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A Fully-Sealed Carbon-Nanotube Cold-Cathode Terahertz Gyrotron

View Article: PubMed Central - PubMed

ABSTRACT

Gigahertz to terahertz radiation sources based on cold-cathode vacuum electron technology are pursued, because its unique characteristics of instant switch-on and power saving are important to military and space applications. Gigahertz gyrotron was reported using carbon nanotube (CNT) cold-cathode. It is reported here in first time that a fully-sealed CNT cold-cathode 0.22 THz-gyrotron is realized, typically with output power of 500 mW. To achieve this, we have studied mechanisms responsible for CNTs growth on curved shape metal surface, field emission from the sidewall of a CNT, and crystallized interface junction between CNT and substrate material. We have obtained uniform growth of CNTs on and direct growth from cone-cylinder stainless-steel electrode surface, and field emission from both tips and sidewalls of CNTs. It is essential for the success of a CNT terahertz gyrotron to have such high quality, high emitting performance CNTs. Also, we have developed a magnetic injection electron gun using CNT cold-cathode to exploit the advantages of such a conventional gun design, so that a large area emitting surface is utilized to deliver large current for electron beam. The results indicate that higher output power and higher radiation frequency terahertz gyrotron may be made using CNT cold-cathode electron gun.

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The field emission properties of the individual CNTs, where (a) I-E curve image from the individual CNT tips and (b) enlarged part indicated by red rectangle. (c) SEM image of the curved CNTs sidewall emitter with a diameter of 33 nm, (d) SEM image of a new emitter formed two CNTs, (e) the I-E curve during the above process occurred. (f,g) SEM images showing continued testing of the new emitter before and after the breakdown event, (h) the corresponding I-E curve recorded during the test process.
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f6: The field emission properties of the individual CNTs, where (a) I-E curve image from the individual CNT tips and (b) enlarged part indicated by red rectangle. (c) SEM image of the curved CNTs sidewall emitter with a diameter of 33 nm, (d) SEM image of a new emitter formed two CNTs, (e) the I-E curve during the above process occurred. (f,g) SEM images showing continued testing of the new emitter before and after the breakdown event, (h) the corresponding I-E curve recorded during the test process.

Mentions: To investigate the emission capability, we study the possible emission sites and their emission performance. The field emission measurements on individual CNT tip and the curved sidewall of a single CNT are performed in a SEM chamber (ZEISS-Supra 55) equipped with a nano-manipulator, which is fixed with a tungsten tip. In the experiment, the CNTs be tested are as far away from the other CNTs as possible to exclude their interference. The distance between the anode probe and the tip of CNTs was typically set to around 1 μm. A picoammeter with a power supply (Keithley 6487) was employed to record the field emission current. The typical vacuum chamber pressure was ~5 × 10−4 Pa. Field emission from sidewall of CNTs has not been paid enough attention to so far. The anode is driven positively using a variable DC voltage power supply. The field emission properties of the individual CNTs are given in Fig. 6. The maximum emission current from single CNT tips before vacuum breakdown ranges from hundreds of nanoamps to several microamps, as Fig. 6a,b show. We also find that there is large probability of electron emission from the sidewalls of CNTs. Figure 6c,d are SEM image of the CNT sidewall emitter with a diameter of 33 nm before and after the vacuum breakdown happened, forming a CNT bundle with two tips. The I-E curve (Fig. 6e) shows that the emission current reached 0.12 μA under the applied field of around 78 MV/m. Also, after the local vacuum breakdown, the CNT only broke locally and became a new emitter formed by two CNTs (Fig. 6d). Figure 6f,g show continued testing of the emitter, and it can reach the current of 0.06 μA (Fig. 6h) before another a local breakdown occurred, after which one CNT still remained. The experimental phenomena above prove that the curved sidewalls of CNTs make up an important part of electron emission sites, in addition to that from the CNTs tips. They enhanced significantly the emission capability of our CNT cold-cathode. We attribute this effect to direct benefits from the defect region in a single nanotube and the direct growth from stainless steel surface, otherwise local breakdown will cause destructive vacuum breakdown to the CNT cold-cathode, as described in our early work18. This new findings change the present comment view, ie field emission sites are from the tips of CNTs. Our finding reveals that field emission sites consist of two parts, tips and sidewalls of CNTs.


A Fully-Sealed Carbon-Nanotube Cold-Cathode Terahertz Gyrotron
The field emission properties of the individual CNTs, where (a) I-E curve image from the individual CNT tips and (b) enlarged part indicated by red rectangle. (c) SEM image of the curved CNTs sidewall emitter with a diameter of 33 nm, (d) SEM image of a new emitter formed two CNTs, (e) the I-E curve during the above process occurred. (f,g) SEM images showing continued testing of the new emitter before and after the breakdown event, (h) the corresponding I-E curve recorded during the test process.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5017026&req=5

f6: The field emission properties of the individual CNTs, where (a) I-E curve image from the individual CNT tips and (b) enlarged part indicated by red rectangle. (c) SEM image of the curved CNTs sidewall emitter with a diameter of 33 nm, (d) SEM image of a new emitter formed two CNTs, (e) the I-E curve during the above process occurred. (f,g) SEM images showing continued testing of the new emitter before and after the breakdown event, (h) the corresponding I-E curve recorded during the test process.
Mentions: To investigate the emission capability, we study the possible emission sites and their emission performance. The field emission measurements on individual CNT tip and the curved sidewall of a single CNT are performed in a SEM chamber (ZEISS-Supra 55) equipped with a nano-manipulator, which is fixed with a tungsten tip. In the experiment, the CNTs be tested are as far away from the other CNTs as possible to exclude their interference. The distance between the anode probe and the tip of CNTs was typically set to around 1 μm. A picoammeter with a power supply (Keithley 6487) was employed to record the field emission current. The typical vacuum chamber pressure was ~5 × 10−4 Pa. Field emission from sidewall of CNTs has not been paid enough attention to so far. The anode is driven positively using a variable DC voltage power supply. The field emission properties of the individual CNTs are given in Fig. 6. The maximum emission current from single CNT tips before vacuum breakdown ranges from hundreds of nanoamps to several microamps, as Fig. 6a,b show. We also find that there is large probability of electron emission from the sidewalls of CNTs. Figure 6c,d are SEM image of the CNT sidewall emitter with a diameter of 33 nm before and after the vacuum breakdown happened, forming a CNT bundle with two tips. The I-E curve (Fig. 6e) shows that the emission current reached 0.12 μA under the applied field of around 78 MV/m. Also, after the local vacuum breakdown, the CNT only broke locally and became a new emitter formed by two CNTs (Fig. 6d). Figure 6f,g show continued testing of the emitter, and it can reach the current of 0.06 μA (Fig. 6h) before another a local breakdown occurred, after which one CNT still remained. The experimental phenomena above prove that the curved sidewalls of CNTs make up an important part of electron emission sites, in addition to that from the CNTs tips. They enhanced significantly the emission capability of our CNT cold-cathode. We attribute this effect to direct benefits from the defect region in a single nanotube and the direct growth from stainless steel surface, otherwise local breakdown will cause destructive vacuum breakdown to the CNT cold-cathode, as described in our early work18. This new findings change the present comment view, ie field emission sites are from the tips of CNTs. Our finding reveals that field emission sites consist of two parts, tips and sidewalls of CNTs.

View Article: PubMed Central - PubMed

ABSTRACT

Gigahertz to terahertz radiation sources based on cold-cathode vacuum electron technology are pursued, because its unique characteristics of instant switch-on and power saving are important to military and space applications. Gigahertz gyrotron was reported using carbon nanotube (CNT) cold-cathode. It is reported here in first time that a fully-sealed CNT cold-cathode 0.22 THz-gyrotron is realized, typically with output power of 500 mW. To achieve this, we have studied mechanisms responsible for CNTs growth on curved shape metal surface, field emission from the sidewall of a CNT, and crystallized interface junction between CNT and substrate material. We have obtained uniform growth of CNTs on and direct growth from cone-cylinder stainless-steel electrode surface, and field emission from both tips and sidewalls of CNTs. It is essential for the success of a CNT terahertz gyrotron to have such high quality, high emitting performance CNTs. Also, we have developed a magnetic injection electron gun using CNT cold-cathode to exploit the advantages of such a conventional gun design, so that a large area emitting surface is utilized to deliver large current for electron beam. The results indicate that higher output power and higher radiation frequency terahertz gyrotron may be made using CNT cold-cathode electron gun.

No MeSH data available.


Related in: MedlinePlus