Research Progress of Coatings Modulating Secondary Electron Emission Characteristics
Secondary electron emission,which occurs typically in both metals and non-metals as a result of high energy primary electrons interacting with matters,is important for a range of scientific and technical applications,including radiation biology,high energy accelerators,particle detectors and electron microscopy.In the fields of microanalysis and accelerator physics,there is a tendency to enhance the performance of electronic devices characterized by increasing secondary electron emission yields of inner materials.When it comes to high-power microwave aircraft component applications,the secondary electron multipacting effect might result in electron avalanche and surface discharge,which can lead to major breakdowns and insulation failures of the devices,which is deemed to be dangerous.Finding a solution to prevent this from happening is highly critical for the stability of device performance.According to properties of the electronic device and its working condition,the modification of secondary electron emission coefficient is classified into two categories:suppression and improvement.Film coating process provides an effective and relatively common surface treatment technology to directly alter surface morphology,chemical composition,and microstructure characteristics.Secondary electron emission process has been shown to be dependent on substrate materials and coating films.It may also display a combined effect of the top film and the bottom substrate,which is determined by coating thickness.In comparison with bulk materials,a more complicated physical phenomenon is indicated.In the beginning of this paper,a discussion of the physical mechanism and theory of coating film secondary electron emission were given.The semi-empirical physical model described the penetration depth of incident electrons and secondary electron yield in different thickness of coating films and various metallic substrates.It was applied for the purpose of getting accurate secondary electron yield determined by different coating properties according to practical measurement data.The benefits and drawbacks of the model were summarized,and optimization direction which included electrical conductivity of dielectric material was given,with focus on higher accuracy and larger scope of application.Second,the modulation of secondary electron emission might be classified into two unique ways,augmentation used in vacuum electronic devices and suppression used in high power microwave components and sources.Representative coating materials of high secondary electron yield materials(＞3)and low secondary electron yield materials(＜2)reviewed with recent research work were introduced,and it was closely related to the properties of emission layer.It was discussed in terms of coating process,microstructure and secondary electron emission performance,respectively.Coating methods,such as chemical vapor deposition(CVD)and physical vapor deposition(PVD),were used to deposit TiN,graphene,and Pt films to decrease secondary electron yield of the metal surface.On the other hand,oxide thin films such as Al2O3、MgO and ZnO films produced by atomic layer deposition(ALD)were successfully utilized as secondary-emitting materials in electron multipliers,photomultiplier tubes,and other devices due to their high secondary electron yield and stable performance.Lastly,based on developments and limitations of electron-emission materials,an appropriate perspective for future trends in the secondary electron emission modulation of coating film was commented.
secondary electron emissionmodulation of secondary electron yieldfilm coating processsurface microstructurenano-film
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