Research On Preparation And Infrared Properties Of Low Lightness And Multicolor Aluminum Pigments

Flaky aluminum pigments?Al flakes?have unique effects,like“color flop”and low infrared emissivity,can be used in low infrared emissivity coatings in the filed of energy saving and military camouflage.However,the coatings containing Al flakes have high lightness which leading to produce light pollution,and reduce the performance of military camouflage in the visible region.If color pigments are added in the coatings to reduce the lightness,the infrared emissivity of coatings will be significantly increased,meanwhile the heat reflective capacity will drop sharply.Therefore,the modification of Al flakes become an urgent problem to reduce their lightness,enrich colors,and have low lightness low infrared emissivity simutaniously.In this dissertation,the scattering of particles is systematically studied by the factors of particle size,orientation and the roughness of coatings.Furthermore,colorful flaky aluminum composite pigments with low lightness and low infrared emissivity are successfully prepared by the method of ball milling and thermal-cracking/hot-flowing through coating metal oxides,such as MnO₂,Co₃O₄ and NiO.The factors of prepartion such as temperature,time,mass ratio,etc.are studied to find their influence on lightness,color,“flop”and reflective properties of composite pigments in visible and infrared region.The research contents and highlights of this dissertation are as follows:?1?According to Mie scattering model,the influence of particle size,thickness and single particle orientation on scattering has been discussed.Besides,based on rough surface reflection model,the relationship between surface roughness and reflectance of coatings is discussed.Meanwhile,we have studied the influence of distribution of pigments on the reflectance of coatings.We find that the key to prepare pigments with low lightness and low infrared emissivity is to control the geometrical size and surface roughness of pigments.?2?A new method of mechanical alloying is proposed to fabricate aluminum composite pigments with low lightness and low infrared emissivity.MnO₂ particles are embedded in Al flakes and dense MnO₂ coatings are formed on Al flakes with strong combination during wet ball milling.The effects of initial MnO₂ amount and milling time on particle size,surface roughness,and even optical properties,such as spectral reflectance,infrared emissivity and lightness of composite pigments are studied.The results show that the best performance of Al/MnO₂ composite pigments can be achieved by the molar ratio of Al:MnO₂=8:1 and milling time of 24h.The L*is only 48.6,meanwhile the infrared emissivity?_8-14?m is 0.30,and the Al/MnO₂ composite pigments are showing good features of low lightness and low infrared emissivity.?3?Al/MnO₂ composite pigments with complete,smooth MnO₂ shell are prepared by thermal-cracking and hot-flowing method.The initial KMnO₄ adding amount and hot-flowing time can significantly affect the thickness of MnO₂ shell and the distribution of MnO₂ particles on the surface of aluminum flakes,and thus affect the“flop”.The results show that when mass ratio of Al:KMnO₄ is 2.3:1 and hot-flowing time is 48h,the Al/MnO₂ composite pigments have better“flop”properties in the observation angles from15°to 115°.The values of L*decrease from 80.09 to 70.88,while infrared emissivity increased only by 0.07.Thus Al/MnO₂ composite pigments show good compatibility in both visible and infrared region.?4?To enrich colors,Al/Co₃O₄ and Al/NiO composite pigments with low brightness and low infrared emissivity are prepared by thermal-cracking and hot-flowing method.It is found that the adding amount of basic cobalt carbonate,basic nickel carbonate in precursors and hot-flowing temperature are the main factors affecting the performance of composite pigmens.When under optimum conditions,the L*of Al/Co₃O₄ composite pigments decrease obviously from 81.4 to 69.2,and the infrared emissivity only increase0.06.The L*of Al/NiO composite pigments,which are showing gray,decrease from 81.4to 75.9,while the emissivity increase only 0.03.They can be as new low lightness pigments used in low infrared emissivity coatings.The brown/gray aluminum composite pigments prepared in this work can be used as new pigments in low emissivity coatings to improve visible/infrared stealth performance.When the infrared emissivity is 0.41,compared to the aluminum flakes,the L*value of composite pigments are reduced from 80.09 to 52.80,only the particle size of pigments is slightly larger.This series of aluminum composite pigments with low lightness and low infrared emissivity,will be expected to be novel pigments to improve the visible/infrared stealth performance of low-E coatings.