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Mechanochemical synthesis of zinc ferrite from zinc oxide and α-Fe2O3
Powder Technology 114 Ž2001. 12–16 www.elsevier.comrlocaterpowtec
Mechanochemical synthesis of zinc ferrite from zinc oxide and a-Fe 2 O 3 Wantae Kim, Fumio Saito ) Institute for AdÕanced Materials Processing, Tohoku UniÕersity 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan Accepted 23 March 2000
Abstract Mechanochemical synthesis of zinc ferrite ŽZnFe 2 O4 . from a powder mixture of zinc oxide ŽZnO. and hematite Ž a-Fe 2 O 3 . by room temperature grinding using a planetary ball mill was investigated. The grinding enables us to obtain the amorphous mixture of the starting materials. Most of ZnO reacts with a-Fe 2 O 3 to convert into insoluble amorphous zinc and iron compounds within 2h-grinding. Prolonged grinding enhances the crystallization of ZnFe 2 O4 from the amorphous compounds. ZnFe 2 O4 crystallized by the grinding for 3 h or more consists of nanocrystalline particles with high specific surface area. q 2001 Elsevier Science S.A. All rights reserved. Keywords: Mechanochemical synthesis; Zinc ferrite; Zinc oxide; Hematite; Planetary ball mill
1. Introduction ZnFe 2 O4 is known as an advanced mixed-metal sorbent for high-temperature desulfurization of coal gas. This removes hydrogen sulfide ŽH 2 S. from hot coal-derived gas streams and can operate at temperatures and pressures matching typical gasifier outlet conditions. Due to the versatility of high performance desulfurization and easy regeneration, kinetics and capacities of the sorbent ZnFe 2 O4 in fixed-bed combustion process have been investigated w1–3x. Catalytic-grade ZnFe 2 O4 is normally manufactured by a heating process of an equal molar mixture of zinc and iron oxides at high temperature w4–7x. ZnFe 2 O4 is characterized by a normal spinel structure in which Zn2q cations with no magnetic moment occupy tetrahedral sites, while all the Fe 3q cations occupy octahedral sites within the lattice of cubic close packing. For the reason that no exchange interaction of each lattice site of ZnFe 2 O4 configuration takes place Žparamagnetic., therefore, structural change and surface chemistry of ZnFe 2 O4 for further utilization are also studied by high-energy ball-milling techniques w8,9 x. Recently, surface activation and mechanochemical solid-state reaction have become promising tools for the synthesis and modification of spinel ferrites. Sepelak et al. w10x used the mechanically modified ZnFe 2 O4 for H 2 S removal. Lefelshtel et al. w11x ) Corresponding author. Tel.: q81-22-217-5135; fax: q81-22-2175135. E-mail address: [email protected] ŽF. Saito..
and Jovalekic et al. w12x reported the formation of ferrite materials by long-time ball milling. However, little information has been offered on the mechanism of ZnFe 2 O4 formation from ZnO and a-Fe 2 O 3 powder mixture during its grinding. The main purpose of this work is to provide experimental information on the mechanochemical synthesis of ZnFe 2 O4 from a powder mixture of ZnO and a-Fe 2 O 3 by its dry grinding conducted using a planetary ball mill at room temperature. This work also involves the crystallizing mechanism of nanocrystalline ZnFe 2 O4 from the mixture during the grinding. Thermal behavior and microstructure of synthesized ZnFe 2 O4 were investigated by monitoring TG–DTA and high-resolution transmission electron microscopy ŽHR-TEM.. It was found that ZnFe 2 O4 was synthesized from the amorphous phase of the starting materials and the formation was promoted by the state of amorphization.
2. Experimental 2.1. Experimental apparatus and procedure Zinc oxide ŽZnO. and hematite Ž a-Fe 2 O 3 . supplied from Wako, Japan were used in this work. These two starting materials were weighed in molar ratio of 1.0, which coincides with the stoichiometric valence of zinc ferrite ŽZnFe 2 O4 ., and mixed thoroughly in acetone using an agate mortar with a pestle prior to grinding. The
0032-5910r01r$ - see front matter q 2001 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 0 . 0 0 2 5 6 - 4
W. Kim, F. Saito r Powder Technology 114 (2001) 12–16
13
mixture prepared prior to grinding is called unground mixture in this work. A planetary ball mill ŽFritsch Pulverisette-7. with a pair of stainless steel pots of 50 cm3 P inner volume containing seven stainless steel balls of 15 mm diameter was used for the grinding of the mixtures. The mixture Ž4.0 g. was put in the mill pot and ground at 790 rpm in rotational speed of the mill. The duration of grinding was varied from 0.5 to 4 h. The grinding was suspended for 10 min after every 10 min grinding to avoid excess temperature increase inside the mill pots during grinding. In order to check the reliability of XRD results, the amount of unreacted ZnO of the ground mixtures was quantified by leaching method as follows: The ground mixture Ž0.2 g. was suspended in 50 ml of acid solution Ž0.1 mol HCl. with stirring for 10 min at room temperature. The solid phase in the suspension was separated from the solution by filtration and washed completely with deionized water. From our preliminary leaching test, the same amount of ZnO was completely extracted within 10 min in the leaching condition employed. Finally, the concentration of zinc in the leaching solution was measured by an inductively coupled plasma ŽICP. analysis. 2.2. Characterization
Fig. 1. XRD patterns of the mixtures ground for various durations of time.
X-ray diffraction ŽXRD. analysis ŽRAD-B System, Rigaku. of the ground mixtures was carried out to check the phases of the mixtures. Morphology of the ground mixtures was observed by a scanning electron microscope ŽSEM, SEM-4100, Jeol.. Specific surface areas Ž S w . of the unground and ground mixtures were measured by a surface area analyzer ŽASAP-2010, Micromeritics.. The samples were also subjected to thermogravimetric and differential thermal analysis ŽTG–DTA, TAS-200, Rigaku. simultaneously in air at the heating rate of 108Crmin. In addition, atomic scale observation on the grain boundary and lattice fringe of ZnFe 2 O4 formed in the ground mixture was performed by a high-resolution transmission electron microscope ŽJEM-3010, Jeol..
3. Results and discussion Fig. 1 shows XRD patterns of the mixtures ground for various durations of time. The characteristic peaks of ZnO disappear in the pattern of the mixture ground for 1 h, while the peaks of a-Fe 2 O 3 are still detected in the mixture ground for 2 h. Accordingly, very weak new peaks are detected in the pattern of the mixture ground for 2 h, indicating that the follow ing reaction occurs mechanochemically. ZnO q Fe 2 O 3
™ ZnFe O 2
4
Ž 1.
Standard Gibbs free energy change DG 2988 calculated in Eq. Ž1. is y3.02 kJ moly1 , implying that the grinding assists the spontaneous reaction forward thermodynami-
cally to form ZnFe 2 O4 from the starting mixture when the interface of the solid surface in the starting materials is closely contacted and sheared by the grinding. It is noted that the peaks of a-Fe 2 O 3 almost disappear and the peaks of ZnFe 2 O4 are developed markedly in the mixture ground for 3 h. From the XRD profiles, it can be deduced that the crystallization of ZnFe 2 O4 is initiated in the amorphous mixture of the starting materials. Consequently, the solidphase diffusion is accelerated by the grinding when the amount of amorphous zinc and iron compounds in the ground mixture increases as the grinding progresses. However, no noticeable change is observed in the patterns of the mixtures ground for 4 h or more except for the slight increment of the peak intensity of the ZnFe 2 O4 formed. Fig. 2 shows S w of the mixtures as a function of grinding time. The S w value increases to 7.4 m2rg in the early stage of grinding within 0.5 h. Prolonged grinding up to 2 h reduces the value to 4.2 m2rg. This behavior can be explained by aggregation of ground fine particles, the aggregation that is attributed to the mechanical activation caused by dry prolonged grinding. On the contrary, the increase trend from 7.7 to 12.3 m2rg can be seen in the mixtures ground for 3 and 4 h, respectively. This may be attributed to the following reasons. The aggregates of the ground particles with relatively weak bonding are reground Ždissociation. into fine particles by prolonged mechanical stresses of friction and compression. Simultaneously, the transformation reaction of amorphous phase into fine crystalline ZnFe 2 O4 occurs on the surface of the ground particles with solid-phase diffusion under the strong grind-
14
W. Kim, F. Saito r Powder Technology 114 (2001) 12–16
Fig. 2. S w of the mixtures as a function of grinding time.
ing energy and the reaction penetrates into the particles interface, resulting in the formation of individual crystals as the grinding progresses. Fig. 3 shows SEM photographs of the mixtures ground for various duration of time. Morphology of the unground mixture ŽA. is presented as a reference. Comparatively coarser and irregular particles of a-Fe 2 O 3 and fine particles of ZnO are observed in the photo. In the mixture ŽB. ground for 2 h, original shapes of the starting materials are changed into the aggregates of fine particles. When the grinding is prolonged 3 h or more ŽC and D., though few aggregates still remained in the mixture ground for 3 h, new fine particles approximately 0.2–0.5 mm in size are observed as a whole in the photos. Although no further
Fig. 4. Free zinc content in the mixtures as a function of grinding time.
distinct information can be obtained from the photographs, the formation of fine particles, coupled with the two results of XRD and S w measurements as shown in Figs. 1 and 2, may be attributed to the crystallization of ZnFe 2 O4 . Fig. 4 shows the free zinc content in the mixtures as a function of grinding time. In the mixture ground for 0.5 h, most of the ZnO Ž83.2% in zinc base. does not react with a-Fe 2 O 3 , whereas only a small portion of ZnO converts to insoluble compounds of amorphous zinc and iron as well as their crystalline ones. This implies that most of the starting materials still retain their own crystal structures in the early stage of grinding within 0.5 h, though the particles undergo not only size reduction but also lattice disordering by mechanical stresses. In the 2h-ground mixture, the free zinc content decreases with an increase in grinding
Fig. 3. SEM photographs of the mixtures ground for various durations of time. ŽA. Unground ŽB. 2 h ŽC. 3 h and ŽD. 4 h grinding.
W. Kim, F. Saito r Powder Technology 114 (2001) 12–16
time, and reaches about 8.1% at 2 h of grinding. This indicates that the solid-state reaction proceeds remarkably as the grinding progresses up to 2 h. However, further grinding does not contribute to the complete reaction of the starting materials; approximately 4.2% of unreacted zinc still remained even in the mixture ground for 4 h. Fig. 5 shows TG–DTA curves of the mixtures ground for various durations of time. In the DTA curves, the remarkable exothermic peak appeared at 7608C in the unground mixture corresponding to the crystallization of ZnFe 2 O4 by heating the starting materials w5x. In addition, a broad exothermic reaction appeared in the temperature range from around 508C to 11008C corresponding to the formation of ZnFe 2 O4 . The former exothermic peak tends to shift towards the lower temperature side with increment of its peak intensity and the latter broad exothermic reactions shorten in the temperature range of around 50–9008C as the grinding progresses. This implies that the grinding operation is favorable for the formation of ZnFe 2 O4 . It is noticeable that the former exothermic peak disappears and only a broad exothermic peak at around 4008C is detected in the mixtures ground for 3 h or more. This broad exothermic reaction seems to be mainly attributed to the recrystallization of ZnFe 2 O4 from the unstable and amorphous phases of ZnFe 2 O4 already formed in the ground mixtures w8x. On the other hand, no distinct variation can be detected in TG curves of the mixtures over the wide range of the grinding time and temperature employed. Fig. 6 shows HR-TEM photographs of the 3h-ground mixture ŽA. and the mixture heated at 4008C for 1 h ŽB.. The microstructural image ŽA. shows the single crystal of ZnFe 2 O4 approximately 25 nm in size. The particle is
15
Fig. 6. HR-TEM photograph of the 3h-ground mixture and the mixture heated at 4008C.
folded and surrounded by other crystals and amorphous phases. This indicates that the ZnFe 2 O4 is crystallized from the amorphous phases of the starting materials. The grain boundary is not clearly observed; anyhow, the lattice distance of the fringe is approximately 0.487 nm, which corresponds to the Ž111. plane of ZnFe 2 O4 . In the ground mixture heated at 4008C ŽB., the grain boundary Žd. of each crystal is clearly observed and most straight fringes are measurable in each single crystal, in which the lattice distances are measured as approximately 0.487 Ža. and 0.298 Žb and c. corresponding to the Ž111. and Ž220. plains of ZnFe 2 O4 respectively. 4. Conclusion Mechanochemical synthesis of ZnFe 2 O4 from a powder mixture of ZnO and a-Fe 2 O 3 was conducted by the planetary ball mill at room temperature in this work. The experimental results obtained from the work are summarized as follows. Fig. 5. TG-DTA curves of the mixtures ground for various durations of time.
1. The grinding of the mixture enables us to obtain the amorphous phase of the starting materials. Especially,
16
W. Kim, F. Saito r Powder Technology 114 (2001) 12–16
most of the ZnO reacts with a-Fe 2 O 3 to form the insoluble amorphous compounds within 2 h. 2. Crystalline ZnFe 2 O4 can be synthesized from the amorphous compounds of the starting materials and the formation is enhanced when the amorphization is fully achieved by prolonged grinding. 3. ZnFe 2 O4 crystallized by the grinding for 3 h or more consists of nanocrystalline particles with high specific surface area.
Acknowledgements One of the authors ŽW. Kim. wishes to thank the Ministry of Education, Science and Culture of Japan ŽMonbusho. for the financial support provided through the scholarship.
References w1x R.E. Ayala, D.W. Marsh, Characterization and long-range reactivity of zinc ferrite in high-temperature desulfurization processes, Ind. Eng. Chem. Res. 30 Ž1991. 55–60.
w2x L.A. Bissett, L.A. Strickland, Analysis of a fixed-bed gasifier IGCC configuration, Ind. Eng. Chem. Res. 30 Ž1991. 170–176. w3x M.C. Wood, S.K. Gangwal, D.P. Harrison, K. Jothimurugesan, Kinetics of the reaction of a zinc ferrite sorbent in high-temperature coal gas desulfurization, Ind. Eng. Chem. Res. 30 Ž1991. 100–107. w4x J.F. Duncan, D.J. Stewart, Kinetics and mechanism of formation of zinc ferrite, Trans. Faraday Soc. 63 Ž1967. 1031–1041. w5x G.A. Kolta, S.Z. El-Tawil, A.A. Ibrahim, N.S. Felix, Kinetics and mechanism of zinc ferrite formation, Thermochim. Acta 36 Ž1980. 359–366. w6x F.J.C.M. Toolenaar, Formation of zinc ferrite, J. Mater. Sci. 24 Ž1989. 1089–1094. w7x I. Halikia, E. Milona, Kinetic study of the solid state reaction between alpha-Fe 2 O 3 and ZnO for zinc ferrite formation, Can. Metal. Q. 33 Ž1994. 99–109. w8x K. Tkacova, V. Sepelak, N. Stevulova, V.V. Boldyrev, Structure–reactivity study of mechanically activated zinc ferrite, J. Solid. State. Chem. 123 Ž1996. 100–108. w9x V. Sepelak, K. Tkacova, V.V. Boldyrev, U. Steinike, Crystal structure refinement of the mechanically activated spinel–ferrite, Mater. Sci. Forum 228–231 Ž1996. 783–788. w10x V. Sepelak, K. Jancke, J. Richter-Mendau, U. Steinike, D. Uecker, A. Gogachev, Mechanically induced reactivity of ZnFe 2 O4 , Kona 12 Ž1994. 87–93. w11x N. Lefelshtel, Y. Nadiv, I.J. Lin, Y. Zimmels, Production of zinc ferrite in a mechano-chemical reaction by grinding in a ball mill, Powder Technol. 20 Ž1978. 211–217. w12x C. Jovalekic, M. Zdujic, A. Radakovic, M. Mitric, Mechanochemical synthesis of NiFe 2 O4 ferrite, Mater. Lett. 24 Ž1995. 365–368.
Mechanochemical synthesis of zinc ferrite from zinc oxide and a-Fe 2 O 3 Wantae Kim, Fumio Saito ) Institute for AdÕanced Materials Processing, Tohoku UniÕersity 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan Accepted 23 March 2000
Abstract Mechanochemical synthesis of zinc ferrite ŽZnFe 2 O4 . from a powder mixture of zinc oxide ŽZnO. and hematite Ž a-Fe 2 O 3 . by room temperature grinding using a planetary ball mill was investigated. The grinding enables us to obtain the amorphous mixture of the starting materials. Most of ZnO reacts with a-Fe 2 O 3 to convert into insoluble amorphous zinc and iron compounds within 2h-grinding. Prolonged grinding enhances the crystallization of ZnFe 2 O4 from the amorphous compounds. ZnFe 2 O4 crystallized by the grinding for 3 h or more consists of nanocrystalline particles with high specific surface area. q 2001 Elsevier Science S.A. All rights reserved. Keywords: Mechanochemical synthesis; Zinc ferrite; Zinc oxide; Hematite; Planetary ball mill
1. Introduction ZnFe 2 O4 is known as an advanced mixed-metal sorbent for high-temperature desulfurization of coal gas. This removes hydrogen sulfide ŽH 2 S. from hot coal-derived gas streams and can operate at temperatures and pressures matching typical gasifier outlet conditions. Due to the versatility of high performance desulfurization and easy regeneration, kinetics and capacities of the sorbent ZnFe 2 O4 in fixed-bed combustion process have been investigated w1–3x. Catalytic-grade ZnFe 2 O4 is normally manufactured by a heating process of an equal molar mixture of zinc and iron oxides at high temperature w4–7x. ZnFe 2 O4 is characterized by a normal spinel structure in which Zn2q cations with no magnetic moment occupy tetrahedral sites, while all the Fe 3q cations occupy octahedral sites within the lattice of cubic close packing. For the reason that no exchange interaction of each lattice site of ZnFe 2 O4 configuration takes place Žparamagnetic., therefore, structural change and surface chemistry of ZnFe 2 O4 for further utilization are also studied by high-energy ball-milling techniques w8,9 x. Recently, surface activation and mechanochemical solid-state reaction have become promising tools for the synthesis and modification of spinel ferrites. Sepelak et al. w10x used the mechanically modified ZnFe 2 O4 for H 2 S removal. Lefelshtel et al. w11x ) Corresponding author. Tel.: q81-22-217-5135; fax: q81-22-2175135. E-mail address: [email protected] ŽF. Saito..
and Jovalekic et al. w12x reported the formation of ferrite materials by long-time ball milling. However, little information has been offered on the mechanism of ZnFe 2 O4 formation from ZnO and a-Fe 2 O 3 powder mixture during its grinding. The main purpose of this work is to provide experimental information on the mechanochemical synthesis of ZnFe 2 O4 from a powder mixture of ZnO and a-Fe 2 O 3 by its dry grinding conducted using a planetary ball mill at room temperature. This work also involves the crystallizing mechanism of nanocrystalline ZnFe 2 O4 from the mixture during the grinding. Thermal behavior and microstructure of synthesized ZnFe 2 O4 were investigated by monitoring TG–DTA and high-resolution transmission electron microscopy ŽHR-TEM.. It was found that ZnFe 2 O4 was synthesized from the amorphous phase of the starting materials and the formation was promoted by the state of amorphization.
2. Experimental 2.1. Experimental apparatus and procedure Zinc oxide ŽZnO. and hematite Ž a-Fe 2 O 3 . supplied from Wako, Japan were used in this work. These two starting materials were weighed in molar ratio of 1.0, which coincides with the stoichiometric valence of zinc ferrite ŽZnFe 2 O4 ., and mixed thoroughly in acetone using an agate mortar with a pestle prior to grinding. The
0032-5910r01r$ - see front matter q 2001 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 0 . 0 0 2 5 6 - 4
W. Kim, F. Saito r Powder Technology 114 (2001) 12–16
13
mixture prepared prior to grinding is called unground mixture in this work. A planetary ball mill ŽFritsch Pulverisette-7. with a pair of stainless steel pots of 50 cm3 P inner volume containing seven stainless steel balls of 15 mm diameter was used for the grinding of the mixtures. The mixture Ž4.0 g. was put in the mill pot and ground at 790 rpm in rotational speed of the mill. The duration of grinding was varied from 0.5 to 4 h. The grinding was suspended for 10 min after every 10 min grinding to avoid excess temperature increase inside the mill pots during grinding. In order to check the reliability of XRD results, the amount of unreacted ZnO of the ground mixtures was quantified by leaching method as follows: The ground mixture Ž0.2 g. was suspended in 50 ml of acid solution Ž0.1 mol HCl. with stirring for 10 min at room temperature. The solid phase in the suspension was separated from the solution by filtration and washed completely with deionized water. From our preliminary leaching test, the same amount of ZnO was completely extracted within 10 min in the leaching condition employed. Finally, the concentration of zinc in the leaching solution was measured by an inductively coupled plasma ŽICP. analysis. 2.2. Characterization
Fig. 1. XRD patterns of the mixtures ground for various durations of time.
X-ray diffraction ŽXRD. analysis ŽRAD-B System, Rigaku. of the ground mixtures was carried out to check the phases of the mixtures. Morphology of the ground mixtures was observed by a scanning electron microscope ŽSEM, SEM-4100, Jeol.. Specific surface areas Ž S w . of the unground and ground mixtures were measured by a surface area analyzer ŽASAP-2010, Micromeritics.. The samples were also subjected to thermogravimetric and differential thermal analysis ŽTG–DTA, TAS-200, Rigaku. simultaneously in air at the heating rate of 108Crmin. In addition, atomic scale observation on the grain boundary and lattice fringe of ZnFe 2 O4 formed in the ground mixture was performed by a high-resolution transmission electron microscope ŽJEM-3010, Jeol..
3. Results and discussion Fig. 1 shows XRD patterns of the mixtures ground for various durations of time. The characteristic peaks of ZnO disappear in the pattern of the mixture ground for 1 h, while the peaks of a-Fe 2 O 3 are still detected in the mixture ground for 2 h. Accordingly, very weak new peaks are detected in the pattern of the mixture ground for 2 h, indicating that the follow ing reaction occurs mechanochemically. ZnO q Fe 2 O 3
™ ZnFe O 2
4
Ž 1.
Standard Gibbs free energy change DG 2988 calculated in Eq. Ž1. is y3.02 kJ moly1 , implying that the grinding assists the spontaneous reaction forward thermodynami-
cally to form ZnFe 2 O4 from the starting mixture when the interface of the solid surface in the starting materials is closely contacted and sheared by the grinding. It is noted that the peaks of a-Fe 2 O 3 almost disappear and the peaks of ZnFe 2 O4 are developed markedly in the mixture ground for 3 h. From the XRD profiles, it can be deduced that the crystallization of ZnFe 2 O4 is initiated in the amorphous mixture of the starting materials. Consequently, the solidphase diffusion is accelerated by the grinding when the amount of amorphous zinc and iron compounds in the ground mixture increases as the grinding progresses. However, no noticeable change is observed in the patterns of the mixtures ground for 4 h or more except for the slight increment of the peak intensity of the ZnFe 2 O4 formed. Fig. 2 shows S w of the mixtures as a function of grinding time. The S w value increases to 7.4 m2rg in the early stage of grinding within 0.5 h. Prolonged grinding up to 2 h reduces the value to 4.2 m2rg. This behavior can be explained by aggregation of ground fine particles, the aggregation that is attributed to the mechanical activation caused by dry prolonged grinding. On the contrary, the increase trend from 7.7 to 12.3 m2rg can be seen in the mixtures ground for 3 and 4 h, respectively. This may be attributed to the following reasons. The aggregates of the ground particles with relatively weak bonding are reground Ždissociation. into fine particles by prolonged mechanical stresses of friction and compression. Simultaneously, the transformation reaction of amorphous phase into fine crystalline ZnFe 2 O4 occurs on the surface of the ground particles with solid-phase diffusion under the strong grind-
14
W. Kim, F. Saito r Powder Technology 114 (2001) 12–16
Fig. 2. S w of the mixtures as a function of grinding time.
ing energy and the reaction penetrates into the particles interface, resulting in the formation of individual crystals as the grinding progresses. Fig. 3 shows SEM photographs of the mixtures ground for various duration of time. Morphology of the unground mixture ŽA. is presented as a reference. Comparatively coarser and irregular particles of a-Fe 2 O 3 and fine particles of ZnO are observed in the photo. In the mixture ŽB. ground for 2 h, original shapes of the starting materials are changed into the aggregates of fine particles. When the grinding is prolonged 3 h or more ŽC and D., though few aggregates still remained in the mixture ground for 3 h, new fine particles approximately 0.2–0.5 mm in size are observed as a whole in the photos. Although no further
Fig. 4. Free zinc content in the mixtures as a function of grinding time.
distinct information can be obtained from the photographs, the formation of fine particles, coupled with the two results of XRD and S w measurements as shown in Figs. 1 and 2, may be attributed to the crystallization of ZnFe 2 O4 . Fig. 4 shows the free zinc content in the mixtures as a function of grinding time. In the mixture ground for 0.5 h, most of the ZnO Ž83.2% in zinc base. does not react with a-Fe 2 O 3 , whereas only a small portion of ZnO converts to insoluble compounds of amorphous zinc and iron as well as their crystalline ones. This implies that most of the starting materials still retain their own crystal structures in the early stage of grinding within 0.5 h, though the particles undergo not only size reduction but also lattice disordering by mechanical stresses. In the 2h-ground mixture, the free zinc content decreases with an increase in grinding
Fig. 3. SEM photographs of the mixtures ground for various durations of time. ŽA. Unground ŽB. 2 h ŽC. 3 h and ŽD. 4 h grinding.
W. Kim, F. Saito r Powder Technology 114 (2001) 12–16
time, and reaches about 8.1% at 2 h of grinding. This indicates that the solid-state reaction proceeds remarkably as the grinding progresses up to 2 h. However, further grinding does not contribute to the complete reaction of the starting materials; approximately 4.2% of unreacted zinc still remained even in the mixture ground for 4 h. Fig. 5 shows TG–DTA curves of the mixtures ground for various durations of time. In the DTA curves, the remarkable exothermic peak appeared at 7608C in the unground mixture corresponding to the crystallization of ZnFe 2 O4 by heating the starting materials w5x. In addition, a broad exothermic reaction appeared in the temperature range from around 508C to 11008C corresponding to the formation of ZnFe 2 O4 . The former exothermic peak tends to shift towards the lower temperature side with increment of its peak intensity and the latter broad exothermic reactions shorten in the temperature range of around 50–9008C as the grinding progresses. This implies that the grinding operation is favorable for the formation of ZnFe 2 O4 . It is noticeable that the former exothermic peak disappears and only a broad exothermic peak at around 4008C is detected in the mixtures ground for 3 h or more. This broad exothermic reaction seems to be mainly attributed to the recrystallization of ZnFe 2 O4 from the unstable and amorphous phases of ZnFe 2 O4 already formed in the ground mixtures w8x. On the other hand, no distinct variation can be detected in TG curves of the mixtures over the wide range of the grinding time and temperature employed. Fig. 6 shows HR-TEM photographs of the 3h-ground mixture ŽA. and the mixture heated at 4008C for 1 h ŽB.. The microstructural image ŽA. shows the single crystal of ZnFe 2 O4 approximately 25 nm in size. The particle is
15
Fig. 6. HR-TEM photograph of the 3h-ground mixture and the mixture heated at 4008C.
folded and surrounded by other crystals and amorphous phases. This indicates that the ZnFe 2 O4 is crystallized from the amorphous phases of the starting materials. The grain boundary is not clearly observed; anyhow, the lattice distance of the fringe is approximately 0.487 nm, which corresponds to the Ž111. plane of ZnFe 2 O4 . In the ground mixture heated at 4008C ŽB., the grain boundary Žd. of each crystal is clearly observed and most straight fringes are measurable in each single crystal, in which the lattice distances are measured as approximately 0.487 Ža. and 0.298 Žb and c. corresponding to the Ž111. and Ž220. plains of ZnFe 2 O4 respectively. 4. Conclusion Mechanochemical synthesis of ZnFe 2 O4 from a powder mixture of ZnO and a-Fe 2 O 3 was conducted by the planetary ball mill at room temperature in this work. The experimental results obtained from the work are summarized as follows. Fig. 5. TG-DTA curves of the mixtures ground for various durations of time.
1. The grinding of the mixture enables us to obtain the amorphous phase of the starting materials. Especially,
16
W. Kim, F. Saito r Powder Technology 114 (2001) 12–16
most of the ZnO reacts with a-Fe 2 O 3 to form the insoluble amorphous compounds within 2 h. 2. Crystalline ZnFe 2 O4 can be synthesized from the amorphous compounds of the starting materials and the formation is enhanced when the amorphization is fully achieved by prolonged grinding. 3. ZnFe 2 O4 crystallized by the grinding for 3 h or more consists of nanocrystalline particles with high specific surface area.
Acknowledgements One of the authors ŽW. Kim. wishes to thank the Ministry of Education, Science and Culture of Japan ŽMonbusho. for the financial support provided through the scholarship.
References w1x R.E. Ayala, D.W. Marsh, Characterization and long-range reactivity of zinc ferrite in high-temperature desulfurization processes, Ind. Eng. Chem. Res. 30 Ž1991. 55–60.
w2x L.A. Bissett, L.A. Strickland, Analysis of a fixed-bed gasifier IGCC configuration, Ind. Eng. Chem. Res. 30 Ž1991. 170–176. w3x M.C. Wood, S.K. Gangwal, D.P. Harrison, K. Jothimurugesan, Kinetics of the reaction of a zinc ferrite sorbent in high-temperature coal gas desulfurization, Ind. Eng. Chem. Res. 30 Ž1991. 100–107. w4x J.F. Duncan, D.J. Stewart, Kinetics and mechanism of formation of zinc ferrite, Trans. Faraday Soc. 63 Ž1967. 1031–1041. w5x G.A. Kolta, S.Z. El-Tawil, A.A. Ibrahim, N.S. Felix, Kinetics and mechanism of zinc ferrite formation, Thermochim. Acta 36 Ž1980. 359–366. w6x F.J.C.M. Toolenaar, Formation of zinc ferrite, J. Mater. Sci. 24 Ž1989. 1089–1094. w7x I. Halikia, E. Milona, Kinetic study of the solid state reaction between alpha-Fe 2 O 3 and ZnO for zinc ferrite formation, Can. Metal. Q. 33 Ž1994. 99–109. w8x K. Tkacova, V. Sepelak, N. Stevulova, V.V. Boldyrev, Structure–reactivity study of mechanically activated zinc ferrite, J. Solid. State. Chem. 123 Ž1996. 100–108. w9x V. Sepelak, K. Tkacova, V.V. Boldyrev, U. Steinike, Crystal structure refinement of the mechanically activated spinel–ferrite, Mater. Sci. Forum 228–231 Ž1996. 783–788. w10x V. Sepelak, K. Jancke, J. Richter-Mendau, U. Steinike, D. Uecker, A. Gogachev, Mechanically induced reactivity of ZnFe 2 O4 , Kona 12 Ž1994. 87–93. w11x N. Lefelshtel, Y. Nadiv, I.J. Lin, Y. Zimmels, Production of zinc ferrite in a mechano-chemical reaction by grinding in a ball mill, Powder Technol. 20 Ž1978. 211–217. w12x C. Jovalekic, M. Zdujic, A. Radakovic, M. Mitric, Mechanochemical synthesis of NiFe 2 O4 ferrite, Mater. Lett. 24 Ž1995. 365–368.