SENS-ERA project, 7/12/2012

Metal-oxide Semiconductor Gas Sensors

Dimitris Davazoglou

NCSR “Demokritos”, Institute of Advanced Materials, Physico-Chemical

Processes and Micro- Nano- Electronics

SENS-ERA project, 7/12/2012

I. INTRODUCTION Applications – Gas Sensors

II. CATALYTIC MATERIALS Metallic oxides

III. RELATED PHENOMENA Resistance variations, gas-chromism, electro-chromism

IV. APPLICATIONS 1. In shipyards, 2. Food, 3. Seamless sensor-display systems

SENS-ERA project, 7/12/2012

I. INTRODUCTION

APPLICATIONS OF GAS SENSORS AND TARGET GASES

MINES: (Methane, Flammable and Explosive Hydrocarbons)SHIPYARDS: (Same)SPACE: (Hydrogen)MILITARY: (Corrosive, Bio, …)SAFETY: Airports (Explosives, Bio, …) Subways (Same) Public Buildings (Same)CHEMICAL Industry (Hydrogen, VOCs, …)OIL Industry (Same)FOOD Industry (VOCs, Ammoniac, Amines, Hydrogen Sulfide, …_DOMESTIC: Air Conditioners (Water Vapors, COx, CFCs, …) Microwave Ovens (VOCs, Vapors)AUTOMOBILE: (COx, NOx, …)

SENS-ERA project, 7/12/2012

FIRST GAS DETECTORSCanaries, Controlled combustion, Heated Platinum Wires

THE PELLISTOR

Resistance Measurement

Problem: High Power Consumption ( 1-10 W) ̴̴��

SENS-ERA project, 7/12/2012

MINIATURIZATION OF DEVICES USING SCREEN PRINTING

Power Consumption: Down to 50 mW

Resistance of the sensing element affected by the chemical environment. Necessity of high temperature (150-350 oC).

SENS-ERA project, 7/12/2012

Power Consumption: Down to 5-10 mW

Still very high for many applications (e.g., in safety and fire detection systems 1 mW or less desired)

IN ALL DEVICES THE CATALYTIC MATERIAL PLAYS THE MAIN ROLE

FURTHER MINIATURIZATION BY MICROELECTRONIC TECHNIQUES (SURFACE MICROMACHINING)

SENS-ERA project, 7/12/2012

II. CATALYTIC MATERIALS: METAL-OXIDES (SnO2, WO3, MoO3, …) superficially doped with noble metals (functionalized )

W

O Basic Building Block: Octahedron W at the center O at the corners

Octahedra connected by sharing the O ions

Other basic building blocks possible dependent on the oxide (e.g., tetrahedra for TiO2, pyramides for MoO3, etc.). All kinds of building blocks are connected by sharing the oxide ions in various ways (corners, acmes, planes).

SENS-ERA project, 7/12/2012

●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●

e-

• Adsorption on surface• Interchange of electrons• Possible gas dissociation • Possible out-diffusion (oxygen loss)• Possible in-diffusion of ions • Desorption

Example: Electronic structure of cubic WO3

Band gap of 3 eV => Insulator, transparent

Surface interaction of a Metal-oxide with a reactive gas phase

SENS-ERA project, 7/12/2012

Influence of the loss of oxygen and (of charge interchange) on the electronic structure of various oxides.They are now conductors. Also, some oxides exhibit coloration.

Influence of the doping on the electronic structure of WO3.

All these processes are reversible. After removal of the gas phase and re-exposure of the oxide to ambient air the electronic structure of the oxide returns to its initial state (insulating and transparent).

SENS-ERA project, 7/12/2012

By monitoring the electrical conductivity => Gas Sensing

By taking advantage of the in-diffusion => Gas-chromic smart windows

An electrochromic Display

By taking advantage of the in-diffusion and electronic interchange => Electro-chromic smart windows and displays

III. RELATED PHENOMENA

SENS-ERA project, 7/12/2012

Characteristics of Gas Sensors

1. Sensitivity = Rs/R0 Rs= Sensor Resistance in various gas concentrations R0= Sensor Resistance in a specific concentration of a target gas

2. Sensor Response.Sensor resistance drops very quickly when exposed to gas, and when removed from gas its resistance will recover to its original value after a short time. The speed of response and reversibility vary according to sensing oxide and the gas involved.3. Selectivity (sensors arrays)4. Initial action5. long term stability6. heater stability7. heater voltage dependency

New catalytic materials needed to improve all the above characteristics

SENS-ERA project, 7/12/2012

IV. APPLICATIONS

1. Detection of Explosive Gases in Shipyards (EPET II Program)

In the tanks of remain hydrocarbons, which during welding explode. Movement in the tank very difficult, so continuous inspection impossible. Proposed solution: Numerous simple sensors positioned in the tank able to emit IR when triggered. IR cannot escape from the metallic tank so it will be detected by one (expensive) IR detector to give an alarm.

IR LED

IR Detector Tank (all metal)

SENS-ERA project, 7/12/2012

2. Electronic Detection of Fish Freshness (EPET II Program)

A Sensing system was devised to detect Ammoniac and other Amines based on WO3 and In2O3. Ammoniac seemed to be the most efficient indicator.

Many kinds of fishes were tested by measuring the emitted gases with standard chemical analysis and with electronic gas sensors for a time period of 5 days. Excellent correlations between the results of the chemical analysis and of the electronic detection were obtained.

SENS-ERA project, 7/12/2012

3. A seamless System Gas Sensor-Display based on WO3 films (GoodFood Project)

The gas sensor was used as a variable resistor, whose value was dropping in contact with a reducing gas (e.g., butane) so the electrochromic display was turning on.

SENS-ERA project, 7/12/2012

OUTLOOK & CONCLUSIONS

• Metal-Oxide Gas Sensors mainly based on SnO2 are mature devices• Open questions: Power, selectivity • Many other oxides remain to be tested• Many deposition methods remain to be tested (low temperature, large area) • The rapidly developing technologies of large area and transparent electronics

offer new opportunities for the further development of this technology with the vision of seamless systems integrating sensors, electronics and displays based on the same metal-oxide film.

• The number of possible applications is limited only by our imagination (food, vehicles, packages, clothing, glazing, portable electronic devices, …)

SENS-ERA project, 7/12/2012

THANK YOU FOR YOUR ATTENTION