I&EC REPORTS & COMMENTS dare Earth lechnology Goes Commercial Acoustically Augmented Diffusion Characterized
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Technology developed in the Manhattan Project, and more recently in laser research, has resulted in a rareearth phosphor said to produce a brighter color television picture with a truer red and a variety of complementary colors hitherto unattainable with conventional red, green, and blue phosphors. Although the uses of rare earths have been extremely limited, General Telephone & Electronics believes that this development marks the beginning of a definite trend to further industrial use of the rare earth elements (lanthanides). These elements have application in several fields such as
glass polishing compounds, laser glass, camera lens glass, and as control materials for nuclear reactors, hut their overall use has been somewhat limited. Because of the great similarity of the lanthanide elements, in their common tripositive oxidation state, their separation from each other has long posed a problem to potential users. Accordingly, rare earths were used as combinations of these metals rather than as individual elements. Prior to 1945, individual lanthanides could be separated into pure form by long drawn out processes such as fractional crystallization, precipitation, or decomposition.
Figur6 1. Relative intensify of thc emitted light from thc two leading redphosphors wed for color television tubes. Figure 2. Thc coIor comdinatcs for the uariow possible phosphors used in color television tubes. Thc l o w left “apex” of fhc curue cone-
The first of these methods depended on slight differences in the solubilities of various salts of these elements, while the fractional precipitation and decomposition methods were based on the slight basicity differences be. tween adjacent tripositive ions. The most effective tool presently available for the separation and purification of the lanthanide elements was developed during 1945 as part of the overall effort devoted to the Manhattan Project. The method is that of ion exchange using synthetic, polymeric, organic resins containing sulfonic or carboxylic
sponds to blue emission, thc upper apex to t hgrccn, and the lower right to the yeIIowhnd h i g h woacIengthr. MiXing matm’als on thc cwm or within th Jidd of thc diagram results in phosphor cmnbinntionr of presm’bed cmissioc pmperficr. VOL 5 6
NO. 1 2
DECEMBER 1964
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I&EC R E P O R T S
groups, the hydrogens of which are replaceable by other cations. Lanthanide ions are fixed on these resins when their aqueous solutions are passed slowly through long adsorption columns. Complexing agents, such as dilute solutions of ammonium citrate-citric acid mixtures, are used to elute the elements from the adsorptive columns. -4s a result, it is now possible to prepare most of these metals in a high state of purity. The lighter lanthanides can be obtained in yields greater than 99% by reduction of the anhydrous trichlorides with calcium in an argon atmosphere. Elements such as europium, yttrium, and samarium can be produced from their fluorides and bromides, with high temperatures, and reducing agents other than calcium. As described by A. K . Levine and F. C. Palilla of the GT&E Laboratories, the new phosphor, a solid solution of europium and yttrium vanadates, is claimed to be superior in both color and brightness to silver activated zinc-cadmium sulfide which until now has been the mainstay of cathode ray excited phosphors and has been used almost universally in picture tubes for color television. Yttrium ortho-vanadate, YVO4, is white in body color and crystallizes in the tetragonal system with a zircon structure. In this structure the vanadium atoms are tetrahedrally coordinated with oxygen atoms, while each yttrium ion is surrounded by eight oxygen neighbors arranged in two groups of four equidistant atoms. The europium tripositive ion with the same valence as the yttrium ion, has an ionic radius of 1.13 A as compared to yttrium’s radius of 1.06 A. I t is thus possible, by the proper admixture of europium, to synthesize YVO4 in such a manner that a desired proportion of yttrium ions are substituted by europium ions. The phosphor contains typically 1 europium ion to 19 yt10
trium ions or about 5 atom ’%. The cathodoluminescent spectrum of (Y,Eu) VO4 (see Figure 1) shows that the light emitted from it, is concentrated in an intense, relatively narrow band peaking at 619 nanometers while the luminescence from (Zn,Cd)S:Ag, on the other hand, is spread over a very wide wavelength interval. The emission from the ZnCd phosphor starts somewhere in the yellow-green region of the spectrum and extends into IR region. Although it peaks in the same region as the europium activated phosphor, the latter has a redder aspect since an appreciable portion of the sulfide wavelength is on the short side where the visibility factor is large. Calculation of the color coordinates (a,b,e, Figure 2) of the emission of (Y,Eu)V04 shows that the area under this curve exceeds that area under the sulfide curve. This difference in areas is said to account for the greater brightness or luminosity claimed for the vanadate phosphor. Further examination of these emission curves shows that the vanadate phosphor has the appearance of a pure spectral red corresponding to 612 nanometers. By comparison the sulfide phosphor is said to be more orange. The triangle formed by joining the coordinate points representing the blue and green emitting phosphors to the red is larger for the vanadate phosphor giving rise to a wider range of derived colors. While the spacing between the coordinate points of the vanadate and sulfide is not large on the diagram, these points are said to lie in a region where the eye is very sensitive to color difference and the apparently small shift adds significantly to proper color rendition. Companion investigations carried out with other orthovanadates show that gadolinium and lutetium orthovanadates are also efficient hosts for red cathodoluminescence from europium, with spectral output and relative emission intensities from
INDUSTRIAL AND ENGINEERING CHEMISTRY
these materials similar to those of the yttrium compound.
ACOUSTIC DIFFUSION The limiting rates imposed by diffusive transport on many processes may be increased by inducing acoustic vibrations of the diffusion medium from an external source. This idea has been attractive for some time but there doesn’t seem to have been any consistent attack on quantitative description of acoustically augmented diffusion until recently. Subjecting a diffusion medium to sonic vibrations has, in most cases, involved single frequencies, a series of harmonics, or at best, a narrow spectrum of frequencies. G. A. Kardashev [Inzh.-Fiz. Zh., Akad. Nauk Belorussk, SSR 7 ( 7 ) ) 96 (1964); CA 61, 10068b (1964)l has studied diffusive processes under the influence of a sonic field and, with an assumed model of the liquid medium, concluded that the augmentation of thermal diffusion follows when there is resonance between the thermal vibrations and the induced acoustic vibrations. T o make the augmentation appreciable, it is necessary to accommodate the entire spectrum of thermal vibrations. The greatest effect of acoustic superimposition occurs, therefore, when the induced sonic vibrations cover a very broad spectrum. Kardashev’s liquid model assumes quasicrystalline heterogeneities of different dimensions in the liquid which are at all times in dynamic equilibrium with the surrounding medium.
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CORRECTIONS
In the September 1964 issue of I & E C , p. 27, Table 11, Formula 4 should read: CFa( CFzhCF3. In the July 1964 issue, p. 66, paragraph 1, the structural description of phenol should indicate that the ortho and para positions are more reactive with respect to formaldehyde crosslinking than are the meta positions.
