Vapor-phase transport

Carpenter, M. D., Gierke, J. S., et al., 1993, Vapor Phase Transport in Physically-Mixed Clay Soils In Proceedings of the 1993 Petroleum Hydrocarbons and Organic Chemicals in Ground Water Prevention, Detection and Restoration. American Petroleum Institute and National Ground Water Association, November 10-12, pp. 479-489. 

Vapor phase transport Requires volatile constituents... 

Vapor Phase Transport Chemical vapor transport is a technique in which polycrystalline starting materials react with a gaseous reactant present in the system to form exclusively gaseous products. These gaseous species then travel to another place in the... 

Palansky J, Kapila S, Manahan SE, et al. 1986. Studies on vapor phase transport and role of dispersing medium on mobility of 2,3,7,8-TCDD in soil. Chemosphere 15 1389-1396. 

Chemical vapor deposition (CVD) The deposition of materials using one or more chemically driven processes that employ vapor-phase transport of the reagents to the deposition zone. 

Gradient sublimation has been used for many years as a means to purify materials and many variations exist and have been proven effective for organic purification. A detailed description of the vapor phase transport method applied to organic materials was given by Ch. Kloc,... 

Zhang, Y. W., Okubo, T. and Ogura, M. Synthesis of mesoporous aluminosilicate with zeolitic characteristics using vapor phase transport, Chem. Commun. 2005, 2719-2720. 

The growth of Pc-based, 1-D nanostructures on gold nanoparticles has also been achieved via vapor-phase transport, in a process in which the gold nanoparticles act as nucleation sites for the perfluorinated CuPc molecules and promote their anisotropic growth leading to very uniform, 1-D structures with high aspect ratio (Fig.28) [212],... 

Interparticle transport may be possible either by surface diffusion across the support or by vapor phase transport. Depending on the supported systems and on the sintering conditions the particles may grow predominantly via one of these possible routes. Supported PcFe deposits have been sintered in experimental conditions close to that of the condensation process (T = 235°C, residual pressure 10 torr). The surface areas of PcFe after 2 and 3 hours of sintering are shown in Table IV. It is seen that the sintering effect is more pronounced with a homogeneous carbon support. 

If we consider the microstructural changes to the particles, it is clear that the spherical particles will alter their shape with time to that of a prolate ellipsoid of revolution by tins transfer of material finm the spherical surface to that of the neck. In fact, the distance between the centers of the particles is not affected by vapor phase transport and only the shape of the pores is changed. Without a decrease in the center-to-center distance there is no densification or sintering. Therefore, this is one example of coarsening during the initial stage sintering. 

There are a number of properties required for compounds to be suitable as precursors for MOVPE these include sufficient vapor pressure to allow suitable growth rates, abihty to be highly purified, controllable deposition behavior, stabihty in storage, and stability with other species during vapor-phase transport. 

In the phases of composition ZrsSbsZ, obtained either by arc melting, vapor-phase transport, or metal flux methods,... 

It is the purpose of this paper to study which mechanism, particle-to-particle or vapor phase transport, is responsible fa- the mobility of vanadium in the FCC unit The approach used in this work is to measure the rate of vanadium transport to a basic oxide "vanadium trap" in fluid bed experiments. By varying the particle size distribution, the collision frequency can be changed and the rate of transport determined. Also, calculations of the mass transfer of a vapor species in a fluid are performed. 

The results of this work show that even though the vapor pressure of vanadium is low, the transfer velocity of vanadium vapor is high and the rate of mass transfer in a fluidized bed is high. A high rate of vanadium transport to traps and a low rate of vanadium transport by transpiration are consistent with the vapor phase transport model. The vapor pressure of the vanadic acid follows a second order Freundlieh isotherm, which reflects a coverage dependent heat of adsorption. The rate of vanadium transfer from catalyst to trap is only weakly dependent on the number density of the catalyst or trap particles. This lack of dependence suggests that inter-particle collisions are not the dominant mechanism for vanadium transfer. Vanadium mobility in FCCU s is a complex issue dependent on many operating variables. 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

The so-called dry gel method [20-22] is another alternative for membrane synthesis where vapors containing templates (i.e., amines) and water are employed to crystallize silica or silica-alumina layers previously deposited onto the support (vapor-phase transport method) or where steam is used to crystallize silica and template or sUica-alumina and template dry layers previously deposited onto the support (steam-assisted crystallization). Using this approach, the reactant consumption is clearly diminished, an important issue for scale-up purposes on the other hand, synthesis time is delayed due to transport-controlled phenomena. 

Matsufuji T, Nishiyama N, Matsukata M, and Uyama K. Separation of butane and xylene isomers with MFI-t5fpe zeolitic membrane synthesized by a vapor-phase transport method. J Membr Sci 2000 178 25-34. 

However, the interpretation of these results in terms of a mechartism is a matter of speculation. Correia and Van Reijen hypothesized that the low rate of sintering after 10 h was due to vapex phase transport of Ni, possibly as NiCl2. This hypothesis was based on two observations (1) XPS analysis indicating traces of chltxide and (2) a dependence of sintering rate on position of sanples in their furnace which was interpreted in terms of vapor-phase transport of... 

In 1990, Xu et al. first reported the transformation of a dry aluminosilicate gel to crystalline MFI by contact with vapors of water and volatile amines, which was named dry gel conversion (DGC).[99] Since then, this method has been extensively studied and a large number of microporous materials with new compositions and structures were prepared. Generally, DGC can be divided into vapor-phase transport (VPT) and steam-assisted conversion (SAC) according to the volatility of the SDAs. For volatile SDAs such as ethylenediamine, a mixture of water and SDA was poured into the bottom of the autoclave and then a dry gel, which does not contain any SDAs, was placed over the liquid surface. Water and SDAs were vaporized at elevated temperature (150 200 °C), reached the dry gel, and initiated the crystallization, which was called VPT. Less volatile SDAs such as tetrapropylammonium hydroxide were usually involved in the dry gel. Only water steam is supplied during the reaction, which was called SAC. 

M.H. Kim, H.X. Li, and M.E. Davis, Synthesis of Zeolites by Water-Organic Vapor-phase Transport. Microporous Mater., 1993, 1, 191-200. 

N. Nishiyama, K. Ueyama, and M. Matsukata, Synthesis of Defect-free Zeolite-alumina Composite Membranes by a Vapor-phase Transport Method. Microporous Mater., 1996, 7, 299-308. 

Specifically for the preparation of zeolite films, zeolite nanoparticles could be used directly to form zeolite films by self-assembly. They could also be combined nicely with the seeded growth and vapor phase transport method to produce high quality polycrystalline zeolite films. There is clear evidence that small zeolite nanoparticles are preferred for producing compact continuous films. 

The tensile strength of tungsten whiskers, grown by vapor-phase transport, was reported as up to 27.5 GPa [1.57]. 

At temperatures >600 °C, gaseous W03 -H20=W02(0H)2 is the most volatile compound formed in the system W-O-H. It is responsible for all vapor-phase transport processes at temperatures where volatilization of oxides can be neglected ... 

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