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Hi-Spec Systems
we are specialised in the design, development and manufacture of Phased Array and Time-of-Flight Diffraction (ToFD) ultrasonic inspection systems.
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Technical Discussions
Wilby Daniels
Wilby Daniels
01:45 Jan-09-2005
Bonded ultrasonic transducers and methods for making

I have heard that it is the type backing material that is critical in the making of a low noise ultrasonic transducer. I have used some different types of epoxies from hysol but I have not had good results. Can anyone assist me with techniques or ideas for other materials to try? I understand too that matching impedance at or near crystal is important.


    
 
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Carlos Valdecantos
Engineering
MTORRES, Ing., Spain, Joined May 2001, 9

Carlos Valdecantos

Engineering
MTORRES, Ing.,
Spain,
Joined May 2001
9
02:46 Jan-10-2005
Re: Bonded ultrasonic transducers and methods for making
Backing must match the crystal impedance (usually high) but, at the same time, must strongly atenuate any pulse going backwards. Unfortunately no single material shows these two features, oppossing each other in some way, at the required level. A classical solution makes use of epoxi resin (low viscosity, not too fast)+metallic powder+vermiculite (globular mica). Tungsten is ideal because its very high density and its powder is commercially available. Cork particles are also useful instead of vermiculite. Appliying such a mixture to the crystal and leaving it to settle leads to a composite backing with high impedance (tungsten rich) close to crystal and high attenuation (vermiculite rich) at the back surface. Centrifugation is sometimes used to get higher impedance at the interface.

Good luck. And happy New Year.

C. Valdecantos




    
 
 Reply 
 
Paul A. Meyer
R & D,
GE Inspection Technologies, USA, Joined Nov 1998, 47

Paul A. Meyer

R & D,
GE Inspection Technologies,
USA,
Joined Nov 1998
47
04:01 Jan-10-2005
Re: Bonded ultrasonic transducers and methods for making
Hello Wilby,
When you excite a piezoceramic, you store energy in it in the form of resonant vibrations. The energy is dissipated as heat, or as it is coupled into adjacent materials. To remove the energy quickly, the adjacent materials should be close to the acoustic impedance of the piezoceramic (ceramics are ~ 20 to 35 Mrayles). Epoxies have impedances about 3 to 5 Mrayles and, therefore, do not damp the ceramic very well. Adding dense particles to the epoxy can increase the impedance, tungsten particles work well. Additionally, the particles scatter the energy in the mixture reducing reflections back into the ceramic. The books "Ultrasonic Testing of Materials" by Krautkramer, and "Ultrasonic Transducers for Nondestructive Testing" by Silk cover these topics.
I hope this helps.
Paul
----------- Start Original Message -----------
: I have heard that it is the type backing material that is critical in the making of a low noise ultrasonic transducer. I have used some different types of epoxies from hysol but I have not had good results. Can anyone assist me with techniques or ideas for other materials to try? I understand too that matching impedance at or near crystal is important.
------------ End Original Message ------------




    
 
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