Study On Properties Of Resistive-type Humidity Sensors Based Nano-oxides

Monitoring and controlling humidity are more and more emphasized on industry, agriculture, daily life, science research etc. The resistance-type humidity sensor is a kind of the chemical sensors, which can be widely used in many fields. It plays a role through adsorption effect of the sensing materials, in other words, the impedance of the resistance-type humidity sensor will change when the sensor is exposed in different water vapor. In this paper, the author has prepared and characterized two kinds of materials sensitive to humidity, and the electrical properties and sensing mechanism to humidity of the two resistance-type humidity sensors based on the two materials have been investigated.In the first chapter, the concept of humidity sensors, the development process, and the direction of current research was introduced comprehensively. At the end of this chapter, we pointed out the significance of our work and summarized the important results obtained.Perovskite-type oxides LaCo_xFe_1-xO₃ was mainly applied in these areas such as solid oxide fuel cells, chemical sensors for the detection of alcohol and oxygen, methane oxidation catalysis. There were reports about the magnetism of this material. But the humidity sensitive properties of this material were rarely reported. In the second chapter, the nanocrystalline LaCo_xFe_1-xO₃ prepared by citrate method was ground to form the humidity sensing paste, and the paste was then screen printed on ceramic substrates with interdigitated electrodes to obtain the humidity sensitive thick films. The humidity response of LaCo_xFe_1-xO₃ showed that LaCo_0.3Fe_0.7O₃ has the highest sensitivity to change in humidity when compared with the other samples, and the optimal calcination temperature is 600°C.In view of the past, many reports have showed that the addition of alkali ions was effective in improving the performance of many different kinds of materials, the alkali ion can be dissolved in water to become the conduction carrier, and play a major role in improving the humidity properties of materials. In the third chapter, the doping of alkali cation was considered to improve the humidity-sensitive properties of material, the results showed that the optimal dopant was K+, the resistance of material reduced after alkali cation was doped into it, the humidity sensor based on it had good response in the whole humidity range, and the sensitivity was enhanced significantly. The shortage of this method was time-consuming and laborious, and the uniformity of doping was limited, moreover, the recovery characteristic of humidity sensor was poor.In the fourth chapter, the K+ was doped by adding to precursor prepared by sol-gel method. The appropriate doping of K⁺ improved the sensitivity of material in the low relative humidity. The resistance of material reduced after K⁺ was doped into it, the humidity sensor based on it had good response in the whole humidity range, and the sensitivity was enhanced significantly. The advantages of this method are saving time and manpower, and easy to achieve uniform doping. At the same time, the recovery characteristic of humidity sensor has also been greatly improved.TiO₂ possess good properties such as chemical and physical stability, good humidity sensitivity and wide sources. Pure TiO2 is an intrinsic semiconductor, the humidity sensor based on the pure TiO2 have high intrinsic resistance, and bring some difficulties to accurate measure. In the past research, the humidity sensitive properties of TiO₂ were usually improved by doping or mixing with other compounds, which was called as TiO₂-based humidity sensitive material, and have been widely investigated. For the material sensitive to humidity, when the other factors about surface of material is roughly same, the greater the surface area, the more the center for absorption of water, and the material possess better humidity sensitive properties. In the fifth chapter, we try to prepared TiO₂ with larger surface area and pore volume by adding tetraethyl orthosilicate to precursor, followed by HF treatment. The sensitivity of synthesized TiO₂ in the low RH was improved significantly, and the humidity sensor had good response to change in humidity in the whole humidity range with little hysteresis and quick response and recovery.The theory of Dielectric Physics was applied to explain the sensing mechanism of the humidity sensors based on the two materials. The complex impedance was obtained, and a circuit was designed to do an experiment named ITIC (the isothermal transient ionic current) to deduce which kinds of the carriers of the sensing materials are domain in different humidity environment. The final results showed that there are two kinds of particles to participate electric conduction in all humidity range, i.e. electron and ion. The electron plays a role more and more weakly, and ion plays a role more and more strongly for electron conduction when relative humidity increases from low to high. The obtained conclusion is identical to the Electron and Proton (Ion) Theory, which is popular currently to explain the humidity sensing mechanism.