Long-Term Corrosion of Eutectic Gallium, Indium, and Tin (EGaInSn) Interfacing with Diamond

At a Glance

Metadata	Details
Publication Date	2024-06-02
Journal	Materials
Authors	Stephan Handschuh‐Wang, Tao Wang, Zongyan Zhang, Fucheng Liu, Peigang Han
Institutions	Shenzhen Institutes of Advanced Technology, Institute of Materials Research and Engineering
Citations	5
Analysis	Full AI Review Included

Executive Summary

This study investigates the long-term stability and corrosion resistance of Eutectic Gallium, Indium, and Tin (EGaInSn, Galinstan) liquid metal when interfaced with various diamond coatings (Nanocrystalline Diamond, Boron-Doped Diamond, and Graphite) over a four-year period.

Interface Stability: The diamond coatings (NCD and BDD) demonstrated excellent corrosion resistance. The liquid metal did not penetrate the diamond layer or corrode the underlying Ti-alloy or Si substrates, confirming diamond’s suitability as a protective barrier.
Liquid Metal Failure Mode: The EGaInSn solidified on all samples after four years, regardless of the substrate material (Ti-alloy or Si).
Mechanism of Solidification: The solidification was definitively attributed to hydrolysis-induced dealloying, not alloying with substrate materials (like Aluminum).
Chemical Reaction: Ambient humidity caused the Gallium (Ga) component to oxidize and hydrolyze, forming solid Gallium Oxide Hydroxide (GaOOH).
Dealloying Result: This process separated the alloy into solid GaOOH crystals and a residual, higher melting point InSn-rich liquid alloy, leading to the overall hardening and failure of the Thermal Interface Material (TIM).
Engineering Implication: The long-term stability of gallium-based liquid metals in thermal management systems is severely limited by ambient humidity, necessitating protective measures or hermetic sealing in high-reliability applications.

Technical Specifications

Parameter	Value	Unit	Context
Liquid Metal Alloy	EGaInSn (Galinstan)	N/A	68.5 wt% Ga, 21.5 wt% In, 10 wt% Sn
EGaInSn Melting Point	ca. 10.5	°C	Fresh Alloy
EGaInSn Surface Tension	ca. 600	mN/m	Fresh Alloy
Aging Duration	4	years	Storage Experiment
Average Summer Temperature	ca. 25	°C	Storage Condition
Average Summer Humidity	ca. 80	%	Storage Condition
NCD Growth Pressure	1.5	kPa	HFCVD Process
BDD Growth Pressure	2	kPa	HFCVD Process
HFCVD Filament Temperature	ca. 2500	°C	Diamond Deposition
Structured NCD Thickness	ca. 3.5	µm	Measured via SEM cross-section
NCD Raman Shift (on Ti)	1335	cm^-1	Indicates compressive stress (-1.58 GPa)
BDD Boron Concentration	ca. 2.8 x 10²¹	cm^-3	High conductivity threshold exceeded
Solidified Phase Identified	GaOOH	N/A	Confirmed by XRD (PDF #54-0910)

Key Methodologies

The experiment involved synthesizing various diamond coatings via Hot Filament Chemical Vapor Deposition (HFCVD), followed by a four-year aging test with EGaInSn, and subsequent material analysis.

1. Substrate Preparation and Seeding

Substrates: Ti-alloy (Ti₆Al₄V) and Silicon (Si) were used.
Pretreatment: Substrates were mechanically ground (Ti-alloy) or chemically oxidized (Si), followed by ultrasonic cleaning.
Seeding (Electrostatic Self-Assembly): Nanodiamond (DND) particles were stabilized (using TMAEMC for smooth NCD or oxalic acid for structured NCD/BDD) and ultrasonically seeded onto the substrates for 30 min.

2. Diamond Coating Synthesis (HFCVD)

Nanocrystalline Diamond (NCD):
- Gases: Methane (CH₄) at 32 sccm, Hydrogen (H₂) at 800 sccm.
- Parameters: Filament T ca. 2500 °C, Pressure 1.5 kPa, 1 h deposition.
Boron-Doped Diamond (BDD):
- Gases: CH₄ (32 sccm), H₂ (400 sccm), Boron source (Trimethyl Borane, TMB) (160 sccm).
- Parameters: Filament T 2500 °C, Pressure 2 kPa, 1 h deposition.

3. Aging and Corrosion Test

Liquid Metal Application: EGaInSn (Galinstan) was deposited (1-2 g) onto the coated substrates and forced to wet the surface.
Storage: Samples were stored in closed (but not air-tight) petri dishes for 4 years under ambient conditions (T: 18-25 °C, Humidity: 60-80%).

4. Post-Aging Analysis

Cleaning: Solidified liquid metal was mechanically removed, followed by cleaning with water, 1 min immersion in 1 mol/L HCl, and a final water rinse.
Characterization Techniques:
- SEM/EDS: Used for surface morphology, cross-sectional analysis (to check for penetration), and elemental mapping (to confirm dealloying).
- XRD: Used on the solidified material to confirm the presence of Gallium Oxide Hydroxide (GaOOH).
- Raman Spectroscopy: Used to verify the integrity, stress, and doping level of the diamond coatings after aging.

Commercial Applications

The findings are critical for the design and reliability of systems utilizing gallium-based liquid metals in conjunction with advanced thermal materials.

High-Power Electronics Thermal Management: Diamond and BDD coatings provide the highest known thermal conductivity (ca. 2300 W/mK) and serve as essential corrosion barriers for liquid metal TIMs in devices like high-power LEDs, CPUs, and GPUs.
Long-Life Communication Devices (e.g., 6G): The demonstrated failure mechanism (hydrolysis) highlights the necessity of hermetic sealing or robust protective coatings to ensure liquid metal stability over the required 10-year lifespan of wireless communication hardware.
Corrosion-Resistant Contacts: Conductive BDD coatings can be used as stable, corrosion-resistant electrical contacts for liquid metals, offering an alternative to traditional refractory metals (Mo, Ta).
Liquid Metal Catalysis and Microfluidics: The observed hydrolysis-induced dealloying has direct implications for the storage and operational lifetime of gallium-based microdroplet devices and SCALMS (Supported Catalytically Active Liquid Metal Solutions), where contact with ambient moisture must be strictly controlled.

View Original Abstract

Thermal transport is of grave importance in many high-value applications. Heat dissipation can be improved by utilizing liquid metals as thermal interface materials. Yet, liquid metals exhibit corrosivity towards many metals used for heat sinks, such as aluminum, and other electrical devices (i.e., copper). The compatibility of the liquid metal with the heat sink or device material as well as its long-term stability are important performance variables for thermal management systems. Herein, the compatibility of the liquid metal Galinstan, a eutectic alloy of gallium, indium, and tin, with diamond coatings and the stability of the liquid metal in this environment are scrutinized. The liquid metal did not penetrate the diamond coating nor corrode it. However, the liquid metal solidified with the progression of time, starting from the second year. After 4 years of aging, the liquid metal on all samples solidified, which cannot be explained by the dissolution of aluminum from the titanium alloy. In contrast, the solidification arose from oxidation by oxygen, followed by hydrolysis to GaOOH due to the humidity in the air. The hydrolysis led to dealloying, where In and Sn remained an alloy while Ga separated as GaOOH. This hydrolysis has implications for many devices based on gallium alloys and should be considered during the design phase of liquid metal-enabled products.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma17112683

References

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