Hybrid Atmosphere Processing of Lead-Free Piezoelectric Sodium Potassium Niobate-Based Ceramics

Abstract

K 0.5 Na 0.5 NbO 3 -based ceramics, a promising group of lead-free piezoelectrics, are challenging to sinter dense while avoiding alkali evaporation. This work explores hybrid atmosphere processing, a new approach where reducing atmospheres is used during heating to avoid coarsening from alkali carbonates and hydroxides, and oxidizing atmospheres is used during sintering to avoid alkali evaporation. Discs of Li 0.06 (K 0.52 Na 0.48 ) 0.94 Nb 0.71 Ta 0.29 O 3 with 0.25 mol% Mn (KNNLTM) were sintered in air, N 2 , 9% H 2 in N 2 , or 9% H 2 in N 2 during heating and air during sintering (hybrid atmosphere processing). The highest density was obtained by sintering in 9% H 2 in N 2 , but resulted in high alkali loss and decomposition of the surface, followed by low piezoelectric response. However, with the hybrid H 2 /air processing it was possible to both avoid surface decomposition and leakage currently associated with alkali evaporation during sintering in H 2 , and to obtain a denser, more phase-pure and small-grained KNNLTM ceramic with a higher piezoelectric response than obtained by sintering in air or N 2 .

Suppression of abnormal grain growth in K0.5Na0.5NbO3: phase transitions and compatibility

“…In detail, the very low density of the samples, which can be presumed from Fig. 2(d) is undesirable, but in general agreement with diverse studies on the application of different sintering atmospheres 58,59 . Recently, it has been shown that sintering in air causes the least weight loss (alkali evaporation) in comparison to other tested atmospheres, but also reaches rather low densities 58 .…”

Textured multilayered piezoelectric structures for energy conversion

“…Differences in sintering shrinkage is also a potential cause of bending and delaminations during sintering [44], which is observed to some extent by the slight curvature of the multilayers (direction of bending agrees with a higher shrinkage of the porous layer versus the dense, figure 3). The overall shape of the tapes' dilatometry curves resemble that of powder of the same composition [40], but the differences in the magnitude of shrinkage is related to the softness and compressibility at low temperature, and extra shrinkage related to the use of pore formers [21]. Here, the choice of PMMA as the pore former made it possible to limit the differences in total shrinkage to ∼10% between the dense and the porous layers, and successfully co-sinter this combination, in contrast to the delamination seen with graphite (20% shrinkage difference) (figures 3(c) and 4(a)).…”

“…Densification of KNN-based ceramics is generally challenging [40,49], which is also reflected in the density of the textured multilayer transducers made in this work. A density of 81%-84% in the textured layers is significantly lower compared to what can be expected in bulk ceramics [24,40]. Low density is known to cause a reduction of the dielectric permittivity and piezoelectric coefficients such as d 33 in the transducers, but does not have such a strong effect on the k t coupling coefficient [50].…”

“…Densification of KNN-based ceramics is generally challenging [40,49], which is also reflected in the density of the textured multilayer transducers made in this work. A density of 81%-84% in the textured layers is significantly lower compared to what can be expected in bulk ceramics [24,40].…”

“…The magnitudes of the saturated (P s ) and remanent (P r ) polarization, the coercive field (E c ) and the gap between the start and end polarization values differ strongly between the samples. For example, P r varies from 6 to 20 μC cm −2 , which is in the typical range for KNN-based samples [8] while the specific KNNTLM composition is expected to have ∼17 μC cm −2 in nontextured KNNLTM [40], and ∼7 μC cm −2 in (100)-oriented single crystal KNNLTM [25]. The polarization decreases in the textured compared to non-textured samples, expected from both due to the alignment of the polar axis off the direction of the electric field and the lower density in these samples compared to the nontextured [41].…”