Rare-Earth Metals H2

Introduction

 Currently, permanent magnets made from Nd₂Fe₁₄B type alloys possess the best magnetic properties with maximum magnetic energies (BH)_max reaching up to 50 MGOe (400 kJ/m³) and extremal values of coercive force and remanence [1]. Permanent magnets made from these alloys are widely used in many civilian applications including electronics, automotive industry, radio engineering and automation. e.g. for the small motors in CD and DVD-ROM drives, zoom lenses, camcorders etc. Improvement of the properties of the magnets will permit miniaturization of the user end products and therefore constitutes a significant advance in engineering.

 There have been several methods proposed to improve the magnetic properties such as the powder metallurgy [2], rapid quenching [3], mechanical alloying [4], hot working [5], sintering, etc. (see Fig. 1 [6]). However, these methods have their limitations. For instance, the magnets produced by pulverizing the cast ingot and then sintering did not show sufficient coercivities [7-8]. Moreover, most of above-mentioned methods led to processing costincrease of permanent magnets. For solution of this problem in last three decades have been proposed some new approaches. For instance, in 1978 by Harris et al. [9] at first was proposed method of hydrogen treatment of hardmagnetic alloys so-called

       Figure 1. The main methods permanent magnets production (after [6]).

Hydrogen Decrepitation (HD) process.Hydrogen Decrepitation of Nd-Fe-B type magnets is a two stage process. Firstly the hydrogen reacts with the Nd-rich phase at room temperature and then at temperature increasing up ~ 160^oC lead to second stage evolution in which the hydrogen is absorbed by Nd₂Fe₁₄B matrix phase. In both cases cracking occurs due to differential lattice expansion and the resulting HD treated alloy is extremely friable. This allows milling times to be reduced considerable and this is the main factor which reduces the overall processing cost of permanent magnet manufacture. Thus, using of the HD process led to increase of permanent magnets coercivity and also led to considerable decreasing processing cost of permanent magnets on 16-25% [10].

 The recently developed in 1989 by Takeshita and Nakayama [11-12] HDDR-process (Hydrogenation- Decomposition-Desorption-Recombination) as a new method of hydrogen treatment of hard magnetic alloys is very promising from this viewpoint [13-15]. This HDDR-process provides a new strategy to improve the properties of permanent magnets via hydrogen-induced phase transformations in Nd₂Fe₁₄B powders. It was found that Nd₂Fe₁₄B magnets produced by HDDR-treatment posses (BH)_max more 8-10% than those by mechanical alloyed magnets [16]. Moreover, currently the HDDR-process is the unique method for production of Nd-Fe-B type anisotropic powders for permanent magnets (see Fig. 1) [17-19].

1. Principles of HDDR process and HIDP-transformations

  Earlier from the viewpoint of Chemistry the conventional HDDR process in Nd₂Fe₁₄B hard magnetic alloys consists 4 of the following main stages: Hydrogenation, Decomposition, Desorption, Recombination (see Fig. 2a). As a rule, on first stage alloy absorbs hydrogen at room temperature (Hydrogenation) and then on second stage at increasing of temperature up 750-900° C occurs process of initial Nd₂Fe₁₄B alloy decomposition on NdH₂, and Fe₂B phases (Decomposition). Subsequent hydrogen evacuation from decomposed alloy (Desorption) leads to development of recombination process of decomposed phases into initial Nd₂Fe₁₄B phase (Recombination).

 The main microstructural change during these HIDP-transformations is the conversion from coarse grains into sub-micron grains (about 0.3-0.5 size, i.e. comparable with the size of single magnetic domains [7-11]) whereby the magnetic characteristics are expected to be significantly improved. This hydrogen-vacuum treatment is perfectly suitable for producing powder samples with extremely high coercivity, which can be used for producing the Nd-Fe-B base magnets by hot high-pressure and bonding.

  On the other hand, from the viewpoint of the Solid-State Physics and Material Science, the HDDR process is based on the following direct and reverse Hydrogen-Induced Diffusive Phase (HIDP) transformations [20-21] (see Fig. 1b).

(1) Direct HIDP-transformation: in a hydrogen atmosphere (~ 0.1 MPa) at temperature range of 600-900, the hydrogen-induced direct phase transformation (decomposition) occurs by the following chemical reaction [21]:

                                          (1)

(2) Reverse HIDP-transformation: the reverse phase transformation (recombination) occurs upon hydrogen evacuation at ~ 10^-2 Torr [21]:

                             (2)

    Moreover, the treatment scheme in the case of HIDP-transformations (see Fig. 2b) which at first was proposed by Rybalka et. al. [21] is principally different from conventional HDDR treatment scheme (see Fig. 2a). In this case, the Nd₂Fe₁₄B alloy is heated up in vacuum to high temperatures first (stage I on Fig. 2b), and then at this constant temperature the reaction chamber is filled with hydrogen to develop the direct HIDP-transformation (stage II on Fig. 2b). Afterwards, when the direct HIDP-transformation is done the hydrogen is evacuated from chamber, which leads to the development of reverse HIDP-transformation (stage III on Fig. 2b). Finally, when reverse HIDP-transformation development is finished the Nd₂Fe₁₄B alloy powder cooled down to room temperature in vacuum (stage IV on Fig. 2b).

Figure 2. a) Scheme of conventional HDDR-process (after Takeshita [6]); b) Scheme of hydrogen treatment based on HIPD-transformations (after Rybalka et al. [21]).

 It has been established that the magnetic properties of permanent magnets strongly depend on their microstuctural features [1]. For instance, the coercive force of permanent magnets is a function of microstuctural features, such as the average grain size of hard magnetic phase, grain size distribution, degree of magnetic isolation of hard magnetic phase etc. [1]. However, the microstuctural features depend on the processing conditions. For instance, as shown in Fig. 3, the permanent magnets’ coercivity is a function of such important kinetic parameters as the transformation time and temperature during the reverse HIDP-transformation in Nd₂Fe₁₄B type alloy [6,22]. This phenomenon of coercivity decrease might be resulting from the abnormal grains growth processes in case if hydrogen-vacuum treatment has been carried out after full finishing of HIDP-transformation in Nd₂Fe₁₄B type alloy. If the hydrogen-vacuum treatment in Nd₂Fe₁₄B type alloy carry out without taking into account the kinetic features, such as the hydrogen pressure, temperature and transformation time according to conventional HDDR scheme treatment (see Fig. 2a) it can lead to the abnormal grains growth of hard magnetic phase (Ф on Fig. 2a) up to tens-hundreds [23].

 Thus, this kinetic factor is one of the main factors that lead to coercivity decrease in Nd₂Fe₁₄B type permanent magnets.                                                                                         Back to Top

 In contrast to the conventional HDDR scheme treatment, if hydrogen-vacuum treatment is carried out in accordance with the HIDP-transformation scheme based on T-T-T (Temperature-Transformation-Time) isothermal kinetic diagrams in Nd₂Fe₁₄B type alloy (II and III on Fig. 2b) it leads to homogeneous microstructure of Nd₂Fe₁₄B alloy with sub-micron grains of Nd₂Fe₁₄B hard magnetic phase (Ф on Fig. 2b) and high degree of its magnetic isolation [24,25]. This fact is prerequisite for improving permanent magnets coercivity [1] made from powder treated by this way.

    Figure 3. Permanent magnets coercivity dependence on temperature and transformation time exposure during reverse HIDP-transformation in Nd₂Fe₁₄B type alloy: a) Takeshita's data [6], b) Liesert's data [22].

  So far, researches on the hydrogen-induced phase transformations in Nd₂Fe₁₄B type alloy earlier was limited in the following aspects:

(1) After the classical work by Davenport and Bain on ferrous alloys [26], it has been well accepted that the isothermal kinetic diagrams are the primary bases for heat treatment of steels, alloys and compounds. For the hydrogen-induced phase transformations in Nd₂Fe₁₄B type alloy, however, there is little data on these critical process parameters. Most of the published hydrogen-vacuum experiments conducted to date have been carried out empirically without taking into account the kinetic features, such as the hydrogen pressure, temperature and transformation time and it led to abnormal growth grains processes and coercivity decrease of permanent magnets.

However, our group has been demonstrated the possibilities of constructing of the T-T-T isothermal kinetic diagrams hydrogen-induced phase transformations (HIDP-transformations) in Nd₂Fe₁₄B type alloy and also was shown that kinetics of these type transformations is strongly depend from temperature treatment and hydrogen pressure [21,27-31]. Moreover, preliminary experiments based on the T-T-T kinetic diagrams lead to microstructure homogenization of Nd₂Fe₁₄B alloy [32] and submicron grains of Nd₂Fe₁₄B hard magnetic phase (0.3-0.8 ) with high degree of its magnetic isolation [24,25].

(2) There has been very little correlation between microstructural changes and processing conditions for the HIDP-transformations.

(3) As a rule, most of the experiments were carried out on laboratory samples without any real world applications.

3. Conclusions

Thus, we believe that in future a systematic investigation of the HIDP-transformation kinetics and microstructure features in Nd₂Fe₁₄B industrial alloy based on treatment scheme according Fig. 2b may lead to a new improved technology of treatment of Nd₂Fe₁₄B hard magnetic alloys, which in future may be extended on many others classes of hard magnetic materials.

1. P. Campbell, Permanent magnet materials and their application, Cambridge University Press, 1994.

2. M. Sagawa, S. Fujimura, N. Togawa et al., J. Appl. Phys., 1984, 55, 2083.

3. J.J. Croat, J.F. Herbst, R.W. Lee et al., J. Appl. Phys., 1984, 55, 2078.

4. L. Schultz, J. Wecker, E. Hellstern, J. Appl. Phys., 1987, 61, 3583.

5. R.W. Lee, Appl. Phys. Lett., 1985, 46, 790.

6. T. Takeshita, J. Alloys Comp., 1995, 231, 51.

7. H.H. Stademaier, N.C. Lui, Matter. Lett., 1986, 4, 304.

8. C.R. Paik, H. Mino, M. Okada, H. Homma, IEEE Trans. Mag. Magn., 1987, MAG-23, 2512.

9. Harris I.R., McGuiness P.J. Proc. XI Int. Workshop on Rare-Earth Magnets and Their Applications, Pittsburg, 1990, 29.

10. O.M. Ragg, G. Keegan et. al., Int. J. Hydrogen Energy, 1997, 22, 333.

11. Takeshita T., Nakayama R. Proc. X Int. Workshop on Rare-Earth Magnets and Their Applications, Kyoto, 1989, 551.

12. Takeshita T., Nakayama R. Proc. XI Int. Workshop on Rare-Earth Magnets and Their Applications, Pittsburg, 1990, 49.

13. X.J. Zhang, P.J. McGuiness, I.R. Harris, J. Appl. Phys., 1990, 69, 5838.

14. T. Takeshita, J. Alloys Comp., 1993, 193, 231.

15. O. Gutfleisch, I.R. Harris, Proc. XV Int. Workshop on Rare-Earth Magnets and Their Applications, Dresden, 1998, 487.

16. V.N. Verbetsky, Doctoral thesis, Moscow State University, Moscow, 1998.

17. R. Nakayama, T. Takeshita, M. Itakura et al., J. Appl. Phys., 1994, 76, 412.

18. T. Takeshita, K. Morimoto, J. Appl. Phys., 1996, 79, 5040.

19. T. Takeshita, R. Nakayama, IEEE Trans. J. Magn. Jap., 1993, 8, 692.

20. V.A. Goltsov, J. Alloys Comp., 1999, 293-295, 844.

21. Rybalka S. B., Goltsov V. A., Didus V. A. and Fruchart D. J. Alloys Comp., 2003, 356-357, 386.

22. Liesert S. These docteur de physique, CNRS, Grenoble, 1998.

23. E. Estevez, J. Fidler, C. Short, I.R. Harris, J. Phys. D: Appl. Phys, 1996, 29, 951.

24. Rybalka S. B., unpublished data.

25. Krayushkina E.Yu., Rybalka S.B., Yurasova V.Yu. Formation of ultrafine nanocrystalline microstructure during hydrogen-induced reversible phase transformations in Nd₂Fe₁₄B type  magnetic alloy // IHISM'15 Junior: Proc. X International School for young scientists named after A.A. Kyrdumov 'Interaction of Hydrogen isotopes with structural Materials', Sarov, Russia. — 2016. — P. 108-118.

26. Davenport E.S., Bain E.S. AIME Technical Publications , 1930, 90 (348), 117.

27. V.A. Goltsov, S.B. Rybalka, V.A. Didus, D. Fruchart, Progress in hydrogen treatment of materials, ed. by V.A. Goltsov, Coral Gables, 2001, 367.

28. Didus V. A. Rybalka S. B., Fruchart D. and Goltsov V. A. J. Alloys Comp., 2003, 356-357, 390.

29. V.A. Goltsov, S.B. Rybalka, A.F. Volkov, Int. J. Hydrogen Energy, 1999, 24, 913.

30. V.A. Goltsov, S.B. Rybalka, A.F. Volkov et al., Metal Physics and Advanced Technologies, 1999, 21, 22.

31. V.A. Goltsov, S.B. Rybalka, A.F Volkov et al., The Physics of Metals and Metallography, 2000, 89, 363.

32. A.A. Didus, S.B.Rybalka, V.A. Didus, A.F.Volkov, Bull. DonGASA, 2001, 3, 15.

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