Materials for planar oxygen sensors and other HTCC multilayer structures Introduction For many years ESL
ElectroScience has supplied materials for the fabrication of a variety of sensors
(eg. platinum paste or ink #5545 and gold paste or ink #8880-H). These include
gas, especially oxygen, pressure, and temperature sensors. Many of these sensors
are used in vehicles, but there are many types that are used to measure/regulate
parameters in industrial, scientific, medical and domestic applications.
Traditionally, thick film pastes have been used on fired ceramic, typically
an alumina substrate, but other ceramics such as zirconia and aluminum nitride
have also been used. Heater/electrode combinations have been manufactured using
pastes for operation at elevated temperatures (~400°C) and these pastes are
fired at high temperatures (~1000°C) to ensure stability of the fired films
under operating conditions. Sometimes the sensors, or elements within the
sensor structure, have been made on thin alumina substrates (250µm or even
thinner), or on fired zirconia (125µm) where ceramic diaphragms having great
flexural strength are required. Much expertise has been acquired by building
prototypes according to customer designs or simple test structures to prove
the feasibility of any given sensor build. During the same period,
and in parallel to the sensor work on fired ceramic, experience has been
gained in the use of ESL green tape materials. These green tapes fall into
a number of categories, including Low Temperature Cofired Ceramic (LTCC) and
High Temperature Cofired Ceramic (HTCC). Compatible pastes are also available
to complete the material systems. There is an increasing
need for some ceramic sensors to operate at temperatures above 400°C and even
as high as 1200°C. This is particularly true for automotive sensors. A variety
of sensors are being made using HTCC where, provided the correct materials are
chosen, higher firing temperatures up to 1550°C may be used to maximize stability
under ever more demanding operating conditions. ESL supplies materials
for planar sensors in HTCC and, in many cases, can make prototypes according
to a customer design. Enquiries are welcomed.
Oxygen sensors The technology has seen
considerable improvement since the advent of oxygen sensors for automobiles
(circa 1976). These sensors are used within the exhaust stream of an automobile
to sense and send signals to the engine control module (ECM), also known as
engine control unit (ECU), where the air/fuel ratio (AFR) can be controlled.
The stoichiometric AFR is 14.7 air/ 1 fuel. Provided that there is no special
engine tuning required for high performance, the ECM controls the ratio of
air to fuel at the stoichiometric level to maximize fuel economy, while still
providing enough power. Emissions are minimized, especially in the short
period after switch on. Reducing the time to ‘light off’, i.e. the time when
the sensor becomes an ionic conductor and control is established, enables
emissions to be greatly reduced. ESL ElectroScience
has supplied materials for the traditional thimble or conical sensors for
decades. It has been known for years that the inclusion of a heater in these
devices accelerates the process for reaching the optimum operating temperature.
Modern planar oxygen sensors are smaller and lower in mass and incorporate a
heater within the sensor structure to enable far more rapid response than the
heated conical counterpart (seconds as opposed to up to a minute). The
reference air channel within the sensor requires fugitive materials compatible
with HTCC tapes and pastes. Multiple layers of HTCC green tapes are used.
Printed layers are formed using the screen-print technique to produce buried,
insulated platinum heaters. All layers are cofired at temperatures up to
~1550°C, using a correct product mix and prolonged firing regimes to ensure
complete burn-out of the thick sensor build and to minimize blistering and
warp. ESL ElectroScience has extensive experience in enabling flat parts to
be constructed, even when using a combination of tapes and pastes.
Generic oxygen sensor structure Successful processing and
firing of multilayer cofired structures as found in oxygen sensors requires
knowledge of dimensional stability during layer build and printing, shrinkage
during firing, and the necessary firing regime to ensure that flat, blister-free
parts are produced. ESL ElectroScience has prototype-building equipment and
expertise in all these processing aspects and can arrange training programs.
Successful organic removal during burn-out depends on many parameters –
materials (both ceramic and metal), size of part, structure, etc. A generic
firing profile for a zirconia based system is given, but this may vary for a
given sensor structure with a specific mixture of ESL tapes and pastes.
42013-A zirconia firing
NOx sensors ESL ElectroScience has
a range of materials that is suitable for the manufacture of the heater/electrode
combination found in planar nitrogen oxide (NOx) sensors. The sensitive layer
applied to the electrode is normally proprietary, but ESL ElectroScience provides
a paste making service using customers’ powders. The same manufacturing
principles employed in the fabrication of oxygen sensors apply but the product
mix may be different, and there may not be a need for a reference air channel
as the measurement of NOx is not necessarily used to control AFR.
Particle/fume sensor
This type of sensor is
used in diesel engines to sense the soot from the exhaust stream. Soot deposited
onto the sensor causes a change in resistance or capacitance of the sensor element.
If the value saturates, the soot may be burned off using an integral heater.
ESL ElectroScience supplies materials for the manufacture of such devices,
and can supply prototypes according to customer design.
Temperature sensors
Printed film
temperature sensors on fired ceramic have existed for many years. These
are capable of measuring temperatures up to several hundred degrees Centigrade. New generation
temperature sensors need to operate at extremely high temperatures (in excess
of 1200°C in some cases) and, consequently, there is a need to use HTCC. ESL
ElectroScience is able to supply materials for this type of sensor. A range of
platinum pastes are available with different levels/types of filler and at
various resistivities, depending on the application.
Pressure sensors
The traditional design
for pressure sensors utilizes a four-resistor Wheatstone bridge on a thin
(down to 125µm) diaphragm. This diaphragm has been made in either alumina
or zirconia. A typical pressure sensor design specifies a thin printing gold
termination with a high gauge factor, low signal/noise ratio, resistor. ESL
ElectroScience supplies 8837-G and
8081-A as thin print termination gold and
3414-A as the gauge factor resistor. The sensing element is attached to the
main body of the pressure sensor using a sealing glass such as ESL
4026-A.
ESL ElectroScience capabilities
ESL ElectroScience
has a range of existing products suitable for the manufacture of high temperature
sensors. The following table is specific to Pt-based planar HTCC oxygen
sensors, but other sensor inks or pastes and green ceramic tapes can be found
on the sensor application page on the ESL web site.
ESL ElectroScience planar oxygen sensor materials ESL ElectroScience can
also modify existing pastes and tapes to meet specific customer requirements,
or manufacture tapes and pastes from customer supplied powders. Lot sizes range
from a few hundred grams to the high volumes required by the automotive industry. ESL can also provide
training at our own facilities (or at our customer’s site) covering most
aspects of sensor manufacturing – materials, equipment, prototype building, etc. Please contact: Alvin Feingold, PhD John Whitmarsh Or other ESL ElectroScience
staff in the USA,
Europe,
China,
Japan, etc.
For more information on the products mentioned above and other ESL materials for sensors, see elsewhere on the ESL web site at
http://www.electroscience.com/. |
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