Carbon nanowalls in field emission cathodes

Abstract

Carbon materials, including various crystalline (diamond, graphite) and noncrystalline (fullerene, nanotubes, graphene, etc.) ordered substances with unique physicochemical properties are of practical interest. Some carbon materials due to the autoemission property are promising for use as an emitting layer of field emission cathodes (autocathodes). The presence of field emission means a decrease in the electric field strength to 1-10 V/μm, which is required for the onset of field emission of electrons. Autocathodes are used in the development of X-ray tubes, microwave devices, electron guns for exciting lasers, cathodoluminescent lighting devices, flat displays and other devices [1][2][3][4][5]. The most promising for the creation of autocathodes with a low electron emission barrier are the so-called carbon nanowalls (CNW) -layers of a plate-like carbon material with a predominant orientation of the plates perpendicular to the substrate [1][2][3].The layers of carbon materials formed by plasma methods, including CNW, are as a rule multiphase layers [6][7][8]. The structure and concentration of crystalline and X-ray amorphous phases depend on the conditions for carbon materials formation and affect their emission properties. The problems of using CNW in autocathodes are associated with the instability of emission parameters (magnitude and density of the cathode current, as well as the degree of electrical current uniformity over the cathode area) due to changes in composition and structure during testing and operation [4,6,8].Before being placed into electrovacuum devices and soldered, autocathodes are always preliminaryly tested in a vacuum chamber for compatibility with the parametres of the device. In some cases, preliminary tests are carried out to achieve such required parameters as autoemission current and its stability in time. Stability tests can be performed both in the voltage stabilization mode [4] and in the current stabilization mode. In the first case we consider cathode current dependence (decrease) on time at a fixed stabilized voltage, in the second case -voltage dependence (growth) on time at a fixed stabilized current. In both cases, the graphs of the dependencies (hereinafter aging curves) objectively characterize the degradation (aging) of the autocathode, regardless of what causes it.Storing the tested autocathodes with CNW layers on open air also leads to a deterioration of their emission properties. This is caused by the fact that during vacuum testing on the surface of CNW plates, the layer of adsorbed hydrocarbons is destroyed. This layer normally prevents adsorption of the components of the air mixture (water and nitrogen molecules, etc.) that impair the emission characteristics of the autocathodes [4, 15]. To recover the emission properties of autocathodes that had passed preliminary tests, they are annealing in vacuum or in an inert gas