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=                              Hafnium                               =
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                            Introduction
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Hafnium is a chemical element; it has symbol Hf and atomic number 72.
A lustrous, silvery gray, tetravalent transition metal, hafnium
chemically resembles zirconium and is found in many zirconium
minerals. Its existence was predicted by Dmitri Mendeleev in 1869,
though it was not identified until 1922, by Dirk Coster and George de
Hevesy. Hafnium is named after , 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
nanometers 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.


Physical characteristics
==========================
Hafnium is a shiny, silvery, ductile metal that is corrosion-resistant
and chemically similar to zirconium in that they have the same number
of valence electrons and are in the same group. Also, their
relativistic effects are similar: The expected expansion of atomic
radii from period 5 to 6 is almost exactly canceled out by the
lanthanide contraction. Hafnium changes from its alpha form, a
hexagonal close-packed lattice, to its beta form, a body-centered
cubic lattice, at 2388 K.  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 cladding of their nuclear fuel rods.


Chemical characteristics
==========================
Hafnium reacts in air to form a protective film that inhibits further
corrosion.  Despite this, the metal is attacked by hydrofluoric acid
and concentrated sulfuric acid, and can be oxidized with halogens or
burnt in air.  Like its sister metal zirconium, finely divided hafnium
can ignite spontaneously in air. The metal is resistant to
concentrated alkalis.

As a consequence of lanthanide contraction, the chemistry of hafnium
and zirconium is so similar that the two cannot be separated based on
differing chemical reactions. The melting 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 40 isotopes of hafnium have been observed, ranging in mass
number from 153 to 192. The five stable isotopes have mass numbers
ranging from 176 to 180 inclusive. The radioactive isotopes'
half-lives range from 400 ms for 153Hf to  years for the most stable
one, the primordial 174Hf.

The extinct radionuclide 182Hf has a half-life of , and is an
important tracker isotope for the formation of planetary cores. 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 between 3.0 and 4.8 ppm of the
Earth's upper crust by mass.  It does not exist as a free element on
Earth, 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 , with
atomic Hf > Zr. An obsolete name for a variety of zircon containing
unusually high Hf content is 'alvite'.

A major source of zircon (and hence hafnium) 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.


                             Production
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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 of 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
fractional distillation of the chloride--have not proven suitable for
an industrial-scale production. After zirconium was chosen as a
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
producing 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 ->[1100~^\circ\text{C}] Hf{} + 2 MgCl2

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
preferentially, and the chemically bound iodine and hafnium dissociate
into the native elements. The hafnium forms a solid coating at the
tungsten filament, and the iodine can react with additional hafnium,
resulting in a steady iodine turnover and ensuring the chemical
equilibrium remains in favor of hafnium production.
: Hf{} + 2 I2 ->[500~^\circ\text{C}] HfI4
: HfI4 ->[1700~^\circ\text{C}] Hf{} + 2 I2


                         Chemical compounds
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Due to the lanthanide contraction, the ionic radius of hafnium(IV)
(0.78 ångström) is almost the same  as that of  zirconium(IV) (0.79
angstroms). Consequently, compounds of hafnium(IV) and zirconium(IV)
have very similar chemical and physical properties. Hafnium and
zirconium tend to occur together in nature and the similarity of their
ionic radii makes their chemical separation rather difficult. 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. Some hafnium compounds in lower oxidation states
are known.

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 (HfO2), with a melting point of 2,812 C and a
boiling point of roughly 5,100 C, is very similar to zirconia, but
slightly more basic. Hafnium carbide is the most refractory binary
compound known, with a melting point over 3,890 C, and hafnium nitride
is the most refractory of all known metal nitrides, with a melting
point of 3,310 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
() possesses the highest melting point of any currently known
compound, . Recent supercomputer simulations suggest a hafnium alloy
with a melting point of 4,400 K.


                              History
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Hafnium's existence was predicted by Dmitri Mendeleev in 1869.
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.

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 he claimed 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. In 1921, Charles R. Bury suggested that element 72 should
resemble zirconium and therefore was not part of the rare earth
elements group. By early 1923, Niels Bohr and others agreed with Bury.
These suggestions were based on Bohr's theories of the atom which were
identical to chemist Charles Bury, the X-ray spectroscopy of Moseley,
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.

Hafnium was one of the last two stable elements to be discovered. The
element rhenium was found in 1908 by Masataka Ogawa, though its atomic
number was misidentified at the time, and it was not generally
recognised by the scientific community until its rediscovery by Walter
Noddack, Ida Noddack, and Otto Berg in 1925. This makes it somewhat
difficult to say if hafnium or rhenium was discovered last.

In 1923, six predicted elements were still missing from the periodic
table: 43 (technetium), 61 (promethium), 85 (astatine), and 87
(francium) are radioactive elements and are only present in trace
amounts in the environment, thus making elements 75 (rhenium) and 72
(hafnium) the last two unknown non-radioactive elements.


                            Applications
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Most of the hafnium produced is used in the manufacture of control
rods for nuclear reactors.

Hafnium has limited technical applications due to a few factors.
First, it's very similar to zirconium, a more abundant element that
can be used in most cases. Second, pure hafnium wasn't widely
available until the late 1950s, when it became a byproduct of the
nuclear industry's need for hafnium-free zirconium. Additionally,
hafnium is rare and difficult to separate from other elements, making
it expensive. After the Fukushima disaster reduced the demand for
hafnium-free zirconium, the price of hafnium increased significantly
from around $500-$600/kg ($227-$272/lb) in 2014 to around $1000/kg
($454/lb) in 2015.


Nuclear reactors
==================
The nuclei of several hafnium isotopes can each absorb multiple
neutrons. This makes hafnium a good material for nuclear reactors'
control rods. Its neutron capture cross section (Capture Resonance
Integral Io ≈ 2000 barns) 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 pressurized water reactors. The German research reactor FRM II uses
hafnium as a neutron absorber. It is also common in military reactors,
particularly in US naval submarine reactors, to slow reactor rates
that are too high. It is seldom found in civilian reactors, the first
core of the Shippingport Atomic Power Station (a conversion of a naval
reactor) 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 thereby improves 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 gates of transistors as
insulators in the 45 nm (and below) 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 (along with ytterbium) are also used
in isotope geochemistry and geochronological applications, in
lutetium-hafnium dating. It is often used as a tracer of isotopic
evolution of Earth's mantle through time. This is because 176Lu decays
to 176Hf with a half-life of approximately 37 billion years.

In most geologic materials, zircon is the dominant host of hafnium
(>10,000 ppm) 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. 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 the air.

The high energy content of 178m2Hf was the concern of a DARPA-funded
program in the US. This program eventually concluded that using the
above-mentioned 178m2Hf 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'.

Hafnium metallocene compounds can be prepared from hafnium
tetrachloride and various cyclopentadiene-type ligand species. Perhaps
the simplest hafnium metallocene is hafnocene dichloride. Hafnium
metallocenes are part of a large collection of Group 4 transition
metal metallocene catalysts  that are used worldwide in the production
of polyolefin resins like polyethylene and polypropylene.

A pyridyl-amidohafnium catalyst can be used for the controlled
iso-selective polymerization of propylene which can then be combined
with polyethylene to make a much tougher recycled plastic.

Hafnium diselenide is studied in spintronics thanks to its charge
density wave and superconductivity.


                            Precautions
======================================================================
Care needs to be taken when machining hafnium because it is
pyrophoric--fine particles can spontaneously combust when exposed to
air. 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.

People can be exposed to hafnium in the workplace by breathing,
swallowing, skin, and eye contact. The Occupational Safety and Health
Administration (OSHA) has set the legal limit (permissible exposure
limit) for exposure to hafnium and hafnium compounds in the workplace
as TWA 0.5 mg/m3 over an 8-hour workday. The National Institute for
Occupational Safety and Health (NIOSH) has set the same recommended
exposure limit (REL). At levels of 50 mg/m3, hafnium is immediately
dangerous to life and health.


                           External links
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* [http://periodic.lanl.gov/72.shtml Hafnium] at Los Alamos National
Laboratory's [http://periodic.lanl.gov/index.shtml periodic table of
the elements]
* [http://www.periodicvideos.com/videos/072.htm Hafnium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [http://www.americanelements.com/hf.htm Hafnium Technical &
Safety Data]
*
[http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+hafnium,+elemental
NLM Hazardous Substances Databank - Hafnium, elemental]
* Don Clark: [https://www.wsj.com/articles/SB119481053795589302 Intel
Shifts from Silicon to Lift Chip Performance] - WSJ, 2007
*
[https://web.archive.org/*/www.intel.com/technology/45nm/index.htm?iid=homepage+marquee_45nm
Hafnium-based Intel 45nm Process Technology]
* [https://www.cdc.gov/niosh/npg/npgd0309.html CDC - NIOSH Pocket
Guide to Chemical Hazards]


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