Luminescent mixed borates based on rare earths

Details Luminescent mixed borates based on rare earths Luminescent mixed borates based on rare earths

1988-02-24

1990-08-07

Cooper, Jack

Compositions

Inorganic luminescent compositions

Compositions containing halogen; e.g., halides and oxyhalides

C09K 1178

Patent

active

049466216

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to new mixed borates of rare earths, to their preparation and to their use, principally as luminophores for fluorescent lighting
It is known that luminophores based on rare earths are employed in fluorescent tubes for lighting. These fluorescent tubes use rays emitted by mercury vapor under low pressure, which is particularly intense in the short ultraviolet for wavelengths of about 254 nm. The luminophores function to convert this ray into visible light. Generally mixtures of luminophores emitting in wavelengths corresponding to green, red or blue are employed. The conditions which must be filled by the luminophores to give a satisfactory light are known; R. M. Leskela, Rare Earths Spectroscopy, World Scientific, pp. 617-628, 1985.
Luminophores based on rare earths provide yields of light superior to those of transition elements such as alkaline earth chloro-phosphates doped with manganese. However, their production cost limits their use. Efforts are directed principally to obtaining less burdensome red or green luminophores than those actually used, because mercury itself emits in the blue.
The most widely employed green luminophore is an aluminate of a formula approaching Ce.sub.0.67 Tb.sub.0.33 MgAl.sub.11 O.sub.19 ; terbium is the luminescent element and cerium is a sensitizer which absorbs mercury rays and transmits the absorbed energy to the terbium.
The most widely used red luminophore is yttrium oxide doped with europium.
For these two luminophores, the preparation temperatures are high; greater than 1500.degree. C. for the aluminate.
A reduction in the production cost of the luminophores can be realized either by reducing the amount of rare earths (particularly europium, terbium and yttrium), or by reducing the preparation temperature.
It has already been proposed to use, as luminescent substances, rare earth borates. Thus, German patent application 2.410.134 envisages the use of a mixed orthoborate of the type, M.sub.3 Ln.sub.2 (BO.sub.3).sub.4, M being an alkaline earth metal and Ln, a rare earth. These products are obtained by heating corresponding metallic oxides and boron oxide, or their precursors, at a temperature of 1150.degree. C.
More recently there has been described luminophores made of mixed borates of the type MgLnB.sub.5 O.sub.10 that are obtained by heating corresponding oxides or their precursors at a temperature of 1000.degree. C. for 15 hours, see B. Saubat et al., Mat. Res. Bull., Vol. 16, pages 193-198, 1981. These mixed borates permit the use of small amounts of terbium relative to cerium, in comparison with the aluminate.
There has now been discovered the existence of a new family of mixed borates of the type MLnB.sub.9 O.sub.16, which exhibits principally the advantage of being able to be prepared, by reaction in the solid state, at a temperature lower than 1000.degree. C. They possess, moreover, good stability during operation of the fluorescent tubes that are produced therefrom.
These new mixed borates are crystalline products which have characteristic x-ray diffraction spectra, distinct from those of various alkaline-earth borates, rare earth borates and known mixed borates. In other words, the new mixed borates of the present invention are defined compounds. On x-ray diffraction, all intense rays are indexed in the hexagonal system. The following table gives for example the parameters for the host lattice (of the type M(II), M(III)B.sub.9 O.sub.16, where M(II) is barium and M(III) is lanthanum, gadolinium or yttrium):


______________________________________ a c M(III) (Angstrom) (angstrom) ______________________________________ La 7.88 15.63 Gd 7.80 15.47 Y 7.78 15.40 ______________________________________
Moreover, the mixed borates of the present invention possess an emission spectrum which is peculiar to them.
The new mixed borates of the present invention exhibit, moreover, the characteristic of being rich in boron oxide. It results from it various advantages, in particular improved stability, a reduction in the amount

REFERENCES:
patent: 4202794 (1980-05-01), Lehman
patent: 4319161 (1982-03-01), Looye et al.
Blasse et al, "J. of Chem. Phys.", vol. 47, No. 6, 1967, pp. 1920-1926.

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