Wide-range Vacuum Measurements from MWNT Field Emitters Grown Directly on Stainless Steel Substrates
© Zhang et al. 2016
Received: 13 October 2015
Accepted: 21 December 2015
Published: 6 January 2016
The field emission properties and the vacuum measurement application are investigated from the multi-walled carbon nanotubes (MWNTs) grown directly on catalytic stainless steel substrates. The MWNT emitters present excellent emission properties after the acid treatment of the substrate. The MWNT gauge is able to work down to the extreme-high vacuum (XHV) range with linear measurement performance in wide range from 10−11 to 10−6 Torr. A modulating grid is attempted with improved gauge sensitivity. The extension of the lower pressure limit is attributed largely to low outgassing effect due to direct growth of MWNTs and justified design of the electron source.
The electron field emission has potential to overcome thermionic emission related problems [1–4] for merits such as room-temperature operation, low power consumption, and quick response capability. Carbon nanotubes (CNTs) possess unique electrical, chemical, and mechanical as well as structural properties and are regarded as the most promising field emission material with low emission fields and good vacuum behaviors [5–14]. The “cold” cathode operation could help to reduce the outgassings, benefiting greatly the ultra-high vacuum/extreme-high vacuum (UHV/XHV) measurement. Many investigations have been conducted to employ the CNT field emission in ionization gauge applications. Murakami et al. presented a Bayard-Alpert type CNT field emission gauge and tested in 10−6 to 10−4 Torr range . In 2004, Dong and Myneni developed an extractor type field emission gauge based on the MWNTs grown on Ni alloy. The gauge presented excellent measurement linearity from 10−10 to 10−6 Torr with the sensitivity of about 3 Torr−1 [16, 17]. Huang et al. replaced thermionic filament of the Bayard-Alpert gauge (BAG) by a line-type CNT cathode, and the gauge performances were studied from 10−4 to 10−7 Torr with the sensitivity of 3.6 Torr−1 . Suto et al. developed the CNT electron source by screen printing and studied its ionization gauge application .The gauge responded linearly from 10−8 to 10−4 Torr with the sensitivity of 13 Torr−1, close to the commercial gauge sensitivity. New gauge designs and structural modulations were also attempted. Sheng et al. constructed a saddle field gauge with a simple ring anode. The gauge was tested from 10−5 to 10−3 Pa with improved sensitivity of 1.7 Pa−1 . A simple triode type of CNT ionization gauge was investigated by several groups, but there were no UHV/XHV measurement results [21–25].
The applications of CNT field emission in pressure measurement have not been well established. Some technical issues, including the outgassings due to the breaking off of the film components and the gas desorption from the CNT cathode, restrict the UHV/XHV applications. In this work, MWNTs are grown directly on the stainless steel substrate (S.S.) without extra catalyst layer by thermal chemical vapor deposition (CVD). Improved field emission performances are presented from the MWNT emitters after anodizing the substrate. The all mechanical assembly MWNT field emission cathode is constructed with low emission fields, high electron transmittances over the gate, and long-term stability. The ionization gauge behaviors based on the MWNT source are investigated with excellent measurement linearity from 10−11 to 10−6 Torr.
The MWNT film is grown directly on the 304 S.S. without extra catalyst layer by the CVD technique from C2H2/Ar source gases at 750 °C. To improve the field emission properties, the S.S. substrate is anodized in the 0.3 mol L−1 oxalic acid solution. The field emission properties are compared for regular and anodized MWNT emitters from the diode setup. By screwing the parts together without any adhesive material, MWNT electron source is developed from totally mechanical assembly with features of low outgassing rates, good emission stability, and the feasibility to replace the MWNT cathode. The MWNT field emission ionization gauge is constructed upon the extractor gauge. The gauge performances are investigated inside a turbo vacuum system with 10−11 Torr background vacuum, and the pressure is calibrated against a commercial IE 514 extractor gauge. The measurement linearities are investigated for N2, H2, and O2, respectively, in wide pressure ranges. Keithley multimeters are used to supply the operation potentials and measure ion and electron currents.
Results and Discussion
The gauge outgassing property, including the electron stimulated desorption (ESD) effect, plays a key role for the measurement accuracy and the XHV feasibility. This MWNT ionization gauge exhibits excellent low outgassing behaviors, leading to the superior measurement performance. Under the emission current of 100 μA, the system pressure may go up about 2 × 10−10 Torr in the 10−11 Torr vacuum. After the electrode cleaning from the higher emission flashing, i.e., 200 μA for a couple of minutes, the pressure rise could be smaller than 7 × 10−11 Torr. The electron bombardment cleans the surface adsorptions effectively. The outgassing properties for different gas species are further investigated. Figure 5b shows the partial pressure variations with increasing the emission current up to 200 μA. Hydrogen accounts for the major gas kind, which increases roughly linearly from 7.01 × 10−12 to 4.86 × 10−11 Torr. The steady rise of the hydrogen partial pressure is believed to be related to H2 desorptions from interiors of the metal electrodes due to the temperature rise by electron bombardments [33, 34]. With increasing the emission to 106 μA, partial pressures of H2O, N2, CO2, and O2 increase from 4.57 × 10−12 to 8.72 × 10−12 Torr (91 %), 3.69 × 10−12 to 6.78 × 10−12 Torr (84 %), 2.46 × 10−12 to 6.68 × 10−12 Torr, and 1.47 × 10−13 to 3.0 × 10−13 Torr, respectively, and reach stable states or drop afterwards. The gentle ascents for these four gases before 100 μA are ascribed to surface outgassings. The low outgassing performance, which enables the extension of the lower pressure limit to 10−11 Torr level, is attributed to several aspects, including the mechanical assembly of the cathode consisting of low outgassing materials, low extraction potentials, and the direct growth of MWNT emitters without extra gas adsorption materials.
The improvement of the sensitivity factor is particularly important to extend the measurement down to UHV/XHV range where ion collection currents are normally extremely small. To evaluate the sensitivity performance by modulating the electron energy, the role of a mesh modulator with 92.2 % physical transparency is investigated. As shown in Fig. 6c, the relations between K and the modulator potential are tested for three anode potentials of 350, 400, and 450 V, respectively. With increasing the modulator potential from 100 to 700 V, the sensitivity factor varies roughly in two plateaus, and a transition leap occurs when the modulator potential is equal to and 50 V higher than the anode potential. Further increase of the modulator potential does not improve the K factor significantly. This leap is most probably related to the prolongations of the electron trajectories because a slightly higher modulator potential assists the electrons move back and forth around the top of the anode grid. Under the anode potential of 400 V, K reaches 2.85 Torr−1, 20 % higher than the value without the modulator. Liu et al. also demonstrated the sensitivity improvement by a shield electrode for the CNT BA gauge . Therefore, the electron energy modulation enhances the gauge measurement sensitivity effectively.
In summary, we have developed a carbon nanotube field emission ionization gauge after growing MWNTs directly on stainless steel substrates. The MWNT cathode presents excellent emission J-E and stability performances after the substrate anodization. The MWNT gauge demonstrates excellent measurement linearity in wide pressure range with the lower measurement limit down to 10−11 Torr, attributed largely to low outgassings due to the direct growth of MWNTs. The modulation of the electron energy benefits the improvement of the measurement performance. This gauge shows potential for UHV/XHV pressure measurements.
This work is supported by the NSF of China (Grants No. 11274244, No. 61125101) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars.
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