Hafnium carbide () is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

hafnium Hafnium is a chemical element; it has symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dm ...

and

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

. Previously the material was estimated to have a melting point of about 3,900 °C. More recent tests have been able to conclusively prove that the substance has an even higher melting point of 3,958 °C exceeding those of

tantalum carbide Tantalum carbides (TaC) form a family of binary chemical compounds of tantalum and carbon with the empirical formula , where ''x'' usually varies between 0.4 and 1. They are extremely hard, brittle, refractory ceramic materials with metallic elec ...

and

tantalum hafnium carbide Tantalum hafnium carbide is a refractory chemical compound with a general formula , which can be considered as a solid solution of tantalum carbide and hafnium carbide. It was originally thought to have the highest melting of any known substan ...

which were both previously estimated to be higher. However, it has a low oxidation resistance, with the oxidation starting at temperatures as low as 430 °C. Experimental testing in 2018 confirmed the higher melting point yielding a result of 3,982 (±30°C) with a small possibility that the melting point may even exceed 4,000°C. Atomistic simulations conducted in 2015 predicted that a similar compound,

hafnium carbonitride Hafnium carbonitride (HfCN) is an ultra-high temperature ceramic (UHTC) mixed anion compound composed of hafnium (Hf), carbon (C) and nitrogen (N). molecular dynamics calculations have predicted the HfCN (specifically the HfC''0.75''N''0.22' ...

(HfCN), could have a melting point exceeding even that of hafnium carbide. Experimental evidence gathered in 2020 confirmed that it did indeed have a higher melting point exceeding 4,000 °C, with more recent

ab initio ( ) is a Latin term meaning "from the beginning" and is derived from the Latin ("from") + , ablative singular of ("beginning"). Etymology , from Latin, literally "from the beginning", from ablative case of "entrance", "beginning", related t ...

molecular dynamics calculations predicting the phase to have a melting point as high as 4,110 ± 62 °C, highest known for any material. Hafnium carbide is usually carbon deficient and therefore its composition is often expressed as (''x'' = 0.5 to 1.0). It has a

cubic Cubic may refer to: Science and mathematics * Cube (algebra), "cubic" measurement * Cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex ** Cubic crystal system, a crystal system w ...

(rock-salt) crystal structure at any value of ''x''. Hafnium carbide powder is obtained by the reduction of

hafnium(IV) oxide Hafnium(IV) oxide is the inorganic compound with the formula . Also known as hafnium dioxide or hafnia, this colourless solid is one of the most common and stable compounds of hafnium. It is an electrical insulator with a band gap of 5.3~5.7 eV. H ...

with carbon at 1,800 to 2,000 °C. A long processing time is required to remove all oxygen. Alternatively, high-purity HfC coatings can be obtained by

chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (electro ...

from a gas mixture of

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

, hydrogen, and vaporized

hafnium(IV) chloride Hafnium(IV) chloride is the inorganic compound with the chemical formula, formula HfCl4. This colourless solid is the Precursor (chemistry), precursor to most hafnium organometallic compounds. It has a variety of highly specialized applications, m ...

. Because of the technical complexity and high cost of the synthesis, HfC has a very limited use, despite its favorable properties such as high hardness (greater than 9 Mohs) and melting point. The magnetic properties of change from

paramagnetic Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...

for ''x'' ≤ 0.8 to

diamagnetic Diamagnetism is the property of materials that are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagn ...

at larger ''x''. An inverse behavior (dia-paramagnetic transition with increasing ''x'') is observed for , despite its having the same crystal structure as .

References

{{Authority control Carbides Hafnium compounds Rock salt crystal structure Refractory materials

See also

References