| Περίληψη: | In this doctoral dissertation, materials for development of electronic devices via printing processes were evaluated. More specifically, two silver-based inks with different sintering requirements (chemical and thermal sintering), a conductive polymer (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)), an ethyl-cellulose stabilized graphene dispersion in cyclohexanone – terpineol and a water-based functionalized reduced graphene oxide ink were used for developing humidity, temperature and strain sensors on paper and polyimide substrate. Also, electrical contact methods were evaluated; pre-patterned copper tracks on polyimide were interfaced with inkjet-printed graphene and silver nanoparticle structures via direct over-printing and commercial connectors.
A comprehensive literature review was performed for identifying possible material – substrate combinations that could lead to cost-effective, repeatable production of sensors at a mass scale via inkjet printing. Two resistive output humidity sensors, one with Ag-nanoparticle based electrodes and one with PEDOT:PSS on paper substrate were developed, demonstrating the usage of paper substrate as a humidity sensing film with a mean resistance change of one order of magnitude per 10 %rH in the range of 0 to 30 %rH and a mean resistance change of one order of magnitude per 20 %rH in the range 30 – 90 %rH.
Ag nanoparticle and PEDOT:PSS-based temperature sensors on paper, and graphene and f-rGO temperature sensors on polyimide substrate were developed and evaluated for their electrical characteristics and their Thermal Coefficients of Resistance (TCR) were extracted as 9.389×10-4 oC-1, -0.0139 oC-1, -1.94x10-3 oC-1 and -1.64x10-2 oC-1, respectively. Graphene-based materials exhibited good thermal cycling endurance and a mean response time of 2.47 s (graphene) and 2.94 s (f-rGO).
Ag nanoparticle and PEDOT:PSS-based strain sensors were evaluated for creating a complete sensing platform alongside the aforementioned sensors; gauge factors of GFAgTensile = 0.4259 and GFPEDOT:PSSTensile = 0.1422 for tensile strain and GFAgCompress = -0.1572 and PEDOT:PSS GFPEDOT:PSSCompress = 0.1448 for compressive experiments. Also, long-term mechanical strain sensing was evaluated with a mechanical XY precision stage; samples were bended for 1000 cycles and resistance was measured in parallel. Ag nanoparticle-based strain sensor outperformed PEDOT:PSS in terms of mechanical endurance.
2D thermal flow sensors were studied and two approaches for fabrication were investigated: firstly, a 2D thermal flow sensor consisting of discrete SMT platinum elements which acted both as micro-heaters and sensing elements, was developed on prelaminated polyimide with copper tracks. Capabilities to detect flow amplitude with two modes of operation (namely, constant current and constant temperature) and angle of attack were assessed via a custom measurement setup. Transfer of technology to a fully printed device was performed via screen printing of active components consisting of activated carbon and BaTiO3 which acted as heaters and sensing elements on PET substrate. The same modes of operation were evaluated and characteristics regarding flow detection capabilities were extracted. For the SMT based flexible device on polyimide, an artificial neural network was also developed for accompanying and correcting the measurements offline. Both flow sensors were capable of detecting flows in the range of 0 to 25 SLPM, while constant temperature mode was evidently more sensitive (12.6 mW/(m/s) versus -2.80 mW/(m/s) for constant current mode) for the SMT sensor.
Different methods for electrical interfacing between printed lines and traditional Cu-based electronics on polyimide was also performed. It was concluded that depending on the application, commercial mechanical connectors such as Amphenol FPC and Clincher can provide adequate electrical interfacing. Also, as also evaluated by finite element analysis, the engagement points and measurement method are of major importance in electrical field formation and current density.
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