寻找可替代铅基材料的无铅压电陶瓷是目前信息功能陶瓷领域的前沿，也是环境发展的迫切需求。本论文重点研究了铌酸钾钠(KNN)基无铅压电陶瓷的化学组成及烧结工艺条件对微观结构和电学性能以及力学性能的影响规律，并在此基础上初步探讨了基于KNN基无铅压电陶瓷的微细有序结构1-3型压电复合材料的制备技术及其电学性能。提出并利用双坩锅法有效地抑制了KNN基无铅压电陶瓷在高温烧结过程中碱金属元素的挥发，利用无压烧结成功地制备出致密度高、压电性能良好的KNN基无铅压电陶瓷。研究表明，KNN基陶瓷的烧结致密化以及电学性能对于烧结温度十分敏感；对于组成一定的KNN陶瓷，提高其压电性能需要针对致密度和显微结构以及抑制挥发等方面综合优化烧结工艺。经过组成和工艺优化，在Li/Ta共掺的KNN体系中获得了最高压电常数d33为206 pC/N的无压烧结陶瓷。详细研究了KNN基陶瓷无压烧结过程中的晶粒长大行为，并观察发现在KNN基陶瓷中存在伴随一种新型的“核－壳”结构的异常晶粒长大现象，且在特定条件下“异常晶粒长大”反而转变为“正常晶粒长大”。利用电子背散射衍射和透射电镜中的选区衍射来证实了新型“核－壳”结构是具有微小取向差的纳米晶粒自组装而成的类单晶，并分析讨论了该“核－壳”结构的形成机理。利用放电等离子体烧结(SPS)技术制备了几乎完全致密且晶粒细小的KNN基无铅压电陶瓷。通过进一步优化后热处理工艺，得到最高压电常数d33为225pC/N的KNN基陶瓷。由于利用SPS制备的KNN基陶瓷具有更致密和细晶的显微结构，其力学性能优于无压烧结样品，适合于微细柱状阵列的机械加工。利用改善后的切割－浇注法制备了微米尺度的1-3型KNN基无铅压电陶瓷/环氧树脂复合材料。通过优化陶瓷含量和复合材料的结构尺寸，制备得到具有单一厚度谐振工作模式、工作频率可达6 MHz、压电电压常数g33均高于200×10-3m2/C的1-3型KNN基无铅压电陶瓷/环氧树脂复合材料厚膜，展示了KNN基无铅压电陶瓷取代PZT铅基陶瓷在超声换能器中得到应用的潜力。
The environment development urgently calls for lead-free piezoelectric ceramics to substitute lead-based piezoelectric ceramics. Special emphasis was put on the effect of composition, sintering process on the microstructure, electrical and mechanical properties of sodium potassium niobate (KNN)-based lead-free piezoelectric ceramics. On this basis, the fabrication process and electrical properties were investigated in the novel 1-3 connectivity KNN-based lead-free piezoelectric ceramics/epoxy piezoelectric composites.A double-crucible method was introduced to efficiently suppress the volatilization of alkali metal oxides during the high-temperature sintering of KNN-based lead-free piezoelectric ceramics, by which KNN-based ceramics with high piezoelectric properties and high relative density were successfully prepared. It was found that the densification process and electrical properties of KNN-based lead-free piezoelectric ceramics were sensitive to sintering temperature. For KNN-based ceramics with constant composition, high performance can be achieved by optimizing relative density, microstructure along with suppressed volatilization of alkali metal oxides. Through optimizing composition and preparation process, the highest piezoelectric coefficient d33 206 pC/N was obtained in Li/Ta co-doped KNN ceramics.Grain growth behavior during the normal sintering of KNN-based ceramics was thoroughly investigated. It was found the abnormal grown grains show a new typical “core-shell” structure, but it turns back to normal grain growth behavior under some special condition. Selected area electron diffraction (SAED) and electron backscattered diffraction (EBSD) analysis confirmed that coarse grains with core-shell structure were single-crystalline-like grains composed by self-assembled nano-grains with small grain orientations.The formation process mechanism of such coarse grains was then discussed.Nearly full dense and fine-grained Li/Ta co-doped sodium potassium niobate lead-free piezoelectric ceramics were prepared by spark plasma sintering (SPS) process. Through tailoring the post-annealing process, highest piezoelectric constant d33 of 225 pC/N was obtained. The mechanical properties of SPSed KNN-based ceramics were higher than those prepared by normal sintering, because of the high relative density and fine-grained structure obtained by spark plasma sintering process.New fine-scale 1-3 lead-free piezoelectric ceramic/epoxy composites were prepared by modified dice and fill method. After optimizing the ceramic content and appropriate structure sizes, 1-3 KNN-based lead-free piezoelectric ceramic/epoxy composites with typical and uncoupled thickness mode were obtained. The working frequency was increased to 6 MHz and the highest piezoelectric voltage coefficients were all higher than 200×10-3m2/C, showing the promising future in ultrasonic transducers application for substituting the PZT-based piezoelectric ceramics.
