"Carbides," In: Ullmann's Encyclopedia of Industrial Chemistry
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Article No : a05_061 Carbides HELMUT TULHOFF, Hermann C. Starck Berlin, Werk Goslar, Goslar, Federal Republic of Germany 1. Survey ............................. 565 2.6. Hafnium Carbide..................... 576 1.1. Saltlike Carbides ..................... 565 2.7. Vanadium Carbide ................... 576 1.2. Metal-like Carbides ................... 567 2.8. Chromium Carbide ................... 577 1.3. Diamond-like Carbides ................ 567 2.9. Molybdenum Carbide ................. 578 1.4. Carbides of Nonmetallic Elements........ 567 3. Mixed Carbides ...................... 579 1.5. Crystal Structure..................... 567 3.1. Tungsten – Titanium Carbide ........... 580 1.6. General Production Processes ........... 568 3.2. Other Mixed Carbides................. 580 1.7. Uses ............................... 569 3.3. Carbonitrides........................ 581 2. Metal-like Carbides of Industrial Importance 569 3.4. Mixed Carbonitrides .................. 581 2.1. Tungsten Carbide .................... 569 4. Carbides of the Iron Group and Manganese 581 2.2. Titanium Carbide .................... 573 5. Complex Carbides .................... 581 2.3. Tantalum Carbide .................... 574 References .......................... 582 2.4. Niobium Carbide ..................... 575 2.5. Zirconium Carbide ................... 576 1. Survey viewed as a diamond-like carbide because of its hardness and other properties resembling those Most of the elements form binary compounds of SiC. with carbon, all of which can be called carbides. Figure 1 surveys the four types of carbides in The properties of these carbides are very differ- the form of a periodic table. Elements that do not ent; therefore, like binary hydrides and nitrides, form binary compounds with carbon, or are not the carbides should be classified into groups. To known to form carbides, are not shown. The avoid too many subdivisions, the following four carbides of the iron group and manganese are types of carbides may be defined: a subgroup of the metal-like carbides. 1. saltlike carbides of metallic elements, e.g., CaC2 1.1. Saltlike Carbides 2. metal-like carbides of metallic elements, e.g., WC Saltlike carbides of metallic elements are the 3. diamond-like carbides, e.g., B4C carbides of the elements of groups 1 – 3 and 4. carbides of nonmetallic elements, e.g., CO 11 – 13 (I – III, both A’s and B’s) of the periodic table, the lanthanides and actinides included. This classification suggests another group: the Exceptions are Ga, In, and Tl, which do not form elements that do not react with carbon, e.g., Sn. carbides, and B4C, which is a typical diamond- Generally, the four groups of carbides can not like carbide. be strictly separated from each other. Numerous The saltlike carbides – also called ionic carbides carbides are in intermediate positions between – are attacked by water to form hydrocarbons. these groups. One example is BeC2 [57788-94- Most of these carbides form acetylene, e.g.: 0]. It is a typical saltlike carbide and is decom- posed by water. On the other hand, it may be CaC2þ2H2O ! CaðOHÞ2þC2H2 Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/14356007.a05_061 566 Carbides Figure 1. Survey of binary compounds of carbon with the elements Vol. 6 Vol. 6 Carbides 567 These carbides can be viewed as salts of water. The metallic character of these com- acetylene and may be called acetylides. The pounds is shown in their high thermal and elec- 2À crystals contain C2 anions. trical conductivity as well as in their metallic The carbides Be2C and Al4C3 form pure luster. methane when hydrolyzed: All the metallic carbides are stable at room temperature and resist attack by dilute acids as Be Cþ4H O ! 2BeðOHÞ þCH 2 2 2 4 well as by alkaline and organic liquids. Their hardness and wear resistance are utilized in the Al4C3þ12 H2O ! 4AlðOHÞ3þ3CH4 cemented carbides (! Hard Materials), which In the crystal lattice of these carbides, the are sintered products of the carbides with cobalt carbon atoms are isolated from each other, in or other metals. Because of their industrial sig- contrast to the C2 groups of the acetylides. The nificance, these carbides are described in more Be2C lattice is antiisotypical to that of CaF2. detail in Chapter 2. The carbide MgC2 can be decomposed by The carbides of Mn, Fe, Co, and Ni are heating to form Mg2C3 and graphite. Hydrolysis generally included in the metal-like carbides, of Mg2C3 yields propyne: although they are really better classified as a group on their own. These carbides are in an Mg2C3þ4H2O ! 2MgðOHÞ þCH3ÀC CH 2 intermediate position between the metal-like In their carbides the lanthanides and actinides carbides and the saltlike carbides. Their crystal are mainly divalent. During hydrolysis they be- structures are quite different from the structures come trivalent, and hydrogen is formed in this of the metal-like carbides and the saltlike car- reaction: bides. The pure compounds are attacked by water M2þþHþ ! M3þþH or dilute acids. This hydrogen reacts with the acetylene also formed to produce a mixture of acetylene, meth- 1.3. Diamond-like Carbides ane, ethylene, and hydrogen. Whereas the saltlike carbides of groups 1 and Diamond-like carbides include, strictly speak- 2 are transparent and are not electrical conduc- ing, only B4C and SiC. They are called diamond- tors, the lanthanide and actinide carbides show like because of their extreme hardness, which is some metallic behavior, an indication of a state exceeded only by diamond itself. Sometimes the intermediate between saltlike and metal-like car- very hard Be2C is included in the diamond-like bides. The electrical conductivity and metallic carbides. However, its hardness cannot be used luster may be due to the fact that the metals are industrially, because of its decomposition by divalent in their carbides and the third valence water. electron is available for metallic bonding. One other subgroup of saltlike carbides should be mentioned: the alkali-metal – graphite com- 1.4. Carbides of Nonmetallic Elements pounds. They are formed by absorption of molten Na, K, Rb, and Cs by graphite. Compositions Such carbides as CO, CS2, and CCl4, the carbides such as MC8,MC16, and MC60 are known. These of nonmetallic elements, have covalent, molecu- compounds are quite likely not chemical com- lar character and are not discussed in this article. pounds, but merely adsorptional compounds, and perhaps better not called carbides. 1.5. Crystal Structure 1.2. Metal-like Carbides The lattice structure of most carbides can be deduced from the structure of their most impor- Metal-like carbides of metallic elements are the tant group, the metal-like transition-metal car- carbides of the transition elements of groups 4, 5, bides. Basically these carbides are cubic or hex- and 6 of the periodic table. These carbides, also agonal closest packings of metal atoms with the called metallic carbides, are not attacked by smaller carbon atoms in the interstitial sites. 568 Carbides Vol. 6 Therefore, the transition-metal carbides can also carbides, M2C3, also contain C2 groups, e.g., be called interstitial carbides. U2C3. In 1931 HA€GG [7] reported that the structure of The structures of the diamond-like carbides the transition-metal carbides is determined by the SiC and B4C differ from all structures described radius ratio r of the metal and carbon atoms thus far. The carbide SiC has an expanded dia- r ¼ rC/rmetal. When r is less than 0.59, the metals mond lattice, whereas B4C crystallizes in a rhom- form the simple structures just described, with bic lattice containing B12 icosahedrons and C3 the carbon atoms located at the octahedral inter- chains. stices. If all interstices are occupied in a body- centered cubic (bcc) metal lattice, the result is the face-centered cubic (fcc) sodium chloride struc- 1.6. General Production Processes ture. All the carbides of transition-metal groups 4 and 5 crystallize in this B1 lattice. There are a number of general methods of pro- Tungsten carbide has a simple hexagonal struc- ducing carbides: ture with all of the trigonal prismatic interstitial sites occupied by carbon. 1. Nearly all carbides can be prepared at high The B1 carbides, principally TiC, ZrC, HfC, temperature by direct reaction from the metal and VC, tend to form defect structures in which powder mixed with lamp black or graphite, e.g.: the interstitial sites are not completely filled. Broad homogeneity ranges are the result, but WþC ! WC some substructures with overlapping homogene- Generally the temperature is in the range ity ranges are indicated [8]. 1000–1500C, and special furnaces are used. When only one-half of the octahedral intersti- A protective atmosphere or vacuum is needed. tial sites are occupied in an hexagonal-closest- 2. Instead of the pure metal, the oxide or hydride packed (hcp) metal structure, the subcarbides – can be carburized with solid carbon: V2C, Nb2C, Ta2C, Mo2C, and W2C – are ob- tained. This is a simplified interpretation, and in Ta2O5þ7C! 2 TaCþ5CO fact the subcarbides are more complex structures, Large amounts of gas result from this as was shown by NOWOTNY [9]. Indeed, these reaction. Both processes 1 and 2 are solid- structures are sometimes called Nowotny phases, state reactions. to contrast them with the simpler H€agg phases. 3. Carbides with high melting points can be When H€agg’s ratio exceeds 0.59, the simple prepared by a modified aluminothermic pro- phases can no longer be formed as before. Close cess: to 0.59 and in the case of low carbon content, 3Cr2O3þ6Alþ4C! 2Cr3C2þ3Al2O3 there are the compounds Cr23C6 and Mn23C6, which can still be viewed as interstitial structures. For higher values of r and higher carbon content, 4. Instead of solid carbon, gaseous carbon com- more complex structures, no longer interstitial pounds, such as CO or CH4, can be used. This compounds, are formed: M3C, M7C3,M3C2. process is important in the steel industry, These stoichiometries are primarily found in the where mainly iron, chromium, and manga- iron group.