Environmental Engineering Reference
In-Depth Information
plane, the transverse phonons are quantized and nonpropagating.
The coupling between the transverse and longitudinal motions
results in the resonance when the frequency of the longitudinal
phononmodeisclosetotheeigenfrequencyofthetransversemode.
This resonance effect results in longitudinal phonon localization,
thus offeringa coherent mechanism to reduce thermal conductivity.
On the other hand, the coating layers increase the entire cross-
section area. Because of the increase of the entire cross-section
area after coating, the strength of the surface scattering is reduced,
leading to the increase of thermal conductivity with the increase of
the coating thickness. Therefore, thermal conductivity of Si-coated
GeNWs is determined by these two competing effects. When the
coatingthicknessislessthanacertaincriticalvalue,thesuppression
of the longitudinal phonon transport is the dominating factor,
corresponding to the reduction of thermal conductivity. However,
whenthecoatingthicknessisgreaterthanacertaincriticalvalue,the
reducedsurfacescatteringdominatesthethermaltransport,leading
tothefactthatthermalconductivityoftheresultantGe/Sicore-shell
NWsbecomes larger than that of pristine GeNW withoutcoating.
These results show that in practical application one can control
thermal conductivity of NWs by coating other atoms. This approach
offers novel avenues and more flexibility for the design and thermal
management in nanostructures. For instance, small diameter NWs
arefavorableforthermoelectricapplicationsduetothelowthermal
conductivity but are more challenging for experimental synthesis.
Through the coating method, the low thermal conductivity feature
close to the very thin NW can be obtained from a much thicker
NW. This point can be seen from Fig. 1.13b. Thermal conductivity of
pristine GeNWs with
D
Ge
=
9.0 nm is close to that of coated GeNWs
with
D
Ge
=
11.3 nm and
D
coating
=
2.7 nm. The resultant core-shell
NW has a cross-sectional side length of 16.7 nm, which is almost
twice of the cross-sectional side length for pristine GeNW with the
similar thermal conductivity.
Moreover,forNWswithagivendiameter,thecoatingmethodcan
preservethelowthermalconductivityfeatureoftheoriginalNWsup
to certain critical coating thickness. For example, for NW with
D
Ge
=
9.0 nm, coating layers less than 2.7 nm can give rise to thermal
conductivity lower than that of pristine GeNW. As this coating
