【作者】 吴莉莉；

【导师】 陈裕泉；

【作者基本信息】 浙江大学 ， 生物医学工程， 2007， 博士

【摘要】 目前气体检测基本运用了传感器的敏感元件对气体的吸附特性来改变其电学特性如电阻、电容、频率等参数,从而实现对气体的检测;但这类传感器普遍存在受外界环境影响大,吸附、脱附时间长,可重复性差,气体选择性小,使用周期短,检测设备复杂,成本高等缺点。为此提出了一种新的气敏传感器设计思想,它利用高电场电离待测气体,根据击穿电压和放电电流的不同来区分气体。然而在常温大气压下,要使气体具有稳定的导电能力需要很高的电压建立足够强的电场,使得这种传感器的使用受到了安全和费用等问题的限制。纳米碳管的出现有效地解决了该问题,在外加直流电压激励下,纳米碳管的尖端会形成很强的非均匀电场,在电压相对较低的情况下能很容易地电离气体且获得较大的放电电流。本文研究了一种基于气体放电原理的定向纳米碳管阵列的气敏传感器。其中定向纳米碳管阵列的制备采用了阳极氧化铝(AAO)模板法,利用氧化铝的纳米孔壁对碳管生长过程中起到的限制作用来保证其定向性。虽然目前已有许多研究小组都能在AAO模板上生长出定向纳米碳管,但通常所需的生长时间都很长。实验中通过交流电沉积法在AAO模板纳米孔底沉积了钴催化剂,采用化学气相沉积(CVD)法数分钟内就能快速生长定向纳米碳管。AAO模板法制得的纳米碳管阵列与铝基底相连,因此在用纳米碳管做电极时,基底作为电极一端引出,可以方便地与外部电路连接。由于纳米碳管和电极是一体的,简化了器件结构和工艺,控制和测量都很方便。纳米碳管气敏传感器工作时以纳米碳管阵列作为阳极,铝板为阴极,聚酰亚胺绝缘薄膜用来调节两极间距离,可以通过使用不同厚度的绝缘薄膜的方法来改变距离,使极间距离在数十至数百微米内可调。实验中在外加电压不超过500V的情况下,该传感器成功地对氩气、空气、氮气和二氧化碳进行了检测;同时也对不同浓度的氮气和氩气的混合气体、氩气和空气的混合气体、氨气和空气的混合气体以及氮气和氨气的混合气体进行了地检测。对于外界的环境温度、湿度的影响也给予了分析,实验结果表明该传感器具有灵敏度高、响应时间快,可靠性高和重复性好等优点,有较大的实用价值。弱信号检测是传感器检测技术正在发展的领域,一般考虑弱信号检测技术时往往把注意力集中到如何抑制噪声上,而检测系统在抑制噪声的同时,被测信号也受到抑制或损害。随机共振理论的提出,为微弱信号检测提供了新的途径。随机共振的本质是由于输入信号、噪声和非线性系统的共同作用,使部分噪声能量转化为信号能量。基于随机共振的弱信号检测技术与传统检测方法的区别在于它是利用噪声而不是消除噪声来达到信号检测的目的。因此随机共振现象在微弱信号检测方面有着独特的优势,受到了广泛的关注。本文所研制的纳米碳管气敏传感器能够对包括惰性气体在内的多种单一气体和确定性混合气体进行检测,然而对于混合气体在某些浓度范围内却无法有效地分辨。鉴于随机共振在弱信号检测方面的成功应用,创新地提出了将随机共振检测系统引入气敏传感器领域来提高检测的灵敏度。文中构建了基于随机共振的气敏传感器检测系统,采用了多层随机共振算法来提高精度,根据信噪比最大值之间的差值完成了对气体不同的浓度的区分,从而提高了传感器的灵敏度。本文所提出的方法灵活有效,可进一步拓宽随机共振的应用领域。本文在纳米碳管气敏传感器和随机共振这两种新器件新理论框架下设计和研究了气体检测的新技术和新方法。实验和数值计算结果表明,该传感器具有结构简单、灵敏度高、响应时间快,可靠性高和重复性好等优点,有较大的实用推广价值。本项目的研究对于微纳技术在气体检测中的应用实施具有极大的现实意义,同时对于随机共振理论的进一步发展及其在弱信号检测中的应用也有一定的价值。更多还原

【Abstract】 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.更多还原