Tetragonal tungsten bronzes Nb_8−xW_9+xO_47−δ: optimization strategies and transport properties of a new n-type thermoelectric oxide

Abstract

Engineering of nanoscaled structures may help controlling the electrical and thermal transport in solids, in particular for thermoelectric applications that require the combination of low thermal conductivity and low electrical resistivity. The tetragonal tungsten bronzes Nb 8Àx W 9+x O 47 (TTB) allow a continuous variation of the charge carrier concentration while fulfilling at the same time the concept of a ''phonon-glass electron-crystal'' through a layered nanostructure defined by intrinsic crystallographic shear planes.The thermoelectric properties of the tetragonal tungsten bronzes Nb 8Àx W 9+x O 47Àd (0 o x o 2) were studied in the temperature range from 373 to 973 K. Structural defects and the thermal stability under various oxygen partial pressure pO 2 were investigated by means of thermogravimetry, HR-TEM, and XRD. Nb 8 W 9 O 47Àd was found stable at 973 K and a pO 2 of E10 À15 atm. The oxygen nonstoichiometry d can reach up to 0.3, depending on the applied atmosphere. By increasing the substitution level x, the electrical resistivity q and the Seebeck coefficient S decreased. For x = 2, q reached 20 mX cm at 973 K, combined with a Seebeck coefficient of approximately À120 lV K À1 . The thermal conductivity was low for all samples, ranging from 1.6 to 2.0 W K À1 m À1 , attributed to the complex crystal structure. The best thermoelectric figure of merit zT of the investigated samples was 0.043, obtained for x = 2 at 973 K, but it is expected to increase significantly upon a further increase of x. The control of the oxygen non-stoichiometry d opens a second independent optimization strategy for tetragonal tungsten bronzes. † Electronic supplementary information (ESI) available: Scanning electron microscopy images of the as-synthesized powder and the consolidated pellet (Fig. S1). A zoomed-in image of a consolidated pellet of the TTB Nb 8 W 9 O 47 (Fig. S2), relative weight of Nb 8 W 9 O 47 as a function of time under different oxygen partial pressures (Fig. S3). See Thermoelectric materials have the potential to reduce energy currently lost in the form of waste heat. However, to make widespread implementation possible, improvements to both the efficiency and toxicity of thermoelectric materials are necessary. Oxides are promising materials for thermoelectric applications because they are composed of environmentally benign elements and chemically stable at high temperature. However, most metal oxides are inferior to thermoelectric alloys because they display low carrier mobilities. Even more importantly, large lattice energies and the small atomic mass of oxygen lead to a high velocity of the elastic waves and therefore to a high lattice thermal conductivity. These inherent characteristics of metal oxides seem to preclude their use as thermoelectrics. Here we show that tetragonal tungsten bronzes Nb 8Àx W 9+x O 47 (TTB) conceptually allow a continuous variation of the charge carrier concentration while fulfilling at the same time the concept of a ''phonon-glas electron-crystal'' through a layered nanostruc...

The Effects of Low Oxygen Activity Conditions on the Phase Equilibria and Cation Occupancy of Strontium Barium Niobate

“…The NbO 2 would therefore only form once the solubility limit for excess Nb in SBN is reached. The defect chemistry associated with the Nb excess is uncertain, but possibilities include Nb sitting on the A‐site, similar to materials such as Nb 16 W 18 O 94 or possibly NbO‐layer intergrowths as are found in multiple reduced niobates . In the absence of any Nb excess in solution, the reaction for NbO 2 formation from an increasing A‐site occupancy would be:…”

Thermoelectrics: From history, a window to the future

“…[558] These systems are structurally related to the Magnéli phases, and they are characterized by the possibility of changing the cation composition, and hence the electron density, without altering the crystal structure. In recent years promising results could be obtained for both the SBN and the fully substituted Nb 8-x W 9+x O 47 , with a maximum zT above 0.2 at 1200 K. [559,560]…”

A chemists view: Metal oxides with adaptive structures for thermoelectric applications

“…The change of the charge carrier concentration can be accomplished by substitution: the number of available electrons (TTBs are also n‐type materials) increases with increasing degree of substitution. Due to the large unit cell and to the presence of both, ordered CS planes and disordered grains of different size, the thermal conductivity of the TTBs is relatively low The intrinsically low thermal conductivity of the TTBs in combination with the adaptability makes them interesting as n‐type thermoelectrics. The control of the oxygen content and the formation of oxygen vacancies – similarly to the mixed valent Magnéli phases – might open an additional path to tune the electronic properties of the TTBs.…”

“…The TTB‐type compounds Nb 8 − x W 9 + x O 47 − δ were studied to assess the thermoelectric properties of transition metal oxides with complex structures , Nb 8 − x W 9 + x O 47 − δ is stable up to 1323 K. The structure is complex and allows electronic tuning by cation substitution x and oxygen deficiency δ . To this end, a series of compounds with low substitution degrees x = 0, 0.075, 0.1, 1, and 2 were investigated.…”