Novel Method for Synthesis of Engineered Particulates with Thin Coatings
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ABSTRACT Particulate coatings have wide ranging applications in several new technologies such as flatpanel displays, sintering of advanced ceramics, rechargeable batteries, etc. In this paper, we show the feasibility of the pulsed laser ablation technique to make very thin, uniformly distributed and discrete coatings in particulate systems so that the properties of the core particles can be suitably modified. Presently, laser ablation techniques have been primarily applied to deposit thin films on flat substrate materials. To deposit discontinuous particulate coatings, the laser induced plume from the target comes in contact with an agitated bed of core particles (size 1-800gzm). The pressure and nature of the background gas (inert or active) controls the cluster size of the particles in the laser plume. Experiments were conducted for laser deposition of silver particles on alumina core particles by excimer laser (wavelength = 248 nm and pulse duration = 25 nanosecond) irradiation of silver targets. The surface coverage and the coating of the film wasf found to depend on the laser parameters (energy density and number of laser pulses) as well as the residence time of the core particles in the plume regime. The films were characterized by wavelength and energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy techniques. INTRODUCTION The pulsed laser deposition (PLD) technique has emerged as one of the most popular methods to deposit complex oxide thin films and deposition of compositionally varying multilayer systems [1-4]. Presently, most of the thin film deposition has been conducted on flat substrates which are normally kept parallel to the substrate at a distance of 3-10 cm from the target. The flux generated by the laser interaction with the target and the ablated material, is composed of active atomic and molecular species. The background gas has been found to play a significant role in the film formation process. Ifthe background gas chamber pressure consists of reactive gases such as oxygen, ammonia, nitrous oxide etc., a higher concentration of molecular species is observed in the plasma. At high background pressures (>200 mtorr), small clusters, (>3 nm) size in the plasma have been reported [5-7]. The formation of these clusters is due to increased collisions of the ablated species in the gas phase. Also, ifthe background gas is reactive (e.g. oxygen etc,) the ablated species may react to form new compound species [7]. Until now, there have been no reports in the literature on deposition of coatings on particulate materials by the pulsed laser deposition technique. Composite particulates which have been surface modified with a thin continuous or discontinuous layer have a large number of applications in existing and emerging technologies [8-9]. An example of a composite particulate material is shown in Fig. 1. This figure shows that the surface of the core particle is modified by the attachment of the secondary particles. The degree of surface coverage will determine whether the coating c
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