REPOSITORY > RESULTS

Abstract

Permanent magnets are materials that generate a magnetic field in free space without electricity or other external energy sources. They are essential components of modern technologies, used in many electrical devices. Nowadays, they are included in technology for renewable sources of energy, such as electrical vehicles, generators and wind turbines.
For such applications, Nd-Fe-B magnets are the most suitable choice due to their high energy product (BH)max. The largest (BH)max value above 400 kJ/m3 was reported for high-performance anisotropic Nd-Fe-B magnets. However, the coercivity at room temperature and its thermal stability is still a weak point of Nd-Fe-B magnets. To withstand high temperatures and demagnetization fields during the operation of a motor, the coercivity at room temperature must be improved. Adding heavy rare earths (HREs) such as Dy and/or Tb results in the formation of a high-anisotropy phase (HRE, Nd)2Fe14B that leads to enhanced coercivity. The drawbacks of using HREs are reduced saturation magnetization, high price and risk of supply, since they are predominantly mined in China. Therefore, high-performance Nd-Fe-B magnets with a reduced amount of HREs or even without HREs needs to be considered.
In this thesis four different topics are introduced with one common denominator – melt-spun Nd-Fe-B ribbons. The first topic relates to a first-author paper entitled “Magnetic properties and microstructure evolution of hot-deformed Nd-Fe-B magnets produced by low-pressure spark-plasma sintering”. In this work the hot-deformed Nd-Fe-B magnets are produced from commercial Nd-Fe-B ribbons. The spark-plasma-sintering technique is used as it enables a deformation process under low pressures at 40 MPa. The initial stages of the hot-deformed process are investigated by means of microstructural observations and magnetic measurements. Hot-deformed magnets with different deformation ratios are produced and their remanence dependence on the deformation ratios was investigated. In the initial stages of the hot-deformation process, the cone-like shape of the magnet is observed for the first time. In addition, at different regions of a cone-like hot-deformed magnet, various degrees of alignment of both the ribbons and the grains are detected. The experimental data are qualitatively interpreted in the frame of a simple Stoner-Wohlfarth model, which reveals reasonable magnetic properties while applying a low pressure.
In the second topic a newly developed, economically efficient method for processing rare-earth Nd-Fe-B magnets based on spark plasma sintering is presented in a first-author paper entitled “Toward Low-Energy Spark-Plasma Sintering of Hot-Deformed Nd-Fe-B magnets”. This technique makes it possible to retain the technologically essential properties of the magnet by consuming about 30% of the energy, compared to the conventional spark-plasma-sintering process. A magnet with an anisotropic microstructure is fabricated from MQU F commercial ribbons with a low energy consumption during the deformation process and compared to the conventionally prepared hot-deformed magnet, which consumed 3-times more energy. Both magnets are post-annealed at 650 °C for 120 min in a vacuum. After the post-annealing process, the low-energy processing (LEP) hot-deformed magnet shows a coercivity of 1327 kAm-1 and remanent magnetization of 1.27 T. In comparison, the high-energy processing (HEP) hot-deformed magnet had a coercivity of 1337 kAm-1 and a remanent magnetization of 1.31 T. A complete microstructural characterization and detailed statistical analyses reveal a better texture orientation for the HEP hot-deformed magnet processed with a higher energy consumption, which is the main reason for the difference in the remanent magnetization between the two hot-deformed magnets. The results show that although the LEP hot-deformed magnet was processed with three times less energy being consumed than in a typical hot-deformation process, it still has only an 8% lower maximum energy product.
The third topic relates to a third first-author paper entitled “Significant coercivity enhancement of hot-deformed bulk magnets by two-step diffusion process using a minimal amount of Dy”, where a two-step grain-boundary-diffusion process is implemented on a hot-deformed Nd-Fe-B magnet with the addition of a minimal amount of Dy. The coercivity of a 5.6-mm-thick Nd-Fe-B hot-deformed magnet was increased from 1.13 T to 2.5 T while maintaining a large remanent magnetization of 1.32 T using Nd50Dy30Cu20 and Nd80Cu20 alloys for the first and second steps of the diffusion process, respectively. Only 0.45 wt.% of Dy was used in the high-coercivity bulk magnet which is only ≈10 % of that used in the conventional Dy-alloyed sintered magnets with a comparable coercivity. In addition to the high room-temperature value of coercivity, an excellent temperature coefficient of coercivity of –0.41 %/oC was realized in the final diffusion-processed hot-deformed magnet. Consequently, this work paves a way towards the development of bulk high-coercivity Dy-lean Nd-Fe-B magnets.
Last, but not least, the fourth topic presents an invention that relates to the improvement of the coercivity of a commercial magnetic melt-spun Nd-Fe-B ribbons with a small intergranular phase fraction and a process for the production of polymer-bonded magnets by the diffusion of a low-temperature eutectic alloy (Nd-Cu). The coercivity of the commercial Nd-Fe-B powder is not satisfactory for certain applications, while commercial powders for the fabrication of polymer-bonded magnets with high coercivity are significantly more expensive due to the added HRE. This invention presented the improvement of the magnetic properties (coercivity) of economically more favourable melt-spun Nd-Fe-B ribbons with a very small proportion of intergranular phase. The solution for coercivity enhancement was the diffusion of low-temperature-eutectic alloy (Nd-Cu), which resulted in a 15 % coercivity improvement of the final polymer-bonded magnet compared to that prepared from standard, low-cost, magnetic, melt-spun Nd-Fe-B ribbons.