Tantalum pentoxide

Tantalum pentoxide is Ta₂O₅, also known as tantalum(V) oxide. Both orthorhombic and hexagonal phases are known. Ta₂O₅ is a high refractive index, low absorption material useful for coatings in the near-UV to IR spectra regions; it decomposes only at temperatures >1470 °C.

Ta₂O₅ is used to make capacitors in automotive electronics, cell phones, and pagers, electronic circuitry; thin-film components; and high-speed tools. In the 1990s, there was a very strong interest to do research on tantalum oxide as a high-k dielectric for DRAM capacitor applications. For example, Elpida Memory (a Japanese company making DRAM) has put in a strong effort to improve ultrathin tantalum oxide films for DRAM applications. Subsequently in the 2000s, this strong interest has dropped very significantly. There is still some interest to use it in MIM capacitors for RF CMOS integrated circuits. Due to its high index of refraction, Ta₂O₅ has been utilized in the fabrication of the glass of many photographic lenses.

It can be synthesized by hydrolysis of its alkoxide:

2 Ta(OCH₂CH₃)₅ + 5 H₂O → Ta₂O₅ + 10 HOCH₂CH₃

Materials to avoid when dealing with tantalum pentoxide are strong oxidizing agents, chlorine trifluoride, bromine monofluoride, lithium and acids.

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History

The metal oxide discovered by Ekeberg was obtained from minerals taken from pegmatite (an igneous rock associated with tantalite or columbite) at Ytterby, Sweden, and Kimoto, Finland. The microlite-pyrochlore mineral series is also a source of tantalum oxide. Microlite contains approximately 70% of tantalum oxide, and pyrochlore contains approximately 10%.

Synthesis

There are several methods available for the syntesis of tantalum oxide. One method involves isolating tantalum pentoxide by crushing a concentrate of the mineral columbite-tantalite and fusing the powdered mineral with sodium hydroxide to give a slurry of mixed insoluble niobic and tantalic acids. The mixed acids are dissolved by passing anhydrous hydrofluoric acid into the slurry and adding the stoichiometric quantity of potassium fluoride to yield a salt. The salts can be separated by a liquid-liquid extraction process, using various ketones as the liquids.

(FeMn)(NbTa)₂O₆ + 6NaOH → Ta₂O₅*7H₂O + Nb₂O₅*7H₂O

1/2Ta₂O₅*7H₂O + 1/2Nb₂O₅*7H₂O + 16KF → K₂TaF₇ + K₂NbF₇ + 2HF + 12KOH Or,

1/2Ta₂O₅ + 1/2Nb₂O₅ + 4KF + 10HF → K₂TaF₇ + K₂NbF₇ + 5H₂O

Once K₂TaF₇ has been isolated, Ta can be extracted by the electrolysis of molten K₂TaF₇. This pure form of tantalum is then exposed to oxygen at temperatures above 1000 degrees Celsius, to form Ta₂O₅.

Ta₂O₅ can be chlorinated to become TaCl₅ by heating a mixture of tantalum pentoxide and tetrachlorocarbon in a sealed glass tube in a high pressure steel reaction vessel.

Ta₂O₅ + 5CCl₄ → 2TaCl₅ + 5COCl₂

Ta₂O₅ can be reduced several different ways, including the use of metallic reductants such as calcium and aluminum.

Ta₂O₅ + 2Al → Al₂O₃ + TaO₂ + Ta

Ta₂O₅ + Ca → CaO + TaO₄ + Ta

These reactions can be further modified to yield pure tantalum via stoichiometry, thermal control and electrolysis.

Structure

Both low and high temperature forms exist. The low temperature form is known as β-Ta₂O₅, and the high temperature form is known as α-Ta₂O₅. The transition point between these two forms has been reported as 1360 °C. The transition is slow but reversible. The structures of both forms consist of chains built from octahedral and pentagonal bipyramidal polyhedra sharing opposite vertices. These chains are further joined by sharing edges to yield the 3D structure.

References

1. http://www.reade.com/products/Oxides/tantalum-pentoxide.html Copyright 1997 Reade Advanced Materials; P. O. Box 15039; 3708 Pawtucket Avenue; Riverside, RI, U. S. A. 02915-0039, All rights reserved
2. http://www.speclab.com/elements/tantalum.htm
3. http://chemlab.pc.maricopa.edu/periodic/Ta.html
4. Fairbrother, F. The Chemistry of Niobium and Tantalum. Elsevier Publishing Company. New York, 1967, pp. 1-28.
5. Wells A.F. Structural Inorganic Chemistry. Oxford, the Clarendon Press. 1947.
6. http://www.linkgrinder.com/Patents/Tantalum_sputte_6955938.html