Department of Metallurgical & Materials Engineering

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IIT Jodhpur

Borophene, a 2D allotrope of boron and the lightest elemental Dirac material, is the latest very promising 2D material owing to its unique structural and electronic characteristics of the X3 and β12 phases. The high atomic density on ridgelines of the β12 phase of borophene provides a substantial orbital overlap, which leads to an excellent electron density in the conduction level and thus to a highly metallic behavior. These unique structural characteristics and electronic properties of borophene attract significant scientific interest. Herein, approaches for crystal growth/synthesis of these unique nanostructures and their potential technological applications are discussed. Various substrate-supported ultrahigh-vacuum growth techniques for borophene, such as molecular beam epitaxy, atomic layer deposition, and chemical vapor deposition, along with their challenges, are also summarized. The sonochemical exfoliation and modified Hummer's technique for the synthesis of free-standing borophene are also discussed. Solution-phase exfoliation seems to address the scalability issues and expands the applications of these unique materials to various fields, including renewable energy devices and ultrafast sensors. Furthermore, the electronic, optical, thermal, and elastic properties of borophene are thoroughly discussed and are compared with those of graphene and its “cousins.” Numerous frontline applications are envisaged and an outlook is presented. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

World University root node

Advanced Materials

Borophene: New sensation in flatland

The present work evaluates the coefficient of friction (CoF), electrical resistivity, and electrical contact resistance (ECR) of the electrodeposited single-layered Cu-SiC nanocomposite coating and five-layered Cu, Cu-SiC functionally graded coating (FGC). Both the coatings have a similar thickness (60 µm) and same composition at the top surface (7 vol.% reinforced SiC nanoparticles), while the FGC has a gradient of composition and microstructure throughout the thickness. The Cu, Cu-SiC FGC has two layers of Cu-SiC with a decrement in the content of SiC nanoparticles from 7 to 2 vol.% followed by three Cu layers with an increasing crystallite size towards the substrate. The electrical resistivity of the Cu, Cu-SiC FGC is measured by the four-wire resistance measurement method and the value is observed to be 50% less than the conventional nanocomposite coating. A linear reciprocating sliding wear test is carried out at 2, 5 and 8 N load at a constant frequency and stroke length of 10 Hz and 2 mm, respectively. The monitored value of CoF is significantly less for the Cu, Cu-SiC FGC than the single-layered coating at 2 and 5 N loads and is nearly equal at 8 N load. It is observed that before wear, the ECR values of both the coatings are higher than the uncoated Cu and after wear the ECR value of Cu, Cu-SiC FGC is the lowest. © 2020, ASM International.

Journal of Materials Engineering and Performance

Reciprocating Sliding Wear of Cu, Cu-SiC Functionally Graded Coating on Electrical Contact

Fe42Mn28Co10Cr15Si5 high entropy alloy (DP-5Si-HEA) exhibited superplastic elongation of 550% at ε˙=10−3s−1 after complete tensile deformation at 700 °C. Pronounced Cr partitioning at testing temperature facilitated σ phase formation which assisted in stabilizing the grain size during deformation. Stable equiaxed fine-grained dual phase microstructure at all strain rates suggested dominance of grain boundary sliding over diffusion creep to maintain the superplasticity at the tested strain rate-temperature combinations. Cavitation along the gage region is evident, however, effective diffusional linkage of the cavities along the superplastic flow resulted in elongations &gt;200% irrespective of strain rate at 700 °C. © 2019

Materialia

Superplasticity in fine grained dual phase high entropy alloy

Transformation induced plasticity (TRIP) leads to enhancements in ductility in low stacking fault energy (SFE) alloys, however to achieve an unconventional increase in strength simultaneously, there must be barriers to dislocation motion. While stacking faults (SFs) contribute to strengthening by impeding dislocation motion, the contribution of SF strengthening to work hardening during deformation is not well understood; as compared to dislocation slip, twinning induced plasticity (TWIP) and TRIP. Thus, we used in-situ neutron diffraction to correlate SF strengthening to work hardening behavior in a low SFE Fe40Mn20Cr15Co20Si5 (at%) high entropy alloy, SFE ~ 6.31 mJ m−2. Cooperative activation of multiple mechanisms was indicated by increases in SF strengthening and γ-f.c.c. → ε-h.c.p. transformation leading to a simultaneous increase in strength and ductility. The present study demonstrates the application of in-situ, neutron or X-ray, diffraction techniques to correlating SF strengthening to work hardening. © 2020, The Author(s).

Fulltext

Scientific Reports

Correlating work hardening with co-activation of stacking fault strengthening and transformation in a high entropy alloy using in-situ neutron diffraction

Effect of strain rate on the tensile behavior of two friction stir-processed dual-phase high entropy alloys was investigated. The dual-phase high entropy alloys have a high fraction of metastable f.c.c. phase. Engineering the f.c.c. phase metastability through alloy chemistry leads to different levels of transformation and impacts the retained f.c.c. phase after deformation. These metastable high entropy alloys show dominance of transformation-induced plasticity (TRIP) irrespective of strain rates, unlike the observations in TRIP steels. © 2020, The Minerals, Metals & Materials Society and ASM International.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Effect of Strain Rate on Deformation Response of Metastable High Entropy Alloys Upon Friction Stir Processing

We have investigated functionalized 2D carbon allotrope, graphdiyne (GDY), as a promising hydrogen storage media. Density functional theory with a range of vdW corrections was employed to study Ni decoration of pristine and boron-doped GDY and the interaction of resulting structures with molecular hydrogen. We showed that boron-doped GDY is thermally stable at 300 K, though, its synthesis requires an endothermic reaction. Also, boron doping enhances Ni binding with the graphdiyne by increasing the charge transfer from Ni to GDY. Ni doping drastically influenced hydrogen adsorption energies: they rise from ~70 meV per H2 molecule on pristine GDY to a maximum of 1.29 eV per H2 becoming too high in value for room temperature reversible applications. Boron doping improves the situations: in this case, after Ni decoration desorption temperature estimation is ~300–500 K. Overall, each Ni adatom on B-doped GDY can bind only one H2 molecule within the needed energy range, which gives low hydrogen uptake (~1.2 wt%). However, doping with boron led to the decrease in the value of hydrogen adsorption energy and good desorption temperature estimations, therefore, codoping of metal atoms and boron could be an effective strategy for other transition metals. © 2020 The Authors

Materials Today Energy

Influence of Kubas-type interaction of B–Ni codoped graphdiyne with hydrogen molecules on desorption temperature and storage efficiency

In this work, we have envisaged the hydrogen evolution reaction (HER) mechanism on Mg3N2 monolayer based on electronic structure calculations within the framework of density functional theory (DFT) formalism. The semiconducting nature of Mg3N2 monolayer motivates us to investigate the HER mechanism on this sheet. We have constructed the reaction coordinate associated with HER mechanism after determining the hydrogen adsorption energy on Mg3N2 monolayer, while investigating all possible adsorption sites. After obtaining the adsorption energy, we subsequently obtain the adsorption free energy while adding zero point energy difference (ΔZPE) and entropic contribution (TΔS). We have not only confined our investigations to a single hydrogen, but have thoroughly observed the adsorption phenomena for increasing number of hydrogen atoms on the surface. We have determined the projected density of states (DOS) in order to find the elemental contribution in the valence band and conduction band regime for all the considered cases. We have also compared the work function value among all the cases, which quantifies the amount of energy required for taking an electron out of the surface. The charge transfer mechanism is also being investigated in order to correlate with the HER mechanism with amount of charge transfer. This is the first attempt on this material to the best of our knowledge, where theoretical investigation has been done to mapping the reaction coordinate of HER mechanism with the associated charge transfer process and the work function values, not only for single hydrogen adsorption, but also for increasing number of adsorbed hydrogen. © 2020 The Authors

International Journal of Hydrogen Energy

Reaction coordinate mapping of hydrogen evolution mechanism on Mg3N2 monolayer

The transition from insulator to conductor can be realized in some materials but requires modification of both the arrangement of atoms and their electronic configurations. This is often achieved by doping. Here we reveal a different mechanism the lattice may adopt to induce such a transition. Experiments showed the surprising finding that limited exposure to subband-gap light caused a permanent transition from an insulator state to a conductor state in the insulating oxide Ga2O3, with a nine orders of magnitude increase in electronic conduction. Furthermore, annealing up to 400 °C did not suppress or decrease the induced conductivity. Photoexcitation by light-induced modification in the charge state of defects and subsequent lattice distortion around them was suggested to be the underlying mechanism behind this transition. Density functional theory calculations confirmed that modifying the charge state of defects leads to redistribution of the localized electrons and massive structural distortion in the surrounding lattice, causing large shifts in the density of states and introducing new states with shallower energy levels. Both experimental and theoretical results revealed the introduction of stable shallow energy levels, explaining the mechanism behind the transition from an insulator to a conductor state by light. We suggest that this mechanism may occur in other wide band-gap metal oxides leading to drastic modification in their electronic properties. Published by the American Physical Society

arXiv preprint arXiv:2003.

Light-driven permanent transition from insulator to conductor in Ga2O3

Light soaking in thin film solar cells shows both beneficial and detrimental impact on efficiency depending upon the absorber–buffer pair. We here numerically simulated the advantageous and detrimental impact of light soaking behavior as experimentally observed in the literature by utilizing suitable defect models for the two cases, respectively. The proposed models are based on the photoconductivity and metastable photo-induced activity of absorber and buffer layers. It is observed that light soaking in absorber layer vis-a-vis photo-induced defect in absorber is beneficial for device performance. Photo-generated defect in absorber acts as hole doping which is turn enhance the built-in voltage. The photoconductivity of the buffer layer is detrimental to the device performance. The photo-generated defects in the buffer cause lowering of fill-factor, current, accelerate interfacial recombination and depreciate overall efficiency. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.

Applied Physics A

Impact of light soaking on absorber and buffer layer in thin film solar cells

Nucleation and growth of chemical conversion coating on Mg–4Zn alloy prepared by immersion in saturated NaHCO3 at ambient temperature and its corrosion behaviour in simulated body fluid (SBF) is investigated in this paper. Compact dense coating obtained up on 24 h of immersion at ambient temperature is found to provide complete coverage of the surface and a high initial corrosion resistance in SBF. With sustained exposure to SBF, the coating dissolves and slowly cracks to expose the substrate to the corrosion medium. The 24 h coated sample shows high levels of bio mineralisation, with flower shaped crystals of phosphates of Ca and Mg on the surface. The combined effects of slow first hydrogen evolution rate and precipitation of phosphates, and similarities to the carbonate coating developed in vivo makes the conversion coating discussed in this paper a suitable choice for Mg alloys for temporary bio implant applications. © 2019 Elsevier B.V.

Journal of Alloys and Compounds

Morphological studies on the development of chemical conversion coating on surface of Mg–4Zn alloy and its corrosion and bio mineralisation behaviour in simulated body fluid