Introductory Chapter: Humidity Sensors

1. Introduction

1.1 Humidity sensors: A brief introduction of materials, mechanism and classifications

Humidity refers to the presence of water vapor in the air. Humidity affects human health and physical qualities of materials [1, 2] and therefore, it is critical to measure and control the humidity. Humidity measurement and control are critical not only for human and animal comfort but also for manufacturing processes and industrial products [3, 4, 5, 6]. Humidity measurement is considered very imperative in different industries such as health care, environmental monitoring, automobile, building air-conditioning, civil engineering, agriculture, semiconductor, pharmaceutical, textile, medical, paper, and process industries [7, 8, 9].

Humidity can be measured in three ways namely absolute humidity, specific humidity, and relative humidity. Relative humidity (RH) is one of the most commonly measured quantities in industry and everyday life [10]. RH is traditionally measured with microporous thin sheets and thin plates piezoelectric quartz sensor. The sensing mechanism of these materials is based on variation in the luminescence and oscillation frequency, correspondingly. These sensors may be expensive or demand high operational power/temperature, as well as a significant maintenance cost, depending on the nature of the materials. Humidity can also be measured using materials that indicate a change in resistance, impedance, or capacitance as a function of humidity. Such materials may be ceramics, low-molecular-weight organic materials, polymers, and composites. Humidity sensors are characterized as capacitive, oscillating, resistive, gravimetric, impedimetric, thermo elemental, hydrometric, or integrated optical based on their sensing method. The design of humidity sensors and nature of the material (sensing material) have an impact on their performance [10, 11, 12, 13, 14].

For a smart sensor, the required properties are the linear response, high sensitivity, wide sensing range, low hysteresis, fast response, high stability (physical and chemical), and low cost [14, 15]. To obtain these required features, several materials (organic, inorganic, and composites) have been studied. During last few decades organic-inorganic nanocomposites have been developed for advanced optic, magnetic and electronic applications. Despite having low stability as compared to inorganic materials, the organic materials have a lot of potential owing to their lightweight, high flexibility, high surface area, and easy fabrication. The merits of both materials (organic-inorganic) may be combined in a single device by using organic-inorganic composites as an active material [5, 10, 16, 17, 18].

There are various types of humidity sensors based on sensing mechanisms, sensing materials, sensors design, fabrication technologies, and applications. Humidity sensors can be fabricated by various types of fabrication techniques such as thick and thin film preparation processes, which are quite flexible and advantageous over other technologies. Similarly, micro-fabrication technology is also one of the useful fabrication techniques. Based on sensing mechanism, humidity sensors can be divided into capacitive, resistive, piezoresistive, magnetoelastic, field effective transistors, bulk acoustic wave, and optical humidity sensors. It has been reported that optical fiber sensors are best for use in harsh weather conditions, while the nanobricks sensors and capacitive sensors, which use impedance and resistive sensors fabricated with ceramic or organic polymer materials have also excellent humidity sensing qualities. Several types of sensing materials are used for sensing humidity like different ceramic materials, resistive polyelectrolytic materials, conductive polymeric materials, low molecular weight organic materials, and graphene-based materials such as graphene, graphene oxide, graphene quantum dots, reduced graphene oxide, and graphene-composites. On the other hand, humidity sensors are classified in different classes, e.g. rigid, flexible, dynamic, and static humidity sensors. The efficiency of these humidity sensors can be increased by investigating and developing new sensitive, durable, and resistive environmental factors sensing elements for the humidity sensors. In this book, the main aspects of humidity sensors have been covered.