The Investigation Of CuO Based Composite Oxides On The Electrical Properties Of (K,Na)NbO3 Lead-free Piezoceramics

Alkali metal niobate-based lead-free piezoceramics(K_0.5Na_0.5NbO₃),abbreviated as KNN,is one of the most promising candidates due to its high Curie temperature,good piezoelectric and electromechanical properties.However,a large number of research of the KNN ceramics focus on improving the piezoelectric properties of KNN ceramics by different sintering methods or sintering additives(especially d₃₃).And there are little attention paid to increasing the mechanical quality factor?Q_m?and its influence mechanism of KNN ceramic.In this paper,the conventional solid-state reaction method is used.And the method of adding CuO based composite oxides sintering additives and doping modification is to improve the density of KNN ceramics,dielectric properties,piezoelectric properties,mechanical quality factor.Furthermore,the influence mechanism of high Q_m values of KNN ceramics has been investigated in detail.The main contents of this paper are as follows:1?B₂O₃-CuO mixed oxides were added to a K_0.48Na_0.52NbO₃-based lead-free piezoceramic?abbreviated as BC-KNN?.The results suggest that the B₂O₃-CuO additives effectively improved the sinterability and Q_m value of the piezoceramic.An optimal Q_m value as high as 2128 was obtained,which is 35 times higher than that of pure KNN ceramic.Interestingly,we found that the Q_m value was sensitive to humidity of the surrounding environment.As the relative humidity?RH?increased from 25%RH to 78%RH,the Q_m value of the BC-KNN ceramics decreased from 2128 to 267.We found that the dependence of the Q_m value on humidity was closely related to the instability of the relative dielectric constant??_r?.The results show that a dense microstructure is critical for maintaining a stable high Q_m performance in a humid environment.2?x mol%?SiO₂-CuO?-K_0.48Na_0.52NbO₃?0.5?x?10??SC-KNN?were prepared by the conventional solid state reaction method.The effect of SiO₂-CuO mixed oxides on the crystalline structure,microstructure,dielectric and piezoelectric behaviors were studied in details.With the compositional range,all the samples were demonstrated orthorhombic symmetry according to the X-ray diffraction analysis.The mechanical quality factor?Q_m?of SC-KNN ceramics was found to achieve a maximum value of 1598 when x=5 sintered at 1110^oC in low relative humidity?RH?.Interestingly,the Q_m,as well as the capacitance?C_o?of KNN ceramics is sensitive to the humidity changes,which verified the stability of KNN can be affected by the humidity.The microstructure revealed that SiO₂-CuO additives were effective to promote grain growth and to form a dense microstructure,which was beneficial for maintaining humidity stability.3?In this study,(Li_xK_0.5-xNa_0.5)(Nb_0.97Sb_0.03)O₃-1mol%CuO?x=0,0.05,0.1,0.2,0.5,abbreviated as CuO-L_x KNNS?lead-free piezoelectric ceramics have been fabricated by a conventional solid-state reaction route.We focused on the comparison of buried powder sintering with exposed sintering.The effect of buried powder sintering method on density,structure and dielectric,piezoelectric properties of CuO-L_xKNNS ceramics has been investigated.It was found that the density increased greatly when the buried powder sintering method was used,which was effective in inhibiting the volatilization of alkali metal oxides compared with exposed sintering method.Piezoelectric and dielectric properties showed a similar tendency with the density.These ceramics showed an obvious phase transition with increasing the Li content.CuO-L_xKNNS ceramics showed orthorhombic symmetry when x=0,but turned to be tetragonal symmetry when x?0.05.The optimal composition of CuO-L_xKNNS with buried powder sintering method was x=0.05 sintered at 1020 ^oC,for which the maximum value of the piezoelectric constant(d₃₃)was 180 pC/N,the density???was 4.40 g/cm³,the room temperature relative dielectric constant??_r?was 270 and the dielectric loss?tan??was 0.07.