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Low-temperature thermal oxidation of FeNdB powders
LOW-TEMPE~TURE C.N. CHRISTODOULOU
May 1988
MATERIALS LETTERS
Volume 6. number 8.9
THERMAL
OXIDATION
OF Fe-Nd-B
POWDERS
‘, J.R. SCHLUP
Department of Chemical Engineering, Kansas State University, Manhattan,
KS 66506, USA
and G.C. HADJIPANAYIS Department qfphysics, Kansas State Universit_v, Manhattan, KS 66506, US..
Received 3 March 1988
Differential scanningcalorimetry (DSC) has been employed to study low-temperature thermal oxidation of Fe-Nd-B powders. The three exothermic reactions observed cause a significant deterioration ofthe hard magnetic properties ofthe resulting magnets. X-ray photoelectron data demonstrate that at least two forms of oxygen are present on such powders.
Fe-Nd-B powders have been used to produce magnets having the highest energy product reported to date [ 1,2]. Preparation of these permanent magnets using powder metallurgy techniques becomes difficult when the particle diameter becomes too small. A drastic decrease in remanence and coercivity of the sintered magnets occurs when the particle size passes through a critical value [ 3 1. Aligned green compacts of these powders do not show this behavior. Therefore, the phenomena occurring during sintering play an important role in establishing hard magnetic properties. In the present study DSC data were obtained to examine changes in an Fe-Nd-B powder as a function of temperature. The relationships between the DSC data and the coercivity and remanence of the aligned green compacts and resulting sintered magnets have been investigated. The Fe-Nd-B alloy investigated in the present study had a nominal composition of Fe,,Mm,,,,Dy,,,B,,,. A 100 g arc-melted ingot was first crushed under argon using a micro-mill. The powder ( < 150 urn) was ballmilled for 5 min under moisture-free ’ Present address: Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh PA 152 13. USA.
0167-577x/88/$ ( Noah-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
toluene using an attritor to achieve a uniform particle size and homogeneous composition. ,4pproximately 80 g samples of the resulting powder were taken from the original batch and milled further under the same conditions for 40 min. A 3 g sample was taken from this batch. This sample was transferred to a die where it was aligned in an 8 kOe magnetic field and pressed at 3.3 kbar perpendicular to the direction of the magnetic field. Half of the aligned green compact was sintered under an oxygen-free atmosphere for 15 min at temperatures between 1000 and 1080°C. The magnetic properties of the aligned green compact and sintered magnet were measured using a vibrating sample magnetometer in fields up to 17 kOe. Differential scanning calorimetry (DSC) measurements were made using a DuPont 910 differential scanning calorimeter. The particle size dist~bution of the powders was obtained using a Jeol 1OOC scanning transmission electron microscope. The particle size distribution for a given powder was obtained by determining Feret’s horizontal diameter for over 500 particles of a given powder. Particle size data were fit to gaussian distribution functions using regression analysis. Gaussian distributions for powders milled for 0, 3.5, and 60 min are shown in fig. 1. Mean particle diameters were 6.1. B.V.
287
Volume
6, number 8,9
LATERAL
LETTERS
May 1988
Time(min)
Fig. 3. Deconvoluted DSC curves 0fFe~~.~Mrn~3.~EZy~.~B,,7 powder with a rni~~~n~ time of 35 min. / I 2
4
6 Particle
ID Diameter
1°C 14 (in mierunsJ
16
1%
20
Fig. 1. Particle size distributions of powders with milling times ofQ min I), 35 min (...), and 60 min(---).
1.7, and 1,l pm, corresponding to milling times of 0, 35, and 60 min, respectively. These same milling times resulted in standard deviations for the gaussian dist~bution functions of 3.5,0.49, and 0.27 urn, respectively. More detailed analysis of the particle size distribution as a function of milbng time can be found in the discussion of the impact of processing variables upon the properties of the resulting magnets [4]. The remanence of the aligned pressed powders initially increases as the particle diameter decreases and then it saturates for average particle diameters greater than 1.5 urn [ 3 1. This is consistent with having powder particles that are single grain and therefore all oriented. The coercivity continues to increase monotonically with increasing miffing time. However, the data in fig. 2 demonstrate that the coercivity of sam-
l
Coercivity
(koe)-as
n Coerc. Ikoel-align.
sintered green camp.
0
Fig.. 2. Coercivity of aligned green compacts and sintered magnets as a function of milling time.
288
pies prepared from powders with diameters less than 1.5 urn decreases upon sintering. Similar effects are observed in the remanence data of these samples. In order to better understand the behavior of the magnetic properties of the resulting sintered magnet, DSC data were obtained for the as-cast alloy and for a sample milled for 35 min (fig. 3). The smaller surface area of the as-cast alloy sample minimized any effects due to surface reactions. No exothermic or endothermic peaks are observed with the as-cast alloy in the temperature range between 25 and 500°C. The exothermic peaks obtained for the powder sample in this temperature region (fig. 3) are due to surface,reactions based upon a comparison with the WC data for the as-cast alloy. Structural transitions are not involved. The DSC data can be explained as the superposition of three exothermic peaks which suggest that three different processes are occurring. The most probable cause of the three different processes is the oxidation reactions of the 14:2: 1,4: 1:4, and rare-earth rich phase present in the as-cast alloy [ $6 1. These three phases have been identi~ed using X-ray diffraction along with optical and electron microscopy and thermomagnetic measurements_ Deconvoluted DSC data are shown in fig. 3 for the powder milled for 35 min. From these data it is obvious that the heat of transformation associated with the first process is the most dominant_ The DSC data provide useful information for the temperature at which such phenomena are initiated. It should be noted that all such transformations have been completed at temperatures less than 400°C. Since sintering results in a decrease in remanence and coercivity, oxidation of the powder is the most likely candidate for the exothermic processes present in the DSC data. X-ray photoelectron spectroscopy
Volume 6. number
MATERIALS
8.9
LETTERS
May 1988
convolution of such spectra and specific assignments to the binding energies obtained. However, the initial XPS data unambiguously demonstrate that more than one surface species of oxygen are present on these powders. . ‘5 c
We would like to acknowledge the assistance of Dr. Martin Stockli in the deconvolution of the DSC curves. The X-ray photoelectron spectra were obtained by Mr. Guy Wilson and Dr. Peter Sherwood. This work was supported by Ford Motor Company Electrical and Electronic Division.
‘& ++Qaemp_: l-
I
I
I
530
Binding Fig 4. X-ray photoelectron 40 min.
Energy
I
528
I
526
(in eV)
0 Is spectrum
of powder
milled for
(XPS) provided direct evidence of oxidation of these powders. Fig. 4 shows the XPS 0 1s spectrum of a powder prepared by milling for 40 min. At least two oxygen species are indicated. Binding energies in the range from 529 to 534 eV are characte~stic of oxygen present as a bulk oxide phase, as oxygen in hydroxyl groups, and as chemisorbed oxygen. Use of well-characterized reference samples will permit de-
References [ 1] G.C. Hadjipanayis, [2] [3] [4] [S] [6]
R.C. Hazelton and K.R. Lawless, Appl. Phys. Letters 43 ( 1983) 797. M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura, J. Appl. Phys. 55 ( 1984) 6. C.N. Christodoulou, J.R. Schlup and G.C. Hadjipanayts. J. Appl. Phys. 6 1 ( 1987) 3760. C.N. Christodoulou, J.R. Schlup and G.C. Hadjipanayis, Mater. Sci. Eng., submitted for publjcation. J.F. Herbst, J.J. Croat, F.E. Pinkerton and W.B. Yelon. Phys. Rrv.B29(1984)4176. D. Givord, H.S. Li and J.M. Moreau, Solid State Commun. 50 (1984) 497.
289
May 1988
MATERIALS LETTERS
Volume 6. number 8.9
THERMAL
OXIDATION
OF Fe-Nd-B
POWDERS
‘, J.R. SCHLUP
Department of Chemical Engineering, Kansas State University, Manhattan,
KS 66506, USA
and G.C. HADJIPANAYIS Department qfphysics, Kansas State Universit_v, Manhattan, KS 66506, US..
Received 3 March 1988
Differential scanningcalorimetry (DSC) has been employed to study low-temperature thermal oxidation of Fe-Nd-B powders. The three exothermic reactions observed cause a significant deterioration ofthe hard magnetic properties ofthe resulting magnets. X-ray photoelectron data demonstrate that at least two forms of oxygen are present on such powders.
Fe-Nd-B powders have been used to produce magnets having the highest energy product reported to date [ 1,2]. Preparation of these permanent magnets using powder metallurgy techniques becomes difficult when the particle diameter becomes too small. A drastic decrease in remanence and coercivity of the sintered magnets occurs when the particle size passes through a critical value [ 3 1. Aligned green compacts of these powders do not show this behavior. Therefore, the phenomena occurring during sintering play an important role in establishing hard magnetic properties. In the present study DSC data were obtained to examine changes in an Fe-Nd-B powder as a function of temperature. The relationships between the DSC data and the coercivity and remanence of the aligned green compacts and resulting sintered magnets have been investigated. The Fe-Nd-B alloy investigated in the present study had a nominal composition of Fe,,Mm,,,,Dy,,,B,,,. A 100 g arc-melted ingot was first crushed under argon using a micro-mill. The powder ( < 150 urn) was ballmilled for 5 min under moisture-free ’ Present address: Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh PA 152 13. USA.
0167-577x/88/$ ( Noah-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
toluene using an attritor to achieve a uniform particle size and homogeneous composition. ,4pproximately 80 g samples of the resulting powder were taken from the original batch and milled further under the same conditions for 40 min. A 3 g sample was taken from this batch. This sample was transferred to a die where it was aligned in an 8 kOe magnetic field and pressed at 3.3 kbar perpendicular to the direction of the magnetic field. Half of the aligned green compact was sintered under an oxygen-free atmosphere for 15 min at temperatures between 1000 and 1080°C. The magnetic properties of the aligned green compact and sintered magnet were measured using a vibrating sample magnetometer in fields up to 17 kOe. Differential scanning calorimetry (DSC) measurements were made using a DuPont 910 differential scanning calorimeter. The particle size dist~bution of the powders was obtained using a Jeol 1OOC scanning transmission electron microscope. The particle size distribution for a given powder was obtained by determining Feret’s horizontal diameter for over 500 particles of a given powder. Particle size data were fit to gaussian distribution functions using regression analysis. Gaussian distributions for powders milled for 0, 3.5, and 60 min are shown in fig. 1. Mean particle diameters were 6.1. B.V.
287
Volume
6, number 8,9
LATERAL
LETTERS
May 1988
Time(min)
Fig. 3. Deconvoluted DSC curves 0fFe~~.~Mrn~3.~EZy~.~B,,7 powder with a rni~~~n~ time of 35 min. / I 2
4
6 Particle
ID Diameter
1°C 14 (in mierunsJ
16
1%
20
Fig. 1. Particle size distributions of powders with milling times ofQ min I), 35 min (...), and 60 min(---).
1.7, and 1,l pm, corresponding to milling times of 0, 35, and 60 min, respectively. These same milling times resulted in standard deviations for the gaussian dist~bution functions of 3.5,0.49, and 0.27 urn, respectively. More detailed analysis of the particle size distribution as a function of milbng time can be found in the discussion of the impact of processing variables upon the properties of the resulting magnets [4]. The remanence of the aligned pressed powders initially increases as the particle diameter decreases and then it saturates for average particle diameters greater than 1.5 urn [ 3 1. This is consistent with having powder particles that are single grain and therefore all oriented. The coercivity continues to increase monotonically with increasing miffing time. However, the data in fig. 2 demonstrate that the coercivity of sam-
l
Coercivity
(koe)-as
n Coerc. Ikoel-align.
sintered green camp.
0
Fig.. 2. Coercivity of aligned green compacts and sintered magnets as a function of milling time.
288
pies prepared from powders with diameters less than 1.5 urn decreases upon sintering. Similar effects are observed in the remanence data of these samples. In order to better understand the behavior of the magnetic properties of the resulting sintered magnet, DSC data were obtained for the as-cast alloy and for a sample milled for 35 min (fig. 3). The smaller surface area of the as-cast alloy sample minimized any effects due to surface reactions. No exothermic or endothermic peaks are observed with the as-cast alloy in the temperature range between 25 and 500°C. The exothermic peaks obtained for the powder sample in this temperature region (fig. 3) are due to surface,reactions based upon a comparison with the WC data for the as-cast alloy. Structural transitions are not involved. The DSC data can be explained as the superposition of three exothermic peaks which suggest that three different processes are occurring. The most probable cause of the three different processes is the oxidation reactions of the 14:2: 1,4: 1:4, and rare-earth rich phase present in the as-cast alloy [ $6 1. These three phases have been identi~ed using X-ray diffraction along with optical and electron microscopy and thermomagnetic measurements_ Deconvoluted DSC data are shown in fig. 3 for the powder milled for 35 min. From these data it is obvious that the heat of transformation associated with the first process is the most dominant_ The DSC data provide useful information for the temperature at which such phenomena are initiated. It should be noted that all such transformations have been completed at temperatures less than 400°C. Since sintering results in a decrease in remanence and coercivity, oxidation of the powder is the most likely candidate for the exothermic processes present in the DSC data. X-ray photoelectron spectroscopy
Volume 6. number
MATERIALS
8.9
LETTERS
May 1988
convolution of such spectra and specific assignments to the binding energies obtained. However, the initial XPS data unambiguously demonstrate that more than one surface species of oxygen are present on these powders. . ‘5 c
We would like to acknowledge the assistance of Dr. Martin Stockli in the deconvolution of the DSC curves. The X-ray photoelectron spectra were obtained by Mr. Guy Wilson and Dr. Peter Sherwood. This work was supported by Ford Motor Company Electrical and Electronic Division.
‘& ++Qaemp_: l-
I
I
I
530
Binding Fig 4. X-ray photoelectron 40 min.
Energy
I
528
I
526
(in eV)
0 Is spectrum
of powder
milled for
(XPS) provided direct evidence of oxidation of these powders. Fig. 4 shows the XPS 0 1s spectrum of a powder prepared by milling for 40 min. At least two oxygen species are indicated. Binding energies in the range from 529 to 534 eV are characte~stic of oxygen present as a bulk oxide phase, as oxygen in hydroxyl groups, and as chemisorbed oxygen. Use of well-characterized reference samples will permit de-
References [ 1] G.C. Hadjipanayis, [2] [3] [4] [S] [6]
R.C. Hazelton and K.R. Lawless, Appl. Phys. Letters 43 ( 1983) 797. M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura, J. Appl. Phys. 55 ( 1984) 6. C.N. Christodoulou, J.R. Schlup and G.C. Hadjipanayts. J. Appl. Phys. 6 1 ( 1987) 3760. C.N. Christodoulou, J.R. Schlup and G.C. Hadjipanayis, Mater. Sci. Eng., submitted for publjcation. J.F. Herbst, J.J. Croat, F.E. Pinkerton and W.B. Yelon. Phys. Rrv.B29(1984)4176. D. Givord, H.S. Li and J.M. Moreau, Solid State Commun. 50 (1984) 497.
289