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1) Ni-Mn-Ga system exhibits magnetic shape memory effect as a giant magnetic field-induced strain (MFIS) [1,2]. 2) MFIS is possible due to the high mobility of twin boundaries coupled with large magneto-crystalline anisotropy [1,3]. 3) MFIS was observed only in modulated five-layered (10M) or seven-layered (14M) martensites in alloys with near-stoichiometric compositions [4]. 4) Non-modulated (NM) martensite with tetragonaly disorted L21 is stable in compositions far from stoichiometry [4]. It does not exhibit significiant MFIS [5]. 5) We performed ab initio calculations to reveal, how increasing concentration of excess Mn affects formation and propagation of twin boundaries. An effect of local arrangement of excess Mn in Ga sublattice has been considered as well.

MATERIALS FOR HUMANITY (MH 21)

First-Principles study of twin formation in Mn-excess Ni-Mn-Ga alloys

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poster

It has been a grand challenge to develop efficient thermoelectric materials for room temperature (300- 500K) applications. Recently 2D Sn2Bi sheet has been synthesized on silicon wafer and exhibits a high band degeneracy, which is favorable for high TE performance. In this work, for the first time we demonstrate that fluorination not only can stabilize Sn2Bi sheet but also can significantly improve the TE performance. Because fluorine has a heavier atomic mass and a larger electronegativity, which can lead to a lower phonon group velocity and larger anharmonicity. Inspired by these advances, we show that fluorinating Sn2Bi sheet can significantly improve the thermoelectric performance at room temperature.

The ultralow thermal conductivity and ultrahigh thermoelectric performance of fluorinated Sn2Bi sheet in room temperature

This work is focused on a theoretical study of the phase stability of solid solutions in Al-Ge and Ag-Pb alloys, in which there are no intermediate phases across the whole composition range. The solubilities of the solid solutions were determined using temperature-dependent free energies of pure elements and respective Al- and Ag-rich solid solutions obtained from ab initio calculations based on density functional theory. All total energy calculations were performed by Vienna Ab initio Simulation Package (VASP) [1] with the help of Projector Augmented-wave potentials [2, 3]. Contributions of vibrational free energy and electron free energy were obtained from Phonopy package [4]. The temperatures, at which the solid solutions become stable, were estimated from in the free energy difference between solid solution and mixture of pure elements according to [5]. For Ag-Pb alloy, cluster expansion method and subsequent Monte Carlo simulations with temperature dependent expansion coefficients [6] were employed in order to calculate the phase boundary as well. Also in both cases, quasi-harmonic approximation was employed to include the effect of volume expansion. Including this effect provides results closer to experimental observations in both cases. The resulting solubilities were compared with experimental ones in phase diagrams obtained from the CALPHAD method. The results correspond very well to the experimental measurements qualitatively, however the calculated temperature values are underestimated. References [1] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993); ibid. 49, 14 251 (1994). [2] P. E. Blöchl, Phys. Rev. B 50, 17953 (1994). [3] G. Kresse and D. Joubert, Phys. Rev. 59, 1758 (1999). [4] A. Togo and I. Tanaka, Scr. Mater., 108, 1-5 (2015) [5] V. Ozolinš and M. Asta, Phys. Rev. Lett., 86, 448 (2001) [6] A. van de Walle and G. Ceder, J. of Phase Equilibria, 23, 348 (2002)

Computational prediction of solubility in Al-Ge and Ag-Pb alloys

Motivation - FeCo alloys, characterized by the highest saturation magnetization, good permeability in magnetic fields, and high Curie temperatures have become indispensable materials for inductive applications such as electrical machines and motors. However, the magnetomechanical properties are ruled by complex temperature-ordering relationships. These effects have been studied experimentally and by ab initio computations, but the microstructural origin is still unconclusive. Alternatively, semi-empirical molecular dynamics (MD) can accurately describe relatively large atomic systems. Interatomic potentials for FeCo alloys are already available but focused on the study of high entropy alloys (HEA), thus there is a lack on the proper description of the thermo-mechanical characteristics. Consequently, a FeCo focused potential is required which can also become an important piece to the study of other FeCo based alloys.

A second nearest-neighbor embedded atom method interatomic potential for Fe-Co alloys

We have performed a quantum-mechanical study of a series of stoichiometric Ni2MnSn structures focusing on pressure-induced changes in their magnetic properties. Motivated by the facts that (i) our calculations give the total magnetic moment of the defect-free stoichiometric Ni2MnSn higher than our experimental value by 12.8% and (ii) the magnetic state is predicted to be more sensitive to hydrostatic pressures than seen in our measurements, our study focused on the role of point defects, in particular Mn-Ni, Mn-Sn and Ni-Sn swaps in the stoichiometric Ni2MnSn. For most defect types we also compared states with both ferromagnetic (FM) and anti-ferromagnetic (AFM) coupling between (i) the swapped Mn atoms and (ii) those on the Mn sublattice. Our calculations show that the swapped Mn atoms can lead to magnetic moments nearly twice smaller than those in the defect-free Ni2MnSn. Further, the defect-containing states exhibit pressure-induced changes up to three times larger (but also smaller) than those in the defect-free Ni2MnSn. Importantly, we find both qualitative and quantitative differences in the pressure-induced changes of magnetic moments of individual atoms even for the same global magnetic state. Lastly, despite of the fact that the FM-coupled and AFM-coupled states have often very similar formation energies (the differences only amount to a few meV per atom), their structural and magnetic properties can be very different.

The impact of pressure and point defects on magnetism in austenitic stoichiometric Ni2MnSn

How to produce starch-based plastics with better mechanical, thermal and barrier properties suitable for food packaging applications.

Incorporation of TiO 2 Nanoparticles in Starch Based Bioplastics for Food Packaging Applications

1) Lignin is a complex polymer found in plants as well as a common by-product from the paper mills. Due to its abundant phenol groups, lignin is often exploited for UV capabilities1,2 . 2) Nanodiamonds (ND) are the result of controlled explosion that were discovered to have promising potential. 3) Lignin and ND were functionalised and coupled as a complex for sunscreen actives (Fig. 1). The relationship and enhanced performances were investigated.

Exploiting industrial waste for UV blocking applications

Anode materials required basic properties like metallic structure, fast and easy diffusion and low redox potential (OCV). Recently, the novel two-dimensional polyalanine C3N predicated as a promising and alternate anode material for LIB at the place of graphite. C3N is a two-dimensional sp 2 hybridized semiconductor material. In our work by using first principles approach, we proposed the C3N/Graphene, C3N/Graphene_BL and BL_C3N/Graphene multilayer heterostructure. For all these multilayer heterostructure, we are able to attain specific capacity in the range of 581 - 484 mAh/gm. Due to small interlayer distance (3.15 Å) as compare to graphite delivers the volume expansion in the range of 18 - 20 %.

First Principles study of sp2 hybridized C3N/Graphene multilayer heterostructure as a promising anode for Li-ion Battery

Typical fabrication methods used for laboratory-scale (<1 cm2) investigations of perovskite solar cells do not scale to large-area (>100 cm2) modules required for commercial application- with crystallization control of the active perovskite absorber being particularly challenging. Here, we show that the addition of alkali salts Cs+ and K+ to the perovskite precursor ink used in the scalable slot-die coating can be exploited to control the crystallization of Cs0.15FA0.85Pb(I0.83Br0.17)3 (CsFA), allowing for large-area fabrication of homogeneous and highly crystalline perovskite films. X-ray photoelectron spectroscopy analysis reveals segregation of potassium ions (K+) at the SnO2/perovskite interface, confirming that the additive initially forms nanoclusters in the precursor solution which serves as nucleation sites for subsequent bottom-up crystallization of the perovskite. The highest efficiencies are achieved with the K+ additive: the champion small cell PCE is 18.94% (0.09 cm2) and the mini-module efficiency is 16.22 % at an active area of 57.5 cm2- the highest efficiencies achieved for slot-die coated methylammonium-free perovskites. The small area devices show significantly enhanced operational stability compared to additive-free controls retaining ~72 % of its initial PCE after 426 days (10,224 h) in 30 % relative humidity (RH) and retaining 82% and 54 % of initial PCE after 1,150 h and 958 h at 65 °C and 85 °C. Later, K(add)-CsFA additive based perovskite minimodule retains ~82% of its initial PCE after 200 days (4,800 h) under 30 % RH and the perovskite solar module retains ~50 % of its initial PCE after 560 h without encapsulation under continuous illumination in the dry air environment. We believe that such a controlled “bottom-up crystallization” approach for perovskite film formation is a promising approach for scalability and reproducibility, key for industrializing perovskite photovoltaic technology.

Functional additive approach for highly efficient slot-die coated perovskite solar cells

Simultaneous improvement of composite strength and toughness has been being in challenge for material researchers for more than a few decades. With natural inspiration and understanding of the nanoscale structure-processing-property relationships, this work presents polymer nanocomposites which were incorporated with the newly-developed filler, composed of three parts synergistically function to improve the mechanical performance of the composite. The filler was designed with (a) the rigid part as inner core and (b) the soft-ductile segment which firmly bonds to the inner core as a middle layer and terminated with (c) the active functional groups, strongly bonding to matrix system. The developed filler showed the synergetic effect of each segment to enhance both strength and toughness of the nanocomposite. One of the studied system is epoxy composite; with only 1 wt% of the newly-developed filler in the epoxy matrix, the impact resistance and the KIc respectively increases ~80%, and ~20%, as compared to the neat epoxy resin. The elongation at break and the maximum strength increased more than 100% and ~50%, respectively. The mechanism of mechanical enhancement was also investigated. The phenomenon indicates that the newly-invented filler can solve the challenge mentioned above and it can be an excellent alternative for the future advanced material industry.

Core-Shell Fillers for Simultaneous Enhancement of Mechanical Performance of Polymer Composites

Electronic structures and complex optical constants including dielectric functions, complex optical conductivities and absorption coefficients of a series of CdSe-based nanoplatelets (NPLs) films are simultaneously unveiled by spectroscopic ellipsometry techniques. Based on our experimental results, the incremental monolayer-thickness in thicker NPLs not only boosts the complex optical constants but also enhances the amplitude of coupling strength of the lowest excitonic transition mainly due to the higher inorganic volume fraction and the improved surface passivation. Overall, the broad tunability of complex optical features by simply controlling the growth of vertical thickness will pave a new path for the design of the next-generation NPLs-containing photonics devices.

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First-Principles study of twin formation in Mn-excess Ni-Mn-Ga alloys

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The ultralow thermal conductivity and ultrahigh thermoelectric performance of fluorinated Sn2Bi sheet in room temperature

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