Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics

Kim, Sang Il,Lee, Kyu Hyoung,Mun, Hyeon A,Kim, Hyun Sik,Hwang, Sung Woo,Roh, Jong Wook,Yang, Dae Jin,Shin, Weon Ho,Li, Xiang Shu,Lee, Young Hee,Snyder, G. Jeffrey,Kim, Sung Wng American Association for the Advancement of Scienc 2015 Science Vol.348 No.6230

<P><B>Squeezing out efficient thermoelectrics</B></P><P>Thermoelectric materials hold the promise of converting waste heat into electricity. The challenge is to develop high-efficiency materials that are not too expensive. Kim <I>et al.</I> suggest a pathway for developing inexpensive thermoelectrics. They show a dramatic improvement of efficiency in bismuth telluride samples by quickly squeezing out excess liquid during compaction. This method introduces grain boundary dislocations in a way that avoids degrading electrical conductivity, which makes a better thermoelectric material. With the potential for scale-up and application to cheaper materials, this discovery presents an attractive path forward for thermoelectrics.</P><P><I>Science</I>, this issue p. 109</P><P>The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit (<I>zT</I>). The <I>zT</I> of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi<SUB>0.5</SUB>Sb<SUB>1.5</SUB>Te<SUB>3</SUB> (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the <I>zT</I> to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices.</P>

Improved trade-off between thermoelectric performance and mechanical reliability of Mg₂Si by hybridization of few-layered reduced graphene oxides

Kim, Gwansik,Kim, Sung Wng,Rim, Hyun Jun,Lee, Hwijong,Kim, Jeongmin,Roh, Jong Wook,Kim, Byung-Wook,Lee, Kyu Hyoung,Lee, Wooyoung Pergamon 2019 Scripta materialia Vol.162 No.-

<P><B>Abstract</B></P> <P>Nanocomposites can simultaneously enhance the thermoelectric and mechanical properties of thermoelectric materials. Here, we fabricated bulks of Mg<SUB>1.96</SUB>Al<SUB>0.04</SUB>Si<SUB>0.97</SUB>Bi<SUB>0.03</SUB> with monodispersed few-layered reduced graphene oxides utilizing ultrasonic-based wet chemical pulverizing-mixing and spark plasma sintering to improve unfavorable trade-off between thermoelectric performance and mechanical reliability, which is important for commercialization. An unexpected high fracture toughness of ~1.88 MPa m<SUP>1/2</SUP> was observed due to the synergetic effect of the deflection of crack propagation, bridging, and sheet pull-out mechanisms, and a high thermoelectric figure of merit ~0.6 was obtained even for a high content (3 vol.%) of reduced graphene oxides.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Direct Observation of Inherent Atomic-Scale Defect Disorders responsible for High-Performance Ti₁₋<i>_x</i> Hf<i>_x</i> NiSn₁₋<i>_y</i> Sb<i>_y</i> Half-Heusler Thermoelectric Al

Kim, Ki Sung,Kim, Young-Min,Mun, Hyeona,Kim, Jisoo,Park, Jucheol,Borisevich, Albina Y.,Lee, Kyu Hyoung,Kim, Sung Wng Wiley (John WileySons) 2017 Advanced Materials Vol.29 No.36

<P>Structural defects often dominate the electronic- and thermal-transport properties of thermoelectric (TE) materials and are thus a central ingredient for improving their performance. However, understanding the relationship between TE performance and the disordered atomic defects that are generally inherent in nanostructured alloys remains a challenge. Herein, the use of scanning transmission electron microscopy to visualize atomic defects directly is described and disordered atomic-scale defects are demonstrated to be responsible for the enhancement of TE performance in nanostructured Ti1-xHfxNiSn1-ySby half-Heusler alloys. The disordered defects at all atomic sites induce a local composition fluctuation, effectively scattering phonons and improving the power factor. It is observed that the Ni interstitial and Ti,Hf/Sn antisite defects are collectively formed, leading to significant atomic disorder that causes the additional reduction of lattice thermal conductivity. The Ti1-xHfxNiSn1-ySby alloys containing inherent atomic-scale defect disorders are produced in one hour by a newly developed process of temperature-regulated rapid solidification followed by sintering. The collective atomic-scale defect disorder improves the zT to 1.09 +/- 0.12 at 800 K for the Ti0.5Hf0.5NiSn0.98Sb0.02 alloy. These results provide a promising avenue for improving the TE performance of state-of-the-art materials.</P>

Long-Range Lattice Engineering of MoTe₂ by a 2D Electride

Kim, Sera,Song, Seunghyun,Park, Jongho,Yu, Ho Sung,Cho, Suyeon,Kim, Dohyun,Baik, Jaeyoon,Choe, Duk-Hyun,Chang, K. J.,Lee, Young Hee,Kim, Sung Wng,Yang, Heejun American Chemical Society 2017 NANO LETTERS Vol.17 No.6

<P>Doping two-dimensional (2D) semiconductors beyond their degenerate levels provides the opportunity to investigate extreme carrier density-driven superconductivity and phase transition in 2D systems. Chemical functionalization and the ionic gating have achieved the high doping density, but their effective ranges have been limited to similar to 1 nm, which restricts the use of highly doped 2D semiconductors. Here, we report on electron diffusion from the 2D electride [Ca2N](+)e to MoTe2 over a distance of 100 nm from the contact interface, generating an electron doping density higher than 1.6 x 10(14) cm(2) and a lattice symmetry change of MoTe2 as a consequence of the extreme doping. The long-range lattice symmetry change, suggesting a length scale surpassing the depletion width of conventional metalsemiconductor junctions, was a consequence of the low work function (2.6 eV) with highly mobile anionic electron layers of [Ca2N](+)e . The combination of 2D electrides and layered materials yields a novel material design in terms of doping and lattice engineering.</P>

Enhanced Thermoelectric Performance of p-Type Bi_0.4Sb_1.6Te₃ by Excess Te Addition

Kim, Tae Wan,Roh, Jong Wook,Moon, Seung Pil,Ahn, Yeon Sik,Park, Hee Jung,Choi, Soon-Mok,Kim, Jong-Young,Kim, Sung Wng,Lee, Kyu Hyoung American Scientific Publishers 2017 Journal of nanoscience and nanotechnology Vol.17 No.10

<P>Thermoelectric properties of p-type Bi0.4Sb1.6Te3 polycrystalline bulks fabricated by spark plasma sintering from the ingots of stoichiometric Bi0.4Sb1.6Te3 and 0.5-3 wt.% excess Te added compositions were investigated in an effort to demonstrate the feasibility of traditional melt-solidification combined with a pressure induced sintering process. We found that the electronic and thermal transport properties of p-type Bi2-xSbxTe3 could be optimized by excess Te addition due to the suppression of uncontrollable formation of Te-vacancies during the fabrication process. Enhanced thermoelectric performance ZT of 1.1 at 300 K was obtained in 1, 2, and 3 wt.% excess Te added Bi0.4Sb1.6Te3. High ZT and superior controllability of the thermoelectric transport parameters achieved in the present study enable the pressure induced sintering process to be utilized for commercial manufacturing process for solid state cooling and power generation modules.</P>

The scalable pinacol coupling reaction utilizing the inorganic electride [Ca₂N]⁺·e⁻ as an electron donor

Kim, Ye Ji,Kim, Sun Min,Hosono, Hideo,Yang, Jung Woon,Kim, Sung Wng The Royal Society of Chemistry 2014 Chemical communications Vol.50 No.37

<P>The scalable pinacol coupling reaction is realized utilizing the inorganic electride [Ca2N](+)center dot e(-) as an electron donor in organic solvents. The bond cleavages of the [Ca2N](+) layers by methanol play a vital role in transferring anionic electrons to electrophilic aldehydes, accompanying the formation of Ca(OMe)(2) and ammonia.</P>

Two dimensional inorganic electride-promoted electron transfer efficiency in transfer hydrogenation of alkynes and alkenes

Kim, Ye Ji,Kim, Sun Min,Cho, Eun Jin,Hosono, Hideo,Yang, Jung Woon,Kim, Sung Wng Royal Society of Chemistry 2015 Chemical Science Vol.6 No.6

<▼1><P>A simple and highly efficient transfer hydrogenation of alkynes and alkenes by using a two-dimensional electride, dicalcium nitride ([Ca<SUB>2</SUB>N]<SUP>+</SUP>·e<SUP>–</SUP>), as an electron transfer agent is disclosed.</P></▼1><▼2><P>A simple and highly efficient transfer hydrogenation of alkynes and alkenes by using a two-dimensional electride, dicalcium nitride ([Ca<SUB>2</SUB>N]<SUP>+</SUP>·e<SUP>–</SUP>), as an electron transfer agent is disclosed. Excellent yields in the transformation are attributed to the remarkable electron transfer efficiency in the electride-mediated reactions. It is clarified that an effective discharge of electrons from the [Ca<SUB>2</SUB>N]<SUP>+</SUP>·e<SUP>–</SUP> electride in alcoholic solvents is achieved by the decomposition of the electride <I>via</I> alcoholysis and the generation of ammonia and Ca(O<SUP>i</SUP>Pr)<SUB>2</SUB>. We found that the choice of solvent was crucial for enhancing the electron transfer efficiency, and a maximum efficiency of 80% was achieved by using a DMF mixed isopropanol co-solvent system. This is the highest value reported to date among single electron transfer agents in the reduction of C–C multiple bonds. The observed reactivity and efficiency establish that electrides with a high density of anionic electrons can readily participate in the reduction of organic functional groups.</P></▼2>

Suppression of bipolar conduction via bandgap engineering for enhanced thermoelectric performance of p-type Bi_0.4Sb_1.6Te₃ alloys

Kim, Hyun-Sik,Lee, Kyu Hyoung,Yoo, Joonyeon,Shin, Weon Ho,Roh, Jong Wook,Hwang, Jae-Yeol,Kim, Sung Wng,Kim, Sang-il Elsevier 2018 Journal of alloys and compounds Vol.741 No.-

<P><B>Abstract</B></P> <P>Substitutional doping is known to be effective when used to enhance the thermoelectric figure of merit <I>zT</I>, and this is generally explained as resulting from a reduction in the thermal conductivity caused by an additional atomic-scale defect structure. However, a comprehensive analysis of the substitutional doping effect on the electrical and thermal properties together has not been undertaken, especially when the bipolar thermal conductivity becomes serious. A previous study by the authors also showed that the <I>zT</I> of Bi<SUB>0.4</SUB>Sb<SUB>1.6</SUB>Te<SUB>3</SUB> thermoelectric alloys was enhanced by indium (In) doping due to the reduction of the total thermal conductivity. Here, we more closely analyze the electrical and thermal transport properties of a series of indium (In)-doped p-type Bi<SUB>0.4</SUB>Sb<SUB>1.6-x</SUB>In<SUB>x</SUB>Te<SUB>3</SUB> (x = 0, 0.003, 0.005, 0.01) using both the single-parabolic-band model and the Debye-Callaway model in an effort to investigate the origin of the observed thermal conductivity reduction more closely. The bipolar contribution to the total thermal conductivity was estimated exclusively based on a two-band model based on a single-parabolic-band model. Furthermore, the lattice thermal conductivity was calculated using the Debye-Callaway model while taking additional In substitutional defects into consideration. The calculations indicated that the significant suppression of bipolar thermal conductivity was achieved as a result of the increased bandgap in Bi<SUB>0.4</SUB>Sb<SUB>1.6</SUB>Te<SUB>3</SUB> caused by In doping. Additional point defects from In doping also reduced the lattice thermal conductivity, but not as much as the bipolar thermal conductivity did. The study suggests that the suppression of bipolar conduction by means of a bandgap modification can be an effective approach for enhancing <I>zT</I> further via a simple In-doping process in Bi<SUB>0.4</SUB>Sb<SUB>1.6</SUB>Te<SUB>3</SUB>.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Thermal conductivity reduction in In-doped Bi<SUB>0.4</SUB>Sb<SUB>1.6</SUB>Te<SUB>3</SUB> is analyzed. </LI> <LI> Bandgap increase by In doping suppresses bipolar conduction significantly. </LI> <LI> Extra point defects by In doping reduces lattice thermal conductivity. </LI> <LI> The <I>zT</I> enhancement in In-doped Bi<SUB>0.4</SUB>Sb<SUB>1.6</SUB>Te<SUB>3</SUB> is mainly due to bipolar suppression. </LI> </UL> </P>

Potential-current co-adjusted pulse electrodeposition for highly (110)-oriented Bi₂Te_3-<i>x</i>Se_<i>x</i> films

Kim, Jiwon,Lee, Kyu Hyoung,Kim, Sung Wng,Lim, Jae-Hong Elsevier 2019 Journal of alloys and compounds Vol.787 No.-

<P><B>Abstract</B></P> <P>Controllable crystal orientation is necessary to obtain high thermoelectric performance in thin films of Bi<SUB>2</SUB>Te<SUB>3</SUB> alloys. In the present study, highly (110)-oriented thin films of n-type Bi<SUB>2</SUB>Te<SUB>3-<I>x</I> </SUB>Se<SUB> <I>x</I> </SUB> with improved composition controllability are prepared through a simple electrodeposition-based process. Using potential-current co-adjusted pulse electrodeposition (PCP-ED) with adjustments to the zero current during the off-time period enables the fabrication of dense Bi<SUB>2</SUB>Te<SUB>3-<I>x</I> </SUB>Se<SUB> <I>x</I> </SUB> thin films with highly (110)-oriented grains by minimizing the ionic gradient (Bi<SUP>3+</SUP>, Te<SUP>2−</SUP>, Se<SUP>2−</SUP>) between the substrate and solution. The power factor of the PCP-ED thin film was much higher than that of the dendritic Bi<SUB>2</SUB>Te<SUB>3-<I>x</I> </SUB>Se<SUB> <I>x</I> </SUB> thin film fabricated by constant-potentiostatic electrodeposition (C-ED) because of the simultaneous enhancement of electrical conductivity and Seebeck coefficient. The high power factor of ∼1920 μW/m⋅K<SUP>2</SUP>, which is the best value among reported n-type Bi<SUB>2</SUB>Te<SUB>3</SUB>-based thin films, was obtained at room temperature after low-temperature annealing at 200 °C by exploiting the crystallinity enhancement and carrier concentration optimization.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Modified pulsed electrodeposition was used to fabricate n-type BiTe<SUB>3-<I>x</I> </SUB>Se<SUB> <I>x</I> </SUB> TE films. </LI> <LI> Fabrication method enabled high compositional and morphological control. </LI> <LI> Annealing improved crystallinity; TE power factor was high at room temperature. </LI> <LI> Further improving crystallinity may further improve TE figure of merit. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Role of alkali metal promoter in enhancing lateral growth of monolayer transition metal dichalcogenides

Kim, Hyun,Han, Gang Hee,Yun, Seok Joon,Zhao, Jiong,Keum, Dong Hoon,Jeong, Hye Yun,Ly, Thuc Hue,Jin, Youngjo,Park, Ji-Hoon,Moon, Byoung Hee,Kim, Sung-Wng,Lee, Young Hee IOP 2017 Nanotechnology Vol.28 No.36

<P>Synthesis of monolayer transition metal dichalcogenides (TMDs) via chemical vapor deposition relies on several factors such as precursor, promoter, substrate, and surface treatment of substrate. Among them, the use of promoter is crucial for obtaining uniform and large-area monolayer TMDs. Although promoters have been speculated to enhance adhesion of precursors to the substrate, their precise role in the growth mechanism has rarely been discussed. Here, we report the role of alkali metal promoter in growing monolayer TMDs. The growth occurred via the formation of sodium metal oxides which prevent the evaporation of metal precursor. Furthermore, the silicon oxide substrate helped to decrease the Gibbs free energy by forming sodium silicon oxide compounds. The resulting sodium metal oxide was anchored within such concavities created by corrosion of silicon oxide. Consequently, the wettability of the precursors to silicon oxide was improved, leading to enhance lateral growth of monolayer TMDs.</P>