The invention relates to a luminescent zeolite of the Faujasite type containing Ce ^,+ ions wherein the Si/Al atomic ratio of the zeolite is in the range 1.0-4.0 and to a method for preparing such a zeolite.

The invention also relates to a luminescent screen and to a low pressure mercury discharge lamp provided with such a screen.

A luminescent zeolite as mentioned in the opening paragraph is known from international patent application WO 95/16759. The known zeolite has excellent properties for use in a luminescent screen. In general a luminescent screen is used to convert excitation energy into radiation within 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 a luminescent screen is generally chosen in dependence of the desired spectral composition of the light radiated by the fluorescent lamp. In many applications it is desirable that the fluorescent lamp radiates only light in the visible part of the spectrum. However, in some applications for instance in lamps for influencing photochemical processes, such as polymerization, lacquer hardening, drying, curing, medical irradiation purposes or suntanning, it is desirable that at least part of the light radiated by the lamp is UV-radiation, more in particular so called UV-A-radiation having wavelengths between 315 and 400 nm. In case the known zeolite is excited by UV- radiation with a wavelength of 254 nm, it emits UV-radiation having an emission maximum between 315 nm and 400 nm. Depending on the amount of Ce ³⁴ ions the quantum efficiency can be very high. The known zeolite is therefore very suitable to be used in the luminescent screen of a low pressure mercury discharge lamp that is used for these photochemical processes. A disadvantage of the known zeolite, however, in case it has been heated to several hundred degrees centigrade (for instance in a process for coating a lamp vessel with a

luminescent layer comprising the known zeolite), is that the quantum efficiency is only high in case the zeolite contains a relatively large amount of Ce ^3"1" ions. In fact the quantum efficiency is only relatively high in case the amount of Ce ⁺ ions per unit cell is higher than 16. This means that in the known zeolite a very large part of the Ce ³⁺ ions does hardly contribute to the luminescent properties of the zeolite so that the Ce ³ * ions incorporated in the zeolite are inefficiently used.

The invention aims to provide a luminescent zeolite that has a relatively high quantum efficiency even in case the zeolite has been heated to several hundred degrees centigrade and its Ce ³⁺ ion content is relatively low.

A luminescent zeolite as mentioned in the opening paragraph is therefore according to the invention characterized in that the zeolite further contains cations S""" wherein n ≥2, and in that the sum of the amount of Cε ³ * ions and the amount of cations S ^B+ per unit cell of the zeolite is at least 16. It was found that the zeolite according to the invention has a relatively high quantum efficiency, also for Ce ³⁺ ion contents far lower than 16 Ce ³⁺ ions per unit cell. It was also found that good results were obtained in case the amount of cations S ^B+ per unit cell of the zeolite is at least 1. Similarly good results were obtained in case the amount of Ce ³ * ions per unit cell is at least 1.

Good results were obtained in case the cations S ⁿ⁺ comprise La ³ * ions. It is possible to prepare a zeolite according to the invention by introducing the cations S ^B+ and the Ce ³⁺ ions into the zeolite at the same time. It has also been found, however, that a comparatively high quantum efficiency was obtained in case the zeolite according to the invention was prepared by a method comprising the following successive steps introducing cations S ^D+ into a zeolite of the Faujasite type wherein the Si/Al ratio is in the range of 1.0-4.0 by means of ion exchange, heating the zeolite, introducing Ce ³⁺ ions into the zeolite by means of ion exchange, and heating the zeolite.

It is also possible for part of the cations S ⁰⁺ to be rare earth ions emitting

in the visible region of the spectrum, such as Tb ³⁺ or Eu ³ *. When such a zeolite is excited by 254 nm radiation, at least part of the excited Ce ³ * ions transfers its excitation energy to the rare earth metal ions emitting visible radiation. Thus such a zeolite emits visible radiation as well as UV-A radiation in case the energy transfer is not complete. The following is a typical example of the preparation of a luminescent zeolite according to the invention.

2 gram of a zeolite of the Faujasite type and approximately of formula Na _M . (Al _M. Si _ιrjS. ,O _jM ) ^,,, 264 H ₂ O was suspended in 18.24 ml H ₂ O. The pH is adjusted to ca. 5 with 0.1 n HCI. To this suspension 1.76 ml of aqueous LaCl ₃ (IM) was added. The resulting mixture is refluxed for 24 hours. The solid is filtered off, washed with H ₂ O (3 x 20 ml), and dried at 120°C in vacuo. The resulting powder is calcined under oxygen at 600 °C for 2 hours (heating-up rate l °C/min.) After cooling the powder is suspended in 18.24 ml H ₂ O, to which 1.76 ml of IM CeCl ₃ are added. The mixture is refluxed for another 24 hours, the solid filtered off, washed with H ₂ O (3 x 20 ml) and dried at 60°C in air. The material obtained is then calcined under oxygen (600°C for 2 hours; heating-up rate 1 ^β C/min) and finally reduced under N ₂ /H ₂ (95:5) for 1 hour (heating-up rate l°C/min). The approximate composition of the reaction product was Na _$.9 La _l} Ce _n (AlO2) _β6.9 (SiO2) _ια5 . _I .264 H ₂ O.

Table I shows the optical properties of a zeolite luminescent material containing Ce ³ * ions (the known zeolite), of a zeolite material according to the invention containing a ³ * ions and of some known luminescent materials having an emission maximum between 300 nm and 400 nm. Both the known zeolite as well as the zeolite according to the invention had been subjected to a heat treatment wherein they were heated to several hundred degrees centigrade. UV radiation with a wavelength of 254 nm was used as excitation energy. More than 90% of this radiation was absorbed. In the table λ^.. is the wavelength for which the emission of the luminescent material shows a maximum and η is the quantum efficiency. It can be seen that the quantum efficiency of the zeolite material containing Ce ³ * ions (the known zeolite) is comparable to that of the non-zeolite luminescent materials. However, the same is true for the zeolite according to the invention despite the fact that its Ce ³ * ion content is relatively low. It can also be seen that the X^ of the zeolite according to the invention differs slightly from that of the other zeolite.

TABLE I

Luminescent material > (nm) η(%)

Na,.. ₉ Ce _l6 (AlO2) _M . ₉ (SiO2) _l05 . ₁ .264 H ₂ O 365 88

Na,.,La,jCe„(AlO2) _M . ₉ (SiO2) _JO i.,.264 H ₂ θ 358 81

SrB ₄ O ₇ :Eu 368 86

(Sr,Ba) ₂ MgSi ₂ O ₇ :Pb 365 70

BaSi ₂ O ₅ :Pb 350 76

In a typical example of a zeolite wherein part of the cations S"* are rare earth ions emitting in the visible region of the spectrum Na _{2l 7} La„ ₉ C j _$ Tb _{3 7} (A- H _j O was prepared by incorporating the La ³ * ions first, then heating the zeolite and thereafter incorporating the Ce ³ * ions and the Tb ³ * ions. After that the zeolite was dried at 60 C and heated at 600 C. When zeolite that had been dried but had not yet been subjected to the heat treatment was excited with 254 nm radiation, the absorptivity was found to be 77.7 %. The quantum efficiency was found to be 83.2 %. Only a neglectable amount of visible radiation was emitted; practically all the radiation emitted by the zeolite was in the UV-A. After the zeolite had been subjected to the heat treatment its absorptivity was found to be 67.3 %. The quantum efficiency of the terbium emission is 20.5 % and the quantum efficiency of the cerium was 38.4 % .