LUMINESCENT SCREEN AND LOW-PRESSURE MERCURY VAPOUR DISCHARGE LAMP COMPRISING SUCH A SCREEN

Abstract: Luminescent screen (1) comprising a luminescent mate­ rial (4) activated by lead or by lead and terbium and/or manganese, the luminescent material having the hexagonal apatite crystal structure and a composition which is defined by the general formula Me a Ln b (AO₄)₆X₂, wherein Me repre­ sents at least one of the elements Ca, Sr, Ba, Mg and Zn, wherein Ln represents at least one of the elements Y and La, and (AO₄) at least one anion from the group (PO₄)³⁻, (SiO₄)⁴⁻ and (BO₄)⁵⁻, and wherein X represents at least one element of the group of oxygen, the halogens and vacancies, and wherein a ≠ 0, b ≠ 0 and 9 ≦ a+b ≦ 10. The material further comprises Gd, which at least partly replaces the Ln.

Description: "Luminescent screen and low-pressure mercury vapour dis charge lamp comprising such a screen." The invention relates to a luminescent screen comprising a luminescent material which has been activated by lead or by lead and at least one of the elements terbium and manganese, the material having the hexagonal apatite crystal structure and a composition which is defined by the general formula MeaLn(AO4)6X2 wherein Me represents at least one of the elements Ca, Sr, Ba, Mg and Zn, Me having partly been replaced by Pb or by Pb and Mn, wherein Ln represents at least one of the elements Y and La, the Tb, if present, replacing a portion of the Ln, wherein (A04) represents at least one anion of the group (SiO4)4- and (BO4)5-, and X at least one element of the group of oxygen, the halogens and vacancies, and wherein a 6 o, b 6 0 and 9 ; a + b 410. In addition, the invention relates to a low-pressure mercury vapour discharge lamp comprising such a luminescent screen. Luminescent silicates of an alkaline earth metal and of Y and/or La are known from Netherlands Patent Application 7005708 (PHN 4817), which silicates have the apatite crystal structure. More specifically, the said Patent Application describes Pb and also Pb and Mn-activated silicates which are defined by the above-mentioned general formula, for example Me2Ln8(SiO4)602. Netherlands Patent Application 7104263 (PHN 5375) which has also been laid open to public inspection describes such silicates in which a portion of the (sio4) is replaced by (po4)3. Netherlands Patent Application 7802632 (PHN 9074) which has been laid open to public inspection discloses luminescent materials which are activated by gadolinium and which furthermore comprise second and third activator elements. In these materials the excitation energy, for example the short-wave ultraviolet radiation of a lowpressure mercury vapour discharge is absorbed by the second activator element and transferred to the third activator element via gadolinium ions, which enables efficient emission of the third activator element. The said Patent Application mentions as an example of such a material a lead and terbium-activated, gadolinium-containing lanthanum silicate with an apatite crystal structure. This known silicate contains as cations only Ln-ions and no Me-ions and therefore does not satisfy the general formula specified in the preamble. It has appeared that the known silicate has a comparatively low quantum efficiency (not more than 50 ). It is an object of the invention to provide novel luminescent materials on the basis of a host lattice with an apatite crystal structure. According to the invention, a luminescent screen of the type defined in the opening paragraph is characterized in that the luminescent material contains gadolinium, the Gd replacing at least partly the Ln. It has been found that on activation by both Pb and Gd, the apatites which contain Me (Ca, Sr, Ba, Mg and/or Zn) and Ln (Y and/or La) form efficiently luminescing materials which, for example on excitation by ultraviolet radiation from a low-pressure mercury vapour discharge, yield in a very efficient manner the characteristic Gdradiation (line emission in the range from 310 - 315 nm). In addition, it has appeared that if terbium or manganese are used as an activator in addition te the lead and the gadolinium, efficiently luminescing materials are obtained having the characteristic green Tb-emission and the Mnband emission with a maximum at approximately 590 nm, respectively. From experiments it appeared that both the phosphate and also the silicate apatites and also the borate-containing and the mixed apatites form suitable host lattices. These host lattices may be either oxy- or halogen-apatites (X representing oxygen or a halogen, F in particular) or mixed oxy-halogen apatites. X may alternatively represent an unoccupied host lattice (vacancy). In addition to the stoichiometric apatites (wherein a+b=10 and which do not have vacancies X) it has appeared that also non-stoichiometric materials (9#a + b#10) form good host lattices. It is a condition that the luminescent materials always comprise Me and Ln (a 6 0, b 6 0). In the luminescent materials of the invention the excitation energy is absorbed by the lead and transferred tithe gadolinium. If furthermore Tb and/or Mn are present, a further, efficient transfer to the said last elements occurs. A great advantage of the materials in accordance with the invention is that the transfer of excitation energy from the lead is so complete that the materials are substantially free from residual emission from the lead in the visible part of the spectrum. This is in contrast with the materials already known from the said Netherlands Patent Application 7005708 and 7104263 mentioned in the foregoing, and was completely unexpected. The luminescent materials in accordance with the invention are particularly suitable for use in the luminescent screen of low-pressure mercury vapour discharge lamps, as the mercury resonant radiation (predominantly 254 nm) generated in these lamps can be well absorbed by the lead. Generally, preference is given to luminescent materials in accordance with the invention on the basis of a stoichiometric host lattice (this is to say in the general formula a + b = 10), as the highest luminous fluxes can usually be obtained with such materials. A first embodiment of a luminescent screen in accordance with the invention is characterized in that the luminescent material has a composition which is defined by the formula Me10-y-p-qPbqLny-r-sGdrTbs(PO4)6-y(SiO4)yF2, wherein 0.25 4 y g 6 0.002 4 p & 1.00 0 z q t 0.25 0.25 L r 0 i s $ 3.0 r+s S y Such a screen comprises fluoro-silicate apatite which may possibly contain phosphate and is stoichiometric (in the general formula: a + b = 10). A second embodiment of a luminescent screen in accordance with the invention, which also comprises a stoichiometric apatite, is characterized in that the luminescent material has a composition defined by the formula Me2+X-y-p-qPbpMnqLn8-x+y-r-sGdrTbs(SiO4)6-x-y(PO4)x(BO4)yO2, wherein 0 # x # 6 o g y # 6-x, if 2x6 0 # y < 2+x, if 0# x#2 0.002#p # 1.00 0 / q # 0.25 p+q 2+x-y 0.25 # r 0 s i 3.0 r+s # 8-x+y. These luminescent materials are oxy-, silicate- and/or phosphate-apatites, which may furthermore contain borate. They are very efficient materials with a high quantum efficiency at short-wave UV-excitation, particularly if the materials do not contain borate. Preference is therefore given to those materials for which it holds that Y = 0. An embodiment of a screen in accordance with the invention which contains a non-stoichiometric apatite and with which high luminous fluxes can also be obtained, is characterized in that the luminescent material has a composition defined by the formula Me1-p-qPbpMnqLn8-r-sGdrTbs(SiO4)6-x-y(PO4)x(BO4)yF2-2c-x+y 0 c+x-y wherein 0 / x # 2 0 # y # 2 0 # c # 1-¸x+¸y 0.002#p # 1.00 0 # q # 0.25 P+q 41 0.25 4 r 0 # s # 3.0 r+s # 8. More specifically, the fluoro-silicate-apatites having as host lattice MeLn8(SiO)F2 appear to be very satisfactory. A still further embodiment of a luminescent screen in accordance with the invention is characterized in that the luminescent material has a composition defined by the formula Me3-p-1PbpMnqLn6-r-sGdrTbs(SiO4)6-x(PO4)xF4-2c-xOc+x-2' wherein 0 # x # 4 2-x # c c 5 2-2x o # 1.00 t q 4 0.25 0.25 # r 0 (s 4 3.0 r+s # 6. Also this screen comprises a non-stoichiometric apatite (in the general formula a + b = 9). Of this group of apatites, the silicates (x = 0) having as host lattice Me3Ln6(Si04)6 in particular appear to furnish high luminous fluxes. Embodiments of luminescent materials suitable for a luminescent screen in accordance with the invention will now be further described with reference to a drawing and a number of examples for the preparation of these materials. In the drawing Figure 1 shows a low-pressure mercury vapour discharge lamp in accordance with the invention, and Figure 2 shows the spectral energy distribution of the emission of a Pb and Gd-activated material in accordance with the invention, Figure 3 shows the emission spectrum of a material activated by Pb, Gd and Tb, and Figure 4 shows the emission spectrum of a material activated by Pb, Gd and Mn. Figure 1 shows schematically and in cross-section a low-pressure mercury vapour discharge lamp having a tubular glass wall 1. Electrodes 2 and 3 are placed, one at each end of the lamp, the discharge being maintained between these electrodes during operation of the lamp. The lamp contains a small quantity of mercury and a rare gas as a starting gas. The wall 1 is coated on its inside surface with a luminescent layer 4 constitutes a luminescent screen which layer 4 comprises a luminescent material in accordance with the invention. The layer 4 can be applied to the wall 1 in a customary manner, for example by means of a suspension which contains the luminescent material. Example 1. A mixture is made of 0.502 g CaCO3 0.414 g CaF2 0.112 g PbO 1.418 g SrC03 3.666 g La203 1.359 g Gd2O3 2.118 g SiO2. This mixture which furthermore contains 2 % by weight of NH4F was heated for 1 hour in air at 1350 C. The product obtained was homogenized whilst 10 % by weight NH4Cl was added and thereafter subjected to a second heating operation for 7 hour in air at 12500C. After cooling and homogenizing, a lead and gadolinium-activated fluoro-apatite was obtained having a composition defined by the formula Ca2Sr1.9Pb0.1La4,5Gd1.5(SiO4)6F2. The apatite crystal structure (also of all the following examples of materials in accordance with the invention) was determined by means of X-ray diffraction analysis. On excitation by short-wave ultraviolet radiation (predominantly 254 nm) the material shows the characteristic Gd-line emission (maximum 312 nm). Figure 2 shows the spectral energy distribution of the emission. In this Figure the wave length > in nm is plotted on the horizontal axis and the relative radiation intensity on the vertical axis in arbitrary units. This material had a quantum efficiency of 70 , the absorption A of the exciting radiation (254 nm) being 82 . The peak height P of the Gd-Emission was 88 % relative to a standard material 0.5La0.487Bi0.013B3O6). Examples 2 to 12 inclusive. In a corresponding manner as specified in Example 1, a number of lead and gadolinium-activated fluoro-silicate apatites of different compositions were obtained. The formulae of these materials as well as the results of measuring the peak height P of the Gd-line emission (in %0 relative to the standard mentioned in Example 1) and of the absorption A (in fo) of the exciting 254 nm-radiation are shown in the following Table. Example Formula P A 2 Ca1.9Sr2Pb0.1La5.9Gd0.1(SiO4)6F2 66 83 3 Ca 1.9Sr2Pb0.1La5Gd(SiO4)6F2 80 82 4 Ca2Sr1 gPbo 1La5Gd(SiO4)6 2 83 83 5 Ca2Sr1.9Pb0.1La4Gd2(SiO4)6F2 81 86 6 Ca2Sr1.9Pb0.1La3.5Gd2.5(SiO4)6F2 66 86 7 Ca2Sr gPbo La3Gd3(SiO4)6F2 50 86 8 Ca2Sr1.9Pb0.1La2.5Y2.5Gd(SiO4)6F2 75 88 9 Ca2Sr1.9Pb0.1La2.25Y2.25Gd1.5(SiO4)6F2 83 86 10 Ca2Sr1.9Pb0.1La2Y2Gd2(SiO4)6F2 80 88 11 Ca2Sr 1.9Pb0.1La1.75Y1.75Gd2.5(SiO4)6F2 62 88 12 Ca2Sr1.9Pb0.1La1.5Y1.5Gd3(SiO4)6F2 42 87 Examples 13 to 40, inclusive. In a similar manner to that described in Example 1, a number of fluoro-silicate apatites, activated by lead, gadolinium and terbium were prepared, the temperature being however 1330 C during the first heating operation, and cooling after the heating operation being effected in a nitrogen atmosphere. The materials obtained all showed the characteristic green Tb-emission. The following Table shows the formulae of these materials and, in addition, the measurements of the luminous efficacy LO (in % relative to a Tb-activated cerium magnesium aluminate which was used as a standard) and the absorption A (in %) of the exciting radiation (predominantly 254 nm). Example Formula LO A 13 Ca2Sr1.9Pb0.1Y0.8Gd5Tb0.2(SiO4)6F2 73 88 14a) Ca2Sr1.9Pbo0.1Y0.6Gd5Tb0.4(SiO4)6F2 80 89 15 Ca2Sr1.9Pb0.1Y0.4Gd5Tb0.6(SiO4)6F2 80 89 16 Ca2Sr1.9Pb0.1Y0.2Gd5Tb0.8(SiO4)6F2 79 89 17 Ca2Sr1.9Pb0.1Y1.8Gd4Tb0.1(SiO4)6F2 74 88 18 Ca2Sr1.9Pb0.1Y1.6Gd4Tb0.4(SiO4)6F2 78 88 19 Ca2Sr1.9Pb0.1Y1.4Gd4Tb0.6(SiO4)6F2 79 89 20 Ca2Sr 1.9Pb0.1Y1.2Gd4Tb0.8(SiO4)6F2 79 89 21 Ca2Sr 1.9Pb0.1Y2.8Gd3Tb0.2(SiO4)6F2 72 87 22 Ca2Sr1.9Pb0.1Y2.6Gd3Tb0.4(SiO4)6F2 77 88 23 Ca2Sr1.9Pb0.1Y2.4Gd3Tb0.6(SiO4)6F2 75 88 24 Ca2Sr1.9Pb0.1Y2.2Gd3Tb0.8(SiO4)6F2 77 89 25 Ca2Sr1.9Pb0.1Y3.8Gd2Tb0.2(SiO4)6F2 69 87 26 Ca2Sr1.9Pb0.1Y3.6Gd2Tb0.4(SiO4)6F2 75 89 27 Ca2Sr1.9Pb0.1Y3.4Gd2Tb0.6(SiO4)6F2 76 89 28 Ca2Sr 1.9Pb0.1Y3.2Gd2Tb0.8(SiO4)6F2 78 89 29 Ca2Sr1.95Pb0.05Gd5.5Tb0.5(SiO4)6F2 73 84 30 Ca2Sr1.93Pb0.07Gd5.5Tb0.5(SiO4)6F2 75 85 31 Ca2Sr1.91Pb0.09Gd5.5Tb0.5(SiO4)6F2 77 86 32 Ca2Sr1.89Pb0.11Gd5.5Tb0.5(SiO4)6F2 77 89 33 Ca2Sr1.87Pb0.13Gd5.5Tb0.5(SiO4)6F2 79 90 34 Ca2Sr 1.85Pb0.15Gd5.5Tb0.5(SiO4)6F2 78 91 35 Ca2Sr1.9Pb0.1La2.5Gd3Tb0.5(SiO4)6F2 72 86 36 Ca3.9Pb0.1La2.5Gd3Tb0.5(SiO4)6F2 70 83 37 Sr3.9Pb0.1La2.5Gd3Tb0.5(SiO4)6F2 72 89 38 2 1.9Pb0.1Y2.5Gd3Tb0.5(SiO4)6F2 78 87 2 1.9 0.1 2.5 3 0.5 4.0 2 39 Ca3.9Pb0.1Y2.5Gd3Tb0.5(SiO4)6F2 76 86 40 Sr3.9Pb0.1Y2.5Gd3Tb0.5(SiO4)6F2 73 89 a) The spectral energy distribution of the emission of the material in accordance with Figure 14 is shown in Figure 3. Examples 41 to 45, inclusive. Five oxy-apatites activated by lead, gadolinium and manganese were prepared in a manner which substantially corresponded to the method described in Example 1. The Mn was added to the firing mix in the form of MnCO and 3 a portion of the Ca in the form of CaHP04. The first 0 heating operation took place at 1325 C. The addition of NH4Cl prior to the second heating operation was omitted, and this second heating operation took place for 1 2 hours 0 at 1325 C, followed by cooling in a nitrogen atmosphere. The materials obtained luminesce in a band (half-value width approximately 75 nm) having a maximum at 590 nm. The following Table shows the formulae of these materials as well as the results of the measurement of peak-height PH of the emission band (in % relative to the height of the Mn-band of a luminescent Sb and Mn-activated halo-phosphate having a colour temperature of 3000 K) and the absorption A (in /0) of the exciting 254 nm-radiation. Example Formula PH A 41 Ca1.85Pb0.15Mn0.1Y4.9Gd3(SiO4)5.9(PO4)0.1O2 100 87 42a) Ca1.9Pb0.1Mn0.1Y4.9Gd3(SiO4)5.9(PO4)0.1O2 73 71 43 Ca3.3Pb0.1Mn0.1Y3.5Gd3(SiO4)4.5(PO4)1.5O2 63 75 44 Ca1.8Pb0.15Mn0.15Y4.9Gd3(SiO4)5.9(PO4)0.1O2 37 54 45b) Ca1.75Pb0.15Mn0.1Y6Gd2(SiO4)4(PO4)(BO4)O2 52 81 a) The spectral energy distribution of the emission of the material in accordance with example-42 is shown in Figure 4. b) Boron added as H3BO3. Duration of the second heating operation: 1 hour. A third heating operation took place, after the addition of 10% by weight of NH4Cl, for 1 hour in air at 13250C. Examples 46 to 49, inclusive. The formulae of four examples of materials in accordance with with the invention, also having my -emission, are shown in the following Table, together with the measurement of peak heigth PH (in % relative to the standard mentioned in examples 41 to 45, inclusive). Example Formula PH 46 Ca3.85Pb0.15Mn0.1Y2.9Gd3(SiO4)5.9(PO4)0.1F2 68 47 Ca6.8Pb0.1Mn0.1Gd3(PO4)3(SiO3)3F2 57 48 Ca2.85Pb0.15Mn0.1Y3.9Gd3(SiO4)5.9(PO4)0.1OF 74 49 Mg3.8Pb0.1Mn0.1Y3Gd3(SiO4)6O 45 Examples 50 to 66, inclusive. A number of examples of luminescent materials in accordance with the invention which are not only activated by Pb and Gd but also by Tb (examples 50 to 65, inclusive) or by Mn (example 66) are shown in the following Table. The luminous efficacy LO of the Tb-activated materials is again shown in ffi relative to the standard mentioned in examples 13 to 40, inclusive. The examples 54 to 66, inclusive, relate to non-stoichiometric apatites (a+b=9). Example Formula LO 50 Zn1.9Pb0.1Y5Gd2.5Tb0.5(SiO4)6O2 63 51 Sr0.9Pb0.1Y5.5Gd3Tb0.5(SiO4)5(BO4)O2 60 52 Ca0.9Pb0.1Y5.5Gd3Tb0.5(SiO4)5(BO4)O2 59 53 Mg0.9Pb0.1Y5.5Gd3Tb0.5(SiO4)5(BO4)O2 56 54 Sr0.9Pb0.1La4.5Gd3Tb0.5(SiO4)6F2 64 55 Ca0.9Pb0.1La4.5Gd3Tb0.5(SiO4)6F2 66 56 Ba0.9Pb0.1La4.5Gd3Tb0.5(SiO4)6F2 58 57 Sr0.9Pb0.1Y4.5Gd3Tb0.5(SiO4)6F2 66 58 Ca0.9Pb0.1Y4.5Gd3Tb0.5(SiO4)6F2 69 59 Ba0.9Pb0.1Y4.5Gd3Tb0.5(SiO4)6F2 60 60 Sr2.9Pb0.1La2.5Gd3Tb0.5(SiO4)6 44 61 Sr2.9Pb0.1Y2.5Gd3Tb0.5(SiO4)6 41 62 Sr2.9Pb0.1Gd5.5Tb0.5(SiO4)6 48 63 Ca2.9Pb0.1La2.5Gd3Tb0.5(SiO4)6 32 64 Ca2.9Pb0.1Y2.5Gd3Tb0.5(SiO4)6 37 65 Ca2.9Pb0.1Gd5.5Tb0.5(SiO4)6 37 66 Ca2.8Pb0.1Mn0.1Y3Gd3(SiO4)6 a) a) Peak height of the Mn2+-emission band 46% relative to the standard mentioned in the examples 41 to 45, inclusive.

Claims: 1. A luminescent screen comprising a luminescent material activated by lead or by lead and at least one of the elements terbium and manganese, the material having the hexagonal apatite crystal structure and a composition which is defined by the general formula MeaLnb(AO4)6X2, wherein Me represents at least one of the elements Ca, Sr, Ba, Mg and Zn, Me having partly been replaced by Pb or by Pb and Mn, wherein Ln represents at least one of the elements Y and La, the Tb, if present, replacing a portion of the Ln, wherein (AO4) represents at least one anion from the group (PO4)3-, (sio4) 4- and (BO4)5 , and X at least one element from the group of oxygen, the halogens and vacancies, and wherein a 6 O, b 6 0 and 9 # a+b # 10, characterized in that the luminescent materials contains gadolinium, the Gd at least partly replacing the Ln. 2. A luminescent screen as claimed in Claim 1, characterized in that the luminescent material has a composition defined by the formula Mea-p-qPbpMnqLnb-r-sGdrTbs(AO4)6X2 wherein 0.002 4 p # 1.0 0 # q # 0.25 0.25 < r 4 b-s 0 C s # 3.0. 3. A luminescent screen as claimed in Claim 1 or 2, characterized in that the luminescent material has a composition defined by the formula Me10-y-p-qPbpMnqLny-r-sGdrTbs(PO4)6-y(SiO4)yF2, wherein 0.25 # y # 6 0.002 p # 1.00 0 # q # 0.25 0.25 # r 0 # s # 3.0 r+s y. 4. A luminescent screen as claimed in Claim 1 or 2, characterized in that the luminescent material has a composition defined by the formula Me2+x-y-p-qPbpMnqLn8-x+y-r-sGdrTbs(SiO4)6-x-y(PO4)x(BO4)yO2, wherein 0 # x # 6 0 # y # 6-x, if 2 < x # 6 0 # y < 2+x, if 0 # x # 2 0.002# p p as 1.00 O # q C 0.25 p+q < 2+x-y 0.25 dr 0 # s # 3.0 r+s # 8-x+y. 5. A luminescent screen as claimed in Claim 4, characterized in that y=O. 6. A luminescent screen as claimed in Claim 1 or 2, characterized in that the luminescent material has a composition defined by the formula Me1-p-qPbpMnqLn8-r-sGdrTbs(SiO4)6-x-y(PO4)x(BO4)yF2-2c-x+y Oc+x-y' wherein 0 # x # 2 0 # y # 2 0 # c # 1-¸x+¸y 0.002 # p # 1.00 0 # q # 0.25 p+q # 1 0.25 # r o < s 4 3.0 r+s 8. 7. A luminescent screen as claimed in Claim 1 or 2, characterized in that the luminescent material has a composition defined by the formula Me3-p-qPbp-MnqLn6-r-sGd3Tbs(SiO4)6-x(PO4)xF4-2c-xOc+x-2, wherein 0 # x 4 2-x 4 c # 2-x 0 0.002 4 p # 1.00 0 # q # 0.25 0.25 g r 0 # s # 3.0 r+s # 6. 8. A low-pressure mercury vapour discharge lamp comprising a luminescent screen as claimed in one or more of the Claims 1 to 7.

Inventors: DE HAIR, JOHANNES THEODORUS WILHELMUS
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1981
Main classification: C09K11
IPC codes: C09K11, H01J61