《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_SiC WHISKER-1 Impurity phases of silicon dioxide in commercial SiC whiskers produced by VLS method

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:101016

Materials 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

V.G. Lutsenko/ Materials Chemistry and Physics 115(2009)664-669 The bromoform(2.9 gcm-3) and mixtures of CHBra-CCl4(2. 15-2.65gcm-3 silicon dioxide phases(2.16, 2.33, and 2.65g cm-3 for a-tridymite, a-cristobalite, d a-quartz, respectively)[12 SiC (3. 21 gcm-3)[13]. Fesi (4.93 gcm-3)[14] .and l01 he concentrates were then dried at 150C(boiling b) mperature for CHBrg is 149.C) ating table of particle iC whiskers in aqueous solution(pH 9-10)was fed as a thin layer transverse to nclined riffled surface of the plat ibrating. Due to gravity. the coarse particles( Sioz. Sic and Fesi) deposited onto the (a) ate surface, while fine particles(sic whiskers )were removed by he suspension that flowed off different sections of as of silicon dioxide with 11 oz particle contents up to 60-80 mass% have been prepared, which were then ied at 110C. The Fesi particles were extracted from concentrates by magnetic separatIo phase analysis(DRON-2. 2 Theta(de -radiation). optical microscopy, scanning electron microscopy(SEM, Superprober 733, Jeol), thermogravimetry(Q-1500D, Hungary), and chemical analysis [15]-The Fig. 1. Difractograms of Sic whiskers with different content of impurity phase a- ties(fe, Al, cristobalite:(a)as-synthesized Sic whiskers with 6.7 mass% Sio2: (b)concentrate nethod (SiOz particles were decomposed by concentrated hF acid and by smelting №1(62mass%SiO2) les was determined by the method of heavy liquids [ 16] through immersing particles in a CHBr3-CCl4 solution with density varied with the step of 0.01 from 2.22 to 2.38 gcm-3 (a DT to magnetic separation(suspensions in CCl4)and examined for the presence of g ES. Jamp-10, Jeol) and high- The Sic whiskers were cleaned from surface film of silicon dioxide in an aque- us solution of hydrofluoric ersing crystals upon mixing for 20 mi 58 the crystals were filtered off, washed in distilled water, and dried. The drying erature was varied from 20 to 150C. The dispersing of Sic whiskers in aqueous I pH 1-12 was carried ou stirring(500rpm) of the whereas to enhance water alkalinity the nh4 oh was used. The sedimentation kinet ics of suspensions of whiskers in aqueous solutions was studied in glass cylinders nder conditions which prevented contact of suspensions with COz in air. TG TG Time(min Time(min 3. Results Fig. 2. Thermograms of a-cristobalite concentrates: (a)concentrate e2(0.374 g. 83 mass% Sioz): (b)concentrate No5(0.604g, 80.1 mass%Sio The concentrates of silicon dioxide contained 60-97 mass%Sio2 SiC whiskers and particles, and less than 1-1.5 mass% carbon phases nd FeSi. The X-ray phase analysis showed that the concentrates surface of macropores in a-cristobalite particles(fig. 2). The con- contained only one crystalline phase of SiO2, namely a-cristobalite tent of water in a-cristobalite particles that were extracted from (Fig. 1). The phases of a-quartz and a tridymite and also particle us sus on the particle of X-ray-amorphous phase Sio, were not detected (table 1) of 8-15 mass%. The thermograms for Sioz concentrates that were The heating curves for concentrates of SiO2 particles exhibited extracted in a bromoform did not show endothermic effect(at about endothermic effect at about 260c due to polymorphic transfor- 100C), and there was no decrease in mass in the temperature range mation of a-cristobalite to B-cristobalite [ 17(Fig. 2). When storing 40-240.C the dried Sio2 concentrates in air(relative humidity >50%)there is The a-cristobalite particles extracted from concentrates increase in the mass of only concentrates extracted from aqueous CHBr3-CCl4 solution exhibited substantial difference in size( from uspensions Heating of those concentrates at temperature in the few micrometers to few hundreds micrometers). The main portion range of 40-240oC(the temperature of maximum reaction devel- of a-cristobalite particles(45-70 mass%)had an effective diame- opment is 102C)resulted in a decrease in their mass due to release ter in the range 63-200 um. The density of a-cristobalite particles of water adsorbed in the micropores and mesopores, and also at the varied in relation to their microporosity from 2. 28 to 2.36g cm-3 Properties of particle concentrates and individual particles of Sioz Conditions for extraction Phase composition, particle size ontent of SiO, and other CHBr3, light fraction ncentrate№2 HBr3-CCl4(2.65 gcm-3). light fraction Cristobalite SiC. Concentrate e 3 CHBr3-CCl4(2.4gcm-3) light fraction a-Cristobalite Particles from concentrate N 1 Grinding, CHBra-CCl4(2.50gcm-3) heavy fraction Sic Particles from concentrate Ne 1 Grinding, CHBra-CCl4(250gcm-3), light fraction a-Cristobalite, SiC, C Particles from trate e 3 CHBr3-CCl4(2. 35 gcm-3), light fraction -Cristobalite; particle size: -5-700 97. Impurities. Fe-<0.1 0-35 Concentrate№4 Concentrating table a-Cristobalite, SiC, Fesi 58-77 Concentrate№5 a-Cristobalite sic 60-8

V.G. Lutsenko / Materials Chemistry and Physics 115 (2009) 664–669 665 The bromoform (2.9 g cm−3) and mixtures of CHBr3–CCl4 (2.15–2.65 g cm−3) were used to extract concentrates of SiO2 on account of difference in densities of silicon dioxide phases (2.16, 2.33, and 2.65 g cm−3 for -tridymite, -cristobalite, and -quartz, respectively) [12], SiC (3.21 g cm−3) [13], FeSi (4.93 g cm−3) [14], and nanographite (50%) there is increase in the mass of only concentrates extracted from aqueous suspensions. Heating of those concentrates at temperature in the range of 40–240 ◦C (the temperature of maximum reaction devel￾opment is 102 ◦C) resulted in a decrease in their mass due to release of water adsorbed in the micropores and mesopores, and also at the Fig. 1. Difractograms of SiC whiskers with different content of impurity phase - cristobalite: (a) as-synthesized SiC whiskers with 6.7 mass% SiO2; (b) concentrate №1 (62 mass% SiO2). Fig. 2. Thermograms of -cristobalite concentrates: (a) concentrate№2 (0.374 g, 83 mass% SiO2); (b) concentrate №5 (0.604 g, 80.1 mass% SiO2). surface of macropores in -cristobalite particles (Fig. 2). The con￾tent of water in -cristobalite particles that were extracted from aqueous suspensions depends on the particle porosity, and it is of 8–15 mass%. The thermograms for SiO2 concentrates that were extracted in a bromoform did not show endothermic effect (at about 100 ◦C), and there was no decrease in mass in the temperature range 40–240 ◦C. The -cristobalite particles extracted from concentrates in CHBr3–CCl4 solution exhibited substantial difference in size (from few micrometers to few hundreds micrometers). The main portion of -cristobalite particles (45–70 mass%) had an effective diame￾ter in the range 63–200 m. The density of -cristobalite particles varied in relation to their microporosity from 2.28 to 2.36 g cm−3 Table 1 Properties of particle concentrates and individual particles of SiO2. Specimen designation Conditions for extraction Phase composition, particle size Content of SiO2 and other impurities, mass% Concentrate № 1 CHBr3, light fraction -Cristobalite, SiC, C 62–78 Concentrate № 2 CHBr3–CCl4 (2.65 g cm−3), light fraction -Cristobalite, SiC, C 75–83 Concentrate № 3 CHBr3–CCl4 (2.4 g cm−3), light fraction -Cristobalite, SiC, C 89–95 Particles from concentrate № 1 Grinding, CHBr3–CCl4 (2.50 g cm−3), heavy fraction SiC 200 m – 10–20 mass%; 97; Impurities: Fe – <0.1; Al – <0.05; Ca – <0.02 Concentrate № 4 Concentrating table -Cristobalite, SiC, FeSi 58–77 Concentrate № 5 Concentrating table, magnetic separation -Cristobalite, SiC 60–81

V.G. Lutsenko/ Materials Chemistry and Physics 115 (2009)664-669 2 60μm 1 50卩μI Fig 3. Morphology of concentrates of silicon dioxide and a-cristobalite particles: (a)concentrate Ne4 (62 mass% Sio2): (b)concentrate Ne5(78 mass% Sio2): (c)concentrate No2(82 mass% SiOz);(d)concentrate No3(93 mass% SiOz);(e)surface of porous implicitly-crystalline aggregate of a-cristobalite;(f) thin-plate polysynthetic twin ( -cristobalite( type 3): 1- porous aggregates of a-cristobalite(type 1): 2-fragmental a-cristobalite particles(type 2). (Table 1). The particle color depends on the fe impurity content, fied in HF solution and dried at 20-70C, and stored for 0.5-17 years and it changes from white to light-yellow. in leaky polyethylene bag contained the layer of silicon dioxide of The a-cristobalite particles are not adhered to the surface of 2-3 nm thick on the wisker surface(Table 2). whiskers and non-fibrous Sic particles (Table 1). The particles can The behavior of suspensions of Sic whiskers in aqueous solu- be distinguished with respect to their morphology as follows: (1) tions is fundamentally different for crystals with oxidized and porous implicitly-crystalline aggregates of rounded or elongated non-oxidized surfaces. The Sic whiskers with non-oxidized surface ticles having effective size 1 to 10 um; (2)fragmented particles: (purified in HF solution)form an aggregately unstable suspension (3)fine-plate polysynthetic twins( Fig 3). that flocculate. The crystals with oxidized surface(oxide layer of The Sic whiskers obtained by VLs method on moving drops 1.7-3 nm thick)form an aggregately stable suspension in alkaline contain on their surface thin layer of X-ray-amorphous Sioz phase medium(pH >8). Shown in Fig. 5 is dependence of the sedimen- (Fig. 4, Table 2). tation volume for deposits of Sic whiskers with different types of As evidenced by AES study, the silicon-to-carbon-to-oxygen surface on the ph of aqueous suspe ratio in initial Sic whiskers was 6: 88: 6. This is in accordance witl atomic concentrations of those elements in surface layers, which 4. Discussion were averaged over values obtained for 20 different SiC whiskers. The hydrocarbons and water adsorbed on whisker surface can also The particles of impurity phase a-cristobalite form as a result of contribute to the sum intensities of carbon and oxygen lines in AES hydrolysis of chlorosilanes and silicon tetrachloride in gas phase spectra. The fact that the position of spectral line of Si( 82eV)cor- responds to the Si-o bond clearly indicates the presence of oxide CH3 SiCl3()+ 2H20(g)= Sio2(g)+3HCI()+CH4(g) phase on the surface of as-synthesized whiskers In the spectro rams of whiskers cleaned in HF solution and dried at 70oC the SiCla(g)+ 2H20(g)= SiO2(g)+4HCI(g) position of Si line corresponds to 92 ev thus indicating presence of Sio2(g)+ Sio2(s)-amorph Si-C bonds (table 2). In the absence of oxide phase an essentially less amount of carbon is on the whisker surface, and some portion SiOz(s)-amorph. Sio2(s)-B-cryctobalite pounds. The annealing of Sic whiskers in air at 850C for 1h, that SiO2(s1-p-cryctobalite-SiO2(s)-a-cryctobalite were purified in HF solution, results in formation of oxide phase on The silicon tetrachloride forms due to reaction of residual cata- the whisker surface. Here, chemical composition of the surface is lyst(alloys of Fe-Si system)with HCl in the lower zone of furnace similar to that of as-synthesized whiskers. The Sic whiskers puri- where the temperature is considerably lower than in the reaction

666 V.G. Lutsenko / Materials Chemistry and Physics 115 (2009) 664–669 Fig. 3. Morphology of concentrates of silicon dioxide and -cristobalite particles: (a) concentrate №4 (62 mass% SiO2); (b) concentrate №5 (78 mass% SiO2); (c) concentrate №2 (82 mass% SiO2); (d) concentrate №3 (93 mass% SiO2); (e) surface of porous implicitly-crystalline aggregate of -cristobalite; (f) thin-plate polysynthetic twin of -cristobalite (type 3); 1 – porous aggregates of -cristobalite (type 1); 2 – fragmental -cristobalite particles (type 2). (Table 1). The particle color depends on the Fe impurity content, and it changes from white to light-yellow. The -cristobalite particles are not adhered to the surface of whiskers and non-fibrous SiC particles (Table 1). The particles can be distinguished with respect to their morphology as follows: (1) porous implicitly-crystalline aggregates of rounded or elongated particles having effective size 1 to 10 m; (2) fragmented particles; (3) fine-plate polysynthetic twins (Fig. 3). The SiC whiskers obtained by VLS method on moving drops contain on their surface thin layer of X-ray-amorphous SiO2 phase (Fig. 4, Table 2). As evidenced by AES study, the silicon-to-carbon-to-oxygen ratio in initial SiC whiskers was 6:88:6. This is in accordance with atomic concentrations of those elements in surface layers, which were averaged over values obtained for 20 different SiC whiskers. The hydrocarbons and water adsorbed on whisker surface can also contribute to the sum intensities of carbon and oxygen lines in AES spectra. The fact that the position of spectral line of Si (82 eV) cor￾responds to the Si–O bond clearly indicates the presence of oxide phase on the surface of as-synthesized whiskers. In the spectro￾grams of whiskers cleaned in HF solution and dried at 70 ◦C the position of Si line corresponds to 92 eV thus indicating presence of Si–C bonds (Table 2). In the absence of oxide phase, an essentially less amount of carbon is on the whisker surface, and some portion of carbon should be related to adsorbed carbon-containing com￾pounds. The annealing of SiC whiskers in air at 850 ◦C for 1 h, that were purified in HF solution, results in formation of oxide phase on the whisker surface. Here, chemical composition of the surface is similar to that of as-synthesized whiskers. The SiC whiskers puri- fied in HF solution and dried at 20–70 ◦C, and stored for 0.5–17 years in leaky polyethylene bag contained the layer of silicon dioxide of 2–3 nm thick on the wisker surface (Table 2). The behavior of suspensions of SiC whiskers in aqueous solu￾tions is fundamentally different for crystals with oxidized and non-oxidized surfaces. The SiC whiskers with non-oxidized surface (purified in HF solution) form an aggregately unstable suspension that flocculate. The crystals with oxidized surface (oxide layer of 1.7–3 nm thick) form an aggregately stable suspension in alkaline medium (pH >8). Shown in Fig. 5 is dependence of the sedimen￾tation volume for deposits of SiC whiskers with different types of surface on the pH of aqueous suspension. 4. Discussion The particles of impurity phase -cristobalite form as a result of hydrolysis of chlorosilanes and silicon tetrachloride in gas phase: CH3SiCl3(g) + 2H2O(g) = SiO2(g) + 3HCl(g) + CH4(g) (1) SiCl4(g) + 2H2O(g) = SiO2(g) + 4HCl(g) (2) SiO2(g) → SiO2(s)-amorph. (3) SiO2(s)-amorph. → SiO2(s)--cryctobalite (4) SiO2(s)--cryctobalite → SiO2(s)--cryctobalite (5) The silicon tetrachloride forms due to reaction of residual cata￾lyst (alloys of Fe–Si system) with HCl in the lower zone of furnace where the temperature is considerably lower than in the reaction

V.G. Lutsenko/ Materials Chemistry and Physics 115(2009)664-669 Table 2 Characteristics of Sic whiskers with different types of surface. Specimen designation Treatment co Data from auger-electron Properties of suspensions of thickness(nm) ectroscopy Extraction in CHBr3-CCla and magnetic 1.7-25 Si-82ev, Si: C: 0=6: 88: 6 Stable suspension(pH >8) SiCw(after purification in HF) HF solution, washing in water, drying at Si-92 Flocculation 20°C( silica gel) SiCw (after purification in HF) HF solution, washing in water, drying at 5i-92ev Flocculation SiCw(after purification in HF and storing) HF solution, washing in water, drying at 1.7-25 Si-82ev Stable suspension(pH >8) 70.C, storing for 7 months Sicw(after purification in HF and storing) HF solution, washing in water, drying at 2 ble suspension (pH >8) 70C, storing for 2 year SiCw (after purification in HF and storing) HF solution, washing in water, drying at Si-82ev Stable suspension(pH >8) 70C, storing for 17 years 2 nm zone(1200-1250°C O2 Fesi(s)+ 6HCi(g)= FeCh(g)+ Sicl(g)+ 3H2(g) Sio FeCl2(g)→FeCl2() The sources for water vapor are the following: H2O impurity hydrogen, nitrogen, and propane-butane mixture; oxygen impurity in nitrogen, which interacts with hydrogen in and forms water vapor; periodical entering of water vapor into the furnace volume upon unloading of the synthesized Sic whiskers. he concentration of silicon chlorides is non-uniform over the furnace volume, and it is apparently at a maximum in the lower section of furnace (the density of silicon chloride vapor is higher than that of the rest components of reaction system, which are sta ble in gas phase; the reaction(6)occurs predominantly in the lower furnace zone). The morphological diversity of a-cristobalite particles is due to, first, hydrolysis of silicon chloride at different temperatures, 「l nd, secondly, hydrolysis of different compounds of silicon chlo- ride(CH3 SiCl3, Sicl4), and, third, different ratios of concentrations of silicon chloride and water vapor. The impurities of Fe, Al, and other elements that occur in silicon dioxide due to partial hydroly sis of vapors of their chlorides during hydrolytic decomposition of silicon chlorides and sio nucleation cause the transformation of X-ray-amorphous silicon dioxide into B-cristobalite which in turn Fig 4. HRTEM image of sic whisk enlarged end face of whisker, and fromsynthesis products Inset 1 shows transforms into low-temperature cristobalite at temperature lower nlarged side surface of whisker. than260° The particles of a-cristobalite have a siloxane surface. When dispersing these particles in aqueous medium, a hydroxylation of siloxane surface of particles and formation of silanol groups occur: ≡Si-0-Si=+HOH→≡Si-OH+HOSi≡ Formation of silanol groups on the surface of particles of hydrox ylated a-cristobalite is the cause for water vapor adsorption in 目 micropores and mesopores in Sioz particles when storing them in An oxidation of sic whiskers in furnace occurs due to reaction of crystal surface with water vapor at temperature that is essentially SiC(s)+ 2H2O(g)= Sio(s)+ CHa(g) The reaction(9)is thermodynamically allowed at temperature lower than 1127C[18-20 upon is promoted by water vapor which is involved in the processes Fig. 5. Dependence of sedimentation volume or iC whiskers deposit on suspension of adsorption-desorption and hydroxylation of siloxane surface of ys: 1-SiC whiskers with oxidized surface(th of SiOz layer 2. 2 nm, drying extremely thin layer of SiOz thus facilitating diffusion of O2 and Co mperature 135C): 2-Sic whiskers after purif in HF solution and drying at and decreasing the activation energy for oxidation. The experimen- tal data confirmed presence of thin oxide X-ray-amorphous film of

V.G. Lutsenko / Materials Chemistry and Physics 115 (2009) 664–669 667 Table 2 Characteristics of SiC whiskers with different types of surface. Specimen designation Treatment conditions SiO2 layer thickness (nm) Data from auger-electron spectroscopy Properties of suspensions of SiCw in aqueous medium As-produced SiCw Extraction in CHBr3–CCl4 and magnetic separation 1.7–2.5 Si – 82 eV, Si:C:O = 6:88:6 Stable suspension (pH >8) SiCw (after purification in HF) HF solution, washing in water, drying at 20 ◦C (silica gel) No Si – 92 eV Flocculation SiCw (after purification in HF) HF solution, washing in water, drying at 70 ◦C No Si – 92 eV, Si:C:N:O = 33.8:62.9:1, 5:1.6 Flocculation SiCw (after purification in HF and storing) HF solution, washing in water, drying at 70 ◦C, storing for 7 months 1.7–2.5 Si – 82 eV Stable suspension (pH >8) SiCw (after purification in HF and storing) HF solution, washing in water, drying at 70 ◦C, storing for 2 years 2–3 Si – 82 eV Stable suspension (pH >8) SiCw (after purification in HF and storing) HF solution, washing in water, drying at 70 ◦C, storing for 17 years 2–3 Si – 82 eV Stable suspension (pH >8) Fig. 4. HRTEM image of SiC whisker extracted from synthesis products. Inset 1 shows enlarged end face of whisker, and inset 2 shows enlarged side surface of whisker. Fig. 5. Dependence of sedimentation volume of SiC whiskers deposit on suspension pH (0.87 mass% of SiC whiskers). Suspension amount 100 ml, sedimentation time 3 days; 1 – SiC whiskers with oxidized surface (thickness of SiO2 layer 2.2 nm, drying temperature 135 ◦C); 2 – SiC whiskers after purification in HF solution and drying at 70 ◦C. zone (1200–1250 ◦C): FeSi(s) + 6HCl(g) = FeCl2(g) + SiCl4(g) + 3H2(g) (6) FeCl2(g) → FeCl2(s) (7) The sources for water vapor are the following: H2O impurity in hydrogen, nitrogen, and propane–butane mixture; oxygen impurity in nitrogen, which interacts with hydrogen in the reaction zone and forms water vapor; periodical entering of water vapor into the furnace volume upon unloading of the synthesized SiC whiskers. The concentration of silicon chlorides is non-uniform over the furnace volume, and it is apparently at a maximum in the lower section of furnace (the density of silicon chloride vapor is higher than that of the rest components of reaction system, which are sta￾ble in gas phase; the reaction (6) occurs predominantly in the lower furnace zone). The morphological diversity of -cristobalite particles is due to, first, hydrolysis of silicon chloride at different temperatures, and, secondly, hydrolysis of different compounds of silicon chlo￾ride (CH3SiCl3, SiCl4), and, third, different ratios of concentrations of silicon chloride and water vapor. The impurities of Fe, Al, and other elements that occur in silicon dioxide due to partial hydroly￾sis of vapors of their chlorides during hydrolytic decomposition of silicon chlorides and SiO2 nucleation cause the transformation of X-ray-amorphous silicon dioxide into -cristobalite which in turn transforms into low-temperature cristobalite at temperature lower than 260 ◦C. The particles of -cristobalite have a siloxane surface. When dispersing these particles in aqueous medium, a hydroxylation of siloxane surface of particles and formation of silanol groups occur: Si O Si + HOH ↔ Si OH + HO Si (8) Formation of silanol groups on the surface of particles of hydrox￾ylated -cristobalite is the cause for water vapor adsorption in micropores and mesopores in SiO2 particles when storing them in air. An oxidation of SiC whiskers in furnace occurs due to reaction of crystal surface with water vapor at temperature that is essentially lower than in reaction zone, by the following reaction: SiC(s) + 2H2O(g) = SiO2(s) + CH4(g) (9) The reaction (9) is thermodynamically allowed at temperature lower than 1127 ◦C [18–20]. An oxidation of the surface of SiC whiskers by oxygen in air upon drying moist crystals at temperature higher than 100–120 ◦C is promoted by water vapor which is involved in the processes of adsorption–desorption and hydroxylation of siloxane surface of extremely thin layer of SiO2 thus facilitating diffusion of O2 and CO and decreasing the activation energy for oxidation. The experimen￾tal data confirmed presence of thin oxide X-ray-amorphous film of

V.G. Lutsenko/ Materials Chemistry and Physics 115(2009)664-669 silicon dioxide of 15-25 nm thick on the surface of sic whiskers decrease in the value of sedimentation volume of sic whiskers with fter drying at 120-150C At the same time, formation of thin Sio2 non-oxidized surface at pH>10 seems to be connected with consid- erable increase in the velocity of oxidation of Sic whisker surface not confirmed by experiment upon oxidation of volumetric SiC sin-(Fig 5). However, even at pH 10-11 the dispersing ability of Sic gle crystals(polytypes 3C, 4H, and 6H)at temperatures 120-150 C whiskers with non-oxidized surface is essentially lower than for in the presence of water vapor [21]. those with oxidized surface Upon long-term storing of SiC whiskers in contact with the air The side surfaces of twinned whiskers with superlattice struc- ambient at room temperature and varying humidity (50-90%), th ture elements are formed by stoichiometric (110)) and polar surface of whiskers oxidizes and the layer of X-ray-amorphous Sioz (100 and 1111) planes for which the ratio of surface areas of thickness up to 3 nm forms. depends on the crystal sub-microstructure and is determined by 4 The structure of hybrid superlattice that is realized in Sic conditions for crystallization of Sic whiskers. The absolute values niskers apparently influences the low-temperature oxidation of of surface charge for planes(110]. [100)C,100 Si,(111c, wisker surface in the presence of water vapor [22-27. The side and (111 Si will be different. All the three types of planes have surface of Sic whiskers with growth direction(111)and differ- semiconductor properties [36]. Also, the values of pHpzc for stoi- ent faceting can be described by three low-index planes (100, chiometric and terminated by C and Si atoms planes(non-oxidized (110, and 1111)[28 It follows from this that the crystal sur- and oxidized) must be different. Therefore, the conditions could face is composed by stoichiometric ((110) plane and planes be realized(ph of medium, surfactants, impurities, etc. )at which terminated by both silicon and carbon atoms. Upon drying Sic nanoareas(planes terminated by Si and C atoms) can co-exist on hiskers at 120-150 C, the water boils thus resulting in formation the side surface of whiskers, which have different sign of surface of saturated water vapor (p/ps=1), and the process of multilayer charge. Thus, to study electric-surface properties of Sic whiskers in adsorption-desorption of water occurs on crystal surface. Here, aqueous electrolytes one should apply the methods of potentiomet the conditions can probably be realized(with respect to pH of ric acid-base titration rather than electrophoretic measurements adsorbed water layer) at which the surface electric charge differs which allow for determination of only sum value of f-potential for in its sign on the planes terminated by silicon and carbon atoms. whiskers but do not give information on the sign mosaicity and the So, it becomes possible separation of the generated electric-hole value of density of surface charge for crystals. Depending on the pairs on the planes with different sign of surface charge and for- ratio for surface areas of planes terminated by silicon and carbon, mation of peroxide radicals. The latter interact with crystal surface and of stoichiometric planes one can observe substantial change in d oxidize it. he phiep value. Seemingly, an essential difference in pHiep values SiC + 6OH= Sio+CO 3H20 (10)(from 3 to 7)obtained in [ 10 11] for SiC whiskers of various com- mercial grades is due to different ratio of surface areas of plane 29]. it was shown by experiment that when single crystals terminated by Si and c atoms and stoichiometric planes on the side of SiC-6H polytype, 90-95% of the surface of which related to the surface of Sic whiskers. planes(0001)Cand(0001)Si, were in contact with aqueous solu- tion(pH9-10)in argon ambient(both when lighting and in dark) 5. Conclusion the ph of medium changed up to value 7.2, and the solution gained buffer properties. The silicon dioxide in Sic whiskers produced by VLS method on A change in medium pH is not due to dissociation and ionization moving drops exists in two phases, namely: a-cristobalite and X of surface silanol groups because the total surface area of single ray-amorphous phase. The impurity ax-cristobalite forms as a result crystal is 1-3 cm2 whereas solution volume is 30 mL. Seemingly, of hydrolysis of chlorosilanes and silicon tetrachloride by water the electrons and holes that are separated on the surface of single vapor. The a-cristobalite particles are not bound with Sic whiskers crystal planes having different sign of surface charge interact with and have siloxane surface and considerably different sizes. With electrolyte ions. One of the products of that reaction is peroxide respect to morphology, the a-cristobalite particles can be divided radicals. This process thus causes the change in solution pH into three types. During dispersing of a-cristobalite particles in Presence of thin silicon dioxide layer on the surface of Sic aqueous solutions, the particle surface is subject to hydroxylat hikers makes it possible to carry out effective dispersing of The X-ray-amorphous silicon dioxide does not form individua the crystals in aqueous solutions and obtain aggregately stable particles. It forms as a result of oxidation Sic whiskers with water suspensions in alkaline medium(Fig. 5). The minimum value of vapor in the lower furnace zone and exists as a film of up to 2.5nm sedimentation volume for Sic whiskers with the layer of amor- thick on the surface of whiskers us SiOz was observed for pH 10. It is known that During drying of Sic whiskers(after dispersing and purifica- values of pH for dispersing in aqueous solutions of Sic whiskers tion in aqueous solutions)at 120-150 C the whiskers oxidize and and powders having oxidized surfaces are in the range 9-11 [30], oxide layer forms of up to 3 nm thick. Upon long-time contact the minimum value of sedimentation volume of oxidized powders of non-oxidized Sic whiskers with air ambient the film of X-ray- was detected with pH 9.5[31]. and minimum viscosity of aqueous amorphous silicon dioxide of 2-3 nm thick forms on the surface of suspensions of Sic with the layer of Sio, was observed with pH whiskers 9.75-10.25 [32]and 10 [6]. This fact is due to decrease in absolute The process of Sic whiskers dispersing in water solutions and value of electro-kinetic potential of dispersed SiC with oxidized sur- aggregative stability of suspensions are determined by the ty face upon increasing pH of medium above 10 33] and start of the of crystal surface and the value of medium pH. The Sic whiskers with oxidized surface form stable suspensions in aqueous solutions After wash cleaning of dispersed Sic in HF solution and removal having pH >8 of the layer of surface silicon dioxide the hydrophobization of Sic surface occurs[7]. However, the surface of SiC is no stable neither in References air nor particularly in aqueous solutions for long period. An incre. in stability of suspensions of SiC purified in solution of HF, and also [1] V.M. Beletskii, V.G. Lutsenko, VL Milkov, D.D. Pokrovskii, A.N. Gribkov, EV decrease in the value of pHiep in long-term contact of SiC with aque- ous solutions was observed [34 which is connected with partial [21 V.G. Lutsenko, V M. Beletskii, A F. Gorovtsov, D D Pokrovskii, T.V. Verkhovlyuk oxidation ofSiC surface and formation of silanol groups [35] .Abrupt S L Shein, Powder Metall. Met. Ceram. 32(1993)170-173

668 V.G. Lutsenko / Materials Chemistry and Physics 115 (2009) 664–669 silicon dioxide of 1.5–2.5 nm thick on the surface of SiC whiskers after drying at 120–150 ◦C. At the same time, formation of thin SiO2 layer even on the planes that are terminated by carbon atoms was not confirmed by experiment upon oxidation of volumetric SiC sin￾gle crystals (polytypes 3C, 4H, and 6H) at temperatures 120–150 ◦C in the presence of water vapor [21]. Upon long-term storing of SiC whiskers in contact with the air ambient at room temperature and varying humidity (50–90%), the surface of whiskers oxidizes and the layer of X-ray-amorphous SiO2 of thickness up to 3 nm forms. The structure of hybrid superlattice that is realized in SiC whiskers apparently influences the low-temperature oxidation of wisker surface in the presence of water vapor [22–27]. The side surface of SiC whiskers with growth direction 111 and differ￾ent faceting can be described by three low-index planes ({100}, {110}, and {111}) [28]. It follows from this that the crystal sur￾face is composed by stoichiometric ({110}) plane and planes terminated by both silicon and carbon atoms. Upon drying SiC whiskers at 120–150 ◦C, the water boils thus resulting in formation of saturated water vapor (p/ps = 1), and the process of multilayer adsorption–desorption of water occurs on crystal surface. Here, the conditions can probably be realized (with respect to pH of adsorbed water layer) at which the surface electric charge differs in its sign on the planes terminated by silicon and carbon atoms. So, it becomes possible separation of the generated electric-hole pairs on the planes with different sign of surface charge and for￾mation of peroxide radicals. The latter interact with crystal surface and oxidize it: SiC + 6OH = SiO2 + CO + 3H2O (10) In [29], it was shown by experiment that when single crystals of SiC-6H polytype, 90–95% of the surface of which related to the planes (0 0 0 1) C and (0 0 0 1) Si, were in contact with aqueous solu￾tion (pH 9–10) in argon ambient (both when lighting and in dark) the pH of medium changed up to value 7.2, and the solution gained buffer properties. A change in medium pH is not due to dissociation and ionization of surface silanol groups because the total surface area of single crystal is 1–3 cm2 whereas solution volume is 30 ml. Seemingly, the electrons and holes that are separated on the surface of single crystal planes having different sign of surface charge interact with electrolyte ions. One of the products of that reaction is peroxide radicals. This process thus causes the change in solution pH. Presence of thin silicon dioxide layer on the surface of SiC whiskers makes it possible to carry out effective dispersing of the crystals in aqueous solutions and obtain aggregately stable suspensions in alkaline medium (Fig. 5). The minimum value of sedimentation volume for SiC whiskers with the layer of amor￾phous SiO2 was observed for pH ∼10. It is known that optimum values of pH for dispersing in aqueous solutions of SiC whiskers and powders having oxidized surfaces are in the range 9–11 [30], the minimum value of sedimentation volume of oxidized powders was detected with pH 9.5 [31], and minimum viscosity of aqueous suspensions of SiC with the layer of SiO2 was observed with pH 9.75–10.25 [32] and 10 [6]. This fact is due to decrease in absolute value of electro-kinetic potential of dispersed SiC with oxidized sur￾face upon increasing pH of medium above 10 [33] and start of the process of re-agglomeration of SiC particles [6]. After wash cleaning of dispersed SiC in HF solution and removal of the layer of surface silicon dioxide, the hydrophobization of SiC surface occurs [7]. However, the surface of SiC is no stable neither in air nor particularly in aqueous solutions for long period. An increase in stability of suspensions of SiC purified in solution of HF, and also decrease in the value of pHiep in long-term contact of SiC with aque￾ous solutions was observed [34], which is connected with partial oxidation of SiC surface and formation of silanol groups [35]. Abrupt decrease in the value of sedimentation volume of SiC whiskers with non-oxidized surface at pH >10 seems to be connected with consid￾erable increase in the velocity of oxidation of SiC whisker surface (Fig. 5). However, even at pH 10–11 the dispersing ability of SiC whiskers with non-oxidized surface is essentially lower than for those with oxidized surface. The side surfaces of twinned whiskers with superlattice struc￾ture elements are formed by stoichiometric ({110}) and polar ({100} and {111}) planes for which the ratio of surface areas depends on the crystal sub-microstructure and is determined by conditions for crystallization of SiC whiskers. The absolute values of surface charge for planes {110}, {100} C, {100} Si, {111} C, and {111} Si will be different. All the three types of planes have semiconductor properties [36]. Also, the values of pHpzc for stoi￾chiometric and terminated by C and Si atoms planes (non-oxidized and oxidized) must be different. Therefore, the conditions could be realized (pH of medium, surfactants, impurities, etc.) at which nanoareas (planes terminated by Si and C atoms) can co-exist on the side surface of whiskers, which have different sign of surface charge. Thus, to study electric-surface properties of SiC whiskers in aqueous electrolytes one should apply the methods of potentiomet￾ric acid–base titration rather than electrophoretic measurements which allow for determination of only sum value of -potential for whiskers but do not give information on the sign mosaicity and the value of density of surface charge for crystals. Depending on the ratio for surface areas of planes terminated by silicon and carbon, and of stoichiometric planes one can observe substantial change in the pHiep value. Seemingly, an essential difference in pHiep values (from 3 to 7) obtained in [10,11] for SiC whiskers of various com￾mercial grades is due to different ratio of surface areas of planes terminated by Si and C atoms and stoichiometric planes on the side surface of SiC whiskers. 5. Conclusion The silicon dioxide in SiC whiskers produced by VLS method on moving drops exists in two phases, namely: -cristobalite and X￾ray-amorphous phase. The impurity -cristobalite forms as a result of hydrolysis of chlorosilanes and silicon tetrachloride by water vapor. The -cristobalite particles are not bound with SiC whiskers and have siloxane surface and considerably different sizes. With respect to morphology, the -cristobalite particles can be divided into three types. During dispersing of -cristobalite particles in aqueous solutions, the particle surface is subject to hydroxylation. The X-ray-amorphous silicon dioxide does not form individual particles. It forms as a result of oxidation SiC whiskers with water vapor in the lower furnace zone and exists as a film of up to 2.5 nm thick on the surface of whiskers. During drying of SiC whiskers (after dispersing and purifica￾tion in aqueous solutions) at 120–150 ◦C the whiskers oxidize and oxide layer forms of up to 3 nm thick. Upon long-time contact of non-oxidized SiC whiskers with air ambient the film of X-ray￾amorphous silicon dioxide of 2–3 nm thick forms on the surface of whiskers. The process of SiC whiskers dispersing in water solutions and aggregative stability of suspensions are determined by the type of crystal surface and the value of medium pH. The SiC whiskers with oxidized surface form stable suspensions in aqueous solutions having pH >8. References [1] V.M. Beletskii, V.G. Lutsenko, V.L. Milkov, D.D. Pokrovskii, A.N. Gribkov, E.V. Zagnitko, Yu.V. Gniloshkurov, E.L. Umantsev, V.M. Gunchenko, A.V. Polyakov, Soviet Powder Metall. Met. Ceram. 25 (1986) 392–395. [2] V.G. Lutsenko, V.M. Beletskii, A.F. Gorovtsov, D.D. Pokrovskii, T.V. Verkhovlyuk, S.L. Shein, Powder Metall. Met. Ceram. 32 (1993) 170–173.

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