Research On The Synthesis Of Ba₂ScAlO₅ By High Temperature Solid-state Reaction And Thermionic Emission Mechanism For Impregnated Cathode

The vacuum electronic equipment is not only applied to national defense and military fields such as early warning radar,precision guidance and missile defense,but also plays an important role in satellite communications,navigation,deep space detection and other civilian fields.Modern vacuum electronic equipment is moving towards high-frequency and high-power fields.Therefore,as an electron source for vacuum devices,thermionic cathodes require higher emission current density.Our team has prepared（Ba,Ca）2ScAlO5 through liquid-phase co-precipitation method,and obtained high emission current density of（Ba,Ca）2ScAlO5 impregnated cathode.Regrettably,the emission mechanism of cathode has not been resolved.In order to accurately control the precursor composition and improve the synthesis efficiency,this thesis operates high-temperature solid-states reaction to synthesize the Ba₂ScAlO₅.Its preparation process,phase structure and microscopic morphology have been systematically investigated.In addition,excess Ba2+was replaced with a part of Ca2+.Experimental results indicate this practice can reduce the melting point of Ba₂ScAlO₅.For the impregnated cathode of Ca-doped Ba₂ScAlO₅,this thesis has successfully verified its superior electron emission capability,and attempts to explain the emission mechanism of cathode.The research results obtained in this thesis are summarized as follows:1.For the co-precipitation,the precursor can be sintered into β-Ba₂ScAlO₅ at about 1200℃.However,the high temperature solid-state reaction needs the sintering temperature to reach about 1500℃.2.For the co-precipitation,there is chemical reaction process for the synthesis of Ba₂ScAlO₅:（Ba,Ca）3Al2O6+Sc2O3+（Ba,Ca）CO3→2（Ba,Ca）2ScAlO5+CO2↑（～1200℃）On the other hand,the high temperature solid-state reaction requires the formation of BaAl2O4,Ba3Al2O6 and Ba2Sc2O5 compounds.And these three compounds react and synthesize Ba2ScAl05 at 1500℃,As follows:BaAl2O4+Ba3Al2O6+2Ba2Sc20s→4Ba₂ScAlO₅（～1500℃）3.The α-Ba₂ScAlO₅（generally,an unstable phase at room temperature）can be obtained through adjusting the ratio of BaO-Sc2O3-Al2O3.Its chemical formula is Ba2.2Sc0.7Al1.3O5.2.In an air sintering atmosphere,in addition to the main phase,there is a small amount of BaAl2Ocompound in the sintered product.4.The impregnated experimental cathode of Ca-doped Ba₂ScAlO₅ shows excellent electron emission capability.For the Ba2.1Ca0.5ScAlO5.6（β phase）cathode,Jmax is 13.40,12.99,12.24 and 11.22 A/cm2,corresponds to 1130,1100,1050 and 1000℃B.When the operating temperature of cathode is 950 and 900℃B,Jdiv is 8.62,6.79 A/cm2.For the Ba2.125Ca0.3cAlO5.125（a phase）cathode,Jmax is 12.45,11.59,10.50 A/cm2,corresponds to 1130,1100 and 1050℃B.When the operating temperature of cathode is 1000,950 and 900℃B,Jdiv is 7.10,4.50,2.08 A/cm2,respectively.The cathode of Ca-doped Ba₂ScAlO₅ can produce a higher and stable emission current density at a lower working temperature,and has a wider space charge limitation area.It can be predicted that Ca-doped Ba₂ScAlO₅ has broad application prospects for thermionic cathodes.5.Regarding the emission mechanism of thermionic cathode,Ba2CaWO6 is an important effective material.The Ca-doped Ba₂ScAlO₅ impregnated cathode has the properties of low operating temperature and high current density,its reason is that a low work function layer composed of Ba3Sc2WO9 compound is formed on the tungsten matrix.In terms of engineering application and theoretical analysis,this thesis will help to fully comprehend the phase structure of scandium-containing aluminate prepared by liquid-phase co-precipitation,solid-state reation or other methods.Some research results also help explain the mechanism of scandate cathodes with low operating temperature and high electron emission capability.