Breaking Through Bottlenecks for Thermally Conductive Polymer Composites:A Perspective for Intrinsic Thermal Conductivity,Interfacial Thermal Resistance and Theoretics

Rapid development of energy,electrical and electronic technologies has put forward higher requirements for the thermal conductivities of polymers and their composites.However,the thermal conductivity coefficient(λ)values of prepared thermally conductive polymer composites are still difficult to achieve expectations,which has become the bottleneck in the fields of thermally conductive polymer composites.Aimed at that,based on the accumulation of the previous research works by related researchers and our research group,this paper proposes three possible directions for breaking through the bottlenecks:(1)preparing and synthesizing intrinsically thermally conductive polymers,(2)reducing the interfacial thermal resistance in thermally conductive polymer composites,and(3)establishing suitable thermal conduction models and studying inner thermal conduction mechanism to guide experimental optimization.Also,the future development trends of the three above-mentioned directions are foreseen,hoping to provide certain basis and guidance for the preparation,researches and development of thermally conductive polymers and their composites.

Interfacial thermal resistance between high-density polyethylene(HDPE) and sapphire

To improve the thermal conductivity of polymeric composites, the numerous interfacial thermal resistance （ITR） inside is usually considered as a bottle neck, but the direct measurement of the ITR is hardly reported. In this paper, a sandwich structure which consists of transducer/high density polyethylene （HDPE）/sapphire is prepared to study the interface characteristics. Then, the ITRs between HDPE and sapphire of two samples with different HDPE thickness values are measured by time-domain thermoreflectance （TDTR） method and the results are -- 2 × 10-7 m2.K.W-1. Furthermore, a model is used to evaluate the importance of ITR for the thermal conductivity of composites. The model＇s analysis indicates that reducing the ITR is an effective way of improving the thermal conductivity of composites. These results will provide valuable guidance for the design and manufacture of polymer-based thermally conductive materials.

Self-Modifying Nanointerface Driving Ultrahigh Bidirectional Thermal Conductivity Boron Nitride-Based Composite Flexible Films

While boron nitride(BN) is widely recognized as the most promising thermally conductive filler for rapidly developing high-power electronic devices due to its excellent thermal conductivity and dielectric properties,a great challenge is the poor vertical thermal conductivity when embedded in composites owing to the poor interracial interaction causing severe phonon scattering.Here,we report a novel surface modification strategy called the "self-modified nanointerface" using BN nanocrystals(BNNCs) to efficiently link the interface between BN and the polymer matrix.Combining with ice-press assembly method,an only 25 wt% BNembedded composite film can not only possess an in-plane thermal conductivity of 20.3 W m-1K-1but also,more importantly,achieve a through-plane thermal conductivity as high as 21.3 W m-1K-1,which is more than twice the reported maximum due to the ideal phonon spectrum matching between BNNCs and BN fillers,the strong interaction between the self-modified fillers and polymer matrix,as well as ladder-structured BN skeleton.The excellent thermal conductivity has been verified by theoretical calculations and the heat dissipation of a CPU.This study provides an innovative design principle to tailor composite interfaces and opens up a new path to develop high-performance composites.

Thermal rectification induced by Wenzel–Cassie wetting state transition on nano-structured solid–liquid interfaces

Thermal rectification refers to the phenomenon by which the magnitude of the heat flux in one direction is much larger than that in the opposite direction.In this study,we propose to implement the thermal rectification phenomenon in an asymmetric solid–liquid–solid sandwiched system with a nano-structured interface.By using the non-equilibrium molecular dynamics simulations,the thermal transport through the solid–liquid–solid system is examined,and the thermal rectification phenomenon can be observed.It is revealed that the thermal rectification effect can be attributed to the significant difference in the interfacial thermal resistance between Cassie and Wenzel states when reversing the temperature bias.In addition,effects of the liquid density,solid–liquid bonding strength and nanostructure size on the thermal rectification are examined.The findings may provide a new way for designs of certain thermal devices.

Unusual thermal properties of graphene origami crease:A molecular dynamics study

Graphene is a two-dimensional material that can be folded into diverse and yet interesting nanostructures like macro-scale paper origami.Folding of graphene not only makes different morphological configurations but also modifies their mechanical and thermal properties.Inspired by paper origami,herein we studied systemically the effects of creases,where sp^(2)to sp^(3)bond transformation occurs,on the thermal properties of graphene origami using molecular dynamics(MD)simulations.Our MD simulation results show that tensile strain reduces(not increases)the interfacial thermal resistance owing to the presence of the crease.This unusual phenomenon is explained by the micro-heat flux migration and stress distribution.Our findings on the graphene origami enable the design of the next-generation thermal management devices and flexible electronics with tuneable properties.

Accomplishment and challenge of materials database toward big data

The history and current status of materials data activities from handbook to database are reviewed, with introduction to some important products. Through an example of prediction of interfacial thermal resistance based on data and data science methods, we show the advantages and potential of material informatics to study material issues which are too complicated or time consuming for conventional theoretical and experimental methods. Materials big data is the fundamental of material informatics. The challenges and strategy to construct materials big data are discussed, and some solutions are proposed as the results of our experiences to construct National Institute for Materials Science（NIMS） materials databases.

Thermal transport in organic/inorganic composites

Composite materials, which consist of organic and inorganic components, are widely used in various fields because of their excellent mechanical properties, resistance to corrosion, low-cost fabrication, etc. Thermal properties of organic/inorganic composites play a crucial role in some applications such as thermal interface materials for micro-electronic packaging, nano-porous materials for sensor development, thermal insulators for aerospace, and high-performance thermoelectric materials for power generation and refrigeration. In the past few years, many studies have been conducted to reveal the physical mechanism of thermal transport in organic/ inorganic composite materials in order to stimulate their practical applications. In this paper, the theoretical and experimental progresses in this field are reviewed. Besides, main factors affecting the thermal conductivity of organic/ inorganic compositcs are discussed, including the intrinsic properties of organic matrix and inorganic fillers, topolo- gical structure of composites, loading volume fraction, and the interfacial thermal resistance between fillers and organic matrix.

The lattice Boltzmann Peierls Callaway equation for mesoscopic thermal transport modeling

The lattice Boltzmann Peierls Callaway(LBPC)method is a recent development of the versatile lattice Boltzmann formalism aimed at a numerical experiment on mesoscale thermal transport in a multiphase phonon gas.Two aspects of mesoscopic thermal trans-port are discussed:the finite phonon mean free path and the interface thermal resistance.Based on the phonon momentum screening length measured in the LBPC computa-tional apparatus,the validity of the Umklapp collision relaxation time in the Callaway collision operator is examined quantitatively.The discrete nature of the spatio-temporal domain in the LBPC method,along with the linear approximation of the exponential screening mechanism in the Callaway operator,reveals a large discrepancy between the effective phonon mean free path and the analytic phonon mean free path when the relaxation time is small.The link bounce back interface phonon collision rule is used to realize the interface thermal resistance between phonon gases with dissimilar dispersion relations.Consistent with the Callaway collision operator for the bulk phonon dynam-ics,the interface phonon collision process is regarded as a linear relaxation mechanism toward the local pseudo-equilibrium phonon distribution uniquely defined by the energy conservation principle.The interface thermal resistance is linearly proportional to the relaxation time of the proposed phonon interface collision rule.

Tuning graphene thermal modulator by rotating

Exploring the thermal transport of graphene is significant for the application of its thermal properties.However,it is still a challenge to regulate the thermal conductivity of graphene interface.We study the interfacial thermal transport mechanism of the bilayer graphene by utilizing the molecular dynamics simulations.During the simulation,the interfacial thermal conductivity is regulated and controlled by lattice matching and tailoring.The lattice mismatched bilayer graphene model,combining the straining and torsion,can increase the interfacial thermal resistance(ITR)about 3.7 times.The variation trend of the ITR is explained by utilizing the vibrational spectra and the overlap factor.Besides,the thermal conductivity is proportional to the overlapping area.Our results show that the tailoring models can regularly control the thermal conductivity in a wide range by twisting the angle between upper and lower layers.These findings can provide a guideline for thermoelectric manage-ment and device design of thermal switch.

Molecular Dynamics Simulations of the Effects of Surface Sinusoidal Nanostructures on Nanoscale Liquid Film Phase-Change

Nanostructures on heat transfer surfaces can enhance micro/nanoscale phase-change heat transfer processes. To understand the enhancement mechanisms provided by nanostructures, evaporation processes of nanoscale liquid films on periodic sinusoidal-shaped surfaces were investigated via molecular dynamics simulations. By changing the amplitudes(A) and periods(T) of the curves describing the sinusoidal shapes, which possess shapes similar to f(x)=h0+Asin(2πx/T), sinusoidal surfaces of different sizes were constructed. An unevaporated region always existed on the copper plate for all surfaces during phase-change processes. Moreover, we calculated the interfacial thermal resistances and the mismatches between the vibrational density of states of the solid and liquid for different surfaces. The results show that the argon temperature changes and evaporation rates during the phase-change process on sinusoidal surfaces are higher than those on flat surfaces, and these results increase with the increase in amplitudes and the decrease in periods within certain limits mainly because of the thermal resistance decrease at the solid-liquid interface. Furthermore, the corresponding mismatches between the vibrational density of states of the solid and liquid also decrease, which indicates that the existence of sinusoidal nanostructures enhances the heat transfer of the phase-change process.

A Molecular Analysis of Critical Factors for Interface and Size Effects on Heat Conduction in Nanoconfined Water Film

Heat conduction of nanoconfined liquid may differ from its bulk because of the effects of size,geometry,interface,temperature,etc.In this study,the roles of some critical factors for the heat conduction of nanoconfined water film are systematically analyzed by using the molecular dynamics method.With decreasing thickness,the normal thermal conductivity of nanoconfined water film between two copper plates decreases exponentially,while the thermal resistance,peak of the radial distribution function,and atomistic heat path increase exponentially.The average bond order,radial distribution function,mean squared displacement,and vibrational density of states are calculated to analyze the effects of structure,distribution,molecular diffusion,and vibration of water molecules on heat conduction especially in a region having no oxygen atoms(which is observed by the near-wall density profile).The results show that phonon scattering is dominant for determining the reduced thermal conductivity in this near-wall region.The thermal conductivity ratio of confined water film to bulk water has a roughly linear relationship with the logarithm of the proportion of the near-wall region.Moreover,the high interfacial thermal resistance is positively correlated to the film thickness,but it has a negligible impact on heat conduction.This work provides insights into the contribution of water molecules near the solid/liquid interface to the heat conduction of nanoconfined liquid for process intensification.

Mechanically exfoliated MoS_(2) nanoflakes for optimizing the thermoelectric performance of SrTiO_(3)-based ceramic composites

As a semiconducting material with relatively low thermal conductivity,MoS_(2) nanoflake has the potential to serve as a modulator for optimizing the performance of thermoelectric(TE)materials.However,the low yield of MoS_(2) nanoflakes prepared by conventional methods has constrained the development of MoS_(2) optimized TE materials.We propose a mechanical exfoliation method for mass production of MoS_(2) nanoflakes using attrition mill.After mixed with La and Nb co-doped SrTiO_(3)(SLNT)powder,the MoS_(2)/SLNT composites are fabricated by spark plasma sintering.It is found that the heterojunctions formed at MoS_(2)/SLNT interfaces with proper band offset can effectively scatter the low-energy electrons,resulting in enhanced Seebeck coefficient without significantly undermining the electrical conductivity.The power factor of composites is improved when the MoS_(2) content is lower than 1.5 vol%.Meanwhile,the thermal conductivity of composites is significantly decreased due to the phonon scattering induced large thermal resistance at MoS_(2)/SLNT interfaces,which is much higher than that in graphene embedded SrTiO_(3) composites.Consequently,a maximum ZT-0.24 is obtained at 800 K in 1.5 vol%MoS_(2)/SLNT composite,which is~26%higher compared with pristine matrix.This work paves the way for application of TE materials modulated by transition metal dichalcogenides.