Electrospray Flame Synthesis of Yttria-Stabilized Zirconia Nanoparticles

Article pubs.acs.org/IECR

Electrospray Flame Synthesis of Yttria-Stabilized Zirconia Nanoparticles Manfred Geier*,†,‡ and Terry Parker† †

Colorado School of Mines, Golden, Colorado 80401, United States ABSTRACT: In this study, the technological feasibility of an electrospray-based flame synthesis process for the production of thin films of nanosized metal oxide particulates was investigated. On the basis of published results on electro-hydrodynamic atomization, a spray system was designed, implemented, and tested. Initial tests focused on synthesis of yttria-stabilized zirconia (YSZ) particulates, and the results showed that aggregates with quasi-monodisperse primary particle sizes below 100 nm with cubic-fluorite crystal phase composition can be manufactured through this production route. These tests utilized organometallic solutions of Zr-n-propoxide and Y-2-methoxyethoxide in a mixture of n-propanol and 2-methoxyethanol, which were electrosprayed into the postflame flow of a multielement diffusion burner. Both theoretical analysis and experimental evidence indicate that controlled synthesis of particulate with narrow particle size distribution is feasible. The radial arrangement of electrospray emitters chosen for the initial experiments appears to be a promising spray configuration for further development.

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INTRODUCTION In a large number of industrial processes, transition- and rareearth metal oxide particulates are deposited on some appropriate substrate as thin layers of dispersed particulates to provide the functionality essential for high process efficiencies.1 In many catalytic processes and sensor and fuelcell technology, these layers consist of nanosized, single-phase crystallites to create a maximum number of active surface sites per unit mass. Clearly, it is desirable to deposit the crystallites on the substrate in a cost-efficient manner with minimum contamination of the environment. Manufacture of these powders via flame synthesis facilitates direct particle deposition without the need for time- and energy intensive wet-chemical process steps. It is therefore an attractive option for large-scale, nonbatch production of such films. Vapor-based synthesis is in general the preferred alternative as it ensures highest possible dispersion of precursor chemicals and therefore may produce the smallest possible particulate sizes in a limited collision environment. However, the precursor chemicals for transition- and rare-earth metal oxides are not sufficiently volatile at ambient conditions; they may decompose upon heating, and potentially precipitate as poorly dispersed macroscopic solids. For that reason, a system that introduces precursors either in liquid or solid form into the flame/ postflame environment with a very high dispersion is necessary for production of nanosized particles of these oxides via flame synthesis. An example of such a process is flame spray pyrolysis, which has been used extensively to produce metal oxide powders.2−5 While enabling relatively high production rates, flame spray pyrolysis relies on atomization methods that generate polydisperse droplet clouds.6 This can be problematic for flame spray pyrolysis of rare-earth and transition metals oxides and other material systems in which low-volatility precursor liquids are sprayed into flames with insufficiently high temperatures to fully convert the droplets to gas-phase monomers. In those systems, the amount of metal atoms in each droplet remains unchanged as the carrier solvent evaporates; that is, each single droplet acts as microreactor © 2013 American Chemical Society

producing a single particle after the carrier liquid has evaporated.7 On one hand, this has the advantage that the chemical composition of product metal-oxide particles is determined by the chemical composition of precursor solution. The product particulate composition is homogeneous if the constituents of the solutions are mixed at the molecular level. However, conversion of the polydisperse droplet clouds yields inevitably polydisperse particulates, sometimes with pronounced bimodality in the size distribution.8 The product particulates usually consist of spheroidal nanosized primary particles fused to fractal-like aggregates.6 Initially monodisperse droplet sizes are therefore desirable if the narrowest possible particulate size distributions are to be produced. The smallest particle sizes with minimum spread require (a) (nearly) monodisperse droplet clouds with nanosized droplets and (b) a flame synthesis system that ensures a low frequency of collisions between particles to minimize coagulational broadening of the size distribution. Electro-hydrodynamic (EHD) atomization is capable of generating monodisperse aerosols with much smaller droplets than other spray methods can produce.9 EHD atomization, first studied almost a century ago,10,11 relies upon a strong electric field to force the liquid column inside a capillary to form a cone and to emit a thin thread of liquid from the tip of that cone. For a range of combinations of flow rates and field strengths, which is specific to the atomized fluid, a stable cone-jet or a set of multiple jets is established,12 which are characterized by the break-up of liquid thread(s) into a cloud consisting of submicrometer, quasi-monodisperse droplets. The electric charges on the droplets result in repulsive forces that delay cloud coalescence, which, combined with the Rayleigh jet break-up into a quasi-monodisperse droplets, makes EHD Received: Revised: Accepted: Published: 16842

July 8, 2013 October 18, 2013 October 23, 2013 October 23, 2013 dx.doi.org/10.1021/ie4021478 | Ind. Eng. Chem. Res. 2013, 52, 16842−16850

Industrial & Engineering Chemistry Research

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size distribution (pulsed cone-jets, cone-ramified jets, and spindle mode), or are stable but yield large droplet sizes and/or sustain only small flow rates (dripping- and microdripping modes and simple jets).12,36 Most favorable for application in synthesis of monodisperse nanosized particulate is stable conejet atomization, which, for that reason, will be the mode assumed in the following analysis. For the theoretical analysis of an ESFS system, several simplifying assumptions are made. These include the assumption of solutions with high concentrations of dissolved precursor salts (for minimum solvent volumes) and high electric conductivities, negligible shielding of space charges produced by the emitted droplet cloud, independence of the sprays from individual emitters from each other, and equal flow rates in all emitters. With these assumptions, scaling laws, whose validity has been confirmed for a variety of liquids and wide conductivity range,37 have been derived. In general, the properties of aerosols produced in the conejet mode depend on the electrode configuration, applied voltage, voltage polarity, flow rate as well as properties of both sprayed liquid and gas into which the liquid is dispersed. For a stable cone-jet, field strength (i.e., voltage for a fixed configuration) and flow rate cannot be varied arbitrarily, but results of several experimental studies suggest that flow rates for this mode may be varied over about 2 orders of magnitude.15,25,37−39 For liquids with sufficiently high electrical conductivity, the structure of the jet becomes nearly independent of the electrostatic parameters (voltage and electrode configuration) once a stable cone has been established. For this case, the scaling laws by Fernández de la Mora38,40 and Gañań -Calvo37,39,41 provide relationships for droplet diameter D and emitted electric current I summarized below. The most influential parameters for both D and I are liquid flow rate Q, density ρ, dynamic viscosity μ, relative permittivity β, electric conductivity K, and the gas−liquid surface tension γ. Note that the needle diameter is irrelevant, although it may have some influence on the range of flow rates for which stable cone-jet sprays can be sustained.34,42 Variation of the field strength influences droplet sizes only to a small extent.38 Current and droplet size follow from dimensional analysis, with the dimensionless parameters β, δ = γε0/ρKQ, and δμ = (ρε0γ2/K)1/3/μ:37

atomization an attractive choice for the intented nanoparticle synthesis process. In stable cone-jet and multiple-jet modes, the droplet sizes decrease with decreasing liquid flow rate, increasing voltage and increasing liquid conductivity, independently of the diameter of the emitter capillary needle. Thus, smallest droplet diameters can be produced with highest liquid conductivities, and it is therefore a natural choice to generate the droplet cloud by electrospraying highly conductive, concentrated ionic solutions of organic or inorganic metal carrier salts. Unfortunately, the flow rates for generation of monodisperse aerosols in a controlled manner have to be kept exceptionally small, limiting the applicability of the process, where a single atomizer is used, to the manufacture of small quantities of powder with high specific surface area and size uniformity. Utilization of electrosprays in materials synthesis has been studied for fabrication of quantum dots and nanofibers13 and nanocrystalline ceramic powders.14−22 Other applications of electrosprays include deposition of thin uniform layers of paint, inkjet printing, insecticide delivery, and mass-spectroscopy.13,23 Recent studies have also explored utilization of EHD in the production of encapsulated droplets,24 targeted drug delivery,25 and for dispersion of liquid fuel in mesoscale combustors.26−31 Initial studies of EHD atomization in a combined process with flame synthesis have been reported by Ahn et al.32 and Oh et al.33 Both groups studied a system with axial injection of the precursor solution into a coannular H2/O2 diffusion flame32 or a CH4/air premixed flame.33 The synthesis process investigated by Ahn et al. uses Fe(CO)5, wheras Oh et al.33 used a dilute solution (0.01 mol/L) of an inorganic precursor (Ce(NO3)3· 6H2O) in an ethanol/diethylene glycol butyl ether mixture. In this note, key design criteria for electrospray flame synthesis (ESFS) systems are evaluated based on results from the literature on EHD atomization. With particular focus on materials systems with nonvolatile precursors, we identify operating conditions for controlled synthesis of quasimonodisperse nanosized metal oxide particulates, and present a novel configuration of an ESFS system. Initial results from an experimental study of the manufacture of nanosized yttriastabilized zirconia (YSZ) are discussed with respect to product particle sizes and phase composition. In contrast to the work by Ahn et al.,32 who used a highly volatile precursor, and Oh et al.,33 we demonstrate manufacture of YSZ by electrospraying a concentrated organic solution of (nonvolatile) metal alkoxides (>2 mol/L) into the flow from a multielement diffusion burner. The results of this study thus provide some guidance for future development of ESFS technology in particular for synthesis of nanosized metal oxides with nonvolatile precursors and negligible amounts of monomer vapors at typical flame temperatures.

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⎛ γ 2ε ⎞1/2 I = ⎜ 0 ⎟ h(δ , β , δμ) ⎝ ρ ⎠

(1)

⎛ γε 2 ⎞1/3 D = ⎜ 02 ⎟ g(δ , β , δμ) ⎝ ρK ⎠

(2)

where ε0 = 8.854 × 10−12 A·s/(V·m) is the dielectric constant of vacuum. Mechanistic analysis reveals that if δμ ≪ 1 and δ ≪ 1 such that δμδ1/3 ≤ 1, the viscosity parameter δμ becomes irrelevant for droplet sizes, currents and droplet charges. Gañań -Calvo et al.37 established the empirical correlations h(β,δ) and g(β,δ) as

THEORETICAL BACKGROUND A sufficiently strong electric field between a liquid-supplied capillary needle and an opposing ground electrode forces the liquid column to form a cone at the tip of the capillary needle. From the apex of that cone, a thin ligament of liquid is ejected, which breaks into charged droplets due to surface instabilities induced by repulsive Coulomb forces. In the stable cone-jet mode,12,34,35 this cloud consists of nearly monodisperse droplets with droplet sizes that can be orders of magnitude smaller than the capillary diameter. Apart from multijet modes, in which increased field strengths produce two or more jets, other modes are either unstable and produce a wide droplet

h(β , δ) = 6.2(β − 1)−1/4 δ −1/2 − 2.0

(3)

g(β , δ) = 1.6(β − 1)1/6 δ −1/3 − 1.0(β − 1)1/3

(4)

From the figures provided in ref 37, the uncertainties in h(β,δ) and g(β,δ) may be estimated as ±1 and ±0.4(β − 1)1/3, respectively. The lower bound for the ratio between the 16843

dx.doi.org/10.1021/ie4021478 | Ind. Eng. Chem. Res. 2013, 52, 16842−16850


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tension (“Coulomb fission”) 44,48 and, subsequently, in polydisperse particulates, potentially with a pronounced second and third modes in the size distribution function, as reported in ref 33 (the maximum charge q a droplet of size D can maintain is given by qmax = 2π(2γε0)1/2D3/237 and is commonly referred to as “Rayleigh limit”). Additional precautions to enhance neutralization of the droplet charges may thus become necessary.44 According to eq 5, the jet velocity for Qmin equals 4.55(β − 1)−1/6(γK/ε0ρ)1/3, which is on the order of 250 m/s. For the typical range of feasible flow rates, the velocity remains in the range 60−100% of the velocity at Qmin. The droplet emission frequency decreases monotonically with increasing flow rates and covers about 1 order of magnitude for the typical range of sustainable flow rates. The frequencies range from about 2.4 GHz to 200 MHz, and it thus may be difficult to utilize a superimposed AC field to stabilize sprays or produce smaller droplet sizes for a given flow rate. Electric currents increase from about 12 nA at the minimum flow rate up to about 160 nA at the maximum flow rate; that is, the output power to be provided by the high-voltage power supply will remain below 1 mW per emitter and of minor concern. For the further discussion it is assumed that these power requirements do not pose a technical problem, and the solution is sprayed at the maximum flow rate for the stable cone-jet mode. Expected particle diameters DP are estimated from the precursor molar concentration C, the initial droplet diameter D, the molecular weight of the oxide W, the oxide mass density ρP, and the valence z (the number of metal atoms (ions) per molecule with molecular weight W), and the assumption that each droplet yields a single particle (according to the microreactor concept7):

diameters of the jet before break-up and droplets generated equals the ratio for break-up of uncharged jets in the Rayleigh regime (i.e., D/Dj ≈ 1.89).43 The fluid velocity just before break-up can thus be estimated from conservation of mass (with evaporation neglected): vj =

⎛ γK ⎞1/3 −1 4 × 1.892 Q 4.55 = ⎜ ⎟ δ g (δ , β)−2 π D2 ⎝ ε0ρ ⎠

(5)

Improvements of spray stability and range of sustainable flow rates may be achieved by superimposing an ac-voltage to the constant high voltage between the electrodes.44−47 Application of frequencies somewhat higher than the natural rate of droplet emission may reduce droplet sizes for a chosen liquid flow rate.47 Assuming a stream of individual droplets released at the tip of the jet, the rate of droplet emission follows as fE =

6Q 6 ⎛ K ⎞ −1 = ⎜ ⎟δ g (δ , β)−3 π ⎝ ε0 ⎠ D 3π

(6)

Figure 1 illustrates the functional dependence of these characteristic parameters on the liquid flow rate. The

⎛ CW ⎞1/3 ⎛ ε 2γCW ⎞1/3 ⎟⎟ g(δ , β) ⎟⎟ = ⎜⎜ 02 DP = D⎜⎜ ⎝ zρP ⎠ ⎝ K ρzρP ⎠ Figure 1. Scaling law predictions of electrospray characteristics of equimolar solutions of n-propanol and 2-methoxyethanol. Material properties were estimated as mole-fraction-weighted averages of solution properties at 300 K and K = 100 μS/cm: density ρ = 882 kg/m3, surface tension γ = 27.3 mN/m, relative permittivity β = 18.5, and dynamic viscosity μ = 1.87 mPa·s.

(7)

The time to produce one unit of mass of metal oxide powder from a single cone-jet follows from the flow rate Q as tm = z/ CWQ, which is, of course, independent of particle size. Figure 2 illustrates the dependences of particle sizes, droplet sizes, and production times on electric conductivity of equimolar solutions of n-propanol and 2-methoxyethanol with 1 mol/L zirconium dissolved in the solutions (the same material properties as for Figure 1 were used). Clearly, the maximum flow rate for stable cone-jets strongly decreases with

calculation results shown in this figure are for equimolar solutions of n-propanol and 2-methoxyethanol with electric conductivity K = 100 μS/cm, typical for the organometallic solutions used during the initial experiments discussed below, and air as the surrounding gas. The minimum flow rate that can be sprayed in the cone-jet mode is on the order of Qmin ∼ (β − 1)1/2γε0/ρK, i.e. δmin = (β − 1)−1/2. This gives hmin(δ,β) = 4.2 and gmin(δ,β) = (β − 1)1/3 from which the smallest emitted current and droplet size can be calculated.37 For the liquid considered flow rates for stable cone-jets range from 0.4 to 40 μL/min, which clearly implies that a large number of emitters will be necessary to support mass flows for practical particulate production rates. However, Figure 1 predicts monotonically increasing droplet sizes between 50 and 500 nm with increasing flow rates. These sizes are considerably smaller than those produced by other means of atomization6 and ensure finest possible dispersion of the precursor liquid. A drawback of small droplet sizes is that time scales of solvent evaporation can be much smaller than the time scales for natural charge neutralization. This results in disintegration of the droplets when repulsive electrostatic forces exceed the stabilizing surface

Figure 2. Predictions based on scaling laws: particle and droplet diameters, liquid flow rate, and time requirements for production of 1 g zirconia as functions of liquid conductivity of equimolar solutions of n-propanol and 2-methoxyethanol. Curves are for a single emitter; production time and particle sizes are based on upper flow rate limits. 16844



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increasing conductivity, and this maximum flow per emitter, for the conductivity considered, is 1000 μL/min. For the conductivity range shown in the plot, and for the maximum flow rate with a 1 mol/L-concentration of precursors, the production times are at best 0.01 min/mg ZrO2. For particle sizes that are approximately 1000 nm, the production rate drops to 1 min/mg and as the desired particle size drops, assuming the maximum flow for stable cone operation, the production rate continues to climb. To put the production into scale, 1 mg of material is required to produce a 10 μm thick film of zirconia that is ∼5 mm in diameter. To be economically viable, production rates must be increased beyond those indicated by Figure 2, which shows results for a single emitter. Nonetheless, Figure 2 delineates conditions for manufacture of ultrafine and nanosized particulates. For example, to produce particles less than 100 nm in size, conductivities must be larger than 160 μS/cm at maximum flow rates of solutions with 1 mol/L precursor concentration, or at least 4 μS/cm at the minimum sustainable flow rate. Droplet sizes decrease as the conductivity rises and are predicted to be smaller than 1000 nm for conductivities higher than 30 μS/cm for any liquid flow rate that produces a stable cone-jet electrospray. Unfortunately, the electric conductivity of a solution depends on the precursor concentrations, more precisely, on the amount of ions present in the solution and the strength of their interactions. Figure 3 depicts measured conductivities for 2-

Figure 4. Predicted particle and droplet diameters, time requirements for production of 1 mg YSZ, and maximum liquid flow rate as functions of metal concentration in solutions of n-propanol and 2methoxyethanol for the maximum sustainable flow rates through a single emitter. For comparison, particle sizes also for the lower bound of sustainable flow rates are shown.

as the effect of reduced droplet sizes due to increasing conductivities is offset by the proportional increase of the amount of metal for particle formation. For this central region of the plot, the primary factor controlling particle size, presuming stable cone flow, is the flow rate. For optimum operation Figure 3 implies that the ESFS system should be operated with concentrations near 0.8 mol/L in order to produce smallest possible particle sizes while keeping production times shortest possible. At maximum, sustainable liquid flow rates the smallest particle sizes possible are ca. 150 nm; at the minimum flow rates, the smallest sizes of about 15 nm might be achievable. Manufacture of 1 mg of 150 nm particles from a single emitter takes around 120 min. An increase in the production rate of particles in a controlled, narrow size range can be achieved by increasing the number of outlet nozzles. If the individual sprays do not affect each other, the production times are reduced in proportion to the number of emitters while similar droplet sizes are produced. This has been demonstrated in configurations with multiple droplet emitters in close packing (capillaries or holes in a plane surface),18,28−30,50−52 or with electrosprays operated in multijet modes.26 Based on the results from the evaluation of relevant scaling laws discussed earlier and the intended application of the ESFS system to manufacturing thin films, characteristic features of an ideal ESFS system are (1) a large number of emitter nozzles and (2) emitters arranged such that the particulate produced is spread around the centerline of a carrier gas flow. Figure 5 shows a schematic of such a spray configuration with nozzles pointed radially to the counter electrode in the centerline and a concentric ring extractor to minimize cross-talk between neighboring sources and to provide some shielding between the individual jets before their break-up. The system further uses a flat-flame burner, for instance, a multielement diffusion burner, to create a vertically directed hot gas flow for combustion of the generated droplets. In the case of nonvolatile precursors dissolved in (volatile) organic solvents, the solvent evaporates from the emitted droplets as the droplets are forced in radial direction toward the center electrode by the electrostatic field. This causes the droplet to shrink and to concomitantly increase both the charge density on the droplet surface and the volumentric concentration of precursor species. It can be expected that collisions with ions in the postflame region, produced by combustion of hydrocarbon fuels and/or by corona discharge at the counter

Figure 3. Conductivity of Zr-n-propoxide and Y-methoxyethoxide dissolved in a solution of 2-methoxyethanol and n-propanol.

methoxyethanol/n-propanol solutions as a function of molar metal concentration, calculated as the sum of (undissociated) Zr-n-propoxide and Y-methoxyethoxide concentrations (the different solutions were produced by diluting the most concentrated solution by adding pure 2-methoxyethanol). The measured conductivities initially increase with increasing metal concentrations, but sharply fall at about 2 mol/L as shortrange interactions become significant at these high concentrations (the solid line is an empirical fit of the equation by Gellings proposed to fit conductometric data of concentrated electrolyte solutions49). Figure 4 illustrates ESFS characteristics as function of the metal concentration of the sprayed solution. Using the empirical relationship between conductivity and solution concentration shown in Figure 3, expected particle and droplet sizes, maximum flow rates for operation in the stable cone-jet mode, as well as production times were computed. As expected from the sharp drop in conductivity at high concentrations, droplet and particle sizes sharply increase at concentrations larger than 2 mol/L. Figure 4 shows that particle sizes remain relatively constant for concentrations between 0.1 and 1 mol/L, 16845



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tally, and will be specific to a particular ESFS system configuration.

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EXPERIMENTAL SETUP With the aim to demonstrate synthesis of nanosized rare-earth and transition metal oxide particles, an ESFS system as laid out in the previous section was implemented and small quantities of YSZ, which is used as an electrolyte in solid-oxide fuel cells (SOFC),1 produced. The apparatus, depicted in Figure 6,

Figure 5. Schematic of an electrospray flame synthesis system.

Figure 6. Photograph of the electrospray flame synthesis system used to manufacture YSZ.

electrode, neutralize most of the charges on the droplets such that drag forces will redirect the droplets to follow the vertical flow of postcombustion gas (note that hydrogen-fueled flames generate over an order of magnitude fewer ions53 and may thus be advantageous in optimizing the overall droplet generation/ neutralization process). Once the solvent has evaporated, the droplet temperature rises rapidly (from the solvent’s boiling point) due to the heat transfer from the carrier gas flow and heat release from oxidation of combustible components of the droplet/particle. The metal(s) form bonds with oxygen atoms and internally rearrange into energetically favorable crystallites and primary particles. In the absence of gas-phase monomer species further growth by monomer deposition cannot occur. The produced particulate finally can be deposited on a substrate or collected by other means. For a nonbatch production system, the feedstock solution has to be delivered continuously to the capillary needles, as, for instance, by means of a metering pump. The results of initial tests will be useful in assessing critical design concerns and will guide further modifications of the design. These concerns include the adequacy of the shielding from flame radiation provided by the extractor electrode. Overheating of the liquid column in the tubes from excessive radiative heat transfer could cause precipitation of the precursor solution inside the capillary tubes, and subsequent clogging of the tubes. Aggressive cooling of the ejector tubes, enhanced shielding of the tubes from radiant transport from the postcombustion flow, and even location of the injection tubes to a preflame location could be considered as a means of controlling the temperature of the liquid in the ejector tubes. As an alternative remedy, additional solvent-saturated, low-temperature gas flow between extractor and needle holder may become necessary. Certainly, optimum levels of the sprayproducing high voltage levels have to established experimen-

consisted of a capillary manifold with 12 stainless steel capillary needles (0.55 mm ID, 25 mm long, 16 mm covered with PTFE tubing for electric insulation) arranged according to Figure 5. The manifold was mounted on the perimeter of an industrial burner (N.M. Knight Inc., P.O. Box 1099 Millville, NJ 08332) that had 77 concentric tube pairs (3.9 mm inner diameter (ID) of outer tubes, 3.2 mm outer diameter (OD) of inner tubes, 0.5 mm tube wall thickness) distributed in a 50 mm by 50 mm square. The tube pairs were arranged in a square pattern in a 6.4 mm by 6.4 mm mesh. A cylindric aluminum tube (62 mm tall, 76 mm OD, and 1.6 mm wall thickness) with 7 mm holes concentric with the capillary needles at a height of 22 mm above the burner face served as the extractor and also provided some radiation shielding to the liquid cones at the tips of the capillary needles. The aluminum tube was water-cooled at the end that rested on the burner face. The counter electrode in the burner centerline consisted of a stainless steel tube with internal cooling by a nitrogen flow and external electrical and thermal insulation with an alumina tube that surrounded the electrode tube in close spacing. The total exposed length of the center electrode was about 30 mm with the insulation extending only up to 25 mm from the burner face. Two high-voltage power supplies (model PS/MK20N02.5, Glassman High Voltage Inc., and model HV35 by Infounlimited) were used to produce the electric potentials on capillaries and center electrode relative to the grounded extractor (+5 kV and −2.4 kV, respectively). The feedstock solution was delivered to the capillary needles at a constant flow rate of 1 μL/s using an electronically controlled metering pump (milliGAT, Global FIA, Inc.). For the production of YSZ, more specifically (Y2O3)0.02(ZrO2)0.98, abbreviated 2YSZ, an organometallic precursor solution was sprayed into the postflame region of a CH4/O2/N2 flame with an adiabatic flame temperature of 2300 K (EQUIL of the Chemkin 3.6 software package, Reaction 16846



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FE-SEM images (Figure 8) reveal several interesting size and morphological features of the produced particulate. As clearly visible in the top two images, the particulate is polydisperse with sizes mostly below 1 μm. Parts b and c, with increasing

Design). The solution was prepared by mixing solutions of yttrium 2-methoxyethoxide in 2-methoxyethanol (0.15−0.18 g/ g) and zirconium n-propoxide in n-propanol (0.7 g/g). Pure 2methoxyethanol was added to prepare an approximately equimolar solution of n-propanol and 2-methoxyethanol. As pure 2-methoxyethanol and n-propanol have boiling points of 125 °C, and 97 °C, respectively, the sprayed mixture is quite volatile. It must be kept in mind, however, that both zirconium n-propoxide and yttrium-2-methoxyethoxide are nonvolatile substances (the vapor pressure of zirconium-n-propoxide at 208 °C is 0.1 mmHg54) that remain in the liquid phase at the boiling temperatures of the solvents. The conductivity of the mixture was about 26 μS/cm. From the chemical composition of the stock solutions, the zirconium concentration in the sprayed mixture was estimated as 2.9 mol/L. In order to collect a powder sample for analysis, a silicon wafer was inserted at a distance of about 60 cm above the burner surface. The ESFS system was operated for ca. 30 min and the layer of particles collected on the wafer analyzed for phase purity using X-ray diffraction (XRD) analysis, and for particle size by analyzing field emission scanning electron microscopy (FE-SEM) images. Reliable XRD analysis of the minute quantities of powder available required transferring powder onto a small strip of adhesive tape, which was supported by a disk of amorphous silicon during the XRD measurement. The XRD analysis was performed at a rate of 200 s/deg with 0.1° increments in 2θ.

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RESULTS AND DISCUSSION The results of the XRD analyses are shown in Figure 7. Comparison of standard XRD results of pure Tosoh powder,

Figure 7. Comparison of crystal phase composition of commercial YSZ powder and electrospray YSZ. Different counting times were necessary to account for the varying amount of powder on the adhesive tapes (15 s/deg (a and b), 60 s/deg (c), and 200 s/deg (d)).

which are based on large quantities rather than small quantities placed on an adhesive tape (Figure 7a) demonstrate that the characteristic features in the diffraction pattern are reproduced by the sample on the tape, albeit with magnitudes considerably reduced. The peaks in the diffraction patterns from the YSZ produced via ESFS compared with commercially available pure Tosoh YSZ (both on adhesive tapes, Figure 7c, d) are also at the same diffraction angles, suggesting that both powders have the same YSZ cubic-fluorite crystal phase structure.55 Analysis of these minute quantities available thus suggests that ESFS is capable of producing tailored YSZ powder with the desired phase composition. This initial result is promising but will certainly have to be substantiated by analyzing larger quantities.

Figure 8. Field emission scanning electron microscopic images of 2YSZ produced via electrospray flame synthesis of samples as obtained by inserting a silicon wafer into the hot, particle-laden flow. Magnifications: (a) 5000, (b) 20 000, and (c) 100 000. 16847



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temperature field that suppressed coalescence of the primary particles to larger spherical particulates.6,57 The cooling rate, however, was not high enough to prevent the partial fusion of the particles, which resulted in the formation of aggregates instead of agglomerates consisting of loosely attached, separable, primary particles. Note that the residence time of the particles in the hot flow (before deposition on the silicon wafer) was on the order of 500 ms, providing ample time for coagulational particle growth. The quality of the collected particles in terms of morphology and size distribution clearly points to the need for further process development to manufacture quasi-monodisperse spheroidal particulates. As discussed earlier, the primary particle sizes are in the expected range or even smaller, which is consistent with the one-to-one conversion of individual droplets to particles. In order to avoid formation of aggregates, which impair the ability to produce uniform films with controlled porosity, the rate of bond-forming collisions among primary particles must be kept at a minimum. This essentially involves reducing particle temperatures (to reduce the propensity for neck formation) and reducing particle collision frequencies. First steps in the further development of the ESFS process will thus include optimization of operating parameters to balance time scales for droplet evaporation and primary particle formation with those for removal of heat from the carrier gas. This may, for instance, require downstream addition of cold gas flow, which would also dilute the carrier flow and thus reduce collision frequencies, or electrostatic recharging of the formed primary particles to stabilize the aerosol for a sufficiently long period. Despite the need for necessary modifications, the initial results presented are promising and provide evidence for the potential of ESFSbased manufacture of thin films of nanosized oxide particles, in particular, of metals without volatile precursor substances and condensable monomer vapors.

magnification, show that the particles are aggregates with welldistinguishable spheroidal primary particles of less than 100 nm in size. According to Figure 4, the primary particle sizes for a 2.9 mol/L solution are expected to be in the range between 46 and 490 nm for the range of flow rates for stable cone-jet droplet generation. The actually introduced liquid flow rate of 5 μL/min per emitter (60 μL/min for all 12 emitters) was above the maximum flow rate for controlled cone-jets (2.9 μL/min) to counteract the tendency of the capillary needles to clog due to premature precipitation of deposits inside the needles. Assuming that the scaling laws are valid up to the applied flow rate of 5 μL/min, the corresponding primary particle size would be around 610 nm, which is clearly much larger than the 100 nm primary particles that were produced. The fact that the produced primary particles are considerably smaller than predicted from the scaling laws is certainly a positive outcome, but indicates that, for the applied flow rate, the scaling laws were either evaluated with incorrect input parameters, or some of the processes that are relevant in the droplet-to-particle conversion process are not captured. One uncertain parameter with potentially strong effect on particle size is the precursor concentration at the location the spray is produced. Evaporation of solvent from the Taylor cone at the tip of the capillary could have caused increasingly higher concentrations beyond 3 mol/L, which, according to Figure 4, would have corresponded to lower conductivities and subsequently to primary particle sizes even larger than 610 nm. It is therefore safe to exclude this effect as a cause for the size discrepancy. Another uncertain parameter is the actual flow rate. Primary particle sizes around 100 nm from a 2.9 mol/L solution are predicted for flow rates around 0.09 μL/min, which is more than half a magnitude smaller than the flow rate used in the experiment. Given the controlled metering of liquid solution to the spray system, such a large level of uncertainty is unlikely. As an anonymous reviewer pointed out, another possible mechanism is the transition from controlled evaporation of solvent from the droplets to a boiling mode in which droplets are disruptively fragmented. The ejected fluid parcels in turn are converted to particulates. Lack of data on thermodynamic and transport properties of the precursor solution and the heat transfer characteristics of the process complicates critical assessment of the relevance of this mechanism, but Figure 8 shows that the primary particles are nearly monodisperse, which seems incompatible with the fragmentation mechanism in question. More realistically, the droplets produced by the electric field lost solvent due to evaporation too quickly and underwent secondary break-up at the Rayleigh limit. The ejected secondary droplets, which are much smaller than the parent droplet, were subsequently oxidized to solid particles. Droplet break-up at the Rayleigh limit has been experimentally confirmed by Achtzehn, et al.,56 and was also suggested by Oh et al.33 as the mechanism responsible for production of the finest particles (the authors observed trimodal size distributions). In our system, Rayleigh disintegration is certainly plausible, as we kept the system configuration simple and omitted precautions to ensure sufficiently fast neutralization of droplet charges. However, the observation that nearly monodisperse primary particle sizes were produced (see Figure 8) implies that Rayleigh disintegration may be a beneficial mechanism in producing nanosized rare-earth and transition metal oxide particulates from concentrated precursor solutions. The formation of aggregates (shown in Figure 8) as opposed to spherules indicates that the particles experienced a

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CONCLUSIONS Based on scaling laws for prediction of the performance characteristics of electrohydrodynamic atomization, utilization of electrosprays in a combined process for flame synthesis of metal-oxide nanoparticles was evaluated. An ESFS apparatus that includes a large number of droplet emitters to boost overall particulate production rates was designed with the aim of manufacturing thin films of rare-earth and transition metal oxide nanoparticles. For analysis of product powder, the proposed configuration was implemented and small quantities of yttria-stabilized zirconia (YSZ) produced, demonstrating the feasibility of a electrospray-based flame synthesis process. The initial experimental results for synthesis of YSZ show that particulate with the correct phase composition for SOFC application can be manufactured via ESFS. Particulates in the form of aggregates with primary particle sizes below 100 nm were produced, providing compelling evidence of the applicability of ESFS for manufacture of high-purity nanosized metal oxide powders. As expected, the production rates are small, making ESFS is ill-suited for manufacture of mass quantities. However, nanosized particulate for deposition on substrates in thin, functional layers can be produced in a nonbatch production mode without the need for timeconsuming wet-chemical separation processes. The process lends itself to multiplexing, that is, increasing the throughput by employing a several hundred emitters in compact packing in an axisymmetric configuration. Development of an ESFS process 16848



Article

(14) van Zomeren, A. A.; Kelder, E. M.; Marijnissen, J. C. M.; Schoonman, J. The production of thin films of LiMn2O4 by electrospraying. J. Aerosol Sci. 1994, 25, 1229−1235. (15) Miao, P.; Balachandran, W.; Xiao, P. Formation of ceramic thin films using electrospray in cone-jet mode. IEEE Trans. Ind. Appl. 2002, 38, 50−56. (16) Rulison, A. J.; Flagan, R. C. Synthesis of Yttria powders by electrospray pyrolysis. J. Am. Ceram. Soc. 1994, 77, 3244−3250. (17) Rulison, A. J.; Flagan, R. C. Electrospray atomization of electrolytic solutions. J. Colloid Interface Sci. 1994, 167, 135−145. (18) Rulison, A. J.; Flagan, R. C. Scale-up of electrospray atomization using linear arrays of Taylor cones. Rev. Sci. Instrum. 1993, 64, 683− 686. (19) Chen, C. H.; Emond, M. H. J.; Kelder, E. M.; Meester, B.; Schoonman, J. Electrostatic sol-spray deposition of nanostructured ceramic thin films. J. Aerosol Sci. 1999, 30, 959−967. (20) Lenggoro, I. W.; Okuyama, K.; Fernández de la Mora, J.; Tohge, N. Preparation of ZnS nanoparticles by electrospray pyrolysis. J. Aerosol Sci. 2000, 31, 121−136. (21) Park, D. G.; Burlitch, J. M. Nanoparticles of anatase by electrostatic spraying of an alkoxide solution. Chem. Mater. 1992, 4, 500−502. (22) Hogan, C. J., Jr.; Biswas, P. Narrow size distribution nanoparticle production by electrospray processing of ferritin. J. Aerosol Sci. 2008, 39, 432−440. (23) Electrospray and MALDI Mass Spectroscopy: Fundamentals, Instrumentation, Practicalities, and Biological Applications, 2nd ed.; Cole, R. B., Ed.; John Wiley & Sons: Hoboken, NJ, 2010. (24) Loscertales, I. G.; Barrero, A.; Guerrero, I.; Cortijo, R.; Marquez, M.; Gañań Calvo, A. M. Micro/nano encapsulation via electrified coaxial liquid jets. Science 2002, 295, 1695−1698. (25) Tang, K.; Gomez, A. Generation by electrospray of monodisperse water droplets for targeted drug delivery by inhalation. J. Aerosol Sci. 1994, 25, 1237−1249. (26) Duby, M.-H.; Deng, W.; Kim, K.; Gomez, T.; Gomez, A. Stabilization of monodisperse electrosprays in the multi-jet mode via electric field enhancement. J. Aerosol Sci. 2006, 37, 306−322. (27) Kyritsis, D. C.; Guerrero-Arias, I.; Roychoudhury, S.; Gomez, A. Mesoscale power generation by a catalytic combustor using electrosprayed liquid hydrocarbons. 29th Symp. (Int.) Combust. 2002, 965− 972. (28) Kaiser, S.; Kyritsis, D. C.; Drobrowolski, P.; Long, M. B.; Gomez, A. The electrospray and combustion at the mesoscale. J. Mass Spectrom. Soc. Jpn. 2003, 51, 42−49. (29) Kyritsis, D. C.; Coriton, B.; Faure, F.; Roychoudhury, S.; Gomez, A. Optimization of a catalytic combustor using electrosprayed liquid hydrocarbons for mesoscale power generation. Combust. Flame 2004, 139, 77−89. (30) Deng, W.; Klemic, J. F.; Li, X.; Reed, M. A.; Gomez, A. Increase of electrospray throughput using multiplexed microfabricated sources for the scalable generation of monodisperse droplets. J. Aerosol Sci. 2006, 37, 696−714. (31) Yuliati, L.; Seo, T.; Mikami, M. Liquid-fuel combustion in a narrow tube using an electrospray technique. Combust. Flame 2012, 159, 462−464. (32) Ahn, K.-H.; Ahn, J.-H.; Jeon, K.-S.; Choa, Y.-H. Synthesis of ultra-fine iron-oxide nano-particles in a diffusion flame with electrospraying assistance. Mater. Sci. Forum 2004, 449−452, 1169−1172. (33) Oh, H.; Kim, S. Synthesis of ceria nanoparticles by flame electrospray pyrolysis. J. Aerosol Sci. 2007, 38, 1185−1196. (34) Cloupeau, M.; Prunet-Foch, B. Electrostatic spraying of liquids in cone-jet mode. J. Electrostat. 1989, 22, 135−159. (35) Jaworek, A.; Krupa, A. Classification of the modes of EHD spraying. J. Aerosol Sci. 1999, 30, 873−893. (36) Grace, J. M.; Marijnissen, J. C. M. A review of liquid atomization by electrical means. J. Aerosol Sci. 1994, 25, 1005−1019. (37) Gañań Calvo, A.; Dávila, J.; Barrero, A. Current and droplet size in the electrospraying of liquids. Scaling laws. J. Aerosol Sci. 1997, 28, 249−275.

for thin-film deposition is clearly still in its early stages, but the initial experiments are promising and encourage refinement of both apparatus design and operating conditions. In particular, modifications to prevent solid precipitation in the emitter needles will have to be considered and the process further investigated using an apparatus that accommodates several hundred droplet emitters.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ‡

Sandia National Laboratories, Livermore, CA 94550, United States Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dan Storjohann for his efforts in the XRD and FE-SEM analyses of the powder samples. Assistance with the conductivity measurements by Christiane Hoppe-Jones and Maureen O’Brien is gratefully acknowledged. The authors further thank the anonymous referees for their insightful comments and suggestions. The study was financially supported through the Colorado Fuel Cell Center (CFCC) and the Center for Commercial Applications of Combustion in Space (CCACS) at the Colorado School of Mines through the NASA Space Product Development Program under Cooperative Agreement Number NCCW-0096.

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REFERENCES

(1) Synthesis, Properties, and Applications of Oxide Nanomaterials; Rodríguez, J. A., Fernández-García, M., Eds.; John Wiley & Sons: Hoboken, NJ, 2007. (2) Tani, T.; Mädler, L.; Pratsinis, S. E. Synthesis of zinc oxide/silica composite nanoparticles by flame spray pyrolysis. J. Mater. Sci. 2002, 37, 4627−4632. (3) Tani, T.; Mädler, L.; Pratsinis, S. E. Homogeneous ZnO nanoparticles by flame spray pyrolysis. J. Nanopart. Res. 2002, 4, 337− 343. (4) Mädler, L.; Kammler, H. K.; Mueller, R.; Pratsinis, S. E. Controlled synthesis of nanostructured particles by flame spray pyrolysis. J. Aerosol Sci. 2002, 33, 369−389. (5) Laine, R. M.; Marchal, J. C.; Sun, H. P.; Pan, X. Q. Nano-α-Al2O3 by liquid-feed flame spray pyrolysis. Nat. Mater. 2006, 5, 710−712. (6) Kodas, T. T.; Hampden-Smith, M. J. Aerosol Processing of Materials; Wiley-VCH: New York, 1999. (7) Helble, J. J. Combustion aerosol synthesis of nanoscale ceramic powders. J. Aerosol Sci. 1998, 29, 721−736. (8) Mädler, L.; Stark, W. J.; Pratsinis, S. E. Flame-made ceria nanoparticles. J. Mater. Res. 2002, 17, 1356−1362. (9) Fernández de la Mora, J.; de Juan, L.; Gamero, M. Electrospray atomization: Fundamentals and its application in nanoparticle technology. J. Aerosol Sci. 1997, 28, S63. (10) Zeleny, J. The electrical discharge from liquid points, and a hydrostatic method of measuring the electric intensity at their surfaces. Phys. Rev. 1914, 3, 69−91. (11) Zeleny, J. Instability of electrified liquid surfaces. Phys. Rev. 1917, 10, 1−6. (12) Cloupeau, M.; Prunet-Foch, B. Electrohydrodynamic spraying functioning modes: A critical review. J. Aerosol Sci. 1994, 25, 1021− 1036. (13) Salata, O. V. Tools of nanotechnology: Electrospray. Curr. Nanosci. 2005, 1, 25−33. 16849



Article

(38) Rosell-Llompart, J.; Fernández de la Mora, J. Generation of monodisperse droplets 0.3 to 4 mm in diameter from electrified conejets of highly conducting and viscous liquids. J. Aerosol Sci. 1994, 25, 1093−1119. (39) Gañań Calvo, A. M. The surface charge in electrospraying: Its nature and its universal scaling laws. J. Aerosol Sci. 1999, 30, 863−872. (40) Fernández de la Mora, J.; Loscertales, I. G. The current emitted by highly conducting Taylor cones. J. Fluid Mech. 1994, 260, 155−184. (41) Gañań Calvo, A. M. Cone-jet analytical extension of Taylor’s electrostatic solution and the asymptotic universal scaling laws in electrospraying. Phys. Rev. Lett. 1997, 79, 217−220. (42) Tang, K.; Gomez, A. Monodisperse electrosprays of low electric conductivity liquids in the cone-jet mode. J. Colloid Interface Sci. 1996, 184, 500−511. (43) Lefebvre, A. H. Atomization and Sprays; Taylor & Francis: Boca Raton, FL, 1989. (44) Cloupeau, M. Recipes for Use of EHD Spraying in Cone-Jet Mode and Notes on Corona Discharge Effects. J. Aerosol Sci. 1994, 25, 1143−1157. (45) Marginean, I.; Parvin, L.; Heffernan, L.; Vertes, A. Flexing the electrified meniscus: The birth of a jet in electrosprays. Anal. Chem. 2004, 76, 4202−4207. (46) Sung, K.; Lee, C. S. Factors influencing liquid breakup in electrohydrodynamic atomization. J. Appl. Phys. 2004, 96, 3956−3961. (47) Huneiti, Z. A.; Balachandran, W.; Machowski, W. W. Harmonic spraying of conducting liquids employing ac/dc electric fields. IEEE Trans. Ind. Appl. 1998, 34, 279−285. (48) Gomez, A.; Tang, K. Charge and fission of droplets in electrostatic sprays. Phys. Fluids A 1994, 6, 404−414. (49) de Diego, A.; Usobiaga, A.; Madariaga, J. M. Critical comparison among equations derived from the Falkenhagen model to fit conductimetric data of concentrated electrolyte solutions. J. Electroanal. Chem. 1998, 446, 177−187. (50) Bocanegra, R.; Galán, D.; Márquez, M.; Loscertales, I. G.; Barrero, A. Multiple electrosprays emitted from an array of holes. J. Aerosol Sci. 2005, 36, 1387−1399. (51) Regele, J. D.; Papac, M. J.; Rickard, M. J. A.; Dunn-Rankin, D. Effects of capillary spacing on EHD spraying from an array of cone jets. J. Aerosol Sci. 2002, 33, 1471−1479. (52) Almekinders, J. C.; Jones, C. Multiple jet electrohydrodynamic spraying and applications. J. Aerosol Sci. 1999, 30, 969−971. (53) Fialkov, A. B. Investigations on ions in flames. Prog. Energy Combust. Sci. 1997, 23, 399−528. (54) Zirconium(IV) Propoxide Solution Material Safety Data Sheet; Sigma-Aldrich: St. Louis, MO, 2012. (55) Minh, N. Q.; Takahashi, T. Science and Technology of Ceramic Fuel Cells: Fundamentals, Design, and Applications; Elsevier Science: Amsterdam, 1995. (56) Achtzehn, T.; Müller, R.; Duft, D.; Leisner, T. The Coulomb instability of charged microdroplets: Dynamics and scaling. Eur. Phys. J. D 2005, 34, 311−313. (57) Friedlander, S. K. Smoke, Dust, and Haze. Fundamentals of Aerosol Dynamics, 2nd ed.; Oxford University Press, Inc.: New York, 2000.

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Electrospray Flame Synthesis of Yttria-Stabilized Zirconia Nanoparticles

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