Cadmium vapor pressure

The work of this laboratory extends the defect treatment to intermetallie compounds. The experiments measure simultaneously both the cadmium vapor pressure and the composition at equilibrium for a series of only slightly different alloy compositions. The precision and the relative accuracy of the measurements are high the absolute values suffer from any starting composition uncertainty and from errors in the absolute vapor pressure of cadmium as determined by other techniques. The experimental method is described elsewhere in this symposium (6). It has proved possible to infer the concentration and identity of lattice defects by analyzing the experimental data following the analytical techniques described below. 

The technique requires the measurement of some property which is proportional to a product concentration—e.g., pH, color, or electrical conductivity. In the cerium-cadmium system the cadmium vapor pressure is one such measurable property. 

Once the principal net reaction and equilibrium constant are established, standard total free energies, entropies, and heat contents become corollary. In the case of the cadmium vapor pressure one can therefore calculate total thermodynamic quantities from the established reactions as well as partial thermodynamic quantities from the vapor pressures directly. 

Initial measurements on CeCd 4 r), as expected, indicated a continuous variation of cadmium vapor pressure with composition as is shown in the upper curve of Figure 1. However, the system soon demonstrated distinct, discontinuous vapor pressure-composition relationships, as illustrated by the remaining curves of Figure 1 and as shown in greater detail and with additional data in the remaining figures. We have designated these secondary structures as microphases. ... 

Table I. Cadmium Vapor Pressures for CeCd 45 Structures in Equilibrium with Other Phases at 639° C.

If two vapor pressures are indicated for a single composition, the structures must be different. Any total composition is made up of parent structure plus defects. If the lines cross, the total compositions and cadmium vapor pressures have become equal, but the cerium vapor pressures, total free energies, and structures in the two microphases are different. 

Thermodynamics. Although we have very accurate relative data for the cadmium vapor pressures, we cannot present equally accurate thermodynamics values for the commonly used standard formation reactions. There are four main sources of difficulty ... 

Since we do not have cadmium vapor pressure across the whole cerium-cadmium system, some standard state other than the pure metal would have to be used for cerium. 

Assume that the system is being used to deposit cadmium onto the wafers. The inlet gas is primarily He, which carries a 1% trace of monatomic Cd vapor. The inlet mixture temperature is at the nominal wall temperature of T — 800°C. The cadmium vapor reacts to form a film on the lower-temperature deposition zone with a sticking coefficient of Y =0.8. All other sections of the reactor walls are presumed to be chemically inert. The process is intended to run at a nominal reduced pressure of p = 0.05 bar. 

Dimethylcadmium is a colorless liquid mp — 4.5°C, bp 106°C. It fumes upon exposure to air but does not inflame, and decomposes sluggishly in water. It can be stored indefinitely at 0°C in darkness (light sensitive) under an inert atmosphere, or under its own vapor pressure, in a storage flask such as those in Fig. 4. As a result of the weak acceptor character of dimethyl-cadmium, only in very few cases are stable coordination complexes formed with donor ligands its chemical and physical properties have been fully reviewed.13 H NMR (C6D6, rel. Me4Si, 5 ppm) — 0.53(s). 

Mercury is directly below cadmium in the periodic table, but has a considerably more varied and interesting chemistry than cadmium or zinc. Elemental mercury is the only metal that is a liquid at room temperature, and its relatively high vapor pressure contributes to its toxicological hazard. Mercury metal is used in electric discharge tubes (mercury lamps), gauges, pressure-sensing devices, vacuum pumps, valves, and seals. It was formerly widely used as a cathode in the chlor-alkali process for the manufacture of NaOH and Cl2, a process that has been largely discontinued, in part because of the mercury pollution that resulted from it. 

At a temperature of about 1050K and a pressure of 1 bar, the cadmium vaporizes, thus in the reduction process of CdO,... 

The metals zinc and cadmium should be avoided because of their high vapor pressures. Metals that include zinc and cadmium alloys such as brass (copper and zinc) and some silver solders (cadmium) should also be avoided for the same reasons. It is possible to obtain cadmium-free silver solder and brazing materials that use tin, lead, and indium for vacuum use. Some steel screws are cadmium-coated and also must be avoided. 

Defect equilibria in intermetallic compounds are inferred from measured changes of vapor pressure with composition and from other experimental information. Equilibria analogous to those in aqueous solution are found in dissociation, com-plexing, and random solution other equilibria connected with the ordering of defects show a distinctly intermetallic flavor. Techniques for calculating the equilibria are described. Cerium-cadmium phase information is collected. 

The existence of structures approximating CeCd28 and CeCd19 and the liquidus for equilibrium with CeCd28 are based on unpublished preliminary studies. The existence of these high cadmium compounds seems clear-cut, since the system showed different two-phase equilibrium pressures on either side of each compound and a continuous variation of vapor pressure with composition within each phase... 

At a selected alloy temperature the vapor pressure of cadmium is determined as a function of alloy composition the cerium solvent has a negligible vapor pressure. The alloy, located in one leg of a sealed inverted U-tube, is subjected to various specific pressures of cadmium from a supply of pure cadmium at selected temperatures in the second leg of the tube. The U-tube is freely suspended at its midpoint and connected to a balance, so that the transfer of cadmium from one leg of the tube to the other can be measured. This gives information as to the change in alloy composition and phase equilibrium. 

A second procedure was adopted for some of the points in run III. When the system was thought to be near equilibrium, the cadmium condensate temperature was allowed to drift at a rate of approximately 0.002 degree per minute in a direction which would tend to decrease the rate of cadmium transfer between alloy and condensate. A reversal of the direction of drift of alloy weight then indicated that the equilibrium vapor pressure of the alloy had been observed. 

The absolute vapor pressure of cadmium is not as well known as the relative values. 

Similarly, although there is a maximum difference in cadmium partial molal free energy of a little less than 2 kcal. per mole between the extrapolated upper structure, a, of Figure 1 and the most stable observed microphases, the difference in total molal free energy is less than 10 cal. per mole. (The calculation is based on the path 133 to 209 with assumed two-phase equilibrium pressures of cadmium. This path closes on microphase H.) The difference in these two values emphasizes that the vapor pressure measurements have reflected the concentration and bonding of cadmium species which are minor fractions of the total cadmium present and which did not much alter the average bonding of the system. 

Herasymenko, P., Acta Met. 4, 1 (1956) Vapor Pressure of Cadmium over Alpha Ag-Cd Alloys and Alpha and Beta Au-Cd Alloys, Office of Ordnance Research Project No. 568, Final Report submitted to Box CM, Duke Station, Durham, N. C. 

Somorjai, G. A., Vapor pressure and solid-vapor equilibrium of CdSe (Cadmium Selenide), J. Phys. Chem., 65, (1961), 1059-1061. Cited on pages 269, 270, 464. 

Cd has been emitted in greatly increased quantities after 1945 in the form of dusts and aerosols into the atmosphere, effluent into freshwater, and as solids from anthropogenic industrial activities (Stoeppler, 1991). Cd has a relatively high vapor pressure. The vapor is oxidized quickly to produce cadmium oxide in the air. When reactive gases or vapor, such as carbon dioxide, water vapor, sulfur dioxide, sulfur trioxide or hydrogen chloride, are present, the vapor reacts to produce cadmium carbonate, hydroxide, sulfite, sulfate or chloride, respectively. These salts may be formed in stacks and emitted into the environment (WHO, 1992b). 

Cadmium amide, Cd(NH3)g, is thermally decomposed in a vapor pressure eudiometer (see Part I, p. 102) at 180 °C while repeatedly removing measured amounts of NH3. The evolution of NH3 ceases after about 36 hours. The Cd3N3 product decomposes if the temperature is raised higher. 

Gaseous discharge lamps which contain internal electrodes also can serve as sources for atomic absorption. They are variously called arc lamps, spectral lamps, vapor lamps, and by the name of the manufacturer, such as Osram lamps and Philips lamps. Gaseous discharge lamps contain an inert gas at low pressure and a metal or metal salt. They are especially suited to metals of relatively high vapor pressure, such as the alkali metals and some other metals such as mercury, cadmium, and lead. 

Detailed data have been presented for dicyclohexylammonium nitrite [31], which is one of the most effective of the vapor-phase inhibitors. This substance is white, crystalline, almost odorless, and relatively nontoxic. It has a vapor pressure of 0.0001 mmHg at 21°C (70°F), which is about one-tenth the vapor pressure of mercury itself. One gram saturates about 550m (20,000ft ) of air, rendering the air relatively noncorrosive to steel. The compound decomposes slowly nevertheless, in properly packaged paper containers at room temperature, it effectively inhibits corrosion of steel over a period of years. However, it should be used with caution in contact with nonferrous metals. In particular, corrosion of zinc, magnesium, and cadmium is accelerated. 

---