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Manganese oxide–manganese tungstate composite humidity sensors
April 2002
Materials Letters 53 (2002) 316 – 320 www.elsevier.com/locate/matlet
Manganese oxide–manganese tungstate composite humidity sensors A.M. Edwin Suresh Raj a, C. Mallika b, O.M. Sreedharan b, K.S. Nagaraja a,* a
Department of Chemistry and Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai 600 034, Tamil Nadu, India b Thermodynamics and Kinetics Division, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, India Received 5 June 2001; accepted 2 July 2001
Abstract Experimental results on composites fabricated from different mole percent of MnWO4 and Mn2O3 for humidity sensing are described. Sintered polycrystalline disks of the composites were subjected to d.c. electrical conductivity measurements over the temperature range of 373 – 573 K in inert atmosphere from which the activation energies were determined. The terminal phases and the composites were subjected to d.c. resistance measurements as a function of relative humidity in the range of 5 – 98% RH, achieved by various water buffers at 298 K. The sensitivity factor, Sf (R5/R98) measured at 298 K revealed that the composite MWMO-82 has the highest sensitivity, greater than 4 103. The response and recovery characteristics of MWMO82 were assessed. D 2002 Published by Elsevier Science B.V. Keywords: Composites; Electrical properties; Humidity sensor; Manganese oxides; Manganese tungstate
1. Introduction The measurement and control of humidity is important in many areas including meteorology, medicine, food production, agriculture and domestic environment. Various methods are employed for the measurement of humidity [1] of which the measurement of relative humidity seems to be very convenient. Of the various materials utilized for humidity detection, the metal oxides, which are physically and chemically stable, have been widely investigated at both elevated and room temperatures [2 – 4]. Investigations on materials based on electrolytic manganese
*
Corresponding author. Tel.: +91-44-8276749 (Office), +9144-3744138 (Residence); fax: +91-44-8231684. E-mail addresses: [email protected], [email protected] (K.S. Nagaraja).
oxide have proved to be highly promising and being operative at room temperature [5– 7]. However, our earlier investigations indicate that materials that possess metastable coexistence can have excellent sensitivity towards moisture [8,9]. The main problem in handling manganese oxides based humidity sensors is their thermal stability as their stoichiometry varies with sintering temperature though they are capable of operating in the d.c. mode unlike the conventional ceramic materials which require an a.c. mode. Hence, the present investigation aims at the development of humidity sensors operating in the d.c. mode based on manganese oxide – tungstate composites.
2. Experimental Technical grade MnO2 (BDH, India) and MnWO4 (prepared by precipitation method) were used as the
0167-577X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 5 7 7 X ( 0 1 ) 0 0 4 9 9 - 2
A.M.E. Suresh Raj et al. / Materials Letters 53 (2002) 316–320 Table 1 Activation energy and sensitivity factor of the composites and terminal phases Mol% MnWO4
Mol% MnO2
Sample code
Ea ðeVÞ
Sensitivity factor ðSfÞ
100 80 60 40 20 0
0 20 40 60 80 100
MWMO-10 MWMO-82 MWMO-64 MWMO-46 MWMO-28 MWMO-01
0.45 0.31 0.38 0.41 0.36 0.30
5 4100 33 704 32 43
317
within 5 mass percent limit of its detection of impurity phases. Electrical conductivity studies were carried out by using a two-probe method. DC resistance was used as a tool to monitor the moisture sensing characteristics of the composites, under static conditions. The controlled humidity environments ranging from 5% to 98% RH were achieved by using anhydrous P2O5 and saturated aqueous solutions of various salts. The response and recovery characteristics were assessed as described earlier [8].
3. Results and discussion starting materials. The composite materials were fabricated from different mol% of MnO2 and MnWO4 (Table 1). A 2% solution of cetyl alcohol in ethanol was added as organic binder and the mixture was ground for 6 h in absolute ethanol, dried and compacted into pellets at a pressure of 100 MPa. The pellets were then heated at a rate of 10 K min 1 to 623 K and kept at this temperature for 1 h to remove the binder. Subsequently, the temperature was raised to 723 K at a rate of 2 K min 1 where it was maintained for 2 h to facilitate sintering followed by furnace cooling of the samples. The phase analysis was carried out by powder XRD (Rigaku Miniflex, Japan) using Cu Ka radiation
The representative XRD of the MWMO-82 composite (Fig. 1) is indicative of the presence of MnWO4 and Mn2O3 phases. Further interaction of MnWO4 and Mn2O3 to form the other inter-oxide is avoided by having the low sintering temperature of 723 K. The electrical conductivity studies on the composites in the temperature range of 373 – 573 K revealed the activation energy for electrical conduction to be in the range of 0.30 –0.45 eV, indicating the semiconducting nature of the composites. The results of resistance measurements as a function of relative humidity at a fixed ambient temperature of 298 K are presented in Fig. 2. The resistance
Fig. 1. Powder X-ray pattern of the MnWO4 – Mn2O3 (MWMO-82) composite.
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A.M.E. Suresh Raj et al. / Materials Letters 53 (2002) 316–320
Fig. 2. log R vs. relative humidity (%) plot of the MnWO4 – Mn2O3 composites and terminal phases.
of Mn2O3 is in the order of 104 V, whereas that of MnWO4 is in the order of 108 V under dry conditions (RH f 5%). Xu and Miyazaki [5] reported that manganese oxide hybrid mixtures are more effective than the either the individual bixbyite (Mn2O3) or hausmannite (Mn3O4) oxides for humidity sensing at room temperature. It could be inferred from the plot of logR vs. RH (%) (Fig. 2) that neither of the terminal phases possess appreciable sensitivity towards moisture when compared to that of the composites. As the humidity is increased, the Rdc value drops and the sensitivity of the elements towards humidity increases. The MWMO-82 composite was found to have the highest sensitivity factor, inferred from the ratio R5/R98, where R5 and R98 are the d.c. resistances at 5% and 98% RH, respectively. The sensitivity factor of the 82 composite is 4100. However, the MWMO-46 also shows appreciable variation in Rdc to changes in humidity and the sensitivity factor amounts to 710. The sensitivity factors of MnWO4 and Mn2O3 are calculated to be 5 and 43, respectively. The sensitivity factor of the bixbyite (Mn2O3) is reported [7] to be 38 at 298 K agreeing well with the present study. The significant feature of these composites are that the variation in Rdc with RH(%) is almost linear in the entire range of study, a prerequisite for commercial humidity sensors.
The composite MWMO-82 is chosen for the evaluation of the response and recovery characteristics. The Rdc in dry air as well as in moist air alternatively helped to establish the response and recovery characteristics. The results (Fig. 3) show that the invariant resistance in dry air is in the order of 107 V. Within about 3 min of purging with moist air, the resistance drops by three orders of magnitude to reach a constant value of approximately 104 V. The time taken for the restoration of the original signal is very long and the sample becomes insensitive to further changes in humidity. However, the original humidity response of the samples can be regenerated by heating the samples to 473 K for 1 h in air. This type of heat refreshment to regain humidity response, lost when irreversible adsorption occurs, is common in porous metal oxide sensors, which depend on capillary condensation of water causing ionic conductivity within the sensing material [10]. The water molecules that are physically adsorbed can be adsorbed or desorbed reversibly at room temperature, whereas the occluded water cannot be removed unless heated to elevated temperatures. Mn2O3 consists of uniform spherical grains and each grain possesses numerous micropores [11]. Such a unique microstructure appears to facilitate the adsorption of water. The variation in Rdc against relative humidity is very fast for MWMO-82 (Fig. 2) in
A.M.E. Suresh Raj et al. / Materials Letters 53 (2002) 316–320
319
Fig. 3. Response and recovery characteristics of the MWMO-82 composite.
the high RH range. This is due to the fact that the pore condensation of water molecules into such micropores will take place only under high RH conditions. Though the electrical conductivity study warrants the materials to be semiconductors, the conduction involves protonic also is evident from the plot of log R vs. RH. The semiconductor based materials employed for moisture sensing as reported [12] encompass not only those exhibiting electronic conduction but also the ones with ionic conductivity at ambient temperatures. It is therefore imperative that complex impedance measurements are warranted to access the ionic conductivity. In the absence of such measurements, guidance is taken from the conductivity measurements for effecting the choice of the materials, since at least under conditions of lower RH, electronic conduction influences the change in resistance. More detailed experimentation is warranted to address the exact mode and mechanism of conduction involved in humidity sensing.
4. Conclusions Composites having different mole percent of MnWO4 and Mn2O3 are fabricated and studied for
humidity sensing applications. The hybrid oxide systems provide resistive type elements, which can respond to humidity on d.c. electrical resistance (R dc) mode at room temperature unlike many other ceramic humidity sensors which work in a.c. resistance (R ac) mode, with excellent linear response in the entire RH range. The composite, MWMO-82 is found to exhibit sensitivity higher than 4 103 and the response time is found to be approximately 3 min. The recovery time is very high, but in a commercial device, a microheater could be suitably incorporated, which would be switched on after exposure to higher humidity and might facilitate restoration of the sensor performance within a reasonable time.
Acknowledgements The authors are thankful to Dr. Baldev Raj, Director, MCR group and Dr. V.S. Ragunathan, Associate Director, MCG of IGCAR for their encouragement and support. Many thanks are also due to Rev. Dr. John Pragasam, Director and Dr. K. Swaminathan of LIFE for their useful discussions. This work is supported by the Department of Atomic Energy, India, through BRNS grant No. 99/37/14/BRNS Cell/276.
320
A.M.E. Suresh Raj et al. / Materials Letters 53 (2002) 316–320
References [1] T. Hubert, MRS Bull. (1999) 49. [2] T. Seiyama, N. Yamazoe, H. Arai, Sens. Actuators 4 (1983) 85. [3] G. Gusmano, G. Montesperelli, E. Traversa, J. Am. Ceram. Soc. 76 (1993) 743. [4] C. Cartalini, M. Faccio, G. Ferri, M. Pelino, Sens. Actuators, B 18 – 19 (1994) 437. [5] C.N. Xu, K. Miyazaki, J. Electrochem. Soc. 139 (1992) L111. [6] K. Miyazaki, C.N. Xu, J. Electrochem. Soc. 141 (1994) L35.
[7] C.N. Xu, K. Miyazaki, T. Watanabe, Sens. Actuators, B 46 (1998) 87. [8] A.M.E. Suresh Raj, C. Mallika, O.M. Sreedharan, K.S. Nagaraja, Mater. Res. Bull. 36 (2001) 837. [9] A.M.E. Suresh Raj, C. Mallika, K. Swaminathan, O.M. Sreedharan, K.S. Nagaraja, Sens. Actuators, B, in press. [10] N. Yamazoe, Y. Shimizu, Sens. Actuators 10 (1986) 379. [11] K. Yoshimura, C.N. Xu, S. Kumamoto, K. Miyazaki, J. Electrochem. Soc. Jpn. 63 (1995) 1006. [12] H. Arai, T. Seiyama, Humidity sensors, in: W. Gopel, J. Hesse, J.N. Zemel (Eds.), Sensors A Comprehensive Survey, vol. 7, VCH, Germany, 1990, p. 999.
Materials Letters 53 (2002) 316 – 320 www.elsevier.com/locate/matlet
Manganese oxide–manganese tungstate composite humidity sensors A.M. Edwin Suresh Raj a, C. Mallika b, O.M. Sreedharan b, K.S. Nagaraja a,* a
Department of Chemistry and Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai 600 034, Tamil Nadu, India b Thermodynamics and Kinetics Division, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, India Received 5 June 2001; accepted 2 July 2001
Abstract Experimental results on composites fabricated from different mole percent of MnWO4 and Mn2O3 for humidity sensing are described. Sintered polycrystalline disks of the composites were subjected to d.c. electrical conductivity measurements over the temperature range of 373 – 573 K in inert atmosphere from which the activation energies were determined. The terminal phases and the composites were subjected to d.c. resistance measurements as a function of relative humidity in the range of 5 – 98% RH, achieved by various water buffers at 298 K. The sensitivity factor, Sf (R5/R98) measured at 298 K revealed that the composite MWMO-82 has the highest sensitivity, greater than 4 103. The response and recovery characteristics of MWMO82 were assessed. D 2002 Published by Elsevier Science B.V. Keywords: Composites; Electrical properties; Humidity sensor; Manganese oxides; Manganese tungstate
1. Introduction The measurement and control of humidity is important in many areas including meteorology, medicine, food production, agriculture and domestic environment. Various methods are employed for the measurement of humidity [1] of which the measurement of relative humidity seems to be very convenient. Of the various materials utilized for humidity detection, the metal oxides, which are physically and chemically stable, have been widely investigated at both elevated and room temperatures [2 – 4]. Investigations on materials based on electrolytic manganese
*
Corresponding author. Tel.: +91-44-8276749 (Office), +9144-3744138 (Residence); fax: +91-44-8231684. E-mail addresses: [email protected], [email protected] (K.S. Nagaraja).
oxide have proved to be highly promising and being operative at room temperature [5– 7]. However, our earlier investigations indicate that materials that possess metastable coexistence can have excellent sensitivity towards moisture [8,9]. The main problem in handling manganese oxides based humidity sensors is their thermal stability as their stoichiometry varies with sintering temperature though they are capable of operating in the d.c. mode unlike the conventional ceramic materials which require an a.c. mode. Hence, the present investigation aims at the development of humidity sensors operating in the d.c. mode based on manganese oxide – tungstate composites.
2. Experimental Technical grade MnO2 (BDH, India) and MnWO4 (prepared by precipitation method) were used as the
0167-577X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 5 7 7 X ( 0 1 ) 0 0 4 9 9 - 2
A.M.E. Suresh Raj et al. / Materials Letters 53 (2002) 316–320 Table 1 Activation energy and sensitivity factor of the composites and terminal phases Mol% MnWO4
Mol% MnO2
Sample code
Ea ðeVÞ
Sensitivity factor ðSfÞ
100 80 60 40 20 0
0 20 40 60 80 100
MWMO-10 MWMO-82 MWMO-64 MWMO-46 MWMO-28 MWMO-01
0.45 0.31 0.38 0.41 0.36 0.30
5 4100 33 704 32 43
317
within 5 mass percent limit of its detection of impurity phases. Electrical conductivity studies were carried out by using a two-probe method. DC resistance was used as a tool to monitor the moisture sensing characteristics of the composites, under static conditions. The controlled humidity environments ranging from 5% to 98% RH were achieved by using anhydrous P2O5 and saturated aqueous solutions of various salts. The response and recovery characteristics were assessed as described earlier [8].
3. Results and discussion starting materials. The composite materials were fabricated from different mol% of MnO2 and MnWO4 (Table 1). A 2% solution of cetyl alcohol in ethanol was added as organic binder and the mixture was ground for 6 h in absolute ethanol, dried and compacted into pellets at a pressure of 100 MPa. The pellets were then heated at a rate of 10 K min 1 to 623 K and kept at this temperature for 1 h to remove the binder. Subsequently, the temperature was raised to 723 K at a rate of 2 K min 1 where it was maintained for 2 h to facilitate sintering followed by furnace cooling of the samples. The phase analysis was carried out by powder XRD (Rigaku Miniflex, Japan) using Cu Ka radiation
The representative XRD of the MWMO-82 composite (Fig. 1) is indicative of the presence of MnWO4 and Mn2O3 phases. Further interaction of MnWO4 and Mn2O3 to form the other inter-oxide is avoided by having the low sintering temperature of 723 K. The electrical conductivity studies on the composites in the temperature range of 373 – 573 K revealed the activation energy for electrical conduction to be in the range of 0.30 –0.45 eV, indicating the semiconducting nature of the composites. The results of resistance measurements as a function of relative humidity at a fixed ambient temperature of 298 K are presented in Fig. 2. The resistance
Fig. 1. Powder X-ray pattern of the MnWO4 – Mn2O3 (MWMO-82) composite.
318
A.M.E. Suresh Raj et al. / Materials Letters 53 (2002) 316–320
Fig. 2. log R vs. relative humidity (%) plot of the MnWO4 – Mn2O3 composites and terminal phases.
of Mn2O3 is in the order of 104 V, whereas that of MnWO4 is in the order of 108 V under dry conditions (RH f 5%). Xu and Miyazaki [5] reported that manganese oxide hybrid mixtures are more effective than the either the individual bixbyite (Mn2O3) or hausmannite (Mn3O4) oxides for humidity sensing at room temperature. It could be inferred from the plot of logR vs. RH (%) (Fig. 2) that neither of the terminal phases possess appreciable sensitivity towards moisture when compared to that of the composites. As the humidity is increased, the Rdc value drops and the sensitivity of the elements towards humidity increases. The MWMO-82 composite was found to have the highest sensitivity factor, inferred from the ratio R5/R98, where R5 and R98 are the d.c. resistances at 5% and 98% RH, respectively. The sensitivity factor of the 82 composite is 4100. However, the MWMO-46 also shows appreciable variation in Rdc to changes in humidity and the sensitivity factor amounts to 710. The sensitivity factors of MnWO4 and Mn2O3 are calculated to be 5 and 43, respectively. The sensitivity factor of the bixbyite (Mn2O3) is reported [7] to be 38 at 298 K agreeing well with the present study. The significant feature of these composites are that the variation in Rdc with RH(%) is almost linear in the entire range of study, a prerequisite for commercial humidity sensors.
The composite MWMO-82 is chosen for the evaluation of the response and recovery characteristics. The Rdc in dry air as well as in moist air alternatively helped to establish the response and recovery characteristics. The results (Fig. 3) show that the invariant resistance in dry air is in the order of 107 V. Within about 3 min of purging with moist air, the resistance drops by three orders of magnitude to reach a constant value of approximately 104 V. The time taken for the restoration of the original signal is very long and the sample becomes insensitive to further changes in humidity. However, the original humidity response of the samples can be regenerated by heating the samples to 473 K for 1 h in air. This type of heat refreshment to regain humidity response, lost when irreversible adsorption occurs, is common in porous metal oxide sensors, which depend on capillary condensation of water causing ionic conductivity within the sensing material [10]. The water molecules that are physically adsorbed can be adsorbed or desorbed reversibly at room temperature, whereas the occluded water cannot be removed unless heated to elevated temperatures. Mn2O3 consists of uniform spherical grains and each grain possesses numerous micropores [11]. Such a unique microstructure appears to facilitate the adsorption of water. The variation in Rdc against relative humidity is very fast for MWMO-82 (Fig. 2) in
A.M.E. Suresh Raj et al. / Materials Letters 53 (2002) 316–320
319
Fig. 3. Response and recovery characteristics of the MWMO-82 composite.
the high RH range. This is due to the fact that the pore condensation of water molecules into such micropores will take place only under high RH conditions. Though the electrical conductivity study warrants the materials to be semiconductors, the conduction involves protonic also is evident from the plot of log R vs. RH. The semiconductor based materials employed for moisture sensing as reported [12] encompass not only those exhibiting electronic conduction but also the ones with ionic conductivity at ambient temperatures. It is therefore imperative that complex impedance measurements are warranted to access the ionic conductivity. In the absence of such measurements, guidance is taken from the conductivity measurements for effecting the choice of the materials, since at least under conditions of lower RH, electronic conduction influences the change in resistance. More detailed experimentation is warranted to address the exact mode and mechanism of conduction involved in humidity sensing.
4. Conclusions Composites having different mole percent of MnWO4 and Mn2O3 are fabricated and studied for
humidity sensing applications. The hybrid oxide systems provide resistive type elements, which can respond to humidity on d.c. electrical resistance (R dc) mode at room temperature unlike many other ceramic humidity sensors which work in a.c. resistance (R ac) mode, with excellent linear response in the entire RH range. The composite, MWMO-82 is found to exhibit sensitivity higher than 4 103 and the response time is found to be approximately 3 min. The recovery time is very high, but in a commercial device, a microheater could be suitably incorporated, which would be switched on after exposure to higher humidity and might facilitate restoration of the sensor performance within a reasonable time.
Acknowledgements The authors are thankful to Dr. Baldev Raj, Director, MCR group and Dr. V.S. Ragunathan, Associate Director, MCG of IGCAR for their encouragement and support. Many thanks are also due to Rev. Dr. John Pragasam, Director and Dr. K. Swaminathan of LIFE for their useful discussions. This work is supported by the Department of Atomic Energy, India, through BRNS grant No. 99/37/14/BRNS Cell/276.
320
A.M.E. Suresh Raj et al. / Materials Letters 53 (2002) 316–320
References [1] T. Hubert, MRS Bull. (1999) 49. [2] T. Seiyama, N. Yamazoe, H. Arai, Sens. Actuators 4 (1983) 85. [3] G. Gusmano, G. Montesperelli, E. Traversa, J. Am. Ceram. Soc. 76 (1993) 743. [4] C. Cartalini, M. Faccio, G. Ferri, M. Pelino, Sens. Actuators, B 18 – 19 (1994) 437. [5] C.N. Xu, K. Miyazaki, J. Electrochem. Soc. 139 (1992) L111. [6] K. Miyazaki, C.N. Xu, J. Electrochem. Soc. 141 (1994) L35.
[7] C.N. Xu, K. Miyazaki, T. Watanabe, Sens. Actuators, B 46 (1998) 87. [8] A.M.E. Suresh Raj, C. Mallika, O.M. Sreedharan, K.S. Nagaraja, Mater. Res. Bull. 36 (2001) 837. [9] A.M.E. Suresh Raj, C. Mallika, K. Swaminathan, O.M. Sreedharan, K.S. Nagaraja, Sens. Actuators, B, in press. [10] N. Yamazoe, Y. Shimizu, Sens. Actuators 10 (1986) 379. [11] K. Yoshimura, C.N. Xu, S. Kumamoto, K. Miyazaki, J. Electrochem. Soc. Jpn. 63 (1995) 1006. [12] H. Arai, T. Seiyama, Humidity sensors, in: W. Gopel, J. Hesse, J.N. Zemel (Eds.), Sensors A Comprehensive Survey, vol. 7, VCH, Germany, 1990, p. 999.