Technical Articles

Piezocomposites Enhance Transducer Performance

Piezoelectric composite materials, manufactured by Morgan Electro Ceramics, dramatically improve the performance of ultrasound and sonar transducers used in medical, biometric, military and industrial applications. By fabricating piezoelectric ceramic into precision pillar arrangements with a polymer matrix filler, the composite configuration will outperform a monolithic component constructed solely of the same piezoelectric ceramic. The main drivers for using composites typically are lower acoustic impedance (i.e. better density matching between the device and substrate) and cleaner frequency sweeps (i.e. higher sensitivity).

The fundamental advantage of the composite design is that it addresses the mismatch between the density of piezoelectric ceramic (6 - 8 g/cm³) and the density of the medium through which the sound waves travel – water and body tissue have a density of approximately 1.0 g/cm³. Such a density mismatch can cause a considerable amount of reflection, which degrades the signal strength and the signal-to-noise ratio. That is, one issue with high performance ceramics is that their acoustic impedance is very high (around 30 Mrayls) compared with say human tissue (1.5 Mrayls). This large difference in acoustic impedance causes losses in the reflected wave. A better match in acoustic impedance would reduce such losses. Finely pitched composites act as one homogeneous material with a lower acoustic impedance (as low as 10Mrayls) and hence provide better resolution of the acoustic device.

By combining the piezoelectric ceramic and a polymer filler (with a density of 1-2 g/cm³), the overall density of a composite structure will better match the medium. As a result, the lower acoustic impedance allows for a higher energy transfer through body tissue or water, and therefore a lower reverberation level on the front face of the acoustic device.

Two Architectures

Morgan Electro Ceramics’ piezoelectric composites are available in two architectures. [See Figure 1] The 1-3 structure is so named because pillars of ceramic are continuous in one dimension (the height or Z-axis), while the polymer is continuous in all three dimensions. In the 2-2 structure, both the ceramic and the polymer filler are continuous in two dimensions (height and length or the X and Z axes), with lengths of ceramic and polymer arranged in parallel.

Figure 1: Piezocomposite architectures: 1-3 structure and 2-2 structure. 

Each type has advantages and disadvantages which make it more or less suitable for a particular application. For example, high-frequency medical applications benefit from the added mechanical strength of the 2-2 structure, where the continuity of the ceramic along the X-axis provides added strength to make up for the very thin ceramic slices. The 1-3 structure, on the other hand, is often employed in sonar applications where the relatively large volume of polymer lowers acoustical impedance to maximize highly efficient transmission. However, there is no hard and fast rule about which architecture is preferred for a particular application, as 2-2 can be used for sonar and 1-3 can be used for medical ultrasound.

Enhanced Efficiency and Sensitivity

In addition to the performance advantages deriving from lower density and lower acoustical impedance, piezocomposite materials also benefit from a damping effect of the polymer which reduces lateral vibration modes, cross coupling and spurious activity, resulting in enhanced thickness mode transmit and receive efficiency and improved transducer bandwidth. Phased array transducers benefit from the low cross coupling among neighboring elements. 

For example, biometric applications such as scanning finger prints and palm prints require high resolution imaging. Morgan Electro Ceramics manufactures very close arrays of piezocomposites that cleanly register individual pixels for accurate image processing. The piezocomposites achieve high resolutions by damping the resonance in the length and width modes, leaving just the thickness mode frequencies, for an exceptionally clean signal.

Figure 2 is a frequency sweep that illustrates the high performance of a piezoelectric composite manufactured by Morgan Electro Ceramics. The energy is focused at a single frequency. A conventional monolithic transducer would exhibit multiple peaks from lateral vibration modes. The Morgan Electro Ceramics piezoelectric composite has eliminated all such interference, improving transmission efficiency and reception sensitivity.

Figure 2: Frequency Sweep of Piezocomposite from Morgan Electro Ceramics

The reduction of lateral vibration modes in piezoelectric composites enhances beam pattern and pulse shape. Moreover, piezocomposites materials can be manufactured in cylindrical, spherical or curved shapes to create transducers that are mechanically focused, as an alternative to acoustic lens. The problem of lens attenuation is obviated and the design and manufacture of the transducer is simplified.

Another benefit of the polymer matrix is higher mechanical strength, compared to monolithic piezoelectric materials. Devices utilizing piezocomposite materials have higher resistance to mechanical shocks, vibrations, and high temperatures. Piezocomposites offer lower overall mass when compared to traditional monolithic devices of the same volume.

Flexible Design Parameters

Piezocomposite materials are able to satisfy the divergent requirements of a wide variety of ultrasound and sonar applications, because of the multiple flexible design parameters. Design engineers can adjust the architecture, manufacturing process, material properties, and proportional content of ceramic versus polymer, to optimize performance for the particular application. 

Design involves tradeoffs among various goals, such as maximizing electromechanical coupling or mechanical strength or electrical capacitance, or closely matching acoustical impedance to the medium or matching electrical impedance to the system.

Matrix Construction. One of the first decisions in the design process is the selection of the matrix architecture. As discussed above, the 1-3 and the 2-2 architectures each have their respective advantages, including lower acoustic impedance for the former and higher mechanical strength for the latter. The shape and dimensions of the component elements allows for frequency tuning and focusing.

Manufacturing Technique. Several different manufacturing techniques are employed in the construction of piezocomposites. Dicing the ceramic material and other specialized machining techniques allows companies such as Morgan Electro Ceramics to achieve extremely tight tolerances.

Piezoelectric Ceramic Material. A wide variety of ceramic material is used in piezocomposites, including several formulations of Lead Zirconate Titanate (PZT). The U.S. Navy has established widely-used classifications of PZT, including Type I, a hard PZT commonly used for transmission, and Type VI, a softer material commonly used for sonar receiving. Morgan Electro Ceramics has developed a unique set of advanced piezoelectric materials, including PZT-5K1-HD, PZT-5K3-HD, PZT-508-HD and single crystal PMN-PT, that significantly improve the transmitting and receiving capabilities of transducers used in medical ultrasound applications. 

Polymer Material. The polymer or “filler” material varies even more than the ceramic and includes epoxy, polyurethane and acrylics. Softer polymers provide better receive sensitivity and lower cross coupling, but harder materials are more durable. Each manufacturer offers its own particular polymer formulations. For many applications, Morgan Electro Ceramics recommends high performance epoxy because it offers excellent thermal resistance, high mechanical strength, and tenacious bonding to the PZT, as well high as moldability with good shape retention and minimal shrinkage.

Volume Fraction. Another key design parameter is the volume fraction – the volume of ceramic to the volume of the total matrix. For receive applications, the volume fraction is typically in the range of 15-25 percent, as the higher polymer content provides greater sensitivity. For transmit application, the volume fraction is usually greater than 50 percent, because the higher ceramic content allows higher energy transfer. Combination transmit/receive applications fall between 30-50 percent.

The outstanding acoustical properties and flexible design parameters of Morgan Electro Ceramics’ piezocomposites allow design engineers to satisfy the market demand for high performance transducers with better resolution and efficiency for evolving ultrasound and sonar applications.