arXiv subjects

Shamim Sk

Publications and source records attributed to Shamim Sk.

12 recordsLinked to original sources

arXiv2024

Thermoelectric properties of marcasite-type compounds MSb$_2$ (M = Ta, Nb): A combined experimental and computational study

Here, we investigate the thermoelectric properties of the marcasite-type compounds MSb$_2$ (M = Ta, Nb) in the temperature range of 310-730 K. These compounds were synthesized by a solid-state reaction followed by the spark plasma sintering process. The Rietveld refinement method confirms the monoclinic phase with space group C2/m for both compounds. The observed values of Seebeck coefficients exhibit non-monotonic behaviour in the studied temperature range, with the maximum magnitude of -14.4 and -22.7 $\mu$V K$^{-1}$ for TaSb$_2$ and NbSb$_2$, respectively at ~444 K. The negative sign of S in the full temperature window signifies the n-type behaviour of these compounds. Both electrical and thermal conductivities show an increasing trend with temperature. The experimentally observed thermoelectric properties are understood through the first-principles DFT and Boltzmann transport equation. A pseudogap in the density of states around the Fermi level characterizes the semimetallic behaviour of these compounds. The multi-band electron and hole pockets were found to be mainly responsible for the temperature dependence of transport properties. The experimental power factors are found to be ~0.09 and ~0.42 mW m$^{-1}$ K$^{-2}$ at 310 K for TaSb2 and NbSb2, respectively. From the DFT-based calculations, the maximum possible power factors for p-type conduction are predicted as ~1.14 and ~1.74 mW m$^{-1}$ K$^{-2}$, while these values are found to be ~1.16 and ~1.80 mW m$^{-1}$ K$^{-2}$ for n-type TaSb$_2$ and NbSb$_2$, respectively at 300 K with the corresponding doping concentrations. The present study suggests that the combined DFT and Boltzmann transport theory are found to be reasonably good at explaining the experimental transport properties, and moderate power factors are predicted.

cond-mat.mtrl-sci↗

arXiv2022

First-principles study of optoelectronic and thermoelectronic properties of the ScAgC half-Heusler compound

Here, we present a theoretical study in the context of photovoltaic (PV) and thermoelectric (TE) applications of ScAgC. The electronic, optical, and thermoelectric properties have been investigated systematically using density functional theory (DFT) and semi-classical Boltzmann transport theory. DFT calculates a direct band gap of 0.47 eV, whereas the $G_{0}W_{0}$ method estimates a band gap of 1.01 eV. We used parabola fitting to estimate the effective mass values for bands B1 to B4 at $\Gamma$-point, which are -0.087 (-0.075), -0.17 (-0.27), -0.17 (-0.27), and 0.049 (0.058) along the $\Gamma$-$X$ ($\Gamma$-$L$) direction, respectively. Furthermore, the optoelectronic properties are calculated and analyzed over an energy range of 0 to 10 eV. The optical conductivity, refractive index, and dielectric function show strong optical transitions in the visible region. The lowest calculated reflectivity is 0.24 at 4.7 eV, and the highest calculated value of the absorption coefficient is $1.7\times10^{6}$ cm$^{-1}$ at $8.5$ eV. At 300 K, we expect a maximum solar efficiency (SLME) of 33\% at a thickness of $1~\mu$m. The lattice part of the thermal conductivity shows a maximum value of 3.8 Wm$^{-1}$K$^{-1}$ at $1200$ K. At 1200 K, for electron doping of $3.9\times10^{21}$ cm$^{-3}$, the maximum value of $S^{2}\sigma /\tau$ is $145 \times 10^{14}$ $\mu$WK$^{-2}$cm$^{-1}$s$^{-1}$, while for hole doping of $1.5\times10^{21}$ cm$^{-3}$, it is $123 \times 10^{14}$ $\mu$WK$^{-2}$cm$^{-1}$s$^{-1}$. The highest $ZT$ at $1200$ K is expected to be $0.53$, whereas the optimal efficiency is predicted to be $8.5\%$ for cold and hot temperatures of $300$ K and $1200$ K, respectively. The collected results suggest that the ScAgC compound is a potential candidate for renewable energy sources such as solar cell and TE applications

cond-mat.mtrl-sci↗

arXiv2022

Density functional study of thermodynamic properties, thermal expansion and lattice thermal conductivity of Fe$_{2}$VAl at high temperature region

Here, we present the phonon calculations for thermodynamic properties, thermal expansion and lattice thermal conductivity of Fe$_{2}$VAl in the temperature range of $300-800$ K and compared with existing experiment. Phonon dispersion is computed using finite displacement method and supercell approach. The positive frequencies of all the phonon modes indicate the mechanical stability of the compound. The specific heat at constant volume and Helmholtz free energy are calculated under harmonic approximation, while calculation of thermal expansion is done under quasi-harmonic approximation. Lattice thermal conductivity ($\kappa_{L}$) is calculated using first-principle anharmonic lattice dynamics calculations. The zero-point energy and Debye temperature are computed as $\sim$21 kJ/mol and 638 K, respectively. The calculated thermal expansions are found to be $\sim$6.3 $\times$ 10$^{-6}$ K$^{-1}$ and $\sim$7.2 $\times$ 10$^{-6}$ K$^{-1}$ at 300 and 800 K, respectively. A significant deviation between calculated ($\sim$48.6 W/m-K) and experimental ($\sim$22.8 W/m-K) values of $\kappa_{L}$ are observed at 300 K. But, as the temperature increases, the calculated and experimental $\kappa_{L}$ come closer with the corresponding values of $\sim$18.2 W/m-K and $\sim$11.0 W/m-K at 800. The possible reasons for the deviation of $\kappa_{L}$ are addressed. The temperature dependent of phonon lifetime is computed in order to understand the feature of $\kappa_{L}$. Present study suggests that DFT based phononic calculations provide reasonably good explanations of available experimental phonon related properties of Fe$_{2}$VAl in the high temperature range of $300-800$ K.

cond-mat.mtrl-sci↗

arXiv2022

Thermoelectric properties of Fe$_{2}$VAl at high temperature region: A combined experimental and theoretical study

Heusler type compounds have long been recognized as potential thermoelectric (TE) materials. Here, the experimentally observed TE properties of Fe$_{2}$VAl are understood through electronic structure calculations in the temperature range of $300-800$ K. The observed value of $S$ is $\sim-$138 $\mu$V/K at 300 K. Then, the $|S|$ decreases with increase in temperature up to the highest temperature with the value of $\sim-$18 $\mu$V/K at 800 K. The negative sign of $S$ in the full temperature window signifies the dominating $n$-type character of the compound. The temperature dependent of electrical conductivity, $\sigma$ (thermal conductivity, $\kappa$) exhibits the increasing (decreasing) trend with the values of $\sim$1.2 $\times$ 10$^{5}$ $\Omega^{-1}$m$^{-1}$ ($\sim$23.7 W/m-K) and $\sim$2.2 $\times$ 10$^{5}$ $\Omega^{-1}$m$^{-1}$ ($\sim$15.3 W/m-K) at 300 K and 800 K, respectively. In order to understand these transport properties, the DFT based semi-classical Boltzmann theory is used. The contributions of multi-band electron and hole pockets are found to be mainly responsible for the temperature dependent trend of these properties. The decrement of $|S|$ and increment of $\sigma/\tau$ $\&$ $\kappa_{e}/\tau$ ($\tau$ is relaxation time) with temperature is directly related with the contribution of multiple hole pockets. Present study suggests that DFT based electronic calculations provide reasonably good explanations of experimental TE properties of Fe$_{2}$VAl in the high temperature range of $300-800$ K.

cond-mat.mtrl-sci↗

arXiv2021

First-principles phonon calculations for lattice dynamics, thermal expansion and lattice thermal conductivity of CoSi at high temperature region

This study presents the first-principles phonon calculations to understand the experimental thermal expansion ($\alpha(T)$) and lattice thermal conductivity ($\kappa_{L}$) of CoSi at high temperature region. Phonon dispersion is computed using finite displacement method and supercell approach by taking the equilibrium crystal structures obtained from DFT. The calculation of $\alpha(T)$ is done under quasi-harmonic approximation. The $\kappa_{L}$ is calculated using first-principle anharmonic lattice dynamics calculations under single-mode relaxation time approximation. Calculated $\alpha(T)$ in the temperature range $0-1300$ K gives the good match with existing experimental data. The calculated value of $\kappa_{L}$ ($\sim$8.0 W/m-K) at 300 K is found to be in good agreement with the experimental value of $\sim$8.3 W/m-K. The temperature dependent of phonon lifetime due to phonon-phonon interaction is calculated to understand the behaviour of $\kappa_{L}$. Present study suggests that ground state phonon dispersion obtained from DFT based methods gives reasonably good explanation of experimental $\alpha(T)$ and $\kappa_{L}$.

cond-mat.mtrl-sci↗

arXiv2021

Experimental and computational approaches to study the high temperature thermoelectric properties of novel topological semimetal CoSi

Here, we study the thermoelectric properties of topological semimetal CoSi in the temperature range $300-800$ K by using combined experimental and density functional theory (DFT) based methods. CoSi is synthesized using arc melting technique and the Rietveld refinement gives the lattice parameters of a = b = c = 4.445 {\AA}. The measured values of Seebeck coefficient (S) shows the non-monotonic behaviour in the studied temperature range with the value of $\sim-$81 $\mu$V/K at room temperature. The $|S|$ first increases till 560 K ($\sim-$93 $\mu$V/K) and then decreases up to 800 K ($\sim-$84 $\mu$V/K) indicating the dominating n-type behaviour in the full temperature range. The electrical conductivity, $\sigma$ (thermal conductivity, $\kappa$) shows the monotonic decreasing (increasing) behaviour with the values of $\sim$5.2$\times 10^{5}$ (12.1 W/m-K) and $\sim$3.6$\times 10^{5}$ (14.2 W/m-K) $\Omega^{-1}m^{-1}$ at 300 K and 800 K, respectively. The $\kappa$ exhibits the temperature dependency as, $\kappa \propto T^{0.16}$. The DFT based Boltzmann transport theory is used to understand these behaviour. The multi-band electron and hole pockets appear to be mainly responsible for deciding the temperature dependent transport behaviour. Specifically, the decrease in the $|S|$ above 560 K and change in the slope of $\sigma$ around 450 K are due to the contribution of thermally generated charge carriers from the hole pockets. The temperature dependent relaxation time is computed which shows temperature dependency of $1/T^{0.35}$. Present study suggests that electronic band-structure obtained from DFT provides reasonably good estimate of the transport coefficients of CoSi in the high temperature region of $300-800$ K.

cond-mat.mtrl-sci↗

arXiv2021

Understanding the Seebeck coefficient of LaNiO3 compound in the temperature range 300-620 K

Transition metal oxides have been attracted much attention in thermoelectric community from the last few decades. In the present work, we have synthesized LaNiO$_{3}$ by a simple solution combustion process. To analyze the crystal structure and structural parameters we have used Rietveld refinement method wherein FullProf software is employed. The room temperature x-ray diffraction indicates the trigonal structure with space group $R \, \overline{3} \, c$ (No. 167). The refined values of lattice parameters are a = b = 5.4615 Å\, $\&$ c = 13.1777 Å. Temperature dependent Seebeck coefficient (S) of this compound has been investigated by using experimental and computational tools. The measurement of S is conducted in the temperature range $300-620$ K. The measured values of S in the entire temperature range have negative sign that indicates \textit{n}-type character of the compound. The value of S is found to be $\sim$ $-$8 $μ$V/K at 300 K and at 620 K this value is $\sim$ $-$12 $μ$V/K. The electronic structure calculation is carried out using DFT+\textit{U} method due to having strong correlation in LaNiO$_{3}$. The calculation predicts the metallic ground state of the compound. Temperature dependent S is calculated using BoltzTraP package and compared with experiment. The best matching between experimental and calculated values of S is observed when self-interaction correction is employed as double counting correction in spin-polarized DFT + \textit{U} (= 1 eV) calculation. Based on the computational results maximum power factors are also calculated for \textit{p}-type and \textit{n}-type doping of this compound.

cond-mat.mtrl-sci↗

arXiv2021

Instrument for simultaneous measurement of Seebeck coefficient and thermal conductivity in the temperature range 300-800 K with python interfacing

Fabrication and characterization of instrument for high-temperature simultaneous measurement of Seebeck coefficient (S) and thermal conductivity ($\kappa$) has been carried out with python automation. The steady-state based Fourier's law of thermal conduction is employed for $\kappa$ measurement. The parallel thermal conductance technique is implemented for heat loss measurement. Introducing the thin heater and insulating heater base minimize the heat loss and make the way easier to arrive at high temperature. Measurement of S is carried out using differential method. Same thermocouples are used to measure temperature as well as voltage for S measurement. Care of temperature dependent S of thermocouple has also been taken. Simple design, small size, lightweightmake this instrument more robust. All the components for making sample holder are easily available in the market and can be replaced as per the user demand. This instrument can measure samples with various dimensions and shapes in the temperature range 300 $-$ 800 K. The instrument is validated using different class of samples, such as nickel, gadolinium, Fe$_{2}$VAl and LaCoO$_{3}$. Wide range of S from $\sim$ $-$20 to $\sim$600 $\mu$V/K and $\kappa$ from $\sim$1.1 to $\sim$23.5 W/m-K are studied. The measured values of S and k are in good agreement with the reported data.

cond-mat.mtrl-sci↗

arXiv2020

Dependency of XC functionals and role of 3s(2p) orbitals of Co(Si) as core/valence states on the vibrational and thermodynamic properties of CoSi

First-principles phonon calculations along with density functional theory (DFT) play an important role to study the dynamical and thermal properties of materials. Here, we investigate the effect of exchange correlation (XC) functionals on the vibrational and thermodynamic properties of CoSi. The role of 3s(2p) orbitals of Co(Si) as core/valence states on the phonon properties of this compound is also studied. Phonon calculations are carried out by finite displacement method with supercell approach using equilibrium crystal structures obtained from DFT calculations. The calculated results are compared with the existing experiment. Three XC functionals, viz., LDA, PBEsol and SCAN are used for calculating the phonon dispersion, phonon density of states (DOS)/partial DOS and thermal properties of this compound. SCAN is found to give the highest phonon frequency of $\sim$56 meV which is in good agreement with the experimental value, while LDA (PBEsol) gives $\sim$54 ($\sim$55) meV. The zero-point energy is calculated as $\sim$ 10 kJ/mol for all the functionals. The Debye temperatures ($Θ_{D}$) are computed as $\sim$626 K, $\sim$638 K and $\sim$650 K for LDA, PBEsol and SCAN, respectively. The $Θ_{D}$ obtained from LDA gives the good agreement with the reported value. The phonon dispersion and phonon DOS are found to be dependent whether 3s(2p) orbitals of Co(Si) are considered as core or valence states. But, this orbital dependency is seemed to be insignificant on the thermal properties of this compound.

cond-mat.mtrl-sci↗

arXiv2020

Exploring the possibility of enhancing the figure-of-merit ( $>$ 2) of Na$_{0.74}$CoO$_{2}$: A combined experimental and theoretical study

Search of new thermoelectric (TE) materials with high \textit{figure-of-merit} (ZT) is always inspired the researcher in TE field. Here, we present a combined experimental and theoretical study of TE properties of Na$_{0.74}$CoO$_{2}$ compound in high-temperature region. The experimental Seebeck coefficient (S) is found to vary from 64 to 118 $μ$V/K in the temperature range $300-620$ K. The positive values of S are indicating the dominating p-type behaviour of the compound. The observed value of thermal conductivity ($κ$) is $\sim$ 2.2 W/m-K at 300 K. In the temperature region $300-430$ K, the value of $κ$ increases up to $\sim$ 2.6 W/m-K and then decreases slowly till 620 K with the corresponding value of $\sim$ 2.4 W/m-K. We have also carried out the theoretical calculations and the best matching between experimental and calculated values of transport properties are observed in spin-polarized calculation within DFT+\textit{U} by chosen \textit{U} = 4 eV. The maximum calculated value of ZT is found to be $\sim$ 0.67 at 1200 K for p-type conduction. Our computational study suggests that the possibility of n-type behaviour of the compound which can lead to a large value of ZT at higher temperature region. Electron doping of $\sim$ 5.1$\times$10$^{20}$ cm$^{-3}$ is expected to give rise the high ZT value of $\sim$ 2.7 at 1200 K. Using these temperature-dependent ZT values, we have calculated the maximum possible values of efficiency ($η$) of thermoelectric generator (TEG) made by p and n-type Na$_{0.74}$CoO$_{2}$. The present study suggests that one can get the efficiency of a TE cell as high as $\sim$ 11$\%$ when the cold and hot end temperature are fixed at 300 K and 1200 K, respectively. Such high values of ZT and efficiency suggest that Na$_{0.74}$CoO$_{2}$ can be used as a potential candidate for high-temperature TE applications.

cond-mat.mtrl-sci↗

arXiv2019

Achieving high figure-of-merit in Nb-doped Na$_{0.74}$CoO$_{2}$ compound at high temperature region

We report the thermoelectric (TE) properties of Na$_{0.74}$Co$_{0.95}$Nb$_{0.05}$O$_{2}$ in the temperature range $300-1200$ K, as a potential candidate for p-type thermoelectric material. The experimental values of Seebeck coefficient (S) are $ \sim $ $82-121$ $ μ$V/K measured in the temperature range $300-620$ K. The positive values of S in the entire temperature range indicates p-type behaviour of the compound. At 300 K the experimental value of thermal conductivity ($ κ$) is $ \sim $ 1.88 W/m-K that increases up to $ \sim $ 420 K, then decreases till 620 K with corresponding value $ \sim $ 1.86 W/m-K. To understand the experimentally observed transport properties, we have calculated S and $ρ$ of this compound. Then, based on theoretical understanding, we have estimated \textit{figure-of-merit} (ZT) up to 1200 K by using calculated S and $ρ$ values with extrapolated experimental $κ$. The value of ZT is found to be $\sim$ 0.03 at 300 K, whereas, the highest value is observed as $\sim$ 1.7 at 1200 K. Finally, we have calculated the efficiency ($η$) by keeping the cold end temperature (T$_{c}$) fixed at 500 K and varying hot end temperature (T$_{h}$) from 500 to 1200 K, respectively. The maximum value of $η$ is found to be $\sim$ 8 %, when T$_{c}$ and T$_{h}$ are fixed at 500 and 1200 K, respectively. This result suggests that Na$_{0.74}$Co$_{0.95}$Nb$_{0.05}$O$_{2}$ compound can be used as a p-leg for making high temperature TE generator (TEG).

cond-mat.str-el↗

arXiv2018

Exploring the best scenario for understanding the high temperature thermoelectric behaviour of Fe2VAl

Heusler-type Fe2VAl compound is a promising thermoelectric candidate with non-magnetic ground state. The present work investigates the Seebeck coefficient (S) of Fe2VAl in the temperature region 300 to 620 K with the help of experimental and theoretical tools. The experimental value of S is observed -130 μV/K at 300 K. Afterthat, the magnitude of S decreases gradually as the temperature increases. At T = 620 K, the value of S is found to be -26 μV/K. In order to understand the behaviour of the experimentally observed S value, the band-structure and density of states calculations are performed by using LDA, PBE, PBEsol, mBJ and SCAN within density functional theory. All the above mentioned exchange-correlation (XC) functionals (except mBJ) predict the semi-metal like behaviour of the compound, whereas the mBJ gives the indirect band gap of 0.22 eV having the well agreement with experimentally observed value. The temperature dependence of S for Fe2VAl is also calculated with the help of all the five mentioned functionals individually. The best XC functional is investigated for searching the new thermoelectric materials by taking Fe2VAl as a case example through this study. The best matching between experimental and calculated values of S as a function of temperature is observed by setting the mBJ band gap with the band-structure of PBEsol or SCAN. Therefore, the present study suggests that the band-structure of PBEsol or SCAN with mBJ band gap can be used for searching the new thermoelectric materials.

cond-mat.mtrl-sci↗