Electrochemical Gas Sensors ECEN 5004 – Digital Packaging Mike Weimer Graduate Research Project.
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Transcript of Electrochemical Gas Sensors ECEN 5004 – Digital Packaging Mike Weimer Graduate Research Project.
Electrochemical Gas Sensors
ECEN 5004 – Digital Packaging
Mike Weimer
Graduate Research Project
IntroductionGas 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 automobilesECEN 5004 – Digital Packaging
Introduction – Automotive O2 SensorsMost widely used application
Detects O2 concentration in exhaust streamPromotes cleaner burning fuel/air mixtureReduces overall pollution
ECEN 5004 – Digital Packaging
• Invented by Bosch (1976)
• First used by Volvo (1976)
• Introduced to U.S. (1980)
• Required in Europe (1993)
Introduction – Airport/Toxin Detection
ECEN 5004 – Digital Packaging
GE EntryScan3 Toxic Gas Sensors
Introduction – Natural Gas Detection
ECEN 5004 – Digital Packaging
MythBusters ‘Flatus Catcher’
Introduction – Natural Gas Detection
ECEN 5004 – Digital Packaging
MythBusters captured and analyzed ‘flatus’Employed a bathtub-based flatus catcherFlatus 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 won’t kill you
Proved flatus is flammableProved ‘pretty girls’ do produce flatus
Introduction – Natural Gas Detection
ECEN 5004 – Digital Packaging
Useful for Natural Gas furnaces and fireplacesLeak detectionParticularly useful during sleep (not able to
smell)
Radon detection (carcinogen)No odorResponsible for 21,000 lung cancer deaths/yr
(U.S.)Usual prevention is plastic sheeting
OperationIncoming vapor reacts with surface or
electrolyteCauses 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
ECEN 5004 – Digital Packaging
Operation
ECEN 5004 – Digital Packaging
Typical Electrochemical Gas Sensor Structure
Operation – Anodic Reactions
ECEN 5004 – Digital Packaging
[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-
Operation – Cathodic Reactions
ECEN 5004 – Digital Packaging
[O2]: O2 + 4H+ + 4e- 2H2O
[NO2]: NO2 + 2H+ + 2e- NO + H2O
[Cl2]: Cl2 + 2H+ + 2e- 2HCl
[O3]: O3 + 2H+ + 2e- O2 + H2O
FabricationThin films are becoming more prevalent
Resistance measurement on film surfaceSnO2 films are widely used - high surface
reactivity
Chemical Vapor Deposition (CVD)Gas-phase techniquePrecursors introduced simultaneouslyDeposition is controlled by exposure timeFilms are granular and non-uniform
ECEN 5004 – Digital Packaging
Fabrication – CVD Films
ECEN 5004 – Digital Packaging
CVD Deposited SnO2 Film
Fabrication – PVD FilmsPhysical Vapor Deposition (PVD)
Solid/Gas-phase techniqueBlock of SnO2 heated to vaporization (thermal
evap.)Films are irregular and non-uniform
ECEN 5004 – Digital Packaging
PVD Deposited SnO2 Film
(Actual Journal image)
Fabrication – Wet Chemistry FilmsWet Chemical Deposition (WCD)
a.k.a. ‘Sol-gel’Substrate submersed in solution to form SnO2
ECEN 5004 – Digital Packaging
WCD Deposited SnO2 Film
(speaks for itself)
Fabrication – ALD FilmsAtomic Layer Deposition (ALD)
Conformal, uniformly-deposited SnO2 thin films
Deposition rate precisely controlled
ECEN 5004 – Digital Packaging
ALD Deposited SnO2 Film
(on Al nanoparticles)
Fabrication – ALD Films
ECEN 5004 – Digital Packaging
ALD Deposited Al2O3 Film
(on Ni particle)
Fabrication – ALD Films
ECEN 5004 – Digital Packaging
Fluidized Bed ALD Reactor
Fabrication – ALD Films
ECEN 5004 – Digital Packaging
Precursors introduced individuallyPrevent gas-phase reactionsUsually 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 cycleOperates from 250 – 400 °C
Fabrication – ALD Films
ECEN 5004 – Digital Packaging
Electrochemical gas sensors fabricated via ALD have superior electrical propertiesUniform film depositionUniform electrochemical propertiesFree of pinholes
Packaging Considerations
ECEN 5004 – Digital Packaging
Sensor selectivity/sensitivityEnvironmental concerns
Corrosive environment (metals)Oxidizing environmentHumidityTemperature
Electrolyte housingChemical inertness of housing
Sensor lifetime
Packaging Considerations
ECEN 5004 – Digital Packaging
Surface AreaHigher sensitivity = larger surface areaHigher sensitivity = shorter lifetime
Package MaterialPlastics (polyethylene, polypropylene)
Chemically inert, inexpensiveMetals (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
ECEN 5004 – Digital Packaging
Sn-plated Cu surface in need of a shave
Packaging Considerations
ECEN 5004 – Digital Packaging
Typical gas sensor packages
Typical Sensitivities
ECEN 5004 – Digital Packaging
* More corrosive/reactive gases tend to have higher sensitivity sensors
Summary
ECEN 5004 – Digital Packaging
Electrochemical gas sensors widely availableToxic gas sensing, automotive applicationsExplosives sensingFlatus testing
Thin film sensors are the next generationAtomic Layer Deposition (ALD)
High sensitivities achievable with correct packagingChemical inertness of housingTemperature/humidity variations
Sensor lifetime
Alliance, Nebraska