Hafnium

Notable characteristics

  Hafnium is a shiny silvery, zirconium. The physical properties of hafnium are markedly affected by zirconium impurities, and these two elements are among the most difficult ones to separate. A notable physical difference between them is their density (zirconium being about half as dense as hafnium), but chemically the elements are extremely similar.

The most notable physical property of hafnium is that it has a very high neutron-capture cross-section, and several isotopes of hafnium nuclei can absorb multiple neutrons. This makes hafnium a good material for use in the control rods for nuclear reactors. Its neutron-capture cross-section is about 600 times that of zirconium. (Other elements that are good neutron-absorbers for control rods are boron.)

Separation of hafnium and zirconium becomes very important in the nuclear power industry, since zirconium is a good fuel-rod cladding metal, with the desirable properties of a very low neutron capture cross-section, and a good chemical stability at high temperatures. However, because of hafnium's neutron-absorbing properties, hafnium impurities in zirconium would cause it to be far less useful for nuclear reactor materials applications. Thus a nearly-complete separation of zirconium and hafnium is necessary for their use in nuclear power.

Hafnium carbide is the most °C.^[1] This has led to proposals that hafnium or its carbides might be useful as construction materials that are subjected to very high temperatures.

The metal is resistant to concentrated silicon.

The nuclear isomer Hf-178-m2 is also a source of cascades of lasers.^[3]

Applications

Hafnium is used to make control rods for nuclear reactors because of its ability to absorb neutrons (its thermal neutron absorption cross section is nearly 600 times that of zirconium), excellent mechanical properties and exceptional corrosion-resistance properties.

Other uses:

• In gas-filled and incandescent lamps, for scavenging oxygen and nitrogen,
• As the electrode in plasma cutting because of its ability to shed electrons into air,
• and in alloys.
• A hafnium-based compound is employed in gate insulators in the 45 nm generation of integrated circuits from Intel, IBM and others ^[4]. Hafnium oxide-based compounds are practical high-k dielectrics, allowing reduction of the gate leakage current which improves performance at such scales.^[5][6]
• DARPA has been intermittently funding programs in the US to determine the possibility of using a nuclear isomer of hafnium (the above mentioned Hf-178-m2) to construct small, high yield weapons with simple x-ray triggering mechanisms—an application of induced gamma emission. That work follows over two decades of basic research by an international community^[7] into the means for releasing the stored energy upon demand. There is considerable opposition to this program, both because the idea may not work^[8], and because uninvolved countries might perceive an imagined "isomer weapon gap" that would justify their further development and stockpiling of conventional nuclear weapons. A related proposal is to use the same isomer to power Unmanned Aerial Vehicles,^[9] which could remain airborne for weeks at a time.

History

 

The 1869 periodic table by lanthanum in that spot.

The existence of a gap in the periodic table for a yet to be discovered element 72 was predicted by Henry Moseley in 1914. Hafnium was named for the Latin name Hafnia for "Copenhagen", the home town of Niels Bohr. It was discovered by Dirk Coster and spectroscopy analysis in Norway.

Hafnium was separated from zirconium through repeated recrystallization of the double tungsten filament. This process for differential purification of Zr and Hf is still in use today.

The Faculty of Science of the University of Copenhagen uses in its seal a stylized image of hafnium.

Occurrence

Hafnium is estimated to make up about 0.00058% of the Earth's upper crust by weight. It is found combined in natural Kroll process.  

A major source of zircon (and hence hafnium) ores are eudialyte or armostrongite, at Dubbo in New South Wales, Australia.^[10]

Precautions

Care needs to be taken when machining hafnium because, like its sister metal zirconium, when hafnium is divided into fine particles, it is Dragon's Breath for a demonstration). Compounds that contain this metal are rarely encountered by most people. The pure metal is not considered toxic, but hafnium compounds should be handled as if they are toxic because the ionic forms of metals are normally at greatest risk for toxicity, and limited animal testing has been done for hafnium compounds.

Isotopes

Main article: Isotopes of hafnium

Compounds

See also: Category:Hafnium compounds

See also

• Nuclear isomer
• Induced gamma emission
• Zircon

References

1. ^ ^{a b} Los Alamos National Laboratory – Hafnium
2. ^ WWW Table of Radioactive Isotopes, Lawrence Berkeley National Laboratory Isotopes Project and Lund University.
3. ^ C. B. Collins et al. (2004). "Nuclear resonance spectroscopy of the 31-yr isomer of Hf-178". Laser Physics Letters 2 (3): 162–167. doi:10.1002/lapl.200410154.
4. ^ Markoff, John. "Intel Says Chips Will Run Faster, Using Less Power", New York Times, January 27, 2007. Retrieved on 2007-09-19. 
5. ^ Fulton, III, Scott M.. "Intel Reinvents the Transistor", BetaNews, January 27, 2007. Retrieved on 2007-01-27. 
6. ^ Robertson, Jordan. "Intel, IBM reveal transistor overhaul", AP, January 27, 2007. Retrieved on 2007-09-19. 
7. ^ Induced gamma emission history, The Center for Quantum Electronics, The University of Texas at Dallas.
8. ^ Schwarzschild, Bertram (May 2004). Conflicting Results on a Long-Lived Nuclear Isomer of Hafnium Have Wider Implications. Physics Today. Retrieved on 2007-09-19.
9. ^ Nuclear-powered drone aircraft on drawing board. Information Clearing House. Retrieved on 2007-09-19.
10. ^ Dubbo Zirconia Project Fact Sheet (PDF). Alkane Resources Limited (June 2007). Retrieved on 2007-09-10.

• van Arkel, A.E., and de Boer, J.H. (1925). "Preparation of pure titanium, zirconium, hafnium, and thorium metal". Zeitschrift für Anorganische und Allgemeine Chemie 148: 345–350.

• WebElements.com – Hafnium
• Hafnium Technical & Safety Data
• NLM Hazardous Substances Databank – Hafnium, elemental
• Intel Shifts from Silicon to Lift Chip Performance
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