An official website of the United States government
High-entropy oxides (HEOs) containing five or more cations have garnered significant attention recently due to their vastly tunable compositional space, along with their remarkable physical and mechanical properties, exceptional thermal stability, and phase reversibility at elevated temperatures. These characteristics position HEOs as promising candidates for structural components and coatings in high-temperature applications. While much of the ongoing research on HEOs centers around understanding processing-structure relationships, there remains a dearth of knowledge concerning their mechanical properties, crucial for their prospective high-temperature applications. Whether in bulk form or as coatings, the efficacy of HEOs hinges on robust mechanical properties across a spectrum of temperatures, to ensure structural integrity, fracture resistance, and resilience to thermal stress. This review offers a succinct synthesis of recent advancements in HEO research, spanning from processing techniques to mechanical behaviors under extreme conditions. Emphasis is placed on three key aspects: (1) Investigating the influence of processing parameters on HEO crystal structures. (2) Analyzing the interplay between crystal structure and mechanical properties, elucidating deformation mechanisms. (3) Examining the mechanical behavior of HEOs under extreme temperatures and pressures. Through this review, we aim to illuminate the effective control of HEOs’ unique structures and mechanical properties, paving the way for their future applications in extreme environments.
more »
« less
Synergistic effects of mixing and strain in high entropy spinel oxides for oxygen evolution reaction
Abstract Developing stable and efficient electrocatalysts is vital for boosting oxygen evolution reaction (OER) rates in sustainable hydrogen production. High-entropy oxides (HEOs) consist of five or more metal cations, providing opportunities to tune their catalytic properties toward high OER efficiency. This work combines theoretical and experimental studies to scrutinize the OER activity and stability for spinel-type HEOs. Density functional theory confirms that randomly mixed metal sites show thermodynamic stability, with intermediate adsorption energies displaying wider distributions due to mixing-induced equatorial strain in active metal-oxygen bonds. The rapid sol-flame method is employed to synthesize HEO, comprising five 3d-transition metal cations, which exhibits superior OER activity and durability under alkaline conditions, outperforming lower-entropy oxides, even with partial surface oxidations. The study highlights that the enhanced activity of HEO is primarily attributed to the mixing of multiple elements, leading to strain effects near the active site, as well as surface composition and coverage.
more »
« less
- Award ID(s):
- 2029425
- PAR ID:
- 10464641
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 14
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract High entropy oxides (HEOs), based on the incorporation of multiple‐principal cations into the crystal lattice, offer the possibility to explore previously inaccessible oxide compositions and unconventional properties. Here it is demonstrated that despite the chemical complexity of HEOs external stimuli, such as epitaxial strain, can selectively stabilize certain magneto‐electronic states. Epitaxial (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)3O4‐HEO thin films are grown in three different strain states: tensile, compressive, and relaxed. A unique coexistence of rocksalt and spinel‐HEO phases, which are fully coherent with no detectable chemical segregation, is revealed by transmission electron microscopy. This dual‐phase coexistence appears as a universal phenomenon in (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)3O4epitaxial films. Prominent changes in the magnetic anisotropy and domain structure highlight the strain‐induced bidirectional control of magnetic properties in HEOs. When the films are relaxed, their magnetization behavior is isotropic, similar to that of bulk materials. However, under tensile strain, the hardness of the out‐of‐plane (OOP) axis increases significantly. On the other hand, compressive straining results in an easy OOP magnetization and a maze‐like magnetic domain structure, indicating the perpendicular magnetic anisotropy. Generally, this study emphasizes the adaptability of the high entropy design strategy, which, when combined with coherent strain engineering, opens additional prospects for fine‐tuning properties in oxides.more » « less
-
The ability to accommodate multiple principal cations within a single crystallographic structure makes high entropy oxides (HEOs) ideal systems for exploring new composition–property relationships. In this work, the high-entropy design strategy is extended to strained single-crystal HEO-manganite (HEO-Mn) thin films. Phase-pure orthorhombic films of (Gd0.2La0.2Nd0.2Sm0.2Sr0.2)MnO3 were deposited on three different single-crystal substrates: SrTiO3 (STO) (100), NdGaO3 (110), and LaAlO3 (LAO) (100), each inducing different degrees of epitaxial strain. Fully coherent growth of the thin films is observed in all cases, despite the high degree of lattice mismatch between HEO-Mn and LAO. Magnetometry measurements reveal distinct differences in the magnetic properties between epitaxially strained HEO-Mn thin films and their bulk crystalline HEO counterparts. In particular, the bulk polycrystalline HEO-Mn shows two magnetic transitions as opposed to a single one observed in epitaxial thin films. Moreover, the HEO-Mn film deposited on LAO exhibits a significant reduction in the Curie temperature, which is attributed to the strong variation of the in-plane lattice parameter along the thickness of the film and the resulting changes in the Mn–O–Mn bond geometry. Thus, this preliminary study demonstrates the potential of combining high entropy design with strain engineering to tailor the structure and functionality of perovskite manganites.more » « less
-
Abstract Nanoparticles supported on carbonaceous substrates are promising electrocatalysts. However, achieving good stability for the electrocatalysts during long‐term operations while maintaining high activity remains a grand challenge. Herein, a highly stable and active electrocatalyst featuring high‐entropy oxide (HEO) nanoparticles uniformly dispersed on commercial carbon black is reported, which is synthesized via rapid high‐temperature heating (≈1 s, 1400 K). Notably, the HEO nanoparticles with a record‐high entropy are composed of ten metal elements (i.e., Hf, Zr, La, V, Ce, Ti, Nd, Gd, Y, and Pd). The rapid high‐temperature synthesis can tailor structural stability and avoid nanoparticle detachment or agglomeration. Meanwhile, the high‐entropy design can enhance chemical stability to prevent elemental segregation. Using oxygen reduction reaction as a model, the 10‐element HEO exhibits good activity and greatly enhances stability (i.e., 92% and 86% retention after 12 and 100 h, respectively) compared to the commercial Pd/C electrocatalyst (i.e., 76% retention after 12 h). This superior performance is attributed to the high‐entropy compositional design and synthetic approach, which offers an entropy stabilization effect and strong interfacial bonding between the nanoparticles and carbon substrate. The approach promises a viable route toward synthesizing carbon‐supported high‐entropy electrocatalysts with good stability and high activity for various applications.more » « less
-
Abstract The enhanced compositional flexibility to incorporate multiple-principal cations in high entropy oxides (HEOs) offers the opportunity to expand boundaries for accessible compositions and unconventional properties in oxides. Attractive functionalities have been reported in some bulk HEOs, which are attributed to the long-range compositional homogeneity, lattice distortion, and local chemical bonding characteristics in materials. However, the intricate details of local composition fluctuation, metal-oxygen bond distortion and covalency are difficult to visualize experimentally, especially on the atomic scale. Here, we study the atomic structure-chemical bonding-property correlations in a series of perovskite-HEOs utilizing the recently developed four-dimensional scanning transmission electron microscopy techniques which enables to determine the structure, chemical bonding, electric field, and charge density on the atomic scale. The existence of compositional fluctuations along with significant composition-dependent distortion of metal-oxygen bonds is observed. Consequently, distinct variations of metal-oxygen bonding covalency are shown by the real-space charge-density distribution maps with sub-ångström resolution. The observed atomic features not only provide a realistic picture of the local physico-chemistry of chemically complex HEOs but can also be directly correlated to their distinctive magneto-electronic properties.more » « less
