Concurrent La and A-Site Vacancy Doping Modulates the Thermoelectric Response of SrTiO3: Experimental and Computational Evidence

Laser-induced transverse voltage effect inc-axis inclined LaxSr1-xTiO3thin films prepared by MOCVD

Light and thermal detectors based on the laser-induced transverse voltage (LITV) effect have garnered significant interest for their rapid and broad spectral response. In this study, we prepared the La-doped SrTiO3(STO) epitaxial thin films on the 12° inclined single crystal LaAlO3(LAO) (100) substrates using our home-designed metal-organic chemical vapor deposition system. Under the illumination of a 248 nm laser, the LITV signals of LaxSr1-xTiO3films were observed and showed dependence on the La doping level, which can be explained by the changes in the light absorption coefficient, thermal conductivity, and optical penetration depth. The optimized LITV signal was observed with a peak voltage of 23.25 V and a decay time of 106 ns under the laser power density of 1.0 mJ mm-2. The high peak voltage and fast response time of LaxSr1-xTiO3show great potential in the field of light and thermal detection.

Recent Status and Developments of Vacancies Modulation in the ABO3 Perovskites for Catalytic Applications

Perovskite oxides (ABO₃ ) have attracted comprehensive interest for wide range of functional applications (especially for chemical catalysis) due to their high design flexibility, controllable vacancies sites creation, abundant chemical properties, and stable crystal structure. Herein, the previous research and potential development of ABO₃ through adjusting the vacancy at different sites (A-site, B-site, and O-site) to enhance catalytic performance are systematically analyzed and generalized. Briefly, the ABO₃ with different vacancies sites prepared by multifarious direct and indirect methods, accompanied with the improved physical-chemical properties, endow them with distinct and intensified development of catalysis application. In addition, the impressive optimization proved by the vacancies sites adjustment over the ABO₃ is studied to continuously facilitate the advance in some common catalysis reactions, further expanding to other optimized functional applications. At last, the constructive suggestions for fine regulation and analysis of vacancies sites over ABO₃ are also put forward.

Unraveling the Impact of Graphene Addition to Thermoelectric SrTiO3 and La-Doped SrTiO3 Materials: A Density Functional Theory Study

We present a detailed theoretical investigation of the interaction of graphene with the SrO-terminated (001) surface of pristine and La-doped SrTiO₃. The adsorption of graphene is thermodynamically favorable with interfacial adsorption energies of -0.08 and -0.32 J/m² to pristine SrTiO₃ and La-doped SrTiO₃ surfaces, respectively. We find that graphene introduces C 2p states at the Fermi level, rendering the composite semimetallic, and thus the electrical properties are predicted to be highly sensitive to the amount and quality of the graphene. An investigation of the lattice dynamics predicts that graphene adsorption may lead to a 60-90% reduction in the thermal conductivity due to a reduction in the phonon group velocities, accounting for the reduced thermal conductivity of the composite materials observed experimentally. This effect is enhanced by La doping. We also find evidence that both La dopant ions and adsorbed graphene introduce low-frequency modes that may scatter heat-carrying acoustic phonons, and that, if present, these effects likely arise from stronger phonon-phonon interactions.

Thickness-Independent Vibrational Thermal Conductance across Confined Solid-Solution Thin Films

We experimentally show that the thermal conductance across confined solid-solution crystalline thin films between parent materials does not necessarily lead to an increase in thermal resistances across the thin-film geometries with increasing film thicknesses, which is counterintuitive to the notion that adding a material serves to increase the total thermal resistance. Confined thin epitaxial Ca_0.5Sr_0.5TiO₃ solid-solution films with systematically varying thicknesses in between two parent perovskite materials of calcium titanate and (001)-oriented strontium titanate are grown, and thermoreflectance techniques are used to accurately measure the thermal boundary conductance across the confined solid-solution films, showing that the thermal resistance does not substantially increase with the addition of solid-solution films with increasing thicknesses from ∼1 to ∼10 nm. Contrary to the macroscopic understanding of thermal transport where adding more material along the heat propagation direction leads to larger thermal resistances, our results potentially offer experimental support to the computationally predicted concept of vibrational matching across interfaces. This concept is based on the fact that a better match in the available heat-carrying vibrations due to an interfacial layer can lead to lower thermal boundary resistances, thus leading to an enhancement in thermal boundary conductance across interfaces driven by the addition of a thin "vibrational bridge" layer between two solids.

Simultaneously Breaking the Double Schottky Barrier and Phonon Transport in SrTiO3-Based Thermoelectric Ceramics via Two-Step Reduction

The low powder factor (PF) of polycrystalline oxide perovskites induced by the resistive grain boundaries or known as double Schottky barrier (DSB) greatly restricts their thermoelectric performance in application. Here, a general protocol including (i) powder and (ii) bulk reduction in H₂/Ar forming gas is demonstrated to break the DSB in La and Nb codoped SrTiO₃. While the powder reduction guarantees a high carrier concentration by fully stimulating the donor doping effect, the bulk reduction effectively lowers the DSB by influencing the point defects at grain boundaries, which is proved by the combination of cathode luminescence spectra and energy-dispersive X-ray spectroscopy in transmission electron microscopy. The Hall mobility can approach 10 cm² V^-1 s^-1 after two-step reduction, which is similar to the level of single crystals. However, the Seebeck coefficient is not compromised, giving rise to high PF values up to 1.70 mW m^-1 K^-1 under proper reduction strength. Meanwhile, the reduction process also promotes mild precipitation of Nb nanoparticles, thus effectively lowering the lattice thermal conductivity by scattering phonons. As a result, a remarkable figure of merit reaching 0.4 at 700 K is obtained, which validates the two-step reduction as a reliable strategy toward "electron crystal-phonon glass" behavior in SrTiO₃-based perovskites.

Advanced Thermoelectric Design: From Materials and Structures to Devices

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Highly dense Sr0.95Sm0.0125Dy0.0125□0.025Ti0.90Nb0.10O3±δ/ZrO2 composite preparation directly through spark plasma sintering and its thermoelectric properties

The Sr_0.95Sm_0.0125Dy_0.0125□_0.025Ti_0.90Nb_0.10O_3±δ/ZrO₂ composite was directly prepared through spark plasma sintering. This approach limited the grain growth and facilitated the achievement of a narrow grain size distribution due to fast sintering and ZrO₂ effects. Thermal conductivity declined to 1.68 W m^-1 K^-1, which is the lowest among the reported values for micro-polycrystalline SrTiO₃-based structures.

Self-Nanostructuring in SrTiO3: A Novel Strategy for Enhancement of Thermoelectric Response in Oxides

Nanostructuring is recognized as an efficient route for enhancing thermoelectric response. Here, we report a new synthesis strategy for nanostructuring oxide ceramics and demonstrate its effectiveness on an important n-type thermoelectric SrTiO₃. Ceramics of Sr_0.9La_0.1TiO₃ with additions of B₂O₃ were synthesized by the mixed oxide route. Samples were sintered in air followed by annealing in a reducing atmosphere. Crystallographic data from X-ray and electron diffraction showed Pm3̅m cubic symmetry for all the samples. High-resolution transmission electron microscopy (HRTEM) showed the formation of a core-shell type structure within the grains for the annealed ceramics. The cores contain nanosize features comprising pairs of nanosize voids and particles; the feature sizes depend on annealing time. Atomic-resolution, high-angle annular-dark-field imaging and electron energy loss spectroscopy in the scanning transmission electron microscopy (STEM-HAADF-EELS) showed the particles to be rich in Ti and the areas around the voids to contain high concentrations of Ti³⁺. Additionally, dislocations were observed, with significantly higher densities in the shell areas. The observed dislocations are combined (100) and (110) edge dislocations. The major impact of the core-shell type microstructures, with nanosize inclusions, is the reduction of the thermal conductivity. Sr_0.9La_0.1TiO₃ ceramics containing grain boundary shells of size ≈ 1 μm and inclusions in the core of 60-80 nm exhibit a peak power factor of 1600 μW/m·K² at 540 K; at 1000 K, they exhibit a low thermal conductivity (2.75 W/m·K) and a power factor of 1050 μW/m·K² leading to a high of ZT of 0.39 ± 0.03. This is the highest ZT reported so far for Sr_0.9La_0.1TiO₃ based-compositions. This nanostructuring strategy should be readily applicable to other functional oxides.

Enhancing the thermoelectric properties of Sr1- xPr2 x/3□ x/3TiO3± δ through control of crystal structure and microstructure

A-site deficient perovskites are among the most important n-type thermoelectric oxides. Ceramics of Sr_{1- x}Pr_{2 x/3}□ _x/3TiO₃ ( x = 0.1-1.0) were prepared by solid-state reaction at 1700-1723 K using highly reducing atmospheres. Samples with the highest Sr content had a cubic crystal structure [Formula: see text]; incorporating Pr with A-site vacancies led to a reduction in symmetry to tetragonal ( I4/mcm) and then orthorhombic ( Cmmm) crystal structures. HRTEM showed Pr_2/3TiO₃ had a layered structure with alternating fully and partially occupied A-sites and a short-range order along the (100) direction. Electrical conductivity was highest in samples of high symmetry ( x ≤ 0.40), where the microstructures featured core-shell and domain structures. This enabled a very high power factor of approximately 1.75 × 10^-3 W m^-1 K^-2 at 425 K. By contrast, at high Pr content, structural distortion led to reduced electron transport; enhanced phonon scattering (from mass contrast, local strain and cation-vacancy ordering) led to reduced, glass-like, thermal conductivity. Carbon burial sintering increased the oxygen deficiency leading to increased carrier concentration, a maximum power factor of approximately 1.80 × 10^-3 W m^-1 K^-2 at 350 K and thermoelectric figure of merit of 0.26 at 865 K. The paper demonstrates the importance of controlling both crystal structure and microstructure to enhance thermoelectric performance. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

Structures, Phase Fields, and Mixed Protonic-Electronic Conductivity of Ba-Deficient, Pr-Substituted BaZr0.7Ce0.2Y0.1O3-δ

The BaZr_0.7Ce_0.2Y_0.1O_3-δ-BaPrO_3-δ perovskite system, of interest for high-temperature electrochemical applications involving mixed protonic-electronic conductivity, forms a solid-solution with a wide interval of Ba substoichiometry in the range Ba(Ce_0.2Zr_0.7)_{1- x}Pr _xY_0.1O_3-δ, 0 ≤ x ≤ 1. Structural phase transitions mapped as a function of temperature and composition by high-resolution neutron powder diffraction and synchrotron X-ray diffraction reveal higher symmetry for lower Pr content and higher temperatures, with the largest stability field observed for rhombohedral symmetry (space group, R3̅ c). Rietveld refinement, supported by magnetic-susceptibility measurements, indicates that partitioning of the B-site cations over the A and B perovskite sites compensates Ba substoichiometry in preference to A-site vacancy formation and that multiple cations are distributed over both sites. Electron-hole transport dominates electrical conductivity in both wet and dry oxidizing conditions, with total conductivity reaching a value of ∼0.5 S cm^-1 for the x = 1 end-member in dry air at 1173 K. Higher electrical conductivity and the displacement of oxygen loss to higher temperatures with increasing Pr content both reflect the role of Pr in promoting hole formation at the expense of oxygen vacancies. In more reducing conditions (N₂) and at low Pr contents, conductivity is higher in humidified atmospheres (∼0.023 atm pH₂O) indicating a protonic contribution to transport, whereas the greater electron-hole conductivity with increasing Pr content results in lower conductivity in humidified N₂ due to the creation of protonic defects and the consumption of holes.