Hyunseung Kim

Bio: Hyunseung Kim is an academic researcher from Chonbuk National University. The author has contributed to research in topics: Piezoelectricity & Dielectric. The author has an hindex of 3, co-authored 6 publications receiving 37 citations.

Progress in lead-free piezoelectric nanofiller materials and related composite nanogenerator devices

Abstract: Current piezoelectric device systems need a significant reduction in size and weight so that electronic modules of increasing capacity and functionality can be incorporated into a great range of applications, particularly in energy device platforms. The key question for most applications is whether they can compete in the race of down-scaling and an easy integration with highly adaptable properties into various system technologies such as nano-electro-mechanical systems (NEMS). Piezoelectric NEMS have potential to offer access to a parameter space for sensing, actuating, and powering, which is inflential and intriguing. Fortunately, recent advances in modelling, synthesis, and characterization techniques are spurring unprecedented developments in a new field of piezoelectric nano-materials and devices. While the need for looking more closely at the piezoelectric nano-materials is driven by the relentless drive of miniaturization, there is an additional motivation: the piezoelectric materials, which are showing the largest electromechanical responses, are currently toxic lead (Pb)-based perovskite materials (such as the ubiquitous Pb(Zr,Ti)O3, PZT). This is important, as there is strong legislative and moral push to remove toxic lead compounds from commercial products. By far, the lack of viable alternatives has led to continuing exemptions to allow their temporary use in piezoelectric applications. However, the present exemption will expire soon, and the concurrent improvement of lead-free piezoelectric materials has led to the possibility that no new exemption will be granted. In this paper, the universal approaches and recent progresses in the field of lead-free piezoelectric nano-materials, initially focusing on hybrid composite materials as well as individual nanoparticles, and related energy harvesting devices are systematically elaborated. The paper begins with a short introduction to the properties of interest in various piezoelectric nanomaterials and a brief description of the current state-of-the-art for lead-free piezoelectric nanostructured materials. We then describe several key methodologies for the synthesis of nanostructure materials including nanoparticles, followed by the discussion on the critical current and emerging applications in detail.

Triboelectrification of nanocomposites using identical polymer matrixes with different concentrations of nanoparticle fillers

Abstract: In this study, we investigate triboelectrification in polymer-based nanocomposites using identical polymer matrixes containing different concentrations of nanoparticles (NPs). The triboelectric surface charge density on polymer layers increased as the difference in nanoparticle filler concentration between the two triboelectric layers escalated, despite the fact that the polymer matrix was the same in both layers. This effect was observed in tests of various polymer types and filler NPs. Our mechanical experiments and finite element analysis simulations confirmed that polymeric triboelectrification is related to the surface viscoelastic deformation that occurs during mechanical contact and separation, and is not dependent on electronic or chemical properties, but rather physicochemical and mechanical properties. This supports the heterolytic scission of covalent bonds in polymer chains as an explanation for the triboelectricity. Accordingly, this study marks a significant contribution to the fundamental understanding of the mechanism of polymeric triboelectrification and will be relevant to the development of triboelectric sensors and energy harvesters.

(K,Na)NbO 3 ‑LiNbO 3 nanocube‑based flexible and lead‑free piezoelectric nanocomposite energy harvesters

Abstract: The research on harvesting the wasted mechanical energy and using it as a useful energy source has been in the spotlight. Conventional ceramic-based devices have disadvantages such as mechanical brittleness and high-temperature processing. To overcome these weaknesses, the piezoceramic materials mixed with polymer matrix have been studied to produce flexible piezoelectric devices. Herein, a piezoelectric composite film is fabricated by mixing lead-free 0.942[Na0.535 K0.480NbO3]-0.058LiNbO3 (KNNLN) nanocube powder with polydimethylsiloxane (PDMS) matrix. KNNLN nanocube powder is synthesized by a solid-state reaction method, which can typically present outstanding stoichiometric composition and perovskite crystalline structure. The cube-shaped particles are thought to show higher stress concentration at corners and edges of the nanocube than spherical particles when external mechanical input is applied. The flexible KNNLN nanocube–PDMS nanocomposite device generates the piezoelectric signals of ~ 28 V and ~ 220 nA during bending motion, which is a high-performance energy harvesting efficiency compared to previous KNN-based flexible piezoelectric composites generators. KNNLN nanocube-based composite can replace toxic lead-based nanocomposites for future flexible devices such as eco-friendly self-powered energy harvester and biocompatible sensor.

Nanointerfacial Layer Effect on Dielectric and Piezoelectric Responses in Chemical Solution-Derived Lead-Free Alkaline Niobate-Based Thin Films.

Abstract: Since nonpiezoelectric interfacial layers even at the nanoscale significantly affect the performance of lead-free piezoelectric thin films, the quantitative characterization of property changes of thin films due to interfacial layers is of great importance and should be precisely undertaken for piezoelectric microelectromechanical system (MEMS) and nanoelectromechanical system (NEMS) devices. In contrast to widely accepted concepts for interfacial layer thickness estimation based on the existing series capacitor model, we find that the interfacial layer thickness at the top and the bottom interfaces is clearly different in chemical solution deposition (CSD)-derived (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 (KNMN) thin films. Interestingly, the thickness of the bottom interface increases linearly with increasing thin-film thickness, while the thickness of the top interface is constant regardless of the thin-film thickness. In this work, nanointerfacial layer effects of CSD-derived KNMN thin films are theoretically and experimentally addressed in a combinatorial way using a modified series capacitor model. The obtained information is used to envisage the origins and the mechanisms of nonpiezoelectric interfacial layers and associated dielectric and ferroelectric properties of KNMN thin films. Our research connects macroscopic properties with microscopic origins and is greatly facilitated by separating intrinsic and extrinsic contributions to phenomenological behaviors, as well as engineering interface-related properties of the films. We believe these studies to be crucial for the further development and applications of KNN-based lead-free piezoelectric devices, which also open the door to future studies on other lead-free piezoelectric material systems for practical MEMS and NEMS applications.

Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

Abstract: We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

Lithium-Rich Core-Shell Cathodes with Low Irreversible Capacity and Mitigated Voltage Fade

Abstract: Lithium-rich layered Ni–Mn–Co oxide materials have been intensely studied in the past decade. Mn-rich materials have serious voltage fade issues, and the Ni-rich materials have poor thermal stability and readily oxidize the organic carbonate electrolyte. Core–shell (CS) strategies that use Ni-rich material as the core and Mn-rich materials as the shell can balance the pros and cons of these materials in a hybrid system. The lithium-rich CS materials introduced here show much improved overall electrochemical performance compared to the core-only and shell-only samples. Energy dispersive spectroscopy results show that there was diffusion of transition metals between the core and shell phases after sintering at 900 °C compared to the prepared hydroxide precursors. A Mn-rich shell was still maintained whereas the Co which was only in the shell in the precursor was approximately homogeneous throughout the particles. The CS samples with optimal lithium content showed low irreversible capacity (IRC), as well as ...