Tantalum carbides form a family of binary chemical compounds of tantalum and carbon with the empirical formula TaCx, where x varies between 0.4 and 1. They are hard, refractory ceramic materials with metallic electrical conductivity, they appear as brown-gray powders, which are processed by sintering. Being important cermet materials, tantalum carbides are commercially used in tool bits for cutting applications and are sometimes added to tungsten carbide alloys; the melting points of tantalum carbides peak at about 3880 °C depending on the purity and measurement conditions. Only tantalum hafnium carbide may have a higher melting point of about 3942 °C, whereas the melting point of hafnium carbide is comparable to that of TaC. TaCx powders of desired composition are prepared by heating a mixture of tantalum and graphite powders in vacuum or inert-gas atmosphere; the heating is performed at temperature of about 2000 °C using an arc-melting setup. An alternative technique is reduction of tantalum pentoxide by carbon in vacuum or hydrogen atmosphere at a temperature of 1500–1700 °C.
This method was used to obtain tantalum carbide in 1876, but it lacks control over the stoichiometry of the product. Production of TaC directly from the elements has been reported through self-propagating high-temperature synthesis. TaCx compounds have a cubic crystal structure for x = 0.7–1.0. TaC0.5 has two major crystalline forms. The more stable one has an anti-cadmium iodide-type trigonal structure, which transforms upon heating to about 2000 °C into a hexagonal lattice with no long-range order for the carbon atoms. Here Z is the number of formula units per unit cell, ρ is the density calculated from lattice parameters; the bonding between tantalum and carbon atoms in tantalum carbides is a complex mixture of ionic and covalent contributions, because of the strong covalent component, these carbides are hard and brittle materials. For example, TaC has a microhardness of 1600–2000 kg/mm2 and an elastic modulus of 285 GPa, whereas the corresponding values for tantalum are 110 kg/mm2 and 186 GPa.
The hardness, yield stress and shear stress increase with the carbon content in TaCx. Tantalum carbides have metallic electrical conductivity, both in terms of its magnitude and temperature dependence. TaC is a superconductor with a high transition temperature of TC = 10.35 K. The magnetic properties of TaCx change from diamagnetic. An inverse behavior is observed despite that it has the same crystal structure as TaCx. Tantalcarbide is a natural form of tantalum carbide, it is cubic rare mineral. Hafnium carbide
Tantalum borides are compounds of tantalum and boron most remarkable for their extreme hardness. The Vickers hardness of TaB and TaB2 films and crystals is ~30 GPa; those materials are stable to oxidation to acid corrosion. TaB2 has the same hexagonal structure as most diborides; the mentioned borides have the following space groups: TaB, Ta5B6, Ta3B4, TaB2. Single crystals of TaB, Ta5B6, Ta3B4 or TaB2 can be produced by the floating zone method. Tantalum boride films can be deposited from a gas mixture of TaCl5-BCl3-H2-Ar in the temperature range 540–800 °C. TaB2 is deposited at a source gas flow ratio of six and a temperature above 600 °C. TaB is deposited at BCl3/TaCl5 = 2–4 and T = 600–700 °C. Nanocrystals of TaB2 were synthesized by the reduction of Ta2O5 with NaBH4 using a molar ratio M:B of 1:4 at 700-900 °C for 30 min under argon flow. Ta2O5 + 6.5NaBH4 → 2TaB2 + 4Na + 2.5NaBO2+ 13H2
Tantalum pentoxide known as tantalum oxide, is the inorganic compound with the formula Ta2O5. It is a white solid, insoluble in all solvents but is attacked by strong bases and hydrofluoric acid. Ta2O5 is an inert material with a high refractive index and low absorption, which makes it useful for coatings, it is extensively used in the production of capacitors, due to its high dielectric constant. Tantalum occurs in the minerals tantalite and columbite, which occur in pegmatites, an igneous rock formation. Mixtures of columbite and tantalite are called coltan. Tantalite was discovered by Anders Gustaf Ekeberg at Ytterby and Kimoto, Finland; the minerals microlite and pyrochlore contain 70% and 10% Ta, respectively. Tantalum ores contain significant amounts of niobium, itself a valuable metal; as such, both metals are extracted. The overall process begins with a leaching step; this allows the metals to be separated from the various non-metallic impurities in the rock. 2O6 + 16 HF → H2 + H2 + FeF2 + MnF2 + 6 H2OThe tantalum and niobium hydrogenflorides are removed from the aqueous solution by liquid-liquid extraction using organic solvents, such as cyclohexanone or methyl isobutyl ketone.
This step allows the simple removal of various metal impurities which remain in the aqueous phase in the form of fluorides. Separation of the tantalum and niobium is achieved by pH adjustment. Niobium requires a higher level of acidity to remain soluble in the organic phase and can hence be selectively removed by extraction into less acidic water; the pure tantalum hydrogen fluoride solution is neutralised with aqueous ammonia to give hydrated tantalum oxide, calcinated to tantalum pentoxide as described in these idealized equations: H2 + 5 H2O + 7 NH3 → 0.5 Ta2O55 + 7 NH4F Ta2O55 → Ta2O5 + 5 H2ONatural pure tantalum oxide is known as the mineral tantite, although it is exceedingly rare. Tantalum oxide is used in electronics in the form of thin films. For these applications it can be produced by MOCVD, which involves the hydrolysis of its volatile halides or alkoxides: Ta210 + 5 H2O → Ta2O5 + 10 EtOH 2 TaCl5 + 5 H2O → Ta2O5 + 10 HCl The crystal structure of tantalum pentoxide has been the matter of some debate.
The bulk material is disordered, being either polycrystalline. As such Xray crystallography has been limited to powder diffraction, which provides less structural information. At least 2 polymorphs are known to exist. A low temperature form, known as L- or β-Ta2O5, the high temperature form known as H- or α-Ta2O5; the transition between these two forms is reversible. The structures of both polymorphs consist of chains built from octahedral TaO6 and pentagonal bipyramidal TaO7 polyhedra sharing opposite vertices; the overall crystal system is orthorhombic in both cases, with the space group of β-Ta2O5 being identified as Pna2 by single crystal X-ray diffraction. A high pressure form has been reported, in which the Ta atoms adopt a 7 coordinate geometry to give a monoclinic structure. Purely amorphous tantalum pentoxide has a similar local structure to the crystalline polymorphs, built from TaO6 and TaO7 polyhedra, while the molten liquid phase has a distinct structure based on lower coordination polyhedra TaO5 and TaO6.
The difficulty in forming material with a uniform structure has led to variations in its reported properties. Like many metal oxides Ta2O5 is an insulator and its band gap has variously been reported as being between 3.8 and 5.3 eV, depending on the method of manufacture. In general the more amorphous the material the greater its observed band gap; these observed values are higher than those predicted by computational chemistry. Its dielectric constant is about ~25 although values of over 50 have been reported. In general tantalum pentoxide is considered to be a high-k dielectric material. Ta2O5 does not react appreciably with either HCl or HBr, however it will dissolve in hydrofluoric acid, reacts with potassium bifluoride and HF according to the following equation: Ta2O5 + 4 KHF2 + 6 HF → 2 K2 + 5 H2OTa2O5 can be reduced to metallic Ta via the use of metallic reductants such as calcium and aluminium. Ta2O5 + 5 Ca → 2 Ta + 5 CaO Owing to its high band gap and dielectric constant, tantalum pentoxide has found a variety of uses in electronics in tantalum capacitors.
These are used in automotive electronics, cell phones, pagers, electronic circuitry. In the 1990s, interest grew in the use of tantalum oxide as a high-k dielectric for DRAM capacitor applications, it is used in on-chip metal-insulator-metal capacitors for high frequency CMOS integrated circuits. Tantalum oxide may have applications as the charge trapping layer for Non-volatile memories. There are applications of tantalum oxide in resistive switching memories. Due to its high refractive index, Ta2O5 has been utilized in the fabrication of the glass of photographic lenses
In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be classified by the chemical bonds type as follows: salt-like, covalent compounds, interstitial compounds, "intermediate" transition metal carbides. Examples include calcium carbide, silicon carbide, tungsten carbide, cementite, each used in key industrial applications; the naming of ionic carbides is not systematic. Salt-like carbides are composed of electropositive elements such as the alkali metals, alkaline earth metals, group 3 metals, including scandium and lanthanum. Aluminium from group 13 forms carbides, but gallium and thallium do not; these materials feature isolated carbon centers described as "C4−", in the methanides or methides. The graphite intercalation compound KC8, prepared from vapour of potassium and graphite, the alkali metal derivatives of C60 are not classified as carbides. Carbides of this class decompose in water producing methane. Three such examples are aluminium carbide Al4C3, magnesium carbide Mg2C and beryllium carbide Be2C.
Transition metal carbides are not saline carbides but their reaction with water is slow and is neglected. For example, depending on surface porosity, 5–30 atomic layers of titanium carbide are hydrolyzed, forming methane within 5 minutes at ambient conditions, following by saturation of the reaction. Note that methanide in this context is a trivial historical name. According to the IUPAC systematic naming conventions, a compound such as NaCH3 would be termed a "methanide", although this compound is called methylsodium. Several carbides are assumed to be salts of the acetylide anion C22–, which has a triple bond between the two carbon atoms. Alkali metals, alkaline earth metals, lanthanoid metals form acetylides, e.g. sodium carbide Na2C2, calcium carbide CaC2, LaC2. Lanthanides form carbides with formula M2C3. Metals from group 11 tend to form acetylides, such as copper acetylide and silver acetylide. Carbides of the actinide elements, which have stoichiometry MC2 and M2C3, are described as salt-like derivatives of C2−2.
The C-C triple bond length ranges from 119.2 pm in CaC2, to 130.3 pm in LaC2 and 134 pm in UC2. The bonding in LaC2 has been described in terms of LaIII with the extra electron delocalised into the antibonding orbital on C2−2, explaining the metallic conduction; the polyatomic ion C4−3, sometimes called sesquicarbide or allylenide, is found in Li4C3 and Mg2C3. The ion is linear and is isoelectronic with CO2; the C-C distance in Mg2C3 is 133.2 pm. Mg2C3 yields methylacetylene, CH3CCH, propadiene, CH2CCH2, on hydrolysis, the first indication that it contains C4−3; the carbides of silicon and boron are described as "covalent carbides", although all compounds of carbon exhibit some covalent character. Silicon carbide has two similar crystalline forms. Boron carbide, B4C, on the other hand, has an unusual structure which includes icosahedral boron units linked by carbon atoms. In this respect boron carbide is similar to the boron rich borides. Both silicon carbide and boron carbide are hard materials and refractory.
Both materials are important industrially. Boron forms other covalent carbides, e.g. B25C; the carbides of the group 4, 5 and 6 transition metals are described as interstitial compounds. These carbides are refractory; some exhibit a range of stoichiometries, e.g. titanium carbide, TiC. Titanium carbide and tungsten carbide are important industrially and are used to coat metals in cutting tools; the long-held view is that the carbon atoms fit into octahedral interstices in a close-packed metal lattice when the metal atom radius is greater than 135 pm: When the metal atoms are cubic close-packed filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the rock salt structure. When the metal atoms are hexagonal close-packed, as the octahedral interstices lie directly opposite each other on either side of the layer of metal atoms, filling only one of these with carbon achieves 2:1 stoichiometry with the CdI2 structure; the following table shows actual structures of their carbides.
The notation "h/2" refers to the M2C type structure described above, only an approximate description of the actual structures. The simple view that the lattice of the pure metal "absorbs" carbon atoms can be seen to be untrue as the packing of the metal atom lattice in the carbides is different from the packing in the pure metal, although it is technically correct that the carbon atoms fit into the octahedral interstices of a close-packed metal lattice. For a long time the non-stoichiometric phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected. In these carbides, the transition metal ion is smaller than the critical 135 pm, the structures are not interstitial but are more complex. Multiple stoichiometries are common; the best known is cementite, Fe3C, present in steels. These carbides are more reactive than the interstitial carbides.
In mathematics and chemistry, a space group is the symmetry group of a configuration in space in three dimensions. In three dimensions, there are 230 if chiral copies are considered distinct. Space groups are studied in dimensions other than 3 where they are sometimes called Bieberbach groups, are discrete cocompact groups of isometries of an oriented Euclidean space. In crystallography, space groups are called the crystallographic or Fedorov groups, represent a description of the symmetry of the crystal. A definitive source regarding 3-dimensional space groups is the International Tables for Crystallography. Space groups in 2 dimensions are the 17 wallpaper groups which have been known for several centuries, though the proof that the list was complete was only given in 1891, after the much more difficult classification of space groups had been completed. In 1879 Leonhard Sohncke listed the 65 space groups. More he listed 66 groups, but Fedorov and Schönflies both noticed that two of them were the same.
The space groups in three dimensions were first enumerated by Fedorov, shortly afterwards were independently enumerated by Schönflies. The correct list of 230 space groups was found by 1892 during correspondence between Fedorov and Schönflies. Barlow enumerated the groups with a different method, but omitted four groups though he had the correct list of 230 groups from Fedorov and Schönflies. Burckhardt describes the history of the discovery of the space groups in detail; the space groups in three dimensions are made from combinations of the 32 crystallographic point groups with the 14 Bravais lattices, each of the latter belonging to one of 7 lattice systems. This results in a space group being some combination of the translational symmetry of a unit cell including lattice centering, the point group symmetry operations of reflection and improper rotation, the screw axis and glide plane symmetry operations; the combination of all these symmetry operations results in a total of 230 different space groups describing all possible crystal symmetries.
The elements of the space group fixing a point of space are the identity element, reflections and improper rotations. The translations form a normal abelian subgroup of rank 3, called the Bravais lattice. There are 14 possible types of Bravais lattice; the quotient of the space group by the Bravais lattice is a finite group, one of the 32 possible point groups. Translation is defined as the face moves from one point to another point. A glide plane is a reflection in a plane, followed by a translation parallel with that plane; this is noted depending on which axis the glide is along. There is the n glide, a glide along the half of a diagonal of a face, the d glide, a fourth of the way along either a face or space diagonal of the unit cell; the latter is called the diamond glide plane. In 17 space groups, due to the centering of the cell, the glides occur in two perpendicular directions i.e. the same glide plane can be called b or c, a or b, a or c. For example, group Abm2 could be called Acm2, group Ccca could be called Cccb.
In 1992, it was suggested to use symbol e for such planes. The symbols for five space groups have been modified: A screw axis is a rotation about an axis, followed by a translation along the direction of the axis; these are noted by a number, n, to describe the degree of rotation, where the number is how many operations must be applied to complete a full rotation. The degree of translation is added as a subscript showing how far along the axis the translation is, as a portion of the parallel lattice vector. So, 21 is a twofold rotation followed by a translation of 1/2 of the lattice vector; the general formula for the action of an element of a space group is y = M.x + D where M is its matrix, D is its vector, where the element transforms point x into point y. In general, D = D + D, where D is a unique function of M, zero for M being the identity; the matrices M form a point group, a basis of the space group. The lattice dimension can be less than the overall dimension, resulting in a "subperiodic" space group.
For:: One-dimensional line groups: Two-dimensional line groups: frieze groups: Wallpaper groups: Three-dimensional line groups. Some of these methods can assign several different names to the same space group, so altogether there are many thousands of different names. Number; the International Union of Crystallography publishes tables of all space group types, assigns each a unique number from 1 to 230. The numbering is arbitrary, except that groups with the same crystal system or point group are given consecutive numbers. International symbol or Hermann–Mauguin notation; the Hermann–Mauguin notation describes the lattice and some generators for the group. It has a shortened form called the international short symbol, the one most used in crystallography
Tantalum is a chemical element with symbol Ta and atomic number 73. Known as tantalium, its name comes from Tantalus, a villain from Greek mythology. Tantalum is a rare, blue-gray, lustrous transition metal, corrosion-resistant, it is part of the refractory metals group, which are used as minor components in alloys. The chemical inertness of tantalum makes it a valuable substance for laboratory equipment and a substitute for platinum, its main use today is in tantalum capacitors in electronic equipment such as mobile phones, DVD players, video game systems and computers. Tantalum, always together with the chemically similar niobium, occurs in the mineral groups tantalite and coltan. Tantalum is considered a technology-critical element. Tantalum was discovered in Sweden in 1802 by Anders Ekeberg, in two mineral samples – one from Sweden and the other from Finland. One year earlier, Charles Hatchett had discovered columbium, in 1809 the English chemist William Hyde Wollaston compared its oxide, columbite with a density of 5.918 g/cm3, to that of tantalum, tantalite with a density of 7.935 g/cm3.
He concluded that the two oxides, despite their difference in measured density, were identical and kept the name tantalum. After Friedrich Wöhler confirmed these results, it was thought that columbium and tantalum were the same element; this conclusion was disputed in 1846 by the German chemist Heinrich Rose, who argued that there were two additional elements in the tantalite sample, he named them after the children of Tantalus: niobium, pelopium. The supposed element "pelopium" was identified as a mixture of tantalum and niobium, it was found that the niobium was identical to the columbium discovered in 1801 by Hatchett; the differences between tantalum and niobium were demonstrated unequivocally in 1864 by Christian Wilhelm Blomstrand, Henri Etienne Sainte-Claire Deville, as well as by Louis J. Troost, who determined the empirical formulas of some of their compounds in 1865. Further confirmation came from the Swiss chemist Jean Charles Galissard de Marignac, in 1866, who proved that there were only two elements.
These discoveries did not stop scientists from publishing articles about the so-called ilmenium until 1871. De Marignac was the first to produce the metallic form of tantalum in 1864, when he reduced tantalum chloride by heating it in an atmosphere of hydrogen. Early investigators had only been able to produce impure tantalum, the first pure ductile metal was produced by Werner von Bolton in Charlottenburg in 1903. Wires made with metallic tantalum were used for light bulb filaments until tungsten replaced it in widespread use; the name tantalum was derived from the name of the mythological Tantalus, the father of Niobe in Greek mythology. In the story, he had been punished after death by being condemned to stand knee-deep in water with perfect fruit growing above his head, both of which eternally tantalized him. Anders Ekeberg wrote "This metal I call tantalum... in allusion to its incapacity, when immersed in acid, to absorb any and be saturated."For decades, the commercial technology for separating tantalum from niobium involved the fractional crystallization of potassium heptafluorotantalate away from potassium oxypentafluoroniobate monohydrate, a process, discovered by Jean Charles Galissard de Marignac in 1866.
This method has been supplanted by solvent extraction from fluoride-containing solutions of tantalum. Tantalum is dark, ductile hard fabricated, conductive of heat and electricity; the metal is renowned for its resistance to corrosion by acids. It can be dissolved with hydrofluoric acid or acidic solutions containing the fluoride ion and sulfur trioxide, as well as with a solution of potassium hydroxide. Tantalum's high melting point of 3017 °C is exceeded among the elements only by tungsten and osmium for metals, carbon. Tantalum exists in two crystalline phases and beta; the alpha phase is ductile and soft. The beta phase is brittle; the beta phase is metastable and converts to the alpha phase upon heating to 750–775 °C. Bulk tantalum is entirely alpha phase, the beta phase exists as thin films obtained by magnetron sputtering, chemical vapor deposition or electrochemical deposition from an eutectic molten salt solution. Natural tantalum consists of two isotopes: 181Ta. 181Ta is a stable isotope.
180mTa is predicted to decay in three ways: isomeric transition to the ground state of 180Ta, beta decay to 180W, or electron capture to 180Hf. However, radioactivity of this nuclear isomer has never been observed, only a lower limit on its half-life of 2.0 × 1016 years has been set. The ground state of 180Ta has a half-life of only 8 hours. 180mTa is the only occurring nuclear isomer. It is the rarest isotope in the Universe, taking into account the elemental abu
Hafnium is a chemical element with 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 Dmitri Mendeleev in 1869, though it was not identified until 1923, by Coster and Hevesy, making it the last stable element to be discovered. Hafnium is named after the Latin name for Copenhagen, where it was discovered. Hafnium is used in electrodes; some semiconductor fabrication processes use its oxide for integrated circuits at 45 nm and smaller feature lengths. Some superalloys used for special applications contain hafnium in combination with niobium, titanium, or tungsten. Hafnium's large neutron capture cross-section makes it a good material for neutron absorption in control rods in nuclear power plants, but at the same time requires that it be removed from the neutron-transparent corrosion-resistant zirconium alloys used in nuclear reactors. Hafnium is a shiny, ductile metal, corrosion-resistant and chemically similar to zirconium.
The physical properties of hafnium metal samples are markedly affected by zirconium impurities the nuclear properties, as these two elements are among the most difficult to separate because of their chemical similarity. A notable physical difference between these metals is their density, with zirconium having about one-half the density of hafnium; the most notable nuclear properties of hafnium are its high thermal neutron-capture cross-section and that the nuclei of several different hafnium isotopes absorb two or more neutrons apiece. In contrast with this, zirconium is transparent to thermal neutrons, it is used for the metal components of nuclear reactors – the cladding of their nuclear fuel rods. Hafnium reacts in air to form a protective film; the metal is not attacked by acids but can be oxidized with halogens or it can be burnt in air. Like its sister metal zirconium, finely divided hafnium can ignite spontaneously in air; the metal is resistant to concentrated alkalis. The chemistry of hafnium and zirconium is so similar that the two cannot be separated on the basis of differing chemical reactions.
The melting points and boiling points of the compounds and the solubility in solvents are the major differences in the chemistry of these twin elements. At least 34 isotopes of hafnium have been observed, ranging in mass number from 153 to 186; the five stable isotopes are in the range of 176 to 180. The radioactive isotopes' half-lives range from only 400 ms for 153Hf, to 2.0 petayears for the most stable one, 174Hf. The nuclear isomer 178m2Hf was at the center of a controversy for several years regarding its potential use as a weapon. Hafnium is estimated to make up about 5.8 ppm of the Earth's upper crust by mass. It does not exist as a free element on Earth, but is found combined in solid solution with zirconium in natural zirconium compounds such as zircon, ZrSiO4, which has about 1–4% of the Zr replaced by Hf; the Hf/Zr ratio increases during crystallization to give the isostructural mineral hafnon SiO4, with atomic Hf > Zr. An obsolete name for a variety of zircon containing unusually high Hf content is alvite.
A major source of zircon ores is heavy mineral sands ore deposits, pegmatites in Brazil and Malawi, carbonatite intrusions the Crown Polymetallic Deposit at Mount Weld, Western Australia. A potential source of hafnium is trachyte tuffs containing rare zircon-hafnium silicates eudialyte or armstrongite, at Dubbo in New South Wales, Australia. Hafnium reserves have been infamously estimated to last under 10 years by one source if the world population increases and demand grows. In reality, since hafnium occurs with zirconium, hafnium can always be a byproduct of zirconium extraction to the extent that the low demand requires; the heavy mineral sands ore deposits of the titanium ores ilmenite and rutile yield most of the mined zirconium, therefore most of the hafnium. Zirconium is a good nuclear fuel-rod cladding metal, with the desirable properties of a low neutron capture cross-section and good chemical stability at high temperatures. However, because of hafnium's neutron-absorbing properties, hafnium impurities in zirconium would cause it to be far less useful for nuclear-reactor applications.
Thus, a nearly complete separation of zirconium and hafnium is necessary for their use in nuclear power. The production of hafnium-free zirconium is the main source for hafnium; the chemical properties of hafnium and zirconium are nearly identical, which makes the two difficult to separate. The methods first used — fractional crystallization of ammonium fluoride salts or the fractional distillation of the chloride — have not proven suitable for an industrial-scale production. After zirconium was chosen as material for nuclear reactor programs in the 1940s, a separation method had to be developed. Liquid-liquid extraction processes with a wide variety of solvents were developed and are still used for the production of hafnium. About half of all hafnium metal manufactured is produced as a by-product of zirconium refinement; the end product of the separation is hafnium chloride. The purified hafnium chloride is converted to the metal by reduction with magnesium or sodium, as in the Kroll process.
HfCl4 + 2 Mg → 2 MgCl2 + HfFurther purification is effected by a chemical transport reaction developed by Arkel and de Boer: In a closed vessel, haf