VALVES EXHAUST GAS
Appendix 2 Appendix 2
3. Magnetic properties of RzFel4B compounds
4.2. Magnets' obtained by splat-cooling
The researchers from General Motors obtained in 1983 isotropic N d - F e - B magnets by splat cooling (Croat et al. 1984a, b). These are manufactured by pushing the melt alloy under pressure, in argon atmosphere, by a quartz aperture, onto a rotating disk. Optimum magnetic properties are obtained for a disk speed which results in grain dimensions of the Nd2Fe~4B phase in the range 200-500 A.
If the disk speed is higher, optimum magnetic properties are obtained by annealing the samples. The thermal treatment determines an increase of the particle size.
PHYSICAL PROPERTIES OF R2Fe14B-BASED ALLOYS 121 The composition dependence of the magnetic properties of Ndl_x(Fe0 95B0.05)x amorphous alloys is plotted in fig. 28 (Croat et al. 1984b)o The maximum energy product for this isotropic magnet is obtained for x = 0.87, namely ( B H ) . . . =
l l 2 . 8 k J / m 3. This composition is rich in neodymium and poor in boron in comparison with the Nd2Fe14B compound.
Hadjipanayis et al. (1983, 1985d, e, 1986, Dickenson et al. 1985) analysed the magnetic properties of melt-spun magnets. By adding Si and B to R - F e - B alloys, amorphous ribbons with energy products up to 104 kJ/m 3 were obtained (Had- jipanayis et al. 1983). The as-quenched R - F e - B alloys are usually magnetically soft, but some of them show large magnetic hysteresis when annealed around 700°C. In case of Pr(Nd)16Fe76B 8 samples, coercivities in excess of 0.8 MA/rn were obtained (Hadjipanayis et al. 1985e). Relatively large anisotropies were observed in Y and Gd containing samples. The partial substitution of Fe by Ni and Mn leads to a significant decrease of the Curie points. The magnetic
MHc [kA/m)
(BH)m~x B HC (kA/m]
( kJ/m 3) 128[
(BH)mox( k J / m 3 / 1
32 ~=====a---'- / BHc
Br
0
1280 _ - / ~ ~ ~ 640 -
0
N dl_x(Feo.g5 Bo.os)×
Q7 0.8 0.9
---,,--X
1.6
1.2 0.8
B r (T) []Z
0
Fig. 28. The magnetic properties of Nd~ x(Fe0.95B0ms)x amorphous alloys.
122 E. BURZO and H.R. KIRCHMAYR
behaviour of R15Fe77B s melt-spun ribbons with R = Y, La, Pr, Nd, Sm, Gd, Tb, Dy and Er was also studied (Aly et al. 1986). The ordering temperatures of these alloys were found to be in the range 90-300°C, well below the crystallization temperatures. Fairly large magnetic hysteresis (120-640kA/m) is observed at 4.2 K for alloys containing Er, Sm, Dy and Tb. The coercivities of melt-spun N d - F e - B magnets decrease drastically by increasing the temperature and are small at temperatures considerably lower than the Curie points (Hadjipanayis and Tao 1985). The magnetic properties of the melt-spun R - F e - B magnets have been correlated with the microstructure and domain structure (Hadjipanayis et al.
1985d, Aly et al. 1986). As in crystalline systems, four different phases with composition Nd2FelgB , NdI+~Fe4B4, ~-Fe and a high Nd-content phase have been found. The major phase observed in all the studied alloys is Nd2Fe14 B, with much finer grains in melt-spun ribbons than in sintered magnets. Herbst et al.
(1985) showed that the mean dimension of the particles obtained by neutron diffraction in Nd0.13Fe0 83B0.08 ribbons is --300,~, while by electron microscopy studies the dimension of the particles, in the range 200 A and 800A, are observed.
Gr6ssinger et al. (1985d) and Eibler et al. (1984) show a maximum in MHc values of Nd15FevvB 8 and Nda4FesaB 5 amorphous alloys for a disk speed v--- 15m/s close to the value of 19m/s reported by Croat et al. (1984a, b). In Pr16(Felo 0 xCOx)76B 8 alloys for a disk speed v = 2 0 - 3 0 m / s a value MHc ~ 1.6 M A / m is obtained (Overfeld and Becket 1984).
Narasimhan (1985a) manufactured P r - F e - B magnets in the form of amorphous ribbon having composition close to that reported by Hadjipanayis et al. (1983).
By increasing the praseodymium content, the intrinsic coercive fields are greater.
An alloy having the composition PrlzFev6Ba2 has
MHc
= 0.512 M A / m . By crystal- lization of the Pr0.15Fe0.60B0 17 alloy a coercive fieldMHc
= 23 M A / m was ob- tained.Tao and Hadjipanayis (1985) studied recrystallization of the rare-earth-iron metalloid alloys. It was pointed out that the RzFeI4B phase is stable for R = Ce to Tm. The recrystallized samples have coercive field up to 0.48 M A / m .
Koon and Das (1984) reported the properties of rapidly quenched R - F e - B alloys (R = La, Pr, Nd, Tb) and Becket (1984) studied melt spun Ra6Fey6BsSi 3 (R = Pr, Nd, Sm) alloys. Coercivities
MHc
> 1.6 M A / m were obtained.Yamasaki et al. (1986) analysed the composition dependence of the anisotropy field of rapidly quenched MMyFel00_y xB~ ribbons with 5 < x < 9 and 15 < y < 17 at substrate speed v = 25 and 30 m/s. The MHc exhibits a tendency to increase with increasing x and y. The largest coercive field was 0 . 7 5 2 M A / m for MM10Fe75B 9 ribbons. Intrinsic coercive forces up to 0.512 M A / m were reported in melt-spun Tb0.135Fe0.s~3B0.052 and DY0.~35Fe0813B0.052 alloys after magnetic hardening by rapid solidification (Pinkerton 1986). The peak coercivities are obtained only within a narrow range of substrate velocities and are associated with a microstructure consisting of small particles of R2Fe~4B with size distribu- tion from 30 to 600 nm.
Jiangao et al. (1985) studied the magnetizations and coercivities of (NdxFe I x)94B6 metallic glasses. Sellmyer et al. (1984) obtained good permanent
PHYS1CAI, PROPERTIES OF R2FeI4B-BASED ALLOYS 123
magnets in case of Nd16Fe76B 6 amorphous alloy after heat treatment at -700°C for 30 min.
The magnetic hardening of melt-spun N d - F e - B and P r - F e - B alloys is achieved by either quenching directly from the melt or by annealing an over- quenched precursor (Croat 1985). Samples with optimum properties have a uniform finely crystalline microstructure with an average particle size less than 50nm. The quenched ribbons may be also consolidated into bulk magnets by conventional powder metallurgy processing and hot press/hot form techniques (Lee et al. 1985). Energy products up to 320 kJ/m 3 have been achieved by hot deformation.
Hadjipanayis et al. (1985a) analysed the hysteresis and crystallization behaviour of an amorphous Nd54Co36B10 alloy. Magnetic measurements on melt-spun ribbons reveal a coercivity of 0.48 M A / m at 4.2 K. The coercivity at 4.2 K of a sample heat treated at 210°C is increased to 0.64 M A / m but is reduced gradually upon annealing at higher temperatures. The origin of magnetic hardening in N d - F e - B alloys was also investigated (Hadjipanayis et al. 1985b). Initial mag- netization studies, magnetic viscosity data, the temperature dependence of coer- civity and magnetic domain studies all indicate domain wall pinning at grain boundaries.
The good magnetic properties of amorphous ribbons were obtained only in N d - F e - B and P r - F e - B systems. The partial substitution of Nd by Dy and Tb leads to the increase of the coercive field, decreasing the remanent induction.
Replacement of Nd by La, Ce or Sm decreases the coercive field. The alloys, having high magnetocrystalline anisotropy, were obtained only by using boron.
The substitution by C, P, Si, Ge or A1 had a negative influence on magnetic properties.
Transmission electron microscopy was used to characterize the microstructure of melt-spun ribbons having the composition Nd13Fesz.6B4.4 (Chen 1985). The microstructure consists of fine grains which are practically single-phase materials.
There are also very thin grain boundary films surrounding the grains. The grain size varies from 100 to 800 ~ , with an average of about 150 ~ . The good magnetic properties are probably due to the fine grained Nd2Fe14B compound (i.e. single domain fine particles).
Mishra (1986) shows that the best magnetic properties of melt-spun magnets are obtained in materials with a two phase microstructure where 20-30 nm size Nd2FelaB grains are completely surrounded by a 1-2 nm thick amorphous film of Nd-rich and B-deficient phase. This phase acts as a pinning site for the magnetic domain walls. The presence of this amorphous phase is explained by the fact that nuclei of Nd2FelmB grains form homogeneously during cooling from the melt (or during annealing). As these grains grow, all the B diffuses into the grains and is consumed while the material outside the Nd2Fe14B grains gets enriched in Nd.
The coercive force is highest when the grain boundary area to grain volume is highest, providing maximum pinning sites. The small grain size makes reverse domain nucleation relatively easy in the melt spun magnets, but the pinning is much more effective due to the two-phase microstructure.
124 E. BURZO and H.R. KIRCHMAYR
Low field dc thermomagnetic measurements on N d - F e - B magnets were per- formed in order to obtain information on the microscopic magnetic behavior (Rao et al. 1985). Heat treating of these materials in the temperature range 400 to 600°C is found to change the constitution of the alloy and significantly modify the magnetic properties.
The effect of changing the melt-spinning roll material on the value of roll velocity corresponding to the peak value of ( B H ) m a x for Fesl.TsNd13.sB4.75 alloy, was studied in correlation with differential scanning calorimetry and microscopic observations (Ogilvy et al. 1984).
Lee (1985) and Lee et al. (1985) analysed the possibilities to manufacture magnets from over quenched melt-spin ribbons of the nominal composition Nd0,4(Fe0.91B0.09)0 86 by hot-pressing at 700-750°C and 100 kPa. The resulting material is only slightly magnetically aligned with B r = 0.8 T, MHc = 1.52 M A / m . Die upsetting, however, to thickness reduction of 50%, produces B r = 1.1 T, MI-Ic = 0.96 M A / m and is accompanied by a lateral plastic flow of the ribbons and a corresponding ribbon thickness reduction resulting in an anisotropic magnet.
The alignment arises from anisotropic grain growth and grain rotation which produces platelet shaped grains oriented perpendicular to the press direction.
A new rapid solidification processing technique, liquid dynamic compaction (LDC), has been used to make bulk permanent magnets of composition NdlsFes7Co20B 8 (Chin et al. 1986). In this process, molten metal is spray atomized onto a cooled substrate in a protective atmosphere. Subsequent heat treatment at 450°C of the LDC material resulted in isotropic permanent magnets with intrinsic coercivity and remanence approaching to MHc = 0 . 6 4 M A / m , and
B r = 0.6 T. Tanigawa et al. (1986) used LDC method in making bulk, oxide free N d - F e - C o - B magnets of high quality directly from the molten state. Intrinsic coercivities MHc = 0.64 M A / m and B r = 0.6 T were achieved after one hour heat treatment at 450°C. Magnets with wider range of compositions were then produced. Processing by LDC method allows for sufficient quench rate control to produce microstructures with grain sizes on the submicron scale or up to several tens of microns. The inter-granular material is enriched in Nd - relative to grains themselves. Later on (Chin et al. 1986), the magnetic hardening of dynamically compacted liquid N d - F e - C o - B magnets was studied. A great improvement in the coercivity occurs after heat treatment around 450°C.
The amorphous R - F e - B alloys obtained by melt-spinning have been also studied by M6ssbauer effect (Gr6ssinger et al. 1985d, Niarchos et al. 1985, Rotenberg et al. 1985). The spectra show slight differences with respect to those obtained from annealed ingots.
In case of rapidly solidified magnets, d < d~ and thus a part of the grains are single-domains (Livingston 1985b). The magnets are isotropic and consequently a fraction of the permeability is determined by magnetization rotation. In order to obtain a peak value of the coercivity it is necessary to have a rotating speed of the disk, which ensures single domain particles. For a higher speed, superparamag- netic behaviour may be observed, while for a lower one, multi-domain particles are evidenced. In both cases, the coercivity decreases. As in crystalline systems,
PHYSICAL PROPERTIES OF R2Fej4B-BASED ALLOYS 125 domain wall pinning has been observed at the grain boundaries. The domain walls move easily inside the grains. T h e slower increase of the initial magnetization with applied field observed in ribbons is due to the randomness of R2Fe,4 B grains orientation. This fact needs much higher fields to align them as c o m p a r e d to those of sintered N d - F e - B magnets. T h e randomness also leads to smaller values of the magnetic induction and lower energy product. A u g e r electron spectroscopic studies in these magnets indicate considerable compositional inhomogeneities at the surface. A f t e r Hilscher et al. (1986) the coercivity for melt-spun ribbons with optimum cooling rate appears to be dominated by domain wall pinning.
4.3. The stability of R - F e - B permanent magnets
T h e absolute values of the t e m p e r a t u r e coefficients of Br, RHc and (BH) .... up to 100°C, are two times greater in N d - F e - B magnets than in SmCo 5 ones and three times as large for ~tHc (Li et al. 1985). The losses which appear may be covered, as result of thermal demagnetization, by heating at T > T c and remag- netizing the sample. For a N d - F e - B m a g n e t having (BH) .... = 256 k J / m 3 and
~H c = 0.904 M A / m , the reversible coefficient of coercive field a~/c = - 1 . 0 9 % / ° C and of the energy product C~(BH)~, x = - 0 . 3 9 8 % t ° C (Potenziani et al. 1985).
In case of N d - F e - B magnets heated at 300°C for 3 h in argon atmosphere, irreversible losses are small ( ~ 0 . 5 % ) (Narasimhan 1985a). The changes in demagnetizing curves by heating the samples are shown in fig. 29. By heating at 150°C, the coercive force diminished considerably.
T h e irreversible losses in r e m a n e n t induction of a N d - F e - B magnet with 8H~ = 0.9 M A / m and B r = 1.15 T after exposure for 3 h at various t e m p e r a t u r e s are plotted in fig. 30. T h e relative small irreversible losses suggest that N d - F e - B magnets may be used up to 100°C. For magnets having B~ = 1.285T, Bite = 0 . 9 1 6 M A / m and (BH) . . . . • 310 k J / m 3, the thermal variations of B,. values in
1 , 2 [ F i i r ~ i ~ I V ~
1,0 f _50oC ~ / ~ /
m
0,6
0,4
X / / / F
096 080 0B4 0.48 022 0.16 0
-~ -H (MAIm)
Fig. 29, Some demagnetizing curves for Nd-Fe- B magnets.126 E. BURZO and H.R. KIRCHMAYR
0
o
_.¢ 25-1
I ; [ I
c._
~>-2
25
I I I _ _ h _ ~
50 75 100 125 150
~ T e m p e r o t u r e (o C )
Fig. 30. The temperature dependence of the ir- reversible losses of the remanent induction.
aBr = - 0 . 1 0 % / ° C and aMHc = - 0 . 6 % / ° C . F o r m a g n e t s with higher coercivities a . r = - 0 . 0 8 9 % / ° C and %,/4~ = - 0 . 5 4 % / ° C were r e p o r t e d ( N a r a s i m h a n 1985a).
Strnat et al. (1985) analysed the magnetic properties of N d - F e - B m a g n e t s in the t e m p e r a t u r e range - 2 0 0 ° C to +200°C. As result of the great decrease of
MHc
values in the t e m p e r a t u r e range 100-150°C, the irreversible losses for r e m a n e n t induction in this t e m p e r a t u r e region are large, especially for high values of the p e r m e a n c e .
N a r a s i m h a n (1985a) analysed the integrity of m a g n e t s after e x p o s u r e at tem- p e r a t u r e s 100 to 150°C in m o i s t u r e conditions. A f t e r 16 h, the m a g n e t s are stable.
It is suggested that the N d - F e - B m a g n e t s m a y be used up to 150°C for a limited time.
T h e t e m p e r a t u r e coefficient of r e m a n e n t induction for hot p r e s s e d and die- upset ribbons is - 0 . 2 5 % / ° C ( L e e et al. 1985). T h e t e m p e r a t u r e coefficient of
MHc
for hot pressed is - 0 . 2 5 % / ° C and for die-upset ribbons - 0 . 4 5 % / ° C . By aging in air at high t e m p e r a t u r e s the losses are smaller than for sintered magnets.
T h e t e m p e r a t u r e coefficient of irreversible polarization losses are smaller, the stronger the coercive field. Partial substitution of Nd by D y increases coercivity and the irreversible polarization losses decreases. T h e polarization losses due to t h e r m a l aftereffect decrease with t e m p e r a t u r e ( R o d e w a l d 1985a, b).
In our opinion the most urgent p r o b l e m s to be solved in the n e a r future are the magnetic stability at elevated t e m p e r a t u r e s up to 200°C and the corrosion in humid a t m o s p h e r e , for a real b r e a k t h r o u g h for these m a g n e t s in a multitude of applications to h a p p e n .
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