Hafnium
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2008-07-14T22:09:53Z
Arkuat
29003
/* Characteristics */ section title according to wikiproject elements
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{{Elementbox_series | [[transition metal]]s }}
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{{Elementbox_appearance_img | Hafnium bits| steel grey }}
{{Elementbox_atomicmass_gpm | [[1 E-25 kg|178.49]][[List of elements by atomic mass|(2)]] }}
{{Elementbox_econfig | [[[Xenon|Xe]]] 4f<sup>14</sup> 5d<sup>2</sup> 6s<sup>2</sup> }}
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{{Elementbox_densityliq_gpcm3mp | 12 }}
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{{Elementbox_boilingpoint | k=4876 | c=4603 | f=8317 }}
{{Elementbox_heatfusion_kjpmol | 27.2 }}
{{Elementbox_heatvaporiz_kjpmol | 571 }}
{{Elementbox_heatcapacity_jpmolkat25 | 25.73 }}
{{Elementbox_vaporpressure_katpa | 2689 | 2954 | 3277 | 3679 | 4194 | 4876 | comment= }}
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{{Elementbox_crystalstruct | hexagonal }}
{{Elementbox_oxistates | 4<br />([[amphoteric]] oxide) }}
{{Elementbox_electroneg_pauling | 1.3 }}
{{Elementbox_ionizationenergies4 | 658.5 | 1440 | 2250 }}
{{Elementbox_atomicradius_pm | [[1 E-10 m|155]] }}
{{Elementbox_atomicradiuscalc_pm | [[1 E-10 m|208]] }}
{{Elementbox_covalentradius_pm | [[1 E-10 m|150]] }}
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{{Elementbox_thermalexpansion_umpmkat25 | 5.9 }}
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{{Elementbox_youngsmodulus_gpa | 78 }}
{{Elementbox_shearmodulus_gpa | 30 }}
{{Elementbox_bulkmodulus_gpa | 110 }}
{{Elementbox_poissonratio | 0.37 }}
{{Elementbox_mohshardness | 5.5 }}
{{Elementbox_vickershardness_mpa | 1760 }}
{{Elementbox_brinellhardness_mpa | 1700 }}
{{Elementbox_cas_number | 7440-58-6 }}
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| na=[[synthetic radioisotope|syn]] | hl=1.87 [[year|y]]
| dm=[[electron capture|ε]] | de=0.350 | pn=172 | ps=[[lutetium|Lu]] }}
{{Elementbox_isotopes_decay | mn=174 | sym=Hf
| na=0.162% | hl=2×10<sup>15</sup> [[year|y]]
| dm=[[alpha emission|α]] | de=2.495 | pn=170 | ps=[[ytterbium|Yb]] }}
{{Elementbox_isotopes_stable | mn=176 | sym=Hf | na=5.206% | n=104 }}
{{Elementbox_isotopes_stable | mn=177 | sym=Hf | na=18.606% | n=105 }}
{{Elementbox_isotopes_stable | mn=178 | sym=Hf | na=27.297% | n=106 }}
{{Elementbox_isotopes_decay | mn=178[[Nuclear isomer|m2]] | sym=Hf
| na=[[synthetic radioisotope|syn]] | hl=31 [[year|y]]
| dm=[[Gamma ray|IT]] | de=2.446 | pn=178 | ps=Hf }}
{{Elementbox_isotopes_stable | mn=179 | sym=Hf | na=13.629% | n=107 }}
{{Elementbox_isotopes_stable | mn=180 | sym=Hf | na=35.1% | n=108 }}
{{Elementbox_isotopes_decay | mn=182 | sym=Hf
| na=[[synthetic radioisotope|syn]] | hl=9×10<sup>6</sup> [[year|y]]
| dm=[[beta emission|β]] | de=0.373 | pn=182 | ps=[[tantalum|Ta]] }}
{{Elementbox_isotopes_end}}
{{Elementbox_footer | color1=#ffc0c0 | color2=black }}
'''Hafnium''' ({{pronEng|ˈhæfniəm}}) is a [[chemical element]] that has the [[element symbol|symbol]] '''Hf''' and [[atomic number]] 72. A [[lustre (mineralogy)|lustrous]], silvery gray [[tetravalence|tetravalent]] [[transition metal]], hafnium resembles [[zirconium]] chemically and it is found in zirconium [[mineral]]s. Hafnium is used in [[tungsten]] [[alloy]]s in [[electrical filament|filament]]s and [[electrode]]s, in [[integrated circuits]] as a gate insulator for [[MOSFET|transistors]], and as a [[neutron]] absorber in [[control rod]]s in [[nuclear power plant]]s.
== Characteristics ==
[[Image:HafniumMetalUSGOV.jpg|thumb|left|Hafnium metal]]
Hafnium is a shiny silvery, [[ductility|ductile]] [[metal]] that is [[corrosion]] resistant and chemically similar to [[zirconium]]. The ''physical'' properties of hafnium are markedly affected by zirconium impurities, and these two elements are among the most difficult ones to separate. A notable physical difference between them is their density (zirconium being about half as dense as hafnium), but chemically the elements are extremely similar.
The most notable physical property of hafnium is that it has a very high neutron-capture cross-section, and nuclei of several hafnium isotopes can each absorb multiple neutrons. This makes hafnium a good material for use in the [[control rod]]s for [[nuclear reactor]]s. Its neutron-capture cross-section is about 600 times that of zirconium's. (Other elements that are good neutron-absorbers for control rods are [[cadmium]] and [[boron]].)
Separation of hafnium and zirconium becomes very important in the nuclear power industry, since zirconium is a good 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.
[[Hafnium carbide]] is the most [[Refraction (metallurgy)|refractory]] [[binary compound]] known, with a [[melting point]] over 3890 [[Celsius|°C]], and hafnium nitride is the most refractory of all known metal nitrides, with a [[melting point]] of 3310 [[Celsius|°C]].<ref name=lanl72>[http://periodic.lanl.gov/elements/72.html Los Alamos National Laboratory – Hafnium]</ref> This has led to proposals that hafnium or its carbides might be useful as construction materials that are subjected to very high temperatures.
The metal is resistant to concentrated [[alkali]]s, but [[halogen]]s react with it to form hafnium tetrahalides.<ref name=lanl72/> At higher temperatures hafnium reacts with [[oxygen]], [[nitrogen]], [[carbon]], [[boron]], [[sulfur]], and [[silicon]].
The [[nuclear isomer]] Hf-178-m2 is also a source of cascades of [[gamma rays]] whose energies total 2.45 [[Electronvolt|MeV]] per decay.<ref>[http://ie.lbl.gov/toi/nuclide.asp?iZA=720778 WWW Table of Radioactive Isotopes], Lawrence Berkeley National Laboratory Isotopes Project and Lund University.</ref> 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 [[Trinitrotoluene|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 [[laser]]s.<ref>{{cite journal |journal=Laser Physics Letters |volume=2 |issue=3 |pages=162–167 |year=2004 |title=Nuclear resonance spectroscopy of the 31-yr isomer of Hf-178 |author=C. B. Collins ''et al.'' |doi=10.1002/lapl.200410154}}</ref>
==Applications==
Hafnium is used to make control rods for [[nuclear reactor]]s because of its ability to absorb [[neutron]]s (its thermal neutron absorption cross section is nearly 600 times that of zirconium), excellent mechanical properties and exceptional corrosion-resistance properties.
Other uses:
*In gas-filled and [[incandescent lamp]]s, for scavenging oxygen and nitrogen,
*As the electrode in [[plasma cutting]] because of its ability to shed electrons into air,
*In [[iron]], [[titanium]], [[niobium]], [[tantalum]], and other metal [[alloy]]s.
*A hafnium-based compound is employed in [[gate (transistor)|gate]] insulators in the 45 nm generation of [[integrated circuits]] from [[Intel]], [[IBM]] and others <ref>{{cite news |url=http://www.nytimes.com/2007/01/27/technology/27chip.html |title=Intel Says Chips Will Run Faster, Using Less Power |first=John |last=Markoff |publisher=New York Times |date=[[January 27]], [[2007]] |accessdate=2007-09-19}}</ref>. Hafnium oxide-based compounds are practical [[high-k dielectric]]s, allowing reduction of the gate leakage current which improves performance at such scales.<ref>{{cite news | last = Fulton, III| first = Scott M. | title = Intel Reinvents the Transistor | publisher = BetaNews | date = [[January 27]], [[2007]] | url = http://www.betanews.com/article/Intel_Reinvents_the_Transistor/1169872301| accessdate = 2007-01-27 }}</ref><ref>{{cite news | last = Robertson| first = Jordan | title = Intel, IBM reveal transistor overhaul | publisher = AP | date = [[January 27]], [[2007]]| url = http://www.washingtonpost.com/wp-dyn/content/article/2007/01/27/AR2007012700152.html | accessdate = 2007-09-19 }}</ref>
*[[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<ref>[http://www.hafniumisomer.org/isomer/IGEhistory.htm Induced gamma emission history], The Center for Quantum Electronics, The University of Texas at Dallas.</ref> into the means for releasing the stored energy upon demand. There is considerable opposition to this program, both because the idea may not work<ref>{{cite web |url=http://www.aip.org/pt/vol-57/iss-5/p21.html |title= Conflicting Results on a Long-Lived Nuclear Isomer of Hafnium Have Wider Implications |date=May 2004 |publisher=Physics Today |first=Bertram |last=Schwarzschild |accessdate=2007-09-19}}</ref>, 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]],<ref>{{cite web |url=http://www.newscientist.com/article/dn3406-nuclearpowered-drone-aircraft-on-drawing-board.html |title=Nuclear-powered drone aircraft on drawing board |publisher=New Scientist|accessdate=2008-06-06}}</ref> which could remain airborne for months at a time.
*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.
== History ==
[[Image:Sigilum Facultatis Naturalis.JPG|left|thumb|150px|The hafnium seal of the [[University of Copenhagen Faculty of Science|Faculty of Science of the University of Copenhagen]]]]
The 1869 periodic table by [[Dmitri Mendeleev|Mendeleev]] had implicitly [[Mendeleev's predicted elements|predicted the existence]] of a heavier analog of [[titanium]] and [[zirconium]], but in 1871 Mendeleev placed [[lanthanum]] (element 57) in that spot.
The existence of a gap in the periodic table for a yet to be discovered element 72 was predicted by [[Henry Moseley]] in 1914. Hafnium was named for the [[Latin]] name ''Hafnia'' for "[[Copenhagen]]", the home town of [[Niels Bohr]]. It was discovered by [[Dirk Coster]] and [[Georg von Hevesy]] in [[1923]] in Copenhagen, [[Denmark]], validating the original 1869 prediction of Mendeleev. Soon thereafter, the new element was predicted to be associated with [[zirconium]] by using the Bohr theories of the atom, and it was finally found in [[zircon]] through [[X-ray]] [[spectroscopy]] analysis in [[Norway]].
Hafnium was separated from zirconium through repeated recrystallization of the 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 hafnium tetra-iodide vapor over a heated [[tungsten]] filament. This process for differential purification of Zr and Hf is still in use today.
The [[University of Copenhagen Faculty of Science|Faculty of Science]] of the [[University of Copenhagen]] uses in its [[Seal (device)|seal]] a stylized image of hafnium.
== Occurrence ==
Hafnium is estimated to make up about 0.00058% of the [[Earth]]'s upper [[crust (geology)|crust]] by weight. It is found combined in natural [[zirconium]] compounds but it does not exist as a free element in nature. [[Mineral]]s that contain zirconium, such as alvite [(Hf, [[thorium|Th]], [[zirconium|Zr]])[[silicon|Si]][[oxygen|O]]<sub>4</sub> [[water|H<sub>2</sub>O]]], [[thortveitite]], and [[zircon]] (ZrSiO<sub>4</sub>), usually contain between 1 and 5% hafnium. Hafnium and zirconium have nearly identical chemistry, which makes the two difficult to separate. About half of all hafnium metal manufactured is produced as a by-product of zirconium refinement. This is done through reducing [[hafnium(IV) chloride]] with [[magnesium]] or [[sodium]] in the [[Kroll process]].
[[Image:Hafnium lump thin film effects.jpg|thumb|left|250px|A lump of hafnium which has been oxidized on one side and exhibits [[Thin-film optics|thin film optical]] effects.]]
A major source of zircon (and hence hafnium) ores are [[heavy mineral sands ore deposits]], [[pegmatite]]s 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 armostrongite, at [[Dubbo]] in New South Wales, Australia.<ref>{{cite web | url=http://www.alkane.com.au/projects/nsw/dubbo/DZP%20Summary%20June07.pdf |format=PDF | title=Dubbo Zirconia Project Fact Sheet | date= June 2007 | publisher=Alkane Resources Limited | accessdate=2007-09-10}}</ref>
== 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 (see [[Dragon's Breath]] for a demonstration). 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 are toxic because the ionic forms of metals are normally at greatest risk for toxicity, and limited animal testing has been done for hafnium compounds.
== Isotopes==
{{main|Isotopes of hafnium}}
Hafnium has five stable isotopes.
==Compounds==
{{Expand|date=December 2007}}
{{seealso|Category:Hafnium compounds}}
==See also==
* [[:Nuclear isomer|Nuclear isomer]]
* [[Induced gamma emission]]
* [[Zircon]]
==References==
<references/>
*{{cite journal |author=van Arkel, A.E., and de Boer, J.H. |year=1925 |title=Preparation of pure titanium, zirconium, hafnium, and thorium metal |journal=Zeitschrift für Anorganische und Allgemeine Chemie |volume=148 |pages=345–350}}
Scerri, E.R., Prediction of the Nature of Hafnium from Chemistry, Bohr’s Theory and Quantum Theory, Annals of Science, 51, 137-150, (1994)
== External links ==
{{Commons|Hafnium}}
{{wiktionary|hafnium}}
*[http://www.webelements.com/webelements/elements/text/Hf/index.html WebElements.com – Hafnium]
*[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]
*[http://online.wsj.com/article/SB119481053795589302.html Intel Shifts from Silicon to Lift Chip Performance]
*[http://www.intel.com/technology/45nm/index.htm?iid=homepage+marquee_45nm]
{{clear}}
{{Compact periodic table}}
[[Category:Chemical elements]]
[[Category:Transition metals]]
[[Category:Neutron poisons]]
[[Category:Hafnium|*]]
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