1-3 piezocomposites for high-frequency ultrasonic transducers

Ultrasound imaging is a central component of medical diagnostics and non-destructive material testing. High-resolution imaging of near-surface structures requires the use of high-frequency ultrasound transducers with operating frequencies between 5 and 40 MHz. 1-3-piezo composites have become the standard for manufacturing such ultrasound transducers. These consist of piezoelectrically active ceramic rods embedded in a piezoelectrically inactive and elastic polymer matrix and offer the following advantages over monolithic transducers:

• Excellent electromechanical coupling, resulting in high sensitivity and wide bandwidths
• Good acoustic impedance matching to aqueous media and biological tissue
• Construction of array transducers with individually driven, acoustically isolated transducer elements

In addition to the industrially established “dice-and-fill” process, in which piezoceramic rods are produced by sawing piezoceramic material, the Fraunhofer IKTS has developed the “soft-mold” process, which enables the fabrication of fine-scale piezoceramic structures using micro-molding techniques.

Due to limitations regarding saw blade width, the geometry of the rod cross-sections, and the strength of the piezoceramic material, the dice-and-fill process is suitable for manufacturing ultrasonic transducers with operating frequencies up to 15 MHz. The higher frequency range up to 40 MHz is covered by the Soft-Mold process, which allows for the fabrication of fine-scale rods with smaller spacing. Furthermore, this process is not limited to prismatic rod cross-sections but allows for greater freedom in designing the composite layout. This is made possible by casting from silicon master molds, which are patterned using dry etching. Flexible polymer molds are used to produce the negatives. This is followed by ceramic molding, drying, demolding, and sintering. As with the dice-and-fill process, the ceramic rod structures are then filled with polymer, cured, and ground down to the desired target dimensions.

Services offered
 

• Development of ultrasonic transducers in the frequency range of 1 to 40 MHz
• Technology development for 1-3-piezo composites using the "dice-and-fill" process (up to 10 MHz) or the "soft-mold" process (up to 40 MHz)
• Customization to meet customer requirements
• Characterization of dielectric and electromechanical properties

Technical equipment
 

• Composite manufacturing using the dice-and-fill process
  • DAD3350 automatic wafer saw
  • Diamond saw blades with widths starting at 15 µm
• Composite manufacturing using the soft-mold process
  • Slurry Production and Processing: Planetary ball mill, vacuum planetary centrifugal mixer (Speed-Mixer)
  • Molding at ambient pressure or under vacuum (up to approx. 100 mbar)
  • Laboratory chamber furnaces for heat treatment
• Polymer infiltration, grinding and polishing techniques, metallization
• Measurement technology: rheometers, optical and electron microscopy, small-signal measurements (dielectric constant, dielectric loss factor, electromechanical coupling factor, electrical impedance spectrum in the range from 1 kHz to 120 MHz), acoustic measurements (sound field, impulse-echo)