Aerosol processes are used routinely for the commercial production of ultrafine particles (dp < 100 nm) and materials fabricated from them, and for pilot and laboratory scale production as well. Aerosol reaction engineering refers to the design of such processes, with the goal of relating product properties to the material properties of the aerosol precursors and the process conditions. The most important process conditions are usually the aerosol volume concentration (volume of particles per unit volume of gas) and the time/temperature history of the system.
Powdered material produced commercially by aerosol reactors is usually in the form of large agglomerates of particles held together by bonds of varying strength. The smallest, roughly spherical elements are called primary particles. These combine to form aggregates held together by necks resulting from sintering. Agglomerates are undispersed clusters of aggregates held together by weak (van der Waals) forces or binders. Agglomerates can be relatively easily broken down into their aggregate components.
Product properties of interest include primary particle size (and/or size distribution) and substructure (grain boundary, pore size and defect concentrations, and crystalline state). Also of great interest are the properties of the aggregates, including fractal dimension and particle bond energies. Methods have been developed for relating particle size to process conditions and material properties. Commercial technologies for the manufacture of fine particles have been for the most part designed with limited recourse to the principles of particle formation and growth. Requirements for product properties for existing commercial applications are not exceptionally demanding compared with anticipated needs for advanced materials. Commercially produced particles are polydisperse. The individual particles are polycrystalline with grain boundaries, and there may be significant necking between particles. It should be possible to exploit the available theory (and foreseeable advances) to permit significant improvements in product properties while retaining the high throughputs of commercial production methods, without excessive cost increments.
Fine particle formation by aerosol processes almost always takes place by gas-to-particle conversion. Condensable molecules produced by physical or chemical processes self-nucleate to form particles. The nuclei may be as small as a single molecule for refractory materials, but subsequent collision and coalescence leads to the formation of larger particles. Many lab-scale studies have been made to demonstrate novel methods for particle synthesis or to elucidate the mechanisms of particle formation, and there is an extensive literature (included in Selected References section below). Table 4.1 shows some of the more common processes that have been used for fine particle production. The most important full-scale commercial systems are flame reactors used for production of pigments and powdered materials and in the manufacture of optical fibers, and pyrolysis reactors for carbon black manufacture. Nanometer particles are produced in pilot scale evaporation/condensation (EC) generators operated at low pressures, usually a few torr. Commercial and pilot scale reactors are discussed briefly below.
The collision/coalescence mechanism that controls particle formation in the widely used flame reactors is probably important in pilot and laboratory reactors as well. Theoretical analysis of the collision/coalescence mechanism has focused on particles smaller than the mean free path of the gas, the range of interest for nanoparticle synthesis. The basic concepts are discussed at the end of this paper, along with the factors that determine the formation and properties of agglomerates.
Comparison of Aerosol Processes for Powder Production
From Pratsinis and Kodas (1993)
The flame reactor is the most widely used for commercial production of inorganic oxide particles by aerosol processes. The aerosol precursor in the form of a vapor is mixed with oxygen fed into a reaction chamber and burned. Inert gases and fuels such as hydrogen or methane may also be present. An important commercial product made this way is pyrogenic silica with silicon tetrachloride vapor as the aerosol precursor. Pyrogenic silica is used as a filler in silicone rubber and in natural and synthetic rubber and to modify the rheological properties of paints, resins, and inks. Annual worldwide silica production in 1991 was an estimated 105 tons.
The stoichiometry of the reaction (but not the true chemical reaction steps) can be represented by the equations
Since the reaction occurs with water vapor, the process is called flame hydrolysis. The gas leaving the furnace, which contains silica particles, gaseous hydrochloric acid, hydrogen, and a small amount of chlorine, is passed through a series of tubes to provide residence time for agglomeration. The agglomerates are collected in cyclone separators, which may be followed by a bag filter. Flame temperature is varied over the range 1850oF to 2000oF by varying air, hydrogen, and silicon tetrachloride concentrations. At the lower end of the temperature range, the product particle size is smaller and the surface area higher. Nominal particle sizes for the various grades range from 7 to 27 nm, and surface areas range from 100 to 380 m2/g. The particles form as a result of the collision/coalescence mechanism discussed later.
The flame process is also used in the production of nanoparticles from other vapor aerosol precursors. Examples are alumina and titania, commercial products produced from AlCl3 and TiCl4, respectively. Mixed oxides are produced from a vapor precursor mixture, for example, 99% SiCl4 and 1% TiCl4. Zirconium oxide is also produced on a pilot scale.
In the fabrication of optical fibers, a silica aerosol generated by the oxidation of silicon tetrachloride vapor in argon or helium passes into a quartz tube preform about a meter long, 25 mm o.d., 19 mm i.d. The silica aerosol deposits on the walls of the tube by thermophoresis. Detailed information on the particle size distribution of the aerosol that forms has not been published, but the size range is said to be 20 to 100 nm. Since the particle size is in the free molecule range, the thermophoretic velocity is almost independent of the diameter. The rotating tube is heated by a traversing oxyhydrogen torch, which sinters the deposited silica aerosol to form a surface layer without deforming the substrate tube. In this way, the core or cladding is built up layer by layer. The composition of the individual layers can be varied between torch traverses to produce the desired refractive index gradient in the fiber.
The principal dopant added to the silica for control of the refractive index gradient is germanium dioxide, with germanium tetrachloride vapor as the aerosol precursor. After mounting the preform vertically and heating it above the glass-softening temperature, the 1 m preform is drawn into a 125 (m diameter optical fiber 50 to 100 km long. This product of aerosol processes is of a very high purity capable of transmitting light over long distances with very little attenuation.
Carbon blacks, the oldest manufactured aerosols, are an amorphous form of carbon used in rubber, pigments, and ink. The surface area of the blacks used in rubber goods is in the range 10-150 m2/g, and the primary particles have an average diameter of 20 to 300 nm. Pigment carbon black particles are smaller, with areas of 300 to 500 m2/g. Carbon blacks are made by feeding a heavy petroleum oil and air into a reactor, where partial combustion of the oil raises the temperature to 1100-1700oC, causing decomposition of the unburned hydrocarbon. The hot reaction products including carbon black are cooled by a water spray, and the particles are collected by cyclones and bag filters.
Reactors designed to produce higher surface area grades are operated at high gas velocities, temperatures, and turbulence. The reactors have three zones: a mixing zone in which the feedstock is introduced as a spray into a gas/air mixture, a cylindrical reactor where carbon is generated by the chemical reactions and particle formation occurs, and a third zone consisting of a water quench. The mechanisms of particle formation are not well understood but appear to be a combination of nucleation and growth followed by particle collision and coalescence, similar to inorganic oxide particle formation.
Evaporation/Condensation Aerosol Generators
In the evaporation/condensation (EC) generator, a solid material, usually a metal, is evaporated into an inert gas, and the vapor is then cooled by mixing to produce an aerosol. A systematic study of aerosol properties was made in a chamber consisting of a glass cylinder, 0.34 m in diameter and 0.45 m high, fitted with water-cooled stainless steel endplates. Metal samples were placed in an alumina crucible mounted in the chamber and heated by radiation from a graphite heater element. An inert gas, usually argon at 0.5 to 4 torr, was introduced into the chamber and the crucible heated at constant temperature and inert gas pressure. Hot metal vapor from the crucible mixed with cool surrounding inert gas to nucleate and form particles in sizes ranging from about 3-100 nm. The particles were collected by thermophoretic deposition at the top of the chamber. This system was capable of producing a few milligrams of powdered material over a reasonable period of operation; it has served as a prototype for similar systems operated at higher production rates.
The effect of metal vapor pressure on median particle diameter for magnesium and zinc particles was studied at argon pressures of 2.5 and 3.5 torr. The median particle diameter was roughly proportional to the vapor pressure for a given inert gas pressure. Increasing the inert gas pressure significantly increased particle size. An extensive set of samples of aluminum particles and a smaller number of samples of magnesium, zinc, and tin was produced. Electron diffraction showed that in all cases the particles were crystalline. Size distributions for aluminum and other particles were measured and correlated by lognormal size distribution functions. It was concluded that particle size was controlled by particle collision and coalescence processes. More experimental studies are needed using a wider range of materials and operating conditions, with well-controlled geometries and flow regimes. The experimental results need to be analyzed in light of our understanding of aerosol formation processes. While the EC generator operated in the batch mode has a low production rate, it offers the possibility of limiting contamination, since chemical reactions are usually excluded or limited.
Industrial flame reactors are operated at high particle concentrations resulting in high rates of particle collision. Particle size is determined by the rates of particle collision and subsequent coalescence. The collision/coalescence mechanism for particle formation is based on a series of steps assumed to proceed as follows:
- A chemical (or physical) process converts the aerosol precursor to condensable molecules.
- The condensable molecules self-nucleate to form a cloud of stable nuclei, which may be single molecules (corresponding to a nucleation process with zero activation energy).
- Stable nuclei collide and, initially, coalesce to form larger particles. The particles may be liquid or solid during the coalescence period.
- Coalescence ceases or slows significantly, leading to the formation of agglomerate structures.
- Coalescence and neck formation may continue for particles within the agglomerate structures.
These processes may go on simultaneously. For example, chemical or physical processes may continue to generate condensable monomer molecules throughout the process of particle formation. In this case, after an initial surge of particle formation, further releases of monomer molecules will deposit on existing particles without generating new particles.
The size of the primary particles depends on the temperature/time history and material properties. At high temperatures and for very small particles, the individual particles grow because particle coalescence occurs almost on contact, resulting in agglomerates of large individual particles, hence small specific surface area. At low temperatures, however, particle coalescence takes place slowly compared to collisions, producing fractal-like agglomerates with a high specific surface area. The two processes, collision and coalescence, can be incorporated in a single theory by including coalescence in the general dynamic equation for the aerosol particle size distribution. The rate of coalescence is introduced into the general dynamic equation by a term for the rate of surface area reduction; the final rate of approach to a spherical shape is proportional to the difference in the areas of the actual particle surface and a sphere with the same volume. A key material property controlling primary particle size is the solid state diffusion coefficient, D, which appears in the proportionality coefficient for the surface area decay. For a given aerosol time-temperature history and volumetric loading of solids, increasing D tends to increase primary particle size. Since solid state diffusion is an activated process, there is a strong temperature effect on particle formation in aerosol reactors.
Fractal concepts have been extensively used to characterize agglomerate formation and structure. However, fractal models provide little information on the strength of the bonds that hold particles together in agglomerates. Understanding of particle bond energies and agglomerate rigidity is important to downstream processing of particles and the manufacture of ceramic compacts. Based on experiment, hypotheses have been advanced on the effects of aerosol material properties and process conditions on particle-particle bond energies. In addition, the effects of temperature on the rate of restructuring of agglomerates have been used to estimate the activation energies of particle restructuring for silver and copper agglomerates.
Ceramics and metals made of consolidated nanoparticles may have significantly different and improved properties compared with materials made from coarser-grained powders. Previous studies of the behavior of nanoparticle consolidates have emphasized the importance of individual particle size. Nanoparticles are usually generated as chain aggregates. We have discovered that chains of nanoparticles stretch under tension and contract when the tension is relaxed. Figure 4.4 shows the stretching and contraction of a titania nanoparticle chain aggregate (NCA), initially about 250 nm long, composed of primary particles that are approximately 7 nm. There is reason to believe this behavior is general for morphologically similar aggregates of other transition metal oxide particles. NCA polymer-like behavior may help explain the ductility of nanoparticle ceramic compacts and the ability of carbon black and silica additives to increase the tensile strength and elastic modulus of rubber.
Aerogels are rigid structures composed of nanometer-sized particles with a void fraction up to 99%. In some ways, they can be considered giant aggregates held together by strong chemical bonds. Aerogels are of value for thermal insulation, for catalysis, and as electrode materials. The traditional route to the preparation of aerogels is to fix the particles in the form of a gel from the solution phase. Subsequent drying of the gel by a supercritical expansion leaves the skeleton of the interlinked particles, that is, the aerogel. A disadvantage of this technique is the long preparation time and expensive drying step. Efforts have been made to use an aerosol process to build an aerogel-like structure. Aerosol particles of different materials (carbon, brass, and steel) were generated by laser evaporation at atmospheric pressure; large, low-density agglomerate structures were produced by exposing the aerosol to an electric field.
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Collision-Coalescence Mechanism of Particle Formation
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Nanoparticle Aggregates and Aerogels
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Fig. 4.4. (a) Initial shape of a titania NCA on the carbon film of an electron micrograph grid. (b) Stretched NCA connecting the edges of the carbon film that has partly evaporated in the electron beam. (c) Contracted NCA that is vibrating (seen as a blur) at one edge of the carbon film after disconnecting from the other edge. A portion of the NCA embedded in the film remains in focus. (a), (b), and (c) show the same NCA. The exposure time was about 4 s.