Advanced Dispenser Cathodes
Controlled porosity reservoir cathodes offer significantly improved performance over conventional impregnated cathodes. Optimized performance requires additional design to achieve reduced barium deposition on adjacent surfaces, increased lifetime, and high current density emission. This paper describes the achievable performance and the associated design requirements.
Migration and escape of barium atoms in a thermionic cathode
A quantitative study of the physical processes on a thermionic cathode that are crucial to both cathode life and noise are investigated. Of particular interest are the transport of the barium atoms from the cathode interior to the cathode surface and beyond. To investigate these physical processes, a model of barium migration to the surface and its subsequent removal by bulk and monolayer evaporation and ion spattering is presented. This model incorporates material and tube environment parameters for the calculation of key terms governing diffusion, evaporation, and sputtering. Estimates of coverage, adsorbate work function values, lifetime, and current density are made, which are in qualitative agreement with the experiment. The model is based on a hopping model of barium diffusion to determine barium concentration within the cathode and the surface coverage ratio. Insofar as possible, all parameters are determined from simple models of the underlying physical processes.
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.