3D thermal and electrically conductive graphene network
A group of researchers from China and the United States synthesized a 3D conductive lattice of gallium-based liquid metal by adding graphene nanofillers into the matrix, for use as a new and improved thermal interface material in electronic circuits. This research was published in the journal Advanced materials technology.
To study: Construction of a 3D conductive network in liquid gallium with improved thermal and electrical performance. Image Credit: BeataGFX / Shutterstock.com
What are Gallium-Based Liquid Metals?
Gallium based liquid metals (ML) are a new class of multifunctional materials with excellent electrical and thermal conductivity, non-volatility and rheology. They have shown potential for emerging applications in robotics, 3D printing, drivers, flexible and portable energy technologies.
A schematic illustration which shows the preparation of the LM-GrP composite by one-step ball milling. Image credit: Wenkui Xing et al., Advanced Materials
One of the most promising applications of LM is its use as thermal management materials. Effective thermal management is urgently needed to dissipate the large amounts of heat generated by integrated circuit chips and printed circuit boards (PCBs).
Thermal Interface Materials (TIMs) are commonly used to fill the air spaces between the chip and the heat sink to improve interfacial thermal conduction. Air is a poor conductor of heat which causes IC chips to overheat under heavy workloads. Comparatively, amorphous LM-based materials exhibit much higher intrinsic thermal conductivities than other conventional TIMs such as polymer-based materials.
Analysis of the composition of the LM-based composite. a) Optical images of the LM-GrP composite as prepared, the vacuum-treated LM-graphite mixture and the hand-mixed LM-graphite. b) XPS spectra of the LM-GrP and Ga 2p composite1/2. c) Schematic illustration of the hydrogen bond interactions of graphene with the oxide layer of LM. d) FTIR spectroscopy of graphite, LM-GrP composite and LM. Image credit: Wenkui Xing et al., Advanced Materials
What is the common problem with liquid metals as TIM?
LMs in practical applications can leak and contaminate other parts of the electronic component due to its fluidity and low wettability. Much research has shown that slight oxidation can overcome high surface tension and low viscosity, but such oxidation reduces the intrinsic thermal conductivity of LM.
Similar to polymer-based composites, mixing a highly heat-conductive filler into the LM matrix can improve effective thermal conductivity.
Although the metal fillers in the LM matrix can give high thermal conductivity, most of the reported metal particles such as Mg, Fe and Cu would form intermetallic compounds which consume LM and sacrifice its fluidity and become too hard for further use. Inert particles with high thermal conductivity such as nickel (Ni) and tungsten (W) would not form alloys with the LM matrix; however, this results in the emergence of microscopic geometric features which magnify the texture of the surface. 2D inorganic nanofillers such as graphene, boron nitride (BN) and graphite flakes which possess high thermal conductivity and smooth surface texture are widely used in polymer based TIMs.
Previously, 3D interconnected polymer-based infill networks were also synthesized for high thermal conductivity. Chemical modification, internal microstructure and adjustment of the volume mixing ratio are used to achieve improved thermal conductivity of the generated composites.
LM-based composite heat dissipation test. a) Schematic illustration of the experimental configuration of heat dissipation in LED modules, where the LM-GrP composite was used as TIM. b) The temperature changed over time when different TIMs were used in the LED modules. c) Comparison of temporal infrared images of LED modules among different TIMs. Image credit: Wenkui Xing et al., Advanced Materials
What does the research say?
The team incorporated 2D inorganic graphene nanofillers into the LM matrix to improve the electrical, thermal and rheological properties of LM-based composites. The uniform dispersion and connection of 2D inorganic nanofillers in an array in the metal matrix is one of the critical steps to connect the charges and improve thermal conduction.
They preferred ball milling as the mixing process, which uses mechanical shear force and enables rapid and large-scale production as well as the introduction of chemical functional groups in the graphene nanoplates in the ball milling cylinder when appropriate liquids, solids or chemical vapors have been used.
The researchers used a simple one-step ball milling approach to obtain a 3D thermal and electrically conductive graphene lattice in a gallium-based liquid metal (LM). In it, 2D graphene nanofillers and their derivatives have been incorporated to form thermally and electrically conductive 3D fill arrays.
They found that the resulting composite exhibited the highest 3D thermal conductivity (44.6 W m-1 K-1) among LM gallium-based composites with inorganic 2D nanoplates and high electrical conductivity (8.3 S.µm-1) among liquid metallic composites based on gallium.
The improved thermal conductivity and wettability of the gallium-based composite lead to viable use as thermal interface materials with elegant texture for heat dissipation of IC chips. Magnetic and electrochemical measurement data confirmed that these LM-based composites can also be controlled under external electric or magnetic fields, potentially helping to expand their application in external field-driven systems.
Electrical and magnetic performance of LM-based composites. a) Schematic illustration of the experimental set-up for oxidizing spreading under tension. b) Oxidizing spreading of a drop in a solution of 1 m NaOH under 5 V i) pure gallium; ii) [email protected] composite. c) The directional movement of pure gallium and [email protected] droplets in letter-shaped “J” tracks. d) Schematic illustration of the movement of [email protected] composite controlled by a magnet. e) The directional travel path [email protected] magnetic fluid guided by a magnet. Image credit: Wenkui Xing et al., Advanced Materials
Wenkui Xing, Shen Chen, Han Wang, Wendong Liu, Jiashu Zheng, Feiyu Zheng, Xiaomin Li, Peng Tao, Wen Shang, Benwei Fu, Jianbo Wu, Chengyi Song, Baowen Li, Tao Deng. Construction of a 3D conductive network in liquid gallium with improved thermal and electrical performance. Av. Mater. Technol. 2021, 2100970. https://onlinelibrary.wiley.com/doi/10.1002/admt.202100970