Lapshuda V. Humidity sensors based on nanocellulose for flexible electronics

Lapshuda V. A. Humidity sensors based on nanocellulose for flexible electronics. – Qualifying scientific work on manuscript rights. Dissertation for obtaining of the scientific degree of Doctor of Philosophy in specialty 153 – Micro- and nanosystem engineering (field of knowledge 15 – Automation and instrument engineering). - National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Kyiv, 2024. The work is devoted to the study of nanocellulose and its modifications for use as a humidity-sensitive layer in flexible sensors of relative air humidity. Scientific and applied research, highlighted in the dissertation, is focused on the practical study of the characteristics of flexible sensors of relative air humidity based on nanocellulose, composites based on nanocellulose and carbonized nanocellulose. Biodegradable materials are increasingly used in modern scientific and technical research of flexible sensors of relative air humidity. Sensors made based on biodegradable materials do not need to be disposed of at the end of their useful life. However, sensors based on such materials are not durable. On the other hand, there is a need for the development of cheap disposable sensors for several applications. For example, it can be in the medical field, sports, everyday life: for monitoring physical activity and/or rehabilitation of athletes, military personnel or people with disabilities (breathing monitoring, sweating registration, etc.). For research in this work, nanocellulose obtained from non-wood raw materials (namely from plants that are agricultural waste) was used. Based on NC and its modifications, solid-state humidity sensors, as well as flexible humidity sensors within two approaches, drop-casting and ‘‘self-standing’’, were developed in the work. The type of developed sensors is resistive and capacitive. In the first chapter, the designs, and physical principles of operation of humidity sensors were investigated. Oil based polymers (PI, PA, PET, PEN, PDMS) often produce flexible substrates for humidity sensors. From the point of view of ecology, the most promising materials based on which it is possible to create flexible humidity sensors are biodegradable polymers (films based on gelatin, chitin, chitosan, polylactic acid, nanocellulose, etc.). In the second section, the influence of design parameters on the characteristics of capacitive sensors based on interdigital electrode and capacitive sensors based on electrodes in the configuration of an expanded capacitor were modeled. It was established that NC adsorbs humidity according to the Langmuir isotherm. As a result, it became possible to simulate the response of the sensor to changes in the level of relative air humidity. In the third chapter, the influence of the static parameters (response, sensitivity, hysteresis, repeatability) and dynamic parameters (response and recovery time, short-term and long-term stability) of relative humidity sensors depending on the mass of NC, raw materials and the method of its synthesis were investigated. Thus, it was established that the NC obtained by the method of oxidation in the TEMPO solution shows much better sensitivity for both resistive and capacitive sensors, regardless of the initial material. The thinnest films provided the highest speed for obtained sensors. In chapter four, flexible sensors based on a PI substrate, an NC substrate, an NC/PVA nanocomposite substrate, and carbonized nanocellulose were investigated. It should be noted, sensors based on NCs and NC/PVA nanocomposites used a substrate as a humidity-sensitive layer. PVA/NC composites were developed to improve the mechanical parameters of the nanocellulose film. It was established that the strength of such films drops sharply, but plasticity increases. From the point of view of electrical parameters, sensors based on NC showed the best sensitivity, but the speed of such sensors remained quite low. To improve speed and other dynamic parameters, a new material based on nanocellulose was synthesized. The material was obtained by pyrolysis of nanocellulose films in a vacuum. As a result of this operation, it was observed a lost up to 80% of mass with a minimal volume change. It was established that this material has a significantly higher action speed than the nanocellulose film. In the fifth chapter, the application of flexible air humidity sensors based on nanocellulose and its modifications for medical applications was investigated. The optimal composite materials is the composite NC/PVA, while pure NC and pure PVA show worse results. Such sensors allow to clearly distinguish the frequency of breaths, detect breath delays and distinguish the force of breath.