High-Pressure Liquid Dispersion and Fragmentation of Flame-Made

The influence of primary particle diameter and the degree of agglomeration of ... Flame technology is the main gas-phase manufacturing route for inorg...
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Langmuir 2006, 22, 4928-4935

Articles High-Pressure Liquid Dispersion and Fragmentation of Flame-Made Silica Agglomerates Robert Wengeler,† Alexandra Teleki,‡ Markus Vetter,†,‡ Sotiris E. Pratsinis,‡ and Hermann Nirschl*,† Institute for Mechanical Process Engineering and Mechanics, UniVersita¨t Karlsruhe (TH), D-76128 Karlsruhe, Germany, and Particle Technology Laboratory, Department of Mechanical and Process Engineering, ETH Zu¨rich, Sonneggstrasse 3, CH-8092 Zurich, Switzerland ReceiVed December 5, 2005. In Final Form: March 16, 2006 The influence of primary particle diameter and the degree of agglomeration of flame-made silica agglomerate suspensions in aqueous solutions is studied by high-pressure dispersion (up to 1500 bar) through a nozzle with a 125 µm inner diameter. These particles were produced (4-15 g/h) by oxidation of hexamethyldisiloxane (HMDSO) in a coflow diffusion flame reactor. Their average primary particle size (10-50 nm) and degree of agglomeration were controlled by varying the oxygen and precursor flow rates. The particles were characterized by nitrogen adsorption, electron microscopy, and small-angle X-ray scattering. Hydrodynamic stresses break up soft agglomerates and yield hard agglomerate sizes in the range of 100-180 nm, as characterized by dynamic light scattering. Soft agglomerates exhibited decreasing light scattering diameters with increasing dispersion pressure, while hard agglomerates were insensitive.

1. Introduction Nanoparticles constitute the disperse phase in nanocomposites improving the optical, mechanical, thermal, or handling properties of coatings or bulk materials such as dental materials.1 Synthesis of nanocomposites often requires redispersion of dry particles in an aqueous or monomer solution.2,3 Nanocomposite properties such as elastic modulus4 strongly depend on nanoparticle characteristics such as chemical composition, particle size and distribution, and their degree of agglomeration.5 Flame technology is the main gas-phase manufacturing route for inorganic nanoparticles6 but often leads to agglomerated ones (e.g., fumed silica and alumina). These particles may be held together physically by van der Waals forces (soft agglomerates) or by stronger chemical or sintering bonds (hard agglomerates).7 The degree of agglomeration strongly affects particle performance and depends on process conditions, particularly, temperature profile and residence time.8 At low temperatures, particle agglomeration takes place typically by physical bonding such as van der Waals forces. Yet, high temperatures result in the coagulation of colliding particles, forming bigger particles, which therefore reduces the particle number density.9 As temperature * Corresponding author. Address: Universita¨t Karlsruhe (TH), 76128 Karlsruhe, Germany. Phone: +49 (721) 608 2400. Fax: +49 (721) 608 2405. E-mail: [email protected]. † Universita ¨ t Karlsruhe (TH). ‡ ETH Zu ¨ rich. (1) Nirschl, H. AdV. Eng. Mater. 2003, 5, 1. (2) Nussbaumer, R. J.; Caseri, W. R.; Smith, P.; Tervoort, T. Macromol. Mater. Eng. 2003, 288, 44. (3) Kickelbick, G. Prog. Polym. Sci. 2003, 28, 83. (4) Mueller, R.; Kammler, H. K.; Pratsinis, S. E.; Vital, A.; Beaucage, G.; Burtscher, P. Powder Technol. 2004, 140, 40. (5) Nirschl, H.; Wengeler, R.; Schaefer, B. Proceedings of the 18th AFS Annual Conference, Atlanta, GA, 2005. (6) Wegner, K.; Pratsinis, S. E. AIChE J. 2003, 49, 1667. (7) Tsantilis, S.; Pratsinis, S. E. Langmuir 2004, 20, 5933. (8) Pratsinis, S. E. Prog. Energy Combust. Sci. 1998, 24, 197.

decreases, particles do not fully coalesce upon collision but form chemical bonding by sinter necks.9 At selected high-temperature residence times, even nonagglomerated particles without sinter necks are formed.8 Yet, physically and chemically bonded agglomerates can only be characterized indirectly from the scattering pattern during flame synthesis.10 For small-angle X-ray scattering (SAXS) interpretation, fractal agglomerates are considered to be hard agglomerates, whereas soft agglomerates are considered to show no fractal regime in the scattering pattern.11,12 Yet, for fractal agglomerates, no distinction of bond strength can be obtained from scattering experiments. Mathematical modeling allows the distinction between hard and soft agglomerates in the aerosol phase by modeling the temperature profile and particle growth dynamics by coagulation and sintering. Froeschke et al.13 impacted agglomerates of nanoscale primary particles on transmission electron microscopy (TEM) grids and measured the strength of particle bonding. A wide distribution of bond strength was determined by the degree of fragmentation enclosing an energy range of several decades. Liquid dispersion of agglomerates requires the exertion of high stresses on the constituent particles.14,15 Stirring at high speed often does not suffice, as physical bonding is strong enough (9) Friedlander, S. K. Smoke, Dust, and Haze: Fundamentals of Aerosol Dynamics, 2nd ed.; Oxford University Press: Oxford, 2000. (10) Beaucage, G.; Kammler, H. K.; Mueller, R.; Strobel, R.; Agashe, N.; Pratsinis, S. E.; Narayanan, T. Nat. Mater. 2004, 3, 370. (11) Hyeon-Lee, J.; Beaucage, G.; Pratsinis, S. E.; Vemury, S. Langmuir 1998, 14, 5751. (12) Kammler, H. K.; Beaucage, G.; Mueller, R.; Pratsinis, S. E. Langmuir 2004, 20, 1915. (13) Froeschke, S.; Weber, A. P.; Kasper, G.; Salas-Vicente, J.; Lopez-Gejo, J.; Bossmann, S. H.; Braun, A. M. Proceedings of PARTEC 2004, Nuremberg, Germany, 2004. (14) Mueller, F.; Peukert, W.; Polke, R.; Stenger, F. Int. J. Miner. Process. 2004, 74S, S31. (15) Pohl, M.; Hogekamp, S.; Hoffmann, N. Q.; Schuchmann, H. P. Chem.Ing.-Tech. 2004, 76, 392.

10.1021/la053283n CCC: $33.50 © 2006 American Chemical Society Published on Web 04/18/2006

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Figure 1. High-pressure dispersion apparatus. The suspension from the reservoir passes from a barrier chamber through the dispersion nozzle with a pressure drop of up to 1500 bar, where suspended particles are fragmented upon expansion.

to withstand imposed stresses.15 Therefore, higher stresses need to be applied, which is commonly done by ball mills, colloid mills, and supersonic14,15 or high-pressure dispersion systems.16,17 A high-pressure dispersion apparatus for abrasive dispersions has been developed. It is used to apply very high stresses on the particles, thus it is able to break up physical bonds as well as weak sinter necks. For the distinction of soft and hard agglomerate sizes, a high-pressure dispersion process provides reproducibly stable suspensions,15,17 where a perikinetic reagglomeration by attractive interparticle forces is eliminated. This study investigates high-pressure dispersion and fragmentation of flame-made agglomerates in liquids. Here, silica particles are produced in a coflow diffusion flame reactor.4 The particle generation is controlled by the reactant flow rates leading to samples with different particle sizes and degrees of agglomeration, which are characterized by nitrogen adsorption and TEM. Redispersion in an aqueous solution is conducted by highpressure dispersion. The resulting liquid suspensions are stabilized and characterized by dynamic light scattering (DLS) and SAXS, exploring the possibility of distinguishing between hard and soft agglomerates by their liquid dispersion at high pressures. 2. Experimental Section 2.1. Flame Synthesis and Calcination. The setup for flame synthesis of SiO2 is described in detail elsewhere.18 Hexamethyldisiloxane (HMDSO; Fluka, purity > 99%), vapor-laden argon carrier gas (0.3 L/min), and methane (0.5 L/min) are delivered through the inner center tube of the diffusion burner, while oxygen (2-10 L/min) is delivered through the second annulus (all gases: Pan Gas, purity > 99.999%). Nitrogen (0.5 L/min) is added between the two streams through the first annulus to slightly lift the flame from the burner, preventing particle deposition.6 Silica production rates range from 4.8 to 14.4 g/h. The temperature of all gas lines and the HMDSO evaporator is set to 75 °C. The nanoparticles are collected on a glass fiber filter by a vacuum pump (Vacuubrand RZ 16). Particles produced at a 2 L/min oxygen flow rate were calcined after particle collection in an oven (Carbolite, CWF 1300) at 400, 600, and 800 °C for 4 h. 2.2. High-Pressure Dispersion. A high-pressure dispersion apparatus is developed for the batch processing of abrasive dispersions (Figure 1). The high pressure is generated by a pneumatic-hydraulic pressure intensifier (Maximator, DU 3440), which transforms (16) Pohl, M.; Schubert, H.; Schuchmann, H. P. Chem.-Ing.-Tech. 2005, 77, 258. (17) Wengeler, R.; Nirschl, H. Proceedings of the 2nd Symposium on “Produktgestaltung in der Partikeltechnologie”, Pfinztal, Germany, 2004; Fraunhofer-IRB-Verlag: Stuttgart, Germany, 2004. (18) Teleki, A.; Pratsinis, S. E.; Wegner, K.; Jossen, R.; Krumeich, F. J. Mater. Res. 2005, 20, 1336.

Figure 2. Primary particle size of flame-made silica at 4.8 g/h (obtained from the SSA measured by nitrogen adsorption for the dry powder and in liquid dispersion by SAXS) as a function of the oxygen flow rate in the burner.

pneumatic pressures in the range of 0-10 bar by a ratio of 1:185 with a maximum hydraulic pressure of 1500 bar. From the suspension reservoir, a sample of 70 mL of dispersion is delivered through valve V1, and the high-pressure intensifier is moved into its upper position with control valve V0. Then the suspension is compressed by applying high pressure. Inside a pressure reservoir (barrier chamber), hydraulic pressure is measured. The applied pressure is reduced slightly when valve V2 is opened, and the suspension is dispersed by passing through the dispersion nozzle. The resulting pressure is calibrated with the applied compression load. Valve V3 enables one to degasify and flush the high-pressure unit with demineralized water or an aqueous solution. The barrier chamber is an important feature of the presented dispersion apparatus. Commercial high-pressure homogenizers are well suited for polymer nanoparticles or emulsions.19,20 Yet, metal oxide nanoparticles are much more abrasive than organic materials. Therefore, preventing liquid dispersion from flowing into the pressure intensifier is important. The dispersion and a pure liquid solution are separated by an aluminum piston. This cylindrical device is movable and sealed in a cylindrical high-pressure reservoir. For high-pressure dispersion, numerous dispersion devices are proposed.21 Here, a straight dispersion nozzle with a simple flow pattern is chosen. The nozzle is made by a zirconia capillary (Kyocera) with an inner diameter dC of 125 µm and length lC of 10 mm. Suspensions are prepared by weighing the particles into a beaker and adding an aqueous solution of tetrasodium pyrophosphate as the stabilizing agent. This suspension is homogenized for 15 min using a magnetic stirrer. The stirring yields the break-up of large flocs in the millimeter range, consisting of entangled agglomerates. Stirring also prevents particles from sedimenting before dispersion, especially for the strongly agglomerated ones. 2.3. Particle and Dispersion Characterization. The specific surface area (SSA) of dry particles is determined by nitrogen adsorption at 77 K using the Brunauer-Emmett-Teller (BET) method (Micromeritics TriStar 3000). The corresponding equivalent particle diameter is dBET ) 6/(Fp SSA), where the density of SiO2 is Fp ) 2.2 g/cm3. The SSA of the particles in liquid dispersion is measured by SAXS (ID02 beamline at European Synchrotron Radiation Facility, Grenoble, France22). The scattered intensity I(q), is measured as a function of scattering wave vector q ) (4π/λ) sin(φ/2), where λ is (19) Mu¨ller, R. H.; Bo¨hm, B. H. L.; Grau, M. J. Pharm. Ind. 1991, 61, 74. (20) Schlu¨cker, E.; Johl, H. Pharma+Food 2000, 1, 28. (21) Schultz, S.; Wagner, G.; Ulrich, J. Chem.-Ing.-Tech. 2002, 74, 901.

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Figure 3. Silica particles made at 4.8 g/h and oxygen flow rates of (a) 2 L/min, (b) 4 L/min, (c) 7 L/min, and (d) 10 L/min and with the reference materials (e) Aerosil 200 and (f) Aerosil OX50. the wavelength of incident radiation and φ is the scattering angle. A wide q range (0.02 nm-1 < q < 2 nm-1) is covered using different sample-to-detector distances (2 and 10 m). The two-dimensional SAXS patterns are recorded with an image-intensified CCD camera.22 The standard data treatment involves various detector corrections and normalizations described in detail elsewhere.22 The resulting normalized two-dimensional images are azimuthally averaged to obtain I(q). The absolute intensity of particle scattering is calculated from sample, empty capillary, and pure liquid scattering curves. The scattering invariant Q, defined as Q ) ∫∞o q2‚I(q)dq ) 2π2‚∆F2‚VP, is proportional to the total particle volume Vp in the scattering sample,

where ∆F2 is the electron density difference between the silica and the aqueous solution. The Porod constant, defined as

(22) Narayanan, T.; Diat, O.; Boesecke, P. Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 467, 1005.

The particle size, dBET, directly agrees with the diameter calculated from S/V by SAXS.12

CP ) lim I(q)‚q4 ) 2π‚∆F2‚SP qf∞

accounts for the total particle surface area SP.23 Therefore, the SSA is calculated from22 SSA )

SP π CP ) ‚ VP‚FP FP Q

(1)

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Agglomerate diameters based on the Brownian motion of the particles in suspension are determined by DLS (Beckmann Coulter, N4 Plus). An autocorrelation function of the resulting scattering intensity fluctuations correlate with the mean diffusion coefficient from which a diffusion equivalent particle diameter (dDLS) is calculated from the Stokes-Einstein equation: dDLS )

kBT 3πηD

(2)

TEM (CM30ST microscope, FEI (Eindhoven), LaB6 cathode, operated at 300 kV, SuperTwin lens, point resolution: ∼2 Å) is used also. For TEM analysis, the product powders are deposited onto a holey carbon foil supported on a copper grid. To assess the colloidal stability of the suspension, the electrostatic interaction potentials are obtained by measuring the ζ potential with a Pen Kem 501 (Collotec Messtechnik GmbH). This measures the electrophoretic movement of particles in the stationary layer of electroosmotic liquid flow in a microchannel. The mean electrophoretic velocity corresponds to the ζ potential of the particles.

3. Particle Synthesis and Calcination Figure 2 shows that the primary particle diameter of silica decreases from 47 to about 13 nm as the oxygen flow rate is increased from 2 to 10 L/min, at a production rate of 4.8 g/h, consistent with the results of Mueller et al.4 These primary particle diameters obtained from the SSA measurement of dry powders by nitrogen adsorption and their SAXS in liquid dispersion agree within 5-15%. Aerosil 200 and OX50 by Degussa are used as reference materials in Figure 3. Aerosil 200 is pyrogenic silica with a primary particle size of 11 nm, as measured by BET. The primary particles are strongly sintered and form large fractal agglomerates. Aerosil OX50 is also pyrogenic silica, with low agglomeration and a primary particle size of 44 nm. Figure 3 shows TEM images of the particles generated at different oxygen flow rates. Particles produced at 2 L/min of oxygen form spherical primary particles and only a few agglomerates. These results agree with those of Mueller et al.,4 who observed nonagglomerated particles by SAXS and TEM also made at low oxygen flow rates (2.5-4.7 L/min). The particle size distribution is wide, with sizes ranging from 50 to 100 nm (Figure 3a). Also, a few very small agglomerated particles with particle sizes as small as 10 nm exist. At higher oxygen flow rates, the primary particles are smaller (Figure 3d) and exhibit strong neckings between them, resulting in hard agglomerates, as confirmed by SAXS here and by Mueller et al.4 These assemble in large flocs as the powders are prepared for TEM. For intermediate oxygen flow rates, some necking between smaller primary particles made at 4 (Figure 3b) and 7 L/min (Figure 3c) is shown. The samples show a wide particle size distribution, which is caused by the radial temperature profile and different particle streamlines in the flame. The reference product, Aerosil 200 (Figure 3e), exhibits strongly agglomerated primary particles with a homogeneous primary particle size of about 10 nm. It is comparable to the silica made here at high oxygen flow rates (Figure 3d). Aerosil OX50 (Figure 3f) consists of weakly agglomerated particles that are mostly nonspherical, with particle sizes in the range of 50 nm. The temperature profile of the flame supported by 2 L/min O2 exhibits a long region of maximum temperature (∼2700 °C), as shown by Teleki et al.18 Thus, the particle residence time in the high-temperature region is sufficient for coagulation and the formation of larger spherical primary particles (Figure 3a). With (23) Glatter, O.; Kratky, O. Small-Angle X-ray Scattering; Academic Press: London, 1982.

Figure 4. Silicas produced at (a) 7.2, (b) 12, and (c) 14.4 g/h production rates and at an oxygen flow rate of 2 L/min. The primary particle size increases, but all particles stay nonagglomerated.

increasing oxygen flow rate, the high-temperature region is reduced drastically.4 Thus, the particle residence time in the region of high temperatures is shortened, and the cooling rate is increased; therefore, the product particles are smaller, and complete coagulation-coalescence is inhibited (Figure 3d). Increasing the precursor (HMDSO) flow rate from 3.25 to 16.25 g/h increases dBET from 47.5 to 94.7 nm, at an oxygen flow rate of 2 L/min. Figure 4 shows the corresponding TEM images. Clearly, the primary particle size increases with increasing precursor flow rate, yet the degree of agglomeration does not change. All TEM pictures show nonagglomerated spherical

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Figure 6. Characteristic capillary Re for measurements at different ∆p and corresponding values of the turbulent pipe flow model (Re at ∆p e 200 from the study of Wengeler et al.27). ∆p is correlated with mean hydrodynamic stresses on the interparticle bonds.

smaller particles seem to start developing the first sinter necks at 600 (Figure 5b) and 800 °C (Figure 5c). Also, former studies predict the coalescence and formation of sinter bridges only for temperatures above 1000 °C.25

4. Liquid Dispersion and Fragmentation 4.1. High-Pressure Process Characteristics. Agglomerates are broken up in laminar and turbulent flows inside and after the capillary. The capillary generates an elongational flow at the inlet that changes, forming a turbulent jet at the outlet where high Reynolds stresses are imposed on the particles. The flow characteristics are assessed by measuring the time for one batch of water passing the dispersion nozzle (Figure 1) at different pressure drops. The viscosity of these low-concentration (0.001 v/v) suspensions is slightly higher than that of water (1.04 mPa‚s at 20 °C). From the volume flow through the capillary, the Reynolds numbers (Re) are calculated and plotted against the pressure drop (∆p) (Figure 6). The high Re shows that the capillary flow is highly turbulent. The data follow well the pressure drop in turbulent pipes:26

l V2 ∆p ) λ Fl dC 2

(3)

with λ being the friction factor according to the Blasius resistance formula26 Figure 5. Silicas (Figure 3a) made at 4.8 g/h and a 2 L/min O2 flow rate and calcined in air at (a) 400, (b) 600, and (c) 800 °C for 4 h.

primary particles since no sinter necks are visible, so the particles form mostly soft agglomerates. At rising precursor flow rates, the maximum temperature increases by a reduced velocity difference in the gas streams, which results in slower mixing.24 The SSA and corresponding primary particle diameters do not change significantly by calcination: the as-prepared silica has a dBET of 47.5 nm, which is unchanged by calcination varying between 48 (after sintering at 600 °C) and 50 nm (400 °C), where differences might be caused by sampling. Correspondingly, the TEM images (Figure 5a) show no particle coalescence, yet the (24) Wegner, K.; Pratsinis, S. E. Chem. Eng. Sci. 2003, 58, 4581.

λ ) 0.3614Re-1/4

(4)

Agglomerates are broken up during high-pressure dispersion by strong hydrodynamic stresses from turbulent flow (Figure 6). Then the Reynolds stresses arising from velocity fluctuations disrupt the agglomerate structure.28 For agglomerates in the size range of the smallest eddies, the velocity fluctuations lead to (25) Sanders, R. Entwicklung einer Apparatur zur Untersuchung von Sintervorga¨ngen an Gasgetragenen Nanopartikeln bei Temperaturen bis 1500 °C. Ph.D. Thesis, TU Clausthal, Clausthal, Germany, 2004. (26) Bird, R. B.; Steward, W. E.; Lightfoot, E. N. Transport Phenomena, 2nd ed.; Wiley: New York, 2002. (27) Wengeler, R.; Ruslim, F.; Nirschl, H.; Merkel, T. Chem.-Ing.-Tech. 2004, 76, 659. (28) Cherry, R. S.; Kwon, K.-Y. Biotechnol. Bioeng. 1990, 36, 563.

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laminar shear stresses, which break up the agglomerates. The mean hydrodynamic stresses in a turbulent pipe flow are29

0.5‚dC 1 τF ) ∆p 2 lC

(5)

corresponding to a turbulent length scale κ of the smallest eddies:29

κ)

ν

xτF/Fl

(6)

which results in κ ) 40 nm for ∆p ) 1000 bar (dC ) 0.125 mm, lC ) 10 mm, Fl ) 1000 kg/m3). The agglomerate primary particle sizes here are in the same size range and smaller than 10‚κ,28 so the agglomerates are stressed by laminar fluid stresses. According to Taylor,30,31 the stress transferred from the fluid, τF, onto the surface of spheres in laminar shear flow results in fluctuating stress fields inside the sphere due to its rotation, which correspond to particle stresses τP of

τP ) 2.5·τF

(7)

Therefore, the mean stress exerted on the interparticulate bonding can be deduced from the pressure drop across the nozzle (Figure 6, right ordinate). Two silica dispersions, one highly sintered (hard agglomerates made at 4.8 g/h and an O2 flow rate of 10 L/min) and the other nonagglomerated (soft agglomerates with a production rate of 4.8 g/h and an O2 flow rate of 2 l/min), have been passed 1-10 times through the nozzle at ∆p ) 1000 bar. Although the initial agglomerate diameter is reduced, multiple passes through the dispersion nozzle do not further reduce the agglomerate size for ∆p ) 200-1000 bar. Of course, the larger the ∆p, the smaller the agglomerate size that it is reached in the first passage. Therefore, the bonds that are weaker than the stresses acting at a given pressure drop are broken after a single passage through the dispersion nozzle, leading to an agglomerate diameter that only can be reduced further by exerting higher stresses. 4.2. Colloidal Stability. The dispersion process can be divided into three sections, namely, wetting, agglomerate break-up, and stabilization. Flame-made particles contain OH groups on their surface32 so that wetting can be achieved simply by stirring the suspension. Its stability depends on the interaction potential between particles, which is obtained by superposition of van der Waals, electrostatic, and Bornian interaction33 potentials. van der Waals as well as Bornian interactions are constant for a given material. The electrostatic interaction can be quantified by the ζ potential, which is a measure of the surface charge density.33 Low ζ potentials (|ζ| < 40 mV) represent unstable suspensions, whereas high potentials (|ζ| > 60 mV) represent stable ones. The suspension stability can be improved by adding surface-active chemical species such as tensides or electrolytes. In Figure 7 the silica colloidal stability (depending on the concentration of tetrasodium pyrophosphate Na4P2O7) has been investigated in terms of the ζ potential and the agglomerate size. The ζ potential of the silica particles is -56.5 mV at a volume concentration of 0.1% in demineralized water without stabilizing (29) Pope, S. B. Turbulent Flows; Cambridge University Press: Cambridge, UK, 2000. (30) Taylor, G. I. Proc. R. Soc. London, Ser. A 1932, 138, 41. (31) Taylor, G. I. Proc. R. Soc. London, Ser. A 1934, 146, 501. (32) Mueller, R.; Kammler, H. K.; Wegner, K.; Pratsinis, S. E. Langmuir 2003, 19, 160. (33) Isrealachvili, J. Intermolecular and Surface Forces; Academic Press: London, 1997.

Figure 7. Zeta potential, |ζ|, and agglomerate diameter as a function of the concentration of the stabilizing agent. |ζ| increases with the addition of Na4P2O7, enhancing the colloidal stability and therefore reducing the agglomerate size.

agent, indicating a rather stable suspension. Yet, the addition of small amounts of stabilizing agent already reduces the agglomerate diameter significantly. For tetrasodium pyrophosphate concentrations of 0.5 mM or above, the suspension is stable with a ζ potential, |ζ|, between 68.0 and 71.3 mV, and the agglomerate size cannot be reduced significantly. For industrial applications besides the stability during processing, the shelf life of the suspension is also important. Therefore, two suspensions were studied over a period of two months. Before particle size measurements, the suspension is stirred slightly to homogenize the suspension since it exhibits a particle concentration gradient toward the bottom of the vessel. However, no sediment has built up. The agglomerate size does not change within the accuracy of the measurements ranging from 210.6 to 211.9 nm for the nonagglomerated SiO2 produced at 4.8 g/h and an oxygen flow rate of 2 L/min. Likewise, the agglomerate size changes from 130.4 to 134.6 nm for the agglomerated SiO2 made with an oxygen flow rate of 10 L/min. Therefore, even a longterm stability of the suspension with respect to the agglomerate size could be shown. As a result, it can be inferred that stabilization with Na-pyrophosphate enables the preparation of stable suspensions that do not change within the time needed for measurements, and their reproducibility does not exceed 2% variation. All of the following experiments were prepared with a 1 mM tetra sodium pyrophosphate solution of deionized water, with a concentration of 0.001 v/v silica, and were dispersed in a single passage. 4.3. Influence of the Degree of Agglomeration. Figure 8 shows the agglomerate size after high-pressure dispersion as a function of the applied pressure drop (∆p) for silicas made at different oxygen flow rates (Figure 2) and the two Aerosil products. The agglomerate size is reduced significantly with the strongest impact from 0 to 200 bar, where agglomerate diameters are reduced by 25-50%. An increase in ∆p from 200 to 1000 bar leads to a further agglomerate size reduction by 10-20%. In the high-pressure range, all curves level off, approaching a finite value. The size of Aerosil 200 and that of agglomerates produced at high oxygen flow rates (4-10 L/min) reach a different but constant value for pressures above 1000 bar. Given the high suspension stability, where reagglomeration of the particles after their

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Figure 8. Agglomerate diameter after dispersion as a function of the applied pressure drop for silica particles produced at 4.8 g/h at different oxygen flow rates. For comparison, the dispersion behaviors of the commercial silicas Aerosil 200 and Aerosil OX50 are shown also.

dispersion can be neglected (Figure 7), this indicates that a limiting, hard agglomerate size has been reached by high-pressure dispersion and fragmentation. Yet, an increase in the pressure drop from 200 to 1000 bar and in the stresses exerted on the bonds from 0.5 × 106 to 2 × 106 Pa, reduces the agglomerate size by 25 nm. The particles that exhibit a low degree of agglomeration (produced at a low oxygen flow rate of 2 L/min), such as Aerosil OX50, do not reach an asymptotic size, indicating that they are composed of soft agglomerates of smaller particles, as increasing pressure can possibly mechanically break further physically bonded particles. These results confirm TEM observations (Figure 3) where Aerosil 200 as well as the agglomerates produced at high oxygen flow rates show high degrees of agglomeration, whereas Aerosil OX50 as well as the silica produced at low oxygen flow rates are weakly agglomerated. The initial agglomerate size increases for increasing primary particle sizes. Figure 8 shows that the largest agglomerates, 277 and 284 nm in size, are obtained at oxygen flow rates of 2 and 4 L/min. Yet, the agglomerate size of Aerosil 200 (dBET ) 11 nm) is equal to the materials obtained here with primary particles of 47 nm, demonstrating the high degree of agglomeration with strong sinter necks between primary particles that cannot be dispersed. Therefore, the Aerosil 200 agglomerates consist of a large number of primary particles, whereas the flame-made silica agglomerates made here consist of fewer primary particles. The strong impact of dispersion, even for the lowest pressure drop of 200 bar, shows that physical bonds by particle-particle interaction due to attractive van der Waals forces are broken. The break-up of physically bonded soft agglomerates leads to hard agglomerates in which primary particles are bonded by sinter necks. These chemical bonds have bond energies that can be 104 times higher than those for van der Waals bonding in the aerosol.13 The break-up of some sinter necks leads to a further agglomerate size reduction at high-pressure drops. The progress in dispersing agglomerates depends on the fraction of particle bonds that can be broken when a certain stress is applied on the particles. Figure 8 clearly shows that, at a low

Wengeler et al.

Figure 9. Diameter of primary particles (circles) and agglomerates of flame-made silicas before (triangles) and after (squares) dispersion at 1400 bar at 4.8 g/h for different oxygen flow rates.

oxygen flow rate, the fraction of bonds that are broken by pressures between 200 and 1400 bar is significantly lower than that for those agglomerates made at higher oxygen flow rates. Yet, even an increase in pressure drop from 1000 to 1400 bar leads to a further break-up of correspondingly strong bonds. At higher oxygen flow rates, a relevant fraction of weak sinter necks develops, which is disrupted by pressure drops of 200-1000 bar, and a fraction of the strong sinter bridges withstands even the highest stresses applied and cannot be broken. Compared to the primary particle size, some necks exist, even for low oxygen flow rates (Figure 9) because dBET cannot be reached by dispersion, as the van der Waals forces between the primary particles are strong. For a silica production rate of 4.8 g/h, the initial agglomerate size decreases with an increasing oxygen flow from dDLS ) 188.0 nm to 136.2 nm (Figure 9). The agglomerate diameter of the stirred product before dispersion and break-up cannot be measured with the same accuracy as the other diameters due to very large agglomerates. The ratio of agglomerates to primary particle diameter increases from 4 to 10 upon increasing the synthesis oxygen flow rate from 2 to 10 L/min, further indicating the high degree of agglomeration of flame-made SiO2 at high oxygen flow rates. 4.4. Influence of the Production Rate. Figure 10 shows that the initial agglomerate diameter increases with increasing SiO2 production rate, as has been seen by Kammler and Pratsinis.34 The corresponding agglomerate size at a dispersion pressure drop of 1400 bar ranges from dDLS ) 188.0 to 259.2 nm at production rates of 4.8 and 12 g/h (Figure 10). Dispersion breakup leads to a constant agglomerate size reduction of about 27%. The pressure dependency of the agglomerate size shows no leveling off at high pressures, indicating weakly (soft) agglomerated particles such as Aerosil OX50. The agglomerate diameters of all of the powders are higher than the primary particle diameter. Yet, the ratio between the agglomerate diameter and the primary particle size decreases from 4 to 2.7 for production rates of 4.8-14.4 g/h. This indicates the low degree of agglomeration of all powders prepared at low oxygen flow rates, regardless of production rate. 4.5. Influence of Calcination. The dispersion of calcined particles made at 4.8 g/h with a 2 L/min oxygen flow rate reveals (34) Kammler, H. K.; Pratsinis, S. E. J. Nanopart. Res. 1999, 1, 467.

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correspondingly, high stresses), the additional sinter necks formed at 400 °C could be broken, as these data (400 °C) converge to as-prepared powders at ∆p ) 800 bar. At higher temperatures (T g 600 °C), calcination leads to sinter necks that cannot be broken by the applied high-pressure dispersion. Therefore, the agglomerate sizes increase compared to those of the as-prepared powders constantly at all ∆p. Particles calcined at 800 °C do not disperse as easily by stirring the suspension (∆p ) 0), and the mean particle size after stirring could not be measured. This study confirms that some sinter necks can be broken by hydrodynamic forces. The strength of the additionally formed sinter necks after calcining at 400 °C can therefore be determined. The critical bond stress equals the mean hydrodynamic stress exerted on the bonds, as calculated by eq 7, being τp ) 1.5 × 106 Pa.

5. Conclusion

Figure 10. Diameter of primary particles (circles) and agglomerates of flame-made silicas before (triangles) and after (squares) dispersion at 1400 bar for different production rates using a 2 L/min oxygen flow rate.

Figure 11. Impact of calcination temperatures on silica particles made at 4.8 g/h and a 2 L/min oxygen flow rate on the agglomerate size after high-pressure dispersion. Low calcination temperatures lead to the formation of sinter necks, which are broken at ∆p ) 1000 bar.

that some sinter necks have been formed, even at the lowest temperature of 400 °C. These result in an increase in the agglomerate size at low dispersion pressure drops from 212 to 222 nm at ∆p ) 200 bar (Figure 11). The dispersion shows a different effect for sinter necks developed at 400 °C compared to those developed at higher temperatures. At high ∆p (and,

The impact of flame production parameters such as oxygen flow rate and production rate on product SiO2 particle sizes determines the particle and agglomerate size in a liquid dispersion. Soft and hard agglomerates can be distinguished by highpressure dispersion. Soft agglomerates with physical bonding break up by high hydrodynamic stresses. Thus, hard agglomerate diameters of flame-synthesized particles could be measured by liquid dispersion. Yet, even for nonagglomerated particles, where no particle bonds can be distinguished by TEM, the agglomerate sizes measured by DLS do not coincide with the primary particle sizes measured by nitrogen adsorption and SAXS, indicative of the presence of strong physical bonds between primary particles. For silicas with a high degree of agglomeration, such as Aerosil 200 and the one produced at high oxygen flow rates, a limiting hard agglomerate size is reached, whereas products with a low degree of agglomeration, such as Aerosil OX50 and the products produced at low oxygen flow rates, lead to further reduction of the agglomerates, even at high-pressure drops, indicative that soft (physical) bonds continue to break up upon further particle stressing. Additional investigations on the calcination of nanoparticles show that, at low temperatures, the weak sinter necks could be broken by high-pressure dispersion at high-pressure drops across the nozzle, and the corresponding critical bond strength can be determined. At higher temperatures, the sinter necks withstand the high-pressure dispersion, and the agglomerate size rises by a factor that is constant for all applied pressures. Acknowledgment. The authors thank Dr. Frank Krumeich at the Laboratory for Inorganic Chemistry, ETH Zurich (Switzerland), for the preparation of TEM images. The use of the ESRF beamline ID02 was possible through the support of the experiments ME 628 and ME 1179. Financial support by the Swiss National Science Foundation (No. 200021-100325) and the priority program of the Land Baden-Wu¨rttemberg is gratefully acknowledged. LA053283N