Zhichun Liu

A Revisit to High Thermoelectric Performance of Single-layer MoS2

Both electron and phonon transport properties of single layer MoS2 (SLMoS2) are studied. Based on first-principles calculations, the electrical conductivity of SLMoS2 is calculated by Boltzmann equations. The thermal conductivity of SLMoS2 is calculated to be as high as 116.8 Wm−1K−1 by equilibrium molecular dynamics simulations. The predicted value of ZT is as high as 0.11 at 500 K. As the thermal conductivity could be reduced largely by phonon engineering, there should be a high possibility to enhance ZT in the SLMoS2-based materials.

Extremely High Thermal Conductivity of Aligned Carbon Nanotube-Polyethylene Composites

The ultra-low thermal conductivity of bulk polymers may be enhanced by combining them with high thermal conductivity materials such as carbon nanotubes. Different from random doping, we find that the aligned carbon nanotube-polyethylene composites has a high thermal conductivity by non-equilibrium molecular dynamics simulations. The analyses indicate that the aligned composite not only take advantage of the high thermal conduction of carbon nanotubes, but enhance thermal conduction of polyethylene chains.

Experimental Investigation of New Flat-Plate-Type Capillary Pumped Loop

A capillary pumped loop is a two-phase heat transfer device which has been increasingly applied in thermal control of satellite or spacecraft and electronic cooling. However, startup performance of a normal capillary pumped loop is poor and hydrodynamic oscillation in the system appears sometime during its operation. A flat-plate-type capillary pumped loop is constructed and tested in the present study, which combines the advantages of a capillary pumped loop and a loop heat pipe by improving both system and components. In the present capillary pumped loop, both evaporator and condenser are designed as a type of flat plate with porous wick, and a reservoir is used to control and adjust operation temperature of the system. In addition, a subcooler is adopted to improve the quality of working fluid flowing back to the evaporator. A plane type of evaporator with a cross channel for the liquid supply was made to reduce the probability of a dry-out point caused by strong evaporation in the case of high heat flux, and therefore to enhance the stability of the system. Also, a plane type of condenser with a porous wick is designed, which is of the three ports for vapor-pipe inlet, liquid-pipe outlet, and an adjusting pipe connected to a reservoir. The startup performance of the system benefits greatly from this three-port design, as liquid in the system can be easily pressed into a reservoir due to resistance reduction. The performance of a capillary pumped loop prototype was tested in startup, persistence operations, and different vacuum. Startup experiments under different heat loads and working conditions were conducted. Excellent startup performance, being stable and easy, has been shown for the plane-type capillary pumped loop system, which validates the new configuration. Pressure oscillation and temperature fluctuation in the system were reduced largely owing to the induction of a porous wick in the plane condenser. During experimental processes lasting more than 10 h, the new type capillary pumped loop shows outstanding thermal behavior and no dry-out phenomena were observed. The testing results indicate that 1) the new capillary pumped loop system shows excellent startup performance without pressure priming and liquid clearing of vapor line; and 2) the new capillary pumped loop system is stable under very severe operational conditions, such as low load, sudden power step, etc.

Coalescence-Induced Jumping of Nanodroplets on Textured Surfaces

Conducting experimental studies on nanoscale droplet coalescence using traditional microscopes is a challenging research topic, and views differ as to whether the spontaneous removal can occur in the coalescing nanodroplets. Here, a molecular dynamics simulation is carried out to investigate the coalescence process of two equally sized nanodroplets. On the basis of atomic coordinates, we compute the liquid bridge radii for various cases, which is described by a power law of spreading time, and these nanodroplets undergo coalescence in the inertially limited-viscous regime. Moreover, coalescence-induced jumping is also possible for the nanodroplets, and the attraction force between surface and water molecules plays a crucial role in this process, where the merged nanodroplets prefer to jump away from those surfaces with lower attraction force. When the solid-liquid interaction intensity and surface structure parameters are varied, the attraction force is shown to decrease with decreasing surface wettability intensity and solid fraction.

Effects of Solid Fraction on Droplet Wetting and Vapor Condensation: A Molecular Dynamic Simulation Study

Recently, numerous studies focused on the wetting process of droplets on various surfaces at a microscale level. However, there are a limited number of studies about the mechanism of condensation on patterned surfaces. The present study performed the dynamic wetting behavior of water droplets and condensation process of water molecules on substrates with different pillar structure parameters, through molecular dynamic simulation. The dynamic wetting results indicated that droplets exhibit Cassie state, PW state, and Wenzel state successively on textured surfaces with decreasing solid fraction. The droplets possess a higher static contact angle and a smaller spreading exponent on textured surfaces than those on smooth surfaces. The condensation processes, including the formation, growth, and coalescence of a nanodroplet, are simulated and quantitatively recorded, which are difficult to be observed by experiments. In addition, a wetting transition and a dewetting transition were observed and analyzed in condensation on textured surfaces. Combining these simulation results with previous theoretical and experimental studies will guide us to understand the hypostasis and mechanism of the condensation more clearly.

Impact of torsion and stretching on the thermal conductivity of polyethylene strands

A single polyethylene chain was reported to have a high metal-like thermal conductivity (TC), which stands in sharp contrast to the thermally insulating feature of common bulk polyethylene materials. This work numerically investigates the impact of torsion and stretching on the TC of polyethylene strands by using equilibrium molecular dynamics simulations. The simulation results show that torsion slightly reduces the TC of a single polyethylene chain. In contrast, the heat conduction of polyethylene strands could be slightly enhanced under torsional loading with a specific torsional angle. Particularly, an apparent improvement of TC of polyethylene strands is achieved by combining torsion and stretching functions. It is found that the TC of torsional polyethylene strands is sensitive to torsional patterns. Our study proposes a specific torsional pattern of polyethylene strands that significantly enhances the heat conduction of the original counterpart. This study will play an essential role in guiding the...

Tailoring Thermal Conductivity of Single-stranded Carbon-chain Polymers through Atomic Mass Modification.

Tailoring the thermal conductivity of polymers is central to enlarge their applications in the thermal management of flexible integrated circuits. Progress has been made over the past decade by fabricating materials with various nanostructures, but a clear relationship between various functional groups and thermal properties of polymers remains to be established. Here, we numerically study the thermal conductivity of single-stranded carbon-chain polymers with multiple substituents of hydrogen atoms through atomic mass modification. We find that their thermal conductivity can be tuned by atomic mass modifications as revealed through molecular dynamics simulations. The simulation results suggest that heavy homogeneous substituents do not assist heat transport and trace amounts of heavy substituents can in fact hinder heat transport substantially. Our analysis indicates that carbon chain has the biggest contribution (over 80%) to the thermal conduction in single-stranded carbon-chain polymers. We further demonstrate that atomic mass modifications influence the phonon bands of bonding carbon atoms, and the discrepancies of phonon bands between carbon atoms are responsible for the remarkable drops in thermal conductivity and large thermal resistances in carbon chains. Our study provides fundamental insight into how to tailor the thermal conductivity of polymers through variable substituents.

Nanodroplets Impact on Rough Surfaces: A Simulation and Theoretical Study

Impact of droplets is widespread in life, and modulating the dynamics of impinging droplets is a significant problem in production. However, on textured surfaces, the micromorphologic change and mechanism of impinging nanodroplets are not well-understood; furthermore, the accuracy of the theoretical model for nanodroplets needs to be improved. Here, considering the great challenge of conducting experiments on nanodroplets, a molecular dynamics simulation is performed to visualize the impact process of nanodroplets on nanopillar surfaces. Compared with macroscale droplets, apart from the similar relation of restitution coefficient with the Weber number, we found some distinctive results: the maximum spreading time is described as a power law of impact velocity, and the relation of maximum spreading factor with impact velocity or the Reynolds number is exponential. Moreover, the roughness of substrates plays a prominent role in the dynamics of impact nanodroplets, and on surfaces with lower solid fraction, the lower attraction force induces an easier rebound of impact nanodroplets. At last, on the basis of the energy balance, through modifying the estimation of viscous dissipation and surface energy terms, we proposed an improved model for the maximum spreading factor, which shows greater accuracy for nanodroplets, especially in the low-to-moderate velocity range. The outcome of this study demonstrates that a distinctive dynamical behavior of impinging nanodroplets, the fundamental insight, and more accurate prediction are very useful in the improvement of the hydrodynamic behavior of the nanodroplets.

Optimal Structural Design of a Heat Sink With Laminar Single-Phase Flow Using Computational Fluid Dynamics-Based Multi-Objective Genetic Algorithm

This paper proposes a general method combining evolutionary algorithm and decisionmaking technique to optimize the structure of a minichannel heat sink (MCHS). Two conflicting objectives, the thermal resistance h and the pumping power P, are simultaneously considered to assess the performance of the MCHS. In order to achieve the ultimate optimal design, multi-objective genetic algorithm is employed to obtain the nondominated solutions (Pareto solutions), while technique for order preference by similarity to an ideal solution (TOPSIS) is employed to determine which is the best compromise solution. Meanwhile, both the material cost and volumetric flow rate are fixed where this nonlinear problem is solved by applying the penalty function. The results show that h of Pareto solutions varies from 0.03707 K W 1 to 0.10742 K W , while P varies from 0.00307 W to 0.05388 W, respectively. After the TOPSIS selection, it is found that P is significantly reduced without increasing too much h. As a result, h and P of the optimal MCHS determined by TOPSIS are 35.82% and 52.55% lower than initial one, respectively. [DOI: 10.1115/1.4037643]

Spontaneous Migration of Polyethylene Molecule Sheathed inside Single-Walled Carbon Nanotube for Nano-Heat Pipe

Development of nanoscale thermal exchanging devices is critical to thermal management in nanoscale. The spontaneous migration of polyethylene molecule sheathed inside single-walled carbon nanotube (SWCNT) are observed. And the multi-factor analysis of spontaneous migration about temperature, mass and potential barrier shows new features about motion mechanisms, and enriches the existing mass transport theory greatly. Here, based on this finding, we report a nano-heat pipe (NHP) composing of a SWCNT and a polyethylene molecule. Using molecular dynamics simulations, the heat exchanging coefficient can reach 450 WK−1 cm−2 at 500 K by NHP arrays with a quantity density of 7 × 1013 cm−2. This study will benefit the designs of NHP and other nanoscale mass transport devices.