HDDR process

Hydrogen decomposition desorption recombination (HDDR) process is the only top-down industrial process used for the preparation of coercive nanoparticles. This process applied to rare-earth transition-metal (RE-TM) alloys consists in heating the concerned alloy under hydrogen until it decomposes into a fine mixture of RE-hydride and TM. The hard magnetic phase is recombined with a much finer microstructure. This process was first developed to convert 100 microns sized cast Nd2Fei4B grains into 200-300 nm crystallites [18, 19]. Later, it has been applied to other RE-TM alloys [20, 21]. Recently, a new variation of this process has been developed towards developing texture in the final materials [22], It is briefly described below. 

The highest remanence available is achieved in Nd2Fei4B/a-Fe nanocomposite materials with typical grain size 20 - 30 nm jU()Mr = 0.98 T, juoHc = 1.1 T, and BHW = 127 kJ/m3 [133], However, due to the difficulty in material processing, the commercial development of these materials remains very limited. Anisotropic NdFeB powders are produced by the HDDR process. Nanosize grains form within micron sized powder particles and they are therefore protected against corrosion. Thanks to the induced texture... 

They found that this treatment gave a disproportionated phase with a coarse lamellar structure. They believed that the appearance of anisotropy after HDDR processing via this reaction path should relate to this anisotropic microstructure. 

This model involves a kinetic selection of phases which appear when the reaction front moves at a velocity in a particular range. The t-FeaB has crystallographic coherence with Nd2Fei4B microspheres embedded in the FeaB. These residual Nd2Fei4B microspheres survive through the entire HDDR process and carry the orientational information. 

From the discussion by Tomida et al. (1999), it is naturally concluded that HDDR processing of lanthanide-transition-metal alloys without boron, such as Sm2Fei7, should always yield isotropic materials because of the absence of the Fe-B phases, in which the nuclei of the main phase continuously exist during HDDR process without changing their initial crystal orientations. 

As a conclusion of the above-mentioned discussions, clearly more work needs to be done in the future concerning the origin of the anisotropy in lanthanide permanent magnets during HDDR processing. 

Fig. 20. The effect of Dy addition on the temperature dependence of coerciv-ities in Nd Fe-Co-Ga-Zr-B magnets prepared by the HDDR process (Ikegami et al. 1998).

The most important reactions in the HDDR method are presented in fig. 14 (Nakamura et al. 1999). In the case of Nd2Fei4B, hydrogen absorption starts at around 923 K (Sugimoto et al. 1997). This is the start of the disproportionation process which occurs according to the following relation (Book and Harris 1995) ... 

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