Research On Solid-state Reaction In Aluminates And Emission Performance

Thermionic cathodes are key part of vacuum electronic devices.There are several kinds of vacuum electronic devices,for example,traveling wave tubes,klystron,gyrotron,magnetron.The cathode may be said to be the heart of vacuum electron device.High speed development of solid state devices based on semiconductor technology has enabled wide applications.Many vacuum electronic devices have been replaced by solid state devices.When it comes to high power and high frequency realm however,vacuum electronic devices have special advantage.Vacuum electron devices are widely used in military applications.A good cathode needs to meet some qualities that include,low working temperature,low activation temperature,low evaporation rate,fast turn-on characteristics,iron-bombardment resistance,long-lifetime,uniform electron emission,and so on.The aim of this work is to study how synthesis methods and components influence the phase structure and electron emission ability.Impregnated Ba-W cathodes is widely used in many kinds of vacuum electron devices,many cathodes with high emission ability are based on this kind of material,so the studying Ba-W cathodes have great engineering value.Import 612 Aluminates have good emission ability.It is known from structural characterization that aluminates with Ba₅CaAl₄O₁₂2 main phase with little BaCO₃ phase have good emission performance.The goal of this thesis is thus to achieve this.Our Aluminates were prepared by ball-milling,pressing,and sintering.The phase structure of the resulting aluminates is then characterized by X-ray diffraction methods.Electron emission performance of the aluminates is tested using the standard program“GJB7675-2002”.Temperature compensation and temperature uncompensation methods were used to determine the electron emission performance.Synthesis methods of emission active materials scandate cathodes are very much like that of aluminates.So scandate is also synthesized and it's phase structure characterized by XRD methods.Series of samples have been synthesized under different sintering emperature,sintering duration,sintering atmosphere,and molar ratio of reactants.The phase structures of the various samples are then characterized to study the effect of preparation method and reactant proportions on the structure and phases present and onelectron emission ability.The following some achieved results:1.From XRD data and Retvield refinement results using Rietica software,the import sample is found to have B5CA+BaCO₃ phase structure.2.The 612 aluminates are also synthesized under air atmosphere and it's structure is characterized.It was found that adding CaO may stabilize the B5CA structure and that adding BaO may increase B3CA phase.Single phase structure of 612aluminates may be obtained by synthesizing the aluminate under atmospheric air.3.When 612 aluminates is sintered under specific oxidizing atmosphere at1200?,the formation of B3CA is prevented,so that same phase structure as import612 aluminates is obtained.4.Under the same test temperature,current emission density of 612 aluminates is greater than 512 aluminates.At 1130?_B,612 aluminates with B5CA and BaCO₃structure have current density(J_div=6.8A/cm²)greater than 612 aluminates with B5CA+B3CA structure(J_div=6.3A/cm²).The emission performance of the 612aluminates with B5CA and BaCO₃ structure matches that of import 612 aluminates.5.When 512 aluminates are mixed with little Sc₂O₃?<7wt.%?and sintered,B5CA phase and scandate?Ba₂ScAlO₅?phase are obtained.Increasing the amount of the Sc₂O₃ added to the reactants,lead to rise in the percentage of scandate?Ba₂ScAlO₅?in the reaction products.Emission performance of aluminates contain Sc₂O₃ is remarkably better than aluminates without Sc₂O₃.For elementary composition of 512aluminates,aluminates containing 3%of Sc₂O₃ have emission current density reaching18A/cm²?1100?_B?which is much greater than the case of 5%and 7%Sc₂O₃ content.Different aluminates with same percentage of Sc₂O₃ have different emission performance.Emission performance of 512 aluminates containing 3%Sc₂O₃ is better than corresponding 612 and 812.