Abstract

A series of powerful piezoelectric materials are available to generate ultrasound in ultrasonic probes. These materials differ in their physical properties. Depending on the application and the required probe characteristics, the one or the other material is more advantageous, sometimes for acoustic or technical reasons, sometimes even for economic reasons. Although the overall characteristics of probes, such as pulse shape, frequency and sound field are given in data sheets and are basically well understood, the knowledge about constructional details with regard to the utilised piezoelectric materials is less common. Knowing about these details however does not only contribute towards a better technical understanding of the basic action of the probe, but it does also help towards understanding the behaviour of the probe under certain operation conditions, e.g. if a probe is coupled to a specimen, which it is not designed for. The choice of the correct type of probe is often made easier through this knowledge.

Content
1. Introduction
2. Piezoelectric materials
3. Use of piezoelectric materials in probes
4. Closing remarks
5. Literature

1. Introduction

Probes form the actual core in all non-destructive ultrasonic inspection procedures: The fact whether a workpiece can be inspected or not depends upon them. In numerous cases, especially if a workpiece has a complicated geometry or the inspection has to be done under unusual conditions, ultrasonic inspection becomes only feasible by use of probes, which have appropriate acoustic properties. In any case is the choice of the correct probe is decisive for the quality and the reliability of inspection results.

Today, ultrasonic probes work almost exclusively according to the piezoelectric effect. Fig. 1 shows the basic construction of four fundamental probe types. In the vertical or straight beam probe (Fig. 1a), the piezoelectric element, which converts electrical energy into mechanical energy and vice versa, is mechanically attached to a backing material, most often called the damping block. The acoustic impedance of the damping block must be close to that of the piezoelectric material in order to suppress ringing resp. to enlarge the bandwidth. The second task of the damping block is to absorb that part of ultrasonic energy, generated by the piezoelectric element, which is going backward. A protective and/or matching layer in front ensures that as much of the acoustic energy as possible is transmitted into the workpiece. It also protects the probe against mechanical damage, while it is moved over a workpiece, which may have a rough surface, or against chemical damage, when chemically agressive fluids are used as couplants.


Fig. 1a: Basic construction of ultrasonic probes.

With the angle beam probe (Fig. 1b), the ultrasound is transfered into the workpiece under at a specified angle. The TR probe (Fig. 1c) consists of one separate transmitter and receiver element each. Their sound field characteristics overlap in the workpiece. The piezo-elements are mounted on plastic wedges, which are generally made of plexiglass, polystyrene or other plastic materials of low acoustic absorption. Usually an acoustic matching layer is placed between the piezoelectric element and the plastic wedge. This ensures good energy transmission from the piezo element into the wedge. It also acts as a medium mechanical damping of the piezoelectric element. These facts lead to a high sensitivity, rather short pulses and a high bandwidth. Therefore, angle beam probes build up like this do not need a separate backing material (damping block) on the rear face of the piezoelectric element, as far as not the extremely short pulses of thickness probes are expected from the probe.

The construction of the "Delay line" probe (Fig. 1d) complies basically with that of the vertical beam probe. However here, the sound is transmitted into the workpiece via an additional delay-line made of plastics with low ultrasonic absorption. The delay line can be either rigidly fixed to the front face of the vertical probe or can be interchangeable. The time delay until the ultrasonic signal enters the workpiece avoids, that echoes from flaws close to the surface appear within the dead zones of the ultrasonic flaw detector, which are caused by the high voltage excitation pulses. Use of delay-line probes is a simple measure to have an excellent near field resolution.


Fig. 1b,c,d: Basic construction of ultrasonic probes.

Depending on the application, probes also differ with respect to the size of the active piezoelectric elements, their frequency, bandwidth and the basic design. Fig. 2 shows a few typical models. The ones on the left are for manual testing, the ones on the right are water-proof probes, used for immersion technique, while the probes in the middle are angle beam probes.

30K , Fig. 2: Probes for different applications.

The sound field characteristics of a probe, which are expected under normal inspection conditions, are generally derived from the diameter and the frequency of the piezoelectric element. Most often the user knows exactly these properties of "his" probe, but he isn´t quite familiar with the details regarding the physical and acoustic properties of the used piezoelectric material. However, this knowledge can not only contribute to a better technical understanding of the basic way in which the probe works, but also gives an insight into the behaviour of the probe under various conditions. This knowledge can be also helpful when trying to choose the correct or under given circumstances best working type of probe.

The following article is therefore intended to introduce the different types of piezoelectric materials currently in use. We aim to explain their strengths and weaknesses and to show, how they can be exploited for various probe designs. Additional application examples are given.

2. Piezoelectric materials

The materials from the early days of ultrasonic technology such as quartz, lithium sulphate or barium titanate are almost never used today. Instead, new powerful piezoelectric materials are available, but their basic acoustic and electric characteristics are very different. Depending on the application, the one or other material is more advantageous due to physical or economic reasons, or simply because of a less complicated manufacturing process.

Table 1 shows the important physical characteristics of piezoelectric materials used today to generate ultrasound. Lead zirconate titanate (PZT) is most probably the best known material. Along with lead titanate (PT) and lead metaniobate (PbNb2O6), it is one of today's ceramic piezoelectric materials.

Table 1 Characteristics of Piezoelectric Materials
Physical Property	Lead Zirkonate Titanate PZT-5	Lead Titanate PT	Lead Metaniobate PbNb2O6	Polyviylidenefluoride PVDF (Copolymere)	1-3 Composite
Acoustic Impedance Z [10 exp 6 kg/m²s]	33,7	33	20,5	3,9	9
Resonant Frequency f [MHz]	< 25	< 20	< 30	160 -10 (55 - 2)	< 10
Coupling Coefficient (thickness mode) kt	0,45	0,51	0,30	0,2 (0,3)	0,6
Coupling Coefficient (radial mode) kp	0,58	< 0,01	< 0,1	0,12 (k31)	~ 0,1
Relative Dielectricity er	1700	215	300	10	450
Maximum Temperature [°C]	365	350	570	80	100

The acoustic impedances of all the piezoceramic materials are basically high and comparable to many metallic or ceramic solids, but they differ. This is as well of technical importance as the differences in their relative dielectricities (r and their coupling coefficients kp for radial vibrations. The coupling coefficient for radial vibrations indicates to what extent a piezo-element converts energy into radial modes, which are perpendicular to the thickness mode. Radial modes should be suppressed, since they cause undesired signal distortions. They are located at low frequencies, because they correspond to the lateral dimensions of the piezoelectric element. Lead titanate is the only piezoelectric material with a neglegibly small kp coefficient.

Piezoceramics are used for up to a maximum of 30 MHz. The electromechanical coupling coefficient kt is a measure of the fracture of electrical energy, which is converted into mechanical energy in the thickness mode, in other words, a type of level for the efficiency of generating ultrasound in that vibrational mode, which is utilized in probes. The coupling coefficient of piezo ceramics is basically high. However, if ultrasound shall be radiated into liquids or plastics, most part of the acoustic energy generated by the piezoceramic element is reflected at the boundary between the piezoelectric material and the medium of propagation. Due to the low acoustic impedance of liquids and plastics (from 1.5 to 3x106 kg/m² sec) and the high impedance of ceramics the reflection coefficient at that boundary is far more than 90%. Therefore, only a fraction of the acoustic energy generated by the piezoelectric element is transferred into the medium of propagation or into the workpiece. Piezoelectric plastics like PVDF or related copolymers are better matched to the low acoustic impedance of liquids or plastics. In addition, they are mechanically flexible. With very thin foils it is even possible to generate ultrasound at frequencies up to 160 MHz. Unfortunately PVDF is less sensitive than ceramics, as can be recognized from the coupling coefficient kt for the thickness mode. Piezoelectric PVDF-foils should only be used to a maximum of 80°C, since depolarisation sets in and they loose their piezoelectricity at higher temperatures.

The so-called 1-3 composite materials [1,2] appear more promising. They have an extraordinarily high coupling coefficient kt , while the couplig coefficient of the transverse (radial) mode is quite low. In addition, they form an in-between stage between piezoelectric ceramics and polymers. Fig. 3 shows the schematic structure of 1-3 composite materials: They consists of ceramic rods, which are arranged in parallel to one another, embedded in a epoxy resin matrix. The rods are generally made of high-density PZT.

7K, Fig. 3. Schematic structure of a 1-3 piezo composite.

The result of such an arragement ist an overall low density and therefore a rather low acoustic impedance compared with piezoceramics . Due to the high difference in acoustic impedance between the ceramic rods and the epoxy resin filling, the acoustical cross coupling between the rods is rather small. This is the main reason for a lower planar coupling coefficient compared with pure PZT and ,accordingly, for a lower tendency towards exciting radial vibrational modes, which can interfer with the main thickness mode.

Due to the epoxy-filled spaces between the ceramic rods, such 1-3 piezo-ceramic elements are also mechanically flexible, at least to a certain degree. However, the temperature range is limited to approx. 100°C. At higher temperatures the resin starts to shrink and disintegrate, the piezoelectric elements therefore lose their mechanical stability.

The values shown in Table 1 for 1-3 piezo-composites are typical, but for a special type of design, only. They can be varied in a wide range by varying the piezo-ceramic material, epoxy resin and the aspect ratio of the rods and spaces in-between. Such a 1-3 piezocomposite behaves like an acoustically homogenous piezoelectric plate vibrating in the thickness mode, if the width of the rods and gaps between is smaller than the wavelength, typically within the range 0.1 to 0.2 mm for a nominal frequency of 4 MHz. This is the reason why current manufacturing processes of 1-3 piezocomposites are still very complicated, and why the probes manufactured in this way are most often too costly for every day use. The applications are therefore usually restricted to cases in which the technical aspects are of major importance. Therefore the use of relatively expensive 1-3 piezo-composite material is not recommended, if commonly used ceramic probes can also provide satisfactory results.

3. Use of piezoelectric materials in probes

The very different properties of the piezoelectric materials mentioned in section 2 are very useful for the design of completely different probe types. Depending on the intended application of the probe, that material is used, whose properties suit best.

As a result, lead metaniobate has the lowest acoustic impedance of all the piezo-ceramics (see Table 1). This is most advantageous with respect to short pulses and wide bandwidth. To achieve this, the acoustic impedance of the damping block generally consisting of homogeneous mixtures of heavy metal powder and plastics must be as close as possible to that of the piezoelectric material. The acoustic impedance of such a mixture is proportional to the volume fraction of heavy metal powder. The production process becomes the more complicated and expensive, the higher the damping block must be filled with metal powder. Lead metaniobate with its rather low acoustic impedance is therefore better suited for the design of high-resolution probes with extremely short pulses (so-called shock waves) which practically consist of only one or two half waves, similar to those produced by PVDF probes, whose damping blocks simply consist of plastics with a high ultrasonic absorption, e.g. nonpiezoelectric PVDF. Fig 4. shows the short and clean pulses, which are produced by lead metaniobate and PVDF immersion probes. The sensitivity of PVDF and lead metaniobate immersion probes are comparable to each other, provided probes of the same center frequency and the same crystal diameter are considered. As a rule, the lead metaniobate probe is only slightly more sensitive.

Lead metaniobate is the only piezoelectric material that can be used for shock-wave probes, which can be directly coupled to steel, metal or ceramics (straight beam direct contact probes). Fig. 5 demonstrates the short and clean pulse shapes of a 5 MHz lead metaniobate probe with 6 mm crystal diameter. Such good results can hardly be achieved with PZT.

12K, Fig. 4. Comparison between the pulse shapes: Immersion probes made of lead metaniobate (bottom) and PVDF (top).

5K , Fig. 5. Pulse shape of a braodband straight beam direct contact vertical probe ( 6 mm crystal diameter, 5 MHz).

The relatively low impedance of lead metaniobates is also useful for the design of broadband angle probes and TR probes. The closer the acoustic impedance of the piezoelectric material is matched to the impedance of the plastic wedge or delay line, the more effectively the wedge itself acts as a mechanical damping. Since the acoustic impedance of lead metaniobate is 6 or 7 times higher than that perspex or polystyrene, an acoustic matching layer is inserted between the piezoelement and the wedge (Figs 1b and c). This leads to a gradual transition between higher and lower impedance, resulting in a better damping of the piezo element as well as an improved transfer of energy from the piezo element to the wedge. The impedance ZM of the matching layer should fulfill the following equation

ZA = root( Zp * Zw)

whereby Zp and Zw represent the acoustic impedance of the piezoelectric material and the wedge. In this case, there is a maximum transfer of energy, i.e. the effect of the matching layer is optimized. As an example, Fig. 6 shows the typical pulse shape and spectrum of a 4 MHz angle probe. The result is representative for the state of the art angle and TR probes today.

Due to the small coupling coeffient kp radial vibrational modes of the lead titanates are neglectable. Therefore, this material can be utilized to produce probes with especially small measurements. Fig. 7 shows a subminiature angle probe consisting of lead titanate of only 2 x 3 mm as the piezoelectric element. Its frequency is 10 MHz. The use of PZT would not be possible here: The final transverse coupling of PZT would results in parasitic and interfering transverse and radial vibrations, having the same amplitude as the thickness vibration. And due to the small lateral dimensions, the frequency of those parasitic vibrations would be in the vicinity of the thickness mode. The result would be considerable and not tolerable distortions in the signal shape, frequency shifts and interfering low-frequency signals. All those influences affect probe properties such as the near and far field resolution and cause a bad signal to noise ratio. Such a subminiature angle probe is used to find defects in electron or laser beam welded seams in specimens with tiny measurements, e.g. thin titanium tubes.

13K, Fig. 6: Pulse shape and the spectrum of a piezoceramic angle beam probe (9 x 8 mm, 4 MHz) with matching layer.

10K, Fig. 7: Subminiature angle beam probe (2 x 3mm, 10 MHz).

In workpieces with large measurements and high ultrasonic absorption, it is important that the probe generates ultrasound of higher intensity than usual, so that even echos from the far end can still be detected. In such cases, lead zirkonate titanate (PZT) is still frequently used due to its high electromechanical coupling coefficient. As an example Fig. 8 shows a low frequency probe, which is used to detect flaws in strongly absorbing and scattering materials like concrete and graphite. It is designed for a nominal frequency of 40 kHz. Here, a special stacking technique is applied, as it has been used before to reduce the resonance frequency of PVDF foils. The principle is described in the literature [3] The thickness d of a bulk PZT element, which produces a frequency of 40 kHz, is roughly 50 mm. However, electrical spark transmitters of common flaw detectors are not capable of driving such a thick piezoelement efficiently. At a given excitation voltage V, the electrical field strength in the material, which is given as voltage divided by thickness, and which is directly responsible for the amplitude of the generated ultrasonic wave, becomes too low. Instead, a number of N thinner elements are stacked on top of each other to achieve a total thickness of 50 mm. Such a piezo stack is shown in Fig. 8 next to the probe. Electrically, the individual disks are switched in parallel, but they have an alternating polarisation sequence. When applying a voltage, the total stack vibrates as though a bulk disk of total thickness d had been energised by a voltage with a factor of N. Such a stacked probe can be used with every ultrasonic instrument, whose receiving amplifier is designed for this frequency range. An additional damping block can be glued onto the back face of that piezostack to get a reasonable pulse shape and spectrum.

26K, Fig. 8: Low frequency probes (transmitter/receiver) using a stacking technique.

8K, Fig. 9: Pulse shape of a pair of 40 kHz low frequency probes.

In the receiver probe it is better to use a bulk PZT element of appropriate thickness, because at a given input pressure the voltage across the electrodes of a piezelectric element is proportional to its thickness. This is why probes of that kind are always used in a transmitter receiver mode with separate transmitter and receiver probes. Fig. 9 shows the pulse shape, which is achieved with such a pair of transmitter receiver probes with center frequencies of 40 kHz each. Because of their short pulses such probes are superior to the commonly used resonant probes. Due to the clear pulse shape with a short rise time automatic thickness gaging with a high level of accuracy is also possible with such probes.

At high frequencies the use of piezoceramics is limited. It is well known, that the resonant frequency of piezoelectric disks vibrating in the thickness mode is reversely proportional to the thickness: Above 20 MHz piezoeceramic disks become thinner than 0.1 mm. They are brittle and hard to handle during the maufacturing process of the probe. Thin foils made from PVDF (polyvinylidenefluoride) are easier to handle. Due to their low acoustic impedance, an effective transfer of ultrasound is only possible into water or plastic materials.

As an example Fig. 10a shows a 30 MHz delay line probe made of piezoelectric PVDF. Here the PVDF foil is glued onto a 4mm thick plastic "delay line" . The other end of the delay line protrudes from the probe casing. The A-screen in Fig. 10b shows the high resolution, that is possible with this probe: Interface and backwall echoes of the 100 µm thin plastic foil used as a test specimen are clearly visible. Time sweep is 50 ns/Div. Even multiple echoes appear without interfering each other.

11K, Fig. 10a: 30 MHz delay-line probe made of PVDF - Actual design

8K, Fig. 10b: 30 MHz delay-line probe made of PVDF - Echoes from of a 100 µm thick plastic foil

Even at "normal" frequencies the high flexibility of PVDF and its low impedance being close to that of water, are useful for the design of special line-focussed immersion probes (Fig. 11). The PVDF foil as the source of ultrasound is precisely bent cylindrically. In this way, even the sound field is cylindrical and therefore well matched to cylindrically shaped workpieces such as tubes and rods. The PVDF foil is located behind a protective layer with a thickness of approx. 120 µm, which also works as an acoustical band pass suppressing the high frequencies produced by the probe. Instead of the nominal foil frequency of approx. 28 Mhz, the actual test frequency is reduced to 4 MHz, a value, which is established in most acceptance test specifications for pipes. Such line focussed PVDF probes have been used since 1990 in high speed test systems for steel pipes. They are described in more detail in other publications [4,5].

8K, Fig. 11: Line-focussed immersion probe made of PVDF.

Due to their relatively low acoustic impedance and their high sensitivity, 1-3 piezo-composites are also very suitable for generating ultrasound in liquids and plastics. However the production of such probes is less complicated than the manufacturing process of the piezo-element itself. Damping resp. backing materials with corresponding low impedance are easily found or produced. They don´t need such high metal powder fillings than the backing materials for piezoceramics. Therefore, broadband piezocomposite immersion probes can easily be produced. Fig. 12 shows the pulse shape and the appropriate frequency spectrum of such a probe. The basic sensitivity of such immersion probes can be 6 dB higher than for normally used piezoceramic probes. It should be mentioned however, that due to the periodic structure of the piezocomposite material, the sound field distribution is not as equal as in the case of homogeneously oscillating bulk piezoelectric elements [6]. A fine structure corresponding to the periodical structure of the piezocomposites may be superimposed to the sound field, generally expected from a bulk piezoelectric element of the same size and frequency. However, mostoften the effect of the fine structure on the ultrasonic inspection results can be neglected. Basically, 1-3 piezocomposites are also suitable for the production of line-focussed immersion probes. Compared with the less expensive PVDF-foils, their use would only be justified, if previously used PVDF probes were not sufficiently sensitive, to find the specified test flaws. But to date, this has not been the case. Fig. 13 shows the pulse shape and spectrum of a line-focussed piezocomposite probe with a focal distance of 85mm. When compared with normally used PVDF probes, the gain in sensitivity here is approx. 18 dB.

9K,
Fig. 12: Pulse shape and spectrum of an immersion probe made from 1-3 piezo-composites.
Fig. 13: Pulse shape and spectrum of a line-focussed immersion probe made from 1-3 piezo-composites.

From the viewpoint of the probe production technology, the use of 1-3 piezo-composites in angle beam and TR probes is particularly advantageous. Since the impedance of the piezocomposite material is only about twice as high as the impedance of plastics, the proper bonding of the piezo-element onto plastics alone causes braodband transmission, provided that the adhesion layer is small enough. The use of 1-3 piezo-composites has proven to be even advantageous in the manufacturing of paint brush TR probes. With such probes the sensitivity should be as high as possible, since the echo height is proportional to the ratio of flaw size to crystal size. If the crystal sizes becomes too large the flaw echo will be lost in the electronic noise. The high sensitivity of the 1-3 piezo-composites could be helpful in this specific case. It should be mentioned however, that similar results can be achieved by piezoceramics and appropriate matching layers.

4. Closing remarks

The currently available piezoelectric materials complement each other in their physical characteristics. In the design of ultrasonic probes, it is very important to use a material whose characteristics suit best. In addition to pure technical aspects, economy also plays a role for the user, as well as for the manufacturer. As a result, PZT has developed into an economical piezoelectric material over the years. This was not the case in the beginning. We can therefore expect that the promising piezo-composites will undergo a similar development. At this time they can already help to solve many special tasks.

5. Literature

1. Hossack, J.A., Auld, B.A. Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12.
2. Chofflet, L., Gauchet, M., Tellier, J.M.: Which Piezoelectric Material for which Transducer? Revue Annuelle LEP 1990, 37-38.
3. Swartz, R.G., Plummer, J.D. : On the Generation of High-Frequency Acoustic Energy with Polyvinylidene Fluoride IEEE Trans. Sonics Ultrason. SU - 27 (1980), 295 - 303.
4. Deutsch, V., Platte, M., Möller, P. : Ultraschallprüfköpfe aus piezoelektrischen Hochpolymeren. Materialprüfung 32 (1990), 333 - 337. (Ultrasonic probes made of piezoelectric high polymers ).
5. Platte, M., Möller, P: Automatisches Ultraschallprüfen von Blechen und Rohren. Bänder, Bleche, Rohre 34 (1993), 25 - 32 (Automatic ultrasonic inspection of plates and pipes).
6. Richter, K.P., Reibold, R., Molkenstruck, W.: Sound field characteristics of ultrasonic composite pulse transducers. Ultrasonics 29 (1991), 76 - 80.

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   Authors:

   M. Lach, M. Platte, A. Ries
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