MAGNETIC MATERIALS WITH PROPERTIES STABLE OVER A WIDE TEMPERATURE RANGE

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1 MAGNETIC MATERIALS WITH PROPERTIES STABLE OVER A WIDE TEMPERATURE RANGE Jacek Ćwik Project ERA.Net RUS Plus financed by the EU 7th FP for research, technological development and demonstration, grant no Meeting No 3 Wroclaw February 2017

2 1. International Laboratory of High Magnetic Fields and Low Temperatures, PAS, Wroclaw, Poland J. Ćwik, Y. Koshkidko, K. Rogacki 2. Baikov Institute of Metallurgy and Material Sciences, Moscow, Russia G.S. Burkhanov, N. Kolchugina 3. VSB-Technical University of Ostrava, Ostrava-Poruba, Czech Republic K. Skotnicová, M. Kursa 4. JSC SPETSMAGNIT, Dmitrovskoe sh. 58, Moscow, Russia A.A. Lukin, A.G. Dormidontov Wrocław,

3 1 OUTLINE 1. Available permanent-magnetic materials 2. Evolution of the phase composition of (Nd,Pr,Dy)-Fe-B magnets during stepped annealing 3. Influence and optimization of heat treatment on the structural and magnetic properties of Nd-Fe-B based magnets for wide temperature range application 4. The physical properties of RNi 2 compounds; a real candidates for a practical cryogenic application

4 1. Available permanent-magnetic materials Comparison of the magnetic properties of several commercially available permanentmagnetic materials [1] H cj - magnetization coercive force H k - critical field (magnetization reversing field, at which 10 % decrease in the magnetization takes place) B r - residual magnetic induction (BH) max - maximum energy product. Nd-Fe-B K [1] K. H. J. Buschow, F.R. De Boer in : Physics of Magnetism and Magnetic Materials, Kluwer Academic /Plenum Publishers, New York

5 2. Evolution of the phase composition of (Nd,Pr,Dy)-Fe-B Addition of HRE is effective to enhance coercivity and consequent thermal stability of the Nd Fe B magnets [1]. FIRST STEP Initial alloys were prepared using; Nd -24 wt %, Pr 6.5 wt %, Dy 0.5 wt %, B 1 wt %, Al 0.2 wt.%, Fe 65.8 wt.% strip-casting alloy. The preparation procedure includes the hydrogen decrepitation of flakes in dry hydrogen (at 100 C for 1 h) and subsequent passivation in gaseous nitrogen atmosphere. Dysprosium hydride DyH 2 (2 wt %) was added after cooling the powder to room temperature. The mixture was subjected to fine milling for 40 min to an average particle size of 3 μm using a vibratory mill and isopropyl alcohol medium. Samples were compacted in a magnetic field and sintered at Т = 1070 C (for 2 h). The sintered blanks were subjected to the following heat treatments: [1] W.Q. LIU, W.Q., YI, X.F., et al., J. Alloys Comp., 2010, v. 501,

6 2. Evolution of the phase composition of (Nd,Pr,Dy)-Fe-B Addition of HRE is effective to enhance coercivity and consequent thermal stability of the Nd Fe B magnets [1]. SECOND STEP No.1; 550 C (1 h) + quenching in gaseous nitrogen No.2; 550 C (1 h) + quenching C (2h) + quenching C + cooling for 2 h to 400 C C (6h) + furnace cooling No.3; 550 C (1 h) + quenching C (2h) + quenching C + cooling for 2 h to 400 C C (6h) + furnace cooling C (1h) + quenching [1] W.Q. LIU, W.Q., YI, X.F., et al., J. Alloys Comp., 2010, v. 501,

7 2. Evolution of the phase composition of (Nd,Pr,Dy)-Fe-B main reflections of the measured pattern correspond to the Nd 2 Fe 14 B contain different oxide phases : NdO with the NaCl-type structure (space group Fm-3m) and NdO 2 with the CaF 2 - type structure (space group Fm-3m) No α-fe phase no.3 exhibits a small peak at 2Θ = 42.7 о - this reflection corresponds to the Nd 1.1 Fe 4 B 4 phase [1] X-ray diffraction pattern for sample no. 2 and 3. Red line corresponds to simulated pattern for the Nd 2 Fe 14 B-type structure. [1] Yu.S. Koshkid'ko et al. METAL 2014, Pages

8 2. Evolution of the phase composition of (Nd,Pr,Dy)-Fe-B along the texture axis magnetized to saturation (in a field of 2 T) field was switched off the magnetized samples were heated, rate of 5K/min in a magnetic field of 0.01T. after heating to 1000 K, the samples were cooled to room temperature at the same rate in a magnetic field of 0.01T. Thermal magnetic analysis of sample no.1-2 performed in a magnetic field of 0.01T (above room temperature). [1] Yu.S. Koshkid'ko et al. METAL 2014, Pages

9 2. Evolution of the phase composition of (Nd,Pr,Dy)-Fe-B two magnetic phases one of the phases is the main 2:14:1 hard magnetic phase with the T C close to that of the Nd 2 Fe 14 B compound, α-fe phase with the Curie temperature of 769 o C Thermal magnetic analysis of sample no.1-2 performed in a magnetic field of 0.01T. above room temperature. [1] Yu.S. Koshkid'ko et al. METAL 2014, Pages

10 2. Evolution of the phase composition of (Nd,Pr,Dy)-Fe-B The maximum related to the spin-reorientation transition of the main hard magnetic phase (2:14:1) takes place at 84 K [1] The spin-reorientation transition of the Nd 2 Fe 14 B compound occurs at T SR =135 K [2]. Decrease in the spin-reorientation transition is related to the substitution of other REMs (Dy, Pr) for Nd in the magnetic sublattice. No anomaly is observed at temperatures corresponding to the T C of the Nd 1,1 Fe 4 B 4 phase,t C =13 K [3]. TMA below room temperature for sample no. 3 measured in a field of 0.01 T. The curves correspond to the heating of sample; bottom curve is the enlarged lowtemperature portion of the curve. [1] Yu.S. Koshkid'ko et al. METAL 2014, Pages [2] HERBST J.F. Reviews of Modern Physics V P [3] A. BEZINGE, H.F. BRAUN, J. MULLER, AND K. YVON. Solid State Communications. Vol.55,No.2, pp ,1985 8

11 2. Evolution of the phase composition of (Nd,Pr,Dy)-Fe-B 9 Summary optimal conditions of heat treatment: No.1; 1-h holding at 550 o C followed by quenching in gaseous nitrogen, magnets exhibit high magnetic properties: B r = 1.29 T, jh c = 1309 ka/m, H k = 1220 ka/m, (BH) max = 322 kj/m 3. The phase composition of the magnets: the principal hard magnetic phase based on the Nd 2 Fe 14 B compound, (Nd,Pr,Dy)-rich phase and (Nd,Pr,Dy)-O oxides.

12 3. Influence and optimization of heat treatment 10 Problem - during the milling process partial oxidation of the phase rich in rare earth metals can occur. This may lead to a deterioration of sintering process of the liquidphase due to the reduced amount of liquid phase decrease of the coercive force Fe 67.05; Nd 19.50; B - 1; Dy 6.00; Pr 6.00; Cu 0.15; Al 0.30 (wt.%) in the form of strips was prepared from flakes ~0.3 mm thick by strip-casting procedure. The next step hydrogen decrepitation were carried out for 1-2 h at a dry hydrogen pressure of 1 atm; the subsequent cooling was realized at room temperature in a nitrogen atmosphere Neodymium hydride (2.5 wt%) was added at the grinding stage, in order to improve sintering of the liquid-phase, powder particle size was 3-4 μm. The pressing force was t/cm 2 at a magnetic field of 2 T (pressing of parts in transverse field (TP)). The sintering was carried out at 1080 o C for 2 h. In search of the optimum conditions of heat-treatment, annealing was performed progressively: No o C No o C No o C (1 h each ) + quenching in gaseous nitrogen [1] K. SKOTNICOVÁ et al. METABK 55(4) (2016)

13 3. Influence and optimization of heat treatment No 1 - Nd 2 Fe 14 B phase No 2 and 3 - REM oxide phases in ternary junctions of samples. According to literature data [1-3], ternary junctions can contain different oxide phases : NdO (the oxygen content is 50 at.%), Nd 2 O 3 (the oxygen content is 60 at.%) NdO 2 (the oxygen content is 67 at.%). SEM image of N0 (a), N1 (b), N2 (c) and N3 (d) samples with marking of analyzed phases [1] KIM, T-H., LEE, S-R., NAMKUMG, S., et al.j. Alloys and Compounds, 2012,537, [2] WANG, S.C., LI, Y.. J.M.M.M. 2005, 285, [3] WATANABE, N., at al. Materials Science and Engineering, 2009,

14 3. Influence and optimization of heat treatment 2Θ = 35.59; and deg. main reflections of the measured pattern correspond to the Nd 2 Fe 14 B some reflections are likely to belong to other phases : - NdO with the NaCl-type structure (space group Fm- 3m) (the oxygen content is 50 at.%), - Nd 2 O 3 with the La 2 O 3 -type structure (space group Pm- 3m) (the oxygen content is 60 at.%) - NdO 2 with the CaF 2 -type structure (space group Fm- 3m) (the oxygen content is 67 at.%) X-ray diffraction pattern for the sample N 0-3 (λ= nm). Experimental - (colored lines) and pattern simulated for the Nd 2 Fe 14 B-type structure (space group P42/mnm) (black lines). 12

15 3. Influence and optimization of heat treatment 13 Magnetic properties of the sample of the permanent magnet N1 at room temperature: a) demagnetization curves, b) the dependence of B on BH. Magnetic properties of the permanent magnet N1 (heat treatment at 500 o C).

16 3. Influence and optimization of heat treatment so-called star-like domains and it is characteristic for uniaxial materials observed on the basal plane of single crystals Nd 2 Fe 14 B [1,2] observed type of domain structure indicates good magnetic texture of a permanent magnet. The specimen surface of a permanent magnet N1observed in a polarized light. Red squares highlight the areas with a magnetic domain structure. Magnetic field 0.07 mt. [1] PASTUSHENKOV, Yu.G., FORKL A., KRONMÜLLER H.J.M.M.M1997, 174, 3, [2] SUBKOVA, A.V., ZEZIULINA, et al. Solid State Phen. 2011, 168,

17 3. Influence and optimization of heat treatment 15 Summary The process has optimized sintering of the liquid-phase by adding neodymium hydride. Optimal modes of heat treatment were determined. It was possible to increase the coercivity force of permanent magnets by forming continuous grain boundaries. As a result of the experiment the samples were obtained of the highcoercivity magnet having the following magnetic properties: H CJ = 1470 ka/m, B r = 1.1 T and (BH) max = 231 kj/m 3.

18 3. Influence and optimization of heat treatment Nd-Fe-B-based magnetic materials attracted high interest due to their possible application even at cryogenic temperatures [1]. limited by the existence of spin reorientation transition (SRT) at 135 K, which is accompanied by a decrease in hysteretic characteristics of magnets [2]. Magnetic anisotropy field : Pr 2 Fe 14 B - 7,2 MA/m Nd 2 Fe 14 B - 5,6 MA/m Crystal structure : tetragonal with a strong magnetocrystalline anisotropy oriented along the crystallographic c axis [001]. Saturation magnetization : Pr 2 Fe 14 B 1.5T Nd 2 Fe 14 B 1.6T [1] Substitution of Pr for Nd in (Nd 1-x Pr x ) 2 Fe 14 B allows to decrease T SR [1] C. Benabderrahmane, et al. Nuc. Ins. and Meth. in Phys. Research A 669 (2012) 1-6 [2]J. F. Herbst, Reviews of Modern Physics 63 (1991) DOI: /RevModPhys [3] G. Marusi, et al. J. Phys: Condens. Matter 2 (1990) The compositional dependence of the spin reorientation temperature for (Nd 1-x Pr x ) 2 Fe 14 B [3] 16

19 3. Influence and optimization of heat treatment [1] K. SKOTNICOVÁ et al. METABK 55(4) (2016) 17 After sintered in vacuum at T from 1080 to 1000 o C, the following heat treatments were performed [1] : N1-500 С, 2 h С, 0,5 h С, 20 h N2, N4-500 С, 2 h N3-500 С, 2 h С, 2 h The chemical composition of sintered magnets in / wt. %. Hysteretic properties of magnets subjected to different heat treatments.

20 3. Influence and optimization of heat treatment [1] K. SKOTNICOVÁ et al. METABK 55(4) (2016) 18 No 1- main magnetic (Nd,Pr) 2 Fe 14 B phase No 3 (Nd,Pr)rich phase observed at grain boundaries No 4 - Nd(Pr)O, Nd(Pr)O 2 or (Nd,Pr) 2 O 3 oxides present at grain triple Junctions No 5 - Ti-based phase SEM image of the microstructure of sintered magnets with 10 wt. % Pr (Nd 0.7 Pr 0.3 ) 2 Fe 14 B.

21 3. Influence and optimization of heat treatment According to the EDX microanalysis the composition of the matrix grains corresponds to (Pr 0.3 Nd 0.7 ) 2 Fe 14 B - 10 wt. % Pr (Pr 0.4 Nd 0.6 ) 2 Fe 14 B - 13 wt. % Pr it is typical of the basal plane of uniaxial hard magnetic materials [1]. this fact indicates the adequate magnetic texture of the magnets Domain structure of the magnet with 10 wt. % Pr (in the thermally demagnetized state) observed on the surface perpendicular to the magnet texture [2]. [1] Yu.G. Pastushenkov, et al., J.M.M.M 174 (1997) [2] K. SKOTNICOVÁ et al. METABK 55(4) (2016) 19

22 3. Influence and optimization of heat treatment [1] K. SKOTNICOVÁ et al. METABK 55(4) (2016) 20 VSM method Heat treatment : 500 o C, 2h spin-reorientation transition temperatures agree well with those available in the literature. The temperature dependence of magnetization measured in a magnetic field of 16 ka/m

23 3. Influence and optimization of heat treatment 21 Summary The stoichiometric composition of the matrix grains in the (Nd, Pr)-Fe- B magnets with 10 and 13 wt. % Pr corresponds to (Pr 0.3 Nd 0.7 ) 2 Fe 14 B and (Pr 0.4 Nd 0.6 ) 2 Fe 14 B compounds. Conducted thermomagnetic analysis to samples of these magnets showed the presence of spin-reorientation transition in temperature 95 and 75 K. (Pr 0.4 Nd 0.6 ) 2 Fe 14 B composition has the spin-reorientation transition temperature close to the liquid-nitrogen boiling temperature and it can be recommended for low-temperature applications. The heat treatment of the magnets for the high-coercivity state consists in the 2-h vacuum annealing at 500 C.

24 4. The physical properties of RNi 2 compounds; a real candidates for a practical cryogenic application 22 The purpose of this work is to experimentally identify regularities of variations of the magnetocaloric effect (MCE) value in high external magnetic field ( H) in the region of the magnetic phase transition temperature. Demonstrate experimentally the theoretical concept on the high magnetic field dependence of MCE for RNi 2 second-ordered phase transitions compounds in the low-temperature range

25 4. The physical properties of RNi 2 compounds; a real candidates for a practical cryogenic application 23 The X-ray diffraction patterns of RNi 2 ( R = Tb, Dy, Ho, Er), theoretical bar diagrams calculated for the ideal cubic C15 structure (Fd3m s.g.) red line. At room temperature the principal reflections of the measured pattern correspond to those expected for the C15- type structure (Fd3m space group)

26 4. The physical properties of RNi 2 compounds; a real candidates for a practical cryogenic application 24 ΔT ad = 8 K ΔT ad = 12 K Temperature dependence of the adiabatic temperature change ΔT ad in TbNi 2 (a) and DyNi 2 (b) for magnetic field change listed. The solid lines represent theoretical results and the filled symbols refer to the experimental data.

27 4. The physical properties of RNi 2 compounds; a real candidates for a practical cryogenic application 25 ΔT ad = 14 K ΔT ad = 10 K Temperature dependence of the adiabatic temperature change ΔT ad in HoNi 2 and ErNi 2 for magnetic field change listed.

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