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 ofV.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