Vapor precursor process

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.

Bismuth is an important element in many of the new high-temperature, oxide superconductors and in a variety of heterogeneous mixed oxide catalysts. Some of the methods employed in the preparation of these materials, namely sol-gel and chemical vapor deposition processes, require bismuth alkoxides as precursors and a number of papers on these compounds have recently been published.1 One synthetic route to bismuth alkoxides, which avoids the more commonly used trihalide starting materials and the often troublesome separation of alkali metal halides, involves the reaction between a bismuth amide and an alcohol according to the following equation ... 

In recent years, the interest for the preparation of alkylzinc amides has been motivated by their potential apphcation as catalyst for polymerization of propylene oxide, and also as precursors for the MOCVD see Metal-Organic Chemical Vapor Deposition) process. 

The liquid solution CCVD process does not deposit droplets (these evaporate in the flame environment) or powders as in traditional thermal spray processes. The CCVD technology is drastically different from spray pyrolysis In spray pyrolysis, a liquid mixture is sprayed onto a heated substrate, while CCVD atomizes a precursor solution into sub-micron droplets followed by vaporization of said droplets. The resulting coating capabilities and properties described hereafter qualifies CCVD as a true vapor deposition process. For example, depositions are not line-of-sight limited and achieve epitaxy, 10 nm dielectric coatings onto silicon wafers in a Class 100 clean room resulted... 

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,... 

Tin(II) f-butoxide, [(Sn(O-Bu02)2l, as well as the corresponding heterometal alkoxides [M Sn(0-BuOsjz] (M = Ca, Sr, Ba), have been employed as precursors for chemical vapor deposition processes, generating either Sn02 or MSnOs (M = Ca, Sr, Ba). ... 

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. 

The thermal decomposition of organic compounds can also be employed to generate small carbon clusters or atoms. The borderline with chemical vapor deposition (CVD) as presented in the next section is not really fix. In both cases, the method is based on the thermal decomposition of organic precursors. Processes both with and without catalyst have been reported. Contrary to the chemical vapor deposition, however, the catalyst (if applied) is not coated onto a substrate, but the substance or a precursor is added directly to the starting material ( floating catalyst ). The resulting mixture is then introduced into the reactor either in solid or in liquid state by a gas stream. From this point of view the HiPCo-process could also be considered a pyrolytic preparation of SWNT, but due to its importance it is usually regarded as autonomous method. 

SiC-precursor processing can be subdivided into gas (chemical vapor deposition, CVD) and condensed phase methods. CVD, because of microelectronic applications, has spawned an entire field. It will be discussed here only very briefly. We will focus more intensively on the use of silicon-containing organometaUic precursors as condensed phase sources of ceramic materials. 

Another LDI instrument that was similar in principle to LAMMA was developed by Perchalski (1985) that featured the additional selectivity of two stages of mass analysis provided by a triple quadrupole mass spectrometer (QqQ). The LDI QqQ was shown to have potential for use as a probe-type analyzer for molecular analysis of mixtures, as demonstrated by the detection of a mixture of nine antiepileptic drugs by monitoring the precursor ion/product ion pair for each drug (Perchalski et al., 1983). The LDI—QqQ, however, was determined to be too slow to adequately characterize molecules ionized by cationization or anionization after desorption by a single-shot laser. Also, the vaporization/ionization process on the LDI—QqQ was unable to ionize polar, nonvolatile, and/or thermally unstable molecules (Perchalski, 1985). 

A boron nitride fiber derived from a borazine precursor fiber has recently been reported [32]. As with boron nitride fibers from B2O3, the final heat treatment of boron nitride fibers from borazine must be carried out in a reactive atmosphere (also ammonia) to chemically complete the conversion of the precursor fiber. This nitridation step renders the reaction sequence a chemical vapor infiltration process. 

Chemical vapor deposition refers to the formation of a nonvolatile solid material from the reaction of chemical reactants, called precursors, being in vapor phase in the right constituents. A reaction chamber is used for this process, into which the reactant gases are introduced to decompose and react with the substrate to form thin film or powders There are several main classification schemes for chemical vapor deposition processes. These include classification by the pressure (atmospheric, low-pressure, or ultrahigh vacuum), characteristics of the vapor (aerosol or direct liquid injection), or plasma processing type (microwave plasma-assisted deposition, plasma-enhanced deposition, remote plasma-enhanced deposition)... 

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