题 目：Thermal Conductivity of Two-Dimensional Materials
时 间：6月2日（周五 ），上午9:30-10:30
Two-dimensional (2-D) materials, such as graphene, black phosphorus and transition metal dichalcogenides, have attracted increased interest due to their potential applications in electronic, optoelectronic, and energy systems. Understanding the phononic thermal properties in 2-D materials could be very important for the design of novel devices using 2-D materials.
In the first part of this talk, the first-principles based Boltzmann transport equation approach is developed to predict a series of novel 2-D materials, including silicene and single-layer transition metal dichalcogenides (TMDs). Their thermal conductivities are found to be highly correlated to their crystal structure and atomic masses. Using the same approach, we also study the layer thickness-dependence of thermal conductivity of MoS2. Unlike conventional thin film materials, whose thermal conductivity is usually suppressed when the thickness decreases due to phonon-boundary scattering, the thermal conductivity of MoS2 decreases when increasing its thickness. It appears that both the phonon dispersion and the anharmonicity changes with the thickness of MoS2.
In the second part of this talk, a variable spot size time-domain thermoreflectance (TDTR) approach is developed to systematically measure both the basal plane and cross plane thermal conductivity, Kr and Kz, of four TMD crystals, MoS2, MoSe2, WS2 and WSe2, over a wide range of temperature at 80 – 300 K. We consistently observed frequency dependence in both the through-plane thermal conductivity Kz and the Al/TMD interface thermal conductance G for all these TMD compounds, which we attributed to the non-equilibrium thermal resistance between different groups of phonons in the substrate during the TDTR experiments.
Dr. Ronggui Yang is a Professor of Mechanical Engineering directing the Nano-enabled Energy Conversion, Storage, and Thermal Management Systems group (NEXT) at the University of Colorado Boulder (CU-Boulder). Dr. Yang received his Ph.D degree focusing on Nanoscale Heat Transfer with Professor Gang Chen in Mechanical Engineering from MIT in February 2006. His innovative research has won him numerous awards including the 2014 ITS Young Investigator in Thermoelectrics from International Thermoelectric Society (ITS), the 2010 ASME Bergles-Rohsenow Young Investigator Award in Heat Transfer, an NSF CAREER Award in 2009, the MIT Technology Review’s TR35 Award and the DARPA Young Faculty Award in 2008. Dr. Yang is also well recognized for his professional services. Dr. Yang is currently the Chair (2015-2017) of the K-9 Technical Committee on Nanoscale Thermal Transport of ASME Heat Transfer Division. He is also an Associate Editor for ASME Journal of Heat Transfer and and Associate Editor for Heat Transfer Research. He was elected ASME Fellow in 2015.