Basic Research Of The Gas Ionization Sensors Based On Carbon Nanotubes

Carbon nanotubes (CNTs) have enormous potential applications for gas sensing because of their remarkable adsorption capacity and electronic properties. Previously, the research of CNTs gas sensors has been focused on transistor type, conductance type and capacitor type, which operate based on the electrical conductance changes of nanotubes on exposure to gases. But they are limited by several factors, such as the inability to identify gases with low adsorption energies, irreversible changes for chemisorption and sensitivity to environmental conditions. Ionization sensors work by fingerprinting the ionization characteristics of distinct gases. With CNTs as electrodes, gases can be ionized at low voltages because of nano-sized CNT tips. However, the research on the CNTs gas ionization sensors is just at the beginning stage and there are some basic questions need to be studied. For these reasons, we have researched the basic principles and the relevant fabrication technologies of CNTs gas ionization sensors, and finally achieved the prototype device to detect different kinds of gases at low voltages.In order to obtain high quality SWNTs to meet the requirement of sensor, we researched the effect of He gas pressure and component of FeS on the synthesis process. Then, the obtained SWNTs were purified by high temperature wet vapour and SWNTs film was fabricated by spin-coating method, I/V characteristics of the film were investigated. Using this kind of CNTs film as anode, we tested and analyzed ionization characteristics of some gases. Finally, a prototype of gas senor with 35μm space between discharge electrodes was fabricated. The main results were generally concluded as the following: SWNTs samples obtained through arc discharge method were analyzed and it was found that gas pressure of He, content of FeS affect the yield of SWNTs. The mechanism were investigated: High pressure of He can accelerate heat transfer and S atoms from FeS make the liquid area on the surface of catalyst particles exist in a larger range of temperature and space, which both benefit growth of SWNTs; S atoms also can stabilize the exist of big size nucleus, from which SWNTs of large diameter grow. Based on theoretical analysis and experiment results, the optimum technical parameters are presented.SWNTs with the purity of 93% were obtained after being purified in high temperature wet vapour for 12 hours, which showed that high temperature wet vapour can oxidize and clean out impurity in the raw CNT samples and hardly hurt SWNTs because of its weak oxidizing property. Random film of SWNTs was fabricated by spin-coating, and the I/V characteristics of the film were investigated. The I/V relationship of this kind of film was non-linear and the conductivity was proportional with film thickness.Gas ionization characteristics of SWNTs film were investigated. With this film as anode, gases such as helium, argon, nitrogen, oxygen, carbon monoxide, hydrogen, methane and air, were ionized at distinct and low voltage (from about 100V to 307V with 150μm inter-electrode distance). The extending tubes served as numerous tiny anodes and formed point-to-plane electrode, which tend to induce gas corona discharge at low voltage. The voltage became lower as the inter-electrode distance was reduced, which is helpful to develop compact and battery-powered device. Threshold voltages varied only slightly with gas concentration. The discharge current varied logarithmically with concentration. Gas identity and concentration can be determined by monitoring the ionization voltage and discharge current of the gas. Gas ionization voltages of SWNTs were lower 30 voltages than MWNTs obtained by CVD, and after plasma treatment, the gas ionization characteristics of MWNTs were improved.At last, the fabrication process and characteristics of the SWNTs ionization gas senor with 35μm space between SWNTs film anode and Si cathode were investigated. The results showed that Ar, N2, CO and H2 can be ionized at 12.5V, 17.5V, 32.5V and 15V respectively. The threshold voltages were unstable at the beginning of test and after 1.5h, the ionization properties of our gas sensor became stable.