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Coercivity of Ndfeb-Based Sintered Permanent Magnets – Experimental and Numerical Approaches

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Coercivity of Ndfeb-Based Sintered Permanent Magnets – Experimental and Numerical Approaches THÈSE Pour obtenir le grade de DOCTEUR DE LA COMMUNAUTE UNIVERSITE GRENOBLE ALPES Spécialité : Physique des matériaux Arrêté ministériel : 25 mai 2016 Présentée par Jérôme FLIEGANS Thèse dirigée par Nora DEMPSEY préparée au sein du Laboratoire LMCM au CEA-LITEN dans l'École Doctorale de Physique Coercivity of NdFeB-based sintered permanent magnets – Experimental and numerical approaches Thèse soutenue publiquement le « 16 Décembre 2019 », devant le jury composé de : M. Viorel POP Professeur, Babes-Bolyai University, Cluj-Napoca, Roumanie, Rapporteur Mme Spomenka KOBE Professeur, Jozef Stefan Institute, Ljubljana, Slovénie, Rapporteur M. Thomas SCHREFL Professeur, Danube University Krems, Vienne, Autriche, Examinateur Mme Sophie RIVOIRARD Ingénieur de recherche, Institut Néel-CNRS, Grenoble, Examinatrice M. Olivier ISNARD Professeur, Institut Néel-CNRS, Grenoble, Président du jury Mme Nora DEMPSEY Directrice de recherche, Institut Néel-CNRS, Grenoble, Directrice de thèse M. Gérard DELETTE Ingénieur de recherche, CEA-LITEN, Grenoble, Encadrant de thèse Acknowledgements First of all, I want to thank Viorel Pop, Spomenka Kobe, Thomas Schrefl, Sophie Rivoirard and Olivier Isnard for accepting to serve in my jury. I also want to thank my thesis director Nora Dempsey and supervisor Gérard Delette for giving me the opportunity to do a PhD thesis. During these 3 years, their support, assistance and scientific expertise were invaluable to my work. Special thanks go to Dominique Givord who helped me a lot during the first 2 years. He was the driving force behind the study of demagnetizing field effects in closed- and open-circuit. I hope that he is proud of the simulation results obtained during the last year. I thank all the people from LMCM: the head of laboratory Céline Delafosse, the engineers and the technicians without whom experiments would not be possible. Specials thanks go to Cyril Rado and Olivier Tosoni for their help and fruitful discussions. I also want to thank all the people from LFM and L2EV laboratories at CEA-LITEN, and from the MNM group at Institut Néel. During these years, many of you became more than colleagues and were part of the wonderful time I had in Grenoble. I thank Romain Soulas and Laure Guetaz from PFNC at CEA-LITEN for their help in microstructural characterization. I also want to thank Yves Deschanels from Institut Néel for his support concerning magnetic characterization. Special thanks go to all the people that were/are part of the office of PhD students. This latter is the perfect example of how colleagues can become really good friends. Last but not least, I am grateful to my family for their amazing support during my very long university studies. And thank you Blandine for your daily support during this last year. Content Introduction ............................................................................................................................................. 1 I. From context to coercivity ............................................................................................................... 4 I.1. History of rare earth permanent magnets ................................................................................ 4 I.2. Intrinsic and extrinsic properties of permanent magnets ......................................................... 5 I.2.1. Intrinsic magnetic properties ............................................................................................. 5 I.2.2. Extrinsic magnetic properties ............................................................................................. 7 I.3. Magnetization reversal mechanisms ......................................................................................... 8 I.3.1. Coherent rotation: the Stoner-Wohlfarth model ............................................................... 8 I.3.2. Nucleation vs pinning controlled magnetization reversal .................................................. 9 I.4. Nd-Fe-B sintered magnets: fabrication and microstructure ................................................... 10 I.4.1. Microstructure of a Nd-Fe-B sintered magnet ................................................................. 10 I.4.2. Industrial production process: sintering .......................................................................... 12 I.4.3. Application fields and limitations ..................................................................................... 13 I.4.4. State of the art: current strategies to improve coercivity ................................................ 14 I.5. Micromagnetic simulations and models ................................................................................. 17 I.5.1. Micromagnetic simulations (Landau-Lifshitz-Gilbert formalism) ..................................... 17 I.5.2. Micromagnetic and global models ................................................................................... 18 I.5.3. Main results of micromagnetic simulations ..................................................................... 22 I.6. The grain boundary diffusion process (GBDP) in Nd-Fe-B sintered magnets ......................... 31 I.6.1. Benefits of core-shell microstructure ............................................................................... 31 I.6.2. State of the art ................................................................................................................. 32 I.6.3. Micromagnetic simulations on core-shell structures ....................................................... 36 I.7. Problematic of the thesis......................................................................................................... 38 II. Fabrication of Nd-Fe-B sintered magnets, characterization and numerical methods .................. 40 II.1. From ribbon to green compact .............................................................................................. 40 II.1.1. Strip-casting ..................................................................................................................... 40 II.1.2. Hydrogen decrepitation to coarse powder ..................................................................... 42 II.1.3. Jet milling ......................................................................................................................... 42 II.1.4. From powder to green compact ...................................................................................... 43 II.2. Sintering furnace .................................................................................................................... 44 II.2.1. Sintering heat treatment ................................................................................................. 45 II.2.2. Post-sinter annealing (PSA) heat treatment.................................................................... 45 II.3. GBDP on Nd-Fe-B sintered magnets ....................................................................................... 46 II.3.1. Sample preparation for GBDP ......................................................................................... 46 i II.3.2. Diffusion heat treatment ................................................................................................. 47 II.4. Characterization methods ...................................................................................................... 48 II.4.1. Magnetic characterization ............................................................................................... 48 II.4.2. Metallography ................................................................................................................. 52 II.4.3. Microstructural characterization ..................................................................................... 52 II.5. Numerical methods ................................................................................................................ 53 II.5.1. FEMME software package ............................................................................................... 53 II.5.2. Flux 3D software .............................................................................................................. 57 III. Coercivity of polycrystalline hard magnets .................................................................................. 64 III.1. Introduction ........................................................................................................................... 64 III.2. Study of collective magnetostatic effects: experimental approach ...................................... 64 III.2.1. Model for the demagnetization field correction ............................................................ 64 III.2.2. Experimental protocol and results ................................................................................. 67 III.2.3. Analysis and model improvement .................................................................................. 69 III.3. Study of collective magnetostatic effects: numerical approach ........................................... 71 III.3.1. Closed-circuit configuration simulation ......................................................................... 71 III.3.2. Open-circuit configuration simulation ........................................................................... 74 III.3.3. Discussion about collective effects ................................................................................ 76 III.3.4. Experimental validation .................................................................................................
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