Barium vapor pressure

These reactions are thermodynamically unfavorable at temperatures below ca 1500°C. However, at temperatures in the range from 1000 to 1200°C a small but finite equiUbrium pressure of barium vapor is formed at the reaction site. By means of a vacuum pump, the barium vapor can be transported to a cooled region of the reactor where condensation takes place. This destroys the equiUbrium at the reaction site and allows more barium vapor to be formed. The process is completely analogous to that used in the thermal reduction of CaO with aluminum to produce metallic calcium (see Calcium AND CALCIUM alloys). 

The largest use for barium is as a getter to remove the last traces of gases from vacuum and television picture tubes. It is ideal for this use because of its combination of high chemical reactivity and low vapor pressure (28—32). In some cases it is used as powder obtained by vaporization ia an electric arc (33). It can also be used as an aluminum ahoy (see Vacuum technology). 

Experimentally, the compound consisting of the preformed ion and its counter ion (such as barium perrhenate for perrhenate emission) does not produce ions when heated—instead, only neutral species sublime. It must be embedded in a suitable matrix and then heated. The limited experimental evidence collected to date indicates that the ion to be emitted must have significantly greater mobility and/or vapor pressure in this matrix than its counter ion, allowing diffusion of the ion of interest. When the temperature gets sufficiently hot the ion migrates to the... 

Other workers have added THF or H2O to the carrier gas stream in efforts to affect the requisite vapor pressure increase in Ba(/3-diketonate)2 compounds. " The objective of all these experiments was to coordinatively saturate the barium atom in the vapor phase by incorporation of the lone pair electrons on the oxygen atoms of the neutral ligands into the bonding sphere of the metal. At the temperatures demanded for vapor transport, the equilibrium for dissociation of these ligands is rather unfavorable therefore, any anticipated gain will be minimal. 

Figure 14 Plot of vapor pressure as a function of molecular weight for a series of barium bis(alkoxides) and bis(fluoroalkoxides) ...

Alternative processes for preparing metallic americium are the reduction of AmFs with barium vapor in high vacuum at about 1300°C, reduction of AmF4 with calcium, and reduction of Am02 with lanthanum or thorium at about ISOO C in high vacuum. The vapor pressure of americium is much higher than that of lanthanum or thorium, so that pure americium is condensed in the colder parts of the apparatus [K2, L2]. Metallic americium dissolves readily in mineral acids. 

One approach to avoiding intermolecular agglomeration is the additional coordination of the alkaline earth metal ions with neutral ligands [110]. Addition of Lewis bases such as free ligand, tetrahydrofuran, ammonia, or amines to the carrier gas has afforded some improvement in vapor pressure characteristics of the most commonly used strontium and barium MOCVD precursors, Sr(dpm)2 and Ba(dpm)2 [111-119]. These effects may be due to saturation of the Lewis acidic metal centers with the gaseous bases, thereby increasing precursor vapor pressure and stability in a transient fashion. 

---