Y. Sakai

Humidity sensors based on polymer thin films

Studies on humidity sensors fabricated with organic polymers for the last 10 years are reviewed. Several useful methods for improving the characteristics of humidity sensors based on polymers are proposed. In the case of a resistive-type sensor, cross-linking of hydrophilic polymers or formation of interpenetrated polymer networks with a hydrophobic polymer makes the hydrophilic polymers durable at high humidities. Graft polymerization is another method of preparing water-resistive humidity sensors. In the case of capacitive-type sensors, cross-linking is also useful to modify the hydrophobic polymer to produce a sensor with small hysteresis, high selectivity and high sensitivity.

Humidity sensor using cross-linked poly(chloromethyl styrene)

Abstract A resistive-type humidity sensor was prepared using a poly(chloromethyl styrene) (PCMS) film which was simultaneously cross-linked and quaternized. The impedance dependence on humidity for samples with various reaction degrees was measured. The sorption isotherm curves of water vapor in the cross-linked films were obtained using the quartz crystal microbalance method for the various samples. It was found that the degree of hysteresis depends on the density of the quaternary ammonium group, which affects the diffusion coefficient of the water molecules in the film.

Effect of humidity on NH3 gas sensitivity of polyaniline blend films

Abstract The effect of humidity on the NH 3 detection capability of polyaniline (PANI)-insulating matrix polymer blend films was investigated. Poly(methyl methacrylate) (PMMA) and polystyrene (PSt) were used as the matrix polymers. The presence of water vapor, functioning as the interference gas, led to the lowering of the NH 3 response of the sensor in a humid atmosphere. Although PMMA had more hydrophilic character than PSt (PMMA sorbed more than eight times as much water as PSt), the influence of humidity on the sensor response was not as large as expected. This unexpected finding was due to the characteristic morphology of the “PANI–PMMA blend film”, that is, the blend film consisted of porous structures which resulted in a composition of the coexistence of the PANI–PMMA blend with the PANI and/or PMMA single species. In such cases, water molecules were able to interact with doped PANI directly, and the water sorbing characteristic of PMMA exerted less influence on the conductivity.

Humidity-sensitive and water-resistive polymeric materials

Abstract Four types of polymers with pyridinium groups were prepared and their electrical properties as humidity sensors were studied. The polymers are quaternized vinylpyridine and styrene copolymers, partially quaternized polyvinylpyridine, polytetrafluoroethylene grafted with quaternized polyvinylpyridine and polyvinylpyridine crosslinked with dibromobutane. The latter two polymers showed excellent water resistivity as well as a sensitivity to humidity.

Characterization of polymers for a capacitive-type humidity sensor based on water sorption behavior

Abstract The water sorption behavior of polymers for a capacitive-type humidity sensor was examined using three different techniques. The sorbed water in hydrophilic polymers with a pyridine or hydroxyl group was highly bound and that in less hydrophilic polymers with a carbonyl or ether group was loosely bound to the polymer as determined from the sorption analysis. Among less hydrophilic polymers, the tendency to form the clustered sorbed water molecules was higher for PEG and EC, which have an ether oxygen as the hydrogen bonding basicity. The results of this study proved that cluster formation of sorbed water of a sensing polymer was closely related to the sensors characteristics. The polymers with a weak hydrophilic site, which have a low hydrogen bond propensity, are suitable for the capacitive-type humidity sensor.

Humidity sensors based on organopolysiloxanes having hydrophilic groups

Abstract Two types of humidity sensors were prepared from organosilicon compounds having hydrophilic groups such as NH2, N + (CH3)3Cl− and SO3H. The first is a pressed silica gel disk grafted with hydrophilic organosilicon compounds. The second type is a sintered alumina plate on which a cross-linked organopolysiloxane film having a hydrophilic group was formed. The impedance was measured in the humidity range 0 – 100%. The impedance varies with the hydrophilic group in the order SO3H N + (CH3)3Cl− N + (CH3)3Cl− > SO3H > NH2 > OH. The grafted silica gel device is quite resistant to water. The humidity-sensing characteristics of the alumina-plate type sensor do not change even after soaking in water for several hours.

Solid-state electrochemical CO2 gas sensors based on sodium ionic conductors

The sensing characteristics of a solid-state electrochemical CO2 gas sensor, expressed as Pt∥O2, Na2O ∥ Na ionic conductor ∥ Na2CO3∥CO2, O2∥Pt were investigated in terms of a two-electron electrochemical reaction. The number of electrons for the cell reaction was higher than 2 and approached 2 with an increase in the operating temperature up to about 500 °C. The introduction of water vapour induced a lowering of the e.m.f. and a prolongation of the response time. The formation of sodium oxides in the Na2CO3 layer was considered as a possible cause of these water effects. The sensing characteristics recovered completely after the water vapour was cutoff. The e.m.f. reduction due to water sorption was depressed by using a densified Na3Zr2Si2PO12. A densified Na3Zr2Si2PO12-based electrolyte is preferable for use as a gas sensor with a fast response and high stability for detection of CO2 in air.

A humidity sensor using graft copolymer with polyelectrolyte branches

Abstract 2-Hydroxy-3-methacryloxypropyl trimethylammonium chloride was grafted on to the pore wall of microporous polyethylene film by a γ-ray pre-irradiation method. A humidity sensor was prepared by depositing gold electrodes on both surfaces of the grafted film. Complex impedance measurements were carried out at various humidities. From the complex impedance analysis, the values of parallel resistance and parallel capacitance were estimated. The parallel resistance decreased as the humidity increased, while the parallel capacitance remained constant. The parallel resistance also decreased as the grafting ratio increased. The value of parallel resistance depends on the amount of sorbed water in the film. The sensor is stable for a long time in ordinary atmospheric conditions. It is even stable in water for more than 2 h. The sensor prepared by γ-ray pre-irradiation at 9.0 × 10 4 R h −1 for 4 h was heat-resistant up to 130°C.

Humidity sensors using KH2PO4-doped porous (Pb, La)(Zr, Ti)O3

Humidity sensing devices were prepared by using fine porous (Pb, La)(Zr, Ti)O3 (PLZT) particles with ferroelectricity instead of insulating metal oxides such as alumina and zircon. The impedance of PLZT with 1 wt %. KH2PO4 was 106 Ω and lower than that of zircon with 3.8 wt % KH2PO4 by a factor of 102 in a dry atmosphere. In addition, the impedance in a humid atmosphere was controlled by the adding of potassium dihydrogen phosphate and changed by about four orders of magnitude in the humidity region 0 to 90% relative humidity at 1 kHz for samples burnt at 700° C. The humidity dependence of impedance is mainly governed by the change of coverage of adsorbed water. The hydrophilicity is affected by the burning temperature and lanthanum content of PLZT used as starting particle for porous PLZT ceramics burnt with KH2PO4. From complex impedance analysis it is confirmed that the resistive component inserted in parallel with the capacitive component decreases steeply with an increase in humidity, while the capacitive component is poorly dependent on the humidity.

Humidity sensor using microporous film of polyethylene-graft-poly-(2-hydroxy-3-methacryloxypropyl trimethyl-ammonium chloride)

An hydrophilic monomer, 2-hydroxy-3-methacryloxypropyl trimethylammonium chloride, was graft-polymerized in a microporous polyethylene film using three different initiation methods, that is, with benzyl peroxide, pre-U.V. irradiation and mutual photo-initiation. Humidity dependence of the impedance was measured for the grafted microporous polyethylene films. Among the three graft polymers, the one prepared by pre photo-irradiation seems to have the most homogeneously grafted layer. The impedance increases as the grafting percentage increases except in the low grafting percentage region. The response time for the abrupt change of humidity becomes longer as the grafting percentage increases. The activation energy for conductivity has a maximum at 40% relative humidity in the plot of activation energy against humidity.