Single Crystal Growth and Domain Control of Potassium Niobate Crystals by Vertical Bridgman method

铌酸钾（KNbO_3: KN）具有较大的压电常数和机电耦合系数，是一种优良的压电材料。因此，KN有望成为替代含铅压电陶瓷的未来压电材料之一，并作为利用表面声波的宽带滤波器件的基板而备受关注。然而，KN是一种不均匀熔融材料，因此，人们普遍认为KN单晶只能通过溶液生长，即顶种溶液生长（TSSG）方法生长。在我们之前的研究中，我们发现可以用垂直布里奇曼（VB）法生长直径为10 mm的KN单晶。本文采用VB法制备了直径为1英寸的KN单晶，并对其电学性能进行了研究。将K_2CO_3粉末与Nb_2O_5按K_2CO_3: Nb_2O_5 = 52.5: 47.5 mol%的比例混合，压缩后煅烧。所得柱状化合物作为原料。以z向KN单晶为种子，采用VB法在空气中生长直径为1英寸的RN晶体。生长采用铂坩埚，生长速度为0.5 mm/h。生长后，晶体在相变温度附近缓慢冷却。将晶体切割成晶片并进行抛光，并用x射线形貌仪对晶片进行检测。极化过程结束后，利用阻抗分析仪对极化后的VB-KN试样的压电特性进行了测试。结果表明，采用VB法制备了多云多畴KN晶体。x射线形貌证实，生长的晶体基本为单晶。经过极化处理后，vb生长的KN单晶表现出与TSSG方法相似的压电性。

Potassium niobate (KNbO_3: KN) is known as a great piezoelectric materials which has large piezoelectric constant and electro-mechanical coupling coefficient. Therefore, KN is expected to be one of the future piezoelectric materials instead of lead-containing piezoelectric ceramics, and it has also been attracting attention as the substrate of filter devices with wide band using surface acoustic wave. However, KN is an incongruent-melting material, therefore, it is widely believed that a KN single crystal can be only grown by solution growth that means the top-seeded solution growth (TSSG) method. In our previous study, we showed that KN single crystals 10 mm in diameter could be grown by vertical Bridgman (VB) method. In this study, growth of KN single crystal 1 inch in diameter was demonstrated by VB method and the electric properties of the grown KN crystal was investigated.Powdered K_2CO_3 and Nb_2O_5 were mixed in a ratio of K_2CO_3 : Nb_2O_5 = 52.5 : 47.5 mol%, and the mixture was compressed and was calcined. The resulting columned compounds were used as the raw materials. A z-directional KN single crystal was used as the seed, RN crystals 1 inch in diameter were grown by VB method in air. A platinum crucible was used for the growth, and the growth rate was 0.5 mm/h. After the growth, the crystal was cooled slowly near temperatures of its phase transformation. The crystals were cut into wafers and polished, and the wafers were examined by X-ray topography. After the polarization process, the piezoelectricity of the polarized VB-KN samples was evaluated by using impedance analyzer. As the results, cloudy multi-domain KN crystals were grown by VB method. It was confirmed by X-ray topography that the grown crystals were almost single crystal. After the polarization process, the VB-grown KN single crystal revealed piezoelectricity similar as that grown by TSSG method.