Phase investigations of manganese-bismuth alloyed in a microwave furnace
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Phase investigations of manganese–bismuth alloyed in a microwave furnace Panita THONGJUMPA1, Thanida CHAROENSUK2, 3, Upsorn BOONYANG2, 4, Phimphaka HARDING2, 4, Chitnarong SIRISATHITKUL1, 2, 3 1. Division of Physics, School of Science, Walailak University, Nakhon Si Thammarat 80160, Thailand; 2. Functional Materials and Nanotechnology Center of Excellence, Walailak University, Nakhon Si Thammarat 80160, Thailand; 3. Thailand Center of Excellence in Physics, Ministry of Higher Education, Science, Research and Innovation, 328 Si Ayutthaya Road, Bangkok 10400, Thailand; 4. Division of Chemistry, School of Science, Walailak University, Nakhon Si Thammarat 80160, Thailand © Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract: Implementation of manganese–bismuth (MnBi) alloys as high-performance permanent magnets is a challenge for physicists and engineers because the ferromagnetic low-temperature phase (LTP) is not exclusively obtained. In this work, melting powered by four commercial magnetrons of 2000–2500 W in a microwave furnace is demonstrated as a new route to alloy MnBi. Under an argon atmosphere, microwave heating transferred to pieces of broken Bi ingots and Mn flakes for 2 h gave rise to products of inhomogeneous composition and morphology. Scanning electron micrographs were classified into three regions according to morphology and elemental composition. Cubic-like clusters characterized as Mn precipitated over light solidified Bi-rich regions, and the MnBi phase was formed in homogeneous regions with a balanced composition between Mn and Bi. A ferromagnetic hysteresis loop was obtained in the ground powder with a coercivity of 40 kA/m. Subsequent annealing at 553 K under a pressure of 414 kPa for 12 h enhanced the MnBi phase with extended regions of balanced composition. It follows that the coercivity was increased to 60 kA/m. However, remanent magnetization was slightly reduced. This MnBi alloyed by microwave radiation can be further used in rare-earth-free magnets. Key words: microwave; manganese; bismuth; scanning electron microscopy; magnetic properties; coercivity Cite this article as: Panita THONGJUMPA, Thanida CHAROENSUK, Upsorn BOONYANG, Phimphaka HARDING, Chitnarong SIRISATHITKUL. Phase investigations of manganese–bismuth alloyed in a microwave furnace [J]. Journal of Central South University, 2020, 27(8): 2220−2226. DOI: https://doi.org/10.1007/s11771-020-4443-6.
1 Introduction Ferromagnetic low-temperature phase (LTP) manganese–bismuth (MnBi) exhibits substantial magnetocrystalline anisotropy and a positive temperature coefficient of coercivity. These magnetic properties coupled with a relatively high
Curie temperature are desirable for permanent magnets at elevated temperatures. The goal of developing these rare-earth-free magnets is to provide an alternative to neodymium–iron–boron magnets with higher maximum energy products ((BH)max) than those of ferrites. Whereas substantial values of (BH)max are already achieved in laboratories [1−3], and the
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