Michael T. Pettes

Two-Dimensional Phonon Transport in Supported Graphene

Heat Flow in Graphene Unsupported graphene sheets show exceptional thermal transport properties, but are these properties maintained when a graphene sheet is in contact with a substrate? Seol et al. (p. 213; see the Perspective by Prasher) measured the thermal conductivity of graphene supported on silicon dioxide and found that, while the conductivity was considerably lower than that of free-standing graphene, it was still greater than that of metals such as copper. A theoretical model suggested that the out-of-plane flexing vibrations of the graphene play a key role in thermal transport. Thus, graphene may help in applications such as conducting heat away from electronic circuits. The thermal conductivity of graphene supported on silicon dioxide remains high, despite phonon scattering by the substrate. The reported thermal conductivity (κ) of suspended graphene, 3000 to 5000 watts per meter per kelvin, exceeds that of diamond and graphite. Thus, graphene can be useful in solving heat dissipation problems such as those in nanoelectronics. However, contact with a substrate could affect the thermal transport properties of graphene. Here, we show experimentally that κ of monolayer graphene exfoliated on a silicon dioxide support is still as high as about 600 watts per meter per kelvin near room temperature, exceeding those of metals such as copper. It is lower than that of suspended graphene because of phonons leaking across the graphene-support interface and strong interface-scattering of flexural modes, which make a large contribution to κ in suspended graphene according to a theoretical calculation.

Ultrathin Graphite Foam: A Three-Dimensional Conductive Network for Battery Electrodes

We report the use of free-standing, lightweight, and highly conductive ultrathin graphite foam (UGF), loaded with lithium iron phosphate (LFP), as a cathode in a lithium ion battery. At a high charge/discharge current density of 1280 mA g(-1), the specific capacity of the LFP loaded on UGF was 70 mAh g(-1), while LFP loaded on Al foil failed. Accounting for the total mass of the electrode, the maximum specific capacity of the UGF/LFP cathode was 23% higher than that of the Al/LFP cathode and 170% higher than that of the Ni-foam/LFP cathode. Using UGF, both a higher rate capability and specific capacity can be achieved simultaneously, owing to its conductive (∼1.3 × 10(5) S m(-1) at room temperature) and three-dimensional lightweight (∼9.5 mg cm(-3)) graphitic structure. Meanwhile, UGF presents excellent electrochemical stability comparing to that of Al and Ni foils, which are generally used as conductive substrates in lithium ion batteries. Moreover, preparation of the UGF electrode was facile, cost-effective, and compatible with various electrochemically active materials.

Thermal Conductivity and Phonon Transport in Suspended Few-Layer Hexagonal Boron Nitride

The thermal conductivity of suspended few-layer hexagonal boron nitride (h-BN) was measured using a microbridge device with built-in resistance thermometers. Based on the measured thermal resistance values of 11-12 atomic layer h-BN samples with suspended lengths ranging between 3 and 7.5 μm, the room-temperature thermal conductivity of a 11-layer sample was found to be about 360 W m(-1) K(-1), approaching the basal plane value reported for bulk h-BN. The presence of a polymer residue layer on the sample surface was found to decrease the thermal conductivity of a 5-layer h-BN sample to be about 250 W m(-1) K(-1) at 300 K. Thermal conductivities for both the 5-layer and the 11-layer samples are suppressed at low temperatures, suggesting increasing scattering of low frequency phonons in thin h-BN samples by polymer residue.

Thermal Transport in Three-Dimensional Foam Architectures of Few-Layer Graphene and Ultrathin Graphite

At a very low solid concentration of 0.45 ± 0.09 vol %, the room-temperature thermal conductivity (κ(GF)) of freestanding graphene-based foams (GF), comprised of few-layer graphene (FLG) and ultrathin graphite (UG) synthesized through the use of methane chemical vapor deposition on reticulated nickel foams, was increased from 0.26 to 1.7 W m(-1) K(-1) after the etchant for the sacrificial nickel support was changed from an aggressive hydrochloric acid solution to a slow ammonium persulfate etchant. In addition, κ(GF) showed a quadratic dependence on temperature between 11 and 75 K and peaked at about 150 K, where the solid thermal conductivity (κ(G)) of the FLG and UG constituents reached about 1600 W m(-1) K(-1), revealing the benefit of eliminating internal contact thermal resistance in the continuous GF structure.

Enhanced thermal conductivity of phase change materials with ultrathin-graphite foams for thermal energy storage

For thermophysical energy storage with phase change materials (PCMs), the power capacity is often limited by the low PCM thermal conductivity (κPCM). Though dispersing high-thermal conductivity nanotubes and graphene flakes increases κPCM, the enhancement is limited by interface thermal resistance between the nanofillers, among other factors such as detrimental surface scattering of phonons. Here, we demonstrate that embedding continuous ultrathin-graphite foams (UGFs) with volume fractions as low as 0.8–1.2 vol% in a PCM can increase κPCM by up to 18 times, with negligible change in the PCM melting temperature or mass specific heat of fusion. The increase in κPCM, thermal cycling stability, and applicability to a diverse range of PCMs suggests that UGF composites are a promising route to achieving the high power capacity targets of a number of thermal storage applications, including building and vehicle heating and cooling, solar thermal harvesting, and thermal management of electrochemical energy storage and electronic devices.

Influence of Polymeric Residue on the Thermal Conductivity of Suspended Bilayer Graphene

The thermal conductivity (κ) of two bilayer graphene samples each suspended between two microresistance thermometers was measured to be 620 ± 80 and 560 ± 70 W m(-1) K(-1) at room temperature and exhibits a κ ∝ T(1.5) behavior at temperatures (T) between 50 and 125 K. The lower κ than that calculated for suspended graphene along with the temperature dependence is attributed to scattering of phonons in the bilayer graphene by a residual polymeric layer that was clearly observed by transmission electron microscopy.

High thermal conductivity of chain-oriented amorphous polythiophene

Polymers are usually considered thermal insulators, because the amorphous arrangement of the molecular chains reduces the mean free path of heat-conducting phonons. The most common method to increase thermal conductivity is to draw polymeric fibres, which increases chain alignment and crystallinity, but creates a material that currently has limited thermal applications. Here we show that pure polythiophene nanofibres can have a thermal conductivity up to ∼ 4.4 W m(-1) K(-1) (more than 20 times higher than the bulk polymer value) while remaining amorphous. This enhancement results from significant molecular chain orientation along the fibre axis that is obtained during electropolymerization using nanoscale templates. Thermal conductivity data suggest that, unlike in drawn crystalline fibres, in our fibres the dominant phonon-scattering process at room temperature is still related to structural disorder. Using vertically aligned arrays of nanofibres, we demonstrate effective heat transfer at critical contacts in electronic devices operating under high-power conditions at 200 °C over numerous cycles.

Thermoelectric and structural characterizations of individual electrodeposited bismuth telluride nanowires

The thermoelectric properties and crystal structure of individual electrodeposited bismuth telluride nanowires (NWs) were characterized using a microfabricated measurement device and transmission electron microscopy. Annealing in hydrogen was used to obtain electrical contact between the NW and the supporting Pt electrodes. By fitting the measured Seebeck coefficient with a two-band model, the NW samples were determined to be highly n-type doped. Higher thermal conductivity and electrical conductivity were observed in a 52 nm diameter monocrystalline NW than a 55 nm diameter polycrystalline NW. The electron mobility of the monocrystalline NW was found to be about 19% lower than that of bulk crystal at a similar carrier concentration and about 2.5 times higher than that of the polycrystalline NW. The specularity parameter for electron scattering by the NW surface was determined to be about 0.7 and partially specular and partially diffuse, leading to a reduction in the electron mean-free path from 61 nm in ...

Four-probe measurements of the in-plane thermoelectric properties of nanofilms

Measuring in-plane thermoelectric properties of submicron thin films has remained a challenging task. Here we report a method based on a suspended microdevice for four-probe measurements of the Seebeck coefficient, thermal conductivity, electrical conductivity, and thermoelectric figure of merit of patterned indium arsenide (InAs) nanofilms assembled on the microdevice. The contact thermal resistance and intrinsic thermal resistance of the 40 nm thick InAs nanofilm sample were measured by using the nanofilm itself as a differential thermocouple to determine the temperature drops at the contacts. The microdevice was also used to measure a 190 nm thick silicon dioxide (SiO(2)) film and the results were compared with those reported in the literature. A through-substrate hole under the suspended microdevice allows for transmission electron microscopy characterization of the nanofilm sample assembled on the device. This capability enables one to correlate the measured thermoelectric properties with the crystal structures of the nanofilm.

Significant Electronic Thermal Transport in the Conducting Polymer Poly(3,4‐ethylenedioxythiophene)

Suspended microdevices are employed to measure the in-plane electrical conductivity, thermal conductivity, and Seebeck coefficient of suspended poly(3,4-ethylenedioxythiophene) (PEDOT) thin films. The measured thermal conductivity is higher than previously reported for PEDOT and generally increases with the electrical conductivity. The increase exceeds that predicted by the Wiedemann-Franz law for metals and can be explained by significant electronic thermal transport in PEDOT.