Enhancement Of Thermal Transport Research Articles

Buoyant Convective Flow and Heat Dissipation of Cu–H2O Nanoliquids in an Annulus Through a Thin Baffle

This article numerically investigates the buoyant convective flow and thermal transport enhancement of Cu–H2O nanoliquid in a differentially heated upright annulus having a thin baffle. For the analysis, the outer and inner cylinders are cooled and heated respectively through insulated top and lower boundaries. Also, the baffle temperature is assumed to be that of the hot cylinder. The finite difference based numerical technique is used to solve the system of equations governing the physical processes. The findings are accessible in terms isotherms, streamlines and Nu number for wider ranges of baffle positions and lengths, Rayleigh numbers, and by considering different nanofluid (NF) volume fractions. The average Nu number is enhanced in addition of the Cu nanoparticle to the base liquid and it is also found the liquid flow and heat transport can be successfully controlled via the appropriate selection of baffle location and length. Principally, the baffle length having 20% of annular width placed at 80% of the annular height has been found to produce higher thermal transport rates as compared to other choices of baffle lengths and positions.

Unusual thermal performance in Cu-60Ag by WC nanoparticles

Nanoparticles have been widely used to tune the mechanical, thermophysical, and chemical properties of materials for widespread applications 1-3. Despite achieving other unprecedented functionalities, metals containing ceramic nanoparticles often suffer a significant loss in thermal conductivity with theoretical upper limits set by the Hashin-Shtrikman (with interfacial interactions) and Weiner (without interfacial interactions) models. Here, we discover that the Cu-60Ag alloy with 10 vol% WC nanoparticles delivers a surprisingly high thermal conductivity, breaking the theoretical upper limits. To understand the underlying mechanisms, thermal transport contributions from phonons and electrons, as well as microstructure, are studied. The study suggests that there exists an unusual enhancement of thermal transport in Cu-60Ag by the less conductive WC nanoparticles. Though thermal analysis including 3ω-method may be needed to decouple influencing factors, this new finding is of significance for development and application of nanoparticles reinforced metals with both superior mechanical strength and high thermal conductivity.

Molecular dynamics simulations of separator-cathode interfacial thermal transport in a Li-ion cell

While Li-ion cells show outstanding electrochemical performance, their poor thermal transport characteristics result in reduced performance as well as significant safety concerns. The heterogeneous interface between cathode and separator plays a vital role in the process of thermal conduction in a Li-ion cell. Recent experiments have shown that the cathode-separator interfacial thermal resistance contributes significantly to total thermal resistance within the cell. In this paper, thermal conductance across the cathode-separator interface is calculated using molecular dynamics (MD) simulations with IFF-CVFF force field. Thermal transport in a pristine heterogeneous interface as well as when bridged with 3-Aminopropyl triethoxysilane (APTES), n-Butyl trimethoxysilane (nBTMS) and 3-Mercaptopropyl trimethoxysilane (MPTMS) molecules is computed. It is shown that molecular bridging at the interface results in up to 250% improvement in interfacial thermal conductance for the APTES case, which is consistent with recent experimental data. These results quantify the key role of the cathode-separator interface in thermal transport within the Li-ion cell, as well as the potential improvement in interfacial thermal transport by molecular bridging. The techniques and results discussed here may help downselect molecular species for interfacial thermal transport enhancement in Li-ion cells.

Thermal transport enhancement of hybrid nanocomposites; impact of confined water inside nanoporous silicon

The thermal transport properties of porous silicon and nano-hybrid “porous silicon/water” systems are presented here. The thermal conductivity was evaluated using the equilibrium molecular dynamics technique for porous systems made of spherical voids or water-filled cavities. We revealed large thermal conductivity enhancement in the nano-hybrid systems as compared to their dry porous counterparts, which cannot be captured by effective media theory. This rise of thermal conductivity is related to the increase in the specific surface area of the liquid/solid interface. We demonstrated that the significant difference, more than two folds, in thermal conductivity of pristine porous silicon and “porous silicon liquid–composite” is due to the liquid density fluctuation close to the “solid–liquid interface” (layering effect). This effect is getting more importance for the large specific surface of the interfacial area. Specifically, the enhancement of the effective thermal conductivity is 50% for a specific surface area of 0.3 (1/nm), and it increases further upon the increase in the surface to volume ratio. Our study provides valuable insights into the thermal properties of hybrid liquid/solid nanocomposites and into the importance of confined liquids within nanoporous materials.

Self-propelled nanofluids a coolant inspired from nature with enhanced thermal transport properties

Self-propelled nanofluids (SPNFs), are suspensions that contain active particles, that self-propel by converting some form of energy to mechanical work. This theoretical investigation considers the heat transfer mechanisms that may exist in an SPNF. Equations describing the effective mass diffusivity of spherical and rod-shaped particles were taken from the literature and used in analysis of particles of different shapes, aspect ratios, swimming velocities and suspended in water and in ethylene glycol. The analysis showed that the effective mass diffusivity of the particles was up to three orders of magnitude higher than the thermal diffusivity of the solvent. The enhancement of the mass diffusivity leads to a significant enhancement of thermal conductivity of up to an order of magnitude higher compared to that of the pure solvent. It is further discussed that in SPNFs, dispersion and clustering of particles and active turbulence mechanisms may add extra enhancement to the thermal transport. Recent experimental investigations supporting our findings are discussed. Combining this enhancement in thermal transport with the reported reduction of the viscosity observed for an SPNF consisting of Artificial Bacterial Flagella (ABF) particles, will help to create a highly efficient coolant. This can help to reduce energy consumption in a wide variety of the economic sectors, especially in the computing and data storage.

Three-Dimensional Thermo-Hydraulic Analysis of Solar Air Heater With Equilateral Prism-Shaped Rib Roughness

Abstract Thermal transport and flow friction characteristics due to roughness on the absorber plate of solar air heater are evaluated by applying three-dimensional finite volume based code. Renormalization group (RNG) k–ɛ model is employed to capture the turbulent nature of the flow. The effect of equilateral prism-shaped rib roughness geometrical parameters in terms of relative roughness height (e/D) and relative roughness pitch (p/e) on heat transfer and flow friction is analyzed. Further, the effect of flow parameter, Re in the range of 4000–18,000 is also explored. Results are elucidated in terms of average Nusselt number, friction factor, turbulent kinetic energy, and eddy dissipation. Results are compared with a smooth absorber plate solar air heater. Thermo-hydraulic performance of the roughened solar air heater is analyzed. Noteworthy augmentation in heat transport is obtained. The thermal enhancement factor is calculated for optimal performance and found to vary from 1.7 to 3.5. However, friction factor and pressure loss for roughened plate is significantly higher than its smooth counterpart. The pressure drop across the test section increases with the rise in roughness height due to flow obstruction. A minimum value of the friction factor enhancement ratio worth 2.13 is obtained. Enhancement in thermal transport and pressure losses are combined by introducing a thermo-hydraulic performance factor (THHP). For the range of parameters investigated, the optimum value of the thermo-hydraulic performance factor is found to be 3.41. Correlations for average Nusselt number and friction factor are offered at the end.

Geometry and orientation effects in non-uniformly heated microchannel heat exchangers using supercritical carbon dioxide

There is growing interest to use supercritical carbon dioxide (sCO2) as a working fluid in thermal management applications. This study investigates the thermal-hydraulic performance of microchannel heat sinks as a function of flow channel geometry and orientation at operating conditions representative of electronics cooling applications. Three different experimental test sections, subject to non-uniform heat flux boundary conditions, were investigated. Two of the test sections contained parallel arrays of rectangular microchannels with a hydraulic diameter of 750 μm and aspect ratios of 1 and 2, respectively. The third test section had a staggered array of diamond shaped micro-pins with a hydraulic diameter, based on the minimum flow area, of 525.2 μm. Data were collected for varying inlet temperature (16⩽Tin⩽50 °C), mass flux (315⩽G⩽1000 kg m−2 s−1), and heat flux (20⩽q″⩽50 W cm−2) at a fixed reduced pressure (PR) of 1.1. A data analysis method using 2-D and 3-D heat transfer models of the test sections was used to calculate the average heat transfer coefficients for each experimental condition. Additionally, a pressure drop model was developed to resolve the total measured pressure drop into its components. The results of this study indicate that the turbulent convective heat transfer was independent of orientation (top versus bottom heating) for square microchannel (aspect ratio = 1) for the conditions investigated. Increasing the channel aspect ratio from 1 to 2 led to an enhancement in thermal transport. Finally, the heat transfer performance of the staggered pin array flow geometry was superior to the rectangular channels, but this enhancement in heat transfer was commensurate with the increase in pressure drop. Based on these results, this paper concludes with general design recommendations for those considering the early adoption of supercritical carbon dioxide for thermal management applications.

(Invited) Engineered Particle-Particle Contacts for Improved Thermal Interface Materials

I present my group’s ongoing work to use liquid metal to overcome thermal transport bottlenecks in polymer composites. The central premise behind this effort is that liquid metal-coated particle inclusions will have enlarged particle-particle contact regions relative to the point-like contacts between non-coated particle inclusions. In effect, the LM functions as a thermally conductive “solder” between particles. Polymer composites play an important role for thermal interface materials (TIMs). TIMs improve thermal transport by filling micro air gaps between the rough surfaces of adjacent components (e.g., between a computer chip and heat spreader). These applications require mechanically compliant materials such as polymers that unfortunately also suffer from low thermal conductivity. Consequently high thermal conductivity solid particles are typically added to the polymer to boost thermal transport. Despite these efforts, TIM conductivities still usually fall below 2 W m-1 K-1because the small point-like contacts between solid filler particles lead to large thermal contact resistances that bottleneck thermal transport. I first present our work on polymer composites with nickel-based particle inclusions. We explore bare nickel particles, nickel particles coated with silver, and nickel particles coated with liquid metal. These coatings impart mechanical compliance to the surface of the hard nickel particles and lead to a 3-fold increase in thermal conductivity of the polymer composite. These studies also reveal that a nanometer-scale native oxide coating on the liquid metal impedes thermal transport. In order to improve thermal transport for liquid-metal coated particles, we next focus on methods to physically break the native oxide. Rupturing this thin oxide allows liquid metal to flow and yields improved contact between particles. To address this challenge, we conduct a series of thermal-mechanical studies on liquid metal droplets. In order to focus on the key physics, we create beds of liquid metal droplets with their native oxides intact (i.e. no polymer matrix and no solid particle core). We then study thermal transport as a function of applied mechanical pressure. We observe an enhancement in thermal transport that accompanies the rupture of the native oxide. We also find that the oxide rupture follows theoretical predictions for failure of thin-walled pressure vessels. Lastly, we chemically modify the native oxide shell to reduces its strength, promote oxide rupture, and improve thermal transport.

Constructal enhancement of thermal transport in latent heat storage systems assisted with fins

The aim of this study is to augment thermal transport in phase change materials (PCM) by an optimum installation of the fins. The study emphasizes to obtain best size and location of the fins to minimize the total melting time for the given fins to PCM volume ratio. Thus, the fins are resized and re-positioned according to the constructal theory. The fundamental principle of the theory is to configure limited numbers of fins at optimal position, where thermal transport potential is maximum. A numerical code based on conjugate heat transfer coupled enthalpy porosity approached is developed. The numerical results show that the reduction in size of fins at low temperature gradient and re-positioning them at high temperature gradient are found to be advantageous. In the present study, the fin to PCM volume is reduced by half based on the constructal theory. The proposed constructal fin design augments thermal performance by 4.38% than conventional configuration. The reduction in fins volume by half has not only increased thermal energy storage, but also decreased fins solid mass that can help to reduce the overall weight of thermal energy storage systems. It is also concluded that the fins should not be placed much near to the bottom surface to gain uniform thermal penetration throughout the enclosure.

A study of the heat transfer properties of CIP doped magnetorheological elastomers

This work investigates the enhancement of thermal transport in polydimethylsiloxane (PDMS) thin films with aligned carbonyl iron particles (CIP) embedded, also known as magnetorheological elastomers (MREs). The flash method is used to measure the effective thermal conductivity of PDMS/CIP thin films in the direction parallel to the magnetic field. Under an external magnetic field, the CIPs form chain-like structures, which become effective thermal paths inside the composites. Hence, a significant increase in the thermal conductivity of aligned MREs has been achieved compared with the composites prepared without a magnetic field. The effects of the volume fraction of CIP and pre-structured magnetic flux density have also been studied. The increase in the CIP volume fraction from 5% to 20% brings about a twofold improvement in the thermal conductivity of the aligned MREs. For 2.5 μm CIP doped MREs with a volume fraction of 10%, when the pre-structured magnetic flux density changes from 0 mT to 100 mT, the thermal conductivity k increases by approximately 50%. However, a further increase in the magnetic field intensity only leads to a slight increase in k. Scanning electron microscopy (SEM) inspections demonstrate that the chain length keeps growing with the strength of the pre-structured magnetic field while the structure transition turns into lateral congregation as the strength of the pre-structured magnetic field reaches 100 mT. A finite element method (FEM) model is proposed to investigate in detail the relationship between the MREs’ thermal conductivity and inner chains. This work provides an effective way to improve thermal conductivity of MREs used for electromagnetic radio-absorbers and builds a connection between the thermal properties of MREs and their inner microstructures.

Nonthermal plasma synthesis of silicon nanoparticles and their thermal transport properties

Silicon-based nanostructured materials have been studied to utilize and observe the novel properties of silicon nanoparticles (SiNPs). Nonthermal plasma was employed to synthesize SiNPs, producing particles below 10 nm with narrow size distribution. The synthesis process of nonthermal plasma is discussed including the parameters that determine the particle size, yield and crystallinity of SiNPs. In this study, nanostructured material of SiNP/polystyrene (PS) nanocomposite has been observed to have a significant decrease in thermal conductivity. In this paper, post-processing of thermal annealing was applied, and the thermal conductivity measurement result shows the increase of thermal conductivity values. Although there is no significant enhancement of SiNP dispersion in the PS matrix, the thermal annealing induced molecular structure change, resulting in thermal transport enhancement at the interfacial region. The preservation of unique properties of nanoscale materials and low-cost fabrication by a silicon ink process at room temperature provide the promising potential of the SiNP-based nanostructured material. In addition, the combination of interfacial region engineering and post-processing provides controllable thermal transport properties for application in heat transfer management.

Can the melting behaviors of solid-liquid phase change be improved by inverting the partially thermal-active rectangular cavity?

The solid-liquid phase-change melting of phase change material (PCM, paraffin wax) is simulated in a rectangular cavity with two isothermally partially active vertical walls. A concept of inverting the cavity at different stages during the melting process is proposed as a possible method for thermal behaviors enhancement in the latent heat thermal energy storage with PCM. The enthalpy-porosity model is employed to account for the corresponding phase change process and flow evolution at the PCM solid-liquid interface, and the numerical model is validated by the existing data in the literature. It is confirmed that inverting the cavity is an effective method to improve the PCM thermal behaviors, and the maximum ∼24.3% increase of the liquid fraction during the PCM melting process for the best case of inverting at 40% standard unit hour. On the other hand, the cavity inverted is also a simple technique to enhance the solid-liquid phase-change melting heat transfer as compared to the available techniques reviewed. Furthermore, the concluded way is also suggested to incline the PCM cavity by a few degrees if a full inversion is not available in some situations. In conclusion, this proof of concept is expected to provide a novel development field of thermal transport enhancement in latent heat thermal energy storage.

Lone-pair electrons induced anomalous enhancement of thermal transport in strained planar two-dimensional materials

Strain engineering is one of the most promising and effective routes towards continuously tuning thermal transport due to the flexibility and robustness. However, previous studies mainly focused on quantifying how the thermal conductivity (κ) is modulated by strain, while the fundamental understanding on the electronic origin has yet to be explored. In this paper, we establish a microscopic picture of the lone-pair electrons driving strong phonon anharmonicity in two-dimensional (2D) materials beyond the traditional three-dimensional (3D) systems. We provide solid explanation for the unexpectedly up to one order of magnitude enlarged κ of a class of planar monolayers with bilateral tensile strain applied, which is in sharp contrast to the strain induced κ reduction in graphene despite their similar planar structures. The anomalous positive response of κ to tensile strain is attributed to the attenuated interaction between the lone-pair electrons and the bonding electrons of neighboring atoms, which reduces phonon anharmonicity and leads to the enlarged κ. Our study uncovers the electronic origin of the strain modulated thermal transport, which would have great impact on future investigations related to energy nanotechnologies and applications.

Understanding the temperature dependence of thermo-physical properties of nanofluid suspensions using non-intrusive dynamic measurements

Thermal diffusivity dependence on temperature and concentration of aqueous colloidal suspensions of Al2O3 in deionized water has been investigated by conducting transient heat transfer experiments. The study has been performed in a purely non-intrusive manner using Mach Zehnder interferometry. Through the dependence of refractive index on temperature, the time sequence of recorded interferograms has been used to generate highly sensitive data of evolving temperature distribution in the thermal diffusion field. Determination of thermal diffusivity from temperature-position data has been carried out by the method of least square. In comparison with deionized water, substantial enhancement in thermal diffusivity with increasing temperature and concentration of nanofluid suspension of Al2O3 is observed through the time sequence of the recorded interferograms as well as through analytical measurements. Additionally, the measurement of ζ-potential, pH and effective viscosity of dilute Al2O3 suspension at various temperatures explains the possible mechanisms at micro-scale levels that are responsible for such an anomalous behaviour of thermal diffusivity. The competing interaction of these factors reveals dependence of thermal diffusivity on particle cluster formation and thereby in the resultant stability of the suspension. The experimental findings indicate that, at specific size and shape, the higher conductivity of solid alumina particles and their cluster formation plays a major role in enhancing the thermophysical properties of the nanofluid Al2O3 suspension over the considered range of temperature and concentration. Also, a reasonably close agreement of the obtained trends of thermal diffusivity values with those reported in the literature and clear visualization of enhancement in thermal transport through interferograms signifies the importance of the proposed non-intrusive measurement technique for reliable measurement of thermal properties of nanofluids.

Unconventional thermal transport enhancement with large atom mass: a comparative study of 2D transition dichalcogenides

2D layered transition dichalcogenides have attracted tremendous attention for their excellent properties and multifarious applications. In particular, NbSe2 and TaSe2 are the canonical systems to study superconductivity and charge density waves. Here, we perform a comparative study of the thermal transport properties of 2D NbSe2 and TaSe2 for both 1T and 2H phases based on first-principles calculations. Usually, the lattice thermal conductivity () is smaller with larger average atom mass. However, it is contrary for the comparison between TaSe2 and NbSe2, despite the heavier Ta than Nb. The abnormally larger of TaSe2 originates from the weakened phonon–phonon scattering due to the combination of large phonon bandgap and bunching of the acoustic phonon branches, which is caused by the larger mass difference. On one hand, the large bandgap hinders the acoustic–optical phonon scattering. On the other hand, the bunching of the acoustic phonon branches hampers Umklapp process by weakening the high frequency acoustic–acoustic phonon scattering. The special characteristics of phonon transport are further conformed by mode level analysis and scattering channels of phonon–phonon scattering. Moreover, lower κL of 1T phase for both Nb and Ta selenides compared to 2H phase are also reported, which stems from the stronger anharmonicity.

Enhancement of thermal transport in Gel Polymer Electrolytes with embedded BN/Al2O3 nano- and micro-particles

While Gel Polymer Electrolytes (GPEs) have been widely investigated for use in next-generation Li-ion cells due to the potential for improved thermal safety, thermal transport within a GPE is still poorly understood. Among all materials in a Li-ion cell, the GPE has the lowest thermal conductivity, and hence determines the overall rate of heat flow in a Li-ion cell. This makes it critical to measure and understand thermal transport in a GPE and investigate trade-offs between thermal and ionic transport. This paper presents measurements of thermal and ionic conductivities in a PVdF-based GPE. The effect of incorporating BN/Al2O3 ceramic nano/microparticles in the GPE on thermal and ionic transport is characterized. Measurements indicate up to 2.5X improvement in thermal conductivity of activated GPE membranes, with relatively minor effect on electrochemical performance of GPE-based single-layer cells. The measured enhancement in thermal conductivity is in very good agreement with theoretical calculations based on the effective medium theory that accounts for thermal transport in a dispersed, two-phase medium such as a GPE. The fundamental insights gained in this work on thermal transport in a GPE and the role of nano/microparticle inclusions may facilitate thermal-electrochemical optimization and design of GPEs for safe, high-performance Li-ion cells.

Thermal transport barrier in carbon nanotube array nano-thermal interface materials

Although carbon nanotubes (CNTs) could theoretically achieve thermal conductivity (kCNT) as high as 100 W/m K, the nature of height non-uniformity could induce enormous thermal contact resistance at CNT array-heat sink interface (Rc), obscuring the quality of CNT arrays compared to commercial nano-thermal interface materials (nTIMs). Direct and experimental extraction of the effect of array height uniformity for thermal transport, Rc, provides a foundation for developing an optimum preparation approach of CNT array-based nTIM. However, this remains a big challenge for even the state-of-the-art interface thermophysical measurement techniques. To remedy this situation, we developed a method for achieving experimental distinction between kCNT and Rc. By focusing on growing CNT arrays with height uniformity, instead of the typical method of focusing on increasing the kCNT, we realized the performance boost by eightfold increase in uniformity and elimination of carbonaceous byproducts on the array canopy and dispersed Fe within walls. Rc is successfully cut down by 60% to 1.7 × 10−5 m2 K/W, while maintaining extremely high kCNT ∼170 W/m K is maintained allowing for fast heat transport within CNTs. This study presents novel quantitative evidence that lowering Rc is the most efficient for thermal transport enhancement for CNT array nTIMs.

Bonding-induced thermal transport enhancement across a hard/soft material interface using molecular monolayers.

Manipulating thermal transport across hard/soft material interfaces is important for composites which are critical for a wide range of applications, including electronic packaging, thermal storage, sensors and medicine. To increase the interfacial thermal conductance (Gint), a previous strategy has focused on using a self-assembled monolayer (SAM) to bridge the phonon spectra mismatch between the materials constituting the interface. Here, we introduce a general strategy aiming for interfaces which are incompatible with the previous strategy. Copper (Cu) and epoxy resin are chosen as representative materials constituting the interface. The proposed strategy relies on using a strongly bonding SAM to covalently connect Cu and epoxy. The thermal measurements show that Gint can be enhanced by as much as 11 fold. An interesting result is found that the Cu/epoxy interface, modified with the SAM used in the previous strategy, shows approximate 2-fold lower Gint. Through a series of experiments, including tensile strength and wettability tests, the formation and characters of bonds in different interface systems are explored and understood. The correlation between bonding characters and Gint is also elucidated. We demonstrate that when the structure of the soft material is complex, interfacial thermal transport should be tuned by covalent bonds rather than by phonon spectra match. Finally, the great potential of the proposed strategy in manipulating the thermal properties of nanocomposites is illustrated here with a theoretical prediction.

Nanostructures Significantly Enhance Thermal Transport across Solid Interfaces.

The efficiency of thermal transport across solid interfaces presents large challenges for modern technologies such as thermal management of electronics. In this paper, we report the first demonstration of significant enhancement of thermal transport across solid interfaces by introducing interfacial nanostructures. Analogous to fins that have been used for macroscopic heat transfer enhancement in heat exchangers, the nanopillar arrays patterned at the interface help interfacial thermal transport by the enlarged effective contact area. Such a benefit depends on the geometry of nanopillar arrays (e.g., pillar height and spacing), and a thermal boundary conductance enhancement by as much as ∼88% has been measured using the time-domain thermoreflectance technique. Theoretical analysis combined with low-temperature experiments further indicates that phonons with low frequency are less influenced by the interfacial nanostructures due to their large transmissivity, but the benefit of the nanostructure is fully developed at room temperature where higher frequency phonons dominate interfacial thermal transport. The findings from this work can potentially be generalized to benefit real applications such as the thermal management of electronics.

Thermal transport enhancement in gold nanofluid containing network like structure

In this report, we have shown that gold nanofluid containing networked gold particles have much more enhancement (≈35%) in thermal transport in comparison to gold nanofluid containing conventional spherical gold nanoparticles under similar condition and loading fraction. This shows that structure or shape of the loading materials has very important effect on thermal conductivity of a nanofluid. Connected structure helps to increase the rate of heat transfer in the medium. The gold nanofluid having network of gold nanoparticles is synthesized in a single step process by pulsed laser ablation. The nanofluid (containing gold network) is stable and in addition to enhanced thermal conduction (compared to conventional gold nanofluids) shows no enhancement of viscosity unlike that seen in many nanofluids containing elongated nanomaterials (like cylinder or ellipsoids) in the dispersed phase which limits the use of nanofluids containing these kind of elongated structures in thermal transport though they have enhanced thermal conductivity than the conventional nanofluids (containing spherical nano particles). We synthesized and investigated the thermal transport in nanofluids containing network of Au nanoparticles instead of elongated nanomaterials in the dispersed phase as it retains the advantage of the spherical nanoparticles yet utilizes the enhanced heat transport by the network structure.