Preparation And Properties Of Transition Metal Ions Doped Phosphate Near-Infrared Reflective Inorganic Pigments

On the one hand,due to the high thermal radiant energy of the sun,with the gradual increase of urbanization and industrialization,the urban heat island effect is becoming more and more serious,and the cooling energy consumption is gradually increasing.On the other hand,due to the heat generation of near-infrared light in sunlight,the plastics used outdoors have serious aging problems,which limit its use period.Therefore,it is a research hotspot to synthesize color environmentally-friendly solar heat reflective materials with high near-infrared reflection performance to alleviate the above phenomenon.In this study,a series of transition metal ions doped BiPO₄ and LaPO₄ near-infrared reflective inorganic pigments were synthesized by hydrothermal method under different reaction conditions.And the phase structure,unit cell parameters,phase transition,particle size,morphology,elemental composition,interplanar spacing,elemental valence,color properties,heat stability,chemical stability,NIR reflectivity and application performance of synthetic powder pigment samples were tested,characterized and evaluated by X-ray powder diffractometer（XRD）,Fourier transform infrared spectroscopy（FTIR）,high-resolution field emission scanning electron microscope（FE-SEM）,high-resolution transmission electron microscopy（TEM/HR-TEM）,X-ray photoelectron spectroscopy analyzer（XPS）,high quality computer color difference meter（Lab）,ultraviolet-visible-near-infrared spectrophotometer（UV-vis-NIR）,thermal analyzer（TG-DTA）,homemade temperature test device and heat aging test chamber and so on.Research and analysis results show that:（1）the synthesized BiP_1-xV_xO₄（x=0.00,0.01,0.05,0.08,0.10,and 0.15）pigments are a monoclinic monazite structure having a size of about 0.25-2μm,and have excellent thermal stability and chemical stability.With the increase of progressive doping of V⁵⁺for P⁵⁺in BiPO₄,the forbidden band width of the pigments decreased from 3.657 to 3.244eV,while the color of the pigments changed from white to yellow.More importantly,the synthesized pigments have high NIR reflectivity（>72%）and NIR solar reflectance（≥75.64%）in the NIR range.Moreover,coatings pigmented with synthetic pigments have a higher NIR solar reflectance（≥78.59%）than conventional pigments of similar colors.（2）In the prepared Cr-doped BiPO₄ green solar heat-reflecting pigments,the incorporation of Cr³⁺causes the phase structure to change from low temperature monoclinic phase（LTMP）to hydrated hexagonal phase（HP）,the particle size decreased and the morphology changed from irregular prism-shaped to rod-shaped.The chromium ions in the synthesized pigments have a valence of+3 and have a high NIR reflectivity（>83%）,NIR solar reflectance（>86%）,and excellent thermal and chemical stability.The decrease in the NIR reflectance of the doped samples is caused by a decrease in the free carrier concentration.Compared with pure polymethyl methacrylate（PMMA）and the previously reported colored PMMA,the NIR reflection properties of the synthetic pigments colored PMMA are greatly improved while having good coloring properties.Moreover,the NIR solar reflectance of PMMA colored with Bi_0.85Cr_0.15PO₄ in shortwave NIR region reaches up to 77.83%,which is close to an increase of 52%compared with pure PMMA.Cr-doped BiPO₄ pigments can significantly improve the anti-aging properties of PMMA.（3）The V-doped LaPO₄ solar heat-reflecting color pigments prepared by experiments are monoclinic phase structures,but the crystallinity decreases with the increase of doping amount,indicating that the lattice defects formed gradually increase.With the increase of V⁵⁺doping amount,the morphology of the pigments gradually changed from a long rod-like structure to a short rod-like structure until the granular structure and the size of the particles decreased significantly.The absorption edge increased from 228 to 550nm,and the forbidden band width decreased from 5.42 to2.25eV,the color gradually changes from white to yellowish green to grayish yellow.Synthetic pigments have excellent NIR reflection properties,high temperature resistance and acid and alkali corrosion resistance.The NIR reflection performance gradually decreases with the increase of the V⁵⁺doping amount,which is caused by the decrease of the free carrier concentration in the crystal caused by the gradual increase of lattice defects.Moreover,the V-doped LaPO₄ pigments can greatly improve the near-infrared reflection performance of PMMA,reduce the accumulation of heat on the surface under solar radiation,lower the temperature,and improve the aging resistance of PMMA.Meanwhile,in the case where the content of the incorporated pigment is not more than 20wt%,the amount of the incorporated is increased in an appropriate amount,and the aging resistance of PMMA is increased.The pigments synthesized in this paper have the potential to be environmentally friendly,energy-saving and inexpensive"cool pigments".Among them,the synthesized Cr-doped BiPO₄ and V-doped LaPO₄ near-infrared reflective pigments are also expected to be used as plastic additives to enhance the near infrared reflection performance and anti-aging properties of plastics.Therefore,whether in the field of solar heat reflective pigments or in the field of plastic modification,it has certain research value.