Sensor and AchuztorsB, 13-14 (1993) 523-524
Humidity sensor with manganese oxide for room temperature Chao-Nan Xu and Kazuhide Miyazaki Department of Chemical Engineering, Faculty of Engineering Ftioka
University, 8-19-I Nanakuma, Jonan-ky Fukuoka 814-01
Recently, the addition of certain oxide catalysts in SnOz-based gas sensors was found to lead to an almost total loss of gas sensitivity to inflammable gases [l] which were the main hindrance to humidity detection. Meanwhile manganese dioxide, being familiarized as a catalyst, has been a versatile dry battery-active material for a long time . The current situation encouraged us to consider the possibility of employing manganese oxides as chemical sensor material. In this paper, we studied the behavior of manganese oxides as humidity sensor.
Electrolytic manganese dioxide chosen from International Common Samples (I.C. MnO, samples) was used as the starting material. Low-valency oxides were obtained by calcining the starting material at a temperature above 600°C. The amount of water and oxygen released during calcination was evaluated by a Earl Fisher moisture titrator and a thermal analysis station, respectively. The specific surface area of each powder sample was measured by BET method after outgassing at 300 “C for 20 min in a dry 30% NJHe flow. To measure the sensing properties, each porous sensor element was fabricated on an alumina tube with Pt wire electrodes, and then placed in a test chamber where the humidity and the other environmental gases were controllable. The electrical resistance was measured by a d.c. method and the sensitivity was defined as the ratio of resistance in dry air to that in the test gas.
Results and discussion Senringproperties Figure 1 shows typical response curves for various manganese oxide sensors, in which Mn,O, and Mn,O, were obtained by calcining MnO, at 600 and 1000 “C
“203 __,,‘____ --___
Fig. 1. Response curves of various manganese oxide sensors to humidity (80% r.h.) at room temperature.
for 2 h, respectively. Quick response as well as excellent reversibility were obtained with either Mn,O, or Mn,O, sensors at room temperature. These MnO, sensors were found to be sensitive to H,O only, and therefore the sensors of this material were able to selectively detect humidity in various ambient atmospheres, such as air, oxygen, nitrogen, and reducing gases. The response curves were repeatable without any further thermal desorption treatment. It has been observed that the low-valency manganese oxide sensors possess much remarkable humidity sensitivity. In some cases, however, their high electrical resistance in dry air, as shown in Fig. 1, made the measurement diflicult. Thus we tried to improve the electrical conductivity in dry gases by doping a conductive material. As a result, we have found that adding MnO, to Mn,Os or Mn,O, can enhance both the conductivity and the mechanical strength of the sensor element. Figure 2 summarizes the correlation between the resistance R and the relative humidity (r.h.) for the Mn,O,+MnO, and Mn,O,+MnO, sensors. It is notable that by adding MnO, to Mn,O, sensors in an amount of up to 20 wt.%, a moderate value of resistance
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524 TABLE 1. The specific surface area (SA) and the amount of adsorbed water on various manganese Manganese oxides
Mab MQQ” MRQ
26.8 2.7 0.3
Adsorbed water’ at 25 ‘C (wt.%)
Adsorbed water at 100 ‘C (wt.%)
4.9 0 0
5.3 0.6 0.6
3.8 0 0
4.2 0 0
‘Evaluated by thermogravimetric measurement. bObtained by calcining MnO, at 2lN, 600, and 1000 “C, respectively.
Fig. 2. Humidity-resistance characteristics for MnsO, + MnO, sensors at room temperature: (0) 0% MnOs; (A) 10 wt.% MrQ; (V) 100 wt.% MnOs, as well as Mn,O,+MnOs sensors; (0) 10 wt.% MnOs; (+) 20 wt.% MnO,, and (-) 50 wt.% MnO,.
sitivity of lower valency manganese oxide and the good reversible response to humidity. Based on these results, it seems that the ion conductivity should be taken into account for humidity detection as studied in other cases . Meanwhile, protons released horn the physisorbed water can be readily mobile in manganese oxides as illustrated in dry battery studies . In the cases of mixtures, it can be reasonably assumed that the water adsorbed by Mn02 might be transferred homogeneously throughout the entire sensor element, Mn,O,+MnO, or Mn30., + Mn02, thus leading to an enhancement in humidity sensitivity especially in the low r.h. range.
Conclusion is obtained and highly selective detection of humidity has been achieved over the whole examined range. Sensing mechanism Table 1 gives the specific surface area and the amount of adsorbed water for various kinds of manganese oxide. It illustrates that the water adsorbed on Mn,O, or Mn,O, can be removed easily by either exposing the oxide(s) to dry atmosphere at room temperature or treating the oxide(s) at a temperature above 100 “C. This reversibility seems to be a result of physical adsorption of water because of its low desorption temperature such as 100 “C. It can be seen from Table 1 that the amount of the reversibly adsorbed water (wet ambient minus dry ambient) per specific surface area increased in the order of MnOz < Mn,O, < Mn30,, for example, 0.015, 0.222, and 2.00 (wt.% mm2) at 25 “C. In conjunction with the humidity sensing properties, such a reversibly adsorbed water should be responsible for the good properties of humidity detection. That is, this would be the reason for the higher humidity sen-
The present work makes it clear that certain manganese oxides and their mixtures have excellent responses to humidity. It has been uncovered that humidity can be continuously detected with a high selectivity and sensitivity by using either Mn,O, +MnO,, or Mn,O., +MnOz system. A sensing mechanism is proposed to interpret the results.
References 1 C. N. Xu, J. Tamaki, N. Miura and N. Yamasoe, Grain size effects on gas sensitivity of porous SnO&ased elements, sensors and AClualOnr, 3 (1991) 147-155. 2 K. Miyazaki, Historical key factors for manufacture of electrolytic manganese dioxide in Japan, Rnc. Sump. HirrMy of Battery Technology, Proc. Vol. 87-14, The Electrochemical Society, 1987, pp. 8389. 3 T. Nitta, in T. Seiyama (ed.), Chenuiwl Sensor Technology, Vol. 1, Kodansha, Tokyo/Elscvier, Amsterdam, pp. 57-78. 4 H. P. Brenet, Charge transfer in electrochemical generators, Elechvchim. Acra, 9 (1964) 659-665.