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manganese ferrite composites
Journal of Alloys and Compounds 278 (1998) 264–269
L
Fe or Co–Fe alloy / manganese ferrite composites ` Pourroy* Genevieve ´ I.P.C.M.S., Groupe des Materiaux Inorganiques, 23 Rue du Loess, F-67037 Strasbourg Cedex, France Received 21 February 1998; received in revised form 28 March 1998
Abstract We describe the procedure to obtain composites made of iron or iron–cobalt alloys and manganese-containing magnetite. A KOH concentration of 14 mol l 21 is required. Iron and manganese based composites are made of metallic iron and manganese-containing magnetite. For Mn / Fe ratios higher than 0.15, hydroxide is also present. When cobalt is involved, hydroxide is encountered for Mn / Fe ratios higher than 0.2. The metal is thus an iron–cobalt alloy which crystallizes in the b.c.c. structure or in the a-Mn structure. This latter disappears by heat treatment above 1808C. At 4008C, cobalt of f.c.c. structure crystallizes. Above 5008C, the composite transforms into ¨ the wustite structure. HRTEM and EDSX analyses show that the metal is coated by ferrite grains. 1998 Elsevier Science S.A. All rights reserved. ¨ Coercive fields Keywords: Composite; Spinel ferrite; Cobalt–iron alloys; Wustite;
1. Introduction Metal–ceramic composites have been widely studied for their mechanical or magnetic properties. Several methods of synthesis have been developed in the last years among which the reduction of transition metal oxides (Fe 2 O 3 , CuO, NiO) in oxide or gels of Al 2 O 3 or SiO 2 , or high energy ball milling of metal and oxides [1–6]. Another method which is based on the disproportionation of Fe(OH) 2 in a concentrated and boiling KOH has been used to obtain Fe 0 / Fe 3 O 4 composites [7]. Indeed, FeO is known to disproportionate into Fe 0 and Fe 3 O 4 below 5708C but if FeO is quenched at 3008C, iron metal is not detected from the corresponding X-ray diffraction pattern [8]. When this disproportionation occurs in concentrated alkaline (KOH) solution, iron and magnetite are both very well crystallized [7]. Metallic iron has got the b.c.c. structure and is imbricated in the magnetite grains. When Co(II) or Ni(II) are implied, the procedure involves the disproportionation of Fe II into Fe 0 and Fe III and a reduction of Co(II) or Ni (II) by Fe 0 . For the former, the resulting composite is made of an iron–cobalt alloy and a cobalt containing magnetite [9–11]. The iron–cobalt
*Corresponding author. Tel.: 133 88107132; fax: 133 88107247.
0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00547-7
alloy is found to be either isomorphous of a-Fe (b.c.c. structure) or crystallized both in the a-Fe and a-Mn structures [12]. Hydroxide-free composites are obtained in a narrow range of KOH concentration (for instance 11.3– 14.5 mol l 21 when [Co] / [Fe]50.33). This range is shifted to larger values when [Co] / [Fe] is decreased. Low KOH concentration results in the formation of hydroxides or amorphous phases, while for high concentration, bCo(OH) 2 precipitates [13]. For nickel-containing composites, a KOH concentration higher than 14 mol l 21 is required. The metal is an iron–nickel alloy of f.c.c. structure. The nickel ferrite is obtained after a thermal treatment in vacuum at 8008C [14]. These composites are made of well-defined octahedral particles. Although their sizes are between 0.1 and 1 mm, they do not oxidize in air because the as prepared particles are coated with a thin layer of hydroxide (3.5 nm) [14,15]. Particle size and resistance to oxidation (up to 3008C) are increased when [KOH] and [Co] / [Fe] increases. Coercive fields are improved for the lowest particle sizes and the highest [Co] / [Fe]. In order to give the most complete view about synthesis of metal-ferrite composite in an aqueous media, we present here the formation of metal-ferrite composites when Mn(II) is involved. The conditions to obtain hydroxidefree composites are described. The magnetic properties and the microstructure are described.
G. Pourroy / Journal of Alloys and Compounds 278 (1998) 264 – 269
2. Experimental details Aqueous solutions of chlorides, FeCl 2 .4H 2 O, MnCl 2 .6H 2 O and CoCl 2 .6H 2 O were prepared at room temperature. Aqueous KOH solutions were refluxed in a stainless steel vessel equipped with a mechanical stirrer on a hot plate. The temperature was measured by means of a stainless steel thermometer placed in the center of the vessel. The metal chloride solution was added to the KOH by means of a peristaltic pump at a rate of 10 ml min 21 . During the following maturation time of 20 min, the heating and stirring were maintained. The black precipitate formed was then filtered from the hot solution, washed with boiling water and ethanol and dried at 408C in air. It was then annealed under argon at 4008C for 15 h. The samples were analyzed by the Induced Coupled Plasma method at the ’Laboratoire Central d’Analyse de Vernaison CNRS’ in order to give the overall Mn to Fe and Co to Fe ratios in the final product (Table 1). X-ray diffraction data were collected at room temperature by use of a D500 Siemens diffractometer equipped with a quartz monochromator (Co Ka150.178897 nm). Observations of the homogeneity of the samples were performed with a JEOL scanning electron microscope. Thermogravimetric (TG) and differential thermal (DTA) analyses were carried out in platinum crucibles in air or in vacuum by using a Setaram 92 apparatus. The temperature was increased at a rate of 1 K min 21 . The variation in weight was calculated after subtracting the weight of empty crucible. Hysteresis curves were measured by means of a Foner apparatus. High resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDXS) were used to obtain microstructural information, to determine individual grain compositions, and to identify phases. To solve the problem of aggregation between the small magnetic particles and to avoid their loss in the pole piece of the microscope, the powders were ground and
265
embedded in Epon 812. Thin samples were obtained using an ultramicrotome equipped with a diamond knife. The 60–90 nm thick sections were collected on holey, carbon coated, 200 mesh copper grids and examined with a TOPCON 002B microscope operating at 200 kV (point to point resolution of 0.18 nm) and equipped with an ultrathin window KEVEX EDX spectrometer. For EDXS analysis, the standard spinel material MnFe 2 O 4 and CoFe 2 O 4 was used to check the validity of the K factors and of the correction parameters used for the deconvolution computation. Fifteen spectra were collected from which the atomic ratio of Fe to Co was found to be 2.0060.01. Thus, thickness and fluorescence have minor effects on the results. Because of the large absorption correction necessary for the low energy X-rays characteristic of oxygen, data from this element was not used for quantitative analysis. Nevertheless, the data were used for fast localization of oxide and metallic phases.
3. Results and discussion
3.1. The precipitates X-ray diffraction analysis performed at room temperature on samples with manganese to iron and cobalt to iron ratios between 0 and 0.2 and [KOH] concentration of at least 14 mol l 21 , shows that in all the samples, a spinel phase and one or two metallic phases occur. Thus, the disproportionation of Fe(OH) 2 leading to a metal and a spinel occurs as previously observed when Co(II) or Ni(II) are involved. Fig. 1 displays the X-ray diffraction patterns of the samples with Mn / Fe50.1 for various Co / Fe ratios. When cobalt is not involved, the composite is made of iron of b.c.c. structure (a50.2867(3) nm) and manganese con-
Table 1 Mn / Fe and Co / Fe ratios determined by chemical analysis, lattice parameters of the metallic phases, weight increase in air between 208C and 9008C on samples after annealing at 4008C in vacuum Sample
Mn / Fe
Co / Fe
1 2 3 4 5 6 7 8 9 10
0.046(2) 0.093(2) 0.040(2) 0.091(2) 0.143(2) 0.195(2) 0.043(2) 0.082(2) 0.140(2) 0.197(2)
0.000(0) 0.000(0) 0.098(2) 0.100(2) 0.107(2) 0.101(2) 0.201(2) 0.200(2) 0.200(2) 0.200(2)
a (nm) b.c.c.
0.2861(2) 0.2859(2) 0.2860(2) 0.2859(2) 0.2849(2) 0.2849(2) 0.2842(2)
a (nm) f.c.c.
Dm /m (%) 208C→9008C
Formula (the indices are given with an ESD of 2)
ss (A m 21 )
Fe 0.29 (Mn 0.15 Fe 2.85 O 4 ) 0.71 Fe 0.30 (Mn 0.29 Fe 2.71 O 4 ) 0.70 (Fe 0.60 Co 0.40 ) 0.39 (Mn 0.13 Fe 2.82 Co 0.05 O 4 ) 0.61 (Fe 0.60 Co 0.40 ) 0.33 (Mn 0.27 Fe 2.64 Co 0.09 O 4 ) 0.67 (Fe 0.60 Co 0.40 ) 0.30 (Mn 0.39 Fe 2.49 Co 0.12 O 4 ) 0.70 (Fe 0.60 Co 0.40 ) 0.28 (Mn 0.51 Fe 2.38 Co 0.11 O 4 ) 0.72 (Fe 0.40 Co 0.60 ) 0.43 (Mn 0.13 Fe 2.71 Co 0.16 O 4 ) 0.57 (Fe 0.40 Co 0.60 ) 0.43 )Mn 0.24 Fe 2.63 Co 0.13 O 4 ) 0.57
0.3550(2) 0.3554(2)
7.04(5) 7.13(5) 7.86(5) 6.78(5) 6.27(5) 5.96(5) 7.65(5) 7.82(5) 6.44(5) 4.97(5)
99(2) 93(2) 110(2) 105(2) 95(2) 95(2) 108(2) 109(2) 102(2) 92(2)
(Fe 0.1 Co 0.9 ) 0.26 (Mn 0.47 Fe 2.37 Co 0.16 O 4 ) 0.74
The formula of sample 9 has not been calculated because of the presence of two metallic phases, Fe 0.4 Co 0.6 of b.c.c. structure and Fe 0.1 Co 0.9 of f.c.c structure, of which concentrations cannot be estimated.
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tional manganese is encountered in an hydroxide form. The decrease of [KOH] concentration below 14 mol l 21 has the same effect. This shows that cobalt and a strong basicity favor the crystallization of the composites: on one side the former participates to the reaction through its reduction by iron, on the other side it dehydrates more easily than Fe(II) and Mn(II) hydroxides. Mn(II) is not reduced by metallic iron according to their potential and is consequently included in the spinel structure after dehydration of part of the manganese hydroxide caused by the strong basicity of the medium. Therefore, only little manganese can be introduced in the spinel phase of the precipitate.
3.2. The heat treated samples Fig. 1. X-ray diffraction patterns of the precipitates with Mn / Fe50.1 and various Co / Fe ratios.
taining magnetite. When cobalt is involved, the metallic phases are iron–cobalt alloys because of the reduction of Co(II) by iron. Their lattice parameter does not depend on Mn to Fe ratio but on cobalt concentration. The b.c.c. lattice parameters a50.2862(3) nm and a50.2849(3) nm respectively for Co / Fe50.1 and 0.2, correspond to the compositions Fe 0.6 Co 0.4 and Fe 0.4 Co 0.6 [16]. The diffraction lines of metallic phases are broader when cobalt is involved because of either a distribution of composition or smaller sizes of the metallic grains. For Co / Fe50.2, part of the metal is crystallized in a structure isomorphous with a-Mn with a50.876(1) nm already encountered in iron– cobalt alloy / cobalt containing magnetite composites [12]. The lattice parameters of the spinel phase are reported in Fig. 2. Despite the scattering of the values, one can see that manganese increases the spinel lattice while cobalt seems to have no effect. In addition, for Mn to Fe ratios above 0.10 when Co / Fe50 and above 0.2 when Co / Fe50.1 or 0.2, hydroxides isomorphous with Fe(OH) 2 or dFe 0.66 Co 0.33 OOH are crystallized showing that the addi-
Fig. 2. Variation of the lattice parameter versus Mn / Fe ratio (j Co / Fe5 0, n Co / Fe50.1, dCo / Fe50.2).
Annealing at 4008C in vacuum has consequences on the structure and the composition of the composites (Fig. 3). First, the lattice parameters of the spinel structure increase resulting from a better crystallization and an homogenisation of the composition (Fig. 4). Secondly, the a-Mn structure disappears as soon as the temperature reaches 1808C; Finally, the iron–cobalt alloy of b.c.c. structure turns into metallic cobalt of f.c.c. structure with a5 0.3552(2) nm completely for Co / Fe and Mn / Fe equal to 0.2, and partially when Co / Fe50.2 and Mn / Fe50.15. However, for Co / Fe50.1 or 0.2 and Mn / Fe50.05 or 0.1, no modification of the lattice parameter of the metallic phase has been observed. Therefore, the reduction of Co(II) by metallic iron has gone on during heating for the highest Co / Fe ratios and consequently, the metal has enriched in cobalt and the spinel phase in iron. No weight loss has been observed by TG measurements in vacuum. In air, the weight increase observed between 1808C and 6008C corresponds to the oxidation of iron, manganese and cobalt into Fe 31 , Mn 31 and Co 21 [17] (Fig. 5). Taking into account the weight increase, the
Fig. 3. X-ray diffraction of the samples annealed at 4008C in vacuum with Mn / Fe50.1 and various Co / Fe ratios.
G. Pourroy / Journal of Alloys and Compounds 278 (1998) 264 – 269
267
0.1 and 0.2 and a Mn / Fe ratio equal to 0.05 and 0.1. One or two endothermic peaks occur corresponding to the ¨ transformation into a wustite phase isomorphous of FeO. For Co / Fe50 and very low Mn to Fe ratio, this temperature is that of pure FeO, i.e. 5708C [18]. When the Mn to Fe ratio increases, the transformation begins at lower temperature, 4908C for Mn / Fe50.1. Cobalt has an opposite effect since it increases the transformation temperature [19]. So, when cobalt is introduced, a large peak occurs at higher temperature. However, as manganese is in the spinel phase and cobalt in the metal, these two metals are not homogeneously distributed within the composite and a small peak at 500–5708C also occurs. Fig. 4. Variation of the lattice parameter versus Mn / Fe ratio (j Co / Fe5 0, n Co / Fe50.1, d Co / Fe50.2) for samples annealed at 4008C in argon.
Fig. 5. TG (———) and DT (....) analysis of the sample Mn / Fe50.15 and Co / Fe50.1 after an annealing at 4008C in argon for 4 h.
chemical analysis and the metal composition given by the lattice parameter, we have calculated the chemical formula of the composites (Table 1). One can note that the metal ratio is higher when cobalt is involved showing that cobalt favors the protection of the composite in the aqueous media. Fig. 6 exhibits DT curves for a Co / Fe ratio equal to 0,
Fig. 6. DT analysis in argon after an annealing at 4008C in argon for 4 h.
3.3. Microstructure The previous samples have been observed by SEM. The powders are made of octahedral particles of various sizes between 0.5 and 3 mm. Imaging with backscattered electrons did not allow to point out the metallic particles (particles with the highest mean atomic number are brighter), therefore they are not located on the surface of the octahedra but included within the oxide (Fig. 7). The metal and the oxide phases are easily observed by TEM. Fig. 8a exhibits metal and oxide grains for the sample Mn / Fe50.1 and Co / Fe50. The metal edge has been observed by HRTEM. EDSX analysis shows that it contains Fe, Mn and oxygen. The interplane distances 4.8 ˚ correspond to spinel oxide (Fig. 8b). The metal is thus A coated by aggregates of small ferrite grains. When cobalt is involved, the ferrite grains are smaller (Fig. 9). A metallic grain with a diameter of about 1 mm is surrounded by small crystals of ferrite. HRTEM allows to observe the edge of the crystals. The metal is coated by small grains of ferrite. One can see that the preparation of samples by the ultramicrotome technique has caused the separation of the metal from the ferrite crystals.
Fig. 7. Secondary SEM micrographs showing typical octahedral particles of the composite with Mn / Fe50.1 and Co / Fe 50.1.
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G. Pourroy / Journal of Alloys and Compounds 278 (1998) 264 – 269
Fig. 8. (a) TEM micrographs of a metallic iron (M) and manganese spinel (S) of the sample with Mn / Fe50.1 and Co / Fe50. (b) HRTEM micrographs of the metallic grain coated with spinel micrograins. The interplane distance 0.48 nm correspond to (111) planes.
Fig. 10. Coercive field of composites versus Mn / Fe ratios.
3.4. Magnetic properties
Fig. 9. Metallic grain surrounded by spinel micrograins for the sample with Mn / Fe50.1 and Co / Fe50.1.
Fig. 10 exhibits the variation of coercive field versus Mn to Fe ratio for each value of cobalt to iron ratio. One can note that they decrease when Mn / Fe increases and that for a given Mn / Fe the coercive field increases when Co / Fe increases. The latter variation is in agreement with previous results, i.e. that the contribution of cobalt anisotropy increases the coercive field. The decrease of mag-
G. Pourroy / Journal of Alloys and Compounds 278 (1998) 264 – 269
netization is due to the decrease of metal ratio in the composite (Table 1).
4. Conclusion Composites made of Fe and Mn-containing spinel ferrite can be obtained in a liquid media by using the disproportionation of Fe(OH) 2 . Only small Mn quantities, up to 10 at % can be introduced in the spinel phase. Mn ratio is increased up to 17 at % when Co(II) is involved. The metal is thus an iron–cobalt alloy of b.c.c. structure. However, when Co / Fe reaches 0.2, part of the metal has the a-Mn structure. By annealing this latter transforms into b.c.c. above 1808C, f.c.c. cobalt appears above 4008C showing that an exchange between oxide and metal occurs, ¨ and the composite transforms into a wustite phase above 5008C. That is in agreement with the microstructure, i.e. the metal is coated by spinel grains.
Acknowledgements We thank G. Ehret for TEM observations.
References ` N. Cornu, J.L. Guille, J. Non-Cryst. Solids 170 (1994) [1] C. Estournes, 287.
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[2] H. Tamagawa, K. Oyama, T. Yamaguchi, H. Tanaka, H. Tsuiki, A. Ueno, J. Chem. Soc. Faraday Trans. 83 (1987) 3189. [3] A. Marchand, B. Barbara, P. Mollard, G. Fillion, X. Devaux, A. Rousset, J. Magn. Magn. Mat. 116 (1992) 64. [4] Ch. Laurent, J.J. Demai, A. Rousset, K.R. Kannan, C.N.R. Rao, J. Mater. Res. 9 (1994) 229. [5] T. Ambrose, A. Gavrin, C.L. Chien, J. Magn. Magn. Mat. 116 (1992) L311. [6] P. Matteazi, G. Le Caer, J. Am. Ceram. Soc. 75 (1992) 2749. ¨ [7] S. Lakamp, A. Malats i Riera, G. Pourroy, P. Poix, J.L. Dormann, J.M. Greneche, Eur. J. Solid State Inorg. Chem. 32 (1995) 159. [8] N.N. Greenwood, A.T. Howe, J. Chem. Soc., Dalton Trans. 1 (1972) 110. [9] A. Malats i Riera, G. Pourroy, P. Poix, J. Alloys Comp. 202 (1993) 113. [10] A. Malats i Riera, G. Pourroy, P. Poix, J. Magn. Magn. Mat. 125 (1993) 125. [11] A. Malats i Riera, G. Pourroy, P. Poix, J. Solid State Chem. 108 (1994) 362. ¨ [12] G. Pourroy, S. Lakamp, S. Vilminot, J. Alloys Comp. 244 (1996) 90. ¨ [13] S. Lakamp, G. Pourroy, Eur. J. Solid State Inorg. Chem. 34 (1997) 295. [14] J.C. Yamegni-Noubeyo, T. Bouakham, G. Pourroy, J. Werckmann, G. Ehret, J. Solid State Chem. 135 (1998) 210. [15] J.C. Yamegni-Noubeyo, G. Pourroy, J. Werckmann, A. Malats i Riera, G. Ehret, P. Poix, J. Am. Ceram. Soc. 79 (1996) 2027. [16] Pearson W.B., Handbook of Lattice Spacings and Structures of Metals, Pergamon, (1964), p. 505. [17] B. Gillot, M. Laarj, S. Kacim, J. Mater. Chem. 7 (1997) 827. [18] G. Chaudron, Compt. Rend. Acad. Sci. 172 (1921) 152. ¨ [19] S. Lakamp, G. Pourroy, J. Solid State Chem. 123 (1996) 109.
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Fe or Co–Fe alloy / manganese ferrite composites ` Pourroy* Genevieve ´ I.P.C.M.S., Groupe des Materiaux Inorganiques, 23 Rue du Loess, F-67037 Strasbourg Cedex, France Received 21 February 1998; received in revised form 28 March 1998
Abstract We describe the procedure to obtain composites made of iron or iron–cobalt alloys and manganese-containing magnetite. A KOH concentration of 14 mol l 21 is required. Iron and manganese based composites are made of metallic iron and manganese-containing magnetite. For Mn / Fe ratios higher than 0.15, hydroxide is also present. When cobalt is involved, hydroxide is encountered for Mn / Fe ratios higher than 0.2. The metal is thus an iron–cobalt alloy which crystallizes in the b.c.c. structure or in the a-Mn structure. This latter disappears by heat treatment above 1808C. At 4008C, cobalt of f.c.c. structure crystallizes. Above 5008C, the composite transforms into ¨ the wustite structure. HRTEM and EDSX analyses show that the metal is coated by ferrite grains. 1998 Elsevier Science S.A. All rights reserved. ¨ Coercive fields Keywords: Composite; Spinel ferrite; Cobalt–iron alloys; Wustite;
1. Introduction Metal–ceramic composites have been widely studied for their mechanical or magnetic properties. Several methods of synthesis have been developed in the last years among which the reduction of transition metal oxides (Fe 2 O 3 , CuO, NiO) in oxide or gels of Al 2 O 3 or SiO 2 , or high energy ball milling of metal and oxides [1–6]. Another method which is based on the disproportionation of Fe(OH) 2 in a concentrated and boiling KOH has been used to obtain Fe 0 / Fe 3 O 4 composites [7]. Indeed, FeO is known to disproportionate into Fe 0 and Fe 3 O 4 below 5708C but if FeO is quenched at 3008C, iron metal is not detected from the corresponding X-ray diffraction pattern [8]. When this disproportionation occurs in concentrated alkaline (KOH) solution, iron and magnetite are both very well crystallized [7]. Metallic iron has got the b.c.c. structure and is imbricated in the magnetite grains. When Co(II) or Ni(II) are implied, the procedure involves the disproportionation of Fe II into Fe 0 and Fe III and a reduction of Co(II) or Ni (II) by Fe 0 . For the former, the resulting composite is made of an iron–cobalt alloy and a cobalt containing magnetite [9–11]. The iron–cobalt
*Corresponding author. Tel.: 133 88107132; fax: 133 88107247.
0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00547-7
alloy is found to be either isomorphous of a-Fe (b.c.c. structure) or crystallized both in the a-Fe and a-Mn structures [12]. Hydroxide-free composites are obtained in a narrow range of KOH concentration (for instance 11.3– 14.5 mol l 21 when [Co] / [Fe]50.33). This range is shifted to larger values when [Co] / [Fe] is decreased. Low KOH concentration results in the formation of hydroxides or amorphous phases, while for high concentration, bCo(OH) 2 precipitates [13]. For nickel-containing composites, a KOH concentration higher than 14 mol l 21 is required. The metal is an iron–nickel alloy of f.c.c. structure. The nickel ferrite is obtained after a thermal treatment in vacuum at 8008C [14]. These composites are made of well-defined octahedral particles. Although their sizes are between 0.1 and 1 mm, they do not oxidize in air because the as prepared particles are coated with a thin layer of hydroxide (3.5 nm) [14,15]. Particle size and resistance to oxidation (up to 3008C) are increased when [KOH] and [Co] / [Fe] increases. Coercive fields are improved for the lowest particle sizes and the highest [Co] / [Fe]. In order to give the most complete view about synthesis of metal-ferrite composite in an aqueous media, we present here the formation of metal-ferrite composites when Mn(II) is involved. The conditions to obtain hydroxidefree composites are described. The magnetic properties and the microstructure are described.
G. Pourroy / Journal of Alloys and Compounds 278 (1998) 264 – 269
2. Experimental details Aqueous solutions of chlorides, FeCl 2 .4H 2 O, MnCl 2 .6H 2 O and CoCl 2 .6H 2 O were prepared at room temperature. Aqueous KOH solutions were refluxed in a stainless steel vessel equipped with a mechanical stirrer on a hot plate. The temperature was measured by means of a stainless steel thermometer placed in the center of the vessel. The metal chloride solution was added to the KOH by means of a peristaltic pump at a rate of 10 ml min 21 . During the following maturation time of 20 min, the heating and stirring were maintained. The black precipitate formed was then filtered from the hot solution, washed with boiling water and ethanol and dried at 408C in air. It was then annealed under argon at 4008C for 15 h. The samples were analyzed by the Induced Coupled Plasma method at the ’Laboratoire Central d’Analyse de Vernaison CNRS’ in order to give the overall Mn to Fe and Co to Fe ratios in the final product (Table 1). X-ray diffraction data were collected at room temperature by use of a D500 Siemens diffractometer equipped with a quartz monochromator (Co Ka150.178897 nm). Observations of the homogeneity of the samples were performed with a JEOL scanning electron microscope. Thermogravimetric (TG) and differential thermal (DTA) analyses were carried out in platinum crucibles in air or in vacuum by using a Setaram 92 apparatus. The temperature was increased at a rate of 1 K min 21 . The variation in weight was calculated after subtracting the weight of empty crucible. Hysteresis curves were measured by means of a Foner apparatus. High resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDXS) were used to obtain microstructural information, to determine individual grain compositions, and to identify phases. To solve the problem of aggregation between the small magnetic particles and to avoid their loss in the pole piece of the microscope, the powders were ground and
265
embedded in Epon 812. Thin samples were obtained using an ultramicrotome equipped with a diamond knife. The 60–90 nm thick sections were collected on holey, carbon coated, 200 mesh copper grids and examined with a TOPCON 002B microscope operating at 200 kV (point to point resolution of 0.18 nm) and equipped with an ultrathin window KEVEX EDX spectrometer. For EDXS analysis, the standard spinel material MnFe 2 O 4 and CoFe 2 O 4 was used to check the validity of the K factors and of the correction parameters used for the deconvolution computation. Fifteen spectra were collected from which the atomic ratio of Fe to Co was found to be 2.0060.01. Thus, thickness and fluorescence have minor effects on the results. Because of the large absorption correction necessary for the low energy X-rays characteristic of oxygen, data from this element was not used for quantitative analysis. Nevertheless, the data were used for fast localization of oxide and metallic phases.
3. Results and discussion
3.1. The precipitates X-ray diffraction analysis performed at room temperature on samples with manganese to iron and cobalt to iron ratios between 0 and 0.2 and [KOH] concentration of at least 14 mol l 21 , shows that in all the samples, a spinel phase and one or two metallic phases occur. Thus, the disproportionation of Fe(OH) 2 leading to a metal and a spinel occurs as previously observed when Co(II) or Ni(II) are involved. Fig. 1 displays the X-ray diffraction patterns of the samples with Mn / Fe50.1 for various Co / Fe ratios. When cobalt is not involved, the composite is made of iron of b.c.c. structure (a50.2867(3) nm) and manganese con-
Table 1 Mn / Fe and Co / Fe ratios determined by chemical analysis, lattice parameters of the metallic phases, weight increase in air between 208C and 9008C on samples after annealing at 4008C in vacuum Sample
Mn / Fe
Co / Fe
1 2 3 4 5 6 7 8 9 10
0.046(2) 0.093(2) 0.040(2) 0.091(2) 0.143(2) 0.195(2) 0.043(2) 0.082(2) 0.140(2) 0.197(2)
0.000(0) 0.000(0) 0.098(2) 0.100(2) 0.107(2) 0.101(2) 0.201(2) 0.200(2) 0.200(2) 0.200(2)
a (nm) b.c.c.
0.2861(2) 0.2859(2) 0.2860(2) 0.2859(2) 0.2849(2) 0.2849(2) 0.2842(2)
a (nm) f.c.c.
Dm /m (%) 208C→9008C
Formula (the indices are given with an ESD of 2)
ss (A m 21 )
Fe 0.29 (Mn 0.15 Fe 2.85 O 4 ) 0.71 Fe 0.30 (Mn 0.29 Fe 2.71 O 4 ) 0.70 (Fe 0.60 Co 0.40 ) 0.39 (Mn 0.13 Fe 2.82 Co 0.05 O 4 ) 0.61 (Fe 0.60 Co 0.40 ) 0.33 (Mn 0.27 Fe 2.64 Co 0.09 O 4 ) 0.67 (Fe 0.60 Co 0.40 ) 0.30 (Mn 0.39 Fe 2.49 Co 0.12 O 4 ) 0.70 (Fe 0.60 Co 0.40 ) 0.28 (Mn 0.51 Fe 2.38 Co 0.11 O 4 ) 0.72 (Fe 0.40 Co 0.60 ) 0.43 (Mn 0.13 Fe 2.71 Co 0.16 O 4 ) 0.57 (Fe 0.40 Co 0.60 ) 0.43 )Mn 0.24 Fe 2.63 Co 0.13 O 4 ) 0.57
0.3550(2) 0.3554(2)
7.04(5) 7.13(5) 7.86(5) 6.78(5) 6.27(5) 5.96(5) 7.65(5) 7.82(5) 6.44(5) 4.97(5)
99(2) 93(2) 110(2) 105(2) 95(2) 95(2) 108(2) 109(2) 102(2) 92(2)
(Fe 0.1 Co 0.9 ) 0.26 (Mn 0.47 Fe 2.37 Co 0.16 O 4 ) 0.74
The formula of sample 9 has not been calculated because of the presence of two metallic phases, Fe 0.4 Co 0.6 of b.c.c. structure and Fe 0.1 Co 0.9 of f.c.c structure, of which concentrations cannot be estimated.
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tional manganese is encountered in an hydroxide form. The decrease of [KOH] concentration below 14 mol l 21 has the same effect. This shows that cobalt and a strong basicity favor the crystallization of the composites: on one side the former participates to the reaction through its reduction by iron, on the other side it dehydrates more easily than Fe(II) and Mn(II) hydroxides. Mn(II) is not reduced by metallic iron according to their potential and is consequently included in the spinel structure after dehydration of part of the manganese hydroxide caused by the strong basicity of the medium. Therefore, only little manganese can be introduced in the spinel phase of the precipitate.
3.2. The heat treated samples Fig. 1. X-ray diffraction patterns of the precipitates with Mn / Fe50.1 and various Co / Fe ratios.
taining magnetite. When cobalt is involved, the metallic phases are iron–cobalt alloys because of the reduction of Co(II) by iron. Their lattice parameter does not depend on Mn to Fe ratio but on cobalt concentration. The b.c.c. lattice parameters a50.2862(3) nm and a50.2849(3) nm respectively for Co / Fe50.1 and 0.2, correspond to the compositions Fe 0.6 Co 0.4 and Fe 0.4 Co 0.6 [16]. The diffraction lines of metallic phases are broader when cobalt is involved because of either a distribution of composition or smaller sizes of the metallic grains. For Co / Fe50.2, part of the metal is crystallized in a structure isomorphous with a-Mn with a50.876(1) nm already encountered in iron– cobalt alloy / cobalt containing magnetite composites [12]. The lattice parameters of the spinel phase are reported in Fig. 2. Despite the scattering of the values, one can see that manganese increases the spinel lattice while cobalt seems to have no effect. In addition, for Mn to Fe ratios above 0.10 when Co / Fe50 and above 0.2 when Co / Fe50.1 or 0.2, hydroxides isomorphous with Fe(OH) 2 or dFe 0.66 Co 0.33 OOH are crystallized showing that the addi-
Fig. 2. Variation of the lattice parameter versus Mn / Fe ratio (j Co / Fe5 0, n Co / Fe50.1, dCo / Fe50.2).
Annealing at 4008C in vacuum has consequences on the structure and the composition of the composites (Fig. 3). First, the lattice parameters of the spinel structure increase resulting from a better crystallization and an homogenisation of the composition (Fig. 4). Secondly, the a-Mn structure disappears as soon as the temperature reaches 1808C; Finally, the iron–cobalt alloy of b.c.c. structure turns into metallic cobalt of f.c.c. structure with a5 0.3552(2) nm completely for Co / Fe and Mn / Fe equal to 0.2, and partially when Co / Fe50.2 and Mn / Fe50.15. However, for Co / Fe50.1 or 0.2 and Mn / Fe50.05 or 0.1, no modification of the lattice parameter of the metallic phase has been observed. Therefore, the reduction of Co(II) by metallic iron has gone on during heating for the highest Co / Fe ratios and consequently, the metal has enriched in cobalt and the spinel phase in iron. No weight loss has been observed by TG measurements in vacuum. In air, the weight increase observed between 1808C and 6008C corresponds to the oxidation of iron, manganese and cobalt into Fe 31 , Mn 31 and Co 21 [17] (Fig. 5). Taking into account the weight increase, the
Fig. 3. X-ray diffraction of the samples annealed at 4008C in vacuum with Mn / Fe50.1 and various Co / Fe ratios.
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0.1 and 0.2 and a Mn / Fe ratio equal to 0.05 and 0.1. One or two endothermic peaks occur corresponding to the ¨ transformation into a wustite phase isomorphous of FeO. For Co / Fe50 and very low Mn to Fe ratio, this temperature is that of pure FeO, i.e. 5708C [18]. When the Mn to Fe ratio increases, the transformation begins at lower temperature, 4908C for Mn / Fe50.1. Cobalt has an opposite effect since it increases the transformation temperature [19]. So, when cobalt is introduced, a large peak occurs at higher temperature. However, as manganese is in the spinel phase and cobalt in the metal, these two metals are not homogeneously distributed within the composite and a small peak at 500–5708C also occurs. Fig. 4. Variation of the lattice parameter versus Mn / Fe ratio (j Co / Fe5 0, n Co / Fe50.1, d Co / Fe50.2) for samples annealed at 4008C in argon.
Fig. 5. TG (———) and DT (....) analysis of the sample Mn / Fe50.15 and Co / Fe50.1 after an annealing at 4008C in argon for 4 h.
chemical analysis and the metal composition given by the lattice parameter, we have calculated the chemical formula of the composites (Table 1). One can note that the metal ratio is higher when cobalt is involved showing that cobalt favors the protection of the composite in the aqueous media. Fig. 6 exhibits DT curves for a Co / Fe ratio equal to 0,
Fig. 6. DT analysis in argon after an annealing at 4008C in argon for 4 h.
3.3. Microstructure The previous samples have been observed by SEM. The powders are made of octahedral particles of various sizes between 0.5 and 3 mm. Imaging with backscattered electrons did not allow to point out the metallic particles (particles with the highest mean atomic number are brighter), therefore they are not located on the surface of the octahedra but included within the oxide (Fig. 7). The metal and the oxide phases are easily observed by TEM. Fig. 8a exhibits metal and oxide grains for the sample Mn / Fe50.1 and Co / Fe50. The metal edge has been observed by HRTEM. EDSX analysis shows that it contains Fe, Mn and oxygen. The interplane distances 4.8 ˚ correspond to spinel oxide (Fig. 8b). The metal is thus A coated by aggregates of small ferrite grains. When cobalt is involved, the ferrite grains are smaller (Fig. 9). A metallic grain with a diameter of about 1 mm is surrounded by small crystals of ferrite. HRTEM allows to observe the edge of the crystals. The metal is coated by small grains of ferrite. One can see that the preparation of samples by the ultramicrotome technique has caused the separation of the metal from the ferrite crystals.
Fig. 7. Secondary SEM micrographs showing typical octahedral particles of the composite with Mn / Fe50.1 and Co / Fe 50.1.
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Fig. 8. (a) TEM micrographs of a metallic iron (M) and manganese spinel (S) of the sample with Mn / Fe50.1 and Co / Fe50. (b) HRTEM micrographs of the metallic grain coated with spinel micrograins. The interplane distance 0.48 nm correspond to (111) planes.
Fig. 10. Coercive field of composites versus Mn / Fe ratios.
3.4. Magnetic properties
Fig. 9. Metallic grain surrounded by spinel micrograins for the sample with Mn / Fe50.1 and Co / Fe50.1.
Fig. 10 exhibits the variation of coercive field versus Mn to Fe ratio for each value of cobalt to iron ratio. One can note that they decrease when Mn / Fe increases and that for a given Mn / Fe the coercive field increases when Co / Fe increases. The latter variation is in agreement with previous results, i.e. that the contribution of cobalt anisotropy increases the coercive field. The decrease of mag-
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netization is due to the decrease of metal ratio in the composite (Table 1).
4. Conclusion Composites made of Fe and Mn-containing spinel ferrite can be obtained in a liquid media by using the disproportionation of Fe(OH) 2 . Only small Mn quantities, up to 10 at % can be introduced in the spinel phase. Mn ratio is increased up to 17 at % when Co(II) is involved. The metal is thus an iron–cobalt alloy of b.c.c. structure. However, when Co / Fe reaches 0.2, part of the metal has the a-Mn structure. By annealing this latter transforms into b.c.c. above 1808C, f.c.c. cobalt appears above 4008C showing that an exchange between oxide and metal occurs, ¨ and the composite transforms into a wustite phase above 5008C. That is in agreement with the microstructure, i.e. the metal is coated by spinel grains.
Acknowledgements We thank G. Ehret for TEM observations.
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