Extremely low secondary electron emission from metal/dielectric particulate coatings
Research on low secondary electron emission coatings is essential for the design and manufacture of space high-power RF devices without multipactor discharge. This paper discusses some of the factors that reduce secondary electron emission for metal-dielectric surfaces. We have studied the total electron yield (TEY) behavior of a particulate coating composed of a mixture of a metal (aluminum) nanoparticulates in solid state contact with a particulate dielectric material (polyimide thermosetting resin). Surface charging, roughness, and volume fraction are utilized as the main parameters to characterize the electron emission behavior, which can effectively be determined by continuous (total dose 42.5 nC/mm2) and pulsed (1,1 fC/mm2/pulse) electron irradiation methods. Apart from the important role played by surface composition in the TEY, the influence of the dielectric volume fraction has demonstrated to be critical to achieve a significant reduction of TEY. Analytical surface techniques such as field emission scanning microscopy and X-ray photoelectron spectroscopy were used, because they are very effective in providing insight into the effect of charging on the surface. Typical TEY parameters of the aluminum foil exposed to the air are: the first crossover energy, E1=20 eV, TEY maximum, σmax = 2.9, and Emax = XXX eV; these contrast with the dielectric values: E1= 30 eV, σmax = 2.5, and Emax = XXX eV. However, it was found that E1 of the particulate sample increased with increasing dielectric volume fraction. An extremely high first crossover energy, E1>1000eV, was obtained after the gold metallization of the metal/dielectric coatings of 0.75 volume fraction. It is also remarkable that TEY was 0.2 for E<1000>eV, the low energy emitted electrons appear to be reabsorbed. Another remarkable fact is that TEY curves measured by using either the continuous or the pulsed methods overlap in the whole primary energy range, dissipate the much larger electron dose of the continuous method as compared to the very low dose of the pulsed method; this result is usually understood as an indication of minimal influence of charging on TEY.