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.