Boron interstitial elements

The octahedral voids in the MnsSi3 type structure can be filled not only by a late transition metal. As an example Guloy and Corbett (1994) have tested this possibility for LasPb3. The voids could be filled with P, S, Cl, As, Se, Sb, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, and Ag atoms and also partially with boron or carbon. Filling of the voids leads to a small increase of the lattice parameters. On the other hand, lead can also act as the interstitial element on the octahedral site. The structural relationships of various M5X4 structures have recently been discussed by Guloy and Corbett (2005). 

The interstitial elements C, N, and B are potent solid-solution hardeners (Irvine et al., 1961), although too much of these elements can lead to embrittlement. Addition of approximately 0.003-0.05% boron is beneficial to the properties of superalloys. The boron segregates to grain boundaries where it inhibits grain-boundary tearing under creep-rupture loading. Small additions of boron also improve the hot workability of superalloys (Ramaswamy et al., 1972). Addition of up to 0.1 % zirconium has a beneficial effect similar to that of boron. 

Multicomponent and Commercial Alloy Bases. A representative selection of the numerous multicomponent and commercial alloys that, with the addition of strengthening elements, have imder-gone RSP is presented in Table 11.5. Strengthening elements represented in the list are interstitial elements (boron), eutectoid formers (silicon), RE elements (lanthanum, cerium, and erbium), the metalloid germanium, and sulfur. The... 

Recall that in contrast to substitutional alloys, where one metal atom substitutes for another in the lattice, interstitial alloys contain atoms of one kind that fit into the holes, or interstitial sites, of the crystal structure of the other. In metals with the interstitial elements hydrogen, boron, nitfogen, or carbon, the aUoy that results retains its metallic properties. 

Thermochemical diffusion heat treatments that involve the introduction of interstitial elements, such as carbon, nitrogen, or boron, into a ferrous alloy surface at elevated temperatures... 

H Werheit, R Schmechel, V Kueffel, T Lundstrdm. Electronic properties of P-rhombohedral boron interstitially doped with transition metal elements. J Alloys Compos 262-263 379, 1997. 

Chemical erosion can be suppressed by doping with substitutional elements such as boron. This is demonstrated in Fig. 14 [47] which shows data for undoped pyrolitic graphite and several grades of boron doped graphite. The mechanism responsible for this suppression may include the reduced chemical activity of the boronized material, as demonstrated by the increased oxidation resistance of B doped carbons [48] or the suppressed diffusion caused by the interstitial trapping at boron sites. 

Other single-crystal x-ray diffraction studies of transition element dopants in jS-rh boron are based on the results of a refinement of the /3-rh boron structure that establishes the occurrence of four new low-occupancy (3.7, 6.6, 6.8 and 8.5%) B positions in addition to the earlier known ones. The dopant elements studied, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Hf and Ta, do not enter B positions in the framework, but they enter the Al, A2, D and E positions. In some cases the doping elements have been studied at several concentrations for each element and for different cooling rates. The percentage occupancies of certain positions are eorrelated with the atomie sizes of the dopants. The bond distances between the polyhedra are shorter than those within the polyhedra. The mechanism of doping for some cases is denoted displacive, rather than interstitial or substitutional, because of competing interactions between the six different partially occupied B positions and dopant atoms. 

Niobium combines with carbon, boron, silicon and other elements at very high temperatures, forming interstitial binary compounds of varying compositions. With carbon, it forms niobium carbide having compositions varying from NbCo.7 to NbC [12069-94-2]. With boron, the products are orthorhombic niobium boride, NbB [12045-19-1], and the hexagonal diniobium diboride, Nb2B2[12007-29-3]. 

An interstitial compound consists of a metal or metals and certain metalloid elements, in which the metalloid atoms occupy the interstices between the atoms of the metal lattice. Compounds of this type are, for example. TaC, TiC, ZrC. NbC, and similar compounds of carbon, nitrogen, boron, and hydrogen with metals. 

Other elements with which iron forms binary compounds, especially at higher temperatures, are boron, carbon, nitrogen, silicon, and phosphorus, l.ike FeO, these compounds often depart slightly or even considerably from daltonide composition, frequently being interstitial compounds, and in higher elements of groups VB and VIB, merging into the interstitial compound-solid solution picture which iron exhibits with the transition metals. 

Solubility is one of the important properties of an alloy. Interstitial impurities (for example, hydrogen, oxygen, carbon, boron, and other chemical elements) in metals and alloys change considerably their physical properties [1-18],... 

Generally speaking most of the shallow impurity levels which we shall encounter are based on substitution by an impurity atom for one of the host atoms. An atom must also occupy an interstitial site to be a shallow impurity. In fact, interstitial lithium in silicon has been reported to act as a shallow donor level. All of the impurities associated with shallow impurity levels are not always located at the substitutional sites, but a part of the impurities are at interstitial sites. Indeed, about 90% of group-VA elements and boron implanted into Si almost certainly take up substitutional sites i.e., they replace atoms of the host lattice, but the remaining atoms of 10% are at interstitial sites. About 30% of the implanted atoms of group-IIIA elements except boron are located at either a substitutional site or an interstitial site, and the other 40% atoms exist at unspecified sites in Si [3]. The location of the impurity atoms in the semiconductors substitutional, interstitial, or other site, is a matter of considerable concern to us, because the electric property depends on whether they are at the substitutional, interstitial, or other sites. The number of possible impurity configurations is doubled when we consider even substitutional impurities in a compound semiconductor such as ZnO and gallium arsenide instead of an elemental semiconductor such as Si [4],... 

As for hydrides, borides, and carbides, different types of nitrides are possible depending on the type of metallic element. The classifications of nitrides are similarly referred to as ionic (salt-like), covalent, and interstitial. However, it should be noted that there is a transition of bond types. Within the covalent classification, nitrides are known that have a diamond or graphite structure. Principally, these are the boron nitrides that were discussed in Chapter 8. 

---