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Final Report Summary - THERMONANO (Low-temperature heat exchangers based on thermally-conducting polymer nanocomposites)

The research activities aimed at producing two heat exchanger prototypes for the two end-uses following a progressive three-stage path:
- Development of fillers and compounds in which a wide range of polymers (PP, PVDF, PA66) and fillers (graphite fibres, carbon nanotubes (CNTs), metal coated nanoparticles or fibres) were modified by the compounding of conductive particles and characterised by an intensive testing and characterisation campaign. A fundamental modelling work was also developed for the design of the optimal microstructure of the heat-exchange material.
- Tailoring of polymer processing techniques (injection moulding, compression moulding, extrusion). This experimental and modelling work line was carried out in close contact with the design of innovative heat exchanger.
- Manufacturing and testing of proof-of-concept heat exchangers for the given end-uses.

A virtual platform based on a multi-level model approach (fundamental modelling of heat transfer, computational fluid dynamics (CFD) design of the THERMONANO heat exchangers, system modelling for the virtual implementation of the heat exchangers in various processes) will allow to drive materials developments and evaluate the impact of the developed technologies in various application contexts.

Polymer-filler compounds were developed by NCYL, CEA, Polito, PISAS and SGLC following different strategies, while thermal characterisation of materials have been carried out at TUBAF. Development was completed at the end of the first year, with the development and selection of both CNTs and metal-coated particles. In this task, Nanocyl experimented the possibility to change some of the parameters of the chosen CNTs by modifying the residual content and its nature, the level of defects, the length and the presence of functional groups but also creates other types of CNT materials. Besides CNT, metallisation of different substrates of various shapes was carried out by PISAS. Elected particles included glass flakes, wollastonite nanofibres, polyamide flakes and cellulose fibres. In all the cases, silver was used for metallisation due to many advantages compared to other metals, including high conductivity, stability, reasonable price of the precursor, and simple coating procedure.

As a first activity, selected conductive particles (both CNTs and metal coated particles) were included in thermoplastic polymers (PP, PVDF, PP) as a first screening of thermal conductivity properties obtainable. Detailed studies were performed investigating full concentration range of the filler as well as degree of dispersion of the conductive particles.

Both CNT and metal nanoparticles showed improvements in term of thermal conductivity lower than expected, when used alone at low loading: for instance, conductivity of about 1 W/mK was obtained when using as much as 20 % CNTs. On the other hand, relatively high loading of metal coated particles was needed to reach significant conductivity of polymer composites (e.g. 2 W/Km for composites with filler concentration 60 wt %). Such high loadings were considered to be insufficient taking into account increasing stiffness (resulting in brittle material) and viscosity (leading to higher energy consumption during processing and expected problems regarding final look of the products, especially if injection moulding would be applied as preferred technology). Therefore, after thorough discussion the option of metal coated particles was abandoned and the experiments with the metallised particles did not continue.

The main bottleneck to the efficiency in thermal exchange was identified into the contact resistance between adjacent nanoparticles, which is associated to a drop in temperature at every contact point between two conductive particles. A detailed discussion of this topic is reported in one the scientific publication held in this project (Z. Han, A. Fina: 'Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review'. Progress in Polymer Science 2011, 36, 914-944).

New designs were developed for the polymer nanocomposites heat exchanger, taking into account the ease of processing of polymers into complex shapes. Details of the designs cannot be given in the public report. These three processing technologies were selected based on a detailed evaluation of cost-effectiveness and performance of the heat exchange surfaces obtainable by several polymer processing technologies. No details about processing conditions can be provided in the public report, due to the intellectual property (IP) policy of the project.