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Nanoscale morphology of vanadium dioxide (VO 2 ) thin films can be controlled to realize smooth ultrathin crystalline films (<10 nm) with a simple way of DC sputtering. Here, crystallization annealing conditions determine whether a continuous ultrathin film or nanoparticle morphology is obtained. The experiments show that ultrathin VO 2 thin films possess both a highly crystal orientation (020) and an obvious metal-insulator transition (MIT). Meanwhile, optical transmittance between the visible and near-infrared regions can be modulated by thin film thickness, and the strain in the VO 2 lattice is also found to be dependent on the thin film thickness. The 8 nm thickness thin film shows that the change of resistance and visible transmittance is 2.8 orders of magnitude and 77.5%, respectively.

To fully realize the superiority of the iodine electric propulsion system in streamlining the size and reducing the operating cost, iodine hollow cathode technology must be developed. Considering the corrosiveness of iodine and the possible impurity of working propellant, the C12A7 hollow cathode with promising chemical ability was developed and tested. The C12A7 hollow cathode with a nominal current of 1-4 A was successfully ignited with iodine from the reservoir outside the vacuum chamber. It was operated at 1 A of anode current with a 1.2 mg/s iodine mass flow rate. Despite involuntary extinguishment, the C12A7 hollow cathode could be restarted repeatedly with a single operation time of up to twelve minutes and a total duration of half an hour. The unexpected fluctuation of iodine flow may be the reason for the short operation time. Experimental results and microscopical observation of the electride emitter show the compatibility of the iodine and electride emitter. For the development and demonstration of future single iodine electric propulsion of Hall thrusters, the iodine storage and supply system with precise control and regulation may be the critical technology.

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Emitter overheating is by far the largest problem limiting the performance of novel C12A7 hollow cathodes. To explore the failure operating point and degradation mechanism of the C12A7 hollow cathode, microscopic analyses of a degraded electride emitter after ten hours of thermal electron emission are presented in this paper. The morphology and composition variation of overheated electride emitters by SEM, EDS, and XRD indicate the melting and decomposition of electride of the surface layer. The monitored temperature of the electride emitter during the C12A7 hollow cathode operation shows that to avoid overheating the electride emitter, the average current density should be about 64 mA/mm2 allowed for the C12A7 hollow cathode in its current configuration. Experiment results of the heaterless C12A7 hollow cathode demonstrate that the Xe ions bombardment can remove the insulating layer and restore the thermionic emission capability for less degraded emitters. Based on experimental results and microscopic characterization, the depletion and degradation mechanism of electride emitters during the hollow cathode operation are discussed.