Precursor-Led Grain Boundary Engineering for Superior Thermoelectric Performance in Niobium Strontium Titanate

Abstract

We present a novel method to significantly enhance the thermoelectric performance of ceramics in the model system SrTi_0.85Nb_0.15O₃ through the use of the precursor ammonium tetrathiomolybdate (0.5-2% w/w additions). After sintering the precursor-infused green body at 1700 K for 24 h in 5% H₂/Ar, single-crystal-like electron transport behavior developed with electrical conductivity reaching ∼3000 S/cm at ∼300 K, almost a magnitude higher than that in the control sample. During processing, the precursor transformed into MoS₂, then into MoO_x, and finally into Mo particles. This limited grain growth promoted secondary phase generation but importantly helped to reduce the grain boundary barriers. Samples prepared with additions of the precursor exhibited vastly increased electrical conductivity, without significant impact on Seebeck coefficients giving rise to high power factor values of 1760 μW/mK² at ∼300 K and a maximum thermoelectric figure-of-merit zT of 0.24 at 823 K. This processing strategy provides a simple method to achieve high charge mobility in polycrystalline titanate and related materials and with the potential to create "phonon-glass-electron-crystal" oxide thermoelectric materials.

Keywords: MoS2; ammonium tetrathiomolybdate; grain boundary resistance; high charge mobility; strontium titanate; thermoelectric.

Figure 1

(a) XRD patterns at 2θ = 20°–100° for SrTi_0.85Nb_0.15O₃-based samples (0, 0.5, 1, and 2M) after sintering. The blue circle shows the primary peak for metallic tungsten, and the red circles show the primary peaks for tungsten and molybdenum. (b) XRD patterns at 2θ = 55°–60° showing the peak positions for 0M, 0.5M, 1M, and 2M.

Figure 2

(a–d) Backscattered electron (BSE) images of four sintered samples, (a) 0M, (b) 0.5M, (c) 1M, and (d) 2M. (e-h) EDS maps for Sr, Ti, and Nb in the 1M sample: (e) BSE image of 1M, (f) Sr mapping, (g) Ti mapping, and (h) Nb mapping. The red arrows identify Ti-enriched SPs.

Figure 3

High-resolution XPS spectra (counts per seconds, CPS, vs binding energy) for 0M and 2M samples: Ti 2p collected from (a) 0M and (b) 2M samples; Nb 3d collected from (c) 0M and (d) 2M.

Figure 4

Charge transport properties of the 0M, 0.5M, 1M, and 2M samples. (a) Seebeck coefficients. (b) Electrical conductivity (the gray dotted line data are for single-crystal SrTiO₃). (c) Power factor values (note that the dashed line data are for a SrTiO₃ composition (N15) containing 0.6 wt % rGO, from Okhay et al.).

Figure 5

STEM high-angle annular dark field (HAADF) images of GB regions in (a) 0M sample and (b) 1M sample and the corresponding Ti EDS maps for (c) 0M sample and (d) 1M sample.

Figure 6

AFM topography and corresponding Kelvin probe (KPFM) scans and the line profiles for the 0M sample (a–c) and the 2M sample (d–f).

Figure 7

Schematic diagrams showing (a) process for forming MoS₂ and its conversion to Mo at the GBs during sintering and (b) principle of lowering the GB barrier to achieve high and single-crystal-like electrical conductivity in going from 0M to 0.5M/1M/2M samples.

Figure 8

(a) Thermal conductivity and (b) lattice and electronic thermal conductivity for the four samples and the N15–0.6rGO sample from Okhay et al.

Figure 9

(a) zT values for the 0, 0.5, 1, and 2M samples as a function of temperature, compared with data for the N15–0.6rGO sample (dashed line, from Okhay et al.). (b) Comparison of power factor values for different Nb-doped strontium titanate materials^,,,,− and their dependence on the zT value. Note that the data points located at the far right side of the figure (gray part) exhibit the highest power factors when comparing samples having the same zT values.