Xiulin Ruan, a professor of mechanical engineering, highlighted that graphene holds the distinction of being the first two-dimensional material ever created by humans. Consisting of a single layer of carbon atoms, each only one atom thick, graphene was initially discovered in 2004 and later earned the Nobel Prize for Physics in 2010. Since its discovery, numerous researchers have dedicated their efforts to studying graphene due to its exceptional and distinctive properties.
Graphene is renowned for its superior electrical conductivity, surpassing any other known material in science, and is recognized for its exceptional material strength. Early assessments by thermal transport researchers also labeled it as the top heat conductor among materials.
According to Zherui Han, a Ph.D. student in Professor Xiulin Ruan’s lab, prior to graphene, diamond was considered the material with the highest thermal conductivity—meaning it could transfer heat most efficiently. However, with the introduction of graphene, mainstream studies indicated that graphene exhibited even superior thermal conductivity compared to diamond.
The metric for measuring thermal conductivity is in watts per meter per Kelvin. Traditionally, a diamond’s thermal conductivity is estimated to be around 2,000 on this scale. However, when scientists began assessing graphene’s thermal conductivity, initial estimates exceeded 5,000. This notably piqued the interest of researchers like Professor Xiulin Ruan, who specializes in heat transfer studies.
(b) Contribution and percentage of out-of-plane acoustic (ZA) phonons to κ at room temperature, with a comparison to first principles calculated at 3ph (phonon renormalization, not considered previously) [9] and molecular dynamics (MD) work [29]. In both plots, the presented 3ph case is computed at N=180 without boundary scattering, and it’s important to note that convergence with N has not been achieved.
(Physical Review)
Professor Xiulin Ruan explained that initial estimates of graphene’s thermal conductivity exceeded 5,000, but subsequent experimental measurements and modeling have led to a more refined understanding. More recent studies have revised the thermal conductivity of graphene to around 3,000, still surpassing that of diamond. However, Ruan and the research team discovered something entirely unexpected in their investigation.
Professor Xiulin Ruan’s research team has forecasted that the thermal conductivity of graphene at room temperature is 1,300 W/(m K). This not only falls short of diamond but also ranks lower than the raw graphite material from which graphene is derived. The findings have been detailed in the publication in Physical Review B.
The divergence between the current study and previous research is attributed to a phenomenon known as four-phonon scattering. Phonons are the quantum-mechanical units used by heat transfer scientists to describe heat movement in solids. Historically, researchers could only account for three-phonon scattering when predicting heat transfer through solids.
In 2016, Professor Xiulin Ruan’s team introduced a comprehensive theory of four-phonon scattering, and a year later, they successfully quantified this four-phonon scattering phenomenon. This groundbreaking work earned Ruan the highest recognition from the International Phononics Society in 2023.
In the context of graphene, the significance lies in its two-dimensional structure, as explained by Zherui Han. Previous studies suggested that the two-dimensionality of graphene would impose restrictions on three-phonon scattering, theoretically making graphene more thermally conductive than bulk materials. However, the key insight from their work is that four-phonon scattering, unlike three-phonon scattering, is not constrained by the 2D nature of graphene; on the contrary, its impact is notably robust. The research reveals that four-phonon scattering emerges as the predominant scattering channel in graphene, surpassing three-phonon scattering. This discovery represents a noteworthy and unexpected outcome in the understanding of thermal properties in graphene.
The journey to this discovery faced a hurdle in the form of computing power availability. Calculating the intricacies of four-phonon scattering necessitated a parallel computing approach, specifically leveraging a computing cluster equipped with one terabyte of memory. This computational task was successfully executed at the Rosen Center for Advanced Computing at Purdue University.
It’s crucial to note that these calculations currently exist in the realm of theory. To validate their findings experimentally, the team collaborates with Professor Li Shi at the University of Texas at Austin, supported by joint National Science Foundation grants. Experimental verification becomes imperative as previous measurements on graphene exhibited substantial error margins that need refinement to validate the theoretical framework. Additionally, the team aims to extend their predictions to the thermal conductivity of graphene comprising multiple layers of atoms, moving beyond the single-layer focus.
Professor Xiulin Ruan acknowledges the skepticism surrounding their current theoretical predictions without experimental validation. Drawing parallels to their experience in 2017 when predicting aspects of boron arsenide, Ruan notes that initial skepticism was eventually dispelled when three crucial experiments confirmed their predictions a year later. Over time, their four-phonon scattering theory has garnered increasing support from experimental evidence. Despite the skepticism, the team is optimistic that similar validation will be achieved for graphene. To foster transparency and collaboration, they have made their software open source, inviting other scientists to scrutinize and test the four-phonon theory.
Zherui Han has shared his four-phonon thermal conductivity solver on GitHub, accompanied by a published paper detailing its application. The software is available for use by any heat transfer scientist interested in conducting similar research.
Zherui Han reflects on the initial perception of graphene as something akin to magic, with widespread belief in its exceptional properties across various domains. However, as thermal researchers, their responsibility is to critically evaluate these claims. Despite graphene’s continued standing as a proficient heat conductor, their research predicts that it doesn’t surpass the thermal conductivity of diamond.
Professor Xiulin Ruan emphasizes the importance of acknowledging exceptions in scientific progress. While cautiously optimistic about their findings, particularly in the context of four-phonon scattering, the team aspires to provide more precise theoretical assessments for materials in the future.
Resources
- ONLINE NEWS Pike, J. & Purdue University. (2023, November 28). Is graphene the best heat conductor? Researchers investigate with four-phonon scattering. Phys.org. [Phys.org]
- JOURNAL Han, Z., & Ruan, X. (2023). Thermal conductivity of monolayer graphene: Convergent and lower than diamond. Physical Review, 108(12). [Physical Review]
Cite this page:
APA 7: TWs Editor. (2023, November 28). Graphene and Heat Conduction: A Four-Phonon Scattering Investigation. PerEXP Teamworks. [News Link]
