Stability and Thermoelectric Properties of Zn4Sb3 with TiO2 Nanoparticle Inclusions.

ß-Zn4Sb3 is a cheap nontoxic high-performance thermoelectric material, which unfortunately suffers from stability issues because of zinc migration in thermal and electrical gradients. Here, the thermoelectric properties and thermal stability of ß-Zn4Sb3 mixed with varying sizes and weight percentages of TiO2 nanoparticles are investigated. Furthermore, the stability of pressed ß-Zn4Sb3-TiO2 nanocomposite pellets is investigated by measuring high-energy synchrotron powder X-ray diffraction (PXRD) data during operating conditions using the Aarhus thermoelectric operando setup (ATOS). Through these studies, it is determined that TiO2 nanoparticle addition in pressed pellets of ß-Zn4Sb3 does not prevent Zn migration, and even though effects are seen in the thermal conductivity and electrical resistivity, the overall zT remains unchanged regardless of TiO2 nanoinclusions. For the present samples, the Seebeck coefficients are unaffected by the addition of nanoparticles, and thus, there is no observed energy-filtering effect. The operando PXRD data reveal that the TiO2 nanoinclusions lower the degradation rate by up to 75%, but all samples eventually decompose. This is corroborated by long-term stability tests performed using a thermal gradient. In conclusion, TiO2 nanoinclusions do not degrade the excellent thermoelectric properties of ß-Zn4Sb3, but the stabilizing effect is not sufficient for establishing long-term operating stability.

Tuneable local order in thermoelectric crystals.

Although crystalline solids are characterized by their periodic structures, some are only periodic on average and deviate on a local scale. Such disordered crystals with distinct local structures have unique properties arising from both collective and localized behaviour. Different local orderings can exist with identical average structures, making their differences hidden to Bragg diffraction methods. Using high-quality single-crystal X-ray diffuse scattering the local order in thermoelectric half-Heusler Nb1-x CoSb is investigated, for which different local orderings are observed. It is shown that the vacancy distribution follows a vacancy repulsion model and the crystal composition is found always to be close to x = 1/6 irrespective of nominal sample composition. However, the specific synthesis method controls the local order and thereby the thermoelectric properties thus providing a new frontier for tuning material properties.

Is RuAs2 a candidate for high temperature thermoelectric applications?

The mineral inspired material RuAs2 shows promise as a thermoelectric material with its high stability and attractive band structure. In order to validate these expectations phase-pure polycrystalline ruthenium arsenide was synthesized and densified using Spark Plasma Sintering. RuAs2 is an n-type semiconductor with an indirect band gap 0.69 eV as estimated from temperature dependent resistivity data, while the band gap calculated with DFT is 0.64 eV. The thermal conductivity and electrical resistivity are both high with room temperature values of 16 W m-1 K-1 and 170 mΩ cm respectively, leading to modest thermoelectric properties for the intrinsic system. Band structure calculations suggest that chemical modification should preferably be done at the As site to improve the intrinsic properties. Synchrotron powder X-ray diffraction and Rietveld structural refinements show RuAs2 to be a stable line phase up to 1000 K in both in air and in vacuum, and both as a powder and as a dense pellet. No indication of preferential orientation or material gradients are observed.

Designing high-performance layered thermoelectric materials through orbital engineering.

Thermoelectric technology, which possesses potential application in recycling industrial waste heat as energy, calls for novel high-performance materials. The systematic exploration of novel thermoelectric materials with excellent electronic transport properties is severely hindered by limited insight into the underlying bonding orbitals of atomic structures. Here we propose a simple yet successful strategy to discover and design high-performance layered thermoelectric materials through minimizing the crystal field splitting energy of orbitals to realize high orbital degeneracy. The approach naturally leads to design maps for optimizing the thermoelectric power factor through forming solid solutions and biaxial strain. Using this approach, we predict a series of potential thermoelectric candidates from layered CaAl2Si2-type Zintl compounds. Several of them contain nontoxic, low-cost and earth-abundant elements. Moreover, the approach can be extended to several other non-cubic materials, thereby substantially accelerating the screening and design of new thermoelectric materials.