Grain Boundary Engineering Nanostructured SrTiO₃ for Thermoelectric Applications

Abstract

Nanostructuring to reduce thermal conductivity is among the most promising strategies for designing next-generation, high-performance thermoelectric materials. In practice, electrical grain boundary resistance can overwhelm the thermal conductivity reduction induced by nanostructuring, which results in worse overall performance. Since a large body of work has characterized the transport of both polycrystalline ceramics and single crystals of SrTiO 3 , it is an ideal material system for conducting a case study of electrical grain boundary resistance. An e ective mass model is used to characterize the transport signatures of electrical grain boundary resistance and evaluate thermodynamic design principles for controlling that resistance. Treating the grain boundary as a secondary phase to the bulk crystallites explains the transport phenomena. Considering that the interface can be engineered by controlling oxygen partial pressure, temperature, and the addition of extrinsic elements into the grain boundary phase, the outlook for SrTiO 3 as a nanostructured thermoelectric is promising, and the zT could be greater than 0.5 at room temperature.

Graphene/Strontium Titanate: Approaching Single Crystal–Like Charge Transport in Polycrystalline Oxide Perovskite Nanocomposites through Grain Boundary Engineering

“…The signature of the grain boundary effect in these materials is electrical conductivity that is activated with temperature (Figure 3a), and a Seebeck coefficient (the absolute value) that increases gradually with temperature (Figure 3b). Essentially, the conductivity is affected by grain boundaries, while the Seebeck coefficient is not. The relative weighted mobility between polycrystalline samples and single crystal samples indicates the strength of the grain boundary effect.…”

“…The depletion of positively charged oxygen vacancies induces a negative potential that perturbs the electronic states in the conduction band. As a result, the grain boundary phase is depleted of free carriers and becomes more resistive ( Figure ) . With the addition of graphene, the formation of oxygen vacancies is promoted in the grain boundary regions adjacent to the interfacial graphene, resulting in a localized rise in carrier concentration .…”

“…As a result, the grain boundary phase is depleted of free carriers and becomes more resistive (Figure 5). [13] With the addition of graphene, the formation of oxygen vacancies is promoted in the grain boundary regions adjacent to the interfacial graphene, resulting in a localized rise in carrier concentration. [29] In this context, graphene may be acting as a chemical reducing agent for the STO matrix.…”

“…Despite extensive efforts, the carrier mobilities of polycrystalline oxide perovskites are usually orders of magnitude less than the carrier mobilities in single crystals . The existence of grain boundaries plays a major role in this mobility reduction ( Figure ) due to grain boundary effects that also exist in other perovskite materials, as well as magnesium antimonide …”

“…[11] Despite extensive efforts, the carrier mobilities of polycrystalline oxide perovskites are usually orders of magnitude less than the carrier mobilities in single crystals. [11] The existence of grain boundaries plays a major role in this mobility reduction [12,13] (Figure 1) due to grain boundary effects that also exist in other perovskite materials, [14] as well as magnesium antimonide. [15] Here, we demonstrate a solution to eliminate the grain boundary effect and improve charge transport in a polycrystalline oxide perovskite by adding graphene.…”