Extreme Low Thermal Conductivity in Nanoscale 3D Si Phononic Crystal with Spherical Pores (1309.4898v1)

Abstract: Thermoelectric material provides a high hope for converting harmful and useless heat into useful energy, electricity. It can also be used as solid state Peltier coolers in integrated circuits, an outstanding challenge for electronic engineers. The desire for a high efficient thermoelectric material has never been so keen. Although many works have been done, we are still far from having a recipe for thermoelectric materials. Nano-structuring provides an effective way to increase figure of merit (ZT) by reducing the thermal conductivity without affecting electronic property.1 Here, we propose a novel nanoscale three-dimensional (3D) Si phononic crystal (PnC) with spherical pores, which can reduce the thermal conductivity of bulk Si by a factor up to 10,000 times at room temperature. The extreme-low thermal conductivity could lead to a larger value of ZT than unity. The thermal conductivity changes little when temperature increases from room temperature to 1100 K. The phonon participation ratio spectra show there are more phonon localizations as the porosity of PnC increases.

Evaluation of Extreme Low Thermal Conductivity in 3D Silicon Phononic Crystals

The paper "Extreme Low Thermal Conductivity in Nanoscale 3D Si Phononic Crystal with Spherical Pores" contributes to the ongoing exploration of phononic crystals (PnCs) to enhance thermoelectric materials. The paper primarily focuses on nanoscale three-dimensional silicon PnCs, engineered with spherical pores, to drastically reduce thermal conductivity—thus, potentially increasing the thermoelectric figure of merit, ZT, beyond unity, a notable threshold for practical thermoelectric applications.

Overview of Key Findings

1. Thermal Conductivity Reduction:
   • The research highlights a drastic reduction in thermal conductivity, obtainable via nanoscale structuring. For example, at a porosity level of 90%, the thermal conductivity was observed to be approximately 0.022 W/m-K. This value represents a diminutive 0.01% of that of bulk silicon, revealing the profound impact of porosity on thermal properties. For comparison, bulk silicon's thermal conductivity at room temperature stands around 156 W/m-K.
2. Temperature Insensitivity:
   • Unlike bulk silicon, whose thermal conductivity decreases with temperature increase due to Umklapp scattering, the reported PnCs exhibit minimal change in thermal conductivity across a temperature range from 300 K to 1100 K. This insensitivity is attributed to boundary scattering induced by the periodicity and size of the pores in the PnC structure.
3. Phonon Localization:
   • The paper employs participation ratio spectra to illustrate that phonons are more localized in the PnCs, leading to lowered thermal conductivity. The quantitative measure derived, "localization ratio," indicates that porosity correlates positively with phonon localization.

Methodology

Thermal conductivity calculations were performed using the Green-Kubo method via equilibrium molecular dynamics (EMD). A focus was placed on ensuring the robustness of results against finite-size effects, as evidenced in the simulations where increasing porosity reliably led to characteristic reductions in conductivity. Additionally, the paper corroborates its findings against classical predictive models, illustrating deviations attributable to nanoscale effects.

Implications

The reported findings suggest substantial potential for the practical deployment of Si PnCs in thermoelectric applications, emphasized by the material's compatibility with existing semiconductor fabrication processes. Furthermore, the environmental and cost-effective attributes of Si—and its established role within the semiconductor industry—position Si PnCs as a compelling candidate for scalable thermoelectric devices.

Prospects for Future Research

The promising results regarding phonon localization and reduction in thermal conductivity furnish a solid foundation for future research. Potential future directions include investigating alternative geometries within the phononic framework or integrating the findings with advanced 3D printing techniques to facilitate large-scale production. Additional exploration into optimizing electrical properties without sacrificing thermal modulation could further bolster the commercial viability of Si PnCs.

In conclusion, the paper significantly advances the understanding of PnCs in thermal management and thermoelectrics, providing crucial insights into phonon behavior modification through strategic nanostructuring. By reducing thermal conductivity and enhancing ZT, silicon PnCs offer a promising pathway towards novel thermoelectric materials.