Study On The Microstructure Of KNN-based Lead-free Piezoelectric Materials

Piezoceramics are widely used in microelectronics,medical diagnostics and the automobile industry utilizing the piezoelectric effect which generates a voltage upon a mechanical strain,or vice versa.Although the dominant piezoceramics have been the Pb?Zr,Ti?O₃?PZT?family for a long time,lead-free piezoceramics were proposed to replace PZT due to toxicity of Pb.Among all thelead-free candidates,potassium-sodium niobate?KNN?materials become one of the most promising candidates for lead-free piezoelectric materials due to its high Curie temperature?Tc?and relatively high piezoelectric constant(d₃₃).This thesis cooperate with corresponding group and study the phase and microstructure of three representative KNN-based materials in details.The study aim to provide suggestions for materials processing and basis for understanding property behaviors.For the KNLN ceramics prepared by conventional sintering process,shifting polymorphic phase transition?PPT?to room temperatures in Li-doped KNN?KNLN?can greatly increase its piezoelectric constant.However,pure phase KNLN ceramics could hardly be obtained since secondary phases were formed inevitably during its synthesis.By examining the phase and microstructure in KNLN ceramics,we found that the secondary phase has a tetragonal tungsten bronze structure and is composed of Li,Na,K,Nb and O elements.The secondary phase mainly appears in the inner regions while disappears in the surface area.Moreover,the orthorhombic–tetragonal phase boundary cannot shift to room temperature when considerable amount of secondary phase exist in KNLN piezoceramics?For KNLN lead-free piezoelectric single prepared by solid-state crystal growth?SSCG?method using faceted KTaO₃ single crystals as a seed.SEM-EBSD,PFM and TEM investigations revealed a specific periodic configuration of?-boundaries and ferroelectric domains in the KNLN single crystal.The single crystal is sliced into several nearly parallel segments by?-boundaries and each segment has a mean thickness of 2-3?m.Neighboring segments show a misorientation of 1.4°/[12 3 2]while the alternate segments have the same orientation.The two segments exhibit different ferroelectric domain structures:periodic?100?domain walls and?11l?domain walls,respectively.The periodic?100?domain wall pattern can be sketched by tetragonal 180°or orthorhombic 90°domains.The as-observed periodic configuration of small-angle?-boundaries and ferroelectric domains could be related to the specific stress fields in the epitaxial crystal growth embedded in a ceramic matrix.For KNLNTS textured ceramics prepared by reactive templated grain growth?RTGG?,we perform detailed structural and compositional characterization of ceramics sintered at four elevated temperatures,i.e.800°C,1100°C,1140°C and 1190°C,in order to reveal the sintering mechanism of RTGG process remains.Upon increasing the temperature up to 1190°C,the sintering system undergoes successive processes,including elemental interdiffusion between the matrix grains and NN templates,formation of a eutectic compound and hence melting of matrix grains,nucleation and epitaxial growth on template surfaces,and finally coarsening of epitaxial crystals by consuming the intermediate liquid phase.Finally,suggestions for rational design and fabrication of textured lead-free piezoceramics using RTGG are proposed.