Chemical vapour transport

Another application is in tire oxidation of vapour mixtures in a chemical vapour transport reaction, the attempt being to coat materials with a tlrin layer of solid electrolyte. For example, a gas phase mixture consisting of the iodides of zirconium and yttrium is oxidized to form a thin layer of ytnia-stabilized zirconia on the surface of an electrode such as one of the lanthanum-snontium doped transition metal perovskites Lai j.Srj.M03 7, which can transmit oxygen as ions and electrons from an isolated volume of oxygen gas. 

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). 

Single crystals from 50-800 pm across can be synthesised by chemical vapour transport. Hematite powder is held for on week in a silica tube with a temperature gradient of 1000-850 °C in an atmosphere of HCl(5.3kPa)/02 (133 Pa) (Moukassi et al., 1984). 

Fig. 4. Preparation of oxide single crystals by chemical vapour transport in a thermal gradient...

Fig. 5. UO2 single crystals prepared by chemical vapour transport with TeCl4...

In CVD, solids are formed from gaseous compounds. In chemical vapour transport, a solid or solids interact with a volatile compound and a solid product is deposited in a different part of the apparatus. It is used both for preparing compounds and for growing crystals from powders or less pure crystalline material. 

Chemical vapour transport has been used to grow magnetite crystals using the reaction of magnetite with hydrogen chloride gas ... 

FIGURE 3.13 Growth of magnetite crystals using chemical vapour transport... 

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 ...

Sm-Se-Sb. Rustamov et al. (1981b) reported from X-ray powder diffraction the crystal structure of the compound Sm3Se9Sb4 obtained by using a chemical vapour transport technique, Eu3S9Sb4 structure type, a = 1.744, b = 0.410, c = 2.492. 

Vapour phase reactions also yield solid products in many instances. In chemical vapour transport reactions, a gaseous reagent acts as a carrier to transport a solid by transforming it into the vapour state ... 

Small crystals have been obtained by chemical vapour transport (CVT) in closed tubes using such chemical transport agents as Zn, ZnCl2, ammonium halides (NH4X), HCl, CI2, NH3, NH4CI, HgCU, H2, Br2, at source temperatures ranging from 800 to 1150°C and AT from 20 to 200°C." ... 

In recent years, several novel cationic species have been prepared by utihzing chemical vapour transport. These cations exhibit unprecedented structural features that find no analogs in the fighter congeners. ... 

Studies of iron sulphides indicate that the Fe S ratio is considerably lowered by increasing the acidities of solutions from which the product is precipitated. Zni Fe,PS3 (0 < x < 1) has been prepared by chemical vapour transport of the elements. A structural study of Cu4Fe5Sg has shown it to be closely related to chalcopyrite. ° Similar studies on FeU2S5 show the iron atom to be octahedrally co-ordinated by sulphur and the uranium atoms show bicapped trigonal-prismatic co-ordination. ... 

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. 

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

Bol] Boldt, R., Reichelt, W., Bosholm, O., Oppermann, H., Investigation on Chemical Vapour Transport of Intermetallic Phases in the System Co/Fe/Si , Z. Anorg. Allg. Chem., 629(10), 1839-1845 (2003) (Experimental, Thermodyn., 12)... 

An Au-catalysed chemical vapour transport and condensation (CVTC) process was used to produce ZnO nanorods and nanowires on Si02 and Si substrates [58], ZnO nanorods with a wide band gap (3.37eV) are regarded as promising candidates for the fabrication of nanoelectronic devices. In this work, EDX spectra of the tip and the body of ZnO nanorods were captured which indicates that Au-Zn alloyed droplets were present at the tips of the fabricated nanorods pointing to a nanorod growth via a vapor-liquid-solid (VLS) mechanism. 

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. 

AlCuS -ZnS (l-x)ZnS XCUAIS2 for O x O.33 F43m I42d Z=4 Zincblende-type -> chalcopyrite-type. X, powder. Chemical vapour transport of appropriate mixtures of Cu, Al, S and ZnS (I2 as carrier). 74Don5 )... 

CdGa,S,, Se. 0gx 4 14 ) Z=1 Thiogallate-type. X, powder, structure qualitative. Prepared from mixtures of the elements in carbon-coated vacuum sealed quartz ampoules. Crystals grown by chemical vapour transport with iodine. Colourless to dark brown. 77Kshl... 

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