Fabrication and characterization of transducers for environmental monitoring based on emerging flexible technologies

This thesis explores the development of flexible transducers designed for environmental monitoring, focusing on leveraging emerging technologies in printed and flexible electronics. With increasing global emphasis on sustainable, lightweight, and adaptable sensing solutions, this research aims to create devices that are not only high-performing but also environmentally resilient, mechanically flexible, and cost-effective. The work integrates innovative materials such as laser-induced graphene (LIG), metal-organic frameworks (MOFs), and conductive polymers, combining them with advanced fabrication techniques like laser scribing and solution processing to realize flexible sensors and energy harvesters suitable for real-world applications. Through the development and characterization of two device classes, metal organic framework (MOF), polymer thermoelectric generators (TEGs) and laser-induced graphene (LIG) thermistors, this dissertation enhances flexible printed transducer technology. Making flexible thermistors on polymer substrates, particularly polyetherimide (PEI) and polyimide (PI), is a key aspect of this research. In the fabrication process, these polymers are transformed into conductive graphene-like structures using a technique called laser scribing, which uses a concentrated laser beam. This approach is particularly appealing for scalable manufacturing since it provides a quick, maskless, and eco-friendly means of directly patterning electrical circuits onto flexible substrates. To obtain the intended morphology and electrical characteristics of the resulting laser-induced graphene, the thesis explores the optimization of laser parameters like power, scan speed, and number of passes. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and scanning electron microscopy (SEM) are some of the methods used to thoroughly examine the morphology and chemical makeup of the LIG patterns. According to these findings, high-quality, porous, and highly conductive graphene structures are produced by the laser scribing technique; the attributes of these structures can be adjusted by adjusting process parameters. These thermistors' electrical and sensing capabilities are then assessed in a range of environmental settings. Because of their exceptional humidity responsiveness, PI-based LIG thermistors are appropriate for environmental sensing applications where humidity monitoring is essential. On the other hand, PEI-based thermistors have remarkable mechanical flexibility, environmental resilience, and temperature sensitivity. The devices' potential for wearable applications is confirmed by mechanical bending tests, which show that they continue to function consistently even after repeated flexing. For sensors used in realworld settings, stability and endurance are crucial, and humidity testing provide additional confirmation of these qualities. The development of lightweight, conformal environmental monitoring systems is made possible by the quick and accurate fabrication of these sensors on flexible substrates. Porous graphene networks were made for thermistors by using a laser to cut thin polymer films (200 μm PEI and 150 μm Kapton PI). Imaging showed a rough, foam-like 3D graphene structure, and testing confirmed the presence of characteristic carbon bands. To get even layers of graphene with suitable electrical resistance (about 10^5 ohms per square), laser settings were optimized (around 2 W power, 70 mm/sec speed, 70 μm spacing). The graphene made from PEI had slightly better quality than that from PI, with a lower defect ratio. Tests also showed that laser treatment removed oxygen and nitrogen groups, increasing the amount of well-organized carbon. This shows high-quality graphene was made on both materials using a simple, maskless laser process. The graphene films act as thermistors, which change resistance with temperature. Across - 30°C to 70°C, PEI-based sensors had a resistance change rate (TCR) of about -0.159%/°C and - 0.139%/°C, while PI-based ones had lower rates. All sensors showed a strong, consistent response. Notably, PEI-LIG thermistors were almost unaffected by humidity, making them easier to calibrate, while PI-LIG sensors were more sensitive to moisture. The PEI sensors changed resistance by only about 7.8% and 22.5% in different designs, and the PI sensors by about 10% and 25%. They also remained durable after 300 bending cycles, maintaining their flexibility. Overall, PEI-LIG sensors are accurate, work well over a wide temperature range, are sensitive to temperature, and are not much affected by humidity. The laser method used to make them is cheap, eco-friendly, and scalable, suitable for creating temperature sensors for environmental or wearable uses. Additionally, the thesis explores the incorporation of functional materials, particularly MOFs, for energy harvesting applications in addition to temperature sensors. Crystalline porous materials with a high surface area, adjustable chemistry, and energy conversion potential are known as metal-organic frameworks. The synthesis of UiO-66 MOFs and their thiolfunctionalized versions (UiO-66(SH)₂) is the main objective here. Because of the ability to modify their chemical structure to enhance electrical conductivity and thermoelectric qualities, these MOFs were selected. Solvothermal techniques are used in the synthesis, and then thiol groups are functionalized to improve charge transfer and interaction with conductive matrices. These MOFs are characterized thoroughly using techniques such as Thermogravimetric Analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR), and elemental mapping, confirming their structural integrity and functionalization. To produce flexible thermoelectric composites, the MOFs are integrated into conductive polymer matrices such as PEDOT: PSS. To guarantee that MOF particles are evenly distributed throughout the polymer matrix, the mixing procedure is improved. Electrical and thermoelectrical characterizations of the resultant composites show notable advancements over pure polymer films. Interestingly, the addition of MOF particles raises the Seebeck coefficient and electrical conductivity, two important metrics for thermoelectric performance. Raman and FTIR tests show strong bonds between the MOF particles and the polymer, helping charge move better and improving thermoelectric performance. Although these MOF-based devices produce less power than traditional rigid ones, their flexibility, stability, and light weight make them good for portable, eco-friendly energy harvesting. The MOFs (UiO-66 and a thiol-functionalized version) were made using a heat-based method called solvothermal synthesis. For example, zirconium propoxide and terephthalic acid reacted in a special liquid at 130°C. After washing, tests confirmed the correct structure of the MOFs. The MOFs were then mixed into conductive inks and printed onto flexible surfaces like paper, cork, PEI, and PI. For example, a layer of pure UiO-66 was drop-cast onto a surface, then a second layer of UiO-66 mixed with a conductive polymer (PEDOT:PSS) was added. Both layered and mixed methods were used to make these films. The result was a flexible composite where rigid MOF crystals are embedded in a stretchy polymer, combining good charge flow with low heat transfer. These films were made using simple printing methods, which are cheap and easy to scale up. The devices showed p-type behavior (positive Seebeck coefficient of about +10 to +20 μV/K). However, they generated very small voltages and power. For example, a device made with layered UiO-66 on paper produced about 2.5 nanowatts at a temperature difference of 68.6 K, while pure PEDOT:PSS made around 5 microwatts. The power increases with temperature difference, but adding MOF reduced the electrical conductivity and power because MOF is more insulating. Mainly, the MOF acts as a stable, low-heat-loss scaffold, while PEDOT:PSS carries the electrical charge. The printed MOF-polymer films showed excellent durability, even though they produced only small amounts of power. The MOF particles helped prevent the films from peeling or cracking when gently bent, keeping them flexible and intact. Similar PEDOT:PSS composites in other studies can withstand hundreds of bends with little change in resistance. Importantly, the UiO- 66 framework makes the composite stable in different environments. Because UiO-66 is very resistant to breaking down in water, the composite doesn’t degrade in humid conditions. Instead, the strong MOF helps stabilize the moisture-absorbing PEDOT:PSS and keeps its electrical properties steady. These UiO-66/PEDOT films are expected to maintain consistent conductivity in normal air humidity without needing special waterproofing. This makes them useful for wearable or outdoor energy harvesting, where environmental stability is important. In conclusion, this work shows that printable MOF/polymer deposition and maskless laser writing can produce workable flexible transducers for energy harvesting and sensing. Structure and function optimization was made possible by the thorough material analysis (SEM, Raman, XPS for LIG; XRD, FTIR, and TGA for MOFs). The integration of UiO-66 with PEDOT:PSS to create flexible TEG devices and the enhanced sensitivity and humidity immunity of PEI-LIG thermistors are important discoveries.