Iron , crystal

It is worth while, now, to analyse the motivation for making metallic single crystals and how, in turn, their production affected physical metallurgy. Initially, metallurgists were concerned to prevent the accidental generation of coarse grains in parts of objects for load-bearing service, and studied recrystallisation with this objective in view. To quote Keith, Iron crystals... were achieved subsequently by... 

Iron crystallizes in a body-centered unit cell Its atomic radius is 0.124 nm. Its density is 7.86 g/cm3. Using this information, estimate Avoga-dro s number ... 

Iron crystallizes in a bcc structure. The atomic radius of iron is 124 pm. Determine (a) the number of atoms per unit cell (b) the coordination number of the lattice (c) the length of the side of the unit cell. 

Views of the iron crystal (a) A molecular view, with the lattice representation (b) the unit cell (c) six unit cells stacked to form part of the crystal. 

This diagram shows the stages in the Haher-Bosch process for manufacturing ammonia. The catalyst is a mixture of MgO, AI2O3, and Si02, with embedded iron crystals. 

There has been considerable speculation concerning the role of carbide in the iron base catalyst. The carbide was originally depicted as an intermediate in the reaction (7), but more recent work indicates the contrary (3). It now appears more probable that the lattice between carbide crystals or between groupings of carbide and relatively fewer oxide or even free iron crystals offers the form of pore structure required for both high activity and selectivity. 

James Johnston patents polygon shaped sugar moulds, cast-iron crystallizing pots coated with tin, zinc, or enamel. The latter would bo good If the enamel did not break off into the sugar. 

Fig. 2. Complex polygonized structure in a single silicon-iron crystal deformed 8% by cold rolling before being annealed 1 hour at 1 100°C...

SPHALERITE BLENDE. Also known as zinc blende, this mineral is zinc sulfide, tZn, Fc)S, practically always containing some iron, crystallizing in the isometric system frequently as tetrahedrons, sometimes as cubes or dodecahedrons, but usually massive with easy cleavage, which is dodecahedral. It is a brittle mineral with a conchoidal fracture hardness, 2.5-4 specific gravity, 3.9-4.1 luster, adamantine to resinous, commonly the latter. It is usually some shade of yellow brown or brownish-black, less often red, green, whitish, or colorless streak, yellowish or brownish, sometimes white transparent to translucent. Certain varieties... 

Iron crystallizes in a body-centered cubic unit cell with an edge length of 287 pm. What is the radius of an iron atom (in picometers) ... 

The copper crystals used in these studies were made from copper of both 99.999% and 99.94% purity, the nickel crystals from metal 99.92% pure nickel plus cobalt, and the iron crystals were made from Armco iron rods of the following approximate composition iron, 99.8 carbon, 0.018 manganese, 0.027 phosphorus, 0.005 sulfur, 0.029 silicon, 0.005 copper, 0.11 %. 

Iron crystallizes in several modifications. At about 910°C, the body-centered cubic a-form undergoes a transition to the face-centered cubic y-form. Assuming that the distance between nearest-neighbors is the same in the two forms at the transition temperature, calculate the ratio of density of y-iron to that of a-iron at the transition temperature. 

Potentially of equal importance is the relationship between strain and catalyst stability. A calculation of the contribution to the total free energy of a catalyst crystal caused by the presence of strain-inducing microscopic precipitates50 showed that the extra free energy increases with the size of the crystal and inhibits it from sintering. This theory is an interesting one since it provides a mechanism which the catalyst scientist can exploit in his search for stable, high surface-area materials. The theory predicts the equilibrium crystallite size of the iron crystals of an ammonia synthesis catalyst with acceptable accuracy. 

Diflusion of Fe2+ ions and electrons to the nuclei and growth of the nuclei to iron crystals of various size... 

There are many different reasons for interest in the surface of a crystal. From a narrow, practical viewpoint, one might mention the economic importance of the reaction of oxygen with the surface of an iron crystal (corrosion or rusting), the effect of surface layers upon the performance of various semiconductor devices, and of course catalytic reactions on crystal surfaces which presumably include all of catalysis. 

There are two types of alloys. In a substitutional alloy, some of the metal atoms in a crystal lattice are replaced by other atoms (usually of comparable size). Examples are brass, in which approximately one third of the atoms in a copper crystal are replaced by zinc atoms, and pewter, an alloy of tin that contains 7% copper, 6% bismuth, and 2% antimony. In an interstitial alloy, atoms of one or more additional elements enter the interstitial sites of the host metal lattice. An example is steel, in which carbon atoms occupy interstitial sites of an iron crystal, making the material stronger and harder than pure iron. Mild steel contains less than 0.2% C and is used for nails, whereas high-carbon steels can contain up to 1.5% C and are used in specialty applications such as tools and springs. Alloy steels are both substitutional and interstitial atoms from metals such as chromium and vanadium substitute for iron atoms, with carbon remaining in interstitial sites. Alloy steels have a variety of specialized purposes, ranging from cutlery to bicycle frames. 

Blast furnace production of iron allows the hot, newly reduced product to trickle through the bed of heated coke to the hearth. Since carbon is somewhat soluble in molten iron, pig iron usually contains from 3 to 4.5% carbon. It also contains smaller percentages of other reduced elements such as silicon, phosphorus, manganese, etc., generated by the same reducing processes that yielded the iron (Table 14.3). Primarily from the effect of the high-carbon content on the iron crystal structure, the blast furnace product is brittle, hard, and possesses relatively low-tensile strength. Hence the crude pig iron product of the blast furnace is not much used in this form. 

Metallic iron crystallizes in a cubic lattice. The unit cell edge length is 287 pm. The density of iron is 7.87 g/cm. How many iron atoms are within a unit cell ... 

FOLLOW-UP PROBLEM 12.4 Iron crystallizes in a body-centered cubic structure. The volume of one Fe atom is 8.38X10 cm, and the density of Fe is 7.874 g/cm. Calculate an approximate value for Avogadro s number. 

Modem ammonia plants operate at 200-300 atm and around 673 K (400.°C), and the catalyst consists of small iron crystals fused into a mixture of MgO, AI2O3, and Si02. The reactant gases in stoichiometric ratio (N2 H2 =1 3) are injected into the reaction chamber and over the catalyst beds. The emerging equilibrium mixture, which contains about 35% NH3 by volume, is cooled to condense and remove the NH3 the remaining N2 and H2, which are still gaseous, are recycled into the reaction chamber. 

Each ion in an iron crystal contributes on average 2.22/ig. In Fe304. however, each Fe ion contributes an average of 4.08/i. How can you rationalize this result ... 

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