Imprinted CNTs field emission cathode for electron brushing

Microchannel plates (MCPs) are the key components for low-level light night vision systems. Electron brushing is a crucial processing for MCPs manufacturing. In this paper, an imprinted carbon nanotubes (CNTs) field emission cathode is fabricated and tested. And the electron brushing experiment based on this cathode is carried out. The result shows that the imprinted CNTs cathode has some advantages and can be used in electron brushing. This research lays the foundation for massive, low cost and highly efficient electron brushing applications in the future.


Introduction
Night vision technology originated from the discovery of photoelectric emission by Hertz in 1887 and the publication of the photoelectric effect by Einstein in 1905.It is a high-tech that converts light to electrons, electrons to image and enhances the image under weak night light at last.The instruments based on night vision technology can greatly improve people's ability in weak light.So, the instruments can be applied to various tasks such as wide-spectrum tracking, all-weather observation, aiming and guidance.Nowadays, night vision technology has wider and more important applications in many fields, such as astronomy, aerospace, shipping, biology, medicine, nuclear physics, satellite monitoring and high-speed photography [1][2][3][4][5][6][7][8] .
Researchers discovered the photocathode on Bi-Ag-O-Cs in 1938, on Na-K-Sb and Na-K-Sb-Cs in 1955 and on K-Cs-Sb in 1963.Based on the above photoelectric cathode materials and the fiber optic panel which was invented by Kapany, the first-generation (1G) of night vision technology was developed.The extremely weak target image is transferred to a photoelectric cathode and excites electrons in the 1G night vision system.The excited electrons are accelerated and focused on a fluorescent screen, then the enhanced image appears.The 1G system is passive with high gain because it has three sections of enhancement.However, 1G systems are destined for some shortcomings, such as bulky volume, heavy weight, and poor imaging effect in bright light.
In 1965, the GaAs activated by Cs were used as a high-quantum efficiency photoelectric cathode, which was discovered by J. J. Scheer and J. Vanlaar.In 1968, A. A. Turnbull and G. B. Evans adopted GaAs activated by Cs-O to achieve a better negative electron affinity (NEA) material, which was the basis of a higher-efficiency photoelectric cathode.The combination of NEA materials and microchannel plates (MCP), which were successfully developed in the early 1970s, gave birth to the second-generation (2G) night vision system [1][2][3][4] .The MCPs with high secondary electron emission channels ensure tens of thousands of electron multiplication and keep a significant gain in 1/3 length of the 1G tubes.
As a key part of the night vision systems, MCPs are composed of millions of 2 um to 5 um hollow channels, as shown in Figure 1.The inner wall of the channels is a high secondary electron emission material.The surface impurity will block the electrons and reduce the secondary electron multiplication.Electron scrubbing uses electron bombardment to remove surface impurities and improves the stability of MCPs' channels.So, it is a key step in the production process of MCPs.The quality of electron scrubbing dominates the performance, stability and life of night vision instruments.In this article, we first develop imprinted CNTs field emission cathode and then apply this cathode to electron brushing.The result verifies that the cathode can be used for MCP electron brushing.

The imprinted CNTs field emission cathode
Field emission is usually represented by the F-N formula [12][13][14][15] : Here, J is the current density, the unit is A/cm 2 .E is the electric field strength on the emitter (V/cm).φ is the work function that determines the height of the potential barrier on the surface of the material (eV).A and B are constants: β is the field enhancement factor, which depends on the geometric shape of the cathode.Here, β is dominated by the aspect ratio of the emitter tip.Simple simplification of Formula (1) leads to: Therefore, ln(J/E 2 ) is proportional to 1/E.The relationship curve is approximately linear known as the F-N curve, which can determine whether the emitter material belongs to field emission.
The imprinted CNTs field emission cathode is fabricated as follows [16][17][18][19][20] .Firstly, single-walled CNTs are mixed with absolute alcohol and sonicated for 2 hours.Then the CNTs ethanol solution is dried to a paste at 60℃.Third, the imprinted mold is manufactured, which is used to print a silver paste layer onto a clean graphite substrate as a transition layer.Fourth, the CNTs paste is further imprinted onto the silver paste transition layer.Fifth, the printed silver paste and CNTs are dried in Ar atmosphere at 200℃ for 20 minutes.Sixth, the imprinted CNTs cathode is annealed at 500℃ for 30 minutes to remove organic impurities.At last, the imprinted CNTs cathode is obtained and the process is shown in Figure 2. Figure 4 shows the field emission results of the imprinted CNTs cathode, where the turn-on field and the threshold field are low at 0.62 V/μm and 1.04 V/μm respectively [19][20][21] .The field enhancement factor β reaches 15800.
Figure 5 shows the long-term field emission stability test of the cathode.When the current density keeps 3.5 mA/cm 2 , the cathode is continuously emitted for 140 minutes without significant fluctuations, which indicates that the imprinted CNTs cathode has good field emission stability and can operate stably for a long time [16,[18][19][20] .

Electron brush experimental
The electronic brushing platform is shown in the following Figure 6.It consists of the electron gun, the MCP and a display screen.The electron beam is amplified through the MCP and then projects onto the display screen [2][3][4] .In general, a thermic electron gun including a cathode, a focusing electrode, a heating, an anode and some support structures is embedded in the electron gun sleeve.The imprinted CNTs cathode does not require heating or controlling structure to form an electron beam shape.As shown in Figure 7, the imprinted CNTs cathode is fixed on a quartz sheet with a stainless grid fixed 0.5 mm away from its surface.The grid is made of stainless lines with a diameter of 0.25 mm and the grid mesh size is 1 mm * 1 mm.The imprinted CNTs field emission cathode is connected to a negative high-voltage power, the grid is grounded.The MCP and the display screen are fixed at a distance of 100 mm from the grid respectively which is connected to a voltage of + 800 V and + 3200 V.When the negative high voltage is -800 V and applied to the imprinted cathode, preliminary lighting can be observed as shown in Figure 8 (a).Due to the unevenness of the grid, part of the electron exceeds the emission threshold and emits.With the continuous increase of negative high voltage, the brightness range of the display screen gradually expands.When it reaches -1000 V, the entire screen is lit up, indicating that all areas exceed the threshold and start to emit electrons, as shown in Figure 8 (b).With the increase of the negative voltage, the brightness of the display screen also increases.When the voltage reaches -1500 V, the display screen shows a strong brightness level, as shown in Figure 8 (d).To avoid damaging the MCP and display screen, the voltage increase is stopped.So, the electron brushing process can be operated at 1000 V -1500 V.

Conclusion
The imprinted CNTs cathode has some virtues, such as easy to manufacture, high emission efficiency, no heating requirement, stable performance and easy atmospheric maintenance.This paper has verified that the field emission cathode has the potential to apply electron brushing.This work provides a new method for realizing large-scale electron brushing based on the imprinted CNTs field emission cathode.In addition, it also lays a foundation for further research on electron brushing technology in the future.

Figure 1 .
Figure 1.Real view and detailed schematic diagram of MCPs.In this article, we first develop imprinted CNTs field emission cathode and then apply this cathode to electron brushing.The result verifies that the cathode can be used for MCP electron brushing.

Figure 2 .Figure 3 .
Figure 2. The process of the imprinted CNTs field emission cathode.
(a) J V.S. field (b) F-N curve

Figure 4 .
Figure 4.The field emission testing of the imprinted CNTs cathode.

Figure 5 .
Figure 5.The long-term field emission test result.

Figure 7 .
Figure 7.The imprinted CNTs cathode, the stainless grid & the platform.When the negative high voltage is -800 V and applied to the imprinted cathode, preliminary lighting can be observed as shown in Figure8 (a).Due to the unevenness of the grid, part of the electron exceeds the emission threshold and emits.With the continuous increase of negative high voltage, the brightness range of the display screen gradually expands.When it reaches -1000 V, the entire screen is lit up, indicating that all areas exceed the threshold and start to emit electrons, as shown in Figure8 (b).With the increase of the negative voltage, the brightness of the display screen also increases.When the voltage reaches -1500 V, the display screen shows a strong brightness level, as shown in Figure8 (d).To avoid damaging the MCP and display screen, the voltage increase is stopped.So, the electron brushing process can be operated at 1000 V -1500 V.

Figure 8 .
Figure 8.The brightness of the screen.