(1)Fabrication and Characteristics of Thin Film Humidity Sensor
1982
<ABSTRACT>
A capacitive humidity sensor utilizing an aluminum-oxide thin film formed by reactive ion plating is fabricated and the characteristic of the sensor is measured by newly designed humidity test chamber.
It is found that the sensor fabricated in this experiment works in quite stable with high reproducibly, especially in the high humidity region, and it is also revealed that the structure of the aluminum-oxide film suitable for the humidity sensor is "amorphous like" one. The capacitance of the sensor varies proportionally to the change of relative humidity between 10 and 80%, and a similar behavior is observed on the mass measurement of absorbed water molecules.
It is considered that the absorbed water molecules will form a condensed phase(liquid water) in micro-capillaries in an aluminum-oxide film. A simple model for water absorption in the film is given and mechanism of condensation to liquid water will be discussed briefly relating to the capacitance change of the film.
(2)
Alumina Thin Film Humidity Sensor
Controlling of Humidity Characteristics and Aging
1983
<ABSTRACT>
A capacitive humidity sensor using a γ-Al２O３ thin film formed by reactive ion plating has been fabricated and humidity characteristic of the sensor can be controlled by varying the condition of depositions such as deposition rates,ambient gas pressures and the ratio of O2 and Ar gas pressures.
As-deposited films of γ-Al2O3 are treated in boiling water,and it is found that γ-Al2O3 changes to γ-Al2O3·H2O(boehmite).
The sensor treated in boiling water has been working in a quite sable state even in a high humidity region.
(3)
Characteristics and Reliability of Al2O3 Thin Film Humidity Sensors
1983
<ABSTRACT>
"Thin film humidity sensor" using alumina films formed by reactive ion plating is fabricated on a glass or a sapphire substrate and the change of relative humidity is detected as a capacitance change. The sensing characteristics (such as linearity, sensitivity, response) of the sensor can be controlled reproducibly by varying the growth conditions of the film.
The irreversible change of humidity characteristic in high humidity region due to the interaction of water molecules is reduced to small by passivating the film surface.
A simple model for water absorption in the film is given and mechanism of the capacitance change will be discussed briefly relating to the condensation of water molecules (liquid water) in the film.
(4)
Mechanism of Moisture Sensing by Thin Film Humidity Sensor
1984
<ABSTRACT>
The simultaneous measurement of capacitance and resistance of sensor, and also mass of absorbed water in a sensor has been done using a newly designed humidity sensor. As the sensing part of the sensor , CaF2,Al2O3, and cellulose polymer films are used.
In polymer films, water molecules are absorbed in the form of physically adsorbed water and the humidity characteristics do not depend on temperature.
In CaF2 and Al2O3 films, it is found that the change of capacitance and resistance relates to the electrolysis process.
The sensing mechanism of relative humidity is discussed in connection with tha result of BET plot and infrared spectra.
(5)
Humidity Sensing Mechanism of Hydrogenated Carbon Films Containing Oxygen
1986
<ABSTRACT>
Hydrogenated carbon films containing oxygen are formed by RF plasma decomposition of amyl alcohol. It is found that the film obtained are applicable to humidity sensing. Then the change of the capacitance(C) and the mass of the absorbed water(mH2O) in the film by varying relative humidity(RH) has measured simultaneously.
From the result of measurement, water molecules are absorbed in the film by physisorption and the temperature dependence can not be found in mH2O-RH characteristics. The hysyteresis in mH2O versus RH and C versus RH characteristic is almost negligibly small(up to +/-0.5%RH). The sensor is in a stable state even kept in ethanol or acetone and/or water vapor(130°C 100%RH).
(6)
Moisture Sensing Materials and Mechanism in Capacitive Humidity Sensors(Japanese)
1992
<ABSTRACT>
Moisture sensing mechanism in capacitive humidity sensors has been investigated in details. The infrared spectrometric analysis revealed that the water molecules were occluded in moisturesensitive films of CaF2 and Al2O3 in liquid phase. They also were occluded in polymethyl methacrylate(PMMA) and cellulose triester(CTE) films in the form of rotationally restrained physisorption water which has little interaction among them.
Relatively large hysteresis and/or complicated temperature dependence were observed in capacitance(C) vs. relative humidity(RH) characteristics for hydrogenated carbon containing oxygen(α-CHO), polyvinylalcohol(PVA), PMMA,CaF2 and Al2O3. On the other hand, temperature dependence and hysteresis in C vs. RH and MH2O(Mass of water which was taken in a unit volume of sensing part) vs. RH characteristics were scarcely observe for CTE.
The desirable properties for misture-sensing materials which can be used over a wide range of temperature and humidity are; (1) the heat of adsorption for the 1st layer of water molecules in equal to that of N-th(N=2,3,4,5,6,,,) layer, (2) no internal water molecules condense and (3) the temperature coefficient of dielectric constant is negligibly small.
(7)
Capacitive humidity Sensor Using polyphenol Resin(Japanese)
1993
<ABSTRACT>
Capacitive humidity sensors using polyphenol resin(PPR) were fabricated. Then, the humidity characteristics of PPR were compared with that of cellulose acetate butyrate(CAB).
It is reported by several researchers that CAB is suitable to a humidity sensitizer of the capacitive humidity sensor.
Relative humidity(RH) vs. capacitance(C) curves are almost linear for PPR72(phenylene content 72%), and are non-linear for CAB and PPR66(phenylene contant 66%). For all materials, temperature coefficient of C-RH curve is less than 1%/°C. Especially the temperature coefficient is negligibly small for PPR66. Hysteresis width against humidity cycle are 0.5%,1.5%,3% and 2% at maximum in RH equivalent for PPR72,PPR66,CAB38(butyl content of 38%) and CAB50(butyl content of 50%),respectively. The response duration time against humidity change is short enough at a relatively low humidity range for all materials. However, it takes 4min,4min,33min and 28min for -1% response in RH equivalent by increasing humidity from RH of 81.2% to RH of 90.7% for PPR72,PPR66,CAB38 and CAB50, respectively.The humidity sensors of PPR72 and PPR66 work stable in 3% accuracy after kept in 90%RH of 80°C for 100days. Furthermore, the sensors of PPR66 can be used even kept in 150°C for 100days.
(8)
A selection procedure of sensing material for capacitive humidity sensor
1995
<ABSTRACT>
We have investigated the moisture sensing mechanism in capacitive humidity sensors. We have revealed that the desirable properties of potential material for capacitive humidity sensors are; (1) the heat of adsorption for the first layer of water molecules is equal to that of Nth (N=2,3,4,5......) layer, (2) no occluded water molecules condense and (3) the temperature coefficient of the dielectric constant is negligibly small.
(9)
Fabrication and Characterization of Thin Film
Humidity Sensor Using Hydrogenated Carbon Containing Oxygen
1989
<ABSTRACT>
This paper presents the fabrication process and the characteristics of a capacitive humidity sensor using hydrogenated carbon containing oxygen. The sensing film of the sensor has been deposited by chemical vapor deposition. The response time of the sensors due to humidity change is within 40 seconds, and also the width of hysteresis is +/-1.5%RH at maximum. The sensor works reproducibly against humidity and temperature cycles(0%RH⇔90%RH,30°C⇔60°C). Durability of the sensor in other various environment is also investigated.
(10)
Mechanism of Moisture Sensing in Organic Polymer
1991
<ABSTRACT>
Absorbing process of water molecules into polyvinylalcohol or polymethylmethacrylate is examined. In a low humidity range, BET equation is applicable to the mass of absorbed water vapor.
It is found that the condensation of absorbed water molecules causes the occurrence of hysteresis in a humidity cycles. Moreover it is presumed that the adequate use of organic polymer containing carbonyl groups as the adsorption site of water molecules enables us to fabricate "a relative humidity sensor"
(11)
Capacitive Humidity Sensor Using Polyphenol Resin(English)
1992
<ABSTRACT>
Capacitive humidity sensors using polyphenol resin(PPR) or cellulose acetate butyrate(CAB) were fabricated. The humidity characteristics of PPR and CAB were investigated in details comparing with each other. Relative humidity(RH) vs capacitance(C) characteristics shows almost linear relationship in the humidity range from 0% to 95.4% for PPR, and shows non-linear for CAB. Hysteresis values against humidity cycles are 0.5% and 3% at maximum in RH equivalent for PPR and CAB respectively. The humidity sensors using PPR work reproducibly after they were kept in RH of 90% for 80°C.
(12)
A Humidity Sensor with Linear Relationship
In Relative Humidity-Electrical Capacitance Characteristics
1994
<ABSTRACT>
A humidity sensor having linear relationship in relative humidity(RH) versus electrical capacitance(C) characteristic is fabricated. As the humidity-sensing part of the sensor, the double layered film by polyphenylene ether sulfone(PES) and cellulose acetate butyrate(CAB) is used. Prior to the fabrication, RH−C characteristics are simulated by using RH-C characteristic for PES and CAB. The "linearity" of RH−C characteristic of the sensor fabricated is as small as +/-0.8%RH in the humidity range from 0%RH to 95%RH at the operating frequency of 400kHz. This characteristic shoes good agreement with the simulated one. The width of the hysteresis in RH−C characteristic in 1.5%RH at maximum. The temperature coefficient of RH−C characteristics is as small as about 0.04%RH/°C at maximum in the temperature range from 30°C to 60°C. The sensors can measure RH in +/-3%RH accuracy after kept in 60°C,90%RH or −24°C,about 100%RH for 200days.
(13)
A research on moisture sensing Mechanism in capacitive humidity sensor and its fabrication1993
<ABSTRACT>
First, moisture sensing mechanism in capacitive humidity sensor is
investigated in detail, and a structure of ideal moisture-sensing material is
proposed using that mechanism. Secondly, two kinds of polyphenol resin
are selected as the ideal materials. In the last section, capacitive humidity
sensors are fabricated using the selected resin. These sensing elements
have characteristics, better than that of the sensors made using cellulose-
acetate-butyrate, despite the fact that cellulose-acetate-butyrate has often
been selected by other researchers as the best material from which to
fabricate capacitive humidity sensors.
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