INTRODUCTION
In nearly two decades since a promising lead-free piezoceramic based on potassium sodium niobate was reported in Nature in 2004 [1], there has been continued research on the topic of lead-free piezoceramics and a rapid and continuing growth in publications on the topic [2, 3]. Several families of candidate materials have been identified including, sodium bismuth titanate (NBT), barium titanate (BT), bismuth potassium titanate (BKT) in addition to the aforementioned potassium sodium niobate (KNN) [3–6]. Evidence of these materials used within industry does exist [5]; however the uptake of lead-free piezoceramics to-date has been slow.
The ubiquity of PZT is partly to blame for the slow uptake of lead-free alternatives, however several other factors exist in addition.
The greatest of these are arguably commercial availability of lead-free alternatives and a (historically justified) perception that lead-free piezoceramics offer poorer performance than lead-based alternatives. In addition, the inclusion of exemption clause 7(c)−1 in the EU RoHS directive exempting the use of lead within piezoelectronic device [7] provides relief from an immediate pressure to transition away from lead-based piezoceramics. However whilst this exemption currently exists, it is periodically reviewed [8] with a view to eventually being removed. Should this exemption be withdrawn there will be a pressing legislative motivation for the use of lead-free alternatives.
In the absence of a ‘push’ towards lead-free materials from legislation, the ‘pull’ of lead-free materials does not yet exist since they are not an attractive proposition either technically or commercially owing to their lacklustre performance. Indeed, a key finding in the various review papers currently published on the topic is the need for further collaboration between researchers and industry in order to steer the development of lead-free materials in a direction which promotes their uptake within industry [9, 10]. Thus, a key question surrounding the use of lead-free piezoceramics is performance. Existing reports show that lead-free materials do not offer as readily the same levels of performance as their lead-based alternatives [3, 11]. Reports on the topic of lead-free piezoceramics commonly focus on materials properties, notably the piezoelectric coefficients. For ultrasonic transducers operating in a thickness mode the piezoelectric charge coefficient d33, thickness coupling factor Kt, Curie temperature Tc and mechanical quality factor, Qm [9, 12] are of particular relevance. However whilst these constants are useful in selecting the appropriate piezoceramic for a particular application they are often only good indicators rather than a precise determination of performance. The piezoelectric charge coefficient, d33, for example, is often seen as the indicator of merit for the performance of ultrasound transducers operating in a thickness mode; however when comparing two materials with different coefficients it is not always clear which will perform better in a given application. It is of course the combination of all materials properties and the design of the transducer which gives the most accurate picture of performance.
This report aims to evaluate the performance of a formulation of NBT-BT piezoceramic (Pz12, CTS Ferroperm, Kvistgaard, Denmark) with a view to facilitating the uptake of lead-free materials within industry by presenting the results of a series of tests on transducers fabricated with this material. The data presented here is intended to yield insight into the applicability of this new material to applications within the industrial, medical and underwater sectors. Relevant applications may include: Non-destructive testing and structural health monitoring (NDT/SHM), flow monitoring, thickness gauging, therapeutic ultrasound, medical and underwater imaging.
MATERIALS AND METHODS
Transducer design
Two immersion transducers were made with the lead-free Pz12 piezoceramic. The design used 25 mm discs of Pz12 at a thickness of 1.2 mm for a thickness-mode resonance at 2 MHz. Each disc had a fired silver electrode coating to allow soldering of electrode wires. Both transducers comprised a single ¼ wave impedance matching layer, stainless steel housing, 1.5 m of integrated RG58 cable with BNC connector as per Figure 1. A 0–3 tungsten-Epoxy composite with impedance of 6.7 MRayl was used for the acoustic impedance matching layer both transducers, this was made with 1.5 μm tungsten and a low viscosity unfilled epoxy.
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Integral electrical impedance matching circuitry was designed to match the transducers to a 50 Ω impedance. One of the transducers was backed with a high-density Tungsten-epoxy composite, as commonly used within transducers requiring high levels of mechanical damping. The second transducer was backed with a low density syntactic foam providing a minimal level of mechanical damping to the piezoceramic.
Two additional transducers were constructed for benchmarking purposes using 25 mm discs of a PZT, Navy Type 1 material; Pz26 (CTS Ferroperm, Kvistgaard, Denmark), but were otherwise identical. Table 1 lists the key properties extracted from the datasheet provided for the Pz12 material alongside those of Pz26 for comparison.
TABLE 1 Materials properties for Pz12 and Pz26.
| Pz12 | Pz26 | |||
| Φ | Relative dielectric constant | 700 | 1330 | |
| Tan ɗ | Dielectric losses | % | 2.7 | 0.3 |
| Tc | Curie temperature | ˚C | 200 | 330 |
| d33 | Piezoelectric charge coefficient | pC/N | 110 | 328 |
| Kp | Planar coupling factor | % | 17 | 56 |
| Kt | Thickness coupling factor | % | 41 | 47 |
| Np | Planar frequency constant | m/s | 2700 | 2210 |
| Nt | Thickness frequency constant | m/s | 2400 | 2038 |
| Z | Acoustic impedance | MRayl | 29 | 32.3 |
| Qm | Mechanical quality factor | >100 | >1000 | |
| ρ | Density | kg/m3 | 5700 | 7700 |
Optimum matching layer impedances calculated for both the Pz12 and Pz26 transducers from values in Table 1 according to a geometric mean formula [13], Equation (1), yielded values of 6.6 and 6.9 MRayl for Pz12 and Pz26 respectively.
Based on a simplified analysis of transmission and reflection coefficients, the chosen impedance of 6.7 MRayl was predicted not to impact the transmission efficiency of both variants of transducer and the single 0–3 composite could be considered practically optimized for both piezoelectric materials.
Acoustic testing—Pulse echo measurements
The transducers with tungsten-epoxy backings were initially tested in a pulse-echo configuration using a pulser-receiver (DPR300, Imaginant, Pittsford, USA) with a 40 mm water path and flat aluminium block of 40 mm thickness (Figure 2). Pulser settings were maintained between testing of the two transducer variants to ensure a side-by-side comparison.
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The received echo off the aluminium plate was recorded for both Pz12 and Pz26 based transducers (Figure 5, placed after Figures 3 and 4 for convenience in combining it with the other results). The corresponding frequency spectrum was then derived from this waveform using a Fast Fourier Transform method (Figure 5).
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The electrical insertion loss (Il) was calculated for each transducer as per (2)
Transducer efficiency
Tests on transducer efficiency and power output were carried out using a radiation force balance (RFB). The transducers with syntactic foam backings were used for these tests due to their applicability to high power applications. The RFB was a suspended target type (Precision Acoustics, Dorchester, UK) [14] using the relevant correction factors specified in IEC 61161 [15]. For these RFB measurements the transducers were driven with a continuous sine wave input from a function generator (33500B, Agilent, Santa Clara, USA) directed through a 55 dB RF amplifier (A150, E&I, Rochester, USA). Measurements of transducer efficiency as a function of frequency were initially performed to determine the most efficient operating frequency for each transducer. Here we are defining efficiency as the acoustic energy produced as a percentage of the electrical energy consumed.
Measurements of supplied electrical power were made using current and voltage probes as per Figure 3. Drive voltage was measured at a T-piece on the output of the amplifier using a high impedance passive probe whilst the current was monitored at the same junction where a short section of coaxial cable core had been exposed from the braid to allow clamping of the current probe (1147B, Keysight Technologies, CA, USA).
From theory, input electrical power can be measured with a combination of probes measuring voltage and current across the terminals of the transducer provided the load is purely real [16]. On the basis that the electrical impedance matching of the transducers was sufficient to approximate their impedance as ( and the drive waveform contained only a single frequency. The power, P, dissipated in a load of complex impedance is given by:
Upon establishing the most efficient operating frequency for each transducer, measurements of efficiency as a function of drive voltage were made at this frequency by varying the drive voltage from 30 Vpp to more than 200 Vpp. At low voltages a 10-s duration of continuous sine wave drive was used to determine acoustic power output. At higher voltages a 5-s continuous drive was used to avoid excessive internal heating of the transducers. For the Pz12 based transducer the low to high voltage threshold was 200 Vpp and for the Pz26 based transducer it was 212 Vpp. Measurements of acoustic output power as a function of drive voltage were made concurrently.
Transmitting voltage response
To provide a benchmark of performance for the underwater acoustics sector, measurements of the transmitting voltage response (TVR) of the transducers was made using a needle hydrophone probe (Precision Acoustics, UK) in an immersion test setup as per Figure 4. Tests were performed with a transducer-hydrophone separation of 1 m to yield results in the industry standard form: dB re. 1μPa/V @ 1m.
Temperature measurements
To characterise the effect of temperature on the materials, discs of Pz12 and Pz26 were bonded to a block of Aluminium using an epoxy resin. A direct electrical connection was made to either face using a high temperature coaxial cable. The discs were driven simultaneously each with its own pulser/receiver unit and the peak-peak amplitude of the rear surface echo from the aluminium block was monitored as the indication of performance. The assembly was immersed in a small water bath to create a stable temperature environment and was placed in an oven to raise the temperature to 75°C whilst measurements of the reflection amplitude were acquired periodically. These simple contact ‘transducers’ were used as transducers in their most basic form to try to isolate the effects of temperature on the piezoelectric material from the effects of temperature on any of the other constituent parts such as the electrical matching network or the degree of wetting of the radiating face. Transducers were also monitored during a subsequent natural cooling back to ambient temperature.
RESULTS
Pulse echo performance
Figure 5 shows the pulse echo performance of the Pz12 based transducer to be 4.3 dB lower in amplitude than the Pz26 based transducer. Pulse duration (–6 dB) was slightly shorter for the Pz26 variant at 1.1 μs versus 1.7 μs for the Pz12. This slight difference was reflected in the bandwidth for both transducers (Figure 5), which at −6 dB level was found to be 42% for the Pz12 variant and 56% for the Pz26 variant. There was a slight difference in centre frequency for the two transducers which can be attributed to differences in the electrical and mechanical matching networks and a natural inter-device variation due to manufacturing tolerances.
Despite the lower comparable amplitude, the Pz12 based transducer had sufficient amplitude that several multipath reflections were clearly visible from within the aluminium test block (Figure 6). The level of performance observed was sufficient to allow the Pz12 based transducer to be used for time of flight ranging in an immersion setup as well as thickness gauging in metallic media. The SNR of the received signal was sufficiently high that thickness gauging could reliably be performed on material with thicknesses in excess of the 40 mm thickness used here. In addition, these measurements were made without the use of receive gain on the pulser/receiver which would allow for an even greater ability to resolve time of flight within thicker or more highly attenuating materials.
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The electrical insertion loss of the Pz12 based transducers was found to be on average 3.1 dB lower than that of the Pz26 (Figure 7). This is a similar figure to the −4.3 dB difference observed in the relative pulse-echo amplitudes of the transducers with the 1.2 dB difference most likely as a result of the slight variation in impedance causing a variation in terminal drive voltage between the two transducers. Notably the electrical insertion loss decreases with increasing drive amplitude and this is consistent for both the Pz12 and Pz26 transducers. The scale of the decrease is such that at higher drive amplitudes the lead-free transducer provides a lower insertion loss than that of the Pz26 variant at lower voltages; driving the lead-free transducer at 350 Vpp yields an equivalent insertion loss to driving the Pz26 transducer with approximately 250 Vpp.
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Power output measurements
Acoustic power measurements (Figure 8) show that the lead-free transducer was able to produce 80 W of acoustic power for an input drive voltage of approximately 280 Vpp. Given the 25 mm active diameter of the piezoceramic, this equates to a 16 W/cm2 equivalent at-surface intensity for the device. Had the transducer incorporated a cooling system such as a forced air or water cooling it would have been possible to exceed the 80 W output power since the experiment was only halted due to internal heating in the transducer rather than due to it reaching a drive saturation. Using the 52% efficiency figure for the Pz12 transducer at its highest drive amplitude, an observed acoustic power output of 80 W indicates a total input power of 154 W and therefore 74 W of power contributing to the thermal energy build-up in the device. The Pz26 transducer was able to produce just under 90 W of acoustic power under the same drive regime and required a 22% reduction in drive voltage to produce the same power output as the lead-free transducer. The ability to achieve a higher power output with the Pz26 transducer is a feature of its higher electro-acoustic efficiency (Figure 9) which means a lower proportion of incident electrical power contributing to transducer heating.
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Efficiency measurements for the Pz12 transducer range from 44% at low drive amplitudes to 52% at high drive amplitudes (Figure 9), whilst the range of efficiencies of the Pz26 transducer spans 50–62%. Higher efficiencies are reported at higher drive levels and this is consistent with the reported electrical insertion loss measurements.
The 52% efficiency figure reported here should not be considered as the upper limit for this material. Further gains could be made by, for example, reducing the density of the backing material or leaving the transducer air-backed, adding additional impedance matching layers or reducing the clamping effect of the housing on the edges of the piezoceramic. Efficiency bandwidth measurements (Figure 10) show the Pz26 transducer to be more efficient over very nearly the entire useable bandwidth of the transducers, even at the peak frequency for the Pz12 transducer. Whilst the pulse-echo bandwidth matches very closely the shape of the efficiency bandwidth for the Pz12 transducer, the efficiency bandwidth of the Pz26 design peaks at 2 MHz whilst the pulse-echo bandwidth peaks at 2.3 MHz. This could be due to differences in the resonant frequency of the piezoceramic and the matching layer/electronics. Under a continuous drive regime (efficiency measurements) it appears that the thickness mode resonance of the piezoceramic has a greater influence than the matching resonances which skew the centre frequency up to 2.3 MHz during pulse-echo drive conditions. This could lead to the observed differences in peak frequency between efficiency data and pulse-echo data.
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TVR measurements
Measurements of the TVR (Figure 11) show a similar picture to the pulse-echo data where the Pz12 transducer was found to perform better than the Pz26 transducer over the small section of bandwidth immediately below the 2 MHz resonance. However, at all other frequencies the Pz26 transducer outperformed the Pz12 by an average of 3.1 dB. This is in-line with electrical insertion loss measurements and pulse-echo data which shows the Pz12 transducer to under-perform by 3.1 dB and 4.3 dB respectively.
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Temperature measurements
Measurements of the performance of the Pz12 and Pz26 materials as a function of temperature (Figure 12) show very little difference between variants over the 10–70 ˚C range. A 25% drop in signal amplitude observed over the 20–70 ˚C range and a further 4% drop in amplitude is seen in the Pz26 from 70–75 ˚C whilst the Pz12 shows an 8% drop over the same range. A similar hysteresis is observed in the data between heating and cooling in both variants. It is possible that some of the variation with temperature can be attributed to the effect of temperature on the adhesive bond between piezoceramic and aluminium block however this is likely to be very small owing to the thinness of the bond line.
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CONCLUSIONS
In all tests the lead-free transducers (Pz12) underperformed against the Pz26 benchmark, however the magnitude of this underperformance was typically only 3–4 dB. Whilst this reduction in performance may be significant in critical applications where sensor systems are working close to their limits, for less demanding applications the lead-free Pz12 piezoceramic appears to be a perfectly functional alternative to Pz26.
Suitable applications may include imaging, NDT, time of flight measurements—thickness monitoring and liquid level sensing in addition to applications requiring the delivery of significant levels of power; therapeutic medical or HIFU for example. The conclusion for usefulness within the latter (HIFU) echoes that of another recent study [18].
Considering the transition from lead-based piezoceramics to lead-free, in many cases it will be possible to reclaim this 3–4 dB front-end signal loss by other means. This may include adding extra gain to receive circuitry for pulse-echo applications or using higher drive amplitudes to achieve the same response. In high power applications additional engineering may be required to accommodate the lower efficiency such as active cooling systems or additional matching layers to improve efficiency. From a commercial viewpoint it is hard to justify investing in these changes whilst lead-based piezoceramics are so readily available. In other instances, the drive to outperform a competitor's product may also add to the hesitancy in the uptake of lower performing materials. However, in the event of new legislation requiring the use of lead-free materials within the EU, these commercial drivers will be superseded.
Notwithstanding any potential forthcoming regulations, existing legislation in some industries may already be working to nullify these commercial drivers. Medical ultrasound devices are already working up to the physical limits imposed by industry standards such as IEC60601-2-37, which imposes strict criteria on acoustic intensity and many other parameters of note [17]. Manufacturers are already capable of producing devices which can greatly exceed these parameters but are required to limit performance in the interests of patient safety. In these cases, for example, the transition to lead-free designs may not lead to any noticeable loss of performance to the end user.
Opportunities for further work exist in the characterisation of additional lead-free piezoceramic formulations as and when they are developed. If formulations can be tailored to target specific applications then there should exist the opportunity to further enhance the performance over the results reported here.
An additional avenue may also include the characterisation of lead-free piezocomposites with a view to assessing performance for applications which are increasingly using 1–3 or 2–2 composites, for example, within underwater acoustics and the sonar industry where transducers with low operating frequencies already make good use of these technologies.
Further characterisation (of these additional materials) will certainly be of benefit in promoting the wider uptake of lead-free piezoceramics.
AUTHOR CONTRIBUTIONS
Thomas Kelley: Funding acquisition, Resources, Supervision, Writing-review and editing, Investigation, Methodology, Validation, Writing-original draft
ACKNOWLEDGEMENTS
The author would like to thank CTS Ferroperm Piezoceramics for providing the Pz12 samples for this work.
CONFLICT OF INTEREST STATEMENT
The author declare no conflicts of interest.
FUNDING INFORMATION
This work was funded by Precision Acoustics.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Abstract
An evaluation of the performance of a recently developed formulation of (lead‐free) Sodium‐Bismuth titanate – Barium Titanate (NBT‐BT) piezoceramic is presented through the construction and testing of several immersion transducers based around a new material formulation “Pz12” from CTS Ferroperm (Kvistgaard, Denmark). Results are intended to yield insight into the applicability of this lead‐free material for use within industrial, NDT, medical or underwater sectors through the reporting of experimentally realised performance characteristics. Results of several performance tests are reported including: pulse‐echo amplitude, electrical insertion loss, transducer efficiency, power output and transmitting voltage response (TVR). Benchmarking is provided against comparable lead zirconate‐titanate (PZT) based transducers using an industry standard Navy Type 1 (Pz26, CTS Ferroperm). Performance of the Pz12 transducers was found to exceed expectations, although was lower than that of the comparable Pz26 transducers. Pulse‐echo amplitudes indicated a −4.3 dB relative amplitude between lead and lead‐free variants with electrical insertion loss and transmitting voltage response both showing an average −3.1 dB between the Pz12 and Pz26. Electroacoustic conversion efficiencies of up to 52% are reported for the Pz12 transducers compared with a 62% maximum efficiency for the Pz26 variant. Temperature stability is also investigated in the 10–70 ˚C range.
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Details
1 Sensors department, Precision Acoustics, Dorchester, DT2 8QH, UK