A High Temperature Capacitive Humidity Sensor Based on Mesoporous Silica

Capacitive sensors are the most commonly used devices for the detection of humidity because they are inexpensive and the detection mechanism is very specific for humidity. However, especially for industrial processes, there is a lack of dielectrics that are stable at high temperature (>200 °C) and under harsh conditions. We present a capacitive sensor based on mesoporous silica as the dielectric in a simple sensor design based on pressed silica pellets. Investigation of the structural stability of the porous silica under simulated operating conditions as well as the influence of the pellet production will be shown. Impedance measurements demonstrate the utility of the sensor at both low (90 °C) and high (up to 210 °C) operating temperatures

On the rational design of mesoporous silica humidity sensors

Mesoporous silica is commonly used as matrix for humidity sensors, which operate on the principle of relative humidity-dependent water uptake and read-out by resistive or capacitive means. Although numerous studies have been dedicated to improving the sensing performance, in particular with conductive additives, the effect of pore structure on sensing behaviour has not been systematically investigated so far. Herein, we showcase the effects of pore size and porosity on resistive sensing behaviour in the 0.5-85% relative humidity (RH) range. We employed evaporation-induced self-assembly (EISA) in combination with sol-gel chemistry to fabricate well-defined mesoporous silica thin films with high degree of structural control. Material architectures with pore sizes of 3 to 15 nm and porosities of 40 to 70% were rationally designed by using structure directing agents (SDAs) with increasing molecular weight and tuning the silica to SDA ratio. We found that a combination of pore size of 15 nm and 70% porosity showcases a particularly high sensitivity (~104 times change in resistance) in the measured range, with quick response and recovery times of 3 and 9 seconds, respectively. Across the various sensors, we identified a clear correlation between the pore size and the linear RH sensing range. Sensors with larger mesopores (~15 nm) exhibited higher sensitivity and linear response in the 65 to 85% RH range than sensors with smaller pores (<8 nm). Additionally, increasing the porosity while retaining the pore size, yields better overall sensitivity across the range. Our findings may serve as guidelines for developing broad spectrum high-performance mesoporous sensors and for sensors specifically engineered for optimal operation in specific RH ranges

Characterization and use of a fiber optic sensor based on PAH/SiO2 film for humidity sensing in ventilator care equipment

Objective: To develop a compact probe that can be used to monitor humidity in ventilator care equipment. A mesoporous film of alternate layers of Poly(allylamine hydrochloride) (PAH) and silica (SiO2) nanoparticles (bilayers), deposited onto an optical fibre was used. The sensing film behaves as a Fabry-Perot cavity of low-finesse where the absorption of water vapour changes the optical thickness and produces a change in reflection proportional to humidity. Methods: The mesoporous film was deposited upon the cleaved tip of an optical fibre using the layer-by-layer method. The sensor was calibrated in a bench model against a commercially available capacitive sensor. The sensitivity and response time were assessed in the range from 5 % relative humidity (RH) to 95 %RH for different numbers of bilayers up to a maximum of nine. Results: The sensitivity increases with the number of bilayers deposited; sensitivity of 2.28 mV/%RH was obtained for nine bilayers. The time constant of the response was 1.13 s ± 0.30 s which is faster than the commercial device (measured as 158 s). After calibration, the optical fibre humidity sensor was utilised in a bench top study employing a mechanical ventilator. The fast response time enabled changes in humidity in individual breaths to be resolved. Conclusion: Optical fibre sensors have the potential to be used to monitor breath to breath humidity during ventilator care. Significance: Control of humidity is an essential part of critical respiratory care and the developed sensor provides a sensitive, compact and fast method of humidity monitoring

Toward a new generation of photonic humidity sensors

This review offers new perspectives on the subject and highlights an area in need of further research. It includes an analysis of current scientific literature mainly covering the last decade and examines the trends in the development of electronic, acoustic and optical-fiber humidity sensors over this period. The major findings indicate that a new generation of sensor technology based on optical fibers is emerging. The current trends suggest that electronic humidity sensors could soon be replaced by sensors that are based on photonic structures. Recent scientific advances are expected to allow dedicated systems to avoid the relatively high price of interrogation modules that is currently a major disadvantage of fiber-based sensors

Solid state, dry zinc/MCM-41/air cell as relative humidity sensor

A zinc–air cell utilizing an inorganic MCM-41 membrane separator, in its dry form, reveals a unique behavior in response to relative humidity variation. The open circuit voltage (OCV) of the cell shows a linear dependence on relative humidity content. This is due to water adsorption–desorption characteristics of the mesoporous MCM-41 material. Comparison between relative humidity data obtained from commercial digital humidity sensor with data deduced from the OCV readings of Zn/MCM-41/air cell shows good sensitivity and high reliability

Polymeric optical fibre sensor coated by SiO2 nanoparticles for humidity sensing in the skin microenvironment

The sensitivity of a low-cost polymeric optical fibre humidity sensor based on transmittance changes due to evanescent wave absorption is reported using test measurements in an environmental chamber and of the skin. The layer-by-layer method was used to coat 30mm of the central unclad section of a multimode polymeric optical fibre with 7 layers of a hydrophilic film consisting of bilayers of poly(allylamine hydrochloride) and SiO2 mesoporous nanoparticles. Sensor characterisation shows a decrease in light transmission as relative humidity increases as a result of refractive index changes of the coating deposited onto the optical fibre which correlates with a commercial capacitive humidity sensor. The sensitivity obtained for the sensor coated with an optimum 7 layers was approximately -3.87x10-3 and -9.61x10-3 in transmittance percentage per RH percentage for the range of ~10% to ~75% RH and 90% to 97% RH, respectively. In addition, a response time of 1.5s can be seen for breath monitoring with the polymeric optical fibre humidity sensor. The proof of concept measurements made on the skin indicate that this sensor has the potential to be used to monitor humidity of the skin microenvironment within a wound dressing which can be used to provide better prognosis of healing

On the Rational Design of Mesoporous Silica Humidity Sensors

Mesoporous silica is commonly used as matrix for humidity sensors which operate on the principle of relative humidity (RH)-dependent water uptake and read-out by resistance (R) monitoring. Although numerous studies have been dedicated to improving sensitivity with conductive additives, the role of the pore architecture on the sensing behavior has not been systematically investigated so far. Herein, the effects of pore size and porosity on resistive sensing performance in the 0.5–85% RH range are showcased. Across various sensors, a clear correlation is identified between mesopore size and linear RH sensing range. Sensors with larger pores (≈15 nm) exhibit linear response in the 65 to 85% RH range with larger slope (ΔlogR/ΔRH) than sensors with smaller pores (<8 nm). Additionally, increasing porosity while retaining pore size, yields better overall sensor performance across the 15–85% RH range. In particular, a combination of pore size around 15 nm and porosity of 70% showcased a large resistance versus RH response (R0/R ≈ 10000) in the measured range, with quick response and recovery times of 3 and 9 seconds, respectively. These findings may serve as guidelines for developing broad spectrum high performance mesoporous sensors and for sensors specifically engineered to operate in specific RH ranges

Nanoparticle Chemical Sensors: A Study on Optical Humidity Sensor Design

The need for sensors capable of operating in harsh environments such as those containing flammable, corrosive or reactive vapors is a niche which thin-film optical devices, with their robustness and ease of maintenance may effectively fill. Two such systems were developed using spin-coating techniques and evaluated for applicability as humidity sensors. The first is based on aggregated silica nano particles. The second is a poly electrolyte multi-layer film impregnated with silver nano particles which exhibited strong surface plasmon response. Ellipsometric experiments performed using a sealed test cell with constant humidity maintained using saturated salt solutions showed that the former responded strongly to changing humidity. The latter possessed interesting hysteresis behavior as analyzed in a climate-controlled glovebox via reflectometry, but proved insufficiently responsive to changing humidity. The silica nano particle substrate was found to be a simple, tunable sensor platform which may be viable for the detection of a wide variety of vapor-phase chemical species

Capacitive and resistive response of humidity sensors based on graphene decorated by PMMA and silver nanoparticles

In this paper, we reported comparative study of the humidity characteristics of graphene/silver nanoparticles composite (Gr-AgNps) and graphene/silver nanoparticles/PMMA composite (Gr-AgNps-PMMA) based efficient humidity sensors. Aqueous solution of Gr-AgNps and Gr-AgNps-PMMA was drop casted over interdigitated copper electrodes with 50 μm gap embedded in the substrates in dust free environment. The band gap obtained from the UV-vis spectra for Gr-AgNps and Gr-AgNps-PMMA based humidity sensors was 4.7 and 4.1 eV respectively. The capacitive and resistive humidity response was studied using LCR meter (GW Instek817). Apparent increase in capacitance was observed (100-10,000 nF) with the increase in the humidity percentage (30-95%RH) at lower frequencies for both the sensors. Resistance of the sensors dropped to zero as the humidity level is increased from 30 to 95%RH in the chamber. The devices were tested for real time stability and for fast response/recovery time. Both the devices showed an excellent stability and response by recording their resistance and capacitance respectively. A lagging of RH decreasing response from RH increasing response was observed at 500 Hz frequency for both the sensors depicted from the hysteresis curve. The humidity response of Gr-AgNps was comparatively better than that of the Gr-AgNps-PMMA based humidity sensors

Recent progress of fabrication, characterization, and applications of anodic aluminum oxide (AAO) membrane: A review

The progress of membrane technology with the development of membranes with controlled parameters led to porous membranes. These membranes can be formed using different methods and have numerous applications in science and technology. Anodization of aluminum in this aspect is an electro-synthetic process that changes the surface of the metal through oxidation to deliver an anodic oxide layer. This process results in a self-coordinated, exceptional cluster of round and hollow formed pores with controllable pore widths, periodicity, and thickness. After the initial introduction, the paper proceeds with a brief overview of anodizing process. That engages anodic aluminum oxide (AAO) layers to be used as formats in various nanotechnology applications without the necessity for expensive lithographical systems. This review article surveys the current status of the investigation on AAO membranes. A comprehensive analysis is performed on AAO membranes in applications; filtration, sensors, drug delivery, template-assisted growth of various nanostructures. Their multiple usages in nanotechnology have also been discussed to gather nanomaterials and devices or unite them into specific applications, such as nano-electronic gadgets, channel layers, and clinical platforms tissue designing. From this review, the fact that the specified enhancement of properties of AAO can be done by varying geometric parameters of AAO has been highlighted. No review paper focused on a detailed discussion of applications of AAO with prospects and challenges. This review paper represents the formation, properties, applications with objective consideration of the prospects and challenges of AAO applications. The prospects may appeal to researchers to promote the development of unique membranes with functionalization and controlled geometric parameters and check the feasibility of the AAO membranes in nano-devices.Comment: 36 pages, 19 figures, 8 table