From the digitized pictures of soot clusters formed after the explosion of a hydrocarbon gas mixed with oxygen, the cluster morphology was determined by two different methods: structure factor and perimeter analysis. We find a hybrid, superaggregate morphology characterized by a fractal dimension of D approximately equal to 1.8 between the monomer size, ca. 50 nm, and 1 microm and D approximately equal to 2.6 at larger length scales up to approximately 10 microm. The superaggregate morphology is a consequence of late-stage aggregation in a cluster-dense regime near a gel point.

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We demonstrate that an aerosol can gel. This gelation is then used for a one-step method to produce an ultralow density porous carbon material. This material is named an aerosol gel because it is made via gelation of particles in the aerosol phase. The carbon aerosol gels have high specific surface area (200-350 m 2 /g), an extremely low density (2.5-5.0 mg/cc) and a high electrical conductivity, properties similar to conventional aerogels. The primary particles of the carbon aerosol gels are highly crystalline with a narrow (002) graphitic X-ray diffraction peak. Key aspects to form a gel from an aerosol are large volume fraction, ca. 10 −4 or greater, and small primary particle size, 50 nm or smaller, so that the gel time is fast compared to other characteristic times.

Silica (SiO 2 ) aerosol gels were formed via Brownian aggregation of silica nanoparticles in a closed reaction chamber. A sudden and quick detonation reaction of pyrophoric silane (SiH 4 ) with either oxygen or nitrous oxide created silica nanoparticles with diameters ranging from ∼22 to 90 nm in the presence of an inert background gas with a volume fraction of ca. 10 −4 , conditions necessary for gelation. The background gas was necessary for quick thermal quenching of freshly formed silica molecules and molten nanoparticles and some control of the particle size could be achieved by variation of the gas. The silica aerosol gels were found to have very low densities in the range 4-15 mg/cm 3 and high specific surface areas of 300-500 m 2 /g. Wide angle X-ray diffraction showed that the nanoparticles were amorphous silica. Neutron scattering showed that they were arranged in networks with a fractal dimension of 1.75 between 10 and 1000 nm length scales. INTRODUCTIONIn previous work, we presented an aerosol gelation method to produce porous materials with high specific surface area and extremely low density (Dhaubhadel et al. 2007). The method involved the gelation of nanoparticles in the aerosol phase to yield a material that we have named an "aerosol gel." Unlike wellknown aerogel materials which begin with a liquid phase sol-gel step, aerosol gels are made in the gas phase. Simply said a cloud of smoke in a volume gels or freezes to form a volume spanning, very light weight, porous body; truly "frozen smoke." The initial aerosol is composed of nanometer-sized particles produced rapidly by exploding in a chamber a hydrocarbon precursor with an oxidizer, e.g., oxygen. The nanometer particles so produced aggregate and then gel on the order of tens of seconds to form the aerosol gel. The carbon materials we made previously have densities as low as 2.5 mg/cc. The current state-of-the-art Address correspondence to C. M. Sorensen, Department of Physics, Kansas State University, Manhattan, Kansas 66506, USA. E-mail: sor@phys.ksu.edu for manufacture of aerogel materials is the sol-gel/supercriticaldrying method (Brinker and Scherer 1990; Hüsing and Schubert 1998). The gas phase aerosol gelation method is a significantly different than this state-of-the-art and hence might offer advantages because (1) there is no need for a supercritical drying step as for aerogels and (2) the aerosol gel method should be applicable to a greater variety of substances. Disadvantages of our method lie in the current need for a detonation which could be hard to scale up, and it is a batch process.Gelation is a consequence of random aggregation of noncoalescing particles to form ramified fractal aggregates. Fractal aggregates have a mass-size scaling exponent, the fractal dimension D f , that is less than the spatial dimension, d, and this inequality represents a fundamental condition for gelation (Sorensen and Chakrabarti 2011). Because of this, the ratio of aggregate mean nearest neighbor separation to aggregate size declines as the aggreg...

We report the results of a study of the kinetics of a dense aggregating aerosol system that show that the predictions of the Diffusion Limited Cluster Aggregation (DLCA) model no longer hold when the system is dense. We studied a soot aerosol using the small angle light scattering technique created by exploding a mixture of a hydrocarbon gas and oxygen in a closed chamber. The soot particles started as individual monomers, ca. 38 nm radius, grew to bigger fractal clusters with time and finally stopped evolving after spanning a network across the whole system volume. This spanning is aerosol gelation. The study of the kinetics of the aggregating system showed that as the system evolved from a cluster dilute to cluster dense system, the aggregation kernel homogeneity λ evolved from the dilute limit, DLCA value of zero to a value 0.42 +/− 0.05 at the gel point. This evolution is consistent with previous simulation and theory. The experimental value at the gel point is nearly equal to the value of 0.5 predicted by simulation and theory. In addition, the magnitude of the aggregation kernel showed an increase with increasing volume fraction.

We have demonstrated that gas phase explosive combustion can lead to nanoparticle aerosols with sufficiently large volume fraction to cause a volume spanning gel to form on the order of ten’s of seconds. The term “aerosol gel” was coined to name these materials. So far we have made aerosol gels of carbon and silica. These aerosol gels are similar to well-known, liquid-phase, sol-gel synthesized aerogels.