Min Gyu Kang

High Output Piezo/Triboelectric Hybrid Generator

Recently, piezoelectric and triboelectric energy harvesting devices have been developed to convert mechanical energy into electrical energy. Especially, it is well known that triboelectric nanogenerators have a simple structure and a high output voltage. However, whereas nanostructures improve the output of triboelectric generators, its fabrication process is still complicated and unfavorable in term of the large scale and long-time durability of the device. Here, we demonstrate a hybrid generator which does not use nanostructure but generates much higher output power by a small mechanical force and integrates piezoelectric generator into triboelectric generator, derived from the simultaneous use of piezoelectric and triboelectric mechanisms in one press-and-release cycle. This hybrid generator combines high piezoelectric output current and triboelectric output voltage, which produces peak output voltage of ~370 V, current density of ~12 μA·cm−2, and average power density of ~4.44 mW·cm−2. The output power successfully lit up 600 LED bulbs by the application of a 0.2 N mechanical force and it charged a 10 μF capacitor to 10 V in 25 s. Beyond energy harvesting, this work will provide new opportunities for developing a small, built-in power source in self-powered electronics such as mobile electronics.

A ferroelectric photocatalyst for enhancing hydrogen evolution: polarized particulate suspension

A particle-based photocatalyst with a permanent internal field prepared by a corona poling method is presented as a novel approach to enhance the hydrogen evolution reaction in a particulate-suspension system. Photocatalytic activity of K0.5Na0.5NbO3 was significantly improved by 7.4 times after the polarization.

High-temperature thermoelectric properties of nanostructured Ca3Co4O9 thin films

We prepared nanostructured Ca3Co4O9 (CCO) thin films by promoting localized epitaxial growth on polycrystalline Al2O3 substrates. The thermoelectric properties of the CCO films were studied in the temperature range 300 to 1023 K. We confirmed that localized epitaxial growth occurred on the seed grains that dominate the (006) plane. The nanostructured CCO thin films were found to have a maximum Seebeck coefficient of 206 μV/K and a power factor (at 920 K) of 0.514 mW/mK2. Moreover, the presence of nanostructure was found to reduce the thermal conductivity, and thus, should enhance the overall performance of CCO films in thermoelectric devices.

Non-Volatile Control of 2DEG Conductivity at Oxide Interfaces

The functionalization of two-dimensional electron gas (2DEG) at oxide interfaces can be realized integrating 2DEG with multifunctional oxide overlayers by epitaxial growth. Using a ferroelectric Pb(Zr0.2 Ti0.8 )O3 overlayer on 2DEG (LaAlO3 /SrTiO3 ), we demonstrate a model system of the functionalized 2DEG, where electrical conductivity of 2DEG can be reversibly controlled with a large on/off ratio (>1000) in a non-volatile way by ferroelectric polarization switching.

Enhanced piezoelectric properties of vertically aligned single-crystalline NKN nano-rod arrays

Piezoelectric materials capable of converting between mechanical and electrical energy have a great range of potential applications in micro- and nano-scale smart devices; however, their performance tends to be greatly degraded when reduced to a thin film due to the large clamping force by the substrate and surrounding materials. Herein, we report an effective method for synthesizing isolated piezoelectric nano-materials as means to relax the clamping force and recover original piezoelectric properties of the materials. Using this, environmentally friendly single-crystalline NaxK1-xNbO3 (NKN) piezoelectric nano-rod arrays were successfully synthesized by conventional pulsed-laser deposition and demonstrated to have a remarkably enhanced piezoelectric performance. The shape of the nano-structure was also found to be easily manipulated by varying the energy conditions of the physical vapor. We anticipate that this work will provide a way to produce piezoelectric micro- and nano-devices suitable for practical application, and in doing so, open a new path for the development of complex metal-oxide nano-structures.

Enhanced piezoluminescence in non-stoichiometric ZnS:Cu microparticle based light emitting elastomers

Piezoluminescence (PZL), also referred to as mechanoluminescence (ML), is a promising energy conversion mechanism for realizing mechanically driven photon sources including handheld displays, lighting, bioimaging and sensing applications. However, the realization of a visible PZL intensity at room temperature from low mechanical stresses has been fundamentally challenging. Herein, we describe a PZL elastomer exhibiting significantly enhanced brightness under ambient conditions. The elastomer consisted of defect-engineered non-stoichiometric Cu-doped ZnS (ZnS:Cu) microparticles in a polydimethylsiloxane (PDMS) matrix. The role of the defect structure was found to be the controlling parameter in the nature of PZL emission. Hydrogenation treatment was designed to induce a controlled concentration of sulfur vacancies that provided the trapped electrons, which had a strong correlation with the PZL performance of ZnS:Cu. An optimum electron concentration was necessary in order to maximize the PZL intensity due to an adequate electron energy transfer ratio between non-radiative recombination (NRR) and thermal radiative recombination (TRR). The light-emitting elastomer with an optimum content of PZL particles maximized the stress-mediated electroluminescence–piezoelectric coupling, enabling visible PZL brightness under indoor light conditions.

Functionally Graded Interfaces: Role and Origin of Internal Electric Field and Modulated Electrical Response

We report the tunable electrical response in functionally graded interfaces in lead-free ferroelectric thin films. Multilayer thin film graded heterostructures were synthesized on platinized silicon substrate with oxide layers of varying thickness. Interestingly, the graded heterostructure thin films exhibited shift of the hysteresis loops on electric field and polarization axes depending upon the direction of an applied bias. A diode-like characteristics was observed in current-voltage behavior under forward and reverse bias. This modulated electrical behavior was attributed to the perturbed dynamics of charge carriers under internal bias (self-bias) generated due to the increased skewness of the potential wells. The cyclic sweeping of voltage further demonstrated memristor-like current-voltage behavior in functionally graded heterostructure devices. The presence of an internal bias assisted the generation of photocurrent by facilitating the separation of photogenerated charges. These novel findings provide opportunity to design new circuit components for the next generation of microelectronic device architectures.

Thermo-Magneto-Electric Generator Arrays for Active Heat Recovery System

Continued emphasis on development of thermal cooling systems is being placed that can cycle low grade heat. Examples include solar powered unmanned aerial vehicles (UAVs) and data storage servers. The power efficiency of solar module degrades at elevated temperature, thereby, necessitating the need for heat extraction system. Similarly, data centres in wireless computing system are facing increasing efficiency challenges due to high power consumption associated with managing the waste heat. We provide breakthrough in addressing these problems by developing thermo-magneto-electric generator (TMEG) arrays, composed of soft magnet and piezoelectric polyvinylidene difluoride (PVDF) cantilever. TMEG can serve dual role of extracting the waste heat and converting it into useable electricity. Near room temperature second-order magnetic phase transition in soft magnetic material, gadolinium, was employed to obtain mechanical vibrations on the PVDF cantilever under small thermal gradient. TMEGs were shown to achieve high vibration frequency at small temperature gradients, thereby, demonstrating effective heat transfer.

Large in-plane permittivity of Ba0.6Sr0.4TiO3 thin films crystallized using excimer laser annealing at 300 °C

We demonstrated a way to fabricate the crystalline Ba0.6Sr0.4TiO3 (BST) thin films using excimer laser annealing technique on the amorphous BST thin films fabricated by sol-gel process. The grain size of the laser-annealed films is larger than that of the conventionally thermal-annealed films. However, an uncrystallized, amorphous layer was observed near the film/substrate interface due to the limited laser absorption depth. The uncrystallized layer has a critical influence on out-of-plane dielectric property of BST films. The significant difference of the relative dielectric permittivity (er) between in-plane (1383) and out-of-plane (184) directions is observed.

Fabrication of 1 μm Thickness Lead Zirconium Titanate Films Using Poly(N-vinylpyrrolidone) Added Sol-gel Method

Copyright 2011 KIEEME. All rights reserved. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Fabrication of 1 μm Thickness Lead Zirconium Titanate Films Using Poly(N-vinylpyrrolidone) Added Sol-gel Method