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Non-destructive testing at 800 C

   
 
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This project identified a new piezoceramic material which can be used at temperatures as high as 800 C, and developed reliable, cost-effective methods for producing this material on a commercial scale.
Bringing together industrial and academic partners from five Member States, the project also developed a high temperature thick film electrode compatible with the new piezoceramic, and built prototype probes and sensors incorporating them.
A new high-temperature probe has also been developed for the non-destructive testing of very hot steel plates and pipes. A patent application has been made, and the probe is being actively promoted on the German market.

Piezoelectric ceramic materials, or piezoceramics, convert electrical energy into mechanical energy, and vice versa. They are the key components of high-precision sensors and ultrasonic probes which play an important role in the maintenance of many critical industrial processes.
Such devices are particularly valuable as a means of monitoring equipment or operations which it would be impossible or dangerous for a human operator to approach, and where non-destructive testing (NDT) methods are required. For example, they provide a means of assessing the integrity of pipe welds in chemical or power plants without incurring the enormous expense of shutting down the entire plant.
However, the industrial use of commercially available sensors is constrained by the temperature range within which they are able to operate. Currently this does not go beyond 600 C, too low for optimal performance in many of the applications where they are most needed. They must also be used at a distance, with inevitable loss of measuring accuracy.
The partners set out to identify a new piezoceramic material which could be used at temperatures as high as 800 C, and to develop reliable and cost-effective methods for producing this material on a commercial scale. It also sought to develop a high temperature thick film electrode suitable for use in conjunction with the new piezoceramic, and to design and build one or more prototype devices.
The work was oriented towards two specific high-temperature applications. The first of these was a vibration control device for use in aircraft engines. The second was a high resolution probe for monitoring wall thickness and crack development in pipes carrying liquid metals or super-heated gases.

Exhaustive research

Prior to the project, the only known piezoelectric materials which could operate in the target temperature range were single crystal ceramics such as tourmaline. However, these had low sensitivity and varied in quality.
The partners took as their starting-point ceramics based on lithium niobate (LiNbO3), whose Curie temperature - the point at which piezoelectric properties are lost - was known to be high. Single crystal lithium niobate has a Curie temperature of 1,100 C, but at 600 C it starts to lose oxygen to the environment, which makes it impossible to use.
The partners' hypothesis was that the oxygen loss might be blocked by replacing some of the lithium with another element. They chose sodium, which offers better piezoelectric sensitivity than lithium, and concentrated their efforts on an exhaustive examination of the Li1-xNaxNbO3 family, with an atomic percentage of sodium between 0 and 25. Their problem was that the more lithium they replaced with sodium, the lower the Curie temperature fell.
They also found that fabrication methods and conditions played an important role in determining the characteristics of the final ceramic. Sol-gel and co-precipitation techniques, as well as the traditional mixed oxide route, were used to prepare precursor powders. Processing studies also included sintering conditions and post-sintering hot isostatic pressing (HIP).
This initial element of the workplan turned out to be considerably more complex than the partners, led by Danish electronic component manufacturer Ferroperm, had anticipated. The involvement of two university materials schools was crucial to their efforts to characterise the material structure and properties of each composition.
While conventional mixed oxide production was tested by Ferroperm itself, the University of Limoges assessed the sol-gel technique, while the University of Leeds focused on the co-precipitation method. In each case, the sensitivity, oxygen loss and other properties of each variant were measured across the relevant temperature range.

Zero oxygen loss

Eventually, the research programme led to the successful identification of a polycrystalline material which can operate at 800 C without oxygen loss, and can be produced economically.
The commercial pricing of the piezoceramic material was critical. However, if the partners were to achieve their overall objective of making piezoceramics accessible to a range of new industrial applications, other sensor components would also have to be upgraded to match the higher temperature performance.
In response, Cerdec, the French industrial partner, successfully developed a platinum paste electrode material. In addition to its compatibility with the new piezoceramic, the paste has excellent adhesion and conductivity, and is one of the very few cadmium-free electrode materials available. Offering environmental as well as technical advantages, Cerdec's electrode has already been commercialised with considerable success.
German partner BAM has also marketed a high-temperature probe which incorporates the materials developed in the course of the research programme. Designed for the non-destructive testing of very hot steel plates and pipes, the probe emits a signal at a specified frequency and identifies cracks as changes in the acoustic response. The company has applied for a patent to protect its innovation.

Early industrial applications

The partners are confident that the project has produced some significant results in the field of materials science. But they are under no illusions about the speed with which this new scientific knowledge is likely to be commercialised, even in the target applications.
BAM, of course, already has a marketable product, and is organising a series of promotional short courses for German small and medium-sized enterprises, as a way of educating potential users about the new technology.
The development of a high-temperature accelerometer, undertaken by an unfunded Swiss partner, Vibro-Meter, illustrates the difficulties of feeding the new piezoceramic material through into industrial applications.
A prototype accelerometer has been built and tested, using a housing designed for use with existing devices. Before the prototype can be put into production, a new housing is needed, optimised to match the temperature performance of the device itself, and this development will be a lengthy and expensive business.

Unique performance

Nevertheless, the new piezoceramic material is almost certain to achieve widespread acceptance, even if industrial take-up is slow. The range of potential applications is enormous, and there is simply no other material available which is capable of operating for extended periods in the 600 to 800 C range.
Ferroperm itself, though it has no plans to patent the new material, is making every effort to promote its use, and hopes to interest the oil-drilling industry in the new technology.

 

Project Title:  
New piezo-electric ceramics with Tc>1000C for operation up to 800C

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-5582

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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