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Low pO2 sintering of sodium potassium niobate based lead-free piezoceramics for multilayer actors with base metal electrodes
Keywords: lead free piezoelectric ceramic, low oxygen partial pressure sintering, base metal electrodes
Lead zirconate titanate (PZT) based materials are actually used for nearly all piezoelectric applications. However, according to the EU Restriction of Hazardous Substances directive (RoHS), lead and cadmium containing materials are supposed to be replaced in electrical and electronic equipment. This led to an enormous research activity for developing new lead free piezoelectric materials in the past decades. Sodium potassium niobate (KNN) based piezoceramics are promising candidates as an alternative to PZT-based materials. Sintering of KNN in air atmosphere is quite challenging, due to its tendency for giant grain growth and the high vapor pressure of sodium and potassium. On the other hand, sintering under low pO2 allows co-firing KNN with base metal electrodes, and possibly suppresses giant grain grow of KNN. We report on an experimental study to develop a sintering procedure of KNN at low pO2 to enable co-firing KNN with base-metal electrodes. Process parameters were optimized to sinter a multilayer actuator with base metal electrodes without oxidizing the metallization, but still providing decent piezoelectric properties of the KNN material. For this study KNN material with chemical composition Na0.52K0.44Li0.04Nb0.8Ta0.2O3 was synthesized via solid state reaction route. Raw materials were mixed with zirconia grinding media in ethanol and calcined for 5 h at 850C. The calcined powder was characterized by XRD and a mixture of an orthorhombic and tetragonal perovskite was observed. The calcined powder was milled in a planetary ball mill and cylindrical pellets were prepared by uniaxial pressing. Alternatively, a KNN tape was manufactured by tape casting. For sintering experiments, KNN multilayer laminates were used. We investigated the phase composition, microstructure and electromechanical properties of KNN samples and multilayers, sintered under low oxygen partial pressure of 10-14 to 10-6 atm at a sintering temperature of 1000-1050C for various dwell times ranging from 5 min to 8 hours. After sintering, an annealing step at 850C under pO2 = 10-8 to 10-1 atm was performed for 1 to 12 hours. This reoxidation step was also characterized by TG-measurements of low pO2 sintered KNN pellets annealed at in nitrogen atmosphere (pO2 = 10-5 atm). Furthermore, we have studied the thermal decomposition behavior of Cu- and Ni-thick film pastes. A combination of thermal analysis and sintering experiments allows to obtain processing conditions for co-firing of KNN with base metal thick film pastes.
T. Reimann, academic staff
Jena University of applied sciences Jena
Jena, Thuringia

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