The high content of ferromagnetic phase content δ is crucial in establishing high performance of MnBi magnets, which is the potential alternative for NdFeB magnetic material in high-temperature applications. Because of the peritectic solidification, the ferromagnetic phase of as-cast MnBi alloys is low in content and always assisted with Mn and Bi phases, and therefore, δ enhancement must be done using the annealing process. Since the ferromagnetic phase of MnBi is formed only at the temperature lower than 340 °C, the isothermal low-temperature anneal can enhance δ only moderately. In this paper, the temperature-gradient-driven annealing process has been applied to accelerate the formation of the MnBi ferromagnetic phase. This annealing process is based on the advection-assisted diffusion process which allows to accelerate the formation of ferromagnetic phase and to enhance its content δ and thus the spontaneous magnetization Ms of MnBi alloys. By this annealing process at 290 °C superposed by the temperature gradient gradT = 1–3 °C/cm for 20 h, in comparison with the isothermal annealed sample, the content δ is enhanced from the value of 60.6 to 89.9 wt% resulting the Ms enhancement from 43.0 to 64.1 emu/g. The mentioned annealing process was described by using the time-dependent one-dimensional diffusion–advection equation. The obtained analytical solution allows controlling the parameters of the process. So, the temperature-gradient-driven annealing process is promising for the large-scale production of high-LTP content MnBi alloys.

中文翻译：

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用于形成MnBi铁磁相的温度梯度驱动退火工艺

高含量的铁磁相含量δ对于建立MnBi磁体的高性能至关重要，这是高温应用中NdFeB磁性材料的潜在替代品。由于包晶凝固，铸态 MnBi 合金的铁磁相含量低，并且总是辅以 Mn 和 Bi 相，因此必须使用退火工艺进行 δ 增强。由于MnBi的铁磁相仅在低于340°C的温度下形成，等温低温退火只能适度提高δ。在本文中，温度梯度驱动的退火工艺已被应用于加速 MnBi 铁磁相的形成。该退火过程基于平流辅助扩散过程，该过程允许加速铁磁相的形成并提高其含量 δ ，从而提高 MnBi 合金的自发磁化强度 Ms。通过在 290 °C 下通过温度梯度 gradT = 1–3 °C/cm 叠加 20 小时的退火过程，与等温退火样品相比，δ 含量从 60.6 的值提高到 89.9 wt%，导致Ms 增强从 43.0 到 64.1 emu/g。所提到的退火过程是通过使用与时间相关的一维扩散-平流方程来描述的。获得的解析解允许控制过程的参数。因此，温度梯度驱动的退火工艺有利于大规模生产高 LTP 含量的 MnBi 合金。