Ind. Eng. Chem. Prod. Res. Dev. 1981, 20,669-674
669
GENERAL ARTICLES
Ceramic Humidity Sensor Tsuneharu Nltta Materials Research Laboratoty, Matsushita Electric Industria1 Company, Limited, Kadoma, Osaka, 571, Japan
A new compact, durable, yet highly sensitiie and reliable ceramic humidii sensor has been developed as a cooking
control device for automatic microwave ovens. The novel ceramic humidity sensor makes use of a new semiconducting porous ceramic material composed of a solid solution MgCr2O4-TiO2 having humklity sensitive effects and a rejuvenating response to heat treatment and B new heat-cleaning device to burn off organic contaminants such as oil and dust on the surface of the sensor in order to maintain optimum sensitivity. A new automatic microwave oven using the newly developed sensor has been developed and produced. The sensor controls the heating time by detecting humidity. It is expected that the ceramic humidity sensor will be applied extensively in industrial and medical equipment as well as in household appliances such as microwave ovens and air conditioners.
With the explosive spread of a new automatic control technology consisting of microcomputers and sensors throughout the industrial world, recent automated systems have required various kinds of sensors, including a humidity sensor. The control of humidity is imperative for equipment such as foodstuff ovens, air conditioners, dryers, etc. Conventional humidity sensors which make use of materials such as electrolytes, organic polymers, alumina thin fiims, and other metal oxides, have limitations in the ambient temperature and humidity range in which they can function accurately. Additionally, their reliability is subject to deterioration due to contamination from oil, dust, and other materials in air. It is because of these shortcomings that humidity sensors have not been extensively used despite a rather large potential demand. Recently, Nitta et al. (1980a,b) have successfully developed a new compact, durable, yet highly sensitive and reliable ceramic humidity sensor. This humidity sensor has been made possible through the development of a new humidity-sensitive ceramic material and through the adoption of a new heating device to burn off organic contaminants such as oil and dust on the ceramic surface in order to maintain optimum sensitivity (Nitta et al., 1978a,b). Subsequently, an automatic microwave oven using the ceramic humidity sensor has been developed and produced as reported by Nitta and Hayakawa (1980). The sensor controls the heating time by detecting humidity. This paper is arranged in three parts. The first part outlines the physical and chemical properties of the sensor ceramics. The second part describes the construction, features, and humidity characteristics of the sensor. Finally, as an application of the sensor, we will briefly exemplify a newly developed automatic microwave oven controlled by humidity detection. Sensor Ceramics Adsorption of water vapor enhances the surface electrical conductivity of metal oxides. There have been many reports of humidity sensors using this humidity sensitive effect. The humidity sensors normally exposed not only 0196-432118111220-0669$01.2510
Table I. Structural Data of Porous Ceramic composition, mol %
MgO (39.4),Cr,O, (39.4), TiO. (21.2) spinel*type single phase 60-70 1-2
phase analysis % theoretical density average grain size, pm specific surface area, ma/g 0.2-0.3 % porosity 30-40 average pore size, A 2500-3000
to water vapor but to atmospheres containing various other components tend to lose their inherent humidity-sensitive properties during use due to several complicated physical and chemical processes between these components and the sensor materials. The original surface state of a contaminated metal oxide surface may be recovered by eliminating almost all of the chemisorbed water as well as the poisoning components from the surface through heating at high temperatures. However, the surface structure of metal oxides in powder form is subject to permanent change by repeated heat-cleaning cycles at high temperatures. On the other hand, a ceramic form sintered at high temperatures is essentially more stable physically, chemically, and thermally than the powder form. The most promising approach seems to find a ceramic material with a surface resistivity reversibly responsive to relative humidity, which is not easily changed by repeated heat-cleaning cycles at high temperatures. Of the various metal oxide ceramics investigated, semiconducting porous ceramics composed of the solid solution MgCr2O4-TiO2 offer the most promise. The binary system MgCrz04-Ti02is very important in refractories because of its good mechanical and thermal properties at high temperature. Since metal oxides with high Cr content are generally difficult to sinter to high densities, the sintered compact tends to exhibit a typical porous structure. The MgCrz04-TiOz system containing up to 35 mol % TiOz exhibits a single phase solid solution with a pure MgCrzO, type spinel structure. Table I gives the structural 0 1981 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 TEMPERATURE ('C) 100 50
500 300 200
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Figure 2. Water adsorption-deaorption curve; RH was initially 50% at 20 O C and almost 0% at 80 "C (heating and cooling rate 1 "C/ min).
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Figure 1. Temperature dependence of resistivity in vacuum.
data of a representative porous sintered compact. The fractured sections exhibit intergranular pores in combination with neck parts. The pore size is distributed over a range of 500 to 3000 8, and the average porosity is about 35%. The pore structure consists of open pores that form a kind of capillary tube. The pore structure can easily exhibit absorption or adsorption and condensation of water vapor. Furthermore, the pore structure is an advantage in discouraging fracture due to thermal shock. The MgCrz04-TiOzwith a MgCrz04-typesingle phase exhibits a p-type semiconduction similar to that of pure MgCr204. The resisitivity of MgCrz04decreases with an increase of MgO. Semiconduction is a result of the formation of Cr3+ion defects due to evaporation of Cr components during firing. However, the resistivity of MgCrz04-TiOz systems is higher than one of pure MgCrzOl. The compositions display high spin susceptibilities which are the same as magnesium-chromium spinels containing Cr2+ions. For reason of the higher resistivity, it is tentatively proposed that the added Ti4+ions and paired Mg2+ ions are located on the octahedral sites, thus causing substitution of Cr2+ ions in the tetrahedral sites and a decrease in Cr3+ion defects. Therefore, it can be understood that MgCrZO4-TiO2compositions show only little variation of resistivity due to the firing conditions, while the resistivity of MgCrz04 cannot be easily controlled within a narrow variation even under the same firing condition. Temperature dependence of the resistivity of a representative composition is shown in Figure 1. Most of the compositions have a linear characteristic for inverse temperature w. log resistivity at 200 to 600 OC. This negative temperature coefficient (NTC) property is an advantage in high temperature thermistor application. The surfaces of most metal oxides are covered by hydroxyl groups in atmospheres containing water vapor, which hydrogen bonds to the hydroxyl groups to increase water adsorption. The surface hydroxyl groups can generally be removed by condensation dehydration at higher temperatures in vacuo. On the other hand, water molecules adsorbed on the present porous ceramics can be desorbed easily and almost perfectly under either a reduced pressure even at room temperature or a slightly increased temperature at ambient pressure, corresponding to nearly 0% RH (relative humidity), respectively. The specific surface area of the porous ceramic is very small compared with that of the
Figure 3. Schematic diagrams of water adsorption.
powder form. The electrical resistivity changes remarkably with the adsorption and desorption of water vapor, as In spite of water adsorption shown in Figure 2. throughout the porous ceramic bulk, a very short time is required to reach equilibrium of water adsorption or desorption. Figure 3 shows schematic diagrams of water adsorption in the present porous ceramic. The porous ceramic adsorbs water vapor easily throughout the controlled pore structure and the adsorbed water vapor condenses within the capillaries between the grain surfaces. The condensation of water vapor tends to occur preferentially on the neck parts of the grain surfaces. The neck part usually exhibits properties closely related to the bulk itself. Therefore, the total conductivity of the porous ceramic may be governed mainly by the neck part. In the first stage of water adsorption, a few water molecules chemisorb on the neck to form hydroxyl groups. The most active surface metal ion in MgCrz04-TiOzmight be assumed to be the chromium ion. The origin of these charge carriers is related mainly to an interaction between the surface Cr3+ions of the neck parts and the adsorbed water. These in turn provide mobile protons by transferring of Cr3+-OH to Cr4+-OH as well as dissociation of Cr-OH. The protons can migrate by hopping from site to site across the surface. In the second stage of water adsorption, more water vapor is adsorbed physically on the hydroxide surfaces to form the hydroxyl multilayers. The formation of multilayers due to physically adsorbed water can be certified by the increase in dielectric constant. The dielectric constant increases with relative humidity, and the change is reversible as shown in Figure 4. Thus, the increase in dielectric constant results in lowering the dissociation energy and promoting dissociation and a higher carrier concentration (Figure 3). Finally, large amounts of water molecules are physically adsorbed not only on the neck parts but also on the flat surfaces and convex parts and continuous electrolyte layers are formed between the opposed electrodes, resulting in increased conductivity. Thus, the resultant smooth variation of conductivity with water content may be due to a protonic conduction mechanism as viewed above.
Ind. Eng. Chem. Prod. Res. Dev.. Vol. 20, No. 4, 1981 671 HEATER COO SENSORCERAME
/
Figure 6. Construction sketch of sensor.
I Od
RELATIVE KMIUTYO.1
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Figure 4. Capacitance isotherms at 20 'C.
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Faure 5. Electrical resistivity change due to adsorption ofvarious kinds of gases.
The water adsorption in the porous ceramic is initially due to chemisorption, but most water adsorption occurs by the physical adsorption mechanism. On the other hand, when the porous ceramic is exposed in an atmosphere containing reducing or oxydizing gases at a high temperature, the gases chemisorb on the grain surface so that electrical conduction is very sensitive to gas adsorption. As shown in Figure 5, the electrical resistivity decreases in oxygen gas and increases in reducing gases such as alcohol, other organic hydrocarbons, carbon monoxide, hydrogen, hydrogen disulfide, etc. The change of electrical resistivity is observed at a high temperature over a wide range of 2 W 5 0 0 "C. The phenomenon of gas adsorption can be explained by chemisorption, that is, electron transfer on the grain surface of a p-type semiconductor. A t a temperature above 500 "C, the bulk itself governs the electrical conduction more effectively than the surface chemisorption because of an increase in the thermally excited carrier concentrations and a decrease in chemisorption. Ceramic Humidity Sensor A new ceramic humidity sensor makes use of the previously described porous ceramic, a porous electrode material, a unique heater structure, and an improved terminal unit. Figure 6 shows a construction sketch of the sensor. The sensor detects water absorbed throughout its bulk, thus making it less sensitive to the effect of surface contamination and more stable than the conventional surface layer type. The sensor ceramic has porous electrode layers
40 60 BO RELATIVE WMIDITY(%.)
20
0
Figure 7. Humiditrresistance Characteristics
such as fired ruthenium oxide (RuOd on both surfaces thereof. The fired RuO, is a porous structure having an average pore size above 1pm, excellent adhesion with the sensor ceramic, and excellent adsorption-desorption of water vapor. A coiled heater, such as a Kanthal wire, is attached around the sensor for the purpose of recovery of sensor sensitivity. The heat cleaning above 500 "C can avoid deterioration caused by extremely severe contaminants such as oil vapor and other organic vapors. The cleaning temperature is self-controlledby the thermistor property of the sensor ceramic. Both sensor and heater are supported on the improved terminal unit. The terminal unit makes use of a ceramic substrate with insulation guard rings. The insulation guard ring structure eliminates the effect of electrical leakage between sensor terminals due to the formation of electrolytes such as salt solution. Figure 7 summarizes the typical humidity-resistance characteristics of the present sensor. As RH is increased from nearly 0% to 10070,the resistance decreases rapidly as shown by the log scale. The hysteresis loop is, within experimental error, less than *0.5% RH. There is no voltage dependence of the RH-R curves except for the influence of Joule heating at the high RH side. The temperature coefficient of the relative humidity between 1"C and 80 "C is about 0.38% RH/"C at 60% RH. This value is close to that of pure water. According to practical test results, the sensor is proved to be capable of detecting humidity at temperatures up to 150 OC. An example of the time response for RH changes from 1%-50%RH and 94%-50% RH is shown in Figure 8. By substracting the time required to set the RH to a desired value, the time response is estimated to be within several seconds.
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20,No. 4, 1981 100
7 -
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Table 11. Specifications of Ceramic Humidity Sensor
I
size of sensor ceramic, mm size of heater (diameter), mm
30
TIME(sec)
Figure 8. Humidity time response characteristics to RH variations from 1 to 50% RH and from 94-50% RH.
4X 4 0.25
Characteristics operating temperature operating humidity sensitivity R1%, sz Rl%/R20% Rl%/R40% Rl%/R60% Rl%/RSO% supply voltage, V response time, s absorption (1-50% RH) desorption (94-50% RH) Heat Cleaning power supply, W resistance of heater, sz thermal time constant, s cleaning stop resistance, k a HUMIDITY SENSOR
100
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X
0.25
1-150 "C 1 4 0 0 % RH 1 . 5 X 10' 75 652 2000 6000
