The melting point of hafnium carbide is the highest melting point in a known single compound
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Author : TRUNNANO
Update time : 2021-02-04 10:14:07
What is hafnium carbide? Hafnium carbide (HfC) is a chemical compound of hafnium and carbon. Its melting point is about 3900°C and it is one of the most refractory binary compounds known. However, its oxidation resistance is very low, and oxidation starts at a temperature of 430°C. This compound may be seen as part of the heat shield in future spacecraft. Carbonization usually lacks carbon, so its composition is usually expressed as HfC x (x = 0.5 to 1.0). It has a cubic (salt rock) crystal structure of any x value.
Hafnium carbide is usually synthesized with hafnium dioxide (HfO2) and carbon in an inert or reducing atmosphere. The reaction temperature is 1900-2300°C. Hafnium carbide can form a solid solution with many compounds (such as ZrC, TaC, etc.). The hafnium (IV) oxide obtained by reducing the hafnium carbide powder with carbon is at 1800~2000°C. It takes a long time to remove all oxygen. Alternatively, a high-purity HfC coating can be obtained from a gas mixture of methane, hydrogen and vaporized chlorinated chloride (IV) by chemical vapor deposition. Due to the technical complexity and high synthesis cost of HfC, although it has advantageous properties such as high hardness (>9 Mohs) and melting point, HfC has very limited uses. The magnetism of HfC x changes from paramagnetism with x≤0.8 to diamagnetism with larger x. Although TaC x has the same crystal structure as HfC x, the opposite behavior (para-paramagnetic transition with increasing x) is observed.
What is the use of hafnium carbide? Hafnium carbide is very suitable for rocket nozzles. It can be used as the nose cone of space rockets that re-enter the atmosphere and used in ceramics and other industries.
How strong is hafnium carbide? W-based and Mo-based alloys dispersed with hafnium carbide have superior tensile and stress fracture properties than those without HfC. On the basis of density compensation, MoHfC wire exhibits better tensile and stress rupture strength than WHfC wire under a pressure of about 1400 K. Hafnium carbide has a density of 12.7g/cm3 and a melting point of 3890°C, which is the highest melting point among known single compounds. The volume resistivity is 1.95×10-4Ω·cm (2900℃), and the thermal expansion coefficient is 6.73×10-6/℃. Generally, hafnium dioxide (HfO2) and carbon are used to synthesize powder in an inert or reducing atmosphere. The reaction temperature of hafnium carbide is 1900-2300℃ and it can form a solid solution with many compounds (such as ZrC, TaC, etc.). It has a high melting point and high elastic coefficient, good electrical and thermal conductivity, small thermal expansion and good impact performance.
Is hafnium carbide poisonous? In animal studies, trichlorooxidation was found to be toxic through the intraperitoneal route. There are no reports of involving industrial poisoning. Carbide: Pure carbon has very low toxicity to humans and can be processed in the form of graphite or charcoal or even safely ingested.
Why does hafnium carbide have a high melting point? Hafnium carbide is very resistant to corrosion because it forms an oxide film on the exposed surface. According to a report from "Chemical World", among the three-element compounds, the mixed carbide of tungsten and hafnium has the highest melting point of any known compound at 7,457 degrees Fahrenheit (4,125 degrees Celsius).
Introduction to Hafnium-What is Hafnium? In nature, hafnium often co-exists with zirconium, and zirconium-containing minerals contain hafnium. Hafnium and zirconium are similar in nature, and hafnium is mainly found in zircon. The amount of HfO2 in zircon used in industry is 0.5-2%. The beryllium zircon in secondary zirconium ore can contain up to 15% HfO2. There is also a metamorphic zircon using stone, which contains more than 5% HfO2. The latter two minerals have small reserves and have not been used in the industry. Hafnium is mainly recovered in the process of producing zirconium.
The smelting of hafnium is basically the same as that of zirconium, generally divided into five steps. The first step is the decomposition of ore. There are three methods: ① Zircon is chlorinated to obtain (Zr, Hf)Cl4. ② Alkaline fusion of zircon, zircon and NaOH melt at about 600 ℃, more than 90% of (Zr, Hf) O2 is transformed into Na2 (Zr, Hf) O3, and the SiO2 becomes Na2SiO3, which is removed by water solution. Na2(Zr, Hf)O3 can be used as the original solution for the separation of zirconium and hafnium after being dissolved in HNO3. However, it contains SiO2 colloid, which makes solvent extraction and separation difficult. ③Sinter with K2SiF6, and get K2(Zr, Hf)F6 solution after water immersion. The solution can separate zirconium and hafnium by fractional crystallization.
The second step is the separation of zirconium and hafnium, which can be separated by solvent extraction with the hydrochloric acid-MIBK (methyl isobutyl ketone) system and the HNO3-TBP (tributyl phosphate) system. The technology of multi-stage fractionation using the difference in vapor pressure of HfCl4 and ZrCl4 melts under high pressure (above 20 atmospheres) has long been studied, which can eliminate the secondary chlorination process and reduce costs. However, due to the corrosion problems of (Zr, Hf)Cl4 and HCl, it is not easy to find a suitable fractionation column material, and it will also reduce the quality of ZrCl4 and HfCl4 and increase the purification cost.
The third step is the second chlorination of HfO2 to prepare crude HfCl4 for reduction. The fourth step is the purification of HfCl4 and magnesium reduction. This process is the same as the purification and reduction of ZrCl4, and the obtained semi-finished product is crude sponge hafnium. The fifth step is vacuum distillation of crude sponge hafnium to remove MgCl2 and recover excess metal magnesium. The finished product is sponge metal hafnium. If the reducing agent uses sodium instead of magnesium, the fifth step is changed to water immersion.
Take extra care when removing the hafnium sponge from the crucible to avoid spontaneous combustion. Large pieces of sponge hafnium must be broken into small pieces of a certain size in order to be pressed into consumable electrodes, and then melted into ingots. Spontaneous combustion should also be prevented when broken. The further purification of sponge hafnium is the same as titanium and zirconium by the iodide thermal decomposition method. The control conditions are slightly different from those of zirconium. The small pieces of sponge hafnium around the iodination tank are kept at a temperature of 600°C, while the temperature of the hot wire in the center is 1600°C, which is higher than the 1300°C when the zirconium "crystal rod" is made. . The processing and forming of hafnium include forging, extrusion, drawing and other steps, the same as the method of processing zirconium.The main purpose of hafnium is to make control rods for nuclear reactors.
Hafnium application Pure hafnium has plasticity, easy processing, high-temperature resistance and corrosion resistance, and is an important material in the atomic energy industry. Hafnium has a large thermal neutron capture section and is an ideal neutron absorber, which can be used as a control rod and protection device for an atomic reactor. Hafnium powder can be used as a rocket propeller. The cathode of X-ray tubes can be manufactured in the electrical industry. The alloy of hafnium can be used as the front protective layer of rocket nozzles and gliding re-entry aircraft, and the alloy of Hf-Ta can be used to make tool steel and resistance materials. Hafnium is used as an additional element in heat-resistant alloys, such as tungsten, molybdenum, and tantalum alloys. HFC can be used as cemented carbide additive due to its high hardness and melting point. The melting point of 4TaC·HfC is about 4215°C, which is the highest melting point compound known.
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