SECURITY FEATURE AND METHOD FOR THE DETECTION THEREOF AND SECURITY OR VALUE DOCUMENT

Abstract

The invention relates to a security feature for a security and/or value document which comprises a mixture of electrically conductive field displacement elements which are electrically insulated within the security or value document, and a zinc sulfide luminophore in the form of particles, which mixture is applied to a security and/or value document by means of a printing technology. The zinc sulfide luminophore has the general chemical formula ZnS: Cu.sub.x, M.sub.y, X.sub.z. Here, M represents one or more elements from a group comprising the chemical elements Co, In and Ni; X represents one or more elements from a group comprising the halides F, Cl, Br and I; 0<x0.002; 0<y0.00015; and 0z0.00050. The particles each have cubic phase fractions and hexagonal phase fractions, the zinc sulfide luminophore emitting a first luminescent radiation in the spectral range between 580 nm and 780 nm in the event of excitation by an electrical field, and the zinc sulfide luminophore emitting a second luminescent radiation in the visible spectral range in the event of thermal stimulation and preceding excitation by means of UV radiation. Furthermore, a security and/or value document having a security feature and a method for detection and/or verification of a security feature having a luminophore are provided.

Claims

1. A security feature for a security document and/or document of value, the security feature comprising a mixture, which is applied by means of a printing technology onto a security document and/or document of value, made up of field displacement elements, which are electrically conductive and electrically insulated inside the security document or document of value, and a zinc-sulphidic luminophore in the form of particles, the zinc-sulphidic luminophore having the following generic chemical formula:
ZnS: Cu.sub.x, M.sub.y, X.sub.z where: M=one or more elements from a group comprising the chemical elements Co, In and Ni; X=one or more elements from a group comprising the halides F, Cl, Br and I; 0<x0.002; 0<y0.00015; and 0z0.00050; the particles case having cubic phase fractions and hexagonal phase fractions, the zinc-sulphidic luminophore emitting a first luminescence radiation in the spectral range between 580 nm and 780 nm upon excitation by an electric field, and the zinc-sulphidic luminophore emitting a second luminescence radiation in the visible spectral range upon thermal stimulation and preceding excitation by means of UV radiation.

2. The security feature according to claim 1, characterized in that the mixture furthermore has a viscosity-determining element.

3. The security feature according to claim 1, characterized in that the hexagonal phase fractions in the individual particles of the zinc-sulphidic luminophore on average lie in the range between 20% and 40%.

4. The security feature according to claim 1, characterized in that the first luminescence radiation has an emission spectrum which consists of an emission band in the deep red spectral range.

5. The security feature according to claim 1, characterized in that the second luminescence radiation is emitted in the green spectral range.

6. The security feature according to claim 1, characterized in that the second luminescence radiation has a maximum with a wavelength in the spectral range between 520 nm and 550 nm.

7. The security feature according to claim 1, characterized in that the second luminescence radiation emitted owing to the thermal stimulation has an integral intensity maximum (thermoluminescence glow curve) in the temperature range between 120 C. and 150 C.

8. The security feature according to claim 1, characterized in that the zinc-sulphidic luminophore also emits the second luminescence radiation when it is optically stimulated after preceding excitation.

9. The security feature according to claim 1, characterized in that the particles have an average grain size of between 2 m and 50 m, particularly between 2 m and 20 m.

10. The security feature according to claim 1, characterized in that the zinc-sulphidic luminophore has the following generic chemical formula:
ZnS: Cu.sub.x, Co.sub.y where 0<x<0.002 and 0<y0.00015.

11. A security document and/or document of value having a security feature according to claim 1.

12. A method for detecting and/or verifying a security feature having a luminophore according to claim 1 in a security document and/or document of value, comprising the following steps: a. exciting the luminophore (10) by means of an electric AC field; b. testing whether, as a consequence of the excitation by means of the electric AC field in step a., a first luminescence radiation is emitted in the spectral range between 580 nm and 780 nm; c. exciting the luminophore by means of a UV radiation; d. stimulating the excited luminophore (10) by means of thermal stimulation or by means of optical stimulation of the luminophore; and e. testing whether, as a consequence of the stimulation, a second luminescence radiation is emitted in the visible spectral range.

13. The method according to claim 12, characterized in that a confirmation signal is respectively generated if the occurrence of the tested first or second luminescence radiation is determined in one of the performed test steps b. and/or e.

14. The method according to claim 12, characterized in that the luminophore is heated to a temperature of up to a maximum of 250 C. for the thermal stimulation.

15. The method according to claim 12, characterized in that in the case of the thermal stimulation in step e., the intensity of the emitted second luminescence radiation is compared with a predetermined thermoluminescence glow curve.

16. The method according to claim 12, characterized in that in the case of the optical stimulation in step e., the intensity of the emitted second luminescence radiation is compared with a predetermined decay curve.

Description

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0104] Further details, advantages and special manifestations of the invention are explained in more detail in the following with reference to preferred embodiments of the invention, with reference to the drawing. In the figures:

[0105] FIG. 1: shows an x-ray diffraction diagram of a zinc-sulphidic luminophore, which is also termed a reference luminophore in the following;

[0106] FIG. 2: shows electroluminescence emission spectra of selected zinc-sulphidic luminophores which are activated with copper exclusively;

[0107] FIG. 3: shows electroluminescence emission spectra of embodiments according to the invention of the zinc-sulphidic luminophore, which are additionally doped with cobalt;

[0108] FIG. 4: shows thermoluminescence glow curves of the zinc-sulphidic reference luminophore and a security feature comprising this luminophore;

[0109] FIG. 5: TSL glow curves of selected variants of the zinc-sulphidic luminophore with different hexagonal phase fractions;

[0110] FIG. 6: shows a relationship between the temperature maxima of the TSL glow curves shown in FIG. 5 and the hexagonal phase fractions of the different variants of the zinc-sulphidic luminophore;

[0111] FIG. 7: shows a thermoluminescence emission spectrum of a security feature;

[0112] FIG. 8: shows the TSL glow curve of the zinc-sulphidic reference luminophore compared to TSL glow curves of electroluminescence luminophores according to the prior art;

[0113] FIG. 9: shows a characteristic decay curve for an optically stimulated luminescence of a security feature; and

[0114] FIG. 10: shows a schematic illustration of an optical arrangement for measuring the spectra and curves shown in FIG. 4 to FIG. 9.

[0115] FIG. 1 shows an x-ray diffraction diagram of a zinc-sulphidic luminophore (reference luminophore) of a security feature. Here, the reference luminophore used is a zinc-sulphidic luminophore which is activated with copper exclusively. In the following, the synthesis of this luminophore with the composition ZnS:Cu.sub.0.0005 that is aimed for is explained by way of example.

[0116] To produce the luminophore, 399.3 g of a high-purity powdered zinc sulphide are mixed intensively with 0.25 g of previously ground CuSO.sub.4 and screened by means of a 100 m screen to improve the homogeneity of the mixture further. Subsequently, the batch mixture is transferred to a corundum crucible and heated to 1,200 C. in a chamber furnace with a rate of 15 K/min. After high-temperature annealing for three hours in a forming gas atmosphere with a hydrogen content of 5%, the furnace is cooled to 600 C. within 90 min. The annealing material is removed and cooled in air to room temperature. This is followed by wet grinding, an etching step using dilute nitric acid and a one-off washing process. Subsequently, the solids obtained are separated by means of filtration and once again mixed with 4 ml of a copper sulphate solution (16 g CuSO.sub.4 per litre) for the purpose of redoping. After repeated intensive homogenization, the material mixing is subsequently tempered during tempering at 500 C. for 180 min. The final processing steps comprise renewed wet grinding, which is required for setting the desired grain size distribution of the synthesized luminescent particles, and the subsequent final washing, drying and screening processes.

[0117] The x-ray diffraction diagram of the reference luminophore, which was measured with the aid of a diffractometer, is illustrated in FIG. 1. It consists of numerous linear interferences, which can in each case be assigned to the two different, cubic and hexagonal structural types of the zinc sulphide however. In this case, the peaks provided with the letters h and the respective Miller indices which are placed in brackets represent the hexagonal phase of the powdered luminophore sample, whilst the reflexes marked with the letter k and the Miller indices relevant in this case represent the proportional cubic crystalline structure of the sample. Overlays of hexagonal and cubic interferences were labelled by the letter combination h+k.

[0118] On the basis of the measured diffractogram and the quantitative phase analysis based thereon, it was possible for the structural status of the reference luminophore to determine relative phase fractions of 35% for the hexagonal and 65% for the cubic crystalline structure.

[0119] The extent of the relative structural phase fractions in the luminophore is influenced to a large extent by, in addition to other factors, the preparation conditions used during the production of the luminophore. However, that also means that the corresponding structural status of the luminophore samples can be modified by changing certain synthesis parameters. This can be drawn by way of example from the following table, in which important data for the characterization of the synthesis conditions and the structure and luminescence properties of selected copper doped zinc-sulphidic luminophores are compiled:

TABLE-US-00001 Main annealing Hexagonal Electro- Thermo- process phase luminescence luminescence T/ t.sub.down/ fraction .sub.max/ Int./ T.sub.max/ Int./ Luminophore C. min % nm % % % Luminophore 1,100 140 4 460 10 40 98 1 Luminophore 1,100 120 8 650 66 78 190 2 Luminophore 1,200 120 20 650 143 120 160 3 Reference 1,200 90 35 650 100 128 100 luminophore Luminophore 4 Luminophore 1,200 40 55 650 90 130 45 5

[0120] In the preceding table, the data for the electroluminescence properties and to the thermoluminescence properties relate on the one hand to the wavelength maximum .sub.max of the corresponding EL emission spectrum in each case and on the other hand to the temperature maximum T.sub.max of the thermoluminescence glow curve recorded for the respective luminophore under comparable conditions. The listed data for the percentage intensities relate to the corresponding measured values that were determined for the reference luminophore, which were set at 100 in each case.

[0121] It is to be highlighted that all of the luminophores listed in the tableexcept for the differences explained specifically herehave the same luminophore composition and that they were all manufactured on the basis of the above-described method under predominantly identical production conditions, i.e. the same form of the batch preparation, the same crucible and furnace geometry, identical annealing time and annealing atmosphere and also comparability of all mechanical and thermal aftertreatment steps. To acquire the different phase compositions by contrast, both the temperatures of the main annealing process and the cooling rates were varied.

[0122] As can be drawn from the table, the high-temperature annealings of the batch mixtures for three hours in each case were carried out at temperatures of 1,100 C. or 1,200 C. The characteristic values listed in the table for the different cooling rates t.sub.down that are set relate to the time intervals between the completion of the main annealing process and the respective achievement of a cooling temperature of 600 C.

[0123] Depending on the variations of the preparation conditions which are undertaken, relative hexagonal phase fractions in the range from 4% to 55% were obtained for the resulting zinc-sulphidic luminophores.

[0124] FIG. 2 shows the emission spectra resulting during the excitation of the luminophore samples listed in the above table with an electric AC field. The high voltage AC field has an excitation voltage 30 kV and an excitation frequency of 30 kHz. It emerges from the electroluminescence emission spectra that the luminophore samples, which were produced at comparatively low annealing temperatures and low cooling rates and which for this reason have comparatively low hexagonal structural fractions are characterized, as in the case of luminophore sample 1 (emission spectrum 1), by an exclusively blue electroluminescence or else, as in the case of luminophore sample 2 (emission curve 2), by a blue electroluminescence which is still present at least proportionally.

[0125] As the emission curves 3, 4 and 5 of FIG. 2 show, exceptionally wide-banded emissions in the deep red spectral range with intensity maxima in the region of 650 nm, which are preferred for the formation of the security features according to the invention, clearly dominate from a relative hexagonal phase fraction of approximately 10% and in particular from a relative hexagonal phase fraction of approximately 20%. The intensities of the measured electroluminescences clearly increase up to a hexagonal phase fraction of approximately 20%, in order thereafter to decrease slightly in the case of an even greater extent of the hexagonal structural characteristics.

[0126] FIG. 3 shows electroluminescence emission spectra 6 to 8 of zinc-sulphidic luminophores according to the invention, wherein these luminophores have an additional cobalt co-doping in addition to the copper activation. For the purposes of comparison, the emission curve 4 of the reference luminophore, which is doped exclusively with copper, was also recorded in FIG. 3. It also emerges from FIG. 3 that the additional incorporation of cobalt ions in the copper-doped ZnS fundamental lattice of the zinc-sulphidic luminophore enables an increase of the efficiency of the electroluminescence and a further stabilization of the special emission characteristics.

[0127] In this case, the production of the co-doped luminophore samples according to the invention (emission curves 6 to 8) under the same conditions, as were also used for the synthesis of the reference luminophore, which is activated exclusively with copper (emission curve 4). As in the case of the reference luminophore, uniform values of 500 ppm were also in turn set for the molar fractions of the copper activator ions, whilst for the molar fractions of the cobalt ions, values of 5 ppm (emission curve 6), 10 ppm (emission curve 7) or 20 ppm (emission curve 8) were determined.

[0128] FIG. 4 shows a comparison of the exclusive thermoluminescence glow curves which were determined for the powdered reference luminophore and a security feature having this reference luminophore, wherein this security feature was positioned on a banknote substrate. In this comparison, the thermoluminescence glow curve of the powdered reference luminophore is illustrated with a continuous line, whilst the thermoluminescence glow curve of the security feature is illustrated with a dashed line.

[0129] In both cases, the samples to be tested were first heated to 250 C., in order to be able to remove energies stored randomly in this manner, possibly by means of a corresponding day-light excitation. This primary baking procedure, which aims to secure a high reproducibility of the subsequent standard TSL measurements or OSL measurements, was used in all investigations relating to this.

[0130] After the cooling has taken place, the samples prepared in such a manner, were excited under defined conditions with the aid of a 340 nm laser, in order to effect as complete as possible a filling of the traps responsible for the thermoluminescence of the samples. After that, in all cases, a pause of 20 seconds was complied with, in order subsequently to start with the reading out of the stored light sums by means of targeted heating of the samples using a heating rate of 5 K/s up to a final temperature of 250 C. The integral intensities of the radiation emitted by the samples as a consequence of the supplied thermal energy could be detected with the aid of a TSL/OSL reader of the company RIS (Model DA-15). The graphical representation of its temperature dependence leads to the curves shown in FIG. 4.

[0131] It becomes clear that the two curves have practically the same temperature maxima at approximately 130 C. and that they also differ from one another only slightly with regards to the curve shape. This means that the exclusive thermoluminescence characteristics of the luminophore are also retained if the luminophore is processed to form the security feature, particularly if it is applied onto and/or into corresponding documents of value and security documents, such as for example banknotes, identity cards, passports and driving licences, or else also bank or credit cards.

[0132] The two thermoluminescence glow curves shown are glow curves standardized in terms of intensity. The differences with respect to the measured intensities are comparatively small however. In the case of otherwise identical measuring conditions, the intensities of the thermoluminescence signals measured for one and the same luminophore depend on the thickness of the respective luminophore layer.

[0133] In the comprehensive investigations carried out in this context, it was furthermore possible to show that the exclusive TSL signals as well as possibly the signals for the optically stimulated luminescence of the luminophore of the security feature according to the invention can also be securely verified in the case of the solid concentrations which are to be considered typical for security inks which contain pigment and the print designs obtained using these inks.

[0134] Likewise, reference is made once more at this point that it was also possible to ensure in these investigations that all substrate materials usually used for the creation of documents of value and security documents have a sufficiently high stability in order to withstand the thermal treatment up to temperatures of 250 C. required for the repeated reading out of the thermoluminescence signals without damaging these materials and the security features placed onto and into them.

[0135] The security feature comprises a mixture, which is applied by means of a printing technology onto a security document or document of value, made up of field displacement elements, which are electrically conductive and electrically insulated inside the security document or document of value, and a zinc-sulphidic luminophore 6, 7, 8 according to the invention in the form of particles.

[0136] The mixture comprising the field displacement elements and the zinc-sulphidic luminophore 6, 7, 8 according to the invention can for example be applied onto the security document and/or document of value by means of a gravure, offset or screen printing method.

[0137] The mixture may furthermore comprise an element which is decisive for the viscosity of the mixture, e.g. a binder. In particular, the mixture may be a printing means, for example a printing dye or printing ink, which is applied onto the security document and/or document of value by means of a printing technology. Here, the printing means, particularly the viscosity of the printing means, can be adapted to the printing technology used, particularly regarding the processing properties of the printing means to the printing technology.

[0138] FIG. 5 shows the TSL glow curves 1 to 5 of the zinc-sulphidic luminophores 1 to 5 described in the above table. In this case, the curves were not standardized, so that the intensity differences determined under the same measuring conditions are indicated.

[0139] Furthermore, it becomes clear that the temperature maxima of the glow curves of the investigated luminophores are displaced to higher temperatures under the influence of the increasing, preparatively set hexagonal phase fractions listed in the above table.

[0140] FIG. 6 shows a relationship between the temperature maxima T.sub.max of the TSL glow curves shown in FIG. 5 and the hexagonal phase fractions of the luminophores investigated. In this case, it is shown that for the temperature maxima of the glow curves, values in the preferred range of between 120 C. and 150 C. in the sense of the invention are first achieved from a hexagonal phase fraction of approximately 10%.

[0141] FIG. 7 shows a thermoluminescence emission spectrum which comprises the above-described reference luminophore (luminophore 4 in the table). Surprisingly, this thermoluminescence emission spectrum is, in contrast to the that for the stationary electroluminescence of the luminophore, characterized by a comparatively narrow-banded emission with an emission maximum of approximately 540 nm.

[0142] FIG. 8 once again shows the TSL glow curve of the reference luminophore compared to TSL glow curves of electroluminescence luminophores A, B, C, D according to the prior art, as are used for example in thick film electroluminescence displays. The reference luminophore has an electroluminescence in the deep red range of the electromagnetic spectrum. The electroluminescence luminophores B and D by contrast show an electroluminescence in the blue spectral range of visible light, whilst the luminophores A and C emit in the green range after excitation with the aid of electric AC fields. The previously known electroluminescence luminophores A-D are EL pigments of different manufacturers. All illustrated standardized TSL glow curves were measured under the same conditions.

[0143] In contrast to the characteristic thermoluminescence glow curve of the zinc-sulphidic reference luminophore, the glow curves of all electroluminescence luminophores A, B, C, D included in the comparison have temperature maxima which are settled only slightly above a temperature of 50 C. Differently from in the case of the zinc-sulphidic luminophore of the security feature according to the invention, the comparatively flat traps which are responsible for the occurrence of thermoluminescence effects in this low temperature range can already be emptied by means of the addition of relatively low energies without additional stimulation, that is to say for example even by means of corresponding fluctuations of the room temperature in the form of weak intensity afterglow processes.

[0144] The use of the zinc-sulphidic luminophore with phase relationships which are influenced in a targeted manner in the security feature according to the invention by contrast opens up the possibility of using the exclusive thermoluminescence characteristic as a stand-alone or additional criterion for the authenticity verification of the documents of value and security documents equipped with the security feature according to the invention.

[0145] Alternatively to the addition of thermal energy, the electrons stored in the characteristic lattice traps of the zinc-sulphidic luminophore of the security feature according to the invention after corresponding excitation can however, by means of a targeted optical stimulation, also be freed from the traps and returned to the electronic base state with emission of a corresponding luminescence radiation.

[0146] Unlike for the glow curves which are characteristic for the thermoluminescence, specific decay curves are measured for the optically stimulated luminescence, which can according to the invention likewise be used as an authenticity criterion.

[0147] FIG. 9 shows a characteristic decay curve for an optically stimulated luminescence of a security feature which comprises the above-described zinc-sulphidic reference luminophore, the thermoluminescence glow curve of which is shown in FIG. 4. The security feature is positioned on a banknote substrate. After the filling of the traps with renewed use of the 340 nm laser excitation source and keeping a corresponding pause of 20 seconds, the reading out of the stored light sum took place by means of the optical stimulation with the aid of an intensive 750 nm laser radiation. Orientation experiments that were carried out previously had shown that the largest signal/noise ratio can be achieved when using a laser wavelength of this type.

[0148] The spectral distribution of the light emitted after optical stimulation of the zinc-sulphidic luminophore of the security feature according to the invention matches that which was determined in the corresponding TSL measurements. Considering the wavelength maximum of this emission found at approximately 536 nm and the excitation wavelength of 750 nm, the radiation conversion as a consequence of the optical stimulation of the security feature according to the invention may be classified as anti-Stokes luminescence.

[0149] The exact shape of the decay curves resulting from the optical stimulation of luminophores is influenced by various factors, which also include the laser power. The decay curves measured under defined conditions represent exclusive luminophore characteristic curves however, which can be verified with high reliability at a high read out rate and without any thermal stressing of the security feature according to the invention.

[0150] FIG. 10 shows a schematic illustration of an optical arrangement for measuring the spectra and curves shown in FIG. 4 to FIG. 9. The zinc-sulphidic luminophore or the security feature forms a sample 10. The arrangement comprises a heating device 11, using which the sample 10 can be heated for the purpose of thermal stimulation. The heating device 11 can be controlled using a heating control 12. A thermocouple 13 is arranged on the heating device 11 in order to be able to measure the temperature generated by the heating device 11.

[0151] The arrangement additionally comprises a laser 14, using which the sample 10 can be optically excited. The laser 14 is tunable and controllable using a laser control 16. The laser 12 can also be used to fill the special lattice traps, which are responsible for the occurrence of the exclusive TSL or OSL effects of the security feature according to the invention, by means of excitation. During this excitation, the shutter 18 in front of the light detection device 17 remains closed. The optical filters 19 are selected such that the wavelengths emitted by the sample 10 in the case of the respective thermal or optical excitation can be measured with high efficiency, whilst all other wavelengths are blocked.

[0152] The arrangement furthermore comprises a light detection device 17 which may for example be formed by a photomultiplier tube. An optical shutter 18 and one or more optical filters 19 are arranged between the light detection device 17 and the sample 10. The shutter 18 is activated using a shutter control 21. A high-voltage unit 22 is used for supplying the light detection device 17 with a high voltage. An output signal of the light detection device 17 is amplified using an amplifier 23 and passed to a computer 24. The computer 24 is otherwise used for controlling the heating control 12, the shutter control 21 and the high-voltage unit 22. An output signal of the thermocouple 13 is likewise passed to the computer 24.

[0153] The features disclosed in the preceding description, the claims, and the drawing may be of significance both individually and in any desired combination for the implementation of the different embodiments.

REFERENCE LIST

[0154] 1 Emission spectrum of the electroluminescence of a zinc-sulphidic luminophore 1 which is activated with copper exclusively [0155] 2 Emission spectrum of the electroluminescence of a zinc-sulphidic luminophore 2 which is activated with copper exclusively [0156] 3 Emission spectrum of the electroluminescence of a zinc-sulphidic luminophore 3 which is activated with copper exclusively [0157] 4 Emission spectrum of the electroluminescence of a zinc-sulphidic luminophore 4 (reference luminophore) which is activated with copper exclusively [0158] 5 Emission spectrum of the electroluminescence of a zinc-sulphidic luminophore 5 which is activated with copper exclusively [0159] 1 Thermoluminescence glow curve of the luminophore 1 [0160] 2 Thermoluminescence glow curve of the luminophore 2 [0161] 3 Thermoluminescence glow curve of the luminophore 3 [0162] 4 Thermoluminescence glow curve of the luminophore 4 (reference luminophore) [0163] 5 Thermoluminescence glow curve of the luminophore 5 [0164] 1 Temperature maximum of the glow curve of the luminophore 1 [0165] 2 Temperature maximum of the glow curve of the luminophore 2 [0166] 3 Temperature maximum of the glow curve of the luminophore 3 [0167] 4 Temperature maximum of the glow curve of the luminophore 4 (reference luminophore) [0168] 5 Temperature maximum of the glow curve of the luminophore 5 [0169] 6 Electroluminescence emission spectrum of a luminophore 6 which is additionally co-doped with 5 ppm cobalt [0170] 7 Electroluminescence emission spectrum of a luminophore 7 which is additionally co-doped with 10 ppm cobalt [0171] 8 Electroluminescence emission spectrum of a luminophore 8 which is additionally co-doped with 20 ppm cobalt [0172] 10 Sample [0173] 11 Heating device [0174] 12 Heating control [0175] 13 Thermocouple [0176] 14 Laser [0177] 15 - [0178] 16 Laser control [0179] 17 Light detection device [0180] 18 Optical shutter [0181] 19 Optical filter [0182] 20 - [0183] 21 Shutter control [0184] 22 High voltage unit [0185] 23 Amplifier [0186] 24 Computer