Development of materials and composites for >25 MHz single element transducers

Miniature hollow spheres with diameters of 1 to 6 mm and wall thicknesses of 20 to 120 μm have been fabricated in the lead titanate (PT) family of piezoceramics for use as pre-focused transducers. Spheres are produced using a new sacrificial core technique that produces 100s of spheres with a more uniform wall thickness than previously reported. Shells produced from these spheres were found to have a wall thickness variation of 10% or more. Despite this variation, bulk properties were estimated from capacitance and impedance data. Shells tested in this work had free dielectric constants near 280 with loss < 2% and d₃₃ values of 68 pC/N. Coupling coefficients averaged 50% with mechanical quality factors of < 15. The operating frequency range for composites having 1-3 connectivity has also been expanded into the 15-70 MHz range using fine scale piezoceramic fibers. Composites were fabricated with volume fractions from 10 to 45% with diameters up to 4.5 mm. Also, the composite dielectric constants could be tailored through volume fraction or fiber composition. The thickness dependence of the properties gave indications to radial mode/thickness mode interactions at pillar aspect ratios near 1.7 to 1 thickness to diameter. Coupling coefficients (k_t) of ≥ 58% with mechanical quality factors < 15 were reported and transducers showed bandwidths up to 89% with insertion losses as low as 22 dB.