The Influence of Zirconium Addition and Process Parameters On
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Journal of Magnetism and Magnetic Materials 238 (2002) 56–64 The influence of zirconium addition and process parameters on the magnetic properties of Pr-Fe-B sintered magnets R.N. Faria Instituto de Pesquisas Energeticas! e Nucleares IPEN-CNEN, CEP 05422-970, Sao* Paulo–SP, Brazil Received 19 June 2001; received in revised form 23 August 2001 Abstract Sintered magnets based on the compositions Pr16Fe76B8 and Pr16Fe75.5B8Zr0.5 were produced using the hydrogen decrepitation process. Sintered magnets prepared under specific processing conditions from the zirconium-free alloy exhibited excellent remanence (1.22 T), intrinsic coercivity (1.22 T) and energy product (278 kJmÀ3). The squareness factor of magnets prepared from the Pr16Fe75.5B8Zr0.5 alloy was improved considerably (0.96). This investigation also shows the remarkable influence of zirconium addition on the intrinsic coercivity of these permanent magnets. r 2002 Elsevier Science B.V. All rights reserved. Keywords: PrFeB magnets; Zirconium addition; Hydrogen decrepitation 1. Introduction the addition of Zr [4]. None of these investigations have reported about the influence of zirconium Recent work has shown that addition of addition on the magnetic properties of PrFeB zirconium to Pr-based magnets, produced via the sintered magnets. In this investigation, sintered hydrogenation disproportionation, desorption, permanent magnets with compositions of and recombination process was an effective way Pr16Fe76B8 and Pr16Fe75.5B8Zr0.5 were prepared to developremanence and coercivity in these via the hydrogen decrepitation (HD) process. The materials [1]. Addition of Zr to nanocrystalline influence of zirconium addition, sintering tem- PrFeB-based magnets led to a dramatic decrease in perature and cooling rate from the sintering size of the hard magnetic grains (o10 nm), with temperature on the magnetic properties of these remarkable influence on the magnetic properties of HD magnets has been investigated. single phase magnets [2]. Addition of Zr to A feature of melt-spun Nd–Fe–B alloy and sintered NdFeB permanent magnets reduced mechanically alloyed Sm–Fe–N alloy is the inverse coercivity from 0.9 to B0.6 T [3]. Conversely, correlation between their remanence and coerciv- intrinsic coercivity of mechanically milled PrFeB ity [5]. Similar magnetic behavior has been powders was enhanced from B1.6 to 1.8 T with observed in various strontium hexaferrites [6]. An empirical correlation between remanence and E-mail address: [email protected], [email protected] coercivity for Pr-based magnets that exhibited (R.N. Faria). optimum magnetic properties has been reported 0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0304-8853(01)00701-6 R.N. Faria / Journal of Magnetism and Magnetic Materials 238 (2002) 56–64 57 [7]. In this study, magnetic properties of the where /cos yS is the degree of easy-axis align- Pr16Fe76B8 and Pr16Fe75.5B8Zr0.5 HD sintered ment of the Pr2Fe14B matrix phase (f). It is 0.5 for magnets have been compared to those in which an isotropic magnet and increases to 1 for a the inverse correlation between remanence and perfectly aligned magnet (good practical magnets coercivity of PrFeB-type magnets were observed should have cos y around 0.92–0.95 [12]). The (only for Br and m0 iHcX1 T). The maximum volume fraction (0pf p1) of the Pr2Fe14B matrix observed magnetic properties and the calculated phase in an ideal magnet is unity (to achieve high properties of these magnets have been compared. densification, practical magnets are produced with Various Pr–Fe–B-type magnets, prepared by the higher amounts of rare-earth and the volume standard powder metallurgy (PM) route, the HD fraction is maintained around 0.82–0.85). The process, hot pressing (HP), hot rolling (HR) and packing factor or relative density (P=magnet die upsetting (DU), have also been included for density/theoretical density) for an ideal permanent comparison. magnet is also unity (in practical magnets pro- duced by powder metallurgy it can reach 0.96 [12]). Is is spontaneous polarization of the Pr2Fe14B 2. Experimental matrix phase (and is dependent on the temperature coefficient b). Two commercial alloys in the as-cast state were The spontaneous magnetization (Ms) is given by studied. To produce Pr-based HD magnets [8,9], the ratio of the magnetic moment and the volume. B35 g of the cast alloy ingot was placed in a Pr2Fe14B has a tetragonal crystal structure with À10 stainless steel hydrogenation vessel. This was then lattice constants a ¼ 8:81  10 m and c ¼ À10 2 À28 3 evacuated to backing-pump pressure. Hydrogen 12:27  10 m [13] (V ¼ a c ¼ 9:52  10 m ). was introduced to a pressure of 1 bar, and this The magnetization in units of Bohr magnetons À24 À1 resulted in decrepitation of the bulk material. The (mB ¼ 9:27  10 JT ) per formula unit at room vessel was then evacuated to backing-pump temperature (300 K) is 31.9 mB/f.u. [13]. Four pressure for 30–40 min, the material transferred Pr2Fe14B formula units in a cell [14] with these to a roller ball mill under a protective nitrogen dimensions gives a total magnetic moment of À21 atmosphere and milled for 20 h using cyclohexane 127.6 mB (4  31.9  mB=1.183  10 ). There- as the milling medium. The resultant fine powder fore, the spontaneous magnetization of Pr2Fe14B 5 À1 À21 À28 was then dried for 1 h and transferred to a small is 12.42  10 Am (1.183  10 /9.52  10 ). cylindrical rubber tube under nitrogen atmo- The spontaneous polarization is then 1.56 T À7 À1 sphere. The fine powder was aligned by pulsing (Is ¼ Ms m0; m0 ¼ 4p  10 TmA ). Reported three times in a 4.5 T magnetic field, pressed spontaneous polarization for the Pr2Fe14B matrix isostatically at 1000 kg cmÀ2, vacuum sintered for phase is 1.56 T [15] and 1.575 T (at 293 K) [16]. The 1 h at 10601C followed by slow cooling (SC) in the differences in these values can be attributed to the furnace (B3.51C minÀ1) or fast cooling (FC) temperature coefficient of Is: The highest rema- outside the furnace. For comparison, some sam- nence of a PrFeB permanent magnet (ideal) ples were sintered at 1100oC for 1 h and rapidly can be considered to be around 1.58 T cooled to room temperature. (/cos yS ¼ 1; f ¼ 1 and P ¼ 1). Magnetic characterization of the sintered The molecular mass of Pr2Fe14B is 1074.47 g PrFeB(Zr) magnets was carried out using a (2  140.9+14  55.847+10.811) and four formu- À28 3 permeameter. Measurements were carried out la units in a cell with a volume of 9.52  10 m after saturation in a pulsed field of 4.5 T. In order resulted in theoretical density of approximately À3 23 À28 to compare with the experimental results, the 7.5 g cm (4  1074.47/6.02  10 /9.52  10 = À3 theoretical remanence of the HD magnets was 7.499 Mg m ). Room temperature X-ray density À3 estimated from the expression [10,11] for Pr2Fe14B is 7.53 g cm [17]. Previously calcu- lated and observed densities [14] for Pr2Fe14B were Br ¼ hicos y fPIs; ð1Þ 7.513 and 7.51 g cmÀ3, respectively. The calculated 58 Pr 3. Results and discussion Pr studied heremagnets are [22–30] compared and the in HDcooling Table rate). sintered 2. magnets dramatically The withbe Zr addition observedSmall (and changes but in change remanenceintrinsic intrinsic of in these coercivity, magnets coercivity can comparedcoercivity. to changed SC rapidsquareness factor produces cooling. (SF)[20,21]. at samples the expense with Zirconiumvalues of intrinsic higher increases evensintering when significantly temperature, thethe led the density to increasebeen higher was in reported that remanence unaffected grain inpowders the size, case milled of for due Sm–Co different magnets, times to [9]. increase It has in also density for Nd sintered at 1060 magnetic properties of theoverall Pr magnetic properties.sintered The permanent slightly magnet higher exhibited excellent (7.6 g cm showed very good energy product (278 kJ m intrinsic coercivity.[9], Thus, but the thefrom a TD Pr totally desorbed magnet (TD)alongside powder showed a (286 kJ sintered m a permanent much magnet lower prepared higher than that ofthe the magnet magnet sintered sintered at at 1100 to 1060 room temperature. The remanence (1.22 T) of density ( than that of thetrinsic latter coercivity (1.37 of T). the Inand former both (1.22 slowly cases, T) the was cooled lower (1.18 T). Conversely, the in- Pr coercivity hasbehavior with been respecthydrogen to atmosphere previously remanence during sintering and [19].probably Similar intrinsic reported to for 0.97–0.98. the HD Thiscalculated process high value that and packing the produces packing density a factor, around is due 16 16 16 Table 1 shows the magnetic properties of the The magnetic properties of various permanent Fe Fe Fe 76 76 76 B B B À r 8 8 8 3 ) of the HD magnets was close to the ) [18]. HD magnet sintered at 1100 HD sintered magnets produced with and Pr 2 1 Fe C/1100 14 16 B has a slightly higher value R.N. Faria / Journal of Magnetism and Magnetic Materials 238 (2002) 56–64 Fe 1 75.5 C and fast or slow cooled 1 B C and fast cooled was 8 15 Zr Fe 0.5 79 B 16 HD magnets 6 Fe magnet [23] 1 76 C for 1 h B 8 HD À À 1 C 3 3 ) ) Table 1 Magnetic properties of Pr16Fe76B8 and Pr16Fe75.5B8Zr0.5 HD sintered magnets À3 Alloy Sintering Br (T) m0 iHc (T) m0 bHc (T) SF (ratio) (BH)max r (g cm ) temperature–cooling rate (70.02) (70.02) (70.02) (70.02) (kJ mÀ3)(75) (70.03) Pr16Fe76B8 11001C-FC 1.22 1.22 1.06 0.82 278 7.39 Pr16Fe75.5B8Zr0.5 10601C-FC 1.20 0.67 0.66 0.96 266 7.33 Pr16Fe76B8 10601C-SC 1.18 1.37 1.10 0.80 254 7.35 Pr16Fe76B8 10601C-FC 1.16 1.21 0.93 0.68 253 7.34 Pr16Fe75.5B8Zr0.5 10601C-SC 1.12 1.02 0.86 0.61 227 7.35 R.N.