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 |
High Thermoelectric Performance in Metastable Silicon: A 6CCVD Material Science Analysis
Section titled âHigh Thermoelectric Performance in Metastable Silicon: A 6CCVD Material Science AnalysisâThis document analyzes the findings of the research paper âHigh thermoelectric performance in metastable phase of silicon: a first-principles studyâ (arXiv:2203.16613v1) and connects the underlying material science principlesâspecifically the modulation of thermal conductivity via structural disorderâto 6CCVDâs advanced MPCVD diamond capabilities.
Executive Summary
Section titled âExecutive SummaryâThis study demonstrates that the metastable R8 phase of Silicon (Si-XII) is a highly promising thermoelectric material, achieving significantly higher figures of merit (ZT) than stable diamond-cubic Si (Si-I). The core findings and implications relevant to advanced material engineering are:
- ZT Enhancement: Optimal ZT values for n-type Si-XII reach 0.63 at 500 K, nearly two times larger than the best results for Si-I, confirming Si-XIIâs potential for thermoelectric power generation.
- Thermal Conductivity Reduction: The superior performance is primarily attributed to the one magnitude lower lattice thermal conductivity (kL) of Si-XII (e.g., 16.83 W/(m K) at 300 K) compared to Si-I (131.07 W/(m K)).
- Mechanism: The low kL is caused by stronger phonon scattering in the Si-XII structure, analogous to the effect of grain boundaries and defects in Polycrystalline Diamond (PCD).
- Anisotropy: Si-XII exhibits highly anisotropic heat transport (kx/kz = 1.88), requiring precise orientation control for optimal device integration.
- Material Analogy: The principle of using structural modification (metastable phase) to intrinsically suppress kL is directly applicable to 6CCVDâs Polycrystalline Diamond (PCD) and Boron-Doped Diamond (BDD) products, which are engineered to balance thermal and electrical properties.
- Methodology: The results were derived from first-principles calculations combining Density Functional Theory (DFT) and Boltzmann Transport Theory.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum ZT (n-type Si-XII) | 0.63 | Dimensionless | 500 K, x-axis |
| Optimal Carrier Concentration (500 K) | 4.8 x 1019 | cm-3 | n-type Si-XII, x-axis |
| Optimal Carrier Concentration (300 K) | 2.6 x 1019 | cm-3 | n-type Si-XII, x-axis |
| Lattice Thermal Conductivity (Si-XII, 300 K) | 16.83 | W/(m K) | x-axis |
| Lattice Thermal Conductivity (Si-XII, 300 K) | 8.95 | W/(m K) | z-axis |
| Lattice Thermal Conductivity (Si-I, 300 K) | 131.07 | W/(m K) | Isotropic |
| Anisotropy Ratio (Si-XII, kx/kz) | 1.88 | Dimensionless | 300 K |
| Band Gap (Si-XII, HSE06) | 0.22 | eV | Indirect Semiconductor |
| Band Gap (Si-I, HSE06) | 1.17 | eV | Indirect Semiconductor |
| Temperature Range Studied | 300 - 500 | K | Thermoelectric performance |
Key Methodologies
Section titled âKey MethodologiesâThe theoretical investigation relied on advanced computational physics techniques to model phonon and electron transport:
- Density Functional Theory (DFT) Implementation: Calculations were performed using the VASP package, employing the Projector Augmented-Wave (PAW) method.
- Functional Selection: The Perdew-Burke-Ernzerhof (PBE) Generalized Gradient Approximation (GGA) was used, supplemented by the Heyd-Scuseria-Ernzerhof 2006 (HSE06) hybrid functional to achieve accurate band gap values (critical for semiconductor transport).
- Basis Set and Convergence: A plane-wave basis set cutoff energy was limited to 600 eV. Energy and force convergence criteria were set strictly at 10-6 eV and 10-3 eV/Ă , respectively.
- Phonon Transport Simulation: The Boltzmann Transport Equation (BTE) for phonons was solved using the ShengBTE package to determine lattice thermal conductivity (kL).
- Electron Transport Simulation: The BTE for electrons was solved using the BoltzTrap2 package to calculate electrical conductivity (Ï) and Seebeck coefficient (S), which together determine the Figure of Merit (ZT = S2ÏT / (kL + ke)).
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe research highlights the critical role of intrinsic structural modification in achieving low thermal conductivity (kL) for high ZT. While this study focuses on silicon, 6CCVD specializes in controlling the crystal structure and doping of MPCVD diamond to achieve tailored thermal and electrical properties, directly addressing the engineering challenges implied by this research.
Applicable Materials for Thermoelectric Applications
Section titled âApplicable Materials for Thermoelectric ApplicationsâThe goal of achieving high ZT requires maximizing electrical conductivity (Ï) while minimizing thermal conductivity (kL).
| Research Requirement | 6CCVD Material Solution | Technical Rationale |
|---|---|---|
| Low Lattice Thermal Conductivity (kL) | Polycrystalline Diamond (PCD) | SCD has extremely high kL (~2000 W/mK). PCD, due to its high density of grain boundaries, intrinsically reduces kL by orders of magnitude, mimicking the phonon scattering mechanism observed in Si-XII. |
| High Electrical Conductivity (Ï) | Heavy Boron-Doped Diamond (BDD) | Boron doping transforms diamond from an insulator into a p-type semiconductor/metal. 6CCVD offers BDD materials with tunable carrier concentrations (up to 1021 cm-3) necessary to reach the optimal doping levels (e.g., 1019 cm-3 range) identified in the Si-XII study. |
| Integrated TE Element | Boron-Doped Polycrystalline Diamond (BDD-PCD) | Combining the low kL of PCD with the high Ï of BDD creates a robust, high-temperature thermoelectric material platform superior to conventional Si-based systems. |
Customization Potential for Experimental Research
Section titled âCustomization Potential for Experimental ResearchâTo transition theoretical findings like these into experimental devices, precise material engineering is essential. 6CCVD offers comprehensive customization services:
- Custom Dimensions: We provide PCD plates and wafers up to 125mm in diameter, allowing for large-scale experimental synthesis and device fabrication, overcoming the size limitations often associated with metastable or nanostructured materials.
- Thickness Control: We offer precise thickness control for both SCD and PCD layers from 0.1”m up to 500”m, crucial for optimizing thermal gradients and minimizing parasitic resistance in TE devices.
- Advanced Metalization: The fabrication of TE devices requires robust electrical contacts. 6CCVD provides in-house metalization services, including deposition of Au, Pt, Pd, Ti, W, and Cu, ensuring low-resistance interfaces for carrier injection and collection.
- Surface Finish: For high-precision bonding and integration, 6CCVD guarantees ultra-smooth polishing: Ra < 1nm for SCD and Ra < 5nm for inch-size PCD.
Engineering Support
Section titled âEngineering SupportâThe optimization of the Thermoelectric Figure of Merit (ZT) requires balancing complex, interdependent material parameters (S, Ï, kL, ke). 6CCVDâs in-house PhD team specializes in MPCVD diamond growth and characterization and can assist researchers with:
- Material Selection: Guiding the choice between SCD, PCD, or BDD to meet specific thermal management or power generation requirements.
- Doping Optimization: Fine-tuning Boron doping levels to achieve the optimal carrier concentration required for maximum ZT in similar Thermoelectric Power Generation projects.
- Anisotropy Management: Providing materials with controlled crystal orientation or structure to leverage or mitigate anisotropic transport effects, as highlighted in the Si-XII analysis.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
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.