Diptashikha Das, Author at Adamas University

Half Hausler alloys – A Promising Thermoelectric Material for Future Energy Harvesting

Introduction:

Thermoelectricity deals with the conversion of waste heat into electricity by exploiting the inherent coupling between thermal and electrical properties. It becomes attractive predominantly in the recent perspective that unprecedented need for energy clashes with the necessity of reducing carbon emission owing to the environmental issues.

Decreasing of fossil fuel enforces the mankind to think about sustainable supply of energy to the world’s population. Thermoelectric (TE) phenomena, involving the conversion between thermal and electrical energy, and providing a method for heating and cooling materials, are expected to play an important role in meeting the energy challenge of the future. In addition, TE materials have potential application in the field of solid state cooling viz., peltier cooler. Unfortunately, the application of TE devices is very limited due to its low conversion efficiency. As of now TE devices have niche applications for space missions, laboratory equipment and medical applications, where energy availability, reliability, predictability and silent operation of the modules are more important than its cost and energy efficiency.

There exist various thermoelectric materials for different use purpose. Half-Heusler (maximum ZT ~ 1.0 for both n- and p-type at 500–800⁰C) is environmentally friendly, mechanically and thermally robust though cost may be an issue if a lot of Hf is needed. Therefore, they have attracted intensive research interest.

Structure and physical properties of Half-Heusler materials:

Half-Heuslers consists of XYZ as the main chemical composition, where X can be a transition metal, a noble metal, or a rare-earth element, Y is a transition metal or a noble metal, and Z is a

Fig 1: Crystal structure of a half-Heusler. The blue, green and pink dots correspond to X, Y, and Z atoms in half-Heuslers

main group element. As shown in Fig.1, XYZ forms a MgAgAs type of structure (space group F-43m), where X, Y, and Z atoms form three interpenetrating face-centered-cubic sublattices by occupying Wyckoff positions 4b (1/2, 1/2, 1/2), every other of 4c (1/4, 1/4, 1/4), and 4a (0, 0, 0) positions, respectively. The remaining (1/ 4, 1/4, 1/4) positions are empty. With such an atomic configuration, the strong hybridization of d states of the X and Y atoms induces a band gap in half-Heuslers. Based on calculation, the gap values vary from about 0.1 eV to 3.7 eV based on different compositions, the valence electron count (VEC) per unit cell and the average atomic number. There are theoretical calculations which show that half-Heuslers with 18 VEC per unit cell are stable and have a band gap in the range of 0–1.1 eV, which is suitable for moderate temperature thermoelectric applications. However, high thermal conductivity (6.7–20Wm^-1 K^-1 at room temperature) of half-Heusler is a major disadvantage for thermoelectric applications.

Thermoelectric Figure of Merit – Optimization

Being a solid state device with no moving parts; thermoelectric (TE) devices are silent, reliable, durable and scalable. The devices use just two type of legs i.e. n and p-type leg in series. Both the refrigeration and power generation may be accomplished using the same module. It is noteworthy to mention that NASA has used this principal to provide hundred watts of electrical power for deep space probes such as Voyager I and II and the Cassini mission to Saturn. However, commercial application of the device is limited by the efficiency of the device. The performance of a TE material is quantified in figure of merit, ZT=σS²T/κ; the term S²σ is called power factor (PF), where S, σ and κ are thermopower, electrical conductivity and thermal conductivity, respectively. Recent days, the best TE materials available have ZT ≈ 0.9. It is acceptable only for niche application, but economically competitive and commercial viable thermoelectric refrigerator require ZT ≈3. ZT can be enhanced either by maximize the PF through optimal doping and band engineering or reduce the lattice κ by nanostructuring through phonon engineering. Strategies to accomplish this goal have focused on increasing the average energy per carrier through energy filtering, carrier localization in narrow bands in quantum confined structures or the introduction of resonant levels. At the same time, as mentioned above, electrical conductivity, s (=1/r) must be optimized by properly adjusting the concentration of charge carriers (n) and maximizing their mobility. M. S. Dresselhaus et. al. also advocated for low dimensional materials as next generation TE materials, where two ideas are dominant. Firstly, the introduction of nanoscale constituents would introduce quantum confinement effects to enhance the PF. Secondly, the many internal interfaces found in nanostructures would be designed so that k would be reduced more than the s, based on differences in their respective scattering length. Experiments confirmed that the scattering of long and mid wavelength phonons can substantially enhance ZT by employing a high density of nanometer-sized grain boundaries. From this viewpoint, there are two representative methods of controlling and enhancing the performance of a TE material: nanostructuring as an intrinsic method and composite as an extrinsic method.

Why Half Hausler?

Half-Heuslar alloys (HH) with valence electron count 18, are very attractive to exploit as potential mid-temperature TE materials due to their narrow band gap and sharp slope of the density of states near the Fermi level. HH compounds, MNiSn (M=Ti, Zr or Hf) have become important TE material for converting heat into electricity in the temperature range 500⁰C to 800⁰C. Because of their high thermoelectric performance, low toxicity, relatively inexpensive elemental composition, robust thermal and mechanical properties, these materials exhibit semiconducting transport properties although they are ‘intermetallic’ compounds. Hence, it may be considered as rigid band semiconductor within Zintal compounds framework. It is noteworthy to mention that recently ZT ~1.5 has been obtained at 700K for n-type Mg₃(SbBi)₂ Zintal compound. However, the substitution of atoms can effectively decrease the lattice thermal conductivity via point defect phonon scattering and may tune the band structure, thus electrical properties of the material. Further, phonon scattering at the grain boundary due to presence of nano particle decreases the thermal conductivity in HH alloys. The recent approach to enhance the efficiency of the Hausler alloy is to synthesis a mixed Hausler and half-Heusler alloy with coherent phase boundary.

Resurgence of Research on Half Hausler alloys as a Thermoelectric material

Recently half-Heusler alloy are in the centre of the focus for the mid temperature waste heat management. Half-Heusler alloy draws the attention of researcher as potential candidate of TE material in the temperature range 300-700⁰C. The impediment to achieve high ZT in MNiSn ( Zr, Hf, Ti) half-Hausler alloy is high thermal conductivity. Reduction of thermal conductivity is achieved by incorporation of nano-phase in the matrix of MNiSn alloy. Power-factor might be modified by doping at the Sn-site and thermal conductivity by isoelectronic substitution of Zr on the Hf-site of the MNiSn (M=Zr, Hf, Ti). Theoretically it can be shown that mass disorder by alloying helps to reduce the thermal conductivity. It is noteworthy to mention that MCoSb is p-type thermoelectric material and counterpart of MNiSn for device fabrication. MCoSb composite having half-Heusler matrix with different size and CoSb impurity phase enhances ZT. High ZT of MCoSb half-Heusler alloy is also achieved by nano-composite approach using ball milling and hot pressing. Introduction of metallic phase nano-inclusion and full Heusler in the half-Heusler matrix is the new topic and worth further exploration to achieve high ZT via band structure engineering, scattering and energy filtering.

References:

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Thermoelectricity: A new way to turn waste heat into useful energy

Availability of sustainable energy to the world’s population is a vital societal problem of the 21^st century, as fossil fuel supply or any other non-renewable energy sources are decreasing and world energy demand is increasing. Reuse of waste heat might be one of the best solutions.

Thermoelectric (TE) phenomena enable the conversion between thermal and electrical energy and can offer a distinct method for heating and cooling materials. Therefore, this can be a potential solution to meet the ongoing energy challenge. Thus, TE materials can be used to provide efficient harvesting of electricity from the waste heat in case of power generation. Also, devices fabricated of TE materials, the TE devices are known to be a potential candidate to convert heat into electricity. Apart from power generation, one of the main applications of thermoelectric material lies in the field of solid state cooling. TE devices thus provide direct conversion of heat into electricity and vice versa and hold substantial potential for heat management, precise temperature control and energy harvesting from ever-present temperature gradient. TE devices also have many advantages like no moving parts, scalable and reliable. They can also be operated in hostile environment with minimal maintenance. Use of TE material is limited because of their low conversion efficiency.

Now a days, TE devices are used in niche applications for space missions, laboratory equipment and in medical applications, where energy availability, reliability, predictability and silent operation of the modules are more important than its cost and energy efficiency. The efficiency of a TE material is dictated by a dimensionless figure of merit ZT (which is explicitly related with the efficiency) is given by:ZT=(S²/rk)T; where T is the absolute temperature, S is the Seebeck Coefficient or thermoelectric power, r is electrical resistivity and k is the thermal conductivity of the material. Even though in principle there is no limitation to the maximum TE conversion efficiency apart from the Carnot limit, current TE material struggle to display high S, low r and k simultaneously due to the fact that these are interrelated material property. Nevertheless, the basic science involved in achieving good thermoelectric material is very rich and attracting researchers nowadays.

Advancement in Thermoelectric Materials:

A standard way to improve the TE properties is to modify its chemical composition either by synthesizing solid solutions or alloys or, alternatively, by preparing new chemical compounds. Advancement in TE study involves materials design and intricate tuning of structure–property relationships in complex solids. The advanced thermoelectric material must achieve certain minimum value of the related TE parameters to be potential TE material. However, materials with simple electronic band structure; i.e.; general metals are poor TE materials. Hence, early study on TE materials emphasizes in narrow band gap semiconductors.

Bi₂Te₃, Sb₂Te₃ and PbTe based systems are the conventional thermoelectric materials. However, improvement in ZT is negligible, over the last half century. Bi₂Te₃ based material shows ZT~0.6 under normal condition at room temperature. The best commercially available thermoelectric material is (Bi_1-xSb_x)₂(Se_1-yTe_y)₃ having ZT around 1. Highly doped Bi₂Te₃ based material show ZT~1.40. Whereas ZT=1.5 is obtained for PbTe at 773K. However, modification in chemical composition, introduction of nano-structure and quantum confinement can improve the TE properties.

Worldwide resurgence in activity promotes novel thermoelectric material with high ZT. These are basically PGEC (Phonon Glass Electron Crystal). However, effort also employed to enhance the ZT through reducing the dimension and synthesizing micro-nano compound bulk material.

Fig. 1.Phonon glass–electron crystal (PGEC) prototype.

PGEC material:
A theoretically ideal choice for best TE material is PGEC. PGEC possesses “glass-like” thermal conductivity and electrical conductivity similar as metal. An emblematic picture of the material is presented in Fig 1. The cage atoms form a regular periodic crystal lattice along which electrons (or holes) can move fairly freely, ideally approaching the so-called electron crystal. The central rattler atom is commonly bigger, heavier and more loosely bound compared with the cage atoms. These materials are cage like open structure. L is reduced significantly by introducing heavy atoms at the interstitial voids or cage of PGEC. Half-Heusler alloy and β-Zn4Sb3 alloy exhibit PGEC properties. Some typical PGEC materials are Skutterudites (viz. CoSb3) and clathrates (viz. Sr8Ga16Ge30).

i) Skutterdite materials:
PGEC thermoelectric material includes substitutionalskutterdite and field-type skutterdite. Thermal conductivity in substitution type materials are decreased by lattice defect, created due to solid solution. ZT value of Co4-xFexSb12, substitutionalskutteride increases with x and highest value obtained at x=0.65. Further, field types are doped by rare earth material in cubic structure. Phonon scattering by the doped material causes decrease in thermal conductivity. The doping element may vary in wide range. It may be single element, binary element or multi element. The vibration of the doping elements in the lattice cavity causes scattering of phonon with different frequency range.

ii) Clathrates:
In the clathrates material, guest cation vibrates and scattering of phonon causes decrease in thermal conductivity. Further, strong bonding effect increases the electrical conductivity. Hence these materials have high ZT values. A typical clathrates material is YbxBa8-xGa16Ge30 and MNiSn (M=Zr, Hf, Ti) is a half heuslar type clathrates material.

a) Oxides thermoelectric materials:
These materials are important to use at high temperature. High temperature stability, oxidation resistance, safety and long term durability made them attractive as high temperature thermoelectric material. Oxide thermoelectric material includes, cobalt oxides, perovskite compounds, transparent conductive oxides and novel oxides.

b) Low dimensional materials:
Quantum confinement causes due to decrease in the dimension of the material. Low dimensional materials may be classified as quantum well, quantum wire and quantum dot. Decrease in dimension modifies the S, σ and ρ. Further, Quantum confinement efficiently scatters phonon. Which in turn affect the ZT. Nano inclusion in bulk material is also used as better thermoelectric material than bulk one.

c) Phonon-liquid electron-crystal material (PLEC material):
Reducing the κ is one of the major challenges in recent TE research. Recently, it has been proposed that transverse phonon can be suppressed by liquid like diffusion in supersonic conductors. According to PLEC model, liquid do not propagate transverse modes which even leads to lower value of κL than in glass. It was first used to describe in Cu2Se. Cu ion moves freely (liquid like behavior) and scatter phonons strongly. Further, it has the excellent electrical properties which make it a good TE material.

Uses of Thermoelectric Materials:

Thermoelectric materials are used in niche cooling applications, for example to maintain very stable temperatures in lasers and optical detectors, and they are often found in office water coolers. They are also used in space exploration to convert heat from a radioactive material into electricity.

The current focus on energy sustainability and stricter legislation on the emissions of CO2 imposed on automobile manufacturers has sparked a great deal of interest in these fascinating materials.

Only about one third of the fuel energy is converted into mechanical energy in an internal combustion engine with the remainder lost as heat. A thermoelectric generator harvests waste heat from the exhaust gases, which are at a temperature of 300-500ºC, and turns this into electricity. State of the art modules generate about 1 kW, which can be used to power the electrical equipment in the car. This allows for a smaller alternator, which reduces the roll friction, leading to an increase in fuel efficiency and reduced CO2 emissions.

A “Day-Dream”:
All around our world, there are many sources of natural temperature differences, and if that could be tapped into electrical energy… by using thermoelectric phenomena… That may be the temperature difference between an attic and the outside air, or between the dessert sun on a hot day and the temperature 3-6 feet down (may be some kind of geo loops probably pretty easy to dig, in sand!), etc. Let us imagine that, harvesting energy would result in moderating temperature on each side. If that were to occur, it seems that a homeowner might simultaneously expect to get electricity and a cooler attic in the summer out of the deal. Then, there are ocean currents, carrying huge volume of cool or warm water into other ocean areas, and industrial processes that use lakes to cool down their systems could employ cooling/ energy recovery operations, too..