Highly-stretchable and water impermeable thermally-grown silicon dioxide thin film with wavy structures

To ensure chemical stability and long-term operation, organic electronic devices require encapsulation layer with low water vapor transmittance rate because organic components in organic electronic device are vulnerable to humidity. Encapsulation of commercialized OLEDs are rigid glass and epoxy resin, which are not suitable for flexible devices requiring high flexibility. TFE (thin-film encapsulation) technique has been studied for flexible device encapsulation. Amorphous materials are selected for TFE materials because they are dense and transparent and do not have fast diffusion paths like grain boundary. Thermally-grown silicon dioxide, oxidized from single crystal silicon substrate at high temperature, has ultra-low water vapor transmittance rate due to high density without pinholes and defects. However, the thermally-grown silicon dioxide thin films have a low elastic limit (\u3c 1%) and show brittle fracture alike typical amorphous materials. For that reasons, it is necessary to improve the mechanical properties of the thermally-grown silicon dioxide thin film for flexible encapsulation. In this study, we tried to improve the stretchability by applying the wavy structure to thermally-grown silicon dioxide and developed the wavy structure texturing of single crystal silicon substrate by using photo-lithography and various etching process. we fabricated a highly-stretchable wavy thermally-grown silicon dioxide TFE by oxidizing wavy textured crystalline silicon substrate. Also, we carried out cyclic tensile test of submicron scale wavy thermally-grown silicon dioxide films and defined the elastic limit, and the stretchability. And then, we analyze the enhancement of stretchability by finite element analysis on the wavy and flat thermally-grown silicon dioxide TFE and discussed about the correlation between the improvement of stretchability and wavy structure

Cu2Se-based thermoelectric cellular architectures for efficient and durable power generation

Thermoelectric power generation offers a promising way to recover waste heat. The geometrical design of thermoelectric legs in modules is important to ensure sustainable power generation but cannot be easily achieved by traditional fabrication processes. Herein, we propose the design of cellular thermoelectric architectures for efficient and durable power generation, realized by the extrusion-based 3D printing process of Cu2Se thermoelectric materials. We design the optimum aspect ratio of a cuboid thermoelectric leg to maximize the power output and extend this design to the mechanically stiff cellular architectures of hollow hexagonal column- and honeycomb-based thermoelectric legs. Moreover, we develop organic binder-free Cu2Se-based 3D-printing inks with desirable viscoelasticity, tailored with an additive of inorganic Se-8(2-) polyanion, fabricating the designed topologies. The computational simulation and experimental measurement demonstrate the superior power output and mechanical stiffness of the proposed cellular thermoelectric architectures to other designs, unveiling the importance of topological designs of thermoelectric legs toward higher power and longer durability

Development of Highly Stretchable and Impermeable Thin Film Encapsulation by Applying Wavy Structure to Thermally Grown Silicon Dioxide

Department of Materials Science and EngineeringIn recent, Stretchable displays are being actively researched as a next-generation display following flexible displays. Organic light-emitting diodes (OLEDs) has excellent display performance and a simple and thin device structure, making it suitable for use in flexible displays. However, encapsulation materials with water vapor transmission rate (WVTR) below 10-5 g/m2/day must be used for long-term reliability of OLEDs, but these encapsulation materials are consisting of an amorphous inorganic thin film with low elastic deformation limit and brittle fracture, such as SiO2, Al2O3, SiNx. Thus, development of stretchable encapsulation should be preceded to develop a stretchable display. Fortunately, stretchability can be increased by structure design of material. These stretchable structures allow large deformation of system under small local strain below the elastic deformation limit of constitutive material. Among various stretchable structures, a three-dimensional wavy structure is best structure for stretchable encapsulation materials because it is impossible to have a structure with opened area that can be a fast water vapor permeation path in a large area. This research aims to develop the high stretchable and ultra-low permeable encapsulation material was developed by applying a wave structure to thermally grown silicon dioxide (SiO2). Thermally grown silicon dioxide deposited by oxidation of single crystalline silicon wafer at 1100??? has amorphous structure, high density, and no defects. Thus, it has the high fracture strength, high elastic deformation limit and the superior barrier properties among inorganic thin films. Furthermore, various wavy structures can be applied to the thermally grown silicon dioxide, through surface texturing of the silicon wafer. To apply a wavy structure to silicon dioxide, a process for etching a silicon single crystalline wafer in a wavy structure was developed. By controlling the size of the wavy structure and the thickness of SiO2, a stretchable encapsulation material with a stretchability of up to 20% and WVTR of 10-6 g/m2/day was carried out. By theoretically analyzing the mechanical behavior of wavy structured film, the universal deformation model was carried out. This model expects the stretchability depending on the characteristics of wavy structured inorganic thin film. The wavy structured SiO2 with the optimized structure was fabricated by designing the most optimal wavy structure and film thickness for high stretchability and ultra-low permeability. Through comparing the WVTR at acceleration condition of the wavy structured SiO2 before and after cyclic deformation with 90% of stretchability during 1000 cycles, it was confirmed that the barrier properties and reliability of the wavy structured SiO2 at the cyclic deformation. Furthermore, the barrier properties and lifetime of barrier at atmospheric condition were discussed by Arrhenius approach.clos

Increase in stretchability of thermally grown silicon dioxide film

Silicon dioxide film has become one of the candidates for encapsulation layer due to its low water permeability. Although stretchable organic devices have been widely studied, research related to stretchable encapsulation are insufficient. In this research, we studied nanomechanics to increase stretchability of silicon dioxide thin film. One way to increase stretchability is forming wrinkles on a film by using elastomer as a substrate. Before forming wrinkles, mechanical properties of the flat film should be analyzed as a stretchability of wrinkled film is determined by mechanical properties of the film and the substrate. High elastic deformation limit of the flat film makes highly stretchable wrinkled film. Therefore, we applied Griffith theory to increase elastic deformation limit of the film, which means we studied to decrease the size of the silicon dioxide film. For this reason, first, thickness of silicon dioxide was decreased to apply thickness effect that the size and distribution of defects decrease as thickness decreases. Second, thermal oxidation was used to grow silicon dioxide since this method fabricate silicon dioxide film having rare defects. To analyze the thickness-dependence on elastic deformation limit we evaluated mechanical properties of 50, 100, and 200 nm-thick freestanding thermally grown silicon dioxide film. Precise mechanical properties were evaluated by performing in-SEM tensile tests and push-to-pull devices were used to control thin specimen

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Thickness-dependent tensile behavior of thermally-grown SiO2

Silicon dioxide film can be used as components in devices such as insulator and encapsulation material because it has high electrical resistivity and low water permeability. Although stretchable organic devices have been widely studied, we found that researches related to stretchable SiO2 are insufficient, so we studied stretchable SiO2 film. Many researchers have increased stretchability by applying wrinkles on films. To design wrinkle structure of SiO2, mechanical properties of a SiO2 thin film were required. Because critical strain on wrinkled SiO2 should be lower than yield strain of flat SiO2 film. In this research, we performed tensile tests of 100, 200 and 500 nm-thick freestanding thermally grown SiO2 thin films and analyzed thickness-dependence. We fabricated micro-scale samples using focused ion beam technique and attached to a push-to-pull device using manipulation system. To observe tensile behaviors precisely, in-situ tensile tests were performed using pico-indenter installed in SEM

Tension-compression asymmetry in plasticity of nanoporous gold

Mechanical behavior of nanoporous gold (np-Au) has been extensively studied by nanoindentation test, but systemic analysis on the deformation of np-Au in both tension and compression is not fully carried out. In this study, we investigated mechanical behavior of np-Au in tension and compression. By preparing micro-scale samples without grain boundaries and cracks, we could focus on the mechanical properties only. Three np-Au samples were prepared and the mechanical tests were performed using in-situ push-to-pull (P-to-P) devices for tensile testing and with the nanoindenter for compressive testing. We found two significant things related with tension-compression asymmetry; (1) the tensile yield strengths are higher than the compressive yield strengths and (2) the size effect exponent n in ??y ??? dL-n where ??y is the yield strength and dL is average ligament size is greater in compression. We discuss the effect of loading mode on deformation and size effect in strength

Thickness-dependent elastic deformation limit of thermally-grown SiO2 thin films

In long-term stability, however, most of these organic devices are vulnerable to moisture, oxygen,etc. To prevent the moisture in the air, highly dense film is required as a barrier. In this research, we studied thermally grown SiO<SUB>2</SUB> film as an encapsulation film as we expected thermally grown silicon dioxide film to have ultra-low water permeability and high elasticity due to its rare defects, high density and high uniformity. We increased elastic deformation limit of thermally grown silicon dioxide film to apply on stretchable devices. To increase elastic deformation limit, the thickness of thermally grown SiO<SUB>2</SUB> films were decreased to observe thickness effect. Although thick SiO<SUB>2</SUB> films show very short elastic deformation limit around 0.7%, thinner SiO<SUB>2</SUB> films have smaller size and number of defects, and these features are characterized as high elastic deformation limit. Tensile tests were performed on those films to discuss thickness-dependent on elastic deformation limit