Thermally Driven and Ball-Milled Hematite to ... - Springer Link

Hyperfine Interactions 148/149: 163–175, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Thermally Driven and Ball-Milled Hematite to Magnetite Transformation J. D. BETANCUR1, J. RESTREPO1, C. A. PALACIO1, A. L. MORALES1, J. MAZO-ZULUAGA1, J. J. FERNÁNDEZ2, O. PÉREZ2, J. F. VALDERRUTEN3 and A. BOHÓRQUEZ3 1 Instituto de Física, Universidad de Antioquia, A. A. 1226, Medellín, Colombia 2 Instituto de Química, Universidad de Antioquia, A. A. 1226, Medellín, Colombia 3 Departamento de Física, Universidad del Valle, A. A. 25360, Cali, Colombia

Abstract. In this work, a study on the dynamics of transformation from hematite (α-Fe2 O3 ) to magnetite (Fe3 O4 ) by following two solid-state reaction methods is carried out. One of the procedures consists of a thermal treatment under a 20% H2 and 80% N2 atmosphere at 375◦ C, whereas the second method involves a planetary ball mill to induce the transformation. The phases evolution as a function of the thermal treatment time ranging from 0 up to 25 min every 2.5 min, and from 0 up to 6 hours every hour in the case of the milling method, was followed by using room-temperature Mössbauer spectroscopy and X-ray diffraction analysis. Results evidence a well-behaved structural transformation for which highly stoichiometric Fe3 O4 as a single phase was obtained for treatment times above 12.5 min in the case of the thermally treated samples. Differently from this a less stoichiometric magnetite characterized by a distribution of hyperfine fields for milling times above 3 hours in the case of the ball milled samples was obtained. For reaction times below 12.5 min, two interpretation models based on the presence of an anion-deficient magnetite Fe3 O4−δ and the presence of maghemite accounting for the intermediate states during the thermal transformation are also presented and discussed. Key words: iron oxides, Mössbauer spectroscopy, ball-mill, thermal treatment, magnetite.

1. Introduction Among the family of iron oxides and their technological importance, the Fe3 O4 system has become of long standing interest due to its rich variety of applications like industrial pigment, precursor for magnetic fluids, etc., in addition to being an important corrosion product [1]. Hence, alternative and faster methods of synthesis with the consequent possibility of reducing costs of preparation, should be considered. In this respect, in obtaining magnetite several chemical methods have been proposed [1–3], however, due to the great amount of variables (e.g., pH, reaction time, atmosphere, temperature, velocity, bubbling, washing and drying methods, etc.) involved in those syntheses, most of them fail both to guarantee a suitable control of the final product and parameters like grain size, purity, cristallinity, stoichiometry as well as to visualize the intermediate states through which the

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J. D. BETANCUR ET AL.

transformation evolves from a precursor material, besides being very time consuming. Regarding the intermediate states several chemical formulae for the nonstoichiometric magnetite depending on how the vacancies are supposed to be localized on the A and B sites have been proposed [4–6]. For instance, if the vacancies 3+ are distributed on B sites, the employed formula is (Fe3+ )A [Fe2+ (1−3x)Fe(1+2x) []x ]B 2.5+ 3+ 2− 3+ O2− 4 which can be rewritten as (Fe )A [Fe(2−6x) Fe5x []x ]B O4 or in the case of vacancies occupying both sites to an equal amount, the formula (Fe3+ (1−0.5x) []0.5x )A 2.5+ 3+ 2− [Fe(2−6x)Fe5.5x []0.5x ]B O4 is considered [6]. In a more general fashion if φA and φB represent the fractional concentration of vacancies of A and B sites, respectively [13], obeying φA + φB = 1 with 0 φB 1, the following general formula can be written: 2.5+ 2− 3+ Fe3+ (1) Fe1−(1−φB )x [](1−φB )x A Fe(2−6x) (6−φB )x []φB x B O4 yielding the following expression for the oxidation parameter x: x(φB , R) =

2 − 1.06R 6 + (4.94 + 0.06φB )R

(2)

R(φB , x) =

2 − 6x , 1.06(1 − (1 − φB )x) + (6 − φB )x

(3)

or

where R is the area ratio A(FeB2.5+ )/A(Fe3+ ) and where the Mössbauer fractions 2.5+ 3+ 2.5+ f (Fe3+ A )/f (FeB ) = 1.06 and f (FeB )/f (FeB ) = 1.0 have been used [2]. Figure 1 shows the x–R projection for the two extreme φB values from which x is practically insensitive to φB and differences are only noticed within a 0.8% of precision (see inset in Figure 1). In this way, an average x value can be computed for a given R value. However, an interesting problem arises when experimentally the obtained R value becomes greater than 1.89 corresponding to a stoichiometric magnetite (x = 0), above which the oxidation parameter would become negative. As we will see, this is our case for thermally treated samples along the first stages during the transformation for which two different approaches are addressed. Additionally we present results and differences of employing two solid-state reaction methods (thermal treatment and ball milling) with emphasis on the transformation dynamics as the reduction hematite to magnetite takes place. 2. Experimental 2.1. THERMAL METHOD Starting from 30 mg of hematite (Merck) and by using a Heatech furnace with quartz tube, the sample is submitted initially to a constant flow (100 cm3 /min) of inert atmosphere (N2 ) at room temperature and during 40 minutes to purge the system. Subsequently, the furnace is heated up to 375 ± 1◦ C [7] when a H2

THERMALLY DRIVEN AND BALL-MILLED HEMATITE TO MAGNETITE TRANSFORMATION

165

Figure 1. Area ratio R dependence of the oxidation prameter x for the two extreme φB values from which x is practically insensitive to φB and differences are only noticed within a 0.8% of precision (see inset in figure).

(20 cm3 /min) and N2 (80 cm3 /min) atmosphere is implemented. This change of atmosphere is performed by means of electronic mass flow drivers. Thus, exposure times from 0 up to 25 minutes every 2.5 minutes were considered. Samples corresponding to 5 and 15 minutes were duplicate in order to evaluate the reliability of the method. 2.2. BALL MILLING The method used for preparation of the material studied has been reported previously [8]. Samples were prepared by milling pure (>99.99%, Merck) hematite powders (8.1 g) in addition to 6 ml of destilled water in an evacuated (2 mbar) Fritsch-Pulverisette 5 planetary ball mill at 280 rpm. Milling times of 1 up to 6 hours were considered and the average powder-to-balls weight ratio was taken to be 1 : 30. The phase distribution present on the as-milled and the thermally treated materials was identified from the analysis of X-ray diffraction (XRD) data. These measurements were performed in a Bruker AXS-Advanced diffractometer with Kα-Cu radiation, 0.014◦ (2θ) of step and 1 second of counting in the range of 17◦ to 82◦ (2θ). Finally, powders for 57 Fe Mössbauer spectroscopy were obtained from each sample. The Mössbauer fitting process was carried out by using the Normos-Distri [9] program and only in the case of the as-milled samples hyperfine field distributions with up to 48 sextets were considered in order to account for the atomic disorder induced by the milling process.

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3. Results and discussion Figures 2 and 3 show the reaction time evolution of the room temperature Mössbauer and X-ray diffraction spectra, respectively. As is shown from these figures, the reduction of hematite (H) takes place even in the first 2.5 minutes of exposure at the employed atmosphere and temperature (375◦ C), for which a well-defined component different to hematite (labeled as M) is distinguishable. Thus, as the treatment time is increased and up to 12.5 minutes, results reveal the presence of at least two phases. Above this time and within the resolution of the employed techniques, the magnetite seems to be the only phase. Summarizing the obtained results, data concerning the hyperfine parameters of the three sextets employed in the Mössbauer fitting process corresponding to the presence of H and M (FeB2.5+ and Fe3+ ), the relative areas A(FeB2.5+ ) and B(Fe3+ ), the R ratio, as well as the crystallite size computed from XRD measurements are presented in Table I. As is shown in this table, R values greater than 1.89 for treatment times in the range 2.5 t 10 min are obtained. Moreover, R increases as t decreases, i.e., as earlier stages in the transformation are considered. In this case if the constraint 0 x 1/3 is imposed, two different approaches can be addressed without considering negative x values. The first approach is based on an anion-deficient magnetite Fe3 O4−δ model which can be described by the following equation: 2.5+ 2− 3+ Fe3+ (4) Fe(1−2δ)−(1−φB)x [](1−φB )x A Fe(2+4δ−6x) (6−φB )x−2δ []φB x B O4−δ , where the charge balance is obeyed. In this way, R can increase beyond 1.89 as δ increases preserving the constraint for x. If this is the case, the first stages during the reduction involving a hexagonal to cubic structural change would be characterized by an atomic rearrangement for which the number ratio FeB2.5+ /Fe3+ A diminishes as the treatment time increases up to 10 min. Above this time a R value of 1.48 for t = 12.5 min is obtained, indicating a stabilization of the transformation characterized at this stage by a non-stoichiometric magnetite of the form Fe2.968(2)O4 with x = 0.032 ± 0.002. Finally, for treatment times above 12.5 min, where magnetite is the only phase, the stoichiometry improves with an average R value of 1.67 ± 0.09 (see Table I) corresponding to a magnetite of the form Fe2.984(2)O4 with x = 0.016 ± 0.002. The second approach explaining greater R values during the first 10 minutes of transformation could be the presence of maghemite γ -Fe2 O3 which could eventually be accompanied by magnetite according to the following equations: α-Fe2 O3 γ -3Fe2 O3 + H2 α-4Fe2 O3 + H2 α-3Fe2 O3 + H2

→ → → →

γ -Fe2 O3 , 2Fe3 O4 + H2 O, γ -Fe2 O3 + 2Fe3 O4 + H2 O, 2Fe3 O4 + H2 O,

(5) (6) (7) (8)

where the thermally activated structural change and represented by the first equation would result as a consequence of the heating process when temperature is


167

Figure 2. Room-temperature Mössbauer spectra for the thermally treated samples. Spectra reveal the presence of magnetite as the only phase for treatment times above 12.5 min.

Bhf (T) α-Fe2 O3 Bhf (T) Fe3+ Bhf (T) Fe2.5+ δFe (mm/s) α-Fe2 O3 δFe (mm/s) A (×100%) δFe (mm/s) Fe2.5+ (mm/s) α-Fe2 O3 (mm/s) Fe3+ (mm/s) Fe2.5+ A (×100%) α-Fe2 O3 A (×100%) Fe3+ A (×100%) Fe2.5+ 2.5+ R A(FeB )/A(Fe3+ ) x Fe3−x O4 D311 (nm) Fe3−x O4

51.5(1) – – 0.38(1) – – 0.35(3) – – 1.00 0 0 – – – –

0 51.6(1) 49.0(1) 45.8(1) 0.38(1) 0.27(1) 0.67(1) 0.33(3) 0.31(3) 0.42(3) 0.72(6) 0.096(6) 0.20(6) 2.22 – – –

2.5 51.6(1) 49.0(1) 45.9(1) 0.38(1) 0.27(1) 0.67(1) 0.29(3) 0.29(3) 0.39(3) 0.33(5) 0.22(5) 0.45(5) 2.05 – – 58(5)

5 51.6(1) 49.0(1) 45.9(1) 0.38(1) 0.28(1) 0.67(1) 0.31(3) 0.32(3) 0.40(3) 0.27(4) 0.25(4) 0.48(4) 1.94 – – 57(5)

7.5 51.6(1) 49.1(1) 45.9(1) 0.38(1) 0.29(1) 0.67(1) 0.32(3) 0.33(3) 0.40(3) 0.20(3) 0.28(3) 0.52(3) 1.90 – – 58(5)

51.5(1) 49.1(1) 45.8(1) 0.37(1) 0.30(1) 0.67(1) 0.33(3) 0.37(3) 0.43(3) 0.14(3) 0.35(3) 0.51(3) 1.48 0.032(2) Fe2.968 O4 61(5)

t (min) 10.0 12.5 – 49.0(1) 45.9(1) – 0.28(1) 0.67(1) – 0.32(3) 0.37(3) – 0.37(3) 0.63(3) 1.71 0.013(2) Fe2.987 O4 60(5)

15.0

– 49.2(1) 45.9(1) – 0.29(1) 0.67(1) – 0.38(3) 0.42(3) – 0.38(3) 0.62(3) 1.62 0.020(2) Fe2.980 O4 60(5)

17.5

– 49.2(1) 45.8(1) – 0.29(1) 0.67(1) – 0.35(3) 0.39(3) – 0.37(3) 0.63(3) 1.67 0.016(2) Fe2.984 O4 58(5)

25.0

2.5+ Table I. Hyperfine parameters derived from the Mössbauer spectra using three sextets corresponding to hematite and magnetite (Fe3+ and FeB sites), in addition to the average grain size obtained from XRD measurements. Relative areas per crystalline site, R values, oxidation parameter and chemical formula are also included. Numbers in parenthesis correspond to the estimated error bars

168 J. D. BETANCUR ET AL.


169

Figure 3. X-ray diffraction patterns for the whole considered treatment time range. In agreement with the Mössbauer results, diffractograms exhibit exclusively magnetite reflections for times greater than 12.5 min.

increased from room temperature up to 375◦ C when the hydrogen flow is just implemented. Consequently this maghemite would become reduced to magnetite as sketched by Equation (6). Alternatively, the reduction of the precursor hematite could be leading to both maghemite and magnetite as represented by Equation (7). Thus, taking into account the presence of a component of 46.0 T suggesting the presence of magnetite (the Fe2.5+ contribution), those reactions involving the presence of maghemite and magnetite and since the hyperfine parameteres of maghemite are practically indistinguishable from those of the Fe3+ sites in magnetite at room temperature, would imply that the area A(Fe3+ ) measured in those samples having R greater than 1.89 is indeed a sum of two contributions which would lead to a magnetite even much more defective than the reported on the first model. If we assume that no maghemite is involved during the transformation then Equation (8) would describe the hematite to magnetite reduction leading us to the first approach above.

170


Figure 4. XRD example showing the way as the most intense peak (3 1 1) was fitted with two pseudo-Voigth functions taking into account the hematite (−1 1 0) and magnetite (3 1 1) components.

In order to evaluate the rate of transformation as a function of the treatment time, the respective H and M relative areas were determined from both techniques. In particular, an example showing the way as the most intense peak in the X-ray diffraction patern was fitted for 2.5 min is shown in Figure 4. Results of the relative areas are therefore shown in Figure 5 and they suggest that the kinetics of transformation takes place at greater velocity during the first stages (short times). This fact is easily understandable by assuming that the velocity of transformation should be proportional to the amount of the remanent precursor (H) material, from which the following expression is established: −

∂AH ∝ AH , ∂t

(9)

where AH corresponds to the relative area of hematite. Consequently a behaviour law described by a single decreasing exponential function for the hematite is followed, suggesting probably a percolation-type or avalanche-type phenomenology. Analogously other expression for the final product is obtained: AM = 1 − e−t /τ ,

con τ = 3.5 ± 0.1 min,

(10)

where τ (= 3.5 ± 0.1 min) is a characteristic time describing the kinetics of reaction. Such behaviour laws for both H and M, are shown in Figure 5 and they fit reasonably well to the XRD experimental results. However, although both employed techniques exhibit a similar behavior concerning the relative areas, the observed discrepancies can be attributed to the local character of the Mössbauer spectroscopy in contrast to the bulk character of the XRD measurements. If this is the fact, Mössbauer could be indicating that microscopically the transformation is taking place in well-defined stages with different characteristic times as is shown in Figure 5, labels I and II.


171

Figure 5. Treatment time dependence of the relative areas obtained from both XRD and Mössbauer techniques for the thermally treated samples.

Regarding the as-milled samples, Figure 6 shows the room temperature Mössbauer spectra and their corresponding hyperfine field distributions (HFD). The fitting process was carried out employing three independent HFD plus one sextet associated to hematite which appears only up to 3 hours of milling in clear correspondence with the XRD patterns (Figure 7). In contrast to the thermally treated samples, the magnetites obtained via ball milling are of poor crystallinity. This fact is reflected in the three employed HFD assigned as follows: two distributions centered at around 49 T and 46 T corresponding to magnetite, Fe3+ and Fe2.5+ sites, respectively, and a lower field distribution centered at around 32 T which could be addressed as a consequence of both a broad distribution of particle size accordingly with the broadening of the XRD peaks which in turn could be also attributed to a lattice distortion (Figure 7, and Table II) and a distribution of defects and vacancies originated by the milling process. In this last respect, and according to the obtained HFD, the Fe2.5+ site component becomes broader and less intense in comparison to the Fe3+ site component which remains narrow and almost symmetric independently of the milling time, suggesting a preference of the vacancies for the octahedral sites. This fact has been already reported to occur in partly oxidized magnetites together with the introduction of vacancies [11] and in some doped magnetites for which the dopant elements prefer the B-sites [12].

172


Figure 6. Room-temperature Mössbauer spectra and their corresponding hyperfine field distributions as a function of the milling time for the ball-milled samples.


173

Figure 7. X-ray diffraction patterns for the whole considered milling time range. In agreement with the Mössbauer results, diffractograms exhibit exclusively magnetite reflections for times greater than 3 hours.

In addition, the obtained R values increase from 0.41 for 1 hour up to 0.80 for 6 hours (Table II) in a well-behaved fashion indicating a tendency to obtain more stoichiometric magnetites as the milling time increases. This fact, contrary to the thermally treated samples, suggest a crossover between a “non-stoichiometric maghemite” and a very non-stoichiometric magnetite (Figure 1). The transformation mechanics could be throught as one in which oxygen bonds on the α-Fe2 O3 surface are considered to be broken by the mechanical activation [8]. Finally, concerning the kinetics of transformation, the milling time dependence of the Mössbauer relative areas is shown in Figure 8 from which a characteristic time of τ = 0.86(5) hours was estimated. 4. Conclusions The obtained results suggest that both employed synthesis methods based on solidstate reactions (thermal treatment under controlled atmosphere and ball milling),

174


Table II. Hyperfine fields, relative areas derived from Mössbauer and average grain size in addition to lattice parameter obtained from XRD measurements. Areas ratio, oxidation parameter and chemical formula are also included. Numbers in parentheses correspond to the estimated error bars t (hours)

Bhf (T) α-Fe2 O3 Bhf (T) Fe3+ Bhf (T) Fe2.5+ Bhf (T) Dist. A (×100%) α-Fe2 O3 A (×100%) Fe3+ A (× 100%) Fe2.5+ A (×100%) Dist. R 2.5+ A(FeB )/A(Fe3+ ) x Fe3−x O4 D311 (nm) Fe3−x O4 a(A)

1

2

3

4

5

6

51.5(1)

51.5(1)

51.6(1)

–

–

–

48.9(2)

48.9(2)

48.8(2)

48.7(2)

48.8(2)

48.8(2)

44.0(2)

44.1(2)

44.4(2)

44.5(2)

44.5(2)

44.5(2)

32.5(2)

32.1(2)

32.8(2)

32.0(2)

32.0(2)

31.8(2)

0.30(6)

0.13(6)

0.01(6)

0

0

0

0.43(6)

0.51(6)

0.55(6)

0.50(6)

0.49(6)

0.48(6)

0.17(6)

0.23(6)

0.31(6)

0.37(6)

0.38(6)

0.39(6)

0.10(6)

0.13(6)

0.12(6)

0.13(6)

0.13(6)

0.13(6)

0.41

0.45

0.56

0.73

0.77

0.80

0.195(2) Fe2.805 O4 31(6)

0.185(2) Fe2.815 O4 31(6)

0.160(2) Fe2.840 O4 29(6)

0.127(2) Fe2.873 O4 26(6)

0.120(2) Fe2.880 O4 25(6)

0.116(2) Fe2.884 O4 24(6)

8.39(1)

8.39(1)

8.39(1)

8.36(1)

8.36(1)

8.33(1)

constitute an important and fast alternative for obtaining magnetite from hematite. In the case of the thermally treated samples, magnetite is obtained as the only phase after just 15 minutes under the suitable environment (20% H2 + 80% N2 , 375◦ C) with high crystallinity and stoichiometry. Contrary to this, the ball-milled magnetites exhibit a broad distribution of crystalline and semicrystalline sites in addition to a corresponding distribution of vacancies and grain size induce by the milling process. In this case, magnetite is the only phase above 3 hours of milling. Finally, the present work allowed to evaluate the way as the transformation takes place through the intermediate states by computing the degree of oxidation and the evolution of the relative areas which closely follows an exponential-type behavior.


175

2.5+ Figure 8. Mössbauer relative areas (α-Fe2 O3 , Fe3+ , FeB ) as a function of the milling time for the as-milled samples.

Acknowledgements This work was supported by the Scientific Research Development Committee (CODI) of the Universidad de Antioquia. One of the authors (J.D.B) would like to thank Dr. César Barrero and the project IN378CE-CODI and COLCIENCIAS for the invaluable help. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13.

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