Metal Iodate-Based Energetic Composites and Their Combustion and Biocidal Performance.

The biological agents that can be weaponized, such as Bacillus anthracis, pose a considerable potential public threat. Bacterial spores, in particular, are highly stress resistant and cannot be completely neutralized by common bactericides. This paper reports on synthesis of metal iodate-based aluminized electrospray-assembled nanocomposites which neutralize spores through a combined thermal and chemical mechanism. Here metal iodates (Bi(IO3)3, Cu(IO3)2, and Fe(IO3)3) act as a strong oxidizer to nanoaluminum to yield a very exothermic and violent reaction, and simultaneously generate iodine as a long-lived bactericide. These microparticle-assembled nanocomposites when characterized in terms of reaction times and temporal pressure release show significantly improved reactivity. Furthermore, sporicidal performance superior to conventional metal-oxide-based thermites clearly shows the advantages of combining both a thermal and biocidal mechanism in spore neutralization.

Bionanoparticles as candidate reference materials for mobility analysis of nanoparticles.

We propose bionanoparticles as a candidate reference material for determining the mobility of nanoparticles over the range of 6 × 10(-8)-5 × 10(-6) m(2)V(-1)s(-1). Using an electrospray differential mobility analyzer (ES-DMA), we measured the empirical distribution of several bionanoparticles. All of them show monomodal distributions that are more than two times narrower than the currently used calibration particles for mobility larger than 6 × 10(-8) m(2)V(-1)s(-1) (diameters less than 60 nm). We also present a numerical method to calculate corrected distributions of bionanoparticles by separating the contribution of the diffusive transfer function. The corrected distribution is about 20% narrower than the empirical distributions. Even with the correction, the reduced width of the mobility distribution is about a factor of 2 larger than the diffusive transfer function. The additional broadening could result from the nonuniform conformation of bionanoparticles and from the presence of volatile impurities or solvent adducts. The mobilities of these investigated bionanoparticle are stable over a range of buffer concentration and molarity, with no evidence of temporal degradation over several weeks.

Aggregation kinetics of colloidal particles measured by gas-phase differential mobility analysis.

We demonstrate the utility of electrospray gas-phase ion-mobility analysis as a new method to investigate nanoparticle flocculation, or aggregation. Au nanoparticle (Au-NP) solutions were sampled via electrospray (ES), followed by differential ion-mobility analysis (DMA) to determine the particle mobility distribution. Multimodal size distributions obtained with ES-DMA indicated the presence of single Au-NPs (monomer) as well as larger Au-NP clusters such as dimers, trimers, and tetramers under specific solution conditions. The fraction of each aggregate species as a function of time was quantitatively characterized, from which the degree of aggregation, aggregation rate, and stability ratio at different ionic strengths were determined. The latter enabled the extraction of a surface potential (or surface charge density) of 64 +/- 2 mV for 10 nm Au-NPs, which is in good agreement with values obtained from other methods, thus validating our approach. Our results show that ES-DMA is a valuable tool for quantitatively probing the early stages of colloidal aggregation or as a preparatory tool for the size election of aggregates.

Gas-phase ion-mobility characterization of SAM-functionalized Au nanoparticles.

We present results of a systematic examination of functionalized gold nanoparticles (Au-NPs) by electrospray-differential mobility analysis (ES-DMA). Commercially available, citrate-stabilized Au colloid solutions (10-60 nm) were sized using ES-DMA, from which changes in particle size of less than 0.3 nm were readily discerned. It was found that the formation of salt particles and the coating of Au-NPs by salt during the electrospray process can interfere with the mobility analysis, which required the development of sample preparation and data correction protocols to extract correct values for the Au-NP size. Formation of self-assembled monolayers (SAMs) of alkanethiol molecules on the Au-NP surface was detected from a change in particle mobility, which could be modeled to extract the surface packing density of SAMs. A gas-phase temperature-programmed desorption (TPD) kinetic study of SAMs on Au-NPs found the data to be consistent with a second-order Arrhenius-based rate law, yielding an Arrhenius factor of 1.0 x 10 (11) s (-1) and an activation energy approximately 105 kJ/mol. For the size range of SAM-modified Au-NP we considered, the effect of surface curvature on the energetics of binding of carboxylic acid terminated SAMs is evidently negligible, with binding energies determined by TPD agreeing with those reported for the same SAMs on planar surfaces. This study suggests that the ES-DMA can be added to the tool set of characterization methods used to study the structure and properties of coated nanoparticles.

In-flight kinetic measurements of the aerosol growth of carbon nanotubes by electrical mobility classification.

We describe an on-the-fly kinetic study of gas-phase growth of multiwalled carbon nanotubes. The methodology employs electrical mobility classification of the CNT, which enables a direct measure of CNT length distribution in an aerosol reactor. The specific experiment employs two mobility classification steps. In the first step we mobility classify the catalyst particle, in this case Ni, created by pulsed laser ablation, to generate a stream of monodisperse particles. This then determined the diameter of the CNT, when a hydrocarbon/H2 mix is added in a heated aerosol reactor. A second electrical mobility classification step allows us to determine the length distribution of the CNTs. We found that CNT growth from ethylene required the addition of small amounts of water vapor, whereas growth from acetylene did not. We show that acetylene, which always has small amounts of acetone present when purchased, can provide the oxygen source to prevent catalyst coking. By varying the temperature of the growth, we were able to extract Arrhenius growth parameters. We found an activation energy for growth approximately 80 kJ mol(-1) from both acetylene and ethylene, which is considerably lower than previous works for substrate-grown CNTs (E(a) = 110-150 kJ mol(-1)). Furthermore, we observed that our aerosol CNT growth rates were about 2 orders of magnitude higher than those for substrate-grown CNTs. The dominant growth mechanism of CNT previously proposed is based upon bulk diffusion of carbon through nickel particles. However, on the basis of the lower activation energy found in this work, we proposed that the possible mechanism of gas-phase growth of CNT is correlated with both surface (E(a) = 29 kJ mol(-1)) and bulk diffusion (E(a) = 145 kJ mol(-1)) of carbon on nickel aerosol particles. Finally, the experimental approach described in this work should be amenable to other nanowire systems grown in the aerosol phase.

Tuning the reactivity of energetic nanoparticles by creation of a core-shell nanostructure.

This article presents a novel method for tuning the reactivity of nanoenergetic materials by coating a strong oxidizer nanoparticle (potassium permanganate; approximately 150 nm) with a layer of a relatively mild oxidizer (iron oxide). The measured reactivity for a nano-Al/composite oxidizer could be varied by more than a factor of 10, as measured by the pressurization rate in a closed vessel (psl/micros), by changing the coating thickness of the iron oxide. The composite oxidizer nanoparticles were synthesized by a new aerosol approach in which the nonwetting interaction between iron oxide and molten potassium permanganate aids the phase segregation of a nanocomposite droplet into a core-shell structure.

Understanding the difference in oxidative properties between flame and diesel soot nanoparticles: the role of metals.

The purpose of this paper is to address the differences observed in the oxidative kinetics between flame and diesel derived soots. In particular, it has been observed that flame soot has a significantly higher activation energy for oxidation than does diesel soot. The hypothesis tested in this paper is that metals, possibly coming from lubricating oils, within diesel generated soot particles may be responsible for this effect. This is supported by the fact that addition of metal additives to diesel fuel is shown to have no effect on the activation energy of soot oxidation. The subject of this paper lies in testing the hypothesis by adding metal directly to a flame and extracting oxidation kinetics. Using a high temperature oxidation tandem differential mobility analyzer (HTO-TDMA) we extract particle size dependent kinetics for the oxidation of flame-derived soot doped with and without iron. We found that indeed addition of iron to a flame reduced the activation energy significantly from approximately 162 +/- 3 kJ/mol to approximately 116 +/- 3 kJ/mol, comparable with diesel engine generated soot with an activation energy approximately 110 kJ/mol. These results are consistent with the idea that small quantities of metals during diesel combustion may play an important role in soot abatement.

In-flight size classification of carbon nanotubes by gas phase electrophoresis.

We demonstrate the use of gas phase electrophoresis to size classify CNTs grown in a continuous aerosol process. The separation process occurs at atmospheric pressure and involves electrostatic mobility separation which classifies fibres on the basis of equivalent projected surface area. This implies that one can, for diameter-controlled CNTs, obtain an on-the-fly determination of the CNT length distribution during CNT synthesis, or alternatively have a method for producing size separated CNTs. The method should be generic to any fibre based material.

Size-resolved kinetic measurements of aluminum nanoparticle oxidation with single particle mass spectrometry.

Aluminum nanoparticles are being considered as a possible fuel in advanced energetic materials application. Of considerable interest therefore is a knowledge of just how reactive these materials are, and what the effect of size on reactivity is. In this paper we describe results of size resolved oxidation rate using a recently developed quantitative single particle mass spectrometer (SPMS). Aluminum nanoparticles used were either generated by DC Arc discharge or laser ablation, or by use of commercial aluminum nanopowders. These particles were oxidized in an aerosol flow reactor in air for specified various temperatures (25-1100 degrees C), and subsequently sampled by the SPMS. The mass spectra obtained were used to quantitatively determine the elemental composition of individual particles and their size. We found that the reactivity of aluminum nanoparticles is enhanced with decreasing primary particle size. Aluminum nanoparticles produced from the DC Arc, which produced the smallest primary particle size (approximately 19 nm), were found to be the most reactive (approximately 68% aluminum nanoparticles completely oxidized to aluminum oxide at 900 degrees C). In contrast, nanopowders with primary particle size greater than approximately 50 nm were not fully oxidized even at 1100 degrees C (approximately 4%). The absolute rates observed were found to be consistent with an oxide diffusion controlled rate-limiting step. We also determined the size-dependent diffusion-limited rate constants and Arrehenius parameters (activation energy and pre-exponential factor). We found that as the particle size decreases, the rate constant increases and the activation energy decreases. This work provides a quantification of the known pyrophoric nature of fine metal particles.

Method of measuring charge distribution of nanosized aerosols.

In this paper, we present the development of a method to accurately measure the positive and negative charge distribution of nanosized aerosols using a tandem differential mobility analyzer (TDMA) system. From the series of TDMA measurements, the charge fraction of nanosized aerosol particles was obtained as a function of equivalent mobility particle diameter ranging from 50 to 200 nm. The capability of this new approach was implemented by sampling from a laminar diffusion flame which provides a source of highly charged particles due to naturally occurring flame ionization process. The results from the TDMA measurement provide the charge distribution of nanosized aerosols which we found to be in reasonable agreement with Boltzmann equilibrium charge distribution theory and a theory based upon charge population balance equation (PBE) combined with Fuchs theory (N.A. Fuchs, Geofis. Pura Appl. 56 (1963) 185). The theoretically estimated charge distribution of aerosol particles based on the PBE provides insight into the charging processes of nanosized aerosols surrounded by bipolar ions and electrons, and agree well with the TDMA results.

Internal pressure and surface tension of bare and hydrogen coated silicon nanoparticles.

We present a study of internal pressure and surface tension of bare and hydrogen coated silicon nanoparticles of 2-10 nm diameter as a function of temperature, using molecular dynamics simulations employing a reparametrized Kohen-Tully-Stillinger interatomic potential. The internal pressure was found to increase with decreasing particle size but the density was found to be independent of the particle size. We showed that for covalent bond structures, changes in surface curvature and the associated surface forces were not sufficient to significantly change bond lengths and angles. Thus, the surface tension was also found to be independent of the particle size. Surface tension was found to decrease with increasing particle temperature while the internal pressure did not vary with temperature. The presence of hydrogen on the surface of a particle significantly reduces surface tension (e.g., drops from 0.83 J/m(2) to 0.42 J/m(2) at 1500 K). The computed pressure of bare and coated particles was found to follow the classical Laplace-Young equation.

Ultrahigh surface area nanoporous silica particles via an aero-sol-gel process.

We describe a new salt-assisted aero-sol-gel approach to produce spherical nanosized mesoporous silica particles. As an alternative to expensive templating mediums in prior works, salt (NaCl) was employed as a templating medium because it is thermally stable, recyclable, and easily leached. Furthermore, we demonstrate the ability to carry out traditional sol-gel chemistry within an aerosol droplet. The role of salt in sol-gel chemistry and aerosol processing was investigated as a function of hydrolysis time. It was verified that salt accelerates the kinetics of silica gelation, and simultaneously becomes an excellent templating medium to support nano-sized pores inside silica structures in the aerosol processing route. The presence of salt results in a roughly ten-fold increasing in the pore specific surface area and pore volume, subsequent to leaching of the salt matrix. The surface area and pore volume of the as-produced nanoporous silica particles was found to increase with increasing sol-gel hydrolysis time.

Hybrid monte carlo method for simulation of two-component aerosol coagulation and phase segregation.

The paper presents the development of a hybrid Monte Carlo (MC) method for the simulation of the simultaneous coagulation and phase segregation of an immiscible two-component binary aerosol. The model is intended to qualitatively model our prior studies of the synthesis of mixed metal oxides for which phase-segregated domains have been observed in molten nanodroplets. In our previous works (J. Aerosol Sci.32, 1479 (2001); Chem. Eng. Sci.56, 5763 (2001); submitted for publication) we developed sectional and monodisperse models where the internal state of the aerosol particles was described. These methods have certain limitations and it is difficult to include additional physical effects into the framework. Our new approach combines both constant volume and constant number Monte Carlo methods. Similar to our previous models, we assume that the phase segregation is kinetically controlled. The MC approach allows us to compute the mean number of enclosures (minor phase) per droplet, average enclosure volume, and the width of the enclosure size distribution. The results show that asymptotic behavior of enclosure distribution exists that is independent of initial conditions, which is very close to the continuum self-preserving distribution. Temperature is a key parameter because it allows for a significant change in the internal transport rate within each droplet. In particular, increasing the temperature significantly enhances the Brownian coagulation rate and lowers the number of enclosures per droplet. As a result, the MC results indicate that the growth of the minor phase can be moderated quite dramatically by small changes in system temperature. These results serve to illustrate the utility of this synthesis approach to the controlled growth of nanoparticles through the use of a majority matrix to slow down the encounter frequency of the minor phase and therefore its particle size.

Dynamic light scattering and angular dissymmetry for the in situ measurement of silicon dioxide particle synthesis in flames.

Particle size measurements have been made of silica formation in a counterflow diffusion flame reactor utilizing dynamic light scattering and angular dissymmetry methods. The results suggest that the techniques compare quite favorably in conditions of high signal to noise. However, the dynamic light scattering technique degrades rapidly as the signal strength declines, resulting in erroneously small particle diameters. As a general rule dynamic light scattering does not seem to possess the versatility and robustness of the classical techniques as a possible on-line diagnostic for process control. The drawbacks and limitations of the two techniques are also discussed.