Vapour transport methods

Low levels of structural Ge" have also been observed in natural hematite from the Apex mine, Utah (Bernstein Waychunas, 1987) and to achieve charge balance, incorporation of two Fe for one Ge", i.e. similar to the two Fe" for one in ilme-nite, has been suggested. Synthetic, single crystals of Ge substituted hematite have also been grown by a chemical vapour transport method (Sieber et al. 1985). A range of elements including Zr, Ge, Hf, V, Nb, Ta, W and Pb has been used as low level dopants (2 10 - 0.2 g kg ) to improve the semiconductor behaviour of hematite anodes (Anderman Kermedy, 1988). The increase in unit cell c from 1.3760 to 1.3791 nm and in a from 0.50378 to 0.50433 nm indicated that Nd (as an inactive model for trivalent actinides of similar ionic size (Am r = 0.0983 nm Nd " r = 0.098 nm)) was incorporated in the structure (Nagano et al. 1999). 

Crystals of silica can be grown using the chemical vapour transport method with hydrogen fluoride as a carrier gas. The reaction involved is ... 

Table 3.8. Some crystals grown by chemical vapour transport method ...

Vapour transport methods are used in the synthesis of materials as exemplified by the reaction of MgO and Cr Oj another example is the formation of NiCr O involving the CrOj (g) species ... 

CUjTaSe is formed by the reaction of Cu, Ta and Se in the presence of gaseous I. In Table 2.1, we list a few examples of the chemical transport system. Table 2.2 lists some crystals grown by the chemical vapour transport method. 

Many methods have been used for synthesis of nano-sized particles vapour transport [2], precipitation of homogeneous solution [3], synthesis by sol-gel process [4], freeze-drying... 

Zhang and co-workers reported partial conversion of a mesoporous starting material (SBA-15) into a mesoporous aluminosilicate with zeolitic characteristics in a so-called vapour phase transport method.[82] In this process, Al is firstly introduced onto the mesoporous surface, followed by a filling of the mesopores with a carbonaceous species, and finally a partial recrystallization of aluminosilicate in the vapour of the SDA is conducted. The advantage of this method, compared with the hydrothermal recrystallization method of Kloetstra et al., lies in the fact that the mesopore structure collapses to a lesser extent as the crystallization is limited to the surface of the mesoporous precursor. 

The transport method has been used for the determination of the vapour pressure of solutions, and in this case many precautions are necessary to obtain good results. Regnault found that liquids exert a lower vapour pressure in the presence of indifferent gases than in the presence of their own saturated vapour alone. This result was confirmed by Shaw, and by Campbell. The idea that vapour pressure corresponds with solubility in a vacuumis useful here. 

The transport method can be used to determine the molecular weight Mi of the substance carried over, when the molecular weight M2 of the streaming gas is known. 5 If wi, W2 are the weights of substance and carrier gas passed, p the vapour pressure of the substance, and P the total pressure (gas+vapour), then ... 

To avoid pinholes and cracks, Matsukata et al. [46] optimized the density of the precursor phase by using a dry gel instead of a solution. This was achieved through the use of a slipcasting method. A dry porous alimina plate of 2.2 cm was dipped into a gel whereby the support surface was covered with an amorphous aluminosilicate phase. After drying, the sample was exposed to template vapor, triethylamine, ethylenediamine and steam (the Vapour-phase Transport Method). However, the zeolite layer (20 pm thick) consisted of a mkture of ZSM-5 and Ferrierite. Nitrogen and oxygen permeation were studied. 

Hardly any research has been performed on ferrierite in zeolite membrane configurations. Matsukada et al. [50,51] prepared a ferrierite-based membrane by the frequently used Vapour-phase Transport Method. By using ethylenediamine, triethylamine and steam (under hydrothermal conditions), a porous alumina support, covered with the proper aluminosilicate gel, was transformed into a alumina supported (30 pm thick) ferrierite layer. No permeation with 1,3,5-triisopropylbenzene coirld be observed, proving the layer to be defect-free. Fluxes of small gases were found in the order of 10" -10 mol.m. s. Pa and decreased in the order H2>He>CH4>N2>02>C02... 

In another study by Nishiyama et al. [53], the Vapour-phase Transport method was applied on alumina supports. No permeation of 1,3,5-triisopropylbenzene (kinetic diameter 0.85 nm) could be observed through the 10 pm thick membrane. Mordenite has parallel channels with an elliptical pore dimension of 0.65 x 0.7 nm. Pervaporation of benzene-p-xylene (molar ratio 0.86) at 22°C resulted in a separation factor of 164 (total flux 1.19 10" mol.m s ). The theoretical value based on the gas-liquid equilibrium amounts to 11.3. Apparently, the mordenite-based membrane shows high selectivity for aromatic hydrocarbons. 

The vapour-phase transport method was also successfully used for the preparation of membranes and bulk material of the following zeolites MCM-22 membrane (used for the alkylation of toluene with MeOH to p-xylene),[68] FER-membrane (for the separation of benzene/p-xylene),[69] EMT crystals using a crown ether as structure-directing agent,[70] and zeolite beta (BEA).[71] In the latter two examples, a conversion of agglomerated nanoparticles to highly crystalline zeolite was observed. 

Preparation Methods. Actinide metal preparation is based on methods known or developed to yield high purity material by metallothermic reduction or thermal dissociation of prepurified compounds. Electrolytic reduction is possible from molten salts, but not from aqueous solutions. Further purification of the metals can be achieved by electrorefining, selective evaporation or chemical vapour transport. 

In vapour generation methods for atomic spectrometry, the detection of the analyte is achieved in the gaseous phase the amount of gas produced is normally much larger than separations based on collection in a liquid acceptor stream, and a carrier gas is usually used to assist the efficient separation and transport of the gaseous analyte to the detector... 

A different approach is used in the chemical vapour transport technique (CVT). In this method, the solid is chemically transformed into a volatile... 

E was taken from the linear fit of the data obtained for cell IX. The vapour pressure calculations are compared with the corresponding measurements using the transportation method. The agreement betw n the two methods of measuring the iodine pressure is fairly good (see Fig. 5), the deviations being almost within the reported limits of error for the transportation method. 

It may be pointed out that the static e.m.f method used for the determination of low vapour pressures of iodine seems to be superior to the dynamic transportation method. 

In the vapour-phase transport method (Matsukata et al., 1994), the precursor is deposited on the support as a gel layer which is converted to zeolite under the action of the heated (water and template) vapour. This method is convenient because it minimizes the use of the structure-directing agent, which can be quite expensive. The disadvantages of this method are the necessity of depositing a very thick gel layer on the support in order to achieve the zeohte film formation and the presence of cracks in the zeolite layer (Tsay and Chiang, 2000). 

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