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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. The element is obtained only by separation from
zirconium, with most of the world's hafnium production coming from
processes that also produce zirconium. These processes make use of
heavy mineral sands ore deposits, which include the minerals zircon,
rutile, and ilmenite, among others.
Hafnium is most often used in alloys with nickel, and was used in
larger quantities to produce the control rods used in nuclear
reactors. 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. It is ductile, and is also used in filaments and
electrodes. Some semiconductor fabrication processes use its oxide
for integrated circuits at and smaller, and superalloys used for
special applications can contain hafnium in combination with niobium,
titanium, or tungsten.
Pure hafnium is not toxic, but is extremely flammable to the point of
being pyrophoric--capable of spontaneous combustion in air. Several
industrial processes involved in the production of hafnium have
by-products that can be hazardous when released into the environment,
and several hafnium compounds have hazards of their own. One nuclear
isomer of hafnium, 178m2Hf, was the source of a controversy for its
potential use as a weapon, but it has never been successfully produced
for practical use.
Physical characteristics
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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, roughly three orders of magnitude greater than
that of zirconium, and that the nuclei of several different hafnium
isotopes readily absorb two or more neutrons apiece. Because zirconium
is practically transparent to thermal neutrons, it is commonly used
for the metal components of nuclear reactors--especially the cladding
of their nuclear fuel rods.
Chemical characteristics
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Hafnium reacts in air to form a protective film of hafnium oxide in
the monoclinic phase 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
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At least 40 isotopes of hafnium have been observed, ranging in mass
number from 153 to 192. The five stable isotopes have mass numbers
from 176 to 180 inclusive; the primordial 174Hf has a very long
half-life of years.
The extinct radionuclide 182Hf has a half-life of , and is an
important tracker isotope for the formation of planetary cores. No
other radioisotope has a half-life over 1.87 years.
The longest-lived nuclear isomer 178m2Hf (31 years) was at the center
of a controversy for several years regarding its potential use as a
weapon. Because of its high energy compared to the ground state 178Hf,
the isomer was put under scrutiny as being capable of induced gamma
emission, which could be weaponized to produce large amounts of gamma
radiation all at once. Applications of the isomer have been frustrated
due to the difficulty of producing it without the product being
immediately destroyed as well as its extremely high cost.
Occurrence
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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 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--did not prove 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.
Other methods to purify hafnium from zirconium include molten salt
extraction and crystallization of fluorozirconates. 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 tetrahalides are precursors to
various organohafnium compounds, and hafnium(IV) chloride in
particular is used in microelectronics manufacturing as a source of
hafnium oxide in atomic layer deposition, much in the same way as
zirconium(IV) chloride.
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. Hafnium carbonitride has the highest known melting
point for any material, which is confirmed to be above 4000 C by
experiment, while calculations predict its melting point to be 4110 C.
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.
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 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.
Applications
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Much of the hafnium produced is used in the manufacture of control
rods for nuclear reactors and as an additive in nickel alloys to
increase their heat resistance.
Hafnium has limited technical applications due to a few factors. It is
very similar to zirconium, a more abundant element that can be used in
most cases, and 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. Hafnium products, such as
tubes and sheets of the metal, could be purchased at /kg($170/lb) in
2009.
Nuclear reactors
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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
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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. An alloy that includes as little as 1%
hafnium can withstand temperatures that are higher than the same
alloy without hafnium.
Microprocessors
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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
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Isotopes of hafnium and lutetium 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. Mass
spectrometry also makes use of these ratios to date garnet formed
through igneous events.
Other uses
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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. 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
tougher recycled plastic.
The high energy content of 178m2Hf was the concern of a DARPA-funded
program in the US. This program eventually concluded that using the
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 and difficulty to
manufacture. See hafnium controversy.
Hafnium diselenide is studied in spintronics thanks to its charge
density wave and superconductivity.
Toxicity and safety
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{{Chembox
| Name = Hafnium powder
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Hafnium is a pyrophoric material, and as such fine particles can
spontaneously combust upon exposure to air. Hafnium powder is often
wetted with at least 25% water by weight to be considered safe - the
metal is insoluble in water. Machining hafnium is particularly
hazardous because of the potential for fine particles of the metal to
be produced and immediately introduced to frictional force. Compounds
that contain this metal are rarely encountered by most people. The
pure metal is not considered toxic, though it has been observed to
accumulate in the liver when injected into rats. 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. Hafnium tetrachloride and
hafnium tetrabromide, which are often part of industrial processes
that use the element, are of particular note, with both compounds
releasing acidic fumes on contact with water (hydrochloric and
hydrobromic acid, respectively). Additionally, hafnium tetrachloride
has been observed as causing liver damage at high exposure levels.
People can be exposed to hafnium in the workplace by breathing,
swallowing, skin, and eye contact. In the United States, 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.
Because the mineral zircon is often associated with traces of the
radioactive elements uranium and thorium, the chemically destructive
processes used to separate zirconium from hafnium have potential to
release these radioactive elements and their decay products into the
environment along with other reaction wastes. Additionally, synthesis
pathways that involve liquid-liquid extraction introduce ammonium
chloride and sulfate into reaction mixtures, which as effluent can
reduce available oxygen in water sources or produce cyanides if it
comes into contact with thiocyanate-containing compounds.
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]
* [
https://www.nlm.nih.gov/toxnet/index.html 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]
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Hafnium
