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Oxidation behaviour of Nd(Fe,Mo)12 and Nd(Fe,Mo)12Ny compounds: a Mössbauer investigation
Journal of Magnetism and Magnetic Materials 242–245 (2002) 1338–1340
Oxidation behaviour of Nd(Fe,Mo)12 and Nd(Fe,Mo)12Ny . compounds: a Mossbauer investigation J.M. Le Bretona,*, G. Khe! lifatia, M. Jurczykb a
Groupe de Physique des Mat!eriaux, UMR CNRS 6634, Facult!e des Sciences, Universite! de Rouen, place Emile Blondel, 76821 Mont-StAignan Cedex, France b Institute of Materials Science and Engineering, Poznan University of Technology, 60-965 Poznan, Poland
Abstract . The oxidation behaviour of the 1:12 phase in Nd–Fe–Mo and Nd–Fe–Mo–N powders is investigated by Mossbauer spectrometry. It appears that, in both unnitrided and nitrided powders, the 1:12 phase is better resistant against oxidation than the Nd2Fe14B phase. The same oxidation products are formed (a-Fe, Fe3O4, a-Fe2O3) but in different proportions, the iron oxides being the main oxidation products. r 2002 Elsevier Science B.V. All rights reserved. . Keywords: Oxidation; Mossbauer spectroscopyFRE-TM compounds; Permanent magnetsFrare earth
The use of intermetallic rare earth-transition metal compounds in technological applications is frequently limited by their poor oxidation resistance, because, of the high affinity of rare earths for oxygen. It is thus of importance to study their oxidation behaviour. Due to the improvement of their intrinsic magnetic properties by gas-solid reaction with light atoms, some RFe12xMxYy (R=rare earth, M=Ti, Cr, Mo, V..., Y=N, C) compounds are promising candidates for permanent magnets applications [1]. Surprisingly, the oxidation behaviour of the 1:12-type alloys has not been investigated yet. In this paper, preliminary results on the oxidation of Nd(Fe,Mo)12 and Nd(Fe,Mo)12Ny compounds are given and compared to the well-known oxidation mechanism of the Nd2Fe14B phase. Alloys with Nd12Fe76Mo12 nominal composition were prepared by arc melting of the elements in argon atmosphere. The ingots were crushed coarsely into particles with a size of less than 100 mm. Next, the crushed powders were milled for 48 h in a Spex 8000 Mixer mill, and annealed at 7301C for 30 min under high purity argon to form the Nd(Fe,Mo)12 phase. The socalled annealed powder is a very fine powder, with *Corresponding author. Tel.: +33-2-35-14-67-66; fax: +332-35-14-66-52. E-mail address: [email protected] (J.M. Le Breton).
particles smaller than 10 mm, containing nanostructured grains (grain size of about 50 nm) [2]. The nitride was prepared by nitrogenation of the annealed powder at 4501C for 25 h using 100 kPa pressure of nitrogen. Annealed and nitrided powders were oxidized for times up to 4 days at 3001C in air. In order to characterize the influence of the milling process on the oxidation behaviour, a piece of the initial Nd12Fe76Mo12 alloy was crushed in a mortar, sieved to less than 20 mm, and oxidized under the same conditions. In addition, this allows to compare the behaviour of the Nd(Fe,Mo)12 phase with that of the Nd2Fe14B phase (prepared and oxidized under the same conditions [3]). Both unoxidized and oxidized powders were characterized by scanning electron microscopy (SEM), X-ray diffrac. tion (XRD) and transmission Mossbauer spectrometry (TMS) at room temperature. TMS was performed using a conventional 57Co source in a rhodium matrix. The XRD analysis of the annealed powder shows that it contains both Nd(Fe,Mo)12 and Nd-rich phases. The composition of the magnetic phase, determined from the measured lattice parameters, is NdFe10.35Mo1.65, in agreement with the nominal composition of the initial alloy. SEM observations of the nitrided powders revealed that they consist of particles with diameter less than 10 mm, with a few particles having a size greater than 20 mm (Fig. 1). The biggest particles appear to be aggregates of smaller particles.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 9 6 9 - 6
J.M. Le Breton et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1338–1340
-10
1339
Velocity (mm / s) 0
+10
1.00
0h
Fig. 1. SEM micrograph of the unoxidized Nd–Fe–Mo–N powder.
Velocity (mm / s) -10
0
Absorption (%)
0.98 1.00
2h 0.98 1.00
+10
1.00
24 h 0.98 1.00
0h 0.99 1.00
96 h
Absorption (%)
0.98
2h 0.99 1.00
24 h 0.99 1.00
96 h 0.99
. Fig. 2. Room temperature Mossbauer spectra of the annealed Nd–Fe–Mo powder oxidized at 3001C for times indicated. The contribution of the 1:12 phase is displayed in the spectra of the oxidized powders. The arrows indicate the positions of the main lines of the a-Fe2O3 and Fe3O4 oxides.
. Some Mossbauer spectra of annealed and nitrided powders after different oxidation times are shown in Figs. 2 and 3. One can see that upon oxidation, the
. Fig. 3. Room temperature Mossbauer spectra of the nitrided Nd–Fe–Mo–N powder oxidized at 3001C for times indicated. The contribution of the 1:12 phase is displayed in the spectra of the oxidized powders. The arrows indicate the positions of the main lines of the a-Fe phase.
intensity of Nd(Fe,Mo)12 contribution decreases (Fig. 2), showing the progressive oxidation of the corresponding phase. The same behaviour is observed for Nd(Fe,Mo)12Nx (Fig. 3). The contributions of a-Fe, Fe3O4 and a-Fe2O3 are detected in both cases. It is found that the oxidation products of Nd(Fe,Mo)12 and Nd(Fe,Mo)12Nx are the same as those of Nd2Fe14B, but differ in their relative proportions, the main contributions being here those of the iron oxides (a-Fe is the main oxidation product in the case of Nd2Fe14B [4]). . For each powder, the Mossbauer relative intensity of the Nd(Fe,Mo)12 phase was measured for each treatment time, and the unreacted volume fraction of the phase (namely R1:12 ) was obtained, R1:12 being defined as follows: R1:12 ðtÞ ¼ ð%1:12 ðtÞ=%1:12 ðt ¼ 0ÞÞ 100; where . %1:12(t) is the Mossbauer relative intensity of the . Nd(Fe,Mo)12 phase contribution to the Mossbauer spectrum after time t at a given temperature. The R1:12 ðtÞ curves obtained for the different powders investigated are shown in Fig. 4. From these curves, a
J.M. Le Breton et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1338–1340
1340 1 0.9
initial powder <20µm
0.8
R1:12
0.7
annealed powder
0.6
nitrided powder <20µm
0.5 0.4
nitrided powder
0.3 0.2
Nd2 Fe14B powder <20µm
0.1 0 0
100000
200000
300000
400000
500000
t (s)
Fig. 4. Unreacted volume fraction of the 1:12 phase (R1:12 ) for the powders oxidized at 3001C. The curve corresponding to the Nd2Fe14B powder is obtained from reference [3].
comparison of the different powders can be drawn. First, the Nd(Fe,Mo)12 phase in the initial powder (sieved to less than 20 mm) is oxidized with a slower kinetics than the Nd2Fe14B phase, showing that Nd(Fe,Mo)12 is intrinsically much more resistant against oxidation than Nd2Fe14B. Second, the Nd(Fe,Mo)12 phase in the annealed powder is less resistant than in the initial powder, and this is attributed to a reduction of the Nd(Fe,Mo)12 grain size after the milling/annealing process. Third, the nitrided Nd(Fe,Mo)12Nx phase appears to be less corrosion resistant than the unnitrided phase in the annealed powder. As the nitriding process did not induce a significant change in the grain size [2], this could be related either to a change of the structure of the intergranular phases, or to the fact that the nitrided phase is intrinsically less resistant against oxidation than the unnitrided one. Further investigations are needed to conclude. However, these results
clearly show that the nitrided phase is more resistant than the Nd2Fe14B phase. Finally, it must be noted that the R1:12 ðtÞ curves of the nitrided powder sieved to less than 20 mm and unsieved are not significantly different, indicating that the oxidation kinetics of the nitrided phase do not depend on the particle size, but most likely on the grain size inside the particles, in agreement with SEM observations (Fig. 1). . The Mossbauer investigation of the oxidation behaviour of Nd–Fe–Mo and Nd–Fe–Mo–N powders provides evidence for a better resistance against oxidation of the 1:12 phase (both unnitrided and nitrided material) than the Nd2Fe14B phase. The determination of the kinetic parameters of the oxidation process is in progress, and preliminary results indicate that they are related to the diffusion of oxygen in iron oxides, these phases being the main oxidation products. This work was supported by the INCO-Copernicus project IC15 CT 96-0758 of the European Commission, Brussels. The authors are thankful to B. Beaudouin (Universit!e d’Amiens) for his help during the SEM observations.
References [1] H. Fujii, H. Sun, in: K.H.J. Buschow (Ed.), Handbook of Ferromagnetic Materials, Vol. 9, North-Holland, Amsterdam, 1995, p. 303. [2] M. Jurczyk, J. Alloys Compound 235 (1996) L1. [3] S. Steyaert, J.M. Le Breton, J. Teillet, J. Phys.: Condens. Matter 8 (1996) 10721. [4] J.M. Le Breton, S. Steyaert, J. Phys.: Condens. Matter 11 (1999) 4941.
Oxidation behaviour of Nd(Fe,Mo)12 and Nd(Fe,Mo)12Ny . compounds: a Mossbauer investigation J.M. Le Bretona,*, G. Khe! lifatia, M. Jurczykb a
Groupe de Physique des Mat!eriaux, UMR CNRS 6634, Facult!e des Sciences, Universite! de Rouen, place Emile Blondel, 76821 Mont-StAignan Cedex, France b Institute of Materials Science and Engineering, Poznan University of Technology, 60-965 Poznan, Poland
Abstract . The oxidation behaviour of the 1:12 phase in Nd–Fe–Mo and Nd–Fe–Mo–N powders is investigated by Mossbauer spectrometry. It appears that, in both unnitrided and nitrided powders, the 1:12 phase is better resistant against oxidation than the Nd2Fe14B phase. The same oxidation products are formed (a-Fe, Fe3O4, a-Fe2O3) but in different proportions, the iron oxides being the main oxidation products. r 2002 Elsevier Science B.V. All rights reserved. . Keywords: Oxidation; Mossbauer spectroscopyFRE-TM compounds; Permanent magnetsFrare earth
The use of intermetallic rare earth-transition metal compounds in technological applications is frequently limited by their poor oxidation resistance, because, of the high affinity of rare earths for oxygen. It is thus of importance to study their oxidation behaviour. Due to the improvement of their intrinsic magnetic properties by gas-solid reaction with light atoms, some RFe12xMxYy (R=rare earth, M=Ti, Cr, Mo, V..., Y=N, C) compounds are promising candidates for permanent magnets applications [1]. Surprisingly, the oxidation behaviour of the 1:12-type alloys has not been investigated yet. In this paper, preliminary results on the oxidation of Nd(Fe,Mo)12 and Nd(Fe,Mo)12Ny compounds are given and compared to the well-known oxidation mechanism of the Nd2Fe14B phase. Alloys with Nd12Fe76Mo12 nominal composition were prepared by arc melting of the elements in argon atmosphere. The ingots were crushed coarsely into particles with a size of less than 100 mm. Next, the crushed powders were milled for 48 h in a Spex 8000 Mixer mill, and annealed at 7301C for 30 min under high purity argon to form the Nd(Fe,Mo)12 phase. The socalled annealed powder is a very fine powder, with *Corresponding author. Tel.: +33-2-35-14-67-66; fax: +332-35-14-66-52. E-mail address: [email protected] (J.M. Le Breton).
particles smaller than 10 mm, containing nanostructured grains (grain size of about 50 nm) [2]. The nitride was prepared by nitrogenation of the annealed powder at 4501C for 25 h using 100 kPa pressure of nitrogen. Annealed and nitrided powders were oxidized for times up to 4 days at 3001C in air. In order to characterize the influence of the milling process on the oxidation behaviour, a piece of the initial Nd12Fe76Mo12 alloy was crushed in a mortar, sieved to less than 20 mm, and oxidized under the same conditions. In addition, this allows to compare the behaviour of the Nd(Fe,Mo)12 phase with that of the Nd2Fe14B phase (prepared and oxidized under the same conditions [3]). Both unoxidized and oxidized powders were characterized by scanning electron microscopy (SEM), X-ray diffrac. tion (XRD) and transmission Mossbauer spectrometry (TMS) at room temperature. TMS was performed using a conventional 57Co source in a rhodium matrix. The XRD analysis of the annealed powder shows that it contains both Nd(Fe,Mo)12 and Nd-rich phases. The composition of the magnetic phase, determined from the measured lattice parameters, is NdFe10.35Mo1.65, in agreement with the nominal composition of the initial alloy. SEM observations of the nitrided powders revealed that they consist of particles with diameter less than 10 mm, with a few particles having a size greater than 20 mm (Fig. 1). The biggest particles appear to be aggregates of smaller particles.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 9 6 9 - 6
J.M. Le Breton et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1338–1340
-10
1339
Velocity (mm / s) 0
+10
1.00
0h
Fig. 1. SEM micrograph of the unoxidized Nd–Fe–Mo–N powder.
Velocity (mm / s) -10
0
Absorption (%)
0.98 1.00
2h 0.98 1.00
+10
1.00
24 h 0.98 1.00
0h 0.99 1.00
96 h
Absorption (%)
0.98
2h 0.99 1.00
24 h 0.99 1.00
96 h 0.99
. Fig. 2. Room temperature Mossbauer spectra of the annealed Nd–Fe–Mo powder oxidized at 3001C for times indicated. The contribution of the 1:12 phase is displayed in the spectra of the oxidized powders. The arrows indicate the positions of the main lines of the a-Fe2O3 and Fe3O4 oxides.
. Some Mossbauer spectra of annealed and nitrided powders after different oxidation times are shown in Figs. 2 and 3. One can see that upon oxidation, the
. Fig. 3. Room temperature Mossbauer spectra of the nitrided Nd–Fe–Mo–N powder oxidized at 3001C for times indicated. The contribution of the 1:12 phase is displayed in the spectra of the oxidized powders. The arrows indicate the positions of the main lines of the a-Fe phase.
intensity of Nd(Fe,Mo)12 contribution decreases (Fig. 2), showing the progressive oxidation of the corresponding phase. The same behaviour is observed for Nd(Fe,Mo)12Nx (Fig. 3). The contributions of a-Fe, Fe3O4 and a-Fe2O3 are detected in both cases. It is found that the oxidation products of Nd(Fe,Mo)12 and Nd(Fe,Mo)12Nx are the same as those of Nd2Fe14B, but differ in their relative proportions, the main contributions being here those of the iron oxides (a-Fe is the main oxidation product in the case of Nd2Fe14B [4]). . For each powder, the Mossbauer relative intensity of the Nd(Fe,Mo)12 phase was measured for each treatment time, and the unreacted volume fraction of the phase (namely R1:12 ) was obtained, R1:12 being defined as follows: R1:12 ðtÞ ¼ ð%1:12 ðtÞ=%1:12 ðt ¼ 0ÞÞ 100; where . %1:12(t) is the Mossbauer relative intensity of the . Nd(Fe,Mo)12 phase contribution to the Mossbauer spectrum after time t at a given temperature. The R1:12 ðtÞ curves obtained for the different powders investigated are shown in Fig. 4. From these curves, a
J.M. Le Breton et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1338–1340
1340 1 0.9
initial powder <20µm
0.8
R1:12
0.7
annealed powder
0.6
nitrided powder <20µm
0.5 0.4
nitrided powder
0.3 0.2
Nd2 Fe14B powder <20µm
0.1 0 0
100000
200000
300000
400000
500000
t (s)
Fig. 4. Unreacted volume fraction of the 1:12 phase (R1:12 ) for the powders oxidized at 3001C. The curve corresponding to the Nd2Fe14B powder is obtained from reference [3].
comparison of the different powders can be drawn. First, the Nd(Fe,Mo)12 phase in the initial powder (sieved to less than 20 mm) is oxidized with a slower kinetics than the Nd2Fe14B phase, showing that Nd(Fe,Mo)12 is intrinsically much more resistant against oxidation than Nd2Fe14B. Second, the Nd(Fe,Mo)12 phase in the annealed powder is less resistant than in the initial powder, and this is attributed to a reduction of the Nd(Fe,Mo)12 grain size after the milling/annealing process. Third, the nitrided Nd(Fe,Mo)12Nx phase appears to be less corrosion resistant than the unnitrided phase in the annealed powder. As the nitriding process did not induce a significant change in the grain size [2], this could be related either to a change of the structure of the intergranular phases, or to the fact that the nitrided phase is intrinsically less resistant against oxidation than the unnitrided one. Further investigations are needed to conclude. However, these results
clearly show that the nitrided phase is more resistant than the Nd2Fe14B phase. Finally, it must be noted that the R1:12 ðtÞ curves of the nitrided powder sieved to less than 20 mm and unsieved are not significantly different, indicating that the oxidation kinetics of the nitrided phase do not depend on the particle size, but most likely on the grain size inside the particles, in agreement with SEM observations (Fig. 1). . The Mossbauer investigation of the oxidation behaviour of Nd–Fe–Mo and Nd–Fe–Mo–N powders provides evidence for a better resistance against oxidation of the 1:12 phase (both unnitrided and nitrided material) than the Nd2Fe14B phase. The determination of the kinetic parameters of the oxidation process is in progress, and preliminary results indicate that they are related to the diffusion of oxygen in iron oxides, these phases being the main oxidation products. This work was supported by the INCO-Copernicus project IC15 CT 96-0758 of the European Commission, Brussels. The authors are thankful to B. Beaudouin (Universit!e d’Amiens) for his help during the SEM observations.
References [1] H. Fujii, H. Sun, in: K.H.J. Buschow (Ed.), Handbook of Ferromagnetic Materials, Vol. 9, North-Holland, Amsterdam, 1995, p. 303. [2] M. Jurczyk, J. Alloys Compound 235 (1996) L1. [3] S. Steyaert, J.M. Le Breton, J. Teillet, J. Phys.: Condens. Matter 8 (1996) 10721. [4] J.M. Le Breton, S. Steyaert, J. Phys.: Condens. Matter 11 (1999) 4941.