| High infrared emissivity coatings have attracted the attention of many researchers all over the world because of their high application value in the field of energy saving and environmental protection.At present,the research on high infrared emissivity materials mainly focuses on spinel,cordierite,perovskite and multi-metal composite systems.Composite of materials is an effective way to improve the properties of materials.In addition to causing doping effect,it is also possible to produce new types of materials that are completely different from the original system.In this paper,B2O3 was added to the nickel-chromium system?Cr2O3 and NiO?for compounding.The agglomerated powder was prepared by rotary spray granulation technology and then calcined in muffle furnace at 1400°C to obtain Ni2Cr?BO3?O2 with a ludwigite crystal structure.The prepared Ni2Cr?BO3?O2 powder was sprayed onto316L stainless steel substrates by atmospheric plasma spraying?APS?to obtain a high infrared emissivity coating and then the performance of the coating was tested and analyzed.The results show that the infrared emissivity of Ni2Cr?BO3?O2 coating in the2.5-25?m is 0.893 and further increases to 0.903 after coating a layer of La1-xSrxCoO3-?on the Ni2Cr?BO3?O2 coating.In addition to the anti-corrosion effect,the La1-xSrxCoO3-?layer can further improve the absorption property of the coating as an anti-reflection layer.Compared with NiCr2O4,Ni2Cr?BO3?O2 has higher infrared emission properties,mainly due to the more complex and asymmetric crystal structure of Ni2Cr?BO3?O2.In Ni2Cr?BO3?O2 structure cell,there are four kinds of cation positions and each position is occupied by two different elements.The site occupancies percentage of the divalent metal ions(Ni2+)in each cation position are 12%in M1(in other words,there is 12%of M1 positions in the octahedral occupied by Ni2+and 88%of M1 positions occupied by Cr3+),89%in M2,94%in M3 and 100%in M4.Due to the different radii of Ni2+and Cr3+,a large degree of lattice distortion occurs in the structure cell of Ni2Cr?BO3?O2,which reduces the symmetry of unit cell and enhances the lattice vibration activity.And the phonon absorption of infrared radiation by Ni2Cr?BO3?O2 material will be enhanced.In the crystal structure of ludwigite,the valence state of metal ions often changes,such as 2Me3+?Me2++Me4+,which is prone to electronic transition.In addition,there are four electrons outside the boron atom in the planar group BO3 of the Ni2Cr?BO3?O2unit cell,each B-with three of O atom to form covalent bonds and the remaining electron is easier to occur electronic transition,which makes the Ni2Cr?BO3?O2material have strong electron absorption for infrared radiation.These two reasons make Ni2Cr?BO3?O2 an excellent high-infrared emissivity material.The TG-DSC test shows that the Ni2Cr?BO3?O2 material was formed at a temperature of about 1400°C.The SEM image of the surface of the coating shows that there are many 1-10?m holes on the surface of the coating,which is the typical characteristics of plasma spray coating.The size of these holes matches the wavelength of infrared radiation,which can be regarded as an ideal source of black body radiation.There is no peeling of the coating in the SEM image of cross section,and the bonding between the coating and the substrate is good.The corrosion test of the salt solution of the coating indicates that the coating has good corrosion resistance and its performance don't decrease after 90 days immersion in salt solution.Then,the coating treated by salt solution is placed in a high temperature furnace at 800?for 30 days,and the performance is reduced less than 5%,which still meets the requirements of use. |
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