Key parameters in thermally conductive polymer composites

The thermal conductivity of polymer composites is measured for several tubular carbon nanofillers (nanotubes, fibres, and whiskers). The highest enhancement in the thermal conductivity is observed for functionalized multiwalled carbon nanotubes (90% enhancement for 1 vol. %) and Pyrograf carbon fibres (80%). We model the experimental data using an effective thermal medium theory and determine the thermal interface resistance (RK ) at the filler-matrix interface. Our results show that the geometry of the nanofibres and the interface resistance are two key factors in engineering heat transport in a composite

Thermal properties of metal matrix composites with planar distribution of carbon fibres

High thermal conductivity (TC) and a tunable coefficient of thermal expansion are essential properties for heat management materials operating in a wide temperature range. We combine both properties in a composite with a low‐density metal matrix reinforced with pitch‐based carbon fibres. The thermal conductivity of the metal matrix was increased by 50%, the thermal expansion coefficient was reduced by a factor of five. The samples were produced by powder metallurgy and have a planar random distribution of fibres, leading to high performance in two dimensions

Full Crystallographic Imaging of Hexagonal Boron Nitride Monolayers with Phonon-Enhanced Sum-Frequency Microscopy

Hexagonal boron nitride (hBN) is an important 2D material for van der Waals heterostructures, single photon emitters, and infrared nanophotonics. The optical characterization of mono- and few-layer samples of hBN however remains a challenge as the material is almost invisible optically. Here we introduce phase-resolved sum-frequency microscopy as a technique for imaging monolayers of hBN grown by chemical vapor deposition (CVD) and visualize their crystal orientation. A combination of femtosecond mid-infrared (IR) and visible laser pulses is used for sum-frequency generation (SFG), which is imaged in a wide-field optical microscope. The IR laser resonantly excites a phonon of hBN that leads to an ~800-fold enhancement of the SFG intensity, making it possible to image large 100x100 µm2 sample areas in less than 1 s. Implementing heterodyne detection in combination with azimuthal rotation of the sample further provides full crystallographic information. Through combined knowledge of topography and crystal orientation, we find that triangular domains of CVD-grown monolayer hBN have nitrogen-terminated zigzag edges. Overall, SFG microscopy can be used as an ultra-sensitive tool to image crystal structure, strain, stacking sequences, and twist angles, and is applicable to the wide range of van der Waals structures, where location and identification of monolayer regions and interfaces with broken inversion symmetry is of paramount importance

Tris(1,2-dimethoxyethane-<i>O</i>,<i>O</i>′)sodium pentaphenylcyclopentadienide

The title compound, [Na(C4H10O2)3](C35H25), is the first pentaphenylcyclopentadienide salt with an isolated anion. Solvent-separated sodium cations and pentaphenylcyclopentadienide anions alternate with each other in stacks parallel to thebaxis and are arranged in segregated stacks along theadirection.</jats:p

Formation of surface relief grating in polymers with pendant azobenzene chromophores as studied by AFM/UFM

We studied peculiarities of the structural reconstruction within holographically recorded gratings on the surface of several different amorphous azobenzene-containing polymers. Under illumination with a light interference pattern, two processes take place in this type of polymer. The first process is the light-induced orientation of azobenzene units perpendicular to the polarization plane of the incident light. The second one is a transfer of macromolecules along the grating vector (i.e. perpendicular to the grating lines). These two processes result in the creation of a volume orientation grating (alternating regions of different direction or degree of molecular orientation) and a surface relief grating (SRG)-i.e. modulation of film thickness. One can assume that both orientation of molecules and their movement might change the local mechanical properties of the material. Therefore, formation of the SRG is expected to result also in modulation of the local stiffness of the polymer film. To reveal and investigate these stiffness changes within the grating, spin-coated polymer films were prepared and the gratings were recorded on them in two different ways: with an orthogonal circular or orthogonal linear polarization of two recording light beams. A combination of atomic force microscopy (AFM) and ultrasonic force microscopy (UFM) techniques was applied for SRG development monitoring. We demonstrate that formation of the phase gratings depends on the chemical structure of polymers being used, polymer film thickness, and recording parameters, with the height of grating structures (depth of modulation) increasing with both the exposure time and the film thickness. UFM images suggest that the slopes of the topographic peaks in the phase gratings exhibit an increased stiffness with respect to the grating depressions

Azobenzene-containing polymers for surface relief gratings

Well defined azobenzene containing (co)polymers have been synthesized by atom transfer radical polymerization and subsequent polymeranalogeous reaction for introduction of the chromophores. Four different series were prepared varying only one of the following material parameters at a time, while keeping the others constant. Varied were the molecular weight, the azobenzene content, the glass transition temperature and the molecular architecture of the copolymers. With these four series of (co)polymers the influence of the molecular parameters on the photoinduced formation of surface relief gratings was investigated