Reduction of metal oxides by mechanical alloying method

SOLID STATE ELSEWIER

Solid State Ionics

101-103

IONICS

(1997) 25-31

Reduction of metal oxides by mechanical

alloying method

Kazuto Tokumitsu* Department of Materials

Science,

Tokyo Unitwsit~.

Hongo

73- 1. Bunkyo-ku. Tokyo 113, Japan

Abstract The reduction of metal oxides is usually carried out at high temperature in appropriate conditions with respect to equilibrium thermodynamics. However using the high activity of fresh surfaces enables mechanically activated reduction. After ball-milling of silver oxide, Ag,O, copper oxide. CuO, and hematite, Fe,O,, respectively, with carbon powder, metallic silver and copper could be obtained at room temperature and hematite was reduced to magnetite. The reduction processes were investigated by X-ray diffraction and “Fe Mossbauer spectroscopy. Ke,v~ords: Reduction;

Mechanical

activation;

Ball-milling:

Mechanical

alloying;

Silver oxide: Copper oxide; Iron oxide:

“Fe Miissbauer

spectroscopy Mrrterials:

Ag,O: CuO FezO,

1. Introduction Mechanical

alloying

by milling

of metallic

pow-

used to synthesize new materials. In Benjamin 1970, succeeded in elaborating strengthened superalloys by oxide dispersion [ 11. But this success influenced only the field of powder metallurgy. By the way, a lot of new alloys in the form of supersaturated solid solutions, amorphous alloys and quasi-crystals have been rapidly developed using new techniques, for example, liquid quenching and sputtering. Yermakov et al. 121 and Schwarz and Koch [3] have reported the possibility of getting these non-equilibrium alloys by a ballmilling technique. Milling has been also studied in tribochemistry [4]. This method has two advantages: ders

has been

*Correspondence. 8363.

Tel.:

+81-3

3812 21 I I: fax:

+81-3

3815

0167.2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOl67-2738(97)00277-4

1. In preparing alloys at room temperature. The process requires neither melting nor cooling of alloys. In other words it is independent from the melting points of elements, so high melting temperature and thermally unstable alloys can be prepared. 2. In elaborating alloys in the form of powder samples. This is industrially useful for material forming. This method is now extended to nanostructured materials and multilayered alloys.

articles have been reported about mechanical alloying [5- 121. In the above mentioned way, ball-milling is the basic step for this process. However it is also possible to apply this technique to some chemical reactions, e.g. reactions with liquid hydrocarbons [ 131 and chemical refining [14]. The fresh surfaces created by ball-milling are more active than usual ones or clean surfaces obtained by ultraMany

26

K. Tokumitsu I Solid State Ionics

high vacuum technique. Ball-milling is the only method to create fresh surfaces repeatedly at any time. The scientific and technological fields of mechanical alloying are close to mechanochemistry or tribochemistry. In this paper the reduction of silver oxide, copper (II) oxide and hematite by ball-milling with carbon powder will be presented.

101-103

1600

1400 .u . 5 1200 ;;i f; a 1000 E K 800 ’

2. Experimental Milling of metal oxide powders was carried out by a SPEX 8000 mixer mill. The vial was made of yttria-toughened zirconia (YTZ). The balls of 5 mm in diameter were also made of YTZ. Metal oxide and carbon powders were mixed in the vial, sealed and maintained under an argon atmosphere at constant room temperature. The purity of the metal oxide powders was 99.9 at.%, the used amounts were 0.05, 0.1 and 0.02 mole for Ag,O, CuO and o-Fe,O, respectively and the molar ratios between metal oxides and carbon were calculated for complete reduction, i.e. 2/ 1, 2/ 1 and 2/3 for Ag,O, CuO and c-u-Fe,O, respectively. The internal pressure of the vial increased during reduction by transformation of carbon into CO, gas. Milling was stopped regularly in order to avoid friction heat. The crystal structures were determined by X-ray diffraction (XRD) analysis using the MoKa, CuKcx and FeKol radiations for silver oxide, copper oxide and iron oxide reduction respectively. Mossbauer spectra of 57Fe were performed at room temperature with a constant acceleration spectrometer with a s7Co source in a rhodium matrix.

3. Results and discussion 3.1. Silver oxide (Ag,O) Fig. 1 shows the phase diagram of the silveroxygen system [ 141 for the low oxygen contents. Two kinds of silver oxides are reported, Ag,O, and Ag,O,. The former is a Cu,O-type structure with the ionic state Ag+. The latter is monoclinic with two ionic states, Ag+ and Ag3+. Fig. 2 shows the changes in the XRD patterns of

(1997) 25-31

Ag

Ag+G

600 1 0

2

1 3

oxygen

0

1

Ag

Atomic percent

Fig. 1. Phase diagram

of the silver-oxygen

system after

[ 151.

I

10

20

30

40

10

20

30

40

20 I degree Fig. 2. Changes in the X-ray diffraction patterns (MoKa radiation) with milling time for Ag,O powder milled with carbon powder at room temperature.

Ag,O powders as a function of milling time. The diffraction peaks remained broad during the first 20 min but, suddenly, after 22 min, Ag,O completely reduced to metallic Ag while the internal pressure increased. Three possibilities can be considered for the reduction of silver oxide: decomposition 1. Thermal Ag,O+Ag+O,. 2. Direct reaction of Ag,O Ag,O+C+Ag+CO,.

of with

Ag,O, carbon,

i.e.: i.e.:

K. Tokumitsu

3. Reaction of Ag,O with CO i.e.: C+Ag+CO, (2) Ag+CO+Ag+CO>.

I Solid State lonics

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103 (1997)

25-31

21

(1) Ag?O+

It is well known that Ag,O is thermally unstable and decomposes above 460 K into metallic silver [ 161. If the interface temperature of powders increased above 460 K during milling, the first case would be possible. However carbon was not retained after reduction, so this case is not probable. As for the third case, the reduction of Ag,O would be started as soon as the milling process would be initiated and not only after 22 min and continuously as the reduction of Ag,O with CO gas is possible at temperatures as low as at -40°C [ 171. In any case, additional analysis of the gas composition would be necessary to conclude.

hh h hh

h n-----lcu

J\, y

L

40

60

80

40

60

80

28 / degree

3.2. Copper oxide (GO,

tenorite)

Fig. 3 shows the phase diagram of the copperoxygen system [18]. Two oxides are reported, Cu,O and CuO. The former is cubic with the ionic state cu’. The latter is monoclinic, exists as natural tenorite, dissociates into Cu,O over 800°C in air, melts between 1060°C to 1140°C in rapid heating and its ionic state is Cu’+. Fig. 4 shows the changes in the XRD patterns of CuO powders with milling time. The diffraction lines [( 11 I), (200) and (220)] of Cu,O began to appear after 10 h of milling, and the diffraction lines [( 11 1) and (200)] of metallic Cu were partially observed. The intensity of CuO peaks decreased with milling

1400 I

Fig. 4. Changes in the X-ray diffraction patterns (CuKa radiation) with milling time for CuO powder milled with carbon powder at room temperature.

time increasing and the only peaks after 40 h were due to Cu,O and metallic Cu. Then the intensity of Cu,O peaks decreased and only metallic Cu peaks [( 11 l), (200) and (220)] remained visible after 150 h. The presence of non-stoichiometric compounds, The cue, -x and Cu,O, _x, was not confirmed. pressure in the vial increased suggesting the formation of CO2 gas. The reduction of copper oxide can be considered as essentially due to mechanical activation since the decomposition temperature of CuO occurs above 1000°C and by assuming that the temperature of powders cannot be increased up to this value by friction heat due to milling. The standard Gibbs energies (in kJ) are as follows: [19]. 1. AG;= -50.9+0.0255 T log T-0.186 T for 2 CuO+O.5 C+Cu,O+O.5 CO,, 2. AG;= -27.6+0.0164 T log T-0.21 1 T for Cu? 0+0.5 C-+2 CuO+O.5 CO,, 3. AG;= -73.4+0.0420 T log T-0.310 T for 2 CuO+C+2 Cu+CO,.

CU Fig. 3. Phase diagram

Atomic percent oxygen of the copper-oxygen

system after

0

[ 181.

Assuming that CO does not coexist and the partial pressure of CO, is equal to 1 atm, the free energy

28

K. Tokumitsu

I Solid State Ionics 101-103

(1997) 25-31

changes for these reductions can be roughly estimated. These values at 300 K are - 88 kJ, -79 kJ and - 135 kJ respectively. So, the reduction of CuO with carbon can occur at room temperature. It can be suggested that the milling operation alters the activation process and facilitates those reductions. 3.3. Iron oxide (a-Fe203,

hematite)

Fig. 5 shows the phase diagram of the ironoxygen system [20]. Three iron oxides have been reported: Fe0 (wiistite), Fe,O, (magnetite) and Fe,O, (hematite). Wustite is of NaCl-type and magnetite is of spine1 type. There are two hematites, one is a stable a-phase with corundum structure (hematite) and another is an unstable y-phase (maghemite) of spine1 type and transforming into o-phase above 300-350°C. Fig. 6 shows the changes in XRD patterns for a-Fe,O, powders with milling time. The (220) diffraction peak of Fe,O, began to appear after 10 h of milling. The more intense peak of Fe,O,, (3 1 l), is not easily evidenced because of overlapping with the (110) peak of o-Fe,O,. The intensity of Fe,O, peaks decreased with milling time increasing and the only peaks of Fe,O, were observed after 100 h of milling. Fig. 7 shows the XRD pattern of o-Fe,O, milled for 100 h. All peaks were ascribed to Fe,O, but the presence of Fe0 and metallic Fe was not confirmed within this milling time. No increase of the pressure in the vial after reduction was detected

i-111

40

80

100

40

60

80

100

28 I degree Fig. 6. Changes in the X-ray diffraction patterns (FeKa radiation) with milling time for a-Fe,O, powder milled with carbon powder at room temperature.

20

1800 :

60

Fe304

40

60

80

100

12

20 / degree Fig. 7. X-ray diffraction patterns (FeKa) for ol-Fe,O, milled with carbon powder at room temperature.

0 Fe

20

40

60

Atomic percent oxygen

Fig. 5. Phase diagram

of iron-oxygen

system after [20].

0

powder

as the change of pressure due to the Fe,O,+Fe,O, reaction is extremely low (4.8%). The 57Fe Mossbauer spectra of the milled powders were measured to study the micro-process of reduction and the micro-formation of Fe0 or metallic Fe not easily detected by XRD. The spectrum of a-Fe,O, is a magnetically splitted sextet due to Fe+j

K. Tokumitsu

/ Solid State lonics

with an internal field of 515 kG at room temperature [21]. The spectrum of Fe,O, is composed of two sextets corresponding with the two kinds of Fe sites in the spine1 structure with the following arrangement: [Fe3+],Aj[Fe7+Fe3+],Bj. The outer sextet corresponds with the tetrahedral A-site [Fe+‘] and the inner sextet with the octahedral B-site [Fe”* and Fe3+]. Their internal fields are 491 kG and 453 kG respectively at room temperature [22]. Fig. 8 shows the changes in the Mossbauer spectrum of cx-Fe,O, powders with milling time. A new absorption overlapped inside the sextet of CYFe,O, after 10 h, increased with milling time and was clearly evident after 20 h. This absorption became stronger than that of o-Fe,O, after 40 h and was composed of two sextets. The absorption due to a-Fe,O, disappeared after 100 h of milling and two sextets were obtained which were characteristic of Fe,O, because their internal fields were about 491

I

I

’ a-Fe,0

’ Fe

-p.n_r-r7.n...; :. ; ..

1

101-103

(1997) 25-31

29

kG and 453 kG. The presence of non-stoichiometric compounds (Fe,O,+, or Fe,O,-,, FeO,_,) were not conhrmed. The standard Gibbs energies (in kJ) of the reactions are as follows [23]: 1. AG):=52.4-0.141 T for 3 Fez03+0.5 C-+2 Fe,O,+0.5 CO,, 2. AC;= 115.2-0.126 T for Fe,O,+O.5 C-+3 FeO+0.5 CO,, 3. AGj:=63.0-0.063 T for FeO+0.5 C+Fe+0.5 CO,. At 300 K the values are 10.1, 77.4 and 44.1 kJ respectively with the same assumptions as with copper oxides. The reduction of Fe,O, does not easily proceed at room temperature as the free energy changes are positive. The reduction from Fe,O, to FeO, in particular, is difficult because of the large free energy change of 77.4 kJ. It is evident that the equilibrium: CO, + C = 2C0 must be considered in the reduction of iron oxides.

3

-. ._ .... f

4. Role of milling Table milling:

1 shows the functions

involved

in ball-

M

Mixing and dispersion are the basic way of milling useful for the automatic mortar. Crushing of powders leads to a decrease in size of crystallite grains or particles. Introduction of strains and defects, e.g. vacancies or dislocations, by strong deformations results in both increase of free energy and stimulation of atomic diffusions. Creation of fresh surfaces by crushing of powders causes high chemical reactivity.

Table I Functions

Velocity

/ mm-s-1

Fig. 8. MGssbauer spectra measured at room ol-FezO, powders milled with carbon powder.

temperature

for

(I ) (2) (3) (4) (5)

of milling

Mixing and dispersion Crushing and pulverization Introduction of strains and defects Creation of fresh surfaces Elastic deformation

30

K. Tokumitsu

I Solid State Ionics 101-103

Table 2 Effects of milling (1) (2) (3) (4) (5) (6)

5.

Nano-structured materials and composites Stimulation of diffusion and change of free energy Alloying Amorphization Induction of chemical reactions Multi-layered structure

In metallic elements, elastic deformation is expected. Then milling is regarded as micro-rolling or micro-forging.

It is needless to say that these functions are not independent and work at the same time during milling. Table 2 shows the effects of ball-milling. Through the function of mixing and dispersion, uniformly dispersed alloys can be synthesized independently from their gravities. Through the function of pulverization, non-structured materials can be attained. The introduction of defects stimulates the atomic diffusion enabling elaboration of any alloy at room temperature. This is helpful for the preparation of thermally unstable materials. Moreover increasing defects and interfaces destroys the crystallographic symmetry and changes the free energy leading to amorphous materials. The creation of fresh surfaces induces many chemical reactions. Through the elastic deformation, with metals, it is possible to form multi-layered structures. Among these functions and effects, the creation of fresh surfaces is the most useful to induce the chemical reactions. Clean surfaces can also be created by the ultra-high vacuum technique but these surfaces are rather less active than the fresh surfaces and do not lead to as improved kinetics of chemical reactions as milling which might be a significant method from the point of view of its reproducibility, easiness of use and versatility.

5. Conclusions The reduction of metal oxides by ball-milling with carbon powders was examined at room temperature. Metallic silver and metallic copper were obtained from Ag,O powders and CuO powders respectively

(1997) 25-31

and Fe,O, from a-Fe,O,. oxide was detected.

No non-stoichiometric

Acknowledgements I would like to thank Prof. T. Uomoto, Institute of Industrial Science, Tokyo University and Prof. K. Morita, Tokyo University for their useful discussions. This research was supported by the Grant-inAid for Scientific Research (C) No.06805066 from the Ministry of Education, Science and Culture of Japan.

References [II J.S. Benjamin, Metall. Trans. 1 (1970) 2943. 121 A.Ye. Yermakov, Ye. Ye. Yurchikov, VA. Barinov, Phys. Met. Metall. 52 (1981) 50. [31 R.B. Schwarz, CC. Koch, Appl. Phys. Lett. 49 (1986) 146. 141 G. Heinicke, Tribochemistry, Akademie-Verlag. Berlin, 1984. ISI P.H. Shingu, K.N. Ishihara, A. Otsuki, Mater. Sci. Forum 179-181 (1995) 383. 161 K.N. Ishihara, T. Matsumoto, A. Otsuki, P.H. Shin&u, Solid State Phenomena 42-43 (1995) 77. L71 R.B. Schwarz, W.L. Johnson (Eds.), Solid State Amorphizing Transformations, Elsevier Seq., 1988. PI E. Artz, L. Schultz (Eds.), New Materials by Mechanical Alloying Techniques, DGM, 1989. [91 F.H. Fores, J.J. de Barbadillo (Eds.), Structure Applications of Mechanical Alloying, ASM, 1990. LlOl P.H. Shin&u (Ed.), Mechanical Alloying, Materials Science Forum 88-89 (1992). [Ill PH. Shingu (Ed.), Special issue on Mechanical Alloying, Mater. Transact. JIM 36 (1995). [I21 D. Fiorani, M. Magini (Eds.), Synthesis Mechanically Alloyed and Nanocrystalline Sci. Forum 235-238 (1997).

and Properties of Materials, Mater.

1131 K. Tokumitsu, Z. Physikal. Chem. 183 (1994) 443. [141 P.G. McCormick, Mater. Transact. JIM 36 (1995) 161. 1151 T.B. Massalski (Ed.), Binary Alloy Phase Diagram, ASM, 1986, p. 49. K. Hesselmann (Eds.), Ther1161 0. Knacke, 0. Kubaschewski, mal Properties of Inorganic Substances, Springer, 1991, p. 11. 1171 I. Nakamori, H. Nakamura, T. Hayano, S. Kagawa, Bull. Chem. Sot. Japan 47 ( 1974) 1827. 1181 T.B. Massalski (Ed.), Binary Alloy Phase Diagram, ASM, 1986, p. 943. C.B. Alcock (Eds.), Metallurgical Ther(191 0. Kubashewski, mochemistry, Pergamon, 1979, p. 379.

K. Tokumitsu / Solid State lonics [20] T.B. Massalski (Ed.), Binary Alloy Phase Diagram, ASM. 1986, p. 1087. 1211 N.N. Greenwood, T.C. Gibb (Eds.), MGssbauer Spectroscopy, Chapman and Hall, 1971, p, 240.

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[22] N.N. Greenwood, T.C. Gibb (Eds.), MGssbauer Spectroscopy, Chapman and Hall, 1971, p. 251. 1231 0. Kubashewski, C.B. Alcock (Eds.), Metallurgical Thermochemistry. Pergamon, 1979, p. 38 1,