Vaporized precursors

In chemical vapor deposition (CVD) reactive vapor precursors react to produce solid materials in the gas phase or at the solid-gas interface on the substrate surface at appropriate temperatures. Typical precursors used in the CVD process are metal hydrides, metal chlorides, and metal organic compounds. In the case that the precursor species are metal organic compounds, the process is called metal-organic chemical vapor deposition (MOCVD). The precursor molecules are introduced into a reactor sometimes with a carrier gas and decompose by means of heat, irradiation of UV light, or electrical plasma formed in the gas. Thermal CVD is the most commonly used method. This technique has an advantage that refractory materials can be vapour-deposited at relatively low temperatures,... 

For the initial formation of a solid phase on a substrate surface from vapor precursors through heterogeneous nucleation, as is schematically illustrated in Figure 20.2, the critical nucleus size, r, and the corresponding energy barrier, AG, are given by the following equations ... 

Aerosol-assisted CVD introduces rapid evaporation of the precursor and short delivery time of vapor precursor to the reaction zone. The small diffusion distance between the reactant and intermediates leads to higher deposition rates at relatively low temperatures. Single precursors are more inclined to be used in AACVD therefore, due to good molecular mixing of precursors, the stoichiometry in the synthesis of multicomponent materials can be well controlled. In addition, AACVD can be preformed in an open atmosphere to produce thin or thick oxide films, hence its cost is low compared to sophisticated vacuum systems. CVD methods have also been modified and developed to deposit solid phase from gaseous precursors on highly porous substrates or inside porous media. The two most used deposition methods are known as electrochemical vapor deposition (EVD) and chemical vapor infiltration (CVI). 

The flame process is also used in the production of nanopaiticles from other aerosol precursors. Examples are alumina and titania, commercial products produced from the vapors of AICI3 and TiCU, respectively. Mixed oxides are produced from a vapor precursor mixture—-for example. 99% SiCLj and 1% TiCU. Zirconium oxide is also produced on a pilot scale. 

Particle fonnation is thought to have proceeded as follows Metal oxide molecules formed as the aerosol vapor precursor reacted near the jet orifice. The oxide molecules collided to form particles that grow by the colli.sion-coalescence mechanism until the temperature fell to the point where coalescence was quenched. Particle coalescence was probably driven by solid-state diffusion and. perhap.s. surface diffusion. Metal oxides with higher diffusion coelTicients would be expected to fomt larger primary particles because they continue to coalesce at lower temperatures during the cooling period. 

Quantities of airborne particles in industrialized regions of the Northern Hemisphere have increased markedly since the Industrial Revolution. Atmospheric particles (aerosols) arise both from direct emissions and from gas-to-particle conversion of vapor precursors. Aerosols can affect climate and stratospheric ozone concentrations and have been implicated in human morbidity and mortality in urban areas. The climatic role of atmospheric aerosols arises from their ability to reflect solar radiation back to space and... 

This component generates and supplies the vapor precursors that will be delivered to the reactor chamber. The delivery of precursors depends on the source temperature, carrier gas flow rate, and pressure of the reaction chamber. 

Methods Used tor Liquid or Vapor Precursor Process.482... 

METHODS USED FOR LIQUID OR VAPOR PRECURSOR PROCESS... 

The generic apparatus used in a vapor precursor process is very similar to that used in spray pyrolysis, except that the precursor material is introduced to the reactor as a vapor (see Figure 2.1, 2. la). If the precursor is a liquid, carrier gas is typically bubbled through it. If the precursor is a solid, then the carrier gas is often passed through a heated, packed bed of the material. The vapor-laden carrier gas then flows to a furnace reactor, where thermal decomposition of the precursor occurs and particle formation results. Product powder is collected or measured at the reactor outlet. Flame processes also fall into the vapor precursor/thermal decomposition category of gas-phase powder synthesis. The only difference is that the thermal energy is provided by combustion as opposed to an external source. 

Figure 2.4 Schematic representation of product formation in a vapor precursor process.

Figure 6.36 shows a variety of gas-phase techniques that have been used to synthesize 0-D nanoparticles. Radio frequency plasma sources have long been used for quantitative analysis by atomizing component species in liquid or solid samples - a technique referred to as inductively-coupled plasma atomic emission spectroscopy (ICP-AES). The extreme energy of an ICP may also be exploited to vaporize precursor sources to afford the growth of nanoparticles (Figure 6.36a). In this system, the nanoparticle size/morphology would be mostly controlled by the concentration of precursor in the plasma, and the rate of cooling - a function of its distance from the plasma source.

Figure 2.9 shows a CVD system consisting of a gas metering system, a heated reaction chamber, and a system for the treatment and disposal of exhaust gases [13]. The aluminum or silicon vapor precursor (e.g., AICI3) in a carrier gas is introduced into the reaction chamber that is heated to the desired temperature in advance. This vapor precursor will be chemsorbed on the pore surface to form intermediate species (e.g., -O-AICI2). Subsequently, the pores are evacuated by vacuum to remove all the precursors in the gas phase, and then exposed to water... 

Diffusion coatings can also be formed by pack cementation. In this technique, the diffusion coatings are formed by heating the surface in contact with the material to be diffused (i.e. solid state diffusion) or by heating in a reactive atmosphere where the reactive gas reacts with the solid material to be diffused, thus forming a vapor (vapor precursor) that decomposes on the heated surface and provides the material that diffuses into the surface (similar to CVD... 

Particulates formed by gas phase nucleation of vaporized material or decomposition of chemical vapor precursors. 

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