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Iridium Oxide Sensors for Industrial Lubricants
Platinum Metals Rev., 2004, 48, (2)
58
These materials have volume resistivity of 1 ´ 10
13
to 1 ´ 10
14
W cm. Thus, agitation in nonaqueous
solutions containing organic substances results in a
significant buildup of excess electric charge over
leak charge. This can result in static charges of sev-
eral tens of thousand or hundreds of thousand
volts that could lead eventually to damage or
explosion of the glass-lining materials, even if the
glass-lined devices are electrically earthed. It is
standard practice to embed or wind platinum or
tantalum wires in or around the glass lining mate-
rials, but such treatment primarily has a local effect
and is inadequate.
An example in (2) describes how the addition of
0.5 wt.% of platinum fibre of diameter 0.5
m
m and
length 2 mm to porcelain enamel reduced the vol-
ume resistivity to 1.3 ´ 10
3
W cm. This can
effectively prevent electrostatic buildup. If, howev-
er, platinum powder is used, 20 wt.% of platinum
powder must be added to achieve a volume resis-
tivity of 4.7 ´ 10
3
W cm.
A container that had to be glass-lined every
three months to repair damage caused by static dis-
charge was replaced with a container made of
electrically conductive enamel, using the said
method. After five years, the container remains
serviceable and exhibits no problems.
References
1 S. Shimizu, K. Mori and E. Sakuma, Japanese Appl.
11-226,627; 1999
2 Y. Iizawa and M. Akazawa, Japanese Appl. 10-
081,544; 1998
The Author
Kenya Mori is a Chief Researcher at TKKs Technical Center in
Kanagawa. His main professional interests are in developing
precious metals for industrial materials.
Engine oil lubricates and protects engines against
wear. Engine oils comprise a base oil and additives (1)
to improve the performance and long term stability of
the oil, such as antioxidants, antiwear and corrosion
inhibitors, detergents (surfactants), dispersants and
viscosity modifiers. The working life of any engine oil
or industrial lubricant may depend on its base oil for-
mulation and the additives, and the engine size and its
operating conditions.
In use, engine oils change chemically due to oxi-
dation and contamination by ethylene glycol, fuel,
soot, water, worn metal, etc. Industrial lubricant is
degraded by exposure to high temperature, air, alco-
hols, glycol, NOx and water. The additives interact
with both the oil contaminants and oxidative by-
products of oil degradation to render them harmless.
However, continuous monitoring of the chemical
condition and degradation of the oils, by an online
sensor to indicate the necessary oil changes, could
make engines more efficient and safer. Engine oil
breakdown is closely related to the level of acidity:
increase in total acid number (TAN) (oxidative degra-
dation), and level of basicity: decrease in total base
number (TBN) (degradation of antioxidants, disper-
sants and detergents), in the oil.
Acidity/basicity measurements by potentiomet-
ric testing is standard practice and iridium oxide
(IrO
2
) shows promise for measuring pH range and
sensitivity, ion and redox interference, and hystere-
sis effects.
Now, a team from Case Western Reserve
University and the Lubrizol Corp., U.S.A., have run
tests with chronopotentiometric (CP) sensors having
IrO
2
as working electrode, and have detected TAN
and TBN in a diesel oil (2). The sensors were both
conventional (a macro-scale) and miniaturised
(microelectromechanical system (MEMS)) devices.
In diesel oil drains the sensors showed good cor-
relation between the TBN and TAN numbers and
their individual voltage outputs. Conventional IrO
x
sensors displayed greater sensitivity to changes in
TAN and TBN than the MEMS sensors.
A CP sensor (a “melt Ir oxide sensor”) consisting
of an Ir wire electrode, oxidised in a Li
2
CO
3
melt to
form a Li
x
IrO
y
film on its surface, had a large increase
in sensitivity due to the Li
x
IrO
y
responding to car-
boxylic acids, and also to esters through a second
surface reaction catalysed by Li.
The sputter-formed CP sensor gave a better
response to oxidative degradation of oil due to its
higher sensitivity to ketones and carboxylic acids. The
differences in reaction mechanisms between the Ir
oxide and the components of the solution gave oppo-
site responses to changes in basicity in aqueous and
non-aqueous systems. However, as long term stabili-
ty and durability is a problem it is concluded that
work is needed to improve design and fabrication.
References
1 A. J. J. Wilkins, Platinum Metals Rev., 2003, 47, (3),
140
2 M. F. Smiechowski and V. F. Lvovich, Sens. Actuators
B: Chem., 2003, 96, (1–2), 261
Iridium Oxide Sensors for Industrial Lubricants