Fines, coercive

Maeda, Y. (1978) High coercive y- Fe203 fine particles. The electronics and tele-communications laboratories, NNT. E.C.L. Techn. Publ. 

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. 

Recent research on more coercive media with a low noise ratio involved addition of Zn to the Co alloy system [76-79]. Addition of Zn to the cobalt alloy very effectively produces a film with a fine particle structure, which results from codeposition of elements which are hardly soluble in the matrix. Such codeposition causes segregation and hence produces a film microstructure consisting of fine particles. The fine particulate structure lowers the noise ratio and increases the coercivity of the medium. 

Asada, S., Preparation of fine acicular iron particles with high coercivity by reduction method, Nippon Kagaku Kaishi, 1985, 22, 1985. 

However, moderately fine powders of the copper-modified alloys, when properly heat treated, do not suffer the described aging loss of Hc nor do compacts made from them (Cremer et al. 1982, R.M.W. Strnat and Luo 1982). The reason is that their coercivity is caused by a homogeneous intra-granular precipitate that impedes the movement of domain walls wherever they are located. So, Hc does not strongly depend on the surface condition of these particles. If, however, the precipitate is dissolved by a homogenization heat-treatment, these alloys act just like SmCo5. This is also illustrated in fig. 14, showing early experimental results on a Sm-Co-Cu-Fe alloy near the 1-5 composition. It is equally true of the newer 2-17 alloys, which usually contain other minor elements, such as Zr, in addition to Cu. 

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