Reaction ball milling of systems involving ionic bonds

Materials Science and Engineering A304–306 (2001) 434–437

Reaction ball milling of systems involving ionic bonds V. Varghese a , A. Sharma a,b , K. Chattopadhyay a,∗ a

b

Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India Department of Metallurgy, Regional Engineering College, Thiruchirappalli 620 015, India

Abstract The paper reports an attempt to understand the mechanism of mechanochemical synthesis by mechanical milling. Towards this goal, a systematic investigation of the mechanochemistry of the ionic compounds has been carried out. We have studied the nature of replacement reactions in solid CuSO4 ·5H2 O with Fe, Mg and Sn. The study focuses on structural characterization at different stages of milling to gain insight into the process of synthesis leading to the formation of nanocrystalline copper. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ionic bond; Mechanochemical; Nanocrystalline; Ball milling

1. Introduction In recent studies high energy ball milling processes were receiving attention pertaining to the feasibility of solid state reactions at room temperature. Various kinds of chemical reactions like, the double decomposition, oxidation–reduction, displacement reactions have been induced by reaction ball milling [1]. The aim of the present work is to explore the role of different elements in a model displacement reaction involving CuSO4 . The result of this systematic study on mechanochemical reaction is expected to throw light on the mechanism of such transformation. The results of the mechanochemical reactions between Fe and CuSO4 ·5H2 O, Mg and CuSO4 ·5H2 O and Sn and CuSO4 ·5H2 O will be presented. The systems have been selected in such a way that there are sufficient electrochemical driving forces for the displacement reactions as shown in Table 1. Cu in CuSO4 ·5H2 O is in +2 oxidation state. Fe, Mg and Sn also show +2 oxidation state. However, these are also representative metals from different blocks of the periodic table like, d (transition metals), s and p blocks, respectively, and have different electronic structures. Thus, they exhibit different properties and behavior under similar conditions. The calculation based on the thermochemical data supports double decomposition reaction in CuSO4 ·5H2 O and Fe and CuSO4 ·5H2 O and Mg systems at room temperature [2]. However, the existence of SnSO4 ·5H2 O is not reported in the literature. The crystal structure of CuSO4 ·5H2 O ∗ Corresponding author. Present address: Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India.

¯ and FeSO4 ·5H2 O and MgSO4 ·5H2 O are is triclinic (P1) isostructural to CuSO4 ·5H2 O [3,4]. Thus, the feasibility of the double decomposition reaction is also supported structurally.

2. Experimental Electrolytically pure Fe mixed with CuSO4 ·5H2 O (99%) in 1.15:1 molar proportion was milled (SPEX 8000) in a hardened steel vial with three stainless steel balls (13.75 mm diameter, net weight = 39.5 g) under toluene in 8:1 ball to powder ratio and very small amounts of the reaction mixture were withdrawn at various times for further analysis. Fe was added in slight excess to the reaction mixture to ensure the complete reduction of copper sulfate to copper. An inert medium of toluene was used to provide uniform suspension of powder particles inside the vial and also, to avoid the formation of a layer of Cu on colliding balls. The reaction was repeated under similar conditions of milling with equimolar proportion of CuSO4 ·5H2 O and Mg and CuSO4 ·5H2 O and Sn. The samples were characterized by powder X-ray diffraction technique in Guinier geometry using HUBER DIFFRACTIS 583. For iron containing samples Fe K␣1 and for rest of the samples, Cu K␣1 radiations were used. The time dependence of the grain size of Cu was calculated using Scherrer’s equation from Cu (1 1 1) after the correction for instrumental broadening. The thermal stability of reaction mixture was analyzed using (Polymer Laboratories) TGA–DTA in 99.99% argon gas. The stability of toluene during milling was checked by proton NMR.

0921-5093/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 4 4 3 - X

V. Varghese et al. / Materials Science and Engineering A304–306 (2001) 434–437 Table 1 Electrochemical driving force for the displacement of Cu by different metals from CuSO4 ·5H2 O under standard conditions [2] Electrochemical reaction

1ER0 (V)

Cu+2 (l) + Fe (s) → Fe+2 (l) + Cu (s) Cu+2 (l) + Mg (s) → Mg+2 (l) + Cu (s) Cu+2 (l) + Sn (s) → Sn+2 (l) + Cu (s)

−0.7492 −2.7152 −0.4766

435

and FeSO4 ·4H2 O remained stable through out milling. The increase of the amount of water in toluene during initial milling stage was confirmed by proton NMR of toluene. The toluene was otherwise unaffected by the milling. The overall chemical reaction can be written as follows: Fe (s) + CuSO4 · 5H2 O (s) → FeSO4 · 5H2 O (s) + Cu (s) → FeSO4 · 4H2 O (s) + Cu (s)

3. Results and discussion In CuSO4 ·5H2 O and Fe system, the mixture of CuSO4 ·5H2 O and elemental Fe powder undergoes chemical reaction during high energy ball milling as clearly indicated in Fig. 1. As the milling proceeds, the intensity of CuSO4 ·5H2 O and Fe peaks gradually diminishes while ¯ and Cu peaks increases. Obvithat of FeSO4 ·5H2 O (P1) ously, this result suggests the slow double decomposition reaction, favoring the electrochemical displacement during the milling process. The intensity of CuSO4 ·5H2 O becomes negligible at 11 h of milling (Fig. 1). The completion of double decomposition reaction between 9 and 11 h of milling is supported by the quantitative estimation of Fe in the reaction mixture with time using XRD. The intensity of FeSO4 ·4H2 O increases slowly compared to FeSO4 ·5H2 O peaks, until 9 h of milling and above that the amount of FeSO4 ·4H2 O increases faster. FeSO4 ·5H2 O is completely converted into FeSO4 ·4H2 O by 16 h of milling (Fig. 1). The reaction mixture was further milled to a total of 30 h

Fig. 1. The XRD patterns showing the formation of FeSO4 ·5H2 O and Cu with milling time from the starting mixture of Fe and CuSO4 ·5H2 O. On later hours FeSO4 ·5H2 O disappears and FeSO4 ·4H2 O appears. (䊊) CuSO4 ·5H2 O, ([ ]) Fe, (䉫) Cu, (#) FeSO4 ·5H2 O and (∗) FeSO4 ·4H2 O.

(1) The double decomposition reaction took place to form an isostructural sulfate as expected and Cu was displaced by Fe in a steady state manner. The similarity in the chemical behavior of the transition metals is explicitly evident in this reaction. Its interesting to note that the higher hydrate of a crystal can be dehydrated to a lower hydrate in the presence of toluene during milling. The average grain size of Cu particles during milling is 27 nm. The grain size of Cu remained unaltered with milling time. In CuSO4 ·5H2 O and Mg system, the mixture of CuSO4 ·5H2 O and elemental Mg powder undergoes chemical reaction during high energy ball milling as shown in Fig. 2. As the milling proceeds, the intensity of the CuSO4 ·5H2 O and Mg peaks gradually diminishes while the peaks of the intermediate structure and Cu2 O become more intense. Interestingly, metallic Cu is not formed at any time point of the reaction although; it was expected from

Fig. 2. The XRD pattern showing the formation of intermediate structure and Cu2 O during the initial hours of milling from the starting mixture of Mg and CuSO4 ·5H2 O. Intermediate decomposes to MgSO4 ·4H2 O on further milling. (䊊) CuSO4 ·5H2 O, (䉫) Mg, (#) Cu2 O, (I) intermediate and (∗) MgSO4 ·4H2 O. 24 h hour sample was milled in WC bowl in planetary mill.

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V. Varghese et al. / Materials Science and Engineering A304–306 (2001) 434–437

Fig. 3. Molecular structure of CuSO4 ·5H2 O. The octahedral co-ordination around Cu in CuSO4 ·5H2 O comprises four H2 O molecules and two SO2− 4 ions. The fifth water molecule is not directly linked to the metal ion [5].

the analysis of thermochemical data and structural similarity between MgSO4 ·5H2 O and CuSO4 ·5H2 O in support of double decomposition reaction. Instead, one observes an intermediate phase. The intensity of the intermediate is negligible after 24 h of milling while the presence of MgSO4 ·4H2 O is evident. During this time, the liberation of gas was noticed. The overall reaction for this system can be represented as follows: Mg (s) + CuSO4 · 5H2 O (s)

as follows:

→ Intermediate (s) + Cu2 O (s) + H2 (g) → MgSO4 · 4H2 O (s) + Cu2 O (s)

Fig. 4. The XRD pattern showing the evolution of phases as a function of milling time from an equimolar mixture of Sn and CuSO4 ·5H2 O. Five minute pattern shows SnSO4 peaks along with Cu after drying. (∗) SnSO4 , (䉫) Cu, (#) ␤-brass.

Sn (s) + CuSO4 · 5H2 O (s) (2)

The entirely different reaction path followed by this system can be attributed to the chemical reactivity of Mg towards water. The fifth water molecule is not directly bonded to the metallic ion as shown in the molecular structure of CuSO4 ·5H2 O, Fig. 3. The remaining four water molecules are arranged in a square planar fashion around the metal ion with direct bonding [5]. Mg might be reacting with the fifth water molecule to liberate hydrogen and to form an intermediate structure as shown by the XRD pattern. The affinity of Mg towards water can be rationalized on the basis of bond energies of the metal to water ligands in both CuSO4 ·5H2 O and MgSO4 ·5H2 O [4]. The bond energy is high in the case of CuSO4 ·5H2 O [4] and hence the local chemical environment decides the reaction path. The average grain size of Cu2 O formed in steady state manner during milling is 19 nm. In CuSO4 ·5H2 O and Sn system, the mixture of CuSO4 ·5H2 O and elemental Sn powder undergoes chemical reaction during high energy ball milling as indicated in Fig. 4. As the milling proceeded, the intensity of the CuSO4 ·5H2 O and Sn peaks disappeared after 5 min, while the peaks CuSO4 , ␤-brass and Cu appeared. The presence of water in toluene was confirmed by proton NMR of toluene. Initially the reaction mass settled down at the bottom of the vial in a thick colloidal form. The XRD pattern of the colloid did not show any peaks. The sample was dried and the XRD pattern showed the presence of SnSO4 , ␤-brass and Cu, Fig. 4. So the reaction can be represented

→ SnSO4 (s) + ␤-brass (s) + Cu (s) + H2 O (toluene) → SnSO4 (s) + Cu (s) + H2 O (toluene)

(3)

There is no water of hydration associated with the product sulphate, SnSO4 . Water coming out from CuSO4 ·5H2 O in the initial stages of the reaction might dissolve CuSO4 and hence the reaction could take place faster. The average grain size of Cu obtained for this system is 54 nm after 5 min of milling, when the reaction is practically over. The grain size is large due to the high conversion rate during milling. However, the grain size reduces on further milling. The results of the grain size analysis of the metal or metal oxide formed during milling are given in Table 2. The average grain size does not exceed nanoscale due to the continuos fracturing taking place in the system during milling. However, it is noteworthy that the average grain size of Cu could vary with the kinetics involved in the displacement of Cu. For the given process conditions, the kinetics of the displacement reaction can vary with the variation in the chemical nature of the reactants as well as products. The displacement of Cu was faster in CuSO4 ·5H2 O and Sn system due Table 2 Grain size of Cu/Cu2 O formed from different systems in solid state under similar conditions of reaction ball milling Reaction system

Grain size (nm)

Fe (s) + CuSO4 · 5H2 O Mg (s) + CuSO4 · 5H2 O Sn (s) + CuSO4 · 5H2 O

27 (Cu) 19 (oxide) 54 (Cu)

V. Varghese et al. / Materials Science and Engineering A304–306 (2001) 434–437

to lowering in water of hydration in the product, SnSO4 . There is no difference in the amount of water of hydration in the displacement step of CuSO4 ·5H2 O and Fe system. Hence, the rate of displacement reaction is slower than that of CuSO4 ·5H2 O and Sn system. The average grain size of Cu reduces from 54 to 27 nm as the rate decreases.

4. The grain size of Cu decreases when the displacement of Cu decreases. For the cess conditions, the rate of displacement be altered by appropriate selection of system.

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rate of the given proof Cu can a reaction

Acknowledgements 4. Conclusions 1. Reaction ball milling is a viable route for the slow and steady preparation of nanostructured copper, through the double decomposition reaction between CuSO4 ·5H2 O and Fe in solid state at room temperature in toluene medium. 2. Even though similar reaction is possible with Mg, the reaction route and the products are different in the CuSO4 ·5H2 O and Mg system, while, even with the smallest electrochemical driving force, the reaction is fastest in the CuSO4 ·5H2 O and Sn system. 3. A higher hydrate can loose water of hydration due to milling under toluene to form a lower hydrate.

One of the authors, AS, thanks Indian Academy of Science for the award of a Summer Fellowship at the Indian Institute of Science to carry out this work.

References [1] B.S. Murthy, S. Ranganathan, Int. Mater. Rev. 43 (1998) 1–40. [2] R. Weast (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Ohio, 1975, pp. D-61–D-71, D-120–D-122. [3] J.L. Jambor, R.J. Traill, Can. Mineral. 7 (1963) 751–763. [4] W. H. Baur, Acta Cryst. 15 (1962) 815–826. [5] A.F. Wells, Structural Inorganic Chemistry, 3rd Edition, Oxford University Press, London, 1962, p. 587.