Power ultrasound

Sketch showing a Langevin transducer, commonly used in high power ultrasound systems. The device has a resonance at a thickness mode along its cylindrical axis. Four white ceramic piezoelectric discs are held in the centre under compression by a high tensile steel bolt (black) running through the centre of the Langevin device. Beryllium/copper shim is used between the piezoelectric discs for making electrical connections. At resonance the flat end faces are anti-nodes making the cyclinder a half-wavelength long. The device is designed to have a high mechanical quality factor, Q. The piezoelectric discs pump energy into the resonator at the frequency of resonance and in the absence of any loss of energy the amplitude becomes larger and larger. In use the Langevin transducer is generally coupled to a waveguide that will convey the mechanical energy to a point where it is used and this is the main energy loss mechanism - the load. The waveguide is commonly a resonator and tuned to the same frequency as the Langevin transducer.

Power ultrasound is used for processing materials: welding materials (for example shoe manufacturing), cutting (food industry), cleaning surfaces (silicon wafers), lysing (breaking open) biological cells, moving particles by radiation pressure (wood pulp, pharmaceutical particles), creating aerosols (nebulizers) and causing chemical reactions by sonochemistry.

Cambridge Ultrasonics has clients who are manufacturers of power ultrasound equipment and who want to use power ultrasound for processing materials.

Transducer reliability

A manufacturer of power ultrasound equipment had a problem with the reliability of its popular range of transducers used for plastic welding. These are known as Langevin type transducers and they are the equivalent of lasers in the field of ultrasound. In a Langevin transducer a mechanical resonator is pumped with energy by two piezoelectric discs forming part of the resonator. A Langevin resonator is more efficient than a laser because it can be pumped synchronously at its operating frequency giving high electro-mechanical conversion efficiency, of typically 90%. By contrast Nd:YAG lasers with equivalent powers (1 kW) are only about 5% efficient requiring water cooling. With a Langevin transducer water-cooling is not generally needed but some air cooling is helpful, which makes electrical isolation simpler to achieve.

However, if the Langevin transducer is not well made then more than 5% of the energy becomes heat and if the heat generated exceeds the capacity of the cooling system to remove it then thermal runaway starts and the transducer is damaged. The client was suffering too many overheating failures. Poor assembly practice by a subcontractor was under suspicion and Cambridge Ultrasonics was asked to investigate. We devised a manufacturing process, using best practise, with vector impedance measurement of the piezoelectric components before and after assembly and clean assembly conditions. We found that about 5% of the client's piezoelectric components (actually ferroelectric) had negligible piezoelectric properties and 10% were in a group that indicated poor piezoelectric properties; the remaining 85% were good quality. We assembled using Langevin transducers using only good quality piezoelectric components with vacuum processing of adhesives and gave them to the client to test. The client's tests showed that final quality was much improved.

Lysing biological cells

Cambridge Ultrasonics acted as a broker between two clients in a project investigating lysing (breaking open) of biological cells. One client had high power technology that was already used for processing biological materials at high flow-rates to homogenize and kill bacteria. The other client was interested in lysing different biological cells but also at high flow-rates. Cambridge Ultrasonics arranged for demonstration equipment to be shipped from client 1 to client 2 and we went to set-up the equipment at client 2's premises. Some minor changes were needed to the processing unit. Once they were made Cambridge Ultrasonics performed feasibility tests. The tests were successful and gave client 2 a solution to its processing problem.

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Power Ultrasound