Fluoropolymer-based capacitive carbon dioxide sensor

Introduction This paper systematically investigates the sensing performance of amine-functionalized, microfabricated capacitive carbon dioxide (CO2) sensors as a function of sensor temperature and sensing film thickness. The interdigitated capacitor structure was heated directly by an integrated resistive heater. Using thin sensing films, sensor sensitivities around 1% per 1,000ppm CO2 are obtained at response times of <2 minutes. Background Monitoring of carbon dioxide, a greenhouse gas, using miniaturized sensors is of significant interest for indoor air quality and health monitoring applications [1, 2]. However, CO2 molecule are chemically inert and, thus, pose a challenge for solid-state gas detectors. As a result, optical sensing using nondispersive infrared (NDIR) sensors has become the standard for CO2 sensing. However, NDIR sensors are generally large and costly compared to solid-state sensors. To address this challenge, amino-functionalized sensing films in combination with a variety of transducers have been investigated for CO2 sensing, but exhibit small sensitivities at room temperature [3, 4]. This work systematically investigates the CO2 detection capabilities of amino-functionalized, microfabricated capacitive sensors as a function of the sensor temperature and sensing film thickness. Method Each silicon die contains four 1mm2 interdigitated electrode structures as capacitive sensors, surrounded each by a resistive heater and temperature sensor. The simple 2-mask fabrication process starts with growing a 1µm thermal oxide on top of the silicon wafer for electrical insulation, followed by deposition and patterning a 200nm aluminum film to define the interdigitated electrodes, the resistive heater, and a resistive temperature sensor. The wafer was then passivated using a 20nm thick SiO2 film deposited by plasma-enhanced atomic layer deposition and contact pads were patterned and etched. Figure 1 shows a picture of the sensor die with 4 sensors and a schematic of one sensor with heater and temperature sensor around it. The sensing film was prepared by mixing 3-amino-propyltrimethoxysilane and propyltrimethoxysilane with a volume-ratio of 70:30. The mixture was spray-coated on the sensor and cured at 120˚C for 16 hours in a vacuum oven. The capacitive sensors were mounted in a dual-in-line (DIL) packages and wire bonded. To increase the thermal resistance to the DIL package the sensor chips were mounted on 500µm Kapton tape at its four corners as shown in Figure 2a. The packaged sensors exhibit a heating efficiency of approximately 0.2˚C/mW (Figure 2b). Results and Conclusions An Environics 2040 gas mixing system was used to expose the packaged sensors to defined concentrations of CO2 gas. A commercial NDIR sensor was mounted in line with the capacitive gas sensor and used as a reference sensor. Figure 3 shows a typical response of a capacitive sensor coated with a 500nm thick sensing film when exposed to different CO2 concentrations. This specific measurement was performed at 85˚C die temperature and shows a decrease in capacitance with increasing CO2 concentration. Similar measurements were performed using sensors with three different film thicknesses (100nm, 200nm and 500nm) at die temperatures ranging from 50-120˚C. For the capacitor coated with a 100nm sensing film, Figure 4 shows the capacitance change measured at 3700ppm CO2 concentration as well as the t90 response time as a function of the sensor temperature. A maximum capacitance change was observed at 85˚C and exhibited an average response time t90=11minutes. Faster response times were found at higher temperatures at the expense of sensor sensitivity. Figure 5 compares the capacitance change at 3,700ppm CO2 as a function of temperature for different film thicknesses. While large sensor sensitivities can be obtained for thicker films, the response times increase. For example, for a 500nm thick sensing film, t90≈26minutes with a signal change of ≈17%/1,000ppm.

Pressure sensors have been of great interest in Internet of Things (IoTs) applications including smart window, displays, security systems, mobile phones, and prospective electronics skin (e-skin). In particular, sensing high pressures (> 2 kPa) as well as low pressures (< 30 Pa), such as blood pressure, human touch, and sound pulse, is of great importance for low-powered IoT sensors. These pressure sensing capabilities have been dramatically accelerated through the developments of both resistive and capacitive sensing elements. The resistive sensors are based on the change in electrical resistance upon pressure, and recently, 1-D or 2-D nanomaterials employed for the efficient pressure-sensitive percolation of their networks have allowed for high sensitivity of pressure sensors. Although the advance in the resistive pressure sensors is significant with their simple device construction and high pressure sensitivity, they still suffer from high power consumption much greater than that of the capacitive pressure sensors with the operation voltage of approximately 1 V. The capacitive sensors utilizing the capacitance change upon pressure on the other hand are advantageous owing to their low power consumption, fast response time and compact circuit layout with vertically stacked device architecture. The low pressure sensitivity of a capacitive sensor, however, is detrimental because most of pressure responsive materials such as elastomers and gels have low dielectric constants, which leads to low capacitance change, and thus less responsive to small pressure. Sensors adopting field effect transistor architecture show the high sensitivity because small capacitance change was readily converted into large alteration in channel current of the device, in particular, combined with topologically micropatterned gate dielectric layer. The high driving voltage over 50 V and complicated device architecture including semiconducting channel and three terminal electrodes still limit their usefulness. Ionic gels of polymer composites with ionic liquids (ILs), non-volatile positive and negative charged ion pairs, exhibit high specific capacitance (> 5 μF/cm2) arising from nanometer-thick electrical double layer (EDL) at ionic liquid/electrode interface, giving rise to a pressure sensor with high sensitivity of approximately 1 kPa−1. We envisioned that an ionic gel with high dielectric constant could become more sensitive to pressure when it is topographically micropatterned. A topographically patterned ionic gel sandwiched between two electrodes contains periodic air gap that is likely to decrease when pressurized, making an effective capacitance of the device drastically increase upon even small pressure. Here we present highly sensitive capacitive pressure sensors with topologically patterned arrays of pyramidal ionic gels between two Indium Tin Oxide (ITO) electrodes deposited on mechanically flexible poly(ethylene terephthalate) (PET) substrates. Micropatterned pyramidal ionic gels (MPIGs) exhibits significant variation in capacitance as a function of pressure, giving rise to a pressure sensor with its sensitivity of approximately 40 kPa−1 at the pressure range lower than 400 Pa and 13 kPa−1 at the range between 0.5 kPa and 5 kPa. Our device is suitable for sensing the broad pressure range up to 50 kPa with its sensitivity greater than 2 kPa−1, allowing for efficient detection of various pressure sources from a few Pa to tens of kPa including sound wave, light weight object, jugular venous pulse, radial artery pulse and finger touch. Figure 1

Capacitive humidity sensing performance of naphthalene diimide derivatives at ambient temperature

We present on the design of highly-sensitive capacitive sensors using conductive textile electrodes and polyurethane (PU) foams as the dielectric layer for wearable sensing applications. Previous works involve complex processes in the fabrication of flexible, stretchable, and composite dielectrics using additional fillers or microstructures. In this work, we demonstrate a simple and cost-effective fabrication technique using polyurethane foam as the dielectric material to form capacitive sensors that are sensitive to stretching, bending and pressure. The magnitude of change in capacitance (10-60%) is increased due to the combined effect of micropores in the dielectric foam and the air gaps at the interface between the textile electrodes and dielectric layer. With the use of microporous PU foam, the change in capacitance under a mechanical load is not only due to the change in the thickness of the dielectric layer but also due to the change in the relative permittivity. Hence the proposed textile capacitive sensors can capture critical information when deployed in different locations on the body demonstrated via a shoe insert, speech detection, breathing and heart rate monitoring.

Comparative investigation of interdigitated and parallel-plate capacitive gas sensors based on Cu-BTC nanoparticles for selective detection of polar and apolar VOCs indoors

The interfacial charge on spin-coated films of poly(tetrafluoroethylene-co-2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole) (Teflon AF) was studied by streaming potential and streaming current measurements in diluted aqueous solutions of potassium chloride, potassium hydroxide, and hydrochloric acid. ζ potential and surface conductivity were derived from electrokinetic data determined at varied concentrations of the electrolytes by means of the novel microslit electrokinetic setup (ref 1: J. Colloid Interface Sci. 1998, 208, 329). The results obtained revealed the high relevance of unsymmetrical (preferential) adsorption of ions as the origin of charge formation at unpolar polymer materials in aqueous environments. The preferential adsorption of hydroxide ions (OH-) was found to predominate as compared to the adsorption of hydronium ions (H3O+) at similar concentrations, i.e., in solutions of neutral pH. No effect of preferential adsorption was induced by chloride (Cl-) and potassium (K+) ions. For the...

Capacitive flexible sensors often encounter instability caused by temperature fluctuations, electromagnetic interference, stray capacitance effects, and signal noise induced by ubiquitous vibrations. The challenge lies in achieving comprehensive anti-jamming abilities while preserving a simplistic structure and manufacturing process. To tackle this dilemma, a straightforward and effective design is utilized to achieve comprehensive and robust anti-jamming properties in capacitive sensors. Electrospinning thermoplastic polyurethane (TPU) fiber mats soak with ionic liquid (IL) to create a co-continuous structure (TPU@IL) with high ionic conductivity and dielectric constant, which acts as the sensing units. The sensing mechanism of the TPU@IL with multiple electrode pairs encapsulated by polyethylene terephthalate (PET) is systematically elucidated. The optimal dual-electrode pair design for capacitive and resistive sensors, which have different sensitivities to temperature and stress, simultaneous realizes temperature-stress dual-mode sensing. Remarkably, the sensitivity curve of the TPU@IL/PET capacitive sensor exhibits an intriguing rarely reported S-shape with an adjustable step stress point. No liquid leakage even during extensive stress-strain cycling (>4000 cycles). Despite a slight compromise in sensitivity and response time, the TPU@IL/PET sensor demonstrates exceptional electromechanical stability, reliability, and powerful anti-jamming abilities against various interferences. A simple yet innovative sensor design enhances the performance and applicability of capacitive sensors in challenging environments.

This study focuses on the design of a semi-cylindrical capacitive sensor for intravenous (IV) drip detection. A main objective is to explore the capacitance of the semi-cylindrical capacitive sensor in response to drops of the IV fluid inside the drip chamber. The different sizes of semi-cylindrical capacitive sensors made of copper plates were constructed to be the IV drip detectors. They were put on the drip chamber of IV administration sets for detecting drops of the IV fluid appearing inside the drip chamber. Three sizes of the IV set were involved in the investigation: 15, 20, and 60 drops/ml. The sensor capacitances were measured, and the capacitance waveforms were observed. The relationships among the sensor sizes, the drip sizes, and the IV fluid levels inside the drip chamber were explored. The results showed that changes of the sensor capacitances are consistent with the basic principle of capacitor. The larger sensor size can produce the greater changes in capacitance of the semi-cylindrical capacitive sensor in response to the drops of IV fluid. The detected changes in sensor capacitance are approximately in the range of 0.02, 0.03, and 0.04 pF for the drip sizes 60, 20, and 15 drops/ml, respectively. Several factors have influence on the morphology of the capacitance waveform including the sensor size, the drip size, and the IV fluid level inside the drip chamber. In order to obtain the capacitance waveform appropriate for furthering processing, the design of the semi-cylindrical capacitive sensor must make a trade-off between the sensor size and the IV fluid level inside the drip chamber. The semi-cylindrical capacitive sensor would be an alternative effective device for the IV drip detection. The effective processing method is also required for identifying drops of the IV fluid from the detected capacitive waveform.

This paper reports the development of a wireless pressure sensing coil system by embedding a capacitive micro sensor onto a wire. The coil system, consisting of an inductor (coil itself) and multiple capacitors (sensors), formed a passive LC telemetry configuration for wireless sensing. The capacitive sensor, utilizing the movements of a liquid in a sealed chamber for hydraulic amplification, enabled high capacitance changes and amplified signal outputs. The capacitive sensor also produced digitized signal outputs as the liquid movement crossed a finite number of the interdigitated electrodes. Such characteristics were modeled and confirmed by establishing an equivalent fluidic circuit. The capacitive sensor was fabricated by grafting and polishing a copper wire piece on a silicon wafer, then performing standard microfabrication on the polished wire surface and filling a liquid into a sensor chamber. The fabricated pressure sensing coil system demonstrated a pressure sensitivity of 11.8 kHz/mmHg under a clinical pressure range of 0~206.9 mmHg in an ex vivo test with a water flowing tube, meeting the requirements for wireless detection of vessel restenosis. The sensitivity corresponded to the capacitance changes of 0.052 fF/mmHg and liquid displacement of 0.39 μm/mmHg. The fabricated coil system held a dimension of 2.5 mm in diameter and 7.2 mm in length in the form of a wound wire (copper, 260 μm in diameter). The embedded capacitive sensor consisted of a hydraulic chamber of 400×100×1 2 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and a microchannel of 500×20×1 2 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . [2020-0364]

Soil volumetric water content () is a vital parameter to understand several ecohydrological and environmental processes. Its cost-effective measurement can potentially drive various technological tools to promote data-driven sustainable agriculture through supplemental irrigation solutions, the lack of which has contributed to severe agricultural distress, particularly for smallholder farmers. The cost of commercially available sensors varies over four orders of magnitude. A laboratory study characterizing and testing sensors from this wide range of cost categories, which is a prerequisite to explore their applicability for irrigation management, has not been conducted. Within this context, two low-cost capacitive sensors—SMEC300 and SM100—manufactured by Spectrum Technologies Inc. (Aurora, IL, USA), and two very low-cost resistive sensors—the Soil Hygrometer Detection Module Soil Moisture Sensor (YL100) by Electronicfans and the Generic Soil Moisture Sensor Module (YL69) by KitsGuru—were tested for performance in laboratory conditions. Each sensor was calibrated in different repacked soils, and tested to evaluate accuracy, precision and sensitivity to variations in temperature and salinity. The capacitive sensors were additionally tested for their performance in liquids of known dielectric constants, and a comparative analysis of the calibration equations developed in-house and provided by the manufacturer was carried out. The value for money of the sensors is reflected in their precision performance, i.e., the precision performance largely follows sensor costs. The other aspects of sensor performance do not necessarily follow sensor costs. The low-cost capacitive sensors were more accurate than manufacturer specifications, and could match the performance of the secondary standard sensor, after soil specific calibration. SMEC300 is accurate (, , and of 2.12%, 2.88% and 0.28 respectively), precise, and performed well considering its price as well as multi-purpose sensing capabilities. The less-expensive SM100 sensor had a better accuracy (, , and of 1.67%, 2.36% and 0.21 respectively) but poorer precision than the SMEC300. However, it was established as a robust, field ready, low-cost sensor due to its more consistent performance in soils (particularly the field soil) and superior performance in fluids. Both the capacitive sensors responded reasonably to variations in temperature and salinity conditions. Though the resistive sensors were less accurate and precise compared to the capacitive sensors, they performed well considering their cost category. The YL100 was more accurate (, , and of 3.51%, 5.21% and 0.37 respectively) than YL69 (, , and of 4.13%, 5.54%, and 0.41, respectively). However, YL69 outperformed YL100 in terms of precision, and response to temperature and salinity variations, to emerge as a more robust resistive sensor. These very low-cost sensors may be used in combination with more accurate sensors to better characterize the spatiotemporal variability of field scale soil moisture. The laboratory characterization conducted in this study is a prerequisite to estimate the effect of low- and very low-cost sensor measurements on the efficiency of soil moisture based irrigation scheduling systems.

Part 2 of this two-part paper presents the experimental assessment of a micromachined capacitive incremental position sensor for nanopositioning of microactuator systems with a displacement range of 100 µm or more. Incremental sensing in combination with quadrature detection reduces the requirements for dynamic range for the sensor. Two related concepts for the position sensor are presented. In the incremental capacitance measurement mode (ICCM), the periodic change in capacitance between two periodic geometries S1 and S2 is measured to determine the relative displacement between S1 and S2 with a gap distance of ∼1 µm. In the constant capacitance measurement mode (CCMM), the distance between S1 and S2 is controlled to keep the mutual capacitance constant. Integration of the concepts with conventional comb-drive microactuators in a two-mask surface-micromachining process has been demonstrated. The changes in capacitance are measured using a synchronous detection technique with custom-made electronics. For quasi-static displacements over a range of 32 µm, an estimate for the displacement reproducibility is ∼25 nm for ICMM and ∼10 nm for CCMM, which includes hysteresis, drift and noise and errors in the actuation voltages. CCMM also shows a better performance in terms of nonlinearity and this confirms the conclusions based on the analysis and simulation results presented in part 1. The measurement method and implementation are demonstrated in quasi-static and dynamic experiments and can serve as an important tool to characterize the performance of the capacitive sensor, microsystem and setup. The feasibility of nanometer precision over a long displacement range is demonstrated and this proves the high potential of the two capacitive incremental position sensing concepts.

Stretchable and transparent touch sensors are essential input devices for future stretchable transparent electronics. Capacitive touch sensors with a simple structure of only two electrodes and one dielectric are an established technology in current rigid electronics. However, the development of stretchable and transparent capacitive touch sensors has been limited due to changes in capacitance resulting from dimensional changes in elastomeric dielectrics and difficulty in obtaining stretchable transparent electrodes that are stable under large strains. Herein, a stretch-unresponsive stretchable and transparent capacitive touch sensor array was demonstrated by employing stretchable and transparent electrodes with a simple selective-patterning process and by carefully selecting dielectric and substrate materials with low strain responsivity. A selective-patterning process was used to embed a stretchable and transparent silver nanowires/reduced graphene oxide (AgNWs/rGO) electrode line into a polyurethane (PU) dielectric layer on a polydimethylsiloxane (PDMS) substrate using oxygen plasma treatment. This method provides the ability to directly fabricate thin film electrode lines on elastomeric substrates and can be used in conventional processes employed in stretchable electronics. We used a dielectric (PU) with a Poisson's ratio smaller than that of the substrate (PDMS), which prevented changes in the capacitance resulting from stretching of the sensor. The stretch-unresponsive touch sensing capability of our transparent and stretchable capacitive touch sensor has great potential in wearable electronics and human-machine interfaces.

Thin PZT film is being developed for use in microelectronics, electromechanical and optoelectronic applications. Thin Pb(ZrTi)O/sub 3/ film capacitor devices were fabricated using RF sputtering techniques. The multiple-layer configuration of Si/SiO/sub 2//Ti/Pd or Pt was used as the substrate and bottom electrode. The top electrode was either Pd or Pt. At room temperature, the typical dissipation factor of this PZT film capacitor was 0.083 at 1 kHz. These PZT film capacitors had a parallel resistance of 1.15 mega-ohm. The film capacitor has an energy storage density of 0.023 /spl mu/F/cm/sup 2/. With the thickness of the film being 12,000 /spl Aring/, the dielectric constant was calculated to be 32. The insulation resistance was 138 giga-ohm. The resistivity was calculated to be 8.1/spl times/10/sup 10/ ohm-cm. These film capacitors were tested at 50 volts and the corresponding leakage current was at 1/spl times/x10/sup -5/ amp/cm/sup 2/. The breakdown strength at this stage was 4.2/spl times/10/sup 5/ V/cm. Annealing at 400/spl deg/C increased the value of the dielectric constant about 38%. The dissipation factor was decreased to 0.023 at 1 kHz. The parallel resistance increased from 1.15 to 3.01 mega-ohm. The insulation resistance after annealing was increased to 2180 giga-ohm. The resistivity was increased to 1.28/spl times/10/sup 13/ ohm-cm. The energy storage density of this film capacitor was increased from 0.023 to 0.031 /spl mu/F/cm/sup 2/. These film capacitors produced to-date had little dependence on frequency from 400 Hz to 100 kHz. The PZT sample with a thickness of 4.2 /spl mu/m exhibited a dielectric constant of 107 before annealing and 354 after annealed at 600/spl deg/C. The dissipation factor was reduced from 0.12 to 0.015 before and after annealing at 600/spl deg/C.

Capacitance sensor for measuring void fraction in small channels

For decades, the revolution in design and fabrication methodology of flexible capacitive pressure sensors using various inorganic/organic materials has significantly enhanced the field of flexible and wearable electronics with a wide range of applications in aerospace, automobiles, marine environment, robotics, healthcare, and consumer/portable electronics. Mathematical modelling, finite element simulations, and unique fabrication strategies are utilized to fabricate diverse shapes of diaphragms, shells, and cantilevers which function in normal, touch, or double touch modes, operation principles inspired from microelectromechanical systems (MEMS) based capacitive pressure sensing techniques. The capacitive pressure sensing technique detects changes in capacitance due to the deformation/deflection of a pressure sensitive mechanical element that alters the separation gap of the capacitor. Due to advancement in state‐of‐the‐art fabrication technologies, the performance and properties of capacitive pressure sensors are enhanced. In this review paper, recent progress in flexible capacitive pressure sensing techniques in terms of design, materials, and fabrication strategies is reported. The mechanics and fabrication steps of paper‐based low‐cost MEMS/flexible devices are also broadly reported. Lastly, the applications of flexible capacitive pressure sensors, challenges, and future perspectives are discussed.