High-pressure thermal conductivity and compressional velocity of NaCl in B1 and B2 phase
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-10-29 |
| Journal | Scientific Reports |
| Authors | WenâPin Hsieh |
| Institutions | Institute of Earth Sciences, Academia Sinica, National Taiwan University |
| Citations | 22 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study provides critical, high-pressure, high-temperature (HP-HT) thermal and elastic data for Sodium Chloride (NaCl), a widely used pressure medium and calibrant in Diamond-Anvil Cell (DAC) experiments.
- Critical Data for DAC Modeling: Precisely determined the thermal conductivity (Î) and compressional velocity (Vp) of NaCl up to 66 GPa at room temperature, and Î up to 773 K at high pressure. This data is essential for accurately modeling heat conduction and temperature distribution in NaCl-loaded DACs.
- Thermal Conductivity Behavior: Î increases rapidly in the B1 phase (up to 50 W m-1 K-1 at 28.4 GPa) but drops significantly (approximately 70%) upon transitioning to the B2 phase (~30 GPa), reaching a minimum of 16 W m-1 K-1.
- Validation of LS Equation: The Leibfried-Schlömann (LS) equation, a common model for predicting Î, was confirmed to accurately describe the pressure dependence of NaClâs thermal conductivity across both the B1 and B2 phases over a large compression range (up to 35% volume compression).
- Dominant Mechanism: The high P-T thermal conductivity follows a typical T-1 dependence, confirming that anharmonic three-phonon scattering is the predominant mechanism controlling heat transport in both B1 and B2 NaCl crystals.
- Elastic Properties: The compressional velocity (Vp) scales approximately linearly with density (Ï) in both phases, confirming the applicability of Birchâs law across the studied density range.
- Methodology: The results were achieved by coupling ultrafast optical pump-probe techniques (Time-Domain Thermoreflectance and Picosecond Interferometry) with standard and externally-heated DACs (EHDAC).
Technical Specifications
Section titled âTechnical SpecificationsâThe following table summarizes the key measured and derived parameters for NaCl in its B1 (face-centered cubic) and B2 (body-centered cubic) phases.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum Pressure Studied | 66 | GPa | Room Temperature (RT) |
| Maximum Temperature Studied | 773 | K | High Pressure (HP) |
| Thermal Conductivity (Ambient) | 5 | W m-1 K-1 | NaCl B1 Phase |
| Thermal Conductivity (Max B1) | 50 | W m-1 K-1 | At 28.4 GPa |
| Thermal Conductivity (Min B2) | 16 | W m-1 K-1 | At 35 GPa (Post-transition) |
| Thermal Conductivity (Max B2) | 33 | W m-1 K-1 | At 66 GPa |
| Î Reduction (B1 to B2 Transition) | ~70 | % | Across the ~30 GPa transition |
| Temperature Exponent (n) | -0.98 (±0.16) to -0.92 (±0.1) | N/A | High P, T dependence (Π~ Tn) |
| GrĂŒneisen Parameter (g) | 5.5 (±0.2) | N/A | Derived for NaCl B1 Phase |
| GrĂŒneisen Parameter (g) | 5.5 (±0.5) | N/A | Derived for NaCl B2 Phase |
| Birchâs Law Slope (b1) | 2.32 (±0.04) | km s-1 / (g cm-3) | Vp vs. Ï in NaCl B1 Phase |
| Birchâs Law Slope (b2) | 2.72 (±0.12) | km s-1 / (g cm-3) | Vp vs. Ï in NaCl B2 Phase |
| Brillouin Frequency Range | 19 to 40 | GHz | Measured across 1 to 66 GPa |
Key Methodologies
Section titled âKey MethodologiesâThe experiments combined high-pressure apparatus with ultrafast optical metrology to measure thermal and elastic properties in situ.
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High-Pressure Setup:
- Standard Diamond-Anvil Cells (DACs) were used for room temperature measurements up to 66 GPa.
- An Externally-Heated DAC (EHDAC) was used for simultaneous high P-T measurements (up to 773 K).
- Sample Assembly: A thin sheet of borosilicate glass (~10 ”m thick), coated with a ~90 nm Al film (used as a transducer), was loaded into the DAC along with polycrystalline NaCl powder as the pressure medium and sample of interest.
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Thermal Conductivity Measurement (TDTR):
- Technique: Time-Domain Thermoreflectance (TDTR), an ultrafast optical pump-probe method.
- Principle: A pump beam heats the Al film, and a probe beam monitors the temporal evolution of the surface reflectivity change (related to temperature).
- Analysis: The ratio of the in-phase (Vin) and out-of-phase (Vout) signals (-Vin/Vout) was fitted using a bi-directional thermal model to extract the thermal conductivity of the NaCl layer.
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Compressional Velocity Measurement (TDSBS):
- Technique: Time-Domain Stimulated Brillouin Scattering (TDSBS), a picosecond interferometry method.
- Principle: Measures the Brillouin frequency (f) generated by inelastic light scattering from acoustic phonons (sound waves) propagating through the NaCl.
- Calculation: Compressional velocity (Vp) was calculated using the relationship f = 2NVp/λ, where N is the refractive index (estimated via the Lorentz-Lorenz relation) and λ is the laser wavelength (785 nm).
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Pressure and Temperature Calibration:
- Pressure: Determined using ruby fluorescence (Mao scale). At pressures >50 GPa, results were cross-checked against Raman spectra of the ruby and diamond anvils.
- Temperature (EHDAC): Monitored using an R-type thermocouple. Thermal pressure variations were determined in situ by monitoring the ruby fluorescence shift and applying established temperature calibrations.
Commercial Applications
Section titled âCommercial ApplicationsâThe findings and methodologies presented have significant implications for fields requiring precise material characterization under extreme conditions.
- High-Pressure Research and Metrology:
- DAC Experiment Optimization: The precise thermal conductivity data for NaCl (the most common pressure medium) allows experimentalists to create far more accurate thermal models for laser-heated and externally-heated DACs, reducing systematic errors in temperature determination of samples under study.
- Pressure Standard Refinement: Improves the reliability and accuracy of NaCl as a primary pressure calibrant across a wide range of P-T conditions.
- Geophysics and Earth Science:
- Mantle Dynamics Modeling: Validation of the LS equation provides confidence in predicting the thermal conductivity of key mantle minerals (like MgO and bridgmanite) at conditions relevant to the Earthâs deep interior, which is crucial for modeling heat flux and mantle convection.
- Seismic Interpretation: The confirmed applicability of Birchâs law for Vp vs. density in NaCl provides a benchmark for interpreting seismic velocity profiles and density structures within planetary bodies.
- Advanced Materials Characterization:
- Extreme Environment Testing: The successful coupling of TDTR/TDSBS with EHDAC establishes a robust, high-precision methodology for measuring thermal and elastic properties of any material (e.g., ceramics, semiconductors, or novel phases) under previously inaccessible P-T conditions.
- Fundamental Physics:
- Phonon Engineering: Provides empirical data confirming the dominance of three-phonon anharmonic scattering in ionic crystals under extreme compression, aiding in the development and validation of advanced thermal transport theories.