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KINETICS OF THE FORMATION OF AMORPHOUS Nd-Fe-B PARTICLES BY CHEMICAL REDUCTION
M. López Quintela, J. Rivas, I. Winter, W. Knoche
To cite this version:
M. López Quintela, J. Rivas, I. Winter, W. Knoche. KINETICS OF THE FORMATION OF AMOR-
PHOUS Nd-Fe-B PARTICLES BY CHEMICAL REDUCTION. Journal de Physique Colloques, 1990,
51 (C4), pp.C4-299-C4-305. �10.1051/jphyscol:1990437�. �jpa-00230797�
KINETICS OF THE FORMATION OF AMORPHOUS Nd-Fe-B PARTICLES BY CHEMICAL REDUCTION
M.A. L ~ P E Z QUINTELA,
J.RIVAS, I.
WINTER"and W. KNOCHE"
Departments of Physical Chemistry and Applied Physics, University of Bantiago de Compostela, SP-15706 Santiago de Compostela, Spain
Department of Physical Chemistry, University of Bielefeld, 0-4800 Bielefeld l,
F.R.G.Abstract
-
F i n e and u l t r a f i n e Nd-Fe-B p a r t i c l e s h a v e b e e n obtained f r o m metallic c a t i o n r e d u c t i o n by b o r o h y d r i d e ions both in water and microemulsions. In t h i s way a m o r p h o u s p a r t i c l e s o f d i f f e r e n t s i z e s ranging from 4-50 nm to 100-400 nm c a n be obtained. In t h i s work w e will report o n t h e k i n e t i c s o f t h e growth o f t h e p a r t i c l e s studied by stopped-flow with UV-Vis detection. Quantum e f f e c t s a r e suggested for t h e p a r t i c l e s with s i z e s about 5 nm.1
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INTRODUCTIONMagnetic m a t e r i a l s a r e widely used in modern technology. They a r e fundamental e l e m e n t s in a r e a s such a s information, t e l e c o m m u n i c a t i o n s and electronic devices. T h e development o f magnetic materials c o n d i t i o n s in a d e c i s i v e way t h e a d v a n c e in t h e s e areas. In particular, important a d v a n c e s in t h e s e f i e l d s a r e related with t h e investigation and development o f f i l m s and magnetic p a r t i c l e s /l/.
E v e n though the s t u d y o f u l t r a f i n e p a r t i c l e s i s known s i n c e t h e beginning o f colloidal c h e m i s t r y , introduced by the English chemist Thornas Graham in 1861, t h e interest for f i n e and u l t r a f i n e magnetic p a r t i c l e s h a s not taken hold until t h e last f e w y e a r s in which t h e technology demanded m a t e r i a l s o f higher performance and s m a l l e r s i z e s / 2 / .
For t h e p r e p a r a t i o n o f magnetic p a r t i c l e s o f a m o r p h o u s a l l o y s t h e r e are, fundamentally four basic p r o c e d u r e s /3/:
-rapid quenching
-deposition o f a t o m s in vaporized s t a t e -solid s t a t e r e a c t i o n
-chemical r e a c t i o n s in solution.
Among t h e m e t h o d s w e mention, t h e rapid quenching method is t h e most widely used. T h i s method c o n s i s t s in rapidly cooling the fused material ( s e e f i g u r e 1 ) . T h i s r e s t r i c t s t h e production o f a l l o y s to t h o s e with a c o m p o s i t i o n which is c l o s e to t h e eutectic point (E).
O n the other hand, with t h e chemical method i t is p o s s i b l e to o b t a i n a l l o y s with a n y c o m p o s i t i o n w i t h i n t h e interval c o r r e s p o n d i n g to the temperature at which t h e chemical reaction i s carried out, and what is m o r e important, it permits the direct production o f f i n e and u l t r a f i n e p a r t i c l e s with s i z e s s m a l l e r t h a n Z O . 1 pm / 4 / . P a r t i c l e s o f 1
-
10 nm a r e e a s i l y obtained with t h i s procedure.Recently a n original method w a s developed by t h e a u t h o r s /5-7/ f o r obtaining f i n e and u l t r a f i n e Nd-Fe-E p a r t i c l e s of controlled size. In t h i s work w e present a preliminary s t u d y o f t h e f o r m a t i o n k i n e t i c s for t h e Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990437
C4-300 COLLOQUE DE PHYSIQUE
magnetic particles in different experimental conditions.
R quenching A
r
L
R+L
v
A+LFig. l - Phase diagram o f a simple R 5 system. E = eutectic point. T = glass 9
/' /
'
AABtemperature. Compositions o f metallic glasses which c a n be obtained by rapid quenching and chemical reactions a r e shown.
2- EXPERIMENTAL PROCEDURE
/
+ * \
, l r/i//xJ
L
E
T h e f o l l o w i n g p r o c e d u r e w a s d e v e l o p e d f o r obtaining the m a g n e t i c particles o f
Nd-Fe-8. R solution with an adequate ratio of the metallic cations ~ e = + and
~d'+ was mixed with another o f borohydride. There were two ways o f mixing the solution, o n e was the dropwise addition of the reduction agent solution onto the one with the metallic ions and the other was by rapidly and directly mixing them using o f a "tee-junction". This way, we obtained fine particles with different ratios Nd/Fe/B and with sizes between 100 and 400 nm 7 In order to control the growth o f the particles we carried out the reaction in the presence o f microemulsions / S / . In this case, w e mixed two identical microemulsions, o n e containing the reduction agent and the other the metallic salts. W e found that the real s i z e of the particles ( 2 4
-
5 0 nm) was determined by the size of the droplets which made up the microemulsian. In this work the microemulsions w e used were "water-in-oil" microemulsions composed o f h e p t a n e / w a t e r / a e r o s o l - O T (sodium bis-(2-ethyl, hexyl) sulphosuccinate) with a concentration o f 0 . 0 5 M in aerosol-OT (ROT) and different ratios R = CHZ03/CAOTl. For this type o f microemulsions the s i z e ofA t
- .
T
B
\ chem. reaction
\
the water droplets may be calculated to a first approximation using the equation: r (radius o f the droplet)/nm = 0.175 R + 1.5 /E/.
The composition o f the samples obtained was studied by chemical analysis, ICP (induced coupled plasma) and X-ray fluorescence.
T h e structure was determined by the X-ray diffraction technique (CuK
cY
radiation) observing that the particles obtained were in the amorphous phase.
After exposing these particles to thermal treatments in an Ar atmosphere, they evolved to the crystalline state.
T h e kinetic experiments were performed with a combination o f a stopped-flow developed in our laboratory and a diode array spectrophotometer (HP 84526).
The basic magnetic properties o f the samples obtained w e r e measured with a SQUID magnetometer in the 2
<
T(K)<
300 temperature range. The magnetic materials a r e soft and thev present, at room temperature, values for the coercivity field o f H = 170 O e and for the saturation magnetization o fC
3- EXPERIMENTAL RESULTS
In order to understand the mechanism for the formation o f these magnetic particles we have carried out different types of studies.
In the first place, we performed experiences b y means o f DV-Vis spectrophotometry in the wavelength range 200
<
h(nm)<
500. According to the sizes o f the particles w e mention before, w e c a n differentiate three behaviors depending o n the size/wavelength ratio 9 - 1 1 In figure 2 w e represent these three regions:1 ) Rayleigh region: a/h
< <
1 (a=radius o f a n spherical particle).2 ) Mie region: a / h = 1 3 ) Optical region: a/X
>>.
1.1
-
Optical region
Fig. 2
-
Effective cross-section ( U ) o f an spherical particle. a = radius of the particle. X = wavelength.Therefore, for the particles obtained in water ( a 2 100-400 nm) w e must expect a Mie behavior from the spectrophotometric observation. In figure 3 w e clearly observe a cor~tinuous drop in the absorbance superimposed with resonances of the Mie type. For particles which are smaller than these wavelengths. such a s those obtained in microemulsions f a / &
<
1 ) we c a n expect a monotonous drop in the absorbance (Rayleigh region) due to the fact that the effective cross-section o f the particles decreases.Fig. 3- f?bsorption spectrum o f colloidal NdFeB obtained in water showing the existence o f M i e resonances.
C4-302 COLLOQUE DE PHYSIQUE
In figure 4 we present a characteristic spectrum obtained for the ultrafine particles. As i t can be seen, the behavior is not of the Rayleigh type and two maxima appear in this interval of wavelengths which can be attributed to the existence of quantum effects due to the small size of the particles.
These effects, which have been found in other semiconducting ultrafine particles /12-16/, is associated by most authors to the separation of the energy levels in the conducting band of the semiconductor due to the small dimensions of the particles. Because of this they are called Q-particles.
According to this interpretation, the quantum effects should disappear when we increase the size of the particles. This is so, in figure 5 we can observe for particles of sizes around 15 nm this effect disappears and we only observe the continuous band corresponding to the electronic absorption of this semiconducting material.
Fig. 4- Absorption spectra of colloidal NdFeB obtained in microemulsions
( R = 13) showing the-gresence of quantization effects.
1 ) CFeC123 = 1 . 1 4 ~ 1 0 M; CNdCl l = 2 . 2 2 ~ 1 0 - ~ ~ ; CNaBH l = 5 . 2 8 ~ 1 0 - ~ ~ . 2 ) Concentrations reduced bv a factor 6.
Fig. 5 - Absorption spectrum of colloidal NdFeB obtained in microemulsions with R = 30 ( r 6.5 nm) showing the clasical electronic absorption.
d r opirt-
CFeCl l = CNdCl l = 1 . 5 . ~ 1 0 - * ~ ; CNaBH 3 = 7 . 6 ~ 1 0 - ~ ~ .
the concentration of reactives used. 4 s an example, we show in figure 6 the variation of the UV-Vis spectra with the concentration of the reactives.
According with the above discussion the results clearly reflect the increase in size of the particles with the concentration.
Fig. 6- Absorption spectra of colloidal NdFeB obtained in microemulsions ( R = 22) showing the increase in the particle size with the reactant concentrations. [FeCl2l/[NdClS1/CNaBH>: a ) 1.9/0.37/8.8; b ) 2.7/2.7/13.6; c ) 8.2/2.7/25; d ) 13.6/2.8/36.0 ( x ~ o - ~ M ) .
3.2- Kinetic measurements
Figure 7 and figure 8 show the UV-Vis absorption spectra during the formation of the particles in microemulsions and water respectively.
We have systematically observed in all the experiments performed that the shape of the curves and the positions of the maxima do not vary with time during the formation of the particles. This seems to indicate that within the mechanism that can be used to explain the formation of the particles (see scheme l), the limiting step is the chemical reaction for the formation of
the nuclei and that the final growth of these is faster.
NaBHI particle
nucleation
'
NdaFebBc growth'
(NdaFebBc)x Scheme 1- Proposed mechanism for the formation of colloidal NdFeB.COLLOQUE DE PHYSIQUE
Fig. 8- Time-resolved a b s o r p t i o n s p e c t r a o f colloidal N d F e B obtained in a q u e o u s solution. CFeClZ1 = CNdCl31 = 1 .5x10-*M; CNaBH 3 = 7.6x10-"M.
4
Fig. 7-Time-resolved a b s o r p t i o n s p e c t r a o f colloidal NdFeB obtained in m i c r o e m u l s i o n s ( R = 10). tFeClzl = 8 . 6 ~ 1 0 - ~ ~ ; CNdC133 = 1 . 7 ~ 1 0 - ~ ~ ;
CNaBH43 = 3 . 9 ~ 1 0 - ~ ~ .
A
l
small-angle X-ray scattering by synchrotron radiation (DESY, Hamburg, FRG).
A11 o f these results seem to support the mechanism here proposed.
From the experimental results here reported we c a n then conclude:
1 ) The existence o f quantum effects in ultrafine NdFeB particles and
2 ) The formation o f colloidal NdFeB seems to take place by a chemical
reaction and nucleation which a r e much slower than the growth o f the nuclei.
Finallly, we must point out that these are preliminary results and that more experiments are at the moment being carried out (synchrotron,
time-resolved neutron scattering, etc.) in order to clarify the mechanism o f the formation o f colloidal NdFeB.
Acknowledgments- M.A.L.Q. and I.W. wish to acknowledge the financial support o f the European Community Contract No. SC1-000058. M.A.L.R. and J.R. also acknowledge funding from the Spanish CICYT No. MAT89-0425-C03.
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