Optimizing the Field Emission Properties of Carbon-Nanotube-Based Fibers

Cahay, M.; Zhu, Wei; Ludwick, J.; Jensen, Kevin L.; Forbes, Richard G.; Fairchild, Steven B.; Back, Tyson C.; Murray, Paul T.; Harris, John R.; Shiffler, D.

doi:10.1016/b978-0-12-812667-7.00021-5

“…Recent works made progress to simulate the heat transfer inside emitting single-walled or multi-walled carbon nanotubes, and even carbon nanofibers on a larger scale 45 . Yet, for various reasons, they missed the physics of the Nottingham Inversion Instability.…”

Section: Discussionmentioning

confidence: 99%

Unveiling the Nottingham Inversion Instability during the thermo-field emission from refractory metal micro-protrusions

Mofakhami

¹

,

Seznec

²

,

Minea

³

et al. 2021

Sci Rep

6

0

2

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The electron emission by micro-protrusions has been studied for over a century, but the complete explanation of the unstable behaviors and their origin remains an open issue. These systems often evolve towards vacuum breakdown, which makes experimental studies of instabilities very difficult. Modeling studies are therefore necessary. In our model, refractory metals have shown the most striking results for discontinuities or jumps recorded on the electron emitted current under high applied voltages. Herein, we provide evidence on the mechanisms responsible for the initiation of a thermal instability during the field emission from refractory metal micro-protrusions. A jump in the emission current at steady state is found beyond a threshold electric field, and it is correlated to a similar jump in temperature. These jumps are related to a transient runaway of the resistive heating that occurs after the Nottingham flux inversion. That causes the hottest region to move beneath the apex, and generates an emerging heat reflux towards the emitting surface. Two additional conditions are required to initiate the runaway. The emitter geometry must ensure a large emission area and the thermal conductivity must be high enough at high temperatures so that the heat reflux can significantly compete with the heat diffusion towards the thermostat. The whole phenomenon, that we propose to call the Nottingham Inversion Instability, can explain unexpected thermal failures and breakdowns observed with field emitters.

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“…Recent works made progress to simulate the heat transfer inside emitting single-walled or multi-walled carbon nanotubes or even carbon nanofibers on a larger scale [44]. Yet, for various reasons, they missed the physics of the Nottingham Inversion Instability.…”

Section: /14mentioning

confidence: 99%

Why the Electron Energy Balance During the Thermo-Field Emission From Refractory Metal Micro-Protrusions Can Trigger a Thermal Instability ?

Mofakhami

¹

,

Seznec

²

,

Minea

³

et al. 2021

Preprint

0

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The electron emission by micro-protrusions has been studied for over a century, but the complete explanation of the unstable behaviors and their origin remains an open issue. These systems often evolve towards vacuum breakdown, which makes experimental studies of instabilities very difficult. Modeling studies are therefore necessary. In our model, refractory metals have shown the most striking results for discontinuities or jumps recorded on the electron emitted current under high applied voltages. Herein, we provide evidence on the mechanisms responsible for the initiation of a thermal instability during the field emission from refractory metal micro-protrusions. A jump in the emission current at steady state is found beyond a threshold electric field, and it is correlated to a similar jump in temperature. These jumps are related to a transient runaway of the resistive heating that occurs after the Nottingham flux inversion. That causes the hottest region to move beneath the apex, and generates an emerging heat reflux towards the emitting surface. Two additional conditions are required to initiate the runaway. The emitter geometry must ensure a large emission area and the thermal conductivity must be high enough at high temperatures so that the heat reflux can significantly compete with the heat diffusion towards the thermostat. The whole phenomenon, that we propose to call the Nottingham Inversion Instability, can explain unexpected thermal failures and breakdowns observed with field emitters.

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Atomistic calculations of thermal conductivity in films made from graphene sheets for electron emitter applications

Sami

¹

,

Islam

²

,

Joshi

³

2021

AIP Advances

2

0

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Simulations for the thermal conductivity of carbon fibers (CFs) are carried out based on a molecular dynamics (MD) approach. Our calculations are geared toward providing a general trend for thermal conductivity and its dependence on sample length and to probe the suitability of this material for high power applications from the thermal management standpoint. Since abundant data are not available for carbon fibers, calculations are first carried out in silicon to validate the predictions. The MD results yield a bulk thermal conductivity of 148 W m−1 K−1 for silicon in good agreement with published reports. However, for CF, a much lower bulk value of ∼14 W m−1 K−1 is predicted. This suggests that thermal management would likely be an issue for this material and that CF emitters of lengths below 2 µm should be avoided. The predicted increases in thermal conductivity with temperature may help alleviate the issue to a minor degree. Carbon nanotubes would likely be a better alternative in this context of field emitter arrays.

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Optimizing the Field Emission Properties of Carbon-Nanotube-Based Fibers

Cited by 5 publications

References 153 publications

Unveiling the Nottingham Inversion Instability during the thermo-field emission from refractory metal micro-protrusions

Unveiling the Nottingham Inversion Instability during the thermo-field emission from refractory metal micro-protrusions

Why the Electron Energy Balance During the Thermo-Field Emission From Refractory Metal Micro-Protrusions Can Trigger a Thermal Instability ?

Atomistic calculations of thermal conductivity in films made from graphene sheets for electron emitter applications

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