Host metals

As we have already seen, when an alloy contains more of the alloying element than the host metal can dissolve, it will split up into two phases. The two phases are "stuck" together by interphase boundaries which, again, have special properties of their own. We look first at two phases which have different chemical compositions but the same crystal structure (Fig. 2.5a). Provided they are oriented in the right way, the crystals can be made to match up at the boundary. Then, although there is a sharp change in... 

A phase is a region of material that has uniform physical and chemical properties. Phases are often given Greek symbols, like a or fi. But when a phase consists of a solid solution of an alloying element in a host metal, a clearer symbol can be used. As an example, the phases in the lead-tin system may be symbolised as (Pb) - for the solution of tin in lead, and (Sn) - for the solution of lead in tin. 

Steel is an alloy of about 2% or less carbon in iron. Carbon atoms are much smaller than iron atoms, and so they cannot substitute for iron in the crystal lattice. Indeed, they are so small that they can fit into the interstices (the holes) in the iron lattice. The resulting material is called an interstitial alloy (Fig. 5.48). For two elements to form an interstitial alloy, the atomic radius of the solute element must be less than about 60% of the atomic radius of the host metal. The interstitial atoms interfere with electrical conductivity and with the movement of the atoms forming the lattice. This restricted motion makes the alloy harder and stronger than the pure host metal would be. 

Homogeneous alloys of metals with atoms of similar radius are substitutional alloys. For example, in brass, zinc atoms readily replace copper atoms in the crystalline lattice, because they are nearly the same size (Fig. 16.41). However, the presence of the substituted atoms changes the lattice parameters and distorts the local electronic structure. This distortion lowers the electrical and thermal conductivity of the host metal, but it also increases hardness and strength. Coinage alloys are usually substitutional alloys. They are selected for durability—a coin must last for at least 3 years—and electrical resistance so that genuine coins can be identified by vending machines. 

These carbides, also known as interstitial carbides, are crystalline compounds of a host metal and carbon. The host-metal atoms are generally arranged in a close-packed structure and the carbon occupies specific interstitial sites in that structure. Such a structure sets size restrictions on the two elements in order for the carbon atom to fit into the available sites and the population of these sites (if all are occupied) determines the stoichiometry of the carbide. 

Interstitial nitrides are crystalline compounds of a host metal and nitrogen, where the nitrogen atom occupies specific interstitial... 

Figure 3.19 Schematic representation of surface alloy stability tests. White spheres denote adsorbed hydrogen, black spheres denote solute metal atoms, and gray spheres denote host metal atoms. Adapted from [Greeley and Nprskov, 2007] see this reference for more details.

Kaindl et al. [186] have plotted the isomer shift results for metallic hosts versus the number of outer electrons of the 3d, Ad, and 5d metals and found the transition energy to decrease when proceeding from a to a Ad and further to a 3d host metal in the same column of the periodic table. This systematic behavior is similar to that observed for isomer shifts of y-rays of Fe(14.4 keV) [193], Ru(90 keV), Pm (77 keV), and lr(73 keV) [194]. The changes of A(r ) = (r )e — (r )g for these Mossbauer isotopes are all reasonably well established. Kaindl et al. [186] have used these numbers to estimate, with certain assumptions, the A(r ) value for Ta (6.2 keV) and found a mean value of A(r ) = —5 10 fin with some 50% as an upper limit of error. The negative sign of A(r ) is in agreement with the observed variation of the isomer shift of LiTaOs, NaTaOs, and KTaOs, as well as with the isomer shift found for TaC [186]. 

With the observed temperature shift data for (dSldT)p and calculated (within the framework of the Debye model) numbers for the temperature shift of SOD and with the known thermal expansion coefficient as well as results from Ta Mossbauer experiments under pressure, the authors [191] were able to evaluate the true temperature dependence of the isomer shift, (dSisIdT) as —33 10 " and —26 10 " mm s degree for Ta and W host metal, respectively. 

Although the face-centred cubic structure of metals is close packed, it is still possible for atoms which are much smaller than the host metal atoms to fit into interstitial sites inside the structure, while maintaining the essential properties of metals such as electrical conductivity and heat transport. These interstitial sites are of two kinds. The octahedral interstitial sites have six metal atoms at equal distances from the site, and therefore at the apices of a regular octahedron. The tetrahedral interstitial sites have four nearest neighbour metal atoms at the apices of a regular tetrahedron. A smaller atom can just fit into the octahedral site if the radius ratio is... 

Absorbed on interstitial sites in a host metal At ambient pressure and temperature... 

Palladium hydride is not a stoichiometric chemical compound but simply a metal in which hydrogen is dissolved and stored in solid state, in space between Pd atoms of crystal lattice of the host metal. Relatively high solubility and mobility of H in the FCC (face-centered-cubic) Pd lattice made the Pd H system one of the most transparent, and hence most studied from microstructural, thermodynamic, and kinetic points of view. Over the century that followed many metal-hydrogen systems were investigated while those studies were driven mostly by scientific curiosity. Researchers were interested in the interaction of hydrogen molecule with metal surfaces adsorption and diffusion into metals. Many reports on absorption of in Ni, Fe, Ni, Co, Cu, Pd, Pt, Rh, Pd-Pt, Pd-Rh, Mo-Fe, Ag-Cu, Au-Cu, Cu-Ni, Cu-Pt, Cu-Sn, and lack of absorption in Ag, Au, Cd, Pb, Sn, Zn came from Sieverts et al. [30-33]. 

As seen previously, after reaction (under gaseous or adsorbed hydrogen) of an M R4 organometallic compound (group 14) with the surface of the host metal ( Ms ) various species, Ms[M R ]y, are formed. Depending on % and y and the treatment temperature, these species can be divided in three main groups ... 

Figure 5.2 The six basic hydrogen storage methods and phenomena. From top left to bottom right compressed gas (molecular H2) liquid hydrogen (molecular H2) physisorption (molecular H2) on materials,forexample, carbon with a very large specific surface area hydrogen (atomic H) intercalation in host metals,...

Figure 5.21 Tetrahedral (T) and octahedral (O) interstitial sites occupied by hydrogen atoms [27]. The number of sites per host metal atom and the size, that is, the diameter of the largest possible sphere on the interstitial site, are given in the table.

The hydrogen is, at small hydrogen to metal ratios (H M<0.1), exothermically dissolved (solid solution, a-phase) in the metal. The metal lattice expands proportionally to the hydrogen concentration by approximately 2-3 A per hydrogen atom [31]. At greater hydrogen concentrations in the host metal (H M > 0.1), a strong... 

H-H interaction due to the lattice expansion becomes important and the hydride phase (P phase) nucleates and grows. The hydrogen concentration in the hydride phase is often found to be H M = 1. The volume expansion between the coexisting a- and P-phases corresponds in many cases to 10-20% ofthe metal lattice. Therefore, at the phase boundary high stress is built up and often leads to decrepitation of brittle host metals such as intermetaiiic compounds. The final hydride is a powder with a typical particle size of 10-100 pm (Figure 5.24). 

The semiempirical models mentioned above allow an estimation of the stability of binary hydrides provided that the rigid band theory can be applied. However, the interaction of hydrogen with the electronic structure of the host metal in some binary hydrides and especially in the ternary hydrides is often more complicated. In many cases, the crystal structure of the host metal and therefore also the electronic structure... 

Here Jo and (pg constants and kp is the Fermi wavevector of the host metal. Since the spins are randomly placed in the host metal, some spin-spin interaction will be positive and favor parallel alignment, while other will be negative, thus favoring antiparallel alignment. 

Other transient phases, such as host metal atoms being... 

---