Tantalum hafnium carbide
Encyclopedia
Tantalum hafnium carbide is a refractory
chemical compound
with a general formula Tax
Hf1-x
Cy
, which can be considered as a solid solution of tantalum carbide
and hafnium carbide. Individually, these two carbide
s have the highest melting points among the binary compound
s, 3983 °C (7,201.4 °F) and 3928 °C (7,102.4 °F), respectively, and their "alloy" with a composition Ta4
Hf
C5
is believed to have a melting point of 4215 °C (7619 °F). This value was reported in 1930 and included in the Encyclopædia Britannica
as the highest melting point for any solid. This attribution was reprinted in Britannica up to at least 1995, and from about 1998 was changed to "one of the highest".
Very few, if any, other measurements of melting point in tantalum hafnium carbide have been reported, because of the obvious experimental difficulties at extreme temperatures. A 1965 study of the TaC-HfC solid solutions at temperatures 2225–2275 °C found a minimum in the vaporization rate and thus maximum in the thermal stability for Ta4HfC5. This rate was comparable to that of tungsten
and was weakly dependent on the initial density of the samples, which were sintered
from TaC-HfC powder mixtures, also at 2225–2275 °C. In a separate study, Ta4HfC5 was found to have the minimum oxidation rate among the TaC-HfC solid solutions. Ta4HfC5 was manufactured by Goodfellow company as a 45-micron powder at a price of $9,540/kg (99.0% purity).
Individual tantalum and hafnium carbides have a rocksalt cubic
lattice structure. They are usually carbon deficient and have nominal formulas TaCx and HfCx, with x = 0.7–1.0 for Ta and x = 0.56–1.0 for Hf. The same structure is also observed for at least some of their solid solutions. The density calculated from X-ray diffraction data is 13.6 g/cm3 for Ta0.5Hf0.5C. Hexagonal NiAs-type structure (space group
P63/mmc, No. 194, Pearson symbol
hP4) with a density of 14.76 g/cm3 was reported for Ta0.9Hf0.1C0.5.
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with a general formula Tax
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
Hf1-x
Hafnium
Hafnium is a chemical element with the 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. Hafnium was the penultimate stable...
Cy
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, which can be considered as a solid solution of tantalum carbide
Tantalum carbide
Tantalum carbides form a family of binary chemical compounds of tantalum and carbon with the empirical formula TaCx, where x usually varies between 0.4 and 1. They are extremely hard, brittle, refractory ceramic materials with metallic electrical conductivity. They appear as brown-gray powders...
and hafnium carbide. Individually, these two carbide
Carbide
In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and "intermediate" transition metal carbides...
s have the highest melting points among the binary compound
Binary compound
A binary compound is a chemical compound that contains exactly two different elements. Examples of binary ionic compounds include calcium chloride , sodium fluoride , and magnesium oxide , whilst examples of binary covalent compounds include water , carbon monoxide , and sulfur hexafluoride...
s, 3983 °C (7,201.4 °F) and 3928 °C (7,102.4 °F), respectively, and their "alloy" with a composition Ta4
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
Hf
Hafnium
Hafnium is a chemical element with the 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. Hafnium was the penultimate stable...
C5
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
is believed to have a melting point of 4215 °C (7619 °F). This value was reported in 1930 and included in the Encyclopædia Britannica
Encyclopædia Britannica
The Encyclopædia Britannica , published by Encyclopædia Britannica, Inc., is a general knowledge English-language encyclopaedia that is available in print, as a DVD, and on the Internet. It is written and continuously updated by about 100 full-time editors and more than 4,000 expert...
as the highest melting point for any solid. This attribution was reprinted in Britannica up to at least 1995, and from about 1998 was changed to "one of the highest".
Very few, if any, other measurements of melting point in tantalum hafnium carbide have been reported, because of the obvious experimental difficulties at extreme temperatures. A 1965 study of the TaC-HfC solid solutions at temperatures 2225–2275 °C found a minimum in the vaporization rate and thus maximum in the thermal stability for Ta4HfC5. This rate was comparable to that of tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
and was weakly dependent on the initial density of the samples, which were sintered
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
from TaC-HfC powder mixtures, also at 2225–2275 °C. In a separate study, Ta4HfC5 was found to have the minimum oxidation rate among the TaC-HfC solid solutions. Ta4HfC5 was manufactured by Goodfellow company as a 45-micron powder at a price of $9,540/kg (99.0% purity).
Individual tantalum and hafnium carbides have a rocksalt cubic
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
lattice structure. They are usually carbon deficient and have nominal formulas TaCx and HfCx, with x = 0.7–1.0 for Ta and x = 0.56–1.0 for Hf. The same structure is also observed for at least some of their solid solutions. The density calculated from X-ray diffraction data is 13.6 g/cm3 for Ta0.5Hf0.5C. Hexagonal NiAs-type structure (space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
P63/mmc, No. 194, Pearson symbol
Pearson symbol
The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W.B. Pearson. The symbol is made up of two letters followed by a number. For example:* Diamond structure, cF8...
hP4) with a density of 14.76 g/cm3 was reported for Ta0.9Hf0.1C0.5.