PB systems

Al-Pb. Both lead [7439-92-17, Pb, and bismuth [7440-69-9] Bi, which form similar systems (Fig. 17), are added to aluminum ahoys to promote machinahility by providing particles to act as chip breakers. The Al—Pb system has a monotectic reaction in which Al-rich Hquid free2es partiahy to soHd aluminum plus a Pb-rich Hquid. This Pb-rich Hquid does not free2e until the temperature has fahen to the eutectic temperature of 327°C. SoHd solubiHty of lead in aluminum is negligible the products contain small spherical particles of lead which melt if they are heated above 327°C. 

Figure 3.1. Li-Pb system averaged values (in the temperature range between 400°C and 800°C) of the resistivity of the liquid alloys (Van der Lugt 1996).

Figure 3.2. Li-Pb system trend of the mixing enthalpy, AH, of the liquid alloys (after Predel and Oehme 1979) and dAmixH/dx vs. composition.

A clear indication of the various intermediate phase stability may be obtained from the values of their Afi (AfG). The enthalpies of formation, in the liquid and in the solid state, of divalent metal alloys with Pb, as a typical element in the p-block, have been measured, and their trend discussed, by Sommer et al. (2006). The most exothermic values observed, for instance, in the Ca-Pb and Ba-Pb systems, correspond (for the solid compounds, in kJ/mol of atoms, at 300 K) to — 62 2 (for Ca2Pb) and —13 2 (for BaPb). A relevant compound forming tendency was observed also for the liquid alloys, for which the association model (see 3.2) was successfully applied confirming the existence of strong chemical short-range order. 

Examples of elements exhibiting low or very low solubility (at 600°C) in liquid Sn or Pb are for instance Cr, Nb, Mo, W (with deep minima for Nb, Mo, W). High solubility is shown by Zn, Cd, In, Hg, Tl, Sn, Pb, Bi. Several Sn or Pb systems with these metals give low melting eutectics and form continuous liquid solutions stable at moderate temperatures. 

Antimony(III) trichloride can be used as a catalytic mediator in the electroreduction of carbonyl compounds. The advantage of the procedure is illustrated by the selective reduction of acetophenones (497), leading to the corresponding benzylic alcohols (498) (Scheme 173) [575]. The electroreduction is performed in an EtOH/BuOH(l/1.5)-aq.HCl-(Pt/Pb) system in the presence... 

More advanced techniques are now available and section 4.2.1.2 described differential scanning calorimetry (DSC) and differential thermal analysis (DTA). DTA, in particular, is widely used for determination of liquidus and solidus points and an excellent case of its application is in the In-Pb system studied by Evans and Prince (1978) who used a DTA technique after Smith (1940). In this method the rate of heat transfer between specimen and furnace is maintained at a constant value and cooling curves determined during solidification. During the solidification process itself cooling rates of the order of 1.25°C min" were used. This particular paper is of great interest in that it shows a very precise determination of the liquidus, but clearly demonstrates the problems associated widi determining solidus temperatures. 

Pb from h. The Pb systems are discussed last because of the complexities of the U-Th-Pb system in several aspects. One is that each of the nuclides and h undergoes a long chain of decay to the final stable nuclide decays... 

First the U- Pb system is discussed. undergoes 8 a-decays and 6 p-decays to finally reach the stable nuclide ° Pb (see Table 2-2a). The half-life of is 4.468 X 10 years, and the decay constant is 1.55125 x 10 ° yr . Assuming the changes in the concentrations of the intermediate nuclides (such as... 

Another coupled system is the Sm-Nd system, with two Sm isotopes ( Sm and Sm) undergoing ot-decay to become two Nd isotopes ( Nd and Nd). The half-life of Sm is 106 billion years and that of is 103 million years. In principle, the concepts for the U-Pb system (such as concordia and discordia, Nd-Nd isochron) can also be applied to the Sm-Nd system. However, the Sm-Nd coupled system has not found many applications. One reason is that the half-life of " Sm is so short that it is an extinct nuclide. Secondly, the half-lives of Sm and " Sm are very different, by a factor of 1000 (in contrast, the half-lives of and 235 differ only by a factor of 6.3). Hence, the coupled system has found only limited applications to very old rocks, such as meteorites and very old terrestrial rocks. 

Fig. 12.3. The one-stage Pb evolution model of Holmes-Houtermans according to Eqs. (12.16) and (12.17). The straight lines are isochrones for different values of t. The curved lines are Pb growth curves for U-Pb systems having present-day n ( U/ Pb) values of 7 (solid square), 8 (open circle), and 9 (solid triangle), respectively. Present-day...

In contrast to refractory uranium and thorium, lead is a moderately volatile element. Uranium and thorium are lithophile, while lead can exhibit lithophile, siderophile, or chalcophile behavior. This means that in many cosmochemical situations, it is possible to strongly fractionate the daughter lead from parent uranium and thorium, a favorable situation for radiochronology. On the other hand, lead tends to be mobile at relatively low temperatures and can be either lost from a system or introduced at a later time. As already mentioned, uranium can also become mobile under oxidizing conditions. This means that the U-Th-Pb system is more susceptible to open-system behavior than several other commonly used dating techniques. However, as we discuss below, there are ways to recognize and account for the open-system behavior in many cases. 

The Pb-Pb system can be applied in two ways. If the sample is sufficiently large and diverse that mineral separates with different U/Pb ratios can be produced, an internal isochron can be generated. The equation for the 207pb -206Pb isochron can be obtained by combining Equations (8.30) and (8.31) ... 

One drawback of the Pb-Pb system is that two-stage or three-stage evolution of the U-Pb isotopic system can produce linear arrays on a Pb-Pb isochron plot (Gale and Mussett, 1973). If these arrays are interpreted as single-stage isochrons, incorrect dates may be obtained. In some cases, measurements of the U/Pb ratio can help identify multistage samples. But if several events occurred sufficiently early in solar system history, even a concordia approach may not be able to identify a multistage system (Tera and Carlson,... 

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