DEMONSTRATION OF GAS-PHASE COMBUSTION SYNTHESIS OF NANOSIZED PARTICLES USING A HYBRID BURNER

Pergamon NanoStructured Materials, Vol. 11, No. 7, pp. 955 964, 1999 Elsevier Science Ltd Copyright 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$ see front matter PII S0965-9773(99)00376-1 DEMONSTRATION OF GAS-PHASE COMBUSTION SYNTHESIS OF NANOSIZED PARTICLES USING A HYBRID BURNER Margaret S. Wooldridge, Stephen A. Danczyk and Jianfan Wu Department of Mechanical Engineering and Applied Mechanics, University of Michigan, Ann Arbor, MI 48109-2125 Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123 (Received June 1, 1999) (Accepted July 27, 1999) Abstract A new approach for gas-phase combustion synthesis of nanosized particles using a novel hybrid burner facility is demonstrated. The basis of the synthesis technique is to use both a premixed flame and a diffusion flame to control the synthesis environment. Specific experimental results for silica (SiO 2 ) production from silane/hydrogen/ oxygen/argon (SiH 4 /H 2 /O 2 /Ar) flames are presented. A parametric study of several burner conditions was conducted, and the subsequent effects on the particles produced were determined. Particle morphology was examined using transmission electron microscopy (TEM). The results indicated a broad variation in particle size and structure as a function of the burner operating conditions (in particular, reactant stoichiometry and flame geometry). Particle structures were aggregated with primary particles varying from 6 8 nm in size (high oxygen concentration conditions) to larger more continuous structures with primary particles 18 20 nm in size (low oxygen concentration conditions). Bulk material properties were examined using Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), nitrogen adsorption (BET) and x-ray diffraction (XRD). 1999 Acta Metallurgica Inc. Introduction Gas-phase combustion synthesis is an important methodology for the production of nanosized particles; however, the fundamental mechanisms governing the formation and growth of particles in a flame are not well known. Key concerns regarding the powders produced by combustion techniques include product purity and morphology (i.e. particle size, shape, polydispersity and extent of aggregation) (1,2,3). Previous investigations have yielded results that indicate the parameters with the most significant effect on particle composition and microstructure are the particle residence time, the burner temperature field, and the initial reactant concentrations (see 1 3). In the current work, a novel hybrid burner arrangement is used to control these parameters and explore the potential to explicitly regulate the powder characteristics through the hybrid burner operating conditions. A schematic of the principle of operation of the hybrid burner is shown in Figure 1. The combustion synthesis facility is a hybrid between a diffusion flame and a premixed flame. The premixed flame used in the current work is a flat or one-dimensional flame with properties that are well characterized by laminar flame theory. Figure 1 shows typical calculated species and temperature profiles above the surface of a flat flame burner. Nanosized oxide particles are formed in a diffusion flame, which is generated by injecting gas-phase particle precursor reactants into the premixed flame system. By varying the injection location with respect to the surface of the flat flame burner, the diffusion jet entrains oxidizing gases of differing conditions and composition. In addition, the temperature profile and the species concentration profiles of the premixed flame system can be controlled 955

956 GAS-PHASE COMBUSTION PROCESSING OF NANOPARTICLES Vol. 11, No. 7 Figure 1. Schematic depicting the principle of operation for particle synthesis using a hybrid-burner. Composition and temperature profiles are shown for a stoichiometric H 2 /air premixed laminar flame at 1 atm and T o 298 K, after Glassman Combustion 1996 Academic Press (19). using diluents and different reactant stoichiometries. Mixing of the diffusion flame reactants and species from the premixed flame can also be affected by altering the velocity of the injected particle precursor reactants (e.g., a laminar versus a turbulent jet). In order to facilitate comparison with established particle synthesis methodologies, silica (SiO 2 ) production from silane (SiH 4 ) was chosen as the material system of interest. Flame synthesis of silica powder has been studied extensively (4,5), and modeling data are also available for comparison (6,7,8). The hybrid flame used for the study, shown schematically in Figure 2, consists of a premixed hydrogen/oxygen/argon flame with silane dilute in argon injected at the center of the H 2 /O 2 /Ar flame. The silane/argon mixture forms a diffusion flame where nanosized silica particles are produced. By controlling the location of the SiH 4 /Ar injection with respect to the burner surface, and the absolute and relative reactant flow rates of the primary and secondary flames, the key parameters controlling particle formation can be varied, and the result on the powders produced can be examined. In the current work, details of the hybrid flame facility are described and the effects of several burner operating conditions on powder properties are presented. The microscopic material properties including particle size and morphology are examined using transmission electron microscopy (TEM). Bulk material properties are examined using Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), nitrogen adsorption (BET) and x-ray diffraction (XRD). From the results of the experimental investigation, insight into the gas-phase combustion synthesis of nanosized particles and the silane combustion system is possible. Experimental Apparatus A schematic of the experimental facility used in the study is shown in Figure 2. The facility consisted of the combustion synthesis hybrid burner and a particle sampling system. Hydrogen (99.995%), oxygen (99.995%), and argon (99.995%) were mixed in a plenum region before flowing through the

Vol. 11, No. 7 GAS-PHASE COMBUSTION PROCESSING OF NANOPARTICLES 957 Figure 2. Experimental schematic of the hybrid-burner combustion synthesis facility. stainless steel, porous plug of a flat flame burner (McKenna Products). The H 2 /O 2 /Ar mixture formed a flat flame (the primary flame) across the cylindrical surface of the burner (60 mm in diameter). An annulus coflow of nitrogen (5 mm in width) was used to shroud the flame and minimize entrainment of room air. The burner was customized to allow axial injection of particle precursor reactants via a port (6.4 mm diameter) at the center of the burner. The particle producing flame (the secondary flame) was generated by injecting SiH 4 (Matheson semiconductor grade, 99.995%) dilute in Ar through a stainlesssteel tube (0.8 mm inner diameter, 1.6 mm outer diameter) placed at the center of the port. The injection tube height (h j ) relative to the burner surface could be varied to control the exit conditions encountered by the SiH 4 /Ar mixture. The area between the injection tube and the burner port was sealed. All reactant flow rates were controlled by calibrated rotometers. The particles produced by the hybrid flame burner were sampled for ex-situ analysis. The samples were obtained directly from the diffusion flame using a rapid probe insertion technique, a wellestablished technique for obtaining particles representative of the sampling location in combustion systems (9). Particles were deposited by thermophoresis onto a cooled TEM grid ( 3 mm diameter, copper gilder grids, 1000 mesh) placed at the end of a probe controlled by a pneumatic cylinder. The cooled grid served to chemically quench the particles, thus preventing further reaction from occurring after deposition. Grids without carbon films were used in order to prevent possible contamination of the particles by carbon containing species. The particle sampling apparatus was affixed to a vertical translation stage to allow sampling at a range of locations above the surface of the burner. The evolution of particle morphology throughout the secondary flame could therefore be examined. Particles were also sampled by deposition onto an aluminum plate placed in the exhaust region of the burner. Bulk powder analysis was conducted using the material accumulated on the plate. The plate was placed in the exhaust region of the secondary flame to ensure the particles were fully reacted, and indicative of particles that would be collected in a large-scale production combustion synthesis facility.

958 GAS-PHASE COMBUSTION PROCESSING OF NANOPARTICLES Vol. 11, No. 7 TABLE 1 Experimental Conditions Flow rates [ml/min] Case Injection height [mm] h j Premixed flame conditions (primary flame) Diffusion flame conditions (secondary flame) Shroud flow H 2 O 2 Ar SiH 4 Ar N 2 A 1.6 4580 5610 1920 29.3 903 4000 B 1.6 4580 1700 1920 29.3 903 4000 C 6.6 4580 5610 1920 29.3 903 4000 D 6.6 4580 1700 1920 29.3 903 4000 Results and Discussion Four burner conditions (cases A D, listed in Table 1) were investigated in the current study. The conditions correspond to fuel rich and fuel lean conditions and to two injection heights h j (1.6 mm and 6.6 mm). Here, fuel refers to both the silane and the hydrogen present in the two flames. All the gas flows were maintained at fixed flow rates throughout the study with the exception of the oxygen flow rate, which was altered to create the fuel rich and fuel lean conditions. For the fuel lean conditions (cases A and C), there is sufficient O 2 to fully oxidize both the H 2 from the primary flame and the SiH 4 from the secondary flame according to the overall stoichiometric reactions H 2 1 2O 2 3 H 2 O(g) SiH 4 2O 2 3 SiO 2 (s) 2H 2 O(g) with surplus O 2 in the exhaust gases. The fuel rich conditions (cases B and D) were intentionally chosen such that there was insufficient O 2 to fully oxidize either of the two fuels. The fuel rich conditions may lead to incomplete oxidation of the silicon and formation of Si(s) or SiO(s). Part of the motivation for choosing the fuel rich conditions was to investigate the possible formation of these products. In all the cases studied, the velocity and concentrations of the precursor flame reactants were held constant, resulting in a slightly transitional jet (Re 1800). Jet break up began at approximately 25 mm above the surface of the burner. Microscopic Material Analysis Particles were sampled from the diffusion flame for TEM analysis at a range of axial locations (H 6 31 mm for the lean flames and H 6 41 mm for the rich flames, where H is the distance above the surface of the burner, see Figure 2). Particles were also sampled from the exhaust region of the secondary flame. Figure 3 shows typical micrograph images of samples obtained from the diffusion flame for the low injection height (h j 1.6 mm) and fuel rich conditions (case B). Images from H 26 mm, H 36 mm and from the exhaust region (H 100 mm) are shown in Figures 3a, 3b, and 3c, respectively. All images show aggregate particle morphology with primary particles 18 20 nm in size. The size of the primary particles and the degree of aggregation do not change significantly as a function of location in the secondary flame, as Figure 3 indicates. As will be shown below for the other experimental conditions, the particles formed in the secondary flame do not have sufficient time at high

Vol. 11, No. 7 GAS-PHASE COMBUSTION PROCESSING OF NANOPARTICLES 959 Figure 3. Transmission electron micrographs of particles sampled from the flame and the exhaust region for fuel rich conditions and a low injection height (case B): (3a) H 26 mm, (3b) H 36 mm, (3c) final product (H 100 mm). (The black regions in the images are grid bars.) temperature conditions for collapse of aggregates into larger spherical shapes to occur. No aggregates were present at heights H 11 mm for the case B conditions. Figure 4 shows typical micrograph images of samples obtained from the diffusion flame using the low injection height (h j 1.6 mm) and fuel lean conditions (case A). Images from H 11 mm, H 26 mm and from the exhaust region (H 100 mm) are shown in Figures 4a, 4b, and 4c, respectively. The primary particles formed using fuel lean conditions are markedly smaller, 6 8 nm in size, than those formed using fuel rich conditions. No aggregates were observed below H 11 mm for the case A conditions. Micrograph images obtained from samples using the high injection heights yielded similar trends to those shown in Figures 3 and 4. The fuel rich particles were in the form of larger aggregates with more Figure 4. Transmission electron micrographs of particles sampled from the flame and the exhaust region for fuel lean conditions and a low injection height (case A): (4a) H 11 mm, (4b) H 26 mm, (4c) final product (H 100 mm). (The black regions in the images are grid bars.)

960 GAS-PHASE COMBUSTION PROCESSING OF NANOPARTICLES Vol. 11, No. 7 continuous structures in comparison to the fuel lean particles. However, the average primary particle size was slightly larger (by 2 nm) than those obtained from the corresponding low injection height conditions. For all four of the flame conditions studied, the average size of the primary particles remained constant or decreased slightly at increasing distances from the burner surface. In other words, particles sampled from the diffusion flame were quite representative of the final product formed. The results for particle morphology are consistent with the theory that homogeneous nucleation and agglomeration should be the dominant particle formation and growth mechanisms for oxygen rich conditions. Hence, conditions A and C lead to branchy aggregates with small constituent particles. For oxygen poor conditions (or fuel rich conditions), the particle agglomeration rate should be reduced due to the decrease in the number of primary particles and effects resulting from sintering and/or surface growth should be more apparent. Hence, conditions B and D lead to larger aggregates with less distinctly identifiable primary particles. Electron energy loss spectroscopy (EELS) was also used to examine the elemental composition of the particles (i.e. to determine Si:O ratios). Details of the analysis are provided in Dufner et al. (10). Element maps yielded a surprising result. Significant carbon signatures were obtained in addition to silicon and oxygen signatures. There were no carbon containing species in the flame and TEM grids without carbon films were used to obtain the particle samples. Based on the carbon maps, the location of the carbon appeared to be on the surface of the particles. As a consequence, carbon dioxide adsorption onto the surface of the particles was proposed as a possible source of C. Discrete EELS measurements made at individual locations on particles confirmed the presence of carbon. Unfortunately, the presence of the carbon (and the potential source of additional oxygen) precluded accurate determination of Si:O ratios. Bulk Material Analysis The powders collected on the accumulation plate varied in color from white (fuel lean conditions, A and C) to light brown or tan (fuel rich conditions B and D). The white powders had a fine texture and the tan powders had a more coarse texture. Powders produced using identical reactant conditions yet different injection heights were similar in appearance. Using a counterflow SiH 4 /H 2 /O 2 /N 2 diffusion flame, Chung et al. (11) observed similar changes in the powder color as a function of flame stoichiometry. Low SiH 4 concentrations produced white particles, and high SiH 4 concentrations produced brown particles. Their examination of the particles (by x-ray fluorescence and solubility in hydrofluoric acid) led to the interpretation that the brown powders included SiO(s) and Si(s) particles. X-ray diffraction analysis was used in the current study to determine if crystalline phases were present in the powders produced by the hybrid burner (although none were expected). The results indicated only amorphous material. The XRD patterns were similar for powders produced using either case A or B burner conditions. The results are consistent with those obtained for commercial fumed silica particles (12). The powders produced using the four hybrid burner conditions were also analyzed by FTIR spectroscopy. The particle samples were pressed into KBr pellets, and the infrared absorption spectra were acquired over a wavenumber range of 400 4000 cm 1. The spectra for the fuel lean and fuel rich powders were similar regardless of injection height, therefore Figure 5 shows representative results obtained for samples produced using conditions B and C. Based on characteristic frequencies associated with various vibrational modes (13,14,15), the peaks shown in Figure 5 are attributed to Si-O-Si bend (460 cm 1 ), Si-O-Si bend (800 cm 1 ), Si-O-Si stretch (1080 cm 1 ) and SiO-H stretch (3650 cm 1, bulk hydroxyl groups; 3750 cm 1, free hydroxyl groups). The features at 1630 cm 1 and 3450 cm 1 are due to physically adsorbed water molecules (14,16). The majority of the features are consistent

Vol. 11, No. 7 GAS-PHASE COMBUSTION PROCESSING OF NANOPARTICLES 961 Figure 5. FTIR spectra of as-produced powders (obtained using burner conditions B and C). The transmission intensities have been offset for visual clarity. between the two samples, with the exception of the shift of the peak at 940 cm 1 (case B) to 965 cm 1 (case C). These features may be due to non-bridging stretch of surface Si-O groups (950 cm 1 ) (14) or to Si-OH stretch of silanol groups (970 cm 1 ) (16,17). The spectra do not show indications of Si-H bonding, which would result in broad absorption at 2100 cm 1 (16). The FTIR spectra are similar to those obtained for SiO 2 produced by a premixed disilane/oxygen/nitrogen flame (18). The spectra are also more representative of silica gel produced by precipitation from aqueous solution than of SiO 2 produced by flame hydrolysis (pyrolysis of silicon tetrachloride, air and hydrogen in a turbulent premixed flame via SiCl 4 2H 2 O 2 3 SiO 2 (s) 4HCl). To determine the extent of surface adsorption, the powder samples were also examined via thermal gravimetric analysis. The results, shown in Figure 6, indicate 4 5% (by mass) adsorbed species for both powder samples studied (A and B). Additional powder samples were heated in an oven at 400 C for approximately two hours and then re-examined by FTIR spectroscopy. The resulting spectra for the case B and C powders are shown in Figure 7. As expected, the features associated with physically adsorbed water are significantly reduced in intensity for both samples. In addition, the features at 940 cm 1 and 965 cm 1 are absent, and the shoulder present on the as-produced case B spectra is no longer distinguishable. The loss of these features suggests the peaks were due to Si-OH vibration as heating of silica can lead to removal of surface hydroxyls via (16).

962 GAS-PHASE COMBUSTION PROCESSING OF NANOPARTICLES Vol. 11, No. 7 Figure 6. TGA time-histories for heating rates of 10 C/min of as-produced powders (obtained using burner conditions A and B). (This figure has been digitally reproduced from the original figures.) Figure 7. FTIR spectra of powders after heating at 400 C for 2 hours. Original powders were obtained using burner conditions B and C. The transmission intensities have been offset for visual clarity.

Vol. 11, No. 7 GAS-PHASE COMBUSTION PROCESSING OF NANOPARTICLES 963 TABLE 2 Powder Characteristics Reactant conditions Specific surface area [m 2 /g] Average pore diameter [nm] Average particle diameter* [nm] Fuel lean (Cases A and C) Fuel rich (Cases B and D) 224 380 9.4 5 9 66 110 5.5 19 31 *Average particle diameter as determined from specific surface area, an assumption of spherical particles and a density of SiO2 2.2 gm/cm 3. Weak features at 2350 cm 1 were also observed in some of the spectra obtained from the heated samples. The features are consistent with carbon dioxide adsorption, which supports the hypothesis that CO 2 is the source of the C atoms identified in the EELS analysis. Nitrogen adsorption (BET) was used to determine the average surface area of the powders produced. The results are summarized in Table 2. As expected based on the micrograph images, the fuel lean conditions lead to high specific surface areas. It is interesting to note that the average particle diameter determined from the specific surface area (see Table 2) is consistent with the corresponding micrograph images, even with the relatively poor assumption of spherically-shaped particles. In summary, the results of the materials analyses indicate production of amorphous silica with properties similar to those of silica gel with large surface areas, which are highly adsorptive. Current work includes examining additional material systems of interest, in particular materials for catalyst applications where these properties are especially desirable. Conclusions A novel combustion synthesis facility for producing nanosized particles from gas-phase precursor reactants has been demonstrated. The hybrid burner facility has been used to successfully show that the burner operating conditions can be used to significantly influence particle microstructure. The range of particle sizes and morphology observed also indicate a robust system for control of particle characteristics. In particular, reactant concentrations can be used in conjunction with precursor injection location for both coarse and fine control of particle morphology. Additional advantages associated with powder synthesis via the hybrid burner include continuous production, as opposed to batch production, operation at atmospheric pressures and the capability to use a diverse range of precursor reactants. The latter aspect is a benefit when pyrophoric or highly reactive precursor reactants, such as silane, are preferred. In addition, material analysis of the silica produced using the hybrid burner indicates potential application as an alternative to sol gel synthesis methods, and application for production of high surface area oxides. Acknowledgments The authors would like to thank Professor Kalyan Annamalai for his assitance with the combustion studies and Dr. Carl Dufner, Professor Abraham Clearfield and Professor Levi Thompson for their assistance with the materials analyses. The financial support of the National Science Foundation is gratefully acknowledged, Dr. Farley Fisher, Program Monitor.

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