High-pressure thermal conductivity and compressional velocity of NaCl in B1 and B2 phase

At a Glance

Metadata	Details
Publication Date	2021-10-29
Journal	Scientific Reports
Authors	Wen‐Pin Hsieh
Institutions	Institute of Earth Sciences, Academia Sinica, National Taiwan University
Citations	22
Analysis	Full AI Review Included

6CCVD Technical Documentation: High-Pressure Thermal Transport in Ionic Crystals

Reference: Hsieh, W.-P. (2021). High-pressure thermal conductivity and compressional velocity of NaCl in B1 and B2 phase. Scientific Reports, 11:21321.

Executive Summary

This research utilizes advanced ultrafast optical techniques coupled with Diamond Anvil Cells (DAC) to characterize the thermal and elastic properties of Sodium Chloride (NaCl) under extreme pressure-temperature (P-T) conditions. The findings are critical for accurately modeling heat transfer in high P-T DAC experiments, a core requirement for materials physics and geosciences.

Core Achievement: Precise determination of the thermal conductivity ($\Lambda$) and compressional velocity ($V_p$) of polycrystalline NaCl up to 66 GPa and 773 K.
Methodology: Combination of Time-Domain Thermoreflectance (TDTR) and picosecond interferometry (Brillouin scattering) within an Externally-Heated DAC (EHDAC) setup.
Thermal Behavior: Thermal conductivity increased rapidly in the B1 phase (up to 50 W m^-1 K^-1 at 28.4 GPa) but dropped significantly (~70%) upon transition to the B2 phase (~30 GPa).
Fundamental Validation: Confirmed that $\Lambda$ follows a typical $T^{-1}$ dependence at high pressures, validating the Leibfried-Schlömann (LS) equation across a wide compression range (35% volume compression in B1, 20% in B2).
Elastic Properties: Compressional velocity ($V_p$) was found to scale approximately linearly with density, confirming the applicability of Birch’s law across both B1 and B2 phases.
Material Requirement: The success of this high-precision thermal measurement relies fundamentally on the optical quality and mechanical integrity of the diamond anvils used in the DAC.

Technical Specifications

The following hard data points were extracted from the experimental results, focusing on the extreme conditions achieved and the measured material properties:

Parameter	Value	Unit	Context
Maximum Pressure Achieved	66	GPa	Room temperature measurements
Maximum Temperature Achieved	~773	K	High P-T measurements (EHDAC)
Ambient Thermal Conductivity ($\Lambda_0$)	5	W m^-1 K^-1	NaCl B1 phase
Maximum $\Lambda$ (B1 Phase)	50	W m^-1 K^-1	Measured at 28.4 GPa
$\Lambda$ Drop Across B1-B2 Transition	~70	%	Transition around 30 GPa
Temperature Dependence Exponent ($n$)	-0.98 to -0.92	N/A	$\Lambda(T) \propto T^n$ (Close to $T^{-1}$)
Grüneisen Parameter ($g$)	5.5 (±0.2)	N/A	Derived for B1 phase
Grüneisen Parameter ($g$)	5.5 (±0.5)	N/A	Derived for B2 phase
TDTR Transducer Film Thickness	~90	nm	Aluminum (Al) film
Reference Substrate Thickness	~10	µm	Borosilicate glass (D 263^° T eco)
Laser Spot Size (Radius)	~7.6	µm	TDTR measurement geometry
Brillouin Frequency Range	19 to 40+	GHz	Measured up to 66 GPa
Compressional Velocity Slope ($b_1$)	2.32 (±0.04)	N/A	Birch’s Law coefficient, B1 phase

Key Methodologies

The experiment combined state-of-the-art high-pressure techniques with ultrafast optical metrology to achieve precise thermal and elastic characterization under extreme conditions.

Sample Preparation and Loading: A thin sheet of borosilicate glass (~10 µm thick) was polished and coated with a ~90 nm thick Al film (acting as the TDTR transducer). This assembly, along with polycrystalline NaCl powder and ruby spheres (for pressure calibration), was loaded into a symmetric piston-cylinder DAC (300 µm culet size, Re gasket).
High P-T Generation: An Externally-Heated DAC (EHDAC) was utilized, surrounding the diamond anvils with a ring heater to achieve stable and homogeneous high-temperature conditions up to ~773 K.
Pressure and Temperature Monitoring: Pressure was determined primarily by ruby fluorescence. Temperature-induced pressure variations (thermal pressure) were monitored in situ using calibrated ruby fluorescence shifts.
Thermal Conductivity Measurement (TDTR): Time-Domain Thermoreflectance was performed using a Ti:sapphire oscillator split into pump and probe beams. The pump beam heated the Al film, and the probe beam detected the temporal evolution of the reflectivity change. Thermal conductivity ($\Lambda$) was derived by fitting the ratio of the in-phase ($V_{in}$) and out-of-phase ($V_{out}$) components to a bi-directional thermal model.
Compressional Velocity Measurement (Brillouin Scattering): Time-domain stimulated Brillouin scattering (picosecond interferometry) was used to measure the Brillouin frequency ($f$). The compressional velocity ($V_p$) was calculated from $f$ using the refractive index ($N$) derived from the Lorentz-Lorenz relation.

6CCVD Solutions & Capabilities

This research demonstrates the critical need for high-performance diamond materials and precision fabrication services in extreme P-T research. 6CCVD is uniquely positioned to supply the necessary components to replicate, extend, and advance these demanding experiments.

Applicable Materials for Extreme P-T Experiments

The DAC experiments described require diamonds with exceptional optical transparency, low defect density, and high mechanical strength to withstand pressures up to 66 GPa and temperatures up to 773 K.

Research Requirement	6CCVD Material Solution	Technical Specification
High-Pressure Anvils	Optical Grade Single Crystal Diamond (SCD)	SCD with low birefringence and high purity (Type IIa) ensures maximum optical access for pump-probe techniques (TDTR, Brillouin scattering).
Thermal Substrates	High Purity SCD or PCD Wafers	SCD or PCD plates up to 500 µm thick, offering superior thermal management compared to borosilicate glass, enabling higher accuracy in thermal modeling.
High-Strength Substrates	Thick SCD Substrates	Substrates up to 10 mm thick for robust support in EHDAC setups, ensuring mechanical stability under simultaneous high P-T conditions.
Metal Transducer Layer	Custom Metalized Diamond	SCD or PCD wafers pre-coated with the required transducer layer (e.g., Al, as used in the paper) or advanced multi-layer stacks (Ti/Pt/Au) for enhanced adhesion and stability.

Customization Potential for DAC Integration

6CCVD’s in-house fabrication capabilities directly address the precision requirements of DAC and ultrafast optical setups:

Precision Dimensions: We offer custom diamond plates and wafers up to 125 mm (PCD) and specialized geometries required for DAC culets (e.g., 300 µm culet size used in this study). Our laser cutting services ensure precise shaping for piston-cylinder assemblies.
Ultra-Smooth Polishing: Achieving accurate TDTR measurements requires extremely low surface roughness. We guarantee Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD, minimizing scattering and maximizing optical coupling efficiency.
Integrated Metalization: The experiment relied on a 90 nm Al film. 6CCVD provides internal metalization services, including Au, Pt, Pd, Ti, W, and Cu, allowing researchers to receive ready-to-use diamond components with optimized transducer layers for TDTR or electrical heating elements for EHDAC.

Engineering Support

The complexity of coupling TDTR and Brillouin scattering with EHDAC requires deep expertise in material selection and thermal modeling.

Thermal Modeling Assistance: 6CCVD’s in-house PhD team specializes in the thermo-physical properties of CVD diamond. We can assist researchers in selecting the optimal diamond material (SCD vs. PCD) and thickness to minimize thermal gradients and enhance the accuracy of thermal conductivity measurements in similar high P-T thermal transport projects.
Global Logistics: We provide reliable global shipping (DDU default, DDP available) for sensitive, high-value diamond components, ensuring prompt delivery to research facilities worldwide.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41598-021-00736-2