4:00pm - 4:30pmInvited
Progress and Challenges in the Development of Robust Shape Memory Ceramics at Small Scales
1School of Materials Science & Engineering, Nanyang Technological University, Singapore; 2Temasek Laboratories @ NTU, Nanyang Technological University, Singapore; 3Department of Materials Science & Engineering, Massachusetts Institute of Technology, United States
Shape memory ceramics (SMCs) have been reported to exhibit superelasticity by virtue of a martensitic phase transformation, but it often fails by cracking at low strains and after only several applied strain cycles. In our work, we have mitigated the cracking problems by creating oligocrystalling structures in micro scales and uncovered the crucial structural conditions that lead to highly reproducible superelasticity.
This talk will cover our recent advances in the studies of the small-scale SMCs, including (1) first demonstration of superelasticity in small volume SMCs based on pillars; (2) crystal orientation and size effect; (3) superelasticity behavior in single particles; (4) cycling effect; and (5) superelasticity and energy dissipation in SMC powder compacts. Our work shows that superelasticity over hundreds of strain cycles, with large specific energy dissipation and compression strain up to ~8% is achievable in the SMC pillars and particles, while the behavior is governed by the crystal orientation and size. Lastly, this talk will discuss on the challenges in scaling-up and engineering applications of SMCs.
4:30pm - 4:45pmOral
Phase Field Modelling of Martensitic Transformations in Nanocrystalline Shape Memory Alloys of Bi-Modal Grain Size Distribution
1Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR), Singapore; 2National University of Singapore, Singapore
Polycrystalline shape memory alloys (SMAs) show much lower recoverable strains (memory) than those made as single crystals. This becomes critical especially for nanocrystalline SMAs for which it has been shown that the underlying martensitic transformations may even become completely suppressed once the grain size falls below a critical value. This would mean that the material would lose its shape memory behaviour.
We suggest utilizing this undesirable property of small grains supressing the transformation to form a shape memory composite-like structure with a bi-modal grain size distribution. The smaller grains remain stabilized with the austenite and the larger grains transform into the martensite creating a two-phase composite naturally designed by its own grain size distribution.
To study these structures we use a phase field model of martensitic transformations into which we have incorporated a difference between the grain boundary energies of the austenite and the martensite phase. This allows us to model the experimentally observed phenomenon of suppression. We study the mechanical response of stress-loaded systems with a bi-modal grain size distribution and we analyse the stress-strain hysteresis and the maximum recoverable strain as a function of grain size and grain size distribution. We compare these to structures with unimodal grain size distribution.
During the loading, we observe a detwinning process in which one martensitic variant grows at the expense of the other. However, we also observe that some of the larger grains may unexpectedly become stabilized with the austenite and some smaller grains transform into the martensite. The results suggest new pathways of martensitic transformations influenced by the bi-modal distribution which may help improve the microstructural design of nanoscale shape memory devices.
4:45pm - 5:00pmOral
Oxidation Mechanism and Mechanical Properties of Y-doped CrAlSiN Coatings for High Temperature Applications
1Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research (A*STAR), Singapore; 2Data Storage Institute, Agency for Science, Technology and Research (A*STAR), Singapore
There has been an increasing demand for protective coatings with high temperature compatibility, especially for applications at extreme conditions, such as tools for high speed dry machining, liquid forging dies, and turbine blades for jet engines, in which the thermal stability and oxidation resistance are key to the coating performance, in addition to the high hardness and toughness that are usually required. In this paper, the effects of Y doping on the microstructure, mechanical properties, and oxidation resistance of CrAlN based hard coatings at elevated temperatures are investigated. Y doped CrAlN and CrAlSiN coatings were deposited in a hybrid HIPIMS and magnetron sputtering disposition system, and their microstructure, chemical compositions and mechanical properties were studied by HRTEM, EDX and nanoindentation respectively. The as deposited coatings were annealed in air at 1100°C, and the coating oxidation and change in coating microstructure and composition were characterized by GIXRD, and HRTEM and EELS mapping. It was found that appropriate amounts of Y addition reduces the thickness of oxides scales on the coating surface, however, excessive Y doping may give rise to the formation of porous, non-protective oxide scales, deteriorating the oxidation resistance. This could be attributed to the different effects of Y on the diffusion mechanisms of Cr, Al, and O within the coating and at the interfaces. The kinetics of the oxidation will be discussed and ways to enhance coating oxidation resistance are suggested.
5:00pm - 5:15pmOral
Effects of Mechanical Stress on the Crystallography of Martensitic Transformation of Micron-sized Shape Memory Ceramics
1Temasek Laboratories @ NTU, Nanyang Technological University, Singapore; 2School of Materials Science and Engineering, Nanyang Technological University, Singapore
Shape memory ceramics can experience significant shape deformation and recovery by virtue of mechanical- or thermal-induced martensitic phase transformation. The in-depth understanding of the martensitic transformation crystallography under mechanical or thermal stimulus is, therefore, of great importance for the development of robust shape memory ceramics. In this work, single crystal zirconia samples, in both a pristine condition and after micro-compression, have been studied with synchrotron scanning micro X-ray diffraction (μXRD) through a heating-cooling cycle of 25-600°C. The results show that the post-compressed zirconia follows a different crystallographic path from that in the pristine condition, resulting in distinct monoclinic variant preferences. Their characteristic martensitic transformation temperatures, which define the working temperature of shape memory ceramics, were also found to be different. The underlying mechanism for these phenomena has been discussed based on thermodynamics of martensitic transformation and stress-induced dislocation effects. Our findings provide an important guideline to tailor the thermo-mechanical properties of shape memory ceramics for future scientific research and engineering applications.
5:15pm - 5:45pmInvited
Halide Perovskite from Photovoltaics to Memristor
Sungkyunkwan University, South Korea
Halide perovskite comprising organic cation in A site of AMX3, is proved to be excellent photovoltaic material and now extended to other applications such as light emitting diode, X-ray imaging, memristor etc. Since the first report on the solid-state perovskite solar cell with power conversion efficiency (PCE) of 9.7% in 2012 by our group, its certified PCE now reaches 22%. In this talk, methodologies to get high efficiency perovskite solar cell are introduced. Adduct method is found to be highly efficient way to get high quality perovskite film reproducibly. Grain boundary healing process further improves power conversion efficiency (PCE). Nonstoichiometric precursor induces self-formed grain boundary layers that lead to long carrier life time of perovskite, which eventually results in high open-circuit voltage and fill factor. Moreover, self-formed grain boundary plays important role in charge transporting, where charge conduction is pronounced. Grain boundary healing process yields PCE as high as 20.4%. Organic-inorganic halide perovskite extends its functionality from PV to light emitting diode (LED) and non-volatile resistive random access memory (ReRAM) applications. We have demonstrated a highly efficient green LED with external quantum efficiency exceeding 8% based on type I monocrystalline MAPbBr3 film and a millivolt switching ReRAM with ~107 on-off ratio.
5:45pm - 6:00pmOral
Stress Evolution of Silicon Nano-wires Anode of Li-ion Battery Using In-situ Synchrotron X-ray Microdiffraction
1Singapore University of Technology and Design, Singapore; 2Advanced Light Source (BL 12.3.2), LBNL, Berkeley, United States
Li-ion battery is the most promising candidate for energy storage applications because of high energy density and longer life cycle. Silicon is an attractive anode material for Li-ion batteries because of low discharge potential and the higher theoretical charge capacity (i.e.,4,200 mAh/g). Silicon nanowires (SiNWs) anode is a better choice because of high strength and larger surface area. As silicon expands in volume significantly during lithiation and delithiation, which causes high stresses in SiNWs anode and leads to fracture and eventual failure events. To utilize high capacity SiNWs anode in Li-ion batteries, it is important to study stress evolution and crystal deformation during lithiationa and delithiation.
In-situ experiment of SiNWs anode of Li-ion battery was conducted using synchrotron X-ray microdiffraction to study stress evolution and crystal deformation of SiNWs anode. In our study we found that SiNWs were going through changes in mechanical stress during lithiation. Furthermore, morphology of SiNWs was also studied before and after lithiation by SEM. Fattening of SiNWs was observed after lithiation. This study will help ensure reliability and to overcome failure events of SiNWs anode of Li-ion batteries.
6:00pm - 6:15pmOral
Salinity Effect on Interface Adhesion Mechanics of Polyolefin-based Encapsulant Solar Photovoltaics Module
1Center for Solar Photovoltaics, Surya University, Indonesia; 2Buddhi Dharma University, Indonesia
The application of solar photovoltaics (PV) technology in the tropical regions (i.e. Indonesia) makes a lot of sense as it may yield higher energy output due to constant and high intensity of incoming solar radiation. As Indonesia is archipelago country, developing floating solar farm within Indonesian’s sea can boost the country’s capability to become maritime axis. Before implementing this application, it is very essential to study module reliability under extreme tropical environments (i.e. high temperature, humidity and salinity) as module delamination has occurred very frequent under long term field exposure as reported in some literatures. In this talk, the authors report the study of interface adhesion mechanical degradation on photovoltaics module (with conventional backsheet and polyolefin encapsulant layers) under extreme tropical environment over several time periods. Mechanical characterization in single cantilever beam is utilized to quantify the adhesion strength between backsheet and encapsulant layer. Salinity effect on interface adhesion degradation will be explored. Therefore, understanding the interface strength on photovoltaics module is very crucial to enable robust and reliable solar PV technology in the tropical regions.