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st 21 International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia Preparation of silica-iron oxide composite particles by microwave plasma Dong-Wook Kim1, Satoshi Kodama1, Hidetoshi Sekiguchi1 and Dong-Wha Park2 Department of Chemical Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan 2 Department of Chemical Engineering and Regional Innovation Center for Environmental Technology of Thermal Plasma (RIC-ETTP), INHA University, Yonghyun-dong, Nam-gu, Incheon, 402-751, Republic of Korea 1 Abstract: The silica-iron oxide multi-composite particles were prepared by the decomposition of vapor precursors under atmospheric Ar-O2 microwave plasma. The morphologies of the prepared particles were varied by controlling the experimental conditions. Thermodynamic approach proposed that partial pressure, Fe/Si ratio and injecting position of precursors have a crucial influence on the morphologies of the final particles. Keywords: Microwave plasma, Particle, Silica, Iron oxide 1. Introduction For some decades, numerous researches reported particle preparation processes and those mechanism under thermal plasma and microwave plasma [1, 2]. Especially recent researches frequently deal with multi-component particle such as doped ceramic, coated material and spinel since their unique properties have attracted large interest in application field [3-6]. Compared to single-component system, multi-component system would lead to the formation of more various product by altering the experimental conditions (e.g. ratio of components, injection method). However the particle generation phenomena were not yet explained due to difficulties in predicting behaviors of various chemical species in high temperature region. To prepare the desirable multi-composite particles from plasma processing, further understanding of the multi-component system in the plasma is required. In our previous research, silica coated iron oxide particles were prepared by decomposition of iron pentacarbonyl [Fe(CO)5] and tetraethyl orthosilicate [C8H20O4Si] under DC thermal plasma [7]. In the present research, the same experiment was performed using the microwave plasma and the results were compared with that using thermal plasma. The microwave plasma has relatively lower temperature (generally Tmax<4000K), in which most of phase-transition temperature of iron species and silicon species is distributed, than the thermal plasma. Therefore application of the microwave plasma was expected to cause the different particle generation mechanism from that in the thermal plasma. To explain the experimental results obtained from both plasmas and to investigate the mechanism, the thermodynamic approach was introduced in terms of nucleation temperature based on gas-liquid equilibrium and phase diagram of Fe-Si-O system. 2. Experiments The silica-iron oxide composite were prepared by the microwave Ar-O2 plasma. The microwave plasma was generated with 2.45 GHz microwave generator and the power was 900W at all experiments. The experimental apparatus was described in Fig.1. The vapor precursors, iron pentacarobonyl [Fe(CO)5] and tetraethyl orthosilicate [C8H20O4Si] were injected into quartz tube with argon carrier gas. The molar ratio of Fe to Si was controlled by changing the temperature of the water bath and the flow rate of the carrier gas. The total argon flow rate of the swirling gas and the carrier gas was maintained as 9.3 L/min. In addition, the precursors were injected from two injecting positions; axial injection and swirl injection. The prepared particles were collected at the filter. The detailed experimental conditions were summarized in Table 1. Fig.1. Experimental apparatus Table 1. Experimental conditions Name Fe/Si ratio Injecting position Ex1 3 swirl Ex2 3 axial Ex3 1 axial Ex4 0.333 axial Power Ar flow rate O2 flow rate 900 W 9.3 L/min (discharge gas + carrier gas) 0.7 L/min st 21 International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia 3. Results and discussion Fig.2 showed TEM images of the products obtained from the thermal plasma (a) and from the microwave plasma (b)-(e). A particle shown in Fig. 2(a) has external layer and core. EDS analysis implied that the external layer was almost composed of Si and O, meaning the presence of the silica whereas the core part was almost composed of iron oxide. Although the presence of few Fe in external layer and Si in core left the possibility of the presence of fayalite (Fe2SiO4), brownish color of the products assured that the majority is γ–Fe2O3. Although γ–Fe2O3 is metastable phase, some researches have reported that γ–Fe2O3 can be obtained by microwave plasma [8, 9]. The similar morphologies were observed in Fig.2(c) although the thickness of the silica layer is thinner than that in Fig.2(a). The particles in Fig. 2(b), (d) and (e) did not have any external layer but the mixture of the individual particles of the silica and the iron oxide are observed. In many researches, the supersaturation was introduced to explain the particle generation in the plasma [8]. In contrast, the thermodynamic approach was introduced to explain the difference of the particle morphologies in this study. Here the condensation temperature of the vapor Feor Si-related species was considered as the nucleation temperature. First, typical equilibrium diagram of 75Fe(CO)5-25C8H20O4Si-700O2-9200Ar was calculated using the commercial program Factsage 5.3.1 and was sketched in Fig.3. FeO(liq) is formed as a prior condensed iron species and silica is nucleated as SiO2(liq). It is noted that only the species related to the nucleation was sketched in Fig.3. In this calculation, the partial pressure is substituted for mole fraction. The condensation temperatures were calculated to evaluate the effect of the Fe/Si ratio and the mole fraction of the precursors. Fig.4(a), (b) and (c) showed the condensation temperatures as a function of the mole fractions of the precursors when Fe/Si=3, 1 and 0.333. In the experiments the injected flow rate of the precursors was maintained as 0.012 L/min and that of the carrier gas ranged from 0.5 to 0.8 L/min, that is the mole fraction of the precursors in the carrier gas was 0.016~0.025. When Fe/Si=3 (Fig.4(a)), FeO has slightly higher condensation temperature than SiO2 at low mole fraction of the precursors. However the difference increases as the mole fraction increases. As the rapidly quenched system from the high temperature region was considered in the present research, the higher nucleation temperature means a prior nucleation. Once FeO is condensed, the partial pressure of FeO around the condensed particle decreases. Therefore Si-rich phase becomes easily condensed on the surface of the as-condensed particle. As a result, the final particle has Si-rich external layer and Fe-rich core as shown in Fig.2(a) and (c). However Ex1(Fig.2(b)) did not show any external layer in spite of Fe/Si=3. It’s because precursors couldn’t pass the high temperature region of the plasma. The tangentially injected precursors was considered to pass close to the inner surface of the quartz tube. The gas temperature of the plasma jet was shown in Fig.5. Since the inner radius of the quartz tube was 4.5mm, the temperature of that position might be further lower than 2700K. If the temperature of the injecting zone is comparable to the nucleation temperature of SiO2, SiO2 and FeO may nucleate simultaneously, forming the individual condensed particles. Fig.2. TEM images of the silica-iron oxide composite particles obtained from (a) thermal plasma, (b) Ex1, (c) Ex2, (d) Ex3 and (e) Ex4 Fig.3. Thermodynamic equilibrium diagram of 75Fe(CO)5-25C8H20O4Si-700O2-9200Ar system st 21 International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia Fig.5. Gas temperature of the microwave plasma jet calculated from OH spectrum. (z : axial distance from waveguide [mm]) When Fe/Si=1 (Fig.4(b)), the differences of the condensation temperature are not so large at the all of mole fraction regions. Therefore the similar result with Ex1 was obtained by the simultaneous condensation of FeO and SiO2. When Fe/Si=0.333 (Fig.4(c)), SiO2 has higher nucleation temperature than FeO against all mole fractions. Therefore the formation of the iron oxide coated silica particles was expected by the early condensation of SiO2. However it was difficult to confirm the iron oxide external layer by our facility problem. Thus further analysis is required to confirm the presence of the Fe-rich external layer. In summary the microwave plasma in Fe-Si-O system prepared the various silica-iron oxide composite by controlling Fe/Si ratio or the injection position of the precursors. Although the further research is required, it will be worthwhile to predict the final product in the multi-component through the thermodynamic consideration. Fig. 4. Condensation temperature of FeO and SiO2 with Fe/Si ratio of (a) 3, (b) 1, (c) 0.333 and difference of T(FeO) between T(SiO2): dash line : the mole fraction of the precursors at the experimental point 4. Conclusions The silica-iron oxide composite particles were prepared by the atmospheric Ar-O2 microwave plasma. The molar ratio of Fe to Si in the precursors and the injecting position of the precursors were controlled. When only Fe/Si=3 with axial injection, the iron oxide particles having thin silica layer were observed. The silica coating was considered as a result of prior condensation of the iron oxide followed by condensation of silica-rich phase on the as-condensed iron oxide core. The thermodynamic calculations indicated that the difference of the condensation st 21 International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia temperature between iron oxide and silica is obtained in presence of high Fe/Si ratio and high mole fraction of the precursors when Fe/Si=3. Nevertheless the silica coating wasn't performed with the swirl injection of the precursors in spite of the proper conditions. It is considered that the temperature of the region where the swirl-injected precursors passed is lower than the nucleation temperature of the silica, so that the silica and the iron oxide simultaneously nucleated without the prior nucleation of the iron oxide. When Fe/Si=1, the mixture of the individual silica particles and iron oxide particles were prepared by the simultaneous condensation of FeO and SiO2. When Fe/Si=0.333, the preparation of iron oxide coated silica particles were expected but further discussion is required to confirm it. 5. References [1] D. Vollath, J. Nanopart. Res., 10, 39 (2008). [2] G. Vissokov, Iv. Grancharov, Tsv. Tsvetanov, Plasma Sci. Technol., 5, 2039 (2003) [3] D.W. Kim, D.W. Park, Surf. Coat. Technol., 205 S201 (2010). [4] D.W. Jung, D.W. Park, Appl. Surf. Sci., 255, 5409 (2009). [5] Ilona Mohai, Lorand Gal, Janos Szepvolgyi, Jeno Gubicza, Zsuzsa Farkas, J. Eur. Ceram. Soc., 27, 941 (2007). [6] S. Rizk, M.B. Assouar, C. Gatel, M. Belmahi, J. Lambert, J. Bougdira, Diamond & Related Materials, 17, 1660 (2008). [7] D.W. Kim, T.H. Kim, S. Choi, K.S. Kim, D.W. Park, Adv. Powder Technol., 23, 701 (2012). [8] D. Vollath, D. V. Szabo,R.D. Taylor, J.O. Willis, K.E. Sickafus, Nanostructured Materials, 6, 941 (1995). [9] D. Vollath, D. V. Szabo,J. Mater. Res., 12, 2175 (1997). [10] S. L. Girshick, C.P. Chiu, Plasma Chem. Plasma Process.,9, 355, 1989.