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Atomically thin Bi2O2Se has emerged as a new member in two-dimensional (2D) materials with ultra-high carrier mobility and excellent air-stability, showing great potential for electronics and optoelectronics. In addition, its ferroelectric nature renders ultra-low thermal conductivity, making it a perfect candidate for thermoelectrics. In this work, we experimentally investigate the thermoelectric performance of 2D Bi2O2Se over a wide temperature range (20 - 300 K). We observe a gate-tunable transition from polar optical phonon (POP) scattering to piezoelectric scattering, which facilitates the capacity of drastic mobility engineering in 2D Bi2O2Se. Consequently, high power factor of more than 400 uWm-1K-2 over an unprecedented temperature range (80 – 200 K) is achieved, corresponding to the persistently high mobility arising from the highly gate-tunable scattering mechanism. This finding provides a new avenue for maximizing thermoelectric performances by changing scattering mechanisms and carrier mobility over a wide temperature range.

MATERIALS FOR HUMANITY (MH 21)

Gate-tunable Polar Optical Phonon to Piezoelectric Scattering in Few-layer Bi2O2Se for High Performance Thermoelectrics

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technical paper

Metal lead halide perovskites are promising candidates for optoelectronic applications due to their high absorption coefficient, thin emission line width, solution processability, high photoluminescence efficiency, and wide color tunability. When nanostructured these perovskites offer the additional advantage of strong quantum confinement.1 In this pioneering work, triple cation perovskite Csx(MA0.17FA0.83)1−xPbBr3 (x = 0–0.15) nanocrystals were synthesised. These proved to have superior phase and environmental stability, less sensitivity to the processing conditions so high reproducibility, and high efficiency when deployed in LEDs and solar cells.2 Although these of lead halide perovskites show excellent properties they contain toxic lead. This motivated the study of stable and lead-free cesium copper iodides as optoelectronic materials.3 By synthesising these compounds as nanocrystals using a hot-injection route the phase and composition were prescribed such that Cs3Cu2I5 nanoplates gave blue emission and CsCu2I3 nanorods yield yellow emission. By mixing these components pure white emission from lead-free metal halide nanocrystals was demonstrated for the first-time. 
References:
1. J. Butkus, P. Vashishtha, K. Chen, J. K. Gallaher, S. K. Prasad, D. Z. Metin, G. Laufersky, N. Gaston, J. E. Halpert and J. M. Hodgkiss, Chemistry of Materials, 2017, 29, 3644-3652.
2. P. Vashishtha, S. A. Veldhuis, S. S. H. Dintakurti, N. L. Kelly, B. E. Griffith, A. A. M. Brown, M. S. Ansari, A. Bruno, N. Mathews, Y. Fang, T. White, S. G. Mhaisalkar and J. V. Hanna, Journal of Materials Chemistry C, 2020, DOI: 10.1039/D0TC02038A.
3. P. Vashishtha, G. V. Nutan, B. E. Griffith, Y. Fang, D. Giovanni, M. Jagadeeswararao, T. C. Sum, N. Mathews, S. G. Mhaisalkar and J. V. Hanna, Chemistry of Materials, 2019, 31, 9003-9011.



Metal Halide Nanocrystals for Optoelectronics

The search for batteries with higher energy and power densities than the current Li–ion batteries is a high priority research area in the continuing development of energy storage systems. Among the various possibilities, lithium–sulfur (Li–S) batteries have drawn a strong following because of the high theoretical specific capacity (1675 mAh g–1) and low cost (<$150 ton–1) of sulfur. The actual energy density of Li–S batteries as implemented to date is however notably lower than the theoretical value because of several technical challenges, such as the dissolution of reaction intermediate products (polysulfides), the polysulfide shuttle between the sulfur cathode and lithium anode, and the need for excess electrolyte. The use of catalysts to accelerate the polysulfide conversion to their end discharge product (Li2S) can inhibit the loss of polysulfides to the electrolyte and has the appeal of a root–cause solution. While a great deal of attention has been focused on the search for new polysulfide catalysts, relatively few have focused on strategies which can maximize the performance of catalysts developed to date. This talk introduces several strategies which can improve the catalytic activity and the sustainable use of catalysts in lithium–sulfur batteries.

Strategies for Enhancing the Polysulfide Conversion Catalysis in Rechargeable Lithium-Sulfur Batteries

Natural ceramic composites present complex microstructures that lead to tortuous crack path and confer them high toughness. Current microreinforced composites do not yet reach the level of complexity found in natural microstructures. To achieve this, we developed an automated set up for controlling 2D microplatelet orientation using magnetic fields. The setup has 4 degrees of freedom and allows fine control of platelet angles in 3D as well as position. With our setup, we can thus build microstructured porous ceramic materials (alumina) with local orientation of microplatelets, at a concentration of about 40 vol%. Other work in our group has shown that the same setup can be used to process microstructured ceramics up to 95 vol%.
Among the infinite possible microstructures, a nacre-like helicoidal arrangement was selected due to its ability to tilt and twist the crack path. Indeed, we used an analytical model based on the laminate theory to compare energy dissipation due to mixed modes of failure. Nacre-like helicoidal arrangement was predicted to exhibit the highest energy dissipation. Then, we fabricated the hybrid microstructured porous specimen fabricated and infiltrated them with a silicone matrix. Using compression tests, our hybrid dissipated 1.6 times more energy as compared to nacre-like samples of same composition. The fabrication strategy proposed here could thus be a simple route to increase the toughness of microplatelet reinforced composites.



Energy dissipation in composites with hybrid nacre-like helicoidal microstructures

The solutionization of Inconel 718 (IN718) is often terminated by rapid cooling at a rate of air cooling or faster. However, in this study, a slow cooling method utilizing furnace cooling was employed for selective laser melted (SLM) micron-TiC reinforced IN718 to investigate the interactions between the TiC particles and the matrix. Four holding temperatures of 975, 1075, 1175, and 1275 °C were chosen while the holding time was kept constant at 1 hour. It was found that the addition of micron-TiC facilitated the formation of nano-sized Ti-rich precipitates in as-printed samples. The difference in microhardness between reinforced and non-reinforced IN718 increased with increasing holding temperature, from 14.9% higher at 975 °C to 50.3% higher at 1275 °C. This was mainly attributed to the prevention of solute segregation to the grain boundary and the reduction of the susceptibility to hot cracking after adding TiC particles. The results indicate that the solute materials (Nb, Mo, and Si) were absorbed by the Ti-rich particles through Ostwald ripening mechanism instead of diffusing to the grain boundaries. The degree of absorption increased with increasing holding temperature. Moreover, grain coarsening at high solutionizing temperature was reduced substantially with the presence of Ti-rich particles due to Zener pinning. Thus, this provides a better understanding for choosing a suitable heat treatment profile for IN718 composite.

Mitigation of solute segregation after solutionization in selective laser melted Inconel 718 / micron-TiC composite

Electrification could provide many benefits towards the environment like lessening the dependence on non-renewable resources and reducing emissions when applied in the space of transportation. However, their battery packs take up a significant amount of space, decreasing the cargo capacity of EVs and payload of electric planes. Therefore, electrification of transportation options is driven largely by the advancement of battery technology and light-weighting composites with the goal of achieving a higher energy while reducing weight and saving space. 
To that end, the concept of providing structural power in the form of multifunctional structural batteries has been deemed as a potentially viable strategy to achieve both high energy density, light-weighting and space savings simultaneously. However, the scientific problems facing the development of multifunctional materials for structural batteries are especially hard to solve due to the seemingly incongruent material requirements for energy storage and structural stability. Structural battery constituents include the structural electrodes and the structural electrolytes. In particular, achieving good mechanical and conductivity properties on structural electrolyte is especially challenging due to the complicated electrochemical processes involved in batteries and the stringent mechanical requirements of structural components. In this work, a sulfonated polyether ether ketone, an engineering thermoplastic, grafted with polyethylene glycol (SPEEK-g-PEG) was studied for its multifunctional capabilities. A conductivity of 10-6 S cm-1 was achieved with SPEEK-g-PEG (SPEEK grafted with PEG 750) free-standing film paving the way for the use of engineering thermoplastics as multifunctional structural battery electrolytes.

Towards the design of multifunctional composites for structural battery applications

Polymer composites reinforced with a thin film made of carbon nanotubes (CNTs), also called as buckypapers, have been first reported in 2004 [1]. The CNTs in the films form an entangled, interconnected and porous network hold together by the van der Waals forces. The films retain most of the properties associated with CNTs, including high specific surface area, electrical and thermal conductivities, while they allow to bypass the agglomeration and processing issues faced by traditional processing methods based on melt compounding [2]. The reported applications of buckypapers include batteries, fuel cells, filters, strain sensing and lightning striking protection thus incorporating them into composites could allow for new multifunctional materials [3]. The main drawback of the buckypapers is their preparation method, which is currently based on filtration or Chemical Vapour Deposition, therefore making the preparation of the films slow, expensive and size restricted [4]. In this talk a new approach towards manufacturing of buckypaper composites will be presented. The method relies on making the thin films using a dispersion of carbon nanotubes directly on the epoxy layer, thus saving the time required for vacuum filtration. With the aid of data from Dynamic Mechanical Analysis tests, this presentation will explain the advantages and limitations of this manufacturing method. A future outlook of the buckypaper composites will be given, focusing on the multifunctionality, scalability and additive manufacturing of these materials.

Development of multi-layered epoxy composites with carbon nanotube buckypaper reinforcement

Carbon fiber veils are beneficial materials to functionalize the adhesively bonded joints which are damaged when they need to be debonded. The effects of three different carbon fiber veils on the mechanical, thermal and electrical characteristics of epoxy adhesive systems were examined. In comparison to the neat epoxy configurations, carbon fiber veil interleaving enhanced storage modulus, thermal diffusivity and lap shear strength (LSS) of the adhesive joints while lowering specific heat capacity (Cp) and glass transition temperature (Tg). Fourier-transform infrared spectroscopy (FTIR) analysis showed no chemical changes occurred in epoxy after heating at 100°C and interleaving a carbon fiber veil layer between two epoxy film adhesive layers. Surface roughness and water contact angle measurements were conducted to investigate the wettability of GFRPadherends. ANSYS coupled thermal-electric simulations displayed good agreement with Joule heat experiments. Thermomechanical debonding via Joule heating provided high debonding effectiveness in terms of requiring low force and time while providing no fiber-tearing on the surface of the adherends and heating the bonded region of the joints selectively.

Debonding of carbon fiber veil interleaved adhesively bonded glass fiber reinforced polymer (GFRP) joints

Fiber reinforced thermoplastic (FRTP) composites are gaining popularity in many industries because of their potential to be recycled, among other reasons. Filament winding is a manufacturing process suitable for the fabrication of axisymmetric components. For the filament winding of FRTP composites, the consolidation process can be carried out in-situ, where the FRTP is heated during the winding process. This enables the manufacturing process to be completed in a single step, thus allowing for higher productivity. In this study, the filament winding of glass/polyamide-6 composite using a customized in-situ consolidation setup is investigated. Instead of heating only at the nip point, infrared heaters were used to supply heat across the length of the composite component. This setup is intended to increase the exposure time of the thermoplastic to the heat source. The effectiveness of the in-situ consolidation setup is studied by evaluating the physical and mechanical properties of the consolidated FRTP components. First round of results show that the customized process can produce composite specimens with low void content (below 5%) at a line speed of 2 m/min.

Filament winding of glass/polyamide-6 commingled fibers with customized infrared in-situ consolidation

Prediction of mechanical properties using multiscale manufacturing simulation and micromechanical models is the focus of this paper. An integrated platform is designed to cover the whole chain of analysis and perform the properties' calculation and transfer them between the modules in a smooth mapping procedure. The manufacturing step is the particular problem of coupled dual-scale deformation-flow process such as the one arising in RTM, Vacuum Assisted Resin Infusion (VARI) and Vacuum Bag Only (VBO) prepregs. A finite element formulation of porous media theory framework is employed to predict the element-wise local volume fractions and the deformation of a preform in a press forming process. This formulation considers coupling effects between macro-scale preform processes and mesoscale ply processes as well as coupling effects between the solid and fluid phases. The outcome of the manufacturing simulation is the deformed mesh and element wise volume fractions to be used in micromechanical model to calculate mechanical properties. Structural performance of the "deformed" geometry is then evaluated in mechanical analysis. Structural performance of composites is directly dependent on its ingredient's properties, ply configurations and the manufacturing effects. These are evaluated in the integrated simulation chain and are defined as inputs. The contribution is concluded with a numerical example, where a compression-relaxation test of a planar fluid filled prepreg at globally un-drained condition is considered followed by a mechanical loading analysis. The development is user friendly and interactive and is established to enable design and optimization of composites.
This research is supported by the Singaporean Agency for Science, Technology and Research (A*STAR) under Polymer Matrix Composites Programme (PMCP) project (WP1-1) through the grant no. R265000673305.


Integrated simulation chains for composites mechanical performance and manufacturing effect of defects

The toughness of epoxy resin has a significant influence on the performance of fiber-reinforced composites. Pure epoxy resin has poor toughness and usually needs to be toughened by adding elastomer. However, the epoxy resin is usually a liquid with very low viscosity at the mixing temperature. When the high molecular weight elastomer is directly used, the viscosity of the two materials is quite different, and it is difficult for the shear force to be transmitted from the matrix phase to the dispersion phase. On the other hand, when using a liquid rubber type with low viscosity to toughen, not only the toughening effect is not good, but also the heat resistance and strength of epoxy resin will be significantly reduced. Therefore, to prepare high-performance fiber-reinforced composite materials, we use dynamic covalent bonds to construct a reversible molecular weight polyurethane elastomer toughening agent to achieve reversible changes in the viscosity of the elastomer at different temperatures and its applying for epoxy resins toughening. The reversible bond polyurethane elastomer has low viscosity at higher temperature and is easy to disperse in epoxy resin; after the temperature decreases, the toughening agent returns to high molecular weight elastomer. The phase separation of the composite material system can be controlled on a larger scale by changing the temperature. This technology is expected to be further promoted and applied in multiphase composite material systems.

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Gate-tunable Polar Optical Phonon to Piezoelectric Scattering in Few-layer Bi2O2Se for High Performance Thermoelectrics

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Metal Halide Nanocrystals for Optoelectronics

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Metal Halide Nanocrystals for Optoelectronics