Izabela Firkowska

Nanoplatelet Size to Control the Alignment and Thermal Conductivity in Copper–Graphite Composites

A controlled alignment of graphite nanoplatelets in a composite matrix will allow developing materials with tailored thermal properties. Achieving a high degree of alignment in a reproducible way, however, remains challenging. Here we demonstrate the alignment of graphite nanoplatelets in copper composites produced via high-energy ball milling and spark plasma sintering. The orientation of the nanoplatelets in the copper matrix is verified by polarized Raman scattering and electron microscopy showing an increasing order with increasing platelet size. The thermal conductivity k along the alignment direction is up to five times higher than perpendicular to it. The composite with the highest degree of alignment has a thermal diffusivity (100 mm(2) s(-1)) comparable to copper (105 mm(2) s(-1)) but is 20% lighter. By modeling the thermal properties of the composites within the effective medium approximation we show that (i) the Kapitza resistance is not a limiting factor for improving the thermal conductivity of a copper-graphite system and (ii) copper-graphite-nanoplatelet composites may be expected to achieve a higher thermal conductivity than copper upon further refinement.

The Origin of High Thermal Conductivity and Ultralow Thermal Expansion in Copper–Graphite Composites

We developed a nanocomposite with highly aligned graphite platelets in a copper matrix. Spark plasma sintering ensured an excellent copper-graphite interface for transmitting heat and stress. The resulting composite has superior thermal conductivity (500 W m(-1) K(-1), 140% of copper), which is in excellent agreement with modeling based on the effective medium approximation. The thermal expansion perpendicular to the graphite platelets drops dramatically from ∼20 ppm K(-1) for graphite and copper separately to 2 ppm K(-1) for the combined structure. We show that this originates from the layered, highly anisotropic structure of graphite combined with residual stress under ambient conditions, that is, strain-engineering of the thermal expansion. Combining excellent thermal conductivity with ultralow thermal expansion results in ideal materials for heat sinks and other devices for thermal management.

Effect of carbon nanotube surface modification on thermal properties of copper–CNT composites

Carbon nanotubes (CNTs) were functionalized by polymer wrapping (CNT–PSS) and oxidation (CNT–COOH), followed by reduction of copper ions in hydrogen atmosphere, producing copper decorated carbon nanotubes (CNT–f@Cu). Thus synthesized hybrid nanostructures were used as conductive fillers to tailor the heat transport capabilities of a copper matrix. Thermal properties, i.e. thermal diffusivity and thermal conductivity, of copper composite were measured and compared with those containing pristine and functionalized carbon nanotubes. Experimental results revealed that thermal diffusivity and conductivity of the composites decrease with increasing content of carbon nanotubes. However, composites enriched with nanohybrids where Cu nanoparticles were covalently bonded to carbon nanotubes had thermal conductivity four times higher than those containing the same content of pristine CNTs. The experimental results were analysed using Nans model which accounts for contributions from thermal interface resistance at metal–CNT boundary as well as aspect ratio of carbon nanotubes.

Filler geometry and interface resistance of carbon nanofibres: 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.

Biocompatible Nanomaterials and Nanodevices Promising for Biomedical Applications

Nanotechnology applied to biology requires a thorough understanding of how molecules, sub-cellular entities, cells, tissues, and organs function and how they are structured. The merging of nanomaterials and life science into hybrids of controlled organization and function is possible, assuming that biology is nanostructured, and therefore man-made nano-materials can structurally mimic nature and complement each other. By taking advantage of their special properties, nanomaterials can stimulate, respond to and interact with target cells and tissues in controlled ways to induce desired physiological responses with a minimum of undesirable effects. To fulfill this goal the fabrication of nano-engineered materials and devices has to consider the design of natural systems. Thus, engineered micro-nano-featured systems can be applied to biology and biomedicine to enable new functionalities and new devices. These include, among others, nanostructured implants providing many advantages over existing, conventional ones, nanodevices for cell manipulation, and nanosensors that would provide reliable information on biological processes and functions.

Carbon nanotubes based engineering materials for thermal management applications

We developed innovative solutions for reaching high performance in carbon-nanotube-filled engineering materials. Electrospinning was applied to improve the thermal conductivity in polymer composites via the alignment of nanotubes in a polymer matrix. Alignment was achieved by flow-confinement and charge-induced alignment during electrospinning. Additionally, the use of liquid crystal polymer as a matrix increased the degree of alignment leading to the remarkable increase of the thermal conductivity in composites by a factor 33. We developed the reduction from method to produce metal-matrix composites filled with carbon nanotubes. We were able to engineer the coefficient of thermal expansion (CTE) of the copper composite, for example 3 wt% of carbon nanotubes added to copper yielded CTEs comparable with ceramics and semiconductors. In situ thermal polymerization of natural oils (plant and fish) was applied to produce nanotubes-based thermal greases. This method creates novel, environmentally friendly thermal grease with excellent thermal conductivity (increased by a factor 12), that is easy to handle compound and to remove. Such thermal greases can be applied to surfaces by various methods, including screen printing, and demonstrate good thermal stability, reduced thermal expansion, and no pumping-out effect.