Abstract
The phase boundary structure of (K,Na)NbO3 piezoelectric ceramic was modified by doping with Bi(Na,K,Li)ZrO3 and BiGaO3 through normal solid-state sintering. Rietveld refinements by X-ray diffraction revealed that the Bi(Na,K,Li)ZrO3/BiGaO3 co-doping in (K,Na)NbO3 led to a multi-phase structure at room-temperature, effectively moving the rhombohedral-orthorhombic (R-O) and orthorhombic-tetragonal (O-T) polymorphic phase transition temperatures close to the room temperature region. Increased levels of doping also generated a structural transition, i.e., triphasic R-O-T to diphasic R-T (T-rich) and finally to R-T (R-rich), contributing to shrinkage of the O phase as well as the increase of R phase fraction. A sensitive influence of the BiGaO3 doping (0.001 mole fraction level) on the structural properties such as the phase and microstructure was shown, resulting from the effect of the super-tetragonal structure of BiGaO3. The d33 property was strongly dependent on the phase and its volume fraction, in addition to the grain sizes. Eventually, enhanced and balanced properties of the piezoelectric coefficient and Curie temperature (d33 = 309 pC/N, TC = 343 °C) were obtained when the doped ceramic had a T-rich (86%) R-T structure.
Highlights
The recent great improvement of the piezoelectric coefficient d33 of polycrystalline (K,Na)NbO3(KNN) lead-free ceramics has been made through the advent of emerging rhombohedral-tetragonal (R-T) [1,2,3,4,5,6,7] and rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundaries [8,9,10]
All the ceramics were found to have a pure perovskite structure without second phases, showing that BNKLZ and BG dopants diffused into the KNN lattice to form homogeneous solid solutions in the investigated concentration ranges
There was a dramatic change of phase structure with the variation of the BNKLZ and BG concentrations, as observed by the expanded X-ray diffraction (XRD) patterns in the 2θ range of 44–47◦
Summary
The recent great improvement of the piezoelectric coefficient d33 of polycrystalline (K,Na)NbO3. Perovskite oxides containing Bi3+ ion, such as BiMO3 or (Bi,M)ZrO3 , are of special interest in lead-free piezo-/ferroelectrics and multiferroic materials [11,12] This is because the Bi3+ ion has a stereochemically active 6s2 lone pair, which results in structural distortion of the prototypical cubic phase owing to ion-off centering in its perovskite compounds, as is the case for the Pb2+ ion in lead-based piezoelectrics. Compared to other members of the simple BiMO3 family, this compound has been predicted to have huge tetragonal distortion; the a lattice constant is 3.64 Å and the c/a ratio is 1.3 [17,18] Doping with this material is of concern with respect to both the structural modification and piezoelectricity of KNN. Room-temperature phase structure, including constituent phases and their quantities, was accurately identified by Rietveld refinement method based on X-ray diffraction (XRD)
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