Research On Carbon Nanotubes Gas Sensor And Its Detection System Based On Stochastic Resonance

At present gas sensors mostly utilize adsorption-desorption characteristics of sensitive elements to detect gases. The electronic properties of sensitive elements are modified after absorbing gases, such as resistances, capacities and frequency, etc. However this kind of gas sensor has the disadvantages of long adsorption and desorption time, bad repeatability and selectivity, short usage time, complex detection device, large cost, and it can be affected by the environment easily. So a new different gas sensor was proposed in this project, which was used the principle of gas discharge to distinguish the gases. As different gases have their unique breakdown voltages and discharge currenst, it is possible to identify one gas or certain mixture gases. But the usage of this new gas sensor has been limited by its high voltage and insecurity. Carbon nanotubes (CNTs) are used to solve this problem effectively in this project. As controlled DC voltage is applied to the CNTs electrode, the sharp tips of CNTs can generate high electric fields, so it was easy to ionize gas and get larger current at lower voltages.A novel aligned CNTs gas sensor based on gas discharge was discussed in this project. The method of anodic aluminum oxide (AAO) template was adopted to grow aligned CNTs. The aligned CNTs can be guaranteed since the nanometer pores in the template will limit its direction during the growth of the CNTs. Although many research teams got aligned CNTs on the AAO template, they spent much longer time on its growth. A faster growth method was used in this experiment, which deposited cobalt catalyst in the bottom of template by the method of AC electrolytic deposition. We grew aligned CNTs in a few minutes by chemical vapour deposition (CVD) method. Due to adopting the method of AAO template to grow aligned CNTs, the electrode and CNTs were integrated, so the gas sensor simplified its structure and processing technics.The aligned CNTs worked as anode and aluminium as cathode in this gas sensor. The polyimide insulation film was utilized to adjust the distance between the electrodes. Thus the distance between two electrodes could be adjusted with a range of 20-200μm by the thickness of the film. The gas sensor succeeded in detecting argon, air, nitrogen and carbon dioxide with the applied voltage of less than 500V. Meanwhile the sensor can distinguish the gaseous mixture of nitrogen and argon, argon and air, ammonia and air, nitrogen and ammonia. The influences of temperature and humidity were discussed in detail, and the performance of the sensor was also evaluated. Experimental results show that the CNTs gas sensor has the merits of small size, fast response, good sensitivity and selectivity, and it is very convenient to be operated at room temperature, so it is feasible in gas detection.Detection of weak signal is a new developing technology in the sensor detection field. Commonly, much attention of weak signal detection technology has been focused on seeking the way to reduce or remove noise. But the problem is that measuring signal is reduced or even lost as reducing the background noise when the frequency of signal approximates that of noise. Yet the theory of stochastic resonance (SR) provides a new way for weak signal detection. SR is a nonlinear phenomenon where the synergistic effects of noise and nonlinearity systems lead to an enhanced response of a weak periodic signal with an addition of noise in optimal intensity. Thus the energy of noise can be transformed into signal, and the weak signal can be detected easily. Nowadays, using and leading but not reducing noise in the non-linear subject has already been a new research direction, which has attracted much attention.The gas sensor designed in this dissertation can identify single gas or certain mixture gases, but it can't effectively distinguish some gases mixed in certain concentration. In order to solve this problem and improve the sensitivity, the detecting system of SR was introduced in the gas sensor originally. The gas sensor detecting system was set up, which included the algorithm of modulation and multiplayer SR. The difference value between the max SNR values can be used to identify the mixture gas of different concentration. The detecting method proposed in this project is very effective and it can be used in other application fields of SR.The new techniques and algorithms of gas detecting were studied in this project, which involved the academic fields of CNTs and SR. The experimental and numerical computation results show that the CNTs gas sensor based on SR has the significance in the field of nanometer technology. Meanwhile it will promote the development of SR theory, and play a role in the application of weak signal detection.