Melting potential temperature

Figure 5.18 Adding a chemical to a host (mixing) causes its chemical potential /i to decrease, thereby explaining why a melting-point temperature is a good test of purity. The heavy solid lines represent the chemical potential of the pure material and the thin lines are those of the host containing impurities...

It is now time to draw all the threads together, and look at the temperature at which the thin lines intersect. It is clear from Figure 5.18 that the intersection temperature for the mixture occurs at a cooler temperature than that for the pure material, showing why the melting point temperature for a mixture is depressed relative to a pure compound. The depression of freezing point is a direct consequence of chemical potentials as defined in Equation (5.12). 

In ZnCl2-EMIC (1 1) melt containing Cu(I), the electrodeposition of Cu-Zn alloys on tungsten and nickel electrodes was carried out [180]. The composition of the Cu-Zn deposit was changed by deposition potential, temperature, and Cu(I) concentration in a plating bath. 

The Zn-Cd alloys were obtained from ZnCl2-EMIC melt containing Cd(II), during reduction on Pt, Ni, or W electrodes at the potentials sufficient to reduce Zn(II) to metal [183]. The composition of Zn-Cd alloys was dependent on the deposition potential, temperature, and Cd(II) concentration. 

Figure 2. Cyclic voltammograms of NaCl-KCl-CCh melt at temperature 750 °C and C02 pressure 1.0 105 Pa, potential scan rate - 0.1 V s"1 (a) - on Pt electrode at various potential...

Figure 3 Schematic diagram showing how original lithospheric thickness and mantle potential temperature affect the amount of melt produced (melt thickness) and how these factors relate to continental flood basalts (CFB), volcanic rifted margins (VRM), off-ridge and ridge-centered oceanic plateaus (OP), and midocean ridges (MOR).

Calculations based on radioactive heat production show that the Archaean mantle was hotter than the modern mantle. High mantle potential temperatures calculated from the ultramafic lavas - komatiites, common in the Archaean - led to the assumption that the Archaean mantle was substantially hotter than the modern mantle. However, the recent proposal that komatiites are the product of cooler, wet mantle melting weakens the argument for a very hot Archaean mantle, and there are now good grounds for arguing that the temperature of the Earth s mantle has declined by only 100-200°C since the mid-late Archaean. 

Larger melting friangle RigRef mantle potential. temperature... 

Mantle potential temperature Melt thickness is also controlled by melting temperature. This is because hot mantle will intersect the mantle solidus at a greater depth than cooler mantle, and as already established, melt thickness is a function of the depth of the melt column. However, since mantle temperatures increase with depth there needs to be... 

Melt thickness therefore can be expressed as a function of the mantle potential temperature. A potential temperature of 1,280°C equates to a melt thickness of about 7 km (normal ocean floor), whereas a mantle potential temperature of 1,480°C equates to a melt thickness of about 27 km (see Fig. 3.14). Clearly, these principles are important when considering melting processes in the early Earth, since many geoscientists believe that mantle temperatures were hotter in the Archaean (Section 3.2.3). 

If, as many suppose, the Archaean mantle had a higher potential temperature than the modern mantle, it is important to examine the implications of this for melt production during the early history of the Earth. The relationship between mantle potential temperature and melt thickness during adiabatic melting was outlined in Section 3.1.4.3 and may be briefly summarized by stating that as mantle potential temperature increases so will the melt production, as expressed in the depth of the melt column and the melt thickness. This is illustrated in Fig. 3.26, which shows how deeper, higher-temperature melting should lead to the formation of a thicker oceanic crust. 

The coohng history of the mantle Figure 3.25 shows the secular cooling curve for the mantle from Richter (1988), the calculated potential temperatures for Archaean komatiites and basalts, and an estimated temperature for the Archaean subcontinental mantle. Two important conclusions follow. First, it is clear that the Archaean mantle had both hot and cool regions. Potential temperatures calculated from dry komatiite melting temperatures imply an anomalously hot mantle source,... 

Figure 26-15. Relative potentials of a Zr02 electrode with 1 bar oxygen partial pressure in glass-forming melts with temperature-independent (A, B) and temperature-dependent (C, D) standard Seebeck coefficients. Reference potential 9, = 0. (a) Fiolax klar (b) (Na20)oo7 (K20)o 78(CaO)o, (Si02)o.737 (c) (Na20)o,s6(CaO)ojB7(Si02) .737 (d) BK7 (e) phos-phate-ba optical glass.

Protein-based polymers have the potential to surpass the polyesters and other polymers because they can be directly produced in microorganisms and plants by recombinant DNA technology resulting in the capacity for diverse and precisely controlled composition and sequence. This is not possible with any other polymer, and it increases range of properties and the numbers of applications. Remarkably, with the proper design of composition, protein-based materials can be thermoplastics, melting at temperatures as much as 100°C below their decomposition temperatures. Therefore, they can be molded, extruded, or drawn into shapes as desired. Aspects of protein-based materials as plastics is also considered below. 

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