Xizhu Zhang

Correlation Between Emission Behavior and Surface Features of Scandate Cathodes

Special emission behaviors have been observed historically and recently on scandate cathodes. For a better understanding of the phenomena, we investigated the correlation between emission properties of scandia-doped dispenser cathodes and their surface features. The influence of external electric fields on the semiconductor layer of the cathodes is suggested to be mainly responsible for the special emission characteristics.

Emission mechanism of high current density scandia-doped dispenser cathodes

The emission mechanism of scandia-doped dispenser (SDD) cathodes is characterized based on systematic investigation of microstructure, surface behavior, evaporation, and emission. High current density SDD cathodes have been developed in recent years to meet the requirements of advanced vacuum electron devices. Space charge limited current densities of over 100 A/cm2 have been reliably reached at a temperature of 950 °Cb. SDD cathodes are composed of a tungsten matrix with submicron structure and uniformly dispersed materials containing Sc, Ba, Ca, Al, and O as nanosized particulates in the matrix. The authors conclude that the copious electron emission from this cathode type results from a layer of Ba–Sc–O on W formed after activation on top of the matrix. This layer is about 100 nm thick and maintains a stable composition over its lifespan. Its emission can be explained in terms of a semiconductor layer with characteristics that differ from a metal-like emission model. The Ba–Sc–O layer is formed by codi...

Preparation and emission property of scandia pressed cathode

Abstract Scandia pressed cathodes were prepared by powder metallurgy method using mixed powder of scandia doped tungsten and barium-calcium aluminates obtained by different mixing methods. The element distribution uniformity greatly affected the emission property. The powder prepared by ball-milling had small particle size and uniform distribution of different elements. This kind of powder was favorable for the chemical reaction among Ba, Ca aluminates, scandia and tungsten to take place sufficiently to form active substance which can improve the emission property. The emission current density of the cathode prepared with ball-milling powder was about 8 times higher than that prepared by manually mixing method, 4 times higher than Os coated M-type cathode. The higher content of the active substance on the cathode surface prepared by ball-milling led to the better emission performance of the cathode.

High current density Sc 2 O 3 -W matrix dispenser cathode

Sc2O3-W matrix dispenser cathodes have been prepared by powder metallurgy method and tested in Pierce electron guns. The emission current density can reach 72 A/cm2 at 900°C and over 100 A/cm2 can be achieved at a temperature higher than 950°C. The emission improves and then keeps stable with time throughout the life testing period of 330 h at a continuous loading of 88 A/cm2 pulsed current density with a pulse width of 10 μs and duty cycle of 0.2%. The cathode surface is covered by a semiconductor multilayer composed of Ba, Sc and O. The emission behavior of the cathode can be explained by a semiconductor model.

INVESTIGATION OF INFLUENCE OF SURFACE NANOPARTICLE ON EMISSION PROPERTIES OF SCANDIA-DOPED DISPENSER CATHODES

The microstructure of a fully activated scandia doped dispenser (SDD) cathode has been studied by scanning electron microscope (SEM). The observation results display that nanoparticles appear at the growth steps and the surface of tungsten grains of the fully activated SDD cathode. To study the influence of the nanoparticles on the emission, the local electric field strengths around the nanoparticles have been calculated by Maxwell 2D code and Comsol. The calculation results show that the local electric field strengths are enhanced by 1.1 to 3.8 times to average value based on different model conditions. The highest field strength is about 1.54 × 105 V/cm at an average field strength of 40 KV/cm, which is related to a space-charge limited (SCL) current density of 100 A/cm2 in the experimental configuration. This implies the field strength is not high enough to cause field emission.

Characterization of scandate cathode emission

Characterization of emission properties of scandia doped dispenser (SDD) cathodes with Longo-Vaughan equation has been discussed in this paper.

Surface characteristics of scandate dispenser cathodes during life

To find the changes of surface compositions during life time of the Scandia-doped impregnated dispenser cathodes, two cathodes have been tested at an activation temperature of 1150°CB-Mo and an operation temperature of 1000°CB-Mo , respectively, in an AES system up to more than 7,000 hours. Surface compositions were monitored in-situ at intervals. The tested results have been reported and discussed in this paper.

17.6: Evaporation behavior of scandia doped dispenser cathode

The evaporated substance of Scandia doped impregnated dispenser cathode is mainly composed of Ba and BaO and Ba evaporated more easily than BaO from the surface of the scandate cathode, which is consistent with the result of impregnated Ba-W cathode. However, the evaporation rate of total substance and the evaporated amount of Ba and BaO from the scandate dispenser cathode are lower than those from impregnated Ba-W and M -type cathodes.

P3-37: Emission characteristics of Sc- doped dispenser cathodes in electron gun open structures

Emission properties of Scandia Doped Dispenser (SDD) cathodes tested in Pierce Gun open structures have been reported in this paper. 80 - 100 A/cm2 Space - Charge Limited ( SCL) current densities have been obtained at designed anode voltages and normal operating temperatures for cathodes with either flat surfaces or concave surfaces. The influences of anode effects on cathodes, which appear in close - spaced diodes, are discussed in the paper.

Investigation of influence of surface activator element on scandate cathodes work function

The surface analysis of scandate cathodes has revealed that active substances of Ba, Sc and O formed a layer on the surface of the cathodes. By using First-principles calculation method, the W and Ba-Sc-O-W systems were calculated to study the effect of active elements on the work function of the cathode.