[ Ultrasonic Transducer Workshop ]
[ Follow Ups ] [ Post Followup ] [ Help ]
Re: To all: Is there any disadvantage for composite transducers?
Posted by: Fleury Gerard , E-mail: imasonic@imasonic.com, on September 28, 1996 at 20:44:15:In Reply to: Re: To all: Is there any disadvantage for composite transducers? posted by : Bill Grandia on September 24, 1996 at 08:51:11:
Thank you to Rolf Diederich for his very interesting and very pertinent questions.
Thank you also to Bill Grandia for his explainations about the different technologies and possible limitations of composite transducers.
Please, forgive me for not having replied before,I was
out of the office this week.Due to the wide subject both general ideas and many practical aspects should be discussed .Today I will
only give main answers and I suggest we go into more detailed aspects with on going questions of workshop participants:Considering the different design of active piezocomposite materials.I will only discuss the case of transducers made of active materials with a so called
1-3 connectivity ( for more details on this aspect see articles from R.E Newnham or W.A.Smith ) .
Most applications in NDE require large bandwidth and
high sensitivity to provive good signal to noise ratio and temporal resolution.
Moreover in many applications the active part of the
transducer is acoustically loaded by a low impedance
medium even when the material or the piece to be inspected has a high impedance : for instance in the
case of immersion probes and probes incorporating (or associated with) a shoe, a wedge or a delay line.
made of polymer materials.In these numerous cases
transducers with an active part made of 1-3 piezocomposite material can provide better sensitivity
in a wider freqency band than transducers produced with other presently known technologies. Such transducers can bring an important improvement in inspection quality ( see for instance results given in workshop articles from G.Splitt and G.Fleury ) in applications where signal to noise ratio and bandwidth
are critical parameters .Of course composite transducers have some limitations and yet Bill Grandia noticed some of them .
The case of applications where the transducer is
used in direct and intimate contact with a high impedance material is one of them . Due to their relatively low acoustic impedance, piezocomposite materials may be not appropriate to build such transducers.
The case of applications" requiring High Q transducers" should be discussed into more details :
Is the High Q required to create a narrow band or
is it only a way to design an efficient transducer (at
a given frequency) and /or to transmit high intensity
ultrasound ?If the application imposes a narrow band system ,this narrow band can be obtained by different ways :
. Narrow band transducer is one way to do it and then
composite technology may be not appropriate to get
it .However in such case the frequency will be fixed
by transducer design,dependant from manufacturing tolerances and may be will vary with temperature or
transmitted power ( which creates heating effects
inside the transducer). Narrow band of the system can be obtained by electronic filtering associated with a medium or wide band transducer and then the frequency can be more easily choosen and controlled.
If the application requires an efficient transducer,then
composite transducer can be extremely useful with a high transmit/receive efficiency quite close to what can produce narrow band transducers and again with the possibility to adjust and/or control the frequency.Finally, if the application requires power generation
then specially designed composite transducers can
produce very high level of intensities in CW, high duty
cycle burst or impulse mode .Are composite transducers more expensive than conventional ones ? Such question requires special
attention about the basis of comparisons :.Is the comparison to be done on standardised probes manufactured with the same general specifications excepted acoustical properties? Then, as far as I know, differences in prices are quite low between composite transducers and conventional transducers and I am not able to say if the difference is due to the composite aspect or is due to other aspects as for instance general quality or associated services .
.On special transducers dedicated to special applications the analysis is still more difficult to do
but again as far as I know differences in price are not important in comparison with the gain on global functional performances.
Limitations in frequency :Piezocomposite technology based on 1-3 active
materials is physically limited in frequency due to the
microstructure of materials .As far as I know the upper
limit is a center frequency of 15 Mhz for standard transducers and 18 Mhz for special fabrications .There are many other things ( array applications ,low frequency transducers,aspherical focusing,high temperature, ...) to discuss about composite transducers but excepted the above comments I do not see many restrictions for the use of
such transducers in NDE applications.Of course these
transducers are presently a small part of the total market of transducers but they bring so much advantages in many applications that I am sure they
will play a much more important role in the future .Gerard Fleury