Vapors deposition

Chemical vapor deposition (CVD) is chemical synthesis of solids from gaseous precursors or reactants. The solid product is obtained in the form of a coating or a powder. It is used for synthesis of coatings that modify electrical, optical, and chemical properties of materials or improve their surface properties, such as hardness, wear, and corrosion resistance. The combination of metals with ceramic CVD coatings is a particularly successful one. CVD is the technique used for improving the durability of tools and machine parts. It will be discussed here in more detail than other methods of synthesis because it is representative. 

Composite powders, i.e., powder particles that are built up of alternating layers of different materials or particles with surfaces of a material other than the one of the interior are occasionally necessary as components in composites. Powder particles with modified surfaces can easily be made by means of CVD in a fluidized bed, which is of considerable interest because the nature of the interface between component phases in composites determines their performance. Whiskers and fibers 

CVD is one of the most universal techniques for solid state synthesis virtually any material (even the most refractory and inert ones) can be synthesized with CVD under unusually mild process conditions. Important solids made by CVD include the elements Si, C, B, W, Al, other refractory metals, and the transition metals. Binary compounds deposited by CVD are  

Refractory carbides of silicon, boron, and group 4-6 elements Borides of group 4-6 transition elements and rare earths Silicides of group 4-6 metals, in particular Ti, Mo, W Simple oxides of Al, Si, Sn, and group 4-6 metals Mixed oxides such as perovskites 

We will see below that a CVD coating deposited under the right conditions has a good step coverage, i.e., the layer thickness is the same everywhere on a rough or profiled surface. 

Chemical vapor deposition (CVD) involves the formation of a coating by the reaction of the coating substance with the substrate. The coating species can come from a gas or gases or from contact with a solid as in the pack-cementation diffusion process described in Chapter 5. TTie process is more precisely defined as the deposition of a solid on a heated surface by a chemical reaction from the vapor or gas phase (Ref 54). In general, three processing steps are involved in any CVD reaction (1) the production of a volatile carrier compound, (2) the transport of the gas to the deposition 

erosion-, and corrosion-resistance applications extensively utilize CVD coatings, as do rq plications diat require low friction characteristics. Table 23 lists the properties of typical CVD coating materials for these applications. Some materials, such as titanium diboride, titanium 

Titanium carbide 31.4 4.5 17 7.6 High wear and abrasion resistance, low friction 

Titanium nitride 20.6 3.0 33 9.5 High lubricity stable and inert 

Silicon carbide 27.4 4.0 125 3.9 High conductivity, shock resistant 

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. 

4 SOME THOUGHTS ON CARBON-CARBON PROCESSING 14.4.1 Chemical vapor deposition 

In this process, a hydrocarbon gas (CH4, C2H6, CsHg, CeHg) is thermally degraded onto a hot carbon surface depositing pyrolytic carbon and releasing H2. Liebermann has reported 

Zone 1 - Chemical zone at low temperatures, the chemical reaction is slow and the surface reaction rate is uniform on all the pore siufaces. It is the rate controlling and preferred reaction. 

Zone 2 - Pore diffusion control - as the temperature is increased, there is a rapid increase of chemical reaction, which eventually exceeds the rate of supply of the precursor gas into the pores, causing excessive deposition at the outer pore regions and ultimately blocking the pores. 

Zone 3 - Gaseous diffusion control at exterior of solid - since the precursor gas can no longer penetrate the pores, the gas can only produce a coating on the outside. 

Silicon oxide films can be deposited by the pyrolytic oxidation of a silane or alkoxysilane in a chemical vapor deposition (CVD) system. ° In a process using silane 

Typically, this reaction is carried out at atmospheric pressure in a cold-wall CVD system. The growth rate by the silane process is rather high, usually 8-17 A/s. 

CVD is typically used to form thin films but it can also be used to make nanoparticles. Pt-Ru particles 2 nm in diameter have been formed by vapor deposition using commercially available single metal preeursors, namely platinum(II)-acetylacetonate and ruflienitnn(lll)-aeetylaeetonate [48]. The precursors were adsorbed onto carbon black by sublimation and subsequently decomposed at 320 °C in H2 or N2. While particle size was virtually independent of sublimation temperature, the Pt Ru ratio decreased as sublimation temperature increased from 170 to 240 °C. A maximal Pt Ru ratio, equaling the Pt Ru ratio of the precursor salts, was reached at 220 °C. The composition is affected by the sublimation temperature, as the vapor pressures of the two precursors are influenced by the sublimation temperature. This example illustrates how the vapor pressure of the precursors can act as a limitation upon CVD alloy formation [21]. CVD is also limited by processing temperature and mass-transfer kinetics [21]. One final disadvantage of CVD to note is that precursors can be highly toxic and therefore difficult to work with [49]. 

Mechanical methods, such as cathodic sputtering, and thermal methods, like evaporation, are grouped together under the term physical vapor deposition [50]. All PVD experiments have four essential components (1) they occur in vacuum, they have (2) a source to supply the material, called a target, (3) a substrate on which the film is deposited, and (4) an energy supply to transport the material from the source to the substrate. The types of PVD vary in how energy is imparted to the particles and how the energized particles are transported to the substrate. 

Evaporation PVD occurs by heating the material to be deposited. Under vacuum a molecular beam of the eatalyst material is formed thermally. In a molecular beam, atoms and moleeules move in a well-defined direction without colliding [51]. When the system is oil-free and the substrate is atomically clean before deposition, moleeular beam epitaxial (MBE) deposition is possible. Depositing alloys is diffieult with evaporation PVD, sinee the vapor pressures of the various elements may be different and therefore the deposited film becomes richer in the less volatile speeies [51]. 

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Viswanathan R, Thompson D L and Raff L M 1984 Theoretical investigations of elementary processes in the chemical vapor deposition of silicon from silane. Unimolecular decomposition of SiH J. Chem. Phys. 80 4230 0... 

Kent A D, Shaw T M, Moinar S V and Awschaiom D D 1993 Growth of high aspect ratio nanometer-scaie magnets with chemicai vapor deposition and scanning tunneiiing microscopy Science 262 1249... 

Jensen K F and Kern W 1991 Thermal chemical vapor deposition Thin Film Processes II ed J L Vossen and W Kern (San Diego, CA Academic) chapter III-1, pp 283-368... 

Metal organic chemical vapor deposition pOCVD)... 

PECVD. See Plasma-enhanced chemical vapor deposition. 

The linear polymer of PX, poly(p-xylylene) (PPX) (2), is formed as a VDP coating in the parylene process. The energetics of the polymerization set it apart from all other known polymerizations and enable it to proceed as a vapor deposition polymerization. 

During the vapor deposition process, the polymer chain ends remain truly aUve, ceasing to grow only when they are so far from the growth interface that fresh monomer can no longer reach them. No specific termination chemistry is needed, although subsequent to the deposition, reaction with atmospheric oxygen, as well as other chemical conversions that alter the nature of the free-radical chain ends, is clearly supported experimentally. 

Si02, BaTiO capacitors sol—gel, sputtering, chemical vapor deposition (CVD)... 

D. Kuppers and H. Lydtin, Preparation of Optica/ Waveguides with the Aid ofP/asma Activated Chemica/ Vapor Deposition atEow Pressures in Topics in Current Chemist, Vol. 89, Springer-Vedag, Berlin, 1980, pp. 108—130. 

Fluorination of tungsten and rhenium produces tungsten hexafluoride, WF, and rhenium hexafluoride [10049-17-9J, ReF, respectively. These volatile metal fluorides are used in the chemical vapor deposition industry to produce metal coatings and intricately shaped components (see Thin films,... 

Germanium difluoride can be prepared by reduction (2,4) of GeF by metallic germanium, by reaction (1) of stoichiometric amounts of Ge and HF in a sealed vessel at 225°C, by Ge powder and HgF2 (5), and by GeS and PbF2 (6). Gep2 has been used in plasma chemical vapor deposition of amorphous film (see Plasma TECHNOLOGY Thin films) (7). 

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