Magnesium vapor pressure, high temperature

Maclaurin series, A-65 to 67 Madelung constant, 12-32 Magnesium see also Elements electrical resistivity, 12-39 to 40 electron configuration, 1-18 to 19 heat capacity, 4-135 history, occurrence, uses, 4-1 to 42 ionization energy, 10-203 to 205 isotopes and their properties, 11-56 to 253 magnetic susceptibility, 4-142 to 147 molten, density, 4-139 to 141 thermal conductivity, 12-203 to 204 thermal properties, 12-201 to 202 thermodynamic properties, 5-1 to 3 vapor pressure, 6-61 to 90 vapor pressure, high temperature, 4-136 to 137... 

Many elements evaporate, but many such as chromium (Cr), cadmium (Cd), magnesium (Mg), arsenic (As), and carbon (C) sublime, and many others such as antimony (Sb), selenium (Se), and titanium (Ti), are on the borderline between evaporation and sublimation. For example, chromium, which has a vapor pressure of 10 Torr 600°C below its melting point, is generally vaporized by sublimation. Carbon cannot be melted except under high hydrostatic pressure. Materials such as aluminum, tin, gallium, and lead have very low vapor pressures at temperatures above the points at which they are just-molten. For example, tin has a vapor pressure of lO Torr 1000°C above its melting point. Aluminum and lead have vapor pressures of 10 Torr at about 500°C above their melting points. 

Significant vapor pressure of aluminum monofluoride [13595-82-9], AIF, has been observed when aluminum trifluoride [7784-18-1] is heated in the presence of reducing agents such as aluminum or magnesium metal, or is in contact with the cathode in the electrolysis of fused salt mixtures. AIF disproportionates into AIF. and aluminum at lower temperatures. The heat of formation at 25°C is —264 kJ/mol(—63.1 kcal/mol) and the free energy of formation is —290 kJ/mol(—69.3 kcal/mol) (1). Aluminum difluoride [13569-23-8] h.3.s been detected in the high temperature equihbrium between aluminum and its fluorides (2). 

Calcium and particularly magnesium show excessive vapor pressure at steelmaking temperatures. Table I, but this alloyability problem has been somewhat alleviated bj - submerging the introduction of the calcium alloy under 3 meters of liquid steel (31) However, independently of vapor pressure, it is the low solubility of calcium in liquid iron which limits the effective substitution of >feiS by CaS to extremely low sulfur levels, particiilarly when the manganese content is high, 2% for example (32). 

Harris discusses the findings of Styris and Redfield, who studied the effect of magnesium nitrate on the determination of A1 (95,96). At high temperature, MgO is formed and steadily evaporates. This maintains a steady vapor pressure of MgO in the GFAA tube. The presence of MgO serves to keep A1 as the oxide by establishing the following equilibrium ... 

For metals a sublimation temperature is chosen that corresponds to a vapor pressure of 10 Torr. This vapor pressure is not sufficient for all cases. Platinum and boron, e.g., have a vapor pressure of 10" Torr at 2100°C, yet platinum evaporates four times faster than boron at this temperature. The same difference in evaporation rate between metals and nonmetals is observed for osmium and carbon. Both have a vapor pressure of 10" Torr at 2650°C but their evaporation rates are as different as in the case of platinum and boron. Apparently the activation energy for evaporation of nonmetals is higher than for metals. Ease of evaporation or a high vapor pressure does not guarantee fast deposition rates even for metals. Although magnesium and zinc are volatile, they are difficult to deposit because they do not condense easily as their closed outer electron shell confers helium-like properties on their gaseous atoms. 

---