Metals energetics

Metallization layers are generally deposited either by CVD or by physical vapor deposition methods such as evaporation (qv) or sputtering. In recent years sputter deposition has become the predominant technique for aluminum metallization. Energetic ions are used to bombard a target such as soHd aluminum to release atoms that subsequentiy condense on the desired substrate surface. The quaUty of the deposited layers depends on the cleanliness and efficiency of the vacuum systems used in the process. The mass deposited per unit area can be calculated using the cosine law of deposition ... 

The pressure generated by the combustion of metallized pyrolants is sometimes lower than that generated by the combustion of non-metallized energetic materials. 

When a metallized energetic material is burned as a propellant igniter in a rocket chamber, a consequence of the aforementioned production of metal oxides as hot condensed particles is that there is very tittle associated pressure increase. However, the surface of the propellant grain in the chamber is ignited by the hot particles and a stable burning pressure is established. Typical metallized pyrolants used as igniters are shown in Table 11.1. 

Most metals react exothermically with oxygen to form an oxide. Figure 3.4 shows how the value of AG for this process varies with temperature for a number of metals (and for carbon), and it can be seen that in all cases AG becomes less negative as the temperature is increased. However, the decomposition of these metal oxides into the metal and oxygen is an endothermic process, and Figure 3.4 shows that this process does not become even energetically feasible for the majority of metals until very high temperatures are reached. 

Let us now consider the reduction of a metal oxide by carbon which is itself oxidised to carbon monoxide. The reaction will become energetically feasible when the free energy change for the combined process is negative (see also Figure i.i). Free energies. 

Reference to Figure 3.4 shows that the reduction is not feasible at 800 K. but is feasible at 1300 K. However, we must remember that energetic feasibility does not necessarily mean a reaction will go kinetic stability must also be considered. Several metals are indeed extracted by reduction with carbon, but in some cases the reduction is brought about by carbon monoxide formed when air, or air-oxygen mixtures, are blown into the furnace. Carbon monoxide is the most effective reducing agent below about 980 K, and carbon is most effective above this temperature. 

When using logjoK against l/T graphs, in order to find the temperature at which reduction becomes energetically feasible it is necessary to determine the temperature at which the equilibrium constant for the reduction indicates a displacement of the reaction in favour of the metal. 

There are many interacting parameters and possible feedstock—process—product combinations, but all are not feasible from a practical standpoint eg, the separation of small amounts of metals present in biomass and the direct combustion of high moisture content algae are technically possible, but energetically unfavorable. 

Dinitrogen has a dissociation energy of 941 kj/mol (225 kcal/mol) and an ionisation potential of 15.6 eV. Both values indicate that it is difficult to either cleave or oxidize N2. For reduction, electrons must be added to the lowest unoccupied molecular orbital of N2 at —7 eV. This occurs only in the presence of highly electropositive metals such as lithium. However, lithium also reacts with water. Thus, such highly energetic interactions ate unlikely to occur in the aqueous environment of the natural enzymic system. Even so, highly reducing systems have achieved some success in N2 reduction even in aqueous solvents. 

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