Enhanced Piezoelectric Properties of Lead-free Ferroelectric Single Crystals with Engineered Domain Configurations

在本研究中，我们的目标是(I)将工程畴结构引入到无铅铁电单晶中，（II）通过工程畴结构增强压电性能。为此，选择钛酸钡单晶作为典型的无铅铁电体，详细研究了工程畴结构与压电性能之间的关系。因此，通过本研究发现了以下四点。(1)工程畴结构的概念是通用的，这一概念可以应用于所有的铁电晶体。(2)工程畴构型有多种，并取决于晶体结构，如四方、正交、菱形相和晶体取向[100]、[110]和[111]。(3)在沿[001]方向取向的正交钛酸钡单晶中，发现其压电性能略大于PZT陶瓷。(4)在具有工程结构的钛酸钡单晶中，压电性能随着结构域尺寸的减小而增大。而钛酸钡的居里温度低至130℃，相变在5℃左右，对压电性能的温度依赖性较差。因此，本研究选用铌酸钾作为室温和200℃以上居里温度较高的正交结构的陶瓷导电材料。迄今为止，通过本研究，对铌酸钾单畴晶体的压电特性进行了澄清。目前，我们研究了在铌酸钾晶体中引入工程化畴结构，可以预期，在具有工程化畴结构的铌酸钾单晶中可以观察到更高的压电性能。

In this study, our objectives are (I) an introduction of the engineered domain configurations into lead free ferroelectric single crystals and (II) occurrence of enhanced piezoelectric property by the engineered domain configurations. For these objectives, barium titanate single crystals were chosen as a typical lead-free ferroelectrics, and the relationship between the engineered domain configurations and piezoelectric property was investigated in detail. As a result, the following four points were found through this study. (1) a concept of the engineered domain configurations is universal, and this concept cam be applied into all of ferroelectric crystals. (2) the engineered domain configurations have several kinds and depend on the crystal structures such as tetragonal, orthorhombic and rhombohedral phase and crystallographic orientations such as [100], [110] and [111]. (3) In orthorhonbic barium titanate single crystals oriented along [001] direction, it was found that its piezoelectric properties was slightly larger than those of PZT ceramics. (4) In barium titanate single crystals with the engineered domain configurations, it was found that the piezoelectric properties become to larger with decreasing domain sizes. However, for barium titanate, its temperature dependence of the piezoelectric properties was not good because its low Curie temperature of 130℃ and phase transition around 5℃. Therefore, as tile feroelectrics with orthorhombic structure at room temperature and higher Curie temperature over 200℃, potassium niobate was chosen in this study. Up to date, the unknown piezoelectric properties of potassium niobate single-domain crystals was cleared through this study. At present, we investigated the introduction of the engineered domain configurations into potassium niobate crystals, and it can be expected that the much higher piezoelectric properties will be observed in the potassium niobate single crystals with the engineered domain configurations.