In an article recently published in The Journal of Physical Chemistry C, researchers discussed the unexpected superparamagnetic behavior in nanocrystalline high-entropy alloys based on Niobium.
Study: Unexpected Superparamagnetic Behavior in Nanocrystalline Niobium-Based High-Entropy Alloys. Image Credit: sakkmesterke/Shutterstock.com
High-Entropy Alloys (HEAs)
By carefully expanding their search and combining available metals across the periodic table, scientists and engineers investigated a wide range of design options for materials, notably complex alloys.
High-entropy or multi-principal element alloys (MPEAs), which may meet the requirements of future aerospace, energy, and nuclear technologies, have recently received the greatest attention in the evolution sequence. Initially, the idea of high-entropy alloys was solely applicable to metallic materials or alloys. However, it has been continuously developing to investigate new substances, such as materials based on carbide, oxide, and nitride, with extensive technological application in numerous different domains.
Refractory High-Entropy Alloys (RHEAs)
A new application area for nanocrystalline refractory high-entropy alloys for harsh environments, particularly those in the defense, aerospace, and energy sectors, is made possible by the material's exceptional creep resistance at higher temperatures. The research community hopes to develop more sophisticated, nanoscale high-entropy alloys for multifunctional device applications after realizing the potential of nanostructured high-entropy alloys.
The best use for refractory high-entropy alloys is in high-temperature applications due to their extraordinarily high melting point. Refractory high-entropy alloys are yet to be applied in other domains or for other purposes, particularly at the nanoscale levels.
Even though there have been few attempts to document the controlled growth and microstructure of nanostructured high-entropy alloys, it has been difficult to demonstrate high-entropy alloy nanoparticles with ultra-narrow size distribution and improved functionality.
Nanoparticles of Refractory High-Entropy Alloys
In this study, the authors reported the development of superparamagnetic behavior in nanoparticles made of the refractory high entropy alloy Nb-Cr-Ta-V-W using a simple mechanical pulverization method called high-energy ball milling. Refractory high-entropy alloy nanoparticles' structure, composition, shape, and elemental distribution have all been assessed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and detailed X-ray diffraction (XRD), and the results were compared to the bulk.
Characterization of the Refractory High-Entropy Alloy
The nano-refractory high-entropy alloy retained the parent crystal structure and phase, according to the XRD and SEM data combined with EDS investigations. Still, the size rapidly shrank with milling time. With an increase in milling time to 16 hours, the average crystallite size decreased from 32 to 12 nanometers. The refractory high-entropy alloy nanoparticles showed superparamagnetic nature while the bulk samples behaved diamagnetically.
The team demonstrated the superparamagnetic properties of the refractory high-entropy alloy nanoparticles by magnetic measurements taken in cryogenic and ambient temperatures. While the magnetic domain size followed the opposite trend, the saturation magnetization value increased exponentially with milling duration and stabilized after 8 hours of pulverization. This superparamagnetic pattern of the refractory high-entropy alloy nanoparticle resulted from a significant drop in the magnetic domain size.
The researchers concentrate on the peculiar magnetic behavior and nanoscale manufacturing of Nb-based refractory alloys that could lead to new developments in the field of materials for extreme environment applications. The kinetic energy was transferred into the host crystal during the mechanical pulverization processes, breaking it into smaller particles. Furthermore, the sharp decrease in crystallite size positively encouraged the formation of even smaller magnetic domains, encouraging superparamagnetism over a wide temperature range. Based on XRD studies of phase structure, there was no discernible difference in the crystalline structure between the bulk and nanoscale refractory high-entropy alloy samples.
Superparamagnetic Behavior of the Refractory High-Entropy Alloy
The blocking temperature (TB) was below 50, 96, 85, and 125 K for the ball-milled samples at 2, 8, 4, and 16 h, respectively. The zero field cooled (ZFC) and FC curves coincided from 50 to 400 K. This supported the superparamagnetic behavior of the nanoscaled samples that was seen at 50 K and at normal temperature. The bulk sample's nonmagnetic behavior was supported by the ZFC/FC curve, which displayed below zero magnetization from 50 K to 400 K in the presence of a 200 Oersted applied magnetic field. For the samples milled at 2, 4, 8, and 16 h, respectively, the ball-milled samples show coercivity (HC) near zero and saturation magnetization (MS) of 5, 6, 7, and 7 electromagnetic units/gram.
Conclusions and Future Perspectives
In conclusion, this study showed that the refractory high-entropy alloy nanoparticles exhibited unexpected superparamagnetic properties in contrast to their bulk equivalent. The magnetic domain size differed significantly between the bulk and the nanoscaled refractory high-entropy alloy samples. The superparamagnetic behavior of the nanoscaled samples was caused by the shrinkage of the magnetic domain size.
The superparamagnetic behavior of the refractory high-entropy alloy nanoparticles was confirmed by magnetic measurements with a closed hysteresis loop (M x H) at both room temperature and cryogenic temperatures. The observed M x H results of the nonmagnetic behavior of the bulk sample and the superparamagnetic behavior of the nanoscaled samples were both supported by the M x T measurements.
The authors mentioned that these impressive, magnetic characteristics expand the potential of refractory high-entropy alloys for low-cost magnetic applications in tough environmental settings.
Das, D., Getahun, Y., Escobar, F. S., et al. (2022) Unexpected Superparamagnetic Behavior in Nanocrystalline Niobium-Based High-Entropy Alloys. The Journal of Physical Chemistry C. https://pubs.acs.org/doi/10.1021/acs.jpcc.2c03111