- Email: [email protected]
Controlled combustion synthesis of nanosized iron oxide aggregates
Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 1851–1858
CONTROLLED COMBUSTION SYNTHESIS OF NANOSIZED IRON OXIDE AGGREGATES Z. ZHANG and T. T. CHARALAMPOPOULOS Mechanical Engineering Department Louisiana State University Baton Rouge, LA 70803, USA
Nanosized chainlike aggregates were synthesized in an iron pentacarbonyl–carbon monoxide–air diffusion flame system. The size and shape of the chainlike aggregates and primary particles were characterized in terms of measurable morphological parameters such as diameter of primary particles and aspect ratio. The influences of operating conditions on the parameters have been investigated systematically by employing thermophoretic sampling and transmission electron microscopy (TEM). The morphological parameters were found to be strong functions of operating conditions, residence time, and locations within the flame and could be controlled by adjusting the concentration of Fe(CO)5 seeded in the flame through the variation of carrier gas flow rate and/or temperature of additive. X-ray diffraction measurements coupled with thermodynamic equilibrium composition predictions were used to identify the chemical states of the particulate components. The results indicated that the aggregates formed in this flame consisted predominantly of primary particles of Fe2O3. In addition, the physical and chemical mechanisms of formation of chainlike aggregates are reviewed. Our observations suggest that, while the magnetic forces are necessary for providing a self-alignment mechanism, the flame temperature may also be critical for the formation of chainlike aggregates.
Introduction Knowledge of the shape and size of aggregates formed in flames is significant in many areas of research and practical applications. Information about the size and shape of both isotropic and anisotropic particulates is important (1) for predicting growth and oxidation of particulates in the combustion systems, (2) for identifying the agglomeration mechanisms of particulates, (3) for efficient control of fibrous aerosols in the industrial hygiene, (4) for quality control of materials synthesized in flame reactors, and (5) for air pollution–control applications [1]. On the other hand, the study of the iron oxide and its properties under formation conditions is particularly useful in ferrofluids applications. Several studies have been carried out for determining the morphological features of aggregates formed in combustion processes; see Refs. 1–5 and references therein. A number of studies have dealt specifically with the mechanisms of formation of chainlike aggregates in combustion processes [6– 11]. The formation of chainlike iron oxide aggregates in a CO-air diffusion flame was studied more recently [2–5], employing both ex situ and in situ techniques. The characteristics of the chain aggregates were determined for samples taken in the postflame region of the flame at different concentrations of the seeding vapor and variable O2:CO mixing ratio. However, the effects of operating conditions on the
morphology variation within the flame were not addressed. In addition, the physical and chemical mechanisms governing the formation of chainlike aggregates remain unclear [6]. Thus, the objectives of the present study were (a) to synthesize chainlike aggregates in an iron pentacarbonyl vapor seeded CO/air diffusion flame, (b) to identify the most critical operating parameters among temperature, feeding flow rate, and feeding ratio of the metal additive to fuel for the dominant generation of chainlike aggregates, (c) to characterize the primary particle size and aspect ratio of the aggregates as a function of residence time and of additive concentration, (d) to determine the chemical compositions of the particulates from experimental measurement and theoretical simulation, and (e) to investigate the mechanism for the formation of chainlike aggregates both from physical and chemical perspective.
Experimental The experimental system consists of a diffusiontype carbon monoxide–air flame centered around a concentric stainless steel tube. Regulated fuel, carbon monoxide (CO), is supplied through the inner tube (1/4 inch in diameter). The metal additive, iron pentacarbonyl vapor Fe(CO)5, is introduced into the CO-air flame by diverting a small fraction of bulk
1851
1852
MATERIALS SYNTHESIS
Fig. 1. Radial temperatures for CO-air diffusion flame as function of position above the burner surface.
fuel flow through a column of liquid Fe(CO)5 stored in a temperature-regulated stainless steel cylinder. At the outlet of the cylinder, partially saturated vapor is diluted by main fuel flow and directed to the burner for combustion. The concentration of iron pentacarbonyl vapor introduced into the flame may be controlled by adjusting either the cylinder temperature or the carrier gas flow rate. The system is calibrated so that the actual mass flow rate of iron pentacarbonyl vapor delivered to the fuel, corresponding to different carrier flow rates and cylinder temperature, is known before it enters the flame with an accuracy of 53.6%. The particulates produced in the flame were extracted through a thermophoretic sampling system, which eliminates the potential postsampling agglomeration of particles [12,13] and were deposited on a 200-mesh copper grid coated with carbon film that is stable under electron beam and can withstand relatively higher temperature conditions. The thermophoretic sampling system consists of a double-acting pneumatic cylinder, a solenoid-controlled directional four-way valve, and a computer-controlled variable time-delay relay circuit. A hall effect switch mounted on the pneumatic cylinder is used to calibrate the position and motion of the piston/ probe assembly. A piston speed of 0.86 m/s and a transition time 4 5 1 ms for the entry of the probe into the flame were achieved with the present system. The vertical position of the probe is fixed, whereas the burner system can be translated in the vertical direction. A pulse generator attached to the gear of the burner translation mechanism monitors the vertical displacement of the burner so that the height of the probe with respect to the burner surface can be controlled to within 1/10 of a millimeter. A slotted plate is placed between the flame and probe to deflect the air current induced by the motion of the probe and to block the heat transfer from the flame to the probe. A platinum versus platinum–
10% rhodium (S type) thermocouple with a bead diameter of 0.3 mm was used to measure the flame temperature at various axial and radial locations. Figure 1 shows the radiation-corrected radial temperature profiles of the unseeded CO/air diffusion flame at various heights above the burner surface. Each point on these curves represents an average value of three measurements. A maximum deviation of 2.7% from the average value occurred at a radial position of 4 mm and a height of 35 mm above the burner surface. A detailed description of the experimental system, its components, control, and calibration procedures are presented in Ref. 14. Results and Discussion Morphology Characterization Experiments for the synthesis of chainlike aggregates and the determination of size and morphology of primary particles and chainlike aggregates were performed. Chainlike aggregates were produced in the CO-air diffusion flame seeded with Fe(CO)5 vapor. The configurations of the flame under unseeded and seeded conditions are shown in Figs. 2a and 2b. Seeding the flame with Fe(CO)5 causes bright yellow emissions, which is thermal radiation from the iron oxide particles formed within the flame. The unseeded flame is a predominantly blue flame with a distinct conical white flame front. Yellow regions occurring in the upper portions of the unseeded flame may be attributed to formerly deposited solids in the honeycomb stabilizer of the flame. Nevertheless, the occurrence of such emissions was very sporadic and thus could not influence the measurements of the results in any way. The size and shape of the chainlike aggregates and primary particles were characterized in terms of the diameter of primary particles and aspect ratio. The
COMBUSTION SYNTHESIS OF CHAIN-LIKE AGGREGATES
Fig. 2. (a) CO-air diffusion flame. (b) Flame seeded with Fe(CO)5 vapor.
sampled particles and aggregates were grouped and analyzed statistically employing TEM technique so that quantitative results on the primary particle size distribution and aggregate size distribution as a function of position in the flame for various concentrations of Fe(CO)5 were obtained. Typical structures of primary particles and aggregates are shown in the TEM photograph (see Fig. 3). The image of the iron oxide primary particles appears to be hexagonal or
1853
polyhedric, which is consistent with the microstructure of Fe2O3 [15]. On the other hand, the aggregates exhibit for the most part straight-chain configuration. The variation of the morphological parameters of iron oxide aggregates with concentrations of iron pentacarbonyl vapor seeded in the flame was investigated by adjusting the cylinder temperature and the carrier flow rate. The first set of experiments was performed by fixing a total fuel flow rate 450 cc/min and a carrier flow rate 20 cc/min and by setting the cylinder temperatures at 238, 358, 458, and 558C, respectively. A higher cylinder temperature corresponds to a higher mass flow rate of Fe(CO)5 vapor, resulting in a higher concentration of additive in the flame. Under the operating conditions, the frequency distributions of diameters of the primary particles and aspect ratio of chainlike aggregates for samples collected from the center of the flame at 35 mm above the burner surface are presented in Fig. 4. The differential frequency distributions of diameter and aspect ratio are roughly bell-shaped in the linear plot. As cylinder temperature increases, the frequency distributions shift toward larger values of diameter and aspect ratio. It should be noted that at cylinder temperature 458 and 558C, branched chainlike aggregates were observed. Note that an alternate way to vary the concentration of Fe(CO)5 vapor seeded into the flame is by adjusting the carrier gas flow rate. The experiments were also performed at a fixed cylinder temperature of 238C. The carrier gas flow rates were 15, 20, 25, and 35 cc/min, corresponding to 0.5, 0.9, 1.5, and 2.6% of Fe(CO)5 vapor to fuel by weight for a total fuel flow rate of 450 cc/min. Analysis of the results for samples collected from the center of the flame at a height of 25 mm above the burner surface for distributions of diameters and aspect ratios yielded the same trends as those discussed previously. These trends are consistent with the expectation that increasing both cylinder temperature and carrier gas flow rate in turn increased the concentration of seed-
Fig. 3. TEM photographs.
1854
MATERIALS SYNTHESIS
Fig. 4. Frequency distributions of measured diameter (a) and aspect ratio (b) along with ZOLD predictions (Eqs. [1]–[3]) as functions of heated cylinder temperature.
ing vapor. Higher concentrations of Fe(CO)5 vapor result in increased concentration of primary particles. Therefore, the collision probabilities between particles for particle growth and interparticle aggregation are also expected to increase, yielding larger primary particles and longer chainlike aggregates. The variations of morphological parameters with residence time were assessed by collecting samples within the flame along the centerline at heights of 4, 15, 25, and 35 mm above the burner surface under operating conditions of a total fuel flow rate of 450 cc/min and a carrier flow rate of 25 cc/min with evaporator cylinder temperature maintained at 238C. The distributions of the sizes of primary particles and aggregates are shown in Fig. 5. At lower positions in the flame, all primary particles possess smaller diameters and the distribution is narrower. As the residence time increases, the particles grow, while the distribution becomes broader as a result of the continuous formation of new primary particles and subsequent aggregation. Because of the mag-
Fig. 5. Frequency distributions of measured diameter (a) and aspect ratio (b) along with ZOLD predictions (Eqs. [1]–3]) as functions of height above the burner surface.
netic nature of Fe2O3 [15], chainlike aggregates were observed at short residence times (about 4 mm above the burner surface). However, the ill-defined aggregate shapes precluded any reliable quantification of the aspect ratio distribution in the range of 4–15 mm above the burner surface. It should also be noted that when the flame temperature at 4 mm above the burner surface was above the Curie point of Fe2O3 ('950 K) [16], no chains were observed throughout the flame. On the other hand, for positions higher than 15 mm above the burner surface, more well-defined chainlike structures were observed. The experimental data were curve fitted using the method of least squares to a zero-order logarithmic distribution (ZOLD) function [1], which is defined as
1
exp 1 P(a) 4
r2o 2
2
1
1lnaa 2
2
m
2
exp 1 (1) amro(2p)1/2 2r2o The parameter am is the modal value of a, which
COMBUSTION SYNTHESIS OF CHAIN-LIKE AGGREGATES
1855
TABLE 1 Calculated average diameter, aspect ratio, and standard deviation using ZOLD function with variable additive concentration and height above the burner surface Variable Temperature
Variable Carrier Flow Rate
Variable Height
T (8C)
d (nm)
l/d
r
Q (cc/min)
d (nm)
l/d
r
H (mm)
d (nm)
l/d
r
23 35 45 55
57.4 60.97 61.31 62.16
7.08 8.16 8.63 9.09
4.0 6.0 6.1 6.2
15 20 25 35
45.89 51.03 55.13 59.56
4.93 5.52 — 6.38
2.6 2.6 — 2.4
4 15 25 35
21.7 45.61 54.76 60.52
— 4.17 5.93 6.75
— 2.5 3.2 2.4
represents d in the case of particle diameter distribution and (l/d) in the case of aspect ratio distribution. The parameter ro is a measure of the width and skewness of the distribution. The calculated ZOLD curves based on the inferred am and ro values from the least-squares analyses are also shown, along with the experimental data as determined by sampling and TEM analyses. The parameters am and ro are related to the mean value through the equation [17]: lna 4 lnam ` 1.5r2o
(2)
whereas the standard deviation is given by r 4 am(exp(4r2o) 1 exp(3r2o)1/2)
(3)
The mean diameters of primary particles, mean aspect ratio of chainlike aggregates, and standard deviations were calculated for all operating and sampling conditions mentioned previously, and are summarized in Table 1. The results revealed a definite increase in both the mean primary particle diameter and the mean aspect ratio with increasing height above the burner surface and with increasing concentration of Fe(CO)5 vapor seeded into the flame. Note that the largest increase of mean values was encountered at lower positions in the flame (short residence times) and lower concentrations of the additive. The experimental data of aspect ratio distributions show excellent agreement with the ZOLD function representation, whereas the diameter distributions, for the most part, follow closely the ZOLD function with maximum deviations occurring at the lower and upper portions of the distributions. The somewhat large values of r exhibited by all distributions may be attributed to any one or combination of the following reasons: (a) The flame is of diffusion type and continuous formation of small particles and aggregates is possible at both radial and axial locations of the flame. (b) The particles formed in the reaction zone, within which the particle volume fraction is high, may be transported to the central portions of the flame through thermophoresis since temperature gradients exist in radial direction (approximately
750 K/mm). (c) The thermophoretic probe could have been contaminated at high particle volume fraction locations during the insertion and/or extraction of the sampling assembly. Chemical Composition Although knowledge of chemical states of particle components under flame condition is important for complete understanding of physical and chemical mechanisms through which the chainlike aggregates are formed, it was not possible, at this time, to identify the chemical composition of combustion products under in situ conditions. Powder X-ray diffraction (XRD) has been proven to be an important standard tool for identification of material compositions. As part of the study, the chemical composition of the particulates collected from Fe(CO)5-seeded flame was identified using Xray diffraction. The measured X-ray diffraction pattern and characteristic peaks from database (International Unit for Diffraction Data, Powder Diffraction File) are presented in Fig. 6. It was found that the composition of the particulates is Fe2O3 with two different microstructures: hematite and maghemite. The sample used for X-ray testing was collected at 35 mm above the burner surface. The equipment used was SIEMENS D5000 X-ray diffraction machine, which has a resolution on the order of 0.5% by weight. It also distinguishes coating material with thickness on the order of 5 nm or even lower, provided that the atoms of mixture are known. However, the shortcoming of the approach may be that the samples could not be isolated entirely from air during the sampling and transportation. Thus, particle oxidation might have taken place before and during the measurement. Nevertheless, since it is generally thought that local thermodynamic equilibrium exists at various points throughout a flame [18], the thermodynamic equilibrium calculations of chemical composition were carried out to predict the distributions of possible chemical species in this flame. Specifically, the computer code developed by
1856
MATERIALS SYNTHESIS
Fig. 6. Measured powder X-ray diffraction pattern for particulates collected from Fe(CO)5 seeded CO-air diffusion flame at 35 mm above the burner surface. Blue lines: hematite; green lines: maghemite. Wavelength used was 1.5406 A˚.
Gordon and McBride [19] was employed for the prediction of chemical compositions of particles formed in the Fe(CO)5-seeded CO-air diffusion flame. Chemical equilibrium compositions of carbon monoxide flames seeded with 1.5% iron pentacarbonyl by weight were calculated for a range of fuel equivalence ratios and over a range of flame temperatures. The predicted fractional distribution of iron species as a function of temperature has been presented in Ref. 20 and demonstrates that nonsignificant ionic iron exists (whereas the dominant ironcontaining species is Fe2O3 within the flame temperature range studied). It should be noted that the equilibrium calculations are from thermodynamic viewpoint, which gives no indication of the kinetics of such reactions. Moreover, there may be a number of potential pathways through which iron or iron oxides may be formed and a number of factors, such as fuel-equivalence ratio, temperature, concentration of iron additive, flame configuration, type of fuel, and/or other combustion operating conditions, by which the chemical states of iron species may be affected. Thus, the precise chemical composition of all pertinent reaction products may not be precisely predictable at this time. Mechanism of Chain Formation As noted earlier, a few studies have addressed specifically the formation of chainlike aggregates. It may be argued that most flame-generated aggregates are
formed from thermal coagulation, leading to randomly structured aggregates [9]. However, under certain conditions and specific types of materials, the random motion may be ordered by other forces such as electrostatic and/or magnetic to form well-defined, straight chainlike aggregates. So far, chainlike aggregates have been observed at later stages of the formation or postflame region, and it is believed that the origin of their formation was the result of electrostatic interactions [6–11]. In this study, chainlike aggregates were observed at short residence times (approximately 4 mm above the burner surface) for temperatures lower than about 950 K, which is considered the Curie point for Fe2O3 [16]. Since Fe2O3 is magnetic material, this observation suggests that the magnetic properties may play a relatively more important role during the particle interactions and chain formation. Nevertheless, as will be explained in the section that follows, other system parameters such as flame temperature may play a role in the formation. The magnetic properties of Fe2O3 are complex in that its fundamental antiferromagnetism superimposed by a weak parasitic ferromagnetism results most likely from defects of cystallization. When the iron-oxide particle sizes are below 3 lm with an aspect ratio of 10, they are magnetized as single-domain particles in the presence of a magnetic field [21]. The mean diameter of particles produced in this flame ranges from 20 to 65 nm, or even smaller at early stages of formation; thus, they possess a net magnetic moment. If each particle is treated as a
COMBUSTION SYNTHESIS OF CHAIN-LIKE AGGREGATES
dipole of magnetic moment, apparently the magnetic dipole interactions between particles are stronger than thermal interactions in this flame particle system such that the particles are aligned by the magnetic forces through a self-alignment mechanism. This implies that the most stable configuration of three magnetic dipoles energetically is a chain, and the end of the chain is the energetically most favorable position for an approaching fourth particle to be attached, provided there is no mechanism for deforming the chain. Still, as the flame temperature increases, thermal interactions may prevail and provide a mechanism to deform the chain. Thus, aligned chains may no longer constitute the most stable configuration. Instead, either branched chains are formed or the chains break up to form clusters, depending on the relationship of combustion temperature and Curie temperature of the particles. It is noteworthy that in the present study, when the flame became turbulent, only clusterlike aggregates were observed. Currently, the effects of magnetic forces or dipole moments are not understood well enough to permit a quantitatively precise assessment. Clearly, further studies are required to elucidate the interactions of magnetic and temperature effects and their influence on the morphology and dynamics of the aggregates. Work is underway in this laboratory to study the effects of magnetic fields and temperature on particle interaction/growth. Summary The results of the present study may be summarized as follows: a. The CO-air diffusion flame seeded with Fe(CO)5 vapor was tested and dominantly chainlike aggregates were produced. The size and shape of chainlike aggregates and primary particles were characterized in terms of measurable morphological parameters under different operating conditions. The average diameter of primary particles and average aspect ratio of chainlike aggregates can be controlled by varying the concentration of iron pentacarbonyl vapor seeded into the flame. b. For the concentration range investigated, namely 0.75–2.8% iron pentacarbonyl vapor to fuel by weight, the average diameter of primary particles and aspect ratio increased by about 30% at 25 mm above the burner surface. c. The average diameter of primary particles increased by 8.0% and the average aspect ratio by 28% as the temperature of the heated cylinder increased from 23 to 558C. The total increase of average diameter was about 179% from 4 to 35 mm and the aspect ratio 62% from 15 to 35 mm. d. Higher concentrations and residence times favor the formation of longer chainlike aggregates. The
1857
preceding observations were repeated with identical results with maximum deviation of 5% from the mean values by performing the same measurements. The data, statistically analyzed for the average 1000 particles and 500 aggregates from each run, are expected to provide representative picture of the particulate morphology of iron oxide aggregates formed in this type of flame reactor. e. Both X-ray diffraction measurements and thermodynamic equilibrium calculations showed that the chemical composition of the particles formed in the flame is consistent with Fe2O3. In addition, our observations indicated that, while magnetic forces are essential for providing a self-alignment mechanism, the flame temperature may also be critical for the formation of chainlike aggregates. Acknowledgment This research was supported by the Air Force Office of Scientific Research Grant F49620-0477 and the National Science Foundation Grant CTS-9528598.
REFERENCES 1. Charalampopoulos, T. T., Prog. Energy Combust. Sci. 18:13–45 (1992). 2. Kasper, G., Shon, S.-N., and Shaw, D. T., J. Am. Ind. Hyg. Assoc. 41:288–296 (1980). 3. Kasper, G. and Shaw, D. T., Aerosol Sci. Technol. 2:369–381 (1983). 4. Fu, T. H., Cheng, M. T., Wong, F. C. H., and Shaw, D. T., Aerosol Sci. Technol. 15:19–29 (1991). 5. Cheng, M. T., Xie, G. W., and Shaw, D. T., Aerosol Sci. Technol. 14:74–81 (1991). 6. Jacobs, I. S. and Bean, C. P., Physical Review 100:1060–1067 (1955). 7. Green, H. L. and Lane, W. R., Particulate Clouds: Dust, Smokes and Mists, Wiley, New York, 1964, p. 33. 8. Zebel, G., in Aerosol Science, Vol. 1 (Davies, C. N., Ed.), Wiley, New York, 1966, p. 45. 9. Howard, J. B., Twelfth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1954, pp. 877–887. 10. Wersborg, B. L., Howard, J. B., and Williams, G. C., Fourteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1954, pp. 929–940. 11. Whitby, K. T. and Liu, Y. H., in Aerosol Science, Vol. 1 (Davies, C. D., Ed.), Wiley, New York, 1966, p. 83. 12. Dobbins, R. A. and Megaridis, C. M., Langmuir 3:254–266 (1987). 13. Rosner, D. E. and Mackowski, D. W., Combust. Sci. Technol. 80:87–101 (1991). 14. Zhang, Z., Waguespack, G., and Charalampopoulos, T. T., submitted to Rev. Sci. Instrum. (1996).
1858
MATERIALS SYNTHESIS
15. CRC Handbook of Chemistry and Physics, Vol. 60, CRC Press Inc., Boca Raton, FL, 1979, p. E121. 16. Niel, L., Rev. Modern Phys. 25:58–63 (1953). 17. Kerker, M., The Scattering of Light and Other Electromagnetic Radiation, Academic Press, New York, 1969, p. 356. 18. Alkemade, C. T. J. and Zeegers, P. J. T., Wiley-Interscience 9:3–125 (1972).
19. Gorden S. and McBride, J., NASA SP-273, 1976, p. 1. 20. Zhang, Z., M.S. Thesis, Mech. Eng. Dept., Louisiana State University, 1995. 21. Morrish, A. H. and Yu, S. P., J. Appl. Phys. 26:1049– 1055 (1955). 22. Beischer, D. and A. Winkel, Naturwissenschaften 26:27–420 (1937).
CONTROLLED COMBUSTION SYNTHESIS OF NANOSIZED IRON OXIDE AGGREGATES Z. ZHANG and T. T. CHARALAMPOPOULOS Mechanical Engineering Department Louisiana State University Baton Rouge, LA 70803, USA
Nanosized chainlike aggregates were synthesized in an iron pentacarbonyl–carbon monoxide–air diffusion flame system. The size and shape of the chainlike aggregates and primary particles were characterized in terms of measurable morphological parameters such as diameter of primary particles and aspect ratio. The influences of operating conditions on the parameters have been investigated systematically by employing thermophoretic sampling and transmission electron microscopy (TEM). The morphological parameters were found to be strong functions of operating conditions, residence time, and locations within the flame and could be controlled by adjusting the concentration of Fe(CO)5 seeded in the flame through the variation of carrier gas flow rate and/or temperature of additive. X-ray diffraction measurements coupled with thermodynamic equilibrium composition predictions were used to identify the chemical states of the particulate components. The results indicated that the aggregates formed in this flame consisted predominantly of primary particles of Fe2O3. In addition, the physical and chemical mechanisms of formation of chainlike aggregates are reviewed. Our observations suggest that, while the magnetic forces are necessary for providing a self-alignment mechanism, the flame temperature may also be critical for the formation of chainlike aggregates.
Introduction Knowledge of the shape and size of aggregates formed in flames is significant in many areas of research and practical applications. Information about the size and shape of both isotropic and anisotropic particulates is important (1) for predicting growth and oxidation of particulates in the combustion systems, (2) for identifying the agglomeration mechanisms of particulates, (3) for efficient control of fibrous aerosols in the industrial hygiene, (4) for quality control of materials synthesized in flame reactors, and (5) for air pollution–control applications [1]. On the other hand, the study of the iron oxide and its properties under formation conditions is particularly useful in ferrofluids applications. Several studies have been carried out for determining the morphological features of aggregates formed in combustion processes; see Refs. 1–5 and references therein. A number of studies have dealt specifically with the mechanisms of formation of chainlike aggregates in combustion processes [6– 11]. The formation of chainlike iron oxide aggregates in a CO-air diffusion flame was studied more recently [2–5], employing both ex situ and in situ techniques. The characteristics of the chain aggregates were determined for samples taken in the postflame region of the flame at different concentrations of the seeding vapor and variable O2:CO mixing ratio. However, the effects of operating conditions on the
morphology variation within the flame were not addressed. In addition, the physical and chemical mechanisms governing the formation of chainlike aggregates remain unclear [6]. Thus, the objectives of the present study were (a) to synthesize chainlike aggregates in an iron pentacarbonyl vapor seeded CO/air diffusion flame, (b) to identify the most critical operating parameters among temperature, feeding flow rate, and feeding ratio of the metal additive to fuel for the dominant generation of chainlike aggregates, (c) to characterize the primary particle size and aspect ratio of the aggregates as a function of residence time and of additive concentration, (d) to determine the chemical compositions of the particulates from experimental measurement and theoretical simulation, and (e) to investigate the mechanism for the formation of chainlike aggregates both from physical and chemical perspective.
Experimental The experimental system consists of a diffusiontype carbon monoxide–air flame centered around a concentric stainless steel tube. Regulated fuel, carbon monoxide (CO), is supplied through the inner tube (1/4 inch in diameter). The metal additive, iron pentacarbonyl vapor Fe(CO)5, is introduced into the CO-air flame by diverting a small fraction of bulk
1851
1852
MATERIALS SYNTHESIS
Fig. 1. Radial temperatures for CO-air diffusion flame as function of position above the burner surface.
fuel flow through a column of liquid Fe(CO)5 stored in a temperature-regulated stainless steel cylinder. At the outlet of the cylinder, partially saturated vapor is diluted by main fuel flow and directed to the burner for combustion. The concentration of iron pentacarbonyl vapor introduced into the flame may be controlled by adjusting either the cylinder temperature or the carrier gas flow rate. The system is calibrated so that the actual mass flow rate of iron pentacarbonyl vapor delivered to the fuel, corresponding to different carrier flow rates and cylinder temperature, is known before it enters the flame with an accuracy of 53.6%. The particulates produced in the flame were extracted through a thermophoretic sampling system, which eliminates the potential postsampling agglomeration of particles [12,13] and were deposited on a 200-mesh copper grid coated with carbon film that is stable under electron beam and can withstand relatively higher temperature conditions. The thermophoretic sampling system consists of a double-acting pneumatic cylinder, a solenoid-controlled directional four-way valve, and a computer-controlled variable time-delay relay circuit. A hall effect switch mounted on the pneumatic cylinder is used to calibrate the position and motion of the piston/ probe assembly. A piston speed of 0.86 m/s and a transition time 4 5 1 ms for the entry of the probe into the flame were achieved with the present system. The vertical position of the probe is fixed, whereas the burner system can be translated in the vertical direction. A pulse generator attached to the gear of the burner translation mechanism monitors the vertical displacement of the burner so that the height of the probe with respect to the burner surface can be controlled to within 1/10 of a millimeter. A slotted plate is placed between the flame and probe to deflect the air current induced by the motion of the probe and to block the heat transfer from the flame to the probe. A platinum versus platinum–
10% rhodium (S type) thermocouple with a bead diameter of 0.3 mm was used to measure the flame temperature at various axial and radial locations. Figure 1 shows the radiation-corrected radial temperature profiles of the unseeded CO/air diffusion flame at various heights above the burner surface. Each point on these curves represents an average value of three measurements. A maximum deviation of 2.7% from the average value occurred at a radial position of 4 mm and a height of 35 mm above the burner surface. A detailed description of the experimental system, its components, control, and calibration procedures are presented in Ref. 14. Results and Discussion Morphology Characterization Experiments for the synthesis of chainlike aggregates and the determination of size and morphology of primary particles and chainlike aggregates were performed. Chainlike aggregates were produced in the CO-air diffusion flame seeded with Fe(CO)5 vapor. The configurations of the flame under unseeded and seeded conditions are shown in Figs. 2a and 2b. Seeding the flame with Fe(CO)5 causes bright yellow emissions, which is thermal radiation from the iron oxide particles formed within the flame. The unseeded flame is a predominantly blue flame with a distinct conical white flame front. Yellow regions occurring in the upper portions of the unseeded flame may be attributed to formerly deposited solids in the honeycomb stabilizer of the flame. Nevertheless, the occurrence of such emissions was very sporadic and thus could not influence the measurements of the results in any way. The size and shape of the chainlike aggregates and primary particles were characterized in terms of the diameter of primary particles and aspect ratio. The
COMBUSTION SYNTHESIS OF CHAIN-LIKE AGGREGATES
Fig. 2. (a) CO-air diffusion flame. (b) Flame seeded with Fe(CO)5 vapor.
sampled particles and aggregates were grouped and analyzed statistically employing TEM technique so that quantitative results on the primary particle size distribution and aggregate size distribution as a function of position in the flame for various concentrations of Fe(CO)5 were obtained. Typical structures of primary particles and aggregates are shown in the TEM photograph (see Fig. 3). The image of the iron oxide primary particles appears to be hexagonal or
1853
polyhedric, which is consistent with the microstructure of Fe2O3 [15]. On the other hand, the aggregates exhibit for the most part straight-chain configuration. The variation of the morphological parameters of iron oxide aggregates with concentrations of iron pentacarbonyl vapor seeded in the flame was investigated by adjusting the cylinder temperature and the carrier flow rate. The first set of experiments was performed by fixing a total fuel flow rate 450 cc/min and a carrier flow rate 20 cc/min and by setting the cylinder temperatures at 238, 358, 458, and 558C, respectively. A higher cylinder temperature corresponds to a higher mass flow rate of Fe(CO)5 vapor, resulting in a higher concentration of additive in the flame. Under the operating conditions, the frequency distributions of diameters of the primary particles and aspect ratio of chainlike aggregates for samples collected from the center of the flame at 35 mm above the burner surface are presented in Fig. 4. The differential frequency distributions of diameter and aspect ratio are roughly bell-shaped in the linear plot. As cylinder temperature increases, the frequency distributions shift toward larger values of diameter and aspect ratio. It should be noted that at cylinder temperature 458 and 558C, branched chainlike aggregates were observed. Note that an alternate way to vary the concentration of Fe(CO)5 vapor seeded into the flame is by adjusting the carrier gas flow rate. The experiments were also performed at a fixed cylinder temperature of 238C. The carrier gas flow rates were 15, 20, 25, and 35 cc/min, corresponding to 0.5, 0.9, 1.5, and 2.6% of Fe(CO)5 vapor to fuel by weight for a total fuel flow rate of 450 cc/min. Analysis of the results for samples collected from the center of the flame at a height of 25 mm above the burner surface for distributions of diameters and aspect ratios yielded the same trends as those discussed previously. These trends are consistent with the expectation that increasing both cylinder temperature and carrier gas flow rate in turn increased the concentration of seed-
Fig. 3. TEM photographs.
1854
MATERIALS SYNTHESIS
Fig. 4. Frequency distributions of measured diameter (a) and aspect ratio (b) along with ZOLD predictions (Eqs. [1]–[3]) as functions of heated cylinder temperature.
ing vapor. Higher concentrations of Fe(CO)5 vapor result in increased concentration of primary particles. Therefore, the collision probabilities between particles for particle growth and interparticle aggregation are also expected to increase, yielding larger primary particles and longer chainlike aggregates. The variations of morphological parameters with residence time were assessed by collecting samples within the flame along the centerline at heights of 4, 15, 25, and 35 mm above the burner surface under operating conditions of a total fuel flow rate of 450 cc/min and a carrier flow rate of 25 cc/min with evaporator cylinder temperature maintained at 238C. The distributions of the sizes of primary particles and aggregates are shown in Fig. 5. At lower positions in the flame, all primary particles possess smaller diameters and the distribution is narrower. As the residence time increases, the particles grow, while the distribution becomes broader as a result of the continuous formation of new primary particles and subsequent aggregation. Because of the mag-
Fig. 5. Frequency distributions of measured diameter (a) and aspect ratio (b) along with ZOLD predictions (Eqs. [1]–3]) as functions of height above the burner surface.
netic nature of Fe2O3 [15], chainlike aggregates were observed at short residence times (about 4 mm above the burner surface). However, the ill-defined aggregate shapes precluded any reliable quantification of the aspect ratio distribution in the range of 4–15 mm above the burner surface. It should also be noted that when the flame temperature at 4 mm above the burner surface was above the Curie point of Fe2O3 ('950 K) [16], no chains were observed throughout the flame. On the other hand, for positions higher than 15 mm above the burner surface, more well-defined chainlike structures were observed. The experimental data were curve fitted using the method of least squares to a zero-order logarithmic distribution (ZOLD) function [1], which is defined as
1
exp 1 P(a) 4
r2o 2
2
1
1lnaa 2
2
m
2
exp 1 (1) amro(2p)1/2 2r2o The parameter am is the modal value of a, which
COMBUSTION SYNTHESIS OF CHAIN-LIKE AGGREGATES
1855
TABLE 1 Calculated average diameter, aspect ratio, and standard deviation using ZOLD function with variable additive concentration and height above the burner surface Variable Temperature
Variable Carrier Flow Rate
Variable Height
T (8C)
d (nm)
l/d
r
Q (cc/min)
d (nm)
l/d
r
H (mm)
d (nm)
l/d
r
23 35 45 55
57.4 60.97 61.31 62.16
7.08 8.16 8.63 9.09
4.0 6.0 6.1 6.2
15 20 25 35
45.89 51.03 55.13 59.56
4.93 5.52 — 6.38
2.6 2.6 — 2.4
4 15 25 35
21.7 45.61 54.76 60.52
— 4.17 5.93 6.75
— 2.5 3.2 2.4
represents d in the case of particle diameter distribution and (l/d) in the case of aspect ratio distribution. The parameter ro is a measure of the width and skewness of the distribution. The calculated ZOLD curves based on the inferred am and ro values from the least-squares analyses are also shown, along with the experimental data as determined by sampling and TEM analyses. The parameters am and ro are related to the mean value through the equation [17]: lna 4 lnam ` 1.5r2o
(2)
whereas the standard deviation is given by r 4 am(exp(4r2o) 1 exp(3r2o)1/2)
(3)
The mean diameters of primary particles, mean aspect ratio of chainlike aggregates, and standard deviations were calculated for all operating and sampling conditions mentioned previously, and are summarized in Table 1. The results revealed a definite increase in both the mean primary particle diameter and the mean aspect ratio with increasing height above the burner surface and with increasing concentration of Fe(CO)5 vapor seeded into the flame. Note that the largest increase of mean values was encountered at lower positions in the flame (short residence times) and lower concentrations of the additive. The experimental data of aspect ratio distributions show excellent agreement with the ZOLD function representation, whereas the diameter distributions, for the most part, follow closely the ZOLD function with maximum deviations occurring at the lower and upper portions of the distributions. The somewhat large values of r exhibited by all distributions may be attributed to any one or combination of the following reasons: (a) The flame is of diffusion type and continuous formation of small particles and aggregates is possible at both radial and axial locations of the flame. (b) The particles formed in the reaction zone, within which the particle volume fraction is high, may be transported to the central portions of the flame through thermophoresis since temperature gradients exist in radial direction (approximately
750 K/mm). (c) The thermophoretic probe could have been contaminated at high particle volume fraction locations during the insertion and/or extraction of the sampling assembly. Chemical Composition Although knowledge of chemical states of particle components under flame condition is important for complete understanding of physical and chemical mechanisms through which the chainlike aggregates are formed, it was not possible, at this time, to identify the chemical composition of combustion products under in situ conditions. Powder X-ray diffraction (XRD) has been proven to be an important standard tool for identification of material compositions. As part of the study, the chemical composition of the particulates collected from Fe(CO)5-seeded flame was identified using Xray diffraction. The measured X-ray diffraction pattern and characteristic peaks from database (International Unit for Diffraction Data, Powder Diffraction File) are presented in Fig. 6. It was found that the composition of the particulates is Fe2O3 with two different microstructures: hematite and maghemite. The sample used for X-ray testing was collected at 35 mm above the burner surface. The equipment used was SIEMENS D5000 X-ray diffraction machine, which has a resolution on the order of 0.5% by weight. It also distinguishes coating material with thickness on the order of 5 nm or even lower, provided that the atoms of mixture are known. However, the shortcoming of the approach may be that the samples could not be isolated entirely from air during the sampling and transportation. Thus, particle oxidation might have taken place before and during the measurement. Nevertheless, since it is generally thought that local thermodynamic equilibrium exists at various points throughout a flame [18], the thermodynamic equilibrium calculations of chemical composition were carried out to predict the distributions of possible chemical species in this flame. Specifically, the computer code developed by
1856
MATERIALS SYNTHESIS
Fig. 6. Measured powder X-ray diffraction pattern for particulates collected from Fe(CO)5 seeded CO-air diffusion flame at 35 mm above the burner surface. Blue lines: hematite; green lines: maghemite. Wavelength used was 1.5406 A˚.
Gordon and McBride [19] was employed for the prediction of chemical compositions of particles formed in the Fe(CO)5-seeded CO-air diffusion flame. Chemical equilibrium compositions of carbon monoxide flames seeded with 1.5% iron pentacarbonyl by weight were calculated for a range of fuel equivalence ratios and over a range of flame temperatures. The predicted fractional distribution of iron species as a function of temperature has been presented in Ref. 20 and demonstrates that nonsignificant ionic iron exists (whereas the dominant ironcontaining species is Fe2O3 within the flame temperature range studied). It should be noted that the equilibrium calculations are from thermodynamic viewpoint, which gives no indication of the kinetics of such reactions. Moreover, there may be a number of potential pathways through which iron or iron oxides may be formed and a number of factors, such as fuel-equivalence ratio, temperature, concentration of iron additive, flame configuration, type of fuel, and/or other combustion operating conditions, by which the chemical states of iron species may be affected. Thus, the precise chemical composition of all pertinent reaction products may not be precisely predictable at this time. Mechanism of Chain Formation As noted earlier, a few studies have addressed specifically the formation of chainlike aggregates. It may be argued that most flame-generated aggregates are
formed from thermal coagulation, leading to randomly structured aggregates [9]. However, under certain conditions and specific types of materials, the random motion may be ordered by other forces such as electrostatic and/or magnetic to form well-defined, straight chainlike aggregates. So far, chainlike aggregates have been observed at later stages of the formation or postflame region, and it is believed that the origin of their formation was the result of electrostatic interactions [6–11]. In this study, chainlike aggregates were observed at short residence times (approximately 4 mm above the burner surface) for temperatures lower than about 950 K, which is considered the Curie point for Fe2O3 [16]. Since Fe2O3 is magnetic material, this observation suggests that the magnetic properties may play a relatively more important role during the particle interactions and chain formation. Nevertheless, as will be explained in the section that follows, other system parameters such as flame temperature may play a role in the formation. The magnetic properties of Fe2O3 are complex in that its fundamental antiferromagnetism superimposed by a weak parasitic ferromagnetism results most likely from defects of cystallization. When the iron-oxide particle sizes are below 3 lm with an aspect ratio of 10, they are magnetized as single-domain particles in the presence of a magnetic field [21]. The mean diameter of particles produced in this flame ranges from 20 to 65 nm, or even smaller at early stages of formation; thus, they possess a net magnetic moment. If each particle is treated as a
COMBUSTION SYNTHESIS OF CHAIN-LIKE AGGREGATES
dipole of magnetic moment, apparently the magnetic dipole interactions between particles are stronger than thermal interactions in this flame particle system such that the particles are aligned by the magnetic forces through a self-alignment mechanism. This implies that the most stable configuration of three magnetic dipoles energetically is a chain, and the end of the chain is the energetically most favorable position for an approaching fourth particle to be attached, provided there is no mechanism for deforming the chain. Still, as the flame temperature increases, thermal interactions may prevail and provide a mechanism to deform the chain. Thus, aligned chains may no longer constitute the most stable configuration. Instead, either branched chains are formed or the chains break up to form clusters, depending on the relationship of combustion temperature and Curie temperature of the particles. It is noteworthy that in the present study, when the flame became turbulent, only clusterlike aggregates were observed. Currently, the effects of magnetic forces or dipole moments are not understood well enough to permit a quantitatively precise assessment. Clearly, further studies are required to elucidate the interactions of magnetic and temperature effects and their influence on the morphology and dynamics of the aggregates. Work is underway in this laboratory to study the effects of magnetic fields and temperature on particle interaction/growth. Summary The results of the present study may be summarized as follows: a. The CO-air diffusion flame seeded with Fe(CO)5 vapor was tested and dominantly chainlike aggregates were produced. The size and shape of chainlike aggregates and primary particles were characterized in terms of measurable morphological parameters under different operating conditions. The average diameter of primary particles and average aspect ratio of chainlike aggregates can be controlled by varying the concentration of iron pentacarbonyl vapor seeded into the flame. b. For the concentration range investigated, namely 0.75–2.8% iron pentacarbonyl vapor to fuel by weight, the average diameter of primary particles and aspect ratio increased by about 30% at 25 mm above the burner surface. c. The average diameter of primary particles increased by 8.0% and the average aspect ratio by 28% as the temperature of the heated cylinder increased from 23 to 558C. The total increase of average diameter was about 179% from 4 to 35 mm and the aspect ratio 62% from 15 to 35 mm. d. Higher concentrations and residence times favor the formation of longer chainlike aggregates. The
1857
preceding observations were repeated with identical results with maximum deviation of 5% from the mean values by performing the same measurements. The data, statistically analyzed for the average 1000 particles and 500 aggregates from each run, are expected to provide representative picture of the particulate morphology of iron oxide aggregates formed in this type of flame reactor. e. Both X-ray diffraction measurements and thermodynamic equilibrium calculations showed that the chemical composition of the particles formed in the flame is consistent with Fe2O3. In addition, our observations indicated that, while magnetic forces are essential for providing a self-alignment mechanism, the flame temperature may also be critical for the formation of chainlike aggregates. Acknowledgment This research was supported by the Air Force Office of Scientific Research Grant F49620-0477 and the National Science Foundation Grant CTS-9528598.
REFERENCES 1. Charalampopoulos, T. T., Prog. Energy Combust. Sci. 18:13–45 (1992). 2. Kasper, G., Shon, S.-N., and Shaw, D. T., J. Am. Ind. Hyg. Assoc. 41:288–296 (1980). 3. Kasper, G. and Shaw, D. T., Aerosol Sci. Technol. 2:369–381 (1983). 4. Fu, T. H., Cheng, M. T., Wong, F. C. H., and Shaw, D. T., Aerosol Sci. Technol. 15:19–29 (1991). 5. Cheng, M. T., Xie, G. W., and Shaw, D. T., Aerosol Sci. Technol. 14:74–81 (1991). 6. Jacobs, I. S. and Bean, C. P., Physical Review 100:1060–1067 (1955). 7. Green, H. L. and Lane, W. R., Particulate Clouds: Dust, Smokes and Mists, Wiley, New York, 1964, p. 33. 8. Zebel, G., in Aerosol Science, Vol. 1 (Davies, C. N., Ed.), Wiley, New York, 1966, p. 45. 9. Howard, J. B., Twelfth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1954, pp. 877–887. 10. Wersborg, B. L., Howard, J. B., and Williams, G. C., Fourteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1954, pp. 929–940. 11. Whitby, K. T. and Liu, Y. H., in Aerosol Science, Vol. 1 (Davies, C. D., Ed.), Wiley, New York, 1966, p. 83. 12. Dobbins, R. A. and Megaridis, C. M., Langmuir 3:254–266 (1987). 13. Rosner, D. E. and Mackowski, D. W., Combust. Sci. Technol. 80:87–101 (1991). 14. Zhang, Z., Waguespack, G., and Charalampopoulos, T. T., submitted to Rev. Sci. Instrum. (1996).
1858
MATERIALS SYNTHESIS
15. CRC Handbook of Chemistry and Physics, Vol. 60, CRC Press Inc., Boca Raton, FL, 1979, p. E121. 16. Niel, L., Rev. Modern Phys. 25:58–63 (1953). 17. Kerker, M., The Scattering of Light and Other Electromagnetic Radiation, Academic Press, New York, 1969, p. 356. 18. Alkemade, C. T. J. and Zeegers, P. J. T., Wiley-Interscience 9:3–125 (1972).
19. Gorden S. and McBride, J., NASA SP-273, 1976, p. 1. 20. Zhang, Z., M.S. Thesis, Mech. Eng. Dept., Louisiana State University, 1995. 21. Morrish, A. H. and Yu, S. P., J. Appl. Phys. 26:1049– 1055 (1955). 22. Beischer, D. and A. Winkel, Naturwissenschaften 26:27–420 (1937).