Power of piezoceramics
Researchers at Birmingham University have developed a process to build ultrasound transducers that operate at more than three times the frequency of conventional devices. They believe the process could enable the development of a new breed of medical imaging devices where high resolution and low surface penetration are demanded.
Ultrasound-based diagnostic imaging systems have been used to create images of subcutaneous body structures. Such systems which work at frequencies above the audible range of human hearing employ piezoelectric transducers that are placed on the surface of a body to be imaged. When excited by an electrical stimulus, these transducers create an ultrasound signal that penetrates the body. Reflected signals from features in the body are then detected by the same transducer and converted back into electrical signals that are processed to obtain an image.
Tim Button, professor of functional materials and devices at Birmingham University, claims the most efficient materials currently used to manufacture transducers for such systems are ceramic piezoelectrics.
These are composites that comprise pillars of piezoelectric material encapsulated within a polymer matrix. This design ensures that the transducers have a high electromechanical coupling coefficient, low acoustic impedance and do not produce spurious resonant frequencies that could degrade images.
Fabricating transducers that operate above 20MHz is challenging, not least because it is the thickness of the ceramic material that governs the frequency at which they operate. A device that can operate at 50MHz, for example, would need to be around only 50 microns thick. The commercial fabrication of piezocomposite transducers at this frequency has, to date, been impossible due to the difficulties in scaling down the dice-and-fill manufacturing method used in the production of piezocomposites.
Button said: ’For this reason, developers wishing to build systems that can operate at such high frequencies must turn to either monolithic ceramic transducers, or those based on piezoelectric polymers. But although these are easier to fabricate and deploy, their properties are inferior to those of the optimum piezoceramic-polymer composite material.’
“The fabrication process creates composite transducers that can operate at 25-70MHz”
To resolve this, Button and his team have developed a technique that can be used to manufacture composite transducers that can operate between 25-70MHz. The process uses the same lead zirconate titanate ceramic powder used to build more traditional transducers. This is then mixed with polymers and binders to create a plastic-like green ceramic paste using a viscous powder processing (VPP) technique. The paste can then be embossed into moulds to build transducers of different shapes even those that are curved. The embossed ceramic is then dried and prepared for demoulding. Once extracted from the mould, the polymers and the solvents are removed and the ceramic material is sintered, before being embedded into epoxy resin, after which it is cured and then lapped to the required thickness.
Button said: ’The process creates a dense material almost identical to that produced by conventional processes, with the exception that the fine-scale structure of ceramic material is already encapsulated within it, obviating the need to perform machining processes to produce the pillars of ceramic material.’
The process can also produce pillars of ceramic material that are random in their lateral dimensions. That means that the transducer created using Button’s method produces a composite structure where any unwanted resonances are eliminated.
Button said applications for the high-frequency transducers manufactured using the VPP technique would be in systems that could produce images of surface or near surfaces, where high-resolution imaging is required, yet where the depth of penetration of the acoustic signal is not as important as in more conventional image-processing systems.