| Ferrite has good infrared radiation performance at high temperatures,especially for ferrite with spinel structure which has been proved to have high infrared emissivity in the middle and short wavebands.The investigations on spinel ferrite are mainly focused on optimizing spinel composition to improve emissivity in the literature at present.However,there is a deficiency of research on the mechanism and reasons why the spinel ferrite has high emissivity in the middle and short wavebands,particularly in terms of the characterization of the spinel ferrite,such as its crystal structure and electronic properties,which is important for preparing and optimizing spinel ferrite with high emissivity.In this investigation,the mechanism of spinel ferrite with high emissivity in the 3?5?m waveband was discussed based on the experimental analysis and first-principle calculation from the crystallography and semiconductor physics,and the variation of the emissivity for different spinel ferrites in the 8?14 ?m and 3?5 ?m waveband was also studied to confirm its mechanism.Then,the mechanism of the variation on the spinel ferrite emissivity in the 8?14 ?m and 3?5?m wavebands was verified,and the optimal spinel ferrite was found with the preparation of spinel ferrite by doping.Finally,infrared radiation coating was prepared with the optimal spinel ferrite,and the morphology,thermal shock resistance and emissivity of the infrared radiation coating were investigated to find out the optimal formula.The main conclusions are as followed:(1)The reasons of spinel ferrite with high emissivity in the 3?5?m waveband were analyzed.The 141/amd and Fd-3m structure of CuFe2O4 ferrite were prepared by sintering,and the XPS analysis indicated that oxygen vacancy(VO)had formed in 141/amd and Fd-3m structures during sintering process.The emissivity of Fd-3m structure in the 3?5?m waveband is 0.88 which is higher than that of 141/amd structure(0.71),and it testifies that spinel ferrite(Fd-3m structure)has high emissivity in 3?5 ?m waveband.The optical absorption coefficient of CuFe2O4 with Fd-3m structure in the 3?5?m waveband is lower than that of ferrite CuFe2O4 with 141/amd structure,but the optical absorption coefficient of CuFe2O4-VO with Fd-3m structure in the 3?5?m waveband is higher than that of CuFe2O4-VO with 141/amd structure via the first-principle calculation,which indicates that VO is the main factor that ensures the spinel ferrite(Fd-3m structure)have high emissivity in the 3?5?m waveband.(2)The mechanism of emissivity of spinel ferrite Fe3O4,NiFe2O4,CuFe2O4 and CoFe2O4 in the 8?14?m and 3?5?m wavebands were discussed.It shows that Fe3O4 has the highest emissivity in the 8?14?m and 3?5?m wavebands at 773K.The emissivity in the 8?14?m waveband is 0.976,and the emissivity in the 3?5?m waveband is 0.973.Transition metal atoms of Ni2+,Cu2+,and ion Co3+ occupy the position B in the spinel structure.Therefore,the octahedral force constant(ko)at the position B was calculated.The results show that ko decreases with the increase of the cationic oxygen bond distance,and the smaller of Ko has a higher emissivity in the 8?14?m waveband.Ko and emissivity in the 8?14?m waveband are in negative correlation.The results of first-principle calculation show that Fe3O4 is semimetal,and it has the highest optical absorption coefficient in the 3?5?m waveband.The relationship between band gap and the 3?5?m band emissivity of different spinel ferrites has been verified,and the results prove that the band gap and the emissivity in the 3?5?m waveband are in negative correlation.This indicates that ko and the forbidden bandwidth should be reduced to prepare spinel ferrite with high emissivity in the 8?14?m and 3?5?m wavebands.(3)Spinel ferrite CuxCo1-xFe2O4 was prepared through doping.It demonstrated the mechanism of the variation on the emissivity in the 8?14?m and 3?5?wavebands,and found the optimal properties of CuxC1-xFe2O4 spinel ferrite.When the doping content of CuO is higher than 75%,CuxCo1-xFe2O4 sample appears impurity phase CuFeO2,which reduces the emissivity of CuxCo1-xFe2O4 in the 8?14?m and 3?5?m wavebands The XRD structure refinement was used to analyze the relationship between the emissivity of spinel ferrite from CC1 to CC7 in 8?14?m and Ko,and the band gap test was used to analyze the relationship between the emissivity of 3?5?m and band gap,where ko and band gap are respectively in negative correlation with the emissivity in the 8?14?m and 3?5?m wavebands.It has been determined the raw material of spinel ferrite is Cu0.75Co0.25Fe2O4 of which the emissivity is 0.916 in the 3?5?m waveband and 0.985 in the 8?14?m waveband under 773K.(4)The mixed infrared radiation raw material was prepared by utilizing linear combination of Cu0.75Co0.25Fe2O4 and LaAl1-xNixO3 mixture.When the ratio of LaAl1-xNixO3 mixture to Cu0.75Co0.25Fe2O4 is 6:4,the highest emissivity under 773K is 0.972 in the 3?5?m waveband and 0.973 in the 8?14?m waveband.S-A series infrared radiation samples were prepared respectively according to the formula of infrared radiant coating material,which was obtained by the extreme vertex optimization method.The shape,thermal shock resistance and emissivity of these infrared radiation samples were studied as well.It shows that the optimal infrared radiation coating material is No.3 sample in S-A series.The composition ratio in the formula of the infrared radiant coating material isas followed:mixed raw material:sodium polyacrylate:bentonite:binder(Ludox)=0.70:0.05:0.15:0.10.When the temperature is 773K,the emissivity of infrared radiation coating in the 3?5?m waveband is 0.976 and in the 8?14?m waveband is 0.984.After six times of thermal shock under 1100 0C,the emissivity in the 3?5?m waveband is 0.954,and the attenuation rate is 2.9%;the emissivity in the 8?14?m waveband is 0.98,and the attenuation rate is 0.1%.This study developed a theoretical basis for the preparation and optimization of high emissivity spinel ferrite,and provided the technical support for the research and preparation of high-performance infrared radiation coating. |
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