A vacuum sensor using field emitters made by multiwalled carbon nanotube yarns

doi:10.1016/j.vacuum.2011.04.027

Vacuum

Volume 86, Issue 7, 8 February 2012, Pages 885-888

https://doi.org/10.1016/j.vacuum.2011.04.027 Get rights and content

Abstract

A newly developed vacuum sensor using carbon nanotube (CNT) field-emission has been designed. The fabricated device is an ionization gauge with a silk-like CNT yarn cathode, and the vacuum is indicated by the ratio of the ion current to the electron-emission current. The metrological characteristics of the sensor were studied in a dynamic vacuum system. It showed good linearity ranged from 10⁻⁴ to 10⁻¹ Pa. Taking advantage of the field-emission cathode, the power consumption is only about 5.5 mW. Moreover, comparing it to the conventional thermionic cathode, the CNT yarn cathode is more miniature and a cold cathode with no obvious thermal outgassing effect. Due to these features, the sensor described here could have potential applications in measuring vacuum inside sealed and miniaturized devices.

Highlights

► A vacuum sensor using carbon nanotube yarn field-emission. ► Stable normalized ion current with a variation about ±10%. ► Good linearity in the range from 10⁻⁴ Pa to 10⁻¹ Pa even up to 10 Pa. ► Features of low power consumption, compact structure, and no obvious outgassing. ► Potential application in measuring vacuum inside sealed and miniaturized device.

Introduction

Vacuum metrology plays an important role in the modern industry and the scientific research. For example, in the semiconductor industry, the vacuum condition is closely related to the characteristics of the deposited film. Ionization gauges, which generally consist of cathodes, anodes and collectors, are widely used for measuring vacuum due to ionizing gas molecules. Conventional ionization gauges that use thermionic cathodes have the disadvantages of outgassing, long response time for electron-emission, heat and light generation, and high power consumption. These problems can be avoided if the thermionic cathodes be replaced with the field-emission cathodes. Spindt field-emission cathodes, which are made by tungsten or molybdenum micro-tip arrays, have been applied in the ionization gauges [1], [2], [3]. However, the Spindt tips would deteriorate even in moderate vacuum and can only stably work in a vacuum higher than 10⁻⁶ Pa. Since the discovery in 1991 [4], carbon nanotubes (CNTs) are considered to be very promising field emitters due to their high aspect ratio, mechanical strength, and chemical inertness. Many potential applications have been developed such as field-emission display (FED) [5], lamp [6], and X-ray tube [7]. Recently, CNT field-emission surfaces made by screen-printing CNT pastes were also applied in the ionization gauges [8], [9].

CNT yarns are macroscopic silk-like nanomaterials, in which CNT bundles are joined end to end forming continues yarns. They can be mass produced by drawing from super-aligned CNT arrays and processing with ethanol [10], [11], [12]. CNT yarn field emitters can be efficiently prepared by cutting the continuous yarns into segments and the cross sections are CNT emitters [13]. As the CNT yarns are macroscopic objects, it is easier and more efficient to manufacture field-emission cathodes by CNT yarns compared to as-grown CNT arrays [14] and screen-printing CNT pastes [8], [9]. In this paper, we present a newly designed vacuum sensor with compact structure using CNT yarn field emitters. The metrological characteristics of the vacuum sensor were studied in a dynamic vacuum chamber. It showed good linearity in the range from 10⁻⁴ Pa to 10⁻¹ Pa, and the power consumption was only about 5.5 mW. Additionally, the vacuum sensor has the features of compact structure, a miniature cathode and no obvious outgassing effect, and is promising for measuring vacuum inside sealed and miniaturized devices, such as FEDs.

Section snippets

Experimental

The details for manufacturing CNT yarn field emitters have been described elsewhere [13]. Briefly, the super-aligned CNT arrays were synthesized by chemical vapor deposition (CVD). Then the yarns were drawn from the super-aligned arrays and subsequently pulled forward through droplets of ethanol. After that, the CNT yarns were cutting into segments using knife or laser. The segments were 10 mm long and 100 μm in diameter. Finally, a CNT yarn segment was attached to the end of a copper pole

Results and discussion

The vacuum sensor measures pressure based on the principle of ionization gauges, that is for a constant number of electrons emitted from the cathode accelerating to the anode, a fraction of gas molecules will be ionized and the number of generated ions will be proportional to the number of gas molecules in the chamber and then the pressure [15]. It can be described by the following formula, $\frac{I_{i}}{I_{e}} = K P$ where I_i is the ion current, I_e the electron current, P the pressure, and K a constant called the

Conclusions

A vacuum sensor using CNT yarn field-emission was developed, which has the advantages of compact structure, low power consumption, and no obvious outgassing effect, and the metrological characteristics were studied in a dynamic vacuum chamber. In the voltage-constant mode, the pressure was indicated by the normalized ion current, which was stable with a variation about ±10% regardless of the fluctuation of electron current. The vacuum sensor showed a good linearity in the range from 10⁻⁴ Pa to

Acknowledgments

The authors gratefully thank Professor Pijin Chen for helpful discussions. This work was financially supported by the National Science Foundation of China (10704044, 50825201). This work was also supported by the Doctoral Fund of National Institute of Metrology (ATGQDB0903).

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View full text

A vacuum sensor using field emitters made by multiwalled carbon nanotube yarns

Abstract

Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Vacuum

Bayard-Alpert vacuum gauge with microtips

J Vac Sci Technol B

Field emitter based extractor gauges and residual gas analyzers

J Vac Sci Technol A

Helical microtubules of graphitic carbon

Nature

Fully sealed, high-brightness carbon-nanotube field-emission display

Appl Phys Lett

Field emission from cylindrical carbon nanotube cathodes: possibilities for luminescent tubes

Appl Phys Lett

Carbon nanotubes as electron source in an X-ray tube

Appl Phys Lett

Development of low pressure sensor based on carbon nanotube field emission

Metrologia

A low-vacuum ionization gauge with HfC-modified carbon nanotube field emitters

Appl Phys Lett