2.2.1. Distilled Water Aerosol QCM Experiment

A blank experiment was performed with distilled water. The QCM crystal was cleaned with acetone, dried with N₂ gas, and the crystal frequency was recorded. The crystal was exposed to a distilled water aerosol at one minute’s intervals for a total of eight minutes and the frequency change was recorded each time. The result of this blank experiment is shown in Figure 6. After eight minutes the frequency had decreased by 8.577 KHz. This decrease would be expected for a thickness of liquid layer on the crystal that exceeds the hydrodynamic boundary layer. Consequently the Sauerbrey equation [13] does not hold, and from Figure 6 we conclude it holds only up to four minutes.

2.2.2. QCM Collection of NaCl from an Aerosol Drop

Three different concentrations of NaCl were studied. The three concentrations in weight percent were 0.584 (3 expts), 2.922 (5 expts) and 5.844 (5 expts). After the aerosol drops were collected on the QCM crystal and the crystal was dried under ambient condition. These QCM frequency changes between the wet and dry crystals yielded the average weight% of NaCl in an aerosol drop.

Figure 7 is a plot of the weight% of NaCl in the solution generating an aerosol drop vs. the wt% in the NaCl drops captured by the QCM crystal. The least squares slope of this line is 3.6002 consequently the weight of NaCl in an average aerosol drop is 3.60 times that in the solution from which the aerosol was created. We interpret this to reflect evaporation of water into the ambient flowing air stream transporting the aerosol particles.

Consequently, the weight of water collected from an aerosol using a QCM is a practical way to monitor (A) the spatial distribution aerosol drops in an apparatus and (B) the acceleration of slow kinetic processes because of the increase in concentration of the reactants as the droplets evaporate. To prevent drop evaporation it would be necessary to adjust the transporting mobile gas phase to have the same water partial pressure of as the solutions from which the aerosols are formed.

3. Future Work

3.1. Nebulizer Drop Production Using an Ultrasonic Nebulizer

We plan on using piezoelectric generation of nanoparticles from two piezoelectric sources (see Figure 1) each in conducting containers that are connected at the positive and negative potentials respectively of single controlled current power supply. We choose to use two piezoelectric nebulizers operating at the same frequency because they will produce equal micron size drops with

narrow size dispersions (Equation (1)) that can be charged to any desired magnitude by the controlled current power supply. We anticipate the latter will require relatively little current capacity but must have high voltage compliance. The power supply is a key element in our approach.

The electrical circuit is completed by using an inert carrier gas to merge the two aerosols produced by the two piezoelectric sources in the Reactant Chamber. This arrangement produces equal magnitudes of Reactant and Excess Reactant fluxes with opposite signs, i.e., anodic and cathodic fluxes.

Rajian et al. [14] developed an experimental relationship describing the dependence of the drop sizes produced by using an ultrasonic nebulizer.

(1)

In Equation (1) d_d is the drop diameter [m] that is produced, f is frequency of the ultrasonic transducer [Hz], Q is the liquid consumption rate [m³/s] (and depends on the power level), γ is surface tension of the liquid [N/m]. ρ is density of solid in the drop [kg/m³] and η is viscosity of liquid that produces the aerosol [N s/m²]². This equation shows how the size of drops can be controlled conveniently using experimentally controllable variables.

Wang et al. [15] tested this equation by measuring drop size dispersion resulting by changing the various parameters in Equation. They studied the size dispersion of drops with sizes on the order of microns using a nebulizer operating at 1.6 MHz. As we gain experience, we intend to modify the commercial nebulizers to operate in the 10 MHz range with solutions that have 10⁻⁶ solute volume/solution volume concentrations in order to produce nanoparticles as small as ~10 nanometers.

3.2. Aerosol Formation Phase

Nebulizer Details

One nebulizer will contain a solution of the reactions’ Limiting-Reagent and the other will contain the Excess-Reagent. The conducting containers of the nebulizers’ aerosol will be the electrodes that charge their solutions oppositely and the drops leaving the surface of solutions will be entrained in the inert carrier gas. This will produce aerosols containing oppositely charged, monodisperse drops (see Section 3). The charged aerosols will be transported through the apparatus using the inert carrier gas, Helium and will be mixed in a Reaction-Phase Chamber.

The relative fluxes of the oppositely charged aerosols entering the Reaction Phase Chamber will be adjusted for the chosen charge density conditions by varying current used to generate them in nebulizer reservoirs.

3.3. Reaction Phase

In this Phase, oppositely charged drops are transported by the carrier gas from the two nebulizers and mixed in a Reaction Chamber. There the positive and negative drops containing the components of the nanoparticles merge preferentially to form uncharged larger drops that contain the nanoparticles. The probability of these uncharged drops coalescing further with each other drops are small. Any oppositely charged drops that have not yet coalesced together will be removed by a Filter stage on being transported from the Reaction Phase Chamber to the Collection Phase.

3.4. Filter Phase

The Filter comprises a pair of potential positive and negative electrodes with a high enough potential difference them to collect charged particles, but not large enough to charge the neutral particles containing the nanoparticles. The geometry of these electrodes will be such that the transit time of drops entering and leaving the filter will be long compared to the time required for all charged drops in the carrier gas to reach an electrode Liquids formed from the scavenged droplets will be collected and processed appropriately. Consequently only uncharged droplets containing the nanoparticles will exit to the Processing Phase Chamber.

3.5. Processing Phase

Drops reaching the Processing Phase Chamber will pass through a region that evaporates solvent and other volatile species that may exist in the uncharged drops because they were formed along with the nanoparticles. Additional chemical steps may also be carried out either before or after heating the aerosol to produce the dry nanoparticles that can be collected in a form for their intended use. For an example, Ag(NH₃)₂Cl nanoparticles could be produced in this Processing Phase by exposing an uncharged drop containing nanoparticles of AgCl to NH₃ gas.

Appropriate heat treatment in this zone could convert nanoparticles synthesized in the Reaction Phase to other compounds by thermal treatment, e.g., calcium oxalate, magnesium ammonium phosphate and ferric bicarbonate to nanoparticles of calcium carbonate, magnesium pyrophosphate and Fe₂O₃ respectively.

If desired films of nanoparticles on a substrate could be formed by impacting the carrier gas containing the uncharged drops on a substrate heated to a temperature that produces the desired chemical nano-film composition.

4. Conclusion

Monodisperse nanoparticles can be produced by merging nanodrops in one aerosol with nanodrops in another aerosol. The merging of the two nanodrops yields a mixture that will react to form a product whose mass is determined by the masses of the reactants in the merged drop. Our goal is to assemble a compact desk top apparatus that will carry out the production of the necessary charged aerosols, merging them, and separating the resultant nanoparticles from solvent and reaction byproducts.

REFERENCES