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Ultra-low Thermal Conductivity in Si/Ge Hierarchical Superlattice Nanowire.

Mu X, Wang L, Yang X, Zhang P, To AC, Luo T - Sci Rep (2015)

Bottom Line: Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW.By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved.Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%.

View Article: PubMed Central - PubMed

Affiliation: Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA.

ABSTRACT
Due to interfacial phonon scattering and nanoscale size effect, silicon/germanium (Si/Ge) superlattice nanowire (SNW) can have very low thermal conductivity, which is very attractive for thermoelectrics. In this paper, we demonstrate using molecular dynamics simulations that the already low thermal conductivity of Si/Ge SNW can be further reduced by introducing hierarchical structure to form Si/Ge hierarchical superlattice nanowire (H-SNW). The structural hierarchy introduces defects to disrupt the periodicity of regular SNW and scatters coherent phonons, which are the key contributors to thermal transport in regular SNW. Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW. By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved. Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%. It is concluded that the hierarchical structuring is an effective way of reducing thermal conductivity significantly in SNW, which can be a promising path for improving the efficiency of Si/Ge-based SNW thermoelectrics.

No MeSH data available.


Thermal conductivities of Si/Ge H-SNW-PSi and Si/Ge H-SNW-PGe as a function of their period lengths, LH.The thermal conductivities of Si/Ge H-SNW-RSi, Si/Ge H-SNW-RGe, Si/Ge H-SNW-RSi with amorphous and rough surfaces (Si/Ge H-SNW-RSi-AS and Si/Ge H-SNW-RSi-RS), and Si/Ge H-SNW-RGe with amorphous and rough surfaces (Si/Ge H-SNW-RGe-AS and Si/Ge H-SNW-RGe-RS) are also shown here for comparison. The red dash line represents the thermal conductivity of regular Si/Ge SNW with period of “AB”. All the cases have the same total length: ~1042.6 Å.
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f4: Thermal conductivities of Si/Ge H-SNW-PSi and Si/Ge H-SNW-PGe as a function of their period lengths, LH.The thermal conductivities of Si/Ge H-SNW-RSi, Si/Ge H-SNW-RGe, Si/Ge H-SNW-RSi with amorphous and rough surfaces (Si/Ge H-SNW-RSi-AS and Si/Ge H-SNW-RSi-RS), and Si/Ge H-SNW-RGe with amorphous and rough surfaces (Si/Ge H-SNW-RGe-AS and Si/Ge H-SNW-RGe-RS) are also shown here for comparison. The red dash line represents the thermal conductivity of regular Si/Ge SNW with period of “AB”. All the cases have the same total length: ~1042.6 Å.

Mentions: From the discussion so far, we understand that a large portion of the thermal conductivity in regular SNW, especially for the cases with small period lengths, is contributed by coherent phonons. It is thus possible to reduce the thermal conductivity by either hampering the formation of coherent phonons or scattering the coherent phonons. This can be achieved by using hierarchical structuring, which introduces structural defects into the superlattice. Figure 4 shows the thermal conductivity of periodically defected Si/Ge H-SNW as a function of their period length, LH. For periodically defected H-SNWs, they have periodicity with repeating units of “(AB)mAA” or “(AB)mBB” for Si or Ge defected cases. The blue hollow squares in Fig. 4 represent the thermal conductivities of Si/Ge H-SNW-PSi and the red solid circles represent those of Si/Ge H-SNW-PGe. The total lengths of all the H-SNWs are ~1042.6 Å. The red horizontal dash line indicates the thermal conductivity of regular Si/Ge SNW with period of “AB” and the same total length as the Si/Ge H-SNWs.


Ultra-low Thermal Conductivity in Si/Ge Hierarchical Superlattice Nanowire.

Mu X, Wang L, Yang X, Zhang P, To AC, Luo T - Sci Rep (2015)

Thermal conductivities of Si/Ge H-SNW-PSi and Si/Ge H-SNW-PGe as a function of their period lengths, LH.The thermal conductivities of Si/Ge H-SNW-RSi, Si/Ge H-SNW-RGe, Si/Ge H-SNW-RSi with amorphous and rough surfaces (Si/Ge H-SNW-RSi-AS and Si/Ge H-SNW-RSi-RS), and Si/Ge H-SNW-RGe with amorphous and rough surfaces (Si/Ge H-SNW-RGe-AS and Si/Ge H-SNW-RGe-RS) are also shown here for comparison. The red dash line represents the thermal conductivity of regular Si/Ge SNW with period of “AB”. All the cases have the same total length: ~1042.6 Å.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4644949&req=5

f4: Thermal conductivities of Si/Ge H-SNW-PSi and Si/Ge H-SNW-PGe as a function of their period lengths, LH.The thermal conductivities of Si/Ge H-SNW-RSi, Si/Ge H-SNW-RGe, Si/Ge H-SNW-RSi with amorphous and rough surfaces (Si/Ge H-SNW-RSi-AS and Si/Ge H-SNW-RSi-RS), and Si/Ge H-SNW-RGe with amorphous and rough surfaces (Si/Ge H-SNW-RGe-AS and Si/Ge H-SNW-RGe-RS) are also shown here for comparison. The red dash line represents the thermal conductivity of regular Si/Ge SNW with period of “AB”. All the cases have the same total length: ~1042.6 Å.
Mentions: From the discussion so far, we understand that a large portion of the thermal conductivity in regular SNW, especially for the cases with small period lengths, is contributed by coherent phonons. It is thus possible to reduce the thermal conductivity by either hampering the formation of coherent phonons or scattering the coherent phonons. This can be achieved by using hierarchical structuring, which introduces structural defects into the superlattice. Figure 4 shows the thermal conductivity of periodically defected Si/Ge H-SNW as a function of their period length, LH. For periodically defected H-SNWs, they have periodicity with repeating units of “(AB)mAA” or “(AB)mBB” for Si or Ge defected cases. The blue hollow squares in Fig. 4 represent the thermal conductivities of Si/Ge H-SNW-PSi and the red solid circles represent those of Si/Ge H-SNW-PGe. The total lengths of all the H-SNWs are ~1042.6 Å. The red horizontal dash line indicates the thermal conductivity of regular Si/Ge SNW with period of “AB” and the same total length as the Si/Ge H-SNWs.

Bottom Line: Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW.By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved.Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%.

View Article: PubMed Central - PubMed

Affiliation: Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA.

ABSTRACT
Due to interfacial phonon scattering and nanoscale size effect, silicon/germanium (Si/Ge) superlattice nanowire (SNW) can have very low thermal conductivity, which is very attractive for thermoelectrics. In this paper, we demonstrate using molecular dynamics simulations that the already low thermal conductivity of Si/Ge SNW can be further reduced by introducing hierarchical structure to form Si/Ge hierarchical superlattice nanowire (H-SNW). The structural hierarchy introduces defects to disrupt the periodicity of regular SNW and scatters coherent phonons, which are the key contributors to thermal transport in regular SNW. Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW. By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved. Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%. It is concluded that the hierarchical structuring is an effective way of reducing thermal conductivity significantly in SNW, which can be a promising path for improving the efficiency of Si/Ge-based SNW thermoelectrics.

No MeSH data available.