Yun Goo Lee

Bio: Yun Goo Lee is an academic researcher from Pennsylvania State University. The author has contributed to research in topics: Electric field & Ferroelectricity. The author has an hindex of 6, co-authored 16 publications receiving 94 citations. Previous affiliations of Yun Goo Lee include Korea University & Korea Institute of Science and Technology.

Design and Experimental Investigation of Thermoelectric Generators for Wearable Applications

Abstract: A critical challenge in using thermoelectric generators (TEGs) for charging the portable or wearable electronics has been their limited outputs, as available temperature differential on human body (∆Text) is typically less than 10 K. Furthermore, the thermal resistance (Rth) at the TEG–air interface often overwhelms Rth of TEG itself, which makes the temperature differential within the TEG merely a small fraction of ∆Text. Here, the designs of TEG systems for wearable applications based both on theory and systematic experiments are studied. First, this study fabricates the TEGs having different fill factors (equivalently, varied internal Rth of the TEGs) and finds an optimum fill factor that is determined by both thermal matching condition and the electrical contact resistance. Next, to investigate the effects of heat sink and external air flow, this study combines plate fin heat sinks with the TEGs and evaluates their performance under three different convection conditions: natural convection, and convection with either parallel or impinging flow. Lastly the effect of Rth at the skin–TEG interface is studied. Although the TEG system produces an output power of 126 µW cm−2 (∆Text = 7 K) on a smooth heat source (Cu heater), it generates reduced power of 20 µW cm−2 (∆Text = 6 K) on wrist (uneven heat source).

Enhanced electro-optic beam deflection of relaxor ferroelectric KTN crystals by electric-field-induced high permittivity

Abstract: Most applications of a ferroelectric-based electro-optic (EO) beam deflector have been limited by the high applied voltage. In this Letter, we report a dramatically increased EO beam deflection in relaxor ferroelectric potassium tantalate niobate (KTN) crystals by using the electric-field-enhanced permittivity. Due to the existence of the electric-field-induced phase transition in relaxor ferroelectric materials, the dielectric permittivity can be substantially increased by the applied electric field at a certain temperature. Both the theoretical study and the experimental verifications on the enhanced beam deflection and EO effect in the case with the electric-field-induced high permittivity were conducted. The experimental results confirmed that there was a three-fold increase in the deflection angle, which represented a dramatic increase in the deflection angle. By offering a wider deflection range and a lower driving voltage, such a largely enhanced beam deflection is of great benefit to the KTN deflector.

Harman Measurements for Thermoelectric Materials and Modules under Non-Adiabatic Conditions

Abstract: Accuracy of the Harman measurement largely depends on the heat transfer between the sample and its surroundings, so-called parasitic thermal effects (PTEs). Similar to the material evaluations, measuring thermoelectric modules (TEMs) is also affected by the PTEs especially when measuring under atmospheric condition. Here, we study the correction methods for the Harman measurements with systematically varied samples (both bulk materials and TEMs) at various conditions. Among several PTEs, the heat transfer via electric wires is critical. Thus, we estimate the thermal conductance of the electric wires, and correct the measured properties for a certain sample shape and measuring temperature. The PTEs are responsible for the underestimation of the TEM properties especially under atmospheric conditions (10-35%). This study will be useful to accurately characterize the thermoelectric properties of materials and modules.

Correction of the Electrical and Thermal Extrinsic Effects in Thermoelectric Measurements by the Harman Method

Abstract: Although the Harman method evaluates the thermoelectric figure-of-merit in a rapid and simple fashion, the accuracy of this method is affected by several electrical and thermal extrinsic factors that have not been thoroughly investigated. Here, we study the relevant extrinsic effects and a correction scheme for them. A finite element model simulates the electrical potential and temperature fields of a sample, and enables the detailed analysis of electrical and thermal transport. The model predicts that the measurement strongly depends on the materials, sample geometries, and contact resistance of the electrodes. To verify the model, we measure the thermoelectric properties of Bi2-Te3 based alloys with systematically varied sample geometries and either with a point or a surface current source. By comparing the model and experimental data, we understand how the measurement conditions determine the extrinsic effects, and, furthermore, able to extract the intrinsic thermoelectric properties. A correction scheme is proposed to eliminate the associated extrinsic effects for an accurate evaluation. This work will help the Harman method be more consistent and accurate and contribute to the development of thermoelectric materials.

High-performance compliant thermoelectric generators with magnetically self-assembled soft heat conductors for self-powered wearable electronics.

Abstract: Softening of thermoelectric generators facilitates conformal contact with arbitrary-shaped heat sources, which offers an opportunity to realize self-powered wearable applications. However, existing wearable thermoelectric devices inevitably exhibit reduced thermoelectric conversion efficiency due to the parasitic heat loss in high-thermal-impedance polymer substrates and poor thermal contact arising from rigid interconnects. Here, we propose compliant thermoelectric generators with intrinsically stretchable interconnects and soft heat conductors that achieve high thermoelectric performance and unprecedented conformability simultaneously. The silver-nanowire-based soft electrodes interconnect bismuth-telluride-based thermoelectric legs, effectively absorbing strain energy, which allows our thermoelectric generators to conform perfectly to curved surfaces. Metal particles magnetically self-assembled in elastomeric substrates form soft heat conductors that significantly enhance the heat transfer to the thermoelectric legs, thereby maximizing energy conversion efficiency on three-dimensional heat sources. Moreover, automated additive manufacturing paves the way for realizing self-powered wearable applications comprising hundreds of thermoelectric legs with high customizability under ambient conditions.

The Design of a Thermoelectric Generator and Its Medical Applications

Abstract: Growing energy demands are driving people to generate power in every possible way. New energy sources are needed to plug the energy gap. There is a growing interest in distributed energy generation due to its remarkable advantages such as flexibility, reliability, adaptability and minimal transmission losses. Thermoelectric generators (TEGs) are one such distributed power source that relies on thermal energy for electricity generation. The current review focusses on the design and optimization of TEGs to maximize the power output from the available thermal sources. The basic principle of thermoelectricity generation and suitable architecture for specific applications are explained with an overview of materials and manufacturing processes. Various cooling techniques to dissipate heat from the cold side and their influence on overall efficiency are reviewed in this work. Applications of TEGs for powering biomedical sensors have been discussed in detail. Recent advancements in TEGs for various implantable devices and their power requirements are evaluated. The exploitation of TEGs to generate power for wearable sensors has been presented, along with published experimental data. It is envisioned that this study will provide profound knowledge on TEG design for specific applications, which will be helpful for future endeavours.