A higher order control volume based finite element method to prodict the deformation of heterogeneous materials

Materials with obvious internal structure can exhibit behaviour, under loading, that cannot be described by classical elasticity. It is therefore important to develop computational tools incorporating appropriate constitutive theories that can capture their unconventional behaviour. One such theory is micropolar elasticity. This paper presents a linear strain control volume finite element formulation incorporating micropolar elasticity. Verification results from a micropolar element patch test as well as convergence results for a stress concentration problem are included. The element will be shown to pass the patch test and also exhibit accuracy that is at least equivalent to its finite element counterpart

Deoxyribonucleic Acid as a Universal Electrolyte for Bio-Friendly Light-Emitting Electrochemical Cells [in press]

In the search for bio and eco‐friendly light sources, light‐emitting electrochemical cells (LECs) are promising candidates for the implementation of biomaterials in their device architecture thanks to their low fabrication complexity and wide range of potential technological applications. In this work, the use of the DNA derivative DNA‐cetyltrimethylammonium (DNA‐CTMA) is introduced as the ion‐solvating component of the solid polymer electrolyte (SPE) in the active layer of solution‐processed LECs. The focus is particularly on the investigation of its electrochemical and ionic conductivity properties demonstrating its suitability for device fabrication and correlation with thin film morphology. Furthermore, upon blending with the commercially available emissive polymer Super Yellow, the structure property relationship between the microstructure and the ionic conductivity is investigated and yields an optimized LEC performance. The large electrochemical stability window of DNA‐CTMA enables a stable device performance for a variety of emitters covering the complete visible spectral range, thus highlighting the universal character of this naturally sourced SPE

Analysis of the tilted flat punch in couple-stress elasticity

This paper was accepted for publication in the journal International Journal of Solids and Structures and the definitive published version is available at https://doi.org/10.1016/j.ijsolstr.2016.01.017.In the present paper we explore the response of a half-plane indented by a tilted flat punch with sharp corners in the context of couple-stress elasticity theory. Contact conditions arise in a number of modern engineering applications ranging from structural and geotechnical engineering to micro and nanotechnology. As the contact scales reduce progressively the effects of the microstructure upon the macroscopic material response cannot be ignored. The generalized continuum theory of couple-stress elasticity introduces characteristic material lengths in order to describe the pertinent scale effects that emerge from the underlying material microstructure. The problem under investigation is interesting for three reasons: Firstly, the indentor's geometry is simple so that benchmark results may be extracted. Secondly, important deterioration of the macroscopic results may emerge in the case that a tilting moment is applied on the indentor inadvertently or in the case that the flat punch itself is not self-aligning so that asymmetrical contact pressure distributions arise on the contact faces. Thirdly, the voluntary application of a tilting moment on the flat punch during an experiment gives rise to potential capabilities of the flat punch for the determination of the material microstructural characteristic lengths. The solution methodology is based on singular integral equations which have resulted from a treatment of the mixed boundary value problem via integral transforms and generalized functions. The results show significant departure from the predictions of classical elasticity revealing that valuable information may be deducted from the indentation of a tilted punch of a microstructured solid

Sustainable Materials and Process Techniques for Engineering Solution-Based Organic Light-Emitting Devices

Advances in organic light emitting devices are crucial for the development of the display and solid state lighting (SSL) technologies. This dissertation is organized and pursued in three main projects to meet some problems in the field. Printing technologies can be the key to next-generation affordable, flexible, large area displays and lighting elements by eliminating vacuum processing. In the first part of the thesis, the conventional gravure printing technique was adapted for the processing of emissive layers in the small molecule based organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs). The homogeneous printed layers were granted by either modifying the functional ink properties or altering the printing process parameters. Different functional inks comprising the small molecule as an emissive material were formulated by adjusting viscosity, surface tension, and solvent drying kinetics of the inks. As for the process parameters, the gravure cell parameters such as line screen and tone values were altered to control the overall transfer volume of the ink and the thickness of the printed layers. In both cases, the electrically inert polymers were used as host materials to modify the rheological behavior of the ink while suppressing the aggregation of the small molecule in a solid film. The thin film characteristics of printed layers were analyzed in both qualitative and quantitative ways. The printed films were successfully implemented in the active layer of efficient small molecule based electroluminescent devices on flexible plastic foil. The optical and electrical device performance were considered as well as the effect of the printing process in comparison to spin-coated pristine small molecule based reference devices. The quality and performance of the printed emissive layers in both device type showed that the gravure printing method can be an alternative solution for wet-processing roll-to-roll (R2R) manufacturing in the future. White light-emitting diodes draw particular attention in the field, due to their potential application as the backlight in displays or as energy efficient luminaires for SSL. Even though polymer OLEDs are well-suited for wet-based continues R2R fabrication, evaporation of low work function cathodes and therewith encapsulation remain as major obstacles. In the second part of the work, a novel hybrid device architecture was suggested for the color-tuning and white light emission in polymer light-emitting diodes. The single component polymer LEC layer performed as the electron injection layer as well as the second emissive layer on top of a conventional polymer OLED stack. The hybrid structure maintained a sufficient charge carrier injection from an air-stable cathode, due to the unique operation principles of LECs. As a proof of charge transport at the intersection of two emissive layers, dual color emission was simultaneously observed in a bilayer device configuration. A color-tuning in emission was obtained by changing the thickness of the LEC layer. The emission of hybrid devices was shifted from yellow to white light emission region of the CIE color chromaticity diagram, resulting in OLEDs with the high color temperature values. The results demonstrated that this approach showed a promising potential to achieve color-tuning and white light emission from solution processed OLEDs bearing air-stable cathodes. Sustainable bioelectronics is an emerging technology which to replace conventional electronics with disposable counterparts in the future. Thus, bioinspired and bioderived materials usage in organic electroluminescent devices gained much attention in the last years. In the last part of the thesis, we investigated biodegradable natural and naturally derived polymers such as gelatin, deoxyribonucleic acid (DNA) as the ion-solvating polymers in the emissive layer of polymer LECs. Notably, we focused on DNA and DNA-lipid complex based polyelectrolytes due to the unique hybrid ionic/electronic conductivity behavior of DNA. Different solid polymer electrolytes (SPE) were tested with varying additives of salts at different ratios towards improving the ionic conductivity. Additionally, the electrochemical stability window of SPEs was defined to eliminate nonreversible electrochemical side reactions during device operation. The optoelectrical device characteristics, as well as lifetime measurements, were obtained to determine the stability of LECs. Furthermore, the surface morphology of the active layers was investigated to characterize the phase separation between SPE and emissive polymer and aggregations in thin films, which have a significant influence on the device performance. Biosolid polymer electrolytes were successfully implemented in LECs as promising materials of bio-based LECs

Sustainable Materials and Process Techniques for Engineering Solution-Based Organic Light-Emitting Devices

Advances in organic light emitting devices are crucial for the development of the display and solid state lighting (SSL) technologies. This dissertation is organized and pursued in three main projects to meet some problems in the field. Printing technologies can be the key to next-generation affordable, flexible, large area displays and lighting elements by eliminating vacuum processing. In the first part of the thesis, the conventional gravure printing technique was adapted for the processing of emissive layers in the small molecule based organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs). The homogeneous printed layers were granted by either modifying the functional ink properties or altering the printing process parameters. Different functional inks comprising the small molecule as an emissive material were formulated by adjusting viscosity, surface tension, and solvent drying kinetics of the inks. As for the process parameters, the gravure cell parameters such as line screen and tone values were altered to control the overall transfer volume of the ink and the thickness of the printed layers. In both cases, the electrically inert polymers were used as host materials to modify the rheological behavior of the ink while suppressing the aggregation of the small molecule in a solid film. The thin film characteristics of printed layers were analyzed in both qualitative and quantitative ways. The printed films were successfully implemented in the active layer of efficient small molecule based electroluminescent devices on flexible plastic foil. The optical and electrical device performance were considered as well as the effect of the printing process in comparison to spin-coated pristine small molecule based reference devices. The quality and performance of the printed emissive layers in both device type showed that the gravure printing method can be an alternative solution for wet-processing roll-to-roll (R2R) manufacturing in the future. White light-emitting diodes draw particular attention in the field, due to their potential application as the backlight in displays or as energy efficient luminaires for SSL. Even though polymer OLEDs are well-suited for wet-based continues R2R fabrication, evaporation of low work function cathodes and therewith encapsulation remain as major obstacles. In the second part of the work, a novel hybrid device architecture was suggested for the color-tuning and white light emission in polymer light-emitting diodes. The single component polymer LEC layer performed as the electron injection layer as well as the second emissive layer on top of a conventional polymer OLED stack. The hybrid structure maintained a sufficient charge carrier injection from an air-stable cathode, due to the unique operation principles of LECs. As a proof of charge transport at the intersection of two emissive layers, dual color emission was simultaneously observed in a bilayer device configuration. A color-tuning in emission was obtained by changing the thickness of the LEC layer. The emission of hybrid devices was shifted from yellow to white light emission region of the CIE color chromaticity diagram, resulting in OLEDs with the high color temperature values. The results demonstrated that this approach showed a promising potential to achieve color-tuning and white light emission from solution processed OLEDs bearing air-stable cathodes. Sustainable bioelectronics is an emerging technology which to replace conventional electronics with disposable counterparts in the future. Thus, bioinspired and bioderived materials usage in organic electroluminescent devices gained much attention in the last years. In the last part of the thesis, we investigated biodegradable natural and naturally derived polymers such as gelatin, deoxyribonucleic acid (DNA) as the ion-solvating polymers in the emissive layer of polymer LECs. Notably, we focused on DNA and DNA-lipid complex based polyelectrolytes due to the unique hybrid ionic/electronic conductivity behavior of DNA. Different solid polymer electrolytes (SPE) were tested with varying additives of salts at different ratios towards improving the ionic conductivity. Additionally, the electrochemical stability window of SPEs was defined to eliminate nonreversible electrochemical side reactions during device operation. The optoelectrical device characteristics, as well as lifetime measurements, were obtained to determine the stability of LECs. Furthermore, the surface morphology of the active layers was investigated to characterize the phase separation between SPE and emissive polymer and aggregations in thin films, which have a significant influence on the device performance. Biosolid polymer electrolytes were successfully implemented in LECs as promising materials of bio-based LECs