The structure optimization of the carbon nanotube film cathode in the application of gas sensor

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Abstract

The carbon nanotube (CNT) film is used to fabricate the novel carbon nanotube film cathode (CNTFC) gas sensor, and the working voltage of the novel sensor at atmospheric pressure decreases to several hundred volts from the value of more than thousand volts of the traditional discharge gas sensor, however it is limited by that the space charge in the gap is very hard to diffuse after the breakdown of the gap. This makes the gap in conductance and next self-sustaining dark discharge impossible. To solve the problem, experimental study and simulation calculation were conducted to optimise the structure of CNTFC. Five CNTFCs with different structures were designed and the experiments were conducted to study discharge principle. It shows that the CNTFC could be chosen as a gas sensor, when it has the three characteristics of low working voltage, working at self-sustaining dark discharge state, and the space charge in the gap being easy to diffuse. The electric field distributions of the five CNTFCs were studied and calculated by using the finite element method. The results of the simulations and the experiments show that it is reasonable to use the CNTFC with the above three characteristics as a novel gas sensor.

Introduction

Gas sensors operate by a variety of fundamentally different mechanisms [1], [2], [3], [4], [5], [6], [7], [8], [9], and there are generally two kinds [10], [11] of gas sensors: one kind is based on the chemical mechanism and the physical mechanism [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], the other on the physical mechanism [24], [25]. Among all gas sensors, ionization gas sensors are mainly used as the gas detector in advanced gas analyzer such as chromatograph and mass spectrograph to realize the high-precision measurement of gas concentration after the gas mixture is separated. However, the traditional ionization sensors are limited by high working voltage, high vacuum environment, and their huge and bulky architecture.

The novel MEMS ionization micro-sensor [26] (reported in 1998) with the gap distance 50 μm decreased the breakdown voltage of several gases from the value of more than thousand volts of the traditional discharge gas sensor to the range of 350–550 V at atmospheric pressure. However, because the single tip with the curvature radius of micrometer order was used as the cathode, the electric field distribution was very nonuniform and the corona discharge at high voltage was very easy to occur. Later, when it was used to take place of flammable ionization detector (FID) of chromatograph used in National Weather Bureau of Japan and measure the trace SO2 in the atmosphere, it worked at corona discharge state and its working voltage increased up to 2000 V in condition of the gap distance 50 μm.

The novel carbon nanotube film cathode (CNTFC) gas sensor with CNT array film as the cathode [27] (reported in 2001), worked at self-sustaining dark discharge state and the breakdown voltage is low. In condition of the gap distance 91 μm of the CNTFC, the breakdown voltages of several gases decreased to less than 220 V at atmospheric pressure and room temperature. After the breakdown of the gap occurred, the large quantity of space charge existed in the gap, and the space charge in the gap made the insulation resistance between cathode and anode decreasing from the infinite to the finite. It was very difficult to clear the space charge in the gap at atmospheric pressure except taking apart CNTFC, cleaning the anode plate and fabricating CNTFC again. Thus, the next self-sustaining dark discharge experiment could not be conducted if the space charge was not cleared out of the gap.

The carbon nanotube film anode (CNTFA) gas sensor with CNT array film as the anode [28] (reported in 2003), worked at corona discharge state, and could be used to identify several single gases through the measured breakdown voltages. However, CNTFA worked at vacuum environment and thus needed huge vacuum system. What was proposed and conducted in the end was to combine the CNTFA with chromatograph.

Among the three novel discharge gas sensors, CNTFC gas sensor is superior to the other two sensors in such characteristics as working at atmospheric pressure, self-sustaining dark discharge and low working voltage. To solve the problem of difficult diffusion of the space charge in the gap after the breakdown and make the continuous measurement possible, the structure optimization of CNTFC is studied.

Section snippets

Establishment of the gas mixing system and the test set-up

To study the discharge principle of the CNTFC in the gas, a gas mixing system and the test set-up were established. The gas mixing system was used to offer the gas mixture in a controlled flux and in an environment gas—N2, and the test set-up was used to measure the discharge IV characteristic of the CNTFC.

CNTFC gas sensor fabricated in reference [27]

It was supposed in reference [27] that CNT array film was used as the cathode, and a novel gas sensor with the CNT film cathode (CNTFC) was fabricated to identify the gas variety. The sensor works at self-sustaining dark discharge, which is different with the corona discharge and the glow discharge. Because the CNT film was used as the cathode, and the tip of nanotube supplies very high electric field intensity, the breakdown voltage decreases from thousand volts of the traditional discharge

Experiment study of CNTFC structure optimization

In the experiment study of CNTFC structure optimization, five different CNTFC structures are designed, and the experiments are conducted to study their discharge principle. The CNTFCs used in the electrodes are the CNT films grown by TCVD on Si substrates with different sizes.

Simulation analysis of the electric field distribution in the gap of CNTFC

There have been many studies on the effect of the structures of CNT and CNT film on the electric field distribution, but they all aim at increasing the field electron emission current [31], [32], [33]. And because different structures of CNT and CNT film have different calculation results, the calculation results are not usable for different structures of CNT film cathodes. Thus, the electric field distributions in the gaps of the five CNTFCs are calculated and analyzed on the ANSYS software

Conclusion

  • (1)

    The CNT film is used as the cathode to fabricate the electrode. The breakdown voltages of the CNT film cathode decrease from the value of more than thousand volts of the traditional discharge gas sensor to several hundred volts, in the gap distance with micrometer order at the atmospheric pressure. The current at the breakdown is microampere order, increasing three orders in comparison with that of the novel MEMS discharge gas sensor in reference [26].

  • (2)

    The experiment studies of the five

Acknowledgements

The authors would like to thank the National Nature Science Foundation Committee of China. This subject is supported by the National Nature Science Foundation of China (50077016), (60276037) and (60036010), and also by the National Study and Development Plan Subject on High Technology of China (2001AA313090).

Zhang Yong is an associate-professor with Xi’an Jiaotong University, China, where she received her doctor degree in 2004. Her interests are nanometer sensor system, intelligent sensor system, modern measurement and control instruments, multi-information fusion technology of multi-sensors, signal processing, and integrated virtual instruments.

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    Zhang Yong is an associate-professor with Xi’an Jiaotong University, China, where she received her doctor degree in 2004. Her interests are nanometer sensor system, intelligent sensor system, modern measurement and control instruments, multi-information fusion technology of multi-sensors, signal processing, and integrated virtual instruments.

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