2010 International Conference on Enabling Science and Nanotechnology (ESciNano), 1-3 December, 2010, KLCC, MALAYSIA

Student Paper

Ionization Gas Sensor Using Aligned Multiwalled Carbon Nanotubes

(MWCNTs) Array.

Atieh Ranjbar Kermany*a, Norani Muti Mohamedb and Balbir Singh Mahinder Singhb

"Electrical and Electronics Engineering, b Fundamental & Applied Sciences Department

Universiti Teknologi PETRONAS (UTP), 31750 Bandar Seri Iskandar, Tronoh, Perak, MALAYSIA *Email: atieh842003@yahoo.com

Carbon nanotubes (CNTs) have drawn a lot of interest as the sensing element in sensor

technology because of their unique electronic properties and remarkable mechanical properties. CNTs' extremely high surface-to-volume ratio makes it a very good candidate for the adsorption

of gas molecules. Gas sensors are divided into two types, namely; physical gas sensors and chemical gas sensors. In the case of chemical gas sensors, it is difficult to detect some gases

which have low chemical adsorption energy like inert gases since these sensors operate by variation in the sensing element's resistance. Physical gas sensors which operate by ionization

mechanism [1, 2], work by fingerprinting the ionization characteristics of distinct gases. Although these ionization sensors have better selectivity and response time, but the ones with traditional electrodes are still huge and bulky. With CNTs as the sensing elements providing

billions of sharp nanotips, breakdown voltage can be lowered such that safe operated sensor with

small size, good selectivity and sensitivity can be realized. CNT -based gas sensors can also be

operated in room temperature which will result in safer environment. The paper reports the fabrication and successful testing of ionization gas sensor using aligned multiwalled carbon

nanotubes (MWCNTs) array [3] featuring the electrical breakdown for several gas species namely

air, argon and 2% of hydrogen in air. The effect of the electrode separation on the electrical breakdown was also investigated.

The testing of breakdown voltage was carried out in a vacuum chamber. Fig. 1 shows the

sensor setup with aluminium plate as the cathode and CNTs array as the anode. Both electrodes are connected to a power supply and an ammeter to obtain the breakdown voltage. Certain

amount of gas was purged into the chamber using gas flowmeter and the breakdown voltage was measured. Fig. 2 demonstrates the Current-voltage plot for electrical breakdown of argon at

electrode separation of 140 �m. The breakdown voltage for several gases at room temperature

and for different electrode separations is tabulated in Table 1 and illustrated in Fig. 3. From

Fig. 3, the breakdown voltage was found to decrease as the electrode spacing was reduced from 140 �m to 80 �m. This is attributed to the increase in the electric field as the gap is reduced.

Fig. 4 shows the comparison between the breakdown voltage of argon, air and 2% hydrogen in

air with the electrode separation of 80 �m. Among the three gases, argon has the lowest

breakdown voltage whilst air has the highest value. This can be explained through electron mean

free path, 't where the higher the electron mean free path, the lower would be the onset of

discharge current. The onset of discharge current or electrical breakdown would be in the order of

V Ar < Vair(2%H2)< Vair which respond to the order of mean free path; 'tAr < 't air(2%H2) < 'tair. The

mixed gas of air with 2% hydrogen has a lower onset of discharge current because the mean free path of hydrogen is higher than that of nitrogen and oxygen in air.

Successful testing of few gases using ionization gas sensor will pave the way for more

testing on other pure and mixed gases. With the use of MWCNTs, the breakdown voltage can be further reduced by reducing the electrode separation. This suggests that portable nanotube

sensors based on ionization mechanism could be realized.

ESciNano 2010 - http://www.fke.utm.my/mine/escinano2010

978-1-4244-8854-4/10/$26.00 ©2010 IEEE

2010 International Conference on Enabling Science and Nanotechnology (ESciNano), 1-3 December, 2010, KLCC, MALAYSIA

References [1] Y. Xing, Z. Zhao-ying, W. Ying, Z. Jin and Z. Ying-ying. "A carbon nanotube-based sensing

element," Optoelectronics Letters, vol. 3, no. 2, pp. 81-84, 2007. [2] Norani Muti Mohamed, Hendrayana Thaha, "Carbon Nanotubes Gas Sensor," Proceedings

of UK: Malaysia Engineering Conference 2008 (Publication for British Library), ISBN: 978-0-9559952-0-02008.

[3] M.K.Lai, N.M. Mohamed, K.M.Begam, "The Role of Ah03 Buffer Layer in the growth of

Aligned CNTs," Advanced Materials Research, vol. 32, pp 29-32, 2008.

v

Fig. 1. The setup of ionization gas sensor. The inset is the aligned MWCNTs array

Argon 0 .0014 0.0012

0.00 1

j 0 .0008 0 .0006 0 .0004 0.0002

0

vv

Fig. 2. Current-voltage plot for electrical breakdown of argon at electrode separation

of 140 !lm

Table 1. Breakdown voltage for different gases and different interelectrode separations.

electrode's gap 80

100 120 140

4S0 400

3S0 300

�2S0 �� 200 � ISO

100 SO

o 80 100 120

V(V) 140

Argon

172 207 246 279

�a(gon __ %2 Hydrog"" in Air .....-Alr

Fig. 3. Effect of interelectrode separation on breakdown voltage value.

%2 Hydrogen in Air

Air

212 249 241 300 295 336 350 392

V(V)

300

I 250

200

150 .Gas

100

�O

Argon %2H inair Ai r

Fig. 4. Breakdown voltage comparison of 80 !lm electrode gap for different gases.

ESciNano 2010 - http://www.tKe.utm.my/mine/escinano2010