Demonstration of Copper Co-Fired (Na, K)NbO₃ Multilayer Structures for Piezoelectric Applications

A Li and Ta modified (Na, K)NbO₃ piezoelectric ceramic has been successfully co-fired with inner copper electrodes in a reduced atmosphere. Highly dense NKN ceramics (95% relative density, 4.64 g/cm³) were obtained by sintering the samples in a low oxygen partial pressure (low pO₂) atmosphere at 1050°C. The poly(propylene carbonate) binder system was used to permit a clean burnout at low temperature in N₂ atmosphere, and also prevent the electrode copper particles from undergoing any oxidation. No interdiffusion of copper, chemical reactions, and/or carbon residues were observed in the grains, grain boundaries, or at the electrode-ceramic interface of the co-fired samples from a detailed transmission electron microscopy (TEM) analysis. Dielectric and piezoelectric properties were characterized from those co-fired prototyped samples. The samples displayed high relative dielectric permittivity above 800, with low dielectric loss about 3.6%. A normalized strain coefficient d33 (max. strain/max. electric field) of d33 = 220 pm/V was obtained under unipolar converse electromechanical measurement at 20 kV/cm. This paper presents the feasibility of co-firing a Cu inner electrode with NKN ceramics toward multilayer lead-free piezoelectric applications, providing an engineering route to narrow the performance differences between soft lead-based piezoelectrics and lead-free materials.