Crucibles protective layer

Based on these analyses on the SiC coating, the growth mechanism of the SiC layer on diamond is considered as follows. In the early stage of the SiC formation on diamond, a very thin SiC layer is formed on the diamond surface according to reaction (10.2) between diamond and SiO(g). Once the SiC layer is formed, this reaction does not proceed due to the protective layer of SiC. The carbon sheet and felt in an alumina crucible act as the carbon source. The reaction of C02(g) with these carbon sources will produce further CO(g) and deposit SiC(s) by reaction (10.7). Thin j3-SiC whiskers are observed on the surface of the SiC-coated diamond, suggesting the vapor growth of SiC. 

Obtained by fusing electrolytic Cu with pure Te in a crucible under a protective layer of NaCl and KCl. 

The alloy components, weighed out with the above considerations in mind, are combined by fusion in crucibles or ampoules. Some method must always be devised to minimize losses due to burnii or vaporization. In simple cases, where open vessels are used, this is achieved by covering the charge with a protective layer of a salt or salt mixture which also melts in the process. Alternatively, the mixture may be protected by a blanket of an inert gas crucibles may be closed by a lid and ampoules by fusii the constricted neck. If the closure is gas-tight, some inert or reducing gases may be included and a vacuiun may even be maintained. 

From a rather preliminary study of the phase equilibria at 400 °C (the Al-rich region was studied at 500 °C) Zarechnyuk (1963) and Altunina et al. (1963) attempted to construct an isothermal section (see fig. 2a). Samples were prepared from 99.99% pure Al and 99.567% pure Ce by melting in an AljOj crucible under a protective layer of a mixture of KCl + LiCl. The alloys were annealed for 50-100 h at 400 °C, and 25 samples in the Al-rich region at 500 °C. Phase composition was checked in some cases by chemical analysis. Phase equilibria were determined as based upon X-ray powder and metallographic analyses. Etching was possible in mixtures of HNOj, HF and glycerin. Microhardness values were obtained at a load of 100 g. 

At a composition (in a/o) Ce2oAl35Si45 a ternary compound was observed, which tentatively was indexed hexagonal with c/a 1.17 H = 290 ( 10) kg/mm. The ternary phase was suspected to correspond to the compound CcjAljSij mentioned by Brauer and Haag (1952), who obtained this phase in an unsuccessful attempt to prepare CeSij by reaction of a CeAl melt with excess Si at 900°C in an AljOj crucible under a protective layer of NaCl. After 1 h at 900 0 and subsequent cooling to 600 0 in 2.5 h the obtained alloy was boiled in 2n-NaOH. The remainder was of metallic luster and well crystallized. Indexing was possible on the basis of a hexagonal cell a = 6.242, c = 7.304 (converted from kX units). 

A special flux consisting of the halides of alkali metals with the addition of sodium and aluminium fluorides was employed both to protect the aluminium melt from oxidation and to pre-heat the specimen to the required temperature. First, the flux was melted in a 26 mm inner diameter alumina crucible. Melting started at about 350°C. The height of the flux column was around 15 mm. Aluminium pieces were then melted under the flux layer. The amount of aluminium taken was equivalent to a volume of... 

A fireclay crucible is charged with 400 g. of Al, It is then heated to 1200°C, and 300 g. of nickel cubes are added at once to the Al melt. Nickel cubes are especially useful in this case since the material is porous and thus quickly dissolved at the initial, comparatively low temperature. The nickel dissolves in a vigorous reaction which raises the temperature of the melt to 1500°C. The alloy must be prepared imder a salt melt layer or in an inert atmosphere to protect the Al from oxidation. 

Rhodium is a hard and wear resistant PGM. Advanced optical equipment is plated with thin layers of rhodium in order to get good corrosion and wear protection. Rhodium-alloyed platinum is used in perforated crucibles for making glass fibers. Organic rhodium compounds have been found to be effective catalysts (homogenous) in industrial hydrogenation and in pharmaceutical production of, for instance, the drug L-Dopa for the treatment of Parkinson s disease [32.8]. 

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