Emission current stabilization

The emission current stability of a material is important for practical applications. It demonstrates the fluctuation and degradation of the emission current density during prolonged FE. 

The tube-current stabilizer is usually put in the grounded return output circuit of the high-voltage transformer. The stabilizer functions by properly adjusting the a-c heating current through the filament (x-ray tube cathode), and in this way regulating the electron emission. 

Bormashov V.S., Tchesov R.G., Baturin A.S., Nikolski K.N., Shehsin E.P. (2004) The current stability of field emission cathodes of carbon nanotubes under ion bombardment. IFES 04 Abstracts book, Graz, Austria, 109. 

Field emission cathodes were examined in the long duration mode for 10 hours under fixed value of current 50 pA. The test objective was in stabilization of the emission current value by means of the high-voltage power supply. During the experiment measurements of voltage, supplied on equipment, and emission were performed every second. 

The excellent FE properties of CNTs, such as low-voltage operation, good stability, stable and high emission current, and large field enhancement factor, have opened new application fields in CNT-based devices. The use of CNTs as a source of electrons under an applied electric field is one of the most promising applications. As shown in Figure 8.5, it is easy to control the FE pattern of CNTs to... 

FIGURE 8.4 Emission stability of an SWCNT strand under a vacuum of 5 x KE Pa at room temperature with an initial emission current of 200 xA (corresponding to aeurrent density of 14 A/em ) in 175 h. (Reprinted with permission from Liu, C. et al.. Field emission properties of macroscopie single-waUed carbon nanotube strands. Applied Physics Letters, 2005, 86(22) 223114. Copyright (2005), Ameriean Institute of Physics.)... 

A CNT-based x-ray tube has also been demonstrated [103-108]. Yue et al. deposited CNTs on a substrate with an iron adhesive layer using dielectrophoretic deposition. The metal layer was found to increase not only the emission current but also the emission stability [109]. In this study, the... 

The emission from an individnal CNT can constitnte as a point emitter [111-114]. CNTs exhibit significantly better emission stability than current tnngsten-based field emitter, and this stability can be achieved at a moderate temperature rather than at room temperatnre. A CNT-based point electron emitter was fabricated nsing cavity-confined dielectrophoresis by Lim et al. [114] for which the emission current of an individnal MWCNT was stable np to 10 pA and reached 2 mA (1.7 x 10 A/cm ). At a low electric field, the current fluctuated in a stepwise manner. At higher current emission, the stability was interfered with plateau-like fluctuations that are associated with fleld-indnced unraveling and Joule heating of the cap structures of MWCNTs. 

The following data on emission currents from carbide tips were obtained at room temperature immediately after flash heating a tip. As it has been found that the stability of the total field emission currents is almost equivalent to that of the local emission currents, measured emission currents are usually the total fleld emission if there is no indication in the text. 

It has also been established that the current stability of TiC tips is independent of the tip axis. Emission currents of the TiC<110> tip (18-20) at 1600°C (temperature region 2), which are very important for applications are shown in Fig. 6(18). The steplike and spikelike fluctuations peculiar to carbide tips are seen. As emission currents are relatively large, the stable temperature regions 1 and 3 cannot be defined in this experiment. Also, the TiC< 111 > tip (24) has the same current feature as < 100> and < 110> tips. 

After this procedure, the effects of the surface processing appear. The first effect is a change in the FE pattern, the second is an increase in the emission currents under the constant-voltage condition, and the third is stabilization of the emission currents. These effects are always reproduced by the same procedure. This is a general phenomenon among transition metal carbides. 

The surface-processed tip shows highly stable emission compared with a W tip. Therefore the usual evaluation of current fluctuations is not adequate. Here we define a new term, stable emission current (9). This is the maximum current whose fluctuation amplitude is less than 1% in the initial 20 min after applying the voltage. In contrast to the surface-processed TiC<110> tip, the emission stability of the NbC<110> tip depends on the ethylene exposure (10). These experimental data are shown in Fig. 15. The ordinate is stable emission current (/) times the environmental pressure (F), which means the degree of the stability. The stability of... 

Figure 16 shows the stable emission current versus pressure relation for the surface-processed TiC<110> and NbC<110> tips (10). Data represented by open circles (a) are for the TiC<110> tip and data shown by full circles (b) and a solid line (c) are for the NbC<110> tips. The data of (a) and (b) show that log I is proportional to -log P. The current fluctuations are proportional to the product of I and P. The solid line in Fig. 16c is a calculated stable emission current for the 25,000 L ethylene-processed NbC<110> tip using data for the stable emission current of 24 pA at 2.1 X 10 Pa. The stable emission current of the 25,000 L ethylene-processed NbC<110> tip is about 50 pA at 1 x 10 Pa. Figure 16 indicates that the stable emission current for the ethylene-processed NbC tip is larger than that for the ethylene-processed TiC<110> tip. That is, the NbC tip is more stable than the TiC tip. This difference in emission stability is attributable to lack or existence of monolayer graphite on the (100) surface. Monolayer graphite can be formed on the NbC(lOO) surface over 25,000 L ethylene exposure, but it is quite difficult to form monolayer graphite on the TiC(lOO) surface. 

S Yamamoto, H Hosoki, S Fukuhara, M Futamoto. Stability of carbon field emission current Surf Sci 86 734, 1979. 

The current-voltage and luminance-voltage characteristics of a state of the art polymer LED [3] are shown in Figure 11-2. The luminance of this device is roughly 650 cd/m2 at 4 V and the luminous efficiency can reach 2 lm/W. This luminance is more than adequate for display purposes. For comparison, the luminance of the white display on a color cathode ray tube is about 500 cd/m2l5J. The luminous efficiency, 2 lm/W, is comparable to other emissive electronic display technologies [5], The device structure of this state of the art LED is similar to the first device although a modified polymer and different metallic contacts are used to improve the efficiency and stability of the diode. Reference [2] provides a review of the history of the development of polymer LEDs. 

Spectrum analysis, 61-64 Spot tests, 64, 226, 228, 229 Stabilizer for tube current, 246 Stainless steel films, analysis by x-ray emission spectrography, 230 Stainless steels, analysis by x-ray emission spectrography, 255, 256 Standard counting error, 269-281, 284-288... 

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