Investigation Of Preparation Of Gas Sensors Based On Carbon Nanotubes And Their Characteristics

The development of nanotechnology has created huge potential to build high sensitive, low cost, portable sensors with low power consumption. As the new and developing nanomaterials, carbon nanotubes(CNTs), with excellent conductivity, uniquely hollow structure and extremely high surface-to-volume ratio, which make a significant contribution to the adsorption of gas molecules, is a ideal nanomaterials for gas detection. Generally, only at high temperatures, semiconductor oxides could exhibit semiconductor characteristic; however, high temperatures always generate more energy consumption and rapidly reduce the service life of devices, even in some cases become the explosion source which threats to security.In this dissertation, the topic was focused on the fabrication and the investigation of sensitive characteristics of gas sensors based on CNTs. To obtain well performace for gas detection, the preparation technology of CNT arrays films, which meet the requirement of gas sensitivity, was investigated and optimized; the measurement of gas sensing properties was subsequently conducted. Meanwhile, composite materials of the purified CNTs and semiconductor oxide were studies, as well as the effect of the increasing of doping amounts of CNTs on the sensing properties. The main research contents and results are as follows:1) CNT arrays, directly grown on Si substrate deposited with 2.5nm of Al thin layer used as dispersing agent and 1.2nm of Fe thin layer used as cata lyst, were fabricated by microwave plasma chemical vapor deposition(MWPCVD) method. And CNT arrays, of which length inceased over time, evidently illustrated vertical alignment and entexcellent uniformity, and displayed P-type semiconductor behaviors. CNT arrays with 60 seconds of growth time showed a best, but still low sensitivity.2) CNT arrays with 60 seconds of growth time were annealed, accounting for the removal of by-products such as amorphous carbon and other functional groupps on CNTs surface, finally resulting in the improvment of gas sensitivity. They showed high sensitivity to NH3 and NO2, but low sensitivity to other gases, such as CH4. With a outstanding device structure, CNT gas sensor with interdigital electrodes(IDE), compared with the continuous film, exhibited a better gas-sensing properties; however, to a certain extent, the excellent conductivity of CNTs limits the advantage of the structure of IDE.3) CNTs were acidized by acid mixtures, mixed by concentrated sulphuric acid and nitrating acid by a ratio of 3:1;as a consequence, functional groups which could enhance the ability of absorbtion of gas molecules were introduced into CNTs. CNT/WO3 composite materials were prepared by acidized CNTs and WO3 fabricated by sol-gel method. Due to the excellent conductivity, the resistance of tungsten oxide dramatically decreases with the increasing of doping amounts of CNTs, resulting in the change of semiconductor characteristic of tungsten oxide. It was demonstrated that tungsten oxide doped with 1% CNTs versus to NO2, showed best sensitivity up to 7.4%; CNT/WO3 performed higher sensitivity to NH3 and NO2 was measured.4) As a N-type semiconductor, zinc oxides, were produced by sol-gel method. The resistance of zinc oxide dramatically decreases with the increasing of doping amounts of CNTs, resulting the change of semiconductor characteristic of zinc oxide. It was demonstrated that zinc oxide doped with 2% CNTs versus to ethyl alcohol, showed best sensitivity up to 9.8%. It was found that CNT/ZnO showed higher sensitivity to ethyl alcohol than to other gases.