Moisture sensitive element

Abstract: A moisture-sensitive element comprising a moisture-sensitive film (3) which consists of a polymer whose water sorption amount is between 10 and 40 mg/g at a temperature of 30°C and a relative humidity of 90 %RH. The element has a low temperature dependancy, a small hysteresis and a quick response and accordingly can be employed in a wide range of temperature as well as in a severe condition such as high humidity, high temperature, cyclically fluctuating humidity, low humidity, moisture condensation, water dipping and so on.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a capacitive moisture sensor which employs an organic polymer as moisture sensitive material. Description of the Prior Art A conventional capacitive moisture sensor of the above-mentioned type uses an organic polymer, e.g., celluloseacetatebutyrate (hereinafter simply referred to as "CAB") as a moisture-sensitive material and utilizes an electrical capacitance change in a moisture-sensitive film formed of such moisture-sensitive material to detect humidity, as disclosed in Published Japanese Patent Application No. 62-88951. CAB having 17% of butyryl base typically used as moisture-sensitive material in an above-mentioned type of conventional moisture-sensitive element has a water sorption amount of approximately 70 - 90 mg/g in an equilibrated condition at a temperature of 30°C and a relative humidity of 90 %RH. The relationship between the water sorption amount and relative humidity will be discussed later in detail. Such moisture-sensitive element having a moisture-sensitive film made of CAB presents 2 - 4 %RH of hysteresis (difference in moisture sensitivity characteristic between moisture absorption and desorption processes) at about 25°C and at measuring intervals of 5 minutes. Therefore, it is not possible to reduce the hysteresis to less than 1 %RH, as considered ideal. Also, the hysteresis is temperature dependent and increased by 2 - 3 %RH at about 40°C compared with the value derived at 25°C if other conditions are the same. Further, if the above-mentioned type of moisture sensitive element is left in the same atmosphere for a long time, the effect of humidity to which the element has previously been exposed causes a change in the hysteresis and the moisture sensitivity characteristic. Specifically, if the element is left, e.g, at a relative humidity of 11 %RH and a room temperature for a long time, the hysteresis is increased from an initial value of 2 %RH 4 - 6 %RH. If the same element is left at a relative humidity of about 84 %RH and a room temperature for a long time, the hysteresis is decreased from the initial value, i.e. 2 %RH to about 1 %RH. However, with respect to the moisture sensitivity characteristic, the output is increased by 5 - 7 %RH in a relative humidity ranging from 10 to 90 %RH, that is, a drift occurs in the output. Further, a ratio of an electrical capacitance value at a relative humidity of 90 %RH to that at a relative humidity of 10 %RH (C 90 /C 10 ) is fluctuated while the element is used for a long time. Further, high temperature and high humidity environment, moisture condensation, water dipping, a temperature and humidity cycle and so on also cause the moisture sensitivity characteristic of the element to drift largely in the positive direction. These problems are caused by the fact that a water sorption amount (water absorption ratio) of the moisture-sensitive material employed in the above-mentioned conventional moisture-sensitive element is too large to be an ideal capacitance type or impedance type moisture-sensitive element. Thus, interaction of water molecules absorbed in the moisture-sensitive material causes a change in response of the element and an equilibrated water sorption amount in a measured humidity atmosphere, thereby incurring a drift in the output of the element. US 3582728 discloses a capacitive humidity sensing element which employs a moisture sensitive dielectric core and electrode layers integrally bonded to opposite surfaces of the core. The core and a matrix of the electrode layers should be capable of absorbing at least 1% of their own dry weight of moisture when exposed to 95 per cent humidity at 21°C (70°F). EP-A1-0010771 discloses a moist-are-sensitive element with the features of the preamble of Claim 1. This element includes a moisture-sensitive plastics material film between a gold electrode and another metal film. This plastics material film may be of polysulfon. Properties and syntheses of polysulfon and polyethersulfon are discussed in H.G. Elias et al "Neue Polymere Werkstoffe fur die industrielle Anwendung", 2nd edition, p 212-219 (Carl Hanser Verlag, Munich, 1983). Moisture and Humidity, Proceedings of the 1985 International Symposium on Moisture and Humidity, Washington D.C. April 15-18, 1985 discloses the features in the preamble of claim 1. OBJECTS AND SUMMARY OF THE INVENTION In view of the foregoing problems, it is an object of the present invention to provide a moisture-sensitive element which has a small hysteresis in a variety of using conditions from a high temperature to a low temperature. It is another object of the present invention to provide a moisture-sensitive element which is capable of generating a stable output in a variety of using conditions such as high humidity, high temperature, cyclically fluctuating humidity, low humidity, moisture condensation, water dipping and so on. The present invention provides a capacitive moisture sensor comprising: a moisture-sensitive film made of a polymer material, the thickness of the moisture-sensitive film lying within the range of 0.5 µm to 10 µm; and two electrodes between which the moisture-sensitive film is sandwiched; the moisture-sensitive film having a water sorption in a range between 10 and 40 mg/g when in equilibrium at a temperature of 30°C and a relative humidity of 90%, and the capacitive moisture sensor further comprising a substrate onto which the two electrodes and the moisture-sensitive film are mounted, and characterised in that the moisture-sensitive film consists of a polymer including polyethersulfon or polysulfon as a principal component. Another object of the present invention is to provide a moisture-sensitive element which has a low temperature dependency and accordingly can be employed in a wide range of temperature from a high temperature to a low temperature. It is a further object of the present invention to provide a moisture-sensitive element which shows a small hysteresis and a quick response in a usable range from a high temperature to a low temperature and from a high humidity to a low humidity. It is a further object of the present invention to provide a moisture-sensitive element which is capable of generating a stable output in any severe condition such as high humidity, high temperature, cyclically fluctuating humidity, low humidity, moisture condensation, water dipping and so on. To achieve the above objects, another aspect of the present invention provides a moisture-sensitive element comprising a moisture-sensitive film which consists of a polymer including polyethersulfon as a principal component. These and other objects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view showing an embodiment of a moisture-sensitive element according to the present invention; Figure 2 is a top plan view of the moisture-sensitive element shown in Figure 1; Figure 3 is a graph showing the relationship between the water sorption amount and hysteris of moisture-sensitive elements; Figure 4 is a graph showing the relationship between the water sorption amount and the capacitance ration of moisture-sensitive elements; Figure 5 is a graph showing the relationship between the water sorption amount and the relative humidity of moisture-sensitive elements; Figure 6 is a graph showing the moisture sensitivity characteristic of a conventional moisture-sensitive element (CAB including 17% of butyryl base); Figures 7a and 7b are graphs showing the temperature dependency of moisture-sensitive elements using polyethersulfon and polysulfon, respectively; Figure 8 is a graph showing a moisture sensing characteristic of the moisture-sensitive element according to the present invention; and Figure 9 is a graph showing a moisture sensing characteristic of a prior art moisture-sensitive element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of a moisture-sensitive element according to the present invention will hereinafter be described with reference to Figures 1 and 2. In these drawings, an insulating substrate made, e. g., of alumina, glass, thermally oxidized silicon or the like, is provided, and on the upper surface thereof a lower electrode 2 made e.g. of platinum is formed. Also, a moisture-sensitive film 3 is stacked on and across the lower electrode 2. This film 3 is made of a polymer moisture-sensitive material whose principal component is polyethersulfon (hereinafter simply referred to as "PES") or polysulfon. An upper electrode 4 made, e.g. of gold, is formed on the moisture-sensitive film 3. In other words, the moisture-sensitive film 3 is sandwitched between the lower and upper electrodes 2, 4. Also, leads 2a, 4a are coupled to the lower and upper electrodes 2, 4, respectively, to detect a change in an electrical capacitance corresponding to the relative humidity of the moisture-sensitive film 3. The moisture-sensitive film 3 is made of a polymer moisture sensitive material which includes polysulfon or polyethersulfon. Such polymer materials are respectively coated on a quartz vibrator in a thickness of approximately 10µm or less to form moisture-sensitive films. The moisture-sensitive films are then located in a constant humidity atmosphere to measure respective water sorption amount at about 30°C based on a change in an oscillating frequency of the quartz vibrator. Each of the above-mentioned polymer materials is also coated on the lower electrode 2 deposited on the insulating substrate 1 in a thickness of about 10µm or less to form the moisture-sensitive film 3 on which the upper electrode 4 is formed as mentioned above. With this structure, the relationship between the electrical capacitance ratio and the relative humidity is measured at about 30°C. The same measurements are also performed with the moisture-sensitive film 3 made of conventionally used celluloseacetatebutyrate. It should be noted that a water sorption amount and moisture sensitivity characteristic subtly fluctuate due to a forming method of the polymer film, a thickness of the film, an annealing method, a forming method of the upper electrode 4 and so on, as shown in Figure 6. It was found, from the results of the above measurement, that the celluloseacetatebutyrate film (including 17% of butyryl base) has a water sorption amount of 70 - 90 mg/g, a hysteresis of 2 - 4 %RH and an electrical capacitance ratio (C 90 /C 10 ) of 1.21 - 1.42, while the celluloseacetatebutyrate film (including 50% of butyryl base) has a water sorption amount of 40 - 60 mg/g, a hysteresis of 1 - 2 %RH and an electrical capacitance ratio (C 90 /C 10 ) of 1.10 - 1.20. With a polymethacrylic acid methyl film, a satisfactory result was obtained, where a water sorption amount is 10 - 30 mg/g, a hysteresis 0.5 - 1.0 %RH and C 90 /C 10 1.06 - 1.15. Good results were also obtained by a polyethersulfon film and a polysulfon film. The former has a water sorption amount of 10 30 mg, a hysteresis of 0.5 - 1.5 %RH and C 90 /C 10 of 1.12 - 1.17 while the latter has a water sorption amount of 10 - 15 mg/g, a hysteresis of 0.2 - 1.0 %RH and C 90 /C 10 of 1.05 - 1.08. It is apparent from the measurement results that the amount of water molecules existing in a polymer material causes a large change in the moisture sensitivity characteristic. Specifically, in a relative humidity range below 60 %RH, the water sorption amount is 35 - 40 mg/g and therefore the hysteresis is 0.3 - 0.4 %RH, where the linearity of the moisture sensitivity characteristic is quite satisfactory. However, in a relative humidity range above 60 %RH, the water sorption amount exceeds 40 mg/g, whereby interaction occurs among water molecules and therefore the hysteresis is gradually increased. For example, when measurements were performed at a relative humidity of 10 %RH, 70 %RH and 10 %RH, respectively, a maximum water sorption amount was 40 - 45 mg/g and the hysteresis 1.2 - 1.5 %RH. Meanwhile, when measurements were performed at a relative humidity of 10 %RH, 80 %RH and 10 %RH, respectively, a maximum water sorption amount was 50 - 55 mg/g and the hysteresis 1.5 - 2.1 %RH. It can be seen from these results that the hysteresis becomes larger as the water sorption amount is increased. Such larger hysteresis causes the linearity of the sensor output to be deteriorated. It is apparent from the above results that there is a close correlation between the water sorption amount of a polymer material and the moisture sensitivity characteristic. If the hysteresis is expected to be, e.g., below 1 %RH, the water sorption amount should be not more than 40 mg/g. It should be understood that the data shown here is a typical one, and the hysteresis varies depending upon a manufacturing method, manufacturing conditions, a measuring method, measuring conditions, humidity to which the element has previously been exposed and the like. Therefore, the characteristic curves shown in Figure 3 and 4 may have a certain width. Incidentally, when a moisture-sensitive element as shown in Figures 1 and 2 is made of a polymer material having a water sorption amount of 10 - 40 mg/g at a relative humidity of 90 %RH, its capacitance ratio (C 90 /C 10 ) is 1.05 - 1.2. In the above described embodiment, the moisture-sensitive element is used at a temperature between -30 and 100 °C, so that there will not occur problems relative to the thermal characteristic of polymer materials constituting the moisture-sensitive film 3, e.g., destroying the upper electrode 4 or largely changing the sensor characteristic will not occur. With respect to the dielectric breakdown strength, it has been found that a polymer film having a thickness of 0.5 - 10 µm is durable against a voltage of ±1.5 - ±5 V applied thereto. Further, when the element was measured in combination with an oscillating circuit, no problem occurred with respect to the volume inherent resistivity. As described above, the present invention can provide an improved moisture-sensitive element which achieves less than one per cent of hysteresis and elimination of the temperature dependency. Also, the moisture-sensitive element of the present invention can maintain stable output and electrical capacitance ratio even if it is left in the same atmosphere for a long time. Further, a drift of the moisture sensitive characteristic will be quite small even if it is left in a high temperature and high humidity atmosphere, moisture condensation, water dipping, temperature and humidity cycle, and so on. The graphs of Figures 3, 4 and 5 were obtained with the five materials stated hereinabove namely celluloseacetatebutyrate (including 17% of butyryl base), celluloseacetatebutyrate (including 50% of butyryl base), polyethersulfon, polymethacrylic acid methyl, and polysulfon. The water sorption amount and the hysteresis subtly fluctuate due to a polymer film forming method, an annealing method and a forming method of the upper electrode. Possible range of the water sorption amount and the hysteresis of the respective materials are indicated by rectangles in the attached Figure 3. As can be seen, the curve on Figure 3 is a combination of possible ranges of the water sorption amount and the hysteresis of the five above-mentioned materials. Figures 4 and 5 were derived in the same manner as Figure 3. Table 1 shows characteristics of the above mentioned polymer materials. A specific manufacturing process of the abovementioned moisture-sensitive element will be explained below. First, 10 - 40 grams of PES powder is dissolved in a mixed solvent typically made up of 20 milliliters of dimethylformamide, 80 milliliters of cyclohexanon and 25 milliliters of methylethylketone to provide PES solution. Next, this PES solution is coated on the lower electrode 2 formed on the insulating substrate 1 by a spin coating method and then dried in a nitrogen atmosphere at room temperature to form the moisture-sensitive film 3 having a thickness ranging from 0.5µm to 5µm. The spinner typically rotates at a speed of 500 - 5000 r.p.m. After this drying operation at room temperature, the element is annealed at a temperature between 160°C and 240°C for at least one hour. Next, the upper electrode 4 having a thickness ranging from 50 to 1000Å is deposited on the insulating substrate 1 having the moisture-sensitive film 3 thus formed thereon, e.g. by a vapor deposition method or a sputtering method. The metal used for the electrode 4 is not limited to gold, and any anti-corrosion metal such as palladium, platinum, chrome or the like may be used in place of gold. The lower electrode 2 is formed on the insulating substrate 1 by depositing platinum by a vapor deposition, sputtering method or the like in a thickness of 1000 - 10000 Å. Since the moisture-sensitive film 3 is formed, as mentioned above, by dissolving PES powder in a solvent mixture made up of dimethylformamide, cyclohexanon and methylethylketone, coating the solvent in a small amount on the lower electrode previously formed on the insulating substrate 1, and drying the coated solvent at room temperature, the solvent is evaporated to leave a thin polymer in a high density, thereby making it possible to largely reduce a water sorption amount to 0.2 - 0.4 wt% and also the hysteresis. A similar effect can be produced if a single solvent having a strong polarity such as dimethylformamide, cyclohexanon and methylethylketone is used in place of the above-mentioned solvent mixture. In a second embodiment of the present invention, the moisture-sensitive film 3 is formed of polysulfon in place of the above-mentioned PES. In this case, an annealing process is carried out at a temperature between 140°C to 200°C for at least one hour, after a drying process at room temperature, to thereby produce the same effect as the first embodiment. The polysulfon and PES respectively have a constitutional formula as shown below: In comparison of these two materials with each other, the polysulfon has C(CH 3 ) 2 while the PES has SO 2 at the corresponding position, so that the polysulfon has a water sorption amount and a sensitivity 1/3 as much as the PES. However, such reductions of the water sorption amount and sensitivity will not cause any problem in practice, and it is therefore possible to provide a moisture-sensitive element made of polysulfon which has a low temperature dependency and few drift in its output even in a high-temperature and high-humidity condition. Figures 7a and 7b are respectively a graph showing the relationship between electrical capacitance and relative humidity of the moisture-sensitive film 3 of the first and second embodiments. It should be noted that the measurements of the both moisture-sensitive films were made at a frequency of 100 KHz and a temperature of 10°C, 25°C and 40°C by means of a LCZ meter. As is apparent from these drawings, satisfactory moisture-sensitive characteristics with quite low temperature dependency were provided. Since a detected output does not suffer from fluctuation due to temperature, a temperature compensation circuit is not necessary. Also, Figures 7a, 7b shows a satisfactory result that the hysteresis is below 1 %RH when measured approximately two minutes after a constant humidity bath becomes stable. Figure 8 shows the relationship between elapsed time and drift amounts in outputs from moisture-sensitive elements manufactured according to the present invention at a relative humidity of 10, 30, 50, 60, 70 and 90 %RH. Figure 9 shows the same characteristics derived by a moisture-sensitive element employing a conventional celluloseacetatebutyrate as a moisture-sensitive material which was left in the same condition as the above for purpose of comparison. As is apparent from these drawings, the moisture-sensitive element of the present invention provides a stable moisture-sensitive characteristic with less drift in comparison with the conventional moisture-sensitive elementas shown in Figure 9. Also, the hysteresis is below 1 %RH when measured approximately two minutes after a constant humidity bath becomes stable. From these measurement results, it can be said that the moisture-sensitive element of the present invention has good reproductivity and a stable capacitance ratio even if it is used in a high temperature and high humidity atmosphere or even if it is left in the same atmosphere for a long time. Further, it is appreciated that the moisture-sensitive element can reversibly recover its initial characteristic when it is left in a high temperature and high humidity atmosphere and then brought back to a room temperature atmosphere. In the above embodiments, a sandwich type moisture-sensitive element is explained by way of example, however, the present invention is not limited to this type. It will be no saying that the same effects as mentioned above can be produced if the present invention is applied to a comb type moisture-sensitive element which has a pair of comb-type thin film electrodes formed on an insulating substrate opposite to each other and a moisture-sensitive film covering the comb-type thin film electrodes. Further, in the above embodiments, humidity is detected on the basis of an electrical capacitance change with respect to a relative humidity of the moisture-sensitive film. Alternatively, humidity can be detected on the basis of an impedance change with respect to the relative humidity. The moisture-sensitive film in the above embodiments can also be suitably employed for a moisture sensor in which the moisture-sensitive film is formed on a quartz vibrator and which detects humidity on the basis of a resonance frequency offset caused by water absorption of the moisture-sensitive film as well as a moisture sensor in which the moisture-sensitive film is formed on a surface elastic wave element and which detects humidity on the basis of a change in speed of a wave passing through the surface elastic wave element. Since many changes could be made in the above construction and many apparently widely differing embodiments of the present invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims: A capacitive moisture sensor comprising: a moisture-sensitive film (3) made of a polymer material, the thickness of the moisture-sensitive film (3) lying within the range of 0.5 µm to 10 µm; and two electrodes (2,4) between which the moisture-sensitive film (3) is sandwiched; the moisture-sensitive film having a water sorption in a range between 10 and 40 mg/g when in equilibrium at a temperature of 30°C and a relative humidity of 90%, and the capacitive moisture sensor further comprising a substrate (1) onto which the two electrodes (2,4) and the moisture-sensitive film (3) are mounted, and characterised in that the moisture-sensitive film (3) consists of a polymer including polyethersulfon or polysulfon as a principal component. A capacitive moisture sensor according to claim 1, characterised by a moisture-sensitive film (3) which is formed of polyethersulfon and annealed at a temperature ranging between 160°C and 240°C. A capacitive moisture sensor according to claim 1, characterised by a moisture-sensitive film (3) which is formed of polysulfon and annealed at a temperature ranging between 140°C and 200°C.

Inventors: ABE TORU, KUROIWA TAKAAKI, MIYAGISHI TETSUYA, ABE, TORU, KUROIWA, TAKAAKI, MIYAGISHI, TETSUYA
Applicants: YAMATAKE HONEYWELL CO LTD, YAMATAKE-HONEYWELL CO. LTD.
Application year: 1990
Main classification: G01N27
IPC codes: G01N27