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Rhyee, Jong-Soo,Lee, Kyu Hyoung,Lee, Sang Mock,Cho, Eunseog,Kim, Sang Il,Lee, Eunsung,Kwon, Yong Seung,Shim, Ji Hoon,Kotliar, Gabriel Macmillan Publishers Limited. All rights reserved 2009 Nature Vol.459 No.7249
Thermoelectric energy harvesting—the transformation of waste heat into useful electricity—is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500–900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit. Here we report a binary crystalline n-type material, In<SUB>4</SUB>Se<SUB>3-δ</SUB>, which achieves the ZT value of 1.48 at 705 K—very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.
Colors of graphene and graphene-oxide multilayers on various substrates
Jung, Inhwa,Rhyee, Jong-Soo,Son, Jong Yeog,Ruoff, Rodney S,Rhee, Kyong-Yop IOP Pub 2012 Nanotechnology Vol.23 No.2
<P>We investigated the colors of graphene and graphene-oxide multilayers that were deposited on various dielectric layers. In particular, the effects of the material thickness, the types of dielectric layers, and the existence of a back silicon substrate were analyzed. The colors of graphene-oxide layers on a SiO<SUB>2</SUB>/Si substrate were found to periodically change as the material thickness increased. However, the colors of graphene layers on the same substrate became saturated without a similar periodic change. The calculated colors corresponding to the material thicknesses were verified by optical microscopy and profilometry. We believe that these results demonstrate the possibility of utilizing color as a simple tool for detecting and estimating the thicknesses of graphene and graphene-oxide multilayers.</P>
Lee, Min Ho,Park, Jong Ho,Park, Su-Dong,Rhyee, Jong-Soo,Oh, Min-Wook Elsevier 2019 Journal of alloys and compounds Vol.786 No.-
<P><B>Abstract</B></P> <P>We investigated he thermoelectric (TE) properties and microstructures of Na-doped PbTe compounds, synthesized by various milling and sintering processes. It has been generally accepted that spark plasma sintering (SPS) inhibits grain growth due to the short sintering time, which is beneficial to the nano bulk composite. In contrast, we observed abnormal grain growth for SPS, unlike hot press (HP) sintering. Considering the Gibbs free energy change during sintering, the HP sintering increases internal energy while the SPS increases entropy energy, which causes the abnormal grain growth. Lattice strain and Fourier transform analyses of the transmission electron microscope images showed that the SPS sintered samples had more significant lattice strains and defects than the HP sintered samples. The low thermal conductivity of the SPS sintered samples was not from grain boundary phonon scattering but from phonon scattering by lattice strains and defects. The lattice strains and defects decreased electrical conductivity as well, resulting in deterioration of the power factor and thermoelectric performance. This suggests that the lattice strains and defects are more critical factors for enhancing thermoelectric performance than particle and grain size control.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Abnormal grain growth driven by entropy energy change is significant in spark plasma sintering. </LI> <LI> SPS sintered samples have more significant lattice strains and defects than those of hot press. </LI> <LI> Phonon scattering by lattice strains and defects decreases lattice thermal conductivity in SPS. </LI> <LI> The lattice strains and defects decreases electrical conductivity as well. </LI> </UL> </P>
Han, Mi-Kyung,Ahn, Kyunghan,Kim, HeeJin,Rhyee, Jong-Soo,Kim, Sung-Jin Royal Society of Chemistry 2011 Journal of materials chemistry Vol.21 No.30
<P>Compounds of Cu<SUB><I>x</I></SUB>Bi<SUB>2</SUB>Te<SUB>3</SUB> (0 ≤ <I>x</I> ≤ 0.1) were studied to measure the effect of Cu intercalation on their thermoelectric properties. HR-TEM images of freshly fractured surfaces of Cu<SUB>0.07</SUB>Bi<SUB>2</SUB>Te<SUB>3</SUB> showed that Cu nanoparticles formed in the van der Waals gaps between Te layers in Bi<SUB>2</SUB>Te<SUB>3</SUB>. Such nanoparticles acted as electron donors, changing the native p-type character of Bi<SUB>2</SUB>Te<SUB>3</SUB> to n-type. They also acted as phonon scatters, reducing thermal conductivity. Cu<SUB>0.07</SUB>Bi<SUB>2</SUB>Te<SUB>3</SUB> had high electrical conductivity of 681 S cm<SUP>−1</SUP> and Seebeck coefficient of −236 μV K<SUP>−1</SUP> with low thermal conductivity of 1.0 W m<SUP>−1</SUP> K<SUP>−1</SUP>, thus enhancing the figure of merit, <I>ZT</I>, to 1.15 at approximately 300 K. The intercalation of metal nanoparticles between Te layers in Bi<SUB>2</SUB>Te<SUB>3</SUB> is a promising strategy for the preparation of highly efficient n-type thermoelectric materials.</P> <P>Graphic Abstract</P><P>Compounds of Cu<SUB><I>x</I></SUB>Bi<SUB>2</SUB>Te<SUB>3</SUB> (0 ≤ <I>x</I> ≤ 0.1) were studied to measure the effect of Cu intercalation on their thermoelectric properties. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c1jm10163c'> </P>
Lee, Min Ho,Kim, Ka-Ryeong,Rhyee, Jong-Soo,Park, Su-Dong,Snyder, G. Jeffrey The Royal Society of Chemistry 2015 Journal of Materials Chemistry C Vol.3 No.40
<▼1><P>Sb2Te3/Ag2Te (ST/AT) composites with ST/AT molar ratios of 1/1, 2/1, 4/1, 8/1, 16/1, and 32/1 were synthesized, and high <I>ZT</I> values were achieved compared with other Pb-free p-type chalcogenide thermoelectric materials.</P></▼1><▼2><P>We investigated the thermoelectric properties of Sb2Te3/Ag2Te (ST/AT) composites with ST/AT molar ratios of 1/1, 2/1, 4/1, 8/1, 16/1, and 32/1. The extrinsic composites, synthesized by wet ball milling of two separate powders of Sb2Te3 and Ag2Te, can be differentiated from the intrinsic composites by high temperature phase separation because of the low temperature synthesis process. The thermoelectric properties of the composites show a systematic behaviour of decreased electrical and thermal conductivities with increasing Ag2Te dispersion concentration. The composite with ST/AT ratio = 1/1 exhibits an extremely low lattice thermal conductivity with a high power factor over a wide temperature range, resulting in a high <I>ZT</I> value of 1.5 at 700 K, which is a significantly enhanced value of <I>ZT</I> compared with those of other Pb-free p-type chalcogenide thermoelectric materials.</P></▼2>