PODUCTS EVIEW Metal oxide (SnO

2

) semiconductor industrial gas sensor The conductivity of Tin dioxide (SnO

2

) metal oxide semiconductor materials changes according to gas concentration changes. This is caused by adsorption/desorption of oxygen and reaction between surface oxygen and gases. These reactions cause a dynamic change of electric potential on SnO

2

crystal and results in the decrease of sensor resistance under the presence of reducing gases such as CO, methane, hydrogen. The figures below indicate the sensing mechanism of the SnO

2

gas sensor.

(Sensing element) (Sensor unit)

Sensing material 2 mm SnO

2

particles: sub-micron to several ten micron Microscopic image of (secondary particles) SnO

2

particles (In the presence of CO) Low conductivity (high resistance)High conductivity (low resistance) O

- -

2

+

CO

‘

CO

2

+

e O

-

+

CO

‘

CO

-

2

+

e O

2-

+

CO

‘

CO

-

2

+

2e

O

Grain boundary

O 2 COCOO 2

O

2 O 2 2 -

O

2 -

O

2 -

O

2 -

O

- -
2 -

O

2 -

O

-
2 -

O

2

O

2

O

-

O

2 -

e

-
2

e

- - -

SnO

e e

-

e

- -

e

-

e

-

e

-

e

-

e

-

e

- -

e

-

e e e

2-

SnO

e

2-

SnO

e

-

e

-

e

- -

e

-

e

-

e

- - 2-

2

industrial crystals(several hundredangstrom ) Sensing element SnO

e

- -

e

-

e

-

e

-

e

-

SnO

2-

SnO

2-

e

-

e

- - -

e

-

e

-

e e e

- - -

e

-

e e

-

e

-

e

-

e

-

SnO

2-

e e e

-

e

-

e e

-

e

- - -

e

-

e

-

e

-

e

-

e

-

O O

e

-
2

O

2

O

-
2 - -

O

-

O

- - -

e

-
2

O O

2 2

O

2 -

O

2 2

O

2 2 COCOO O O O 2 O 2 2 2 O 2 2

O

- - - - 2

+ e

‘

O

2-

O

2-

‘

O

2

+ e O

2

+ e

‘

O

2-

O

2-

‘

O

2

+ e 1/2O

- - - - - - - - 2

+ e

‘

O O

‘

1/2O

2

+ e 1/2O

2

+ e

‘

O O

‘

1/2O

2

+ e 1/2O

- - 2

+ 2e

‘

O

2-

O

2-

‘

1/2O

- 2- - 2

+ 2e 1/2O

2

+ 2e

‘

O

2-

O

‘

1/2O

2

+ 2e Potential barrier changes

e

-

e

-

e

-

e

-

e

-

e

-

e

-

e

-

e

-

e

-

e

-

e

-

e

-

e

-

in air n the presence of CO

Sensor resistance in air is kept at high level: e.g. 100 k In the presence of reducing gases, sensor resistance decreases with increase of gas concentration. .

Sensor esistance (log scale)

101001000110010(k) (ppm) air level educing gas Gas concentration (log scale)

11