali resistance. Finally, the application practical of traditional iron oxide yellow pigment and three prepared pigments, i.e., Bi₃YO₆, Bi_3-xFe_xYO₆ (x = 0.4) and Bi₃Y_1-xTb_xO_6+δ (x = 0.05) were studied and compared by infrared thermal imaging. The test results showed that Bi₃YO₆ still had a strong near-infrared reflection effect after being painted. Compared with traditional iron yellow pigment, Bi₃YO₆ could reduce the surface temperature of galvanized sheet by 20 °C under the illumination of infrared light. The series of Fe (III) and Tb (IV) doped pigments prepared in this paper have revealed some promising application values.

Introduction

The continuous consumption of global petroleum energy and rising energy costs have been driving the development of new technologies that increase global energy efficiency, one of these techniques was to use special near-infrared reflective pigments [[1], [2], [3]]. The solar spectrum is a continuous spectrum composed of different wavelengths, which are divided into visible light and invisible light. The invisible light can be divided into two types: infrared rays outside the red light zone and ultraviolet rays outside the purple light zone. As for the total radiation from sun, the visible light region accounts for about 50% of the total radiation energy, the infrared region accounts for about 43%, and the radiation in the ultraviolet region accounts for about 7% of the total [4]. Therefore, the infrared rays have a huge thermal effect [5,6]. The main function of near-infrared reflective pigment is to reflect the near-infrared band that is invisible to humans but has considerable energy, thereby reducing the heat accumulated on buildings, storage tanks, etc. When exposed to solar radiation, building and storage tank coated with these pigments could greatly reduce the usage of cooling system such as air conditioning and circulating cooling water, which contribute to saving energy and relevant costs [1,7]. The United States was one of the first countries to strive for energy efficiency. Shepherd reported a type of pigment that had a solar reflectance of 0.25, which was much higher than that of 0.05 of the ordinary black pigments [8,9]. Researchers at the US Solar Energy Research Institute said they had successfully developed an insulating paint. When the paint was used for building insulation, it might lead to a temperature drop of 5.5 °C to 14.5 °C. The paint could also prevent heat transfer, accordingly, 1% to 2% of heating costs in winter could be saved in terms of housing energy consumption [10].

Compared with organic pigments, inorganic pigments have revealed greater advantages in chemical corrosion resistance and weather resistance. Inorganic pigments with high near-infrared reflectivity possessed great potential to alleviate the global energy crisis [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. Until now, there have been a considerable number of inorganic pigments containing Cd, Co, Cr, Hg, Pb, Sb, Se and some other toxic elements harmful to human health and the environment, so their uses were restricted [[21], [22], [23]]. In ISO 4621:1986 and BS 318–1998, the soluble Cr in the pigment was required to be ≤ 0.02%; China's national standard GB/T20785-2006 enacted in 2006 also imposed stringent requirements on the content of soluble heavy metal elements in inorganic oxidized pigments [[24], [25], [26]]. In the recent years, rare earth based near-infrared reflective pigments have been regarded as an effective alternative to traditional toxic inorganic pigments due to their low toxicity, high near-infrared reflectivity and color diversity [[27], [28], [29]].

Up till the present moment, there has been no reports on the application of bismuth and yttrium mixed oxide in near-infrared reflective pigments. A mixed oxide of bismuth and yttrium – BiYO₃ was usually used as a photocatalyst to promote the degradation of organic matter and the reduction of carbon dioxide. The effects can be attributed to its high catalytic activity and good stability, as well as a good lattice match with the matrix [[30], [31], [32], [33]]. In this study, a series of Bi₃YO₆ pigments doped with Fe₂O₃ and Tb₄O₇ have been prepared. Even in the deep orange and red with lower L* values, they still have higher near-infrared reflectance than that of ordinary pigment. Practical applications of the novel pigments with near-infrared reflection ability and color shaping ability have revealed their promising prospects.