ElectroScience - Thick-Film Materials & Ceramic TapesElectroScience - Thick-Film Materials & Ceramic Tapes

Spotlight

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 Image of 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.

42020 zirconia firing

Heating rate °C/hr Target temperature °C Hold time, hrs at target temp. Air flow
60 100 1 on
60 150 2 on
10 220 2 on
13.5 350 1 on
10 360 1 on
13.5 400 0 on
60 600 1 on
60 800 0 off
300 1250 0 off
70 1450 1.5 off

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

Application ESL Product Number Material Comments
Heater 5571 Pt paste  
Heater 5559-B Pt paste Higher resistance compared to 5571
Heater Leads 5574 Pt paste An option to reduce heater lead resistance
Electrode 5570 Pt paste Porous
Via Fill 5575 Pt paste  
Insulator 4492 or 4530 Alumina paste Insulates the heater from the sensor circuit
Porous Overcoat 4569 Zirconia paste To protect the outer electrode
Air Channel 4441 Fugitive paste Used to form reference channel
Air Channel 49000 Fugitive tape Used to form reference channel
Sensor Body 42020 5 mol% YSZ tape Electrolyte and substrate

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:

Samy Palanisamy
Assistant Director - R&D
ppalanisamy@ElectroScience.com
Tel: +1 (610) 272-8000 ext. 221

John Whitmarsh
Technical Service Manager
jwhitmarsh@esleurope.co.uk
Tel: +44-118-9182432

Or other ESL ElectroScience sales and technical experts in the USA, Europe, China, Japan, Korea, Taiwan, India, and S.E. Asia. 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/.