Thermal conductivity reduction in core-shell nanowires

Abstract

Nanostructuring of thermoelectric materials bears promise for manipulating physical parameters to improve the energy conversion efficiency of thermoelectrics. Using nonequilibrium molecular dynamics, we investigate how the thermal conductivity can be altered in core-shell nanocomposites of Si and Ge. By calculating the phonon vibrational density of states and performing normal mode analysis, we show that the thermal conductivity decreases when phonon-transport becomes diffusion-dominated and unveil a competition between modes from the various regions of the nanocomposite (core, interface, and shell). The effects of nanowire length, cross section, and temperature on thermal conductivity are explicitly considered. Surprisingly, the thermal conductivity variation with nanowire length is much weaker than in pure nanowires. Also, the thermal conductivity is almost independent of temperature in the wide region between 50 and 600 K, a direct result of confinement of the core by the shell. These results suggest that core-shell nanowires are promising structures for thermoelectrics.

Impacts of Atomistic Coating on Thermal Conductivity of Germanium Nanowires

“…For instance, by etching the surface of silicon NWs, it has been experimentally demonstrated that thermal conductivity of Si NW can be reduced more than two orders of magnitude compared with bulk Silicon [5,6]. Moreover, remarkable reduction of thermal conductivity in core-shell [7][8][9][10], tubular [11,12], and surface-decorated [13] NWs has also been reported through various kinds of interface and surface engineering.…”

“…For instance, by etching the surface of silicon NWs, it has been experimentally demonstrated that thermal conductivity of Si NW can be reduced more than two orders of magnitude compared with bulk Silicon [5,6]. Moreover, remarkable reduction of thermal conductivity in core-shell [7][8][9][10], tubular [11,12], and surface-decorated [13] NWs has also been reported through various kinds of interface and surface engineering.Previous studies [7][8][9] on the reduction of thermal conductivity in core-shell NWs mainly focus on Si/Ge core-shell NWs, which is quite intuitive as Ge is a low thermal conductivity material compared to Si [14]. However, in experimental realizations, the NWs synthesised are Ge/Si core-shell NWs [1][2][3].…”

“…Recent studies have shown that thermal conductivity in core-shell NWs is not sensitive to temperature [10,23]. In this work, we mainly focus on the effect of diameter and coating thickness on thermal conductivity.…”

Tunable mechanical and thermal properties of ZnS/CdS core/shell nanowires

“…In recent years, many experimental investigations have been carried out to synthesize and characterize different kinds of core/shell structures such as ZnO/ZnS [13], ZnO/CdS, [14] GaN/GaP, ZnO/TiO2 [15,16], CdSe/CdS [17,18], PbSe/CdSe [19,20], ZnS/CdS [21][22][23][24][25], CdS/ZnS [21,26,27], Ge/Si [28]. Both the classical molecular dynamics (MD) and density functional theory (DFT) methods have been used extensively to study the electronic [29][30][31][32][33][34][35], optical [29,36] and thermal [37][38][39][40][41][42][43][44][45][46][47][48] properties of the core/shell nanostructures. In contrast, mechanical properties of the core/shell nanostructures have so far not been investigated in details.…”

Interface bond relaxation on the thermal conductivity of Si/Ge core-shell nanowires

“…In comparison to the individual silicon nanowires, Si/Ge CSNWs have evident advantages because the covering layer offers the possibility of a partial and internal charge separation, an efficient passivation of the surface trap states and so on. 7 Up to date, significant progresses for Si/Ge CSNWs both experimentally [8][9][10][11][12][13][14] and theoretically [15][16][17][18][19][20][21][22][23][24][25][26][27][28] have demonstrated that the thermal transport properties of Si/Ge CSNWs can be modified at room temperature compared to that of the bare Si or Ge nanowires. Classically, the standard macroscopic approach describing heat transport in semiconductors is the well-known Fourier's law, i.e., J = −κ∇T, where J and ∇T are the heat flux density in the material and the temperature gradient, respectively.…”