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Gas chemical sensors
Yusheng (Alvin) ZhouSchool of Materials Science and
Engieering04/07/2014
ECE 6200
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Outline
Background
Working mechanisms of gas sensors
Structures of MEMS based sensors
Advanced nanostructure to enhance sensitivity
References
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Gas sensors and their applications
• Combustible & Flammable (eg. CxHx, H2, CO)
• Toxic (eg. NH3, H2S, NOx)
• Oxygen (O2)
• Pollution (eg. Freon, SO2, CO2)
Gas sensors measure the concentration of certain gases
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Working mechanisms
Mechanisms
Optical Absorbance, reflectance, luminescence, fluorescence, refractive index etc.
Electrochemical Electrochemical reactions occurring at a sensing electrode and changes the electric signal in proportional to concentration
Electrical The change of electrical properties caused by the interaction of the analyte. Ig. carrier concentration in semiconductors
Mechanical (Mass sensitive)
Transform the mass change of the gas to a change in the sensor’s mechanical properties. Eg. resonant frequency of a cantilever beam
Others Thermoconductivity, magnetic, thermoelectric, etc.
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Optical—infrared gas sensing
• Pros: very high selectivity • Cons: complex structure, high cost
Each gas specie has its own absorption spectrum
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Electrochemical-- solid-state galvanic cells
• Pros: low power requirements, excellent repeatability and accuracy• Cons: Solid electrolyte materials (ionic conductors ) are not
commonly used in microfabrication, short life span
Na + NO2 + 0.5 O2 = NaNO3
(350°C)
Generic structure NO2 sensor
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Electrical—conductivity or work function
•Semiconductor metal oxide sensor
(Eg. Pd, Pt)•Metal–insulator–semiconductor transistor
•Pros: Simpler fabrication, high sensitivity, low cost•Cons: Longer response time and high operating temperature (300-500 C)
“c” is the concentration of the target gas, “K” is a measurement constant, and “n” has values between 0.3 and 0.8. The positive sign is used for oxidizing gases and the negative sign for reducing gases.
(Eg. SnO2)
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Mechanical– cantilever mass sensor
Materials from bottom to top:SiO2, Si3N4, Ti/Pt, PZT, Pt/Ti, SiO2 and Au
Zeolite (MFI) is deposited for detecting Freon
Pros: Can be potentially highly sensitive with smaller beamCons: Needs external system to detect cantilever deflection
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Advanced structures for electrical approaches
Nanostructures (CNT, nanowires ) based (Conductivity or FET)
Porous silicon
Large surface area enhance the gas absorption and thus sensitivity (below 1 ppm)
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References• T. Hübert, et al. Hydrogen sensors – A review, Sensors and Actuators B, 157 (2011) 329–352• M. Noro, K. Suzuki, N. Kishi, H. Hara, T. Watanabe, H. Iwaoka,CO2/H2O gas sensor using a tunable
Fabry-Perot filter with wide wavelength range, in: Proceedings of the 16th IEEE Annual International Conference on Micro Electro Mechanical Systems, Kyoto,Japan, January 19–23, 2003, pp. 319–322.
• Karin Potje-Kamloth, Semiconductor Junction Gas Sensors. Chemical Reviews, 2008, Vol. 108, No. 2• MEMS based sensor for in-door environment applications• G. Korotcenkov & B. K. Cho Porous Semiconductors: Advanced Material for Gas Sensor Applications,
Critical Reviews in Solid State and Materials Sciences, 35:1, 1-37, (2010) • N. S. Ramgir, Y. Yang and M. Zacharias. Nanowire-Based Sensors. Small 2010, 6, No. 16, 1705–
1722• Eric L. Brosha, Rangachary Mukundan, and Fernando H. Garzon. YSZ-Based Mixed Potential
Sensors for the Detection of Explosives. Electrochemical and Solid-State Letters, 11,12, 92-95 2008• Dentcho V. Ivanov. Advanced Sensors for Multifunctional Applications. JOM-e, 52 (10) (2000)• Marisa Luisa Grilli, etc. Electrochemical NO x Sensors Based on Interfacing Nanosized
LaFeO3Perovskite-Type Oxide and Ionic Conductors J. Electrochem. Soc. 2001, 148, 9, H98-H102• Jia Zhou, etc. Self-excited piezoelectric microcantilever for gas detection. Microelectronic
Engineering, Volume 69, Issue 1, August 2003, Pages 37–46
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QUESTIONS ?
THANK YOU!
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Mechanical– Acoustic gas sensors
Cons: very vulnerable to interference from coexisting gases, including humidity.