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Kim, Kwanlae,Kim, Gwansik,Kim, Sang Il,Lee, Kyu Hyoung,Lee, Wooyoung Elsevier 2019 JOURNAL OF ALLOYS AND COMPOUNDS Vol.772 No.-
<P><B>Abstract</B></P> <P>The feasibility of using Bi<SUB>2</SUB>Te<SUB>3</SUB>-based alloys in low-grade heat thermoelectric power generation has been intensively investigated via a substitutional doping approach over the last decade. However, the comprehensive and quantitative understanding of the electronic and thermal transport parameters of doped Bi<SUB>2</SUB>Te<SUB>3</SUB>-based alloys including their carrier concentration (<I>n</I> <SUB>c</SUB>), carrier mobility (<I>μ</I> <SUB>Hall</SUB>), density of state (DOS) effective mass (<I>m</I> <SUB>d</SUB> <SUP>∗</SUP>), and electronic (<I>κ</I> <SUB>ele</SUB>), lattice (<I>κ</I> <SUB>lat</SUB>), and bipolar thermal (<I>κ</I> <SUB>bp</SUB>) conductivities is still elusive. The understanding of these parameters is a prerequisite for designing the modules for real-time applications. In this study, we investigated the effect of Pb, Ag, and Cu doping on the thermoelectric transport parameters of <I>p</I>-type Bi<SUB>0.52</SUB>Sb<SUB>1.48</SUB>Te<SUB>3</SUB> (BST) both theoretically and experimentally. The thermoelectric transport properties of BST and their temperature dependences could be systematically tuned in a low-temperature range by controlled doping of Pb, Ag, and Cu mainly because of the increased concentration of the majority hole carriers. In addition, a <I>zT</I> value of 1 could be obtained over the wide temperature range of 300–400 K by optimizing the doping elements and contents because of the synergetic effect of the suppression of bipolar conduction at higher temperatures and the gradual increase in <I>m</I> <SUB>d</SUB> <SUP>∗</SUP> with the doping content at <I>n</I> <SUB>c</SUB> < 10<SUP>20</SUP> cm<SUP>−3</SUP>.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We fabricated doped Bi<SUB>2</SUB>Te<SUB>3</SUB>-based alloys using melting and SPS process. </LI> <LI> Understanding of the electronic and thermal transport parameters. </LI> <LI> The generated hole carrier improved the thermoelectric performance. </LI> <LI> Suppression of bipolar conduction while maintaining a high carrier mobility. </LI> <LI> The modified band structure (band flattening and Fermi level tuning). </LI> </UL> </P>
Kim, Gwansik,Lee, Hwijong,Rim, Hyun Jun,Kim, Jeongmin,Kim, Kwanlae,Roh, Jong Wook,Choi, Soon-Mok,Kim, Byung-Wook,Lee, Kyu Hyoung,Lee, Wooyoung Elsevier 2018 Journal of alloys and compounds Vol.769 No.-
<P><B>Abstract</B></P> <P>We prepared Sn nanoparticle-embedded Mg<SUB>1.96</SUB>Al<SUB>0.04</SUB>Si<SUB>0.97</SUB>Bi<SUB>0.03</SUB> nanocomposites and measured their thermoelectric properties and fracture toughness to elucidate the trade-off relationship between thermoelectric and mechanical properties. When Sn nanoparticles (50–150 nm) were introduced at the grain boundaries of the thermoelectric Mg<SUB>1.96</SUB>Al<SUB>0.04</SUB>Si<SUB>0.97</SUB>Bi<SUB>0.03</SUB> matrix, the fracture toughness improved because of the inhibition of crack propagation. However, the power factor deteriorated due to the decrease in carrier mobility. We found that interface (between thermoelectric matrix and nanoparticles) density is a critical factor to determine the mechanical properties as well as thermoelectric transport properties. Optimized values of figure of merit (∼0.66 @ 873 K) and fracture toughness (1.10 MPa m<SUP>1/2</SUP>) were obtained for 0.9 vol % Sn nanoparticle-embedded Mg<SUB>1.96</SUB>Al<SUB>0.04</SUB>Si<SUB>0.97</SUB>Bi<SUB>0.03</SUB> nanocomposite.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We prepared nanocomposites by nanometal decoration and SPS. </LI> <LI> The importance of interface density for improving TE and mechanical properties. </LI> <LI> The effect of the grain growth inhibition by Sn nanoparticles. </LI> <LI> Improved fracture toughness was obtained due to inhibiting crack propagation. </LI> <LI> The importance of precisely controlled nanophase design and synthesis process. </LI> </UL> </P>
Kim, Kwanlae,Kim, Gwansik,Lee, Hwijong,Lee, Kyu Hyoung,Lee, Wooyoung Elsevier 2018 Scripta materialia Vol.145 No.-
<P><B>Abstract</B></P> <P>Herein, we report the results of a systematic study on the effect of Pb doping on the thermoelectric transport properties of <I>p</I>-type BiSbTe alloys to validate its potential applications for low-temperature power generation. The maximum power factor (~4.4mWm<SUP>−1</SUP> K<SUP>−2</SUP>) at 300K was obtained using 0.31at.% Pb-doped Bi<SUB>0.52</SUB>Sb<SUB>1.48</SUB>Te<SUB>3</SUB> and was found to originate from an enlarged density of states effective mass as a result of the band engineering effect. The maximum efficiency of thermoelectric power generation (<I>η</I> <SUB> <I>max</I> </SUB>) could be enhanced by 150% at Δ<I>T</I> =220K when the Pb concentration was optimized.</P> <P><B>Graphical abstract</B></P> <P>The maximum power factor (~4.4mWm<SUP>−1</SUP> K<SUP>−2</SUP>) at 300K was obtained using 0.31at.% Pb-doped Bi<SUB>0.52</SUB>Sb<SUB>1.48</SUB>Te<SUB>3</SUB> and was found to originate from an enlarged density of states (DOS) effective mass as a result of the band engineering effect.</P> <P>[DISPLAY OMISSION]</P>
Kim, Kwanlae Materials Research Society of Korea 2017 한국재료학회지 Vol.27 No.9
In the present study, domain evolution processes of a near-morphotropic PZT ceramic during poling was studied using vertical piezoresponse force microscopy (PFM). To perform macroscopic poling in bulk polycrystalline PZT, poling was carried out in a stepwise fashion, and PFM scan was performed after unloading the electric field. To identify the crystallographic orientation and planes for the observed non-$180^{\circ}$ domain walls in the PFM images, compatibility theory and electron backscatter diffraction (EBSD) were used in conjunction with PFM. Accurate registration between PFM and the EBSD image quality map was carried out by mapping several grains on the sample surface. A herringbone-like domain pattern consisting of two sets of lamellae was observed; this structure evolved into a single set of lamellae during the stepwise poling process. The mechanism underlying the observed domain evolution process was interpreted as showing that the growth of lamellae is determined by the potential energy associated with polarization and an externally applied electric field.
Kim, Kwanlae The Korean Institute of Electrical and Electronic 2017 전기전자재료학회논문지 Vol.30 No.9
Ferroelectric properties are governed by domain structures and domain wall motions, so it is of significance to understand domain evolution processes under mechanical stress. In the present study, in situ piezoresponse force microscopy (PFM) observation under compressive stress was carried out for a near-morphotropic PZT. Both $180^{\circ}$ and $non-180^{\circ}$ domain structures were observed from PFM images, and their habit planes were identified using electron backscatter diffraction in conjunction with PFM data. By externally applied mechanical stress, needle-like $non-180^{\circ}$ domain patterns were broadened via domain wall motions. This was interpreted via phenomenological approach such that the total energy minimization can be achieved by domain wall motion rather than domain nucleation mainly due to the local gradient energy. Meanwhile, no motion was observed from curvy $180^{\circ}$ domain walls under the mechanical stress, validating that $180^{\circ}$ domain walls are not directly influenced by mechanical stress.
Piezoresponse Force Microscopy를 이용한 Pb(Zr,Ti)O<sub>3</sub> 세라믹의 단계적 Poling에 의한 강유전체 도메인 진화 과정 관찰
김관래,Kim, Kwanlae 한국전기전자재료학회 2019 전기전자재료학회논문지 Vol.32 No.1
Ferroelectric material properties are strongly governed by domain structures and their evolution processes, but the evolution processes of complex domain patterns during a macroscopic electrical poling process are still elusive. In the present work, domain-evolution processes in a PZT ceramic near the morphotropic phase-boundary composition were studied during a step-wise electrical poling using piezoresponse force microscopy (PFM). Electron backscatter diffraction was used with the PFM data to identify the grain boundaries in the region of interest. In response to an externally the applied electric field, growth and retreat of non-$180^{\circ}$ domain boundaries wasere observed. The results indicate that ferroelectric polarization-switching nucleates and evolves in concordance with the pattern of the pre-existing domains.