Mechanical activation of aluminothermic reduction of NiO by high energy ball milling

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Abstract

The effect of mechanical activation on NiO–Al reaction has been investigated by carrying out high energy ball milling using toluene as the process controlling agent. NiO reduction reaction progresses gradually and a maximum of 70% reduction has been observed in 20 h of milling. Amorphous alumina forms as the product of NiO reduction and it transforms to stable α-Al2O3 at 1000 °C via the formation of metastable transition γ-Al2O3. The activation energy of NiO reduction in unmilled powder is 277 kJ/mol whereas in 20 h milled powder the activation energy has been reduced to 150 kJ/mol.

Introduction

High energy ball milling has been known to activate the solid–solid and even solid–liquid chemical reactions during milling. Since the mechanical energy supplied during milling is utilized for the chemical reactions it has been called mechanochemical synthesis/reactive milling. In reactive milling, the intensive mechanical treatment of crystalline solids lead to new surfaces, plastic deformation and accumulation of structural defects, which may induce reactions with a kinetic and thermodynamic behavior very different from that of thermally initiated reactions. Two different type of reaction kinetics have been reported in the literature depending on the milling conditions [1], [2]:

  • I.

    Self-propagating combustion reaction, which needs a critical milling time for the combustion reaction to be ignited if the reaction enthalpy is very high.

  • II.

    Gradual reactions, which have been otherwise called progressive type reaction since the reaction extend through a small volume during each collision.

Several mechanochemical reactions have been studied for past few years for making in situ nanocomposites and compounds [3], [4], [5]. Most of the reactions studied have been displacement reactions, where the metal oxides are reduced by more reactive metal to a pure metal. In situ nanocomposites obtained by reactive milling are expected to have improved strength and toughness since the reinforcement and matrix are nanocrystalline and the reinforcement will have thermodynamic compatibility with the matrix [1]. The solid state reaction of NiO–Al has been investigated initially by Matteazzi and Le Caer [3], using planetary ball mill to synthesis Al2O3/Ni nanocomposite. The resulted products after the NiO reduction were Ni and α-Al2O3 in 1.5 h of dry milling in argon atmosphere. NiO reduction reaction by Al also has been utilized in the process of development of NiAl based nanocomposite powders with in situ Al2O3 reinforcement [6], [7], [8]. In the present study, the NiO–Al reduction with the stoichiometric composition has been investigated in planetary ball mill using toluene as process controlling agent in which the phase formation and reaction mechanism are expected to be different from dry milling. X-ray diffraction (XRD) has been used for analyzing the phase evolution during milling and heating. The non-isothermal kinetic study has been carried out for unmilled and milled powder using differential scanning calorimetry (DSC). Transmission electron microscopy has also been used to analyze the as milled powder.

Section snippets

Experimental details

Reactive milling was carried out in a planetary ball mill (Fritsch pulverisette-5) for NiO–Al powder mixture with stoichiometric composition which consists of 80.6 wt.% NiO and 19.4 wt.% Al. The stoichiometry of the starting powder corresponded to the equation:3NiO+2Al3Ni+Al2O3

Milling was carried out at the ball to powder ratio of 10:1 using tungsten carbide vials and tungsten carbide balls of 10 mm diameter. Toluene was used as process controlling agent to avoid oxidation and excessive cold

Results

The DSC result of unmilled NiO–Al powder mixture with stiochiometric composition (Fig. 1) shows one endothermic peak at 660 °C and exothermic peak at 1020 °C corresponding to Al melting and NiO reduction respectively. This indicates that the reduction of NiO occurs at high temperature following the Al melting (confirmed by the XRD analysis that is not shown). Fig. 2 shows the XRD pattern of NiO–Al powder mixture as a function of milling time. After 5 h milling the decrease in the reactants (NiO,

Discussion

The particle refinement and the induced defect densities accelerate the diffusion process as a result the reduction of NiO has been carried away during milling. In addition, the repeated welding and fracturing allows fresh surfaces come into contact repeatedly allowing the reaction to proceed without the interference of the product layer [1]. Moreover reduction of NiO by Al is gradual (progressive) extending to a very small volume during each collision instead of a self-propagating combustion

Conclusions

The effect of mechanical activation on the reduction of NiO by Al has been investigated by subjecting the NiO and Al powder with stoichiometric composition to high energy ball milling. The reduction of NiO occurs gradually in toluene medium. The amorphous Al2O3, which is formed as a result of the NiO reduction transforms to stable α-Al2O3 at 1000 °C via the formation of metastable transition γ-Al2O3. The calculated activation energy for NiO reduction in unmilled powder is 277 kJ/mol and it is

References (18)

  • C. Suryanarayana

    Prog. Mater. Sci.

    (2001)
  • P.M. Botta et al.

    Scripta Mater.

    (2003)
  • K.I. Moon et al.

    J. Alloys Compd.

    (1998)
  • S.Z. Anvari et al.

    J. Alloy. Compd.

    (2009)
  • V. Udhayabanu et al.

    Intermetallics

    (2010)
  • T. Venugopal et al.

    Mater. Sci. Eng.

    (2005)
  • V. Bhosle et al.

    Mater. Sci. Eng.

    (2003)
  • L. Lu et al.

    J. Mater. Process. Technol.

    (1997)
  • H.X. Zhu et al.

    Mater. Sci. Eng.

    (2000)
There are more references available in the full text version of this article.

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