Thermoanalytic characterization of binder burnout and sintering of lead-free piezoelectric KNNLT multilayer laminates with Ni electrodes

Abstract

For competitive lead-free piezoceramic multilayer actuators (MLA) it is required to include base metal electrodes (BMEs). However, thermal processing of KNN-based multilayers with Ni electrodes necessitates precise control of the oven atmosphere in order to control the phase composition and functional performance of the piezoceramics and electrodes. Sintering of KNN-based MLAs with BMEs is performed at low oxygen partial pressure followed by a reoxidation treatment to reduce the concentration of oxygen vacancies and increase the resistivity of the ferroelectric, and simultaneously, the BME must not be oxidized. Within this complex thermal treatment protocol, the debindering of the multilayer laminates is a very important process step and proper binder removal is critical to avoid formation of defects or flaws and oxidation of electrodes. We have studied the debindering behavior of a KNN-based MLA with Ni electrodes fabricated from a PVB-based slip formulation using thermal analysis and complementary binder burnout experiments under reducing atmospheres. It is shown that binder burnout in reducing atmosphere allows for protection of Ni layers, but incomplete binder decomposition occurs at lower pO2. On the other hand, after debindering at 350 °C in air complete binder removal and little Ni oxidation only was observed, allowing to use these binder burnout conditions to prepare MLAs.