Gandi, Appala Naidu

Mg–Sn–Gd alloys can be considered prospective contenders for creep applications owing to the formation of thermally stable phases. In the present investigation, the solidification behaviour of Mg–Sn–Gd alloys was analysed from cooling curves obtained from thermal analysis within the temperature range of 700–300 °C. The phase evolution as a function of Sn and Gd content was studied by varying the ratio of Sn to Gd (Sn/Gd) at three levels (0.5, 1, and 2) up to a maximum concentration of 3 wt% Sn and Gd, respectively. The phases in the as-cast microstructure were compared with the phases predicted by a commercially available thermodynamic database, and a deviation was observed. The results indicated that the addition of Sn and Gd promoted the formation of a ternary MgSnGd phase. Moreover, the presence of the MgSnGd phase in the microstructure led to significant grain refinement, and its role as a potential grain refiner has been recognized.

Herein, the symmetry of the experimentally observed soft phonons in the body-centered cubic β-phase (Im (Formula presented.) m) of titanium is analyzed. The harmonic phonon dispersion relations are calculated using the first-principles calculations. Using the group-theoretical methods, the symmetry of the calculated unstable phonons is determined. The symmetries of the unstable phonons observed at wave vectors (Formula presented.) (N) and (Formula presented.) ((Formula presented.)) are the same as the symmetries of the (Formula presented.) and (Formula presented.) irreducible representations, respectively. Transformations of the β-phase due to the atomic motion of unstable phonons and the subsequent structure relaxation are discussed. One possible way to explain the transformation of the β-phase to the hexagonal close-packed α-phase ((Formula presented.) /mmc) is through an orthorhombic structure (either Cmcm or Pnnm). The atomic motion of an unstable (Formula presented.) phonon results in the orthorhombic structure and following structure relaxation transforms the orthorhombic structure to the α-phase. Similarly, the transformation of the β-phase to another hexagonal close-packed ω-phase (P6/mmm) can be considered to be happening through a trigonal structure (either P (Formula presented.) m1 or P3m1). The atomic motion of an unstable (Formula presented.) phonon forms the trigonal structure and subsequent structure relaxation transforms the trigonal structure to the ω-phase. The space group of the intermediate phase is a common subgroup of the space groups of the initial β-phase and the final α/ω-phase. Therefore, the β–α/ω transformation can be described as an unstable phonon-induced reconstructive transformation of the second type. There is no activation energy barrier along each of the four energy-minimizing paths, and the transformation strains are accommodated.

The model presented in our work was able to reproduce the Raman activity that is relevant to clarify the origin of the experimentally observed Raman activity in the high frequency region due to amorphization. Most of the points raised by Helmut Werheit are part of the debate in existing literature regarding the Raman spectra observed in B4C samples. In response to the comments by Helmut Werheit, here more inputs are provided to this discussion by focusing on (a) composition, (b) crystal structure, (c) Raman spectra.

The influence of pressure on the Raman spectra of (B11C)CBCp crystal is studied using first-principles calculations. The calculated spectra agree well with the reported experimental spectra. Reported spectra of recovered samples after depressurization amorphization show activity near ∼1340 cm−1, ∼1520 cm−1, and ∼1810 cm−1. Corresponding phonon modes are identified in the spectra calculated at 100 GPa. These activities are consistently present even at lower pressures up to 0 GPa and is shown to be essential due to the monoclinic symmetry of the polar structure. However, the Raman activity corresponding to the ∼1810 cm−1 phonon was not reported up to 50 GPa during pressurization due to its low intensity. The changes in the Raman activity with the pressure are related to the changes in the polarization. The amorphization induced high pressure in the surrounding crystalline region is responsible for the experimentally observed activities in recovered samples after depressurization amorphization.

Temperature dependence of structural, mechanical, and thermodynamic properties of γ-TiAl is modeled using an extended quasi-harmonic approximation and first-principles calculations. In the first step, the volumes are estimated as a function of temperature following the quasi-harmonic approximation. The lattice parameters are further optimized at fixed volumes in the second step. Modeled mechanical properties (bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and hardness) agree with the experimentally reported mechanical properties. Similarly, the modeled thermodynamic properties (entropy, heat capacity at constant pressure, Gibbs free energy) are in good agreement with the thermodynamic properties reported from experiments and CALculation of PHAse Diagrams approaches. This study suggests that further optimization of the degree of freedom in the unit cell improves the model accuracy of properties estimated following the quasi-harmonic approximation.

The conversion of waste heat into valuable green energy by employing thermoelectric devices has received significant attention worldwide due to the rapid depletion of fossil fuels and the availability of enormous waste heat resources. In this context, the surge for new thermoelectric (TE) materials development with high TE performance is the need of the hour. Half-Heusler (HH) materials are considered the best candidate TE materials in the mid-temperature range (673–873 K) apart from the silicides. Furthermore, double half-Heusler (DHH) materials embracing two aliovalent HH alloys are considered to be potential candidates for TE devices due to their inherent low lattice thermal conductivity. A few DHH alloys have been synthesized in the current study, in particular Ti2FeNiSb2, MgTiNi2Sb2, and Nb2FeNiSn2. Furthermore, the thermoelectric transport properties have been measured and compared with conventional HH compounds, such as TiCoSb, ScNiSb, and NbCoSn. The Ti2FeNiSb2 exhibited the higher Seebeck coefficient of − 120 μV/K among the three compounds at 813 K. As a result, with an increased power factor and reduced thermal conductivity, the Ti2FeNiSb2 DHH has exhibited a figure-of-merit (ZT) of ~ 0.1 at 813 K. This enhancement was mainly due to the aliovalent substitution (1:1) of Fe and Ni at the Co-site in the DHH compound, significantly reducing the lattice thermal conductivity and maintaining the band gap. Furthermore, the synthesized compounds possess a net valence value equal to zero with a valence electron count equal to 18, and these compounds have excellent thermal stability. These results are discussed in detail, delineating the underlying physics to support the experimentally realized results. Suitable aliovalent dopants can further improve the thermoelectric performance of DHH with the optimization of process parameters. Finally, the enhancement of ZT for DHH materials has been suitably corroborated with the appropriate structural and microstructural characterizations.

Organic–inorganic hybrid materials are of significant interest owing to their diverse applications ranging from photovoltaics and electronics to catalysis. Control over the organic and inorganic components offers flexibility through tuning their chemical and physical properties. Herein, it is reported that a new organic–inorganic hybrid, [Mn(C 2 H 6 OS) 6 ]I 4 , with linear tetraiodide anions exhibit an ultralow thermal conductivity of 0.15 ± 0.01 W m −1 K −1 at room temperature, which is among the lowest values reported for organic–inorganic hybrid materials. Interestingly, the hybrid compound has a unique 0D structure, which extends into 3D supramolecular frameworks through nonclassical hydrogen bonding. Phonon band structure calculations reveal that low group velocities and localization of vibrational energy underlie the observed ultralow thermal conductivity, which could serve as a general principle to design novel thermal management materials.

The influence of surfaces and interfaces on the Raman spectra of boron carbide crystal is studied employing first-principles calculations. Surfaces based on {10-11} planes and {01-12} planes are considered. The average energy of the {01-12} surfaces (3.26 J/m2) agrees with the experimentally reported surface energy (3.21 J/m2). Two peaks are observed at ∼270 cm−1 and ∼320 cm−1 in the calculated Raman spectra for the supercells built on the {01-12} planes. This result suggests that the experimentally observed Raman peaks at these two frequencies are more likely to originate from the surface. Two Raman peaks experimentally observed at ∼1330 cm−1 and ∼1520 cm−1 in the amorphized samples were reproduced in the Raman spectra calculated for the supercells built on the {10-11} planes. Therefore, the experimentally observed new Raman activity at ∼1330 cm−1 and ∼1520 cm−1 is more likely to originate from the interface between the amorphous and crystalline regions.

The application of proton exchange membrane water electrolyzer (PEMWE) technology has long been limited by the excessive energy consumption and poor catalyst durability because of the harsh corrosive and oxidative conditions that are related to the anodic oxygen evolution reaction (OER) in acidic electrolytes. Herein, we circumvent this challenge by adopting alternative hydrazine oxidation reaction (HzOR) as the anodic half-reaction, integrated with the cathodic hydrogen evolution reaction (HER) for sustainable hydrogen production. To this end, we further developed a PtCo alloy nanosheets electrocatalyst that can efficiently catalyze both the HzOR and HER with ultralow potentials. Specifically, the overall hydrazine splitting driven by the PtCo alloy requires only 0.28 V at 10 mA cm−2 along with outstanding stability of more than 3000 h. We further proposed a PEM hydrazine electrolyzer (PEMHE) design to promote the practical application. The device can not only produce hydrogen with a high yield rate of 1.87 mmol h−1 cm−2 at a practical current density of 100 mA cm−2 with a long durability of 60 h, but also effectively decontaminate hydrazine sewage with the hydrazine removal efficiency up to 100%. Our work provides a new solution to simultaneous mass hydrogen fuel production and hydrazine hazard removal from acidic waste water at minimized energy consumption.

Metal-ion batteries are widely used energy storage devices. Commercial cathode materials show high voltages but low capacities, limiting energy densities. Although phosphorene has been prepared and delivers a high capacity, the low voltage is problematic for cathode. The structural, electronic, and electrochemical properties of F-decorated phosphorene for metal-ion batteries are investigated using first-principles calculations. The F atoms are converted during lithiation/sodiation/potassiation. Importantly, F-decorated phosphorene delivers a capacity of 536 mAh/g and voltages of 3.46, 3.09, and 2.95 V for lithiation, sodiation, and potassiation, respectively, which leads to high energy densities of 1856, 1657, and 1582 mWh/g. Graphene coating improves stability of F-decorated phosphorene.