Tantalum continued oxidation

Now, then [continued Berzelius], it is clear that the columbic acid of Mr. Hatchett, having been composed of oxide of tantalum and tungstic acid, which communicated to it a part of its specific properties, it is clear, I say, that Mr. Hatchett shares the discovery of tantalum in almost the same manner as MM. Fourcroy and Vauquelin share with Mr. Tennant the honor of having discovered osmium ( Thomson s System, Ed. IV, Vol. 1, p. 200), and I suppose that you will not refuse to render the same justice to the work of the Swede Ekeberg that you have just rendered to the Englishman Tennant. ... 

Another example is the siliciditing of tantalum, basically an oxidation— reduction reaction. The packing is sodium duoride and siUcon. After deposition, the coating diffuses continuously into the substrate, according to the following reactions ... 

The second type of behaviour (Fig. 1.89) is much closer to that which one might predict from the regular cracking of successive oxide layers, i.e. the rate decreases to a constant value. Often the oxide-metal volume ratio (Table 1.27) is much greater than unity, and oxidation occurs by oxygen transport in the continuous oxide in some examples the data can be fitted by the paralinear rate law, which is considered later. Destructive oxidation of this type is shown by many metals such as molybdenum, tungsten and tantalum which would otherwise have excellent properties for use at high temperatures. 

The way in which ammonia solution is added to tantalum or niobium strip solutions is also important for the quality of the precipitated hydroxides and final oxides. The traditional method by which ammonia is poured into a container of strip solution and the mixture agitated is not optimal. According to this method, the first portion of ammonia is added to a solution of high acidity, the pH of which continues to drop gradually with each addition of ammonia, until the final addition of ammonia is made into a low-acidity solution. This procedure leads to a relatively slow increase in pH that can cause contamination of the hydroxide with crystalline oxyfluoride compounds. 

Continuous (barrier, passivation) films have a high resistivity (106Q cm or more), with a maximum thickness of 10 4cm. During their formation, the metal cation does not enter the solution, but rather oxidation occurs at the metal-film interface. Oxide films at tantalum, zirconium, aluminium and niobium are examples of these films. 

Tantalum pentoxide [1314-61-0], Ta is prepared by calcining tantaUc acid or hydrated tantalum oxide [75397-94-3], Ta2Os H20, at temperatures between 800 and 1100°C. This oxide hydrate is produced by adding gaseous or aqueous ammonia to the solvent extraction produced aqueous tantalum solution in a continuous (29) or batch process. 

Complexation studies (continued) polymer backbone, 1, 258 polymerization monitoring, 1, 259 pulse radiolysis initiation, 1, 535 in thin film and wire depositions, 1, 259 valence and oxidation number, 1, 18 t/-Complexes with niobium, 5, 66, 5, 68 with tantalum, 5, 108... 

The trihydride structure of MCp 2H3 is unique to niobium and tantalum. The bent-metallocene fragment of these metals appears to be ideal for accommodating three hydrides because of the favorable oxidation state of +5 and perhaps because the void created at the equatorial girdle has the right size for three hydrides. The compounds can be easily handled and yet show interesting reactivity. The trihydrides continue to serve as important starting compounds for a wealth of chemistry. Scheme 7.12 shows their transformation to some useful precursors and to heterometallic clusters [56b,57,58]. 

High chlorine production yield and suitable lifetime are achieved with mixed metal oxide electrodes (MMO), initially developed for the chlor-alkali process [4]. They consist of a base metal, a titanium plate, or grid for saltwater chlorination coated with a combination of at least two metal oxides including titanium, ruthenium, iridium, or tantalum [5]. Elaborated proprietary production processes are used to achieve reliable electrodes. The chlorine faradaic yield under typical pool working conditions (3 g/L NaCl, current density 50 mA/cm ) is around 60 % [6]. Good quality electrodes achieve several years of service time (several thousands of hours of continuous operation) with a 3 h reversal time and at 60 % of the nominal maximum power. Lifetime is reduced by high current densities and by frequent polarity reversal. Operation at... 

Niobium and tantalum are present in the earth s crust as oxides in minerals that form a continuous series between the two extreme minerals mobile (Fe,Mn) (Nb.Ta) with 78% Nb Oj and tantalite (Fe,Mn)(Ta,Nb)20g with 84% Ta Oj. A common name for all minerals within the series is columhite (Figure M32). 

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