Research On Oxidation Resistance And Electrical And Thermal Transport Properties Of ZrB₂-based Ceramics

As the major radar scattering resource,the engine nozzle is the most critical point that restricts the omnidirectional stealth of the aircraft.It needs to reduce the radar cross section?RCS?of engine nozzle through high temperature radar stealth technology due to the high temperature condition.At present,the radar absorbers,which have a property of absorption by the electrical loss mechanism,are adopted by researchers at home and abroad to achieve high temperature radar stealth.However,the absorbing properties of these radar absorbers are very poor unless these radar absorbers have a very large thickness.Therefore,they are difficulty to be practice applied in the short time.The application of metamaterial,as a new technical approach to achieve high temperature radar stealth,is expected to solve the above problems.The metal materials used in the graphic metal array units of metamaterial cannot withstand the high temperature environment of the engine nozzle.The high-temperature resistant conductive material,as a stable electromagnetic radiation control material,is the most critical factor for achieving high-temperature absorption.Due to the unique characteristics of high melting point,high electrical conductivityand thermal conductivity,excellent oxidation resistance and good thermal shock resistance,the ZrB₂-based ceramics are considered as an ideal replacement for metal materials in metamaterial.However,the inevitable oxidation for ZrB₂ ceramics when exposed to the oxygenated atmosphere at high temperature can lead to failure of cloaking.The oxidation has also greatly restricted the application of ZrB₂ in other field.The methods,which have been reported to improve the oxidation resistance of ZrB₂ ceramics,will certainly bring about changes of their electrical and thermal properties,and thus have an impact on the RCS reduction.In this dissertation,studies have been carried out on the oxidation behavior and kinrtics of ZrB₂-based ceramics,the electrical and thermal properties of ZrB₂-based ceramics,the plasma spray deposition and characterization of ZrB₂-based coatings,in order to overcome these major existing problems.The further enhanced oxidation resistance of ZrB₂-based ceramics,the better adjustable electrical of ZrB₂-based ceramics and thermal properties,the more practical preparation method of ZrB₂-based coatings all provide material and technical support for the omnidirectional stealth of futuristic aircraft.The major works of this dissertation are as follows:1.We use an effective method for doping suitable compounds into ZrB₂ in order to provide a responsive anti-oxidation.The protective layers are formed on the surfaces of ZrB₂ ceramics when they are exposed to high temperature and oxidation environment.The incorporation of SiC and WC into ZrB₂ matrix is achieved using spark plasma sintering?SPS?.For comparison,the oxidation behavior of pressureless sintered ZrB₂ceramics and single-doped ZrB₂ ceramics are also studied.We can get that the oxidation resistance of spark plasma sintered SiC and WC co-doped ZrB₂ ceramics is significantly improved due to the complementary effect of multiple antioxidative mechanisms.There are three types of antioxidative mechanisms:?1?SiC improves the oxidation resistance of ZrB₂ ceramics by forming a silica protective layer on the surface of the ceramics;?2?WC improves the oxidation resistance of ZrB₂ ceramics by forming a dense internal oxidation layer;?3?the pores,act as the permeability channels for oxygen,in the ZrB₂matrix are eliminated due to the co-doping of SiC and WC,which is helpful to improve the oxidation resistance.The adoption of the complementary effect of multiple antioxidative mechanisms,which is achieved by the SiC and WC co-doping,is one of the innovations of this paper.2.The protection layers are coated on the surface of ZrB2 before oxidation treatment in order to provide a preventive anti-oxidation.The SiO₂ protection layer produced by the hydrolysis-polycondensation of tetraethyl orthosilicate is coated on the surface of ZrB₂ particles to prepare the ZrB₂@SiO₂ composite particles via means of sol-gel methods.The stable oxide layers are coated on the surface of ZrB₂ particles to prepare the ZrB₂@Me_x O_y?ZrB₂@Al₂O₃,ZrB₂@Y₂O₃ and ZrB₂@ZrO₂?composite particles via the chemical co-precipitation method.For the first time,ZrB₂ particles are synthesized in fluoride inclusions,so the fluoride layers are coated on the surface of ZrB₂ particles in situ to prepare the ZrB₂@MeF_x?ZrB₂@YF₃,ZrB₂@CeF₃,ZrB₂@SrF₂and ZrB₂@LaF₃?composite particles.This is another innovation of this paper.The oxidation behaviors of these coated ZrB₂ ceramics are studied and we get that the coating of protection layer play a positive effect on the oxidation resistance of ZrB₂ceramics.Therefore,the coating of protection layer is a practical new method to improve the oxidation resistance of ZrB₂ ceramics.3.ZrB₂-based coatings with uniform thickness and strong adhesion are deposited on the stainless steel substrates by using the plasma spraying method.The feasibility of the plasma spraying to coated ZrB₂-based coatings on the engine nozzle has been verified.The microstructures and oxidation behaviour of plasma-sprayed ZrB₂-based coatings are investigated.Results revealed that ZrB₂-based coatings displayed much worse oxidation resistance than ZrB₂-based block ceramics,since there were more pores in ZrB₂-based coatings,which provided channels for oxygen diffusion.4.The secondary action of anti-oxidation processing is to cause the change of microstructures of ZrB₂-based ceramics,and then the electrical and thermal properties will change.Electrical conductivity and thermal conductivity of ZrB₂-based ceramics with different microstructures are measured under by four-probe method and laser flash method,respectively.The relationship between microstructures and electrical and thermal properties in ZrB₂-based ceramics is investigated in detail.Increase in volume fraction of SiC can cause effective electrical conductivity of ZrB₂-based ceramics to decrease,while it has an opposite effec for the thermal conductivity of ZrB₂-based ceramics.It is because that SiC is as an insulator for electrical transport,but it is a conductor for thermal transport.The increasing of the internal interfaces area and oxide impurity on the internal interfaces can cause the decrease of the electrical conductivity and thermal conductivity for ZrB₂-based ceramics.It is because that internal interfaces in polycrystalline ZrB₂-based ceramics act as scattering sites for electrical and thermal transport.The microstructures of ZrB₂-based ceramics can be changed through different preparation technology to get the adjustable electrical of ZrB₂-based ceramics and thermal properties.Then,ZrB₂-based ceramics can meet the design requirements for metamaterial with different functions.