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 reaction666 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