PROPERTIES AND AGING OF SOLEF PVDF PIEZO- AND PYROELECTRIC FILMS FOR MEDICAL TRANSDUCERS

The development of a high quality piezoelectric film implies adequate choices for base material, beta-phase transformation, polarisation conditions and deposition of stable electrodes. In PVDF, a proper balance between amorphous and the crystalline phase will lead to a polymer particularly suited for piezoelectric applications. This balance is also important for obtaining a high content of beta-phase. The preferred polarisation method is corona discharge. In terms of mechanical and electrical properties of finished material, the SOLEF biaxial film shows definite advantages over the monoaxial competitive products. Measurements of aging characteristics at 70 degrees and 90 degrees C for periods over 200 days indicate a decrease of the d*(33) piezoelectric constant of about 15%, with a strong tendency to stabilization afterwards. When the metallisation process by thermal evaporation or ion beam is carried out without temperature control (such as cooling of the sample), lower values of piezoelectric properties are obtained. Adhesion of the metal layer (aluminium mainly) has often been a problem in the past. Different proprietary methods have been investigated which provide adhesions well above those indicated as a limit by the standard scotch test. Numerous applications are currently investigated. Medical echographic transducers giving very narrow pulse responses are one of these commercial products now regularly produced.