Thermoelectric properties of Ge doped n-type Ti_xZr_1−xNiSn_0.975Ge_0.025half-Heusler alloys

Abstract

Several compositions of the n-type, Ti x Zr 1-x NiSn 0.975 Ge 0.025 (x = 0, 0.1, 0.15 and 0.25), half-Heusler (HH) alloys were synthesized by reacting elemental powders at high temperature using induction melting. The resulting ingots were annealed at 900 °C for 2 weeks and mechanically alloyed to achieve fine grain size. X-ray powder diffraction suggested the formation of products with HH structure. However, electron microscopy studies revealed phase separation into Ti-rich and Zr-rich domains for the ingot with x = 0.25. The effect of band gap engineering through isoelectronic substitution of Sn by Ge and mass fluctuation arising from the intermixing of Ti and Zr in the HH structure on the electronic and thermal transports in the temperature range from 300 K to 775 K was investigated. A large reduction in the lattice thermal conductivity (4.9 W/Km to 2.8 W/Km at 300 K) was observed with increasing Ti concentration. Surprisingly, the thermopower and electrical conductivity for the sample with x = 0.25 simultaneously increase with rising temperature from -120 µV/K and 510 S/cm at 300 K to -160 µV/K and 840 S/cm at 775 K. The combination of large reduction in lattice thermal conductivity via mass fluctuation at the Ti/Zr site with optimization of the thermopower and electrical conductivity through Ge substitution at the Sn site significantly improved the figure of merit from 0.05 to 0.48 at 775 K for the sample with x = 0.25.

Band Structure Modification and Mass Fluctuation Effects of Isoelectronic Germanium-Doping on Thermoelectric Properties of ZrNiSn

“…It is clear from Table that the carrier concentration ( n ) varies marginally and remains within the range of (3.38 ± 0.2) × 10 19 cm –3 , which may be ascribed to the isoelectronic substitution of Ge at Sn site of ZrNiSn HH. Meanwhile, a significant improvement in carrier mobility (μ) was observed with Ge-doping which majorly contributes toward σ enhancement with Ge-doping at room temperature in ZrNiSn based HHs, which is in good agreement with those of previous reports. , However, in 3.5% Ge-doping, comparatively low enhancement in μ is consistent with σ at room temperature (see Figure a); moreover larger enhancement in σ at higher temperature may be due to the combined optimization of carrier mobility and concentration. Furthermore, to understand the electrical transport behavior and implication of Ge-doping in ZrNiSn HH alloys, we estimated the density of states effective mass at RT using the Pisarenko relationship m * = (3 eh 2 /8π 2 k B 2 )­(S/ T )­(3 n /π) 2/3 , where m * is the density of states effective mass, e the electronic charge, h Planck’s constant, and k B the Boltzmann constant, and the data is given in Table .…”

“…Here, reduction in the experimental energy band gap as observed in Ge-doped ZrNiSn HH samples can be ascribed to the augmentation in the carrier concentration by thermally excited carriers and increased μ which jointly enhances σ. 56 The comparison of experimental band gap (see Table 3) and the theoretically calculated one using DFT (see Figure 4) exhibits contradictory behavior with doping of Ge, which may be an outcome of theoretical overestimation and synthesis defects inducing atomic disorders 38,58 as observed previously in HHs.…”

“…The electronic transport of TE materials is well-defined by weighted mobility, μ w = μ­( m */ m e ) 3/2 , displayed in Table , demonstrates the combined effect of carrier mobility and effective mass. , The estimated energy band gap, we refer to it as experimental band gap (shown in Table ), was calculated using the Arrhenius equation, σ = σ 0 exp­(− E g /2 k B T ), where σ is the temperature dependent electrical conductivity, σ 0 the constant, E g the energy band gap, k B the Boltzmann constant, and T the absolute temperature. The linear fitting, ln­(σ) versus (−1/2 k B T ) is given in Figure b, where the slope of the linear fitted line gives the value of E g (shown in Table ).…”

Synthesis and Thermoelectric Properties of Ni-Doped ZrCoSb Half-Heusler Compounds

“…Therefore, the reduction of κ L can be attributed to the substitution of Ni at the Co site. Similar results were also found in the TiNiSn, ZrNiSn and TiCoSb systems [22][23][24]. In addition, the κ L shows temperature dependence of T −1 above 520 K, just as illustrated by blue dot line, indicating that phonon-phonon scattering is the dominant scattering for the Ni-doped ZrCo 1−x Ni x Sb half-Heusler compounds in the temperature range from 520 to 850 K. Below 520 K, the κ L deviates the T −1 and follows the T −0.5 behavior, suggesting the point defect scattering plays a great role in scattering mechanism.…”

Thermal conductivity reduction by isoelectronic elements V and Ta for partial substitution of Nb in half-Heusler Nb(1−x)/2V(1−x)/2TaxCoSb

“…To reduce the lattice thermal conductivity k L , the effectiveness of isoelectronic alloying on the M sublattice in HH compounds of MNiSn and MCoSb (M ¼ Hf, Zr, Ti) has been successfully demonstrated. 16,17,22,23,[45][46][47][48] Point defect disorder caused by strain and mass differences between alloying atoms and host atoms acts as scattering centers for phonons, which reduces k L .…”