Preparation of Luminescent Materials

Traditionally, luminescent materials are prepared by mix and fire techniques: small grains of reactants (generally with diameters in the pm range or smaller) are thor­oughly mixed (either dry or in a suspension) and heated. The heating temperatures typically exceed 1000 °C. To facilitate the reaction and to obtain luminescent materials of sufficient crystallinity (which generally improves the efficiency of the luminescence

process significantly), a flux agent or a melting salt can be added. This generally re­sults in lower reaction temperatures and improved crystallinity. Such agents can also be used to optimize the grain size of the luminophores, whereby the optimum size of the particles depends on the specific application.

A suitable flux promotes crystallinity of the luminescent material being manufac­tured and also the reactivity, by dissolving at least one of the reactants. Fluxes can be divided into two types: a pure non-volatile liquid, e. g., a molten salt or a molten oxide (melting salt), and a volatile liquid or gas (volatile flux).

A melting salt generally shows no reaction with the starting materials, it is always a melt. One can use large amounts (up to e. g. 30% by weight of the phosphor) and the material does not decompose or evaporate during phosphor synthesis. Examples are Na2MoO4, Na2B4O7, Na2SiO3 or Na4P2O7.

A flux often reacts with the starting material, sometimes a melt, when used in small amounts (e. g. less than 10% by weight of the phosphor) and always shows decomposition or evaporation during phosphor synthesis. Frequently used fluxes are NH4Cl, NH4Br and AlF3.

Reactivity can be improved by using starting materials which decompose during heating, like carbonates or hydroxides. After decomposition (in the examples given CO2 or H2O leave the reaction mixture), one ends up with a much more reactive mixture, as a result of the larger specific surface area.

Moreover, co-precipitation can be used. Using co-precipitation, a reaction mixture where the starting materials are mixed on an atomic scale can be obtained. By dissolving the reactants and co-precipitating them one obtains an intimately mixed starting mixture. Just to mention an example: Y(NO3)3 and Tb(NO3)3 are dissolved in H2O and co-precipitated by adding to excess oxalic acid (2 to 1 on a molar basis), dissolved in hot water. Alternatively, the oxides can be dissolved in hot dilute nitric acid (4 M). The oxalates obtained are converted into the oxides by heating at 800 °C. This method can be used if an insoluble salt can be identified for each of the reaction constituents. Other possible co-precipitation routes involve sulfates or hydroxides.

Spray drying methods can also be used. Here, one first dissolves the reactants in a medium, preferably H2O. Small droplets are transported in a gas stream and heated, resulting in a very fast evaporation of H2O. As a result, an intimate reaction mixture is obtained.

An interesting combination of methods is the polysulfide flux method, which can be used for the preparation of Ln2O2S (Ln: trivalent rare-earth ion)-based luminescent materials (e. g. Y2O2S:Eu or Gd2O2S:Pr) [5.228]. In this method, the mixed oxides of the metals are mixed with excess sulfur and an alkali metal carbonate. On heating, the alkaline carbonate decomposes and reacts with sulfur to form a liquid polysulfide flux. The oxides react with the polysulfide flux to form the oxysulfide. Flux residues can be removed by washing the reaction product in water.

Different routes have to be chosen in cases where the concentration of dopants is very low, e. g. in the ZnS phosphors for CRT application. Here, the dopant concen­trations are of the order of 100 ppm (by mole). Minute amounts of these activator ions can be precipitated onto ZnS grains, e. g. by preparing a suspension of ZnS in

water and adding soluble activator salts. The activator is precipitated onto the ZnS by adding e. g. (NH4)2S.

Some reactions require a reducing atmosphere to incorporate activator ions in an oxidation state which is lower than the maximum oxidation state, e. g. Eu2+, or to prevent oxidation of the host lattice, e. g. in the case of the preparation of ZnS phosphors. This can be achieved by executing the reaction in diluted H2 or in CO. A CO atmosphere is easily obtained by heating graphite grains in a closed vessel, in which a second, smaller, vessel containing the reaction mixture is placed.


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