Research On Fabrication And Infrared Radiation Properties Of ZrB₂ Thin Films

As one of the most important military equipments,the stealth aircraft are designed to avoid detection using a variety of technologies,which could promoto the survivability in modern wars.With the development of omni-directional stealth technology and the application of aircraft engines which have larger thrust to aircraft weight ratio,there is an urgent need for low infrared emissivity materials at high temperature.As a kind of the high-temperature conductive ceramics,Zirconium diboride(ZrB₂)exhibits high-melting temperature(3245?) as well as excellent electrical conductivity(1.0×10⁷S/m)combining metallic with covalent characteristics,which suggests ZrB₂ could be applied as low infrared emissivity materials at high temperature.The research has been focused on ZrB₂ thin films conducting a series of experiments,which includes the mechanism of low infrared emissivity properties,the fabrication of ZrB₂ and ZrB₂-based thin films and the effect of high temperature on infrared emissivity properties of ZrB₂ thin films.This dissertation has illustrated the physical mechanism of low infrared emissivity properties of ZrB₂ based on the density functional theory(DFT)using CASTEP software and discussed the relationship among substrate roughness,crystalline structure,film morphology,chemical composition with infrared emissivity properties of ZrB₂ thin films,Ag-ZrB₂ composite thin films and Al-Mo-ZrB₂ composite thin films.Moreover,this thesis has investigated and discussed the effect of temperature on the electrical resistivity and infrared emissivity properties of ZrB₂ thin films.The main contents in this dissertation are shown below:1.Based on the theoretical calculation and analysis,ZrB₂ is composed of graphite-like B sheets and hexagonal Zr layers,combining metallic bonds,covalent bonds and ionic bonds.The covalent bonding is respective for the stability and excellent mechanical properties while the metallic behavior is mostly due to the intersection of the Zr3d electrons in valence band with Fermi energy level.The optical interaction of ZrB₂ and electromagnetic waves can be described through Drude-Lorentz model.Therefore,the inner mechanism of low infrared emissivity of ZrB₂ is due to the presence of free electrons.The plasmonic resonance frequency and the damping factor in Drude-Lorentz model could influence the infrared emissivity properties.Therefore,increasing the concentration of free electrons could decrease the plasmonic resonance frequency,leading the increase of infrared reflectivity and decrease of infrared emissivity.Weakening the scattering effect of electrons could increase the lifetime of free electrons which could also decrease the infrared emissivity.2.This dissertation has carried out a systematic investigation on the effect of film thickness and substrate roughness on the infrared emissivity of ZrB₂ thin films.The relationship among crystalline structure,film morphology,chemical composition,substrate roughness and infrared emissivity has been illustrated with a series of instruments including SEM,XRD,XPS,FTIR.Particularly,the relationship between the electrical resistivity and infrared emissivity of ZrB₂ thin films was discussed using Hagen-Rubens equation.The infrared emissivity in 3?5?m and 8?14?m waveband has decreased to 0.15 and 0.05 with the film increasing from 100 nm to 600 nm,exhibiting excellent low infrared emissivity properties.Moreover,the normal infrared reflectivity of ZrB₂ thin films on Ni-based substrates is decreasing with the increase of surface roughness,especially in the shorter infrared waveband.3.This dissertation has investigated the infrared emissivity properties of ZrB₂thin films at high temperature.On the one hand,the crystalline quality of ZrB₂thin films has increased with the increasing annealing temperature(700??1000?)and annealing time(4h?10 h).The infrared emissivity of thin films were gradually decreasing due to weakened scattering effect of free electrons,exhibiting excellent thermal stability and low infrared emissivityproperties.On the other hand,the liner increase of electrical resistivity and infrared emissivity of ZrB₂ thin films with increasing testing temperature has been reported here,which is originated from the lattice vibration.The infrared emissivity of ZrB₂thin film in 8?14?m waveband was 0.329 at 1000? in vacuum,which suggests that ZrB₂ thin films can be applied as low infrared emissivity materials at high temperature.Besides,due to the mismatch of coefficient of thermal expansion between ZrB₂ thin films and Ni-based substrates,the infrared emissivity of films has dramatically increased after annealing at high temperature.Through the interlayer between ZrB₂ thin films and substrates,the problem has been solved well.4.Ag-ZrB₂ nano-composite thin films have been fabricated using magnetron co-sputtering technique and the Al-Mo-ZrB₂ nano-composite thin films have been fabricated through a lab-designed combinatorial magnetron sputtering system.The visible reflectivity and infrared emissivity of Ag-ZrB₂ thin films could be adjusted with the metallic contents,which could possess a low visible reflectivity and infrared emissivity.The thermal stability and infrared emissivity of Al-Mo-ZrB₂ composite thin films could be changed with the contents of Al,Mo,ZrB₂.The research suggested that high-temperature stability and low infrared emissivity could be achieved through double-layer-structure metal and refractory metal compound composite.The research work in this dissertation plays a guiding role in the preparation of high-temperature infrared low emissivity ZrB₂ films,providing theoretical foundation and experimental support for the application of ZrB₂ films in the field of high-temperature infrared stealth materials.Also,the research plays a significant role in the research on the infrared emissivity of transition metal compounds and is beneficial for expanding a new class of high-temperature low infrared emissivity materials.