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Diamond graphite nanocomposite will increase the durability of terahertz emitters
Scientists of the Saratov branch of the V.A. Kotelnikov Institute of Radio Engineering and Electronics of the Russian Academy of Sciences, Saratov National Research State University named after N.G. Chernyshevsky and NPP Almaz for the first time discovered the effect of restoring a diamond graphite cathode when the voltage polarity changes in the interelectrode gap, which will create more durable terahertz emitters.

Mastering the terahertz (THz) range of electromagnetic waves between the microwave and infrared ranges is one of the key problems of electronics development. Coherent THz radiation sources have broad prospects of application in such areas as security (remote detection of explosives), wireless information and communication systems for high-speed data transmission, radio astronomy, spectroscopy, medicine, etc. There are various approaches to mastering the THz range, for example, using solid-state or quantum electronics devices. However, to achieve power levels of the order of tens of watts and above, electric vacuum devices (EVP) are optimal.

The reliability and durability of the EVP are largely determined by the characteristics of the cathodes, that is, the sources of electron emission. Currently, most EVPs produced in the world use thermionic metal-porous cathodes (MPCs). However, the durability of the MPC is not high enough. Other disadvantages of thermal cathodes include a long readiness time and a low maximum current collection density.

A promising direction for improving THz band devices is the replacement of thermionic cathodes with field cathodes. Field emission is the emission of electrons by conducting bodies under the influence of an external electric field of sufficiently high intensity. In field cathodes, electrons overcome the potential barrier at the emitter boundary not due to the kinetic energy of thermal motion, that is, not as a result of cathode heating, as in thermionic emission, but by quantum tunneling through a barrier reduced by an external electric field. Of course, a cathode that does not need to be heated can serve longer and more reliably.

One of the most promising materials for creating this type of cathodes are nanocarbon film structures, in particular, diamond graphite nanocomposites, which are graphite matrices with diamond nanocrystallites embedded in them. In this study, diamondographic film structures with a thickness of about 100 nm deposited on polycore (corundum-based ceramics) plates were used as field cathodes.

The investigation of the operation of the diamond graphite cathode was carried out during 8 test cycles with a total duration of over 13.5 hours. During the tests, conditions with an emergency shutdown of the supply voltage and vacuum pumping facilities were simulated.

The graph above shows the current-voltage characteristics (VAC) of the cathode measured before (curve 1) and after (curve 2) 8 test cycles. Despite the unfavorable factors associated with periodic disconnections of the supply voltage and deterioration of the vacuum, the field emissivity of the cathode did not deteriorate during the tests.

However, the study showed that part of the carbon phase of the cathode degrades during operation and is deposited on the anode. During the experiment, a voltage negative relative to the cathode was applied to the anode, that is, the cathode, in fact, turned into an anode, and the anode became a cathode, that is, a source of electrons. After that, the voltage was applied to this pair again according to the usual operating scheme and the cathode VAC was measured.

The graph above shows the change in the cathode VAC before (1) and after (2) the deposition of the emission material from the anode to the cathode. An improvement in the emission capacity of the cathode after recovery was found. The positive effect was manifested in a decrease in the threshold for the onset of field emission and an increase in the steepness of the VAC curve, which allows obtaining similar currents at lower electric field strengths. This effect was discovered for the first time.

This effect can be used both to create a field cathode with improved emission characteristics, and to restore its emission ability during long-term operation as part of an EVP. The results of the study can be used to predict the service life of field diamond-graphite electron sources.

For more information, see the article "Durability of high-current field electron sources based on nanocomposite diamond-graphite film structures", R. K. Yafarov, A.V. Storublev, "Microelectronics", 2022, T. 51, No. 2, pp. 95-100.

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