Thermal Conductivity Of Nanocomposites Research Articles

On the synthesis and thermal conductivity of nanocomposites with multiwalled carbon nanotubes

A method for growing arrays of vertically aligned carbon nanotubes and synthesizing a polymer composite based on vertically aligned, horizontally aligned, and unaligned carbon nanotubes is described. The thermal conductivity of polymer composites is studied by the laser-flash method. It is found that filling of the polymer with vertically aligned carbon nanotubes leads to an increase in the thermal conductivity coefficient of the epoxy resin by a factor of 10 and more.

Thermal properties of PA6 nanocomposites by addition of graphene non-covalently functionalized with dendronized polyamide

The novel graphene non-covalently functionalized with amino-terminated dendronized polyamide (DPA) was prepared, then reduced and filled into the PA6 to prepare a series of nanocomposites. The structures and thermal properties of products were investigated by FTIR, XPS, XRD, TEM, SEM, TG, thermal conductivity measurements. The results revealed that mediated by the DPA, the amino-terminated dendronized polyamide functionalized graphene nanosheets (DPA-GNs) show good solubility in the organic solvents and more compatible with organic polymer matrix, which results in a certain promotion in thermal stability and thermal conductivity of nanocomposites.

Thermal Conductivity Prediction of Thermally Conductive, Electrically Insulating Materials under van der Waals Interactions

The thermal transport phenomenon in small-scale heterogeneous composites is essentially controlled by van der Waals interactions. In this article, thermal conductivity of nanocomposites with 33 wt% crystallized silicon dioxide is four times higher than that of epoxy (EP) resin composites. Nanocomposites with 33 wt% boron carbide exhibit seven times higher thermal conductivity than pure EP. Pal and Lewis-Nielsen multiscale models were used to infer that distance-associated van der Waals interactions vary between composites with different weight fractions. Such variation consequently affects the thermal conductivity of the composites. Scanning electron microscope images of crystallized silicon dioxide/EP composites provide evidence of our reasonable and accurate inferences with regard to the thermal conduction mechanism. Experimental values confirm that the Pal model is superior to the Lewis-Nielsen model. The observed enhancement in thermal conductivity indicates important implications for the development of highly and thermally conductive electrically insulating materials. Results of this study can also be considered to improve modeling for thermal conductivity under van der Waals interactions.

Phonon Transport in SiGe-Based Nanocomposites and Nanowires for Thermoelectric Applications

ABSTRACTSilicon-germanium (SiGe) superlattices (SLs) have been proposed for application as efficient thermoelectrics because of their low thermal conductivity, below that of bulk SiGe alloys. However, the cost of growing SLs is prohibitive, so nanocomposites, made by a ball-milling and sintering, have been proposed as a cost-effective replacement with similar properties. Lattice thermal conductivity in SiGe SLs is reduced by scattering from the rough interfaces between layers. Therefore, it is expected that interface properties, such as roughness, orientation, and composition, will play a significant role in thermal transport in nanocomposites and offer many additional degrees of freedom to control the thermal conductivity in nanocomposites by tailoring grain size, shape, and crystal angle distributions. We previously demonstrated the sensitivity of the lattice thermal conductivity in SLs to the interface properties, based on solving the phonon Boltzmann transport equation under the relaxation time approximation. Here we adapt the model to a broad range of SiGe nanocomposites. We model nanocomposite structures using a Voronoi tessellation to mimic the grains and their distribution in the nanocomposite and show excellent agreement with experimentally observed structures, while for nanowires we use the Monte Carlo method to solve the phonon Boltzmann equation. In order to accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite and at the boundaries of nanowires, we employ a momentum-dependent specularity parameter. Our results show thermal transport in SiGe nanocomposites and nanowires is reduced significantly below their bulk alloy counterparts.

Radiation resistant metal decorated MWCNTs/PMMA nanocomposite films with enhanced thermomechanical properties

UV/O3 radiation and chemical resistant nanocomposite films of functionalized/metal decorated multiwall carbon nanotubes (MWCNTs) with polymethylmethacrylate (PMMA) are synthesized. Silver nanoparticles are decorated on the surface of UV/O3 functionalized MWCNTs by both reduction and in situ growth from AgNO3 aqueous solution. Microscopic studies reflect the better dispersion of UV/O3 functionalized/silver decorated MWCNTs in polymer matrix contributing in enhancement of thermal stability, thermomechanical strength, glass transition temperatures, and thermal conductivity of nanocomposites even at 0.25 wt% MWCNTs additions. The thermal stability of nanocomposite film (0.25 wt% loading), prepared by using a surfactant (Sodium dodecyl sulfate) is increased to about 27 � C while the thermomechanical properties are raised up to 76% at 100 � C. Thermal and thermomechanical behavior of pre- and post-UV/O3 irradiated nanocomposite films are compared with neat polymer. The results reveal that UV/O3 functionalized MWCNTs can effectively disperse the radiation and have a dramatic reinforcement effect on the nature of the degradation of PMMA matrix. POLYM. COMPOS., 00:000–000, 2014. V C 2014 Society of Plastics Engineers

Influence of graphene functionalized with zinc dimethacrylate on the mechanical and thermal properties of natural rubber nanocomposites

An effective approach is developed to synthesize zinc dimethacrylate functionalized graphene (ZDMA‐GE) as reinforcing nanofiller for natural rubber (NR). The morphology and structure of ZDMA‐GE were characterized to confirm the exfoliation and functionalization of GE. The as‐prepared nanocomposites were investigated by transmission electron microscopy, mechanical analysis, crosslinked network analysis, and the analysis of thermal conductivity. The results demonstrated that there is strong interfacial interaction between GE and rubber matrix due to good dispersion and special two‐dimensional structure of GE. The crosslink density of the nanocomposites is greatly improved with the introduction of ZDMA‐GE because of the homopolymerization and graft polymerization of ZDMA. It is also noticed that the tensile strength, tear strength, and modulus at 300% elongation of NR nanocomposites with 15 phr ZDMA‐GE have been improved by 133, 42, and 174%, respectively. The thermal conductivity of nanocomposites with 40 phr ZDMA‐GE is enhanced 1.3 times as that of the pure NR. This remarkable improvement is attributed to the formation of covalent crosslinked network and ionic crosslinked network, good dispersion of GE, and efficient interfacial interaction between GE and NR matrix. This method provides the potential applications of functionalized GE in the polymer composites. POLYM. COMPOS., 36:1775–1785, 2015. © 2014 Society of Plastics Engineers

Thermal and thermomechanical behavior of amino functionalized and metal decorated MWCNTs/PMMA nanocomposite films

The homogeneous nanocomposites (NC) films of amino modified and metal decorated multiwall carbon nanotubes (MWCNTs) with polymethylmethacrylate (PMMA) were synthesized through in‐situ free radical polymerization. Silver metal nanohybrids (Ag/MWCNTs) were prepared by two strategies, that is, reduction of metal salt in presence of sodium dodecyl sulfate and in‐situ growth from AgNO3 aqueous solution. The amino functionalization by ball milling enhanced the dispersion of MWCNT in monomer and produced a new class of radiation resistant NC. These synthesized films were characterized by FTIR, TGA, TEM, EDX, TC, DMA, and optical microscopy to ascertain their structural morphologies, thermal stability, and mechanical strength. Microscopic studies reflect the homogeneous mixing of amino functionalized and metal decorated MWCNTs in polymer matrix contributing in the enhancement of thermal stability, thermo‐mechanical strength, glass transition temperatures, and thermal conductivity of NC even at 0.25 wt% addition of modified nanofiller. The thermal stability of NC film at 0.25 wt% loading was increased around ≂50°C and the raise of thermo‐mechanical properties was observed up to 85% at 100°C in the presence of adsorbed surfactant. Thermal and thermomechanical behavior of pre and post UV/O3 irradiated NC films has been compared with neat polymer. The results revealed that amino modified nanofiller embedded network in polymer matrix can effectively disperse the radiation and has a dramatic reinforcement effect on the nature of degradation of PMMA matrix. POLYM. COMPOS., 35:1807–1817, 2014. © 2013 Society of Plastics Engineers

Enhancement of Thermal Conductive Pathway of Boron Nitride Coated Polymethylsilsesquioxane Composite

We report here in the fabrication of enhanced thermal conductive pathway nanocomposites of boron nitride (BN)-coated polymethylsilsesquioxane (PMSQ) composite beads using isopropyl alcohol (IPA) as a mixing medium. Exfoliated and size-reduced boron nitride particles were successfully coated on the PMSQ beads and explained by surface charge differences. A homogeneous dispersion and coating of BN on the PMSQ beads using IPA medium was confirmed by SEM. Each condition of the composite powder was carried into the stainless still mould and then hot pressed in an electrically heated hot press machine. Three-dimensional percolation networks and conductive pathways created by exfoliated BN were precisely formed in the nanocomposites. The thermal conductivity of nanocomposites was measured by multiplying specific gravity, specific heat, and thermal diffusivity, based upon the laser flash method. Densification of the composite resulted in better thermal properties. For an epoxy reinforced composite with 30 vol% BN and PMSQ, a thermal conductivity of nine times higher than that of pristine PMSQ was observed.

Interfacial thermal conductance-rheology nexus in metal-contacted nanocomposites

Tailoring high interfacial thermal conductance is crucial for many applications and is often challenging even for interfaces comprised of high thermal conductivity materials. Here, we report the correlation between the rheological behavior of a gold-nanowire-filled polydimethylsiloxane nanocomposite and its interface thermal conductance with copper. At a critical filler fraction, an abrupt increase in the nanocomposite thermal conductivity is accompanied by a liquid-solid transition and a multifold decrease in interface conductance. These concurrent changes are attributed to nanowire percolation network formation and pre-cure polymer gelation that inhibits the formation of conformal void-free interfaces. These findings will be important for designing processing sequences to realize high thermal conductance interfaces.

Experimental and numerical study of the effective thermal conductivity of nano composites with thermal boundary resistance

The thermal interface resistance at the macro scale is mainly described by the physical gap between two (inter) faces and constriction resistance due to this gap. The small gaps and surface geometry mismatch between the two material faces makes up the majority of thermal interface resistance (Rc) at the macro scale. There are various models to predict Rc at macro scale. Although Rc represents thermal resistance accurately for macro size contacts between two metals, it is neither suitable nor accurate to describe interface resistance of a modern composite Thermal Interface Material (TIM) containing micron to nano-sized particles. The thermal discontinuity at a perfectly bonded interface of two dissimilar materials is termed as thermal boundary resistance (Rb) or Kapitza resistance. It is necessary to understand feasibility of using nanoparticles in composite TIM by having better understanding of thermal boundary resistance at that scale. The phenomenon of thermal boundary resistance is an inherent material property and arises due to fundamental mechanisms of thermal transport. For metal–matrix particulate composites, Rb plays a more important role than Rc. The free flowing nature of the polymer would eliminate most of the gaps between the two materials at their interface. This means almost all of the thermal resistance at particle/matrix interface would occur due to Rb. Here, the thermal boundary resistance for silica nanoparticles embedded inside epoxy resin is studied. The bulk conductivity of the sample is measured, and Rb is back calculated using the Hasselman–Johnson’s (H–J) equation. The numerical validation of the equation is also presented, including extrapolation study to predict effective conductivity of the nanocomposite TIM.

On thermal and electrical properties of multiwalled carbon nanotubes/copper matrix nanocomposites

Abstract This study reports on the investigation of thermal and electrical properties of multiwalled carbon nanotubes reinforced copper (MWCNT/Cu) nanocomposites. The nanocomposites were fabricated with different weight fractions of MWCNTs by powder metallurgy technique followed by hot forging as secondary process. The thermal properties of hot forged nanocomposites were measured by Laser flash technique at 473 K. The results indicated that the thermal conductivity of MWCNT/Cu nanocomposites decreased with the increase in MWCNTs content. The drop in the thermal conductivity of nanocomposites is mainly attributed to the interface thermal resistance, scattering of phonons on dislocations, random distribution of MWCNTs and the kinks formed in MWCNTs during the fabrication. The decrease in electrical conductivity of nanocomposites is attributed to the grain refinement caused by the incorporation of MWCNTs.

Thermal conductivity of core-shell nanocomposites for enhancing thermoelectric performance

The differential effective medium method (DEM) is presented from a physical viewpoint and employed to calculate the lattice thermal conductivity of nano-bulk composites comprising core-shell particles. Extended from the average-T-matrix single-particle approximation, DEM incorporates multiparticle effect essential for the study of core-shell nanocomposites (CSN). Interparticle boundary scattering in addition to intraparticle boundary scattering in CSN is found to add to the reduction of thermal conductivity of nanocomposites. Thus, CSN hold the promise of improving the thermoelectric dimensionless figure of merit ZT above that of monolithic nano-bulk phases. Si and SiGe based CSN serve as illustrative examples.

Numerical Calculation of the Effective Thermal Conductivity of Nanocomposites

The effective thermal conductivity (K eff) of carbon nanotube (CNT) composites is affected by the thermal boundary resistance (TBR), the dispersion pattern, and geometry distribution of the CNTs. Traditional effective medium theories assume that CNTs are perfectly dispersed without considering TBR. In this work, we report the development of a new algorithm using CNTs with 3-D worm-like geometry and different persistence lengths. We describe how to obtain K eff using simulations of these realistic CNT configurations with off-lattice Monte Carlo simulation. The results are compared with straight cylinder models without effects of persistence length.

A molecular dynamics study of effective thermal conductivity in nanocomposites

In this study, non-equilibrium molecular dynamics (NEMD) simulations are performed to determine the effective thermal conductivity of nanocomposites embedded with a variety of nanoparticles. The effects of orientation and arrangement of asymmetric nanoparticles, thermal property mismatch at the interface, interface density per unit volume of nanocomposite, and polydispersity of nanoparticles on the effective thermal conductivity of nanocomposites are investigated. Simulation results are compared with existing model predictions based on the effective medium approach. Results indicate that, with the same particle volume fraction, the nanocomposites that have a larger interfacial area perpendicular to heat flow or a larger interface density yield a smaller effective thermal conductivity. In addition, a larger mismatch at the interface between the host material and nanoparticle inclusions leads to a smaller effective thermal conductivity. Finally, it is found that the effective thermal conductivity of nanocomposites decreases with increasing polydispersity of embedded nanoparticles.

Thermal conductivity of nanocomposites with high volume fractions of particles

Abstract The model proposed by Ordonez-Miranda et al. [Appl Phys Lett 2001;98(233): 111], for the thermal conductivity of composites with low volume fractions of nanoparticles is extended for high volume fractions of spherical and cylindrical nanoparticles. In the dilute limit of macro/micro-sized particles, the obtained results agree with the ones previously reported. In contrast, when the radius of the particles is of the order of the mean free path of the energy carriers, it is shown that the dependence of the composite thermal conductivity on the collision cross-section per unit volume of the particles and the average distance that the energy carriers can travel inside the particles becomes stronger when the volume fractions of particles increases and is cancelled out for high enough interfacial thermal resistances. The predictions of the proposed analytical approach are in good agreement with reported experimental data and could be highly useful for guiding the design of particulate nanocomposites where the interactions among the particles and the interfacial scattering of the electrons and phonons need to be considered.

Thermal Transport Properties of Ag-Based Nanocomposites Containing MWCNTs

ABSTRACTVariation in thermal conductivity of Ag-based composites by introduction of multi-walled carbon nanotubes (MWCNTs) was investigated. The Ag/MWCNT nanocomposite powder was successfully prepared when appropriate surfactants were used via a sonoprocess. The nanocomposite powder was subsequently cured at 280-300 ºC in air. After curing, the thermal conductivity of the nanocomposites was compared with the electronic contribution to thermal conductivity that was estimated from experimental values of the electrical conductivity. The thermal conductivity of Ag/MWCNT nanocomposites was much higher than the electronic contribution. Therefore, the increase in thermal conductivity of the Ag-based nanocomposites is attributed to phonon transfer along the percolation network of MWCNTs.

Multiscale modeling of cross-linked epoxy nanocomposites to characterize the effect of particle size on thermal conductivity

A sequential multiscale model to characterize the size effects of nanoparticles on the effective thermal conductivity of SiC/epoxy nanocomposites is developed through non-equilibrium molecular dynamics (NEMD) simulations and continuum micromechanics. Even at the fixed volume fraction condition of a spherical nanoparticle, a significant particle size effect on the thermal conductivity of SiC/epoxy nanocomposites has been demonstrated using NEMD simulations. The main contributions of the particle size dependency are Kapitza thermal resistance at the interface between the particle and matrix, and the formation of highly densified polymer sheathing (adsorption layer) near the particle. To account for these two effects in a continuum regime, both the Kapitza interface and the effective interphase are defined in a micromechanics model, and a four-phase multiscale bridging method is suggested. The thermal conductivity of the effective interphase is implicitly obtained from the four-phase micromechanics model. The accuracy and the relative concentration effect of the particle, Kapitza interface, and the effective interphase are discussed via finite element analysis (FEA). By defining the conductivity of the effective interphase as a function of the particle radius, the proposed bridging model accurately reproduced the particle size dependency observed from NEMD simulations. Using the proposed multiscale model, a parametric study is performed to examine the effect of the Kapitza thermal interface and the effective interphase on the overall thermal conductivity of nanocomposites.

Advance polymeric carbon nanocomposite films with enhanced thermo-mechanical properties

The homogenous nanocomposite films of UV/O3 oxidized multiwall carbon nanotubes (MWCNTs) subsequently modified with aniline moiety were synthesized with polymethylmethacrylate (PMMA) through free radical polymerization. The phenylamine functional groups present on the surface of MWCNTs providing an anchoring sites for deposition of Ag metal nanoparticles (NP).The in situ free radical polymerization of MMA in the presence of these well dispersed nanotubes gave a new class of radiation resistant nanocomposite films. The synthesized materials were characterized by FT-IR, TGA, TEM, EDX, TC, DMA, universal testing machine, and optical microscopy to ascertain their structural morphologies, thermal stability, and mechanical strength. The microscopic and structural properties reflect the homogenous mixing of modified MWCNTs in polymer matrix contributing in enhancement of thermal stability, thermo-mechanical strength, glass transition temperatures, and thermal conductivity of nanocomposites even at 0.25 wt% addition of modified nanofiller. Thermal and thermo-mechanical behavior of pre- and post-UV/O3 irradiated nanocomposite films have been compared with neat polymer. The results revealed that modified nanofiller network can effectively disperse the radiation and has a dramatic reinforcement effect on the nature of degradation of PMMA matrix. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers

Correlation of Thermal Conduction Properties With Mechanical Deformation Characteristics of a Set of SiC–Si3N4 Nanocomposites

New developments in high temperature ceramic materials technology have focused on obtaining nanocomposite materials with nanoscale features for an optimal control of thermal and mechanical properties. One example is the silicon carbide (SiC)–silicon nitride (Si3N4) nanocomposites with nanosized SiC particles placed either in microsized Si3N4 grains or along Si3N4 grain boundaries (GBs). This work focuses on analyzing the influence of GBs, interfaces, and impurities on thermal and mechanical properties of a set of SiC–Si3N4 nanocomposites at three different temperatures (300 K, 900 K, and 1500 K). Nanocomposite thermal conductivity values predicted in this study are smaller in comparison to the bulk Si3N4 values (∼30 W/m K). Even with the volume fraction of SiC phase being limited to maximum 40%, it is shown that the thermal conductivity values could be reduced to less than those of the bulk SiC phase (∼3 W/m K) by microstructural feature arrangement. Nanocomposite phonon spectral density values show a short rage structural order indicating a high degree of diffused phonon reflection. Visual analyses of the atomistic arrangements did not reveal any loss of crystallinity in the nanocomposites at high temperatures. This indicates that structural arrangement, not the phase change, is a factor controlling thermal conduction as a function of temperature. The nanocomposite deformation mechanism is a trade-off between the stress concentration caused by SiC particles and Si3N4–Si3N4 GB sliding. The temperature increase tends to work in favor of GB sliding leading to softening of structures. However, microstructural strength increases with increase in temperature when GBs are absent. GBs also contribute to reduction in thermal conductivity as well as increase in fracture strength. Replacement of sharp GBs by diffused GBs having C/N impurities, lowered thermal conductivity, and increased fracture strength. Decrease in SiC–Si3N4 interfaces by removal of SiC particles tends to favor an increase in thermal conductivity as well as fracture resistance. Overall, it is shown that for high temperature mechanical strength improvement, judicious placement of SiC particles and optimal control of GB atomic volume fraction are the main controlling factors.

The role of straining and morphology in thermal conductivity of a set of Si–Ge superlattices and biomimetic Si–Ge nanocomposites

The ability to alter the thermal and mechanical properties of nanostructures by tailoring nanoscale morphology has led to vast activity in applications such as high figure of merit (ZT) thermoelectric, microelectronic and optoelectronic devices. Two types of nanostructures that have gained significant attention are Si–Ge superlattices and Si–Ge biomimetic nanocomposites, in which one phase is distributed in the other phase in a staggered biomimetic manner similar to biological materials. A systematic comparison of the atomistic factors that affect their thermal behaviour under different extents of straining at a range of temperatures remains to be performed. In this investigation, such analyses are performed for a set of Si–Ge superlattices and Si–Ge biomimetic nanocomposites using non-equilibrium molecular dynamics (NEMD) simulations at three different temperatures (400, 600 and 800 K) and at strain levels varying between −10% and 10%. Analyses indicate that the nanoscale morphology differences between the superlattices and the nanocomposites lead to a striking contrast in the phonon spectral density, interfacial thermal boundary resistance and thermal conductivity. In the case of the nanocomposites, morphology variation at the nanoscale and the tensile or compressive straining at temperatures from 400 to 800 K do not have a significant effect on the changes in thermal conductivity values. Such factors, however, strongly influence the thermal conductivity of superlattices. The thickness of the nanocomposites, however, is found to influence the thermal conductivity values significantly under straining, with the effect of straining increasing with increasing nanocomposite thickness. A relation based on the effective medium approach is shown to fit the NEMD calculated nanocomposite thermal conductivity values.