Design,Fabrication and Properties of Self-doped Me-MeO_x Nanocomposite Photothermal Conversion Coatings

Author:Wang Wei

Supervisor:wen huai xing


Degree Year:2019





Photothermal conversion technology is utilized worldwide to convert solar energy into thermal energy.This includes solar water heating,solar air drying,concentrated solar power,among which,the key issue is the design of solar selective absorber coatings(SSACs)applied to the thermal-harvesting surface of the heat collecting device.To acquire high optical absorption,these traditional SSACs are often more complicated and material cost.This is because co-sputtering with multiple sputtering targets during the fabrication process is required,inevitably imiting the actual industrial productions.Based on the concept of self-doping,a novel self-doped Me-MeOx(Me=W or Mo)SSAC is fabricated by using single metal target with limited reactive sputtering,which exhibits excellent optical absorptivity and thermal stability.The as-abtained SSAC presents simple fabrication processes,low material cost,optical adjustability and structural extensibility,which is promising for low-cost and large-scale photothermal technology in mid-and high-temperature applications.The novel self-doped photothermal technology is used as wearable technology that regulating personal thermal,which can overcome the traditional passive heating way using joule heating and reflecting infrared(IR)wavelength,and actively absorb sunlight to warm the human body.It also reflects the IR thermal radiation of human body to maintain the temperature.Such result shows very effective application value in outdoor emergency heating fabrics,and broadens the application of SSACs in flexible and wearable personal thermal conversion technology.The specific works are as follows.(l)The intrinsic absorption properties of monolayer WOx films are studies,and the dielectric WOx,Si02 and A12O3 films are introduced as the anti-reflective(AR)layer to investigate the evolution of the optical absorption of the doped WOx films.The thermal stabilities of the as-fabricated SSACs are discussed.The results show that the doped WOx film possesses excellent optical absorption,and the highest a value is 0.83 and ε value is 0.05.Moreover,the dielectric WOx film shows the anti-reflective effect,and the corresponding a of doped WOx film is rasied to 0.91 and ε remains at 0.054.The thermal annealing results show that the as-obtained SSACs have poor thernal stability due to the diffusion and oxidation of the element,whereas the coatings show excellent thermal tolerance in vaccum at 400℃.(2)By combining the self-doping concept,the intrinsic absorptions and the AR properties of WOx films,the novel self-doped W-WOx based SSACs are successfully fabricated.And the optical properties and phase compositions are studied.The failure mechanism induced by the diffusion and oxidation at operating temperature is discussed.The results show that the as-fabricated SSACs possess excellent optical absorptivity,where a value is 0.93 and 0 value is 0.058,and η is 90%,which can be attributed to the intrinsic absorption of the WOx film,the optical interference effect between the interface layers,and the near field enhancement effect of the metal W layer.In addition,the thermal annealing results indicate that the W-WOx based SSACs have excellent thermal tolerance in air at below 300℃.The dominant failure mechanism is mainly attributed to the diffusion of W nanoparticles,and then oxidized significantly as the annealing temperature rose.This causes changes in the crystal structure and composition of the SSACs,resulting in degradation in the spectral absorption.(3)The optical absorption of monolayer MoOx film at differemt pressure is analyzed.The AR effects of Si02 and MoOx dielectric films are investigated,and the thermal stabilities of the coatings under different atmospheres are discussed.The results indicate that the non-stoichiometric MoOx film exhibites excellent absorption properties,the highest a is 0.83 and ε is 0.05.Similarly,the dielectric MoOx film can also act as a novel AR layer in the design of SSAC.The thermal annealing results also show that the coatings show excellent thermal stability in vacuum at 450℃,whereas the coatings show poor thermal stability due to the diffusion and oxidation of the element.(4)Combining with the aforementioned analyses,a self-doped Mo-MoOx based multilayer SSAC with more stable structural property is fabricated,and the optical constant,crystal structure and phase composition are studied.The thermal stabilities of the coatings under different atmospheres are discussed.The results show that the coating has excellent optical absorption in visible and near-IR region,where a value is 0.93 and ε value is 0.055,and η is 92%.Moreover,as thermal annealing in air,the surface of the SSAC is severely oxidized,and the SSACs show poor thermal tolerance.When annealing in vacuum at 450℃,the optical properties of the coatings do not show significant change,indicating the outstanding thermal stability.(5)Based on the above results,the Me-MeOx SSACs are fabricated on flexible PET and PI sheets as well as cotton cloth and polyester fibre,and the corresponding optical absorption,bending properties and the photothermal conversion effect are analyzed and discussed.The results show that the coatings on flexible polymers and fabrics show excellent light absorbing ability and bendable nature,where a values are 47%~52%and 82%~83%respectively,and both ε values are 30%~35%.Under one sun illumination(1 KW/m2),the surface temperature of the coatings on flexible polymers and fabrics is 78℃ and 90℃,showing good soalr thermal conversion effect.In addition,under natural sunlight the fabric sample also shows obvious endothermic effect,and the steady temperature is up to 10~11℃,and the temperature difference is close to 6℃compared with the blank fabric.In summary,the self-doped Me-MeOx SSACs possess excellent optical absorption properties and thermal stability,which are suitable for mid-and high-temperature solar thermal conversion technology.In addition,the SSACs on flexible ploymer and wearable fabric show excellent bendable nature and light-to-heat conversion efficiency.Such result shows significant important industrial application value for flexible and wearable energy related personal thermal management,and it also broadens the application of traditional SSAC in solar thermal technology and provides the theoretical basis and practical experience.