Cu-Zn system

The SME process can be illustrated by the Cu—Zn system, one of the first SMAs to be studied. A single orientation of the bcc P-phase on cooling goes through an ordering process to a B2 phase. In a disordered alloy, the lattice sites are randomly occupied by both types of atoms, but on ordering the species locate at particular atomic sites, yielding what is called a supedattice. When the B2 phase is cooled below the Mp it transforms to... 

An alloy system is all the alloys you can make with a given set of components "the Cu-Zn system" describes all the alloys you can make from copper and zinc. A binary alloy has two components a ternary alloy has three. 

TA2 mode ([IlO] polarization) visibly depends on the influence of the electrons on the lattice vibrations (see Fig. 7). Without V2, the stiffness of the TAi mode is enlarged. Therefore, we can conclude that this TA2 mode is very sensitive to changes in the electronic structure and measures the stability of the B2 phase. This may be a hint of the temperature or stress dependence of the Cu-Zn system in the martensitic region (Zn < 42 at%) close to the stoichiometric concentration CuZn... 

Cu-Zn system, shape-memory effect in, 22 343. See also Copper entries CVD-deposited films, properties of,... 

Figure 2.42. The Cu-Zn system phase diagram and microstructure scheme of the diffusion couple obtainable by maintaining Cu and Zn blocks in contact for several days at 400°C. Shading indicates subsequent layers, each one corresponding to a one-phase region. The two-phase regions are represented by the interfaces between the one-phase layers (adapted from Rhines 1956).

The Cu-Zn system (see Figure 2.7) displays a number of intermediate solid solutions that arise due to limited solubility between the two elements. For example, at low wt% Zn, which incidently is the composition of alloys known as brass, the relatively pure copper a phase is able to accommodate small amounts of Zn as an impurity in the crystal structure. This is known as a terminal solid phase, and the solubility limit where intermediate solid solutions (such as a + /S) begin to occur is called the solvus line. Some of the three-phase transformations that are found in this diagram include a peritectic (5 - - L -> e) and a eutectoid (5 -> y - - e). Remember that these three-phase transformations are defined for equilibrium coohng processes, not heating or nonequihbrium conditions. 

The Cu-Zn system (brass) is complex as shown in Figure 9.1. The a phase is a ccp solid solution of Zn in Cu. The (3-brass is body-centered cubic, the composition corresponding to CuZn. Each phase exists over a range of Cu/Zn ratios corresponding to a solid solution with Zn or Cu added to the compound. The y-brass, CupZns, has a complex cubic structure and e-brass, CuZn3, has an hep structure. Hume-Rothery found that many intermetallic compounds have structures similar to (3-, y-, and e-brass at the same electron-to-atom ratio as the corresponding brass compounds. Some examples of these so-called electron... 

Ca/Na, and Cu/Zn mixtures were tested for the cation separation and concentration, and chromate/chloride mixture for the anion separation. For Ca/Na system, concentration factor obtained was 55 at Ca/Na selectivity equal 39. For the Cu/Zn system, the concentration factor was 50 at Cu/Zn selectivity equal 70. For potassium chromate/sodium chloride system, the concentration factor was 75 at chromate/chloride selectivity equal 30. 

Seco, A., et al.. Absorption of heavy metals from aqueous solutions onto activated carbon in single Cu and Ni systems and in binary Cu-Ni, Cu-Cd and Cu-Zn systems. J. Chem. Tech. Biotech., 68(1) pp. 23—30, 1997. 

The shift reaction is exothermic and thus the equilibrium is favored by low temperatures (Figure 6.2.4). Thus, the reaction temperature should be kept as low as possible, but is limited by the activity of the catalyst. The Fe-Cr shift catalyst is sufficiently active only above about 300 °C. Catalysts based on copper and zinc are active enough at about 200 °C but these catalysts are very sensitive to poisoning and require extremely pure gases, typically with less than Ippm H2S. In practice, the water-gas shift reaction is carried out in two adiabatic fixed-bed reactors with intermediate cooling between both converters. The first high-temperature shift reactor operates with a Fe-Cr catalyst, and the second low-temperature shift reactor contains the more active Cu-Zn system. At the exit of the second shift reactor, the CO2 present in the converted syngas is removed in a gas scrubber, usually by chemical absorption in aqueous amine solutions, for example, mono- or diethanolamine (Section 3.3.3). 

Calculated mixing enthalpies of a, 0, /, , and ry—phases of the Cu-Zn system are shown in Fig. 3. As standard states we used the pure fee Cu and hep Zn with theoretically determined c/a ratio 1.82, which compares well with experimental c/a= 1.856 . One can see that calculations correctly reproduce the principal features of the Cu-Zn phase diagram . Considering the random alloys alone we observe that for the Cu-rich alloys the fee structure is stable. When Cu concentration increases, the fee alloys are almost degenerate with the hep alloys, followed by the region where bcc alloys are more... 

The low-temperature solid phase may be an intermediate solid solntion (e.g., e in the preceding reaction), or it may be a terminal solid solution. One of the latter peritectics exists at about 97 wt% Zn and 435°C (815°F) (see Figure 9.19), where the tj phase, when heated, transforms into e and liquid phases. Three other peritectics are foimd for the Cu-Zn system, the reactions of which involve p, 8, and y intermediate solid solutions as the low-temperature phases that transform upon heating. 

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