Abstract
Humidity sensors are widespread in many industrial applications, ranging from environmental and meteorological monitoring, soil water content determination in agriculture, air conditioning systems, food quality monitoring, and medical equipment to many other fields. Thus, an accurate and reliable measurement of water content in different environments and materials is of paramount importance. Due to their rich surface chemistry and structure designability, carbon materials have become interesting in humidity sensing. In addition, they can be easily miniaturized and applied in flexible electronics. Therefore, this short review aims at providing a survey of recent research dealing with carbonaceous materials used as capacitive and resistive humidity sensors. This work collects some successful examples of devices based on carbon nanotubes, graphene, carbon black, carbon fibers, carbon soot, and more recently, biochar produced from agricultural wastes. The pros and cons of the different sensors are also discussed in the present review.
Highlights
Gas sensors are miniaturized analytical devices that can deliver real-time and on-line information on the presence of a target gas
When air is fully saturated with water, the pressure exerted by the contained water vapor is defined as the saturation water vapor pressure (Ps) that is dependent on the temperature
In the last two decades, various resistive-type humidity sensors based on polymers [22], sulfonated polyimides (SPIs) [23], poly(2-acrylamido-2-methylpropane sulfonic acid) [24], poly(2-acrylamido-2-methylpropane sulfonate) [25], and poly(4-vinylpyridine)/poly(glycidyl methacrylate [26]) and/or carbon nanotubes [27] have been investigated
Summary
Gas sensors are miniaturized analytical devices that can deliver real-time and on-line information on the presence of a target gas. Semiconductor metal oxides (SMOs) are sensitive towards different gases Their working principle is based on the variation of their electrical properties: on the surface of the grains of an n-type SMO gas sensor, oxygen molecules can adsorb. The active materials are the most important part in high-performance flexible sensors and should be both flexible and electrically conductive To this aim, recently, carbon films have attracted great attention for their potential applications as humidity sensors because of their large sensing area and high chemical inertness [4,12]. This paper reviews different experimental activities dealing with carbonaceous materials used as capacitive or resistive humidity sensing materials with a special focus on biochar It discusses their advantages and underlines some limits and drawbacks, suggesting useful strategies for minimizing them
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