Facile synthesis of nanostructured n-type SiGe alloys with enhanced thermoelectric performance using rapid solidification employing melt spinning followed by spark plasma sintering

Avinash Vishwakarma,Sivaiah Bathula,Nagendra S. Chauhan,Ruchi Bhardwaj,Bhasker Gahtori,Avanish K. Srivastava,Ajay Dhar 한국물리학회 2018 Current Applied Physics Vol.18 No.12

SiGe alloy is widely used thermoelectric materials for high temperature thermoelectric generator applications. However, its high thermoelectric performance has been thus far realized only in alloys synthesized employing mechanical alloying techniques, which are time-consuming and employ several materials processing steps. In the current study, for the first time, we report an enhanced thermoelectric figure-of-merit (ZT)∼1.1 at 900 °C in ntype Si80Ge20 nano-alloys, synthesized using a facile and up-scalable methodology consisting of rapid solidification at high optimized cooling rate∼3.4×107 K/s, employing melt spinning followed by spark plasma sintering of the resulting nano-crystalline melt-spun ribbons. This enhancement in ZT>20% over its bulk counterpart, owes its origin to the nano-crystalline microstructure formed at high cooling rates, which results in crystallite size ∼7 nm leading to high density of grain boundaries, which scatter heat-carrying phonons. This abundant scattering resulted in a very low thermal conductivity ∼2.1 Wm−1K−1, which corresponds to ∼50% reduction over its bulk counterpart and is amongst the lowest reported thus far in n-type SiGe alloys. The synthesized samples were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy, based on which the enhancement in their thermoelectric performance has been discussed.

Mechanistic Insight into the Chemical Exfoliation and Functionalization of Ti₃C₂ MXene

Srivastava, Pooja,Mishra, Avanish,Mizuseki, Hiroshi,Lee, Kwang-Ryeol,Singh, Abhishek K. American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.36

<P>MXene, a two-dimensional layer of transition metal carbides/nitrides, showed great promise for energy storage, sensing, and electronic applications. MXene are chemically exfoliated from the bulk MAX phase; however, mechanistic understanding of exfoliation and subsequent functionalization of these technologically important materials is still lacking. Here, using density-functional theory we show that exfoliation of Ti3C2 MXene proceeds via HF insertion through edges of Ti3AlC2 MAX phase. Spontaneous dissociation of HF and subsequent termination of edge Ti atoms by H/F weakens Al-MXene bonds. Consequent opening of the interlayer gap allows further insertion of HF that leads to the formation of AlF3 and H-2, which eventually come out of the MAX, leaving fluorinated MXene behind. Density of state and electron localization function shows robust binding between F/OH and Ti, which makes it very difficult to obtain controlled functionalized or pristine MXene. Analysis of the calculated Gibbs free energy (Delta G) shows fully fluorinated MXene to be lowest in energy, whereas the formation of pristine MXene is thermodynamically least favorable. In the presence of water, mixed functionalized Ti3C2Fx(OH)(1-x) (x ranges from 0 to 1) MXene can be obtained. The Delta G values for the mixed functionalized MXenes are very close in energy, indicating the random and nonuniform functionalization of MXene. The microscopic understanding gained here unveils the challenges in exfoliation and controlling the functionalization of MXene, which is essential for its practical application.</P>

Breakthrough behaviour of NBC canister against carbon tetrachloride: a simulant for chemical warfare agents

Avanish Kumar Srivastava,D. Shah,T.H. Mahato,Beer Singh,A. Saxena,A.K. Verma,S. Shrivastava,A. Roy,S.S. Yadav,A.R. Shrivastava 한국탄소학회 2012 Carbon Letters Vol.13 No.2

A nuclear, biological, chemical (NBC) canister was indigenously developed using active carbon impregnated with ammoniacal salts of copper (II), chromium (VI) and silver (I), and high efficiency particulate aerosol filter media. The NBC canister was evaluated against carbon tetra chloride (CCl4) vapours, which were used as a simulant for persistent chemical warfare agents under dynamic conditions for testing breakthrough times of canisters of gas masks in the National Approval Test of Respirators. The effects of CCl4 concentration, test flow rate, temperature, and relative humidity (RH) on the breakthrough time of the NBC canister against CCl4 vapour were also studied. The impregnated carbon that filled the NBC canister was characterized for surface area and pore volume by N2 adsorption-desorption isotherm at liquid nitrogen temperature. The study clearly indicated that the NBC canister provides adequate protection against CCl4 vapours. The breakthrough time decreased with the increase of the CCl4 concentration and flow rate. The variation in temperature and RH did not significantly affect the breakthrough behaviour of the NBC canister at high vapour concentration of CCl4, whereas the breakthrough time of the NBC canister was reduced by an increase of RH at low CCl4 vapour concentration.

Isolation of pristine MXene from Nb₄AlC₃ MAX phase: a first-principles study

Mishra, Avanish,Srivastava, Pooja,Mizuseki, Hiroshi,Lee, Kwang-Ryeol,Singh, Abhishek K. The Royal Society of Chemistry 2016 Physical Chemistry Chemical Physics Vol.18 No.16

<P>Synthesis of pristine MXene sheets from MAX phase is one of the foremost challenges in getting a complete understanding of the properties of this new technologically important 2D-material. Efforts to exfoliate Nb4AlC3 MAX phase always lead to Nb4C3 MXene sheets, which are functionalized and have several Al atoms attached. Using the first-principles calculations, we perform an intensive study on the chemical transformation of MAX phase into MXene sheets by inserting HF, alkali atoms and LiF in Nb4AlC3 MAX phase. Calculated bond-dissociation energy (BDE) shows that the presence of HF in MAX phase always results in functionalized MXene, as the binding of H with MXene is quite strong while that with F is weak. Insertion of alkali atoms does not facilitate pristine MXene isolation due to the presence of chemical bonds of almost equal strength. In contrast, weak Li-MXene and strong Li-F bonding in Nb4AlC3 with LiF ensured strong anisotropy in BDE, which will result in the dissociation of the Li-MXene bond. Ab initio molecular dynamics calculations capture these features and show that at 500-650 K, the Li-MXene bond indeed breaks leaving a pristine MXene sheet behind. The approach and insights developed here for chemical exfoliation of layered materials bonded by chemical bonds instead of van der Waals can promote their experimental realization.</P>

Atomistic Origin of Phase Stability in Oxygen-Functionalized MXene: A Comparative Study

Mishra, Avanish,Srivastava, Pooja,Carreras, Abel,Tanaka, Isao,Mizuseki, Hiroshi,Lee, Kwang-Ryeol,Singh, Abhishek K. American Chemical Society 2017 The Journal of Physical Chemistry Part C Vol.121 No.34

<P>Oxygen-functionalized MXene, M2CO2 (M = group III-V metals), are emergent formidable two-dimensional (2D) materials with a tantalizing possibility for device applications. Using first-principles calculations, we perform an intensive study, on the stability of fully O-functionalized (M2CO2) MXenes. Depending on the position of O atoms, the M2CO2 can a.) O atom occupies a site which is exactly on the top of the metal exist in two different structural phases. On one side of MXene, the atom from the opposite side. On the other side, the O atom can occupy either the site on the top of the metal atom of the opposite side (BB' phase) or on the top of the C atom (CB phase). We find that for M = Sc and Y the CB phase is stable, whereas for M = Ti, Zr, Hf, V, Nb, and Ta the stable phase is BB'. The electron localization function, the atom-projected density of states, the charge transfer, and the Bader charge analyses provide a rational explanation for the relative stability of these two phases and justify the ground state structure by giving information about the preferential site of adsorption for the O atoms. We also calculate the phonon dispersion relations for both phases of M2CO2. The BB'-Sc2CO2 and the CB-Ti2CO2 are found to 'be dynamically unstable. Finally, we find that the instability of BB'-M2CO2 (M = Sc and Y) originates from the weakening of M-C interactions, which manifest as a phonon mode with imaginary frequency corresponding to the motion of C atom in the a-b plane. The insight into the stability of these competing structural phases of M2CO2 presented in this study is an important step in the direction of identifying the stable phases of these 2D materials.</P>