Sintering and properties of lead-free (K,Na,Li)(Nb,Ta,Sb)O3 ceramics
Introduction
The search for alternative lead-free piezoelectric materials is now being focused on modified bismuth titanates,1, 2, 3 alkali niobates and systems in which a morphotropic phase boundary, MPB, occurred. Among them (Na0.5K0.5)NbO3, KNN, has been considered a good candidate for lead-free piezoelectric ceramics because of its strong piezoelectricity.4 KNN exhibits a MPB at around 50% K and 50% Na separating two orthorhombic phases and, as for lead titanate zirconate piezoceramics (PZT), an increase in properties for compositions close to this MPB.4 However, the major drawback of KNN ceramics is the need for special handling of the starting powders, sensitivity of properties to non-stoichiometry, and complex densification process.5 The alkaline carbonates shown rather good water solubility and thus the water processing could introduce deviations in stoichiometry during the ceramic processing and could provoke ceramic degradation of the sintered samples. Excess of alkaline elements in the ceramic samples easily reacts with the moisture in the air and shows deliquescence.5 One of the reasons for the poor sinterability of the KNN system is the low melting point of KNbO3,6 approximately at 1058 °C, moreover alkaline metal elements that were included in these materials easily evaporated at high temperatures. The evaporation of one of the constituents of the MPB provoke thus a compositional fluctuation of the composition as occurred in well known piezoelectric perovskite systems as PZT7 that resulted in poorer properties. The sinterability of these materials should be improved by sintering aids as CuO5, 6, 7, 8 because of its low melting point. The sintering aid enters in B composition of the perovskite structure and the A site vacancies thus suppresses the formation of the hygroscopic secondary product.9
Recently, it was reported exceptionally high piezoelectric properties in the system (K,Na)NbO3–LiTaO3–LiSbO310 prepared by a complex processing method, with d33 values over 400 pC/N in textured ceramics. Similar composition with 4 mol% of Lithium and 10 mol% of tantalum substituted KNN ceramic prepared by simple pressureless solid state sintering without aid additives or special powder handling reach interesting properties with d33 ∼160 pC/N without antimony.11 In addition of the good electromechanical properties comparable to those obtained in hard PZT, two aspects need to be clarified in these materials11: how the proximity of ambient temperature to the tetragonal-orthorhombic phase transition temperature would affect high reported properties? and which effect originated the high dielectric losses that are sometimes observed in KNN based materials?
The aim of this work is to study piezoelectric ceramic based in such a system by the conventional mixed oxide and carbonate route. The effect of the A/B ratio was investigated concerning the sintering process and piezoelectric properties.
Section snippets
Experimental
Two different composition were studied in this work: stoichiometric (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 and non stoichiometric (K0.38Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O2.97, hereinafter will be named as S and NS, respectively. Both compositions were prepared to evaluate the effect of the excess B sites ions (A/B = 0.94). The ceramics were prepared by solid state synthesis from K2CO3, Na2CO3, Li2CO3, Nb2O5, Ta2O5, and Sb2O5. The raw materials used are all of high-purity grade. In order to
Results and discussion
Fig. 1 shows the FE-SEM micrographs and the particle size distribution of the calcined powder NS. As it can be seen, the calcined powders were ∼500 nm agglomerates formed by smaller plate-like particles ranging from 50 to 70 nm. Stoichiometric composition had similar morphology, particles size distribution and agglomeration state (not shown). The small particle size was attributed to the low temperature process and will improve the reactivity of the powder. Regular synthesis process in KNN
Conclusions
Lead-free piezoceramics in the system (K,Na,Li)(Nb,Ta,Sb)O3 had been prepared following a low temperature synthesis solid state route. The composition with excess B cations sintered in the presence of liquid phase that promotes grain growth and thus favours the solid solution of the perovskite. The samples showed a relaxor type behaviour that diminished because of the compositional homogeneity improvement that took place during the liquid phase assisted sintering in the non-stoichiometric
Acknowledgements
The Spanish CICYT under contract MAT2004-04843-C02-01 financially supported this work. F. Rubio-Marcos was supported by a grant from FPI-CAM-FSE program.
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