Evaluating Lead Zirconate Titanate Materials in Single-Element Piezo Transducers
What is the real performance difference between traditional dense PZT, porous PZT and composite PZT in single-element transducers?
With so many choices available in lead zirconate titanate (PZT) materials and processing techniques, making a decision on the direction to pursue can be difficult. What is the real performance difference between traditional dense PZT, porous PZT and composite PZT in practice? Identical single-element transducers were used as a test bed to evaluate the differences in performance of these three styles of PZT materials in four different operational categories.
Transduction, by definition, is the conversion of energy from one form to another. When discussing piezoelectric transducers specifically, transduction is the conversion of mechanical to electrical energy and/or vice-versa. This is achieved by engineering the transducer around an active piezoelectric ceramic element.
The piezo transducer’s ability to convert mechanical energy to electrical energy and vice-versa relies mostly on the mechanical design of the transducer and how well the piezo element is optimized within the design. While the piezo element itself makes the generation and detection of sound waves possible, the mechanical design of the transducer allows for efficient transfer of the sound energy to and from the transducer and the medium through which the sound waves propagate.
Transducers that generate audible-range sound waves can be used for applications such as speakers, buzzers and alarms. Those that generate ultrasonic-range sound waves (above 20 KHz) can be used to measure distance, determine rate of flow, monitor fluid level, atomize various liquids, perform medical imaging, weld plastics or metals, monitor structural health, and find fish underwater.
For simplicity’s sake, this article will focus only on single-element transducers, as they are often basic in design and a cost-effective option for signal generation and sensing applications. By definition, single-element transducers contain one active ceramic element. This often results in a simple design and therefore a reduced cost for the end user. These transducers can be easily designed as transmitters, receivers, or both simultaneously.
As shown in Figure 1, single-element transducers are made up of four primary components: the piezoceramic element, the backing layer, the matching layer, and the housing/cable assembly. (For a more in-depth discussion, see “Transducer Basics: Ceramics in Transduction,” Ceramic Industry, June 2017.)
When designing a single-element piezo transducer, the type of PZT material used is among the first things to consider. What environment will the transducer be exposed to? Environmental considerations include excessive temperature, aggressive atmosphere, chemical exposure, etc. This could drive a piezo choice based on the Curie temperature or other material properties. What about the type of electronics ultimately driving the transducer? This may require choosing a piezo material based on electrical impedance or other electrical properties.
The focus of this article is not to compare different PZT compositions necessarily, but three different styles of PZT components all based on the same basic composition: traditional dense PZT, porous PZT and composite PZT. Materials chosen for the comparison featured high Curie temperatures, relatively low impedance, excellent aging stability, and temperature stability over a broad range.*
The dense PZT used typically features 96%+ of theoretical density, electroded with standard Ag electrodes (see Figure 2). The composite PZT is comprised of 250-µm-diameter fibers, randomly arranged and supported in an epoxy matrix. The PZT fibers make up approximately 65% of the volume of the component, with 35% being the supportive epoxy matrix. Electrodes were CuSn. One major drawback of the composite-type material is that the maximum operating temperature is generally lower than that of a dense or porous PZT component.
It seems counterintuitive to induce porosity in a material that most everyone wants to be as dense as possible. Similar to composites, porous PZT has a lower acoustic impedance than typical dense PZT. The efficiency of a transducer is dramatically improved when the acoustic impedance of the transducer is matched as closely as possible to the acoustic impedance of the medium that the transducer is designed to operate within.
With typical dense PZT having acoustic impedance in the 30-35 MRayl range, multiple acoustic impedance matching layers are generally required to operate the transducer efficiently in liquid or air mediums (most liquids fall in the 0.5-2.5 MRayl range, excluding liquid metals). Porous PZT materials have an acoustic impedance of only 16-17 MRayl. This results in fewer matching layers for similar efficiencies, as well as an increased bandwidth, which dramatically increases the usable range of any transducer.
The density of PZT and its acoustic impedance are certainly related. Reducing the density of the component results in reduced acoustic impedance, but the relationship is not exactly linear. In most applications, a fugitive can be added to the material and removed later via combustion. However, sintering profiles must be optimized to control the pore size and the pore distribution (see Figure 3).
Since we are only evaluating changes in the piezo component material, it was decided for this test that all other variables relating to the transducers should be fixed. Identical “Peek” housings were machined in-house for multiples of each of the three types of transducers. The matching layer would be the transducer housing face, which would be hand-worked into ¼ wave matching for all samples. The piezo bond to the housing would be made at the same time, using the same material and at the same bond thickness. All of the piezo elements would be 10 mm in diameter, have a wraparound electrode, and be air backed in the transducer.
The thickness of the piezo components would vary to achieve the same 2-MHz resonance frequency for each piece. Since most single-element transducers operate at resonance, this (2 MHz) would be a meaningful target frequency as it is commonly used in flow sensing, level detection, velocity metering, and other applications.
The performance of each of the three types of piezo materials was evaluated in four operational categories: electrical impedance, sensitivity, acoustic power output, and total bandwidth.
A basic direct electrical impedance measurement (in water) was taken at resonance using an HP4294 impedance analyzer.
Sensitivity was measured using a pulse echo test in water (comparative measurement) by generating pulses around the 2 MHz operating range and maximizing peak-to-peak voltage output. Drive conditions included a 20-cycle tone burst at 2 MHz (10 Vpp into 50 ohm load) at 4-in. spacing to a stainless steel reflector. Data is presented in dB gain, with a less negative value indicating higher sensitivity.
Acoustic Power Output
Acoustic power output was measured using an Ohmic acoustic power meter driven at 1 W electrical input.
Bandwidth was measured using a pulse echo test in water by maximizing sensitivity (voltage output PP) and sweeping up and down in frequency until sensitivity was half of its maximum (-6 dB) in both the positive and negative sweep direction, then summing the total measured range and dividing by the center operating frequency, resulting in % bandwidth.
A summary of the average results can be seen in Table 1. When comparing the porous material to the standard dense material, an increase in sensitivity of 10% with an increase in total bandwidth of over 60% is observed. The same or similar is true when comparing the composite material to the dense PZT. An increase in sensitivity of approximately 10% and in total bandwidth of about 60% was observed with the composite PZT-based transducers as well.
The results of the comparisons show that both composite and porous PZT materials exceed the performance of traditional dense PZT materials in single-element transducers. With significantly wider bandwidths, slightly higher sensitivities, and similar electrical impedances, composite and/or porous PZTs are worth considering for many different applications. Porous PZT provides all of these advantages with no real performance disadvantage, while composite PZT exhibits the same performance benefits but may have limitations in use temperature due to its epoxy-based matrix when compared to traditional dense PZT.
*APC-850, traditional dense; a Navy II fiber composite; and APC-860, porous