Significantly enhanced phonon mean free path and thermal conductivity by percolation of silver nanoflowers.

Soft thermal interface materials (TIMs) composed of thermally conductive fillers and polymer matrixes have been widely employed for thermal management in electronic and energy devices. However, the thermal conductivity (κ) of TIMs is significantly smaller than the intrinsic κ of fillers due to the large interfacial thermal contact resistance between fillers. Here we achieve a very efficient thermal percolation network of flower-shaped silver nanoparticles (silver nanoflowers, Ag NFs) in soft polyurethane (PU) matrix TIMs. A record high κ (42.4 W m-1 K-1) is achieved compared with soft isotropic TIMs in the literature. Ag nanoflake-PU and Ag nanosphere-PU TIMs provide significantly smaller κ (7.9 and 15.0 W m-1 K-1) at an identical filler concentration (38 vol%). Surprisingly, the phonon transport of the Ag NF-PU TIM dramatically increases (κlat = 22.2 W m-1 K-1) compared with Ag nanoflake-PU and Ag nanosphere-PU (κlat = 0.2 and 1.2 W m-1 K-1) TIMs. Kinetic theory reveals that the phonon mean free path (39.6 nm) is significantly increased for the Ag NF-PU TIM by the active coalescence of metallic Ag NFs. The hierarchically structured Ag NFs construct an excellent thermal percolation network in soft isotropic TIMs.

Transparent conductive film fabrication using intercalating silver nanoparticles within carbon nanotube layers.

Carbon nanotubes have received attention as alternative materials to indium tin oxide for application in transparent conductive films. Their electrical conductivity, however, still has to be improved. In this study, a layer-by-layer self-assembly process was demonstrated using nano-silver-coated carbon nanotubes, which help improve electrical conductivity. The method was based on the pi-pi interaction between the side walls of the carbon nanotubes and nano-silver clusters that were functionalized with benzyl mercaptan. The self-assembled nano-silver cluster monolayer on the surface of the nanotubes can reduce the interfacial contact resistance, thereby leading to high conductivity at a high transparency. The compound was characterized via scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and the four-point probe method.

Flexible, transparent single-walled carbon nanotube transistors with graphene electrodes.

This paper reports a mechanically flexible, transparent thin film transistor that uses graphene as a conducting electrode and single-walled carbon nanotubes (SWNTs) as a semiconducting channel. These SWNTs and graphene films were printed on flexible plastic substrates using a printing method. The resulting devices exhibited a mobility of ∼ 2 cm(2) V(-1) s -1), On/Off ratio of ∼ 10(2), transmittance of ∼ 81% and excellent mechanical bendability.

Electron tunneling of hierarchically structured silver nanosatellite particles for highly conductive healable nanocomposites.

Healable conductive materials have received considerable attention. However, their practical applications are impeded by low electrical conductivity and irreversible degradation after breaking/healing cycles. Here we report a highly conductive completely reversible electron tunneling-assisted percolation network of silver nanosatellite particles for putty-like moldable and healable nanocomposites. The densely and uniformly distributed silver nanosatellite particles with a bimodal size distribution are generated by the radical and reactive oxygen species-mediated vigorous etching and reduction reaction of silver flakes using tetrahydrofuran peroxide in a silicone rubber matrix. The close work function match between silicone and silver enables electron tunneling between nanosatellite particles, increasing electrical conductivity by ~5 orders of magnitude (1.02×103 Scm-1) without coalescence of fillers. This results in ~100% electrical healing efficiency after 1000 breaking/healing cycles and stability under water immersion and 6-month exposure to ambient air. The highly conductive moldable nanocomposite may find applications in improvising and healing electrical parts.

Unusual strain-dependent thermal conductivity modulation of silver nanoflower-polyurethane fibers.

Thermal management of stretchable and wearable electronic devices is an important issue in enhancing performance, reliability, and human thermal comfort. Here, we constructed a unique experimental setup which investigated the strain-dependent thermal conductivity. The thermal conductivity of flower-shaped silver nanoparticle (silver nanoflower)-polyurethane (Ag-PU) composite fibers was systematically investigated as a function of strain. The strain-dependent temperature distribution of the Joule-heated fiber was measured using an infrared camera, and the thermal conductivity was obtained from the 1-dimensional Fourier's conduction model. There was a monotonic decrease in both lattice and electronic thermal conductivity with stretching at 25 °C. However, there was an initial increase in lattice and total thermal conductivity in the low strain region (<10%), when the fiber was stretched at 45 °C, although the electronic thermal conductivity decreased monotonically. The softening of the polymer at increased temperatures enhanced Poisson's ratio. Resultantly, the fiber cross-sectional area and radial-direction inter-particle distance between silver nanoflowers decreased. This could increase the thermal transport in conductive fibers by modulating the interfaces between silver nanoflowers and polyurethane. A further stretching decreased the lattice thermal conductivity due to the significantly increased axial distance between silver nanoflowers and the decreased filler fraction. The weft-knitted fabric also demonstrated an increased thermal conductance in the low strain region (≤30%) at 45 °C.

Ultrahigh Thermal Conductivity of Interface Materials by Silver-Functionalized Carbon Nanotube Phonon Conduits.

An ultrahigh thermal conductivity (κ = 160 W m(-1) K(-1) ) of thermal interface materials is achieved with a high enhancement factor (96). A small amount (2.3 vol%) of 1D multiwalled carbon nanotubes (MWNTs) with high κ constructs effective phonon transport pathways between microscale silver-flake islands, and a solid phonon transport junction is realized by the coalescence of silver nanoparticles pre-functionalized on the MWNTs.

Silver-plated carbon nanotubes for silver/conducting polymer composites.

Carbon nanotubes (CNTs) have advantages as conductive fillers due to their large aspect ratio and excellent conductivity. In this study, a novel silver/conducting polymer composite was developed by the incorporation of silver-plated CNTs. It is important to achieve a homogeneous dispersion of nanotubes and to improve the interfacial bonding to utilize the excellent properties of reinforcements in the matrix material. The homogeneous dispersion of nanotubes was achieved by an acid treatment process, and the interfacial contact was improved by electroless silver plating around nanotubes. The resistivity of the silver/conducting polymer composite was decreased by 83% by the addition of silver-plated single-walled carbon nanotubes. Conductive bumps were also screen-printed to demonstrate the capability of the composite as electrical interconnects for multi-layer printed circuit boards.