Vapor pressure rare earth elements

Figure 4 shows vapor pressure curves of rare-earth metals[24], clearly showing that there is a wide gap between Tm and Dy in the vapor pressure-temperature curves and that the rare-earth elements are classified into two groups according to their volatility (viz.. Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu, non-volatile elements, and Sm, Eu, Tm, and Yb, volatile elements). Good correlation between the volatility and the encapsulation of metals was recently... 

A search for new efficient techniques of rare earth element separation and purification from calcium is a current problem, as production of high-purity rare earths is of great importance in advanced technology and material science. This problem may be solved by vacuum sublimation of volatile compounds when the difference in vapor pressure of the components present is used. This technique of purification was tested for Nd and Ca separation in vacuum. The well-known volatile and thermally stable dipivaloylmethanates were taken as starting substances. It was found that the addition of pivalic acid to the Nd(dpm)3 + Ca(dpm)2 mixture caused an increase in the separation efficiency and led to pure Nd(dpm)3 in the sublimate . ... 

Raoult s law A law stating that the partial pressure of a solvent over a solution, Psolutioiu b given by the vapor pressure of the pure solvent, P,oivent> times the mole fraction of a solvent in the solution. Asoivent Psolution. solventAsolvent (Section 13.5) rare earth element See lanthanide element. (Sections 6.8 and 6.9)... 

The major breakthrough occurred in 1953 when the Ames Laboratory team (Daane et al. 1953) reported the preparation of samarium, europium and ytterbium in high purity and high yields by the reduction of their oxides with lanthanum metal in a vacuum. With the preparation of samarium metal, finally, 126 years after the first rare earth element was reduced to its metallic state, all of the naturally occurring rare earths were now available in their elemental state in sufficient quantity and purity to measure their physical and chemical properties. The success of this reaction is due to the low vapor pressure of lanthanum and the extremely high vapor pressures of samarium, europium and ytterbium (Daane 1951, 1961, Habermann and Daane 1961). It is interesting to note that this same technique has been the method of choice for the preparation of some transplutonium metals (Cunningham 1964). 

Compounds of the high vapor pressure non-metallic elements and the gaseous elements (X) can be prepared by direct combination of the gas with solid rare earth metals at fairly low temperatures. Many times the reactions do not go to completion because of the formation of an impervious RX coating which prevents or slows down the reaction. For the sulfides and selenides a few milligrams of I2 helps break up this coating and the reaction goes to completion in the matter of a few hours, compared to weeks or months when no I2 is present (Takeshita et al. 1982). 

Table 1 shows some of the physical properties of the more common rare earth elements and, for comparison, those of reactive metals and iron (Beaudry and Gschneidner 1978, Grayson 1985, Bingel and Scott 1973). Figure 1 shows the temperature dependence of the vapor pressure of these elements (Barin 1989). The melting points and vapor pressures of rare earths differ from those of iron to such an extent that problems of dissolution... 

When the boiling points of metallic impurities are much lower than the boiling point of the main metal, they can simply be distilled away in most cases. The rate and the extent of the removal by distillation of these impurity elements depend upon their partial pressures over the main metal/melt. As an example, let the feasibility of distilling magnesium and magnesium chloride from titanium and calcium from the rare earths be considered. In the firstcase, at 900 °C, the pertinent vapor pressure values are P = 4 10-11 torr, PMg = 105 torr... 

Some elements, such as the rare earths and the refractory metals, have a high affinity for oxygen, so vaporization of these elements in a normal vacuum of about 10 4 Pa, would lead to the formation of at least a surface layer of oxide on a deposited film. The evaporation of these elements therefore requires the use of ultra-high vacuum techniques, which can produce a pressure of 10-9 Pa. 

A similar situation exists for alloys where a component pressure is not measurable under the temperatures for measurable vapor pressures of other components. Examples are Hf in Ni-Al-Hf [85]), Cr in Mn-Cr [104], and rare earth (RE) in Mg-RE alloys [105]. The latter study was based on a Knudsen cell without the use of a mass spectrometer. Nonetheless the approach is applicable to a mass spectrometric study. Depending on the alloy system, several approaches can be taken. In some cases the effect of controlled additions of the low-pressure component on a fixed ratio of the measured elements is the only required information [100,102]. Albers et al. [85] and Zaitsev et al. [104] did a Gibbs-Duhem integration to obtain the activities of the low vapor pressure component. Pahlman and Smith [105] assumed Raoultian behavior in the terminal RE-Mg solution and moved across the phase diagram to derive the activity of the RE component in each two-phase region. 

The discussion above reveals that there has been considerable attention paid to Al-based QC and CMA surfaces. Naturally, the question arises as to the surface science of the non-Al-based QCs and CMAs. The two main QC famihes in this category are i-Zn-Mg-RE (rare earth) and i-Cd-Yb. The difflculty in applying surface science techniques to either of these families is that surface preparation in UHV is extremely difflcult due to the presence of the high-vapor-pressure elements... 

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