Kiyama, Masao

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Title Particle Growth of Co[x]Fe₃[-x]O₄ with x?0.2 by AirOxidation of Aqueous SuspensionsAuthor(s)Kiyama, MasaoCitationBulletin of the Institute for Chemical Research, KyotoUniversity (1982), 60(3-4): 247-253Issue Date1982-10-15URL http://hdl.handle.net/2433/77002RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityBull. Inst. Chem. Res., Kyoto Univ., Vol. 60, No. 3-4, 1982        Particle Growth of  Co.Fe3,04 with  x<0.2 by           Air Oxidation of Aqueous Suspensions                                Masao KIYAMA*                               Received May 15, 1982   The conditions were studied for promoting the particle growth of  CoxFe3_.04 with x<0.2 by oxidation with air at  70°C of  Fe(OH)3 suspensions containing slight amounts of Co(II). The particle growth by oxidation is accelerated notably with increasing concentrations of both excess NaOH and Fe(II) in the starting suspension. The presence of extremely fine  Fe304 particles in the starting suspension retards the particle growth by oxidation. The samples consist of multi-, single-, and sub-domain  (or superparamagnetic) particles, each being inhomogeneous in Co(II) concentration, with their intermingled particle ratio as well as the inhomogeneity as governed by the conditions for the particle growth.    KEY WORDS: Air  oxidation/ Particle  growth/ Cubic  particle/ Single-                  domain  particle/ Coercivity/  Co„Fe3-x04/    The precipitates consisting of cubic Fe304 particles can be prepared by oxidation of Fe(OH)2 suspensions, provided that the conditions for oxidation are suitably chosen. The appropriate conditions of oxidation have been investigated for the formation of  Fe3O4.1 The temperature at which  Fe3O4 is formed is lowered with decreasing concentration of either excess NaOH or iron (II) salt in the starting sus-pension, whereas each concentration range suitable for the formation of Fe304 becomes wider with increasing oxidation temperature beyond 50°C. The Fe304 samples with greater than 0.01  um in mean particle sizes could successfully be prepared by selecting the oxidation conditions, and their magnetic properties were dependent on the particle sizes. It has been well known that the substitution of slight amounts of Co(II) for Fe(II) in  Fe3O4 causes a magnetically harder material. The precipitates of  Co.Fe3-.04 with  0.05<x<0.2 were also prepared in the presence of slight amounts of Co(II) under oxidation conditions similar to those for Fe304 formation.    This paper deals with oxidation conditions for the growth of particles of the compositions of  Co.Fe3_.04 with x<0.2 and their magnetic properties.                       EXPERIMENTAL    The starting suspension, varying in the concentrations of iron(II) and excess  NaOH, were prepared as follows: Acidic solutions containing  different concentrations of Fe(II) were prepared by dissolving  FeC12.6H20 or  FeSO4•7H20 (both of analytical grade) in water. To every 1 L (1  L=1 dm3) solution, a NaOH solution whose *  *tu  Sig: Laboratory of Solid State Chemistry, Institute for Chemical Research, Kyoto  University, Uji, Kyoto 611.  ( 247  )                                      M.  KIVAMA concentration had been suitably controlled was added in various  20H-/Fe(II) ratios (R). Each suspension was diluted with water to 3 L in a 4 L flask. The oxidation of each 3 L suspension was carried out at 70°C by bubbling CO2-eliminated air into at a constant rate of 300 L/h. The construction of the flask for the air-oxidation experiments has previously been  described."     To investigate the transformation into Fe304 in the course of oxidation, a 40-50 cm3 sample was taken several times from each suspension during oxidation. After sampling each filtered precipitate in muddy form was subjected to X-ray, electron microscopic, and chemical examinations. Oxidation for the suspensions with  R<1 was stopped when the pH dropped sharply with the disappearance of dark blue (or green) or whitish precipitate in the suspension, whereas the alkaline suspensions with R> 1 were oxidized until Fe(OH)2 was completely oxidized. A number of alkaline suspensions were also prepared by using acidic solutions containing  Fe(II) and either of Fe(III) or Co(II).     The oxidation products were filtered, washed with water, and treated with acetone. The black slurries prepared by oxidation in the absence or presence of Fe(III) in the starting suspensions were, respectively, dried at 70°C under reduced pressure, or at room temperature in air.    All powdery samples thus obtained were examined by X-ray diffraction using Mn filtered FeKa radiation. The samples consisting only of  Fe304, as revealed by X-ray analysis were further examined by electron microscopic observation, magnetic measurement at room temperature using a vibrating sample magnetometer in magnetic field up to 10 kOe  (1  0e =1000/47r A  m-1), BET surface area determination using nitrogen, and chemical analysis. The mean particle size, D, of Fe304 samples was estimated from the BET surface area (m2/g) by assuming that the sample consisted of cubic particles with 5.2 specific gravity. The average particle size, d, for the samples with D <0.1  pm was further estimated from the line broadening of the X-ray diffraction peak for the (311) plane of Fe304 crystal structure.    The cobalt and iron ion contents of samples were determined by atomic ab-sorption after the samples had been dissolved in a HC1 solution. The Fe(II) content in Fe304 samples was decided by titrating with  KMnO4 after they had been dissolved in a mixed solution of H2SO4 and H3PO4 under a nitrogen atmosphere.                  RESULTS AND DISCUSSION    Aqueous suspensions each containing 0.24 M  (1  M=1 mol  dm-3) Fe(II) were prepared by varying R values in the range 0.3-2.5. On adding a NaOH solution to a  FeC12 solution a whitish (or with a tint of blue) precipitate was formed. When the amount of NaOH added fell short of reaching the equivalence point (i.e., R<l), a bluish white neutral suspension was obtained. When it was equal to or more than that for the equivalence point, a weak or a strongly alkaline suspension was obtained.    When air was bubbled into, a color change took place of a neutral suspension to dark blue. The dark blue precipitates as examined by an X-ray analysis and electronmicroscopy proved to consist of hexagonal plate-like particles with the same  (  248  )                             Particle Growth of  Co,,Fe3,04 crystal structure as that of green rust  I.2) When R<0.7, with the progress of oxidation the  formation of a black, ferromagnetic precipitate, Fe304, could be observed in the dark blue suspension with pH 6.0+0.3. For R=0.9 or 1.0, the formation of Fe304 took place at pH 7.5-9.0 in the presence of Fe(OH)2 with or without green rust I. As the precipitates of  Fe(OH)2 and green rust I completely transformed themselves into Fe304, the pH sharply dropped to 4 or less. The white precipitate  (Fe(OH)2) in the alkaline suspensions was observed to gradually change without the formation of green rust I to the black, and ferromagnetic precipitate.    It has been found that (a) all the filtered Fe304 samples in a muddy form has the lattice constant,  ao, nearly constant at 8.38 A, irrespective of R values, (b) the dried samples prepared for  R>  1.1 consist of cubic particles with 0.2-0.8  pm in size, with 8.37-8.38  A in  ao and contain Fe(II)  30-31% of the entire Fe ions, whereas (c) those for R<1 consist of spherical and cubic particles with 0.05-0.15  ,um in size, with 8.36 A in  ao containing Fe(II)  26-27%.   0,50                      0,40 -                     0.30 -                  G20 -                 E           a. 0.15- •         /*- •                     0.10                                           •  0.05  -                                          1.0           Fig. 1. Mean particle size, D, of Fe304 samples prepared by oxidation                   of suspensions with various R values containing 0.24M  FeCl2.                   Transformation into Fe304 by oxidation took placein the presence                   of green rust I  (-  -  -), a mixture of green rust I and Fe(OH)2  (—  •  —), or Fe(OH)2 ( ).    A plot of D values of Fe304 samples thus obtained vs. R values (Fig.  1) demon-strates that the D values are affected by the Fe(OH)2 concentration in the starting suspension when  R<0.7, and that increasing the concentration of an excess NaOH accelerates the growth of Fe304 particles (Fig. 2) as a result of retardation in the speed of Fe304 formation caused by oxidation.    Three kind suspensions with  R  =0.5, 0.9, and 1.5, varying in the Fe(II) con-centration in the range of 0.046-0.48 M were prepared from solutions of FeC12 and  (  249  ) M.  KIYAMA        ilatt *11117511; ":- "6:                                  Att )10,                                           . _                                  tV? 41" 1: 4 •                                  '4* tAA,'                   r •                                                  •      10- Ili It       - •                    111 re,' ,                           ,    0.5.f            — •  (  a  )  (  b  )                Fig. 2. Electronmicrographs of  Fe2O4  prepared by oxidation of                       suspensions containing 0.24 M  Fe(I  I)  with  R=2.5  (a),                     and 1.1 (b).  0.20.   01 5- -                  E                                            4-             °0.10-/ A                 •  /'  I  /4)(r-A  .`•••41 ° 0.05-                 0.1 0.2 0.3  0.4 0.5  Fe(OH)2 Concn./M                Fig. 3. Mean particle size,  13, of  Fes04 prepared by oxidation of                        suspensions containing different  Fe (OH), concentrations                      with  R=2-0.5  (•), 0.9  (/), and  1.5 (0).  NaOH, and subjected to oxidation. For the  R<1, the amount of iron (II) pre-cipitates such as Fe(OH)2 increases with increasing R at a given Fe(II) concentration, and is calculated from R values. The D values are plotted against the concentration of Fe(OH)2 at three R values in Fig. 3. The growth of Fe304 particles by oxidation is promoted with increasing the concentrations of both excess NaOH and Fe(II) in the starting suspension.    Similar experiments were conducted at 0.24 M  Fe(II) using  FeSO4 in place of FeC12. No particular differences could be found in the conditions for promoting the growth of Fe304 particles in alkaline media  (R>1.0) caused by oxidation. When R<0.7, fine needle-like  a-FeO(OH) particles were formed besides cubic Fe304 particles as had been  described))  ( 250  )                             Particle Growth of  CoxFe3-x04    As has been known, Fe(III) easily reacts with Fe(II) in an alkaline medium to form extremely fine Fe304 particles.3) It was found that the presence of such extremely fine Fe304 particles in the starting suspension tends to retard the growth of Fe304 particles caused by oxidation. In order to study the effect of Fe(III) con-centration in the starting suspension on the particle growth, alkaline suspensions containing 1.0 M excess  NaOH, 0.28 M Fe(II), and different amounts of Fe(III) in the range 0.046<M<0.260 were prepared from solutions of  FeSO4 and  FeC13, and subjected to oxidation.    Table I. Properties of typical  Fe304 samples prepared by oxidation at  70°C of suspensions            containing 1 M excess  NaOH, 0.288 M Fe(II), and different Fe(III) concentrations   Sample Fe(III)/mol  dm-3 D/A  d/A.  Hc/Oe M/emu g-1  A  0.060 322 740 120 65  B  0.  102 230 357 95 62  C  0.  152 145 236 62 49  D  0.200 129 109 20  44                       :^11Y:11:411: ;. ..::::.';::1•.,x-p. t :,:iiiaroxk.  ''Ilfl'41                                                                  •         '-10 . , .--. #f ' .  l'  4  •  •               ' , -1•• 10.t.,' 1' i ,*.4,14;• &I"..  .14' '                                                   -.4.           • - .4. t 7;'4„, :Y- „iizil. itit , :           4., 44 "oat: oti," _,C. , t. Atm,. t' '            r .,    Sitt     ilo                                                    ,...p:                     "43,4 I 1                    .3 AIL..i.MOM&....111-XII**..1                                            Az04/u1                       *-'t"...4!It'  4.... 4 14. , At  :.,  (  a  )  (  b  )               Fig. 4. Electronmicrographs of  Fe304prepared by oxidation of                       suspensions containing  1 M excess  NaOH, 0.29 M Fe(II),                     and either 0.06 M (a) or 0.15 M  Fe(III) (b).   Properties of typical samples are given in Table I, and electron micrographs of typical two Fe304 samples are shown in Fig. 4. The increase in the Fe(III) con-centration in the starting suspension up to 0.06 M was found to cause a remarkable decrease in D, but a slight decrease in d. These samples consist of extremely fine particles with 0.01-0.02 pm in size in addition to cubic particles with 0.05-0.1 pm in size (Fig. 4(a)). Further increase in the Fe(III) concentration resulted in the decreases both in D and d.    Their coercivities, Hc, which were independent of the packing densities of cubic Fe304 particles, are plotted against D in Fig. 5. For the samples with  D>0.1 pm their magnetic saturation values,  Ms, ranging 85-80  emu/g  (1  emu/g=47  10-7Wb-m  (  251  )                                     M. KIYAMA       ..............   200-  •-                                                         •                                                     •                                                      •• • -  a)  O   u100•  -- •  I ••      0 0.1 0.2 Q3  Dium              Fig. 5. Coercivity, Hc, of Fe304 as a function of mean particle size                     D of Fe304 samples.  kg-1), were independent of D, whereas their  Hc values increased near to 200 Oe with decreasing D to 0.1  pm as a result of increasing single-domain particles. Further decrease in D from 504 to 119 A causes decreases in  He to 13 Oe and in magnetisation, M, at 10 kOe to 44 emu/g due to an increase in sub-domain (or superparamagnetic) particles.    Alkaline suspensions of hydroxides of Fe(II) and Co(II) in the form of a solid solution of  Co./3Fei,/3(OH)2 were prepared by adding an excess NaOH to an acidic solution containing  FeSO4 and  CoSO4 in the  Co(II)/Fe(II) ratios of 0.016 and 0.071, and subjected to oxidation. D values of  Co.Fe3-.04 samples with  x=0.05 and 0.20 could be controlled in the range  0.1<D<0.5  tan by selecting conditions similar to those for promoting the growth of Fe304 particles. As has been known, the difference in the lattice constant between Fe304 and cobalt ferrite is appreciably small. The fine particles with the  Co.Fe3_„04 composition  (x<0.2) could be also prepared by epitaxial coating of cobalt ferrite on the extremely fine Fe304 particles in an alkaline medium.    The  He values of typical  Co.Fe3-.04 samples with three D values prepared by        Table II. Magnetic properties of  Co„Fe3_x04 precipitates prepared under various                  conditions  Sample  Co(II)  ±Fe(II)/mol  dm-3  R  Di/2m  x  Hc/Oe M/emu  g-1  E  0.360  2.5  0.45  0.20 266 85  F  0.360  1.  1  0.  12  0.20 633 83  0.  098  2.5  0.025  0.20 205 75  H  0.360  1.1  0.10  0.05 250 83  I  0.098  2.5  0.025  0.05 175 76   Samples G and I prepared at 0.29 M excess NaOH in the presence of Fe304 particles with   D=0.025 pm.  (  252  )                             Particle Growth of  CoxPea-x04 varying the conditions are given in Table II. Samples G and I given in the table were prepared by subjecting alkaline suspensions containing  Coyi3Fei__,/3(OH)2 and fine  Fe304 particles with D =0.025 pm and with  Hc  =103 Oe. The Fe304 particles which had been prepared in the presence of Fe(III) in the alkaline suspension by oxidation as mentioned above were used in a slurry form. Samples G and I consist of 27 vol% of  CoyFe3_y04 with  y  =0.73 and 0.18, respectively, as a result of calculation. The  Hc values become greater by coating of  CoyFe3_,,04 on the Fe304 particles and the increment in  He is affected by the Co content. The  Hc increment is presumably due to the participation of a surface magnetic anisotropy as previously described for needle-like particles of  r-Fe203.4)    It has been inferred that a Co(II) concentration gradient exists within each particle in the  Co.Fe3-.04 samples prepared even in the absence of Fe304 seeds, depending on the oxidation conditions and that  He values of the  Co.Fe3-.04 samples with  D  =0.1 pm vary with heat treatment at 350°C in  vacuo as a result of the diffusion of the metal ions in each particle.5)      Table III. Coercivity,  He/0e, of  CoxFe3_x04 samples subjected to heat treatment in                    vacuo Sample  E  F  G  H  I  250°C 930 450 290 240   350°C 280 1500 670 350 260     Five samples E-I, each enclosed in glass tubes under reduced pressure of  10-3 Pa were heat treated at temperatures between 250 and 350°C for 5 h and then allowed to cool to room temperature. The  He values of the samples except for sample E begin to increase at 250°C depending on x (Table III), whereas no difference in M values of five samples (A-E) was detected before and after heat treatment.    The  He value of sample E was found to increase at 560 Oe with heat treatment at 200°C in air for 20 h. The  He increment with heat treatment must be attributed to the fact that the contribution of the crystal anisotropy of  Co(II) due to the diffusion of the metal ions in each particle becomes predominant, and the diffusion rate is governed by the concentration of cation vacancies in each particle.                   ACKNOWLEDGMENT    The author wishes to thank Professors T. Takada, Y. Bando, and T. Shinjo for their encouragements during this work. He is also indebted to Dr. K.  Watanabe and Messers T. Asai and N. Nagai for their assistance in the experiments.                             REFERENCES  (1) M. Kiyama, Bull. Chem. Soc.Jpn., 47, 1646 (1974)  (2) J. D. Bernal, E. R. Dasgupta, and A. L. Mackay, Clay  Min. Bull., 4, 15 (1959)  (3) M. Kiyama, Bull. Inst. Chem. Res. Kyoto Univ., 47, 607 (1969).  (4) H.-L. Lo and W. Gung,  J.  Appl. Phys., 50, 2414 (1979)  (5) M. Kiyama, Bull Chem. Soc.  Jpn., 51, 134 (1978)  (  253  )

Particle Growth of Co[x]Fe₃[-x]O₄ with x≤0.2 by Air Oxidation of Aqueous Suspensions

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Particle Growth of Co[x]Fe₃[-x]O₄ with x≤0.2 by Air Oxidation of Aqueous Suspensions

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