Hafnium (pronounced /ˈhæfniəm/, HAF-nee-əm)
is a chemical element with the symbol Hf
and atomic number 72. A lustrous, silvery gray, tetravalent transition
metal, hafnium chemically resembles zirconium
and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev
in 1869. Hafnium was the penultimate stable
isotope element to be discovered (rhenium
was identified two years later). Hafnium was found by Dirk
Coster and Georg von Hevesy in 1923 in Copenhagen,
Denmark, and named Hafnia after the Latin
name for "Copenhagen".
Detailed description
Hafnium is used
in filaments, electrodes, and some semiconductor fabrication processes for integrated
circuits at 45 nm
and smaller feature lengths. Its large neutron capture cross-section makes hafnium a good
material for neutron absorption in control rods in nuclear
power plants.
Some superalloys used for special applications contain
hafnium in combination with niobium, titanium, or tungsten.
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History
In his report on The
Periodic Law of the Chemical Elements, in 1869, Dmitri Mendeleev had implicitly predicted the existence of a heavier analog of titanium and
zirconium. At the time of his formulation in 1871, Mendeleev believed that the
elements were ordered by their atomic masses and placed lanthanum (element 57) in the spot below
zirconium. The exact placement of the elements and the location of missing
elements was done by determining the specific weight of the elements and
comparing the chemical and physical properties.[2]
The X-ray
spectroscopy
done by Henry
Moseley in 1914 showed a
direct dependency between spectral line and effective
nuclear charge.
This led to the nuclear charge, or atomic number of an element, being used to
ascertain its place within the periodic table. With this method, Moseley
determined the number of lanthanides and showed the gaps in the atomic
number sequence at numbers 43, 61, 72, and 75.[3]
The discovery of
the gaps led to an extensive search for the missing elements. In 1914, several
people claimed the discovery after Henry Moseley predicted the gap in the
periodic table for the then-undiscovered element 72.[4] Georges Urbain asserted that he found element 72 in
the rare
earth elements
in 1907 and published his results on celtium in 1911.[5] Neither the spectra nor the chemical
behavior matched with the element found later, and therefore his claim was turned
down after a long-standing controversy.[6] The controversy was partly due to the
fact that the chemists favored the chemical techniques which led to the
discovery of celtium, while the physicists relied on the use of the new
X-ray spectroscopy method that proved that the substances discovered by Urbain
did not contain element 72.[6] By early 1923, several physicists and
chemists such as Niels Bohr[7] and Charles R. Bury[8] suggested that element 72 should
resemble zirconium and therefore was not part of the rare earth elements group.
These suggestions were based on Bohr's theories of the atom, the X-ray
spectroscopy of Mosley, and the chemical arguments of Friedrich Paneth.[9] [10]
Encouraged by
these suggestions and by the reappearance in 1922 of Urbain's claims that
element 72 was a rare earth element discovered in 1911, Dirk Coster and Georg von Hevesy were motivated to search for the new
element in zirconium ores.[11] Hafnium was discovered by the two in
1923 in Copenhagen, Denmark, validating the original 1869 prediction of
Mendeleev.[12][13] It was ultimately found in zircon in Norway through X-ray spectroscopy
analysis.[14] The place where the discovery took
place led to the element being named for the Latin name for
"Copenhagen", Hafnia, the home town of Niels Bohr.
Characteristics
Hafnium is a
shiny, silvery, ductile metal that is corrosion-resistant and chemically similar to
zirconium.[20] The physical properties of hafnium
metal samples are markedly affected by zirconium impurities, and especially the
nuclear properties, as these two elements are among the most difficult ones to
separate because of their chemical similarity.[20]
A notable
physical difference between these metals is their density, with zirconium having about one-half
the density as hafnium. The most notable nuclear properties of hafnium is its high
thermal neutron-capture
cross-section,
and that the nuclei of several different hafnium isotopes readily absorb two or
more neutrons apiece.[20] In contrast with this, zirconium is
practically transparent to thermal neutrons, and it is commonly used for the
metal components of nuclear reactors - especially the claddings of their nuclear fuel rods.
Chemistry
As a tetravalent
transition metal, hafnium forms various inorganic
compounds, generally in
the oxidation state of +4. The metal is resistant to concentrated alkalis, but halogens react with it to form hafnium
tetrahalides.[27] At higher temperatures, hafnium
reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon.[27] Due to the lanthanide
contraction
of the elements in the sixth period, zirconium and hafnium have nearly
identical ionic
radii. The ionic
radius of Zr4+ is 0.79 Ångström and that of Hf4+ is
0.78 Ångström.[27]
This similarity
results in nearly identical chemical behavior and in the formation of similar
chemical compounds.[27] The chemistry of hafnium is so
similar to that of zirconium that a separation on chemical reactions was not
possible, only the physical properties of the compounds differ. The melting
points and boiling points of the compounds and the solubility in solvents are the major differences
in the chemistry of these twin elements.[28]
Occurrence
Hafnium is
estimated to make up about 5.8 ppm of the Earth's upper crust by weight. It does not exist as a
free element in nature, but is found combined in solid solution for zirconium in natural zirconium compounds such as zircon, ZrSiO4, which usually has a about
1 - 4 % of the Zr replaced by Hf. Rarely, the Hf/Zr ratio increases during
crystallization to give the isostructural mineral 'hafnon' (Hf,Zr)SiO4,
with atomic Hf > Zr.[30] An old (obsolete) name for a variety
of zircon containing unusually high Hf content is alvite.[31]
A major source of
zircon (and hence hafnium) ores are heavy mineral sands ore deposits, pegmatites particularly in Brazil and Malawi, and carbonatite intrusions particularly the Crown
Polymetallic Deposit at Mount Weld, Western Australia. A potential
source of hafnium is trachyte tuffs containing rare zircon-hafnium silicates eudialyte or armstrongite, at Dubbo in New South Wales, Australia.[32]
Production
The heavy mineral sands ore deposits of the titanium ores ilmenite and rutile yield most of the mined zirconium, and
therefore also most the hafnium.[33]
Zirconium is a
good nuclear fuel-rod cladding metal, with the desirable properties of a very low
neutron capture cross-section and 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
applications. Thus, a nearly complete separation of zirconium and hafnium is
necessary for their use in nuclear power. The production of hafnium-free
zirconium is the main source for hafnium.[20]
Applications
Nuclear reactors
The nuclei of
several hafnium isotopes can each 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 cadmium and boron.) Excellent mechanical properties and
exceptional corrosion-resistance properties allow its use in the harsh
environment of a pressurized
water reactors.[35] The German research reactor FRM
II uses hafnium as
a neutron absorber.[38]
Alloys
Hafnium is used
in iron, titanium, niobium, tantalum, and other metal alloys. An alloy used for liquid rocket thruster nozzles, for example the main
engine of the Apollo
Lunar Modules
is C103, which consists of 89% niobium, 10% hafnium and 1% titanium.[39]
Small additions
of hafnium increase the adherence of protective oxide scales on nickel based
alloys. It improves thereby the corrosion resistance especially under cyclic
temperature conditions that tend to break oxide scales by inducing thermal
stresses between the bulk material and the oxide layer.[40][41][42]
Microprocessors
The electronics
industry discovered that hafnium-based compound can be employed in gate insulators in the 45 nm
generation of integrated
circuits from Intel, IBM and others.[43][44] Hafnium oxide-based compounds are
practical high-k
dielectrics,
allowing reduction of the gate leakage current which improves performance at
such scales.[45][46]
Other uses
Due to its heat
resistance and its affinity to oxygen and nitrogen, hafnium is a good scavenger
for oxygen and nitrogen in gas-filled and incandescent lamps. Hafnium is also used as the
electrode in plasma
cutting because of its
ability to shed electrons into air,[47]
The high energy
content of 178m2Hf is the concern of a DARPA funded program in the US. This program should
determine the possibility of using a nuclear isomer of hafnium (the above mentioned 178m2Hf)
to construct high yield weapons with X-ray triggering mechanisms—an application
of induced
gamma emission.
That work follows over two decades of basic research by an international
community[48]
into the means for releasing the stored energy upon demand. There is
considerable opposition to this program[49]
because uninvolved countries might perceive an "isomer weapon gap"
that would justify their further development and stockpiling of nuclear
weapons. A related proposal is to use the same isomer to power Unmanned
Aerial Vehicles,[50]
which could remain airborne for months at a time.
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