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ThermoTex Report Summary

Project ID: 637624
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - ThermoTex (Woven and 3D-Printed Thermoelectric Textiles)

Reporting period: 2015-06-01 to 2016-11-30

Summary of the context and overall objectives of the project

Imagine a world, in which countless embedded microelectronic components continuously monitor our health and allow us to seamlessly interact with our digital environment. One particularly promising platform for the realisation of this concept is based on wearable electronic textiles. In order for this technology to become truly pervasive, a myriad of devices will have to operate autonomously over an extended period of time without the need for additional maintenance, repair or battery replacement. The goal of this research programme is to realise textile-based thermoelectric generators that without additional cost can power builtin electronics by harvesting one of the most ubiquitous energy sources available to us: our body heat.

Current thermoelectric technologies rely on toxic inorganic materials that are both expensive to produce and fragile by design, which renders them unsuitable especially for wearable applications. Instead, in this programme we will use polymer semiconductors and nanocomposites. Initially, we will focus on the preparation of materials with a thermoelectric performance significantly beyond the state-of-the-art. Then, we will exploit the ease of shaping polymers into light-weight and flexible articles such as fibres, yarns and fabrics. We will explore both, traditional weaving methods as well as emerging 3D-printing techniques, in order to realise low-cost thermoelectric textiles.

Finally, within the scope of this programme we will demonstrate the ability of prototype thermoelectric textiles to harvest a small fraction of the wearer’s body heat under realistic conditions. We will achieve this through integration into clothing to power off-the-shelf sensors for health care and security applications. Eventually, it can be anticipated that the here interrogated thermoelectric design paradigms will be of significant benefit to the European textile and health care sector as well as society in general.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The aim of the project is the development of materials and processing routines to fabricate flexible thermoelectric devices with organic semiconductors. The project is organised in 4 workpages that have the following sub-goals:
WP1 – development of materials for WP2 and WP3
WP2 – use of textile manufacturing techniques (e.g. weaving and embroidery) to pattern thermoelectric devices
WP3 – use of 3D printing to pattern thermoelectric devices
WP4 – fabrication of prototype thermoelectric generators.

In the following I will describe the progress in each work package during the first reporting period. WP1 is divided into 4 tracks to improve materials with improved thermoelectric performance.
Track 1 focuses on hybrid nanocomposites of nanofillers such as carbon nanotubes and a polymer matrix. A major breakthrough (achieved in collaboration with ICMAB-CSIC, Spain) was the development of a nanocomposite processing strategy that allows to change the type of majority carriers. We were able to use UV light to switch an initially p-type material to an n-type nanocomposite. This is relevant because it allows us to simplify the patterning process needed to define the n- and p-type legs of thermoelectric devices. Also, since the n-type material is stable at ambient conditions, our work addresses a current bottleneck in the field, i.e. the poor durability of n-type semiconductors. This work was published in Advanced Materials (DOI: 10.1002/adma.201505521); a patent application has been filed on the described concept.

Track 2 deals with new dopant strategies. We have developed a new design concept for conjugated polymers that display improved miscibility and stability with commonly used molecular dopants. We attach ethylene glycol side chains to the conjugated backbone. The increase in polarity improves the processability and environmental stability of the polymer with polar molecular dopants. The result is a drastic improvement in thermoelectric performance. For instance, we are now able to process a polythiophene and model dopants such as F4TCNQ from organic solvents and obtain an electrical conductivity of 100 S/cm. A manuscript that summarizes this work is currently under review.
Track 3 aims at improving the understanding of structure-property relationships that govern doping of organic semiconductors. We chose to study a model system, the commonly studied conjugated polymer P3HT and F4TCNQ, during the first part of the project. We explore a doping strategy that is based on exposure of the polymer to vapor of the dopant. This approach allows us to disentangle the process of solid-state structure formation (i.e. we can spin-coat samples without the presence of dopant molecules that disrupt packing of the semiconductor) from the doping process. As a result we have considerably improved our understanding of the impact of parameters such as the degree of structural order, the structural perfection of the polymer (regio-regularity) and the molecular weight of the polymer on the electrical conductivity of doped samples. Importantly, this approach now allows us to achieve record conductivities even with the P3HT:F4TCNQ model system. A first manuscript about this work is close to submission.

Track 4 focuses on semiconductor:insulator polymer blends. The addition of a commodity polymer has the purpose to dilute the more expensive semiconductor, and at the same time improve its mechanical properties. We have studied one model system comprised of P3HT, F4TCNQ and polyethylene oxide (PEO). Careful tuning of the processing conditions allowed us to prepare ternary blends that, at a content of more than 60% insulator, displays the same thermoelectric performance as the neat doped conjugated polymer. Importantly, the mechanical properties of the resulting blend is significantly improved, which is critical for the development of materials that we need for WP2 and WP3 (See Figure to the right; a mechanically robust tape composed of the F4TCNQ:P3HT:PEO ternary blend). This work was reported in a recent publication (DOI: 10.1002/advs.201600203). We are now in the process of combining the knowledge gained from Tracks 2 and 4 to further improve the quality of our materials.

WP2 deals with the fabrication of tough thermoelectric fibres and their integration into textiles. We decided to carry out an initial study with the current champion p-type material (PEDOT:PSS) until better materials from WP1 become available. We prepared yarns with exceptional wear and wash resistance through dyeing of silk from the silkworm Bombyx mori with the conjugated polymer:polyelectrolyte complex PEDOT:PSS. Importantly, we have developed a robust and scalable dyeing process that results in up to 40 m long yarns. These yarns maintain their bulk electrical conductivity of about 14 S/cm when experiencing repeated bending stress as well as mechanical wear during sewing. This aspect is critical because it allows us to integrate these fibres into textiles with standard manufacturing techniques. Through a collaboration with the Swedish School of Textiles at the University of Borås we are now in the process of evaluating the use of these fibres in large(r) volume automated embroidery and weaving machines. Another advantage of the developed fibres is the high degree of ambient stability, which is paired with the ability to withstand both machine washing and dry cleaning. A summary of this work has been submitted for publication.

The availability of the PEDOT:PSS dyes at this stage of the project has allowed us to prepare a first small-scale prototype, as envisioned in WP4. We fabricated an in-plane thermoelectric module that comprises 26 p-type legs is demonstrated by embroidery of dyed silk yarns onto a piece of felted wool fabric.

Finally, it is worth mentioning that the ThemoTex team has written a tutorial review in Chemical Society Reviews, where we present our vision for thermoelectric plastics (DOI: 10.1039/c6cs00149a). This effort to formulate a coherent set of research questions was an excellent opportunity to build a well-functioning team, from which the project will no doubt benefit during the remaining 3.5 years.

The ThermoTex team has presented the progress of the project on 17 occasions during the first 18 months of the project, of which 5 were public outreach activities (cf. Appendix 1). Three study visits to Imperial College London, IIT Milan and the CHESS synchrotron in Ithaca, US were carried out.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The ThermTex team has carried out five outreach activities so far, including one exhibition at Chalmers that was well received. We have noticed that the concept of electronic textiles and in particular the possibility to generate electricity with textiles is a concept that can be easily conveyed to a general audience. Thus, we use this opportunity to explain to the general public what type of work is carried out at universities. It should also be mentioned that one patent application was filed on a materials concept that was developed with colleagues at ICMAB-CSIC in Spain during the first part of the project. We are now in the process of identifying stakeholders that can benefit from this new technology.
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