A study of the preparation conditions of FeB amorphous alloy powders produced by chemical reduction

Journal of Non-Crystalline Solids 124 (1990) 139-144 North-Holland

139

A study of the preparation conditions of Fe-B amorphous alloy powders produced by chemical reduction J. Jiang, I. D6zsi, U. G o n s e r a n d X. Lin a Universitiit des Saarlandes, Werkstoffwissenschaften, 1)-6600 Saarbriicken, FRG Institut fiir Physikalische Chemie, D-6600 Saarbriicken, FRG Received 18 April 1990

Ultrafine amorphous F e - B alloy powders were produced by chemical reduction. The experimental conditions (FeSO a, K B H 4 solution concentrations, volume ratio of the FeSO 4 to K B H 4 solutions, and reaction temperature) determining the structure and properties of the amorphous powders were investigated. The optimal conditions for production were determined.

1. Introduction Amorphous alloys are usually prepared as ribbons or films by melt spinning or by sputtering techniques. Recently, it was shown that ultrafine amorphous alloy powders can be prepared by chemical reduction performed at room temperature [1-5]. This result is important because with this method amorphous powders can be produced in large quantities. Because of the practical interest of these materials (magnetic recording media, catalysts, etc.) these powders have been extensively investigated in the past few years [1-5]. As early as 1953 Schlesinger et al. [6] prepared crystalline Fe-Co-base alloy powders by reducing FeSO 4 and CoC12 with potassium borohydride K B H 4. Recently, amorphous Fe-Co-base alloy powders were prepared by using a similar method [1]. The structure of the powder samples changes with differing conditions during preparation, However, the correlation between the conditions and the structure of the powder samples is not known. Since the chemical reaction is complex and is not precisely known, the optimal condition could be found only by a systematic change of the preparation conditions. In this paper we present results on the influences of various conditions on

the structure of the F e - B powder samples. Four parameters were varied systematically: the concentration of the FeSO 4 and KBH 4 solutions; the volume ratio of the FeSO 4 to KBH 4 solutions; and the reaction temperature. An optimal condition was determined for the production of the amorphous F e - B alloy powders.

2. Experimental The powder samples were prepared by the chemical reduction of FeSO 4 aqueous solution with potassium borohydride KBH 4. The process of sample preparation is shown in fig. 1. The aqueous solution of the metal salt, FeSO 4, was added in drops to an aqueous solution of the potassium borohydride, KBH 4, and vigorously stirred. Black precipitates formed and were collected on a filter, washed with distilled water and acetone and dried in a slow stream of air. Table 1 shows the various parameters applied during the preparation of the samples. The MiSssbauer spectra of the samples were measured at room temperature on a conventional constant acceleration type spectrometer. The absorbers were made b y pressing the powders gently between two synthetic

0022-3093/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)

140

J. J i a n g et al. / F e - B a m o r p h o u s alloy p o w d e r s

t:? Ice end Wafer

Reduction

Fittering ond Washing

Drying

Fig. 1. Schematic illustration of the preparatory steps for the production of the amorphous F e - B alloy powders by chemical reduction.

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J. Jiang et al. / Fe-B amorphous alloy powders

mol FeSO 4 solution, the sample A1 was found to contain a larger amount of the crystalline phase and a smaller quantity of the amorphous component. A least-squares fitting of the spectrum shows three subspectra. The hyperfine parameters are given in table 2. On the basis of the hyperfine parameters and the X-ray diffraction pattern of sample A1, three components can be identified: a-Fe; 7 - F e 2 0 3 ; and an F e - B amorphous phase. The hyperfine parameters of the 7-Fe20 3 in the sample - isomer shift (0.26 m m / s ) and quadrupole splitting (0.78 m m / s ) - agree well with the reported data for 7-Fe20 3 particles with 6 nm average diameters [7]. These data indicate that the y-Fe203 phase exists in small particles. With a decrease ( < 1 mol) of the concentration of the FeSO 4 solution, the relative intensity of the amorphous component increased and the fraction of a-Fe and y-Fe203 components significantly decreased. The powder sample with the largest intensity of the amorphous component was obtained by using 0.1 mol FeSO 4 solution (as shown in fig. 2 for A3). In this case an a-Fe phase was not be detected. The M6ssbauer spectrum of the sample A3 could be fitted by a broadened sextet component with a hyperfine field distribution typical of amorphous alloys. The mean hyperfine field is 230 kOe. The amorphous state of this sample was also verified by its X-ray diffraction pattern, as shown in fig. 3, where no sharp diffraction peaks could be resolved. When the concentration of the FeSO 4 solution further decreased (fig. 2 for A4), the spectrum showed a broad splitting with a 227 kOe mean hyperfine field corresponding to an amorphous phase. It is worth noting that a paramagnetic doublet subspectrum also appeared in

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fig. 2 for A3 and A4 samples, suggesting the existence of the ferric oxide component. These results indicate that the concentration of the FeSO 4 solution plays a very important role. In fig. 4 the change of the amorphous fraction as a function of the FeSO 4 solution concentration determined by the relative area of the Mi~ssbauer spectral components is shown. It was assumed that the Debye-Waller factor is the same for the various components. One can conclude that a predominating amorphous fraction could be produced by using a relatively low concentration of the FeSO 4 solution (0.1 mol). Schlesinger et al. [6] obtained crystalline alloy powders using highly concentrated FeSO 4 and CoC12 solutions (1 mol).

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100

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J. Jiang et al. / F e - B amorphous alloy powders

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t r u m c o n s i s t s of two c o m p o n e n t s , o n e ferric o x i d e a n d the o t h e r F e - B a m o r p h o u s c o m p o n e n t (fig. 5 for B1). T h e a m o r p h o u s f r a c t i o n d e c r e a s e d w i t h increasing KBH 4 concentration. The a-Fe phase was d e t e c t e d w h e n a 0.5 m o l c o n c e n t r a t i o n o f the K B H 4 s o l u t i o n was u s e d (fig. 5 for B3). T h e m e a n h y p e r f i n e field of the a m o r p h o u s c o m p o n e n t i n fig. 5 for B3 was 254 k kOe, w h i c h is l a r g e r t h a n the v a l u e for s a m p l e B2 (230 kOe). It is k n o w n t h a t the r e l a t i o n of the m e a n h y p e r f i n e field to the b o r o n c o n t e n t i n the F e - B a m o r p h o u s p o w d e r s a m p l e s is s i m i l a r to t h a t of F e - B a m o r p h o u s alloy r i b b o n s a m p l e s p r o d u c e d b y the m e l t s p i n n i n g t e c h n i q u e [8]. T h e m e a n h y p e r f i n e field decreases w i t h i n c r e a s i n g b o r o n c o n t e n t . A s we disc u s s e d in o u r earlier p a p e r [5], the h y p e r f i n e field is d e p e n d e n t o n v a r i o u s factors, b u t it c a n also b e r e l a t e d to the b o r o n c o n t e n t of the F e - B a m o r p h o u s s a m p l e . Based o n this a p p r o x i m a t i o n , the d e c r e a s e of the h y p e r f i n e field m a y i n d i c a t e

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J. Jiang et al. / Fe-B amorphous alloy powders

t h a t the b o r o n c o n t e n t in s a m p l e B3 is lower t h a n in s a m p l e B2. O n e c a n c o n c l u d e that the high a m o r p h o u s f r a c t i o n c a n n o t be o b t a i n e d b y using a relatively high K B H 4 s o l u t i o n c o n c e n t r a t i o n ( > 1 mol). O n the o t h e r h a n d , one should r e m e m b e r that the a - F e p h a s e a p p e a r e d b y using a relatively low K B H 4 s o l u t i o n c o n c e n t r a t i o n . Therefore, an o p t i m a l K B H 4 c o n c e n t r a t i o n (1 mol) should be u s e d for o b t a i n i n g the l a r g e s t f r a c t i o n of a m o r p h o u s phase. F u r t h e r , we c o n s i d e r e d a n o t h e r p a r a m e t e r , the v o l u m e r a t i o of the F e S O 4 to K B H 4 solutions. F i g u r e s 7 a n d 8 show the MiSssbauer s p e c t r a a n d the c o n t e n t s of the a m o r p h o u s cornp o n e n t in the s a m p l e s p r e p a r e d with various v o l u m e ratios, respectively. It can b e seen that a m o r p h o u s F e - B alloy p o w d e r s a m p l e s can be p r e p a r e d o n l y with large v o l u m e ratios ( > 0.6) of the F e S O 4 to K B H 4 solutions. T h e m e a n h y p e r fine fields of the a m o r p h o u s c o m p o n e n t s for the s a m p l e s C2 a n d C3 are 198 k O e a n d 243 kOe, respectively. This c a n again be d u e to the different b o r o n c o n t e n t s of the samples. F r o m the i s o m e r shift values the s a m e c o n c l u s i o n can b e d r a w n . I n the s a m p l e C3 the i s o m e r shift is 0.12(1) m m / s a n d for the s a m p l e C2 it is 0.25(1) m m / s . F o r the a m o r p h o u s F e - B alloy system [8], the i s o m e r shift was f o u n d to d e c r e a s e with d e c r e a s i n g b o r o n content. A small a m o u n t of a - F e c a n also b e f o u n d in s a m p l e s C1 a n d C2, the reason for which at this m o m e n t is n o t known. F i g u r e s 9 a n d 10 show the M S s s b a u e r s p e c t r a a n d c o n t e n t s o f the a m o r p h o u s c o m p o n e n t , f o r the s a m p l e s ( D 1 - D 5 ) , respectively, p r e p a r e d at vari.I.00

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144

J. Jiang et al. / Fe-B amorphous ahoy powders

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ous r e a c t i o n t e m p e r a t u r e s . T h e s p e c t r a in fig. 9 show two c o m p o n e n t s , the a m o r p h o u s p h a s e a n d a p a r a m a g n e t i c ferric oxide d o u b l e t c o m p o n e n t . T h e l a t t e r increases with increasing r e a c t i o n temperature. Therefore, a low r e a c t i o n t e m p e r a t u r e ( < 279 K) s h o u l d b e used to a v o i d the f o r m a t i o n of the ferric oxide. F i g u r e 11 shows the c h a n g e of the m e a n h y p e r f i n e field a n d i s o m e r shift values of a m o r p h o u s F e - B c o m p o n e n t s as a f u n c t i o n of the r e a c t i o n t e m p e r a t u r e . It is seen that the b o r o n c o n t e n t d e p e n d s s t r o n g l y on the r e a c t i o n t e m p e r a ture. W h e n the t e m p e r a t u r e increases, the b o r o n c o n t e n t also increases.

4. Conclusions T h e results p r e s e n t e d in this p a p e r show that c e r t a i n p r i o r c o n d i t i o n s s h o u l d b e a d o p t e d in o r d e r tO p r e p a r e a m o r p h o u s F e - B alloy powders. Since the r e a c t i o n m e c h a n i s m is not sufficiently u n d e r -

stood, o n l y some e m p i r i c a l l y d e t e r m i n e d o p t i m a l p a r a m e t e r values can be o b t a i n e d . U s i n g a relatively high c o n c e n t r a t i o n of F e S O 4 s o l u t i o n ( > 0.1 mol) a larger crystalline c o m p o n e n t results. A larger a m o u n t of ferric o x i d e c o m p o n e n t forms when the s a m p l e s are p r e p a r e d b y using a low FeSO4 s o l u t i o n c o n c e n t r a t i o n a n d low v o l u m e ratios of F e S O 4 to K B H 4 solutions, high conc e n t r a t i o n of the K B H 4 s o l u t i o n a n d at high reaction t e m p e r a t u r e . T h e a - F e p h a s e forms at a relatively low K B H 4 s o l u t i o n c o n c e n t r a t i o n . T h e b o r o n c o n t e n t of the a m o r p h o u s F e - B alloy comp o n e n t varies with the c o n c e n t r a t i o n of the F e S O 4 a n d K B H 4 solutions, the v o l u m e r a t i o of F e S O 4 to K B H 4 solutions, a n d the r e a c t i o n t e m p e r a t u r e . A n o p t i m u m c o n d i t i o n for the p r e p a r a t i o n of the a m o r p h o u s F e - B alloy p o w d e r s c a n b e c o n c l u d e d to be as follows. (i) c o n c e n t r a t i o n of the F e S O 4 solution: = 0.1 mol (ii) c o n c e n t r a t i o n of the K B H 4 solution: = 1.0 mol (iii) q u a n t i t y r a t i o of F e S O 4 to K B H a solutions: > 0.6 (iv) r e a c t i o n t e m p e r a t u r e : < 279 K

References [1] J. van Wonterghem, S. Marup, C.J.W. Koch, S.W. Charles and S. Wells, Nature 322 (1986)622. [2] D. Buchkov, S. Nikolov, I. Dragieva and M. Slavcheva, J. Magn. Magn. Mater. 62 (1986) 87. [31 A. Inoue, J. Saida and T. Masumoto, Metall. Trans. 19A (1988) 2315. [41 A. Corrias, G. Ennas, G. Licheri, G. Marongiu, A. Musini, G. Paschina, G. Piccaluga, G. Pinna and M. Magini, J. Mater. Sci. Lett. 7 (1988) 407. [5] J. Jiang, I. D6zsi, U. Gonser, and J. Weissmiiller, J. NonCryst. Solids 116 (1990) 247. [6] H. Schlesinger, H.C. Brown, A.E. Finholt, J.R. Gilbreath, H.R. Hoekstra and H.R. Hyde, J. Am. Chem. Soc. 75 (1953) 215. [7] J. Jing, PhD Thesis, Universitiit des Saarlandes (1989). [8] S. Wells, S.W. Charles, S. Morup, S. Linderoth, J. van Wonterghem, J. Larsen, and M.B. Madsen, J. Phys. C, to be published.