Controlling the Thermoelectric Behavior of La-Doped SrTiO3 through Processing and Addition of Graphene Oxide

Abstract

The addition of graphene has been reported as a potential route to enhance the thermoelectric performance of SrTiO₃. However, the interplay between processing parameters and graphene addition complicates understanding this enhancement. Herein, we examine the effects of processing parameters and graphene addition on the thermoelectric performance of La-doped SrTiO₃ (LSTO). Briefly, two types of graphene oxide (GO) at different oxidation degrees were used, while the LSTO pellets were densified under two conditions with different reducing strengths (with/without using oxygen-scavenging carbon powder bed muffling). Raman imaging of the LSTO green body and sintered pellets suggests that the added GO sacrificially reacts with the lattice oxygen, which creates more oxygen vacancies and improves electrical conductivity regardless of the processing conditions. The addition of mildly oxidized electrochemical GO (EGO) yields better performance than the conventional heavily oxidized chemical GO (CGO). Moreover, we found that muffling the green body with an oxygen-scavenging carbon powder bed during sintering is vital to achieving a single-crystal-like temperature dependence of electrical conductivity, implying that a highly reducing environment is critical for eliminating the grain boundary barriers. Combining 1.0 wt % EGO addition with a highly reducing environment leads to the highest electrical conductivity of 2395 S cm^-1 and power factor of 2525μW m^-1 K^-2 at 300 K, with an improved average zT value across the operating temperature range of 300-867 K. STEM-EELS maps of the optimized sample show a pronounced depletion of Sr and evident deficiency of O and La at the grain boundary region. Theoretical modeling using a two-phase model implies that the addition of GO can effectively improve carrier mobility in the grain boundary phase. This work provides guidance for the development of high-performance thermoelectric ceramic oxides.

Keywords: SrTiO3; composite; grain boundary; graphene oxide; thermoelectric.

Figure 1

Raman spectroscopy characterization of REGO in LSTO matrix before and after sintering: (a) typical Raman spectrum and (b) G band intensity mapping of the hybrid powder of 1 wt % REGO and LSTO before sintering; (c) a typical Raman spectrum from where carbon was present and (d) G band intensity mapping of the sintered sample prepared from this hybrid powder.

Figure 2

Temperature dependency of the thermoelectric properties of LSTO-based composites with resistive grain boundaries: (a) electrical conductivity, (b) Seebeck coefficient, (c) power factor, and (d) thermal conductivity. The samples were sintered under H₂–Ar flow in the absence of sacrificial powder.

Figure 3

Thermoelectric properties of LSTO samples with conductive grain boundaries; (a) electrical conductivity, (b) Seebeck coefficient, (c) power factor, and (d) thermal conductivity. The samples were sintered under H₂–Ar flow and in the presence of sacrificial powder.

Figure 4

Weighted mobility as a function of temperature for (a) samples with RGBs and (b) samples with CGBs. (c) m = μ_w/κ_lattice ratio for both sets of samples with RGBS and CGBs.

Figure 5

Dimensionless figure of merit for LSTO-based composites: Samples with (a) RGBs and (b) CGBs.

Figure 6

(a) TEM image of high power factor 1.0EGO-M-HA sample showing grain boundaries. (b) HRTEM image for a grain boundary presented in (a). (c, d) Related SAED and FFT data from the upper grain areas in (b) and (e) FFT data from the grain boundary in (b). (f) HAADF and (g) BF images of the grain boundary.

Figure 7

(a) HAADF image of the grain boundary. EELS data were collected from the area denoted by the green rectangle. (b) Ti L_3,2 and O K spectra obtained from the grain (integration over the rectangular red box) and grain boundary (blue box) in the smaller HAADF image. The spectra are background-subtracted but otherwise not normalized or scaled to allow for a direct comparison. Raw data are shown as a scatter plot, with overlaid solid lines displaying smoothed spectra (Savitzky–Golay, 20 data points) as a guide to the eye. (c) HAADF signal during data acquisition and EELS maps for Sr, Ti, O, and La from the (green box) region shown in (a).

Figure 8

Experimental electrical conductivity data (solid symbols) and results from two-phase model fitting (solid lines) for (a) LSTO-REGO composite samples prepared without a carbon sacrificial bed and (b) LSTO-EGO samples prepared with a sacrificial carbon bed. The fitting parameters employed are presented in Table S1 (supporting material).