Optimized Design And Thermal Stability Of Photothermal Conversion Coatings Based On ZrN

For solar selective absorbing coatings applied to parabolic trough collector concentrating,cermet coating is the main researched system.During the service time of cermet coatings on the high temperature,the oxidization of metal and dielectric and diffusion of N and O between sub-layer results in degradation.The transition metal nitride(TiN,CrN,NbN etc)as the absorbing units improves the oxidization resistance of cermet coatings,further.As the absorbing layer in cermet coatings,aluminum-based ternary Me-Al-N films(Me=Cr,Ti,Zr and Hf etc)deposited by physical vapor deposition would form the columnar structure.There exist many micro-porous and the grain boundary in the columnar structure.The micro-porous and the grain boundary accerlerate the element diffusion because they service as the diffusion channel of atom,which deteriorates the high-temperature thermal stability.In the equilibrium thermodynamic conditions,stoichiometric fcc-TiN and hcp(?)-Si3N4 are immiscible.The Me-Si-N system will form the the quasi-binary nc-TiN/a-Si3N4 nanocomposites("nc-" stand for nanocrystalline,"a-" for X-ray amorphous).The formation of a nanocomposite consisting of TiN nano-crystals and amorphous Si3N4 layers in between block the grain boundary decreses the defect(grain boundary,micro pores,etc).Additionally,this structure weakens the diffusion of N and O.It also improves the thermal stability of cermet coatings.A certain amount of aluminum added in the MeN will be completely soluble in the fcc ZrN leading to form the ternary single-phase MeAlN compounds.The Me-Al-Si-N system will form nc-MeAlN/a-Si3N4 nanocomposites.In this paper,we optimized the deposition parameters of single films(including the ZrN,ZrAIN and AlOxNy).Then,Cu/Zr0.3Al0.7N/Zr0.2Al0.8N/Al34O60N6 was optimized and prepared,and their thermal stability of the two coatings was investigated.Based on the above investigation,the Cu/Zr0.32Al0.63Si0.05N/Zr0.21Al0.73Si0.06N/Al31Si3O60N6 cermet coatings were designed by the addition Si to ZrAlN.And then,the thermal stability of the ZrAlSiN cermet coatings was studied.Aim to develop the high thermal stability coating and simplify the deposition procedure,the Cu/Zr0.2Al0.8N/ZrN/AIN/ZrN/AlN/AlOxNy stacks coatings were designed and deposited by replacing the semitranmittance metal by ZrN,and their thermal stability was investigated.The obtained findings are as follow:(1)The influence of ion-atom arrival ratio on structure and optical properties of ZrNx films deposited by ion beam assisted deposition(IBAD)was investigated.The deposited films are mainly consisted of ZrN with little ZrOxNy.The stoichiometric ZrN is synthesized at ion-atom arrival ratio of 1.5.A strong(220)texture is formed in the ZrNx film deposited at ion-atom arrival ratio of 2.4 and 5.0.As the ion-atom arrival ratio decrease,the electrical resistivity and refractive index decrease,the extinction coefficient increases.Then,the influence of aluminum content on structure and optical properties of Zr1-xAlxN films was studied.For the Zr1-xAlxN films with x?0.5.the Zr1-xAlxN films crystallizes in the rock salt type of structure.In the composition range 0.5<x<0.8,the deposited Zr1-xAlxN films consist a two-phase mixture of cubic ZrAlN(NaCl type)and amorphous AIN.For the high Al content(x?0.8),the deposited films are composited of amorphous AIN and little ZrAlN.As the aluminum content increase in Zr1-xAlxN films,the electrical resistivity increases.The dielectric character is weakened,the mettlic character is reinforced.(2)The effect of nitrogen-oxygen ratio on composition,micro-structure and optical properties of AlOxNy films deposited by IBAD was investigated.As the ratio of nitrogen to oxygen decreases,the content of nitrogen in the films decreases,whereas,the content of oxygen increases;at the same time,the amorphous phase transfers to AlON crystal phase.The chemical state of nitrogen and oxygen element keeps constant,however,the chemical state of aluminum element evolves from nitride stated to oxidized state.The zero value for extinction coefficient of the films indicates the deposited films are dielectric materials.And the refractive index of films within the whole wavelength range decreases with decreasing the ratio of nitrogen to oxygen.The instinct reason of refractive index decrease is due to the increase of Al-O bond in the AlOxNy films.(3)The Cu/Zr0.3Al0.7N/Zr0.2Al0.8N/Al34O60N6 cermet coatings deposited by IBAD obtains the high absorptance(a=0.953)and low emittance(?100?=0.044,?400?=0.079).In the Cu/Zr0.3Al0.7N/Zr0.2Al0.8N/AlOxNy cermet coatings,the Zr0.3Al0.7N sub-layer is consisted of ZrAIN and AIN nano-crystal with little AIN amorphous,the Zr0.2Al0.8N sub-layer is composed of amophous and little ZrAIN crystal,the anti-reflection layer of Al34O60N6 is amorphous.(4)After annealing at 400 ? for 192 h in vacuum(5.0×10-2)ambient,the photo-thermal conversion property and the surface morphology of ZrAIN cermet coatings keeps stable.After annealing at 600 ? for 192 h,the absorptance is stable,whereas,the emittance increases from 0.079 to 0.141 obviously.After annealing treatment at 600 ?,the four-layer structure of coating is stable.The surface morphology is stable after annealing at 600 ? for 192 h.The spinodal decomposition of cubic ZrAIN in the Zr0.3Al0.7N sub-layer is happened during annealing process.Then,the decomposition and phase transition lead to the escape of ntrigen atom.The nitrogen atom diffuses along the grain boudrary seriously.This process leads to the change of optical constants for Zr0.3Al0.7N and Zr0.2Al0.8N sub-layer and the deviation of coating reflectance comparing to the original design.After annealing,the decrease number of the free electron in the copper results in the emittance increase.(5)In the Cu/Zr0.32Al0.63Si0.05N/Zr0.21Al0.73Si0.06N/Al31Si30O60N6 cermet coatings,the Zr0.32Al0.63Si0.05N sub-layer possesses nc-ZrAlN/a-(Si3N4 and AIN)composite structure.The structure of Zr0.21 Al0.73Si0.06N and Al31Si3060N6 is amorphous.The high absorptance(?=0.945)and low emittance(?100?=0.048,?400 ?=0.084)is achieved.(6)In the vacuum(5.0×10-2)ambient,the photo-thermal conversion property of coatings is stable after annealing treatment at 400 ? for 192 h.After annealing at 600 ?for 192 h,the photo-thermal conversion property(?/?400?)decrease due to the emittance increase from 0.084 to 0.144(?400 ?).After annealing treatment at 600 ?,the four-layer structure of coatings keeps stable.The micro-structure of sub-layers and surface morphology are also stable.The diffusion in absorbing layer and anti-reflection layer is little.The formation of AIN and Si3N4 amorphous in interface supresses the spinodal decomposition of ZrAIN phase.It prevents the escape of nitrogen atomo and improves the coating thermal stability.The diffusion between copper and slicon results in the decrease of free electron in the metal infrared layer,which leads to the emittance increase.(7)Among the Cu/Zr0.2Al0.8N/ZrN/AlN/ZrN/AlN/Al34O62N4 stacks coating,the ZrN and AIN sub-layer are consisted of the amorphous and nano-crystal,the Al34O62N4 and Zr0.2Al0.8N sub-layer are amorphous.The deposited coatings exhibit the absorptance of 0.9416 and the emittance of 0.058 at 100 ?.(8)In the vacuum(5.0×10-2 Pa)ambient,the photo-thermal conversion property of Cu/Zr0.2Al0.8N/ZrN/AlN/ZrN/AlN/Al34O62N4 after annealing at 400 ? for 144 h keeps stable.After annealing at 600 ? for 144 h the emittance increases obviously.The photo-thermal conversion property(?/?100?)decreases from 16.2 to 8.72 obviously.After annealing at 600 ? for 144 h in vacuum ambient,the coating structure keeps stable,and the micro-structure of sub-layers in the coatings is also stable.The surface morphology is stable.The nitrogen diffusion in the ZrN sub-layer close to the surface and in the AIN sub-layer in the middle leads to the reflectance increase in the range of 500 nm to 1000 nm.The diffusion bettween copper and slicon leads to the emittance increase.