Bipolar Coagulation Process for the Production of Powders - American

Ind. Eng. Chem. Res. 1999, 38, 631-638

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Bipolar Coagulation Process for the Production of Powders D. Camelot, J. C. M. Marijnissen, and B. Scarlett* Particle Technology Group, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

A new process is presented where the controlled coagulation of droplets is achieved through electrical forces. The charged droplets are produced by electrosprays operated in the cone-jet mode. The coagulation of oppositely charged droplets leads to the neutralization of the electrosprays. Furthermore, this coagulation enables controlled mixing on a microscale and subsequent chemical reactions inside the combined droplets. This process has many potential applications in the production of “tailored” particles, particles of uniform size and composition. 1. Introduction The production of particles with specific properties has become a key issue in many areas of powder technology. Particle properties such as size, size distribution, surface (coated particles), porosity, morphology, and homogeneity all influence the macroscopic behavior of powders. The actual processes for the production of powders, such as milling, are not energy efficient and usually do not lead to homogeneity of the particles. In this paper a novel process is presented which is called bipolar coagulation. It enables the production of particles with controlled properties. This process has Patent No. 1003442 (Borra et al.1). It is based on the controlled coagulation of oppositely charged droplets through electrical forces. Thus, two sprays of oppositely charged droplets are directed toward each other, and coagulation of the droplets takes place through the electrical attraction between them (Figure 1). The concept of powder production by bipolar coagulation is to induce the coagulation of droplets having the same but opposite charge, the same size, and appropriate stoichiometry. Droplets which do not coagulate, or combinations other than doublets, have a different charge, and mass and can easily be separated from the product. The charged droplets are produced through electrohydrodynamic atomization of solutions or melts (EHDA), a process usually referred to as “electrospraying”. This spray may contain primary, secondary, or satellite droplets depending upon the break-up conditions of the jet. The process is optimized to achieve the coagulation of two primary droplets of opposite charge. Depending on the nature of the solutions which are sprayed, different processes can take place in the coagulated droplets such as chemical reactions, micromixing, polymerization, and coprecipitation which can result, in turn, in different products. To control the charge of the mixed droplets and the ratio of the precursors to be mixed, the charge and size of the primary droplets must be controlled. This can be achieved by electrospraying the solutions in the cone-jet mode (Figure 2). A derivation for the coagulation rate of oppositely charged particles was made by Fuchs2 in 1934, but the term bipolar coagulation was introduced by Eliasson and Egli.3 The intention was to improve the collection * To whom correspondence should be addressed. Tel.: 31 (0) 15 2783577. Fax: 31 (0) 15 2784452. E-mail: [email protected].

Figure 1. Concept of bipolar coagulation.

Figure 2. Cone-jet mode.

efficiency of an electrostatic precipitator for particles between 0.1 and 1 µm. This model was later improved by assuming a log-normal distribution of the particles (Otto et al.4). None of these models can apply to the process presented here. They are all based on thermal coagulation with correction for a low level of charge on the particles. In the process presented, the droplets carry a high charge and exhibit negligible Brownian motion. Several authors have reported the use of one electrospray for powder production. Slamovich5 reported the production of zirconium acetate powders using an electrospray. The particles were collected on an earthed plate. There was no attempt to neutralize the droplets before their collection. Vercoulen et al.6 produced powdered SnO2 using the Delft Aerosol Generator (DAG). In this device a corona needle was used to neutralize the droplets (Meesters et al.7). In this work chemical analysis of the powders was not rigorous so that the

10.1021/ie980435j CCC: $18.00 © 1999 American Chemical Society Published on Web 02/10/1999

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actual production of SnO2 was not proven. Rulison and Flagan8 achieved the synthesis of Yttria powder by electrospray pyrolysis. They neutralized the aerosol produced by the electrospray using the method described by Meesters et al.7 and they also used a commercial aerosol neutralizer (TSI). Chen et al.9 reported the production of monodisperse powders ranging from 4 nm to 1.8 µm using simple sucrose solutions. They discharged their electrospray with a radioactive neutralizer. Recently, Lenggoro and Okuyama10 produced ZnS particles using an electrospray. In these papers there is no report that neutralization of the charge was controlled or quantified. An important factor was introduced by Chen et al.,9 who stated that “The droplets were electrically neutralized to a level below the Rayleigh charge limit so as to prevent droplet break up during evaporation.” This statement refers to the fact that the surface of an evaporating droplet decreases until the charge density leads to further instability or breakup. In the next section of this paper, the droplet production through electrospraying and the bipolar coagulation process are described. In the third section, experiments which demonstrate various features of the bipolar coagulation process are presented. Experiments related to the neutralization of the droplets are reported. Furthermore, the mixing and chemical reactions inside droplets were studied. Finally, some conclusions of this work as well as possible applications of this process are presented. 2. Droplet Production and the Bipolar Coagulation 2.1. Characteristics of the Electrosprays. The EHDA of liquids consists of the nebulization of a liquid into charged droplets under the influence of an electric field. This process can function in many different modes depending on the voltage applied to the liquid, the liquid flow rate through the capillary or nozzle, and the liquid properties. This was shown by Cloupeau and PrunetFoch11 and Grace and Marijnissen.12 Both emphasize that most of the reported research has been directed toward the cone-jet mode for three reasons. First, the size distribution produced by the spray is monodisperse (primary droplets) or bimodal (primary and secondary droplets). When primary and secondary droplets are produced, they can easily be separated from one another because of their different electrical mobilities. Second, a very large range of primary droplet diameters can be produced, from the submicron range to tens of microns. Third, this mode is achievable for a very wide range of liquid properties in terms of conductivity, viscosity, and surface tension. These properties of the cone-jet mode are of primary interest for the production of homogeneous powders. The mechanism of droplet production in the cone-jet mode is the following. The tangential component of the electric field at the liquid surface creates a force accelerating the charges which are close to the surface toward the tip of the cone and thus creating a jet (Fernańdez de la Mora and Loscertales13 and Cloupeau and PrunetFoch11). The breakup of this jet is assumed to take place by the classical mechanism of instability, Rayleigh breakup, enhanced by the electrical charge. Many authors have reported the production of monodisperse droplets with electrosprays operating in the cone-jet mode. It has to be noted that the breakup of the jet

usually gives rise to a bimodal size distribution, e.g., Gomez and Tang,14 depending on the wavelength of the breakup. Very long wavelengths may also result in tertiary satellites. The smaller droplets undergo a segregation effect because of their charge and velocity which drives them to the outside of the spray (Hartman15). Extensive study of the cone-jet mode has led to the derivation of scaling laws by different authors (Fernańdez de la Mora and Loscertales,13 Ganãn-Calvo, and16 Ganãn-Calvo et al.17). The charge distribution of the droplets produced by EHDA of liquids in the cone-jet mode is considered to be narrow. Thus, the droplet charge is obtained by dividing the total spray current by the number of main droplets (which is the total flow rate divided by the volume of the main droplet). This is an approximation in which the mass and charge carried by the satellite droplets are neglected. Gomez and Tang14 made the assumption of a narrow size distribution and justified it a posteriori with measurements of the droplet size and velocity in the center of the spray. The velocity distribution was found to be narrow, and it was concluded that this was only possible if the charge distribution was also narrow. De Juan and Fernańdez de la Mora18 also studied the charge distribution of electrosprayed droplets. They measured the mobility and the size of the droplets using a DMA (differential mobility analyzer) and an aerosizer. They found that the distribution of charge was 2.5 times broader than the distribution of size. The distribution of charge had a range of about a factor 4. The droplets were segregated by mobility and the sizes then measured in the aerosizer. They found that the charge-tomass ratio was nearly constant; q ∝ d3. They also mentioned that the charge carried by the satellites could not be neglected and, thus, that I/Q (where I is the current and Q the flow rate) could not be used to determine the droplet charge. Using the scaling laws introduced by the same author and the ratio I/Q leads to q ∝ d3/2. However, their finding was reported only for one type of liquid which had been specially formulated to meet the requirements imposed by the use of the DMA. Hartman15 developed a physical model of EHDA for the cone-jet mode. They found for ethylene glycol that q ∝ d1.5. This result was obtained by measuring the droplet velocities in a spray with a laser Doppler anemometer and by using these measurements to verify the model. Subsequent simulations and experiments showed that, depending on the break-up mechanism of the jet, the charge of the primary droplet was proportional to the diameter with the power either 3/2 or 2 (Hartman15). A study was conducted in order to have a better insight into the droplet production in our process. The liquid used during all of the experiments was 1-butanol. The electrospraying of this liquid in the cone-jet mode was observed using a high-speed spray imaging system (HSSIS from Oxford Laser). It consists of a highdefinition digital camera and a pulse diode laser. The camera records digital pictures of 1008 × 1008 points. It is connected to a long-distance microscope lens enabling resolution of 1 mm detail to be achieved when taken 5 cm away from the object. The minimum laser pulse duration is 1 µs. The experiments consisted of observing the jet breakup mechanism for different butanol flow rates through

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Figure 3. Flow-rate variation for butanol jet breakup (15 × 10-6 S/m).

Figure 4. Flow-rate variation for butanol jet breakup (30 × 10-6 S/m).

the nozzle and also for different butanol conductivities. The different conductivities of the solutions were obtained by addition of salts to the solution (LiCl). Digital pictures of the jet breakup were taken, and the droplet sizes were measured. Four different conductivities were studied, for each with 10-15 different flow rates. The four different conductivities used for the solutions were 15, 30, 50, and 60 S/m. The flow rates were varied from 1 to 25 mL/h. More than 500 digital pictures were made. A selection is given in Figures 3 and 4 to illustrate the main trends. In Figure 3, photographs of the jet breakup for different butanol flow rates through the nozzle are displayed. From the left to the right the flow rates are 1, 3, 5.5, and 12.5 mL/h. The conductivity of the solution in that case is 15 × 10-6 S/m. In Figure 4 the photographs displayed correspond to butanol with a conductivity of 30 × 10-6 S/m and from left to right a flow rate of 5.5 and 10 mL/h. These photographs indicate four trends. First, the increase of droplet size with increasing flow rate. In Figure 3 from left to right the primary droplet sizes are 12, 22, 31, and 43 µm. Second, the increase of droplet size with decreasing conductivity. The third photograph from the left of Figure 3 and the first from Figure 4 have the same flow rate. The primary droplet sizes are respectively 22 and 25 µm. Third, the occurrence of secondary droplets and even satellites (tertiary droplets) with increasing flow rate. Fourth, the occurrence of asymmetric breakup of the jet with increasing flow rate for a given conductivity. This is shown in the second photograph of Figure 4 and occurs for every butanol conductivity if the flow rate is high enough. Contrarily for sufficiently low flow rate, the breakup of the jet is axisymmetric. This study allowed us always to operate the electrosprays in the cone-jet mode, with axisymmetric breakup of the jet and without the occurrence of secondary droplets. 2.2. Bipolar Coagulation. The charged droplets are produced by two electrosprays operated in the cone-jet mode. This allows the use of the scaling laws to adjust the parameters such as conductivity, surface tension, liquid flow rate, and the voltages to ensure similar mean droplet size and charge for both sprays. In an ideal case both electrosprays would produce droplets of the same size and with the same but opposite charge. The concentration of the reactants in each spray is adjusted so that the required stoichiometry is achieved when the coalescence of two primary droplets takes place. This ideal case is hardly achievable. Nonetheless,

a careful formulation of the solutions and precise measurement of their conductivities will ensure an approximate similarity of both sprays in terms of both charge and size of the droplets. Later in this paper it is shown that the case where one positive droplet meets one negative droplet is the most frequent. Any other combination results in droplets which have a net charge. These charged droplets are collected in an electrostatic precipitator, which is positioned at the exit of the reactor. This procedure ensures that the product contains only nearly neutral particles with the required composition. Thus, the main features of bipolar coagulation are the mixing of the reactants, the chemical reaction inside the droplets, and the neutralization of the charged electrosprays. These three steps have been investigated through a number of experiments. 3. Experimental Validation of the Bipolar Coagulation The aim of these experiments was to demonstrate the features of bipolar coagulation. The neutralization of the sprays by each other, the micromixing of reactants, and the occurrence of chemical reactions inside the droplets were studied. 3.1. Neutralization of the Electrosprays. An experiment was conducted in order to determine whether the neutralization of the sprays is effective. The charge remaining on the coagulated droplets was determined. 3.1.1. Experiments and Results. An electrostatic precipitator was placed between the reactor and a particle counting device [condensation particle counter (CDC) TSI model 3022]. The size distribution of the droplets before and after the precipitator was measured with an aerosizer (Amherst Process Instruments) (Figure 5). The voltage applied to the electrostatic precipitator was varied, and the corresponding particle concentrations and size distributions were recorded. This allowed an assessment of the charge on the coagulated droplets to be made. A clean and constant nitrogen flow conveys the droplets produced through the precipitator. In this experiment, pure 1-butanol solutions were sprayed from both sides. In Figure 6 the relative number of droplets measured by the CPC (in percent) is plotted as a function of the voltage difference in the precipitator. At 500 V a lower limit for the relative number of droplets was reached. This indicates that about 53% of the coagulated droplets was nearly neutral. In Figure 7 the size distribution of the droplets entering the electrostatic precipitator is shown on the curve marked “before precipitator”. This distribution has

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Ind. Eng. Chem. Res., Vol. 38, No. 3, 1999 Table 1. Charge Remaining on the Coagulated Droplets

∆V in the precipitator (V)

max charge per passinga droplet (C)

40 60 90 120 200 500 800

1.31 × 1010-14 8.79 × 10-15 5.86 × 10-15 4.39 × 10-15 2.63 × 10-15 1.05 × 10-15 6.59 × 10-16

a

Figure 5. Arrangement for the measurement of particle charge.

Figure 6. Collection of droplets in the precipitator versus voltage.

Figure 7. Size distribution of the droplets before and after the precipitator.

three modes. The first mode, the smallest size, is attributed to droplets formed by the coalescence of two secondary droplets. The second mode is attributed to droplets formed by the coagulation of some secondary droplets and one primary droplet. Finally, the main peak is attributed to nearly neutral, coagulated primary droplets. The second curve in Figure 7 shows the size distribution at the exit of the electrostatic precipitator when operated with a voltage of 90 V. The first two modes have disappeared, indicating the higher charge-to-mass ratio of those droplets compared with the neutral, coagulated primary droplets. In addition, the mean diameter of the main mode has shifted, indicating a higher electrical mobility of the small particles. 3.1.2. Charge on Coagulated Droplets. The mean diameter of the coagulated primary droplets at the exit of the reactor was 5.71 µm. Calculations showed that Stokes law can be applied to describe particle motion

fraction of the fraction of charge of a droplets passinga droplet relative to the passinga initial droplet through the charge (%) precipitator (%) 2.77 1.85 1.24 0.93 0.55 0.22 0.14

79.5 75.0 66.8 63.1 56.2 53.1 53.1

Droplet not collected in the precipitator.

in the precipitator and that in the precipitator droplet evaporation is negligible. Thus, the maximum charge carried by a droplet which was not collected has been calculated for different values of ∆Vprecipitator. Comparison of this charge with the charge on a primary droplet indicates the extent of the neutralization in the process. These results are listed in Table 1. When ∆Vprecipitator ) 40 V, 79.5% of the droplets exiting the reactor were not collected. With their sizes shown, the maximum charge carried by those droplets passing was calculated. This is 1.31 × 10-14 C. Similarly, 53.1% of the droplets exiting the reactor have a maximum charge of 6.6 × 10-16 C. Evaporation of the droplets has taken place in the reactor. The initial diameter of the primary coagulated droplet was calculated using the mean diameter at the reactor exit and evaporation theory for moderately volatile liquid droplets (Hinds19). Then, dividing this volume by 2 indicates the size of a primary droplet to be 33.3 µm. The charge on this droplet was calculated by dividing the total current of the butanol spray (67.6 nA) by the flow rate (2.77 × 10-9 m3/s) and multiplying by the volume of the primary droplet (1.933 × 10-14 m3). The charge of a primary droplet was thus 4.73 × 10-13 C. This is 60% of the Rayleigh charge limit for such a droplet. Consequently, 6.6 × 10-16 C was found to be the maximum charge for 53.1% of the coagulated droplets. This charge represents 0.14% of the charge of a primary droplet and 1.2% of the Rayleigh limit for such droplets. These results show that the neutralization of an electrospray by another is effective. This level of neutralization and the width of the size distribution in Figure 7 indicate that the controlled coagulation of oppositely charged droplets is achievable. Further proof of the effectiveness of this mechanism was obtained through the study of the micromixing of compounds in the particles produced. 3.2. Micromixing in Droplets. The coagulation of two droplets creates a mixing of chemical compounds on a microscale. This ensures good mixing in a powder on the scale of one particle. 3.2.1. Experiments and Results. Particles containing zirconium and titanium in a given ratio were produced. The experimental setup used is shown in Figure 8, while in Figure 9 the concept of these experiments is shown. The conductivity of the solutions was measured with a WTW microprocessor conductivity meter LF96. It was assumed that the surface tension, viscosity, density, and relative electrical permittivity of the solutions were the same as those for the solvent because


Figure 8. Experimental arrangement for micromixing.

Figure 9. Concept of micromixing experiments. Table 2. Results of Mixing Experiments

expt

initial nTi/nZr in the electrosprays

mean nTi/nZr measured in the particles

average absolute deviation from the mean

a b c d e

1/10 1/2 1/1 2/1 10/1

0.085 0.509 0.98 1.96 9.17

0.012 0.078 0.25 0.36 2.13

the concentrations used were very low. The solvent used was 99.8% anhydrous 1-butanol (Aldrich). The solutes were titanium tetrabutylate (Aldrich) of 99% purity and zirconium tetrabutylate available as a solution of 80% mass in 1-butanol (Aldrich) without an indication of its purity. The particles produced through bipolar coagulation were driven in a nitrogen flow through a tubular furnace (Carbolite tubular furnace) where pyrolysis of the zirconium and titanium precursors took place, causing the formation of solid particles. Those particles were collected downstream on a ceramic membrane for subsequent analysis. The particles were analyzed by energy-dispersive X-rays (EDX) coupled with transmission electron microscopy (TEM). The analysis was made on single particles. Thus, the quantitative atomic ratio of the elements in a single particle was determined. The values of the atomic ratio are very sensitive to experimental parameters such as shape, morphology, and density which introduce a deviation in the results. For the mixing experiments two solutions were sprayed. On the one side was a solution of Zr(OBu)4/1BuOH with a molar concentration nZr, and on the other side was a solution of Ti(OBu)4/1-BuOH with a molar concentration nTi. In Table 2 the average measured values of the atomic ratio with their average absolute deviation are given for different nTi/nZr ratios. In each case 100 particles were measured. As can be seen, in four cases out of five the ratio measured is lower than the theoretical ratio.

Figure 10. EDX analysis on powders with different Ti/Zr ratios: (a) 1/10, (b) 1/2, (c) 1/1, (d) 2/1, and (e) 10/1.

The results of the EDX analysis for the five powders are shown in Figure 10. As can be seen, in each case a few particles are present with an atomic ratio deviating significantly from the mean. This variation indicates that in each powder particles can be found which contain mainly Ti or Zr. The size of these particles is also smaller than the others. 3.2.2. Discussion and Conclusions Concerning Mixing in the Particles. The assumption by which the coagulation of droplets takes place between two droplets of the same size and of the same charge is an idealized concept of the process. There are, of course, factors which introduce a deviation from the ideal. First, the variation of solute concentration in both solutions results in a variation of the conductivity of the solutions. This affects the mean size of the primary droplets.

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Table 3. Concentration and Conductivity of the Solutions for Mixing Experiments conc (M)

expt a b c d e

titanium/ butanol solution 10-3

5 × 10-3 10-2 10-2 10-2

zirconium/ butanol solution 10-2

10-2 10-2 5 × 10-3 10-3

ratio Ti/Zr 1/10 1/2 1/1 2/1 10/1

conductivity (µS/m) Ti/BuOH Zr/BuOH solution solution 29 27.8 30.2 28.2 30.2

30.5 30.5 30.1 24 28.4

Second, the size distribution of the primary particles is narrow but is not monodisperse, and this means that particles of slightly different size can coagulate. A difference of 1% in the size of two coagulating primary droplets will lead to a variation of 3% from the initial nTi/nZr ratio in the resulting particle. This means that small variations of the solution properties can have a large impact on the final ratio of the product in the particles. This is probably the main reason for the deviation found between the expected atomic ratio and the average measured value. Concerning the particles with an atomic ratio deviating significantly from the mean, their smaller sizes give an indication of the mechanism of their formation. These particles are probably formed from secondary droplets of the spray. Even when the sprays are operated with a low flow rate in order to avoid the formation of these secondary droplets, some are still created. Study of digital pictures of the jet breakup in our conditions implies that there might be 1 secondary droplet formed for every 12 primary droplets. During EHDA of liquids, the secondary droplets are segregated to the outside of the spray because of their higher electrical mobility. This means that these secondary droplets are moving in areas of the spray of lower concentration, and they thus have a lower chance of coagulating than the primary droplets which are concentrated at the center of the sprays. When they do not coagulate, these secondary droplets are subjected to Rayleigh explosion at a certain stage. Thus, they evaporate and their charge-to-surface ratio increases until disruption of the surface of the droplet occurs and it breaks up into small charged fragments. Hence, the hypothesis suggested for the formation of particles with an atomic ratio significantly deviating from the mean is as follows. On one side a secondary droplet containing either titanium or zirconium meets some highly charged small fragments resulting from the explosion of a secondary droplet from the other side. A droplet with a lower charge is formed which contains mainly one of the solutes and little of the other. This droplet leads to a particle with a significantly deviating atomic ratio. During these experiments particular attention was paid to the properties of the solution. The aim was to have in both sprays, in each experiment, the same properties. Thus, both sprays were operated with the same flow rate. To compensate for the differences in the conductivities of the solution due to differences of the solute concentrations, the conductivities were adjusted in each case by adding LiCl salt to the solution. However, small variations in the conductivities of the solutions still persist. This is due to the fact that only extremely small amounts of LiCl are needed. The conductivities of the solutions used in the five experiments of mixing are given in Table 3. According to these conductivities, the droplets sprayed from the Ti/BuOH solution should be bigger than the

droplets formed in the Zr/BuOH spray in experiments a and b. This again should lead to a measured ratio that is bigger than the initial ratio. As seen in Table 2, the opposite was found. However, for experiments d and e the difference in conductivity corresponds to the expected ratio. These results seem to indicate that here the conductivity is not the parameter most influencing the atomic ratio. Critical factors are probably the slight difference in the flow rate of the two sprays, the measuring method itself, and in particular the algorithm used to calculate the ratio of the atomic species inside the particles. However, it appears possible to mix two species on a microscale using the bipolar coagulation process. The fact that every particle measured contained atoms of both metals (Ti and Zr) supports the concept of bipolar coagulation. When it is ensured that the electrosprays operate in the cone-jet mode, it is possible to mix products in a desired ratio. 3.3. Microreaction in the Droplets. The coagulation of two droplets can lead to chemical reactions in the resulting droplet. This is the microreactor concept. In most of the chemical reactions, the limiting factor is the diffusion of the reactants. Performing this reaction in a micron-sized environment should result in faster mixing and higher conversion. This feature of the bipolar coagulation is very attractive, but it raises two problems. First, the evaporation of the coagulated droplet must be carefully controlled. Evaporation causes an increase in concentration of the reactants and might induce their precipitation before they completely react. Second, each of these coagulated droplets must experience the same physical conditions in terms of pressure and temperature in order to ensure uniformity of the reaction kinetics. This means that the transport of the coagulated droplets must take place in a controlled environment. 3.3.1. Hydrolysis and Condensation Reactions of Metal Alkoxides. The reactions that were chosen to test the reaction in droplets are the hydrolysis and condensation of metal alkoxides. These reactions have been widely studied in the development of the sol-gel process. Certain characteristics of these reactions make them suitable for the study of bipolar coagulation. It is reported by Bradley et al.20 that the hydrolysis of the metal alkoxides by water is almost instantaneous. The fast kinetics are interesting because it can be assumed that when a droplet containing a metal alkoxide coagulates with a droplet containing water, a hydrolysis reaction will take place.

M(OR)n + nH2O f M(OH)n + nROH Actually, not every metal alkoxide exhibits fast kinetics in hydrolysis; it can vary considerably from one metal cation to another. For these experiments the metal alkoxides used were titanium tetrabutylate (Ti(OBu)4) and zirconium tetrabutylate (Zr(OBu)4). For these compounds hydrolysis is reported to be fast and will thus be a good indication of the reaction taking place in the droplets. However, for demonstration purposes, it was necessary to ensure that the hydrolysis could not take place in other ways, in particular, to ensure that the carrier gas was moisture-free and the solvent was anhydrous. Anhydrous butanol was used as the solvent for these metal alkoxides. Its volatility offered an additional advantage. Brinker and Scherer21 have noted that the


quick evaporation of the solvent allows the formation of denser particles. Thus, quick evaporation of the butanol was expected to lead to the formation of dense particles. The use of metal alkoxides as precursors presented another advantage. It is known that the conductivity of the precursor solution has a great influence on the electrospray. High conductivities lead to a smaller range of flow rate for the cone-jet mode, thus limiting the total spray output. In the case of the metal alkoxide solutions, the conductivity remains very low (∼20 × 10-6 S/m) even for very high volume concentrations (up to 50%). Thus, the concentrations can be varied widely while the sprays are still operating in the stable cone-jet mode. This allows control over the final particle size. Metal hydroxides which are formed by hydrolysis react together. Two types of reactions take place: alcoxolation and oxolation. These reactions are called condensation reactions. They are considered to be an inorganic polymerization of the metal hydroxides and lead to a network of metal oxide. The condensation reactions will take place simultaneously with the hydrolysis reactions but, nevertheless, their kinetics are much slower than the kinetics of the hydrolysis (Brinker and Scherer21). The residence time in the total setup varied from 30 s to 1 min. There was not enough time for the metal hydroxides formed through hydrolysis to condense, which means that the particles formed contained partially reacted metal hydroxides. The inhomogeneities induced by these partial reactions hindered the analysis of the particles. Thus, it was necessary to heat the particles produced in order to obtain completed metal oxide particles. A high-temperature tubular furnace was placed after the reactor. The particles formed in the reactor were conveyed in a nitrogen flow through the furnace and were subsequently collected on a ceramic membrane. The temperature attained was sufficient to decompose the metal hydroxides into the oxides. 3.3.2. Experiments and Results. In one spray there was Zr(OBu)4 in 1-butanol. The other spray contained water in 1-butanol. The two solutions were sprayed with the same voltage, with identical flow rate but with opposite polarity. The nitrogen flowing through the process was filtered and dried. The temperature of the tubular furnace was varied from 400 to 800 °C. The residence time of the droplets in the reactor was varied. The particles produced were analyzed by EDX. Some SEM pictures of the powders were taken. The variation of the residence time in the reactor had a considerable effect on the particle morphology. This variation was exploited by changing the length of the reactor. The residence time after the reactor, namely, in the tubular furnace, was not changed. When the residence time in the reactor was 13 s, spherical particles of 1-2 µm were collected after the furnace (Figure 11a). For a residence time of 2.5 s the size of the particles collected on the filter is approximately 100 nm (Figure 11b). The particles were spherical and had a small dispersity of size. These characteristics are consistent with a mechanism of formation involving homogeneous precipitation from the gas phase. When the residence time before the furnace was short, the zirconium precursor entered the furnace in a liquid state, evaporated in the furnace, and recondensed at its exit before reaching the filter. The hypothesis of Rayleigh explosion in the oven due to precursor evaporation

Figure 11. Residence time: (a) 13 s; (b) 2.5 s.

Figure 12. EDX analysis on produced ZrOx particles as a function of furnace temperature.

has to be ruled out because of the low charge on the coagulated droplets. In the case of a long residence time, the process of solvent evaporation did not take place as indicated by the formation of micron-sized particles. This implies that a solid was formed in the droplet because the reaction had time to take place. The results obtained with different temperatures of the tubular furnace are shown in Figure 12. In this graph the atomic ratio of zirconium and oxygen is plotted versus the particle size. A constant ratio could be expected for a given oven temperature but, in any case, the ratio appears to increase with the particle size. This is due to the EDX analysis of single-micron-sized particles. The radiation emitted by the oxygen has a lower energy than that from the zirconium. Part of the oxygen signal is absorbed by the surrounding zirconium atoms, and the bigger the particle, the more it is absorbed. This explains the apparent increase of the ratio of zirconium to oxygen (Zr/O) with particle size. In Figure 12, it can be seen that for a given size the ratio increases with temperature from 400 to 600 °C but that there is less difference between the 600 and 800 °C experiments. The increase of this ratio with temperature is due to a more complete reaction in the particle. The metal hydroxides formed by hydrolysis of the precursor metal organic react toward the metal oxide in the furnace. The fact that the product is similar at

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600 and 800 °C tends to indicate that the oxide is already formed at 600 °C. The entire process of metal oxide formation which occurs in the particles can only take place if the precursor has already been hydrolyzed. This is clearly so and proves that the reaction between Zr(OBu)4 and water took place in the coagulated droplets. These sets of experiments thus demonstrate the feasibility of the process of bipolar coagulation. In addition, a simple mass balance was performed on the process. It led without any optimization to a 30 mass % efficiency. Most of the losses are attributed to droplet collection on the earthed counter electrodes of the EHDA system. Their polarization and an improved design will lead to an increase of the efficiency. 4. Conclusions A new process referred to as bipolar coagulation has been presented. The principle is based on the controlled coagulation of droplets through the use of electrical forces. The original intention of this process was the improvement of the characteristics of powders produced by spray processes. This process not only improves the characteristics of known powders but also creates new opportunities for powder production in general. This is due to the features of this process which have been demonstrated in this paper: mixing on a microscale and reactions occurring inside the droplets. The potential of the bipolar coagulation process has been demonstrated by a number of experiments. The atomization of the solutions in the cone-jet mode is controlled through electrical measurements on the sprays. It is possible to ensure that both sprays are producing droplets with equal charge and size and with a narrow size distribution. Furthermore, the neutralization of one spray by the other is established. The low level of charge of the coagulated droplets is an indication of this mechanism. This mechanismsthe coagulation of one positive main droplet with its negative counterpart leading to coagulated dropletssis confirmed through the micromixing experiments. It is shown that it is possible to mix two reactants on a microscale with a desired stoichiometry. Eventually, it is shown that a fast chemical reaction, in this case the hydrolysis of zirconium tetrabutylate by water, can be performed inside the droplets. It must be mentioned that the use of only two electrosprays limits considerably the throughput of the process. At most, a few tens of milligrams per hour can be produced. However, the scaling up of this process is achievable. It has already been shown that arrays of electrosprays can be operated.8,15 Moreover, this process is aiming for the production of very specialized products with a high added value. Considering its features, the possible applications and extensions of this process of electrostatic coagulation are considerable in the field of particle technology. For example, it should be possible to coat particles. The coating could be made by using on one side a highly evaporative solvent, leading quickly to a particle (a polymer for example), and on the other side a less evaporative solvent droplet which would collide (because of electrical attraction) with the already solid particle. Then the precipitation of the second solution droplet would take place at the surface of the particle formed from the first solvent, leading to coating of a particle. Bipolar coagulation could also be applied to polymerization in droplets by contacting two droplets,

one containing a solution of the monomer and the other an inductor of the polymerization. Processes such as coprecipitation or liquid-liquid extraction could also benefit from the use of bipolar coagulation. The process is particularly attractive in that it can be used as part of an integral process. In principle, the droplets produced can be passed directly, with a net charge if required, to the next stage of a manufacturing process. Literature Cited (1) Borra, J.-P.; Camelot, D. M. A.; Hartman, R. P. A.; Marijnissen, J. C. M.; Scarlett, B. Apparatus and processes to produce homogeneous or multi layer (coated) powders with controlled size, structure and chemical composition. Dutch Patent 1003442, 1996. (2) Fuchs Uber die Stabilitat und Aufladung der Aerosolen. Z. Phys. 1934, 89, 736. (3) Eliasson, B.; Egli, W. Bipolar CoagulationsModelling and Applications. J. Aerosol Sci. 1991, 22 (No. 4), 429. (4) Otto, E. et al. A Log-normal Model to Describe Bipolar Coagulation. J. Aerosol Sci. 1995, 26S, S645. (5) Slamovich, E. B. Spherical Zirconia Particles via Electrostatic Atomisation: Fabrication and Sintering Characteristics. Mater. Res. Soc. Symp. Proc. 1988, 121, 257. (6) Vercoulen, P. H. W.; Camelot, D. M. A.; Marijnissen, J. C. M.; Pratsinis, S.; Scarlett, B. SnO2 Production by an Electrostatic Spray Pyrolysis Process. In Synthesis and Measurement of Ultrafine Particles; Marijnissen, J. C. M., Pratsisnis, S., Eds.; Delft University Press: Delft, The Netherlands, 1993. (7) Meesters, G. M. H.; Vercoulen, P. H. W.; Marijnissen, J. C. M.; Scarlett, B. Generation of micron-sized droplets from the Taylor cone. J. Aerosol Sci. 1992, 23, 37. (8) Rulison, A. J.; Flagan, R. C. Synthesis of Yttria Powder by Electrospray Pyrolysis. J. Am. Ceram. Soc. 1994, 77 (12), 3244. (9) Chen, D.-R.; Pui, D. H. Y.; Kaufman, S. L. Electrospraying of Conducting Liquids for Monodisperse Aerosol Generation in the 4 nm to 1.8 µm Diameter Range. J. Aerosol Sci. 1995, 26, 963. (10) Lenggoro, I. W.; Okuyama, K. Preparation of NanometerSized Zinc Sulphide Particles by Electrospray Pyrolysis. J. Aerosol Sci. 1997, S28, S351. (11) Cloupeau, M.; Prunet-Foch, B. Electrohydrodynamic Spraying Functioning Modes: a Critical Review. J. Aerosol Sci. 1994, 25 (No. 6), 1021. (12) Grace, J. M.; Marijnissen, J. C. M. A Review of liquid Atomisation by electrical Means. J. Aerosol Sci. 1994, 25 (No. 6), 1005. (13) Fernańdez de la Mora, J.; Loscertales, I. G. The Current Emitted by Highly Conducting Taylor Cones. J. Fluid Mech. 1994, 260, 155. (14) Gomez, A.; Tang, K. Charge and Fission of Droplets in Electrostatic Sprays. Phys. Fluid 1994, 6, 404. (15) Hartman, R. P. A. ElectroHydroDynamic atomizationin the cone-jet mode. Ph.D. Dissertation, Delft University of Technology, Delft, The Netherlands, 1998. (16) Ganãn-Calvo, A. M. The Size and Charge of Droplets in the Electrospraying of Polar Liquids in the Cone Jet Mode, and the Minimum Droplet Size. J. Aerosol Sci. 1994, 25S, S309. (17) Ganãn-Calvo, A. M.; Davila, J.; Barrero, A. Current and droplet size in the electrospraying of liquids. Scaling laws. J. Aerosol Sci. 1997, 28, 249. (18) De Juan, L.; Fernańdez de la Mora, J. Charges and Size Distributions of Electrosprays Drops. J. Colloid Interface Sci. 1997, 186, 280. (19) Hinds, W. C. Aerosol Technology; John Wiley & Sons: New York, 1992. (20) Bradley, D. C.; Mehrotra, R. C.; Gaur, D. P. Metal Alkoxides; Academic Press: London, 1978. (21) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: London, 1990.

Received for review July 8, 1998 Revised manuscript received December 14, 1998 Accepted December 15, 1998 IE980435J

Bipolar Coagulation Process for the Production of Powders - American

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