Computational Design of Auxetic Shells

Recent advances in material science and digital fabrication provide promising opportunities for product design, mechanical and biomedical engineering, robotics, architecture, art, and science. Engineered materials and personalized fabrication are revolutionizing manufacturing culture and having a significant impact on various scientific and industrial works. As new fabrication technologies emerge, effective computational tools are needed to fully exploit the potential of digital fabrication. This thesis introduces a novel computational method for design and fabrication with auxetic materials. The term auxetic refers to solid materials with negative Poisson ratio â when the material is stretched in one direction, it also expands in all other directions. In particular, we study 2D auxetic materials in the form of a triangular linkage which exhibits auxetic behavior at the macro scale. This stretching, in turn, allows the flat material to approximate doubly-curved surfaces, making it attractive for fabrication. We physically realize auxetic materials by introducing a specific pattern of cuts into approximately inextensible material such as sheet metal, plastic, or leather. On a larger scale, we use individual rigid triangular elements and connect them with joints. First, this thesis focuses on a regular triangular linkage. When deformed into a curved shape, the linkage yields spatially-varying hexagonal openings. However, the global coupling of the linkage elements makes manual, incremental approach unlikely to succeed when trying to approximate a given curved surface. Thus, we leverage conformal geometry to enable complex surface design. In particular, we compute a global conformal map with bounded scale factor to initialize an otherwise intractable non-linear optimization. Constraint-based optimization is used to find the final linkage configuration that closely approximates a target 3D surface. Furthermore, we develop a computational method for designing novel deployable structures via programmable auxetics, i.e., spatially varying triangular linkage optimized to directly and uniquely encode the target 3D surface in the 2D pattern. The target surface is rapidly deployed from a flat initial state via inflation or gravitational loading. The thesis presents both inverse and forward design tools for interactive surface design with programmable auxetics. This allows the user to efficiently approximate a given shape and directly edit and adapt the auxetic linkage structure to explore the design alternatives. In addition, our solution enables simulation-based form-finding that uses deployment forces for interactive exploration of feasible shapes. The resulting designs can be easily manufactured via digital fabrication technologies such as laser cutting, CNC milling, or 3D printing. Auxetic materials and deployable structures enable scientific, industrial, and consumer applications across a wide variety of scales and usages. We validate our computational methods through a series of physical prototypes and application case studies, ranging from surgical implants, through art pieces, to large-scale architectural structures.

PRISMS: An Integrated, Open-Source Framework for Accelerating Predictive Structural Materials Science

Zhouji Huang, Lin Qi, Zheng Wang

The Center for Predictive Integrated Structural Materials Science (PRISMS Center) is creating a unique framework for accelerated predictive materials science and rapid insertion of the latest scientific knowledge into next-generation ICME tools. There are three key elements of this framework. The first is a suite of high-performance, open-source integrated multi-scale computational tools for predicting microstructural evolution and mechanical behavior of structural metals. Specific modules include statistical mechanics, phase field, crystal plasticity simulation and real-space DFT codes. The second is the Materials Commons, a collaboration platform and information repository for the materials community. The third element of the PRISMS framework is a set of integrated scientific “Use Cases” in which these computational methods are linked with experiments to demonstrate the ability for improving our predictive understanding of magnesium alloys, in particular, the influence of microstructure on monotonic and cyclic mechanical behavior. This paper reviews progress toward these goals and future plans.

Springer Nature2018

Fabrication and Characterization of Functional ALD Metal Oxide Thin Films for Solar Applications

Ludmilla Steier

Motivated to revolutionize our today's fossil fuels based energy production, my work concentrated on the investigation of promising low-cost materials for photoelectrochemical hydrogen production and photovoltaic electricity generation. Hydrogen presents a fully scalable energy storage solution while photovoltaics have the biggest potential for clean electricity generation. Both are combined in the hydrogen-based economy that will be introduced in Chapter 1. A clear way to achieve this revolutionary technological and societal goal is through fundamental understanding of the complex electronic properties of the most promising low-cost semiconductors offering strong visible light absorption. The modern fields of photoelectrochemical (PEC) water splitting and photovoltaics have a lot in common: materials, scientific concepts and theoretical background. In other words, their complementarity was a strong motivation for the interdisciplinary work presented in this thesis. The main focus in this thesis is on hematite, which is a promising low-cost material offering visible light absorption and the chemical robustness for photoelectrochemical water oxidation. However, it has two major drawbacks: firstly, for a semiconductor, hematite has extremely low electron and hole mobilities. This makes it challenging to collect charges that are photo-generated deep within the hematite layer and far away from the surface. Secondly, water oxidation appears to be limited by trap states located in the mid band gap region. Chapter 3 addresses these drawbacks showing that doping of hematite from the underlayer, surface passivation from annealing treatments and/or overlayers are all key parameters to consider for the design of more efficient iron oxide electrodes. By better understanding the underlying principles of over- and underlayers, I was able to design multilayered hematite photoanodes comprised of functional thin films to obtain a significant reduction in the water oxidation overpotential. Whereas hematite thin film electrodes were fabricated by ultrasonic spray pyrolysis in Chapter 3, I introduce a new atomic layer deposition (ALD) route towards crystalline, highly photoactive, phase pure and impurity-free hematite films in Chapter 4. With this thin film model system I could precisely demonstrate that only the 10 nm thick space charge region of hematite is photoactive, which presents a major challenge when considering that around 60-70 nm are needed to achieve sufficient light absorption as shown in Chapter 5. In light of this charge transport limitation, I propose and demonstrate new host-guest electrode designs that would be indispensible for the realization of high performance ALD hematite photoanodes. To complement these studies, I demonstrate in Chapter 5 the basis for an optoelectronic modeling of the hematite PEC device that helps identifying and eliminating major optical and electronic losses in the PEC cell. In Chapter 6 my work on ALD SnO2 as an electron selective layer (ESL) led to major advances in low-cost flat hybrid organic-inorganic perovskite solar cells. The tin oxide layer is found to resolve the problem of an energy band misalignment encountered with the previously used ALD TiO2 ESL in addition to stabilizing the photovoltaic performance. Hysteresis-free solar conversion efficiencies of up to 18% have been achieved for flat devices paving the way towards low-cost flat film perovskite solar cells that will easily surpass the photovoltaic performance of polycrystalline silicon solar cells in the near future.

EPFL2016

Beyond Developable: Computational Design and Fabrication with Auxetic Materials

Sofien Bouaziz, Bailin Deng, Mina Konakovic Lukovic, Mark Pauly

We present a computational method for interactive 3D design and rationalization of surfaces via auxetic materials, i.e., flat flexible material that can stretch uniformly up to a certain extent. A key motivation for studying such material is that one can approximate doubly-curved surfaces (such as the sphere) using only flat pieces, making it attractive for fabrication. We physically realize surfaces by introducing cuts into approximately inextensible material such as sheet metal, plastic, or leather. The cutting pattern is modeled as a regular triangular linkage that yields hexagonal openings of spatially-varying radius when stretched. In the same way that isometry is fundamental to modeling developable surfaces, we leverage conformal geometry to understand auxetic design. In particular, we compute a global conformal map with bounded scale factor to initialize an otherwise intractable non-linear optimization. We demonstrate that this global approach can handle non-trivial topology and non-local dependencies inherent in auxetic material. Design studies and physical prototypes are used to illustrate a wide range of possible applications.

Assoc Computing Machinery2016

Fabrication and Characterization of Functional ALD Metal Oxide Thin Films for Solar Applications

Ludmilla Steier

EPFL2016