ALBERTO BOLLERO REAL

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Synopsis Nanocrystalline permanent magnets present unusual magnetic properties because of surface/interface effects different from those of bulk or microcrystalline materials. This is mainly due to the grain size and the presence or absence of intergranular phases. The first part of this work presents results of a systematic investigation of the relationship between microstructure and magnetic properties in isotropic nanocrystalline (Nd,Pr)(Fe,Co)B permanent magnets. Highly coercive (Nd,Pr)FeB-type magnets have been produced using high energy ball milling and melt-spinning techniques. The influence of small amounts of additives, Dy and Zr, and the substitution of Nd by Pr on the microstructural and magnetic properties are shown. An assessment of the hot deformation behaviour for these materials has been carried out. The lowest deformation stresses necessary for texturing were obtained for the as-milled Pr-containing alloys. Intensive milling of an alloy with starting composition Pr 9 Nd 3 Dy 1 Fe 72 Co 8 B 6.9 Zr 0.1 yielded, after subsequent annealing treatment, nearly single-phase magnet powders with a maximum energy product (BH) max ∼ 140 kJm -3 . Co is shown to have a beneficial effect on the intrinsic magnetic properties but also on the microstructure, with a mean grain size of 20 nm. Furthermore, intensive milling has shown to be a very versatile technique to produce high-performance nanocomposite magnets by blending this latter alloy with different fractions of soft magnetic α-Fe. Addition of 25wt.% α-Fe led to an optimum combination of magnetic properties with a very high (BH) max value of 178 kJm -3 due to an effective exchange-coupling between the hard and the soft magnetic phases. Reversibility measurements of the demagnetisation curves give important information on the magnetisation processes in exchange-coupled magnets. The intergrain interactions between the crystallites of the nanocomposite structure have been analysed in the second part of this study. Demagnetisation recoil loops of the nanocomposite magnets have shown relatively open minor loops due to the exchange-spring mechanism. Information about the intergrain interactions during demagnetisation can be obtained by plotting the deviation of the demagnetising remanence from the Wohlfarth-model (“δJ-plot”). Exchange-coupling phenomena have been studied by analysing the evolution of the corresponding δJ values i
when varying (i) the α-Fe content, (ii) the annealing temperature, i.e. the grain size and (iii) the measurement temperature. Measurements of the reversible and irreversible changes in polarisation have provided valuable insight into the mechanisms responsible for exchange-coupling in these nanocomposite permanent magnets. Low temperature measurements did not reveal any sign of spin reorientation for these Pr-based nanocomposite magnets. The temperature dependence of the magnetic properties has been studied in the temperature range from -200ºC to 200ºC. The work concludes showing the possibility of using a mechanically activated gas-solid reaction to obtain an effective grain refined microstructure starting from stoichiometric Nd 2 (Fe 1-x Co x ) 14 B alloys (x = 0, 0.25, 0.5, 0.75 and 1). These compounds have been milled under enhanced hydrogen pressure and temperature leading to their disproportionation into NdH 2 + δ and bcc-(Fe,Co) (0 ≤ x ≤ 0.75) or fcc-Co (x = 1). This reactive milling leads to the disproportionation of the thermodynamically very stable Nd 2 Co 14 B alloy. This reaction is not possible via the conventional hydrogenation disproportionation desorption and recombination (HDDR) process. Grain sizes of recombined Nd 2 (Fe,Co) 14 B materials were found to be 40-50 nm – approximately an order of magnitude smaller than those of conventional-HDDR processed alloys. The recombined Nd 2 Co 14 B alloy shows on average slightly smaller grain sizes than the Nd 2 Fe 14 B compound. A more effective exchangecoupling leading to enhanced remanences, possibly due to the slightly smaller grain size, has been observed for Nd 2 Co 14 B powders recombined at 600-700 °C. ii
Page 1 and 2: Alberto Bollero Real Isotropic Nano
Page 3: Die vorliegende Dissertationsschrif
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 and 14: 1 Introduction J s = 0.5 can not be
Page 15 and 16: 2 Magnetism of R-T intermetallic co
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
Page 51 and 52: 5.2 Structural characterisation whe
Page 53 and 54: 5.3 Characterisation of magnetic pr
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6.1 Thermal characterisation The te
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6.2 Microstructural characterisatio
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6.3 Characterisation of magnetic pr
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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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