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 bandstructure of PBEsol or SCAN with mBJ band gap can be used for searching the new thermoelectric materials.

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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 Å . The measured values of Seebeck coefficient (S) shows the non-monotonic behaviour in the studied temperature range with the value of ∼ −81 μV/K at room temperature. The |S| first increases till 560 K (∼ −93 μV/K) and then decreases up to 800 K (∼ −84 μV/K) indicating the dominating n-type behaviour in the full temperature range. The electrical conductivity, σ (thermal conductivity, κ) shows the monotonic decreasing (increasing) behaviour with the values of ∼5.2×105 (12.1 W/m-K) and ∼3.6×105 (14.2 W/m-K) Ω−1 m−1 at 300 K and 800 K, respectively. The κ exhibits the temperature dependency as, κ ∝ T0.16 . 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 σ around 450 K are due to the contribution of thermally generated charge carriers from the hole pockets. The temperature dependent relaxation time (τ ) is computed by comparing the experimental σ with calculated σ/τ and it shows temperature dependency of 1/T0.35 . Further this value of τ is used to calculate the temperature dependent electronic part of thermal conductivity (κe) and it gives fairly good match with the experiment. 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.

Fabrication and characterization of an instrument for the high-temperature simultaneous measurement of the Seebeck coefficient ( S) and thermal conductivity ( κ) have been carried out with Python automation. The steady-state-based Fourier’s law of thermal conduction is employed for κ measurement. The parallel thermal conductance technique is implemented for heat loss measurement. Introducing a thin heater and insulating heater base minimizes the heat loss and makes it easier to arrive at high temperatures. Measurement of S is carried out using the differential method. The same thermocouples are used to measure the temperature as well as voltage for S measurement. Care of temperature dependent S of the thermocouple has also been taken. Simple design, small size, and lightweight make this instrument more robust. All the components for making a sample holder are easily available in the market and can be replaced as per the user’s demand. This instrument can measure samples with various dimensions and shapes in the temperature range 300–800 K. The instrument is validated using different classes of samples, such as nickel, gadolinium, Fe2VAl, and LaCoO3. A wide range of S values from ∼−20 to ∼600 μV/K and κ values from ∼1.1 to ∼23.5 W/m K are studied. The measured values of S and κ are in good agreement with the reported data.

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