7440-58-6 Usage
Description
Hafnium is a silvery, ductile metallic transition element with the symbol Hf and atomic number 72. It is found with zirconium and is extracted by the formation of the chloride and reduction by the Kroll process. Hafnium is physically and chemically very similar to zirconium, making it difficult to separate the two elements. It has a close-packed hexagonal structure and is not as ductile or easily worked as zirconium. Hafnium has a high neutron-absorption cross section, making it useful in various applications.
Uses
Used in Nuclear Industry:
Hafnium is used as a neutron-absorbing control material in nuclear reactors due to its high neutron-absorption cross section (105 barns). This attribute, along with its strength and resistance to corrosion, makes it superior to cadmium for making control rods, especially for the type of nuclear reactors used aboard submarines.
Used in Vacuum Tubes:
Hafnium is used as a "getter" in vacuum tubes and other applications that require gas removal. It absorbs any trace oxygen or nitrogen in the tube, thus extending the life of the vacuum tube.
Used in Light Bulb Filaments:
Hafnium's qualities make it ideal for filaments in light bulbs. When mixed with rare-earth metals, it forms a "sparking" misch metal.
Used as an Alloying Agent:
Hafnium is used to a lesser extent as an alloying agent for several other metals, including iron, titanium, and niobium.
Used in High-Temperature Refractory Materials:
Similar to zirconium, hafnium is alloyed with niobium and carbide to produce high-temperature refractory materials for furnaces, jet components, and plasma cutters.
Used in Microprocessor Chips:
Hafnium oxide is increasingly being used to augment or replace silicon oxide-based microprocessor chips in certain applications, as well as in cathodes and capacitors.
Physical Properties:
Hafnium occurs as a close-packed hexagonal alpha-form and a body-centered cubic beta modification. It has an electrical resistivity of 35.5 microhm-cm at 20°C, a magnetic susceptibility of 0.42x10^-6 emu/g at 25°C, and a thermal neutron absorption cross-section of 105 barns/atom. Hafnium has a work function of 3.5 eV, a modulus of elasticity of 20x10^6 psi, and a tensile strength of 58,000 psi at 25°C. It is insoluble in water, dilute mineral acids, and nitric acid at all concentrations but is soluble in hydrofluoric acid, concentrated sulfuric acid, and aqua regia.
Occurrence:
Hafnium is the 47th most abundant element on Earth, more abundant than gold or silver. It is always found together with zirconium in nature, and both metals are refined and produced by the Kroll process. Baddeleyite (ZrO2) and zircon (ZrSiO4) are treated with chlorine and a carbon catalyst, producing a mixture of zirconium and hafnium tetrachlorides. These are reduced using sodium or magnesium, resulting in the production of both metals. The molten metals are separated by the process known as fractionation, which depends on their different melting points and densities.
Industrial Uses:
Pure hafnium is a lustrous, silvery metal that is not as ductile or easily worked as zirconium. However, hafnium can be hotand cold-rolled using similar techniques as those used for zirconium. All zirconium chemicals and alloys may contain some hafnium, and hafnium metal usually contains about 2% zirconium. The electric conductivity of hafnium is about 6% that of copper, and it has excellent resistance to a wide range of corrosive environments. Due to their similar chemical properties, zirconium and hafnium always occur together in nature. However, they differ greatly in their ability to absorb neutrons, leading to their use in different ways in nuclear reactors. Zirconium, with a low neutron-absorption cross-section, is desirable as a structural material in water-cooled nuclear reactor cores, while hafnium, with a high neutron-absorption cross-section, can be used as a neutron-absorbing control material in the same nuclear reactor cores.
History, Occurrence and Uses
Hafnium was discovered in 1922 by Coster and deHevesy. They named it for Hafnia, the Latin word for Copenhagen. It is found in all zirconium ores, such as zircon, (ZrSiO4) and baddeleyite (ZrO2). It occurs in the earth’s crust at about 3 mg/kg. Its average concentration in sea water is 7 ng/L.
Hafnium is used in control rods for nuclear reactors. It has high resistance to radiation and also very high corrosion resistance. Another major application is in alloys with other refractory metals, such as, tungsten, niobium and tantalum.
Production
Hafnium is obtained commercially from mineral zircon, which is zirconium orthosilicate [14940-68-2]. Zircon usually contains hafnium oxide, HfO2, in an amount that ranges between 1 to 2%. Zircon sand is separated from heavy mineral fractions from alluvial deposits by various electrostatic and magnetic separation processes. The sand is then ground and heated with caustic soda at 600°C or with soda ash at 1,000°C, or fused with lime at elevated temperatures to separate silicates.
Alternatively, zircon may be decomposed by heating with chlorine in the presence of coke at 1,100°C. In the caustic fusion process, pulverized fusion cake is washed with water to remove water-soluble sodium silicate and unreacted caustic soda, leaving behind insoluble hydrous zirconium oxide. Hydrous zirconium oxide is soluble in most acids. It is dissolved in hydrochloric acid and filtered to remove unreacted ore and silica. When the chlorination process is applied, the products are zirconium tetrachloride, hafnium tetrachloride, and silicon tetrachloride. Silicon tetrachloride is more volatile than the other two chlorides and, therefore, zirconium tetrachloride and hafnium tetrachloride can be removed from silicon tetrachloride by condensing under controlled heating. The condensed tetrachlorides are dissolved in water and filtered to remove insoluble matter.
Reactions
The chemical properties of hafnium are very much similar to those of zirconium. In aqueous solutions, the metal exists in tetravalent state. The electrode potential for the reaction Hf→ Hf 4+ + 4eˉ is –1.70V. The metal in bulk form does not react with most reagents at ordinary temperatures. However, the powdered metal or hafnium sponge may readily burn in air after ignited with a spark. When heated at 360°C under water pressure, the metal is oxidized to hafnium oxide, forming a thin, protective, surface oxide layer. A similar surface hafnium oxide layer forms in nitric acid, which protects the metal from acid attack.
Reaction with hydrofluoric acid at ordinary temperatures yields hafnium tetrafluoride, HfF4.
Reaction with hydrogen occurs around 700°C. Hafnium absorbs rapidly, forming a hydride which probably has a composition HfH1.86.
Hafnium metal reacts very slowly in concentrated sulfuric acid at ordinary temperatures. At acid concentration above 70% and under boiling conditions, sulfuric acid readily attacks the metal.
Isotopes
There are 44 known isotopes for hafnium. Five are stable and one of the unstableisotopes has such a long half-life (Hf-174 with a 2.0×10+15 years) that it is includedas contributing 0.16% to the amount of hafnium found in the Earth’s crust. The percentagecontributions of the 5 stable isotopes to the element’s natural existence on Earth areas follows: Hf-176 = 5.26%, Hf-177 = 18.60%, Hf-178 = 27.28%, Hf-179 = 13.62%,and Hf-180 = 35.08%.
Origin of Name
Named after Hafnia, the Latin name for the city of Copenhagen, Denmark.
History
Hafnium was thought to be present in various minerals and concentrations many years prior to its discovery, in 1923, credited to D. Coster and G. von Hevesey. On the basis of the Bohr theory, the new element was expected to be associated with zirconium. It was finally identified in zircon from Norway, by means of X-ray spectroscopic analysis. Hafnium was named in honor of the city in which the discovery was made. Most zirconium minerals contain 1 to 5% hafnium. It was originally separated from zirconium by repeated recrystallization of the double ammonium or potassium fluorides by von Hevesey and Jantzen. Metallic hafnium was first prepared by van Arkel and deBoer by passing the vapor of the tetraiodide over a heated tungsten filament. Almost all hafnium metal now produced is made by reducing the tetrachloride with magnesium or with sodium (Kroll Process). Hafnium is a ductile metal with a brilliant silver luster. Its properties are considerably influenced by the impurities of zirconium present. Of all the elements, zirconium and hafnium are two of the most difficult to separate. Their chemistry is almost identical; however, the density of zirconium is about half that of hafnium. Very pure hafnium has been produced, with zirconium being the major impurity. Natural hafnium contains six isotopes, one of which is slightly radioactive. Hafnium has a total of 41 recognized isotopes and isomers. Because hafnium has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used for reactor control rods. Such rods are used in nuclear submarines. Hafnium has been successfully alloyed with iron, titanium, niobium, tantalum, and other metals. Hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory of all known metal nitrides (m.p. 3310°C). Hafnium is used in gas-filled and incandescent lamps, and is an efficient “getter” for scavenging oxygen and nitrogen. Finely divided hafnium is pyrophoric and can ignite spontaneously in air. Care should be taken when machining the metal or when handling hot sponge hafnium. At 700°C hafnium rapidly absorbs hydrogen to form the composition HfH1.86. Hafnium is resistant to concentrated alkalis, but at elevated temperatures reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Halogens react directly to form tetrahalides. The price of the metal is about $2/g. The yearly demand for hafnium in the U.S. is now in excess of 50,000 kg.
Characteristics
As the first element in the third series of the transition elements, hafnium’s atomic number(72Hf ) follows the lanthanide series of rare-earths. The lanthanide series is separated out ofthe normal position of sequenced atomic numbers and is placed below the third series on theperiodic table (57La to 71Li). This rearrangement of the table allowed the positioning of elementsof the third series within groups more related to similar chemical and physical characteristics—for example, the triads of Ti, Zr, and Hf; V, Nb, and Ta; and Cu, Ag, and Au.
Air & Water Reactions
Highly flammable. The dry powder reacts with moisture to produce hydrogen, a flammable gas. The heat from this reaction may be sufficient to ignite the hydrogen. HAFNIUM does not appreciably react with large quantities of water.
Reactivity Profile
Metals, such as HAFNIUM METAL(reactivity similar to zirconium), are reducing agents and tend to react with oxidizing agents. Their reactivity is strongly influenced by their state of subdivision: in bulk they often resist chemical combination; in powdered form they may react very rapidly. Thus, as a bulk metal HAFNIUM is somewhat unreactive, but finely divided material may be pyrophoric. The metal reacts exothermically with compounds having active hydrogen atoms (such as acids and water) to form flammable hydrogen gas and caustic products. The reactions are less vigorous than the similar reactions of alkali metals, but the released heat can still ignite the released hydrogen. Materials in this group may react with azo/diazo compounds to form explosive products. These metals and the products of their corrosion by air and water can catalyze polymerization reactions in several classes of organic compounds; these polymerizations sometimes proceed rapidly or even explosively. Some metals in this group form explosive products with halogenated hydrocarbons.
Hazard
Although the metal hafnium is not harmful, its powder and dust are both toxic if inhaledand explosive even when wet.
Health Hazard
Fire will produce irritating, corrosive and/or toxic gases. Inhalation of decomposition products may cause severe injury or death. Contact with substance may cause severe burns to skin and eyes. Runoff from fire control may cause pollution.
Fire Hazard
Flammable/combustible material. May ignite on contact with moist air or moisture. May burn rapidly with flare-burning effect. Some react vigorously or explosively on contact with water. Some may decompose explosively when heated or involved in a fire. May re-ignite after fire is extinguished. Runoff may create fire or explosion hazard. Containers may explode when heated.
Flammability and Explosibility
Highlyflammable
Check Digit Verification of cas no
The CAS Registry Mumber 7440-58-6 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 7,4,4 and 0 respectively; the second part has 2 digits, 5 and 8 respectively.
Calculate Digit Verification of CAS Registry Number 7440-58:
(6*7)+(5*4)+(4*4)+(3*0)+(2*5)+(1*8)=96
96 % 10 = 6
So 7440-58-6 is a valid CAS Registry Number.
InChI:InChI=1/Hf
