Thermionic cathodes are used in critical civilian and military components including radar, communications, materials processing, electronic warfare, and high-energy physics research technologies. See J. H. Booske, “Plasma physics and related challenges of millimeter-wave-to-terahertz and high power microwave generation,” Physics Of Plasmas 15, 055502 (2008) (“Booske 2008”).
In traditional cathodes, increased electron emission is generally achieved by increasing the operating temperature, but results in degradation of the cathode by the depletion of surface barium (Ba) through evaporation, effectively decreasing the lifetime of the cathodes.
Scandate-based cathodes share the same backbone as common thermionic emitters such as the dispenser B-type cathode composed of pressed and sintered tungsten powder impregnated with a precise compositional mixtures of an emissive mix comprising BaO, CaO, and Al2O3. See J. L. Cronin, “Modern Dispenser Cathodes,” IEE PROC., Vol. 128, Pt. I, No. 1. pp. 19-33 (1981).
Academic and industrial research have established that scandate-based cathodes have the potential to be the next generation electron emitter cathodes based on the demonstration of substantially improved emission properties over other thermionic electron emitters. The best scandate-based cathodes reach a current density of 52 A/cm2 at 850° C., see Wang 2008, supra, though some scandate-based cathodes systems have displayed current densities of ˜400 A/cm2 at temperatures of 965° C. (an 800% improvement in emission over traditional cathodes). See G. Gärtner et al., “Emission properties of top-layer scandate cathodes prepared by LAD,” Applied Surface Science 111 (1997) 11-17. Such improvements can lead to longer lifetimes and vastly improved device characteristics for devices which require a large supply of emitted electrons and high power densities, such as THz-regime vacuum electronic devices, high resolution display tubes, and pick-up tubes. See Booske 2008, supra; J. H. Booske et al., “Vacuum Electronic High Power Terahertz Sources,” IEEE Transactions On Terahertz Science And Technology, Vol. 1, No. 1, September 2011 (“Booske 2011”); and S. Yamamoto, et al., “Application of an Impregnated Cathode With W-Sc2O3 to a High Current Density Coated Electron Gun,” Applied Surface Science 3/4 (1988) 1200- 1207 (“Yamamoto 1988”).
“Traditional” scandate cathodes simply augment the compositional mixture of oxides to include small amounts of Sc2O3. See A. van Oostrom and L. Augustus, “Activation and Early Life of Pressed Barium Scandate Cathode.” Applications of Surface Science 2 (1979) 173-186. The cathodes are produced by sintering the powder mixtures or impregnating a partially sintered tungsten metal matrix with the emissive mix. Though these early studies revealed the enhanced emission of scandate-based cathodes by demonstrating current densities of ˜10 A/cm2 at 950° C., such preparation techniques have been shown to produce cathodes with non-uniformity and instability in electron emission, see R. M. Jacobs et al., “Intrinsic defects and conduction characteristics of Sc2O3 in thermionic cathode systems,” Phys. Rev. B 86, 054106 (2012); van Oostrom, supra; and J. Wang, W. Liu, L. Li, Y. Wang, Y. Wang, and M. Zhou, “A Study of Scandia-Doped Pressed Cathodes,” IEEE Transactions on Electron Devices, Vol. 56, No. 5, pp. 799-804 (2009) (“Wang 2009”), and do not provide enough processing control to allow consistent reproducibility in cathode behavior. See Gärtner, supra.
High electron emission scandate-based cathodes systems were discovered approximately 50 years ago. See U.S. Pat. No. 3,358,178 Figner et al., “Metal-Porous Body Having Pores Filled with Barium Scandate”; and van Oostrom, supra. However, they have failed to make the transition from laboratory demonstration to industrial production in all but a few limited cases, see S. Fukuda et al., “Performance of a high-power klystron using a BI cathode in the KEK electron linac,” Applied Surface Science 146 1999 84-88; and J. Li et al., “Investigation and application of impregnated scandate cathodes,” Applied Surface Science 215 (2003) 49-53, as a result of observed non-uniformity and instability in electron emission, see J. W. Gibson, “Investigation of Scandate Cathodes: Emission, Fabrication, and. Activation Processes.” IEEE Transactions on Electron. Devices, Vol. 16, No. 1, January 1989. More recent studies have highlighted the obvious need for control over the microstructural uniformity of the scandate nanostructure and the location of scandate relative to the tungsten metal matrix. See Wang 2009, supra; see also J. Wang et al., “Sc2O3-W matrix impregnated cathode with spherical grains,” Journal of Physics and Chemistry of Solids 69 (2008) 2103-2108 (“Wang 2008”).
The most recent attempts to evenly distribute scandate have endeavored to co-dope tungsten with scandium, resulting in various distributions of nano-scale scandate particles on sub-micron tungsten powders. The best emission arises from cathodes comprised of sub-micron tungsten with the “most even” distribution of scandate nanopowders. See Wang 2008, supra. While these appear to be the “best” cathodes, the publications often state that dozens of cathodes were tested before optimal emission was achieved, suggesting poor control over the process of distributing scandate. Better control over the scandate coating and overall microstructural design of the cathode (such as tungsten powder size and scandate thickness) might lead to even greater improvements in emission. Furthermore, thin film studies on model cathode systems have identified the need to have the scandate as a separate nanometer thick layer in between the emissive mix and the tungsten, see C. Wan et al., “Tungstate formation in a model scandate thermionic cathode,” J. Vacuum Science & Technology B 31(1), 011210 (2013), for enhanced electron emission. Therefore, it appears that the “best” cathodes should actually have conformal and uniform nanometer thick scandate directly on tungsten powders (sub-micron or nano).
Various attempts have been made to mitigate the issues describe above, including use of different powder mixtures, see J. Hasker et al., “Scandium Supply After Ion Bombardment of Scandate Cathodes,” IEEE Trans. on Electron. Dev. Vol. 37, No. 12, December 1990, 2589-2594 (“Hasker 1990”), and coating the top of the cathode with tungsten (W) and various scandates. See Yamamoto 1988, supra; see also J. Hasker et al., “Properties and Manufacture of Top-Layer Scandate Cathodes,” Appl. Sur. Sci. 26 (1986) 173-195 (“Hasker 1986”); and S. Yamamoto et al., “Work Function Measurement of (W-Sc2W3O12)-Coated Impregnated Cathode by Retarding Potential Method Utilizing Titaniated W(100) Field Emitter,” Japanese Journal of Applied Physics, Vol. 28, No, 5, May 1989, pp. L865-L867 (“Yamamoto 1989”).
Recent studies suggest that the emission uniformity of scandate cathodes primarily depend on the distribution of the scandate, with a more uniform distribution leading to more uniform emission. See A. Shih et al., “Interaction of Sc and O on W,” Applied Surface Science, 191 (2002) 44-51; and J. Wang et al., “Preparation and emission property of scandia pressed cathode,” Journal of Rare Earths, Vol. 28, Spec. Issue, December 2010, p. 460 (“Wang 2010”). Therefore, state-of-the-art techniques employ liquid-liquid doping techniques in an effort to evenly distribute scandium by precipitating scandium-“doped” tungsten or tungsten oxide (then reducing the tungsten oxide with hydrogen). See Wang 2008, supra, and Wang 2010, supra. The scandium/tungsten powder can then be sintered and impregnated with the traditional oxide mixture.
However, electron microscopy reveals that nanoparticles of scandium oxide actually co-precipitate on the surface of the tungsten powder see Wang 2008, supra, rather than “dope” the tungsten. More importantly, microscopy reveals that, while nanoparticles cling to many of the tungsten particles, there are tungsten particles void of scandium oxide.
It has been theorized that a Ba—Sc—O monolayer formed on the tungsten substrate is responsible for the high emission density, see S. Yamamoto, “Fundamental physics of vacuum electron sources,” Rep. Prog. Phys. 6 (2006) 181-232 (“Yamamoto 2006”) and Y. Wang et al., “Emission mechanism of high current density scandia-doped dispenser cathodes,” J. Vacuum Sci. and Tech. B 29 04E106 (2011) (“Wang 2011”), suggesting that the order or layering of the Ba (i.e. emissive mix) is not critical.
However, more recent systematic studies on model thin-film scandate cathodes reveal that the best thermionic electron emission is observed from areas initially composed of 200 nm of BaO deposited on 200 nm of Sc2O3 deposited on tungsten. See Wan et al., supra. A reversed thin-film cathode structure, where Sc2O3 was deposited onto BaO was determined be a poor emitter and heating of that surface produced residual surface coverage of bulk crystals. For the BaO on Sc2O3 on W, at the end of the cathode life (since they are thin film cathodes there is no replenishment of emitting material) the deposition/emission area was completely devoid of thin film BaO, Sc2O3, of observable bulk oxide, or tungstate material. It is suggested that the scandate acts as a barrier between the BaO and tungsten that prohibits the formation of any barium tungstate, which reduces the emissive properties. Importantly, the key similarity between the co-precipitated “doping” process and the thin-film studies is the nanostructure of the scandate material which resides in between the BaO (or oxide mixture) and the tungsten metal frame.
Interestingly, the exact role of the scandate in the enhanced electron emission process is not well understood. A few experimental attempts to replace scandium with another similar element, such as europium and other rare earths, have resulted in reduced emission. See J. Wang et al., “A study of Eu2O3, Sc2O3 co-doped tungsten matrix impregnated cathode,” Journal of Physics and Chemistry of Solids 72 (2011) 1128-1132; and S. Yamamoto et al., “Electron Emission Properties and Surface Atom Behavior of Impregnated Cathodes with Rare Earth Oxide Mixed Matrix Base Metals,” Applications of Surface Science 20 (1984) 69-83 (“Yamamoto 1984”). Since conventional theory expects that elements from the same group (i.e., column in the periodic table) should behave similarly and that the Lanthanide series also exhibit similar behaviors, the finding that Eu does not mimic Sc in these cathodes systems suggests that identifying an alternative material will be difficult.
Though a theoretical consensus has yet to be determined, careful review of experimental work identifies two critical elements for optimal and consistent emission, the uniformity of the scandate material that acts as a barrier between the emissive mix and the tungsten surface and its nano-sized scale. Furthermore, modifications to the scandate thickness and the tungsten particle size may even improve the scandate cathode emission properties.