Hee Seok Kim

Hee Seok Kim
University of Washington Tacoma | UW Tacoma · School of Engineering and Technology

Doctor of Philosophy

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28
Publications
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Introduction
Skills and Expertise

Publications

Publications (28)
Article
Full-text available
The thermoelectric efficiency formula (η_max), derived by Ioffe, provides a direct connection between the material dimensionless figure of merit ZT and the maximum efficiency from heat into electricity in an ideal device with fixed terminal temperatures. However, for a thermoelectric power generator (TEG) system made for practical use, it could mee...
Article
Operant conditioning of Hoffmann's reflex (H-reflex) is a non-invasive and targeted therapeutic intervention for patients with movement disorders following spinal cord injury. The reflex-conditioning protocol uses electromyography (EMG) to measure reflexes from specific muscles elicited using transcutaneous electrical stimulation. Despite recent ad...
Article
Full-text available
Stress is one of the main causes that increase the risk of serious health problems. Recent wearable devices have been used to monitor stress levels via electrodermal activities on the skin. Although many biosensors provide adequate sensing performance, they still rely on uncomfortable, partially flexible systems with rigid electronics. These device...
Article
Full-text available
Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human–machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubte...
Article
Full-text available
Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human–machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubte...
Article
The Seebeck coefficient is one of the three important properties in thermoelectric materials. Since thermoelectric materials usually work under large temperature difference in real applications, we propose a quasi-steady state method to accurately measure the Seebeck coefficient under large temperature gradient. Compared to other methods, this meth...
Article
Full-text available
Improvements in thermoelectric material performance over the past two decades have largely been based on decreasing the phonon thermal conductivity. Enhancing the power factor has been less successful in comparison. In this work, a peak power factor of ∼106 μW·cm −1 ·K −2 is achieved by increasing the hot pressing temperature up to 1,373 K in the p...
Article
Till date, the dimensionless figure of merit (ZT) of p-type skutterudites lags its n-type counterpart. In this work, we report improved thermoelectric performance of La/Ce double filled p-type skutterudites. The samples were synthesized by hot pressing nano-powder made by ball milling annealed ingots of La 0.68 Ce 0.22 Fe 4Àx Co x Sb 12 with varyin...
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Motivated by good thermoelectric performance of Bi-based Zintl compounds Ca1−xYbxMg2Biy, we further studied the thermoelectric properties of Zintl compound CaMg2Bi1.98 by dopingNa into Ca as Ca1−xNaxMg2Bi1.98 via mechanical alloying and hot pressing. We found that the electrical conductivity, Seebeck coefficient, power factor, and carrier concentra...
Article
It has been previously shown that Zintl compounds Ca1-xYbxZn2Sb2 and EuZn2Sb2 could be good candidates as thermoelectric materials. However, the conventional synthesis process via melting-solidification-annealing introduces impurities and vacancies, resulting in abnormal high carrier concentration and ultimately low thermoelectric properties. Here...
Article
Good thermoelectric materials should have high engineering figure-of-merit (ZT)eng, not peak ZT, to achieve high conversion efficiency. In this work, we achieved a good (ZT)eng by optimizing the carrier concentration to improve the room temperature ZT using deep lying dopant, indium, in PbTe1−ySy. It was found that a room temperature ZT as high as...
Article
Full-text available
The nanostructuring approach has significantly contributed to the improving of thermoelectric figure-of-merit (ZT) by reducing lattice thermal conductivity. Even though it is an effective method to enhance ZT, the drastically lowered thermal conductivity in some cases can cause thermomechanical issues leading to decreased reliability of thermoelect...
Article
Thermoelectric properties of materials with compositions on and near the band crossing line connecting Mg2Sn0.765Ge0.22Sb0.015 and Mg2Sn0.685Si0.3Sb0.015 in the Mg2Sn-Mg2Ge-Mg2Si system are investigated. Although ZTs are very similar, power factors are different. On the line, the power factor decreases from Mg2Sn0.765Ge0.22Sb0.015 to Mg2Sn0.685Si0....
Article
Bi-based Zintl compounds, Ca1-xYbxMg2Bi2 with the structure of CaAl2Si2, have been successfully prepared by mechanical alloying followed by hot pressing. We found that the electrical conductivity, Seebeck coefficient, carrier concentration, and thermal conductivity can be adjusted by changing the Yb concentration. All Ca1-xYbxMg2Bi2 samples have lo...
Article
Full-text available
While considerable efforts have been made to develop and improve thermoelectric materials, research on thermoelectric modules is at a relatively early stage because of the gap between material and device technologies. In this review, we discuss the cumulative temperature dependence model to reliably predict the thermoelectric performance of module...
Article
Full-text available
The maximum conversion efficiency of a thermoelectric module composed of p- and n-type materials has been widely calculated using a constant property model since the 1950s, but this conventional model is only valid in limited conditions and no Thomson heat is accounted for. Since Thomson heat causes the efficiency under- or over-rated depending on...
Article
Constantan alloy (Cu–Ni) has been known for a long time in thermocouples due to its thermal power property. In this study, we show an enhancement in thermoelectric performance of Cu 56 Ni 42 Mn 2 alloy by introducing nanoscale twins into its microstructure. Comparing to arc-melted ingot (without nanoscale twins), the ball milled and hot pressed (BM...
Article
The figure of merit (ZT) has been used for more than half a century to guide the research of thermoelectric materials. In this perspective viewpoint, we emphasize that the power factor (PF) is more important than the thermal conductivity for thermoelectric power generation for a given ZT. We also propose the newly defined engineering parameters (ZT...
Article
Historically, a material parameter B incorporating weighted mobility and lattice thermal conductivity has guided the exploration of novel thermoelectric materials. However, the conventional definition of B neglects the bipolar effect which can dramatically change the thermoelectric energy conversion efficiency at high temperatures. In this paper, a...
Article
Mg3−xNaxSb2 has been prepared successfully by mechanical alloying plus hot pressing to investigate the effects of Na doping on the thermoelectric properties. Thermoelectric properties were characterized by the Seebeck coefficient, electrical resistivity, thermal conductivity, and thermoelectric figure of merit (ZT) from 298 to 773 K. Transport meas...
Article
Full-text available
The formula for maximum efficiency (ηmax) of heat conversion into electricity by a thermoelectric device in terms of the dimensionless figure of merit (ZT) has been widely used to assess the desirability of thermoelectric materials for devices. Unfortunately, the ηmax values vary greatly depending on how the average ZT values are used, raising ques...
Article
Thermoelectric power generation (TEG) represents one of the cleanest methods of energy conversion available today. It can be used in applications ranging from the harvesting of waste heat to conversion of solar energy into useful electricity. Remarkable advances have been achieved in recent years for various thermoelectric (TE) material systems. Th...
Article
Thermoelectric power generation is one of the most promising techniques to use the huge amount of waste heat and solar energy. Traditionally, high thermoelectric figure-of-merit, ZT, has been the only parameter pursued for high conversion efficiency. Here, we emphasize that a high power factor (PF) is equivalently important for high power generatio...
Article
Full-text available
Based on the fact that Hf is much more expensive than other commonly used elements in HfCoSb-based half-Heusler materials, we studied the thermoelectric properties of the p-type half-Heusler Hfx(ZrTi)1-xCoSb0.8Sn0.2 by reducing Hf concentration. A peak ZT of ~1.0 was achieved at 700 °C with the composition of Hf0.19Zr0.76Ti0.05CoSb0.8Sn0.2 by keepi...
Article
MgAg0.97Sb0.99 was found to be potentially a new class of thermoelectric materials with ZT values above 1 in the temperature from 100 to 300 °C. In this report, we systematically studied the effect of Na doping in Mg, Mg1−xNaxAg0.97Sb0.99, on the thermoelectric properties and found Na was effective to increase the carrier concentration and power fa...
Article
Full-text available
A segmented thermoelectric (TE) generator was designed with higher temperature segments composed of n-type Mg2Si and p-type higher manganese silicide (HMS) and lower temperature segments composed of n- and p-type Bi–Te based compounds. Since magnesium and silicon based TE alloys have low densities, they produce a TE module with a high specific powe...
Article
Full-text available
A segmented linear-shaped thermoelectric generator was designed with n-type Mg2Si and p-type higher manganese silicide as higher temperature segments and n-type and p-type Bi–Te based compounds as low temperature legs. A new design of a dovetail-shaped AlN–Cu composite as an electrode enabled linear-shaped thermoelectric generator to be securely bo...