Heinrich J.G., Aldinger F. (Eds.) Ceramic Materials and Components for Engines
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100
290
0“
0
80
m
70
-
c
a
.s
1
50
E
40
30
m
60
II]
recovering
the
ahaust
gis
eeergy
by
the
use
of
T.C.G.
F1-q
to
reco~a
the
exhaust
energy,
the
chernid
the
recovering
system
has
be
studied
According
to
eaimation
tile
thermal
ef6ciw
willbe
imploved
about
65%
OT
more.
0
5
10
15
20
25
30 35
AddiQonal Rate
of
Phosphonc Acid
(Z)
Fig.16 Separating Rate
of
COzwhen Carbon Graphite Deposited
Phosphoric Acid
is
used
for
Separating Materials
ued
Cell
G/T.
S/T
Combined
Thormal
Power
Nuclear
Steam
Turbine
1kW
10
100
1
MW
10
100
1000
Power
Fig.17
Thermal Efficiency on Many Kind
of
Engines
on
the
other
hand,
we
inteed
ill
use
dle
ceramics
turbo
compound
mginepl-;acticayl
fbrthe
cqpedonm
to
supply
very
cheap
electric
power
m
the
atyaniq
because
the
engine
systemhas
such
excellentkh
as
compt,
simple,
low
NOx
aaissioDs
and
high
efficiency
as
well
as,
we
intehd
to
apply
all
of
the
techwlogies
for
the
vehicle
needing
dean
exhaust
emissions
ad
high
theamal
efficiency
which
is
sponsor
by
Japzln
MlTI
in
order
to
achieve
the
target
of
the
truck
with
NO.l
level
of
fuel
conslrmption
aud
low
anissionS
m
the world
Conclusion
The DPF
made
of
Sic
fiber
will
be
used
fbr
the
did
vehicles
used
m
Tokyo
mtropohu
city.
We
eshmted
that
the
eon
volume
is
about
1300
thousand-3300
thousand
kits
per
years.
Thz
movanent
obhgatmg
the
installation
of
DPF
is
exteding
h
Tokyo
to
all
of
Japan
The
prachcal
use
ofthe
dcs
engine
is
progressing
for
the
-on
system
to
sqply
the
electric
power
to
the
office
m
the
city
area
When
we
fbkhed
to
conform
the
References
1)
T.
Sakagdu
et
aL
‘~velopment
of
High
Durabday
Diesel Patticulate
Filter
by
using
Sic
Fiber”
SAE
paper
No.
1995Lo1-0463
2)
D.
W
Dickey
“The
E€kt
of
Insulated
Cornbushan
S~on~InjedionDi~~P~,
Ermssians
and
Gmbustion”
SAE
paper
No.
89029
H
Kawannna
and
H
Matsuoka
“Low
Heat
Fkyction
En+
w-h
Thema
Struchae”
SAE
papa
No.950978
H
Kawauuna,
A
Egashmo
and
S.
Selayama
“Combustion
and
(hmbushon
Chamber
For
a
Low
HeatRejection&&”
SAEpaperNo.
W506
H
Ka-
“Ceramics
Engine
far
Eeagy
Saving”
6th
Sympmium
of
Cemmics
Mateds
and
Components
for-
IWmA~itaKqrNoteLectme.
A
Higashut0
H
sasalo:
ad
H
Ka-
“Compression
Ignition
Comtnmon
m
a
l+dmhmd
and
Heat
insulated
bgme
Using
a
Homogeneous
Nand
Gas
Mixture”
SAE
paperNo.
2oooO14330
32
BETA CERAMIC
IN
ZEBRA@
AND
NAS BATTERIES
Cord-H. Dustmann
CH-6821 Stabio, Switzerland
ABSTRACT
Batteries
as
electro-chemical energy storage devices
a~
characterised by the electrode material and the
electrolyte. BETA batteries use V-Al203 ceramic
fbr
the electrolyte
as
the key component in combination
with sodium (Na)
for
the negative electrode and NiCl2
(ZEBRA@ battery)
or
sulphur (NAS battery)
for
the
positive electrode. The V-cerarnic is conductive for
sodium ions, an insulator for electrons and inert
for
the
electrode material. Batteries of this type
are
in
production in Switzerland (ZEBRA@ battery) and in
Japan (NAS battery).
ZEBRA@ batteries
are
designed for a power to energy
ratio of 2
for
electric vehicles
or
hybrid vehicles. The
cells have an open circuit voltage of 2,58V
and a
capacity
of
32Ah.
NAS batteries are designed
for
load levelling with a
power to energy ratio
of
0,125, The cells have an open
circuit voltage
of
2,08V and a typical capacity
d
632Ah.
In
Europe and Japan there is production experience
d
more than
500
000
electrolyte tubes which provide the
confidence
for the next phase
of
industrialisation with
millions
of
p”
tubes produced annually.
INTRODUCTION
600 million vehicles that are in operation worldwide
obtain their energy from crude oil. The transition to
regenerative energies
for
mobility requires the
availability
of
storage devices
for
electricity and
Hz.
The infrastructure for electricity already exists and the
batteries are available now
as
descibed in this paper.
1/3
of
the primary energy consumed worldwide
is
converted to electricity
(14
TWh annually)
for
instantanious use because there is very little storage
capacity. Power plant capacity is necessary
for
peak
power demand which leaves large investments in
equipment unused in low load periods. NAS batteries
are developed
for
load
levelling in order to reduce this
problem.
Both fields represent an enormous demand for a low
cost and effective device for
the
storage of electricity.
Electrochemical solutions are an important option.
ELECTROCHEMICAL ENERGX
STORAGE
All electrochemical energy storage devices use anode
and cathode materials that should be light and
offer
as
much
as
possible free energy out of their reaction. This
reaction is controlled by the electrolyte which
separates
the electrodes
(
fig.1
).
Charger
Load
N1
+
2NaCl-2Na
+
NICC
Fig.
1.
ZEBRA@ Electrochemical reaction
The requirements on the electrolyte material are the
following:
It has to be stable long term in contact with both
electrode materials
0
It
has to be a low resistance conductor for the
related ions and a high resistance insulator
for
electrons
It
must not release hazardous material
or
gases even
under worst case abuse, short circuit
or
crash
conditions
It must be available in large quantities
for
low cost
without a cost burden
It has
to
fit into the battery recycling process
p”-A1203 ceramic does meet these requirements
for
sodium batteries which have a theoretical specific
energy
of
790
Whkg. This is very high compared
to
the corresponding values
of
the well known systems
Pb/H2S04/Pb0
with
16
1
Wh/kg and
Cd/KOH/NiOOH
with 208 Whkg.
The energy content
of
P-batteries
is
determined by the
volume of the cathode whereas the power is determined
by the
surface
of
the electrolyte Two different types
of
these &batteries are in use:
1.
ZEBRA@ batteries are designed
for
automotive
applications in electric and certain types
of
hybrid
electric vehicles that require a power to energy
ratio
of
about 2 (1,2)
33
2.
NAS batteries
are
designed for load levelling
applications optimized for high energy content
with a power to energy ratio of
0,125
(3,4)
mBRA@
BA'ITERY
SYSTEM
The ZEBRA@ Battery system (fig2) is divided in the
subgroups: p"-ceramic electrolyte, cells, battery box
with thermal insulation and peripheral equipment.
Air
Cooling
cdls
&to
16
baltefy
unitsvl parsld
Fig.
2.
ZEBRA@Battery System
P"CERAMIC ELECTROLYTE
The p"ceramic electrolyte (fig3) is the key component
of
the battery. The battery
perfiormance
and the reliability
is dependent on it. The fact that the ceramic electrolyte
is a perfect insulator for electrons results in
a
100% Ah
efficiency for the battery. Since the first report on the
extraordinary high conductivity for sodium p-alumina
in 1967 (5) intensive
R&D
work has prepared maturity
for
commercialization (6).
Fig. 3.
p"-
alumina Electrolyt
The processes used for production of these tubes
are:
Calcination of boehmite produces a mixture of
y-
6-
and 6-alumiNAS. This calcined powder
is
mixed with
sodium carbonate and lithium hydroxide to produce a
coarse powder, which is then calcinated at
-1200°C
to
produce p"-alumina granules. Upon dispersing this
p"-
alumina in water and milling to a particle size of
1-2
Pm
The slip is then spray-dryed to produce a fi-ee-flowing
powder. This powder is isostatically pressed into the
desired monolith tubular shape and sintered in a batch
furnace
inside a spinel crucible to prevent
soda
loss.
Finally the tube is cut to length and sealed to an
a-
alumina ring with a sealing glass.
After final inspection this component is delivered to the
cell assembly line.
ZEBRA@
CELL DESIGN
98
The cell case which is connected to the negative pole is
made out of steel by deep drawing. It is laser welded to
the outer nickel component of the TCB seal. The inner
nickel component
of
the TCB seal is laser welded to
the current collector which is the positive pole (fig.
4)
in contact with the positive electrode, a mixture
of
Ni
powder and salt (NaCI) impregnated with a molten salt
(NaAlc1.1). This liquid electrolyte has several
functions:
Fig.
5.
Liquid electrolyt reaction
1.
2.
3.
4.
5.
Sodium ion conductivity between the inner
surface
of the p"-electrolyte and the NiC12 inside the
cathode in order to utilize its total volume
Overcharge protection following the overcharge
reaction in fig.5. This protects the ceramic due to
the availability
of
additional sodium and stops the
charge current
by
the higher open circuit voltage
Overdischarge protection following the overdis-
charge reaction in fig.5.
Short circuit of the cell in the case
of
a ceramic
fiacture
by the formation of metallic aluminium
following the overdischarge reaction of fig.5. This
makes the battery tolerant against cell failures up
to about 5%.
Contribution to battery safety. In case of a heavy
accident there must not be any additional danger
from the battery. This was demonstrated in a test
in which the battery was crashed against a pole
with 50 km/h (fig.6). As soon as the ceramic
breaks the liquid electrolyte reacts with the liquid
sodium and the resulting NaCl and A1 passivates
34
the cathode. This reaction produces only 213 of thermal
load compared to the normal discharge reaction (7). A
complete review on battery safety is given in
(8).
Fig.
6.
Battery safety 212 crash test
=BRA' BATTERY DESIGN
ZEBRA@cells can be connected in parallel and in
series. The series connected cells get never out
af
balance because of the
100%
Ah efficiency and the
parallel connected cells balance themself automatically
by redistribution of the current due to the internal
resistance of the cells.
The smallest battery has about 105 cells and the largest
one has 528 cells at present. Fig.7 shows the standard
battery Z5C with 216 cells.
Type
25-278 25-557
ML-64
ML-32
Capacity Ah
64
32
Rated Energy kWh 17.8 17.8
Open Circuit Voltage
0-15%
DOD V 278.6 557
Max. discharge current A 224 112
Cell
Type/N"
of
cells
ML3
I216
195
94
Weight with
BMi
kg
Specific energy
without BMI
Wh/kg
Energy density
without
BMI
Wh/l
148
Specific power W/kg 169
Power density W/I 265
Peak power kW 32
80%
DOD.
213
OCV.
30s, 335%
Ambient temperature "C
-40
to +70
Thermal
loss
W
<
120
at
270'C
internal temperature
The cells
are
assembled in a double walled vacuum
insulated box. The volume between the inner and the
outer wall is filled with an insulation material with the
very low heat conductivity of only
0,006
W/mK. This
material also cames the atmospheric pressure
so that
the rectangular shape of the box can be made using 0,5
mm stainless steel sheet metal.
An ohmic heater and
an
air cooling fan
are
operated by
the battery controller
BMI
in order to keep the internal
operating temperature in the limits of 270°C and
350°C. The outside surface of the box is only 5
-
10°C
above ambient. The main contactors and the charger
are
integral parts of the battery system.
BA'ITERY LIFE AND RECYCLING
The average lifetime of a battery is
1000
cycles
OF
100% charge and discharge and 10 years.
9
years and
1500 cycles have been demonstrated. Ageing effects
known
are only a rise of the internal resistance due to
slow cathode morphology change and ceramic cracks
due to small non detected production faults.
Up
to
5%
cell failures are tolerated.
For
recycling the cells
are
sold to steel production
plants where the contained
Ni
and steel is utilized in
the stainless steel production process
in
which the salt
and ceramic content contribute to the slag.
APPLICATIONS
This battery system is used in the DaimlerChrysler
VITO as a van for citylogistic and postal services
as
well
as
in different bus types. Fig.
8
shows the
example of a small city bus which is available in
an
electric, a hybid and a diesel version.
Hybrid
Capacity 10+1 Seating
29 standing
Drive Brushless DC
Drive power 50kW
Type
of
Battery 2xz5c
Energy Content 35.6 kWh
Battery Voltage
(OW 279 V
Weight
of
Battery
400
kg
Maximum
Speed
E
50
kmlh
Electric
10+1 Seating
29 standing
Brushless DC
50kW
5xz5c
89
kWh
279 V
1000
kg
60
kmlh
Fig. 7
Z5
standard battery with main
data
Fig.
8.
Autodromo bus
35
The electric bus
offers
zero
emission transportation
for
up to 150
km
per day with overnight charge and up to
350 km with interim charging during the day.
Type
Outer Diameter (mm)
Length (mm)
Thickness (mm)
ACC. Production 1998
table I).Due to the specially designed safety tube the
amount
of
sodium available
for
the reaction with
sulphur is limited in the event of a break in the
p”-
alumina tube. The battery safety was demonstrated
for
stationary
use.
ML3
T5
+
33
058
220
477
1.25 1.7
130.000
70.000
NAS
BATTERY
SYSTEM
The other battery system that utilizes b”-mamic for the
electrolyte is the NAS system developed by NGK
Insulators in Japan in cooperation with Tokyo Electric
Power Company (4). The basic reaction is
The main
differences
to the ZEBRA@system
are
that
the cathode is sulphur instead
of
NiCh and the
absence
of the liquid electrolyte.
The subgroups of the NAS battery
are
the p”-.....ic
electrolyte, the cells the battery box and the ACDC
converter with the
transformer
for the integration into
the
6,6
kW,
30
utility network
for
load levelling.
2 Na
+
3
S
e
Na&
NAS CELL DESIGN
The NAS cells (fig.
9)
are
designed
for
very high
energy content in stationary applications.
Fig. 9.Nas Cell
The cell case which is the positive pole is made out
d
deep drawn aluminium and inside the electrolyte is the
sodium electrode which is the negative pole. The
ceramic electrolyte has impressive dimensions (fig.
10,
Fig.
10.
NAS
PI-
alumina Electrolyt
b*-
Alumina-Tube Dimension
ZEBRA
NAS
Table 1
NAS BATTERY APPLICATIONS
384
NAS
cells
are
connected in parallel and in series
for
a
50
kW, 400 kWh module (fig.
11).
Fig.
11
50
kW modul NAS Battery
For a
6
MW/48 MWh load levelling power plant 120
of
these modules
are
assembled in 6 blocks. All
together
20
plants with
15,7
MW/125,6 MWh total
are
commissioned until March 2000. This system is
designed for an 8 h charge over night, 4
h
interval, 8 h
discharge during daytime peak power and
4
h interval
36
before the next charge starts. For this mode of operation
cooling is not necessary.
CONCLUSION
Batteries using p"-alumina for the electrolyte
are
under
development since 30 years for NAS and
20
Years
for
ZEBRA (NaNiClz). Both systems have now reached
maturity for industrialization and first production
experience has been gained. Within the next years they
have to prove their competitiveness to other options
for
electric energy storage.
REPERENCES
(1)
J.
Coetzer:
,,A
New High-Density Battery
System" J. Power Sources 18:377
-
380 (1986)
(2)
J. Coetzer, J.L. Sudworth: ,,A Second Generation
Sodium/Nickel Chloride ZEBRA@Cell" Electric
Vehicle Symposium 13, Osaka, Oct. 1996
(3)
J.L. Sudworth, A.R. Tilley: ,,The Sodium/Sulfur
Battery" London, Chapman and Hall
(1
985)
(4)
Communication from NGK Insulators, Ltd.,
Nagoya, Japan
(5) Y.F.J. Yao, J.T. Kummer: J. Inorg. Nucl. Chem.
29 (1967)
p.
2453
(6) J.L. Sudworth, P. Barrow,
W.
Dong, B. Dunn,
G.C. Farrington, J.O. Thomas: ,,Toward
Commercialisation of the Beta-Alumina Family
d
Ionic Conductors" MRS Bulletin March 2000
(7)
A. van Zyl, C.-H. Dustmann: ,,Safety Aspects
of
ZEBRA High Energy Batteries" Proceedings
d
the WEVA Conference for Electric Vehicle
Research, Development and Operation, Nov. I995
in Paris
(8)
D. Trickett: ,,Current Status of Health and
Safety
Issues of SodiudMetal Chloride (ZEBRA)
Batteries, NREL/TP-46025553, November I998
(9) E. Kodema, A. Okuno, F. Kiuchi,
Y.
Kurashima,
T. Mima,
K.
Mori:"Development
of
Compact
Sodium Sulfur Battery", Proc.
of
Electrical
Energy Storage Applications and Technologies,
Chester, June 16
-
18, 1998.
37
This Page Intentionally Left Blank
OXYGEN SENSORS
FOR
LEAN COMBUSTION ENGINES
E.
Ivers-Tiffbe', K. H. Hardtl',
W.
MenesMou' and
J.
Riegel**
(*)
Universitat Karlsruhe(TH), Karlsruhe, Germany
(**)
Robert
Bosch
GmbH, Stuttgart
ABSTRACT
Oxygen sensors are used in automotive applications to
control the air-fuel ratio in order to reduce emissions
and fuel consumption. The three way catalyst system
(TWC) represents the most effective system for the
emission control at this time. In TWC systems, the air-
fuel ratio is kept at the stoichiometric point lambda=]
and is controlled by potentiometric zirconia sensors
(Nernst type sensors). New control strategies with linear
lambda control at lambda=l, for direct injection engines
and other lean bum engines operating with air excess
(lambda> 1) need alternative sensor concepts. Hence,
current limiting electrochemical pumping cells
(amperometric sensors) based on zirconia have been
developed for engine control applications. The paper
will give an overview about potentiometric and
amperometric sensors based on zirconia and will
present new research works in resistive type oxygen
sensors based on semiconducting metal oxides as a
future option.
INTRODUCTION
The lambda
(1)
closed loop control together with the
lambda sensor and the three-way catalyst (TWC)
represent today's most effective concept for the
reduction of toxic emissions in spark ignition (SI)
engines. Since 1976, when Bosch started with the
world's first
ZrO,
based oxygen sensor to go into
operation in vehicle exhaust emission control systems,
worldwide a few hundred millions of lambda sensors
have been produced.
To meet the new exhaust emission requirements
LEV
(low emission vehicle),
EU4
and
ULEV
(ultra low
emission vehicle) and fkture regulations like
SULEV
(super ultra low emission vehicle) the two-stage
controller using a conventional thimble type
ZrO,
oxygen sensor seems to be at its limit. Today the main
emissions appear during warm up phase and, especially
with aged catalysts, at maximum deviation from
h
=
1
due to the oscillation of the conventional two-stage
controller behavior. Therefore new emission control
strategies have been developed and were applied in
vehicles during the last few years
[
1,2,3,4].
One strategy keeps the two-stage controller but
starts
the closed-loop mode during warm-up phase. The
engine is preferentially slightly lean driven or uses a
secondary air pump to enable fast light
off
of the
catalyst. This requires fast light off capability of the
upstream sensor (115
s
resp. 40
s)
which needs to be
much better than the capability of the conventional
thimble type oxygen sensor.
Another strategy uses the linear h-control to keep the
conversion rate of aged catalysts high by reducing
the
deviation
from
the ideal
h
=
1 point. Together with lean
warm-up and secondary air pump concepts these
strategies require a linear wide range lambda sensor
with fast light off capability and high accuracy.
Offering less fuel consumption lean bum engines and
recently gasoline direct injection (GDI) engines were
developed which need linear lambda control outside
h=l. Current lean burn and GDI systems don't operate
exclusively at lean condition but also at h=l for better
acceleration and emission conversation due to three-
way-catalysts. If NOx-storage catalysts are used under
lean condition, cyclic rich phases have to guarantee the
NOx-regeneration. Wide range sensors can improve the
quality by closed loop control during this phases. While
simple limiting current oxygen sensors only can operate
in lean exhaust gas wide range lambda sensors have a
measuring range from
h=0.7
to air. The functional
principles of such universal oxygen sensors are already
described [5, 6, 7,
8,
91 and Bosch started with series
production of
its
wide range lambda sensor
LSU
in
1998.
The
ZrO,
planar technology
is
the basis for the
universal wide range oxygen sensor with high
performance for lean bum applications further
miniaturization and a platform for the next generation
of exhaust gas sensors for NOx, HC, etc. [8,9].
Resistive type oxygen sensors based on semiconducting
metal oxides are an alternative to the above-mentioned
sensors because they have also a good oxygen
sensitivity and can be manufactured using a low-cost
screen printing technique.
A
resistive gas sensor for
internal combustion engines was proposed by
Logothetis
[lo]
for the first time. He suggested
semiconducting TiO,. However its decisive
disadvantage is its insufficient chemical stability.
A
further disadvantage of TiO, as well as of many other
semiconducting oxides is its high temperature
dependence of the conductivity
[l
I]. For this reason, a
family of semiconducting oxides has been investigated
where the temperature dependence is low, for example
[14]. In this paper we present a thick film sensor based
on Sr(Fe,Ti)O, whose temperature dependence is
suppressed by
an
adequate adding of iron [15]. Such
sensors give new aspects for applications of resistive
type sensors in lean exhaust gas.
cO,-m,o
~1,
s~MP,J~,.,o,
[
131, ~a~e,.,~~.~0,
39
PLANAR
POTENTIOMETRIC
OXYGEN SENSORS
The planar technology of
ZrO,
oxygen sensors is quite
similar to the multi-layer-technology of electronic
circuits
(MCM
-
multi-chip-modules). All operating
layers are arranged in plane, consecutive surfaces. This
technique allows the integration of complex functions
for example heating and sensing layers, gaps, cavities
and channels with determined porosity in a compact
3-
dimensional monolithic design.
THICKFILM TECHNOLOGY
OF
PLANAR
ZIRCONIA OXYGEN SENSORS
In a tape casting process
[
161
ZrOz
ceramic green sheets
are manufactured by adding an organic binder phase to
the Zirconia powder (fig.1). These green tapes are
punched into separate sheets. On each side of the sheets
in a screen-printing process (thick-film) individual
function groups (e.g. Pt heater, Alz03 insulation layers,
leads, electrodes, electrochemical cells and porous
ceramic layers) are attached to arrange the desired layer
structure. These layers can consist of various inorganic
materials. By punching and drilling channels and holes
(e.g. reference air duct, contact holes) can be produced.
The layout for several individual sensing elements in a
parallel arrangement is printed on one substrate sheet. A
number of sheets can be stacked and laminated together
to form complex composite structures. After separation
the sensor elements are sintered.
2%
Sinteri
Fig. 1. Process scheme for manufacturing of planar
zirconia oxygen sensors
[
171.
Mechanical and thermal shock resistance, long term
phase stability and high ionic conductivity are
demanded to ensure adequate thermal and mechanical
stability of the ceramic elements and to fulfill the
requirements for automotive application over a life time
of 15 years and 150000 miles. The properties of the
different materials e.g. tape casting performance,
thermal coefficient of expansion and cofiring capability
of ZrO,, A1,0, and Pt layers must be accurately adapted
~71.
PLANAR LAMBDA=l SENSOR
LSF
The planar lambda=l sensor LSF4 is the realization of
the conventional heated
ZrO,
oxygen sensor in planar
technology
[9,
171. Fig.
2
illustrates a simplified version
of the layer structure.
-
Porous
protective
layer
L-
Outer ewde
Sensor
sheet
--a
Inner electrode
"
,.
'1
Airdudsheet
Insulation
layer
'.
Heater
.
.9.
.
-
\
Heatersheet
Fig.
2.
Thick film technology of planar zirconia Nernst
sensor LSF (Bosch) [9, 171
Fig.
3
shows a cross cut of the sensing cell [9, 171.
Compared to the conventional thimble type zirconia
oxygen sensor the planar sensor LSF operates as a ZrO,
solid electrolyte galvanic oxygen-concentration cell
with the advantages
-
fast light
off
capability (-10
s)
and reduced electrical
power consumption
(5
-
7
W)
on account of an
integrated heater and lower thermal mass
-
small element size (-59x4~1 mm') and reduced
weight
-
basic technique for new functions, complex planar
designs (e.g. universal, HC and NOx-sensors) and
Mer miniaturization.
ZrO~-Ceramic
Electrodes
Insulation
EEtl
Porous
protective
layer
0.98
1.0
1.02A
Normalized
AIF
ratio
Fig.
3.
Cross
cut of planar lambda=l sensor LSF
(Bosch)
Because of
the
temperature dependence in the rich
region and the very low voltages and flat curve at the
lean region, this galvanic lambda=l sensor only can be
used near the stoichiometric point with high accuracy.
continuous h=l-control to lean bum control, diesel
control,
CNG
engines, burners and measuring devices
(Lambdameter LA3). Due to its fast response time
cylinder balancing has been demonstrated to be
feasible. This wide range oxygen sensor LSU4 is in
series production for linear lambda control and lean
burn control since 1998.
RESISTIVE OXYGEN SENSORS
The principle of operation of resistive sensors is based
on
the dependence of the electrical conductivity of a
metal oxide on the oxygen partial pressure in the
ambient gas atmosphere at elevated temperatures.
Electrical conductivity
(J
of semiconducting oxides can
be expressed generally by
The first term, the activation energy for conduction E,,
the Boltzmann constant
k,
and the temperature T,
describes the temperature dependence of the electrical
conductivity. Oxygen partial pressure dependence is
described by the second term, including m as a constant
which depends on the dominant type of bulk defects
and corresponds to the sensor sensitivity.
Figure
7
arranges some semiconducting oxides
according to temperature dependence (EA) and
sensitivity (m).
24
0
112
1
I6
1
18
Fig. 7. Sensitivity (m) and temperature dependence (Ed
of some semiconducting oxygen sensor materials.
The ternary metal oxide strontium titanate is a
promising material for resistive high temperature (T
>
700
"C)
oxygen sensors. SrTiO, is able to tolerate large
levels
of
dopants without phase transformations, and the
perovskite crystal structure is stable in a large
temperature (T
<
1200
"C)
range [22, 231. This makes
an adjustment of the electronic properties by doping the
parent structure oxide with alternative metal ions
possible and renders some interesting combinations of
sensor properties.
Figure 8 shows the electric conductivity of SrTi,-,Fe,O,
bulk ceramics for x
=
0.01 and x
=
0.35
as a function of
the oxygen partial pressure PO, and temperature T. The
electric behavior at high temperatures is well
understood on a defect chemical basis and published in
numerous papers e.g. [24].
0
-I
-2
x=0935
~~~
-5~"'I'"'I'~'"'"'I"'
-10
-5
0
5
lO!aPO,
1
Pa)
Fig.
8.
Conductivity versus oxygen partial pressure and
temperature for Sr(Ti,-,Fe,)O, ceramics.
The PO,-dependence of
(J
can be divided into
3
ranges
of the PO,, see figure 8. In the case of very small PO,
(<
lo4
Pa) the material is a n-type conductor. The
conductivity decreases with increasing PO, (m
<
0)
and
shows a strong temperature dependence for all iron
contents x (EA
>
1 eV). A conductivity minimum at
mean values of PO, Pa
-
1O-I pa), that almost
shows partial pressure independence, indicates traces of
ionic conductivity for x
=
0.01.
For an application of SrTi,,Fe,O, as a temperature
independent oxygen sensor for lean bum engines, the
partial pressure range PO,
>
10' Pa (m
>
0)
is of
interest. In this p-type range the isothermal data lines
are very much closer together than in the n-type range.
This can be explained. In metal oxides at a fixed oxygen
partial pressure of the ambient atmosphere oxygen
leaves the lattice with increasing temperature. This
leads to an increase of electrons (n) or due to the
generation-recombination
(characterized by the band
gap E,) to
a
decreasing hole concentration
@J).
The
latter effect is opposite to the thermal promotion of the
dominating charge carriers
(p?),
which leads to an
increasing conductivity. Equation
3
describes the
temperature dependence of conductivity (characterized
by E,), that is determined in the p-type range by the
reduction enthalpy of the oxygen vacancies
AHRd
and
the energy of
generation-recombination
of electronic
charge carriers
E,
[25].
L
Since the effective band gap
E,
of SrTi,~,Fe,O, depends
strongly on the iron content
x
the temperature
dependence of conductivity can be adjusted by the iron
content x
[26,
271. In the case of x=0.35, the
temperature dependence of conductivity can even be
eliminated
(E,
=
0)
consequently, this iron content is the
material base for an ideal resistive oxygen sensor for a
use in a lean-bum engine @0,=103 Pa
-
2.104 Pa). The
42