High thermoelectric performance in metastable phase of silicon - A first-principles study
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-04-18 |
| Journal | Applied Physics Letters |
| Authors | Yongchao Rao, Changying Zhao, Shenghong Ju |
| Institutions | Shanghai Jiao Tong University |
| Citations | 11 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study uses first-principles calculations and Boltzmann transport theory to investigate the thermoelectric (TE) performance of metastable R8 phase silicon (Si-XII) compared to stable diamond-cubic silicon (Si-I).
- Core Value Proposition: Metastable Si-XII is identified as a highly promising, earth-abundant material for thermoelectric power generation, overcoming the extremely low ZT (Figure of Merit) of stable bulk silicon (Si-I).
- Thermal Conductivity Breakthrough: Si-XII exhibits a lattice thermal conductivity (kL) that is one magnitude lower than Si-I. At 300 K, kL drops from 131 W/mK (Si-I) to as low as 8.95 W/mK (Si-XII, z-axis).
- Mechanism: The significant kL reduction is attributed to stronger phonon scattering resulting from flatter phonon branches and lower group velocity in Si-XII, enhancing three-phonon scattering processes.
- Optimal Performance: The maximum ZT achieved for n-type Si-XII along the x-axis is 0.63 at 500 K, a value comparable to traditional TE materials (0.2-0.5 eV band gap range).
- Anisotropy: Si-XII is an anisotropic system, showing superior electrical conductivity (Ď) and ZT along the x-axis for n-type doping, driven by smaller electron effective masses in that direction.
- Feasibility: Since Si-XII persists at ambient pressure, it is feasible for synthesis and integration into high-performance silicon-based TE modules.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum ZT (n-type) | 0.63 | Dimensionless | Si-XII, 500 K, x-axis, Carrier Conc. 4.8 x 1019 cm-3 |
| Maximum ZT (n-type) | 0.43 | Dimensionless | Si-XII, 400 K, x-axis |
| Maximum ZT (p-type) | < 0.4 | Dimensionless | Si-XII, 500 K, x-axis |
| Lattice Thermal Conductivity (kL) | 131.07 | W/mK | Stable Si-I, 300 K |
| Lattice Thermal Conductivity (kL) | 16.83 | W/mK | Metastable Si-XII, 300 K, x-axis |
| Lattice Thermal Conductivity (kL) | 8.95 | W/mK | Metastable Si-XII, 300 K, z-axis |
| Thermal Anisotropy Ratio (kx/kz) | 1.88 | Dimensionless | Si-XII at 300 K |
| Band Gap (Eg) | 1.17 | eV | Stable Si-I (Indirect, HSE06) |
| Band Gap (Eg) | 0.22 | eV | Metastable Si-XII (Indirect, HSE06) |
| Optimal Carrier Concentration (500 K) | 4.8 x 1019 | cm-3 | n-type Si-XII |
| Si-I Structure | Diamond-Cubic (DC-Si) | Fd3m | Stable phase |
| Si-XII Structure | R8 Phase | R3 | Metastable phase |
Key Methodologies
Section titled âKey MethodologiesâThe study relied entirely on first-principles calculations combined with transport theory, avoiding physical synthesis experiments.
- Density Functional Theory (DFT) Implementation: Calculations were performed using the VASP package, utilizing the Projector Augmented-Wave (PAW) method.
- Structure Optimization: The Perdew-Burke-Ernzerhof (PBE) form of the Generalized Gradient Approximation (GGA) functional was used for initial geometry optimization. Convergence criteria were strict: 10-6 eV for energy and 10-3 eV/Angstrom for force.
- Accurate Electronic Properties: The Heyd-Scuseria-Ernzerhof 2006 (HSE06) hybrid functional was employed to calculate accurate electronic band gaps (Eg) and electronic transport properties, necessary because GGA typically underestimates Eg in semiconductors.
- Phonon Transport Modeling: The lattice thermal conductivity (kL) was calculated by solving the Boltzmann Transport Equation (BTE) for phonons using the ShengBTE package, focusing on three-phonon scattering processes.
- Electron Transport Modeling: Electrical conductivity (Ď) and Seebeck coefficient (S) were calculated by solving the BTE for electrons using the BoltzTrap2 package. Electron relaxation times were predicted using the deformation potential theory.
- Doping Simulation: Thermoelectric performance (ZT) was evaluated across a wide range of carrier concentrations (1018 to 1021 cm-3) for both p-type and n-type doping at temperatures of 300 K, 400 K, and 500 K.
Commercial Applications
Section titled âCommercial ApplicationsâThe enhanced thermoelectric performance of metastable Si-XII, combined with siliconâs abundance and compatibility with existing manufacturing, makes it highly relevant for next-generation energy conversion devices.
- Waste Heat Recovery (WHR): Si-XII is ideal for converting low-grade waste heat (300 K to 500 K) from industrial machinery, automotive systems, and HVAC units into usable electricity, improving overall energy efficiency.
- Cost-Effective Thermoelectric Generators (TEGs): By utilizing earth-abundant silicon, Si-XII offers a non-toxic, scalable, and potentially cheaper alternative to traditional TE materials like bismuth telluride (Bi2Te3) and lead telluride (PbTe).
- Integrated Micro-Electronics and Sensors: The material can be integrated directly into silicon-based microchips to harvest thermal energy generated by the chip itself, providing localized power or improving thermal management in high-density electronic packages.
- High-Stability Power Sources: Siliconâs inherent thermal and mechanical stability makes Si-XII suitable for creating robust, long-lasting TE modules for remote sensors or distributed power generation where maintenance is difficult.
- Anisotropic Device Design: The strong anisotropy (kx/kz = 1.88) allows engineers to design TE devices where heat flow and charge transport are optimized along specific crystallographic axes for maximum ZT output.
View Original Abstract
In this work, both thermal and electrical transport properties of diamond-cubic Si (Si-I) and metastable R8 phases of Si (Si-XII) are comparatively studied by using first-principles calculations combined with the Boltzmann transport theory. The metastable Si-XII shows one magnitude lower lattice thermal conductivity than stable Si-I from 300 to 500 K, attributed from the stronger phonon scattering in three-phonon scattering processes of Si-XII. For electronic transport properties, although Si-XII with smaller bandgap (0.22 eV) shows a lower Seebeck coefficient, the electrical conductivities of anisotropic n-type Si-XII show considerable values along the x axis due to the small effective masses of electrons along this direction. The peaks of the thermoelectric figure of merit (ZT) in n-type Si-XII are higher than that of p-type ones along the same direction. Owing to the lower lattice thermal conductivity and optimistic electrical conductivity, Si-XII exhibits larger optimal ZT compared with Si-I in both p- and n-type doping. For n-type Si-XII, the optimal ZT values at 300, 400, and 500 K can reach 0.24, 0.43, and 0.63 along the x axis at carrier concentrations of 2.6Ă1019, 4.1Ă1019, and 4.8Ă1019 cmâ3, respectively. The reported results elucidate that the metastable Si could be integrated to the thermoelectric power generator.