Oxygen-dominant Phosphors

Oxygen-dominant phosphors are generally produced by solid-state reactions in which the components of the phosphor (mostly oxides) are intimately mixed and fired at 500-1500 °C. The desired phosphor is formed by solid-state reaction below the melting point of the compound during a firing period of 3-16 h. 

The Tamman rule, which states that a measurable rate of change of position of the lattice components begins at two-thirds of the absolute melting point, gives an indication of the necessary firing temperature [5.352], 

The activators are added in the form of oxides, oxalates, carbonates, or other compounds which readily decompose at higher temperatures. 

The oxidation state of the activators can be influenced by the choice of the furnace atmosphere. As in the case of the sulfide phosphors, contamination by Fe, Co, or Ni greatly reduces the quantum yield. 

As well as higher quantum yields, oxygen-dominant phosphors stand out because of their good temperature stability. For most substances the quantum yield only decreases noticeably above 250 °C. 

Y AljGa Ojj is a green phosphor with high brightness that is used in projection 

In the 1990s a breakthrough was achieved in the development of long-lived afterglow phosphors. It was discovered that by co-doping rare earth aluminates, especially strontium, with europium and dysprosium gave phosphors with around ten times the afterglow of copper activated zinc sulfide and also with ten times the 

Strontium thiogallate SrGa S iCe, doped with 4 mol% of cerium, gives a very good blue phosphor 455 nm), which has found to be useful in thin-film electroluminescent devices (see section 3.8.3.2). 

Yttrinm and europium oxides form mixed crystals without any vacancies due to the very similar size of their ionic radii. Y OjiEu is used with around 3 mol% europium to give a very intense red phosphor (/L 612 run) that is used in colour CRTs. 

Zn orthosilicate ZUjSiO iMn is a green phosphor (X 525 nm) used in fluorescent lamps, CRTs and plasma display panels. It demonstrates a low resistance to bum out, is excited by low-energy electrons and shows a linear increase in brightness with electron beam intensity. YjSiOjiCe is a highly resistant blue CRT phosphor used in projection TV tnbes (X 400 60 nm depending on the Ce content). 

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Oxygen-dominant phosphors are made by a solid-state reaction between the intimately mixed components of the phosphor, generally the oxides or oxide forming derivatives, at 500-1500 °C. For instance, Y2Al3Ga20j2 Tb + is made by mixing stoichiometric qnantities of the oxides, containing 5-7 mol% of terbium, with 20% BaFj in ethanol. The mixture is dried and then fired for 2 h at 1500 °C. The resultant flnx is washed with 20% nitric acid. 

For example, in ZnS Cu, if the Cu-cation is not stabilized in the Cu state, it acts as a killer of luminescence. This is particularly true in oxygen-dominated phosphor compositions. This insight has important consequences since the method of preparation thus becomes important. If these cations become stabilized in the wrong valence state, they become energy traps and dissipaters of excitation energy. 

Hersh, H.N., and H. Forest, 1970, Proposed mechanism of cathodoluminescence in some oxygen dominated phosphors, in Proc. Int. Conf. on Luminescence, Delaware, 1969, ed. W. Ferd(North-Holland, Amsterdam) pp. 862-868. 

There are two types of ener transfer from a point in a lattice to a luminescent center. For semi-conducting phosphors like ZnS, migration of electrons in the conduction band, or holes in the valence band, conve3rs the excitation energy to localized luminescent centers. Excltons (bound electron-hole pairs) is another mechanism. For insulators (which are generally oxygen dominated compositions), the excitation energy can be transferred from an excited point in the lattice, or from an excited luminescent center to other unexcited centers in the lattice by Quantum Mechanical resonance. 

However, there is also one other class of host which we have not yet addressed. This class comprises those anions which are not oxygen-dominated. Thus, we find that phosphors can be formed from ... 

A generalized table of oxygen dominated hosts, which is an extension of the classification proposed by Johnson (1966) and DeKalb ( 970), is shown in table 37E.1. The emphasis in our studies has been to evolve analytically useful phosphors with a high degree of reproducibility in their preparation, freedom from interelement effects (see 5.1) and with the best possible limits of detection. In the periodic table shown in fig. 37E.6 the 57 elements enclosed by boxes when converted to an appropriate phosphor host support the luminescence of one or more of the lanthanides down to at least the 100 ppm level. 

Four core samples were selected for carbon and oxygen isotope analysis following identification of the mineralogy under bulk-rock XRD. Only samples dominated by dolomite cement were chosen for isotope analysis, as identified by XRD (Table 3). The samples were crushed to a fine dry powder. Carbon dioxide was extracted from the powdered samples by reaction with 100% phosphoric acid at... 

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