Controllable Synthesis,Microwave Absorption And Mechanism Research Of Hydrogrnated TiO₂ And Its Composite

With the rapid development of electronic information,the electromagnetic?EM?interference and radiation caused by EM wave pose a serious threat to electronic communication equipment and people's life.Recently,radar stealth technology has become the commanding point of national military security.Therefore,EM wave absorbing materials play a key role in the fields of information security and homeland security.Traditional EM wave absorbing materials usually have shortcomings such as high density,narrow absorption band width and poor stability,which are difficult to meet the current needs for application.Therefore,the exploration of new microwave wave absorbing materials with light weight,excellent performance and chemical stability have great practical significance and theoretical value.Traditional titanium dioxide?TiO₂?as a kind of photocatalytic material has been widely used in the field of environment and clean energy.However,the single TiO₂material has poor response in microwave region due to its weak surface electron polarization.Thus,TiO₂ is not among the candidate material for traditional microwave absorption.In recent years,hydrogenation method has become an innovative and effective pathway to optimize the property of materials,and black hydrogenated titanium dioxide?H-TiO₂?has become one of the hot topics due to its unique microstructure and excellent physical and chemical properties.Compared with the reported single dielectric absorption material,H-TiO₂ has more excellent dielectric properties,which provides a solid foundation for the application of H-TiO₂ in microwave absorption field.Up to now,several methods have been developed to synthesize the H-TiO₂,including high press-H₂gas hydrogenation,electrochemical method,and high temperature metal reduction and plasma assisted hydrogenation,and high temperature metal reduction.However,these reported routes need high temperature and/or high pressure,which are difficult to control the crystal size and be suitable for practical applications.Especially,with the increasing degree of hydrogenation,black H-TiO₂ material easily spontaneous combust in the air.Accordingly,in order to study and utilize the black H-TiO₂ effectively,a facile and economical scheme for synthesizing the black TiO₂ is still in high demand.The key for controllable preparation of hydrogenated black TiO₂ is to find a simple,convenient,safe and controllable of morphology and structure method.Based on the full understanding of H-TiO₂ materials and the international hot issues,the controllable preparation,characterization,microwave absorption properties and mechanisms of H-TiO₂ materials have been systematically and in-depth studied in this work.We developed a new“sealing-transfer”method for preparing strongly hydrogenated H-TiO₂,the method of controlling the proportion of reducing agent and the parameters of heat treatment can effectively control the microstructure and properties of the sample.We fabricated a strongly hydrogenated H-TiO₂ nanoparticles,H-A-TiO₂@Ni composites and B-TiO₂/C,and we also studied the effect of hydrogenation degree on the dielectric properties of B-TiO₂ materials.The microwave absorption mechanism of H-TiO₂ was investigated.Furthermore,new composites based on H-TiO₂ with various microwave loss mechanisms and excellent properties have been further developed.This study broadens new ideas for new microwave absorption materials and further broadens the application of H-TiO₂ composites in microwave absorption and other fields.This dissertation includes seven chapters.The first chapter is the introduction,including the research background and the importance of this paper,and discovery,microstructure characteristics,performance and application of H-TiO₂ materials.Then,the preparation process,characterization methods of H-TiO₂,the application research progress and current situation in different fields are summarized.In addition,the application research status of TiO₂ and H-TiO₂ composite materials was also summarized,and the research status and progress in microwave absorption materials are mainly introduced.Finally,the importance,main content and innovation of this work are introduced.In the second chapter,the experimental materials and methods used in this paper are introduced in detail,the testing methods and instruments are described in detail.The experimental methods include three parts:preparation of strong hydrogenated B-TiO₂particles by"seal-transfer"hydrogenation method,synthesis of H-A-TiO₂@Ni microspheres with core-shell structure by sol-gel method combined with"seal-transfer"hydrogenation method;H-TiO₂/C composites were prepared by chemical vapor deposition?CVD?and reduction agent hydrogenation.In addition,the microstructure,surface element composition and morphology of H-TiO₂ and related composites were also introduced.Finally,the forming technology and electromagnetic parameter testing technology of absorbing material are introduced.The third chapter of this paper mainly introduces a method of preparing H-TiO₂nanomaterials by"seal-transfer"hydrogenation method.A kind of strongly hydrogenated B-TiO₂ nanoparticles with was prepared by controlling the ratio of reductant,reaction temperature and reduction time,as well as the subsequent sample transfer treatment process.The method of"seal-transfer"hydrogenation treatment can not only avoid spontaneous combustion of H-TiO₂ material,but also be simple and safe.In this chapter,we not only investigate the effect of hydrogenation process parameters on the sample,but also systematically study the microstructure,surface chemical composition and morphology of H-TiO₂.It was found that there was a large amount of oxygen vacancies and T³⁺ions on the surface of H-TiO₂,and"amorphous-crystalline"core-shell structure was formed on the inner and outer surfaces.Finally,the formation mechanism of H-TiO₂materials was put forward.In the fourth chapter,two kinds of H-TiO₂ composite for maicrowave absorption with different loss mechanisms were synthesized on the basis of the third chapter of the thesis:1)a kind of microwave absorbing material whose shell is H-A-TiO₂ and the core is Ni microsphere were prepared through the combination of sol-gel method and"seal-transfer"hydrogenation method;2)the carbon-deposited B-TiO₂ nanoparticles compositeformicrowaveabsorptionwerepreparedbyone-step hydrogenation-carbonization method.We have systematically analyzed the effect of the preparation process on the morphology and microstructure of the products.At the same time,the morphology,chemical composition and microstructure of the composites were analyzed by SEM,HRTEM,XRD,XPS and other characterization methods.In the fifth chapter,the electromagnetic parameters and microwave absorption properties of H-TiO₂ and H-TiO₂ composites wre studied systematically.We also studied and analyzed systematically the effects of hydrogenation degree and experimental parameters on microwave absorptivity,and the optimum conditions of microwave performance were found out.It was found that the dielectric properties and EM wave absorption properties of H-TiO₂ samples were significantly improved after hydrogenation,which was nearly 10 times larger than that of pristine TiO₂ materials.In order to further improve the microwave absorption properties and reduce the thickness of the materials,the prepared H-A-TiO₂@Ni and H-TiO₂/C composites can effectively improve the microwave absorption properties.Compared with single H-TiO₂,the H-A-TiO₂@Ni and B-TiO₂/C composites are considered as excellent absorbing materials with"light weight,strong absorption and wide frequency".In the sixth chapter,the microwave loss mechanism of H-TiO₂ and its composites was studied,and the microwave absorption properties of three kinds of materials were analyzed with impedance matching law.The results indicated that the enhanced microwave absorption properties of single H-TiO₂ material are due to the enhancement of surface polarization.Under the action of external EM field,the free electrons or ions on the surface of H-TiO₂ gather at the interface,which lead to loss relaxation and interface polarization and enhanced EM wave attenuation.Unlike the single loss mechanism of H-TiO₂ materials,the microwave loss mechanism of H-A-TiO₂@Ni composites are not only due to the surface polarization of the materials but also comes from the impedance matching and the magnetic loss of the magnetic Ni core.Finally,the microwave absorption mechanism of amorphous carbon-coated H-TiO₂ composites was also studied.The present H-TiO₂/C composites exhibited improved and superior EM wave attenuation abilities due to the introduction of large number of interfaces,conductive loss and preferable matching impedance after carbon coating.Chapter seven is a full summary.Finally,a brief introduction of participated projects,published papers,honors and awards and personal resume were given.