Metal-oxide Semiconductor Gas Sensors

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Nov 1, 2013 (3 years and 8 months ago)

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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,
CO
x
, CFCs, …)


Microwave Ovens (VOCs, Vapors)

AUTOMOBILE: (
CO
x
,
NO
x
, …)


SENS
-
ERA project, 7/12/2012

FIRST GAS DETECTORS

Canaries, 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

o
C)
.

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 (SnO
2
, WO
3
, MoO
3
, …)


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

TiO
2
,

pyramides

for

MoO
3
,

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 WO
3


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

WO
3
.

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 = R
s
/R
0

R
s
= Sensor Resistance in various gas concentrations


R
0
= 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 action

5.
long term stability

6.
heater stability

7.
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 WO
3

and In
2
O
3
. 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 WO
3

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

SnO
2

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