| X-ray imaging is widely used in medical testing and industrial nondestructive testing.As the core component of X-ray imaging system-X-ray tube,over the years due to the shortcomings of hot electron emission cathode limitations,such as: hot electron emission cathode volume,resulting in Joule heat,low current density and other defects.X-ray tube has been hindered the further development in the field of X-ray imaging,particularly,in the medical field dual-energy CT imaging,often requires that X-ray tube has small focal spot size,high current density,transient high power,high speed pulse camera,low effective radiation dose.Carbon nanotube field emission X-ray tube is considered to be a very good solution because it has the characteristics of fast start speed,low working temperature,high-resolution imaging,high-speed camera function and low dose.However,when carbon nanotubes are used in the field emission field,the strong electric field shielding effect of carbon nanotubes,besides,the carbon nanotubes and the substrate are difficult to bond together in the form of chemical valence bonds,resulting in poor electrical contact performance,the resistance of carbon nanotubes and substrate is high,so is Joule heat,when the emission current is high,it is easy to lead to ignition phenomenon.In order to improve the reliability of the carbon nanotube field emission cathode,by making the graphene and the carbon nanotubes into a composite structure field emission cathode to solve the electrical contact emission of the carbon nanotubes and the substrate,which greatly improves the emission of the stability,and can get a high field emission current density.According to the characteristics of the field emission cathode,X-ray tube with similar focusing characteristics can be designed to meet the two kinds of anode of high voltage at 80 kV and 120 kV simultaneously to satisfy the requirements of dual energy CT.And we successfully achieved DR(Digital Radiography)imaging and CT(Computed Tomography)imaging,which has a high resolution.The main works of this paper are as follows:1?Composite structure cold cathode array preparation.Including the preparation of carbon nanotube arrays by microwave plasma chemical vapor deposition(MW-PCVD).Two kinds of schemes were used to synthesize carbon nanotubes and graphene: 1.The catalysts were coated with nickel,copper and MW-PCVD was used to prepare graphene,the carbon nanotube array is grown on graphene by CVD method;2,the grown graphene is transferred to the substrate,and then the carbon nanotubes array are grown by MW-PCVD.The results show that the maximum field emission current of the composite structure is 300 mA and the field emission current density is 3.75 A/cm2,and it has a good the stability.2?X-ray tube structure design and optimization.The influence of the parameters of the X-ray tube on the size of the anode focal spot is simulated by the electron beam simulation software,and the similar focusing characteristics are achieved under different anode voltages to meet the requirements of the dual-energy CT system.3.X-ray tube preparation and testing.According to the simulation design requirements,the corresponding electronic gun parts are machined,the brazing process is used to realize the welding of the gate and the grid.The carbon nanotube X-ray tube is prepared by ultra-high vacuum encapsulation process and the current emission characteristics and imaging test are carried out.The test results show that the maximum anode current reaches 110 mA,which achieves high resolution DR imaging and CT imaging with imaging resolution of 0.1 mm. |
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