Electrochemical Gas Sensors

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Electrochemical Gas Sensors
Graduate Research Project
Mike Weimer
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Introduction

• Gas sensors used in several applications
• Detection of toxic vapors
• HCl
• Cl2
• H2S
• O3
• Explosives/narcotics detection
• Airport sensors (GE EntryScan3)
• Police/Government narcotics detection
• Nuclear detection at U.S. ports
• Radon / Natural Gas detection (Methyl Mercaptan)
• O2 sensors on automobiles

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Introduction Automotive O2 Sensors

• Most widely used application
• Detects O2 concentration in exhaust stream
• Promotes cleaner burning fuel/air mixture
• Reduces overall pollution

Invented by Bosch (1976) First used by
Volvo (1976) Introduced to U.S. (1980)
Required in Europe (1993)
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Introduction Airport/Toxin Detection
GE EntryScan3
Toxic Gas Sensors
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Introduction Natural Gas Detection
MythBusters Flatus Catcher
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Introduction Natural Gas Detection

• MythBusters captured and analyzed flatus
• Employed a bathtub-based flatus catcher
• Flatus contained in a Flatulence Containment Unit
  (FCU)
• Methyl Mercaptan (CH4S) Highly Toxic, Highly
  Smelly
• Methane (CH4) Highly Flammable
• Hydrogen Sulfide (H2S) Flammable and Toxic
• Proved though toxic, flatus inhalation wont
  kill you
• Proved flatus is flammable
• Proved pretty girls do produce flatus

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Introduction Natural Gas Detection

• Useful for Natural Gas furnaces and fireplaces
• Leak detection
• Particularly useful during sleep (not able to
  smell)
• Radon detection (carcinogen)
• No odor
• Responsible for 21,000 lung cancer deaths/yr
  (U.S.)
• Usual prevention is plastic sheeting

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Operation

• Incoming vapor reacts with surface or electrolyte
• Causes changes in current or resistance
• Current FET-type devices (micro fuel cells)
• Resistance Film-based devices
• Anomalies in current/resistance ? concentration
• Multi-layered design for high sensitivity
• 1st Layer Hydrophobic Membrane
• 2nd Layer Electrodes
• 3rd Layer Electrolyte

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Operation
Typical Electrochemical Gas Sensor Structure
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Operation Anodic Reactions
CO CO H2O ? CO2 2H
2e- H2S H2S 4H2O ? H2SO4 8H 8e-
NO NO 2H2O ? HNO3 3H 3e-
H2 H2 ? 2H 2e- HCN 2HCN
Au ? HAu(CN)2 H e-
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Operation Cathodic Reactions
O2 O2 4H 4e- ? 2H2O NO2
NO2 2H 2e- ? NO H2O Cl2 Cl2
2H 2e- ? 2HCl O3 O3 2H
2e- ? O2 H2O
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Fabrication

• Thin films are becoming more prevalent
• Resistance measurement on film surface
• SnO2 films are widely used - high surface
  reactivity
• Chemical Vapor Deposition (CVD)
• Gas-phase technique
• Precursors introduced simultaneously
• Deposition is controlled by exposure time
• Films are granular and non-uniform

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Fabrication CVD Films
CVD Deposited SnO2 Film
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Fabrication PVD Films

• Physical Vapor Deposition (PVD)
• Solid/Gas-phase technique
• Block of SnO2 heated to vaporization (thermal
  evap.)
• Films are irregular and non-uniform

PVD Deposited SnO2 Film (Actual Journal image)
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Fabrication Wet Chemistry Films

• Wet Chemical Deposition (WCD)
• a.k.a. Sol-gel
• Substrate submersed in solution to form SnO2

WCD Deposited SnO2 Film (speaks for itself)
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Fabrication ALD Films

• Atomic Layer Deposition (ALD)
• Conformal, uniformly-deposited SnO2 thin films
• Deposition rate precisely controlled

ALD Deposited SnO2 Film (on Al nanoparticles)
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Fabrication ALD Films
ALD Deposited Al2O3 Film (on Ni particle)
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Fabrication ALD Films
Fluidized Bed ALD Reactor
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Fabrication ALD Films

• Precursors introduced individually
• Prevent gas-phase reactions
• Usually deposited using SnCl4 H2O2
• SnOH SnCL4 ? SnOSnCl3 HCl
  A
• SnCl H2O2 ? SnOH HCl ½ O2
  B
• Resulting SnO2 film deposits at 0.1 nm/AB cycle
• Operates from 250 400 C

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Fabrication ALD Films

• Electrochemical gas sensors fabricated via ALD
  have superior electrical properties
• Uniform film deposition
• Uniform electrochemical properties
• Free of pinholes

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Packaging Considerations

• Sensor selectivity/sensitivity
• Environmental concerns
• Corrosive environment (metals)
• Oxidizing environment
• Humidity
• Temperature
• Electrolyte housing
• Chemical inertness of housing
• Sensor lifetime

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Packaging Considerations

• Surface Area
• Higher sensitivity larger surface area
• Higher sensitivity shorter lifetime
• Package Material
• Plastics (polyethylene, polypropylene)
• Chemically inert, inexpensive
• Metals (aluminum, tin)
• Lightweight, inexpensive, less porous
• Apparently several metals grow whiskers (even Al)
• Whisker growth inside package can alter
  sensitivity and cause false concentration reports

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Sn-plated Cu surface in need of a shave
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Packaging Considerations
Typical gas sensor packages
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Typical Sensitivities
More corrosive/reactive gases tend to have
higher sensitivity sensors
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Summary

• Electrochemical gas sensors widely available
• Toxic gas sensing, automotive applications
• Explosives sensing
• Flatus testing
• Thin film sensors are the next generation
• Atomic Layer Deposition (ALD)
• High sensitivities achievable with correct
  packaging
• Chemical inertness of housing
• Temperature/humidity variations
• Sensor lifetime

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Alliance, Nebraska