Thermal conductivity of iron and nickel during melting - Implication to the planetary liquid outer core

At a Glance

Metadata	Details
Publication Date	2022-12-13
Journal	Pramana
Authors	Pinku Saha, Goutam Dev Mukherjee
Institutions	Indian Institute of Science Education and Research Kolkata
Citations	4
Analysis	Full AI Review Included

Thermal Conductivity of Fe and Ni in LHDAC: Implications for Planetary Cores

Technical Analysis and 6CCVD Material Solutions

Executive Summary

This research utilizes the Laser Heated Diamond Anvil Cell (LHDAC) technique and COMSOL simulations to directly measure the thermal conductivity ($\kappa$) of Fe and Ni at extreme high-pressure (HP) and high-temperature (HT) conditions, providing critical data for planetary geophysics.

Core Application: Direct measurement of thermal transport properties of Fe and Ni, crucial for modeling the heat loss and magnetic field generation (Geodynamo) in planetary liquid outer cores (e.g., Mercury and Mars).
Methodology: Thermal conductivity ($\kappa$) was determined by measuring the temperature gradient across the sample surface in a single-sided LHDAC and matching the profile using finite-element simulations (COMSOL).
Key Finding (Fe): Thermal conductivity of molten Fe is constant at 60-70 ± 20 W m^-1 K^-1 across the studied pressure range (5-8 GPa) near the Mercury core-mantle boundary conditions.
Key Finding (Ni): Thermal conductivity of molten Ni is constant at 65-70 ± 20 W m^-1 K^-1 across the studied pressure range (4-22 GPa).
Melting Observation: A sharp decrease (25-40% decrement) in $\kappa$ was observed upon melting, attributed to the loss of long-range order and the formation of a liquid-solid interface at the hotspot boundary.
Material Requirement: The success of the experiment hinges on the use of high-quality, optically transparent Single Crystal Diamond (SCD) anvils capable of withstanding pressures up to 22 GPa and temperatures exceeding 2100 K.

Technical Specifications

The following hard data points were extracted from the experimental setup and results:

Parameter	Value	Unit	Context
Pressure Range Studied (Ni)	4 to 22	GPa	Melting curve measurements
Fe Thermal Conductivity ($\kappa$) at Melting	60-70 ± 20	W m^-1 K^-1	Constant over 5-8 GPa range
Ni Thermal Conductivity ($\kappa$) at Melting	65-70 ± 20	W m^-1 K^-1	Constant over 4-22 GPa range
Fe Melting Temperature Observed	~1975, 2035, 2098	K	At 5, 7, and 8.5 GPa, respectively
Diamond Anvil Culet Flat Diameter	300	µm	Used in Almax-Boehler DAC design
Sample Thickness (Fe/Ni Plate)	~15	µm	Polycrystalline compacts
PTM/Insulator Thickness (NaCl)	~12	µm	Thermal insulation from diamond culet
Hotspot Radius ($r_1$) (Measured)	9 ± 2	µm	Input for COMSOL simulation
Laser Wavelength ($\lambda$)	1.070	µm	Diode-pumped Ytterbium fiber optic laser
Laser Maximum Power	100	W	YLR100-SM-AC-Y11
Total Uncertainty in $\kappa$ Determination	~30	%	Estimated from propagated errors

Key Methodologies

The experiment relied on precise control of pressure, temperature, and geometry within the LHDAC setup:

High-Pressure Apparatus: A plate-type Diamond Anvil Cell (DAC) (Almax-Boehler design) was used, equipped with 300 µm culet SCD anvils.
Gasket Preparation: T301 stainless steel gaskets were preindented to 50 µm thickness, and a central hole (~110 µm diameter) was drilled via Electric Discharge Machining (EDM).
Sample Preparation: Thin plates (~15 µm thick) of polycrystalline Fe and Ni were compacted using a 300 ton hydraulic press.
Sample Loading: Fe/Ni samples were sandwiched between NaCl discs (~12 µm thick, 90-110 µm diameter) which served as both the Pressure Transmitting Medium (PTM) and thermal insulation.
Heating System: Single-sided heating was achieved using a Continuous Wave (CW) diode-pumped Ytterbium fiber optic laser ($\lambda$ = 1.070 µm).
Temperature Measurement: Sample surface temperature was measured using spectroradiometry (650-900 nm range) by fitting Planck’s radiation function. The temperature gradient was mapped by translating a 50 µm pinhole across the magnified sample image with 1 µm resolution.
Thermal Conductivity Determination: The steady-state temperature distribution was simulated using COMSOL Multiphysics finite-element software. The thermal conductivity ($\kappa$) of the sample was varied in the simulation until the computed temperature profile matched the experimentally measured gradient.

6CCVD Solutions & Capabilities

The successful execution of high-pressure, high-temperature experiments like this requires diamond materials with exceptional purity, mechanical integrity, and surface finish. 6CCVD specializes in providing the necessary MPCVD diamond components to replicate and advance this research.

Applicable Materials

To achieve the extreme conditions (up to 22 GPa and 2100 K) and maintain the optical quality required for LHDAC measurements, Optical Grade Single Crystal Diamond (SCD) is the ideal material.

Optical Grade SCD: Essential for LHDAC experiments, offering high transparency in the infrared (1.070 µm laser) and visible/near-infrared (650-900 nm spectroradiometry) ranges. Our SCD material ensures minimal absorption and scattering, reducing temperature measurement errors (currently ±50 K).
High Purity: SCD provides the necessary mechanical strength and thermal stability to withstand the immense pressures and thermal gradients generated during laser heating without failure or degradation.

Customization Potential

6CCVD’s advanced MPCVD growth and processing capabilities directly address the specific needs of high-pressure research:

Research Requirement	6CCVD Capability	Technical Advantage
Anvil Geometry: 300 µm Culet	Custom Dimensions: We supply SCD plates/wafers up to 125mm, cut and polished to precise culet sizes (e.g., 300 µm, 500 µm, 1000 µm) tailored for specific DAC designs (e.g., Almax-Boehler).	Optimized pressure generation and stability for specific experimental ranges.
Surface Quality: Optical Access	Ultra-Low Roughness Polishing: SCD polishing to Ra < 1 nm.	Minimizes laser scattering and improves the accuracy of spectroradiometric temperature measurements across the sample surface.
Advanced Heating/Sensing:	Custom Metalization: Internal capability for depositing thin films (Au, Pt, Ti, W, Cu) directly onto the diamond culet face.	Enables advanced LHDAC setups, such as integrated resistive heaters or electrical contacts for simultaneous resistivity measurements (Wiedemann-Franz law validation).
Material Thickness: SCD Anvils	Thickness Control: SCD material available from 0.1 µm up to 500 µm, and substrates up to 10 mm thick.	Ensures robust anvil performance and optimal thermal management in the LHDAC setup.

Engineering Support

The determination of thermal conductivity in this study relies heavily on complex finite-element modeling (COMSOL) which requires accurate material inputs and boundary conditions. 6CCVD’s in-house PhD team specializes in the material science of diamond under extreme conditions. We can assist researchers in selecting the optimal SCD grade and geometry for similar High-Pressure Geophysics and Planetary Core Dynamics projects, ensuring the diamond material itself does not become the limiting factor in experimental accuracy or pressure range.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1007/s12043-022-02471-3