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One-Step Flame Synthesis of Ultrafine SiO2-C Nanocomposite Particles with High Carbon Loading and Their Carbothermal Conversion Andri Vital,*,† Jo1 rg Richter,† Renato Figi,‡ Oliver Nagel,‡ Christos G. Aneziris,§ Johannes Bernardi,| and Thomas Graule† Laboratory for High Performance Ceramics and Laboratory for Analytical Chemistry, Empa, Swiss Federal Laboratories for Materials Testing and Research, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland, Institute for Ceramics, Glass and Construction Materials, Technical UniVersity Bergakademie Freiberg, Agricolastrasse 17, 09596 Freiberg, Germany, and Center for Transmission Electron Microscopy, Vienna UniVersity of Technology, Wiedner Hauptstrasse 8-10, 1040 Vienna, Austria
Submicron-sized silica-carbon composite particles with a high carbon loading were synthesized in a single process step by oxidation of hexamethyldisiloxane vapor in an atmospheric oxygen-fuel diffusion flame. By metered addition of a fuel gas to the flame, the carbon content could be controlled from 1.4 to 14 wt % and the specific surface area was varied from 31 to 121 m2/g. The primary particles in the powders had a size of 20 to 100 nm, were irregularly shaped, and formed arborescent aggregates. The distribution of silicon and carbon within the particles was analyzed by transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS) and image filtering (EF-TEM). It is shown that, independent of the composition, the individual primary particles consist of nanosized silica and carbon clusters, and form a homogeneous nanocomposite. Heat treatment of the flame-made powders and their mixture with carbon black at 1400 to 1500 °C under argon atmosphere led to fibrous β-SiC/SiO2 composites. The amount of nanofibers was dependent on the carbon fraction in the original SiO2-C powder. 1. Introduction Thermal routes for nanoparticle synthesis are widely applied since they yield high-purity products at high process yields and high production rates.1 Within this category, flame aerosol processes predominate in the manufacture of nanopowder commodities such as carbon black (C), fumed silica (SiO2), and titania (TiO2). Fumed SiO2 nanoparticles are used as a raw material for optical fibers; as a catalyst carrier and polishing medium in microelectronics;2 to control thickening, viscosity, and thixotropy of paints, lacquers, cosmetic creams, and lotions; to improve the mechanical properties of rubbers and polymers by forming silica networks;3 as an antisettling agent to stabilize suspensions;4 and to optimize the performance of silicon seals as well as dental composites.5 Carbon black is an amorphous carbon of semigraphitic structure which, when compounded with rubber, increases the rubber’s tensile strength, modulus, abrasion, and tear resistance.6 It is also used as a pigment in printing inks, coatings, plastics, fibers, papers, cements, and concretes.7 In the metals industry it is used during the metal smelting process as lubricating and reducing agent and as a carbon source, and in the production of refractory castables it is used to improve the work of fracture, thermal shock behavior, and corrosion resistance.8,9 Mixtures of fumed silica and carbon powder are widely applied. In the ceramic industry, carbothermal reduction of SiO2 with carbon is the prominent process for production of silicon carbide (SiC). Intimate contact and homogeneous mixing of the raw materials is the prerequisite for a high product yield.10 The tire industry has shown that tread compounds made with a * To whom correspondence should be adddressed. Tel.: +41 44 823 43 66. Fax: +41 44 823 41 50. E-mail:
[email protected]. † Laboratory for High Performance Ceramics, Empa. ‡ Laboratory for Analytical Chemistry, Empa. § Technical University Bergakademie Freiberg. | Vienna University of Technology.
silica-carbon dual phase filler (Ecoblack)11 have properties superior to those of rubber compounds made with physical mixtures of the two constituents, and this progress enabled the development of so-called “green tires”.12 Therefore, a one-step synthesis of SiO2-C composite nanoparticles with intensely mixed constituents and precise control of the composition would be advantageous as it circumvents powder blending steps and related issues such as a limited attainable homogeneity of the mixture. Further, for specific applications, tailoring of the nanoparticles is required by independently adjusting their specific surface area and composition. High carbon loading of the silica is required in all the applications listed above. Koc10 used a high-temperature-cracking process at elevated pressure to coat silica nanoparticles with carbon. Up to 30 cycles, each lasting more than 1/2 h, were necessary to achieve a carbon fraction of 36.8 wt % for the subsequent conversion to SiC. Spicer3 synthesized SiO2-C nanoparticles with up to 30 wt % carbon content in an oxygen-acetylene (C2H2) premixed flat flame using silicon tetrachloride (SiCl4) as the precursor. However, reproducibility of the carbon content in the powders was poor. Kammler13 investigated the combustion of hexamethyldisiloxane (HMDSO) vapor with O2 and O2/N2 mixtures in a diffusion flame without addition of a fuel gas. The maximum carbon content achieved was 4.8 wt % at an oxygen-to-fuel ratio (lambda, λ) of 0.4, and the author assumed that the carbon coats the SiO2 particles. In a further study Kammler4 produced SiO2-C nanoparticles in turbulent hydrogen-oxygen diffusion flames with HMDSO vapor as precursor. The maximum carbon content reported was only 1.4 wt %, and the author proposed that silica core particles were locally coated with carbon nanoparticles. Herlin-Boime14 performed thermal conversion of siloxane vapors into composite nanoparticles using a CO2 laser in an inert argon atmosphere. Independent of the precursor, changes in laser focusing resulted in different Si/C/ O-carbon and SiC-Si/C/O-carbon composite nanoparticles. In all reports the distribution of Si, O, and C within the
10.1021/ie061374p CCC: $37.00 © 2007 American Chemical Society Published on Web 05/15/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 Table 1. Thermodynamic Data of the Fuel Gases
oxygen demand per mole of fuel [mol/mol] combustion enthalpy [kJ/mol] adiabatic flame temperaturea [K] thermal diffusivity19 at 400 K, 1 atm [cm2/s] a
Figure 1. Schematic of the experimental setup for the synthesis of silicacarbon composite nanoparticles in a co-flow diffusion flame at atmospheric pressure.
nanoparticles has only been postulated or analyzed spectroscopically but not determined by elemental mapping. Further, precise control of a carbon content higher than 5 wt % has not been achieved in a single-step flame process yet, and carbothermal conversion has not been investigated. The present study reports on the one-step flame synthesis of submicron-sized SiO2-C nanocomposite particles with a carbon content up to 14 wt %. Hexamethyldisiloxane was used as the silicon precursor and methane, isobutane, or acetylene, featuring different discriminative carbon bonding, oxygen demand for complete oxidation, and combustion enthalpy, was added to control the carbon content in the particles. The influence of the fuel gas on particle composition and specific surface area as a function of the overall oxygen-to-fuel ratio in the flame has been systematically studied. High-resolution transmission electron microscopy (HR-TEM) was applied to obtain information on particle morphology and crystallinity. The total carbon and oxygen contents were analyzed by hot gas extraction methods. Furthermore, the distribution of silicon and carbon within the nanoparticles was determined with energy-dispersive X-ray analysis (EDX), electron energy loss spectroscopy (EELS), and energy-filtered imaging in a transmission electron microscope (EF-TEM). Results of the high-temperature conversion of the SiO2-C particles are presented. 2. Experimental Section 2.1. Powder Synthesis. A schematic of the experimental setup is shown in Figure 1. The equipment consists of a delivery unit for the liquid precursor and the gases, an evaporator, the flame reactor, and a filter for powder collection. The gas flow rates of the fuel gases (acetylene (C2H2), >99.6%; isobutane (i-C4H10), >99.95%; methane (CH4), >99.995%; Carbagas, Switzerland), the oxidant (oxygen, >99.95%; Carbagas, Switzerland), and the carrier gas and lift gas (nitrogen, >99.995%; Carbagas, Switzerland) were controlled by mass flow controllers (MFCs) from Bronkhorst High-Tech B.V., Netherlands. The liquid precursor hexamethyldisiloxane (HMDSO, (O(Si(CH3)3)2, >98.5%; Fluka, Switzerland) was metered by a liquid flow controller (LFC) (Bronkhorst High-Tech B.V., Netherlands) and directed into the evaporator (Hovacal, Germany) equipped with an ultrasonic atomizer and two heating circuits. The gaseous precursor was drawn from the evaporation chamber with nitrogen carrier gas and transported to the flame reactor via a heated tube. All gas supply lines (heated tubes, type IHH 203/ 205, Wisag, Switzerland) to the flame reactor, the reactor itself,
methane (CH4)
acetylene (C2H2)
isobutane (C4H10)
2.0
2.5
6.5
-802.5 3054 0.405
-1256.2 3341 0.190
-2648.8 3101 ∼0.08
At stoichiometric conditions; O2 as oxidant.
and the evaporator were kept at 150 °C to avoid recondensation of the HMDSO vapor. The stainless steel burner is of the co-flow diffusion type consisting of three concentric tubes. The central tube has an inner diameter of 4.8 mm, and the gaps between the center tube and the middle tube and the middle tube and the outer tube are 0.3 and 0.85 mm, respectively. All three tubes have a wall thickness of 0.4 mm. A single diffusion flame was used for all experiments, with the fuel gas and the precursor vapor/carrier gas mixture being supplied via the center tube. Oxygen was provided through the outer annulus. Nitrogen was delivered through the inner annulus, thereby separating the oxidant from the fuel/precursor gas mixture immediately at the burner mouth and lifting the flame away from the burner. This prevents particle deposition in the burner mouth and thus makes it possible to run a continuous process. The flame was surrounded with a Pyrex glass cylinder (inner diameter 250 mm, height 350 mm), which was sealed to the base plate to exclude ambient influences, such as extraneous air in the flame zone, and to avoid particle emission. A hood was placed directly above the glass cylinder to direct the particle-laden aerosol to the filter unit located 55 cm from the flame reactor. Particle samples of about 0.5-1 g were collected on a glass microfiber filter (GF/A 50, 150 mm diameter, Whatman, U.K.) with the aid of a vacuum pump (Trivac 15V, Leybold, Switzerland). The carrier gas, lift gas, fuel gas, and oxygen flow rates were kept constant at 86, 45, 41, and 514 L/h, respectively, for all experiments (standard conditions). The precursor flow rate was varied from 138 to 435 g/h, resulting in a particle production rate between 102.0 and 321.5 g/h for pure SiO2 as product powder. Two special series were performed: one without fuel gas and a HMDSO flow rate up to 735 g/h, and a second with 21 L/h acetylene, 10 to 21 L/h oxygen, and 38 g/h HMDSO (referred to below as the special acetylene series). The overall flame stoichiometry is characterized by the air number (λ) defined as
λtotal )
n˘ O2
∑i
(1) n˘ fueli(n˘ O2/n˘ fueli)st
where the molar flow rates n˘ ’s can be calculated from the feed gas flow rates, assuming ideal gases. The fuels (subscript “fuel”) include all the oxygen-consuming species (subscript “i”), i.e., HMDSO and the fuel gas added. The subscript “st” in the denominator denotes the stoichiometric oxygento-fuel ratio for complete oxidation of the fuels. Values of λtotal < 1 represent oxygen-lean (substoichiometric) flames, whereas λtotal > 1 indicate oxygen-rich (overstoichiometric) flames. In the series performed in this study, λtotal was controlled by adjusting the volume and mass flow rates of the reactants. The oxidation reactions of the fuel gases and characteristic reaction data are given in Table 1. The air number is the recipro-
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cal value of the fuel equivalence ratio, which is also used in the literature.15 2.2. Powder Characterization. The specific surface area (SSA) of the powders was determined by a five-point N2 adsorption isotherm obtained from BET (Brunauer-EmmettTeller) measurements using a Beckman-Coulter SA3100 (Beckman-Coulter, Switzerland). Prior to measurement the powder samples were degassed for 3 h at 150 °C under flowing nitrogen to remove water from the particle surface. The total carbon content of the SiO2-C powders was measured using a hot-gas extraction device LECO CS-400 (Leco Corp., St. Joseph, MI). High-purity iron, tungsten, and tin powders were added to the SiO2-C particles to improve the coupling of the induction heating. A constant oxygen flow was supplied to the heating chamber to ensure complete oxidation of the powder, especially conversion of carbon to CO2. The CO2 gas in the exhaust was detected photometrically and quantified by an infrared cell. A certified synthetic carbon reference material (LECO No. 502-029) was used for the calibration. Determination of the oxygen mass fraction was carried out by a hot-gas extraction method with a LECO TC-500 (Leco Corp., St. Joseph, MI). A 10 mg sample of the powder, mixed with 2 g of nickel powder as aggregate, was melted in a tin container placed inside a double graphite crucible. The oxygen from the sample powder forms CO with the graphite crucible and is subsequently converted to CO2 at the copper-chipping catalyst column. Measurement and quantification of the CO2 concentration was performed by an infrared measuring cell. Calibration was performed with a certified SiO2 reference sample (LECO No. 502-502-139). A transmission electron microscope (TEM) (Philips CM 30, Philips, Eindhoven, Netherlands) was used for the morphological analysis. A few milligrams of a powder was dispersed in 1015 mL of isopropyl alcohol (>99.5%, Fluka, Switzerland), and a few drops of this dispersion were allowed to dry on a copper grid coated with a holey carbon film (200 mesh, Plano GmbH, Wetzlar, Germany). High-resolution TEM (HR-TEM) was performed on an analytical transmission electron microscope with a field emission gun (FEI TECNAI F20 S-TWIN) equipped with a high angle annular dark field detector (HAADF) and a Gatan imaging filter (GIF 2001) in order to detect crystal domains and/or cluster structures within the particles. Energy-dispersive spectroscopy (EDS) line scans (EDAX UTW) permitted the determinaton of the microscopic particle composition. Electron energy loss spectroscopy (EELS) and image filtering (EF-TEM) were performed for elemental mapping of silicon and carbon, and identification of the bonding of silicon with carbon and oxygen. X-ray diffraction (XRD) measurements were performed with a PANalytical PW 3040/60 X’Pert PRO instrument using Nifiltered Cu KR radiation of wavelength 1.5418 Å. A 2θ scan range from 5° to 80°, a scanning step size of 0.01°, and a scintillation counter detector was used. Proprietary software from Philips X’Pert high score plus was applied for peak fitting. The powders after annealing were analyzed with a scanning electron microscope (SEM) (Tescan, Vega 5136 MM, Germany). The samples were sputtered with a gold layer. 2.3. Carbothermal Conversion. High-temperature conversion of the powders was performed in a Carbolite CTF 16/75 tube furnace. The powder samples were placed in sintered SiC crucibles. A heating and cooling rate of 6 °C/min was used, and the dwell time was 2 h at a given temperature (1300, 1400, 1450, and 1500 °C). Two hours prior to sample loading, during
Figure 2. Total carbon content of SiO2-C powders as a function of HMDSO mass flow rate and fuel gas supplied to the flame.
the heating, cooling, and dwell period, the furnace was purged with a constant flow (10 L/min) of argon (99.9997%). Heat treatment of the as-produced powders and their mixtures with carbon black (Printex U, Degussa, Germany) were investigated. The powder blends were homogenized in a polyethylene container by mixing for 2 h with 0.5 mm silica beads in a Turbula mixer. The admixed carbon amount complemented the existing carbon fraction to 37.5 wt % (stoichiometric SiO2/C mixture for full conversion to SiC by the carbothermal reaction). 3. Results and Discussion 3.1. Influence of the HMDSO Flow Rate and Fuel Gas on Carbon Content. Figure 2 shows the total carbon content of the SiO2-C powders as a function of the HMDSO mass flow rate. Qualitatively the carbon fraction increased with the precursor flow rate for all series; however, compared to the experiments without fuel gas and methane, the carbon concentration was significantly higher when acetylene was added and appreciably lower when isobutane was admixed to the HMDSO vapor/carrier gas. The special acetylene series with low HMDSO, acetylene, and oxygen flow rates yielded particles with by far the highest carbon content (13 to 14 wt %). At HMDSO flow rates up to 200 g/h, the particles produced without extra fuel gas, with methane, and with isobutane have a similar composition (1 to 3 wt % C). Addition of acetylene at these low HMDSO flow rates resulted in a higher carbon content (4 to 5 wt %). At HMDSO flow rates >200 g/h, particles from isobutane-containing flames exhibited a considerably lower carbon content (2.5 to 3.5 wt %) than particles from the other series. Again, particles generated in the HMDSO vapor/carrier gas flame had approximately the same carbon fraction as those produced with methane added, whereas the acetylene addition yielded a steep increase in carbon content up to 10.5 wt %. In the present study, HMDSO is not only the origin for silicon but also a carbon source and a fuel. When the HMDSO mass flow increased from 138 to 435 g/h, the HMDSO vapor concentration increased significantly from 18 to 41 vol % in the series without fuel gas addition and from 13 to 32 vol % for the series with fuel gas addition, whereas the fuel gas concentration decreased little from 28 to 22 vol %. According to the literature, the higher the flow rate of the fuel gas in the center and the higher the fuel concentration, the higher the amount of soot formed.16,17 The data in Figure 2 support this conclusion drawn from flames operated with hydrocarbon fuels; hence it is also valid for HMDSO vapor.
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The increase in carbon content with increasing HMDSO flux is high at low HMDSO mass flow rates (up to 250 g/h) and decreased at higher precursor flow rates as HMDSO became the major carbon source. The precursor vapor supplied 5.1 to 9.2 mol/h carbon at 138 to 250 g/h HMDSO, respectively, and increased to 16.0 mol/h at 435 g/h precursor, while the fuels inserted the constant amounts of 1.83, 3.66, and 7.32 mol/h carbon for methane, acetylene, and isobutane, respectively. Based on this consideration, isobutane was expected to contribute the greatest carbon amount; however, of all hydrocarbon gases, acetylene is known to have the greatest tendency to form soot in diffusion flames.16 It has been established that soot formation in flames with hydrocarbon fuel gases mainly proceeds via the inception of C2H2 formation, thus explaining the essentially higher carbon content of the particles produced when C2H2 was added to the flame.18,19 In contrast, methane is reported to show little sooting tendency and is assumed to leave the diffusion flame unreacted.16 It must be taken into account that addition of the fuel gases reduced the HMDSO vapor concentration and decreased the residence time of the gases in the flame by a factor of 0.71 to 0.78 as the total volume flow was increased when compared to the conditions without fuel gas admixed. According to Glassman,17 a longer growth path (increased residence time) leads to more soot and this effect might explain the slightly higher carbon fraction found in powders produced in the flame without than in powders produced with methane. However, the far lower carbon content of the powders produced in the presence of isobutane cannot be explained with these arguments, and therefore other factors must have an important influence on the tendency for soot to form. In diffusion flames with the fuel gas in the center surrounded by the oxidant, combustion (oxidation) of the fuel with the oxidant gas readily proceeds at their interface (flame front), resulting in a high lateral temperature, which theoretically corresponds to the adiabatic flame temperature (Table 1). With increasing distance from the burner exit, the flame front gradually diffuses to the center of the gas stream. Therefore, combustion of the fuel gases in the center cannot occur at the outset, which causes the temperature of the gases along the flame center line to be initially much lower than at the cylindrical flame front and to approach the front’s temperature only with increasing distance from the burner mouth.16,20 Consequently, the gases exiting the center of the burner are pyrolyzed but not oxidized directly after discharge and might form soot nuclei, if they contain carbon. Calculations of the temperatures on the flame axis for airstarved methane-air diffusion flames (1.05 < λtotal < 2) indicated a decrease in temperature with decreasing air number.19 Measured and calculated temperature profiles in ethylene-air diffusion flames with varying fuel concentrations18 supported this finding and have been confirmed for HMDSO vaporoxygen double diffusion flames in the range 0.4 < λtotal < 6.25, where a distinct drop of the inner flame front temperature for λtotal < 1 was detected.13 From these observations it can be deduced that in diffusion flames heat is transported from the hot lateral flame front to the cold center by radiation and conduction. On the basis of these considerations, Glassman17 suggested that the thermal diffusivity of the gases in the center tube governs sooting behavior of a flame. The thermal diffusivity influences the center gas temperature and therefore the rate of pyrolysis, the concentration of soot nuclei, and the growth period of the particles in the flame, all of which together determine
the total mass of soot formed. Glassman concluded that the higher the thermal diffusivity, the higher the pyrolysis rate of the fuel and the faster the formation and greater the number of soot nuclei. This results in a longer growth path and therefore promotes the formation of soot. Furthermore, dilution effects and the flame temperature must also be considered. For the fuel gases added in the present study, CH4 has the highest thermal diffusivity, which is about 5 times higher than that of isobutane (Table 1).17 At a given HMDSO mass flow rate, the volume fraction of the fuel gases (and nitrogen) is constant, and thus the pyrolysis process is only influenced by the thermal diffusivity and the flame temperature. Since the adiabatic flame temperatures of the different fuel gases do not differ significantly (Table 1), the various flames differ only in the thermal diffusivities of their respective fuel gases. In agreement with Glassman, isobutane with the lowest thermal diffusivity in the study yielded silica powder with the least carbon content. The special acetylene series supported the abovementioned conclusions. In this case, nitrogen predominated with 77 vol % (thermal diffusivity of 0.367 cm2/s)17 and the HMDSO and the fuel fraction were small compared to the other series; thus heat conduction to the flame center was fast and pyrolysis enhanced. In addition, the gas flow rates were much smaller than in the other series and therefore the soot growth path far longer. Finally, the C2H2/HMDSO ratio was about 2 times higher than in the other series with C2H2 addition at a low HMDSO mass flow rate (138 g/h). This explains the high carbon fraction in the powder. For HMDSO pyrolysis Herlin-Boime14 proposed a decomposition mechanism which includes cleavage of Si-C, C-H, Si-O, and Si-H bonds by thermal homolysis and abstraction, followed by radical formation and interaction. Subsequent polymerization and agglomeration reactions occur, resulting in various polyoxocarbosilane compounds. However, the data points for the special acetylene series do not fit into the general devolution of the carbon content against HMDSO flow data in Figure 2 since several process parameters were varied when compared to the other series. The data points in Figure 1 lie in a vertical line since the powders were prepared at a constant HMDSO flux and with varying oxygen flow rate, yielding powders with different carbon contents. The same applies for the data of Kammler.13 Therefore, the influence of the variation of process parameters other than the precursor flow rate on the carbon content of the powders cannot be depicted in a graph where the carbon content is shown against the precursor flow rate. 3.2. Influence of Air Number on Carbon Content. A crucial parameter characterizing flames is the air number, or its reciprocal value, the fuel equivalence ratio. The propensity of a diffusion flame for formation and escape of soot not only depends on the fuel gas nature,19 fuel gas concentration, and gas velocity, but also to a great extent on the air number.18 In Figure 3 the carbon content of the SiO2-C nanoparticles is plotted against the total air number (λtotal) for the different fuel gases. Irrespective of whether fuel gas was present, the carbon content increased with decreasing air number and the carbon fraction varied with the fuel gas species at a given air number. The data reveal that carbon-containing particles formed even at a high oxygen excess without fuel addition. Also, Roper19 showed that for air numbers greater 1 soot escaped laminar methane-air flames but earlier and in higher concentrations as the air number was lowered toward unity. In the present study substoichiometric flames yielded darkgray to black powders with a maximum carbon content of 10.5
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Figure 3. Total carbon content of SiO2-C powders against total oxygen to fuel ratio (λtotal) in the flame.
wt % at λtotal ) 0.62 with acetylene addition and 14 wt % C at λtotal ) 0.06 in the special acetylene series. The pure HMDSO vapor as well as the HMDSO/isobutane series showed a linear increase in the carbon concentration with decreasing air number, however with a significant parallel shift. In contrast, the HMDSO/acetylene series showed a distinct increase in the carbon fraction at about λ ) 1.25 and a similar, yet less pronounced leap in the carbon content is noticeable in the HMDSO/methane series at λ near 1.25. In contrast to Figure 2, the data from the special HMDSO/acetylene series can be fitted to the other series and fall on a notional extrapolation line of the HMDSO/acetylene series. The shift of the graphs in Figure 3 has to be attributed to the differences in oxygen demand of the fuels (Table 1) and their propensities to form soot. The influence of variations in gas flow dynamics can be neglected as congruent flow conditions in between the series were assured by control of the gas flow rates. Granted that the fuel gas added does not influence soot generation, the graph for the HMDSO/N2 gas mixtures in Figure 3 would be shifted to the left along with an increase in the negative slope. Based on their oxygen demand, isobutane would induce the greatest shift and methane the smallest. Given the case that the fuel does contribute to soot generation, the data will additionally be shifted in the vertical direction. As can be seen in Figure 3, the data for the acetylene series lie above and to the right of the curve for the pure HMDSO/N2 mixture, which implies that acetylene contributed much carbon to the product powder, whereas the other gases did not. Further, the data of the special acetylene series lie approximately in line with the standard acetylene series. This indicates that the carbon fraction in the silica nanoparticles resulting from the HMDSO/C2H2 flames is governed by the air number. Spicer3 proposed that the pyrolysis products of the silica precursor act as nuclei for soot formation and that this effect became more pronounced with decreasing air number. The results of the present study support his conclusion. Moreover, from our data can be deduced that addition of a fuel, which induces early soot formation at high λ values, amplifies the carbon content in the product powders at low λ values. According to above details, depiction of the carbon content against λ (Figure 3) instead of the precursor flow (Figure 2) rate allows incorporation of several process parameters and thus drawing of conclusions regarding the influence of process conditions on particle composition, which is not possible for the latter graph. Comparison of the series without fuel gas and
Figure 4. Specific surface area of SiO2-C powders as a function of total oxygen to fuel ratio (λtotal) in the flame.
the series of Kammler13 shows a significant difference because the oxygen flow rate was varied in the latter study, and therefore fluid dynamics, reactant mixing conditions, and temperature profile changed as well.21 Furthermore, Kammler used a double diffusion flame configuration with two flame fronts, which resulted in an increased oxidation rate of the fuel when compared to the single diffusion flame operated in the present study. These two factors explain the lower carbon content in Kammler’s particles and the relatively small slope of the corresponding data in Figure 3. 3.3. Specific Surface Area (SSA) against Air Number. Figure 4 depicts the specific surface area of the SiO2-C powders as a function of the total air number. With the sole exception of the HMDSO/acetylene experiments, an increase of the SSA with decreasing λtotal was the distinct tendency. For HMDSO/ acetylene the opposite trend was observed. The powder produced without additional fuel at λtotal ) 0.42 had a SSA of 45 m2/g at 8.3 wt % C, and the powder made at reduced HMDSO/C2H2/ O2 flow rates (λtotal ) 0.06) with 14 wt % C showed the maximum attained SSA of 121 m2/g. For the powder produced in the flame without fuel gas addition under oxygen-rich conditions (λtotal ) 2.25), the lowest SSA of 31.4 m2/g (C content of 1.36 wt %) was measured. The curves for the series without fuel and with methane addition nearly overlap, and all the SSA values of these two series are lower than those for the powders made with isobutane and acetylene addition. The slope of the curve for the isobutane series is the steepest of all the series. Kammler13 and Spicer3 identified that the specific surface area of the powder increased when less oxygen was added to the flame and when the fuel equivalence ratio was increased, either of which corresponds to lowering the air number. The data for the powders in the present study are in agreement with this conclusion. The SSAs of the powders produced with isobutane were high, and the graph has a steep negative slope (Figure 4). Due to the low thermal diffusivity of isobutane, the heat conduction was reduced, and this led to slow sintering of the particles compared to the other series.21 The increase of the SSA with decreasing λ (increasing HMDSO amount) is strongly affected by the HMDSO concentration, which amplifies the collision rate and results in fine particles and therefore higher SSA values. Overlapping of the SSA against λ curves for the series without fuel and methane demonstrates that particles with the same SSA but different compositions can be produced by conducting the synthesis process either with or without methane. 3.4. Morphology. Significant differences in powder morphology as a function of the powder composition and SSA could
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Figure 5. Representative TEM (a, c) and HR-TEM (b, d) pictures of SiO2-C nanoparticles deposited on a TEM grid with a holey carbon film. (a, b) Particles from a HMDSO vapor flame without fuel addition. The powder is dark brown, has a specific surface area of 45 m2/g, and contains 8 wt % carbon. (c, d) Particles made at low HMDSO/C2H2/O2 flow rates (special acetylene series) with a specific surface area of 120 m2/g and a carbon content of 14 wt %. Dark areas in the TEM and HR-TEM pictures occur due to particle overlapping and are not associated with structural effects.
not be identified by TEM. Representative pictures of particles produced with the parameter set HMDSO/no fuel (SSA 45 m2/ g, 8 wt % C) are shown in Figure 5a,b and particles resulting from the special acetylene series (SSA 121 m2/g, 14 wt % C) in Figure 5c,d. The powders consist of large, highly ramified aggregates with a branch length up to several micrometers in at least one dimension. The aggregates are made of primary particles 20 to 150 nm in size. Crystalline domains in the particles could not be identified by high-resolution TEM (Figure 5b,d), and the particles are amorphous. This has been confirmed by XRD. Particular microstructural arrangements of the constituents, for instance a core-shell morphology, could not be observed, with the exception of a few small areas with graphenelike structure found in the particles made with acetylene. It is well-known that high-temperature pyrolysis of C2H2 leads to graphene formation.22 3.5. Elemental Structure. Elemental mapping of the samples with 8 and 14 wt % C with EF-TEM showed that silicon and carbon were homogeneously distributed within the primary particles as well as in the particle aggregates (Figure 6). Neither areas on particle surfaces enriched in an element nor larger accumulations of one constituent were detected such as a silica core or carbon dots as suggested by Kammler.13 EDS line scans revealed that carbon never appears alone and is always associated with the presence of silicon and oxygen. At a few points along the line scan, carbon was missing, indicating the presence of silica domains. Further, with EELS analysis a peak at 105 to 106 eV (Si L23 edge) attributable to SiC4 bonds could not be detected, but SiO4-x suboxide bonds from disproportionated SiOx were found.23 The predominant peaks at 108.5 and 114.5 eV belong to SiO4 bonds, and therefore SiO2 was identified as the major phase, while SiC and silicon oxycarbides (Si-C-O) could not be detected. For verification, the oxygen content of the powders was analyzed. From the measured oxygen and carbon contents, assuming that the particles consist of silicon,
Figure 6. TEM images and elemental maps for Si and C of SiO2-C nanoparticles. (a-c) Electron image and elemental maps for 8 wt % carbon (45 m2/g) powder from HMDSO vapor flame without fuel addition and (d-f) corresponding pictures for 14 wt % carbon (121 m2/g) powder from HMDSO/acetylene flame (special acetylene series). The element analyzed appears bright in the elemental maps. At the bottom left hand corner in (a)-(c) and near the lower edge of (d)-(f), the rim of a strut of the carbon film appears.
carbon, and oxygen, the silicon content can be calculated from [Si] wt % ) 100 wt % - [C] wt % - [O] wt %. From this the O/Si molar ratio was determined to be 1.89 ( 0.10, which is close to the value for stoichiometric SiO2 (O/Si ) 2). The slight oxygen deficiency can be assigned to the presence of disproportionated SiOx. From these analyses it can be concluded that carbon in the samples exists as free carbon and the silicon is present as silica. Both constituents appear as nanosized clusters, and they form a homogeneous nanocomposite. This arrangement of SiO2 and C is irrespective of the particle composition. Herlin-Boime14 found that SiC-Si/C/O-graphite composites formed during laser pyrolysis of HMDSO vapor in argon atmosphere, when the reaction temperature was >2000 °C, while a temperature of 910 °C in the reaction zone resulted in Si/C/ O-carbon composites (Si1C0.8O0.5 polyoxocarbosilanes). Temperature measurements in diffusion flames similar to the ones operated in the present study quoted center gas temperatures clearly below 2000 °C.13,24 These findings correlate well with composition of the Si1CxO2 (where 0e x e 0.78) composite particles produced in the present study, as formation of SiC was not observed. In contrast to Herlin-Boime, the flame synthesis was performed in an oxygen-containing atmosphere, which allowed controlled partial to complete oxidation of the reactants. Therefore, the composition of the flame-made powders was shifted to higher oxygen contents when compared to the powders prepared by Herlin-Boime. 3.6. Carbothermal Reduction. As-produced powders with 8 and 14 wt % C, mixtures thereof with the stoichiometric
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Figure 7. Weight loss of flame-made powders with 8 and 14 wt % carbon contents, their stoichiometric mixture with carbon black (+C), and the stoichiometric blend of silica OX50 with carbon black as a function of temperature.
amount of carbon black added (totally 37.5 wt % C according to eq 2), and a blend of OX50 (fumed nano silica, 50 m2/g, Degussa, Germany) with 37.5 wt % carbon black were heat treated. The weight loss of the powder mixtures against the annealing temperature is shown in Figure 7.
SiO2(s) + 3C(s) ) SiC(s) + 2CO(g)
(2)
The overall carbothermal reduction of SiO2 by C and full conversion to SiC according to eq 2 leads to a theoretical weight loss of 58.3 wt %. Equation 2 proceeds via formation of SiO(g) and CO2(g) as intermediates (eqs 3-7), which cause an increased weight loss if they are carried away and do not react with carbon to form SiC(s) and CO(g), respectively.25
SiO2(s) + C(s) ) SiO(g) + CO(g)
(3)
SiO(g) + C(s) ) SiC(s) + CO(g)
(4)
SiO2(s) + CO(g) ) SiO(g) + CO2(g)
(5)
2SiO2(s) + SiC(g) ) 3SiO(g) + CO(g)
(6)
CO2(g) + C(s) ) 2CO(g)
(7)
The weight loss of the OX50 + carbon black mixture at 1500 °C accounted for 59 wt %, which corresponds to the calculated value and implies that full conversion to SiC occurred. This was confirmed by XRD (Figure 8), as the pattern only showed β-SiC peaks while no silica could be detected. By comparison, the weight losses of the as-produced powders (8 and 14 wt % C) are significantly higher at T g 1400 °C than the theoretical values of 12.4 and 21.8 wt %, respectively. The XRD patterns (Figure 8) for the sample with 14 wt % C showed broad β-SiC peaks with a low intensity. A broad diffraction line at 2θ ≈ 21.9° was observed, which may arise from residual amorphous silica, as excess silica was present in the starting material, or a silicon oxycarbide (SiOC) glass phase.26 At 1500 °C a sharp peak was noticeable at 2θ ) 21.8°, which can be assigned to β-SiC with a d-spacing of 4.114 Å26 or to cristobalite as observed for polycarbosil(ox)anes pyrolyzed at 1500 °C.27 The increased weight loss of the as-produced powders is attributed to entrainment of SiO(g) due to the lack of carbon in the raw powder. The difference in weight loss (≈11.5 wt %) between the samples with 8 and 14 wt % C corresponds to the deviation determined from their carbon content by eq 2. Therefore, the
Figure 8. X-ray diffraction patterns of pure flame-made powder with 14 wt % carbon content, with carbon black admixed (+C), and of OX50 (SiO2) mixed with carbon black after heat treatment at 1400 and 1500 °C. For reference the pattern of high-quality β-SiC submicron powder (Grade BF12, >96% β-SiC, 12 m2/g, H.C. Starck, Germany) is shown (dashed line).
loss of mass is dependent on the carbon fraction in the powders, though not on the specific surface area. For the mixtures with carbon black added, β-SiC was the prominent phase at T g 1400 °C, and the intensity of the broad diffraction line decreased greatly, although it was still present even at 1500 °C. Again the sample with 8 wt % C from the flame-made particles showed a significantly lower mass loss than the powder with 14 wt % C, and the deviation is slightly higher than for the powder without carbon black addition. For both powders the increase in weight loss due to the extra carbon black was approximately the same compared to the as-produced powders. However, in contrast to OX50 + C, the weight loss of both powder mixtures at 1500 °C was less than the theoretical 58.3 wt %. This means that the stoichimetric carbon added did not lead to full conversion of the flame-made powders to SiC such as occurred for OX50, and the effect increased with decreasing carbon content in the as-produced particles. It is known that the carbothermal reduction mechanism takes place by a solid-solid reaction at the interface of SiO2 with C, where the gaseous intermediates SiO(g) and CO(g) escape from the particle surface.25 As shown, the flame-made, silica-rich particles consist of intermixed silica and carbon clusters. Due to this microstructure, generation and escape of SiO(g) and CO(g) by the carbothermal reduction are enhanced. The process can readily take place at the surface and gradually proceed to the center of the particles, leaving behind porous silica-rich SiO2/ SiC/SiOC particles, as the starting material consisted of excess silica. The higher the carbon content in the as-produced flame particles, the more this mechanism is promoted. The enhanced SiO(g) generation explains the pronounced formation of SiC when carbon black was admixed. Apparently the carbothermal reduction of the residual porous particles is inhibited. It has been established that the rate of carbothermal reduction depends on the size of the reacting particles.25 Here, the specific surface area of the composite particles with 14 wt % C was more than 2.5 times higher than that of the powder with 8 wt % C. Moreover, due to the elemental structure of the flame-made particles, it is also plausible that a silicon-rich silicon oxycarbide phase (SiOC) formed in the particles’ interior during heat treatment as the evolution of SiO(g) and CO(g) was not possible. The SiOC phases have been shown to convert only partially to SiC during annealing.27 Analysis of the samples with spectroscopic methods is necessary to allow full understanding of the
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Figure 9. SEM pictures of (a, b) pure flame-made powder with 14 wt % carbon content heat treated at 1500 °C and (c, d) the same powder with the stoichiometric amount of carbon black replenished and annealed at 1500 °C for 2 h in an argon atmosphere.
phase composition; however, this is not in the scope of the present study. SEM pictures of powders (Figure 9) revealed significant differences between the heat-treated pure flame-made particles with 14 wt % C and their heat-treated counterparts with carbon black added. The same was observed for the flame powders with a carbon content of 8 wt %. The as-produced powders resulted in composites of submicron-sized fibers and powder lumps (Figure 9a) after annealing at 1400 and 1500 °C. The fibers are up to several micrometers long and have a diameter of about 100 to 200 nm. They can be found throughout the sample in hollows and pores of the powder and form bundles and agglomerates locally (Figure 9b). By contrast, no fibers could be found in the powder mixture with carbon black added (Figure 9c,d). Therefore, the amount of nanofibers was dependent on the carbon fraction in the starting SiO2-C powder mixture. The presence of the fibers was also noticeable from the powder flow and compression behavior: while the heattreated powder mixtures were free flowing and compaction was very difficult to achieve, the powders with fiber intergrowth did not flow evenly and formed lumps, but were easy to compact. Intergrowth of long, submicron fibers during carbothermal reduction of silica has been observed in only a few cases. Koc10 reported on the formation of bent fibers of amorphous silica and a few SiC whiskers after heat treatment of carbon-coated
silica nanoparticles at 1300 °C. He assigned the fiber growth to a high concentration of SiO(g) and hence the reverse reaction of eq 5. Silicon carbide fiber growth was observed in S/SiC composites during tetramethylsilane vapor infiltration into the pyrolyzed and impregnated C/Si/SiC starting composite. The fibers were carbon-rich SiC as the atmosphere was oxygen free and the reaction chamber was evacuated frequently.28 The fibers improved the mechanical properties of the composite essentially. Li29 prepared nanometer silicon carbide whiskers from binary carbonaceous silica aerogels at temperatures >1300 °C. Recently, Raman30 synthesized silicon carbide nanofibers from pitch blended with sol-gel derived silica. All routes required multiple-step preparation of the Si-O-C precursors. In the present study fiber growth occurred if the raw material lacked carbon (
