Phase equilibria, formation, crystal and electronic structure of ternary compounds in Ti–Ni–Sn and Ti–Ni–Sb ternary systems

Magnetically Enhanced Thermoelectric Performance of Ti0.75NiSb+x mol % Fe (x = 0-5) Nanocomposites

Ti_0.75NiSb is a half-Heusler compound with low lattice thermal conductivity due to a large number of cation vacancies. However, the higher carrier concentration limits the improvement of its thermoelectric performance. In this paper, magnetic Fe nanoparticles with a size of 30 nm are composited into Ti_0.75NiSb in the form of the second phase. The charge transfer between Fe nanoparticles and Ti_0.75NiSb leads to a decrease in carrier concentration. The strong interaction between the magnetic moment and carriers enhances the electron scattering, so that the scattering factor increases and the mobility decreases. The combined effect results in an increase of about 10% in the Seebeck coefficient and a decrease by about 14% in the electronic thermal conductivity at 873 K for the composite Ti_0.75NiSb+2 mol % Fe. Meanwhile, the magnetic Fe nanoparticles provide additional scattering centers, leading to a decrease in lattice thermal conductivity. As a result, a zT value of 0.4 at 873 K is achieved for the composite Ti_0.75NiSb+2 mol % Fe, which is 21% higher than that of Ti_0.75NiSb. This work demonstrates that the compositing magnetic nanoparticles Fe can enhance the thermoelectric performance of Ti_0.75NiSb.

Physical properties of {Ti,Zr,Hf}2Ni2Sn compounds

Physical properties, i.e. electrical resistivity (4.2-800 K), Seebeck coefficient (300-800 K), specific heat (2-110 K), Vickers hardness and elastic moduli (RT), have been defined for single-phase compounds with slightly nonstoichiometric compositions: Ti_2.13Ni₂Sn_0.87, Zr_2.025Ni₂Sn_0.975, and Hf_2.055Ni₂Sn_0.945. From X-ray single crystal and TEM analyses, Ti_2+xNi₂Sn_1-x, x ∼ 0.13(1), is isotypic with the U₂Pt₂Sn-type (space group P4₂/mnm, ternary ordered version of the Zr₃Al₂-type), also adopted by the homologous compounds with Zr and Hf. For all three polycrystalline compounds (relative densities >95%) the electrical resistivity of the samples is metallic-like with dominant scattering from static defects mainly conditioned by off-stoichiometry. Analyses of the specific heat curves C_pvs. T and C_p/T vs. T² reveal Sommerfeld coefficients of γ_Ti2Ni2Sn = 14.3(3) mJ mol^-1 K^-2, γ_Zr2Ni2Sn = 10(1) mJ mol^-1 K^-2, γ_Hf2Ni2Sn = 9.1(5) mJ mol^-1 K^-2 and low-temperature Debye-temperatures: θLTD = 373(7)K, 357(14)K and 318(10)K. Einstein temperatures were in the range of 130-155 K. Rather low Seebeck coefficients (<15 μV K^-1), power factors (pf < 0.07 mW mK^-2) and an estimated thermal conductivity of λ < 148 mW cm^-1 K^-1 yield thermoelectric figures of merit ZT < 0.007 at ∼800 K. Whereas for polycrystalline Zr₂Ni₂Sn elastic properties were determined by resonant ultrasound spectroscopy (RUS): E = 171 GPa, ν = 0.31, G = 65.5 GPa, and B = 147 GPa, the accelerated mechanical property mapping (XPM) mode was used to map the hardness and elastic moduli of T₂Ni₂Sn. Above 180 K, Zr₂Ni₂Sn reveals a quasi-linear expansion with CTE = 15.4 × 10^-6 K^-1. The calculated density of states is similar for all three compounds and confirms a metallic type of conductivity. The isosurface of elf shows a spherical shape for Ti/Zr/Hf atoms and indicates their ionic character, while the [Ni₂Sn]^n- sublattice reflects localizations around the Ni and Sn atoms with a large somewhat diffuse charge density between the closest Ni atoms.

Thermoelectric transport properties of (Ti1-cAlc)NiSn half-Heusler alloy

The influence of Al on the thermoelectric properties of the half-Heusler (HH) TiNiSn compound is reported. The research combined ab initio Density Functional Theory (DFT) calculations with experimental microstructure evaluation and measurements of the transport properties up to 750 K. It is shown that Al addition to the Ti sub-lattice results in an increase of the absolute value of the Seebeck coefficient and electrical resistivity in polycrystalline TiNiSn, while preserving the n-type behavior of the ternary compound, in addition to a significant reduction of the thermal conductivity. In (Ti_0.99Al_0.01)NiSn, upon 1% Al substitution of Ti, an improvement of 17% in the thermoelectric figure of merit (0.42 at 723 K) compared to pure TiNiSn was observed. Theoretical lattice thermal conductivity calculations are applied to shed light on the different scattering mechanisms in this class of materials. It is shown that the major contribution to the lattice thermal conductivity reduction is stimulated by the presence of Sn-rich inclusions, in addition to an influence of mass fluctuation scattering due to substitution of Ti by Al. Although it is shown that in the widely applied polycrystalline TiNiSn, an addition of the acceptor Al dopant could not fully compensate n-type electronic active defects (e.g. grain boundaries) for obtaining p-type materials, the currently reported results pave a route for thermoelectric optimization of MNiSn (M = Ti, Ni, Sn) n-type half-Heusler compounds.

Long- and short-range structures of Ti1−xHfxNi1.0/1.1Sn half-Heusler compounds and their electric transport properties

Experimental study reveals the apparent ordered arrangement of excess Ni at the nominally vacant sublattice in thermoelectric Ti_1−xHf_xNi_1.0/1.1Sn half-Heusler compounds.

Synthesis of Pure NiTiSn by Mechanical Alloying: An Investigation of the Optimal Experimental Conditions Supported by First Principles Calculations

Synthesis of NiTiSn by a mechanical alloying process followed by a high temperature thermal annealing was studied. Experiments were conducted varying parameters like the provided energy, the mechanical alloying reaction time, as well as the annealing temperature and duration. Based on the careful investigation of the phases present in the samples by systematic X-ray diffraction (after mechanical alloying and after annealing) and selected microscopy analyses, a reaction mechanism is proposed supported by theoretical calculations at the DFT (Density Functional Theory) level. An energy window to prepare directly NiTiSn has been evidenced. Highly pure NiTiSn has also been obtained by conversion from a multicomponent precursor obtained by low energy mechanical alloying.

Extrinsic doping of the half-Heusler compounds

Controlling the p- and n-type doping is a key tool to improve the power-factor of thermoelectric materials. In the present work we provide a detailed understanding of the defect thermochemistry in half-Heusler compounds. We calculate the formation energies of intrinsic and extrinsic defects in state of the art n-type TiNiSn and p-type TiCoSb thermoelectric materials. It is shown how the incorporation of online repositories can reduce the workload in these calculations. In TiNiSn we find that Ni- and Ti-interstitial defects play a crucial role in the carrier concentration of TiNiSn. Furthermore, we find that extrinsic doping with Sb can substantially enhance the carrier concentration, in agreement with experiment. In case of TiCoSb, we find ScTi, FeCo and SnSb being possible p-type dopants. While experimental work has mainly focussed on Sn-doping of the Sb site, the present result underlines the possibility to p-dope TiCoSb on all lattice sites.

Panoscopically optimized thermoelectric performance of a half-Heusler/full-Heusler based in situ bulk composite Zr(0.7)Hf(0.3)Ni(1+x)Sn: an energy and time efficient way

All scale hierarchical architecturing, matrix/inclusion band alignment and intra-matrix electronic structure engineering, the so called panoscopic approach for thermoelectric materials has been demonstrated to be an effective paradigm for optimizing high ZT. To achieve such hierarchically organized microstructures, composition engineering has been considered to be an efficient strategy. In this work, such a panoscopic concept has been extended to demonstrate for the first time in the case of half-Heusler based thermoelectric materials via a composition engineering route. A series of new off-stoichiometric n-type Zr0.7Hf0.3Ni1+xSn (0 ≤x≤ 0.10) HH compositions have been modified to derive HH(1 -x)/full-Heusler (FH)(x) composite with an all scale hierarchically modified microstructure with FH inclusions within the matrix to study the temperature dependent thermoelectric properties. The structural analysis employing XRD, FE-SEM and HR-TEM of these materials reveal a composite of HH and FH, with hierarchically organized microstructures. In such a submicron/nano-composite, the electronic properties are observed to be well optimized yielding a large power factor; α(2)σ (∼30.7 × 10(-4) W m(-1) K(-2) for Zr0.7Hf0.3Ni1.03Sn) and reduced thermal conductivity (∼2.4 W m(-1) K(-1) for Zr0.7Hf0.3Ni1.03Sn) yielding a high ZT∼ 0.96 at 773 K for composition Zr0.7Hf0.3Ni1.03Sn which is ∼250% larger than the normal HH Zr0.7Hf0.3NiSn (ZT∼ 0.27 at 773 K). The enhancement in ZT of these composites has been discussed in terms of primary electron filtering, electron injection and several phonon scattering mechanisms such as alloy scattering, point defect scattering, and grain boundary scattering. The Bergman and Fel model is used to calculate effective thermoelectric parameters of these composites for comparing the experimental results.

An ab initio study of the thermoelectric enhancement potential in nano-grained TiNiSn

Novel approaches for the development of highly efficient thermoelectric materials capable of a direct conversion of heat into electricity, are being constantly investigated. TiNiSn based half-Heusler alloys exhibit a high thermoelectric potential for practical, renewable power generation applications. The main challenge of further enhancement of the thermoelectric efficiency of these alloys lies in the reduction of the associated high lattice thermal conductivity values without adversely affecting the electronic transport properties. The current manuscript theoretically investigates two possible routes for overcoming this limitation in TiNiSn alloys. On the one hand, the influence of nano-grained structure of TiNiSn on the electronic structure of the material is theoretically demonstrated. On the other hand, the potential for thermal conductivity reduction upon increasing the Ni fraction in the intermetallic TiNiSn compound via the formation of metallic TiNi2Sn nanoparticles is also shown. Using the applied approach, a useful route for optimizing both the electronic and thermal properties of half-Heusler TiNiSn, for practical thermoelectric applications, is demonstrated.

On the constitution and thermodynamic modelling of the system Ti–Ni–Sn

Experimental and calculated phase equilibria for the system Ti–Ni–Sn.