Electrochemical Gas Sensors ECEN 5004 – Digital Packaging Mike Weimer Graduate Research Project

Introduction Gas sensors used in several applications Detection of toxic vapors HCl Cl 2 H 2 S O 3 Explosives/narcotics detection Airport sensors (GE EntryScan3) Police/Government narcotics detection Nuclear detection at U.S. ports Radon / Natural Gas detection (Methyl Mercaptan) O 2 sensors on automobiles ECEN 5004 – Digital Packaging

Introduction – Automotive O 2 Sensors Most widely used application Detects O 2 concentration in exhaust stream Promotes cleaner burning fuel/air mixture Reduces 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 EntryScan3Toxic 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 catcher Flatus contained in a Flatulence Containment Unit (FCU) Methyl Mercaptan (CH 4 S) – Highly Toxic, Highly Smelly Methane (CH 4 ) – Highly Flammable Hydrogen Sulfide (H 2 S) – Flammable and Toxic Proved though ‘toxic,’ flatus inhalation won’t kill you Proved flatus is flammable Proved ‘pretty girls’ do produce flatus

Introduction – Natural Gas Detection ECEN 5004 – Digital Packaging 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

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 1 st Layer: Hydrophobic Membrane 2 nd Layer: Electrodes 3 rd 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 + H 2 O  CO 2 + 2H + + 2e - [H 2 S]: H 2 S + 4H 2 O  H 2 SO 4 + 8H + +8e - [NO]: NO + 2H 2 O  HNO 3 + 3H + + 3e - [H 2 ]: H 2  2H + + 2e - [HCN]: 2HCN + Au  HAu(CN) 2 + H + + e -

Operation – Cathodic Reactions ECEN 5004 – Digital Packaging [O 2 ]: O 2 + 4H + + 4e -  2H 2 O [NO 2 ]: NO 2 + 2H + + 2e -  NO + H 2 O [Cl 2 ]: Cl 2 + 2H + + 2e -  2HCl [O 3 ]: O 3 + 2H + + 2e -  O 2 + H 2 O

Fabrication Thin films are becoming more prevalent Resistance measurement on film surface SnO 2 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 ECEN 5004 – Digital Packaging

Fabrication – CVD Films ECEN 5004 – Digital Packaging CVD Deposited SnO 2 Film

Fabrication – PVD Films Physical Vapor Deposition (PVD) Solid/Gas-phase technique Block of SnO 2 heated to vaporization (thermal evap.) Films are irregular and non-uniform ECEN 5004 – Digital Packaging PVD Deposited SnO 2 Film (Actual Journal image)

Fabrication – Wet Chemistry Films Wet Chemical Deposition (WCD) a.k.a. ‘Sol-gel’ Substrate submersed in solution to form SnO 2 ECEN 5004 – Digital Packaging WCD Deposited SnO 2 Film (speaks for itself)

Fabrication – ALD Films Atomic Layer Deposition (ALD) Conformal, uniformly-deposited SnO 2 thin films Deposition rate precisely controlled ECEN 5004 – Digital Packaging ALD Deposited SnO 2 Film (on Al nanoparticles)

Fabrication – ALD Films ECEN 5004 – Digital Packaging ALD Deposited Al 2 O 3 Film (on Ni particle)

Fabrication – ALD Films ECEN 5004 – Digital Packaging Fluidized Bed ALD Reactor

Fabrication – ALD Films ECEN 5004 – Digital Packaging Precursors introduced individually Prevent gas-phase reactions Usually deposited using SnCl 4 + H 2 O 2 SnOH* + SnCL 4  SnOSnCl 3 * + HCl [A] SnCl* + H 2 O 2  SnOH* + HCl + ½ O 2 [B] Resulting SnO 2 film deposits at ~0.1 nm/AB cycle Operates from 250 – 400 °C

Fabrication – ALD Films ECEN 5004 – Digital Packaging Electrochemical gas sensors fabricated via ALD have superior electrical properties Uniform film deposition Uniform electrochemical properties Free of pinholes

Packaging Considerations ECEN 5004 – Digital Packaging Sensor selectivity/sensitivity Environmental concerns Corrosive environment (metals) Oxidizing environment Humidity Temperature Electrolyte housing Chemical inertness of housing Sensor lifetime

Packaging Considerations ECEN 5004 – Digital Packaging 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

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 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

Alliance, Nebraska