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08:51 Sep-24-1996
Re: To all: Is there any disadvantage for composite transducers?

a) A solid piezo disk can be broken in small pieces or granules, whereby the spaces between the granules are filled with epoxy. A preferred filler seems to be Spurr epoxy, which in liquid state has a very low viscosity (like water).
A focused composite element can simply be produced by inserting a solid polarized element between concave and convex shaped press-mold halves, hereby submerging the entire mold in liquid epoxy. This method allows the epoxy to flow into the crevices and prevents air from entering into the crushed structure. After the epoxy is cured, the dish-shaped element can be taken out of the mold. Of course, the electrodes need to be restored. There is no need to re-polarize the element, since the granules are staying in their original alignment.
This construction is less popular, since the fill factor for the element is practically fixed. The only feature is that the radial to the thickness mode becomes uncoupled. By this, the effectiveness of the thickness mode vibration is enhanced. There is virtually no reduction in acoustic impedance and dielectric constant. The bandwidth of the element is becomes increased. When a high "Q" element such as PZT5A is used, the signal response of the crushed version more or less behaves like a lead metaniobate element, except that the sensitivity is stronger.

b) A better method is the dicing approach, whereby the element is cut into small pieces and has been described in detail by G. Fleury and G.Splitt.
We need to discuss the disadvantages.

1: The composite element is not desired when a high "Q: transducer is necessary. A "high "Q" means that the transducer has a narrow bandwidth and therefor produces a large amount of ringing. The penetration power of a high Q transducer is strong. When highly attenuative materials are to be inspected, a strong signal is required. Sometimes this can be enhanced by applying a toneburst signal with a frequency equal to the series resonance frequency of the element. At this resonant condition, the electrical impedance of the device becomes minim and therefor the generated sound pressure into the material is strongly enhanced. This is not possible with diced elements. From a receiving standpoint, a strong signal can also be obtained in the narrow band mode. The narrow band application is very popular for the through transmission technique.
2: The composite transducer is less suited as a direct contact transducer for the inspection of steel or ceramics, especially when the transducer needs to be wear resistant.
The lowered acoustical impedance is not impedance matched to the ceramic wearplate or the steel surface.
3: Another characteristic of composite transducers it that the capacitance of the element reduces with the aspect ratio of the total area of the diced elements and the filler epoxy. When this ratio becomes high while trying to make a low acoustic impedance (like good matching to water), the electrical charge produced on the diced element from a reflected signal becomes small. When a long coaxial cable is connected to the transducer, the signal amplitude becomes small. The cable acts now like a parallel capacitance, hereby absorbing the signal to an appreciable extend. Typically, the capacitance of a coaxial cable is 31 pFd / foot. A 10 foot cable therefor is 310 pFd. If the element capacitance is around 300 pFd, the signal amplitude drops to half the value. Of coarse, the can be overcome by using an high input impedance low noise pre-amplifier located very close bythe transducer. It is possible to use such pre-amplifiers in the pulse echo mode by inserting an isolation circuit to protect the amplifier from the large damaging transmitting pulse. The use of such an amplifier greatly enhances the performance of the composite transducer by preserving the high resolution wave shape.
This sumps up the few disadvantages of the composite transducers.

Bill Grandia

00:44 Sep-28-1996
Re: To all: Is there any disadvantage for composite transducers?

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 frequencycan 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

06:53 Oct-01-1996
Re: To all: Is there any disadvantage for composite transducers?

The comments of Bill Grandia have shown that piezocomposite transducers are
suitable for nearly all probes except for hardfaced probes, which will be coupled
directly to steel. But even there could be some advantage depending on crystal
diameter, frequency and wear plate material. A normal PZT transducer will produce
radial ringing which can result in spurious echos in the hardface probe. With a
piezocomposite transducer the radial coupling is reduced to a small amount making
the hardface probe more quiet.

Of course there are some other restrictions for using piezocomposite transducers for
all types of probes.

1. The material is more costly, therefore it will not be used in cheap probes.

2. It is difficult and extremely costly to fabricate high frequency transducers. To avoid
additional ringing the periodicity of the 1-3 piezocomposite matrix should be smaller
than halve the wavelength of shear waves in the epoxy matrix for the highest
frequency within the pass band of the probe. For 15 MHz probe e.g. this results in
very small ceramic posts of 0.04 mm edge length and very small separating epoxy
lines of 0.015 mm (for 20 MHz upper pass band and shear wave velocity of 1100
m/s)!

3. Piezocomposite material is not suitable for temperatures above 130 degree C
approx.

Gerhard Splitt