Structure And Properties Of KNN-Based Lead-Free Piezoceramics

Discipline:Materials Science and Engineering

PhD Experience:Being a PhD is1%inspiration and99%sweat.

This thesis focuses on Li, Ta, and Sb co-doped (K,Na)NbO3 (KNN-LTS) lead-free piezoceramics. Firstly, the “linear combination rule” is used to predict the polymorphic phase transition (PPT) of KNN-based materials, and the PPT boundary composition of the KNN-LTS system is identified, resulting in lead-free piezoelectric ceramics with a piezoelectric constant as high as 359 pC/N, exceeding the average level reported in literature.

The KNN-LTS nanopowders prepared by chemical methods exhibit low sintering temperatures and wide sintering ranges. The sintered ceramics have high density and uniform composition, achieving a piezoelectric constant of 424 pC/N, which is 27% higher than that of ceramics prepared by the same formula solid-state method. The study found that the high piezoelectric activity of ceramics prepared from nanopowders is related to the presence of nanoscale electric domains within the ceramics.

Using X-ray absorption fine structure, the bond lengths between B-site atoms and neighboring oxygen atoms in KNN-based ceramics with different crystal structures were studied, confirming that the increase in spontaneous polarization directions is a key reason for the enhanced piezoelectric performance in the PPT range.

By analyzing the relationship between the electromechanical properties of ceramics and electric domain structures, a connection between the macroscopic performance of ceramics and their microscopic electric domain structures was established, revealing that the high piezoelectric activity at the PPT boundary is primarily due to the flipping motion of electric domains rather than the translational motion of domain walls.

Application of the Linear Combination Rule in KNN System

The PPT composition of n-component systems lies on the line connecting the PPT compositions of two (n-1) component systems.

Radial Distribution Function Diagram and Correspondence with Ta-O Bonds

There are three different peak positions in the single-phase interval, corresponding to three different Ta-O bond lengths; in the coexistence interval of two phases, multiple Ta-O bond lengths are present, confirming that this interval has multiple spontaneous polarization directions.

1. For the first time, the “linear combination rule” was used to obtain the polymorphic phase boundary composition of complex multicomponent KNN systems, achieving a piezoelectric coefficient d33 as high as 359 pC/N in KNN-based ceramics with high piezoelectric activity.

2. The nanopowders prepared by chemical methods enabled low-temperature sintering of Li, Ta, and Sb modified KNN ceramics, resulting in dense, fine-grained, and uniform ceramics, with d33 further increased to 424 pC/N, reaching an internationally advanced level.

3. From the perspectives of atomic structure and electric domain structure, in-depth research was conducted, experimentally confirming for the first time that the increase in spontaneous polarization directions is a crucial reason for the enhanced piezoelectric performance of KNN-based systems in the polymorphic phase boundary; the excellent piezoelectric performance in the polymorphic phase boundary mainly arises from the flipping motion of electric domains rather than the translational motion of domain walls.

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