Hafnium

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 it was not identified until 1923, making it the penultimate stable element to be discovered (rhenium was identified two years later). 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.

One important use of hafnium comes from its being waste. Zirconium is extraordinarily good for cladding nuclear fuel pellets. It is almost transparent to neutrons and excellent metallurgical qualities for reactor service, but even high-purity zirconium is contaminated with hafnium, Hafnium is one of the elements least transparent to neutrons, which makes hafnium-bearing zirconium useless for reactor service. It is necessary to remove Hf from Zr in order to use the latter in reactors, and that leaves the Zr supplier with leftover Hf.

Hafnium is relatively opaque to neutrons because it captures them readily. That makes it useful for reactor control rods, which throttle power output of a reactor by absorbing a portion of the neutron flux in its core. Refining of reactor-grade zirconium generates enough hafnium to meet the demand for control rods.

Nuclear properties

Thirty eight isotopes of Hf have been observed, ranging fro ¹⁸⁸Hf down to ¹⁵³Hf, as well as 26 isomers. These lie within the band of predicted isotopes that goes from ²⁴⁰Hf to ¹⁴⁴Hf. Isotopes ¹⁸¹Hf and heavier all decay by beta emission with the expected pattern of decreasing stability as A rises. Isotopes ¹⁷³Hf and lighter decay by emitting positive charge. Positive beta decay (positron emission or electron capture) is observed or predicted in all isotopes down to ¹⁴⁶Hf. Alpha decay is also observed in the band ¹⁶³Hf down to ¹⁵⁴Hf, and is dominant in ¹⁵⁷Hf and ¹⁵⁶Hf (for which N = 84). Decay by proton emission is predicted to appear in the lightest isotopes.

Isotopes between ¹⁷⁴Hf and ¹⁸⁰Hf are stable in their ground state, except for ¹⁷⁵Hf [half-life (t12) = 70 d, mode = b⁺]. Most of these stable isotopes have isomers, though. Seventeen of hafnium's known isomers decay to stable ground states. Most have half-lives under 1 sec, but t12(^177m3Hf) = 51.4 min, t12(^179m2Hf) = 25.05 d, and t12(^180m1Hf) = 5.47 hr. Star of this show, though, is ^178m2Hf, with its 31 yr half-life. It should be noted that ¹⁸²Hf - which is positioned relative to the other stable isotopes of Hf the way ²³⁸U is positioned with respect to uranium's other stable isotopes - has a 61 minute isomer. Freshly synthesized Hf is significantly radioactive for a long time.

Hafnium isotopes ¹⁸⁰Hf through ¹⁷⁷Hf mark the ends of beta decay chains extending from the neutron dripline. These isotopes can form by rapid neutron capture (r process) in a supernova or neutron star merger. They can also form via infrequent neutron capture (s process*) in evolved stars and cold bodies which also contain neutron sources. An s process would normally produce ¹⁷⁶Hf, but ¹⁷⁶Lu is stable. At temperature high enough to make high-energy photons available, the reaction gamma + ^AZ --> ^(A-1)Z + n [ g(^AZ,^(A-1)Z)n ] will produce it, as well as ¹⁷⁴Hf.

*The term "s process" refers to infrequent neutron capture, regardless of neutron energy. The term can be confusing because "slow neutron capture" can also refer to capture of low-energy neutrons. This is a nontrivial difference because neutron capture cross sections of a given nuclide vary widely as a function of neutron energy - and the variation differs from nuclide to nuclide. A stellar s process, at temperatures on the order of 10⁸ K (9 keV) will produce different abundances than a terrestrial r process at 300 K (essentially 0 eV).

9-Period Periodic Table of Elements
1	1 H																																																																	2 He
2	3 Li	4 Be																																																																	5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg																																																																	13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	19 K	20 Ca																																																																	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
5	37 Rb	38 Sr																																																																	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
6	55 Cs	56 Ba																																																					57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb	71 Lu	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
7	87 Fr	88 Ra																																																					89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No	103 Lr	104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Cn	113 Nh	114 Fl	115 Mc	116 Lv	117 Ts	118 Og
8	119 Uue	120 Ubn																															121 Ubu	122 Ubb	123 Ubt	124 Ubq	125 Ubp	126 Ubh	127 Ubs	128 Ubo	129 Ube	130 Utn	131 Utu	132 Utb	133 Utt	134 Utq	135 Utp	136 Uth	137 Uts	138 Uto	139 Ute	140 Uqn	141 Uqu	142 Uqb	143 Uqt	144 Uqq	145 Uqp	146 Uqh	147 Uqs	148 Uqo	149 Uqe	150 Upn	151 Upu	152 Upb	153 Upt	154 Upq	155 Upp	156 Uph	157 Ups	158 Upo	159 Upe	160 Uhn	161 Uhu	162 Uhb	163 Uht	164 Uhq	165 Uhp	166 Uhh	167 Uhs	168 Uho	169 Uhe	170 Usn	171 Usu	172 Usb
9	173 Ust	174 Usq
Alkali metal Alkaline earth metal Lanthanide Actinide Superactinide Transition metal Post-transition metal Metalloid Other nonmetal Halogen Noble gas predicted predicted predicted predicted predicted predicted predicted predicted predicted