Energy harvesting devices have been developed with the aim of the powering small electronic devices and achieving self-powered devices. Nanogenerators (NG) based on piezoelectric ma-terials is one of the energy harvesting device, which have considered as a potential candidate to drive electronic devices, such as implantable biosensors, microelectromechanical systems (MEMS), and portable electronics. In the NG applications, the output voltage and power of NG is directly proportional to the piezoelectric coefficient. Therefore, designing material with high piezoelectric coefficient is essential for enhancing output performance to meet the com-mercial applications.
The solid solution of Pb(Mg1/3Nb2/3)O3 and PbTiO3, so-called as PMN-PT, has the highest piezoelectric coefficient (2500 pm/V) near morphotropic phase boundary region (MPB). Particularly, one-dimensional piezoelectric nanostructures are mechanically robust and flexible which leads to high sensitivity to small mechanical disturbances. It has been consid-ered as potential for harvesting energy from the environment and sensing devices at nanoscale. Recently, 0.72PMN-0.28PT nanowires were synthesized by hydrothermal method and the pi-ezoelectric coefficient of these nanowires were measured as 381 pm/V. But, the role of syn-thesis parameters on the phase and morphological evolution of PMN-PT nanowires have not been studied. The performance of single PMN-PT nanowire has not been characterized yet in NG applications.
In this study, the solid solution of lead magnesium niobate (PMN: Pb(Mg1/3Nb2/3O3) and lead titanate (PT: PbTiO3) nanowires were synthesized by the hydrothermal method. The solid solutions of PMN with molar ratio of 35% PT was chosen, since this composition is close to the morphotropic phase boundary region, which is known to exhibit higher piezoelectric coefficient due to their relaxor-ferroelectric behavior. Initially, the PMN-PT stock solution for a hydrothermal reaction was prepared by a sol-gel method using PEG-200 and methanol both as solvent. Then, mixing of stock solution with KOH mineralizer followed by hydrothermal treatment at 200 oC, yields PMN-PT nanowires. The influence of synthesis parameters such as KOH concentration, reaction time, and PEG concentration on phase formation and morpholo-gy of PMN-PT powders were investigated systematically. The optimum reaction parameters to produce perovskite phase of PMN-PT with nanowire morphology were determined. The for-mation of PMN-PT nanowires in the hydrothermal environment was discussed in detail using dissolution-reprecipitation mechanism. The physical characteristics of resultant PMN-PT pow-ders were studied by XRD, Raman, SEM, and TEM. For the first time, the nanogenerator per-formance with single PMN-PT nanowire was measured by the flexible nanogenerator device. The single PMN-PT nanowire produces the output voltage ~ 9 mV and output current of ~ 20 nA during bending/unbending motions.
본 연구에서는 압전 나노 물질의 합성 및 나노구조 형성에 영향을 미치는 다양한 매개 변수를 알아보고자 하였다. PMN-PT는 높은 압전 상수 및 유전 상수를 가지는 물질로 알려져 있으며, 나노 압전 소자에 적용 시 0차원 입자에 비하여 유리한 1차원 나노입자를 합성하고자 하였다.
정방정계 페로브스카이트(Perovskite)상을 가지는 1차원 PMN-PT나노 구조체는 수열반응법에 의하여 합성하였다. 이 과정에서 반응용액인 KOH수용액의 농도, 수열반응 시간, PEG농도 등의 변수에 따른 PMN-PT나노 입자의 결정상 및 형상 변화를 비교？관찰하였다.
KOH수용액의 농도와 PEG의 함량은 정방정계 결정상 형성과 입자 형상 변화에 중요한 역할을 하였다. 일련의 매개 변수 조절 실험 중 12M KOH수용액, 77wt% PEG함량 및 200°C 6시간의 수열반응 조건에서 1차원 나노구조체 형상을 가지는 PMN-PT입자가 얻어졌다. 이 밖의 조건들로부터 얻어진 입자의 경우에는 불균일 형상의 입자와 원하지 않는 결정상의 입자가 얻어졌다.
한편, 균일한 형태의 1차원 PMN-PT나노 구조체를 얻기 위하여 교반수열방법을 통한 합성을 진행하였다. 이로부터, 종횡비가 교반이 없이 수열반응하여 합성한 입자(길이 약 6μm)에 비하여 큰 약 11μm의 길이를 가지는 PMN-PT입자를 얻을 수 있었다.
그러나, 이 공정을 통하여 얻어진 분말에는 정육면체 형상의 입자 또한 혼재되어 있었다. 이렇게 혼재된 두 형상의 입자들은 계면활성제 혼합 후 원심분리를 통하여 분리하였다.
분리된 1차원 PMN-PT나노구조체는 패턴이 전사된 유연 폴리머 소자에 연결하여 굽힘 시의 압전 특성을 측정하였다. 단일 1차원 PMN-PT나노구조체로부터 약 9 mV와 20 nA의 출력전압 및 전류를 얻을 수 있었다.
