학위논문 (박사)-- 서울대학교 대학원 공과대학 재료공학부, 2017. 8. 주영창.As the development of flexible devices has progressed for user's convenience, researches on the rational design of architectures and structures which could provide mechanical functionality to flexible devices, have been actively carried out as well as material development perspective. The control of mechanical properties through structural innovation has the advantage that it provides unprecedented properties beyond the existing material limits and is easy to design predictably. Based on this feature, there is a growing interest in reconfigurable materials that could operate in response to external physical stimuli. Auxetics, which are one of the mechanically reconfigurable materials, is a structure which are able to perform negative Poisson's ratio behavior. Based on the characteristics, auxetics have an excellent expandability and possible to maintain an excellent conformability even on a non-zero Gaussian surface. Therefore, auxetics are an attracting attention as a structural material for a next generation flexible device. In this thesis, the mechanical and electrical performance of flexible devices were improved through proper geometric design of two dimensional auxetic structure, and suggested auxetics as a new paradigm of structural materials for flexible devices.
At first, a platform for flexible devices was proposed, which was capable of large displacement in all directions by using a rotational unit auxetic including a self-similar hierarchical structure. Through the finite element analysis, it was proved that even if the hierarchical auxetic was subjected to complicated deformation, not only the tension but also crumpling, the deformation could be concentrated only in the hinges connecting the individual unit. Based on the deformation characteristics, an omnidirectionally and extremely deformable battery was developed. When the hinge was composed of an elastomer having excellent mechanical reliability, it was possible to deform the hinge in an unexpected three dimensional manner beyond the viewpoint of the conventional two dimensional auxetic view. As a result, the degree of freedom of hinge deformation could be increased to infinity. Also, as the level of the hierarchical structure increased, the strain concentrated on the hinge is relaxed even at the same level of strain, thereby improving the mechanical stability and improving the stretchability and be crumpled easily. In addition, it was possible to design the same hierarchical auxetic in a thin plastic substrate through the cutting process. In this case, the sharp cut pattern could cause tearing, which could result in severe mechanical failure. To improve the mechanical reliability at the hinge, a design to prevent the crack propagation was proposed, thereby confirmed the potential ability of applying the hierarchical auxetic to thin sheets.
Secondly, a hybrid auxetic composite had been developed which could be predictably design the modulus of elasticity and Poisson's ratio, by embedding two dimensional re-entrant auxetic as a composite scaffold into soft material. This composite could have an anisotropic deformation behavior due to the influence of re-entrant structure, which had a negative Poisson's ratio behavior in in-plane and a positive Poisson's ratio in the normal direction. Especially, the thickness of matrix in the composite could be decreased even more compared to the conventional isotropic materials due to the volume conservation. Using the anisotropic mechanical property of composite, a stretchable capacitive strain sensor having improved sensing property was developed which could stretch up to 50 %. The Gauge factor, which is a ratio of change in capacitance to the stretch, of the conventional capacitive strain sensor was limited to 1 due to the geometric factor. Applying the auxetic composite as a dielectric, sharper capacitance change was achieved even under the same degree of stretch compared to the conventional sensor. In addition to the improvement, the composites could represent a good conformability to such as elbows and knees, thus our composites could suggest a new direction in geometry design for wearable sensors.
This thesis suggested auxetics as a new paradigm as a structural material for flexible devices by improving the mechanical and electrical performance of various flexible devices that are currently being developed, by properly customizing the architectural design.Chapter 1. Introduction
1.1. Emergence of soft electronics ......... 1
1.2. Necessity of geometric design for soft electronics .. 4
1.3. Auxetic: A new geometric concept for soft electronics ............ 6
1.4. Thesis objectives ....................................................... 12
1.5. Organization of the thesis ....................................................... 13
Chapter 2. Theoretical Background
2.1. Neo-Hookean solid ..................................... 14
2.2. Theoretical limit in Poissons ratio of istropic material ......... 16
2.3. Auxetic ......... 18
2.3.1. Classification of auxetic ................................................ 18
2.3.2. Mechanical model for re-entrant auxetic ................. 21
2.4. Mechanism of strain sensor ................................................ 23
Chapter 3. Hierarchical Auxetic for Extremely deformable Device Platform
3.1. Introduction ................................................................. 25
3.2. 3-dimensional deformation of soft hierarchical auxetic ............. 29
3.3. Experimental procedure ........................... 37
3.4. FEM for 3-D deformation of hierarchical auxetic .... 41
3.4.1. Tensile deformation model ........................................... 43
3.4.2. Crumple deformation model ........................................... 47
3.4.3. Motif array dependence ........................................... 53
3.5. Experimental realization ............................................... 56
3.4.1. Mechanical reliability confirmation of hinge ............ 56
3.4.2. Omni-directionally deformable batteries ............ 59
3.4.3. Hierarchical auxetic design for thin film application ......... 61
3.6. Summary ................................................................... 65
Chapter 4. Tunable Elastic Property of Soft Materials by Auxetic Composites and Strain Sensing Application
4.1. Introduction .............................................................................. 66
4.2. Experimental procedure ...................................... 71
4.2.1. Fabrication of auxetic composite ...................................... 71
4.2.2. Fabrication and measurement of strain sensor ...................... 75
4.3. Tensile behavior of auxetic composite .... 77
4.4. Auxetic composite design through FEM .... 81
4.4.1 Determination of elastic property of composite element .... 81
4.4.2. Stretch direction dependence of re-entrant auxetic .... 83
4.4.3. Boundary condition for stretch of auxetic composite .... 86
4.4.4. Geometric dependence ............................................ 92
4.4.5. Material dependence ............................................ 101
4.4.6. Thickness dependence of auxetic composite .... 103
4.5. Hybrid auxetic composite for capacitive strain sensor .... 109
4.5.1. Performance of auxetic strain sensor ................................ 110
4.5.2. Capacitance calculation by FEM ................................ 112
4.5.3. Analytic model for predicting gauge factor ........................ 115
4.6. Summary ...................................................................................... 121
Chapter 5. Conclusion
5.1. Summary of results .................................................................... 122
5.2. Future work and suggested research ...................................... 124Docto
