Hot cathode

Abstract: Publisher Summary This chapter describes various aspects of electron probe microanalysis. The principal elements of a conventional electron probe microanalyzer are four in number. An electron optics system, consisting of an electron gun followed by reducing lenses, whose role is to produce at the level of the sample an electron probe with a diameter approximately between the limits of 0.1 and 2 μ. The probe is obtained by forming, by means of electron lenses, a much reduced image of the crossover produced by an electron gun. The gun is of the conventional hot cathode triode type. When the maximum Gaussian diameter compatible with complete occultation is obtained, a measurement is made by means of a Faraday cylinder, of the current carried by the beam. This can then be compared with the theoretical value deduced from the emissivity of the filament and from the spherical aberration of the probe-forming lens. The physical basis of the emission-concentration relation is also elaborated.

Abstract: The cathode spot of a vacuum arc generates a spray of liquid droplets directed almost parallel to the cathode surface as well as a highly ionized plasma jet directed normal to the cathode surface. Theories include droplet ejection by the reaction force of back-streaming ions on the underlying microscopic liquid pool and formation from explosive debris. The droplets have an exponentially decreasing size distribution and velocities ranging from 10 to 800 m s-1. During their motion in the arc, the macroparticles can be further accelerated and deflected, obtain a negative charge, be heated to temperatures of around 2000 °C and evaporate. The macroparticle mass emission rate from the cathode increases with increasing arc current and average cathode surface temperature and decreases with increasing cathode material melting temperature. Cathodes with gaseous surface layers have less macroparticle erosion than clean cathodes. Droplet production can be reduced by maintaining as low a temperature as possible on the cathode surface near the cathode spots by providing effective cooling, by operating at low cathode current densities, by using magnetic fields to provide for directed rapid cathode spot movement and, in reactive deposition, by operating a poisoned cathode. Macroparticle inclusions can be reduced by substrate biasing and by concentrating the plasma flow with magnetic fields and can be eliminated completely by using a curved magnetic plasma duct.

Abstract: The slow activity of cathode materials is one of the most significant barriers to realizing the operation of solid oxide fuel cells below 500 °C. Here we report a niobium and tantalum co-substituted perovskite SrCo0.8Nb0.1Ta0.1O3−δ as a cathode, which exhibits high electroactivity. This cathode has an area-specific polarization resistance as low as ∼0.16 and ∼0.68 Ω cm2 in a symmetrical cell and peak power densities of 1.2 and 0.7 W cm−2 in a Gd0.1Ce0.9O1.95-based anode-supported fuel cell at 500 and 450 °C, respectively. The high performance is attributed to an optimal balance of oxygen vacancies, ionic mobility and surface electron transfer as promoted by the synergistic effects of the niobium and tantalum. This work also points to an effective strategy in the design of cathodes for low-temperature solid oxide fuel cells. Sluggish activity of cathode materials impedes operation of solid oxide fuel cells at low temperatures. Here, the authors report a niobium and tantalum co-doped perovskite cathode exhibiting high electroactivity below 500 °C, and argue that the dopants improve the cathode performance synergistically.