Luminescent alkaline earth orthosilicate, luminescent screen provided with such a silicate and low-pressure mercury vapour discharge lamp provided with such a screen

Abstract: Luminescent alkaline earth metal orthosilicate activated by praseodymium, defined by the formula M 2-3/2x-1/2y Pr x M′ y SiO 4 in which M is at least one of the elements Ca and Sr of which up to 50 mol % may be replaced by Ba and/or of which up to 20 mol % may be replaced by Mg, in which M′ is at least one of the elements Li, Na and K and in which 0.004 ≦ x ≦ 0.10 and 0 ≦ y ≦ 0.10.

Description: The invention relates to a luminescent alkaline earth metal orthosilicate activated by a rare earth metal. The invention also relates to a luminescent screen provided with such a silicate and to a low-pressure mercury vapour discharge lamp provided with such a screen. It is known from British Patent Specification 544,160 that bivalent europium can be used as an activator in orthosilicates of calcium, barium and strontium. Dependent on the selected alkaline earth metal or alkaline earth metal mixture, emission in the green, yellow or blue part of the spectrum occurs upon excitation by ultraviolet radiation. United States Patent 3,260,675 describes the activation of orthosilicates of calcium and magnesium by terbium. Line emission in the green part of the spectrum takes place upon excitation by ultraviolet radiation. The Patent states that the terbium may be partly replaced by other rare earth metal activators, for example, samarium or dysprosium. Activation of Ca₂SiO₄ by dysprosium is known from United States Patent 4,495,085. The luminescent material is intended for use in cathode ray tubes and emits white light upon excitation by an electron beam. The activation of Ca₂SiO₄ by cerium is known from the book by F.A. Kröger, "Some aspects of the luminescence of solids", 1948, page 288. The luminescence colour is blue. It is an object of the invention to provide efficiently luminescing materials which are particularly suitable for use in low-pressure mercury vapour discharge lamps for UV-irradiation purposes having a germicidal activity. Such lamps are used, for example, for disinfecting air, water and medical instruments. According to the invention a luminescent alkaline earth metal orthosilicate activated by a rare earth metal is characterized in that the silicate is activated by Pr³⁺ and is defined by the formula M 2-3/2x-1/2y Pr x M′ y SiO 4 in which M is at least one of the elements Ca and Sr of which up to 50 mol % may be replaced by Ba and/or of which up to 20 mol % may be replaced by Mg, in which M′ is at least one of the elements Li, Na and K and in which 0.004 ≦ x ≦ 0.10 and 0 ≦ y ≦ 0.10. It has surprisingly been found that the praseodymium exhibits an efficient band emission in the ultraviolet part of the spectrum upon excitation of the luminescent silicates according to the invention by 254 nm radiation (mercury resonance line). A limited quantity of the elements Ca and Sr denoted by M may be replaced by Ba (50 mol % maximum) and/or by Mg (20 mol % maximum). There is insufficient luminescence at larger quantities of Ba and Mg. The elements Li, Na and K denoted by M′ may ensure charge compensation as monovalent cations at the partial replacement of bivalent Ca and/or Sr by trivalent Pr. The praseodymium content x is chosen within the abovementioned limits. If x is chosen to be smaller than 0.004, materials are obtained which luminesce insufficiently due to a too small absorption of the exciting radiation. For values of x exceeding 0.10 there is also a too weak luminescence due to concentration quenching. As stated hereinbefore, the lower limit for the content y of the elements Li, Na and K denoted by M′ is set at y = 0 (no charge compensation). In fact, it has been found that efficiently luminescing materials are also obtained without charge compensation. In connection with the charge compensation, the maximum value of the content y follows from the maximum value of the praseodymium content x and is equal thereto. The upper limit of the content y is therefore 0.10. It also follows from the above-mentioned formula that the concentration y of a charge compensator which is present may differ from the praseodymium concentration x in luminescent silicates according to the invention. In such cases efficiently luminescing materials are also obtained. A preferred embodiment of a luminescent silicate according to the invention is characterized in that 0.01 ≦ x ≦ 0.03 and y = x . Optimum luminescent materials are realised with such relatively low praseodymium concentrations while using equimolar quantities of praseodymium and charge compensator. The luminescent silicates according to the invention may be obtained by a solid-state reaction at a high temperature. The starting material is a mixture of the composite oxides or of compounds yielding these oxides at elevated temperatures (for example, carbonates). Small deviations (up to 10 mol %) from the stoichiometry of the compound to be formed are admissible in the starting mixture. Generally, a small excess of one or more of the composite elements and/or a flux may promote the reaction. The starting mixture is heated once or several times for some time at a temperature between 1200 and 1500 o C, preferably in a weakly reducing atmosphere in order to maintain or possibly bring the praseodymium in its trivalent state. Embodiments of luminescent silicates according to the invention, suitable for use in low-pressure mercury vapour discharge lamps, will now be further described with reference to the accompanying drawing and a number of examples of preparation. In the drawing: Fig. 1 shows a low-pressure mercury vapour discharge lamp in a longitudinal section, comprising a luminescent screen provided with a Pr³⁺ activated silicate according to the invention; Fig. 2 shows the spectral energy distribution of the radiation emitted by a Pr³⁺ activated silicate according to the invention under 254 nm excitation radiation. The low-pressure mercury vapour discharge lamp according to Fig. 1 has a tubular glass wall 1. Electrodes 2 and 3 between which the discharge is maintained during operation are arranged within the lamp and at its ends. The lamp contains a small quantity of mercury and a rare gas as a starter gas. The wall 1 serves as a support for a luminescent layer 4 which comprises a Pr³⁺ activated silicate according to the invention. The wall 1 and the luminescent layer 4 form together a luminescent screen. The layer 4 may be provided on the wall 1 in a conventional manner, for example, by means of a suspension which comprises the luminescent material. Example 1. A mixture was made of 10.217 g of CaCO₃ 1.712 g of SrCO₃ 3.485 g of SiO₂ 0.043 g of Li₂CO₃ 0.191 g of Pr₂O₃ This mixture was heated in a furnace for 1 hour at 1400 o C in a weakly reducing atmosphere (a mixture of N₂ and several vol.% of H₂). After cooling the firing product was ground. Subsequently the firing product was again heated for 1 hour at 1400 o C in the same atmosphere and ground again after cooling. A luminescent silicate activated by Pr³⁺ defined by the formula Ca 1.76 Sr 0.2 Pr 0.02 Li 0.02 SiO 4 was thus obtained. Under 254 nm excitation radiation the quantum efficiency was 51% and the UV absorption was 80%. Fig. 2 shows the emission spectrum of this silicate under 254 nm excitation radiation. The wavelength λ (in nm) is plotted on the horizontal axis and the emitted radiation energy E (in arbitrary units is plotted on the vertical axis. The maximum of the emission spectrum is at 284 nm in the UV-B range (280-315 nm). Examples 2 to 25. In a corresponding manner as described in Example 1 a number of Pr³⁺ activated silicates according to the invention was prepared. The Table below states for these silicates the formula, the quantum efficiency Q (in %), the UV absorption A (in %) of the 254 excitation radiation, the wavelength λ max (in nm) at which the emission is maximum and E₁₂₋₈₈ (in nm). The reference E₁₂₋₈₈ indicates the width of the uninterrupted wavelength range within which 76% of the total radiation energy is emitted. The other 24% of emitted energy are found outside this range, evenly distributed over 12% at smaller wavelengths and 12% at larger wavelengths. No. Formula Q(%) A(%) λ max (nm) E₁₂₋₈₈(nm) 2 Ca 1.97 Pr 0.02 SiO₄ 25 67 283 67 3 Ca 1.992 Pr 0.004 Li 0.004 SiO₄ 38 39 271 56 4 Ca 1.988 Pr 0.006 Li 0.006 SiO₄ 46 45 274 60 5 Ca 1.98 Pr 0.01 Li 0.01 SiO₄ 36 63 273 60 6 Ca 1.968 Pr 0.016 Li 0.016 SiO₄ 51 70 275 62 7 Ca 1.96 Pr 0.02 Li 0.02 SiO₄ 49 72 273 63 8 Ca 1.96 Pr 0.02 Na 0.02 SiO₄ 41 58 271 61 9 Ca 1.96 Pr 0.02 K 0.02 SiO₄ 43 58 271 61 10 Ca 1.95 Pr 0.02 Li 0.04 SiO₄ 25 82 273 64 11 Ca 1.92 Pr 0.02 Li 0.10 SiO₄ 26 82 272 63 12 Ca 1.875 Pr 0.05 Li 0.10 SiO₄ 34 86 273 63 13 Ca 1.80 Pr 0.10 Li 0.10 SiO₄ 29 87 282 63 14 Ca 1.66 Sr 0.3 Pr 0.02 Li 0.02 SiO₄ 42 82 284 68 15 Ca 1.46 Sr 0.5 Pr 0.02 Li 0.02 SiO₄ 51 77 283 66 16 Ca 0.98 Sr 0.98 Pr 0.02 Li 0.02 SiO₄ 31 67 281 68 17 Ca 1.76 Ba 0.2 Pr 0.02 Li 0.02 SiO₄ 47 80 284 66 18 Ca 0.98 Ba 0.98 Pr 0.02 Li 0.02 SiO₄ 34 55 284 67 19 Ca 1.76 Mg 0.2 Pr 0.02 Li 0.02 SiO₄ 50 73 281 64 20 Ca 1.36 Sr 0.2 Ba 0.2 Mg 0.2 Pr 0.02 Li 0.02 SiO₄ 53 68 283 67 21 Ca 0.96 Sr 0.4 Ba 0.4 Mg 0.2 Pr 0.02 Li 0.02 SiO₄ 35 63 282 67 22 Ca 1.16 Sr 0.4 Mg 0.4 Pr 0.02 Li 0.02 SiO₄ 31 56 285 68 23 Sr 1.97 Pr 0.02 SiO₄ 30 64 270 61 24 Sr 1.96 Pr 0.02 Li 0.02 SiO₄ 39 53 269 59 25 Sr 0.98 Ba 0.98 Pr 0.02 Li 0.02 SiO₄ 35 34 265 56

Claims: A luminescent alkaline earth metal orthosilicate activated by a rare earth metal, characterized in that the silicate is activated by Pr³⁺ and is defined by the formula M 2-3/2x-1/2y Pr x M' y SiO₄ in which M is at least one of the elements Ca and Sr of which up to 50 mol % may be replaced by Ba and/or of which up to 20 mol % may be replaced by Mg, in which M' is at least one of the elements Li, Na and K and in which 0.004 ≦ x ≦ 0.10 and 0 ≦ y ≦ 0.10. A luminescent silicate as claimed in Claim 1, characterized in that 0.01 ≦ x ≦ 0.03 and y = x . A luminescent screen comprising a support having provided thereon a luminescent silicate as claimed in Claim 1 or 2. A low-pressure mercury vapour discharge lamp provided with a luminescent screen as claimed in Claim 3.

Inventors: SMETS BRUNO MARIA JEAN, VERLIJSDONK JOHANNUS GODEFRIDU, SMETS, BRUNO MARIA JEAN, VERLIJSDONK, JOHANNUS GODEFRIDUS
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1990
Main classification: H01J61
IPC codes: C09K11, H01J61