Wu-Xing Zhou

An important mechanism for thermal rectification in graded nanowires

In the quest for the origin of the different thermal rectifying behavior of two graded nanowires, we reveal the important role that standing waves play in the thermal transport properties of such graded structures. Evidence for the existence of standing waves is given from two angles, and one possible scenario of the origin of the standing wave is presented. The key point is that the formation of the standing wave, which greatly hinders the propagation of phonon waves, occurs only when the narrow end of the nanowire is at a higher temperature than the wide end, making the heat current flow preferably from the wide end to the narrow end.

Phonon wave interference in graphene and boron nitride superlattice

The thermal transport properties of the graphene and boron nitride superlattice (CBNSL) are investigated via nonequilibrium molecular dynamics simulations. The simulation results show that a minimum lattice thermal conductivity can be achieved by changing the period length of the superlattice. Additionally, it is found that the period length at the minimum shifts to lower values at higher temperatures, and that the depth of the minimum increases with decreasing temperature. In particular, at 200 K, the thermal conductivities of CBNSLs with certain specific period lengths are nearly equal to the corresponding values at 300 K. A detailed analysis of the phonon spectra shows that this anomalous thermal conductivity behavior is a result of strong phonon wave interference. These observations indicate a promising strategy for manipulation of thermal transport in superlattices.

Core-shell nanowire serves as heat cable

To analyze the thermal transport properties in core-shell nanowires, we calculate systematically the distributions of heat flux in InAs/GaAs and GaAs/InAs core-shell nanowires by using nonequilibrium molecular dynamics simulations. The results show that for InAs/GaAs core-shell nanowires, the heat current tends to transport in the shell, while for GaAs/InAs core-shell nanowires the heat current tends to transport through the core. Moreover, a simple equation is presented to describe the relationship of the thermal conductance among the core, the tubular shell, and core-shell nanowire. It is suggested that the core-shell nanowires can be served as heat cable.

Conjunction of standing wave and resonance in asymmetric nanowires: a mechanism for thermal rectification and remote energy accumulation

As an important way to control and manage heat transport, thermal rectification has become an elementary issue in the field of phononics and plays a key role in the designing of thermal devices. Here we investigate systematically the standing wave and the accompanying resonance process in asymmetric nanowires to understand the standing wave itself and its great effect on thermal rectification. Results show that the standing wave is sensitive to both the structural and thermal properties of the material, and its great effect on enhancing the thermal rectification is realized not only by the energy-localization nature of the standing wave, but also by the resonance-caused large amplitude and high energy of the standing wave.

The thermal conductivity in hybridised graphene and boron nitride nanoribbons modulated with strain

Thermal transport properties in hybridised graphene and boron nitride ribbons (HGBNRs) under different strains are studied by using reverse nonequilibrium molecular dynamics simulations. It is found that the effect of strains on the thermal conductivity is different for different types of strains. When the tensile and shear strains are applied, the thermal conductivity can be modulated at least up to 50% at room temperature as the strain e ranges from 0 to 0.2. However, when the compressive and flexural strains are respectively applied, the thermal conductivity is insensitive to the variation of the strain. In addition, it is also found that the thermal conductivity of HGBNRs depends sensitively on the dimension of the hybridised ribbon and the relative amount of h-BN to graphene. A brief analysis of these results is given.

Triggering piezoelectricity directly by heat to produce alternating electric voltage

Piezoelectricity has proved itself a promising mechanism for energy conversion and signal sensing by taking advantage of its ability to convert mechanical energy into electricity. Here, we demonstrate that the piezoelectricity in free-standing non-centrosymmetric nanowires can also be triggered directly by heat to produce electricity. The feasibility of the idea is first analyzed by the dynamic theory of crystal lattices and then confirmed by molecular dynamics simulations. The most salient point is that the heat-induced voltage drop across the cross section of the free-standing nanowires alternates periodically with the vibration of the nanowire. Moreover, the electric potential induced by heat here (as large as 0.34 V) is proved to be comparable with the previously reported potentials induced by mechanical energy, and the maximum value can be tuned by controlling the size of the nanowire and the applied heat.

Excellent Thermoelectric Properties in monolayer WSe 2 Nanoribbons due to Ultralow Phonon Thermal Conductivity

By using first-principles calculations combined with the nonequilibrium Green’s function method and phonon Boltzmann transport equation, we systematically investigate the influence of chirality, temperature and size on the thermoelectric properties of monolayer WSe2 nanoribbons. The results show that the armchair WSe2 nanoribbons have much higher ZT values than zigzag WSe2 nanoribbons. The ZT values of armchair WSe2 nanoribbons can reach 1.4 at room temperature, which is about seven times greater than that of zigzag WSe2 nanoribbons. We also find that the ZT values of WSe2 nanoribbons increase first and then decrease with the increase of temperature, and reach a maximum value of 2.14 at temperature of 500 K. It is because the total thermal conductance reaches the minimum value at 500 K. Moreover, the impact of width on the thermoelectric properties in WSe2 nanoribbons is not obvious, the overall trend of ZT value decreases lightly with the increasing temperature. This trend of ZT value originates from the almost constant power factor and growing phonon thermal conductance.

Excellent thermoelectric properties induced by different contact geometries in phenalenyl-based single-molecule devices

We investigated the thermoelectric properties of phenalenyl-based molecular devices by using the non-equilibrium Green’s function method combined with density function theory. The results show that the thermoelectric performance of molecular device can be significantly improved by different contact geometries. The ZT value of the device can reach 1.2 at room temperature, which is two orders of magnitude higher than that of graphene. Moreover, the change of the coupling between molecule and electrodes can also enhance the ZT value. The ZT value can be further optimized to 1.4 at 300 K and 5.9 at 100 K owing to the decrease of electronic thermal conductance and almost unchanged power factor.