Influence of reduction mechanism on the morphology of cobalt nanoparticles in a silica-gel matrix
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Influence of reduction mechanism on the morphology of cobalt nanoparticles in a silica-gel matrix A. Basumallick Department of Metallurgical Engineering, Bengal Engineering College (D.U.), Howrah-711 103, India
G.C. Das and S. Mukherjee Department of Metallurgical Engineering, Jadavpur University, Calcutta-700 032, India (Received 16 June 1999; accepted 7 October 1999)
Cobalt-chloride- and dextrose-containing silica gels were reduced in situ under nitrogen atmosphere in the temperature range of 600 to 950 °C. Analysis of kinetic data on the in situ reduction shows that, in the temperature range of 600 to 750 °C, the contracting geometry type and, in the temperature range of 800 to 950 °C, the nucleation and growth type of mechanisms remain operative. The shape and size of the reduced cobalt nanoparticles in the silica matrix was studied by examining the transmission electron micrographs of the reduced Co/SiO2 samples. The morphology of the reduced metallic particles was found to be influenced by the change in reduction mechanism.
I. INTRODUCTION
During the last few years preparation of glass–metal nanocomposites via the sol-gel route has gained considerable importance.1–5 This new class of materials exhibits remarkable electrical2,3,6 and magnetic7 properties, which has raised the possibility of their commercial exploitation as substrates for semiconduction technology.8 The shape and size of the nanoparticles present in the host matrix influence the physical properties to a great extent. However, it has been reported that the shape and size of the metallic islands in the host matrix vary within wide limits.9 Such wide deviation in shape and size imparts a great deal of inconsistency in the results of physical characterization of these novel materials. Therefore, we feel strongly that, unless the physical properties are tailored to be uniform through precise control of shape and size, the possibility of commercial exploitation of these materials appears to be eclipsed. Control over the morphology of the particles from the fundamental viewpoint can be achieved by controlling the kinetic parameters and thereby by the mechanism(s) of conversion of gels to nanocomposites. Considering the above views in the backdrop, the present paper reports the effect of kinetic parameters, e.g., temperature and time, and conversion mechanism(s) on the morphology of CoCl 2 -containing SiO 2 gels reduced to Co–SiO 2 nanocomposites.
and CoCl 2 in ethyl alcohol. The volume ratio of C2H5OH:TEOS was maintained at 4:1 for all the gel samples. The detailed preparation technique has been described elsewhere.9 The resulting gels were then subjected to isothermal reduction treatment in the temperature range 600 to 950 °C at an interval of 50 °C under N2 atmosphere. The N2 flow rate was maintained at 5 cc/s throughout the experiment. The reduction treatments were carried out in an electrical heating furnace. The desired temperature was maintained by a PID controller with an accuracy of ±1 °C. The
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