Sintered NdFeB Magnets as Anisotropic Bonded Magnets via Hydrogen Decrepitation Process

Abstract
The scrap sintered Nd–Fe–B magnets were recycled as the raw materials for bonded magnets using the hydrogen decrepitation (HD) process. The HD powders have the lowest oxygen and hydrogen content by hydro-genation at 150°C with 1 bar H2 pressure and dehydro-genation at 600°C. The powders with the largest particle size ([380lm) bear the best magnetic properties (Br=110.59emu/g, Hcj=9005.80Oe). The magnetic properties of the resulting bonded magnet prepared from the above powders were Br=7.02 kGs,Hcj=4.56 kOe,(BH)max=7.13 MGOe.

Keywords
Scrap Nd–Fe–B sintered magnet, Recycling, Hydrogen decrepitation, Bonded magnet

sintered Nd–Fe–B magnets anisotropic bonded magnet

sintered Nd–Fe–B magnets anisotropic bonded magnet

Introduction

The Nd–Fe–B sintered magnets have been developed quickly since 1984, and playing a key role in the related field, such as VCM devices, HEV, turbine motor, MRI etc.

By now, China is the biggest Nd–Fe–B production base in the world. However, the amounts of scraped products including leftover waste during machining process and the non-qualified products are above 25 % of the mass of the finished products. Besides, as the electronic information industry rapidly develops, more and more waste products which include much of magnets are built up, therefore it is necessary to take efficient process to recycle the scrap magnets.

A number of routes for recycling sintered Nd–Fe–B magnets have been proposed. Takeda investigated the possibility of extracting Nd metal directly from magnet scrap. Itoh recycled rare earth containing sintered magnets to make isotropic bonded magnets by melt spin-ning, and the values of recycling magnets were nearly comparable to the commercially available MQPB bonded magnet. Kwon studied on recycling sintered magnet scrap to prepare bonded magnet powders by adding several rare earth oxide and fluoride after HD process. Gutfleisch showed that anisotropic resin bonded magnets could be produced by recycling Nd–Fe–B sintered magnets using a combined HD and d-HDDR route. Perigo employed the HDDR process to recycle N42 sintered magnets for iso-tropic powders. Zakotnik pointed out many recycling ways to produce powder that would be suitable for the production of anisotropic bonded or hot pressed magnets.

This study concentrates on one method of recycling Nd–Fe–B sintered magnets; it starts with the use of the HD process to generate powder from scrap magnets. The HD powder is either processed into powder with high coer-civity or the powder is recycled to make anisotropy bonded magnets capable of being used in applications requiring lower costs.

Experimental methods
The scrap sintered Nd–Fe–B magnets are employed in the present study, and crashed by hydrogenation and dehydrogenation process. The oxygen and hydrogen contents of HD powder were measured. The powders from Nd–Fe–B scrap were sieved through different mesh number sieves (40, 80, 100, 200, 400 mesh number). The different particle size powders mixed with 2.5 % epoxy resin were pressed and aligned in the 3 T magnetic field, and then consolidated at 150°C for 10 min. The magnetization hysteresis curves of these powders were measured by vibrating sample magnetometer, respectively. The demagnetization curves of the bonded magnets were measured by BH tracer.

Results and discussion
It is necessary to limit the oxygen and hydrogen content of the powder by modifying the hydrogenation pressure and temperature in HD process. Figure1 shows the oxygen content of Nd–Fe–B powder in different HD process. The lowest oxygen content of powder (Fig.1b) was obtained under 150°C and 1 bar H2 pressure. However, the highest oxygen content of powder (Fig.1c) was obtained under room temperature and 3 bar H2 pressure which has been attributed that higher pressure caused stronger reaction and more amount of heat, so the powder was easy to be oxidized.
The influence of dehydrogenation temperature on the hydrogen content of powder obtained under 150°C and 1 bar H2 pressure was also evaluated. Figure2 shows the hydrogen content of powder at different dehydrogenation temperature. The hydrogen content of powder is more than 2500 ppm when the dehydrogenation temperature is at 550°C. As the dehydrogenation temperature increases to 600°C, the hydrogen content of powder decreases sharply to 66 ppm, which demonstrates that dehydrogenation process proceeds within 550–600°C. The hydrogen content of powder keeps stable with the increase of the dehydrogenation temperature.

The HD powders prepared by optimum hydrogen decrepitation process were sieved into different sizes and the distribution is expected, which is shown in Fig.3. The magnet was typically crushed along grain boundary through HD process. The oxygen contents of these powders were measured, respectively, and presented in Fig.4. The results show that the oxygen content of the powders was much different even if all the powders were made under the same processing technology. The oxygen content reaches 2700 ppm when the particle size is 5–10lm, and it decreases rapidly as particle size increases. Especially, this trend becomes slow with the particle size reaching approximately 150lm; this phenomenon is due to higher specific surface area of finer powder. Therefore, by controlling HD process, the large particle size powder with high coercivity and low oxygen content is obtained.

The magnetic property of different particle size powder prepared by optimum hydrogen decrepitation process is shown in Fig.5. As the particle size increases, the remanence augments gradually, peaks at approximately 160lm, and falls. The reduction of remanence is prob-ably attributed that single-crystalline powder particle transforms into polycrystalline particles when the particle size is bigger than approximately 160 lm. As the parti-cle size of HD powder decreases, the coercivity decreases monotonically due to the increase of the oxygen content. The coercivity could reach 9 kOe when the particle size is more than 380lm. Assuming the powder density is 7.5 g/m-3, the best properties with(BH)max=20.9 MGOe are expected.

Conclusions
Nd–Fe–B anisotropic bonded powder and magnets were prepared from recycling sintered magnets by HD and brief sieving process. The optimized hydrogen and dehydrogenation process are at 150°C with 1 bar H2pressure and 600°C, respectively. The coercivity of Nd–Fe–B powders with the particle size [380lm reaches 9005.8 Oe. The magnetic properties of the resulting bonded magnet prepared from the above powder were Br=7.02 kGs, Hcj=4.56 kOe, (BH)max=7.13 MGOe.
The advantage of recycling technology is simple and short for large-scale production. Besides, the properties of recycling bonded magnets are much better than those of any known ferrite.

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