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)170-173668 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.