Field Emission Cathodes using Carbon Nanotubes

Transcription

1 21st Microelectronics Workshop, Tsukuba, Japan, October 2008 Field Emission Cathodes using Carbon Nanotubes by Yasushi Ohkawa, Koji Matsumoto, and Shoji Kitamura Innovative Technology Research Center, Aerospace Research and Development Directorate, JAXA 1 /22

2 Outline Motivation Demand for small & low-power electron sources Objective To demonstrate feasibility of field emission cathodes (FECs) for space Field Emission Cathodes (FECs) Fundamentals Required performance for FECs Current Research Status FECs using carbon nanotubes FECs using nanostructure emitters Summary 2 /22

3 Motivation Electron sources are required in some space applications Electric propulsion (Ion engines, Hall thrusters, Colloid thrusters, ) Charging control of satellites (ISS, GEO satellites, ) Demand for small & low-power electron sources Small electric propulsion for small satellites Electrodynamic tether system Charging control of satellites X-ray sources for space science Keywords for electron sources: Small, Low-power, Simple Field Emission Cathodes 3 /22

4 Objective To develop Field Emission Cathodes for Space Application To demonstrate the feasibility of Field Emission Cathodes 4 /22

5 Field Emission Cathodes Field Emission of Electrons Electrons are extracted from conductive surface by applying electric field Anode Electrons Cathode (Emitter) Fowler-Nordheim (F-N) equation (Electron current by field emission) j 3 2 e E exp 8πhφ 8π 2m φ 3heE = 3/ 2 5 /22

6 Field Emission Cathodes Field Emission of Electrons j 3 2 e E 8π 2m exp φ 8πhφ 3heE = 3/ 2 Emission current, current A J / V 2 1.E-04 1.E-10 1.E-16 1.E-22 1.E-28 1.E E Voltage, V 1000 / V I-V Characteristics Fowler-Nordheim (F-N) plot Linearity = Field Emission 6 /22

7 Field Emission Cathodes Field Emission Cathodes (FECs) Electrons are extracted from emitter tips with very small tip-radius Gate is used for the extraction Electrons Gate Emitter Electron field on the tips is enhanced by the sharpness Field Enhancement Factor β F = β E j 3 2 e F 8π 2m exp φ 8πhφ 3heF = 3/ 2 7 /22

8 Field Emission Cathodes Keywords for Development Low voltage Short electrode gap, Emitter with low φ and high β High current Large emitter area, High current density (Emitter with low φ and high β) Low drain current Optimization of optics Durability in space environment Durable emitter material, Emitter coating Electrons Gate Emitter 8 /22

9 Current Research Status Two types of Field Emission Cathodes (FECs) Carbon Nanotube Cathodes Carbon nanotubes are used as electron emitters FECs with Nanostructure Emitters Sharp emitter tips are formed by nanostructural processing 9 /22

10 Carbon Nanotube Cathodes Carbon nanotubes (CNTs) are used as electron emitters CNT has Nanometer-scale tube-diameter High β (field enhancement factor) can be expected CNT is tolerable to ion impingement Low-cost manufacturing Electrons Gate Carbon nanotubes 100 μm SEM image of CNT 10 /22

11 Carbon Nanotube Cathodes Selection of CNTs SWNT (Single Wall NanoTube) or MWNT (Multi Wall Nano Tube) SWNT : High β, Expensive MWNT : High durability, Low cost MWNT was selected Fabrication Process Arc discharge method : High durability, High current density CVD method : Easy to make large emitter Arc discharge method was selected Emission current, ma Arc Constant Emission current, ma CVD Decreasing Elapsed time, s Elapsed time, s 11 /22

12 Carbon Nanotube Cathodes Optics Design Requirement: To suppress drain current Answer: Electron trajectory control by Mask Gate Mask Gate Electron emitter (CNT) e - e - e - Electron emitter (CNT) e - e - e - Without Mask With Mask 12 /22

13 Carbon Nanotube Cathodes Optics Design Requirement: To suppress drain current Answer: Electron trajectory control by Mask Electron trajectory Electron emitter Gate Mask Gate Without Mask Anode current: 85% Gate current: 15% Electron emitter With Mask Anode current: 100% Gate current: 0% 13 /22

14 Carbon Nanotube Cathodes Try and Error fabrication 14 /22

15 Carbon Nanotube Cathodes Electron Extraction Tests Vacuum chamber Va Anode Ja Gate CNT Emitter Vg Jg Je P = Pa 15 /22

16 Carbon Nanotube Cathodes Typical Performance Curve Emission current, Je, ma Gate current, Jg, ma No.1 Je No.1 Jg No.2 Je No.2 Jg No.3 Je No.3 Jg No.4 Je No.4 Jg No.5 Je No.5 Jg Emitter Dia. = 4 mm Gap spacing = 0.65 mm Cathode-to-gate voltage, Vg, kv 16 /22

17 Carbon Nanotube Cathodes Endurance Performance Performance degradation is unavoidable but acceptable 2 Current, J, ma Emission current, J e Emitter Dia. = 4.0 mm Gap spacing = 0.65 mm V g = 2.2 kv (Kept constant) Gate current, J g Elapsed time, hour 17 /22

18 FECs with Nanostructure Emitters Field emitter array fabricated by Transfer Mold Method Array size: 10 x 10 mm Emitter size (base) : 1 μm Emitter tip radius : 6 nm Various coatings for high durability Research Institute of Electronics, Shizuoka University Field Emitter Array 1 μm (Collaborative work with Prof. Nakamoto in the Research Institute of Electronics, Shizuoka Univ.) 18 /22

19 FECs with Nanostructure Emitters Typical Performance Research Institute of Electronics, Shizuoka University Needle anode Emitter array I V Characteristics F N Plot 19 /22

20 FECs with Nanostructure Emitters Future plan Research Institute of Electronics, Shizuoka University R&D of emitter coating for high durability Fabrication of Gated Emitter Array Very small gap spacing Very low gate voltage Very low power consumption Gate Insulator Emitter 20 /22

21 Summary Research & development of Field Emission Cathodes has been performed in the Innovative Technology Research Center, JAXA Field Emission Cathodes are applicable to small electric propulsion, electrodynamic tethers, and charging control of satellites Carbon Nanotube Cathodes with ma-level emission and reasonable durability were fabricated and tested Nanostructural Emitter Array was fabricated and fair emission capability was obtained Feasibility of Field Emission Cathodes was demonstrated and further effort is necessary to develop practical devices for space 21 /22

22 Acknowledgment Authors thank Prof. Nakamoto in Shizuoka University for his great contribution to the development of nanostructural emitter array 22 /22