Takaaki Kuroiwa

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

Abstract 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.

A thin-film polysulfone-based capacitive-type relative-humidity sensor

Abstract A capacitive type relative humidity sensor utilizing newly developed cross-linked fluorinated polyimide has been fabricated using thin film technology and evaluated for commercial building and factory applications. The sensor showed a good initial performance with less than 1% r.h. hysteresis and a 0 ± 0.1% r.h./°C temperature coefficient of capacitance along with good stability especially under high temperature and high humidity conditions. Also, because of the inherent chemical resistance of the polyimide, good sensor stability when subjected to volatile environments has been demonstrated.

A Capacitive‐Type Humidity Sensor Using Cross‐Linked Poly(methyl methacrylate) Thin Films

A capacitive‐type humidity sensor was prepared using cross‐linked poly(methyl methacrylate) (PMMA). The electrical capacitance changed linearly with the relative humidity over the range of 10–90% RH and the sensitivity (the ratio of capacitance at 90% RH to that at 10% RH) was about 1.13. The amount of sorbed water on PMMA at 90% RH was 25 mg/g which was one‐third of that on cellulose acetate butyrate. This low hygroscopicity of PMMA was effective in reducing any hysteresis. The decrement of temperature coefficient and high durability for acetone vapor were achieved using the cross‐linked PMMA film with divinyl benzene or ethylene glycol dimethacrylate as the cross‐linking agent.

Capacitive‐Type Humidity Sensors Using Polymerized Vinyl Carboxylate

Capacitive-type humidity sensors were prepared using four kinds of polymerized vinyl carboxylate, i.e., vinyl benzoate, vinyl methacrylate, vinyl crotonate, and vinyl cinnamate. These monomers have different chemical structures and vinyl methacrylate and vinyl crotonate formed a cross-linked film. The sensitivity was enhanced by proceeding the crosslinking reaction. This is due to the increase in the water sorption ability. The temperature coefficient of the sensitivity decreased by using the cross-linked polymer. Although the electrical capacitance of all sensors was affected by acetone vapor, the capacitance of the cross-linked sensor recovered to the initial value after removal of the acetone vapor

Effect of the degree of cross-linking on the characteristics of a PVCA capacitive-type humidity sensor

Abstract A cross-linked PVCA film was prepared on a suitable substrate by using a photoirradiation technique and evaluated as a capacitive-type humidity sensor. The sensing characteristics were considerably affected by the degree of cross-linking in the film. The sensitivity was particularly enhanced up to ca. 95% cross-linking, beyond this the sensitivity decreased. This has been explained by the variation in the ability to adsorb water with the degree of cross-linking. Similar behavior was observed for the adsorption of organic vapors and the adsorption of the larger organic vapors was greatly reduced by using the highly cross-linked polymer. This behaviorr was explained by the molecular sieve effect of the cross-linked film. It was confirmed that the effect of coexistent acetone vapor was considerably smaller for the sensor using highly cross-linked PVCA film.

Effect of Sorbed Water on the Dielectric Properties of Acetylene‐Terminated Polyimide Resins and Their Application to a Humidity Sensor

A cured acetylene-terminated polyisoimide oligomer was used to prepare a capacitive-type humidity sensor. Sorption behavior of water on the cured film was examined by measuring the dielectric constant and water content. The amount of sorbed water was small and the sorbed water did not form clusters. This is due to the hydrophobic nature and rigid structure of the film. The sorption behavior reflects its excellent sensor properties, i.e., low hysteresis, low temperature coefficient, durability against acetone vapor, and long-term stability in a high temperature and humid atmosphere

Drift Phenomenon of Capacitive‐Type Relative Humidity Sensors in a Hot and Humid Atmosphere

The long-term aging effects on capacitive-type relative humidity sensors in a hot and humid atmosphere were investigated to better understand the mechanism of the drift phenomenon. For this purpose, four kinds of polymers, cellulose acetate (CA), cellulose acetate butyrate (CAB), poly(methyl methacrylate) (PMMA), and cross-linked polyimide (PI) were used as sensing materials. For all sensors, the electrical capacitance increased with aging in a 40°C and 90% relative humidity atmosphere. The QCM measurement showed that the drift of the sensor capacitance was caused by the increase in the water sorption ability of the sensing polymers. From XPS data, the increase in the water sorption ability could be attributed to the introduction of the oxygen-containing group at the surface of the sensing polymers. Morphological changes were also suggested by the film thickness measurement. Furthermore, it was confirmed that the magnitude of the drift was affected not only by the increment but also by the state of the sorbed water. Fourier transform infrared data indicated that the sorbed water in the hydrophobic polymers such as PMMA and PI are close to the isolated state and that the state is hardly affected by aging. Because the change in the isolated water does not significantly affect the electrical capacitance, PMMA and PI sensors show little drift even after 250 days aging.

Effect of Sensing Film Thickness on Drift Phenomenon of Capacitive-Type Humidity Sensors

The effect of a sensing films thickness on the drift phenomenon of capacitive-type humidity sensors in a hot and humid atmosphere was investigated. Poly(methyl methacrylate) (PMMA) was chosen as the model sensing material for this purpose. The electrical capacitance measured at 0% relative humidity (RH, C 0 ) decreased, while that measured in a wet atmosphere (C X ) increased by aging the sensors in a 40°C and 90% RH atmosphere. This aging test showed an increase in the resulting sensitivity (C X /C 0 ). The drift of sensitivity can be closely related to the increase of water sorption ability of the sensing polymer. The magnitude of the drift of the water sorption ability was larger for the thicker film. The large increase of film thickness induced by aging was also observed for the thicker film. These results suggest that an irreversible volumetric change in PMMA film caused by the slight swelling that occurs in a hot and humid atmosphere is one of the important contributive factors associated with the drift phenomenon of the water sorption ability of PMMA film.