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Functionalized screen-printed PZT cantilevers for room temperature benzene detection
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Procedia Engineering
Procedia Engineering 00 (2011) 000–000 Procedia Engineering 25 (2011) 1077 – 1080 www.elsevier.com/locate/procedia
Proc. Eurosensors XXV, September 4-7, 2011, Athens, Greece
Functionalized screen-printed PZT cantilevers for room temperature benzene detection R. Vázquez1, R. Lakhmi2, H. Debéda2, F. J. Arregui3, C. R. Zamarreño3, M. Delgado4, C. Lucat2, E. Llobet1* 1 MINOS-EMaS, University Rovira i Virgili, Tarragona, Spain Université de Bordeaux, Laboratoire IMS, 33405 Talence Cedex, France 3 Public University of Navarra, IEE Department 31006, Pamplona, Spain 4 Sensotran, 31 Av. Remolar- 08820 El Prat de Llobregat, Spain
2
Abstract Here we study the performance of screen-printed PZT cantilevers coated with either active carbon or tin oxide for room temperature detection of benzene traces. It is found that the first longitudinal mode of active carbon coated cantilevers is extremely sensitive to benzene. Shifts of -13 and -144 Hz are respectively recorded for 1 and 10 ppm of benzene. These negative shifts indicate that the response mechanism is driven by a mass absorption effect. The sensors regain the baseline values of their characteristic resonance frequencies by cleaning in air and without heating. These results are promising for the development of a hand-held benzene detector.
© 2011 Published by Elsevier Ltd. Keywords: microcantilever; PZT; benzene; active carbon; tin oxide
1. Introduction There is an increasing interest in the use of resonant cantilevers for physical and chemical sensor applications [1]. In particular for chemical sensors, a sensing layer is deposited over cantilevers which transduce physical properties changes related to chemical adsorption into a mechanical response. When the sensor layer is exposed to a gas, the cantilever mechanically responds by bending because of surface stress change and/or by mass change [2][3]. These changes are detected by measuring resonance frequency shifts. * Corresponding author. Eduard Llobet, Tel.: +34977558502; fax: + 34977559605. E-mail address: [email protected].
1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.12.265
1078 2
R. Vázquez al. / Procedia Engineering 25 (2011) 1077 – 1080 Author nameet/ Procedia Engineering 00 (2011) 000–000
The aim of this work is to fabricate and test a self actuated microcantilever sensor for benzene detection. This cantilever is a ceramic PZT cantilever operated in a resonant in plane 31-longitudinal mode. In order to improve the absorption capacity of the cantilever surface, active carbon or tin oxide are used as sensitive layers, instead of polymer layers classically used. These materials are chosen because it is well known that active carbon [4] and tin oxide [5] show a good absorption capacity for Volatile Organic Compounds (VOCs). 2. Fabrication and functionalization of PZT cantilevers Microcantilevers are fabricated in IMS Laboratories from Bordeaux University, using a screen-printing technology associated to a sacrificial layer [6]. Dimensions of these devices are 8x2x0.1mm3 and their structure is shown in Fig. 1. A piezoelectric paste was prepared using piezoelectric PZ26 powder and 5wt% lead borosilicate glassfrit blended with ESL 400 organic vehicle. The sacrificial layer consist of an epoxy-type ink with SrCO3 mineral filler. The gold ink used for electrodes is a commercial gold ink from ESL.
(a)
(b)
Fig. 1. (a) PZT cantilever structure; (b) Photograph of piezoelectric cantilever
First of all, the sacrificial layer is deposited on the substrate and polymerized at 120°C. After that, bottom electrode, PZT layer and top electrode are printed successively after drying at 120°C during 20 min between each layer deposition. Then, samples are fired at 900 °C for 2 hours in air atmosphere before the sacrificial layer removal in a 0.9 mole·l-1 H3PO4 aqueous solution. Finally PZT microcantilevers are poled with an electric field of 5 kVcm-1 at 550 K. In order to functionalize cantilevers for gas sensing application, tin oxide and active carbon are used as an active layer. For tin oxide the dimensions of the nanoparticles are between 10 and 100 nm, and for active carbon particle diameter is 5 μm and pore width is 1 nm. To deposit these materials a drop coating technique is employed using a microdispenser connected to a syringe, filled-up with the material to be deposited. The drop-coated pastes are prepared by mixing the material with a viscous solvent (glycerol) in a mortar for about 10 mn. After the material has been deposited, the solvent is evaporated at 125 °C and finally the device are annealed at 250 °C for 3 hours.
Fig. 2. (a) Top view and (b) side view of PZT cantilever coated with SnO2. (c) Top view of PZT cantilever coated with active carbon
1079 3
R. Vázquez al. / Procedia 1077 – 1080 Eduard et Llobet/ ProcediaEngineering Engineering25 00(2011) (20111) 000–000
SEM images in Fig. 2 show microcantilevers after the deposition process. Film thicknesses of 68 μm for tin oxide and 115 μm for active carbon are estimated.. At Table 1 we can observe that for both materials the resonance frequency decreases due to the mass addition of material over the cantilever. Table 1. Values of the first 31 longitudinal mode before and after coating with active carbon and SnO2 Cantilever
1st mode (Hz)
Uncoated #1
67111
#1 coated with active carbon Uncoated #2
67049 71607
#2 coated with SnO2
67669
3. Sensing properties measurements The sensor is placed in a 115 cm3chamber made of methacrylate. Sensor response is measured for 1 and 10 ppm of benzene. During measurements, chamber temperature is about 23°C with a maximum variation of 1°C. Measurements are carried out using an impedance meter HP4192A controlled with a PC via a LabVIEW program. The resolution of these measurements is 1 Hz. The sequence used for measurements is as follows. First, the sensor was left to stabilize under a constant flow of dry air (100 ml/min) for a few hours before the first data acquisition. After that, a specific concentration of benzene is introduced and a measurement was taken 1 hour after introducing benzene. Finally, the sensor is cleaned in dry air flow during 6 hours before the last measurement. 4. Results The sensitivity of the microcantilever sensor is obtained from the measurement of the resonance frequency variation due to the adsorption of gas molecules onto the active layer. Fig. 3 shows different measurements for 1 and 10 ppm benzene concentration. The decrease of the first mode resonance peak indicates that the cantilever coated with active carbon adsorbs benzene molecules, which increases the mass of the cantilever. The recovery during the cleaning phase in synthetic air is also observed. 89.5
synthetic air 1 ppm of benzene cleaning 6 hours
89.4
89.2 89.1 89
89.4
10 ppm of benzene cleaning 6 hours
89.2 89.1 89
88.9
88.9
88.8 6.68
synthetic air
89.3
Phase (deg)
Phase (deg)
89.3
89.5
88.8 6.7
6.72
6.74
6.76
Frequency (Hz)
6.78
6.8
6.82 x 10
4
6.68
6.7
6.72
6.74
Frequency (Hz)
6.76 x 10
4
1080 4
R. Vázquez al. / Procedia Engineering 25 (2011) 1077 – 1080 Author nameet/ Procedia Engineering 00 (2011) 000–000
(a)
(b)
Fig. 3. Impedance measurement for the first longitudinal mode of an active carbon coated PZT cantilever at 1 ppm of benzene (a) and 10 ppm of benzene (b).
At 10 ppm there is a decrease of the quality factor. Table 2 shows the quite proportional frequency variation with benzene concentrations. Table 2. Frequency shifts recorded in the presence of benzene for the first 31 longitudinal mode of an active carbon and tin oxide coated PZT cantilever Benzene concentration
Δf (Hz) active carbon
Δf (Hz) tin oxide
1 ppm
-13
-33
10 ppm
-144
-43
5. Conclusions We have shown the possibility of benzene detection at room temperature using the first longitudinal mode of screen-printed ceramic microcantilevers. Cantilevers coated with active carbon are more suitable for detecting benzene, since an almost linear frequency shift is obtained in the 1 to 10 ppm region. Acknowledgements Région Aquitaine (France), Generalitat de Catalunya (Spain) and Gobierno de Navarra (Spain) have supported this research through CTP Research Grants. References [1] T. Thundat, P. I. Oden, R. J. Warmack, ‘Microcantilever sensors’, Microscale Thermophysical Engineering, (1997) 185 – 199 [2] M. Maute, S. Raible, F.E. Prins, D. P. Kern, ‘Fabrication and application of polymer coated cantilevers as gas sensor’, Microelectronic Engineering, 46 (1999) 439 – 442. [3] M. Goeders et al., “Microcantilevers: Sensing Chemical Interations via Mechanical Motion”, Chem. Re., 108, 2008, 522542. [4] K. L. Foster, R. G. Fuerman, J. Economy, S. M. Larson, M. J. Rood, “Adsorption characteristics of trace volatile organic compounds in gas streams onto activated carbon fibers”, Chem. Mat., 1992, 4 (5), 1068-1073. [5] M. Mabrook, P. Hawkins, “A rapidly-responding sensor for benzene, methanol and ethanol vapours on films of titanium dioxide dispersed in a polymer operating at room temperature”, Sensors and Actuators B75, 2001, 197-202. [6] C. Lucat, P. Ginet, C. Castille, H. Debéda and F. Ménil, “Microsystems elements based on free-standingthick-films made with a new sacrificial layer process”, Microelectronics Reliability, 48, 6, 2008, 872-875.
Procedia Engineering
Procedia Engineering 00 (2011) 000–000 Procedia Engineering 25 (2011) 1077 – 1080 www.elsevier.com/locate/procedia
Proc. Eurosensors XXV, September 4-7, 2011, Athens, Greece
Functionalized screen-printed PZT cantilevers for room temperature benzene detection R. Vázquez1, R. Lakhmi2, H. Debéda2, F. J. Arregui3, C. R. Zamarreño3, M. Delgado4, C. Lucat2, E. Llobet1* 1 MINOS-EMaS, University Rovira i Virgili, Tarragona, Spain Université de Bordeaux, Laboratoire IMS, 33405 Talence Cedex, France 3 Public University of Navarra, IEE Department 31006, Pamplona, Spain 4 Sensotran, 31 Av. Remolar- 08820 El Prat de Llobregat, Spain
2
Abstract Here we study the performance of screen-printed PZT cantilevers coated with either active carbon or tin oxide for room temperature detection of benzene traces. It is found that the first longitudinal mode of active carbon coated cantilevers is extremely sensitive to benzene. Shifts of -13 and -144 Hz are respectively recorded for 1 and 10 ppm of benzene. These negative shifts indicate that the response mechanism is driven by a mass absorption effect. The sensors regain the baseline values of their characteristic resonance frequencies by cleaning in air and without heating. These results are promising for the development of a hand-held benzene detector.
© 2011 Published by Elsevier Ltd. Keywords: microcantilever; PZT; benzene; active carbon; tin oxide
1. Introduction There is an increasing interest in the use of resonant cantilevers for physical and chemical sensor applications [1]. In particular for chemical sensors, a sensing layer is deposited over cantilevers which transduce physical properties changes related to chemical adsorption into a mechanical response. When the sensor layer is exposed to a gas, the cantilever mechanically responds by bending because of surface stress change and/or by mass change [2][3]. These changes are detected by measuring resonance frequency shifts. * Corresponding author. Eduard Llobet, Tel.: +34977558502; fax: + 34977559605. E-mail address: [email protected].
1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.12.265
1078 2
R. Vázquez al. / Procedia Engineering 25 (2011) 1077 – 1080 Author nameet/ Procedia Engineering 00 (2011) 000–000
The aim of this work is to fabricate and test a self actuated microcantilever sensor for benzene detection. This cantilever is a ceramic PZT cantilever operated in a resonant in plane 31-longitudinal mode. In order to improve the absorption capacity of the cantilever surface, active carbon or tin oxide are used as sensitive layers, instead of polymer layers classically used. These materials are chosen because it is well known that active carbon [4] and tin oxide [5] show a good absorption capacity for Volatile Organic Compounds (VOCs). 2. Fabrication and functionalization of PZT cantilevers Microcantilevers are fabricated in IMS Laboratories from Bordeaux University, using a screen-printing technology associated to a sacrificial layer [6]. Dimensions of these devices are 8x2x0.1mm3 and their structure is shown in Fig. 1. A piezoelectric paste was prepared using piezoelectric PZ26 powder and 5wt% lead borosilicate glassfrit blended with ESL 400 organic vehicle. The sacrificial layer consist of an epoxy-type ink with SrCO3 mineral filler. The gold ink used for electrodes is a commercial gold ink from ESL.
(a)
(b)
Fig. 1. (a) PZT cantilever structure; (b) Photograph of piezoelectric cantilever
First of all, the sacrificial layer is deposited on the substrate and polymerized at 120°C. After that, bottom electrode, PZT layer and top electrode are printed successively after drying at 120°C during 20 min between each layer deposition. Then, samples are fired at 900 °C for 2 hours in air atmosphere before the sacrificial layer removal in a 0.9 mole·l-1 H3PO4 aqueous solution. Finally PZT microcantilevers are poled with an electric field of 5 kVcm-1 at 550 K. In order to functionalize cantilevers for gas sensing application, tin oxide and active carbon are used as an active layer. For tin oxide the dimensions of the nanoparticles are between 10 and 100 nm, and for active carbon particle diameter is 5 μm and pore width is 1 nm. To deposit these materials a drop coating technique is employed using a microdispenser connected to a syringe, filled-up with the material to be deposited. The drop-coated pastes are prepared by mixing the material with a viscous solvent (glycerol) in a mortar for about 10 mn. After the material has been deposited, the solvent is evaporated at 125 °C and finally the device are annealed at 250 °C for 3 hours.
Fig. 2. (a) Top view and (b) side view of PZT cantilever coated with SnO2. (c) Top view of PZT cantilever coated with active carbon
1079 3
R. Vázquez al. / Procedia 1077 – 1080 Eduard et Llobet/ ProcediaEngineering Engineering25 00(2011) (20111) 000–000
SEM images in Fig. 2 show microcantilevers after the deposition process. Film thicknesses of 68 μm for tin oxide and 115 μm for active carbon are estimated.. At Table 1 we can observe that for both materials the resonance frequency decreases due to the mass addition of material over the cantilever. Table 1. Values of the first 31 longitudinal mode before and after coating with active carbon and SnO2 Cantilever
1st mode (Hz)
Uncoated #1
67111
#1 coated with active carbon Uncoated #2
67049 71607
#2 coated with SnO2
67669
3. Sensing properties measurements The sensor is placed in a 115 cm3chamber made of methacrylate. Sensor response is measured for 1 and 10 ppm of benzene. During measurements, chamber temperature is about 23°C with a maximum variation of 1°C. Measurements are carried out using an impedance meter HP4192A controlled with a PC via a LabVIEW program. The resolution of these measurements is 1 Hz. The sequence used for measurements is as follows. First, the sensor was left to stabilize under a constant flow of dry air (100 ml/min) for a few hours before the first data acquisition. After that, a specific concentration of benzene is introduced and a measurement was taken 1 hour after introducing benzene. Finally, the sensor is cleaned in dry air flow during 6 hours before the last measurement. 4. Results The sensitivity of the microcantilever sensor is obtained from the measurement of the resonance frequency variation due to the adsorption of gas molecules onto the active layer. Fig. 3 shows different measurements for 1 and 10 ppm benzene concentration. The decrease of the first mode resonance peak indicates that the cantilever coated with active carbon adsorbs benzene molecules, which increases the mass of the cantilever. The recovery during the cleaning phase in synthetic air is also observed. 89.5
synthetic air 1 ppm of benzene cleaning 6 hours
89.4
89.2 89.1 89
89.4
10 ppm of benzene cleaning 6 hours
89.2 89.1 89
88.9
88.9
88.8 6.68
synthetic air
89.3
Phase (deg)
Phase (deg)
89.3
89.5
88.8 6.7
6.72
6.74
6.76
Frequency (Hz)
6.78
6.8
6.82 x 10
4
6.68
6.7
6.72
6.74
Frequency (Hz)
6.76 x 10
4
1080 4
R. Vázquez al. / Procedia Engineering 25 (2011) 1077 – 1080 Author nameet/ Procedia Engineering 00 (2011) 000–000
(a)
(b)
Fig. 3. Impedance measurement for the first longitudinal mode of an active carbon coated PZT cantilever at 1 ppm of benzene (a) and 10 ppm of benzene (b).
At 10 ppm there is a decrease of the quality factor. Table 2 shows the quite proportional frequency variation with benzene concentrations. Table 2. Frequency shifts recorded in the presence of benzene for the first 31 longitudinal mode of an active carbon and tin oxide coated PZT cantilever Benzene concentration
Δf (Hz) active carbon
Δf (Hz) tin oxide
1 ppm
-13
-33
10 ppm
-144
-43
5. Conclusions We have shown the possibility of benzene detection at room temperature using the first longitudinal mode of screen-printed ceramic microcantilevers. Cantilevers coated with active carbon are more suitable for detecting benzene, since an almost linear frequency shift is obtained in the 1 to 10 ppm region. Acknowledgements Région Aquitaine (France), Generalitat de Catalunya (Spain) and Gobierno de Navarra (Spain) have supported this research through CTP Research Grants. References [1] T. Thundat, P. I. Oden, R. J. Warmack, ‘Microcantilever sensors’, Microscale Thermophysical Engineering, (1997) 185 – 199 [2] M. Maute, S. Raible, F.E. Prins, D. P. Kern, ‘Fabrication and application of polymer coated cantilevers as gas sensor’, Microelectronic Engineering, 46 (1999) 439 – 442. [3] M. Goeders et al., “Microcantilevers: Sensing Chemical Interations via Mechanical Motion”, Chem. Re., 108, 2008, 522542. [4] K. L. Foster, R. G. Fuerman, J. Economy, S. M. Larson, M. J. Rood, “Adsorption characteristics of trace volatile organic compounds in gas streams onto activated carbon fibers”, Chem. Mat., 1992, 4 (5), 1068-1073. [5] M. Mabrook, P. Hawkins, “A rapidly-responding sensor for benzene, methanol and ethanol vapours on films of titanium dioxide dispersed in a polymer operating at room temperature”, Sensors and Actuators B75, 2001, 197-202. [6] C. Lucat, P. Ginet, C. Castille, H. Debéda and F. Ménil, “Microsystems elements based on free-standingthick-films made with a new sacrificial layer process”, Microelectronics Reliability, 48, 6, 2008, 872-875.