ALBERTO BOLLERO REAL

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1 Introduction Nd 2 Fe 14 B system and, afterwards, two similar systems are considered: Nd 2 Co 14 B and Pr 2 Fe 14 B. The main limitation of Nd 2 Fe 14 B is its relatively low Curie temperature which disables high temperature applications because of the rapid decrease of its hard magnetic properties. Substitution of Co for Fe significantly enhances the Curie temperature. At low temperatures, the Nd 2 Fe 14 B phase exhibits another disadvantage: a spin reorientation takes place at temperatures lower than 135 K. Contrary to this Pr 2 Fe 14 B is a very good candidate for low temperature applications since no spin reorientation takes place. Furthermore, the final magnet performance at room temperature for the Pr 2 Fe 14 B system is comparable to that of NdFeB-based magnets. Melt-spinning, mechanical alloying and the hydrogenation-disproportionationdesorption-recombination (HDDR) process are three different methods of processing nanocrystalline NdFeB alloys (Chapter 5). The two former methods are used in a first stage of this work to produce highly coercive powders (Chapters 6). Second, it is shown that intensive milling is a very versatile technique to obtain high-quality isotropic nanocomposite powders by blending a Pr-based starting alloy with elementary powders of a soft magnetic α-Fe phase (Chapter 7). The evolution of the magnetic properties in dependence on the different Fe fractions is studied, and the achieved energy density and Curie temperature of the optimally processed composition make it an excellent candidate for bonded magnets with the additional advantage of a reduced R-content, i.e. lower cost and improved corrosion resistance. An analysis of magnetisation reversal in these Pr-based nanocomposite magnets has been carried out in order to obtain a better understanding of the intergranular interactions present. These results are compared with those of decoupled magnets (Chapter 8). The last part of this work (Chapter 9) shows the possibility of producing nanocrystalline isotropic magnets of Nd 2 Fe 14 B and Nd 2 Co 14 B with an enhanced remanence by a mechanically activated gas-solid reaction. This is achieved by using the reactive milling technique which consists of ball milling in enhanced hydrogen pressures and temperatures. The main problem is the high thermodynamical stability of the Nd 2 Co 14 B parent compound. In the present work the reactive milling technique has been applied to Nd 2 Fe 14 B and Nd 2 Co 14 B alloys. 3
2 Magnetism of R-T intermetallic compounds A brief introduction of some important concepts of magnetism is given in the following with the focus on rare-earth (R) transition-metal (T) intermetallic compounds. For more details, the books of Chikazumi (1997) [1] and Cullity (1972) [2] are very useful for a general introduction to magnetism, and the book edited by Coey (1996) [3] presents a complete overview on rare earth-iron permanent magnets. Herbst and Croat (1991) [4] describe the properties of hard magnets with particular reference to NdFeB magnets. A general review on R-T compounds and processing routes is given by Gutfleisch [5]. 2.1 Hysteresis loop The response of a ferromagnetic material to a magnetic field H is represented by its hysteresis loop. This loop is broad for a good permanent magnet which has to resist demagnetisation by external fields and the field created by its own magnetisation. Ideal and typical hysteresis loops for permanent magnets are shown in Fig. 2.1. Significant parameters associated with the loop are the remanence J r (value of the polarisation when H = 0), and the coercivity H c (reverse field required to reduce the net polarisation to zero – that is, to demagnetise the material). H c has to be distinguished from the coercivity B H c (value of field at which B = 0). The J r value is limited by the saturation polarisation, J s , which can be defined as the maximum polarisation value when the material is magnetised by applying an external magnetic field. 4
Page 1 and 2: Alberto Bollero Real Isotropic Nano
Page 3 and 4: Die vorliegende Dissertationsschrif
Page 5 and 6: when varying (i) the α-Fe content,
Page 7 and 8: liefert wichtige Informationen übe
Page 9 and 10: 6.1.2. Milled and melt-spun alloys
Page 11 and 12: µ 0 H no nucleation field [T] µ 0
Page 13: 1 Introduction J s = 0.5 can not be
Page 17 and 18: 2.2 Magnetic properties 2.2 Magneti
Page 19 and 20: 2.2 Magnetic properties classified
Page 21 and 22: 2.2 Magnetic properties sublattice
Page 23 and 24: 2.2 Magnetic properties where the m
Page 25 and 26: 2.2 Magnetic properties highly stru
Page 27 and 28: 3 The exchange-coupling mechanism A
Page 29 and 30: 3.1 Models enhancement to those obt
Page 31 and 32: 3.2 Wohlfarth’s relation and Henk
Page 33 and 34: 3.3 Magnetisation reversal processe
Page 35 and 36: 3.4 Development of exchange-coupled
Page 37 and 38: 3.4 Development of exchange-coupled
Page 39 and 40: 4 Systems under investigation 4.1 N
Page 41 and 42: 4.2 NdCoB 4.2 NdCoB The tetragonal
Page 43 and 44: 4.3 PrFeB e 1 p 1 Fig. 4.4: Liquidu
Page 45 and 46: 5 Experimental details The differen
Page 47 and 48: 5.1 Nanocrystalline powder processi
Page 49 and 50: 5.2 Structural characterisation and
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Page 53 and 54: 5.3 Characterisation of magnetic pr
Page 55 and 56: 6.1 Thermal characterisation The te
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6.4 Hot workability necessary for N
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6.5 Summary advantage of the Pr-con
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7.1 Characterisation of the base al
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7.3 Summary Magnetisation ( arb. un
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8.1 Initial magnetisation processes
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8.1 Initial magnetisation processes
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8.2 Recoil loops at room temperatur
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8.3 Analysis of exchange-coupling u
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8.4 Summary An analysis of the init
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Fig. 9.1: Dependence of the lattice
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9.1 Reactive milling of a PrFeB all
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9.2 Reactive milling of Nd(Fe,Co)B
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9.3 Summary of J r = 0.91 T after a
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10 Conclusions and future work 10 C
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10 Conclusions and future work the
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10 Conclusions and future work spin
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10 Conclusions and future work (Mö
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REFERENCES [13] O. Gutfleisch, K. K
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REFERENCES [37] J.J. Croat, J.F. He
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REFERENCES [60] J. Bauer, M. Seeger
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REFERENCES [83] S. Hirosawa, Y. Mat
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REFERENCES [106] R.W. Lee, “Hot-p
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List of publications: Some parts of
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Acknowledgments The realisation of
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ALBERTO BOLLERO REAL

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