Formation of Light-weight Low-density Materials via Gas Phase Aerosol Gelation
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Formation of Light-weight Low-density Materials via Gas Phase Aerosol Gelation Christopher M. Sorensen, Rajan Dhaubhadel, Corey S. Gerving, and Amitabha Chakrabarti Department of Physics, Kansas State University, Manhattan, Kansas, USA
ABSTRACT 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. INTRODUCTION In the course of our fundamental studies to understand the physics of dense aerosols [14], we discovered a novel method to produce porous materials with high specific surface area and extremely low density [5, 6]. Our method involves the gelation of nanoparticles in the aerosol phase to yield a material that we have named an “aerosol gel”. Unlike well known aerogel materials which begin with a liquid phase, sol-gel step, our 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 carbon or silica particles produced rapidly by exploding in a chamber any one of number precursor materials such as hydrocarbons or silane with an oxidizer, e.g. oxygen. The nanometer particles so produced aggregate and then gel on the order of ten’s of seconds to form the aerosol gel. The materials we have made have densities as low as 2.5 mg/cc, roughly twice the density of air. The aerosol gel is a new material that has not been described before, and a patent for the production method and material has been awarded [6]. The current state of the art for manufacture of aerogel materials is the solgel/supercritical-drying method [7, 8]. Our gas phase aerosol gelation method is significantly different than this state-of-the-art and hence might therefore 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 Another interesting feature about our method is that during the detonation to quickly make the nanoparticle aerosol, the temperate inside the chamber reaches ca. 5000K during the 30m msec detonation as measured by a two-color pyrometer. This temperature is consistent with thermo-chemical calculations. It is also much hotter than typical flames which are on the order of ca. 2000K. Thus the particles have experienced both a very high temperature and a rapid cooling. Transmission Electron Microscope (TEM) pictures of the carbon aerosol gel show that the primary particles appear to be thin graphitic sheets and are more or less polygonal in structure. Subsequent test are indicating that these materials are a form of graphene. Such high temperatures and rapid quenches may lead to other novel formu
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