Gallium is a very useful element that went along with the advancement of human civilization throughout the 20th century. Gallium is identified as a technical critical element, as it is essential for the manufacture of semiconductors and transistors. In particular, gallium nitride and related fertilizers allowed the discovery of the blue LED, which was the ultimate key in the development of an energy-efficient and durable white LED lighting system. This discovery led to the awarding of the 2014 Nobel Prize in Physics. It is estimated that up to 98% of the demand for gallium comes from the semiconductor and electronics industry.
In addition to its use in electronics, the unique physical properties of gallium have led to its use in other fields. Gallium itself is a metal with a very low melting point and is a liquid at just above room temperature (30 ° C). Also, gallium is able to form several eutectic systems (alloys that have a lower melting point than any of their constituents, including gallium) with several other metals. Both real gallium and those molten metal alloys have high surface tensile densities and are considered “non-spreadable” on most surfaces. This makes them difficult to handle, shape or process, which limits their ability to apply in the real world. However, a recent discovery may have solved the potential for wider use of gallium in a range of functional materials.
The Center for Heterogeneous Carbon Materials (CMCM) research team within the Institute for Basic Science (IBS) in Ulsan, South Korea and the National Institute of Science and Technology of Ulsan (UNIST) has devised a new approach for the incorporation of filler particles in gallium liquid to form a functional fertilizer of molten metal. The introduction of fillers changes the material from a melt state to a paste or putty-like form (with a consistency and “feel” similar to the commercial product “Plasticine”) depending on the amount of granules added. In the case when graphene oxide (GO) was used as a filler, the GO content of 1.6 ~ 1.8% came to a paste-like shape, with 3.6% being the best for putty formation. A mixture of new gallium compounds and the manner in which they were formed is described in a recent article published in the journal Advances in science.
Mixing of grains within a gallium-based molten metal changes the physical properties of the material, allowing for much easier handling. First author Chunhui Wang notes: “The ability for liquid gallium fertilizers to form gallstones or putties is very beneficial. It removes most of the gallium treatment issues for applications. It no longer stains surfaces, can be coated or “painted” on almost any surface, can be designed in a variety of shapes. This opens up a wide range of applications for gallium that have never been seen before. ” The use of this detector has the potential to include situations where soft and flexible electronics are required, such as in accessible devices and medical implants. The study even showed that the mixture can be made into a porous foam-like material with extreme heat, with the ability to stand up a blowtorch for 1 minute without sustaining any damage.
In this study, the team was able to identify the factors that allowed the fillers to mix successfully with gallium liquid. Corresponding author Benjamin Cunning described the prerequisites: “Molten gallium improves the ‘skin’ of oxygen when exposed to air, and this is essential for blending. cooking of the filler and making it stable inside the gallium, but this skin is stable.We have learned that particles of large enough size must be used otherwise mixing cannot take place and a machine cannot to do “.
The researchers used four different materials as fillers in their study: graphene oxide, silicon carbide, diamond, and graphite. Among them, two of them particularly exhibited excellent properties when incorporated into gallium liquid: graphene oxide (rG-O) reduction for electromagnetic interference shielding (EMI) and diamond particles for thermal interface materials. A 13-micron thick coating of Ga / rG-O on a reduced graphene oxide film was able to improve the film’s coating efficiency from 20 dB up to 75 dB, which is sufficient for both commercial (> 30 dB) and military (> 60 dB) applications. However, the most remarkable property of the cement was its ability to impart EMI shielding properties to everyday common material. The researchers showed that a 20-micron thick coating of Ga / rG-O on a simple sheet of paper gave a shielding efficiency of more than 70 dB.
Perhaps the most interesting thermal performance is when diamond grains were introduced into the material. The CMCM team measured the thermal behavior in collaboration with UNIST researchers Dr. Shalik Joshi and Professor KIM Gun-ho, and the “real world” application tests were conducted by LEE Seunghwan and Professor LEE Jaeson. The thermal conductivity test showed that the diamond had a large thermal conductivity of up to ~ 110 W m-1 K-1, with larger filler particles yielding more thermal conductivity. This exceeded the thermal conductivity of the commercially available thermal paste (79 W m-1 K-1) by more than 50%. The application test further confirmed the efficacy of the gallium-diamond mixture as a thermal interface material (TIM) between a heat source and a heat sink. Interestingly, the combination with smaller sized diamond particles showed real cooling capacity in the world despite their lower thermal conductivity. The reason for this difference is that the larger diamond particles are more likely to penetrate through the gallium mass and create air gaps at the interface of the heat sink or heat source and the TIM, reducing their efficiency. (Ruoff notes that there are some ways to resolve this issue in the future.)
Finally, the group has even created and tested a material made from a mixture of gallium metal and commercial silicone putty – better known as “Silly Putty” (Crayola LLC). This latter type of gallium in which a mixture is formed by a completely different method, in which small drops of gallium are scattered throughout the Silly Putty. Although it does not have the impressive EMI shielding capacity of the above Ga / rG-O (the material needs 2 mm of coverage to achieve the same 70 dB wing efficiency), it is compensated by advanced mechanical properties. Because this combination uses a silicone polymer rather than gallium metal as the base material, it is easy to expand as well as affordable.
Professor Rod Ruoff, director of CMCM who came up with the idea of mixing such carbon fillers with molten metal notes: “We first submitted this work in September 2019, and It has been the subject of a number of reports since then.We have found that there is a wide variety of grains that can be incorporated into liquid gallium and have provided a basic understanding of how grain size increases. play a role in successful mixing.We found that this behavior extends to gallium alloys which are liquids at room temperature such as indium-gallium, tin-gallium.
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