Electrochemical Gas Sensors

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

2. 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 ECEN 5004 – Digital Packaging

3. Introduction – Automotive O2 Sensors • Invented by Bosch (1976) • First used by Volvo (1976) • Introduced to U.S. (1980) • Required in Europe (1993) • Most widely used application • Detects O2 concentration in exhaust stream • Promotes cleaner burning fuel/air mixture • Reduces overall pollution ECEN 5004 – Digital Packaging

4. Introduction – Airport/Toxin Detection GE EntryScan3 Toxic Gas Sensors ECEN 5004 – Digital Packaging

5. Introduction – Natural Gas Detection MythBusters ‘Flatus Catcher’ ECEN 5004 – Digital Packaging

6. 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 won’t kill you • Proved flatus is flammable • Proved ‘pretty girls’ do produce flatus ECEN 5004 – Digital Packaging

7. 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 ECEN 5004 – Digital Packaging

8. 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 ECEN 5004 – Digital Packaging

9. Operation Typical Electrochemical Gas Sensor Structure ECEN 5004 – Digital Packaging

10. 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- ECEN 5004 – Digital Packaging

11. 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 ECEN 5004 – Digital Packaging

12. 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 ECEN 5004 – Digital Packaging

13. Fabrication – CVD Films CVD Deposited SnO2 Film ECEN 5004 – Digital Packaging

14. Fabrication – PVD Films PVD Deposited SnO2 Film (Actual Journal image) • Physical Vapor Deposition (PVD) • Solid/Gas-phase technique • Block of SnO2 heated to vaporization (thermal evap.) • Films are irregular and non-uniform ECEN 5004 – Digital Packaging

15. Fabrication – Wet Chemistry Films WCD Deposited SnO2 Film (speaks for itself) • Wet Chemical Deposition (WCD) • a.k.a. ‘Sol-gel’ • Substrate submersed in solution to form SnO2 ECEN 5004 – Digital Packaging

16. Fabrication – ALD Films ALD Deposited SnO2 Film (on Al nanoparticles) • Atomic Layer Deposition (ALD) • Conformal, uniformly-deposited SnO2 thin films • Deposition rate precisely controlled ECEN 5004 – Digital Packaging

17. Fabrication – ALD Films ALD Deposited Al2O3 Film (on Ni particle) ECEN 5004 – Digital Packaging

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

19. 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 ECEN 5004 – Digital Packaging

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

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

22. 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 ECEN 5004 – Digital Packaging

23. Sn-plated Cu surface in need of a shave ECEN 5004 – Digital Packaging

24. Packaging Considerations Typical gas sensor packages ECEN 5004 – Digital Packaging

25. Typical Sensitivities * More corrosive/reactive gases tend to have higher sensitivity sensors ECEN 5004 – Digital Packaging

26. 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 ECEN 5004 – Digital Packaging

27. Alliance, Nebraska