Hafnium, metal Hf, atomic number 72, atomic weight 178.49, is a shiny silver gray transition metal.
Hafnium has six naturally stable isotopes: hafnium 174, 176, 177, 178, 179, and 180. Hafnium does not react with dilute hydrochloric acid, dilute sulfuric acid, and strong alkaline solutions, but is soluble in hydrofluoric acid and aqua regia. The element name comes from the Latin name of Copenhagen City.
In 1925, Swedish chemist Hervey and Dutch physicist Koster obtained pure hafnium salt by fractional crystallization of fluorinated complex salts, and reduced it with metallic sodium to obtain pure metal hafnium. Hafnium contains 0.00045% of the earth’s crust and is often associated with zirconium in nature.
Product name: hafnium
Element symbol: Hf
Atomic weight: 178.49
Element type: metallic element
Physical properties:
Hafnium is a silver gray metal with a metallic luster; There are two variants of metal hafnium: α Hafnium is a hexagonal closely packed variant (1750 ℃) with a higher transformation temperature than zirconium. Metal hafnium has allotrope variants at high temperatures. Metal hafnium has a high neutron absorption cross-section and can be used as a control material for reactors.
There are two types of crystal structures: hexagonal dense packing at temperatures below 1300 ℃（ α- Equation); At temperatures above 1300 ℃, it is body centered cubic（ β- Equation). A metal with plasticity that hardens and becomes brittle in the presence of impurities. Stable in the air, only darkens on the surface when burned. The filaments can be ignited by the flame of a match. Properties similar to zirconium. It does not react with water, dilute acids, or strong bases, but is easily soluble in aqua regia and hydrofluoric acid. Mainly in compounds with a+4 valence. Hafnium alloy (Ta4HfC5) is known to have the highest melting point (approximately 4215 ℃).
Crystal structure: The crystal cell is hexagonal
CAS number: 7440-58-6
Melting point: 2227 ℃
Boiling point: 4602 ℃
Chemical properties:
The chemical properties of hafnium are very similar to those of zirconium, and it has good corrosion resistance and is not easily corroded by general acid alkali aqueous solutions; Easily soluble in hydrofluoric acid to form fluorinated complexes. At high temperatures, hafnium can also directly combine with gases such as oxygen and nitrogen to form oxides and nitrides.
Hafnium often has a+4 valence in compounds. The main compound is hafnium oxide HfO2. There are three different variants of hafnium oxide:hafnium oxide obtained by continuous calcination of hafnium sulfate and chloride oxide is a monoclinic variant; The hafnium oxide obtained by heating the hydroxide of hafnium at around 400 ℃ is a tetragonal variant; If calcined above 1000 ℃, a cubic variant can be obtained. Another compound is hafnium tetrachloride, which is the raw material for preparing metal hafnium and can be prepared by reacting chlorine gas on a mixture of hafnium oxide and carbon. Hafnium tetrachloride comes into contact with water and immediately hydrolyzes into highly stable HfO (4H2O) 2+ions. HfO2+ions exist in many compounds of hafnium, and can crystallize needle shaped hydrated hafnium oxychloride HfOCl2 · 8H2O crystals in hydrochloric acid acidified hafnium tetrachloride solution.
4-valent hafnium is also prone to form complexes with fluoride, consisting of K2HfF6, K3HfF7, (NH4) 2HfF6, and (NH4) 3HfF7. These complexes have been used for the separation of zirconium and hafnium.
Common compounds:
Hafnium dioxide: name Hafnium dioxide; Hafnium dioxide; Molecular formula: HfO2 [4]; Property: White powder with three crystal structures: monoclinic, tetragonal, and cubic. The densities are 10.3, 10.1, and 10.43g/cm3, respectively. Melting point 2780-2920K. Boiling point 5400K. Thermal expansion coefficient 5.8 × 10-6/℃. Insoluble in water, hydrochloric acid, and nitric acid, but soluble in concentrated sulfuric acid and hydrofluoric acid. Produced by thermal decomposition or hydrolysis of compounds such as hafnium sulfate and hafnium oxychloride. Raw materials for the production of metal hafnium and hafnium alloys. Used as refractory materials, anti radioactive coatings, and catalysts. [5] Atomic energy level HfO is a product obtained simultaneously when manufacturing atomic energy level ZrO. Starting from secondary chlorination, the processes of purification, reduction, and vacuum distillation are almost identical to those of zirconium.
Hafnium tetrachloride: Hafnium (IV) chloride, Hafnium tetrachloride Molecular formula HfCl4 Molecular weight 320.30 Character: White crystalline block. Sensitive to moisture. Soluble in acetone and methanol. Hydrolyze in water to produce hafnium oxychloride (HfOCl2). Heat to 250 ℃ and evaporate. Irritating to eyes, respiratory system, and skin.
Hafnium hydroxide: Hafnium hydroxide (H4HfO4), usually present as a hydrated oxide HfO2 · nH2O, is insoluble in water, easily soluble in inorganic acids, insoluble in ammonia, and rarely soluble in sodium hydroxide. Heat to 100 ℃ to generate hafnium hydroxide HfO (OH) 2. White hafnium hydroxide precipitate can be obtained by reacting hafnium (IV) salt with ammonia water. It can be used to produce other hafnium compounds.
Research History
Discovery History:
In 1923, Swedish chemist Hervey and Dutch physicist D. Koster discovered hafnium in zircon produced in Norway and Greenland, and named it hafnium, which originated from the Latin name Hafnia of Copenhagen. In 1925, Hervey and Coster separated zirconium and titanium using the method of fractional crystallization of fluorinated complex salts to obtain pure hafnium salts; And reduce hafnium salt with metallic sodium to obtain pure metal hafnium. Hervey prepared a sample of several milligrams of pure hafnium.
Chemical experiments on zirconium and hafnium:
In an experiment conducted by Professor Carl Collins at the University of Texas in 1998, it was claimed that gamma irradiated hafnium 178m2 (the isomer hafnium-178m2 [7]) can release enormous energy, which is five orders of magnitude higher than chemical reactions but three orders of magnitude lower than nuclear reactions. [8] Hf178m2 (hafnium 178m2) has the longest lifespan among similar long-lived isotopes: Hf178m2 (hafnium 178m2) has a half-life of 31 years, resulting in a natural radioactivity of approximately 1.6 trillion Becquerels. Collins’ report states that one gram of pure Hf178m2 (hafnium 178m2) contains approximately 1330 megajoules, which is equivalent to the energy released by the explosion of 300 kilograms of TNT explosives. Collins’ report indicates that all energy in this reaction is released in the form of X-rays or gamma rays, which release energy at an extremely fast rate, and Hf178m2 (hafnium 178m2) can still react at extremely low concentrations. [9] The Pentagon has allocated funds for research. In the experiment, the signal-to-noise ratio was very low (with significant errors), and since then, despite multiple experiments by scientists from multiple organizations including the United States Department of Defense Advanced Projects Research Agency (DARPA) and JASON Defense Advisory Group [13], no scientist has been able to achieve this reaction under the conditions claimed by Collins, and Collins has not provided strong evidence to prove the existence of this reaction, Collins proposed a method of using induced gamma ray emission to release energy from Hf178m2 (hafnium 178m2) [15], but other scientists have theoretically proven that this reaction cannot be achieved. [16] Hf178m2 (hafnium 178m2) is widely believed in the academic community not to be a source of energy
Application field:
Hafnium is very useful due to its ability to emit electrons, such as as as used as a filament in incandescent lamps. Used as the cathode for X-ray tubes, and alloys of hafnium and tungsten or molybdenum are used as electrodes for high-voltage discharge tubes. Commonly used in the cathode and tungsten wire manufacturing industry for X-rays. Pure hafnium is an important material in the atomic energy industry due to its plasticity, easy processing, high temperature resistance, and corrosion resistance. Hafnium has a large thermal neutron capture cross-section and is an ideal neutron absorber, which can be used as a control rod and protective device for atomic reactors. Hafnium powder can be used as a propellant for rockets. The cathode of X-ray tubes can be manufactured in the electrical industry. Hafnium alloy can serve as the forward protective layer for rocket nozzles and glide re-entry aircraft, while Hf Ta alloy can be used to manufacture tool steel and resistance materials. Hafnium is used as an additive element in heat-resistant alloys, such as tungsten, molybdenum, and tantalum. HfC can be used as an additive for hard alloys due to its high hardness and melting point. The melting point of 4TaCHfC is approximately 4215 ℃, making it the compound with the highest known melting point. Hafnium can be used as a getter in many inflation systems. Hafnium getters can remove unnecessary gases such as oxygen and nitrogen present in the system. Hafnium is often used as an additive in hydraulic oil to prevent the volatilization of hydraulic oil during high-risk operations, and has strong anti volatility properties. Therefore, it is generally used in industrial hydraulic oil. Medical hydraulic oil.
Hafnium element is also used in the latest Intel 45 nanoprocessors. Due to the manufacturability of silicon dioxide (SiO2) and its ability to reduce thickness to continuously improve transistor performance, processor manufacturers use silicon dioxide as the material for gate dielectrics. When Intel introduced the 65 nanometer manufacturing process, although it had made every effort to reduce the thickness of the silicon dioxide gate dielectric to 1.2 nanometers, equivalent to 5 layers of atoms, the difficulty of power consumption and heat dissipation would also increase when the transistor was reduced to the size of an atom, resulting in current waste and unnecessary heat energy. Therefore, if current materials are continued to be used and the thickness is further reduced, the leakage of the gate dielectric will significantly increase, Bringing down transistor technology to its limits. To address this critical issue, Intel is planning to use thicker high K materials (hafnium based materials) as gate dielectrics instead of silicon dioxide, which has successfully reduced leakage by more than 10 times. Compared to the previous generation of 65nm technology, Intel’s 45nm process increases transistor density by nearly twice, allowing for an increase in the total number of transistors or a reduction in processor volume. In addition, the power required for transistor switching is lower, reducing power consumption by nearly 30%. The internal connections are made of copper wire paired with low k dielectric, smoothly improving efficiency and reducing power consumption, and the switching speed is about 20% faster
Mineral distribution:
Hafnium has a higher crustal abundance than commonly used metals such as bismuth, cadmium, and mercury, and is equivalent in content to beryllium, germanium, and uranium. All minerals containing zirconium contain hafnium. Zircon used in industry contains 0.5-2% hafnium. The beryllium zircon (Alvite) in secondary zirconium ore can contain up to 15% hafnium. There is also a type of metamorphic zircon, cyrtolite, which contains over 5% HfO. The reserves of the latter two minerals are small and have not yet been adopted in industry. Hafnium is mainly recovered during the production of zirconium.
It exists in most zirconium ores. [18] [19] Because there is very little content in the crust. It often coexists with zirconium and has no separate ore.
Preparation method:
1. It can be prepared by magnesium reduction of hafnium tetrachloride or thermal decomposition of hafnium iodide. HfCl4 and K2HfF6 can also be used as raw materials. The process of electrolytic production in NaCl KCl HfCl4 or K2HfF6 melt is similar to that of electrolytic production of zirconium.
2. Hafnium coexists with zirconium, and there is no separate raw material for hafnium. The raw material for manufacturing hafnium is crude hafnium oxide separated during the process of manufacturing zirconium. Extract hafnium oxide using ion exchange resin, and then use the same method as zirconium to prepare metal hafnium from this hafnium oxide.
3. It can be prepared by co heating hafnium tetrachloride (HfCl4) with sodium through reduction.
The earliest methods for separating zirconium and hafnium were fractional crystallization of fluorinated complex salts and fractional precipitation of phosphates. These methods are cumbersome to operate and are limited to laboratory use. New technologies for separating zirconium and hafnium, such as fractionation distillation, solvent extraction, ion exchange, and fractionation adsorption, have emerged one after another, with solvent extraction being more practical. The two commonly used separation systems are the thiocyanate cyclohexanone system and the tributyl phosphate nitric acid system. The products obtained by the above methods are all hafnium hydroxide, and pure hafnium oxide can be obtained by calcination. High purity hafnium can be obtained by ion exchange method.
In industry, the production of metal hafnium often involves both the Kroll process and the Debor Aker process. The Kroll process involves the reduction of hafnium tetrachloride using metallic magnesium:
2Mg+HfCl4- → 2MgCl2+Hf
The Debor Aker method, also known as the iodization method, is used to purify sponge like hafnium and obtain malleable metal hafnium.
5. The smelting of hafnium is basically the same as that of zirconium:
The first step is the decomposition of the ore, which involves three methods: chlorination of zircon to obtain (Zr, Hf) Cl. Alkali melting of zircon. Zircon melts with NaOH at around 600, and over 90% of (Zr, Hf) O transforms into Na (Zr, Hf) O, with SiO transformed into NaSiO, which is dissolved in water for removal. Na (Zr, Hf) O can be used as the original solution for separating zirconium and hafnium after being dissolved in HNO. However, the presence of SiO colloids makes solvent extraction separation difficult. Sinter with KSiF and soak in water to obtain K (Zr, Hf) F solution. The solution can separate zirconium and hafnium through fractional crystallization;
The second step is the separation of zirconium and hafnium, which can be achieved using solvent extraction separation methods using hydrochloric acid MIBK (methyl isobutyl ketone) system and HNO-TBP (tributyl phosphate) system. The technology of multi-stage fractionation using the difference in vapor pressure between HfCl and ZrCl melts under high pressure (above 20 atmospheres) has long been studied, which can save the secondary chlorination process and reduce costs. However, due to the corrosion problem of (Zr, Hf) Cl and HCl, it is not easy to find suitable fractionation column materials, and it will also reduce the quality of ZrCl and HfCl, increasing purification costs. In the 1970s, it was still in the intermediate plant testing stage;
The third step is the secondary chlorination of HfO to obtain crude HfCl for reduction;
The fourth step is the purification of HfCl and magnesium reduction. This process is the same as the purification and reduction of ZrCl, and the resulting semi-finished product is coarse sponge hafnium;
The fifth step is to vacuum distill crude sponge hafnium to remove MgCl and recover excess metal magnesium, resulting in a finished product of sponge metal hafnium. If the reducing agent uses sodium instead of magnesium, the fifth step should be changed to water immersion
Storage method:
Store in a cool and ventilated warehouse. Keep away from sparks and heat sources. It should be stored separately from oxidants, acids, halogens, etc., and avoid mixing storage. Using explosion-proof lighting and ventilation facilities. Prohibit the use of mechanical equipment and tools that are prone to sparks. The storage area should be equipped with suitable materials to contain leaks.