Nitrogenation process in Sm2Fe17 under various N2-gas pressures up to 6 MPa

Journal of

ALtOYS A~4D COMPOUND5 ELSEVIER

Journal of Alloys and Compounds 236 (1996) 156-164

Nitrogenation process in

SmzFe17under various N2-gas pressures up to 6 MPa

H. Fujii, K. Tatami, K. K o y a m a Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739, Japan Received 15 August 1995

Abstract In order to clarify the nitrogenation process in Sm2Fet7 under various N2-gas pressures from 0.01 MPa to 6 MPa, we examined the time evolution of nitrogen absorption rates and nitrogen atom distributions in the samples by respectively measuring the increase in mass and by observing the corresponding EPMA profiles. From the results obtained, we could deduce the following characteristic features: the nitrogen atoms rapidly diffuse along the grain boundaries in the bulk sample at first, and then the N atoms penetrate into the grains along directions perpendicular to the grain boundaries. Under low N2-gas pressures below 0.05 MPa, the nitrogenation into the Sm2Fe17 grains is mainly due to diffusion of nitrogen, and a homogeneous phase with variable nitrogen content could be stabilized. With increasing N2-gas pressures, the grain growth of fully nitrogenated phase becomes more dominant than the diffusion during nitrogenation. This grain growth process makes high pressure nitrogenation effective for synthesizing a high quality Sm2Fe17N3 nitride. Keywords: Sm2Fe17; Nitrogenation process; Permanent magnets

1. Introduction One of the methods for fabricating new promising magnetic materials is to interstitially introduce nonmetallic atoms like H, B, C or N with small atomic radius into host metals or compounds. In 1990, the interstitially modified compound SmzFelTN a was discovered by applying the gas-phase interstitial modification technique to Sm2Fe17 [1]. This interstitial nitride was prepared by heating Sm2Fea7 to 670-820 K under an atmosphere of nitrogen gas or nitrogencontaining gas. The crystal lattice expands by 6-7% to accommodate three nitrogen atoms at the interstitial sites, the Curie temperature T~ increases dramatically from 398 K to 752 K, the saturation magnetization (M s = 1.54 T at room temperature) is comparable with that of Nd2Fe14B and the uniaxial magnetic anisotropy ( ~ H A = 26 T at room temperature) is three times as strong as that of Nd2Fe14B [1-4]. Since the discovery of the Sm2Fe17N 3 nitride, worldwide efforts have been made and are being devoted to the study of interstitial modifications of various kinds of intermetallics of rare earth and 3d elements. The study covers not only every aspect of the structural 0925-8388/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0925-8388(95)02080-2

and intrinsic magnetic properties, but also the development of the promising interstitial compounds into hard magnets. In addition, much attention has been paid to the study of the gas-phase interstitial modification process itself [5,6]. So far, gas-phase nitrogenation has been conducted at 673-773 K on a finely-ground powder under moderate N2-gas pressures less than 1 MPa [1-3,7], nitrogen-containing gas NH 3 [1] or mixed gas such as NH 3 + H 2 [8] or N 2 + H 2 [9]. All of these nitrogenation processes more or less give rise to a small but noticeable trace of a-Fe phase in the powder samples during nitrogenation. Recently, we have succeeded in synthesizing the high quality R2Fea7N3+a with 6 = 0.1-0.6 (R = Y, Ce and Sm) by nitriding under high pressure N 2 gas up to 10 MPa [4,10]. The high pressure nitrogenation offers two advantages: the first is to suppress the segregation of a-Fe phase on the powder upon nitrogenation and the second is to reach a full nitrogen content of N = 3 within a relatively short nitrogenation treatment. Although the nitrogenation process has been studied by many different researchers using different experimental techniques, such as Kerr microscopic observations [11], X-ray diffraction studies [5 ], in situ

H. Fujii et al. / Journal of Alloys and Compounds 236 (1996) 156-164

neutron powder diffraction studies [12], micrograph studies applying metallography and electron probe micro-analyzer (EPMA) techniques [13] and so on, whether the metastable nitride is a simple gas-solid solution with a continuous range of intermediate nitrogen contents, or whether it is a two-phase mixture of nitrogen-poor and nitrogen-rich phases, is still an open question. In this paper, we report the results of time evolution of nitrogen contents and nitrogen atom distributions in Sm2Fe17 at several temperatures from 670 to 683 K under various N2-gas pressures from 0.01 to 6 MPa on the plate-shaped SmRFe17 and powder with particle sizes between 32 and 53/zm. This work was performed to clarify the nitrogenation process itself in gas-phase reaction.

157

4

3

o''O-

II - - ~ - - - - " II

o

1

• plate6MPa D plate0.1MPa O powder<20p.m

f - r-L~O

00

i

[

i

48

96

144

i

i

192

t(h) Fig. as a data also

1. Nitrogen concentration in the plate-shaped Sm2Fe17 sample function of reacting times at 733 K u n d e r 0.1 and 6 MPa. The for the powder with the particle sizes of less than 20 /zm are included.

2. Experimental procedures

Sm2Fe17sample The host Sm2Fe17 compound was prepared by induction melting of the constituent elements under an argon atmosphere, which was provided by Suzuki (Minebea Co. Ltd) [14]. The compound was then annealed at 1373 K for 3 days in an argon atmosphere for homogeneity treatment. After confirming that the sample was a single-phase with a rhombohedral Th2Zn17-type structure, three pieces of plate-shaped samples with dimensions of 5 × 3 x 1 mm 3 were cut from the ingot. The rest was pulverized into powder of size 32 to 53/zm. Nitrogenation was performed by changing the reacting times from 1 to 192 h at several constant temperatures from 670 to 780 K under various N2-gas pressures from 0.01 to 6 MPa. The nitrogen concentration in the samples was determined by the increase in mass at each nitrogenation process. In order to check the phases contained in the samples during nitrogenation, X-ray diffraction studies were performed at room temperature using Cu Ka radiation with a Mac-Science MXP3 monochromater. Morphological and microscopic composition analyses were performed using scanning electron microscope (SEM) and EPMA, from which we could deduce the time evolution of spatial nitrogen distribution in Sm2Fe17.

3. Results

3.1. Nitrogenation of plate-shaped Sm2Fe17sample First, we nitrogenated the plate-shaped SmzFe17 sample of approximately 1 mm thickness at 733 K under N2-gas pressures of 0.1 and 6 MPa to understand nitrogen absorption process in the bulk form. In Fig. 1, nitrogen concentration in the plate-shaped

is shown as a function of the reacting time at 733 K under 0.1 and 6 MPa. Also, included are the results of time evolution of nitrogenation for the powder with particle sizes less than 20 # m in diameter. As is evident from Fig. 1, the nitrogen content x reaches a value larger than 3.0 when nitriding the powder sample at 733 K under 6 MPa within 12 h. In contrast, the nitrogen content in the plate sample shows a sigmoidal-shaped curve as a function of the loading time with an incubation period of nearly 24 h, which is characteristic of a self-catalytic reaction. Even in the bulk sample, nitrogenation under 6 MPa at 733 K for 192 h leads to x 1>3.0, indicating complete nitrogenation, whereas the nitrogen content remains below 3.0 in reactions under 0.1 MPa for 192 h. It takes much longer to complete the nitriding of the bulky sample under 0.1 MPa. Therefore, it seems likely that application of high pressure N 2 gas is quite effective for completing nitrogenations of bulk samples. In Fig. 2, X-ray diffraction patterns are shown for the plate-shaped samples after nitriding at 733 K under 6 MPa for various reacting times. Prior to the X-ray diffraction examination, the plate samples were crushed into powder of less than 50/zm diameter. A small peak corresponding to a-Fe segregation is observed around 20 = 44.7 ° even in the host compound, but the peak intensity does not increase upon nitrogenation under 6 MPa at 733 K, irrespective of the reacting times. This indicates that the disproportionation reaction of Sm2Fe17N 3 into SmN, Fe4N and a-Fe does not proceed during nitrogenation at 733 K and 6 MPa. When the nitrogen content reaches x/> 3.0, an almost single-phased 2-17 nitride Sm2Fe17N3+ ~ is found in the X-ray diffraction patterns. In Fig. 3, a comparison is made of the X-ray diffraction patterns of the plate-shaped Sm2Fea7 after nitrogenation at 733 K under 0.1 and 6 MPa. In this

H. Fujii et al. / Journal of Alloys and Compounds 236 (1996) 156-164

158

(a)

SmeFe~rN, plate 733K 6MPa

X=0.1 -4 c6 v

o) c'-

96hrs

¢-

(b)

~÷. i

I

I

20

30

40

20

x=1.7

I

!

50

60

(deg.)

Fig. 2. Powder X-ray diffraction patterns for the plate-shaped sample after nitriding at 733 K under 6 MPa for various reacting times.

(c) Sm2Fe17N x plate ~

733K

x=3.0

96hrs

,g c-ql)

144hrs

I

I

I

!

!

20

30

40

50

60

20

Fig. 4. SEM figures of the cross-section of the plate sample Sm2FetTN x with (a) x = 0.1, (b) 1.7 and (c) 3,0 obtained in nitrogenation at 733 K under 6 MPa.

(deg.)

Fig. 3. Powder X-ray diffraction patterns of the plate-shaped Sm2Fe17 after nitrogenation at 733 K under 0.1 and 6 MPa for 96 and 144 h.

figure, we notice that complete nitrogenation is reached more rapidly under 6 MPa than 0.1 MPa at 733 K, and the nitrogenation under 6 MPa leads to strong suppression of the a-Fe segregation compared with 0.1 MPa. This also indicates that high pressure interstitial modification gives advantages for synthesizing high quality 2-17 nitrides. In order to clarify how nitrogen atoms penetrate the bulky Sm2Fe17 under high pressure N 2 gas, we observed the SEM and EPMA image figures of the cross-section of the plate-shaped samples with different reacting times. The samples were polished by a buffing machine using alumina powder with particle size less than 1 /~m. The results obtained are given in Figs. 4 and 5. We can see in the SEM figure that the

samples with lower nitrogen contents than 3.0 are so brittle that the edge of the sample is broken off, and there are many cracks in the samples. The EPMA profiles show that nitrogen only exists around the cracks but not in the grains for x = 0.1, indicating that the diffusion into the grains is very slow compared with the short circuit diffusion through grain boundaries. For x = 1.7, nitrogen atoms are almost homogeneously distributed inside the plate, but the nitrogen content is low near the surface of the plate; finally, for x = 3.0, nitrogen homogeneously distributes throughout the sample bulk. From these results, we conclude that the nitrogen absorption in the bulk sample proceeds by the following two steps: (1) at the first step, nitrogen atoms diffuse faster along the grain boundaries and cause many cracks; (2) in the next step, the nitrogen atoms are absorbed in the grain interior. Then, the nitrogen absorption rate is faster inside the plate than near the surface, indicating a poisoning of

159

H. Fujii et al. I Journal of Alloys and Compounds 236 (1996) 156-164 i

i

(a) x=0.1

/

•

•

"4": =

2

•/

0

__----

/" A / / ".

///.~"" .ll

1 /",/0

- 1 - - 6MPa --'Z~-- 0 . 1 M P a - - - 0 - - - O,05MPa ~7" 0.01MPa

~/ i

(b)

v

....

I

00

,

I

24

,

~)

48

7

t (h)

x=1.7

Fig. 6. Nitrogen concentration in the Sm2Fe~7powder sample as a function of the reacting time at 733 K under various N2-gas pressures. The particle sizes of the powder are between 32 and 53 ~m.

features: (1) the nitrogen absorption rate decreases with decreasing N2-gas pressures Ps2; (2) under PN2 less than 0.05 MPa, the nitrogen absorption rate becomes independent of pressure, suggesting that the rate determining step in lower Nz-gas pressures is the diffusion process of nitrogen atoms in the Sm2Fe17 grains. In Fig. 7, nitrogen concentration at various temperatures is shown as a function of the reacting time under 0.1 and 6 MPa for the powder sample. It is clear that the nitrogen absorption rate is larger under 6 MPa

(c) x=3.0

4 Fig. 5. EPMA image figures of N element in the cross-sectionof the plate-shaped Sm2Fe17Nx with (a) x=0.1, (b) 1.7 and (c) 3.0 obtained in nitrogenation at 733 K under 6 MPa.

the grain surface near the bulk surface due to the remaining gas in the reactor. Therefore, it is essential to keep lhe surface of the bulky Sm2Fe17 sample clean for homogeneous and faster nitrogenation. Still, we cannot answer the question of why the difference in the nitrogen absorption rate of the plate sample appears between 0.1 and 6 MPa. Hence, we next planned to study the nitrogenation process in the powder Sm2Fe17 sample. 3.2. Nitrogenation process in the p o w d e r sample

Sm2Fe17

In F i g 6, the nitrogen concentration is shown as a function of the reacting time at 733 K under various N2-gas pressures for the powder sample with particle sizes between 32 and 53 /xm diameter, which are composed of an assembly of single crystal particles. In this figure, we notice the following characteristic

(a) 0.1MPa 3

,----~-

"'2 o f I

/,%./"

.~ t'1~-

0

.-'Q" .

- - [ 3 - - 773K

•

- . A . - 733K

.O"

•

- - O - - 713K i

'

i

,

'

i

4

(b) 6MPa 3

~

.~ .......

"'7.

:3 ",-

~'"

,,

#./" .K/"

2

o 1 ~

0

-.A--

733K

- - • - - 673K

•

,

0

z~--

--.43-- 773K

i

24

..e

,

i

48

,

i

72

t (h) Fig. 7. Nitrogen concentration in the Sm2Fe17 powder at various temperatures as a function of reacting times under (a) 0.1 MPa and (b) 6 MPa.

H. Fujii et al. / Journal of Alloys and Compounds 236 (1996) 156-164

160

than 0.1 MPa for all temperatures. Assuming that the powder samples are in a spherical form with average diameter of 42 /~m and the nitrogenation reaction progresses from the surface of the powder, we estimated the average penetration depth of nitrogen dp, the time evolution of which is given in Fig. 8 for three cases. As shown in Fig. 8(c), under PN2 ~<0.05 MPa, dp is proportional to t 1/2 in the whole reacting stage. This result indicates that the diffusion process of nitrogen is dominant in nitrogenation at 733 K f o r PN2 ~<0.05 MPa. Under PN2 >/0.1 MPa, dp is proportional to t 1/2 at the earlier stage, but it is proportional to the reacting time t at the later stages of nitrogenation (see Figs. 8(a) and 8(b)). This suggests that the grain growth of the fully nitrogenated phase becomes dominant at the late nitrogenation stage for PN~ ~>0.1 MPa, though the diffusion of nitrogen atoms is important at the earlier nitrogenation process.

20 (a) 0.1MPa /" /

E .~-~ 10 -o=

-~

°I

- JO-- 713K

Z~'O".... O''"

0 20

~'°

i

I

I

I

(b) 6MPa s

Y

E 10

i~.fk

.o=

..A"

./-'''~"

In order to clarify whether the Sm2Fe17 nitride is a simple gas-solid solution with intermediate nitrogen contents or a two-phase mixture of nitrogen-poor and nitrogen-rich phases, we compared the X-ray diffraction patterns at an intermediate stage in nitrogenation under various N2-gas pressures (Fig. 9). We notice that the separation of the (113), (300) and (204) reflection peaks between the nitrogen-poor SmzFei7N x and the fully nitrided SmzFe17N 3 phases is clear in the nitride with x ~ 1.9 prepared by reacting at 733 K under P~ = 6 MPa, while the separation becomes unclear with decreasing nitrogenized pressure. Therefore, it is concluded that the SmzFe17 nitride is a simple gassolid solution with continuous range of intermediate nitrogen contents in principle, indicating that the critical temperature is below 733 K. In such cases, questions as to the nitrogenation process arise, i.e. why does a two-phase mixture appear in the X-ray diffraction patterns of the nitride prepared under P,~ = 6 MPa? To answer this question, we performed2microscopic composition analyses of the nitrogen atoms in the SmzFe]vN x powder sample using the EPMA equipment. Here, it should be noted that the beam width of EPMA electron probe is several micrometers. Figs. 10 and 11 show the EPMA line profiles and the corresponding SEM figures for the SmzFe17N x powder with intermediate nitrogen contents, obtained by reacting at 733 K for 12 h under PN ----0.1 and 6 MPa respectwely. We can see some diffetZences of nitrogen atom distributions in partly nitrogenized powders under PN. = 0.1 and 6 MPa. For the powder nitrogenized at 733 K for 12 h under PN =0"1 MPa, the N atoms penetrate into the interior from the surface of powder and the nitrogen content gradually decreases with increasing the depth from the surface. However,

--!- 773K -,A--

/"

733K

--O-- 673K

A

Sm2Fel;,N,

........ 7

0 20

o~ -

(c) 733K

::3

6MPa --'A'- 0.1MPa ~ --O--O.05MPa/

El0

lk'"

v

/ "/

j'/"

.~X'"

./"

-0-

-

"2o

..; :.0" 0

0

I

I

I

24

48

72

t(h) Fig. 8. Average penetration depth of nitrogen in the powder sample as a function of the reacting times in nitrogenation process at various temperatures under (a) 0.1 MPa and (b) 6 MPa, and (c) at 733 K under various pressures.

20

30

40

50

60

20 (deg.) Fig. 9. X-ray diffraction patterns of the Sm2Fe17 powder with x = 1.6 to 1.9 at an intermediate stage in nitrogenation under various N2-gas pressures. The patterns for the host and fully nitrogenated samples are included in this figure.

H. Fujii et al. / Journal of Alloys and Compounds 236 (1996) 156-164

3~ xS=h.83 IIII I 12h :5~ 2 ~ x = 1 . 3

t I

i i'

EPMA-line profile

:',i

II

II

161

"~

I I~ ~ \

(a) 0.1MPa

24h x=1.9

// % ",;-----%-;----3'o d~ (gin)

...... N 200cps

""~., ,,,

i

.... 3 1 ~ I~.~

3h x=0.79

(b) 6MPa

lOIJrn

Fig. 10. EPMA line profiles of Fe, Sm and N elements in the Sm2FelTN ' powder obtained by reacting at 733 K for 12 h under 0.1 MPa. The inset shows the corresponding SEM figure.

•~

II~

] / \ \

0

f

~

12h

~

~

~

I ~

24h x=2.4 ~

10 20 dp(~tm)

[

30

Fig. 12. Nitrogen atom distribution as a function of the depth from the surface of powder for various reacting times in nitrogenation at 733 K (deduced from the recording of the corresponding EPMA line profiles of N element) under (a) 0.1 MPa and (b) 6 MPa. The nitrogen concentration c on the surface of powder is normalized at c = 3.0 for all samples.

Pm=6MPa -Fe 20,O00cps . . . . . Sm 2,000cps ....... I'1 200cps

\

I\\\\ OI

profile

~,

',

grain growth of the fully nitriding phase Sm2Fe17N 3 is dominant under high N2-gas pressures.

,' .;,,,/,,, , , ~ . • . ,,; ,,, , i

•

10um

Fig. 11. EPMA line profiles of Fe, Sm and N elements in the Sm2F%7 N powder nitrogenated at 733 K for 12 h under 6 MPa. The inset ~hows the corresponding SEM figure.

for the powder prepared u n d e r PN2 = 6 MPa, the nitrogen content decreases rather sharply at a critical depth from the surface, compared with the profile of nitrogen distribution under PN2 = 0.1 MPa. Systematically, we recorded the EPMA line profiles of the N atom distribution in the Sm2Fe17Nx prepared by changing the reacting times from 6 to 48 h at 733 K under 0.1 and 6 MPa. The results obtained are shown in Fig. 12. Here, the nitrogen concentration c on the surface of powder is normalized at c = 3.0 for all the samples. We can recognize some characteristic features from Fig. 12: (1) the final depth of the nitrogen atoms reached in the powder is independent of the nitriding pressure PN2, for example it reaches about 15 /zm from the surface by heating at 733 K for 12 h; (2) the fully nitrogenized depth at Pr%---6 MPa is almost twice as thick as at PN2 = 0.1 MPa, indicating that

4. Discussion

On the basis of the nitrogen atom distributions in the Sm2Fe17Nx powder observed in this work, we will discuss the nitrogenation process under various N2-gas pressures. In the nitrogen gas-solid reaction in the grain, there are three fundamental reaction processes: (1) nitrogen dissociation process from the N2-gas phase to the N atoms (N 2 ~ 2N) on the surface of powder; (2) nitrogen diffusion into the grain from the surface (Sm2Fe17+l/2N2--~Sm2F%TN x (x<3.0)); (3) grain growth of the fully nitrided phase SmzFelvN 3 from the nitrogen-poor a-phase (SmzFeI7N,---~SmzFelvN3). If the nitrogen dissociation process was the rate determining step for the nitriding of the Sm2Fe17 powder, a nitrogen distribution profile as shown in Fig. 13(a) would be realized, in which the nitrogen content is homogeneous in the powder. This is in contrast to the experimental results shown in Figs. 10 and 11. Next, if the nitrogenation process was mainly due to the diffusion of nitrogen with no phase transformation of Sm2Fe17Nx (x < 3) to Sm2Fe17N3, density profiles of

H. Fufii et al. / Journal of Alloys and Compounds 236 (1996) 156-164

162

(a)

C

. . . . . . . . . . . . . . . . . . . . . . .

R

gas pressures for PN2 ~<0.05 MPa and the average penetration depth d p o f nitrogen in S m 2 F e 1 7 is proportional to t 1/2, where t is the nitrogenized time. These results indicate that the diffusion of the nitrogen atoms is rather dominant under the lower N2-gas pressures, PN2 ~<0.05 MPa, and determines the reaction rate. Hence, using Eq. (2) we calculated the nitrogen concentration c(r) as a function of nitrogenation time t so as to well fit to the experimental points; the results are shown in Fig. 14(a) by the solid line. Then, the diffusion constant D was deduced to be D = 3.1 x 10 -16 m 2 s -1 at 733 K. This value is in good agreement with the estimated value of 3.3 × 1 0 -16 m 2 s -1 from Eq. (3) using D O= 1.02 x 1 0 - 6 m 2 s -1 and Q = 133 kJ mol 1 obtained by Coey and co-workers [5,6]. With increasing the N2-gas pressure PN: the concentration distribution of nitrogen atoms could not be understood by considering only the diffusion process of nitrogen.

~ ....

R

R

0

R

R

0

R

Distance from center of powder r Fig. 13. Nitrogen distribution profiles for three f u n d a m e n t a l reaction processes: (a) nitrogen dissociation process; (b) nitrogen diffusion process; (c) grain growth process of the fully nitrided phase.

3

(a) O.05MPa

"-1 %,,,_

O f

2

f3 t/~

the N atoms in the grain as shown in Fig. 13(b) would be realized. When the nitrogen concentration c varies only along a certain direction denoted r, the nitrogen distribution is obtained by solving the following diffusion equation: Oc Ot - D V 2 c

(1)

0 experiment calculation I

4

i

I

I

,

(b) 0.1MPa ::3

v2

which is subject to the boundary condition c(0, t)= c o = 3.0 on the particle surface. Then, the solution of Eq. (1) is given by the following error function [15]:

o

calculation

oo

c(r, t) = c o

2

J

exp( - uZ)du

(2)

I

I

s

(c) 6 M P a

r/2(Dt)l/2

In the calculation, (Dt) 1/2 is assumed to be much smaller than the sample size. Here, c(r,t) is the nitrogen concentration at the depth r from the surface of powder at the reacting time t, and D is the diffusion constant of the N atoms in Sm2Fe17, which is usually expressed by the hopping-type model, as D = D Oexp( - Q / k T )

I

(3)

Here, D O is the pre-exponential factor, and Q the activation energy for the hopping process. The values a D 0 = 1 . 0 2 × 1 0 - 6 m 2 s -1 and Q = 1 3 3 kJ m o l - were given by Coey [5] and Skomski [6] respectively. In our experiment, we found that the nitrogen absorption rate in SmzFe17 is independent of the N 2-

~

I

o--

2

1

periment

- - calculation

°o

I

24

4; t (h)

Fig. 14. Nitrogen concentration c(t) as a function of nitriding time t at 733 K u n d e r (a) 0.05 MPa, (b) 0.1 M P a and (c) 6 MPa. The solid lines are the calculated curves using the diffusion constant D = 3.1 × 10 -16 m 2 s -1 at 733 K and the velocity of grain growth of the fully nitrided phase (a) V o = 0 for 0.05 MPa, (b) Vo = 4.5 x 10 -11 m S-1 for 0.1 MPa and (c) Vo = 9 . 0 × 10 -I1 m s t for 6 MPa at 733 K.

H. Fujii et al. I Journal of Alloys and Compounds 236 (1996) 156-164

When the grain growth of the fully nitrogenated phase becomes dominant in the nitrogenation process, the distribution of nitrogen atoms in the powder shown in Fig. 13(c) would exist. The experimental results of nitrogen atom distributions in Fig. 12 are quite similar to the model distribution in Fig. 13(c). In the model distribution, with the nitrogen concentration under relatively high nitrogenized pressures PN2 ~> 0.1 MPa, c(r,t) is given by the following equation:

c(r,t) =' Co

exp(-u2)du

Sm2Fe,TN,

,'-7-.

.__>, O3 I:E C

733K

6MPa

forr>~VG(P, T)t I

r/2(dt) 1/2

I 30

20

I 40

(4) Here, VG is the velocity of the grain growth of the /3-phase nitride Sm2F17N3, which increases with increasing the nitriding Nz-gas pressures. This model explains fairly well the experimental results for nitrogenation under PN2 ~>0.1 MPa. That is, the velocity of grain growth of the/3-phase could be estimated to be Vc =4.5 × 10 -11 m s -1 at 733 K a n d PN2 =0.1 MPa from Fig. 12(a), using the relation of the depth of r-phase r t from the surface at the reacting time t, r t

163

20

I 50

I 60

(deg.)

Fig. 15. Powder X-ray diffraction patterns of the host Sm2Fe17 and fully nitrogenated Sm2Fe17N33 intermetallics.

I

I

!

x=3.3

1.5 ~Sm2FelTNx

// //

=VGt.

By substituting the diffusion constant D = 3.1 × 10 -16 m 2 s -1 and the velocity of grain growth of /3-phase VG = 4.5 x 10 -11 m s -1 into Eq. (4), the c(r,O was calculated in the case of PN2 = 0.1 MPa; this is shown in Fig. 14(b) by the solid line. The experimental points are in good agreement with the calculated curve. Furthermore, the c(r,t)in the nitrogen gas-solid reaction process at 733 K under 6 MPa is shown in Fig. 14(c), which was calculated by substituting D = 3.1 x 10 -16 m 2 s -1 and V~ = 9.0× 10 -11 m s - 1 (estimated from r, =: VGt) into Eq. (4). The agreement between the calculated and experimental data is quite good. Thus, we could understand the nitrogenation process in the gas-solid reaction under various N2-gas pressures in Sm2Fe J7. Finally, we tried to synthesize high quality Sm2Fel7N 3 by nitriding the 32 to 53/.tm powder for 84 h at 733 K under PN~ = 6 MPa. This reacting time was deduced by the relation of rt = Vot using 2r, = 42/.tm (average diameter of the powder) and VC = 9.0 × 10 -11 -1 m s . The X-ray diffraction profiles of Sm2Fe17 and Sm2FelTN3.3 thus obtained are shown in Fig. 15. These profiles indicate a high quality nitride with no segregation of c~-Fe phase. The magnetization curves of the aligned Sm2Fe17N3.3 powder are shown in Fig. 16. We notice the appearance of a strong magnetocrystalline anisotropy upon nitrogenation. The magnetization along the hard axis is quite small and linear against the magnetic field. This also confirms that the SmzFe17N3.3

/

0.5,'

. .-

x=0

,"" / '/

..L

/ #

0 ;'-'-"-"-

o

I

0.5

I

1.0

i

1.5

P.oH(T) Fig. 16. Magnetization curves of aligned powder at room temperature for the host Sm2Fe17 and fully nitrogenated Sm2FelTN3~ intermetallics.

fabricated under high pressure N 2 gas is really in a single-phase. The saturation magnetization and the anisotropy field at room temperature are estimated to be M s = 1.54 T and /z0HA/> 18 T respectively; the Curie temperature reaches Tc = 752 K.

5. Summary For clarifying the nitrogenation process in Sm2Fe17 under various N2-gas pressures from 0.01 to 6 MPa, we examined the time evolution of nitrogen absorption rates and nitrogen atom distributions by respectively measuring the increase in mass and by observing the corresponding EPMA profiles in plate-shaped pieces and powder. The nitrides were prepared by reacting the samples at several constant temperatures between 673 and 773 K for various reacting times from 3 to 192

164

H. Fujii et al. / Journal of Alloys and Compounds 236 (1996) 156-164

h under high N2-gas pressure atmospheres. The results obtained are summarized below. (1) In the nitrogen gas-solid reaction, there are two fundamental nitrogenation processes: a diffusion process of nitrogen atoms forming SmzFe17N x (x ~<3) and a grain growth process of the fully nitrogenated phase SmzFelTN3 . (2) The nitrogen atoms rapidly diffuse along the grain boundaries in the plate-shaped sample and cause many cracks. (3) In the next step, the N atoms diffuse into the grain interior along directions perpendicular to the grain boundaries. (4) The nitrogen absorption rate in the plate is larger during nitrogenation under 6 MPa than under 0.1 MPa, indicating an enhancement of the grain growth process under high N2-gas pressures. (5) Since nitrogenation is faster inside the plate than near the surface, it is essential to keep the surface of sample clean to obtain a homogeneous and faster nitrogenation. (6) Under reaction pressures PN2 ~<0.05 MPa, the diffusion process of nitrogen is dominant in nitrogenation at 733 K and leads to a simple gas-solid solution with intermediate nitrogen contents. (7) Under PN2 ~>0.1 MPa, the grain growth of the fully nitrogenated phase Sm2FelvN 3 becomes dominant at the later stages in nitrogenation at 733 K, exceeding the diffusion process of nitrogen atoms, which is important at the earlier stage. (8) The values of the diffusion constant D and the velocity of the grain growth Vc are estimated to be D = 3.1 × 10 -16 m 2 s l at 733 K independent of pressure and VG = 0 under PN2 ~<0.05 MPa, V6 = 4.5 × 10 -11 m s -1 under 0.1 MPa and Vc = 9.0 × 10 -11 m s -1 under 6 MPa at 733 K. (9) The grain growth process makes high pressure

nitrogenation effective for synthesizing high quality Sm2Fe17N3 nitride.

Acknowledgements This work was supported by a Grant for International Joint Research Project from NEDO, Japan. The authors are grateful to S. Suzuki of Minebea Co. Ltd. for providing the Sm2Fe17 host compound.

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