Hafnium

72 lutetium ← hafnium → tantalum Zr ↑ Hf ↓ Rf periodic table
General
Name, Symbol, Number	hafnium, Hf, 72
Chemical series	transition metals
Group, Period, Block	4, 6, d
Appearance	gray steel
Atomic mass	178.49(2) g/mol
Electron configuration	[Xe] 4f14 5d2 6s2
Electrons per shell	2, 8, 18, 32, 10, 2
Physical properties
Phase	solid
Density (near r.t.)	13.31 g/cm³
Liquid density at m.p.	12 g/cm³
Melting point	2506 K (2233 °C, 4051 °F)
Boiling point	4876 K (4603 °C, 8317 °F)
Heat of fusion	27.2 kJ/mol
Heat of vaporization	571 kJ/mol
Heat capacity	(25 °C) 25.73 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 2689 2954 3277 3679 4194 4876
Atomic properties
Crystal structure	hexagonal
Oxidation states	4 (amphoteric oxide)
Electronegativity	1.3 (Pauling scale)
Ionization energies (more)	1st: 658.5 kJ/mol
	2nd: 1440 kJ/mol
	3rd: 2250 kJ/mol
Atomic radius	155 pm
Atomic radius (calc.)	208 pm
Covalent radius	150 pm
Miscellaneous
Magnetic ordering	no data
Electrical resistivity	(20 °C) 331 nΩ·m
Thermal conductivity	(300 K) 23.0 W/(m·K)
Thermal expansion	(25 °C) 5.9 µm/(m·K)
Speed of sound (thin rod)	(20 °C) 3010 m/s
Speed of sound (thin rod)	(r.t.) 78 m/s
Shear modulus	30 GPa
Bulk modulus	110 GPa
Poisson ratio	0.37
Mohs hardness	5.5
Vickers hardness	1760 MPa
Brinell hardness	1700 MPa
CAS registry number	7440-58-6
Notable isotopes
Main article: Isotopes of hafnium iso NA half-life DM DE (MeV) DP 172Hf syn 1.87 y ε 0.350 172Lu 174Hf 0.162% 2×1015 y α 2.495 170Yb 176Hf 5.206% Hf is stable with 104 neutrons 177Hf 18.606% Hf is stable with 105 neutrons 178Hf 27.297% Hf is stable with 106 neutrons 178m2Hf syn 31 y IT 2.446 178Hf 179Hf 13.629% Hf is stable with 107 neutrons 180Hf 35.1% Hf is stable with 108 neutrons 182Hf syn 9×106 y β 0.373 182Ta

Hafnium (chemical symbol Hf, atomic number 72) is a lustrous, silvery gray metal. Chemically, it resembles zirconium and is found in zirconium minerals. It is used in tungsten alloys in filaments and electrodes and also acts as a neutron absorber in control rods in nuclear power plants.

Occurrence

Hafnium metal

Hafnium is found combined in natural zirconium compounds but it does not exist as a free element in nature. Minerals that contain zirconium, such as alvite [(Hf, Th, Zr)SiO₄ H₂O], thortveitite and zircon (ZrSiO₄), usually contain between one and five percent hafnium. Hafnium and zirconium have nearly identical chemistry, which makes the two difficult to separate. About half of all hafnium metal manufactured is produced by a byproduct of zirconium refinement. This is done through reducing hafnium(IV) chloride with magnesium or sodium in the Kroll process.

History

Hafnium (Latin Hafnia for "Copenhagen," the hometown of Niels Bohr) was discovered by Dirk Coster and Georg von Hevesy in 1923 in Copenhagen, Denmark. Based on the Bohr Theory, this element was predicted to be associated with zirconium, and it was finally found in the mineral zircon through X-ray spectroscopic analysis performed in Norway.

This metal was separated from zirconium through repeated recrystallization of double ammonium or potassium fluorides by Jantzen and von Hevesey. Metallic hafnium was first prepared by Anton Eduard van Arkel and Jan Hendrik de Boer by passing tetraiodide vapor over a heated tungsten filament.

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

Notable characteristics

Hafnium is a transition metal that lies in period six of the periodic table, between lutetium and tantalum. In addition, it is located in group four (former group 4B), just below zirconium. It is ductile and corrosion resistant, and its chemical properties closely parallel those of zirconium. The properties of hafnium are markedly affected by zirconium impurities and these two elements are amongst the most difficult to separate. The only notable difference between them is their density—zirconium is about half as dense as hafnium.

This metal is resistant to concentrated alkalis, but halogens react with it to form hafnium tetrahalides. At higher temperatures hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Hafnium carbide is the most refractory binary compound known, and hafnium nitride is the most refractory of all known metal nitrides, with a melting point of 3310 °C.

The nuclear isomer Hf-178-m2 is also a source of cascades of gamma rays whose energies total to 2.45 MeV per decay. It is notable because it has the highest excitation energy of any comparably long-lived isomer of any element. One gram of pure Hf-178-m2 would contain approximately 1330 megajoules of energy, the equivalent of exploding about 317 kilograms (700 pounds) of TNT. Possible applications requiring such highly concentrated energy storage are of interest. For example, it has been studied as a possible power source for gamma ray lasers.

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.

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.
• In alloys with iron, titanium, niobium, tantalum, and other metals.
• Hafnium dioxide is a candidate for High-K gate insulators in future generations of integrated circuits.

Proposed use

• 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 into the means for releasing the stored energy upon demand. There is considerable opposition to this program, both because the idea may not work 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, which could remain airborne for weeks at a time.

Precautions

Care needs to be taken when machining hafnium because when it is divided into fine particles, it is pyrophoric and can ignite spontaneously in air. Compounds that contain this metal are rarely encountered by most people and the pure metal is not normally toxic but all its compounds should be handled as if they are toxic (although there appears to be limited danger to exposed individuals).

See also

• Chemical element
• Periodic table

References

ISBN links support NWE through referral fees

• Chang, Raymond. Chemistry. 9th ed. New York: McGraw-Hill Science/Engineering/Math, 2006. ISBN 0073221031
• Cotton, F. Albert, and Geoffrey Wilkinson. Advanced Inorganic Chemistry. 4th ed. New York: Wiley, 1980. ISBN 0471027758
• Greenwood, N.N. and A. Earnshaw. Chemistry of the Elements. 2nd ed. Oxford, U.K.; Burlington, MA: Butterworth-Heinemann, Elsevier Science, 1997. ISBN 0750633654
• van Arkel, A.E., and J.H. de Boer. Preparation of pure titanium, zirconium, hafnium, and thorium metal. Zeitschrift für Anorganische und Allgemeine Chemie 148 (1925):345–350.

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