Sintering Behavior of Lead‐Free (K,Na)NbO₃‐BasedPiezoelectric Ceramics

Abstract

In this manuscript, we investigate the sintering behavior of (K,Na)NbO 3 (KNN)-based ceramics covering unmodified KNN, (K,Na,Li)NbO 3 (KNLN), and KNLN-xBaTiO 3 (KNLN-BT) systems. Three stages of sintering were identified using detailed temperature and time dependent microscopy. The first stage corresponds to rearrangement of random shape particle to form stack of plate type particles. The second stage is characterized by rapid grain growth promoted by formation of liquid phase. The liquid phase was identified to be Na-deficient composition. In the final stage of sintering, densification slows and microstructural coarsening becomes the dominant process. The fraction control of liquid phase was an important factor to obtain dense and uniform microstructure by completely filling the voids between the plate-shaped particle stacks. II. Experimental ProcedureCeramics in the system (K 0.5 Na 0.5 )NbO 3 (0.5KNN), (K 0.5(1Àx) Na 0.5(1Àx) Li x )NbO 3 (xKNLN, e.g. 0.04KNLN 5 (K 0.48 Na 0.48 Li 0.04 )NbO 3 , and 0.05KNLN 5 (K 0.475 Na 0.475 Li 0.05 ) NbO 3 ), and (1Àx)(K 0.48 Na 0.48 Li 0.04 )NbO 3 -xBaTiO 3 [(1Àx) KNLN-xBT] were synthesized from oxides of 499% purity by conventional solid-state route. The powders of K 2 CO 3 , Na 2 CO 3 , Nb 2 O 5 , Li 2 CO 3 , BaCO 3 , and TiO 2 (all obtained from Alfa Aesar, Ward Hill, MA) were mixed for 24 h in a polypropylene jar with zirconia balls. Mixed powders were dried J. Roedel-contributing editor This work was financially supported by Office

Sintering of sub-micron K 0.5 Na 0.5 NbO 3 powders fabricated by spray pyrolysis

“…44 Evaporation of Na2O, rather than K2O, is often said to cause alkali deficiency in sintered KNN. 23,[45][46][47][48][49] Our calculations clearly show that K is more volatile than Na. A problem during EDS/WDS analysis is, however, misinterpretation of secondary phase grains (with lower solubility of Na than K) as matrix grains and electron beam related decomposition.…”

Development of KNN-Based Piezoelectric Materials

“…Ahn and coworkers have recently reported an extensive study on the sintering mechanism of KNN-based ceramics [35]. In their work three stages of sintering were identified.…”

“…The melting points of these precursors are 851, 891 and 1,520 C respectively. Since the normal calcination temperature of KNN is close to the melting point of two alkali precursors, they always have the problem of volatilization during calcination and hence deficiency of A site ions [30,34,35]. Effect of calcination temperature, dwell time and excess of alkali carbonates on phase formation and microstructure of KNN is studied extensively [30,36,37].…”

“…Significant amount of research is done to understand the problem of nonsinterability in KNN and other members of family [35,42]. Ahn and coworkers have recently reported an extensive study on the sintering mechanism of KNN-based ceramics [35].…”

“…Lower relative density of piezoelectric ceramics not only leads to poor electromechanical coupling factor but also gives rise to high conductivity which makes it difficult to pole. This problem is mainly attributed to high volatilization of alkali elements at sintering temperature, which is higher than 1,100 C for normal solid state sintering [34,35]. Cubical shape of KNN grains also contributes to this problem as high packing density is difficult to achieve with such particle shape [39].…”