【作者】 姚尧；

【导师】 陈向东；

【作者基本信息】 西南交通大学 ， 通信与信息系统， 2013， 博士

【摘要】 湿度做为一个重要的物理量,在人类日常生活和工农业生产中扮演着非常重要的作用。随着科技的不断发展,人们对高性能湿度传感器的需求不断增加,这为湿度传感器行业的发展带来了前所未有的机遇和挑战。近些年,新型纳米材料被广泛报道应用于湿度传感器领域,正逐渐成为湿度敏感材料的主要发展方向及研究热点。本论文围绕一种新兴的二维碳质纳米材料-氧化石墨烯(GO)的湿敏特性开展研究,提出了几种基于GO敏感薄膜的湿度传感器,并探讨了环境湿度对GO电学特性的影响。论文主要工作如下：(1)运用交流复阻抗法和直流Ⅰ-Ⅴ测试两种方法研究了环境湿度因素对GO薄膜电学特性的影响及其机制,并探讨了GO薄膜在阻抗式湿度传感器上的应用。1)采用交流复阻抗法研究了GO薄膜的湿度敏感机制并建立相应的等效电路模型,通过分析不同湿度条件下的GO薄膜的Cole-Cole图和Bode图,可以发现在低湿环境中,GO薄膜的导电机制为材料固有的电子电导和极化,由于GO薄膜固有的电子电导非常微弱,GO材料的极化对复阻抗贡献占主导地位,GO薄膜表现出较大的阻抗。而在高湿环境中,离子电导及水分子参与的极化对GO薄膜导电起主要贡献,GO薄膜表现出弱的导电性；进一步探讨了GO薄膜在阻抗式湿度传感器上的应用,研究结果表明GO薄膜构建的阻抗式湿度传感器在低频激励下(50Hz)具有较好的湿度响应灵敏度,此外这种类型的传感器还具有湿滞小,响应和恢复迅速等特性。2)研究了不同环境湿度条件下GO薄膜的直流Ⅰ-Ⅴ特性,结果表明加载电压幅度影响GO薄膜的Ⅰ-Ⅴ特性曲线特征,在低加载电压区(-2V-2V),GO薄膜的Ⅰ-Ⅴ特性曲线在各个湿度点均表现为近似线性的直线, GO薄膜电阻随着湿度的增加而下降,且薄膜电阻与扫描电压大小无关,而在高加载电压区(-4V--2V以及2V-4V),GO薄膜的Ⅰ-Ⅴ特性曲线特征不仅与外界环境湿度水平有关,还与加载在GO上的电压大小有关,高幅度加载电压导致GO薄膜电阻下降,推测可能的机理为GO薄膜在强电场作用下发生部分还原反应,GO的部分碳原子由Sp3杂化态转向Sp2杂化态,从而使得GO薄膜的电导率得以提高。加载电压幅度的增大以及外界湿度水平的提高都会使得GO薄膜的还原程度提高,导致GO薄膜的电传导能力得到大幅度提高。上述研究工作表明,对于溶液法制备的GO薄膜电子器件,在应用过程中需考虑环境湿度和工作电压因素的影响。(2)由于GO薄膜具有的较强亲水性、大比表面积及较高机械模量等优异特性,提出了一种兼具高灵敏度和高稳定度的GO薄膜修饰石英晶体微天平(QCM)湿度传感器。QCM作为基本换能元件,GO做为湿度敏感层沉积在QCM电极上。首先采用振荡电路法研究了所制备的QCM湿度传感器的湿度敏感特性,结果表明GO薄膜修饰的QCM湿度传感器呈现出较高的湿度响应灵敏度,且在宽湿度范围内(6.4～93.5%RH)具有较好的线性,以及快速的响应及恢复特性,低的湿滞特性及较好的长期稳定性；此外,研究了GO薄膜厚度对传感器湿度敏感性能的影响,发现较厚的GO薄膜修饰的QCM湿度传感器具有更高的灵敏度响应,但其线性度略有下降。还采用阻抗电路法研究了GO修饰的QCM湿度传感器在湿度环境中的谐振行为,并与传统聚合物材料修饰的QCM湿度传感器进行了对比分析。研究发现,GO薄膜修饰的QCM湿度传感器在高湿环境下的品质因数(Q)大大高于聚合物材料修饰的QCM湿度传感器,因而GO材料比较适合于构建具有高稳定度的QCM湿度传感器。(3)基于GO薄膜吸附水分子发生溶胀这一特性,提出了一种易于集成化的GO-硅双层结构MEMS湿度传感器。采用硅微桥做为换能元件,GO材料做为湿敏层涂覆于硅微桥,形成GO-硅的双层结构。基于GO薄膜的MEMS湿度传感器的换能原理如下：GO吸附/脱附水分子产生膨胀变形,GO薄膜的膨胀变形作用于硅微桥使其产生相应的机械形变,集成在硅微桥内部的压阻惠斯通电桥感知这种机械形变并产生电压输出信号。研究发现,基于GO薄膜的MEMS湿度传感器在较宽湿度范围内(10-98%RH)呈较好的湿度线性响应,并具有快速的响应及恢复特性,以及较好的重复性和低湿滞等特性。除此,研究了GO涂层厚度对传感器湿度响应灵敏度及线性度的影响,并讨论了环境温度因素对传感器输出灵敏度的影响。基于GO薄膜的MEMS湿度传感器由于制作工艺与传统的硅集成电路工艺相兼容,可以比较容易集成后端信号处理电路以实现单片式的湿度传感器,以及与其他类型传感器集成实现多参数、智能化传感器。更多还原

【Abstract】 As an important physical quantity, relative humidity (RH) plays an important role in human daily life, industry and agriculture. With the continuous development of science and technology, the increasing demands for high performances humidity sensor bring out unprecedented accident and challenge for humidity sensor industry. In recent years, novel nanostructure materials have frequently been adopted in the application of humidity sensor, and gradually become the development trends and the hot spots of the humidity sensing materials. This dissertation centres on and spreads out from the humidity sensing properties of graphene oxide (GO), which is a novel two dimension carbon nanomaterial. We present several types of humidity sensor based on GO thin films, and investigate the influence of ambient humidity on the electrical properties of GO films. The main content summarized as follows:Firstly, the effects and mechanism of ambient humidity on the electrical properties of GO films is studied by using alternating current complex impedance technique and direct current I-V method.(1) Utilizing alternating current complex impedance technique, the humidity sensing mechanism of GO films is studied, and the correspoding equivalent circuits are proposed. By analyzing the characteristics of the Cole-Cole diagram and Bode diagram of GO films, we find that the conductive mechanisms of GO films at low RH are intrinsic electron conduction and electronic polarization of GO films; Because of the intrinsic electron conduction of GO films is weak, GO films exhibit large impedance and the polarization is dominant in the impedance of GO films. At high RH, however, the conductive mechanisms of GO films are ion conduction and polarization of water, GO films exhibit poor conductivity. We further dicuss the application of GO films in impedance humidity sensor. The results suggest that GO films based impedance humidity sensor show good humidity sensing property at low frequency exciting point (50Hz). In addition, the sensor exhibit low humidity hysteresis, fast response and recovery.(2) The I-V characteristics of GO films at various humidity levels are investigated. The results indicate that the I-V characteristic of GO films under a lower loading voltage (-2V～2V) is different from that of a higher loading voltage (-4V～-2V and2V～4V). In the case of a lower loading voltage, the I-V curves of GO films are linear at various RHs. The resistance of GO films decreases with inceasing RH at a constant loading voltage. The resistance of GO films is independent on the loading voltage. However, in the case of a higher loading voltage, the I-V characteristics of GO films are not only dependent on humidity, but also on the loading voltage, the resistance of GO films decreases with increasing loading voltage. It is infered that the reduction of GO films occurs in the presence of strong electric fields, a part of cabon atoms of GO films transform from sp3hybridization state to sp2hybridization state; it results in the enhancement of the conduction of GO films. The increase in loading voltage and humidity level can also strengthen the reduction of GO films, leading to the large enhancement of the conduction of GO films. The above-mentioned studies indicate that the graphene electrical device arising from solution processed GO shoud consider the effect of humidity and loading voltage.Second, due to these excellent properties of GO films, such as strong hydrophilic, high specific surface area, high mechanical modulus, GO modified quartz crystal microbalance (QCM) humidity sensor with both large humidity sensitivity and high frequency stability is presented. The sensor utilizes QCM as transducer element. GO films as humidity sensing layer is deposited on the electrode of the QCM. At first, we study the humidity sensing properties of GO films modified QCM humidity sensor by oscillation circuit method. The results show that GO films modified QCM exhibit high humidity response sensitivity, linear humidity-frequency response in the wide detection range of6.4-93.5%RH, fast response and recovery, low humidity hysteresis, good long-term stability. In addition, the influence of the thickness of GO films on the humidity sensing characteristics of QCM humidity sensor is discussed; it is found that QCM humidity sensor with the thicker GO films show higher sensitivity response, but worse linearity. Next, the resonant behaviors of GO films modified QCM humidity sensor in various RHs are investigated through impedance circuit method, and are compared with conventional polymer modified QCM. The results indicated that the quality factor (Q) of GO films modified QCM is much higher that of polymer modified QCM in high RH environment, as a result, GO as humidity sensing material is very suitable to realize high stability humidity sensor by combining with QCM transducer.Thirdly, a novel MEMS humidity sensor is presented based on the swelling property of GO films due to water adsorption. The sensor utilizes silicon microbridge as transducer element. GO films as humidity sensing layer is deposited on the silicon microbridge, which forms GO-silicon bilayer structure. The transducer principle of GO films based MEMS humidity sensor is as follow: the adsorption/desorption of water molecule from GO films produce volume swelling/shrinking. This stain applies on silicon microbridge, resulting in the bending of silicon microbridge. The full piezoresistive Wheatstonebridge integrated in silicon microbridge transforms this deformation into a measurable output electrical signal. The experiment results show that GO films based MEMS humidity sensor exhibited excellent linear humidity response in a wide humidity range of10-98%RH, fast response and recovery, good repeatable property and low humidity hysteresis, and so on. Moreover, the dependence of the thickness of GO thin films on response sensitivity and linearity is investigated, and the effect of temperature on the sensor output is also dicussed. Since the fabrication process of GO films based MEMS humidity sensor is compatible with classic silicon integrated circuit process, this approach can easily realize singlechip humidity sensors by integrating signal process circuit on MEMS chip, and form multi-informations and smart sensors by integrating other types of sensor on one chip.更多还原