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Development of Gas Sensors on Microstrip Disk Resonators
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 87 (2014) 1083 – 1086
EUROSENSORS 2014, the XXVIII edition of the conference series
Development of gas sensors on microstrip disk resonators Davide Aloisio and Nicola Donato* Department of Electronic Engineering, Chemistry and Industrial Engineering, University of Messina, Messina, Italy
Abstract Here it is reported about the development of microwave disk resonators and their employment as transducers in gas sensing devices. The sensors were developed by designing, modeling and realizing microstrip resonators on several microwave substrates, and by depositing sensing films on the disk. The transduction mechanism can be explained through the change in the permittivity value, in a low thickness of the sensing film, due to the interaction with the gas target. Furthermore, due to the microwave penetration on the sensing material, the response depends on volume effect, not only on surface one. These devices own intrinsic stability due to the fact that the resonance frequency value is related to the physical dimensions. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-reviewunder underresponsibility responsibility scientific committee of Eurosensors Peer-review of of thethe scientific committee of Eurosensors 2014 2014. Keywords: Gas Sensors; Microstrip Resonators;
1. Introduction The research of new topologies of sensors with increasing low power consumption features is today one of the areas of greatest interest in the market. Low power sensors can be easily connected in sensors networks, with the right balance between sensor performance and battery lifetime. In such a frame relatively new category of sensors can be represented by microwave devices with interesting properties in terms of fast response, really low power, fully compatibility with wireless technologies and room temperature operating value. These devices can be included in conductometric transducers category, with a slightly different mechanism of transduction than traditional ones, because in this case change of permittivity of sensitive layer is involved in the transduction process. So the adsorption of molecules on the surface of the sensing layer and correspondent variation of the permittivity is a
* Corresponding author. Tel.: +390903977502; fax: +390903977464. E-mail address: [email protected]
1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi:10.1016/j.proeng.2014.11.351
1084
Davide Aloisio and Nicola Donato / Procedia Engineering 87 (2014) 1083 – 1086
phenomenon which operates only in the second order as a conductometric transducer [1]. In particular the possibility to balance between the sensing material properties and the resonator configuration for design of the sensor make them very versatile for different applications. Microstrip technology, widely employed in the design of microwave resonators and filters, can be successfully used in the development of such sensors. In this work a resonant microstrip structure with a circular disk geometry was investigated coating it with a sensitive layer. This structure is well known in microwave classic literature and can constitute a good starting point for new sensing layers characterization. 2. Experiments The devices realized in this work show a resonant behavior due to the particular electromagnetic structure realized on a microstrip configuration: a transmission line appropriately sized, knowing substrate parameters, is capacitively coupled through a gap to a circular disc of conductive material whose radius is intimately connected to field modes for which it is possible to achieve a resonance phenomenon. The reason why this phenomenon occurs can be obtained by developing Maxwell's equations applied to the miscrostrip geometry with appropriate assumptions on the boundary conditions. An approximate analysis is carried out by treating the system as a cavity formed by the printed disk conductor and the ground plane. The conductor is assumed to be perfect and magnetic wall condition is applied to the boundary of the disk. Also electric field does not depend on component orthogonal to the disk and magnetic field is assumed zero for this direction, thus giving as solution tyipical TM modes [2]. However, in our approach, for a proper design of the resonator, a preliminar CAD-based simulation was carried out to obtain wanted dimensions and resonant frequency. In this case is important to set the FEM solver, with appropriate boundary condition and grid mesh, to obtain accurate results. In Fig.1 (a), (b) are reported one of the simulated resonator configuration and a typical simulation results.
Fig. 1. A simulated configuration (a) and a typical response (b) Accordingly with the geometry ensuring the motion of an electromagnetic wave in the microwave range through the sensing layer, the reflected wave on the material should be modified, namely attenuated and/or out of phase. The real part of the permittivity (ε’) is linked to a capacitive effect of molecules whereas the imaginary part of permittivity (ε’’) is linked to conductivity. With a CAD electromagnetic simulator it is possible to evaluate small changes in frequency behavior when a layer, modeled with proper dielectric parameters, is added over the structure. So the simulation is really a good approach as a first design step. However, the final response towards gas of the sensor realized can be only obtained a posteriori by measuring the real reflection coefficient of the device with the VNA, Fig.3.
ܵͳͳ ൌ Ȟሺ݂ሻ ൌ
ܴ݂݈݁݁ܿ݁ݒܽݓ݀݁ݐ ݁ݒܽݓݐ݊݁݀݅ܿ݊ܫ
The developed devices, with operating frequency values in a range spanning from 4 to 6 GHz, were characterized by measuring S11 parameter (reflection coefficient) with an Agilent VNA 8753ES. The resonator structure was developed by employing an Arlon 25 N 30 substrate with 35 Pm copper layers. The radius of the disk resonator was dimensioned by using the approximated formula for the calculation of the resonant frequency value of the TM dominant mode, given by:
Davide Aloisio and Nicola Donato / Procedia Engineering 87 (2014) 1083 – 1086
݂ͳͳͲ ൌ
1085
ͳǤͺͶͳܿ ʹߨݎξ߳ݎ
Where f is the resonant frequency of the resonator, c is the speed of light in vacuum, r is the radius of the disk, and ϵr is the relative dielectric constant of the substrate.
Fig. 2. Layouts of the developed resonators. An increasing/decreasing of resonator diameter value brings to a decreasing/increasing of the resonance frequency. A 50Ω microstrip/coaxial line was designed with the aid of the CAD tool to model a reliable connection path with the resonator, an SMA connector was mounted on the input microstrip line to allow the connection with the VNA. The chosen structure was then realized by means of LPKF Protomat S103 prototyping system. A sensing film was then deposited on the resonator layer by inkjet printing of an aqueous solution of Poly-(diallyldimethylammonium chloride) (PDDAC) and MWCNTs. The sensing materials generally adopted can be MWCNT-based films [3] and metal oxides, due to the large variation in the dielectric properties for gas adsorption [4]. In particular the whole frequency domain have a strong relationship with the chemical nature of gases and films and sensitivity can heavily change working in different frequency regions.
gas
gas gas
Incident wave
Sensing film
Reflected wave Fig. 3. Typical resonator configuration working as sensor. 3. Results The prototype was characterized towards ammonia vapors at several concentration values in a range spanning from 0% to 25 %. The samples were placed in a Teflon test chamber with controlled atmosphere. The concentration values are set by means of a fully automated gas control system equipped with certified gas bottles, permeation tubes and a bubbler in a thermostatic bath. In Fig. 4 it is shown how the presence of ammonia brings to a decreasing of the resonance frequency value of the device, In Fig. 5 is reported the resonance frequency shift and the real and imaginary part of S11 vs ammonia at room temperature.
1086
Davide Aloisio and Nicola Donato / Procedia Engineering 87 (2014) 1083 – 1086 _
_
200
0.7
0 (Deg)
0.65 0
32
16
4
24
8
16 (Deg)
0.6 0.55 0.5
24 (Deg)
100
32 (Deg) 4 (Deg)
Increasing of NH3 concentration
8 (Deg)
0.45
0
Increasing of NH3 concentration
0.4 0.35
-100
0.3 0.25 0.2
-200
5.49
5.52
5.55
5.49
5.52
Frequency (GHz)
5.55
Frequency (GHz)
Fig.4. S11 Magnitude (a) and Phase (b) of a Microwave resonant sensor vs. ammonia concentration at 25°C.
5
Equation
y = a + b*x
Weight
No Weighting
-0.18
0.99409
Adj. R-Square
Value
3
Frequency
Intercept
Frequency
Slope
Standard Erro
-0.07662
0.05822
0.11193
0.00431
-0.20
Im(S11)
'F(MHz)
4
-0.16
0.02069
Residual Sum of Squares
2 1 PDDAC+MWCNTs Linear Fit of Frequency
0 -5
0
5
10
15
20
25
30
35
-0.22 -0.24 -0.26 -0.28
increasing of NH3 concentration -0.17
Concentration (%)
-0.16
-0.15
-0.14
-0.13
-0.12
-0.11
Real(S11)
Fig. 5. (a) Frequency shift and (b) real and imaginary part of S11 vs. ammonia concentration at 25°C respectively. 4. Conclusions In this work is reported the development and characterization of microstrip disk resonators and their employment as transducers in gas sensing devices. The sensing films are deposited on resonators by inkjet printing of water based solutions of PDDAC and MWCNTs. The so-realized device is able to detect ammonia vapour in a concentration range up to 25%. The promising results suggest further activities, in order to validate and relate the variation of the response to the change of permittivity of the sensing layer, and to investigate sensing properties of other topologies of microwave resonators and filters. Acknowledgements MUR PON 01-01322-PANREX funding is gratefully acknowledged. References [1] G. Barochi, J. Rossignol, M. Bouvet, Development of microwave gas sensors, Sensors and Actuators B 157 (2011) 374– 379. [2] R. E. Collins, Foundations for microwave engineering, Wiley-Interscience IEEE press, second edition (2001) 490-499. [3] M. P. McGrath and A. Pham, Carbon Nanotube Based Microwave Resonator Gas Sensors, Nanotube and nanowires, selected topics in electronics and systems vol. 44 (2007) 31-49. [4] J. Jouhannaud, J. Rossignol and D. Stuerga, Metal oxide-based gas sensor and microwave broad-band measurements: an innovative approach to gas sensing, Sensors and their Applications XIV (SENSORS07) Journal of Physics Conference Series 76 (2007).
ScienceDirect Procedia Engineering 87 (2014) 1083 – 1086
EUROSENSORS 2014, the XXVIII edition of the conference series
Development of gas sensors on microstrip disk resonators Davide Aloisio and Nicola Donato* Department of Electronic Engineering, Chemistry and Industrial Engineering, University of Messina, Messina, Italy
Abstract Here it is reported about the development of microwave disk resonators and their employment as transducers in gas sensing devices. The sensors were developed by designing, modeling and realizing microstrip resonators on several microwave substrates, and by depositing sensing films on the disk. The transduction mechanism can be explained through the change in the permittivity value, in a low thickness of the sensing film, due to the interaction with the gas target. Furthermore, due to the microwave penetration on the sensing material, the response depends on volume effect, not only on surface one. These devices own intrinsic stability due to the fact that the resonance frequency value is related to the physical dimensions. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-reviewunder underresponsibility responsibility scientific committee of Eurosensors Peer-review of of thethe scientific committee of Eurosensors 2014 2014. Keywords: Gas Sensors; Microstrip Resonators;
1. Introduction The research of new topologies of sensors with increasing low power consumption features is today one of the areas of greatest interest in the market. Low power sensors can be easily connected in sensors networks, with the right balance between sensor performance and battery lifetime. In such a frame relatively new category of sensors can be represented by microwave devices with interesting properties in terms of fast response, really low power, fully compatibility with wireless technologies and room temperature operating value. These devices can be included in conductometric transducers category, with a slightly different mechanism of transduction than traditional ones, because in this case change of permittivity of sensitive layer is involved in the transduction process. So the adsorption of molecules on the surface of the sensing layer and correspondent variation of the permittivity is a
* Corresponding author. Tel.: +390903977502; fax: +390903977464. E-mail address: [email protected]
1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi:10.1016/j.proeng.2014.11.351
1084
Davide Aloisio and Nicola Donato / Procedia Engineering 87 (2014) 1083 – 1086
phenomenon which operates only in the second order as a conductometric transducer [1]. In particular the possibility to balance between the sensing material properties and the resonator configuration for design of the sensor make them very versatile for different applications. Microstrip technology, widely employed in the design of microwave resonators and filters, can be successfully used in the development of such sensors. In this work a resonant microstrip structure with a circular disk geometry was investigated coating it with a sensitive layer. This structure is well known in microwave classic literature and can constitute a good starting point for new sensing layers characterization. 2. Experiments The devices realized in this work show a resonant behavior due to the particular electromagnetic structure realized on a microstrip configuration: a transmission line appropriately sized, knowing substrate parameters, is capacitively coupled through a gap to a circular disc of conductive material whose radius is intimately connected to field modes for which it is possible to achieve a resonance phenomenon. The reason why this phenomenon occurs can be obtained by developing Maxwell's equations applied to the miscrostrip geometry with appropriate assumptions on the boundary conditions. An approximate analysis is carried out by treating the system as a cavity formed by the printed disk conductor and the ground plane. The conductor is assumed to be perfect and magnetic wall condition is applied to the boundary of the disk. Also electric field does not depend on component orthogonal to the disk and magnetic field is assumed zero for this direction, thus giving as solution tyipical TM modes [2]. However, in our approach, for a proper design of the resonator, a preliminar CAD-based simulation was carried out to obtain wanted dimensions and resonant frequency. In this case is important to set the FEM solver, with appropriate boundary condition and grid mesh, to obtain accurate results. In Fig.1 (a), (b) are reported one of the simulated resonator configuration and a typical simulation results.
Fig. 1. A simulated configuration (a) and a typical response (b) Accordingly with the geometry ensuring the motion of an electromagnetic wave in the microwave range through the sensing layer, the reflected wave on the material should be modified, namely attenuated and/or out of phase. The real part of the permittivity (ε’) is linked to a capacitive effect of molecules whereas the imaginary part of permittivity (ε’’) is linked to conductivity. With a CAD electromagnetic simulator it is possible to evaluate small changes in frequency behavior when a layer, modeled with proper dielectric parameters, is added over the structure. So the simulation is really a good approach as a first design step. However, the final response towards gas of the sensor realized can be only obtained a posteriori by measuring the real reflection coefficient of the device with the VNA, Fig.3.
ܵͳͳ ൌ Ȟሺ݂ሻ ൌ
ܴ݂݈݁݁ܿ݁ݒܽݓ݀݁ݐ ݁ݒܽݓݐ݊݁݀݅ܿ݊ܫ
The developed devices, with operating frequency values in a range spanning from 4 to 6 GHz, were characterized by measuring S11 parameter (reflection coefficient) with an Agilent VNA 8753ES. The resonator structure was developed by employing an Arlon 25 N 30 substrate with 35 Pm copper layers. The radius of the disk resonator was dimensioned by using the approximated formula for the calculation of the resonant frequency value of the TM dominant mode, given by:
Davide Aloisio and Nicola Donato / Procedia Engineering 87 (2014) 1083 – 1086
݂ͳͳͲ ൌ
1085
ͳǤͺͶͳܿ ʹߨݎξ߳ݎ
Where f is the resonant frequency of the resonator, c is the speed of light in vacuum, r is the radius of the disk, and ϵr is the relative dielectric constant of the substrate.
Fig. 2. Layouts of the developed resonators. An increasing/decreasing of resonator diameter value brings to a decreasing/increasing of the resonance frequency. A 50Ω microstrip/coaxial line was designed with the aid of the CAD tool to model a reliable connection path with the resonator, an SMA connector was mounted on the input microstrip line to allow the connection with the VNA. The chosen structure was then realized by means of LPKF Protomat S103 prototyping system. A sensing film was then deposited on the resonator layer by inkjet printing of an aqueous solution of Poly-(diallyldimethylammonium chloride) (PDDAC) and MWCNTs. The sensing materials generally adopted can be MWCNT-based films [3] and metal oxides, due to the large variation in the dielectric properties for gas adsorption [4]. In particular the whole frequency domain have a strong relationship with the chemical nature of gases and films and sensitivity can heavily change working in different frequency regions.
gas
gas gas
Incident wave
Sensing film
Reflected wave Fig. 3. Typical resonator configuration working as sensor. 3. Results The prototype was characterized towards ammonia vapors at several concentration values in a range spanning from 0% to 25 %. The samples were placed in a Teflon test chamber with controlled atmosphere. The concentration values are set by means of a fully automated gas control system equipped with certified gas bottles, permeation tubes and a bubbler in a thermostatic bath. In Fig. 4 it is shown how the presence of ammonia brings to a decreasing of the resonance frequency value of the device, In Fig. 5 is reported the resonance frequency shift and the real and imaginary part of S11 vs ammonia at room temperature.
1086
Davide Aloisio and Nicola Donato / Procedia Engineering 87 (2014) 1083 – 1086 _
_
200
0.7
0 (Deg)
0.65 0
32
16
4
24
8
16 (Deg)
0.6 0.55 0.5
24 (Deg)
100
32 (Deg) 4 (Deg)
Increasing of NH3 concentration
8 (Deg)
0.45
0
Increasing of NH3 concentration
0.4 0.35
-100
0.3 0.25 0.2
-200
5.49
5.52
5.55
5.49
5.52
Frequency (GHz)
5.55
Frequency (GHz)
Fig.4. S11 Magnitude (a) and Phase (b) of a Microwave resonant sensor vs. ammonia concentration at 25°C.
5
Equation
y = a + b*x
Weight
No Weighting
-0.18
0.99409
Adj. R-Square
Value
3
Frequency
Intercept
Frequency
Slope
Standard Erro
-0.07662
0.05822
0.11193
0.00431
-0.20
Im(S11)
'F(MHz)
4
-0.16
0.02069
Residual Sum of Squares
2 1 PDDAC+MWCNTs Linear Fit of Frequency
0 -5
0
5
10
15
20
25
30
35
-0.22 -0.24 -0.26 -0.28
increasing of NH3 concentration -0.17
Concentration (%)
-0.16
-0.15
-0.14
-0.13
-0.12
-0.11
Real(S11)
Fig. 5. (a) Frequency shift and (b) real and imaginary part of S11 vs. ammonia concentration at 25°C respectively. 4. Conclusions In this work is reported the development and characterization of microstrip disk resonators and their employment as transducers in gas sensing devices. The sensing films are deposited on resonators by inkjet printing of water based solutions of PDDAC and MWCNTs. The so-realized device is able to detect ammonia vapour in a concentration range up to 25%. The promising results suggest further activities, in order to validate and relate the variation of the response to the change of permittivity of the sensing layer, and to investigate sensing properties of other topologies of microwave resonators and filters. Acknowledgements MUR PON 01-01322-PANREX funding is gratefully acknowledged. References [1] G. Barochi, J. Rossignol, M. Bouvet, Development of microwave gas sensors, Sensors and Actuators B 157 (2011) 374– 379. [2] R. E. Collins, Foundations for microwave engineering, Wiley-Interscience IEEE press, second edition (2001) 490-499. [3] M. P. McGrath and A. Pham, Carbon Nanotube Based Microwave Resonator Gas Sensors, Nanotube and nanowires, selected topics in electronics and systems vol. 44 (2007) 31-49. [4] J. Jouhannaud, J. Rossignol and D. Stuerga, Metal oxide-based gas sensor and microwave broad-band measurements: an innovative approach to gas sensing, Sensors and their Applications XIV (SENSORS07) Journal of Physics Conference Series 76 (2007).