Research On Structure,Electronic Emission Properties And Applications Of Impregnants For Dispenser Cathodes

Vacuum electronic devices?VEDs?have important applications in both national defense and civil fields.In recent years,the development of high power and/or frequency VEDs urgently needs to use good performance dispenser cathodes.Synthesis of high-quality impregnant materials is one of the most effective ways to improve the cathode performance.However,there are few researches on the impregnant materials for the dispenser cathodes,which limits the further improvement of the manufacturing level of thermionic cathodes in China.In this thesis,three kinds of impregnants,barium-calcium-aluminate,barium-calcium-scandium-aluminate and single-phase?Ba,Ca?₂ScAlO₅compounds,were successfully prepared by liquid-phase co-precipitation method.The chemical composition,phase transformation,thermal behaviors and morphology of these impregnants were systematically investigated by means of XRD,TG-DSC,XPS,FESEM,and EDS mapping analysis.In addition,the emission performace of the corresponding cathodes in diodes and VEDs were tested.Some valuable and innovative results are found in these studies and main results can be summarized as follows:1.Some commercial aluminates,such as 512,612,532 and 411 composition,were prepared by co-precipitation method.The results of phase composition analysis showed that the 512 and 612 composition has single?Ba,Ca?₃Al₂O₆and Ba₃CaAl₂O₇structure after sintering at 1200°C,respectively;the 532 composition is a mixture of?Ba,Ca?₃Al₂O₆ and Ba₃CaAl₂O₇ after sintering at 1200°C;the main phase of the 411composition sintering at 1200°C is?Ba,Ca?₄Al₂O₇phase and the impurity phase is Ba₇Al₂O₁₀and Ba CO₃.The test cathodes impregnated with 411 salt has the strongest electron emission ability,followed by the 612 salt,532 salt and 512 salt.2.In order to optimize the synthesis process of 612 aluminate for the application of impregnated dispenser cathodes,the influence of two precipitants,ammonium carbonate?AC?and ammonium hydrogen carbonate?AHC?,on the chemical composition,phase transformation,thermal behaviors and morphology of the precursors was systematically investigated.The results revealed that AC precipitant facilities to form NH₄Al?OH?₂CO₃?AACH?phase and barium calcium carbonate in the precursor,while the product precipitated by AHC comprises amorphous?-AlOOH phase and the carbonate.Both precursors precipitated by the AC and AHC undergo various stages of phase transformations and finally converted to an emission-active material Ba₃CaAl₂O₇ phase after sintering at 1200°C.DC emission current values have been measured and the maximum space charge limited current densities are 8.0 A·cm^-2 at 1130°C_B for the testing cathodes impregnated with the AC aluminates.The higher emission capacity originates from the barium content in the aluminates prepared with different precipitants.3.A series of 6Ba O·1Ca O·2Al₂O₃+x wt%Sc₂O₃?x=1,3,5,7,9?impregnants were synthesized by a co-precipitate method.?Ba,Ca?₃Al₂O₆,Ba₂ScAlO₅,and?Ba,Ca?Al₂O₄ phases were confirmed in the sintering products as well as impregnation residues by the XRD and FESEM results.The concentration of the Ba₂ScAlO₅ and?Ba,Ca?Al₂O₄ phases increase with x,while that of the?Ba,Ca?₃Al₂O₆ phase decreases.The addition of 3 wt%Sc₂O₃ significantly increasing the emission level to 11.4 A·cm^-2at1100°C_B for test cathodes.The relations between electron emission properties and phase concentration in the impregnants also have been investigated and discussed.The high concentration of the ineffective emission-active?Ba,Ca?Al₂O₄ phase leads to the deterioration of cathode emission performance.Therefore,the impregnant materials of8Ba O·1Ca O·2Al₂O₃·1Sc₂O₃ was designed in this thesis,which only contained?Ba,Ca?₄Al₂O₇ and Ba₂ScAlO₅phases.The electron emission capacity of the cathode using this sort impregnant has been improved.The space charge current density of about14.9 A·cm^-2can be recovered at 1100°C_B operating temperature.4.?Ba,Ca?₂ScAlO₅compounds are synthesized and improved emission performance is achieved by virtue of their single-phase structure and low work function.For the preparation of single-phase Ba₂ScAlO₅compound,it is necessary to add 25 at%Ba O to eliminate the impurity phase?Ba,Ca?Al₂O₄ in order to form single-phase structure.The substitution of Ca²⁺for Ba²⁺effectively lowers the melting points of?Ba,Ca?₂ScAlO₅from over 1800°C to 1700°C when the Ca²⁺substitution reaches 20 at%.During sintering,the co-precipitation precursor first reacts to form?Ba,Ca?₃Al₂O₆ near 912°C before reacting with Ba CO₃ and Sc₂O₃ at the higher temperature of 1186°C to generate?Ba,Ca?₂ScAlO₅.Emission tests reveal that the maximum full space charge current density is 9.6 A·cm^-2at 1000°C_B for the cathode impregnated with single-phase?Ba,Ca?₂ScAlO₅.Low work function??1.5 e V?and good emission uniformity make such a cathode promising for applications in high-power microwave devices.5.The band-beam cathodes for W-band extended interaction oscillator VEDs were successfully fabricated by impregnating single-phase?Ba,Ca?₂ScAlO₅ active material.In the practical application test,the cathode can stably withdraw 5.3 A·cm^-2 space charge limiting current at 1000°C_B,and has good electron emission uniformity,which can fully meet the design requirements.In addition,612 aluminate was used to prepare M-type cathodes for Ka band TWT-gyrotron.At 1100°C_B,the M-type cathodes can obtain a full space charge limited current density of 17.6 A·cm^-2 in the close-diode test.The practical test results show that the cathode can stably provide the required 10 A pulse emission current at 980°C,and the reliability and longlife can be guaranteed at a lower working temperature.