Chemical Engineering Journal 153 (2009) 193–198 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej A macrokinetic study of the WO 3 /Zn reaction diluted with NaCl Hyung Il. Won, Hayk. H. Nersisyan, Chang Whan Won ∗ Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, Yuseong, Daejeon 305-764, South Korea article info Article history: Received 7 January 2009 Received in revised form 26 May 2009 Accepted 28 May 2009 Keywords: Combustion Thermal profiles Tungsten nanoparticles Wave structure abstract In this work, experiments with stoichiometric WO 3 + 3Zn mixture, diluted with NaCl, were conducted for nanostructured tungsten synthesis. The reaction samples, preheated until 720K, were self-ignited and reacted in the steady combustion regime. The temperature–time profiles in the combustion wave were collected over the NaCl interval from 1 to 6mol, and the values of the combustion parameters (T c , U c ) were evaluated. From these profiles the spatial distributions of heat generation rate (x) and degree of conversion (x) in the combustion wave were received at different k values. The calculated activation energy for the combustion process was E = 55 ± 2 kJ mol -1 . After the reduction experiments, pure tungsten nanopowder with particle size from 200 to 50 nm was obtained depending on NaCl concentration. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. 1. Introduction Investigation of the combustion mechanism of self-propagating high-temperature synthesis reactions (SHS) is of significant inter- est from both fundamental and practical viewpoints. The current status of computational methods and tools allows the simulation of the combustion process; however, in order to develop an ade- quate combustion model, it is necessary to have experimental data for each individual system, including the combustion mechanism and chemical reaction kinetics. To date, most of the knowledge on the combustion mechanism and chemical reaction kinetics occur- ring during the combustion of SHS systems is derived from studies of the time–temperature profiles and combustion wave structure [1–3]. Generally, the time–temperature profiles of combustion pro- cesses are measured by thermocouples, point and linear pyrom- eters, and 2D thermal video systems [4–6]. However, the most accurate method to obtain data on the combustion wave ther- mal structure is the thermocouple method. A variety of systems (borides, silicides, hydrides, etc.) were investigated by thermocou- ple method and the obtained results contributed to an in-depth understanding of the high-temperature kinetic and reaction mech- anisms [7,8]. Two mathematical approaches have been proposed for the analysis of temperature profiles, one by Zenin [7] and the other by S.D. Dunmead et al. [9,10]. Both of them allow the extraction of the kinetic parameters of rapid combustion reactions. Refractory oxide (WO 3 , Ta 2 O 5 , TiO 2 , etc.)/magnesium mixtures are of interest for the synthesis of metal nanopowders and manufac- ∗ Corresponding author. Tel.: +82 42 821 6587; fax: +82 42 822 9401. E-mail address: cwwon@cnu.ac.kr (C.W. Won). turing compact materials with dramatically improved mechanical properties for aerospace, military, chemical and metallurgy appli- cations. Nanosized tungsten is the most promising among the refractory metals, and the commercial demand of this powder has been growing rapidly. The combustion synthesis of tungsten pow- der from WO 3 + 3Zn system was first reported in Ref. [11]. It was shown that the reduction mechanism of WO 3 by Zn is multistage process including the formation of low oxides of tungsten (WO 2.9 , WO 2.72 and WO 2 ) in the intermediate stages of combustion. The authors suggest that the combustion process in WO 3 + 3Zn system was mainly driven by evaporation of Zn. Therefore, a decrease of combustion temperature from 1340 to 1050K was recorded, when Zn evaporation was suppressed by an inert gas pressure. However, tungsten powder obtained under the optimized reaction conditions was mainly agglomerated and micrometer sized. We have recently reported the synthesis of nanostructured tungsten powder from WO 3 + 3Zn mixture diluted with NaCl [12]. The method reported allowed to synthesize W powder with particle size less than 100 nm and an average oxygen concentration lower than 1.0wt%. In this article, we present an analysis of the kinetics and the com- bustion mechanism of WO 3 + 3Zn + kNaCl system as determined from temperature profiles and XRD analysis data. 2. Experimental WO 3 powder (99.9% pure, particle size 10–50 m; Grand Chem- ical and Material Co., Ltd., Korea), Zn powder (99% pure, particle size 5–10 m; Daejung Chemicals and Metals Co., Ltd., Korea), and NaCl powder (99.5% pure, particle size <50–150 m; Samchun Pure Chemicals Co., Ltd., Korea) were used as starting materials. WO 3 powder was first grinded into a fine powder (≤200 nm) and then thoroughly mixed with Zn and NaCl powders by ball-milling for at 1385-8947/$ – see front matter Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2009.05.042