Preparation of Phosphors

In this section, we will first describe the preparation of phosphors in terms of the parameters involved, followed by measurements usualfy required to ascertain the energy efficiency of the phosphors so- produced, and finally how to make such measurements. 

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In the late fifties, the N. I.H. started the Visiting Program under this, young scientists were invited to work for one or two years at the Institutes under the direction of staff members. Dr. Fletcher actively participated in this program. It was as a Visitor in Dr. Fletcher s laboratory that Dr. Donald L. MacDonald became interested in the preparation of phosphoric esters of carbohydrates, and his work eventually led to the MacDonald method of synthesizing glycosyl phosphates. 

Commercial phosphorus often contains a small quantity of arsenic. This on the treatment with nitric acid is oxidized to arsenic acid, which would contaminate the preparation of phosphoric acid unless removed by hydrogen sulphide. 

The classical Stokes method1 for the preparation of phosphor-amidic acid and its salts entails a three-step procedure with the use of organic intermediates. Ammonium hydrogen phosphor-amidate, NH4HPO3NH2, however, is prepared more conveniently and in higher yield by the simple reaction of phosphoryl chloride with aqueous ammonia. The acid salt is stable and nonhygroscopic, and it is readily converted to the free acid or to other salts of the acid. 

R. Lutz, Preparation of Phosphoric-acid Waste Gypsum for... Building-materials, Zem.-Kalk-Gips 47(12), 690, Dec. (1994). 

Many cases will be encountered during the preparation of phosphors where the activator forms a volatile compound. If we use As3+, Sb3+, Bi3+, Pb2+, T1+ and ZnO or CdO as activators, then the probability is high that volatile compound formation will ensue. This will lead to loss of activator, thereby causing a lower concentration than intended. However, there is also a beneficial side-effect in that the activator becomes better dispersed throughout the host structure when a volatile compound is formed. 

We now return to our phosphor example, BaSi20s Pb. Tliere are at least three (3) methods we can employ in the general preparation of phosphors, these are ... 

In the preparation of phosphors, materials that glow in the dark after being exposed to light ... 

Fig. 199. Preparation of phosphoric acid amides according to Goehrlng and Nledenzu. a) reaction tube h) condensation trap and Storage tube f) glass frit h-i-h stopcocks s) ball and socket joint.

Some of the procedures outlined below have been described previously. " They are designed for the preparation of phosphors that are to be used for quantitative studies of luminescence. Details are presented for the preparation of strontium nitrate, strontium chloride, strontium sulfate, and strontium selenite in a state of high purity for the reduction of the sulfate to the sulfide and the selenite to the selenide and for the conversion of the sulfide and the selenide into typical infrared-sensitive phosphors. 

The advance in the dosimetric properties of phosphors was mostly empirical. More theoretical work on the effects involved in the preparation of phosphors and in the TL emission may lead to more efficient phosphors, hr the present situation it happens that minor changes in the preparation of a phosphor lead to changes by orders of magnitude in the intensity of the dosimetric TL peak (see, for example, Dhopte et al. 1991a). Burlin and co-workers have developed the cavity theory (sect. 3.2.3) to explain the effect of the grain sizes on the sensitivity of TL detectors. Unfortunately, the use of this theory was limited presumably because of additional effects which masked the cavity effect. 

Mitouard, on 12 December 1772, read a paper to the Academy on the residue in the retort in the preparation of phosphorus, which was reported on by Macquer and Lavoisier on 16 December. When it was received is not stated. It was recommended for publication in the Memoires. . . divers Savants but as far as I can find it was not published, and all we know of it is contained in the report. The material was rich in phosphorus, since it inflamed in air, and when burnt in an apparatus which Mitouard described, and with precautions which he describes , it gave more than its weight of phosphoric acid, even when this was concentrated by evaporation. He had described (as Lavoisier was later to do) a simple preparation of phosphoric acid from phosphorus by combining it with the base of nitre (presumably nitric acid). He also discovered what must have been red phosphorus, regarding it as phosphorus itself combined with a portion of white earth . The increase in weight Mitouard attributed to the humidity of the air or to the air itself contained in the vessels in which the combustion was made . 

In the commercial preparation of phosphors, the highly purified host and the required amount of activator are intimately mixed, normally with a flux, such as an alkaU or alkahne earth halide or phosphate, which suppUes a low-temperature melting phase. The flux controls the particle development and aids in the diffusion of the activator into the lattice. This mixture is then fired at high temperature, 1472-2192° F (800-1200°C) on a prescribed schedule in order to develop the desired physical and luminescent properties. 

Because phosphate rock contains Cap2/ hydrofluoric acid is a by-product in the preparation of phosphoric acid. HF is used in aluminum production. 

XEOL of rare earths 445 5.2. Preparation of phosphor hosts 452... 

Various flux materials have been used for the preparation of europium-activated yttrium vanadate phosphor from vanadium and rare earth oxides. These include sodium nitrate and carbonate, and sodium and ammonium vanadates (Martin and Trond, 1967 Kauders, 1967 Yokota et al., 1970). A precipitation method in which the rare earths are precipitated in nitric acidic solution with NH4VO3 is also used in the preparation of phosphors (Martin and Trond, 1967). The precipitate is afterwards calcined to the flnal composition. 

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