Physical Properties of Pure Tantalum

Tantalum is dark, dense, ductile, very hard, easily fabricated, and highly conductive of heat and electricity. The metal is renowned for its resistance to corrosion by acids; in fact, at temperatures below 150 °C tantalum is almost completely immune to attack by the normally aggressive aqua regia.

PropertyTantalum
Atomic number73
Atomic mass180.95 [g/mol]
Melting point2996 [°C]
Boiling point5458 [°C]
Atomic volume1.80 • 10-29 [m3]
Vapour pressureat 1800 °C (2073 K)5 • 10-8 [Pa]
at 2500 °C (2773 K)5 • 10-3 [Pa]
Density at 20 °C16.60 [g/cm3]
Lattice structurebody-centred-cubic
Lattice constant330.3 • 10-12 [m]
Hardness at 20 °Ccold-worked180 - 300 [HV10]
recrystallized80 - 100 [HV10]
Young's modulus at 20 °C186 [GPa]
Poisson's ratio0.34
Linear coefficient of thermal expansion at 20 °C6.4 • 10-6 [m/(m•K)]
Thermal conductivity at 20 °C54 [W/(m•K)]
Specific heat at 20 °C0.14 [J/(g•K)]
Electrical conductivity at 20 °C8 • 10-6 [1/(Ω•m)]
Specific electrical resistance at 20 °C0.13 [(Ω•mm2)/m]
Acoustic velocity at 20 °C
longitudinal wave4100 [m/s]
transverse wave2900 [m/s]
Electron work function4.25 [eV]
Thermal neutron capture cross section2.13•10-27 [m2]
Recrystallization temperature(1 hour at temp)900 - 1450 [°C]
Superconductivity (transition temperature)< 4.5 [K]

Chemical Properties of Pure Tantalum

It is able to form oxides with the oxidation states +5 (Ta2O5) and +4 (TaO2). The most stable oxidation state is +5, tantalum pentoxide. Tantalum pentoxide is the starting material for several tantalum compounds. The compounds are created by dissolving the pentoxide in basic hydroxide solutions or by melting it in another metal oxide. Such examples are lithium tantalate (LiTaO3) and lanthanum tantalate (LaTaO4). In the lithium tantalate, the tantalate ion TaO-3 does not occur; instead, this part of the formula represents linkage of TaO7-6 octahedra to form a three-dimensionalperovskite framework; while the lanthanum tantalate contains lone TaO3-4 tetrahedral groups.

The fluorides of tantalum can be used for its separation from niobium. Tantalum forms halogen compounds in the oxidation states of +5, +4, and +3 of the type TaX5, TaX4, and TaX3, although multi core complexes and substoichiometric compounds are also known. Tantalum pentafluoride (TaF5) is a white solid with a melting point of 97.0 °C and tantalum pentachloride (TaCl5) is a white solid with a melting point of 247.4 °C. Tantalum pentachloride is hydrolyzed by water and reacts with additional tantalum at elevated temperatures by forming the black and highly hygroscopic tantalum tetrachloride (TaCl4). While the trihalogen compounds can be obtained by reduction of the pentahalogenes with hydrogen, the dihalogen compounds do not exist. A tantalum-tellurium alloy forms quasicrystals. Tantalum compounds with oxidation states as low as -1 have been reported in 2008.

Like most of the other refractory metals, the hardest known compounds are the stable nitrides and carbides. Tantalum carbide, TaC, like the more commonly used tungsten carbide, is a very hard ceramic that is used in cutting tools. Tantalum (III) nitride is used as a thin film insulator in some microelectronic fabrication processes. Chemists at the Los Alamos National Laboratory in the United States have developed a tantalum carbide-graphite composite material that is one of the hardest materials ever synthesized. Korean researchers have developed an amorphous tantalum-tungsten-copper alloy that is more flexible and two to three times stronger than commonly-used steel alloys. There are two tantalum aluminides, TaAl3 and Ta3Al. These are stable, refractory, and reflective, and they have been proposed as coatings for use in infrared wave mirrors.