Defect chemistry and thermoelectric behavior of n-type SrxBa1-xNb2O6

Abstract

Complex oxides with an intricate atomic structure and tunable stoichiometry are promising materials for use in high-temperature thermoelectric (TE) generators. Among this class of ceramics, highly disordered relaxor ferroelectrics are particularly interesting due to their very low thermal conductivity (κ), which serves to increase the dimensionless figure of merit (ZT). In this work, the defect chemistry and TE behavior of the n-type relaxor SrxBa1-xNb2O6 (SBN100x) were investigated. A solution combustion synthesis (SCS) route was first devised to fabricate SBN nanoparticles with excellent phase purity. X-ray photoelectron spectroscopy (XPS) data obtained for various SBN compositions confirmed that Sr atoms occupy two positions in the tetragonal tungsten bronze SBN lattice; a higher binding energy state was associated with Sr ions at pentagonal (A2) sites, presumably due to their increased coordination over Sr at tetragonal (A1) positions. Reduction heat treatments were carried out to raise the electrical conductivity (σ) of sintered SBN50 specimens, and a decrease in the average Nb valence was observed through the growth of lower binding energy Nb 3d XPS peaks. Both the Ba 3d photoemission and the A2 component of the Sr 3d spectra show shifting to lower binding energy as the reduction time is increased, indicating possible preferential oxygen vacancy formation adjacent to A2 sites. Electron transport by variable range hopping was identified at low temperatures (105 - 150 K), followed by a transition to small polaron hopping from 230 to ~550 K. Above ~550 K, the temperature dependence of σ becomes metallic-like (i.e., dσ/dT < 0), consistent with an Anderson transition; such a scenario is supported by high-temperature Seebeck coefficient (S) measurements. Based on the identified behavior, polymethylmethacrylate (PMMA) pore formers were utilized with the goal of decreasing κ through enhanced phonon scattering while preserving the electron transport characteristics. With the introduction of PMMA into SBN50 prior to sintering, κ was reduced by an average of 44%, and a slight increase in the power factor (S2σ) was observed. Ultimately, a substantial increase in ZT by ~128% on average (peak value of 0.14 at 873 K) was achieved for SBN50 processed with 5 wt% PMMA.