| Particle composite coating is composed of filler particles,binders and other functional additives.It is a particle dispersion medium,which is widely used in many fields such as radiation refrigeration and infrared stealth,and plays an important role in energy conservation and national defense.Therefore,it is important to theoretically predict the radiation characteristics of particle composite coatings for their applications.In engineering fields such as the design and application of particle composite coatings,it is generally assumed that the size of filler particles is uniform and distributed randomly in the coating.However,in the actual preparation process,the size of filler particles is different,and in the forming process of polymer composite coating,the filler particles may float up or settle down due to the unbalanced effects of buoyance.surface tension and gravity,which leads to the spatial non-uniform distribution,rather than the ideal uniform distribution.Therefore,the effect of the spatial and size distribution of filler particles on the apparent radiation characteristics of the coating is studied in this paper,taking the particle composite infrared stealth and radiation cooling coating as an example.The radiation transfer model of coating considering the size distribution and spatial nonuniform distribution of filler particles is established in this paper.Firstly,the radiation characteristics of single particle and the particle system with log-normal distribution were calculated by Mie scattering theory,and the absorption factor,scattering factor and scattering phase function of the particle system in the binder were obtained.Then,the coating absorption and scattering coefficients are calculated under the assumption of independent scattering.Finally,Monte Carlo method is used to solve the apparent spectral radiation characteristics of the coating such as absorption rate and emissivity under different particle distribution modes.The effects of the size,volume fraction and thickness of the doped particles on the reflectivity and emissivity of the coatings with uniform distribution,floating or settling of particles,andmulti-particle doped particles are discussed.The differences of the radiation characteristics of the coatings with uniform distribution,spatial distribution and size distribution of filler particles are compared and analyzed.The results show that for the particle composite infrared stealth coating,the non-uniform distribution of filler particles in space has a significant impact on the coating emissivity.Compared with the uniform distribution,the particle floating is beneficial to reduce the coating emissivity,and the sedimentation is beneficial to increase the emissivity.When the particle radius is 2 μm,the doping volume fraction is 0.025,and the coating thickness is 200μm,the average emissivity is 0.32,0.45 and 0.49.The larger the nominal particle size of the doped particles is.the lower the emissivity of the coating is,and the less affected by the degree of particle non-uniformity.For particle composite radiation cooling coating,the non-uniform distribution of filler particles in space is beneficial to enhance the radiation cooling performance.Particle size is an important factor affecting the performance of the coating.The cooling performance of the coating doped with 0.2 μm TiO2 particles is better,and the absorption emission ratio is about 0.25.For the same nominal particle size ra,the solar reflectivity of the coating increases and the in frared emittance decreases with the increase of the particle average geometric deviation σ.When ra=0.3 μm,the minimum absorption emission ratio of the coating at σ=0.9 is about 0.36.With the increase of coating thickness and doping volume fraction,the solar reflectance of the coating increases and the infrared emissivity decreases. |
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