The Cocktail Effect in High-Entropy Alloys: A Comprehensive Overview
High-entropy materials emerged as a field of research in 2004, when the first research on high-entropy alloys was published. The scope was soon expanded from high-entropy alloys to medium-entropy alloys, as well as to ceramics, polymers and composite materials. High-entropy alloys (HEAs) have evolved to be one of the most popular materials in the last decade. Their unique configuration and attractive properties make HEAs one of the most promising catalysts.
To fully understand the behavior of these materials, it's important to consider the key concepts that govern their properties. A fundamental understanding on high-entropy materials was proposed in 2006 by the 'four core effects' - high-entropy, severe-lattice-distortion, sluggish-diffusion and cocktail effects - which are often used to describe and explain the mechanisms of various peculiar phenomena associated with high-entropy materials. Throughout the years, the effects have been examined rigorously, and their validity has been affirmed. This perspective discusses the fundamental understanding of the four core effects in high-entropy materials and gives further insights to strengthen the understanding for these effects.
Understanding the Four Core Effects
The "four core effects" provide a framework for understanding the unique properties of high-entropy materials:
- High-Entropy Effect: This refers to the high configurational entropy resulting from mixing multiple elements in near-equimolar ratios, stabilizing the solid solution phase.
- Severe-Lattice-Distortion Effect: The different atomic sizes and bonding characteristics of the constituent elements lead to significant lattice distortion, influencing mechanical and physical properties.
- Sluggish-Diffusion Effect: Diffusion rates are reduced due to the complex chemical environment, affecting phase transformation kinetics and high-temperature stability.
- Cocktail Effect: This refers to the synergistic interactions between the constituent elements, leading to properties that are not simply a weighted average of the individual components.
The Cocktail Effect in Detail
The cocktail effect is a key aspect of HEAs, where the combination of multiple elements results in properties that are superior to those of individual elements or traditional alloys. Meanwhile we explored the cocktail effect among high entropy alloy elements through annealing.
High entropy alloys (HEAs) with adjustable composition and diverse surface active sites show good catalytic performance of methanol oxidation reaction (MOR). However, the synthesis and morphology control of HEA remains a challenge. Herein, we use a fast co-reduction method to synthesize a series of ultrathin wavy nanowires (WNWs) with an average diameter of 3-5 nm, including Pt-based and Pd-based binary alloys, and HEA with different pairs of immiscible elements. The catalytic properties of PtPdAgCuNi WNWs show that the mass activity and stability of PtPdAgCuNi WNWs is higher than Pt and Pd WNWs.
HEA Catalysts: Synthesis, Properties, and Applications
Although a very limited amount of work has been reported, higher activities of HEAs than traditional catalysts have been confirmed. This review firstly summarizes the current synthetic methods of nanostructured HEA catalysts. Then, four core effects of HEAs, namely high entropy, cocktail effect, lattice distortion and sluggish diffusion, are briefly introduced, and their impacts on the catalytic properties of HEAs are highlighted. The research progress in the application of HEAs in heterogeneous catalysis is subsequently reviewed from the perspectives of both theory and experiment. The relationships among metastable microstructures, substitution effects (i.e. strain, ligand and ensemble effects) and d-band center are discussed in detail, and their impacts on the adsorption energy of intermediates in catalytic reactions are emphasized.
Here is a summary of the effects of HEAs on catalytic properties:
| Effect | Impact on Catalytic Properties |
|---|---|
| High Entropy | Stabilizes unique microstructures, enhances surface reactivity |
| Cocktail Effect | Synergistic interactions improve activity and selectivity |
| Lattice Distortion | Alters electronic structure, tunes adsorption energies |
| Sluggish Diffusion | Enhances thermal stability, prevents sintering |
References
The information presented in this article is based on the following research:
- Yeh, J. W. et al. Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes. Adv. Eng. Mater. 6, 299-303 (2004).
- Chen, T. K., Shun, T. T., Yeh, J. W. & Wong, M. S. Nanostructured nitride films of multi-element high-entropy alloys by reactive DC sputtering. Surf. Coat. Technol.
- Cantor, B., Chang, I. T. H., Knight, P. & Vincent, A. J. B. Microstructural development in equiatomic multicomponent alloys. Mater. Sci. Eng. A 375-377, 213-218 (2004).
- Yeh, J. W. et al. Formation of simple crystal structures in Cu-Co-Ni-Cr-Al-Fe-Ti-V alloys with multiprincipal metallic elements. Metall. Mater. Trans. A 35, 2533-2536 (2004).
- Huang, P. K., Yeh, J. W., Shun, T. T. & Chen, S. K. Multi-principal-element alloys with improved oxidation and wear resistance for thermal spray coating. Adv. Eng. Mater.
- Wang, Y. Yao, X. Yu, C. Wang, C. Wu and Z. J. Mater. Chem. B. J. Mater. Chem.