Introduction

Recently, with the rapid development of technology, ferroelectric thin films are widely used in various modern electronic devices, including random access memories, optical switches, accelerometers, etc . And, with the rapid development of wireless sensor networks, piezoelectric energy harvesters using ferroelectric thin films as power sources for wireless sensor nodes are attracting more and more attention. The piezoelectric energy harvester converts the mechanical energy of environmental vibrations into electrical energy by the direct piezoelectric effect of the ferroelectric film. Since most ambient vibrations have low frequencies of about 10-200 Hz , it is necessary to tune the resonant harvester to such low frequencies while keeping the device size small. Therefore, ferroelectric films formed on metal substrates with low Young's modulus are ideal for energy harvesting applications.

Low-cost base metals, such as nickel, aluminum and copper, are easily oxidized at high temperatures in air. In addition, the piezoelectric and energy harvesting properties of ferroelectric thin films are highly dependent on their crystal orientation and thickness. Therefore, it is particularly important to develop a low-temperature film growth process for ferroelectric thin films that can control the crystal orientation and film thickness. Hydrothermal method is a low-temperature synthesis technique for obtaining crystalline materials from aqueous solutions and has been reported for depositing ferroelectric oxides such as Pb(Zr,Ti)O3 (PZT), BaTiO3, BiFeO3 (BF), and (K,Na)NbO3 (KNN), for ferroelectric thin films grown hydrothermally on metallic substrates, (Morita et al.), at a low temperature of 160 °C PZT films were fabricated on titanium substrates by hydrothermal reaction at 160 °C. The hydrothermal method was reported to be capable of depositing thick films of oriented KNN up to 27 μm thick on nickel-based alloy substrates [14-16]. These previous works show that hydrothermal method is a powerful technique for fabricating ferroelectric thin films on low-cost metal substrates.

PZT-based ferroelectrics are considered to be the most promising materials for energy harvesting applications due to their excellent piezoelectric properties. However, the search for alternative lead-free ferroelectric materials has become increasingly important due to the strong concern about the negative environmental impact of lead. Bismuth-containing chalcogenide oxides, such as BF, have been reported to exhibit excellent ferroelectric properties due to the electron configuration of Bi3+ with 6s 2 lone pair electrons, which is the same as that of Pb2+. Recently, our group has developed a hydrothermal synthesis process for fine particles and bulk ceramics of potassium bismuth titanate (Bi1/2K1/2).

TiO3 (BKT) is a ferroelectric body with a tetragonal chalcogenide structure. Subsequent studies of bulk BKT ceramics have shown that BKT has good piezoelectric properties for energy harvesting applications, such as relatively high piezoelectric constants (d33 ~ 95 pC/N), high depolarization temperatures (Td ~ 300 °C), and low dielectric constants (εr ~ 600). However, little has been reported on the preparation and deposition of BKT films. To date, no technology has been developed for BKT thin films on metallic substrates.

In this report, we apply the hydrothermal method to fabricate BKT films on nickel-based alloy metal substrates. LaNiO3 (LN), a conductive chalcogenide oxide, is deposited as a buffer layer on the substrate by sol-gel coating to promote heterogeneous nucleation of BKT. We show that BKT films with a preferred c-axis orientation can be grown on LN buffer substrates by hydrothermal reaction at a low temperature of 150 °C. In this approach, the surface structure and thickness of the BKT films can be controlled by the number of deposition cycles of LN and BKT, respectively. The dielectric and ferroelectric properties of the resulting BKT films are also reported.

Applications

In this paper, we focus on the suitability of ferroelectric thin films fabricated on metallic substrates for use in piezoelectric vibration energy harvesters. In this report, we demonstrate that lead-free ferroelectric (Bi1/2K1/2)TiO3 (BKT) dense thin films with preferred c-axis orientation can be grown on nickel-based alloy substrates by using a hydrothermal synthesis method. A LaNiO3(LN) buffer layer was formed on the substrate by sol-gel spin coating, and then BKT was grown by hydrothermal reaction at a low temperature of 150 °C. It was found that the surface structure and thickness of the final BKT films strongly depend on the number of deposition cycles of LN and BKT, respectively. As a result, highly c-axis oriented BKT films with smooth surfaces and up to 830 nm thickness were obtained. Although the proposed deposited BKT films contain hydroxyl groups in their lattice, they were found to be effectively removed by a post-annealing process at 650°C in air, and the polarization response of the annealed BKT films demonstrated the iron electrolysis conversion in the films.

Image 10 shows the surface and cross-sectional structure of the film as observed by field emission scanning electron microscopy (FE-SEM) using an FEI Sirion microscope at an accelerating voltage of 5 kV. The average grain size was determined by measuring the edge lengths of the 30 cubic grains found in the surface FE-SEM image of the film. The phase determination of the films was performed by X-ray diffraction (XRD) analysis using a Bruker AXS D8-ADVANCE diffractometer with 2θ/θ geometry of Cu Kα radiation. For electrical characterization, the deposited films were post-annealed in air at 650°C for 10 minutes to remove the lattice hydroxyl groups contained in the films. Then, a 1 mm diameter circular gold electrode was sputtered on the film sample as the top electrode, while the Inconel substrate served as the bottom electrode. By using an LCR meter (ZM2371, NF Inc.). Polarization (P) vs. electric field (E) and current density (J) vs. E curves were measured at room temperature by applying a triangular voltage wave with a frequency of 200 Hz using a ferroelectric test system (FCE10-B, TOYO Corp.).

Source

Authors: Masayoshi Yamamoto1, Ryotaro Sakurai, Manabu Hagiwara, Shinobu Fujihara

Institution: Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

Published: Received 15April 2020; Received in revised form 17 August 2020; Accepted 8 September 2020

Keywords: ferroelectrics, thin film growth, hydrothermal method, metal substrates, low temperature synthesis

Journal: Thin SolidFilms

Article source website.Hydrothermalgrowth of c-axis oriented ferroelectric (Bi1/2K1/2)TiO3 films on metal substrates