Polyimide-based Humidity Sensor Research Articles

Design and synthesis of vertical gradient of expansion structure based on polyimide humidity sensing layer with ultra-fast response, high sensitivity, and large deformation driven by humidity

The design of humidity actuators with fast reaction sensitivity (particularly actuation time) while retaining desirable mechanical qualities is critical for advanced smart manufacturing such as soft robots and smart devices, yet it remains a challenge. Here, we fabricated film-based humidity-responsive materials from hydrogenated pyromellitic dianhydrides (HPMDAs) via a simple two-step chemical imidization. The bending of the film is highly sensitive to humidity, and even a small amount of humidity based on a wet cotton swab can be observed from a distance of a few millimeters to bend the film. HPMDAs-based PI (PIs-HPMDAs) not only possess facile fabrication features and excellent properties, such as thermal stability, mechanical properties, and optical properties, but also deliver excellent deformation, including a fast response/recovery (0.02/0.94 s), large deformation (220°), and sufficient bending stability (no detectable degradation after 1200 times). Taking advantage of these benefits, the relative humidity can be converted into a deformation signal of moving object, and it can be further turned into electric signals. Compared with conventional humidity sensors, such the polyimide-based humidity sensor is expected to be used in ambient electrical sensors and relays due to their simple fabrication, rapid response, and high stability. Our strategy of generating and exploiting internal stress in responsive polymers provides a promising platform for humidity actuators.

A nanoforest-based humidity sensor for respiration monitoring

Traditional humidity sensors for respiration monitoring applications have faced technical challenges, including low sensitivity, long recovery times, high parasitic capacitance and uncalibrated temperature drift. To overcome these problems, we present a triple-layer humidity sensor that comprises a nanoforest-based sensing capacitor, a thermistor, a microheater and a reference capacitor. When compared with traditional polyimide-based humidity sensors, this novel device has a sensitivity that is improved significantly by 8 times within a relative humidity range of 40–90%. Additionally, the integration of the microheater into the sensor can help to reduce its recovery time to 5 s. The use of the reference capacitor helps to eliminate parasitic capacitance, and the thermistor helps the sensor obtain a higher accuracy. These unique design aspects cause the sensor to have an excellent humidity sensing performance in respiration monitoring applications. Furthermore, through the adoption of machine learning algorithms, the sensor can distinguish different respiration states with an accuracy of 94%. Therefore, this humidity sensor design is expected to be used widely in both consumer electronics and intelligent medical instrument applications.

Performance Enhanced Humidity Sensor by In-Situ Integration of Nanoforests

In this letter, we report a high performance humidity sensor composed of a nanoforest-based sensing capacitor, a thermistor, a micro-heater, and a reference capacitor. Compared with traditional polyimide-based humidity sensors, sensitivity of the nanoforest-based one is significantly improved by 15 times in a relative humidity range of 10-90% RH. Meanwhile, integration of a micro-heater into the humidity sensor can reduce the humidity response time by 8 s. The use of the reference capacitor and the micro-heater can help to eliminate parasitic capacitance and to obtain higher accuracy. In addition, the fabrication process of this sensor is simple and compatible with CMOS processes, thus such a humidity sensor is suitable for batch production and has potential to replace existing sensors with low performance.

Highly-Sensitive Humidity Sensors for Condition Monitoring of Hybrid Laminates

In recent years fibre-reinforced polymers (FRPs) gained importance in a wider field of application due to such favourable properties as low mass and tailorable mechanical strength. However, water penetrating into the lightweight material can lead to a loss of shear strength and finally to a collapse of the whole mechanical structure. Consequently, the integration of humidity sensors into compound materials is able to promote the reliability via online condition monitoring. An innovative concept is the use of ceramics-polymer-composites, which are well suited for the integration into lightweight structures during inline production. Composite and polyimide based humidity sensors have been manufactured by flexographic printing and spin-coating processes. A 5-fold increase in sensor’s capacity related to a humidity change from 10 to 80 % r.h. manifests the outstanding sensitivity of manufactured composite sensors. In addition, FRP-integrated polyimide sensors showed a significant response to water penetration, whereby the capability of condition monitoring could be confirmed.

A Novel Structure of Polyimide-Based Humidity Sensor Used in Low Temperature Environment

In this paper, a new structure of polyimide-based humidity sensor is developed, which can work better at low temperature environment. In this humidity sensor, the capacitive part is sputtered on a suspended membrane, the lower electrode is also a controllable heater, used to control the temperature on the surface of the sensor through the external circuits. The results of experiment suggest that the sensor has a wide range of sensing ambient relative humidity from 10%RH to 90%RH with a controllable constant temperature and at a lower temperature it shows a good linearity.

Preparation of epoxy/polyelectrolyte IPNs for flexible polyimide-based humidity sensors and their properties

Resistive humidity sensors based on an epoxy interpenetrating polymer network (IPN) polyelectrolyte films, which had strong adhesion to polyimide substrate, were developed for flexible polyimide electrode. Humidity sensitive epoxy IPN films were prepared using two stage cross-linking between the diglycidyl ether of bisphenol-A (DEGEBA) and a phosphate ester curing agent (EPEs), followed by photocuring of copolymer of 2-(cinnamoyloxy)ethyl methacryate (CEMA) and [2-(methacryloyloxy)ethyl] trimethyl ammonium chloride (METAC). Under optimal conditions ([METAC/CEMA]/[DGEBA/EPEs]=2/1), a semi-log graph of film resistance was plotted linearly by four orders of magnitude (from 103 to 107Ω) on increasing humidity over a humidity range of 20–95%RH and exhibited good linearity (Y=−0.0529X+7.8048, R2=−0.9944). Furthermore, reliability testing (in terms of water resistance and high temperature, humidity, and long-term stabilities) of the epoxy IPN/gold electrode/polyimide substrate showed that they are satisfactory for use as flexible humidity sensors.

Nano-grass polyimide-based humidity sensors

This paper deals with a micro-capacitive-type relative humidity sensor with nano-grass polyimide as a dielectric sensing material. Our humidity sensor achieves key performance indices such as quick response, high sensitivity, and stability enabled by the modification of polyimide into nano-grass morphology where the dimension of a typical individual pillar is 387 nm × 40 nm. A low hysteresis operation is also achieved by integrating a micro-heater in the sensing area. The nano-grass morphology is created with an oxygen plasma treatment on polyimide surface which increases surface to volume ratio by more than 280 times larger than that of a simple flat-film. This amplification of the surface to volume ratio leads to the rapid adsorption and desorption of water into the sensing material. Furthermore the oxygen plasma treatment introduces a carbonyl group that facilitates an enhanced affinity of the polyimide surface to water molecules. XPS analysis is used to confirm the emergence of carbonyl groups as a result of the treatment. The total response time of a nano-grass sensor is 11 s which is improved by about 2.5 times than a normal flat-film sensor. The sensitivity of the nano-grass sensor is 0.08 pF/%RH (relative humidity) which is improved by 8 times compared with the flat-film one. In stability test for 200 h, the signal of the nano-grass sensor is fluctuated ±1.0%RH. Theoretical models employing the Looyenga and Dubinin equations are used to explain the behavior of the nano-grass sensor.

Micromachining of Multilayer Thin Films for High-Speed Humidity Sensor Fabrication

This paper reports the microfabrication and characteristics of a novel high-speed polyimide-based humidity sensor using an etch process based on a combination of isotropic and anisotropic etching steps with inductively coupled plasma (ICP) and a localized curing of polyimide films on microhotplate. The polyimide capacitive humidity sensors showed a sensitivity of 0.77 pF/percent relative humidity (%RH), a linearity of 0.9995, and a hysteresis of 0.62%RH, a response time of 3 s. In comparison, the sensors with a conventional structure exhibited a time response of 7 s. High speed is achieved because a device has many holes and moisture is thus absorbed into the top and side surfaces of polyimide.

Fabrication of high-speed polyimide-based humidity sensor using anisotropic and isotropic etching with ICP

A novel high-speed, high-sensitivity capacitive-type humidity sensor is fabricated by using a new microfabrication process involving combination of directional and isotropic etching with inductively coupled plasma (ICP) etcher and a localized curing of polyimide films on a micro-hotplate. The polyimide humidity sensor is found to have a sensitivity of 0.75 pF/%RH, a linearity of 0.995, a hysteresis of 1.32%RH, a time response of 3 s, and a temperature coefficient of 0.22%RH/°C. This high-sensitivity and high-speed device is achieved using the locally-cured polyimide and a sensor structure having many holes and allowing moisture to diffuse through the top and side surfaces of polyimide film.

O2 plasma treated polyimide-based humidity sensors.

The effect of non-plasma and plasma treated polyimide-based humidity sensors is presented. Pure oxygen was used to etch polyimide in a plasma etcher. The sensor treated in a plasma exhibited higher sensitivity and faster response speed against moisture. The plasma treated sensor had 3.4 times the sensitivity and responded almost twice as fast as the non-plasma treated sensor. A further comparison of sensor outputs, sensitivity and response speed are presented. Chemical analysis of the polyimide surface was carried out by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (ATR-FTIR). XPS and ATR-FTIR showed the increase in carbonyl carbon bonds, C=O, after the plasma treatment. Geometrical modification was observed by atomic force microscopy (AFM). It showed considerable surface roughness after the plasma treatment. O2 plasma treatment improved the sensitivity, and reduced the hysteresis of the sensor due to the increase in C=O bonds in the polyimide.

A high-sensitivity polyimide capacitive relative humidity sensor for monitoring anodically bonded hermetic micropackages

This paper presents the design, fabrication and complete characterization of a high-sensitivity polyimide-based humidity sensor for monitoring internal humidity level in anodically bonded hermetic micropackages. This capacitive sensor is 1 mm on a side and utilizes CU1512 polyimide film with a thickness in the range from 300 /spl Aring/ to 1200 /spl Aring/ sandwiched between two metal electrodes to sense moisture. The measured sensitivity for a sensor with a 1200-/spl Aring/-thick film is 0.86 pF/%RH, and for a 300-/spl Aring/-thick sensor is 3.4 pF/%RH. The sensor has been exposed to and survived a one-hour test at 400/spl deg/C, which is the temperature typically used to perform anodic bonding. Measurements show a drift of less than 1% RH at 50% RH and 37/spl deg/C for 48 h, and a hysteresis of <2% RH over a range from 30 to 70% RH for a 1200-/spl Aring/-thick polyimide film sensor. The measured breakdown voltage of the sensor (1200 /spl Aring/ thick) exceeds 20 V and agrees well with other results.

Stability and reliability of capacitive-type relative humidity sensors using crosslinked polyimide films

Newly developed crosslinked polyimide and fluorinated polyimide-based humidity sensors were fabricated for office building and factory applications. Both sensors showed good initial sensor performance. The stability and the reliability of these sensors were evaluated by exposing the sensor elements to various test conditions. The crosslinked polyimide sensor was resistant to chemicals and the crosslinked fluorinated polyimide one exhibited long-term stability in harsh environments. By using these sensors properly, their sensor performance is superior to that of most commercially available capacitive-type sensors.