Optimisation of adhesive-free high-frequency miniature class IV transducers incorporating single-element piezoelectric plates

Tissue biopsy under regional anaesthesia requires accurate localisation of the needle tip during penetration through skin and soft tissue. To enhance needle tip visibility in colour Doppler imaging, ultrasonically actuated needle systems based on bolted Langevin transducers have been developed. However, these systems impose design constraints as the needle length must be tuned to the transducer resonance, and their relatively large size limits the suitability for hand-held procedures. Flextensional transducers offer a compact alternative, enabling large displacement with minimal bulk piezoelectric material. This paper presents an optimisation study of three metal shell geometries for class IV miniature flextensional transducers, excited by hard piezoelectric plates to maximise displacement amplification. The transducers employ a negative interference fit between the piezoelectric plate and metal shell, introducing structural pre-stress without bonding or transition mechanisms. Additionally, the dynamic response and power density of the transducers are evaluated for different active materials, including piezoceramics (Pz54) and piezocrystals (Mn:PIN-PMN-PT). Experimental results show that, for identical dimensions, the Mn:PIN-PMN-PT transducer achieves a lower resonant frequency, reduced and near-resistive impedance, enhanced electromechanical coupling, higher mechanical Q, and increased power density compared with the piezoceramic counterpart.