HYDROGEN DECREPITATION (HD), HISTORY AND APPLICATIONS IN THE PRODUCTION OF PERMANENT MAGNETS

Size: px

Start display at page:

Download "HYDROGEN DECREPITATION (HD), HISTORY AND APPLICATIONS IN THE PRODUCTION OF PERMANENT MAGNETS"

Barnaby Small
6 years ago
Views:

1 International Journal of Advanced Research in Engineering and Technology (IJARET) Volume 7, Issue 5, September-October 2016, pp , Article ID: IJARET_07_05_005 Available online at ISSN Print: and ISSN Online: IAEME Publication HYDROGEN DECREPITATION (HD), HISTORY AND APPLICATIONS IN THE PRODUCTION OF PERMANENT MAGNETS Iessa Sabbe Moosa College of Engineering, University of Buraimi, Sultanate of Oman, Al- Buraimi. ABSTRACT An essential information about history and applications of Hydrogen Decrepitation has been generally reported. Very important technical details about the HD route were summarized, as well as fabrication procedure of Nd 16 Fe 76 B 8 permanent magnet. Some micro strucural of SEM images of hydride and sintered magnets were given to demonstrate the advantage of using the HD process in the production of Rare Earth magnets. In addition, recycling of Nd-Fe-B sintered magnets by employing the HD route has been investigated. This process is very important in the industrial sector for maintaining the environment and providing the raw materials for permanent magnets production with less cost. In reality, the HD method found to have the following advantages in the manufacturing of Rare Earth magnets; Simplicity, Reducing the sintering temperature, and Enhancing the coercivity of the produced magnets. Key words: Hydrogen Decrepitation, maximum energy product, magnet production, Recycling of Nd-Fe-B magnets. Cite this article: Iessa Sabbe Moosa, Hydrogen Decrepitation (HD), History and Applications in the Production of Permanent Magnets. International Journal of Advanced Research in Engineering and Technology, 7(5), 2016, pp HIGHLIGHTS Hydrogen Decrepitation (HD) method in the production of Rare Earth permanent magnets have been investigated in some details. Important technical information and micro structural features of some specimens using SEM have been given. Furthermore, recycling of Nd-Fe-B magnets was given an essential part in this article. 2. INTRODUCTION Magnetic materials play very essential role in the development of modern technology. The outstanding magnetic properties, low cost, and viability of iron made it possible to generate massive amounts of electricity worldwide. The use of magnetic materials to perform vital functions is not limited to electricity generation, but other industries consume appreciable amounts of magnets, such as electric machines, home appliances, communication fields, computers, wind energy field, etc. It is very well known that chemical composition, impurities, metallic and non- metallic inclusions, routes of production, grain size of starting powder, and heat treatment processes are important factors to control magnetic properties of the final 37 editor@iaeme.com

2 Iessa Sabbe Moosa products according to the required task. Hence powder metallurgy technique found to offer advantages in the fabrication process of permanent magnets [1]. Based on the above, all attempt of the workers in the field of permanent magnets focus their effort to get excellent magnetic properties, developing production routes, and discovering new magnetic materials that give required properties and abundant with low cost, unless otherwise specified. During the last 5 decades permanent magnets based upon Rare Earth elements have been widely developed. In 1967 Strnat et al. investigated several inter metallic phases of the type RCo 5 where R represents the following Sm, Y, Ce, Pr,or Y-rich, Ce-rich mischmetal [2]. Very big jump has been occurred when Das in 1969 declared a Samarium Cobalt magnet with a value of (BH) max of about 158 kj/m 3 using powder metallurgy route and magnetic alignment [3]. History and development of permanent magnets has been almost fully reported by the present author [4]. In 1978, Harris et al. patented the Hydrogen Decrepitation (HD) process as a mean of producing powder of SmCo 5 permanent magnet alloy [5]. After then it also applied to decrepitate Nd-Fe-B permanent magnet alloys as very easy and extremely important route to prepare the staring powder for magnets production [6,7]. The aim of this article is to give essential details related with the history and applications of the HD in the fabrication of Rare Earth Magnets either at the first run of fabrication or in the recycling process of these magnets. 3. WHAT IS HYDROGEN DECREPITATION Hydrogen Decrepitation (HD) is a route to crumble alloys which can react with hydrogen gas to form hydrides, and that rendered the materials extremely friable and therefore made them very easy to mill to the required particle size. Thus this method represents an important alternative mean of preparing the starting powder prior for any consequent research or manufacturing processes. During the HD process hydrogen reacts with the Rare Earth rich phases in the ingots of Rare Earth magnets to form hydride compound. Thus volume expansion takes place due to the formation of the hydride and the product is very brittle, so that gaining very fine powder by using any milling technique. More essential technical information about the HD will be given in this article. 4. APPLICATION OF HD ON SAMARIUM- COBALT ALLOYS As mentioned above, in 1978, Harris et al. patented the Hydrogen Decrepitation (HD) process as a new route of producing powder of SmCo 5 alloy. Large lumps can be crumbled up readily to obtain an extremely friable product which is consequently very amenable to further reduction in size according to the plan of research. This production route was again applied to decrepitate the Sm 2 (Co, Fe, Cu, Zr) 17 alloys [8, 9, 10]. For the duration of the 1960 s, the first generation of Rare Earth permanent magnets were announced by Strnat based upon Sm -Co [2]. The form of this new type magnets was SmCo 5 with high magnetocrystalline anisotropy and thus high coercivity compared with the previous types of magnets, such as magnetic steels, Al-Ni-Fe magnets, Alnico, and ceramics magnets. Production of Sm(Co,Fe,Cu,Zr) 7.1 permanent magnets using hydrogen decrepitation and ball milling has been reported by Kwon et al. [11]. Their conclusion was that this combination between HD and ball milling technique is very efficient to manufacture this sort of magnets. 5. DISCOVERY OF ND-FE-B MAGNETS Permanent magnets based upon Nd-Fe-B alloys first emerged during the middle of These magnets could be considered as the third generation of Rare Earth magnets. This subject has been comprehensively reported by Mitchell [12]. At the beginning, and independently, two methods were employed for fabrication of these magnets. The first is a conventional powder metallurgy route that was developed by Sagawa et al. at Sumitomo Special Company Ltd (SSMC) in Japan [13]. They discovered that the new sintered anisotropic magnets based on Nd, Fe, and a little amount of B had excellent magnetic properties, and the typical chemical formula was Nd 15 Fe 77 B 8. The second method of the manufacturing is rapid 38 editor@iaeme.com

3 Hydrogen Decrepitation (HD), History and Applications in the Production of Permanent Magnets solidification to get an isotropic permanent magnet, this route being developed by the General Motors Corp. in USA by Croat et al. [14]. The energy product (BH) max of the magnets fabricated by this technique was much less of those magnets produced by the conventional powder metallurgy method. Typical approximate values of characteristics reported for early magnets by these routes are given in Table1. Table1 Characteristics of magnets made by conventional and rapid solidification routes [13, 14] Properties Sagawa et al., 1984 [13] Croat et al., 1984 [14] Remanence (B r ) 1.23 T 0.82 T Intrinsic Coercivity (H ci ) 969 kam kam -1 Normal Coercivity (H cb ) 880 kam kam -1 Energy Product (BH) max 290 kj/m kj/m 3 Density 7.4 g/cm g/cm 3 Vickers hardness 600 HV 600 HV Curie Temperature (T C ) C C 6. APPLICATION OF HD ON ND-FE-B ALLOYS After the discovery of Nd-Fe-B Rare Earth magnets in 1983, the HD route was immediately re-applied on as-cast cast ingots of Nd 15 Fe 77 B 8 and related compositions. This great work was carried out in the School of Metallurgy and Materials at Birmingham University, UK [6].At the beginning, they showed that H 2 gas could fragment the as-cast alloys if the lumps of these ingots were exposed to hydrogen at a pressure of about 33 bar and ambient temperature. The Research Crew in this Department are pioneers of wide attempt of researches in this field worldwide till date, as exposed in these selected references [15, 16, 17, 18, 19, 20].Comparison between permanent magnets based on Nd-Fe-B alloys produced by the conventional process and the HD route have been reported by Moosa et al. [21]. Their important conclusions were that the HD improves the sintering process because of the volume fraction of the Nd-rich phase, enhances the coercivity, and reduces the sintering temperature below that implied with the conventional route. The two microstrucural types of sintered magnets are shown for comparison purposes in Figure1. (a) (b) Figure 1 High magnification SEI micrographs viewing the microstructure for sintered magnets produced by (a) HD route; (b) conventional route; A- Nd 2 Fe 14 B phase, B- B-rich phase; C- Nd-rich phase [21] editor@iaeme.com

4 Iessa Sabbe Moosa 7. MAGNET PRODUCTION BY USING THE HD ROUTE 7.1. Hydrogen Decrepitation Units There are two HD units, one of them for use at room temperature to decrepitate Nd 15 Fe 77 B 8 and near chemical composition alloys, while the other contained heating elements and cooling systems to enable the HD process to be carried out at elevated temperatures to decrepitate Nd 2 Fe 14 B alloy. Both units are made of stainless steel. An Argon inlet and vacuum outlet are incorporated for obtaining a low oxygen content atmosphere before starting the HD process.fig.2 illustrates the designed units. Figure 2 (a) Unit for HD at room temp., (b) Unit for HD with heating and cooling systems Hydrogen Decerpitation Process Lumps of the as-cast ingots of Nd 16 Fe 77 B 8 were broken into pieces about 1cm in diameter, and then put in a stainless steel basket as shown in Fig.2a. The basket was placed in the HD unit, and the unit was evacuated for about 20min using the rotary pump provided with unit. Following this process, high purity Argon gas was introduced into the vessel of the unit and evacuated again, this was repeated 4-5 times to create very clean atmosphere prior to the HD process. The broken lumps were exposed to hydrogen gas at ambient temperature for 1hour. A range of different pressures can be used starting with 1bar. The process generated heat, so that the hydride was always cooled down to room temperature before removal from the HD unit, and it was milled under cyclohexane by using planetary milling machine. The milling time depends on the required particle size according to the research plan. Particle size of about 3μm can be abstained with a pressure of 10 bar and milling time of about 10h.Fig.3 shows fracture surface and decrepitated surface with different hydrogen pressure editor@iaeme.com

$Hydrogen Decrepitation (HD), History and Applications in the Production of Permanent Magnets Figure 3 SEM micrographs of hydride, (a) fracture surface, 20 bar, (b) decrepitated polished surface, 4$ The hydride powder was placed in a flexible mould and this sealed in a flexible tube and then removed from the glove box prior to magnetic alignment.

The hydride powder was placed in a flexible mould and this sealed in a flexible tube and then removed from the glove box prior to magnetic alignment.

The particles were aligned in a magnetic field of about 800 kam -1 as shown in Figure 4. Figure 4 Magnetic alignment using a flexible bag filled with hydride powder.

5 Hydrogen Decrepitation (HD), History and Applications in the Production of Permanent Magnets Figure 3 SEM micrographs of hydride, (a) fracture surface, 20 bar, (b) decrepitated polished surface, 4 bar after 10 min Magnet Production After milling to get a certain particle size, the hydride powder was dried from cyclohexane in metallic chamber of glove box. The hydride powder was placed in a flexible mould and this sealed in a flexible tube and then removed from the glove box prior to magnetic alignment. Sometimes the product can be removed from the glove box whilst it is still wetted with cyclohexane to protect the hydride form oxidation in the subsequent presses. The particles were aligned in a magnetic field of about 800 kam -1 as shown in Figure 4. Figure 4 Magnetic alignment using a flexible bag filled with hydride powder. The sealed aligned powder was then is ostatically compacted using full automatic is ostatic press, with a pressure of about 200MPa. The green compact was removed from the mould prior to sintering. The specimen was gradually heated to C in vacuum at about 10-6 torr to remove the dissolved hydrogen. The temperature was then raised to about C for 1h, and the specimen was slowly cooled to room temperature under vacuum. Following the sintering, the specimen was given a post-sintering treatment under vacuum at about C for different time and then rapidly cooled to investigate the effect of this process on magnetic properties. After sintering, the specimen was cut using an Isomet low speed diamond saw, to produce a pair of parallel faces. The specimen was then mounted using cold resin and faced by emery paper grade After facing, magnetization was carried out using a pulse magnetizer to get a permanent magnet editor@iaeme.com

6 Iessa Sabbe Moosa The magnet was confined between the magnetic poles of an electromagnet to find the demagnetization curve in the second quadrant. The values of magnetic properties such as B r, H ci, H cb,and (BH) max were printed out. These measurements were made on equipment in the research group headed by professor I. R. Harris in the Department of Metallurgy at the University of Birmingham during the Ph. D study of the author [7]. Magnetic properties and density of a magnet of Nd 16 Fe 76 B 8 are given in Table 2. Table 2 Magnetic properties and density of produced magnet from Nd 16 Fe 76 B 8. Properties Values Condition Remanence (B r ) 1.3 T --- Intrinsic Coercivity (H ci ) 755 kam -1 As-sintered, ~ C Normal Coercivity (H cb ) 1030 kam -1 Post-sintered, C, Ih, rapid cool, Maximum Energy Product (BH) max 310 kj/m 3 particle size ~ 6μm Density 7.45 g/cm RECYCLING OF ND-FE-B MAGNETS Nd-Fe-B permanent magnets are widely used in many of imperative applications, such as electric motors for multi functions, computer hard drives, and generators in gearless wind mill. Recycling is very important in all industrial fields for two reasons, the first is to maintain the environment and the second is to provide the raw materials without relying on original source sin the manufacturing process, which probably reduces the cost of production. The recycling of Rare Earth magnets can reduce the total amount of Rare Earth elements that need for magnets production. A crucial review about recycling of Rare Earths has been reported by Binnemans et al. [22]. They concluded that recycling and re-use of metals is very necessity for a resource efficient economy, and their review showed that during the last years, there has been an intensification of research activities for the development of effective recycling routes for Rare Earths. Recycling of Nd-Fe-B sintered magnets has been investigated fully by many workers in this filed [23, 24, 25, 26]. In 2014, Sprecher et al. investigated the energy and human toxicity cost related with the recycling of Nd-Fe-B permanent magnets from electronics waste using the HD. Their essential comment was that the using of hydrogen assisted process used 88% less energy compared to the primary production process and scored 98% lower on the human toxicity baseline [27]. An extremely important detail associated with the Processing and Characterization of Recycled Nd-Fe-B Sintered Magnets has been reported by Malik [28]. Recently, the use of HD route for separating and recycling of Nd-Fe-B magnets has been studied by What on et al. [29]. They reported that, by the re-sintering process, it was possible to recover greater than 90% of the magnetic properties of the starting material, with significantly less energy that needed in primary magnet fabrication. Large scale recycling of Nd-Fe-B permanent magnets has been investigated by employing the HD technique [30]. 9. CONCLUSION History and applications of hydrogen decrepitation on Rare Earth alloys have been investigated. The most important conclusion was that the process can be carried out at room temperature or with elevated temperature of around C for some alloys, producing extremely friable hydride. From the production point of view, the HD route was found to have the following advantages over the conventional technique: Simplicity in the preparing of the starting powder. It enables the sintering temperature to be reduced editor@iaeme.com

7 Hydrogen Decrepitation (HD), History and Applications in the Production of Permanent Magnets The coercivity can be enhanced because of uniformity distribution of the Rare Earth rich- phase over the grain boundaries. In addition, the post-sintering was found to be very effective in improving the H ci values. Furthermore, the HD process has been intensively applied in the recycling of the sintered Rare Earth magnets, and hence reducing the cost of production and sustaining ecological balance. ACKNOWLEDGEMENTS The author would like to thank Professor I. R. Harris of School of Metallurgy and Materials, University of Birmingham for helpful discussions and the provision of facilities for making the magnetic measurements during the period of Ph. D study of the author. I should also like to thank Mr. Kalyan Baddipudi of the UoB for proofreading. REFERENCE [1] Iessa Sabbe MOOSA, Powder Metallurgy and its Application in the Production of Permanent Magnets. International Journal of Advanced Research in Engineering and Technology (IJARET), 4(6), 2013, pp [2] Strnat K. J., Hoffer G., Olson J., Ostertag W. and Becker J. J., J. Appl. Phys., 38, (1967), pp [3] Das D. K., IEEE Trans. Magn., Magn-5, (1969), pp [4] Moosa I. S., International Journal for Research & Development in Technology, Volume: 2, Issue: 1, (JULY-2014), pp [5] Harris I. R., EVANS J., NYHOLMP. S., UK Patent (October 1979). [6] HarrisI. R, Noble C., and Bailey T., J. Less Common Met., 106, (1985), L1-L4. [7] Moosa I. S., Ph. D Thesis" The Production, Properties and Microstructures of Nd-Fe-B Magnets", (1991), The University of Leeds. [8] Abbas Kianvash, Harris I. R., journal of Materials Science, Vol. 20, issue 2, (1985), pp [9] Harris I. R., J. Less - Common Met., 131, (1987), pp [10] Harris I. R, Proc. 9 th int. workshop on Rare Earth Magnets and their applications, Aug. 31 Sept.2, and 5th Int.Symp. on magnetic anisotropy and coercivity in Rare Earth Transition metals alloys, Sept. 3, ed. C. Herget and R. Poerschke, Germany, (1987), pp [11] Kwon H.W., Harris I. R., J. Appl. Phys. 69, No. 8, (1991), pp [12] Mitchell I. V., " Nd-Fe permanent magnets, their present and future applications', (1984), Elsevier Applied Science Publishers, London and New York. [13] Sagawa M., Fujimura S., Togawa N., Yamamoto H., Matsuura Y., J. Appl. Phys., 55, (1984), pp [14] Croat J. J., Herbst J. F., Lee R. W., Pinkerton F. E., J. Appl. Phys., 55, (1984), pp [15] McGuiness P. J., Ahmed A., Jones D. G. R., Harris I.R., Burns S., Rozenddaal E., J. Appl. Phys., 67, (1990), pp [16] Harris I. R., McGuiness P.J., Less-Commom Met., , (1991), pp editor@iaeme.com

8 Iessa Sabbe Moosa [17] Ragg O. M., Harris I. R., J. Alloys Compd., 256 (1997), pp [18] Ragg O. M, Keegan G. Nagel H., and Harris I. R., Int. J. hydrogen Energy, No. 2/3, (1997), pp [19] Zakotnik A., Williams A. J., Martinek G., Harris I. R, J. Alloys Compd., 450, (2008), L1-L3. [20] Sheridan R. S., Sillitoe R., Zakotnik M., Harris I. R., Williams A. J., J. of Magn. and Magn. Mater., 324, (2012), pp [21] Moosa I. S., Johnson G. W., Nutting J., J. Less- Common Met., 158, (1990), pp [22] Binnemans K., Jones P. T., Blanpain B., Gerven T. V., Yang Y., Walton A., Buchert M., Journal of Cleaner Production, Vol. 51, (2013), pp [23] Abubakar Alkali and Edward Gobina, Hydrogen Permeation Behavior and Annealing in Composite Palladium Membranes at High Temperature. International Journal of Advanced Research in Engineering and Technology (IJARET), 5(4), 2014, pp [24] Masahiro I., Masahiro M., Shunji S., Ken-ichi M., J. Alloys Compd., 374, (2004), pp [25] Zakotnik M., Devlin E., and Harris I. R., Journal of Iron and Steel Research, Vol. 13, Supplement1, (2006), pp [26] Zakotnik M., Harris I. R, Williams A. J., J. Alloys Compd., 450, (2008), pp [27] Zakotnik M, Harris I. R, Williams A. J., J. Alloys Compd., 469, (2009), pp [28] Sprecher B., Xiao Y., Walton A., Speight J., Harris I. R., Kleijn R., Visser G., Kramer G. J., Environmental Science & Technology, 48 (7), (2014), pp [29] Malik John Jamaji Degri, Ph. D Thesis, The Processing and Characterisation of Recycled NdFeB-type Sintered Magnets", 2014, School of Metallurgy and Materials, College of Engineering and Physical Sciences, University of Birmingham. [30] Walton A., Han Yi, Rowson N. A., Speight J. D., Mann V. S. J., Sheridan R. S., Bradshaw A., Harris I. R., Williams A. J., Journal of Cleaner production, 104, (2015), pp [31] Zakotnik M., Tudor C.O., Waste Management, 44, (2015), pp editor@iaeme.com

Most Production routes of Nd-Fe-B permanent magnets

Most Production routes of Nd-Fe-B permanent magnets Iessa Sabbe Moosa, Chiraz ZIDI Abstract- A vital information about the most used production routes of Nd-Fe-B magnets has been reported. Very significant