Preparation Of Magnesium Oxide Materials And Its Daytime Radiative Cooling Spectrum Study

Global warming and energy are two major problems facing the whole world today.In order to enjoy cool in hot summer,air-conditioners are widely used.However,they emit a large number of greenhouse gases and exacerbate global warming.The huge energy consumption makes it very important to find new cooling methods.Radiative cooling is a new cooling method that has attracted much attention.It could transfer energy in the form of heat radiation to outer space through the atmospheric transmittance window by the materials,thus realizing passive cooling.Nighttime radiative cooling materials only need to have high emissivity in 8～13 μm atmospheric window.But daytime radiative cooling puts forward higher requirements.The materials for daytime radiative cooling should have reflectivity larger than 95%in the solar spectral band,with high emissivity in 8～13 μm atmospheric window.At present,the reported daytime radiative cooling materials mostly adopt the complex structure like photonic crystal,multilayer film and microsphere film,which is not conducive to the simple preparation process.Most of them have high emissivity in other infrared band,which would lead to a large amount of energy input from atmospheric radiation and reduce the net radiation flux.Therefore,it is significant to realize spectral selective daytime radiative cooling through a single material in academia and application.According to the theory of solid state spectroscopy,the reststrahlen band of magnesium oxide materials is in 13～25 μm.While in 8～13 μm atmospheric window,it has high emission.Moreover,it could strongly reflect visible and near infrared through structural scattering effect.Therefore,this paper selects magnesium oxide as the research object.Through the prepartion and research of magnesium oxide coating and bulk materials,the daytime radiative cooling spectral performance of magnesium oxide was improved.The magnesium oxide coating with spectral selectivity was prepared by silk-screen printing.This paper did the research of the coating preparation process.We made the research on the powder size,the content of ethyl cellulose and sintering temperature,which influence the microstructure and thickness of the coating.In this process,we obtained optimum preparation parameters.High-purity magnesium oxide was used as ingredient.The powder with smaller particle size was obtained by ball milling,which could improve the sintering activity.Using the powder with smaller particle size,the average polycrystal particles size of the coating increased from 0.52μm to 2.78 μm.Decreasing the content of ethylcellulose could also increase the average polycrystal particles size.The average particle size of the coating increases from 0.46μm to more than 2 μm by increasing the sintering temperature.The emissivity of the magnesium oxide coating reaches 92.06% at the atmospheric window of 8～13 μm because it is in the resonance absorption zone of magnesium oxide material.However,the average free path of phonons in the coating is shorter due to the porous structure and multiple microinterfaces.The intensity of the photon-phonon coupling is higher than that of the interaction between phonons caused by the anharmonic vibration.The coupling is a "real" process,and the intensity of the residual reflection band is low,and the emissivity of 13～25 μm is only 75%～77%.With the increase of polycrystal particles size,the strength of residual reflection band is improved.The solar spectral reflectance of the magnesium oxide coating is 95.61%and the emissivity of the magnesium oxide coating is 77.74% from 3 μm to 8 μm due to the porous enhanced scattering.In order to make infrared radiation selectivity materials,it is necessary to prepare magnesium oxide materials with more dense micro-surface.Therefore,this paper did the research on block radiative cooling materials based on magnesium oxide ceramics.High density magnesium oxide block ceramics were prepared by dry pressing and high-temperature pressureless sintering.We made the research on powder size,pressure,sintering temperature and sintering time were studied respectively.Optimum preparation parameters were obtained by adjusting surface morphology and porosity.The powder with higher sintering activity was obtained by ball milling,as average particle size of polycrystalline increased from 0.49 μm to 6.36 μm,and the porosity decreased by 13.58%.Increasing the pressure,the density of clay body was larger and the contact of powder was closer.After sintering,the average particle size of polycrystalline magnesium oxide bulk material increased from 1.19 μm to 6.36 μm,and the porosity decreased by 4.32%.With the increase of sintering temperature,the material migration rate increases,the average particle size increases from 0.18 μm to6.36 μm,and the porosity decreases by 41.93%.The surface morphology is basically unchanged and the porosity decreases by 0.05% when the holding time is prolonged.The emittance of magnesium oxide bulk material is over 94% from 8 μm to 13 μm.Due to the shallow skin depth and the influence of surface crystal integrity,the intensity of reststrahlen band increases with the increase of surface polycrystalline particle size,and the emissivity of 13～25 μm can be as low as 30%.The solar spectral reflectance of the magnesium oxide block material can reach 96% through the pore structure scattering.The near infrared and 2.5～9μm emissivity are affected by water absorption and pore structure scattering.The water peaks near 1900 nm,2200 nm and3 μm affect the band emissivity.In the visible near infrared and 2.5～9 μm infrared,the high frequency transparent zone of magnesium oxide materials decreases the porosity,which reduces the structure scattering mainly by pores.And the transmittance of transparent zone is improved,which reduces the solar spectral reflectance and emissivity of 8～13 μm.Considering that it is difficult to compare the spectral selectivity of different materials and the cooling test is greatly affected by environmental factors,it is proposed to evaluate the performance of radiant cooling materials by spectral distance,spectral Angle and relative value of radiation power.Compared with literature reports,magnesium oxide block material has excellent daytime radiative cooling performance.