Tantalum fluoride properties

Ta and Nb containing minerals, 4 Ta and Nb oxides dissolution, 258 Tantalum fluoride properties, 25 Tantalum extraction, 285-288... 

FERROELECTRIC PROPERTIES OF NIOBIUM AND TANTALUM FLUORIDE COMPOUNDS... 

Aluminium alloys well with up to about 3-5 per cent, of tantalum, which has no effect, however, on the mechanical strength, ductility, and working properties of aluminium.3 Reduction of tantalum pentoxide by the thermite process yields hard, brittle alloys.1 A substance the composition of which corresponds with the formula TaAls has been obtained by reducing potassium tantalum fluoride, K2TaF7, with aluminium filings at a high temperature. It is described as an iron-grey crystalline powder, of density 7-02, which is scarcely attacked by acids.5... 

Since niobates and tantalates belong to the octahedral ferroelectric family, fluorine-oxygen substitution has a particular importance in managing ferroelectric properties. Thus, the variation in the Curie temperature of such compounds with the fluorine-oxygen substitution rate depends strongly on the crystalline network, the ferroelectric type and the mutual orientation of the spontaneous polarization vector, metal displacement direction and covalent bond orientation [47]. Hence, complex tantalum and niobium fluoride compounds seem to have potential also as new materials for modem electronic and optical applications. 

The most universal method for the synthesis of tantalum and niobium fluoride compounds is based on direct interaction between their pentafluorides, TaF5 or NbFs, and fluorides of other metals. Some physical-chemical properties of these compounds are presented in Table 8 [71, 72]. 

Physicochemical properties of fluoride melts containing tantalum... 

Tantalum and niobium fluoride compounds that crystallize in coordination-type structures also seem to be perspective candidates for the investigation of ferroelectric properties. Ravez and Mogus-Milancovic [404] showed that some fluoride and oxyfluoride compounds with crystal structures similar to the Re03 type exhibit ferroelastic properties. For instance, ferroelastic properties were found in some solid solutions based on Nb02F and Ta02F [405,406]. 

Tantalum powder is produced by reduction of potassium heptafluoro-tantalate, K2TaF7, dissolved in a molten mixture of alkali halides. The reduction is performed at high temperatures using molten sodium. The process and product performance are very sensitive to the melt composition. There is no doubt that effective process control and development of powders with improved properties require an understanding of the complex fluoride chemistry of the melts. For instance, it is very important to take into account that changes both in the concentration of potassium heptafluorotantalate and in the composition of the background melt (molten alkali halides) can initiate cardinal changes in the complex structure of the melt itself. 

It is, therefore, required that all initial compounds be dried properly prior to performing the reduction. This procedure is not at all trivial and refers, first of all, to the diluent salts, and especially to potassium fluoride, KF, which is characterized by a strong hygroscopic property and a tendency to form stable crystal hydrates. The problem of contamination due to hydrolytic processes can usually be resolved in two manners. The first is to apply another tantalum-containing complex fluoride compound that does not undergo hydrolysis. The second involves the adjustment of the reduction process parameters and use of some additives that will "collect" the oxygen present, in the form of water, hydroxyl groups or other compounds. 

The pentahalides of niobium and tantalum are predominantly covalent. Their volatility decreases from the fluorides to the iodides. Nb and Ta belong to the few elements which form stable pentaiodides. Selected physical and thermodynamic properties for the halides are listed in Table 2. All are sensitive to moist air, water or hydroxylic solvents. 

PHYSICAL PROPERTIES The physical properties of metallic tantalum and tantalum pentoxide are provided for illustrative purposes, (metallic tantalum) steel-blue to gray solid or black powder very hard, malleable, ductile metal can readily be drawn in fine wires odorless insoluble in water insoluble in acids, except hydrofluoric and fuming sulfuric acids soluble in fused alkalies MP (2996"C, 5425"F) BP (5425 C, 9797T) DN/SG (16.65 (metal), 14.40 (powder)) CP (25.4 J/K-mol crystal at 25°C) VD (NA) VP (approximately 0 mmHg at 20 C) TS (130,000 psi) EC (8 x 10" over range 20 - 1500 C). (tantalum pentoxide) white, microcrystalline, infusible powder insoluble in water, acids, and alcohol soluble in hydrogen fluoride MP (1800 C, 3272 F) BP (NA) DN/SG (8.2) CP (135.1 J/K-mol crystal at 25"C) VD (NA) VP (approximately 0 mmHg at 20 C). 

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