Role of four-phonon processes in thermal conductivity of two-dimensional materials and thermal-transport enhancement arising from interconnected nanofiller networks in polymer/nanofiller composites.

Recent research has shed light on the importance of four-phonon scattering processes in the thermal conductivity () of 2D materials. The inclusion of 4 phonon scattering processes from first-principles has been shown to lead to a thermal conductivity of ∼1290 W m K in graphene at 300 K, significantly lower than the values predicted to be in excess of 4000 W m K based only on 3 phonon scattering processes. Four phonon processes are shown to be most significant for flexural ZA phonon modes, where the reflection symmetry selection rule (RSSR) is less restrictive for 4-phonon than 3-phonon scattering processes.

Crystal Growth, Structural and Electronic Characterizations of Zero-Dimensional Metal Halide (TEP)InBr Single Crystals for X-Ray Detection.

Recently, metal halides have shown great potential for applications such as solar energy harvesting, light emission, and ionizing radiation detection. In this work, we report the preparation, structural, thermal, and electronic properties of a new zero-dimensional (0D) halide (TEP)InBr (where TEP is tetraethylphosphonium organic cation, CHP). (TEP)InBr single crystals are obtained within a few days of continuous crystal growth time via a solution growth methodology.

Superior enhancement in thermal conductivity of epoxy/graphene nanocomposites through use of dimethylformamide (DMF) relative to acetone as solvent.

This method article describes the fabrication of graphene-epoxy nanocomposites using two different solvents, dimethylformamide (DMF) and acetone, and validates the resulting thermal conductivity improvements. The study compared the two solvents at a filler composition of 7 wt% and found that DMF resulted in more uniform dispersion of graphene nanoparticles in the epoxy matrix, leading to a 44% improvement in thermal conductivity compared to acetone. Laser scanning confocal microscopy (LSCM) imaging showed that DMF-based composites had more evenly dispersed graphene nanoplatelets than acetone-based composites, which exhibited larger graphene agglomerations.

Cryogenic characteristics of graphene composites-evolution from thermal conductors to thermal insulators.

The development of cryogenic semiconductor electronics and superconducting quantum computing requires composite materials that can provide both thermal conduction and thermal insulation. We demonstrated that at cryogenic temperatures, the thermal conductivity of graphene composites can be both higher and lower than that of the reference pristine epoxy, depending on the graphene filler loading and temperature. There exists a well-defined cross-over temperature-above it, the thermal conductivity of composites increases with the addition of graphene; below it, the thermal conductivity decreases with the addition of graphene.

Length dependence thermal conductivity of zinc selenide (ZnSe) and zinc telluride (ZnTe) - a combined first principles and frequency domain thermoreflectance (FDTR) study.

In this study, we report the length dependence of thermal conductivity () of zinc blende-structured Zinc Selenide (ZnSe) and Zinc Telluride (ZnTe) for length scales between 10 nm and 10 μm using first-principles computations, based on density-functional theory. The value of ZnSe is computed to decrease significantly from 22.9 W m K to 1.

Large Enhancement in Thermal Conductivity of Solvent-Cast Expanded Graphite/Polyetherimide Composites.

We demonstrate in this work that expanded graphite (EG) can lead to a very large enhancement in thermal conductivity of polyetherimide-graphene and epoxy-graphene nanocomposites prepared via solvent casting technique. A value of 6.6 W⋅m⋅K is achieved for 10 wt% composition sample, representing an enhancement of ~2770% over pristine polyetherimide (~0.

The superior effect of edge functionalization relative to basal plane functionalization of graphene in enhancing the thermal conductivity of polymer-graphene nanocomposites - a combined molecular dynamics and Green's functions study.

To achieve polymer-graphene nanocomposites with high thermal conductivity (), it is critically important to achieve efficient thermal coupling between graphene and the surrounding polymer matrix through effective functionalization schemes. In this work, we demonstrate that edge-functionalization of graphene nanoplatelets (GnPs) can enable a larger enhancement of effective thermal conductivity in polymer-graphene nanocomposites relative to basal plane functionalization. Effective thermal conductivity for the edge case is predicted, through molecular dynamics simulations, to be up to 48% higher relative to basal plane bonding for 35 wt% graphene loading with 10 layer thick nanoplatelets.

Chemically Edge-Carboxylated Graphene Enhances the Thermal Conductivity of Polyetherimide-Graphene Nanocomposites.

In this work, we demonstrate that edge oxidation of graphene can enable larger enhancement in thermal conductivity () of graphene nanoplatelet (GnP)/polyetherimide (PEI) composites relative to oxidation of the basal plane of graphene. Edge oxidation offers the advantage of leaving the basal plane of graphene intact, preserving its high in-plane thermal conductivity ( > 2000 W m K), while, simultaneously, the oxygen groups introduced on the graphene edge enhance interfacial thermal conductance through hydrogen bonding with oxygen groups of PEI, enhancing the overall polymer composite thermal conductivity. Edge oxidation is achieved in this work by oxidizing graphene in the presence of sodium chlorate and hydrogen peroxide, thereby introducing an excess of carboxyl groups on the edge of graphene.

Thermal conductivity of hexagonal BCP - a first-principles study.

In this work, we report a high thermal conductivity () of 162 W m K and 52 W m K at room temperature, along the directions perpendicular and parallel to the -axis, respectively, of bulk hexagonal BCP (h-BCP), using first-principles calculations. We systematically investigate elastic constants, phonon group velocities, phonon linewidths and mode thermal conductivity contributions of transverse acoustic (TA), longitudinal acoustic (LA) and optical phonons. Interestingly, optical phonons are found to make a large contribution of 30.

Strain tuned high thermal conductivity in boron phosphide at nanometer length scales - a first-principles study.

Breakdown of Fourier law of heat conduction at nanometer length scales significantly diminishes thermal conductivity, leading to challenges in thermal management of nanoelectronic applications. In this work we demonstrate using first-principles computations that biaxial strain can enhance k at a nanoscale in boron phosphide (BP), yielding nanoscale k values that exceed even the bulk k value of silicon. At a length scale of L = 200 nm, k of 4% biaxially strained BP is enhanced by 25% to a value of 150.

Effect of Alignment on Enhancement of Thermal Conductivity of Polyethylene-Graphene Nanocomposites and Comparison with Effective Medium Theory.

Thermal conductivity () of polymers is usually limited to low values of ~0.5 W in comparison to metals (>20 W). The goal of this work is to enhance thermal conductivity () of polyethylene-graphene nanocomposites through simultaneous alignment of polyethylene (PE) lamellae and graphene nanoplatelets (GnP).

High thermal conductivity through simultaneously aligned polyethylene lamellae and graphene nanoplatelets.

The effect of simultaneous alignment of polyethylene (PE) lamellae and graphene nanoplatelets (GnP) on the thermal conductivity (k) of PE-GnP composites is investigated. Measurements reveal a large increase of 1100% in k of the aligned PE-GnP composite using 10 wt% GnPs relative to unoriented pure PE. The rate of increase of k with applied strain for the pure PE-GnP composite with 10 wt% GnP is found to be almost a factor of two higher than the pure PE sample, pointing to the beneficial effect of GnP alignment on k enhancement.

Transition from near-field thermal radiation to phonon heat conduction at sub-nanometre gaps.

When the separation of two surfaces approaches sub-nanometre scale, the boundary between the two most fundamental heat transfer modes, heat conduction by phonons and radiation by photons, is blurred. Here we develop an atomistic framework based on microscopic Maxwell's equations and lattice dynamics to describe the convergence of these heat transfer modes and the transition from one to the other. For gaps >1 nm, the predicted conductance values are in excellent agreement with the continuum theory of fluctuating electrodynamics.

Anisotropy of the thermal conductivity in GaAs/AlAs superlattices.

We combine the transient thermal grating and time-domain thermoreflectance techniques to characterize the anisotropic thermal conductivities of GaAs/AlAs superlattices from the same wafer. The transient grating technique is sensitive only to the in-plane thermal conductivity, while time-domain thermoreflectance is sensitive to the thermal conductivity in the cross-plane direction, making them a powerful combination to address the challenges associated with characterizing anisotropic heat conduction in thin films. We compare the experimental results from the GaAs/AlAs superlattices with first-principles calculations and previous measurements of Si/Ge SLs.

Coherent phonon heat conduction in superlattices.

The control of heat conduction through the manipulation of phonons as coherent waves in solids is of fundamental interest and could also be exploited in applications, but coherent heat conduction has not been experimentally confirmed. We report the experimental observation of coherent heat conduction through the use of finite-thickness superlattices with varying numbers of periods. The measured thermal conductivity increased linearly with increasing total superlattice thickness over a temperature range from 30 to 150 kelvin, which is consistent with a coherent phonon heat conduction process.

Acoustic phonon lifetimes and thermal transport in free-standing and strained graphene.

We use first-principles methods based on density functional perturbation theory to characterize the lifetimes of the acoustic phonon modes and their consequences on the thermal transport properties of graphene. We show that using a standard perturbative approach, the transverse and longitudinal acoustic phonons in free-standing graphene display finite lifetimes in the long-wavelength limit, making them ill-defined as elementary excitations in samples of dimensions larger than ∼1 μm. This behavior is entirely due to the presence of the quadratic dispersions for the out-of-plane phonon (ZA) flexural modes that appear in free-standing low-dimensional systems.

High thermal conductivity in short-period superlattices.

The thermal conductivity of ideal short-period superlattices is computed using harmonic and anharmonic force constants derived from density-functional perturbation theory and by solving the Boltzmann transport equation in the single-mode relaxation time approximation, using silicon-germanium as a paradigmatic case. We show that in the limit of small superlattice period the computed thermal conductivity of the superlattice can exceed that of both the constituent materials. This is found to be due to a dramatic reduction in the scattering of acoustic phonons by optical phonons, leading to very long phonon lifetimes.

Role of disorder and anharmonicity in the thermal conductivity of silicon-germanium alloys: a first-principles study.

The thermal conductivity of disordered silicon-germanium alloys is computed from density-functional perturbation theory and with relaxation times that include both harmonic and anharmonic scattering terms. We show that this approach yields an excellent agreement at all compositions with experimental results and provides clear design rules for the engineering of nanostructured thermoelectrics. For Si(x)Ge(1-x), more than 50% of the heat is carried at room temperature by phonons of mean free path greater than 1   μm, and an addition of as little as 12% Ge is sufficient to reduce the thermal conductivity to the minimum value achievable through alloying.