TiNiSn Alloys Research Articles

Optimization of Thermoelectric Properties in TiNiSn Half-Heusler Alloy by Controlling Microwave Sintering Time Using Microwave Synthesis-Cold Pressing-Microwave Sintering Method

Homogeneous pure TiNiSn Half-Heusler (HH) alloys were successfully prepared in a time-efficient manner using microwave synthesis-cold pressing-microwave sintering (MCM) process in this study. The effects of different microwave sintering time on the composition, microstructure and thermoelectric properties of TiNiSn materials were studied. When the time of microwave sintering was 6 min, the TiNiSn sample was almost pure phase except for a small amount of TiNi2Sn phase. More TiNi2Sn impurity can be found in the TiNiSn sample after microwave sintering of 10 min due to the decomposition of TiNiSn resulting from the over-sintering. The thermoelectric properties of TiNiSn samples prepared by MCM process could be effectively improved by adjusting appropriate sintering time. The TiNiSn alloys sintered for 6 min had the zTmax value of 0.15 at 800 K.

Investigation of mechanical, lattice dynamical, electronic and thermoelectric properties of half Heusler chalcogenides: A DFT study

Favorable physical properties of many Half Heusler alloys (HHAs), along with the possibility of tuning their electronic structure by changing the composition, make the HHAs promising candidates for thermoelectric and spintronic applications. In the present work, we explore HHAs based on various chalcogen atoms, with 18 valence electrons (VEC) per unit cell, namely, A(=Sc,Y,La)B(=Co,Rh)C(=S,Se,Te). Systematic density functional theory (DFT) based investigations have been carried out to probe their energetic, mechanical and lattice dynamical properties. Following that, analyses of band structure and Fermi surface reveal high values of valley degeneracy in the states near the Fermi energy in many of these alloys. Therefore, subsequently, thermoelectric properties (TEP) of these above-mentioned alloys have been studied. The dependence of the TEP on the carrier concentration, chemical potential and temperature for the n- and p-type doped systems have been studied. An enhancement of 50%–100% in the values of power factor (S2σ/τ) for YCoC(=S,Se,Te) and LaCoC(=S,Se,Te) alloys for the p-type doping has been observed as compared to TiNiSn. In comparison to the well-known TiNiSn alloy, for most of the probed HHAs, about 10% enhancement has been found in the maximum values of figure of merit calculated using only the electronic contribution ((ZTe)max). Further, it is essential to calculate the figure of merit (ZT) for total thermal conductivity (κ = κe + κl). Hence, to obtain the lattice contribution of the thermal conductivity, we employ an equation which uses the Gru¨neisen parameters. Subsequently, we calculate the ZT values using κ. We find that these values show enhancement when compared with the value obtained for TiNiSn. The favorable electronic structure and propitious values of power factor and figure of merit make this class of systems promising for possible thermoelectric applications.

Processing and Thermoelectric Properties of TiNiSn Materials: A Review

The half-Heusler alloy TiNiSn is a promising material for high-temperature thermoelectric applications due to its good thermal stability and semiconductor-like electrical properties. Numerous synthesis techniques have been utilized to make TiNiSn, but a comparative study on its thermoelectric properties with respect to processing parameters has not been reported. Based on published experimental data, primary melting methods, such as arc melting or induction levitation melting, do not have a noticeable effect on the final thermoelectric properties, although choice of densification technique and annealing parameters correlate with each thermoelectric property. Thermal conductivity (2.47-6.08 W/m K), governed by lattice scattering effects, was maximized with high density from the spark plasma sintering (SPS) technique as well with low inclusions of metallic impurities and interstitial Ni defects from low-temperature, non-densified processes. Electrical resistivity (4.75-30 µΩ m), inversely related to thermal conductivity, is minimized when density is maximized and chemical defects are favorable from using SPS. All reported TiNiSn alloys contain intermetallic impurities, imparting a global trend of lowered electrical resistivity. Seebeck coefficient (− 273.20 to − 50.71 µV/K), proportional to electrical resistivity, is maximized when charge carrier concentration is minimized by eliminating phase impurities and Ni defects. Porosity has been observed to scatter low-energy electrons, which can increase Seebeck coefficient and electrical resistivity, although no statistical significance is found. An ideal TiNiSn alloy should have intermediate values of all interconnected thermoelectric properties to maximize ZT. Future research work should strive to include quantitative phase analysis to better characterize the contributions of Ni defects and impurity phases toward thermoelectric properties.

Simultaneous increase in thermopower and electrical conductivity through Ta-doping and nanostructuring in half-Heusler TiNiSn alloys

A new series of Ta-doped Ti1−xTaxNiSn (x = 0–0.05) were synthesized through mechanical alloying (MA) followed by spark plasma sintering (SPS) for the first time. MA for 5 h gave rise to Ti/Ti+Ta and Ni3Sn4 phases. With increasing milling time and subsequent SPS, half-Heusler phase formed along with Ni3Sn4 and TiC as secondary phases. To see the effect of temperature on the density of TiNiSn pellets, SPS was carried out at 1073, 1173 and 1273 K. Sintering of TiNiSn at 1073 K produced full-Heusler (TiNi2Sn) and Ni3Sn4as secondary phases and the pellets had long continuous pores. In contrast, SPS at 1273 K gave rise to increased volume fraction of TiNi2Sn impurity phase with almost no porosity. Thus, SPS conditions were set with respect to optimized densification (95 %) and volume fraction of secondary phases (19 %) at 1173 K for 5 h MA of TiNiSn. Ta-doping is expected to decrease the thermal conductivity and improve the ZT of the alloys. The Ta-doped TiNiSn samples were subsequently milled for 5 h and SPS were carried out for them at 1173 K. Increase in Thermopower and electrical conductivity were observed with increasing temperature for all the samples. Due to the simultaneous improvement in electrical conductivity and Thermopower, a maximum power factor of 1.12 and 2.57 mWm−1K−2 at 830 K were observed for TiNiSn and Ti0.95Ta0.05NiSn sintered at 1173 K, respectively. Due to reduced lattice thermal conductivity of 1.8 and 3 Wm−1K−1 for TiNiSn and Ti0.95Ta0.05NiSn sintered at 1173 K, respectively, maximum ZT of 0.17 and 0.31 were obtained at 823 K.

Surface Oxidation of TiNiSn (Half-Heusler) Alloy by Oxygen and Water Vapor.

TiNiSn-based half-Heusler semiconducting compounds have the highest potential as n-type thermoelectric materials for the use at elevated temperatures. In order to use these compounds in a thermoelectric module, it is crucial to examine their behaviour at a working temperature (approximately 1000 K) under oxygen and a humid atmosphere. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were utilized to study the surface composition and oxidation of the TiNiSn alloy at elevated temperatures. It was found that during heating in vacuum, Sn segregates to the surface. Exposing the alloy to oxygen at room temperature will cause surface oxidation of Ti to TiO2 and Ti2O3 and some minor oxidation of Sn. Oxidation at 1000 K induces Ti segregation to the surface, creating a titanium oxide layer composed of mainly TiO2 as well as Ti2O3 and TiO. Water vapor was found to be a weaker oxidative gas medium compared to oxygen.

Effect of C and N Addition on Thermoelectric Properties of TiNiSn Half-Heusler Compounds.

We investigated the thermoelectric properties of the ternary half-Heusler compound, TiNiSn, when introducing C and N. The addition of C or N to TiNiSn leads to an enhanced power factor and a decreasing lattice thermal conductivity by point defect phonon scattering. The thermoelectric performances of TiNiSn alloys are significantly improved by adding 1 at. % TiN, TiC, and figure of merit (ZT) values of 0.43 and 0.34, respectively, can be obtained at 723 K. This increase in thermoelectric performance is very helpful in the commercialization of thermoelectric power generation in the mid-temperature range.

Improved Thermoelectric Performance Achieved by Regulating Heterogeneous Phase in Half-Heusler TiNiSn-Based Materials

With excellent high-temperature stability (up to 1000 K) and favorable electrical properties for thermoelectric application, TiNiSn-based half-Heusler (HH) alloys are expected to be promising thermoelectric materials for the recovery of waste heat in the temperature ranging from 700 K to 900 K. However, their thermal conductivity is always relatively high (5–10 W/mK), making it difficult to further enhance their thermoelectric figure-of-merit (ZT). In the past decade, introducing nano-scale secondary phases into the HH alloy matrix has been proven to be feasible for optimizing the thermoelectric performance of TiNiSn. In this study, a series of TiNiSn-based alloys have been successfully synthesized by a simple solid-state reaction. The content and composition of the heterogeneous phase (TiNi2Sn and Sn) is accurately regulated and, as a result, the thermal conductivity successfully reduced from 4.9 W m−1 K−1 to 3.0 Wm−1 K−1 (750 K) due to multi-scale phonon scattering. Consequently, a ZT value of 0.49 is achieved at 750 K in our TiNiSn-based thermoelectric materials. Furthermore, the thermal stability of TiNiSn alloys is enhanced through reducing the Sn substance phase.

Synthesis of nanocrystalline half-Heusler TiNiSn by mechanically activated annealing

Mechanically activated annealing was employed successfully for the synthesis of nanocrystalline half-Heusler (TiNiSn) alloy for the first time. TiNiSn could be synthesized by annealing, after 5h of mechanical alloying of equiatomic elemental blend, at a temperature as low as 600°C in 2h. The nanocrystalline nature and resultant increased surface area coupled with large number of defects induced by mechanical alloying has resulted in a significant reduction in temperature and time of annealing in comparison to earlier reports (800°C–400h). This could lead to energy efficient low temperature synthesis of nanocrystalline half-Heusler alloys for thermoelectric and magnetic applications.

Phase stability and property evolution of biphasic Ti–Ni–Sn alloys for use in thermoelectric applications

Thermoelectric properties and phase evolution have been studied in biphasic Ti–Ni–Sn materials containing full-Heusler TiNi2Sn embedded within half-Heusler thermoelectric TiNiSn. Materials, prepared by levitation induction melting followed by annealing, were of the nominal starting composition of TiNi1+xSn, with x between 0.00 and 0.25. Phases and microstructure were determined using synchrotron X-ray diffraction and optical and electron microscopy. The full-Heusler phase is observed to be semi-coherent with the half-Heusler majority phase. Differential thermal analysis was performed to determine melting temperatures of the end-member compounds. The thermal conductivity is reduced with the introduction of a dispersed, full-Heusler phase within the half-Heusler material. This leads to an increased thermoelectric figure of merit, ZT, from 0.35 for the stoichiometric compound to 0.44 for TiNi1.15Sn. Beyond x = 0.15 ZT decreases due to a rise in thermal conductivity. Density functional theory calculations using hybrid functionals were performed to determine band alignments between the half- and full-Heusler compounds, as well as comparative energies of formation. The hybrid functional band structure of TiNiSn is presented as well.

Phase separation and antisite defects in the thermoelectric TiNiSn half-Heusler alloys

The half-Heusler TiNiSn alloys have recently gained an attention as promising candidates for thermoelectric applications. Improvement of these alloys for such applications can be obtained by both electronic and compositional optimizations. The latter can result in a miscibility gap, allowing a phase separation in the nano-scale and consequently a thermal conductivity reduction. Combination of ab initio calculations and statistical thermodynamics was applied for studying the relative stability of a number of superstructures in TiNiSn based alloys. The quasi-binary phase diagram beyond T=0 K for TiNiSn–TiNi{sub 2}Sn solid solutions was calculated using energy parameters extracted from the total energy calculations for ordered structures in the Ni sublattice. We demonstrated that a decomposition of the off-stoichiometric Ni-rich half-Heusler alloy into the stoichiometric TiNiSn phase and into Ni deficient Heusler TiNi{sub 2}Sn phase occurs at elevated temperatures—an effect which recently had been observed experimentally. Furthermore, favorable energetic conditions for antisite defects formation were deduced, based on calculations of the energy of formation, an effect which was explained as a cooperative process of partial disordering on the Ni sublattice. The influence of these two effects on improvement of the thermoelectric performance of TiNiSn based half Heusler compounds is discussed. - Graphical abstract: Phase separationmore » and antisite defects in the thermoelectric TiNiSn alloy, are covered as methods for nanostructuring and thereby enhancement of the thermoelectric potential. - Highlights: • Ab initio calculations/statistical thermodynamics was applied for studying the TiNiSn system. • The phase diagram for TiNiSn–TiNi{sub 2}Sn solid solutions was calculated. • Decomposition of the Ni-rich HH into TiNiSn and Ni deficient TiNi{sub 2}Sn phases was observed. • Favorable energetic conditions for antisite defects formation were deduced.« less

Crystallization and martensitic transformation behavior of Ti–Ni–Sn alloy ribbons

Crystallization behavior of rapidly solidified Ti–Ni–Sn alloys amorphous ribbons and martensitic transformation behavior after crystallization were examined by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Glass forming ability of Ti–Ni–Sn alloys increased with increasing Sn content and activation energy for crystallization increased from 151.2 ± 7.0 kJ/mol to 165.1 ± 9.0 kJ/mol with increasing Sn content from 5 at% to 7 at%, above which it almost kept constant. Crystallization occurs in the sequence of amorphous → (Ti,Sn)2Ni and B2 → B2 and Ti3Sn when Sn content is ≤5 at%, amorphous → (Ti,Sn)2Ni → B2 → B2 and Ti3Sn when Sn content is in the range of 5 at% and 10 at%, amorphous → (Ti,Sn)2Ni → Ti3Sn → B2 and Ti3Sn when Sn content is ≥10 at%. The crystallized Ti–Ni–Sn alloys showed the B2-R-B19′ martensitic transformation behavior. Rapid solidification was effective to separate the B2-R transformation from the R-B19′ transformation in Ti–Ni–Sn alloys.

Two-way shape memory effect of TiNiSn alloys developed by martensitic deformation

Abstract Two-way shape memory effect of TiNiSn alloys was developed by martensite deformation. With increasing Sn content, two-way shape memory strain increased. A two-way shape memory strain of 0.83% was obtained in the Ti 56 Ni 38 Sn 6 alloy after 7.6% deformation. The dependence of Sn content on two-way shape memory effect was discussed.

Nanosized precipitates in half-Heusler TiNiSn alloy

The microstructure of half-Heusler TiNiSn alloy has been investigated in this study. A high density of coherent nanosized Heusler precipitates was found within the half-Heusler matrix. Formation of these Heusler precipitates occurs by a phase separation process, where a single phase solid solution (half-Heusler-Heusler) decomposes into an equilibrium two-phase mixture of half-Heusler and Heusler regions. These Heusler nanoprecipitates improve the thermoelectric properties of the alloy.

Martensitic transformation behavior of Ti–Ni–Sn alloys

Abstract Microstructural features and martensitic transformation behavior of Ti–Ni–Sn alloys were investigated by means of electron probe microanalysis (EPMA), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). In a Ti–49.5Ni–0.5Sn alloy, Ti 2 Ni phase embedded in Ti–Ni matrix containing 0.7 at%Sn was observed. In a Ti–49.0Ni–1.0Sn alloy, Ti 2 Ni, unknown phase and Ti–Ni matrix containing 1.1 at%Sn were observed, while Ti 2 Ni and two different Ti–Ni matrices with 2.1 at%Sn and without Sn were observed in a Ti–50.0Ni–1.0Sn alloy. In a Ti–48.5Ni–1.5Sn alloy, Ti 2 Ni, Ti 3 Sn and Ti–Ni matrix containing Sn about 1.2 at% were observed. The B2–R–B19′ transformation occurred in Ti–49.5Ni–0.5Sn, Ti–49.0Ni–1.0Sn and Ti–48.5Ni–1.5Sn alloys, while the B2–B19′ transformation occurred in a Ti–50.0Ni–1.0Sn alloy. Substitution of Sn for Ni of an equiatomic TiNi alloy was effective to induce the R phase transformation.

Microstructures and martensitic transformation behavior of Ti–Ni–Sn alloys

The Ti–Ni–Sn phase in Ti–(50-2 X )Ni– X Sn ( X = 5, 8, 10, 12.5) alloys showed the B2–R–B19′ martensitic transformation on cooling and the B19′–B2 transformation on heating. The B2–R transformation start temperature ( T R ) decreased from 314 to 303 K with increasing Sn content from 5 to 12.5 at.%. Ti–(50-2 X )Ni– X Sn alloys with X less than 12.5 at.% were deformed in a plastic manner up to more than 6%. The shape recovery ratio was about 95% after training, although it was less than 81% before training.

Thermoelectric Performance of Half-Heusler TiNiSn Alloys Fabricated by Solid-Liquid Reaction Sintering

A new fabrication process was proposed for half-Heusler type TiNiSn thermoelectric alloys. Based on the result that the TiNiSn phase can be formed easily at the Sn(Liquid)/TiNi(Solid) interface, the liquid-solid reaction-sintering process was developed using TiNi and Sn powders. The TiNi compound powders were prepared by the atomization method using argon gas atmosphere. We have fabrictaed nearly single-phase TiNiSn alloys and evaluated their thermoelectrical properties; the presnt TiNiSn alloys have large electrical power factor of about 3.5 mWm-1K-2, and the maximum value of dimensionless figure of merit, ZT = 0.67, can be achieved at around 700 K even without tuning of the carrier concentration through alloying elements.

Design of titanium alloy for efficient sintering to low porosity

Few Ti alloys have been designed for ease of sintering. This paper considers the design of alloys for processing using the mixed elemental technique, in which powders are mixed, cold pressed in a die to near net shape and sintered under vacuum at high temperature. The authors describe steps in the process of developing a Ti–Ni–Sn alloy, able to be sintered to near full density at a sintering temperature as low as 1100°C without requiring unusually fine powder or high compaction pressure. Higher sintering temperature allows the Ni content of the alloy to be reduced, but swelling of the alloy probably imposes an upper limit on practical sintering temperature. The increase in green density, and hence sintered density, conferred by Sn in Ti–Sn alloys, and the increase in sintered density due to the high diffusivity of Ni in Ti–Ni alloys, are combined in Ti–Ni–Sn alloys.

Diffusion Paths for the Formation of Half-Heusler Type Thermoelectric Compound TiNiSn

AbstractHalf-Heusler compound TiNiSn is one of the most promising candidates of thermoelectric materials which can be used to directly convert the waste heat to clean electric energy at high temperatures (around 1000 K). Thermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. Half-Heusler compounds have the cubic C1b type ordered structure and show semiconducting behavior when their valence electron count (VEC) is around 18. TiNiSn is the most attractive one not only because it has excellent thermoelectric properties but also it consists of eco-friendly elements which are neither toxic nor costly. However, TiNiSn has a bothersome problem that fabrication of single phase TiNiSn alloy is quite difficult. We have found that TiNiSn phase forms by the ternary peritectic reaction. Thereby, inevitable non-equilibrium solidification results in the formation of impurity coexisting phases which tend to decrease thermoelectric properties. In the present work, to establish the basis of new fabrication processes for TiNiSn alloys, we have started from the investigation on the diffusion paths which are closely related to the formation of TiNiSn phase. The diffusion behavior was evaluated using solid/liquid diffusion couples composed of the binary Ti-Ni intermetallic compounds and Sn liquid phase, where we have selected TiNi, TiNi3 and Ti2Ni as solid phases for instance. The most interesting result is that the single-phase TiNiSn phase layer forms at the TiNi/Sn(L) interface during annealing at 1073 K for only 1 h. Moreover, faceted grains of TiNiSn single-crystal grow at the interface toward the liquid Sn phase. We have confirmed two interesting microstructural features using EBSD analyses. One is that most of these TiNiSn single-crystals have the same crystallographic orientation, and the other is that TiNiSn phase layer formed on the TiNi side of the interface consists of very fine sub-microns grains. While TiNiSn solely forms at the TiNi interface, Heusler TiNi2Sn also forms with TiNiSn at the TiNi3 interface and Ti6Sn5 tends to coexist at the Ti2Ni interface.

The effects of quaternary additions on thermoelectric properties of TiNiSn-based half-heusler alloys

Half-Heusler-type compounds have gained increasing attention as promising thermoelectric materials. In the present work, a focus is placed on TiNiSn with additions of Hf, Zr, Si, or Pt. Nominally stoichiometric TiNiSn alloys were prepared using arc melting and subsequent annealing at 1,073 K for 2 weeks. The thermoelectric properties, such as thermoelectric power, electrical resistivity, and thermal conductivity, were measured in a temperature range from 300 K to 1,000 K. As-cast materials show metallic transport properties, while annealed ones exhibit semiconductor behavior. Microstructures of TiNiSn alloys basically consist of nonequilibrium four-phase; half-Heusler TiNiSn, Heusler TiNi2Sn, metallic Ti6Sn5, and Sn solid solution. The volume fraction of the half-Heusler TiNiSn phase significantly increases by annealing. It is revealed that coexisting metallic phases degrade the thermoelectric properties of half-Heusler TiNiSn. Alloy additions strongly affect not only thermoelectric properties but also phase stability. The thermal conductivity of TiNiSn alloys with alloy additions decreases because of the point-defect phonon scattering.