Improving Magnetic Performance of Sm-Co Magnets through Nano-Structuring -Inorganic Chemical Synthesis-

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

Permanent magnets have been widely used in a large number of modern technologies, i.e., motors, generators, transportation, hard-disk drives, etc. The magnetic strength of a magnet is measured by the figure of merit: the maximum energy product, (BH)max, which is twice the energy stored in the stray field around a magnet. The (BH)max has increased in steps since the discovery of rare-earth permanent magnets. An introduction of rare-earth elements with 4f electrons provides a strong magnetocrystalline anisotropy, drastically increasing their coercivity (Hc), which is the ability to resist demagnetization. Among all known permanent magnets, SmCo5 has the highest magnetocrystalline anisotropy, so it has a great potential to achieve a high Hc through a well-designed nanostructure. Hc of permanent magnets is strongly dependent on the particle size, as the magnetic single-domain theory points out that the Hc of permanent magnets increases as the particle size approaches to the stable single-domain size (SSDS) range. The calculated SSDS range is 740-870 nm, supported by the initial experimental data of this Ph.D. study. However, with further reduced particle size, SmCo5 magnet shows a higher Hc, which means that the critical SSDS range should move to a smaller size, approximately 200-300 nm.
In this Ph.D. dissertation, pivotal studies concentrate on the preparation of SmCo5 fine powders with controllable particle size using a bottom-up approach, and the influence of particle size, phase composition, size distribution, and morphology on Hc. Two chemical methods have been developed to prepare SmCo5 fine-powder based on a reduction-diffusion (R-D) process. Firstly, a combination of the R-D process and a solution combustion method has been developed to prepare Sm-Co particles. The product consists mainly of SmCo5 and an impurity phase of Sm2Co7. The influence of the Co/Sm ratio in the starting materials on the phase composition and particle size of Sm-Co fine powder has been investigated. The best Sm-Co particles have been prepared using a Co/Sm ratio of 4.4/1 and have an average particle size (APS) of 816 nm. It contains the highest weight fraction of SmCo5 phase (76.6 wt.%) and exhibits the largest Hc, 2176 kA m-1 (27.3 kOe). The consolidation of the fine powder into a pellet by spark plasma sintering (SPS) has been studied as well. The SPS-pellet has a (BH)max of 182 kJ m-3 (22.9 MGOe) due to the strong magnetic anisotropy and the retained single-domain size (about 1μm) after consolidation.
Based on the optimized Co/Sm ratio (4.4/1), an improved co-precipitation method has been developed to replace the solution combustion method for better controlling the phase purity and size of SmCo5 particles. The influence of the reaction conditions of the precursor, which mainly has focused on reaction time, on the phase composition, particle size, and magnetic performances of SmCo5 fine powder has been explored. Increasing the reaction time reduces the impurity phase in the final product and increases the APS and the Hc. It was possible to produce pure SmCo5 particles with an APS of 816 nm, which exhibits a Hc of 2631 kA m-1 (33.1 kOe). Although the APS of synthesized SmCo5 fine powder lies in the calculated SSDS region, the size distribution is large. By narrowing the size distribution it should be possible to further increase the Hc.
Based on the R-D process assisted by the solution combustion method, in order to improve the phase purity of SmCo5 particles and decrease their size distribution, a new method was developed. It has been SiO2 nanoparticles (NPs) have been suspended in the water solution, and acts as confining templates in the precursor. The influence of the volume fraction of the SiO2 colloid solution in the precursor has been studied. The results reveal that SiO2 NPs can inhibit the growth of precursor NPs and tune the phase composition, size, and Hc of the final product. The best produced pure SmCo5 particles had an APS of 625 nm and showed the highest coercivity of 2986 kA m-1 (37.5 kOe). The introduction of the SiO2 templates does not narrow the broad size distribution effectively as expected and an extra washing step is required to remove them.
To overcome these two drawbacks, CaCO3/CaO NPs, have been used as a confining matrix. The combustion process produces precursor NPs well distributed in the CaCO3/CaO matrix in one simple step. CaCO3/CaO matrix can efficiently prevent the growth of precursor NPs but also the intergrowth of SmCo5 particles during the R-D process at high-temperature. Thus the CaCO3/CaO matrix can effectively reduce the particle size and size distribution of SmCo5 fine powder simultaneously. Furthermore, some SmCo5 particles with ellipsoidal shape formed because of the confinement of CaCO3/CaO matrix and introduced a shape anisotropy. As a consequence, SmCo5 particles with an APS of 308(6) nm and narrow size distribution are formed and exhibit the highest Hc of 3474 kA m-1 (43.7 kOe). This is an improvement of Hc of 49% compared to the reference sample where no confining templates were used.
Additionally, the developed R-D process assisted by the combustion method has been expanded to prepare Sm2Co7 magnet. It is the first time Sm2Co7 particles have been prepared by a bottom-up approach. By decreasing the Co/Sm ratio in the starting materials, the main phase of the Sm-Co particles becomes Sm2Co7 phase. The sample prepared using a Co/Sm ratio of 2.9/1 contains the highest weight fraction of Sm2Co7 phase (91 wt.%), exhibiting an Hc of 2072 kA m-1 (26.0 kOe). The consolidation of the fine powder into a bulk magnet was carried out by SPS. The results suggest that the metastable phase Sm2Co7 partly decomposed into a stable phase of SmCo5 and an impurity phase of Sm2O3 during the compaction. An enhanced Hc, 3168 kA m-1 (39.8 kOe), has been achieved due to the formation of SmCo5 particles with single-domain size, the Sm2O3 on the surface of Sm-Co particles acting as pinning sites of domain walls, and the rest of the Sm2Co7 phase also introducing phase boundaries. The conversion between different Sm-Co structures provides a new perspective to improve the magnetic performance of rare-earth permanent magnets.
ForlagAarhus University
Antal sider226
StatusUdgivet - 24 apr. 2020

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