Graphene thermal conductivity
Thermal transport in graphene is a thriving area of research, thanks to graphene's extraordinary heat conductivity properties and its potential for use in thermal management applications.
The measured thermal conductivity of graphene is in the range 3000 - 5000 W/mK at room temperature, an exceptional figure compared with the thermal conductivity of pyrolytic graphite of approximately 2000 W⋅m−1⋅K−1 at room temperature. There are, however, other researches that estimate that this number is exaggerated, and that the in-plane thermal conductivity of graphene at room temperature is about 2000–4000 W⋅m−1⋅K−1 for freely suspended samples. This number is still among the highest of any known material.
Graphene is considered an excellent heat conductor, and several studies have found it to have unlimited potential for heat conduction based on the size of the sample, contradicting the law of thermal conduction (Fourier’s law) in the micrometer scale. In both computer simulations and experiments, the researchers found that the larger the segment of graphene, the more heat it could transfer. Theoretically, graphene could absorb an unlimited amount of heat.
The thermal conductivity increases logarithmically, and researchers believe that this might be due to the stable bonding pattern as well as being a 2D material. As graphene is considerably more resistant to tearing than steel and is also lightweight and flexible, its conductivity could have some attractive real-world applications.
But what exactly is thermal conductivity?
Heat conduction (or thermal conduction) is the movement of heat from one object to another, that has a different temperature, through physical contact. Heat can be transferred in three ways: conduction, convection and radiation. Heat conduction is very common and can easily be found in our everyday activities - like warming a person’s hand on a hot-water bottle, and more. Heat flows from the object with the higher temperature to the colder one.
Thermal transfer takes place at the molecular level, when heat energy is absorbed by a surface and causes microscopic collisions of particles and movement of electrons within that body. In the process, they collide with each other and transfer the energy to their “neighbor”, a process that will go on as long as heat is being added.
The process of heat conduction mainly depends on the temperature gradient (the temperature difference between the bodies), the path length and the properties of the materials involved. Not all substances are good heat conductors - metals, for example, are considered good conductors as they quickly transfer heat, but materials like wood or paper are viewed as poor conductors of heat. Materials that are poor conductors of heat are referred to as insulators.
How can graphene’s exciting thermal conduction properties be put to use?
Some of the potential applications for graphene-enabled thermal management include electronics, which could greatly benefit from graphene's ability to dissipate heat and optimize electronic function. In micro- and nano-electronics, heat is often a limiting factor for smaller and more efficient components. Therefore, graphene and similar materials with exceptional thermal conductivity may hold an enormous potential for this kind of applications.
Graphene’s heat conductivity can be used in many ways, including thermal interface materials (TIM), heat spreaders, thermal greases (thin layers usually between a heat source such as a microprocessor and a heat sink), graphene-based nanocomposites, and more.
The latest graphene thermal news:
Researchers from Stanford, NIST, Theiss Research and several others have designed a new heat protector that consists of just a few layers of atomically thin materials, to protect electronics from excess heat.
The heat protector can reportedly provide the same insulation as a sheet of glass 100 times thicker. “We’re looking at the heat in electronic devices in an entirely new way,” said Eric Pop, professor of electrical engineering at Stanford and senior author of the study.
In October 2018, Huawei announced its Mate 20 X smartphone, which was a gaming smartphone that adopted a graphene film cooling technology for heat management purposes. Now, Huawei launched the Mate P30 Pro smartphone, that adopts a graphene film as well.
The Mate 20 X was a bit of a niche phone, but the P30 Pro is more mainstream one (even though it is quite expensive at around $1,200). Besides the graphene cooling film, the P30 Pro sports a 6.47" 1080x2340 flexible AMOLED display, an under-the-display fingerprint sensor, a Kirin 980 chipset, 6/8 GB of RAM, 64/128/256/512 GB of storage a Nano Memory card slot and a high-end quad-camera setup.
China-based Wuxi Graphene Film (owned by Grahope New Materials and The Sixth Element) produces patterned CVD graphene films for heating applications. These films are adopted by several Chinese device makers for different heating products - for example Grahope's graphene eye mask we recently reviewed at Graphene-Info.
The team at Wuxi Graphene Film was kind enough to send a few such films for us to review. These specific films are designed for one of WGF's customers and include a proprietary design which includes a USB connector for easy setup - you just plug these into a USB power source and the films heat up very quickly.
GNM focuses on the R&D of graphene heating technology and the development of graphene products in general. GNM adopts WGF's CVD graphene films in its products - for example in the graphene eye mask we recently reviewed at Graphene-Info.
Researchers at the Norwegian University of Science and Technology (NTNU) in Norway, Sophia University in Japan and SINTEF Industry, Norway have demonstrated the use of graphene as both a growth substrate and transparent conductive electrode for an ultraviolet light-emitting diode.
The team focused on a flip-chip configuration, where GaN/AlGaN nanocolumns were grown as the light-emitting structure using plasma-assisted molecular beam epitaxy. Although the sheet resistance increased after nanocolumn growth compared with pristine double-layer graphene, the experiments showed that the double-layer graphene functioned adequately as an electrode. The GaN/AlGaN nanocolumns were found to exhibit a high crystal quality with no observable defects or stacking faults.