Library notes

Capacitive humidity sensitivity of SnO2:Sn thin film grown on silicon nanoporous pillar array

Effect of electrode configuration on capacitive humidity sensitivity of silicon nanoporous pillar array

Accelerated resistive humidity sensing properties of silicon nanoporous pillar array

Humidity sensors have been extensively used in various fields, and numerous problems are encountered when using humidity sensors, including low sensitivity, long response and recovery times, and narrow humidity detection ranges. Using graphene oxide (G-O) films as humidity sensing materials, we fabricate here a microscale capacitive humidity sensor. Compared with conventional capacitive humidity sensors, the G-O based humidity sensor has a sensitivity of up to 37800% which is more than 10 times higher than that of the best one among conventional sensors at 15%–95% relative humidity. Moreover, our humidity sensor shows a fast response time (less than 1/4 of that of the conventional one) and recovery time (less than 1/2 of that of the conventional one). Therefore, G-O appears to be an ideal material for constructing humidity sensors with ultrahigh sensitivity for widespread applications.

Sodium titanate Na <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> nanorods were synthesized by a hydrothermal method and characterized by X-ray diffraction and scanning electron microscopy. Then, the material was coated on a Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ceramic substrate to fabricate humidity sensors using Ag-Pd as interdigitated electrodes. The sensor shows high humidity sensitivity and fast response and recovery. The impedance changes by five orders of magnitude over a relative humidity (RH) range from 11% to 95%. At the frequency of 100 Hz, response and recovery times are both <;2 s, and the maximum hysteresis is <;3% RH. In addition, the complex impedance at different RH was investigated to understand the sensing mechanism. The results indicate potential applications of sodium titanate for fabricating high-performance humidity sensors.

Effect of interdigital electrode gap on the performance of SnO2-modified MoS2 capacitive humidity sensor

In this study, we fabricated a highly flexible fiber-based capacitive humidity sensor using a scalable convergence fiber drawing approach. The sensor’s sensing layer is made of porous polyetherimide (PEI) with its porosity produced in situ during fiber drawing, whereas its electrodes are made of copper wires. The porosity induces capillary condensation starting at a low relative humidity (RH) level (here, 70%), resulting in a significant increase in the response of the sensor at RH levels ranging from 70% to 80%. The proposed humidity sensor shows a good sensitivity of 0.39 pF/% RH in the range of 70%–80% RH, a maximum hysteresis of 9.08% RH at 70% RH, a small temperature dependence, and a good stability over a 48 h period. This work demonstrates the first fiber-based humidity sensor fabricated using convergence fiber drawing.

The need for humidity sensors in various fields have led to the development and fabrication of sensors for use in industries such as the medical, textile, and laboratories. This chapter reviewed humidity sensors, major types and applications with emphasis on the optical fiber, nanobricks, capacitive, resistive, piezoresistive and magnetoelastic humidity sensors. While optical fiber sensors are best for use in harsh weather conditions, the nanobricks sensors have excellent qualities in humidity sensing. Capacitive sensors make use of impedance and are more durable than the equivalent resistive sensors fabricated with ceramic or organic polymer materials and have short response and recovery times which attest to their efficiency. Piezoresistive sensors have fast response time, highly sensitive and can detect target material up to one pictogram range. Magnetoelastic sensors are very good and can measure moisture, temperature and humidity between 5% and 95% relative humidity range. It was concluded that sensors have peculiar applications.

17

A high-sensitivity optical fiber relative humidity sensor based on microsphere WGM resonator

Characterization and humidity sensing properties of Bi 0.5Na 0.5TiO 3–Bi 0.5K 0.5TiO 3 powder synthesized by metal-organic decomposition

Layers of graphene oxide (GO) deposited on various substrates have been investigated for their capacitive and resistive features. It has been shown that such layers demonstrate large specific capacitances. Here we report the fabrication of a first GO-on-textile capacitive humidity sensor. GO layers are deposited on aluminum silicate textile, and the impedances of the deposits are measured at different atmospheric conditions using various voltage sweeping frequencies. The results demonstrate the dependence of the layer impedance on the humidity level in the surrounding atmosphere. Further, it is clarified that the layer capacitance is humidity dependent. A capacitive humidity sensor is designed and fabricated. The capacitance of a typical device varies from 130 pF, measured in dry synthetic air, to 5000 pF measured in air with 95% relative humidity (RH). Interestingly, these high sensitivities are measured at low frequencies (f=0.01 Hz); sensitivity to humidity decreases as the voltage sweeping frequency increase. For the first time, it is shown that the sensitivity of these capacitive humidity sensors considerably increase when very low frequencies are used for capacitance measurement. The ratio of the capacitance at RH= 95% to the capacitance at RH <; 0.1% both measured at 0.01 Hz is 32 times larger than the same calculated at 100 Hz. The device is suitable for wearable electronic applications.

A simple and new relative humidity (RH) capacitive sensor with a two-layer planar structure based on anodic aluminium oxide (AAO) film serving as a sensing layer has been fabricated. A pair of separated tantalum thin films under the AAO layer served as two coplanar electrodes. AAO film was fabricated by anodising deposited aluminium thin film in oxalic acid. The influence of the pore size and thickness of the AAO film, and the electrode spacing, on the performance of the sensor was investigated. The RH sensor with a thicker AAO film and narrower electrode spacing was more sensitive. By pore widening, the sensitivity of the sensor can be improved. The achieved average sensitivity as high as ∼3.57 pF/RH% was obtained for sensors with an AAO layer of 2 µm thickness and 80 nm pore size. These results were explained using the partial capacitance method and parallel-serial model. In addition, under the humidity range of 20–80%, sensors with 70 nm pores had the highest sensitivity, which was interpreted in terms of effective permittivity and anion concentration. This study provides an easy method to fabricate high-performance humidity sensors with very simple structures for practical applications.

We propose a high-performance porous metal–insulator–metal-type capacitive humidity sensor based on a functional polymer mixed with TiO2 microparticles subjected to inductively coupled plasma (ICP) and reactive-ion etching (RIE) treatments. The humidity sensor is composed of a porous top electrode, a TiO2-containing functional polymer humidity-sensitive layer subjected to two types of oxygen plasma treatment, a bottom electrode, and a glass substrate. The initial O2 ICP dry-etching utilizes higher intensity plasma for deep etching in the inlet holes on the top electrode to increase the contact area and shorten the vapor absorption path, thereby yielding high sensitivity and low hysteresis. Further, the RIE treatment leads to roughening of the polymer etching surface and further improving the performance of the humidity sensor. The functional polymer mixed with TiO2 microparticles exhibits excellent hysteresis over a wide humidity sensing range. The fabricated sensors are tested at various relative humidity (RH) values, achieving an ultra-low hysteresis of 0.64% RH at 60% RH, a high sensitivity of 1.24 pF/% RH, a fast response time of less than 25 s, good temperature dependence, and a stable capacitance value with a maximum error rate of 0.15% over 120 h of continuous testing.

This paper presents a design of a smart humidity sensor. First we begin by the modeling of a Capacitive MEMS-based humidity sensor. Using neuronal networks and Matlab environment to accurately express the non-linearity, the hysteresis effect and the cross sensitivity of the output humidity sensor used. We have done the training to create an analytical model CHS “Capacitive Humidity Sensor”. Because our sensor is a capacitive type, the obtained model on PSPICE reflects the humidity variation by a capacity variation, which is a passive magnitude; it requires a conversion to an active magnitude, why we realize a conversion capacity/voltage using a switched capacitor circuit SCC. In a second step a linearization, by Matlab program, is applied to CHS response whose goal is to create a database for an element of correction “CORRECTOR”. After that we use the bias matrix and the weights matrix obtained by training to establish the CHS model and the CORRECTOR model on PSPICE simulator, where the output of the first is identical to the output of the CHS and the last correct its nonlinear response, and eliminate its hysteresis effect and cross sensitivity. The three blocks; CHS model, CORRECTOR model and the capacity/voltage converter, represent the smart sensor.