The manufacture of UV-emitting SrB4O7:Eu phosphors is a complicated process. Solid-state reaction is difficult due to the large amount of boron-containing reagents required to produce the correct stoichiometry. Boric acid and/or boron oxide reactants will melt at relatively low temperatures, leading to phase segregation and hindering the further reaction needed to produce the phosphor at higher temperatures. Upon cooling the reaction, hard glassy materials are commonly formed. Traditional solid-state syntheses of this phosphor requires repeated cycles of heating and grinding to prepare the phosphor.
A chemical precipitation method is described U.S. Pat. Nos. 4,719,033 and 5,068,055 to circumvent these repeated cycles of heating and grinding, but this method also has its own unique problems. In particular, the method involves adding a SrCO3—Eu2O3 mixture to a concentrated boric acid solution at ˜95° C., whereby an exchange reaction takes place forming (Sr,Eu)B6O10.5H2O and CO2 gas. A carefully controlled excess of the SrCO3—Eu2O3 mixture is added to result in an intimate mixture of (Sr,Eu)B6O10.5H2O and SrCO3—Eu2O3 which is then dried and granulated before being fired in a reducing atmosphere to produce the final SrB4O7:Eu phosphor. However, if the mixture is not intimate enough before firing, the formation of SrB6O10:Eu can occur. SrB6O10:Eu is known to be a very poor UV-emitting phosphor and the presence of this phase drastically reduces the UV emission intensity of the phosphor.
This problem of SrB4O7:Eu phosphor containing the impurity phase SrB6O10:Eu has been encountered in the past and solutions to improve the UV emission intensity of such material have been proposed. U.S. Pat. No. 5,378,398 teaches that mixing SrCO3 or SrF2 or NH4F with such a phosphor material, and then firing said mixture in a reducing atmosphere converts the SrB6O10:Eu phase to SrB4O7:Eu and improves the emission characteristics. In practice, the emission characteristics are dramatically improved, but this treatment results in very sintered, hard cakes of phosphor that are difficult to finish. Even after milling, the particle size of the phosphor can be unacceptably large.