   #copyright

Hafnium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                72              lutetium ← hafnium → tantalum
                Zr
                ↑
                Hf
                ↓
                Rf

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                      Name, Symbol, Number hafnium, Hf, 72
                                         Chemical series transition metals
                                              Group, Period, Block 4, 6, d
                                                     Appearance gray steel
                                              Atomic mass 178.49 (2) g/mol
                               Electron configuration [Xe] 4f^14 5d^2 6s^2
                                   Electrons per shell 2, 8, 18, 32, 10, 2
                                                       Physical properties
                                                               Phase solid
                                      Density (near r.t.) 13.31 g·cm^−3
                                      Liquid density at m.p. 12 g·cm^−3
                                                     Melting point 2506  K
                                                    (2233 ° C, 4051 ° F)
                                                      Boiling point 4876 K
                                                    (4603 ° C, 8317 ° F)
                                          Heat of fusion 27.2 kJ·mol^−1
                                     Heat of vaporization 571 kJ·mol^−1
                          Heat capacity (25 °C) 25.73 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                      P/Pa   1    10  100  1 k  10 k 100 k
                                     at T/K 2689 2954 3277 3679 4194 4876

                                                         Atomic properties
                                               Crystal structure hexagonal
                                                        Oxidation states 4
                                                       ( amphoteric oxide)
                                     Electronegativity 1.3 (Pauling scale)
                                                       Ionization energies
                                           ( more) 1st: 658.5 kJ·mol^−1
                                                    2nd: 1440 kJ·mol^−1
                                                    3rd: 2250 kJ·mol^−1
                                                      Atomic radius 155 pm
                                              Atomic radius (calc.) 208 pm
                                                    Covalent radius 150 pm
                                                             Miscellaneous
                                                 Magnetic ordering no data
                                Electrical resistivity (20 °C) 331 nΩ·m
                       Thermal conductivity (300 K) 23.0 W·m^−1·K^−1
                        Thermal expansion (25 °C) 5.9 µm·m^−1·K^−1
                               Speed of sound (thin rod) (20 °C) 3010 m/s
                                                    Young's modulus 78 GPa
                                                      Shear modulus 30 GPa
                                                      Bulk modulus 110 GPa
                                                        Poisson ratio 0.37
                                                         Mohs hardness 5.5
                                                 Vickers hardness 1760 MPa
                                                 Brinell hardness 1700 MPa
                                             CAS registry number 7440-58-6
                                                         Selected isotopes

                 CAPTION: Main article: Isotopes of hafnium

                             iso      NA    half-life DM DE ( MeV)   DP
                          ^172Hf    syn     1.87 y    ε  0.350     ^172Lu
                          ^174Hf    0.162%  2×10^15 y α  2.495     ^170Yb
                          ^176Hf    5.206%  Hf is stable with 104 neutrons
                          ^177Hf    18.606% Hf is stable with 105 neutrons
                          ^178Hf    27.297% Hf is stable with 106 neutrons
                          ^178 m2Hf syn     31 y      IT 2.446     ^178Hf
                          ^179Hf    13.629% Hf is stable with 107 neutrons
                          ^180Hf    35.1%   Hf is stable with 108 neutrons
                          ^182Hf    syn     9×10^6 y  β  0.373     ^182Ta

                                                                References

   Hafnium ( IPA: /ˈhæfniəm/) is a chemical element in the periodic table
   that has the symbol Hf and atomic number 72. A lustrous, silvery gray
   tetravalent transition metal, hafnium resembles zirconium chemically
   and is found in zirconium minerals. Hafnium is used in tungsten alloys
   in filaments and electrodes and also acts as a neutron absorber in
   control rods in nuclear power plants.

Notable characteristics

   Hafnium metal
   Enlarge
   Hafnium metal

   Hafnium is a shiny silvery, ductile metal that is corrosion resistant
   and chemically similar to zirconium. The properties of hafnium are
   markedly affected by zirconium impurities and these two elements are
   amongst the most difficult to separate. The only notable difference
   between them is their density (zirconium is about half as dense as
   hafnium).

   Hafnium carbide is the most refractory binary compound known and
   hafnium nitride is the most refractory of all known metal nitrides with
   a melting point of 3310 °C. This metal is resistant to concentrated
   alkalis, but halogens react with it to form hafnium tetrahalides. 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 to 2.45  MeV per
   decay. 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 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 lasers.

Applications

   Hafnium is used to make control rods for nuclear reactors because of
   its ability to absorb neutrons (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 lamps, for scavenging oxygen and
       nitrogen,
     * As the electrode in plasma cutting because of its ability to shed
       electrons into air,
     * and in iron, titanium, niobium, tantalum, and other metal alloys.
     * Hafnium dioxide is a candidate for High-K gate insulators in future
       generations of integrated circuits.
     * 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 into the means for releasing
       the stored energy upon demand. There is considerable opposition to
       this program, both because the idea may not work 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, which could remain
       airborne for weeks at a time.

History

   Hafnium (Latin Hafnia for "Copenhagen", the home town of Niels Bohr)
   was discovered by Dirk Coster and Georg von Hevesy in 1923 in
   Copenhagen, Denmark. Soon after, the new element was predicted to be
   associated with zirconium by using the Bohr theory and was finally
   found in zircon through X-ray spectroscope analysis in Norway.

   It was separated from zirconium through repeated recrystallization of
   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 tetraiodide vapor over a heated tungsten
   filament.

   The Faculty of Science of the University of Copenhagen uses in its seal
   a stylized image of hafnium.

Occurrence

   Hafnium is found combined in natural zirconium compounds but it does
   not exist as a free element in nature. Minerals that contain zirconium,
   such as alvite [(Hf, Th, Zr)SiO[4] H[2]O, thortveitite and zircon
   (ZrSiO[4]), 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 by a by-product of zirconium refinement. This is done through
   reducing hafnium(IV) chloride with magnesium or sodium in the Kroll
   process.

Precautions

   Care needs to be taken when machining hafnium because when it is
   divided into fine particles, it is pyrophoric and can ignite
   spontaneously in air. Compounds that contain this metal are rarely
   encountered by most people and the pure metal is not normally toxic but
   all its compounds should be handled as if they are toxic (although
   there appears to be limited danger to exposed individuals).

   Retrieved from " http://en.wikipedia.org/wiki/Hafnium"
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