Assessment of electromechanical properties of screen printed polymer nanopastes

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Abstract

Printed electronics has provided different printing techniques as environmentally friendly and cost-effective manufacturing methods of electronic components. The printed items can be produced on low cost, different types of flexible substrates, even when their surface is corrugated. This opens a new application range of printed electronics and makes them competitive with traditionally manufactured electronics. However, it is necessary to investigate new materials to continue the rapid progress in printed electronics. In our study, the electromechanical properties of polymer nanopastes consisted of carbon nanotubes and graphite platelet nanofibers mixed with a conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were investigated. Their microstructure and the layer morphology were observed using a scanning electron microscope and an optical microscope. The thickness and average roughness of the layers printed on the foil and paper were determined with a contact profilometer. The mechanical durability of the screen printed layers was verified in a cyclic bending test. The highest mechanical durability was exhibited by the polymer pastes containing carbon nanotubes.

Highlights

Carbon nanotubes and graphite nanofibers exhibited a tendency to agglomeration. ► Layer thickness appeared to be used nanopaste- and substrate-dependent. ► EL-P3040/GNF exhibited higher sheet resistance than EL-P3040 and EL-P3040/CNT. ► Multilayer printing caused the layers to be less durable to cyclic bending.

Introduction

Electronic components, such as thin film resistors embedded in printed wiring boards (PWBs), are manufactured using a combination of etching, resist striping, lamination and plating processes. This subtractive technology uses a wide variety of chemicals which are costly to remove. Further, electronics industry is investing significantly on technologies that would allow miniaturization of electronics devices. One of the most prominent new technologies satisfying these needs is printed electronics. Printed electronics is an additive process, which uses various printing techniques, known from graphics industry, e.g. inkjet [1], [2], [3] and screen printing [4], [5], [6], gravure [7], [8]) (see Table 1) for interconnecting or manufacturing components.

Printed components can be produced on rigid substrates as well as on flexible ones (foil, paper, fabrics) what constitutes their substantial advantage.

Development of flexible electronic components requires investigations of new materials characterized with high durability to mechanical stresses. Carbon nanotubes (CNTs) are a promising material of excellent mechanical and thermal properties [9]. The tensile strength of CNTs is 45 GPa, while for comparison high-strength steel alloys breaks at about 2 GPa [10]. CNTs are stable up to 700 °C in air and up to 2800 °C in a vacuum and furthermore, their thermal expansion can be neglected [11]. De et al. [12] carried out investigations of polymer pastes with carbon nanotubes. After 130 bending cycles the printed layer electrical resistance increased by about 1%. Another experiments described in [13] revealed the electrical resistance growth by 9% and by 27% after an exposure of the PMMA/CNT0.5% and PMMA/CNT1% layers 300,000 bending cycles.

Apart from the mechanical durability of the printed components their electrical resistance was also taken into consideration. Results obtained by Carroll et al. [14] and Tsai and Huang [15] showed that an increase in the CNTs concentration in a paste caused a decrease in the layer electrical resistance. The multilayer printing had the same impact on the printed pattern electrical resistance [16], [17].

Graphite nanofibers (GNFs) are a relatively new kind of carbon materials which attracted much attention due to their mechanical and thermal stability, low ohmic resistance and easy mass production. When compared to the carbon nanotubes, GNFs process much more edge sites and active groups on outer walls [18].

Similarly to CNTs, GNFs were investigated to be used as a filler in polymer composites. The electrical, thermal and mechanical properties of poly(methyl methacrylate) reinforced with graphite nanofibers were examined by Seo and Park [19]. Their investigations showed that the GNFs/PMMA composites exhibited higher thermal and mechanical durability compared to the neat PMMA. Further, an electrical percolation threshold was found to be between 1 and 5% (by weight) GNFs contents.

The thermal and electrical properties of PEDOT:PSS/expanded graphite (EG) composite were described by Culebras et al. [20]. The electrical resistance of the PEDOT:PSS/EG films decreased sharply upon addition of expanded graphite, but their thermal stability was not greatly improved. Other studies [21], [22], [23], [24], [25] reported also improvement in the electrical properties of polymer composites reinforced with functionalized graphene sheets.

In this paper, electromechanical properties of pastes contained a conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and carbon nanotubes or graphite platelet nanofibers as an nanoadditive were investigated. Their microstructure, the layer morphology and average roughness were analyzed. The thickness and electrical resistance of printed layers were measured. Their mechanical durability was evaluated in a cyclic bending test.

Section snippets

Experimental

The polymer pastes investigated in this study contained CNTs or GNFs. Both of them were added to the polymer carrier in the form of the commercial paste EL-P3040 (Agfa Gavaert) containing a conductive polymer PEDOT:PSS. These two ingredients were first mixed, then grinded in a mortar and finally three-roll-milled in order to break agglomerates.

The commercial paste EL-P3040 is composed of following ingredients: polymer PEDOT:PSS (2–3%), thermoplastic binder (2%), diethylene glycol (5–10%), ethyl

Microstructure observation

The microstructure of the layers printed with the tested pastes is presented in Fig. 1. The SEM images showed the nanoparticles to be chaotically dispersed in the polymer carrier. Particularly in the case of graphite nanofibers in the EL-P3040/GNF2% layer their clusters were noticeable what could influence electromechanical properties of the investigated samples.

Both carbon nanotubes and graphite nanofibers had a tendency to agglomerate because of the existence of van der Vaals forces. This

Summary

In this paper, electromechanical properties of screen printed polymer pastes with multi-wall carbon nanotubes (CNT) or graphite platelet nanofibers (GNF) were preliminarily assessed. The microstructure, thickness, average roughness, electrical resistance and mechanical durability of the layers printed with the prepared nanopastes were taken into consideration.

The SEM investigations of the printed layer microstructure revealed a strong tendency of carbon nanotubes and graphite nanofibers to

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