Scandia Doped Dispenser Cathodes Research Articles

Robustness Investigation on Nanosized-Scandia-Doped Dispenser Cathodes

In order for practical applications in vacuum electron devices, the robustness of scandia-doped dispenser (SDD) cathode is a major concern. Emission properties after air exposure and gas poisoning have, therefore, been studied. A full restoring of emission has been proved after the cathodes were exposed to air and reactivated for a time much shorter than initial activation. Poisoning experiments demonstrated that the poisoning pressure thresholds of residual gases for the SDD cathode are about half to one order of magnitude higher at equivalent temperatures but with higher emission level in comparison with that of an excellent M cathode. The cathode is easier to recover after poisoning and can operate stably in a nonideal environment. These features are thought to be related to the surface Ba–Sc–O structure which has closely chemical association, better stability for air exposure, relatively high tolerance to gas poisoning, and the semiconductor emission characteristic.

Analysis of Mechanical Strength for Nanosized Scandia Doped Dispenser Cathodes

Analysis of Mechanical Strength for Nanosized Scandia Doped Dispenser Cathodes

Study on the compositions of emission active materials and the emission properties of dispenser cathodes made from Scandia-doped powders

Scandia-doped dispenser cathode provided significantly better performance compared to standard dispenser cathodes in the application of microwave devices. Emission active materials are the key to dispenser cathodes. Scandium doping can enhance the performance of dispenser cathodes. In this paper, active materials are fabricated by co-precipitation method. Comparison is made between the active materials undergoing different heat treatments. XRD and SEM methods are employed to analyze the compositions and microstructure of active material samples. The cathode matrix powders are prepared by the sol–gel method using different recipes of scandium content. The contents of scandium oxide are, respectively, 3.5wt%, 4wt% and 5wt%. Emission tests are carried out in dedicated test vehicles. The results show that there are different aluminates including Ba4Al2O7, CaAl2O4, Ba7Al2O10, Ba3CaAl2O7, and Ba3Al2O6 come into being after heating using different techniques. The cathode impregnated with self-developed emission materials which numbered LN4 can achieve an inflection point emission current density of 122A/cm2 at 1165°C. On the study of different cathode matrix powders, when impregnated with the conventional material (S), the cathode made from 3.5wt% Scandia-doped powders has the best emission performance. The 4wt% cathode sample performs better when the cathodes are impregnated with the self-developed emission materials.

Study of mini-themionic electron sources for vacuum electron THz devices

THz technology has attracted great attention for decades of years. Among the wide research areas of THz technologies, vacuum electron terahertz radiation sources have obvious advantages in high power region. For the THz vacuum electro devices (VED), high current density electron beams with small dimensions are required. Nanosized scandia doped dispenser (SDD) cathodes have the capability to operate stably at pulsed current densities of over 100 A/ cm2 at 950 ℃ so it becomes the most promising cathodes to meet the requirements for THz VEDs. In this paper, we report a new approach for developing miniaturized electron beam sources on normal SDD cathodes. An electron beam of 400 μm in diameter has been directly generated on an SDD cathode by deposition of a Zr/W double-layer anti-emission film and followed by a focused ion beam (FIB) milling. Results indicate that the electron beam is able to provide a space charge limited (SCL) current density of over 50 A/cm2 at the operating temperature of 950 ℃ with proper laminarity and works stably for more than 1000 hours. The beam emission characteristics and the function of the anti-emission film have been discussed and related to the surface analysis results. The approach opens a new way for producing high emission mini-electron sources to satisfy the requirment of THz VEDs.

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.

Preparation and characterisation of scandia doped dispenser cathodes

Scandia doped dispenser cathodes were prepared by powder metallurgy method using mixed powder of scandia doped tungsten, barium–calcium aluminates obtained by sol–gel method. The scanning electron microscope observation results indicate that the cathode has submicron quasi–spherical tungsten grains structure with homogeneous distribution of Sc2O3 and barium–calcium aluminates which dispersed over and among W grains. The cathode exhibits good emission property, i.e. the current density of this cathode can reach more than 40A/cm² at 850°Cb. The cathode has low work function of 1.43 eV. Active substance of Sc, Ba and O diffuse from the inner part of cathode to the surface to form a Ba–Sc–O multilayer having certain ratios of Ba:Sc:O which contains much more Ba than normal dispenser cathode at the surface of Sc2O3–W matrix cathode during the activation period. This active–multi–layer is considered to be responsible for copious emission capability.

Fabrication of Aluminate for Scandia Doped Dispenser Cathode and Emission Property

Barium calcium aluminate (4BaO:CaO:Al2O3) was synthesized via liquid phase co-precipitation and solid phase mixing respectively and sintered in hydrogen atmosphere. Scandia doped tungsten powders were pre- pared in a new process of spray drying combined with two-step hydrogen reduction. During the preparation of scandia doped impregnated cathodes, a series of procedures, including pressing, sintering, impregnating, water cleaning and annealing were taken to obtain the impregnated cathodes. The results show that Ba5CaAl4O12 is formed after high temperature sintering. After impregnation, scandia doped impregnated cathodes with porous sub-micron structure are obtained. For full activation at 1150℃, the cathode surface is covered by a Ba-Sc-O active substance layer with a preferable atomic ratio, leading to its good emission property with space charge limited current density of 37.89 A/cm 2 at 850℃.

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 codiffusion of Sc, Ba, and O (generated inside the W-matrix) to the surface of the W-matrix during activation. Furthermore, this layer guarantees a low evaporation rate of emission-relevant materials and thus enables a longer lifespan.

Emission Characteristics of Nanosized Scandia-Doped Dispenser Cathodes in Open Electron-Gun Structures

The emission properties of nanosized scandia-doped dispenser cathodes tested in open-structure Pierce electron guns are reported in this paper. Over 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and over 80 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> space-charge-limited current densities have been obtained at the operating temperatures of 950 °C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> and 1025°C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> for cathodes with planar emitting surfaces and cathodes with concave emitting surfaces, respectively, at rated anode voltages.

Development of High-Current Sheet Beam Cathodes for Terahertz Sources

Generation of a sheet beam directly from a scandia-doped dispenser cathode has been investigated and optimized by means of beam-profile simulations and measurements. Rectangular beams 600 mum wide and 100 mum thick with current densities over 50 A/cm2 have been generated. The beams are stable for at least several hundred hours with the cathodes operating at 950degCb, making them promising candidates for application in the next generation of terahertz vacuum electron devices such as a 0.5-THz superradiant Smith-Purcell radiation source.

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.

Generation of High-Current-Density Sheet Electron Beams

High-current-density sheet beams directly generated from scandia-doped dispenser cathodes have been studied for application to vacuum electron devices like terahertz sources. Rectangular beams with current densities of over 50 A/cm2 have been developed and demonstrated to maintain for several hundred hours with the cathode operating stably at 950degCb. Beam forming structures have been investigated and optimized by means of beam profile simulation and confirmed by test results.