Hafnium is a chemical element with 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, though hafnium was not identified until
1923, making it the penultimate stable element to be discovered. Hafnium is
named after Hafnia, the Latin name for Copenhagen, where it was discovered.
Hafnium is used in filaments and electrodes. Some semiconductor
fabrication processes use its oxide for integrated circuits at 45 nm and smaller
feature lengths. Some superalloys used for special applications contain hafnium
in combination with niobium, titanium, or tungsten.
Hafnium's large neutron capture cross-section makes it a good material
for neutron absorption in control rods in nuclear power plants, but at the same
time requires that it be removed from the neutron-transparent
corrosion-resistant zirconium alloys used in nuclear reactors.
Characteristics = Physical characteristics =
Hafnium is a shiny, silvery, ductile metal that is corrosion-resistant and
chemically similar to zirconium. The physical properties of hafnium metal
samples are markedly affected by zirconium impurities, especially the
nuclear properties, as these two elements are among the most difficult to
separate because of their chemical similarity.
A notable physical difference between these metals is their density, with
zirconium having about one-half the density of hafnium. The most notable
nuclear properties of hafnium are its high thermal neutron-capture
cross-section and that the nuclei of several different hafnium isotopes
readily absorb two or more neutrons apiece. 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. = Chemical characteristics =
Hafnium reacts in air to form a protective film that inhibits further
corrosion. The metal is not readily attacked by acids but can be oxidized
with halogens or it can be burnt in air. Like its sister metal zirconium, finely
divided hafnium can ignite spontaneously in air, producing an effect similar to
that obtained in Dragon's Breath. The metal is resistant to concentrated
alkalis. The chemistry of hafnium and zirconium
is so similar that the two cannot be separated on the basis of differing
chemical reactions. 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. = Isotopes =
At least 34 isotopes of hafnium have been observed, ranging in mass number
from 153 to 186. The five stable isotopes are in the range of 176 to 180.
The radioactive isotopes' half-lives range from only 400 ms for 153Hf, to 2.0
petayears for the most stable one, 174Hf.
The nuclear isomer 178m2Hf was at the center of a controversy for several
years regarding its potential use as a weapon.
= Occurrence = Hafnium is estimated to make up about
5.8 ppm of the Earth's upper crust by mass. It does not exist as a free
element in nature, but is found combined in solid solution with zirconium in
natural zirconium compounds such as zircon, ZrSiO4, which usually has about
1 – 4% of the Zr replaced by Hf. Rarely, the Hf/Zr ratio increases during
crystallization to give the isostructural mineral "hafnon"SiO4, with
atomic Hf > Zr. An old name for a variety of zircon containing unusually
high Hf content is alvite. A major source of zircon ores is 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. Hafnium reserves have been infamously
estimated to last under 10 years by one source if the world population increases
and demand grows. In reality, since hafnium occurs with zirconium, hafnium
can always be a byproduct of zirconium extraction to the extent that the low
demand requires. Production
The heavy mineral sands ore deposits of the titanium ores ilmenite and rutile
yield most of the mined zirconium, and therefore also most of the hafnium.
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.
The chemical properties of hafnium and zirconium are nearly identical, which
makes the two difficult to separate. The methods first used — fractional
crystallization of ammonium fluoride salts or the fractionated distillation
of the chloride — have not proven suitable for an industrial-scale
production. After zirconium was chosen as material for nuclear reactor programs
in the 1940s, a separation method had to be developed. Liquid-liquid extraction
processes with a wide variety of solvents were developed and are still
used for the production of hafnium. About half of all hafnium metal
manufactured is produced as a by-product of zirconium refinement. The end product
of the separation is hafnium(IV) chloride. The purified hafnium(IV)
chloride is converted to the metal by reduction with magnesium or sodium, as
in the Kroll process. HfCl4 + 2 Mg → 2 MgCl2 + Hf
Further purification is effected by a chemical transport reaction developed by
Arkel and de Boer: In a closed vessel, hafnium reacts with iodine at
temperatures of 500 °C, forming hafnium(IV) iodide; at a tungsten
filament of 1700 °C the reverse reaction happens, and the iodine and hafnium are
set free. The hafnium forms a solid coating at the tungsten filament, and
the iodine can react with additional hafnium, resulting in a steady turn
over. Hf + 2 I2 → HfI4
HfI4 → Hf + 2 I2 Chemical compounds
Hafnium and zirconium form nearly identical series of chemical compounds.
Hafnium tends to form inorganic compounds in the oxidation state of +4.
Halogens react with it to form hafnium tetrahalides. At higher temperatures,
hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. 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
angstrom and that of Hf4+ is 0.78 angstrom.
Hafnium(IV) chloride and hafnium(IV) iodide have some applications in the
production and purification of hafnium metal. They are volatile solids with
polymeric structures. These tetrachlorides are precursors to various
organohafnium compounds such as hafnocene dichloride and
tetrabenzylhafnium. The white hafnium oxide, with a melting
point of 2812 °C and a boiling point of roughly 5100 °C, is very similar to
zirconia, but slightly more basic. Hafnium carbide is the most refractory
binary compound known, with a melting point over 3890 °C, and hafnium nitride
is the most refractory of all known metal nitrides, with a melting point of
3310 °C. This has led to proposals that hafnium or its carbides might be useful
as construction materials that are subjected to very high temperatures. The
mixed carbide tantalum hafnium carbide (Ta
4HfC 5) possesses the highest melting point
of any currently known compound, 4215 °C. Recent supercomputer simulations
suggest a hafnium alloy with a melting point of 4400 K.
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 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. 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.
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. Georges Urbain asserted that he found element 72 in the rare earth
elements in 1907 and published his results on celtium in 1911. 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. The
controversy was partly because 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. By early 1923, several
physicists and chemists such as Niels Bohr and Charles R. Bury 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. 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. Hafnium was discovered by the two
in 1923 in Copenhagen, Denmark, validating the original 1869 prediction
of Mendeleev. It was ultimately found in zircon in Norway through X-ray
spectroscopy analysis. 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. Today, the Faculty of Science of the University of Copenhagen
uses in its seal a stylized image of the hafnium atom.
Hafnium was separated from zirconium through repeated recrystallization of
the double ammonium or potassium fluorides by Valdemar Thal Jantzen and
von Hevesey. Anton Eduard van Arkel and Jan Hendrik de Boer were the first to
prepare metallic hafnium by passing hafnium tetraiodide vapor over a heated
tungsten filament in 1924. This process for differential purification of
zirconium and hafnium is still in use today.
In 1923, four predicted elements were still missing from the periodic table:
43 and 61 are radioactive elements and are only present in trace amounts in the
environment, thus making elements 75 and 72 the last two unknown non-radioactive
elements. Since rhenium was discovered in 1925, hafnium was the next-to-last
element with stable isotopes to be discovered.
Applications Several details contribute to the fact
that there are only a few technical uses for hafnium: First, the close similarity
between hafnium and zirconium makes it possible to use zirconium for most of
the applications; second, hafnium was first available as pure metal after the
use in the nuclear industry for hafnium-free zirconium in the late
1950s. Furthermore, the low abundance and difficult separation techniques
necessary make it a scarce commodity. Most of the hafnium produced is used in
the production of control rods for nuclear reactors.
= 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. Excellent mechanical properties and
exceptional corrosion-resistance properties allow its use in the harsh
environment of pressurized water reactors. The German research reactor
FRM II uses hafnium as a neutron absorber. It is also common in military
reactors, particularity in US naval reactors, but seldom found in civilian
ones, the first core of the Shippingport Atomic Power Station being a notable
exception. = Alloys =
Hafnium is used in alloys with iron, titanium, niobium, tantalum, and other
metals. 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. 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. = Microprocessors =
Hafnium-based compounds are employed in gate insulators in the 45 nm generation
of integrated circuits from Intel, IBM and others. Hafnium oxide-based
compounds are practical high-k dielectrics, allowing reduction of the
gate leakage current which improves performance at such scales.
= Isotope geochemistry = Isotopes of hafnium and lutetium are
also utilized in isotope geochemistry and geochronological applications. It is
often used as a tracer of isotopic evolution of Earth’s mantle through
time. This is because 176Lu decay to 176Hf with a half-life of approximately
37 billion years. In most geologic materials, zircon is
the dominant host of hafnium and is often the focus of hafnium studies in
geology. Hafnium is readily substituted into the zircon crystal lattice, and is
therefore very resistant to hafnium mobility and contamination. Zircon also
has an extremely low Lu/Hf ratio, making any correction for initial lutetium
minimal. Although the Lu/Hf system can be used to calculate a "model age", i.e.
the time at which it was derived from a given isotopic reservoir such as the
depleted mantle, these "ages" do not carry the same geologic significance as
do other geochronological techniques as the results often yield isotopic
mixtures and thus provide an average age of the material from which it was
derived. Garnet is another mineral that contains
appreciable amounts of hafnium to act as a geochronometer. Given the high and
variable Lu/Hf ratios found in garnet make it useful for dating metamorphic
events. = 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. The high energy content of 178m2Hf was
the concern of a DARPA-funded program in the US. This program determined that the
possibility of using a nuclear isomer of hafnium to construct high-yield weapons
with X-ray triggering mechanisms—an application of induced gamma
emission—was infeasible because of its expense. See Hafnium controversy.
Precautions Care needs to be taken when machining
hafnium because, like its sister metal zirconium, when hafnium is divided into
fine particles, it is pyrophoric and can ignite spontaneously in air—similar to
that obtained in dragon's breath. 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 were toxic because the ionic
forms of metals are normally at greatest risk for toxicity, and limited animal
testing has been done for hafnium compounds.
See also Nuclear isomer
Induced gamma emission Zircon
Zirconium References
External links Hafnium at Los Alamos National
Laboratory's periodic table of the elements
Hafnium at The Periodic Table of Videos Hafnium Technical & Safety Data
NLM Hazardous Substances Databank – Hafnium, elemental
Intel Shifts from Silicon to Lift Chip Performance
Hafnium-based Intel 45nm Process Technology
CDC - NIOSH Pocket Guide to Chemical Hazards