Synthesis and electron microscopy of high entropy alloy nanoparticles
Introduction
‘High entropy alloys (HEA)’ is the name given to a class of material that are solid solutions of five or more component atoms in equi-atomic or near equi-atomic ratios [1], [2], [3], [4], [5]. Instead of forming multiple intermetallic phases, these alloys generally adopt simple cubic structures [1], [2], [3], [4], [5]. The stability of the solid solution in HEA is proposed to be due to the increased configuration entropy of the system [1], [2], [3], [4], [5], [6]. The interest in HEAs is due to their remarkable properties such as excellent corrosion and oxidation resistance, high temperature strength, excellent wear resistance etc. [7], [8], [9], [10], [11], [12]. Most of the HEAs synthesized till now are bulk solids which adopt nanocrystalline or amorphous microstructures primarily due to the low rates of nucleation and growth owing to the reduced substitutional diffusion and interaction among the inter-diffusing species during partitioning [1], [4]. There is very limited focus yet on the synthesis of HEAs as nano-solids [13]. Synthesis of isolated HEA nanoparticles by method such as solution phase chemical synthesis technique [14] which is suitable for large scale production of isolated nanoparticles has also not been explored. Nanoparticles have two basic advantages over bulk solids: (a) isolated nanoparticles can be spray painted on surfaces to form ultrathin HEA coatings and (b) nanoparticle coatings can be annealed to cause particle-to-particle sintering which can be used to derive and tune grain size dependent properties. This report provides a methodology for synthesizing isolated HEA nanoparticles. A wet chemical synthesis based method was used to synthesize NiFeCrCuCo nanoparticles. Structural and compositional characterization of as-synthesized nanoparticles was conducted using the electron microscopy techniques.
Section snippets
Experiment
To synthesize the nanoparticles, 0.0922 g of CrCl2·6H2O, 0.0823 g CoCl2·6H2O, 0.0687 g of FeCl2·4H2O, 0.0822 g of NiCl2·6H2O and 0.0589 g of CuCl2·2H2O were mixed into 25 ml of Benzyl Ether. This solution was then transferred into a three neck round bottomed flask fitted with a magnetic stirrer and a reflux condenser. Nanoparticle synthesis reaction occurred under an argon atmosphere. The reaction mixture was heated to 120 °C and kept at this temperature for 15 min. After 15 min, 1.7 ml of oleic acid and
Results and discussion
A representative low magnification and high magnification TEM bright field image of as-synthesized nanoparticles is shown in Fig. 1(a) and (b) respectively. A representative STEM-High Angle Annular Dark Field (STEM-HAADF) image of as-synthesized nanoparticles is shown in Fig. 1(c). It can be observed from Fig. 1(a)–(c) that the nanoparticles are almost spherical in shape. The average size of the nanoparticles calculated from the summation average of sizes of ~500 individual nanoparticles was
Conclusion
Wet chemical synthesis method was used to produce isolated NiFeCrCuCo nanoparticles. Average size of the as-synthesized nanoparticle was 26.7±3.3 nm. Analysis of the electron diffraction pattern revealed that the structure of the as-synthesized nanoparticle was face centered cubic. Compositional analysis of the nanoparticles on a single nanoparticle level reveled that all the component elements were distributed uniformly within the nanoparticle volume.
Acknowledgment
Authors acknowledge the electron microscopy facilities in the AFMM, IISc Bangalore. Research funding from SERB, India (SR/S3/ME/0044/2010), Government of India is also acknowledged.
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