Preparation And Reliability Of Carbon Nanotube/Packfong Composite Block Cold Cathode

Cathode as an electron emission source,plays an irreplaceable role in a vacuum electron system/equipment.Of all kinds of cathodes,the field emission cold cathode is the one featuring high emission efficiency,low delay and small size.Carbon nanotubes(CNTs)film are considered as a common material of the cold cathode due to their good electron emission performance.The films themselves,however,are doomed to have poor damage resistance and tolerance under strong electric field,and therefore they are unable to meet the hash working conditions and high working intensity of certain specific applications.To solve problems mentioned above,a composite material system with bulk properties,in this paper,is prepared,and its main characteristics as a cold field emission cathode are studied,with results shown as follows.(1)The preparation method of the CNTs/packfong composite block cold cathode was studied,in which an conclusion of optimal process parameters in the hot press sintering method was drew.In this paper,the hot press sintering mehod was adopted,in which the CNTs/packfong composite block cold cathode with certain strength was prepared by ball milling CNTs and packfong mixture powder.Based on sintering temperature,sintering pressure and carbon nanotube content in the preparation process,factors that influenced cathode properties were systematically studied,and the electron emission characteristics of the CNTs/packfong composite block cold cathode was compared with those of the CNTs film cathode and of the cathode prepared by non-pressure sintering respectively.It was concluded that the composite cathode with its optimal property,in which the maximum current density was measured at 380μA/cm~2,with the turn-on electric field of 2.93 V/μm,as the electric field was 5.56 V/μm,can be made with the preparation temperature of 800℃,the pressure of 30 MPa and the content of CNTs of 5wt%.In addition,the composite block cold cathode has better damage resistance and tolerance,compared with CNTs film cathode,and by the one prepared through the the hot press sintering process was better than the property of the cold cathode prepared through non-pressure sintering process.(2)The electron beam distribution characteristics and related mechanism of electron emission of the CNTs/packfong composite block cold cathode were revealed through the study of the emission characteristics of CNTs/packfong composite block cathode by means of experiment and simulation.In terms of experiments,the burning marks left by the electron beam on the metal anode plate and the real-time spot caused by the transparent fluorescent plate were obtained by using the metal anode plate and the transparent fluorescent plate respectively during the field emission test,with the electron beam mainly concentrated in the surrounding ring region,while in terms of simulation,COMSOL software was used to simulate the electric field distribution,electron beam tracking and Poincare diagram of anode surface under the corresponding emission conditions,which further proved that the electron beam distribution characteristic of circular cathode emission in this paper is annular a phenomenon from an edge emission mechanism.(3)The evolution of ultimate emission capacity and electron emission stability related to the reliability of CNTs composite block cold cathode under strong electric field were studied,in which a mechanism to activate emission point site for the special material structure of composite block cathode was developed.In the process of the composite block cathode activation method,the property of the cathode tended to be stable following"lighting",and after several"lighting"the field emission performance of the cathode was improved to a certain extent,thus the electron emission point activation mechanism of the cathode material having been developed.In this mechanism,the property of the cathode tended to be stable since the spark cleaned up the surface coverings and substances vulnerable to lighting in the early experience of"lighting",and the field emission performance of the cathode was enhanced,with more emission sites activated during this process.In the process of electron emission stability test,the cathodes prepared under different conditions were tested with the same current.The one prepared under 800℃had better stability and higher current emission density.During a non-stop test of 360 mins,the same sample the voltage fluctuated greatly in the early stage and then gradually became stable.When the current was 0.2 mA,the voltage tended to decline,which indicated that the cathode had excellent reliability in the test of electron emission stability and its emission performance was improved after the long-time passivation.(4)Based on all the results shown above,the research and development of a practical device were carried out.Using a ring cathode drawing offered by a unit and the hot press sintering method,the ring cathode mold was designed,and then the annular cathode that met the given design requirements of the unit was eventually prepared based on the study results of the preparation process.