Peculiarity of Pr-containing permanent magnets

Transcription

1 Peculiarity of Prcontaining NdFe FeB permanent magnets ERA_NET_RUS Development of LowTemperature Hard Magnetic Materials and HighCurrent HighField FeBased Superconductors for Innovative Technologies Russian Partner Tekhma,, Ltd. Leader of the Russian Project Corresponding member, Russian Academy of Sciences Gennadii Burkhanov Participants of the project Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow Cand.. Sci. in Eng. Kolchugina Natalia JSC SPETSMAGNIT,, Moscow Cand.. Sci. in Phys. & Math. Buryakov Il iya Cand.. Sci. in Phys. & Math. Lukin Aleksandr Dr. Sci. in Phys. & Math. Kraposhin Valentin 1

2 Aims of the project Designing new compositions of hard magnetic Nd 2 Fe 14 Bbased materials balancingalloyed alloyed with critical REMs for highenergy permanent magnets, which exhibit properties stable over the wide temperature range (77273 К) and intended for innovative technologies and devices. 2

3 Introduction The substitution of praseodymium for neodymium in the magnetic (Nd x Pr 1x ) 2 Fe 14 B alloys for NdFeBbased permanent magnets allows one to decrease the SRT temperature owing to the absence of the transition for the Pr 2 Fe 14 B compound; to increase the magnetization coercive force owing to the higher magnetic anisotropy field of the Pr 2 Fe 14 B compound (90 koe) as compared to that of the Nd 2 Fe 14 B compound (70 koe). At present, because of features of solidification and sintering processes for Prcontaining 2141based alloys (which are more viscous), Pr additions are used only for compositions highly alloyed with Dy, Tb, Co, Nb, etc. or specific manufacturing processes (casting and subsequent deformation) are applied. 3

4 However, it is known that the substitution of praseodymium for neodymium substantially decreases the reversible temperature coefficient of induction and worsens the corrosion resistance of magnets. To decrease the effect of negative factors, we use only in part substitution of praseodymium for neodymium and compositions additionally alloyed with titanium, copper, and aluminum,, which form nonmagnetic ternary eutectic, binary phases with iron, and borides present within the grainboundary phase and ternary junctions. 4

5 Peculiarities of NdFe FeB and PrFe FeB B magnets The Curie temperature of Pr 2 Fe 14 Bbased magnets is lower than that of NdFe FeB B magnets, the lower both temperature and time stability (in particular, the reversible temperature coefficient of magnetic induction at K is 0,015 %/К and 0,012 %/К for PrFe FeB B and NdFe FeB magnets, respectively). The PrFe FeB B alloys for permanent magnets are more viscous; this fact increases requirements for quenching rates and assumes the application of hydrogen decrepitation,, which is among processes used by us. However, it was shown in [Lin C. H., et al. Hydrogen induced radial anisotropic RFeR FeB B magnet (R = Pr, Dy) ) // J. Appl. Phys V. 69. PP ] 5520] that the hydrogenated phase Pr 2 Fe 14 BН х demonstrates the spinorientation transition at room temperature, which is analogous to that of Nd 2 Fe 14 B. This leads to the fact that the powder cannot be textured along the easy magnetization axis during compacting the hydrogenated powder (the powder behaves as an isotropic powder). This results in the 5 abrupt decrease in the hysteretic properties of magnets.

6 Reported magnets for lowtemperature applications (for liquid nitrogen temperature around 77 K) PrFeBmagnets (most of them are Hitachi s CR53 grade) mixed (Nd x Pr 1x ) 2 Fe 14 Balloys (an individual VAC grade (Nd 0.2 Pr 0.8 ) 2 Fe 14 B). In these, the spinreorientation does not occur. A specific limitation in the use of these high remanent alloys is the limited coercivity,, which requires special precautions already during assembly at room temperature 6

7 VD131TP and VD131DTP Vacuumschmelze has created a new, PrFeBbased magnet grade (VD131TP) and also VD131DTPmagnets, which were produced from VD131TP by subsequent application of the grain boundary diffusion process to those magnets in order to gain improved coercivities without substantial decrease in residual induction. [PROPERTIES, OPTIONS AND LIMITATIONS OF PrFeBMAGNETS FOR CRYOGENIC UNDULATORS, F.J. Börgermann,, C. Brombacher,, K. Üstüner,, Proceedings of IPAC2014, Dresden, Germany ] 7

8 Grainboundary diffusion in the case of VD131TP and VD131DTP magnets An increase of coercivity without significant loss of remanence is achieved by the grainboundary diffusion process. In this process compared to conventional implementation by alloying method the heavy rareearth earth elements (Dy( Dy,, Tb) which are required for effective decoupling of the magnetic grains are transported to the dedicated location inside the magnets microstructure where they operate most effectively. Pure rareearth earth metals or compounds (REM hydrides, fluorides, etc.) are applied on the surface. This transport mechanism is most effective along the existing, Ndrich grain boundary phase in the alloy and should be stopped, before the heavy rareearth elements significantly enter into the grain s bulk consisting of the main phase Pr 2 Fe 14 B. 8

9 VACODYM magnets With the development of VD131TP and VD131DTP, a new alloy series based on PrFeB is available for the construction of cryogenic permanent magnet assemblies to be driven at liquid nitrogen temperature or even lower temperatures 9

10 Aim of the study The aim of the present work is to study the structure and magnetic hysteretic properties of the (Nd,Pr( Nd,Pr)FeB B magnets with 10, 13, and 15 wt % Pr and to demonstrate their possibilities for the low temperature applications. 10

11 Preparation and annealing conditions used at JSC Spetsmagnit Sintered magnets with chemical composition (wt %) (2318)% Nd,, (1015)% 15)% Pr, 0.9% Ti, 0.4% Al, 0.2% Cu, 1.3% B, Febalance were prepared by traditional powder technology. Starting alloys were melted using a Balzers VSG vacuum induction furnace and subsequently cast into a copper watercooled mould (weight of ingot is ~ 8 kg). Ingots were subjected to hydrogen decrepitation and subsequently to milling in a vibratory mill with an isopropyl alcohol medium. PrH 2 addition used for grainboundary structuring for compositions with 10 and 13 wt % Pr grainboundary diffusion combined with the grain boundary structuring Magnets with 15 wt % Pr were prepared without hydride additions. Powders were compacted at a pressure of 30 MPa in a textured magnetic field of 1.6 T. Magnet blanks were sintered in a vacuum at temperatures of 1080 to 1100 C C for 2 h. The following heat treatments were performed: (1) 500 С,, 2 h (optimum heat treatment); it allows us to reach the high hysteretic h properties for both the compositions. 11

12 Scheme of grainboundary diffusion in the case of REM hydride additions REM hydride additions to the powder during fine milling can operates only in the case of Tb, Dy, Ho, Nd hydrides. Diffusion of REM atoms into the main phase grains is possible only in the case of atoms equal or smaller than those of Nd. Pr hydride additions operate (diffusion of Pr takes place) only in the case of low Pr content. 12

13 Lowtemperature hysteretic properties T, K wt % Pr i H c, T 13 wt % Pr Table summarizes the coercive force of the magnets. It should be noted the lower coercive force of the magnet with 13 wt % Pr at room temperature and the more abrupt increase in the coercive force magnitude with decreasing temperature The coercive force is comparable with that of VACODYM magnets 13

14 Roomtemperature hysteretic properties of magnets with 10, 13, and 15 wt % subjected to optimum heat treatment Pr content, wt % / Heat treatment H, j c koe (ka/m) H k, koe (ka/m) B r, kg (T) (BH) max, MG Oe (kj/m 3 ) 10 / 500 С С,, 2 h (optimum) 15.8 (1264) 15.6 (1248) 12.5 (1.25) 38 (302) 13 / 500 С С,, 2 h 14.6 (1166) 14.4 (1143) 12.7 (1.27) 39 (311) 15 / 500 С С,, 2 h (at 77 K) K (1109) 12.6 (1.42) 38 (304) 49 (390) % HoH 2 / 500 С С,, 2 h 19.1 (1514) 11.2 (891) (224) 14

15 Hysteretic characteristics 15 of magnet with 15% Pr

16 Improvement of hysteretic characteristics of PrFeB magnet by HoH2 addition 16

17 Microstructure of magnets with 13 and 15 wt% Pr Peculiarities of the phase composition: The phases found in the structure of magnets both after optimum annealing can be identified as the main magnetic (Nd,Pr)2Fe14B phase (point 1), (Nd,Pr)1.1Fe4B4 phase (point 2) (the phase is absent in magnet with 15%, (Nd,Pr)rich phase observed at grain boundaries (point 3), Nd(Pr)O, Nd(Pr)O2 or (Nd,Pr)2O3 oxides present at grain triple junctions (points 4) and Tibased phase (TiB2) (point 5). It should be noted that the (Nd,Pr)1.1Fe4B4 phase is found after optimum annealing. The formation of the phase is typical of the solidification path of the alloys. Its transformation (in total case) by the reaction with liquid into Nd + Nd2Fe14B takes place at about 700 C. 17

18 Magnetic properties of disk magnets D45х30 mm (for Germany partner) No. B r (T) j H c (ka/m) BH max, kj/m 3 (MG Oe) H 0 (ka/m) Notes 12 Nd 34 Nd 56 Pr (44( 44) 336 (42( 42) 304 (38( 38) 390 (49) Reference samples (measure ments with hysteresis graph MN 50) / / / /496 Field in the pole center (measured by a teslameter) 5 474/ /491 18

19 Acknowledgement The research was supported by the NCBR (Poland) within the project ERA.Net RUS Plus: No. 146 MAGNES financed by the EU 7th FP, grant no and by the Russian Academy of Sciences, program no. III

20 Thank you for the attention 18