Working Principle of SMO Gas Sensors

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Presentation Description

Explains the basics of Semiconducting Metal Oxides based Gas Sensors.

Comments

By: agnivesh1989 (102 month(s) ago)

thanks but how can i download

By: atomic3 (134 month(s) ago)

HI Mehul, Thanks, but still not able to download - says that presenter is not allowing such. Indeed, I'd not pass it on, but rather use some of it in a techical analysis that I'm preparing for a client. I'll definitely attirbute what I use to you, and will share with you what I write. BTW I am a physicist having spent some 45 yeaers in sensor development/ use, am retired from industry having worked previously for a non-pfrofit, the Department of Defense, industry,, and univeristies, and am doing consulting now. MIght I ask the nature of your involvement in SMOs? Best regards, John John P Carrico johnpcarrico9@msn.,com

Presentation Transcript

Working Principle of Semi-conducting Gas Sensors : 

Working Principle of Semi-conducting Gas Sensors

Outline of Presentation : 

Outline of Presentation Applications of Gas Sensors & Emerging Markets Features of SMO Gas Sensors History Basis of MO Semiconductor Gas Sensors Effect of Reducing & Oxidizing Gases Why Metal-Oxides? Sensor Mechanism parameters Influence of Catalysts Importance of nano-size particles Important Characterization parameters Few examples

Applications of Gas Sensors : 

Applications of Gas Sensors Safety Applications*

Slide 4: 

Environmental Control* *FiS Sensors,Japan.

Emerging Markets : 

Emerging Markets Sensing of O2 containing CO2,NOx & SO2. Detection of N2O & Ethylene- -oxide in hospitals. Chemical reagents like NH3, HNO3, HF, & PH3 in silicon machining industries. Auto-damper Systems (ADS) for car ventilation. House-hold applications.

Features of SMO Gas Sensors : 

Features of SMO Gas Sensors  Small Size,  High Sensitivity,  Low Price,  Long Life- Around 10 years compared to,  Maintenance-free 1-2 yrs for catalytic/electrochemical types,  Feasibility to combine control circuit, signal-conditioning.

History : 

History Brattain & Bradeen in 1953 demonstrated change in ‘Contact Potential’ based on changes in ambient conditions.* Oxygen, water vapor, OH radicals, alcohol,etc were found to effect the contact potential. In 1962, Seiyama demonstrated a ZnO film gas sensor based on adsorption & desorption of gases.** Also he found that the process was affective at temperatures near 400C. *- The Bell System Technical Journal,January 1953. **- Analytical Chemistry,Vol 34,No 11.

Basis of MO Semiconductor Gas Sensors : 

Basis of MO Semiconductor Gas Sensors Semi-conducting gas sensors basically operate on bulk changes & surface changes mechanism. 1st class sensitive to only oxygen partial pressure & oxide defect chemistry is of central importance. Operate in the temp range of 700C & above. Typically they are used as Oxygen Sensors in exhausts of combustion engines. Adsorption-desoprtion of Oxygen on the surface the driving mechanism for 2nd class. O2 can be thought of as a surface trap for electrons.

Slide 10: 

Oxygen adsorbates will build a space-charge region i.e electron-depleted layer. Reversible change in resistance based on O2 adsorption-desorption. In temperature range 300-500C can be used to detect impurity/minority gases. The temperature dependence is due to differing stabilities of oxygen- species & different oxidizing temperatures of gases. Why only adsorption of Oxygen is so significant?

Slide 11: 

Oxygen adsorbates will build a space-charge region i.e electron-depleted layer. Reversible change in resistance based on O2 adsorption-desorption. In temperature range 300-500C can be used to detect impurity/minority gases. The temperature dependence is due to differing stabilities of oxygen- species & different oxidizing temperatures of gases. Why only adsorption of Oxygen is so significant?

Effect of Reducing & Oxidizing Gases : 

Effect of Reducing & Oxidizing Gases Space-charge layer will be over the grain boundary area. Consider the following reaction: 2CO + O2-  2CO2 + e- CO will react with the adsorbed O2 species & donate an electron to the surface( reduction process). The surface could be n-type or p-type & determines the change in resistance.

Slide 14: 

Reducing Gases: H2, Hydrocarbons, CO. Oxidizing Gases: Cl, NO2. Oxidizing agents normally don’t react with Oxygen species', chemisorption more dominant. Humidity interference is also a concern. Products of catalytic reactions can accumulate & cause surface poisoning.

Why Metal Oxides? : 

Why Metal Oxides? Certain kinds of surface reactions are to be avoided. Growth of oxide layer, etching of the surface. High operating temperatures & some MOs exhibit stoichiometry changes. Electronic Structure of Metal oxides also important.* Transitional MO with filled d0 & d10 electronic configurations imp for gas sensing applications. Post-transitional MOs like ZnO & SnO2 have filled d10 configurations. d0 configuration found in binary MO likes TiO2,V2O5,WO3. *- G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23

Sensing Mechanism Parameters : 

Sensing Mechanism Parameters Sensing mechanism based on*: i) Receptor Mechanism & ii) Transducer Mechanism. Receptor function based on the  Surface properties  Diffusion of the analyte gas**  Adsorption/Desorption Parameters Transducer function based on interaction with the particles.  Oxidation of analyte gas  Barrrier Height at grain boundaries  Grain Size  Film Thickness**  Band Gap *G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23 **- S&A B 80 (2001) 125-131

Slide 17: 

Parameters for Characterization

Influence of Catalysts : 

Influence of Catalysts Catalysts improve speed of response & selectivity. Two main mechanisms through which catalysts influence sensing mechanism:  Spill Over.  Fermi Energy Control. Normally noble metals like Palladium & Platinum used as catalysts. Why materials like Platinum only?

Slide 19: 

As illustrated, the catalyst will dissociate the molecule & then the atoms will ‘spill over’ the surface.

Slide 20: 

‘Fermi Energy Control’ simply means that the adsorbed oxygen will remove electrons from the catalyst & catalyst will effectively remove from the SC film.

Importance of nano-sized particles. : 

Importance of nano-sized particles. Possible states of materials  Amorphous, glass-state, nanocrystalline, polycrystalline & single crystalline. Nano & polycrystalline form are the most suitable forms. Amorphous & Glassy state are unstable at high temperatures. High cost & difficulties for single crystalline material manufacture limit their applications.

Slide 22: 

Specific surface area increases with a decrease in grain size. Comparability of the grain size (D) with space-charge layer (Ls) increases the sensitivity.* *N. Yamazoe, Sens. Actuators B: Chem. 5 (1991) 7–18.

Examples : 

Examples http://www.grc.nasa.gov/WWW/chemsensors/

Slide 24: 

http://www.hwsensor.com/English/products/sensor/Semiconductor.htm

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