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α-Fe nanocomposite magnets
Journal of Alloys and Compounds 349 (2003) 311–315
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Structure and magnetic properties of Nd 2 (Fe,Co,Al,Cr) 14 B / a-Fe nanocomposite magnets a, b J. Jakubowicz *, M. Giersig a
Institute of Materials Science and Engineering, Poznan University of Technology, M. Sklodowska-Curie 5 Sq, 60 -965 Poznan, Poland b Hahn-Meitner-Institut Berlin GmbH, Department of Solar Energy Research, Glienicker Str. 100, D-14109 Berlin, Germany Received 28 May 2002; received in revised form 25 June 2002; accepted 25 June 2002
Abstract The structure, magnetic properties and corrosion behaviour of two-phase nanocomposite Nd 2 (Fe,Co,Al,Cr) 14 B / a-Fe-type magnets, which have tetragonal / cubic Nd 2 Fe 14 B / a-Fe structure, have been investigated. The magnetic hardening was achieved by high-energy ball-milling (HEBM) of the Nd 2 Fe 14 B-type hard magnetic phase with different vol.% of a-Fe as soft magnetic phase, followed by annealing. Fully dense Nd 2 (Fe,Co,Al,Cr) 14 B / a-Fe type magnets have been produced by hot pressing. Magnets with good corrosion resistance as well as high temperature stability have been produced. The corrosion resistance is improved in the case of Co-, Al-, Cr-doped nanocomposite magnets with a volume fraction of the soft magnetic phase of 37.5 vol.%. Effective protection against corrosion was realised also by surface coating with Zn metal. 2002 Elsevier Science B.V. All rights reserved. Keywords: Permanent magnets; Nanostructured materials; Mechanical alloying; Corrosion; TEM
1. Introduction Permanent magnets based on Nd 2 Fe 14 B compound are widely used in many applications, e.g. motors, electric and computer peripherals. The main problems in NdFeB magnet applications are low temperature coefficients of the remanence a (Jr ) and coercivity b ( J Hc ), as well as low corrosion resistance (comparable with carbon steel). Commonly used sintered NdFeB magnets are composed of several phases: main F (Nd 2 Fe 14 B), h (Nd 11´ Fe 4 B 4 ) and intergranular Nd-rich (Nd 4 Fe) [1]. A high Nd content is necessary to obtain high coercivity, but it deteriorates the corrosion properties. The effect of corrosive environments results in deterioration of the structure and shape of the magnet and consequently its hard magnetic properties. Due to their low Curie temperature (T c 5585 K), the NdFeB magnets have also poor temperature characteristics of technical parameters such as remanence a (Jr ) and
*Corresponding author. Tel.: 148-61-835-1647; fax: 148-61-6653576. E-mail address: [email protected] (J. Jakubowicz).
coercivity b ( J Hc ). Overcoming these features is of primary importance from the applications point of view and also has economic aspects. To avoid the disadvantageous effect of corrosive environments on the magnet structure and its properties, two methods can be applied: (i) coating of the bulk magnet and (ii) alloy modifications. The first method is commonly used and coatings based on Zn, Sn, Al, Ni, Cr, or Cu provide good corrosion resistance. On the other hand, small additives such as Al, Cr, Ti, Ni or Zr not only improve the magnetic properties but also the corrosion resistance, too [2,3]. Recently, considerable attention has been focused on nanocomposite Nd 2 Fe 14 B / a-Fe magnets [4–6], because they have remanences Jr larger than the Stoner–Wohlfarth value Js / 2 (Js , saturation magnetic polarization) and maintain high coercivity. The nanocomposite magnets are composed of a hard phase (Nd 2 Fe 14 B) and a soft phase (a-Fe, Fe–Co or Fe 3 B). Between the nanometre grains (,100 nm) exchange coupling exists, leading to a reduced remanence Jr /Js of about 0.7 and to a higher Curie temperature. When the grain size decreases, a larger volume fraction of the grains comes under the influence of the exchange coupling. The structure plays an important role in magnetic and corrosion properties [7,8]. The main
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advantage of the nanocomposite magnets compared to sintered magnets is the lower content of Nd. It may be concluded that higher and lower content of a-Fe and Nd, respectively, should give better corrosion resistance and temperature stability with respect to sintered magnets. In this work the influence of alloy additives (Co, Al, Cr) and Zn coating on the structure, corrosion behaviour and temperature stability of Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe nanocomposite magnets was studied. Also, a HRTEM analysis was performed.
2. Experimental details Arc melting, followed by homogenization in an argon atmosphere at 1270 K was used to prepare the alloy specimens. Nd 2 Fe 14 B, Nd 2 Fe 12 Co 2 B and Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B-type alloy compositions were used as starting materials for the high-energy ball-milling (HEBM) process. A SPEX 8000 mixer mill was used for the HEBM processing. Powder handling to load the vial was carried out in a glove box with a high-purity argon atmosphere. The remanence-enhanced powders were prepared by HEBM of the rare-earth-containing Nd 2 Fe 14 B powder (precursory crushed to below 100 mm in size) and Fe powder (purity 99.9 at.%, particle size not above 10 mm) in a SPEX mill. The mill was run up to 48 h for every powder preparation. The as-milled powders were heattreated at 950 K for 30 min under high purity argon to form the Nd 2 Fe 14 B phase. Full density magnets were produced by hot pressing at temperatures of 1020 and 1070 K for times of 30 s. Structural characterizations were performed on an XRD diffractometer. The powders were examined at various stages during milling, prior to annealing and after annealing. The structural and chemical compositions were examined using a Philips Cm12 microscope, which was equipped with a 9800 EDAX analyser. The high resolution transmission electron microscopy (HRTEM) images were made under the conditions of minimum phase-contrast artefacts [9]. Axial illumination as well as a ‘nanoprobe mode’ were used for imaging. The latter mode allows the reduction of the beam spot size to 1 nm, enabling electron diffraction of individual small crystals. Magnetic characteristics were measured on a hysteresisgraph and SQUID magnetometer in external fields up to 2.5 MA m 21 . The corrosion resistance test of the nanocrystalline Nd 2 (Fe,Co,Al,Cr) 14 B / a-Fe-type magnets was carried out by corrosion potential measurements (3% NaCl water solution and 50 mV min 21 scanning speed) as well as in a humid atmosphere at 320 K. An efficient protection against corrosion of the nanocomposite magnets was studied through surface coating with a 10 mm layer of Zn.
3. Results and discussion
3.1. Structural properties For all samples, after 48 h high-energy ball milling, the alloy had decomposed into an amorphous phase and nanocrystalline a-Fe with a grain size of about 10–15 nm. Annealing the powder in high-purity argon at 950 K for 30 min results in the formation of the tetragonal Nd 2 Fe 14 Btype phase which coexists with the cubic a-Fe phase, resulting in a two-phase nanocomposite material. The grain size of the magnetically soft a-Fe phase after heat treatment increased to around 35–50 nm (Fig. 1). Magnets without additions (Fig. 1a) have a larger grain size; addition of AlCr suppresses an increase of the grain size (Fig. 1b). At the end of the milling period, a uniform mixing of Fe and Co will have occurred and on annealing the Co will partially substitute for the Fe in the Nd 2 Fe 14 B ¨ phase and also form a soft Fe–Co phase. Mossbauer study confirmed the presence of Co in both the Nd 2 Fe 14 B and a-Fe phases [10]. The bulk magnets were prepared by hot pressing at 1020 K and 1070 K for 30 s. The grain size after hot pressing increases up to 50–90 nm (Fig. 2). The EDAX spectrum confirms the presence of Fe, Co, Nd, B and Cr. Small amounts of Al were not detected. Contaminations such as C, O, Si were introduced during hot pressing in a graphite matrix covered by BN as well as during sample preparation for the TEM measurements. The smallest area for EDAX analysis was |100 nm and hence it was impossible
Fig. 1. The grain size of HEBM (a) Nd 2 Fe 12 Co 2 B / a-Fe and (b) Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe powders with a volume fraction of magnetically soft a-Fe phase of 10%, without annealing and after heat treatment at different temperatures.
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313
Fig. 3. HRTEM micrographs of the bulk Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 10 vol.% a-Fe nanocomposite hot pressed at 1070 K / 30 s.
3.2. Magnetic properties
Fig. 2. TEM micrograph (a) and EDAX spectrum (b) of the bulk Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 10 vol.% a-Fe nanocomposite hot pressed at 1070 K / 30 s.
to determine the exact composition of a single nanograin, because some of the neighbouring grains were also included. Hot pressed Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 10 vol.% a-Fe nanocomposite does not have a two-phase structure, but is a mixture of several phases (Fig. 3). Nd 2 Fe 14 B is the main phase with large grain size (,90 nm). Small amounts of the NdFe 4 B 4 ternary phase were formed also. A fine grained Fe 17 Nd 5 binary phase was found at the triple junctions of the larger grains. XRD analysis confirmed the presence of a-Fe, which is not shown in the HRTEM image. The multiphase structure can result from multistage processing which involves five energetic steps: arc melting, homogenisation, HEBM, heat treatment and hot pressing. The final compaction is important, because new phases can be formed or metastable phases can disappear.
All the powders prepared by the HEBM process are magnetically isotropic. The Nd 2 Fe 14 B nanocrystalline magnet without the soft phase (0% of a-Fe) has a reduced remanence of a 5 Jr /Js 5 0.59 and an intrinsic coercivity 21 of J Hc 5875 kA m . The principal advantages of Co addition to Nd 2 Fe 14 B / a-Fe are an increase in Curie temperature of the hard phase, and an increase in saturation polarization, arising from the higher saturation magnetization of Fe–Co, compared to that of pure a-Fe. The intrinsic coercivity decreases due to lower anisotropy of the Co-doped alloys. Small addition of Al and Cr introduced into the nanocomposites improved the magnetic properties (Table 1). Increase of the a-Fe content results in an increase of Js , Jr and a. Additionally the Curie temperature increases with increasing excess iron content in the nanocomposites. The J Hc maintains an acceptable value. HEBM and heat-treated NdFeCoAlCrB / a-Fe type powders have been compacted by hot pressing to form bulk magnets. Summaries of the magnetic properties of the produced magnets are given in Table 2. Full densification (about 97 vol.%) was achieved for T 5 1070 K. Generally, pressing at higher temperatures results in a decrease in J Hc and an increase in both Jr , and density, r (Table 2, Fig. 4). It is likely that grain growth is occurring at the higher temperatures, leading to a deterioration in the magnetic properties. After hot pressing the coercivity decreases by about 30%. The magnetic properties are temperature dependent (Fig.
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Table 1 Magnetic properties of Nd 2 Fe 14 B / a-Fe-type nanocomposites after HEBM and optimal heat treatment Material composition
a-Fe content (vol.%)
Js (T)
Jr (T)
a (Jr /Js )
J Hc (kA m 21 )
Nd 2 Fe 14 B / a-Fe Nd 2 Fe 12 Co 2 B / a-Fe Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe
0 0 0 10 37.5
1.11 1.19 1.17 1.30 1.48
0.66 0.54 0.61 0.88 1.02
0.59 0.45 0.52 0.68 0.69
875 398 925 811 690
Table 2 Magnetic properties and experimental density at room temperature, for hot pressed nanocomposite magnets Composition
Hot pressing temperature (K)
Jr (T)
J
Hc (kA m 21 )
r (g cm 23 )
Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 10 vol.% a-Fe
1020 1070 1020 1070
0.67 0.69 0.92 0.91
600 500 590 475
7.39 7.60 7.15 7.58
Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 37.5 vol.% a-Fe
5). The temperature coefficients (in the temperature range 293 to 413 K) of a (Jr ) and b ( J Hc ) for the studied magnets are given in Table 3. These coefficients (a (Jr ) and b ( J Hc )) of the Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe magnet with a content of the soft magnetic phase of 37.5 vol.% are 20.09% K 21 and 20.39% K 21 , respectively. These values are better than for sintered magnets. It is mainly due to the high volume fraction of a-Fe and partial replacement of Fe by Co.
Fig. 4. Second quadrant hysteresis loop of the Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 10 vol.% a-Fe after hot pressing at 1020 and 1070 K.
3.3. Corrosion properties The phases that are formed at the grain boundaries obstruct the oxidation of the Nd-rich regions. On the other hand, the precipitates formed inside the F grains might be responsible for the slowing down of the corrosion process inside the grains. A Cr additive of at least 1 at.% is very effective as to corrosion protection. Cr and Co additives lead an increasing tendency for passivation. The corrosion
Fig. 5. Temperature dependencies of the second quadrant hysteresis loops of the Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe magnet with 10 vol.% of a-Fe.
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Table 3 Temperature coefficients of remanent magnetic polarization a (Jr ) and intrinsic coercivity b ( J Hc ) for nanocomposite Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe magnets hot pressed at 1020 K with different volume fractions of a-Fe in comparison with Nd–Fe–B sintered magnet Composition
Content of soft magnetic phase of a-Fe (vol.%)
a (Jr ) (% K 21 ) 293–413 K
b ( J Hc ) (% K 21 ) 293–413 K
Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe Nd–Fe–B sintered magnet a
10 37.5 0
20.11 20.09 20.12
20.42 20.39 20.63
a
Commercial magnets.
Table 4 Results of the corrosion test of galvanic-deposited 10 mm Zn layers on nanocomposite Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe (37.5 vol.% of a-Fe) magnets at 320 K and 95% RH Dipping time (h)
Type of layer
Nd 2 Fe 14 B Dm /m o 3 10 24
Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe (37.5 vol.% of a-Fe) Dm /m o 3 10 24
100
Zn Non-coated Zn Non-coated Zn Non-coated
2 9 2 22 2.5 26
0.3 5 1 10 1 15
300 500
rate of the two-phase nanocomposite Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 37.5 vol.% a-Fe magnet has been estimated from potentiostatic measurements as 0.037 mm year 21 . Effective protection for nanocomposite magnets against corrosion was realised by surface coating with Zn metal. The obtained Zn layers (10 mm) are adherent and very resistant against corrosive agents and humid atmosphere. Table 4 reports the results of corrosion tests. Coating with Zn is responsible for substantial improvement of the oxidation resistance and consequently, excellent magnetic properties of nanocomposite Nd 2 (Fe,Co,Al,Cr) 14 B / a-Fe magnets are revealed after standing in air. The Zn-coated magnets have a more stable remanence and coercivity, than uncoated sintered magnets [11].
4. Conclusion Remanence-enhanced Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe nanocomposite magnets have been produced by HEBM and hot pressing. The addition of small amounts of Al–Cr to the basic Nd 2 Fe 14 B composition improved the remanent magnetic polarization and hysteresis squareness of the studied magnets, which may be related to refined grain size. A remanent magnetic polarization of 1.02 T and an intrinsic coercivity of 690 kA m 21 was achieved for Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 37.5 vol.% a-Fe. Hot pressing produced isotropic magnets with densities of about 7.6 g cm 23 , Jr |0.9 T, J Hc |600 kA m 21 and (BH ) max | 150 kJ m 23 . With increasing volume content of the a-Fe phase the thermal stability of the remanent magnetic
polarization a (Jr ), as well as the intrinsic coercivity b ( J Hc ), increased. HRTEM investigations showed a multi-phase structure. The magnetic and corrosion properties show good results and the demagnetisation curves have a single-phase nature. The corrosion rate was estimated as 0.037 mm year 21 . Effective protection for nanocomposite magnets against corrosion was realised also by the surface coating with Zn metal as well as with Co, Al, and Cr additives.
References [1] K.H.J. Buschow, Magnetism and processing of permanent magnet materials, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 10, Elsevier, 1997. [2] C.W. Cheng, H.C. Man, F.T. Cheng, IEEE Trans. Magn. 33 (1997) 3910. [3] P. Tenaud, F. Vial, M. Sagawa, IEEE Trans. Magn. 26 (1990) 1930. [4] L. Withanawasam, A.S. Murphy, G.C. Hadjipanayis, R.F. Krause, J. Appl. Phys. 76 (1994) 7065. [5] J. Jakubowicz, M. Jurczyk, J. Alloys Comp. 269 (1998) 284. [6] M. Jurczyk, J. Alloys Comp. 299 (2000) 283. [7] J. Jakubowicz, J. Alloys Comp. 314 (2001) 305. [8] J. Fidler, in: Proceedings of the IX International Workshop on Rare-Earth Magnets and their Applications, Bad Soden, Germany, 1987, p. 363. [9] W. Kunath, F. Zemlin, K. Weiss, Ultramicroscopy 16 (1985) 123. [10] J.M. Le Breton, S. Steyaert, M. Jurczyk, J. Teillet, in: L. Schultz, ¨ K.-H. Muller (Eds.), Proceedings of the XV International Workshop on Rare-Earth Magnets and their Applications, Dresden, Germany, Vol. II, 1998, p. 887. [11] A. Walton, J.D. Speight, A.J. Williams, I.R. Harris, J. Alloys Comp. 306 (2000) 253.
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Structure and magnetic properties of Nd 2 (Fe,Co,Al,Cr) 14 B / a-Fe nanocomposite magnets a, b J. Jakubowicz *, M. Giersig a
Institute of Materials Science and Engineering, Poznan University of Technology, M. Sklodowska-Curie 5 Sq, 60 -965 Poznan, Poland b Hahn-Meitner-Institut Berlin GmbH, Department of Solar Energy Research, Glienicker Str. 100, D-14109 Berlin, Germany Received 28 May 2002; received in revised form 25 June 2002; accepted 25 June 2002
Abstract The structure, magnetic properties and corrosion behaviour of two-phase nanocomposite Nd 2 (Fe,Co,Al,Cr) 14 B / a-Fe-type magnets, which have tetragonal / cubic Nd 2 Fe 14 B / a-Fe structure, have been investigated. The magnetic hardening was achieved by high-energy ball-milling (HEBM) of the Nd 2 Fe 14 B-type hard magnetic phase with different vol.% of a-Fe as soft magnetic phase, followed by annealing. Fully dense Nd 2 (Fe,Co,Al,Cr) 14 B / a-Fe type magnets have been produced by hot pressing. Magnets with good corrosion resistance as well as high temperature stability have been produced. The corrosion resistance is improved in the case of Co-, Al-, Cr-doped nanocomposite magnets with a volume fraction of the soft magnetic phase of 37.5 vol.%. Effective protection against corrosion was realised also by surface coating with Zn metal. 2002 Elsevier Science B.V. All rights reserved. Keywords: Permanent magnets; Nanostructured materials; Mechanical alloying; Corrosion; TEM
1. Introduction Permanent magnets based on Nd 2 Fe 14 B compound are widely used in many applications, e.g. motors, electric and computer peripherals. The main problems in NdFeB magnet applications are low temperature coefficients of the remanence a (Jr ) and coercivity b ( J Hc ), as well as low corrosion resistance (comparable with carbon steel). Commonly used sintered NdFeB magnets are composed of several phases: main F (Nd 2 Fe 14 B), h (Nd 11´ Fe 4 B 4 ) and intergranular Nd-rich (Nd 4 Fe) [1]. A high Nd content is necessary to obtain high coercivity, but it deteriorates the corrosion properties. The effect of corrosive environments results in deterioration of the structure and shape of the magnet and consequently its hard magnetic properties. Due to their low Curie temperature (T c 5585 K), the NdFeB magnets have also poor temperature characteristics of technical parameters such as remanence a (Jr ) and
*Corresponding author. Tel.: 148-61-835-1647; fax: 148-61-6653576. E-mail address: [email protected] (J. Jakubowicz).
coercivity b ( J Hc ). Overcoming these features is of primary importance from the applications point of view and also has economic aspects. To avoid the disadvantageous effect of corrosive environments on the magnet structure and its properties, two methods can be applied: (i) coating of the bulk magnet and (ii) alloy modifications. The first method is commonly used and coatings based on Zn, Sn, Al, Ni, Cr, or Cu provide good corrosion resistance. On the other hand, small additives such as Al, Cr, Ti, Ni or Zr not only improve the magnetic properties but also the corrosion resistance, too [2,3]. Recently, considerable attention has been focused on nanocomposite Nd 2 Fe 14 B / a-Fe magnets [4–6], because they have remanences Jr larger than the Stoner–Wohlfarth value Js / 2 (Js , saturation magnetic polarization) and maintain high coercivity. The nanocomposite magnets are composed of a hard phase (Nd 2 Fe 14 B) and a soft phase (a-Fe, Fe–Co or Fe 3 B). Between the nanometre grains (,100 nm) exchange coupling exists, leading to a reduced remanence Jr /Js of about 0.7 and to a higher Curie temperature. When the grain size decreases, a larger volume fraction of the grains comes under the influence of the exchange coupling. The structure plays an important role in magnetic and corrosion properties [7,8]. The main
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advantage of the nanocomposite magnets compared to sintered magnets is the lower content of Nd. It may be concluded that higher and lower content of a-Fe and Nd, respectively, should give better corrosion resistance and temperature stability with respect to sintered magnets. In this work the influence of alloy additives (Co, Al, Cr) and Zn coating on the structure, corrosion behaviour and temperature stability of Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe nanocomposite magnets was studied. Also, a HRTEM analysis was performed.
2. Experimental details Arc melting, followed by homogenization in an argon atmosphere at 1270 K was used to prepare the alloy specimens. Nd 2 Fe 14 B, Nd 2 Fe 12 Co 2 B and Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B-type alloy compositions were used as starting materials for the high-energy ball-milling (HEBM) process. A SPEX 8000 mixer mill was used for the HEBM processing. Powder handling to load the vial was carried out in a glove box with a high-purity argon atmosphere. The remanence-enhanced powders were prepared by HEBM of the rare-earth-containing Nd 2 Fe 14 B powder (precursory crushed to below 100 mm in size) and Fe powder (purity 99.9 at.%, particle size not above 10 mm) in a SPEX mill. The mill was run up to 48 h for every powder preparation. The as-milled powders were heattreated at 950 K for 30 min under high purity argon to form the Nd 2 Fe 14 B phase. Full density magnets were produced by hot pressing at temperatures of 1020 and 1070 K for times of 30 s. Structural characterizations were performed on an XRD diffractometer. The powders were examined at various stages during milling, prior to annealing and after annealing. The structural and chemical compositions were examined using a Philips Cm12 microscope, which was equipped with a 9800 EDAX analyser. The high resolution transmission electron microscopy (HRTEM) images were made under the conditions of minimum phase-contrast artefacts [9]. Axial illumination as well as a ‘nanoprobe mode’ were used for imaging. The latter mode allows the reduction of the beam spot size to 1 nm, enabling electron diffraction of individual small crystals. Magnetic characteristics were measured on a hysteresisgraph and SQUID magnetometer in external fields up to 2.5 MA m 21 . The corrosion resistance test of the nanocrystalline Nd 2 (Fe,Co,Al,Cr) 14 B / a-Fe-type magnets was carried out by corrosion potential measurements (3% NaCl water solution and 50 mV min 21 scanning speed) as well as in a humid atmosphere at 320 K. An efficient protection against corrosion of the nanocomposite magnets was studied through surface coating with a 10 mm layer of Zn.
3. Results and discussion
3.1. Structural properties For all samples, after 48 h high-energy ball milling, the alloy had decomposed into an amorphous phase and nanocrystalline a-Fe with a grain size of about 10–15 nm. Annealing the powder in high-purity argon at 950 K for 30 min results in the formation of the tetragonal Nd 2 Fe 14 Btype phase which coexists with the cubic a-Fe phase, resulting in a two-phase nanocomposite material. The grain size of the magnetically soft a-Fe phase after heat treatment increased to around 35–50 nm (Fig. 1). Magnets without additions (Fig. 1a) have a larger grain size; addition of AlCr suppresses an increase of the grain size (Fig. 1b). At the end of the milling period, a uniform mixing of Fe and Co will have occurred and on annealing the Co will partially substitute for the Fe in the Nd 2 Fe 14 B ¨ phase and also form a soft Fe–Co phase. Mossbauer study confirmed the presence of Co in both the Nd 2 Fe 14 B and a-Fe phases [10]. The bulk magnets were prepared by hot pressing at 1020 K and 1070 K for 30 s. The grain size after hot pressing increases up to 50–90 nm (Fig. 2). The EDAX spectrum confirms the presence of Fe, Co, Nd, B and Cr. Small amounts of Al were not detected. Contaminations such as C, O, Si were introduced during hot pressing in a graphite matrix covered by BN as well as during sample preparation for the TEM measurements. The smallest area for EDAX analysis was |100 nm and hence it was impossible
Fig. 1. The grain size of HEBM (a) Nd 2 Fe 12 Co 2 B / a-Fe and (b) Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe powders with a volume fraction of magnetically soft a-Fe phase of 10%, without annealing and after heat treatment at different temperatures.
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313
Fig. 3. HRTEM micrographs of the bulk Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 10 vol.% a-Fe nanocomposite hot pressed at 1070 K / 30 s.
3.2. Magnetic properties
Fig. 2. TEM micrograph (a) and EDAX spectrum (b) of the bulk Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 10 vol.% a-Fe nanocomposite hot pressed at 1070 K / 30 s.
to determine the exact composition of a single nanograin, because some of the neighbouring grains were also included. Hot pressed Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 10 vol.% a-Fe nanocomposite does not have a two-phase structure, but is a mixture of several phases (Fig. 3). Nd 2 Fe 14 B is the main phase with large grain size (,90 nm). Small amounts of the NdFe 4 B 4 ternary phase were formed also. A fine grained Fe 17 Nd 5 binary phase was found at the triple junctions of the larger grains. XRD analysis confirmed the presence of a-Fe, which is not shown in the HRTEM image. The multiphase structure can result from multistage processing which involves five energetic steps: arc melting, homogenisation, HEBM, heat treatment and hot pressing. The final compaction is important, because new phases can be formed or metastable phases can disappear.
All the powders prepared by the HEBM process are magnetically isotropic. The Nd 2 Fe 14 B nanocrystalline magnet without the soft phase (0% of a-Fe) has a reduced remanence of a 5 Jr /Js 5 0.59 and an intrinsic coercivity 21 of J Hc 5875 kA m . The principal advantages of Co addition to Nd 2 Fe 14 B / a-Fe are an increase in Curie temperature of the hard phase, and an increase in saturation polarization, arising from the higher saturation magnetization of Fe–Co, compared to that of pure a-Fe. The intrinsic coercivity decreases due to lower anisotropy of the Co-doped alloys. Small addition of Al and Cr introduced into the nanocomposites improved the magnetic properties (Table 1). Increase of the a-Fe content results in an increase of Js , Jr and a. Additionally the Curie temperature increases with increasing excess iron content in the nanocomposites. The J Hc maintains an acceptable value. HEBM and heat-treated NdFeCoAlCrB / a-Fe type powders have been compacted by hot pressing to form bulk magnets. Summaries of the magnetic properties of the produced magnets are given in Table 2. Full densification (about 97 vol.%) was achieved for T 5 1070 K. Generally, pressing at higher temperatures results in a decrease in J Hc and an increase in both Jr , and density, r (Table 2, Fig. 4). It is likely that grain growth is occurring at the higher temperatures, leading to a deterioration in the magnetic properties. After hot pressing the coercivity decreases by about 30%. The magnetic properties are temperature dependent (Fig.
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Table 1 Magnetic properties of Nd 2 Fe 14 B / a-Fe-type nanocomposites after HEBM and optimal heat treatment Material composition
a-Fe content (vol.%)
Js (T)
Jr (T)
a (Jr /Js )
J Hc (kA m 21 )
Nd 2 Fe 14 B / a-Fe Nd 2 Fe 12 Co 2 B / a-Fe Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe
0 0 0 10 37.5
1.11 1.19 1.17 1.30 1.48
0.66 0.54 0.61 0.88 1.02
0.59 0.45 0.52 0.68 0.69
875 398 925 811 690
Table 2 Magnetic properties and experimental density at room temperature, for hot pressed nanocomposite magnets Composition
Hot pressing temperature (K)
Jr (T)
J
Hc (kA m 21 )
r (g cm 23 )
Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 10 vol.% a-Fe
1020 1070 1020 1070
0.67 0.69 0.92 0.91
600 500 590 475
7.39 7.60 7.15 7.58
Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 37.5 vol.% a-Fe
5). The temperature coefficients (in the temperature range 293 to 413 K) of a (Jr ) and b ( J Hc ) for the studied magnets are given in Table 3. These coefficients (a (Jr ) and b ( J Hc )) of the Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe magnet with a content of the soft magnetic phase of 37.5 vol.% are 20.09% K 21 and 20.39% K 21 , respectively. These values are better than for sintered magnets. It is mainly due to the high volume fraction of a-Fe and partial replacement of Fe by Co.
Fig. 4. Second quadrant hysteresis loop of the Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 10 vol.% a-Fe after hot pressing at 1020 and 1070 K.
3.3. Corrosion properties The phases that are formed at the grain boundaries obstruct the oxidation of the Nd-rich regions. On the other hand, the precipitates formed inside the F grains might be responsible for the slowing down of the corrosion process inside the grains. A Cr additive of at least 1 at.% is very effective as to corrosion protection. Cr and Co additives lead an increasing tendency for passivation. The corrosion
Fig. 5. Temperature dependencies of the second quadrant hysteresis loops of the Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe magnet with 10 vol.% of a-Fe.
J. Jakubowicz, M. Giersig / Journal of Alloys and Compounds 349 (2003) 311–315
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Table 3 Temperature coefficients of remanent magnetic polarization a (Jr ) and intrinsic coercivity b ( J Hc ) for nanocomposite Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe magnets hot pressed at 1020 K with different volume fractions of a-Fe in comparison with Nd–Fe–B sintered magnet Composition
Content of soft magnetic phase of a-Fe (vol.%)
a (Jr ) (% K 21 ) 293–413 K
b ( J Hc ) (% K 21 ) 293–413 K
Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe Nd–Fe–B sintered magnet a
10 37.5 0
20.11 20.09 20.12
20.42 20.39 20.63
a
Commercial magnets.
Table 4 Results of the corrosion test of galvanic-deposited 10 mm Zn layers on nanocomposite Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe (37.5 vol.% of a-Fe) magnets at 320 K and 95% RH Dipping time (h)
Type of layer
Nd 2 Fe 14 B Dm /m o 3 10 24
Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe (37.5 vol.% of a-Fe) Dm /m o 3 10 24
100
Zn Non-coated Zn Non-coated Zn Non-coated
2 9 2 22 2.5 26
0.3 5 1 10 1 15
300 500
rate of the two-phase nanocomposite Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 37.5 vol.% a-Fe magnet has been estimated from potentiostatic measurements as 0.037 mm year 21 . Effective protection for nanocomposite magnets against corrosion was realised by surface coating with Zn metal. The obtained Zn layers (10 mm) are adherent and very resistant against corrosive agents and humid atmosphere. Table 4 reports the results of corrosion tests. Coating with Zn is responsible for substantial improvement of the oxidation resistance and consequently, excellent magnetic properties of nanocomposite Nd 2 (Fe,Co,Al,Cr) 14 B / a-Fe magnets are revealed after standing in air. The Zn-coated magnets have a more stable remanence and coercivity, than uncoated sintered magnets [11].
4. Conclusion Remanence-enhanced Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / a-Fe nanocomposite magnets have been produced by HEBM and hot pressing. The addition of small amounts of Al–Cr to the basic Nd 2 Fe 14 B composition improved the remanent magnetic polarization and hysteresis squareness of the studied magnets, which may be related to refined grain size. A remanent magnetic polarization of 1.02 T and an intrinsic coercivity of 690 kA m 21 was achieved for Nd 2 Fe 11.49 Co 2 Al 0.17 Cr 0.34 B / 37.5 vol.% a-Fe. Hot pressing produced isotropic magnets with densities of about 7.6 g cm 23 , Jr |0.9 T, J Hc |600 kA m 21 and (BH ) max | 150 kJ m 23 . With increasing volume content of the a-Fe phase the thermal stability of the remanent magnetic
polarization a (Jr ), as well as the intrinsic coercivity b ( J Hc ), increased. HRTEM investigations showed a multi-phase structure. The magnetic and corrosion properties show good results and the demagnetisation curves have a single-phase nature. The corrosion rate was estimated as 0.037 mm year 21 . Effective protection for nanocomposite magnets against corrosion was realised also by the surface coating with Zn metal as well as with Co, Al, and Cr additives.
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