Luminescent screen including a luminescent zeolite

A luminescent screen includes a luminescent zeolite containing rare earths, such as terbium (Tb), and europium (Eu), and an oxide of a transition metal, such as, molybdenum (Mo), tungsten (W), niobium (Nb), and tantalum (Ta). It was found that a zeolite containing both terbium, and tungsten oxide had an absorbability that was approximately 60 times as high as that of a zeolite containing only terbium, thus increasing the luminescent efficiency of the zeolite.

BACKGROUND OF THE INVENTION 
The invention relates to a luminescent screen comprising a bearer provided 
with a luminescent material comprising a luminescent zeolite containing 
rare earth ions. 
The invention also relates to luminescent zeolites for use in such a 
luminescent screen and a method for preparing such zeolites as well as to 
a low pressure mercury discharge lamp comprising such a luminescent 
screen. 
A luminescent screen as described in the opening paragraph is known from 
international patent application WO 95/16759. In general a luminescent 
screen is used to convert excitation energy into radiation with a certain 
range of wavelengths. The excitation energy can for instance consist of an 
electron beam, X-rays or UV- radiation of relatively short wavelengths. 
Among many other applications luminescent screens find application in 
lamps, particularly in low pressure mercury discharge lamps or fluorescent 
lamps. The excitation energy then mainly comprises UV-radiation of 
approximately 254 nm generated by mercury present in the plasma of the 
fluorescent lamp. The composition of the luminescent screen is chosen in 
dependence on the desired spectral composition of the light radiated by 
the fluorescent lamp. In the known luminescent screen the rare earth ions 
contained in the zeolite are Ce.sup.3+ ions. When the known luminescent 
screen is excited by UV radiation with a wavelength of 254 nm it emits 
light having an emission maximum at a wavelength between 300 and 400 nm. 
Such a luminescent screen can for instance be used very effectively in low 
pressure mercury discharge lamps for use in photochemical processes, such 
as polymerization, lacquer hardening, drying, curing, medical irradiation 
purposes or suntanning. The quantum efficiency of the zeolite in the known 
luminescent screen is very high. Because the rare earth ions are enclosed 
by the zeolite structure the known luminescent screen is also very stable 
in different chemical environments and within a wide temperature range. 
Zeolites are also generally very cheap and environment-friendly. Most of 
the time, however, it will be desirable that the fluorescent lamp radiates 
light in the visible part of the spectrum. Such an emission in the visible 
part of the spectrum can be realized by the incorporation of other rare 
earth ions in the zeolite such as e.g. Tb.sup.3+ ions. When a luminescent 
screen provided with a luminescent material comprising a zeolite 
containing Th.sup.3+ ions is excited by means of UV-radiation with a 
wavelength of 254 nm, visible radiation with wavelengths within the range 
450-650 nm is emitted by the luminescent screen. A problem, however, that 
is associated with many zeolites containing rare earth ions emitting 
visible light upon excitation with UV-radiation with a wavelength of 254 
nm, is that the absorption of the exciting radiation is very low so that 
the amount of visible light that is generated out of the exciting 
radiation is relatively low. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a luminescent screen 
that emits visible light upon excitation by UV-radiation with a wavelength 
of 254 nm, that is stable and generates a relatively large amount of 
visible radiation out of the exciting radiation. 
A luminescent screen to the invention is characterized in that the zeolite 
further comprises a transition metal oxide. It has been found that a 
luminescent screen according to the invention generates visible radiation 
with a relatively high efficiency when excited by UV-radiation with a 
wavelength of 254 nm. 
It has been found that relatively good results were obtained in case the 
number of rare earth ions per unit cell of the zeolite is at least 1. 
Similarly relatively good results were obtained in case the number of 
transition metal ions per unit cell of the zeolite is at least 1. 
More in particular the luminescent screen had a comparably high efficiency, 
in case the zeolite is of the Faujasite type. 
Very good results were obtained for luminescent screens wherein the rare 
earth metal ions comprise Th.sup.3+ ions and/or Eu.sup.3+ ions. 
Very good results were also obtained for luminescent screens wherein the 
transition metal oxide comprises one or more oxides chosen from the group 
formed by molybdenum oxide, tungsten oxide, niobium oxide and tantalum 
oxide. 
It has been observed that the luminescent screen had a comparatively high 
efficiency in case the zeolite was prepared by a method comprising the 
successive steps: 
incorporating rare earth ions into a zeolite by means of ion exchange, 
heat treatment of the zeolite, 
incorporating a transition metal compound into the zeolite, and 
conversion of the transition metal compound into transition metal oxide. 
The incorporation of the transition metal compound can be carried out in a 
solution or from the gas phase. Suitable precursors of e.g. tungsten oxide 
are volatile or soluble species, for instance chlorides, oxychlorides or 
carbonyls (WCl.sub.6, WOCl.sub.4, W(CO).sub.6 and the like).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following is a typical example of the preparation of a zeolite 
according to the invention. 
2 gram of a zeolite of approximate formula Na.sub.86.9 (Al.sub.86.9 
Si.sub.105.1 O.sub.384)*264 H.sub.2 O is suspended in 18.24 ml H.sub.2 O, 
and the pH is adjusted to ca. 5 using 0.1 n HCl. 1.76 ml of aqueous 
TbCl.sub.3 (1M) are added. The resulting solution is refluxed for 24 
hours. The solid is then filtered off, washed with H.sub.2 O (3.times.20 
ml) and dried at 120.degree. C. in vacuo. The resulting powder is calcined 
under oxygen at 400.degree. C. for 1 hour (heating-up rate 1.degree. 
C./minute). The resulting first product was of the approximate formula 
Tb.sub.16 Na.sub.38.9 (Al.sub.86.9 Si.sub.105.1 O.sub.384)*264 H.sub.2 O. 
This corresponds to approximately 16 Tb.sup.3+ ions per unit cell. 
1 gram of the powder is brought into an evacuable glass frit, the bottom of 
which holds a small container with 0.5 gram W(CO).sub.6. The container is 
then held in a warming bath from which W(CO).sub.6 is sublimed through the 
frit into the zeolite at a pressure lower than 0.01 mbar. The temperature 
can range from room temperature to 90.degree. C. When all the W(CO).sub.6 
has vanished from the container, the zeolite is taken from the frit and 
placed into a soxhlet to be extracted with THF for up to 24 hours. A white 
material is obtained, which is suspended in 25 ml of water and mixed with 
5 ml of 30% H.sub.2 O.sub.2 and refluxed for one hour. After filtering 
off, the powder is heated at 600.degree. C. for two hours under an oxygen 
stream, which was bubbled through water before contacting the phosphor. 
The resulting second product was of the approximate formula Tb.sub.16 
(WO.sub.x).sub.6 Na.sub.38.9 (Al.sub.86.9 Si.sub.105.1 O.sub.384)*264 
H.sub.2 O, wherein the value of x is not known. 
It was found that the zeolite containing both Tb.sup.3+ ions and tungsten 
oxide (second product) had an absorbability that was approximately 60 
times as high as that of the zeolite containing only Th.sup.3+ ions 
(first product). Under identical excitation conditions the amount of 
visible light in the spectral range between 400 and 650 nm generated by 
the second product is approximately 20 times as high as that generated by 
the first product. A relatively small amount of tungsten oxide thus raises 
the efficiency of the luminescing zeolite surprisingly much. The emission 
spectra of the first and the second product are practically the same.