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Observation of Anisotropy in Thermal Conductivity of Individual Single-Crystalline Bismuth Nanowires
Roh, Jong Wook,Hippalgaonkar, Kedar,Ham, Jin Hee,Chen, Renkun,Li, Ming Zhi,Ercius, Peter,Majumdar, Arun,Kim, Woochul,Lee, Wooyoung American Chemical Society 2011 ACS NANO Vol.5 No.5
<P>The thermal conductivity of individual single-crystalline Bi nanowires grown by the on-film formation of nanowires (ON–OFF) has been investigated. We observed that the thermal conductivity of single-crystalline Bi nanowires is highly anisotropic. Thermal conductivity of nanowires (diameter ∼100 nm) in the off-axis [1̅02] and [110] directions exhibits a difference of ∼7.0 W/m·K. The thermal conductivity in both growth directions is diameter-dependent, which indicates that thermal transport through the individual Bi nanowires is limited by boundary scattering of both electrons and phonons. This huge anisotropy in thermal conductivities of Bi nanowires suggests the importance of direction-dependent characterization of charge, thermal transport, and thermoelectric properties of Bi nanowires.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2011/ancac3.2011.5.issue-5/nn200474d/production/images/medium/nn-2011-00474d_0006.gif'></P>
Tunable thermal conductivity in mesoporous silicon by slight porosity change
Seol, Jae Hun,Barth, David S.,Zhu, Jia,Coso, Dusan,Hippalgaonkar, Kedar,Lim, Jongwoo,Han, Junkyu,Zhang, Xiang,Majumdar, Arun American Institute of Physics 2017 Applied Physics Letters Vol.111 No.6
<P> We report the thermal conductivity of photoelectrochemically synthesized mesoporous silicon (MPS), with ~20-nm diameter pores and 52%-58% porosity. The thermal conductivity of MPS samples with a thickness of a few microns was measured using the three omega (3ω) differential technique. We experimentally demonstrated that the thermal conductivity of MPS varies between 3 and 7W/m K at room temperature and is dependent on the photoelectrochemical etching times used during the MPS synthesis, which induces a slight change in the MPS porosity. Calculations were conducted using the Boltzmann transport equation in the relaxation time approximation, with the results suggesting that the large thermal conductivity reduction in the MPSs was not entirely explained by the pore boundary scattering. Our findings indicate that elastic softening in the mesoporous structure may be responsible for the reduction in the thermal conductivity. </P>
Lee, Eun Kyung,Yin, Liang,Lee, Yongjin,Lee, Jong Woon,Lee, Sang Jin,Lee, Junho,Cha, Seung Nam,Whang, Dongmok,Hwang, Gyeong S.,Hippalgaonkar, Kedar,Majumdar, Arun,Yu, Choongho,Choi, Byoung Lyong,Kim, J American Chemical Society 2012 NANO LETTERS Vol.12 No.6
<P>The strongly correlated thermoelectric properties have been a major hurdle for high-performance thermoelectric energy conversion. One possible approach to avoid such correlation is to suppress phonon transport by scattering at the surface of confined nanowire structures. However, phonon characteristic lengths are broad in crystalline solids, which makes nanowires insufficient to fully suppress heat transport. Here, we employed Si–Ge alloy as well as nanowire structures to maximize the depletion of heat-carrying phonons. This results in a thermal conductivity as low as ∼1.2 W/m-K at 450 K, showing a large thermoelectric figure-of-merit (ZT) of ∼0.46 compared with those of SiGe bulks and even ZT over 2 at 800 K theoretically. All thermoelectric properties were “simultaneously” measured from the same nanowires to facilitate accurate ZT measurements. The surface-boundary scattering is prominent when the nanowire diameter is over ∼100 nm, whereas alloying plays a more important role in suppressing phonon transport for smaller ones.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2012/nalefd.2012.12.issue-6/nl300587u/production/images/medium/nl-2012-00587u_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl300587u'>ACS Electronic Supporting Info</A></P>