Materials Chemistry and Physics 115(2009)664-669 Contents lists available at Science Direct Materials Chemistry and physics LSEVIER journalhomepagewww.elsevier.com/locate/matchemphys Impurity phases of silicon dioxide in commercial Sic whiskers produced by vls method Vadym G Lutsenko V.I. Vernadski Institute of general and Inorganic Chemistry, National Academy of Sciences of ukraine, 32/34 Palladin Avenue, Kyiv 03680, Ukraine ARTICLE FO ABSTRACT Silicon dioxide forms two impurity phases in SiC whiskers, namely a-cristobalite and X-ray-amorphous Sio. The particles of a-cristobalite form as a result of hydrolysis of silicon compounds with chlorine d in revised form 15 August 2008 ccepted 1 February 2009 by water vapor, and these are not bound with SiC surface During synthesis of Sic whiskers, the layer of X-ray-amorphous silicon dioxide of up to 2.5 nm thickness forms on the surface of whiskers through oxidation of crystal surface by water vapor. Upon drying Sic whiskers at temperature above 120C and lIso long-time storing of crystals in contact with air, the surface of whiskers oxidizes and Sio2 film forms of up to 3 nm thickness. The mechanism is proposed for low-temperature oxidation of Sic whiskers in the presence of water vapor. It is shown that aggregately stable suspensions of whiskers form in alkaline Thin films medium in the presence of Sioz film on the surface of crystals. O 2009 Elsevier B V. All rights reserved. 1. Introduction material properties[5-11]. An oxidation of the whisker surface and formation of silicon dioxide film can occur during the synthesis. Commercial Sic whiskers produced from the mixture of drying and heat treatment of crystals, and possibly as a result of 11-4 contain a number of impurity phases, in particular silicon temperature(150-250 C)oxidation of Sic whiskers by oxygen in dioxide, compounds of the system Fe-Si, iron chlorides, and free air(both dry and humid )and the mechanism of formation of Sioz carbon [1]. Practical application of Sic whiskers for reinforcement nanolayers was not discussed previously of both ceramic and metal matrices suggests purification of the The current paper contains the results of study impurity phases synthesized crystals from impurity phases, sorting whiskers, and of silicon dioxide(crystalline and X-ray-amorphous)in commercial modification their surface iC whiskers. This involves, in particular: (i)extraction of particles To optimize the synthesis process and the techniques for of SiO2 impurity phases from Sic whiskers, studying their phase purification Sic whiskers from impurity phases it is necessary and chemical composition, morphological features, state of the to have information on the morphology of impurity phases, the face, and formation mechanism; (ii)extraction of SiC whisk sers from nature of distribution them within the whisker volume, the synthesis products in non-aqueous solvents, studying their mor- phase composition and the formation mechanism of impurity phology and surface state: (ii) purification of the surface of Sic phases. The impurity phases of silicon dioxide are presented whiskers, studying the effect of drying and storage conditions on in commercial Sic whiskers produced in a gas stream by the the growth of oxide film on SiC surface and the mechanism for film vapor-liquid-solid"(VLS)method as the particles of a-cristobalite formation and X-ray-amorphous phase with the latter as a film probably localized on the surface of some SiC crystals [1]. At present, the mechanism for formation of impurity phases(crystalline and x- y-amorphous)of silicon dioxide, the morphology and location The object forinvestigation Plant gas stream of the mixture of chlorosilanes with hyd face(oxidized or purified in HF solution; presence of impurities) carbons(propane, butane )and hydrogen on the moving Fe drops by VLS methodI and whiskers with particles of ceramic matrix in aqueous es The source for Fe was fine FeCh powde also affects substantially the processes of mixing fine Sic parti The as-produced Sic p to 40 mass% of impurit tions and consolidation of ceramic composite materials, and X-ray-amorphous phase)[1, 2]- For in ions, the samples of sic whi selected with high content(25-6.7 mass%)of SiO2. The concentrating and and mixture CHBr3-CCl4, and gravitational separation of suspensions in aqueous E-mail address: vlutsenko33@rambler. ru solutions on the concentrating table. 0254-0584 e front matter o 2009 Elsevier B v. All rights reserved. doi:101016Materials Chemistry and Physics 115 (2009) 664–669 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Impurity phases of silicon dioxide in commercial SiC whiskers produced by VLS method Vadym G. Lutsenko V. I. Vernadski Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, 32/34 Palladin Avenue, Kyiv 03680, Ukraine article info Article history: Received 21 June 2007 Received in revised form 15 August 2008 Accepted 1 February 2009 Keywords: Carbides Oxides Whisker Thin films Oxidation abstract Silicon dioxide forms two impurity phases in SiC whiskers, namely -cristobalite and X-ray-amorphous SiO2. The particles of -cristobalite form as a result of hydrolysis of silicon compounds with chlorine by water vapor, and these are not bound with SiC surface. During synthesis of SiC whiskers, the layer of X-ray-amorphous silicon dioxide of up to 2.5 nm thickness forms on the surface of whiskers through oxidation of crystal surface by water vapor. Upon drying SiC whiskers at temperature above 120 ◦C and also long-time storing of crystals in contact with air, the surface of whiskers oxidizes and SiO2 film forms of up to 3 nm thickness. The mechanism is proposed for low-temperature oxidation of SiC whiskers in the presence of water vapor. It is shown that aggregately stable suspensions of whiskers form in alkaline medium in the presence of SiO2 film on the surface of crystals. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Commercial SiC whiskers produced from the mixture of chlorosilanes with hydrogen and hydrocarbons over a catalyst (Fe) [1–4] contain a number of impurity phases, in particular silicon dioxide, compounds of the system Fe–Si, iron chlorides, and free carbon [1]. Practical application of SiC whiskers for reinforcement of both ceramic and metal matrices suggests purification of the synthesized crystals from impurity phases, sorting whiskers, and modification their surface. To optimize the synthesis process and the techniques for purification SiC whiskers from impurity phases it is necessary to have information on the morphology of impurity phases, the nature of distribution them within the whisker volume, the phase composition and the formation mechanism of impurity phases. The impurity phases of silicon dioxide are presented in commercial SiC whiskers produced in a gas stream by the “vapor–liquid–solid” (VLS) method as the particles of -cristobalite and X-ray-amorphous phase with the latter as a film probably localized on the surface of some SiC crystals [1]. At present, the mechanism for formation of impurity phases (crystalline and X￾ray-amorphous) of silicon dioxide, the morphology and location of those phases are not cleared up. Moreover, the state of SiC sur￾face (oxidized or purified in HF solution; presence of impurities) also affects substantially the processes of mixing fine SiC particles and whiskers with particles of ceramic matrix in aqueous solu￾tions and consolidation of ceramic composite materials, and also E-mail address: vlutsenko33@rambler.ru. material properties [5–11]. An oxidation of the whisker surface and formation of silicon dioxide film can occur during the synthesis, drying and heat treatment of crystals, and possibly as a result of contact with ambient upon storing whiskers. The process of low￾temperature (150–250 ◦C) oxidation of SiC whiskers by oxygen in air (both dry and humid) and the mechanism of formation of SiO2 nanolayers was not discussed previously. The current paper contains the results of study impurity phases of silicon dioxide (crystalline and X-ray-amorphous) in commercial SiC whiskers. This involves, in particular: (i) extraction of particles of SiO2 impurity phases from SiC whiskers, studying their phase and chemical composition, morphological features, state of the sur￾face, and formation mechanism; (ii) extraction of SiC whiskers from synthesis products in non-aqueous solvents, studying their mor￾phology and surface state; (iii) purification of the surface of SiC whiskers, studying the effect of drying and storage conditions on the growth of oxide film on SiC surface and the mechanism for film formation. 2. Experimental The object for investigation was commercial-grade whiskers of SiC (Redkino Pilot Plant, Russia) produced in a gas stream of the mixture of chlorosilanes with hydro￾carbons (propane, butane) and hydrogen on the moving Fe drops by VLS method [1]. The source for Fe was fine FeCl2 powder. The as-produced SiC whiskers contain up to 40 mass% of impurity phases (carbon phases, iron silicides and chlorides, silicon dioxide as -cristobalite and X-ray-amorphous phase) [1,2]. For investigations, the samples of SiC whiskers were selected with high content (2.5–6.7 mass%) of SiO2. The concentrating and extracting of SiO2 particles was performed by two methods, namely: separation in bromoform and mixture CHBr3–CCl4, and gravitational separation of suspensions in aqueous solutions on the concentrating table. 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.02.002