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This paper presents early results of experimental investigation of field electron emission properties of an all-carbon composite material produced from nanodiamond powder. By the means of CVD, diamond grains were covered with onion-like graphitic nano-layers merging at grain junction points and thus solidifying the whole structure. Such composites combine features that might make them very promising cold cathode materials: high conductivity, mechanical strength and extremely large number of uniformly distributed sp3-sp2 interfaces. The role of interface areas can be crucial for attaining to efficient and stable emission from cathodes without high geometric field amplification. Flat diamond films demonstrate excellent emission characteristics, and their active emission centers are known to be associated with defects and graphite inclusions. On the other hand, electron emission from graphitic and metal surfaces is often localized at dielectric contaminant films or grains.
Emission parameters of the composite achieved in these early experiments are typical for many low-aspect-ratio nano-carbons: threshold current of 1 nA from a 0.05 cm2 sample appears at 3-5 kV/mm field, and maximum extracted current is as high as 0.1-1 mA. Current characteristics in Fowler-Nordheim coordinates deviate from linearity, which can be associated with presence of different types of emission sites at the surface. Emission activity is enhanced by thermo-field treatment (extraction of small current from a specimen heated to 300-850°C), as it is usual for multi-phase nano-carbon emitters. The thermo-field processing shows to be effective only when electrons from the specimen directly hits the anode. If a part of current is extracted through an aperture in the anode, the sample area opposite the aperture remains practically unactivated. We explain this phenomenon by an important role of ions originating at the anode in the process of formation of active emission centers. Our next steps will consist in more accurate selection of the composite parameters and activation routine that would result in further improvement of emission properties of developed cathodes.
The work is supported by the RFBR (grant 02-05-17206).
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