EP3999355A1

EP3999355A1 - Security feature for a security document or value document, comprising at least one luminescent substance that can be excited in the ultraviolet spectral range and emits in the infrared spectral range

Info

Publication number: EP3999355A1
Application number: EP20734880.6A
Authority: EP
Inventors: Roland HEISE; Oliver Muth; Starick Detlef; Thomas JÜSTEL; Viktor Anselm
Original assignee: Bundesdruckerei GmbH
Current assignee: Bundesdruckerei GmbH
Priority date: 2019-07-19
Filing date: 2020-06-19
Publication date: 2022-05-25
Also published as: WO2021013448A1; DE102019119687A1

Abstract

The invention relates to a security feature for a security document or value document, comprising at least one luminescent substance that can be excited in the ultraviolet spectral range and, after excitation has been carried out, emits in the infrared spectral range. The invention further relates to a security document or value document comprising a security feature according to the invention.

Description

Security feature for a security or value document, with at least one fluorescent substance which can be excited in the ultraviolet spectral range and which emits in the infrared spectral range

The invention relates to a security feature for a security or value document, with at least one luminescent material which can be excited in the ultraviolet spectral range and which emits in the infrared spectral range after excitation has taken place. In addition, the invention relates to a security or value document with a security feature according to the invention.

Background of the invention

Luminescent organic and / or inorganic materials have long been used in diverse ways as security features in security and value documents, such as banknotes, passports, ID cards, driver's licenses, etc., but also in product protection.

Numerous luminescent security features are known from the technical and patent literature, which are used in different ways as copy protection and / or for the authenticity verification of value and security documents. It is also known to combine linearly and / or band-shaped emitting phosphors to form security features that exclusive luminescence codes can be assigned to the resulting complex emission spectra.

This is described, for example, in the publications EP 1 647 946 A1 and / or EP 1 805 727 B1, in which numerous phosphors for creating code-forming, luminescent security features are named and described, for which different types of radiation conversion are characteristic. Both UV-UV, UV-VIS, VIS-IR, IR-IR and IR-VIS radiation converters are described in detail. Phosphors which can be excited in the ultraviolet spectral range and which emit in the infrared spectral range after being excited are, however, neither named nor suggested in the cited publications. Further research in patent and specialist literature also led to the result that there are apparently very few phosphors that can be excited in the UV and especially in the UV-A range and with sufficient efficiency in the infrared, preferably in the near infrared spectral range ( NIR) luminesce.

In EP 1 373 605 B1, in connection with the description of luminescent security fibers, reference is made, inter alia, to the commercially available fluorescent material IR-CD 139 (YVO ₄ : Nd) from Honeywell Seelze GmbH, which evidently emits emissions when excited with the aid of mercury vapor lamps Has NIR range.

Definitions

The electromagnetic radiation emitted by a physical system during the transition from an excited state to the ground state is referred to as luminescence. As a rule, luminescence relates to the conversion of higher-energy radiation into lower-energy radiation (down-conversion), the difference between the wavelength of the absorbed radiation and the wavelength of the emitted radiation being referred to as the Stokes shift. Depending on the character of the stimulating radiation and the spectral range of the emitted electromagnetic radiation, different types of luminescence (for example photoluminescence, cathodoluminescence, X-ray luminescence, electroluminescence etc.) are distinguished.

On the other hand, so-called anti-Stokes luminescence (up-conversion) is also described and technically used as a special case of luminescence, in which case, for example, after multi-level infrared (IR) -induced excitation, an emission in a higher-energy spectral range, for example in the area of the visible light.

Phosphors are organic or inorganic chemical compounds which show luminescence phenomena when excited by electromagnetic or particle radiation or when excited by means of electric fields. To make this possible, the basic phosphor grids (phosphor matrices) formed by the chemical compounds act as radiation centers Activator ions and, if necessary, additional coactivator ions incorporated. These phosphors are often in the form of solids, in particular in the form of luminescent pigments.

The wavelength range of the electromagnetic radiation which is arranged between that of the X-rays and that of the microwaves is referred to as optical radiation. It thus includes the range of UV radiation, that of visible light and that of infrared radiation and thus the wavelength range between 100 nm and 10 ⁶ nm (1 mm).

Ultraviolet (UV) radiation relates to the wavelength range from 100 to 380 nm. A distinction is usually made between so-called UV-A radiation (380 to 315 nm), UV-B (315 to 280) nm and UV-C Radiation (280 to 100 nm).

Visible light (VIS) is that section of the electromagnetic spectrum that can be perceived by the human eye. For the normal observer, this range covers the wavelengths between 380 and 780 nm.

There are different approaches in the specialist literature for subdividing the wavelength range of infrared (IR) radiation, which extends from 780 nm to 10 ⁶ nm (1 mm). As a rule, a distinction is made between near infrared (NIR) (780 to 3000 nm), medium (3000 nm to 50 mm) and far IR (50 mm to 1 mm), whereby the NIR range often falls into the IR-A range. (780 to 1400 nm) and the IR-B range (1400 to 3000 nm).

An emission spectrum describes the spectral intensity distribution of the electromagnetic radiation emitted by the phosphors at a fixed excitation wavelength. Such an emission spectrum can consist of emission lines and / or emission bands.

An excitation spectrum illustrates the dependence of the intensity of the radiation emitted by a phosphor at a fixed wavelength on the wavelength of the excitation radiation. The measured intensity is influenced by both the efficiency for the absorption of the excitation radiation and the efficiency of the radiation conversion. Object of the invention

Both in the area of ensuring the authenticity of security and value documents and in the area of product protection, there is an increased interest in security features that have a high security level (level 2+ or level 3 characteristics), but at the same time with a possible can be verified with little technical effort.

Finding and using efficiently luminescent UV-NIR converters would take this desire into account and significantly expand the possibilities for using luminescent security features.

Numerous UV radiation sources are available for UV excitation, including easy-to-use handheld devices, for example in the form of UV-emitting LEDs. The acquisition of the NIR luminescence signals can take place with the aid of the same sensors that are also used for the detection of visible radiation. For example, comparatively inexpensive silicon detectors are preferred, which are characterized by the fact that they have their greatest spectral sensitivity in the range between 800 and 950 nm.

It would also be particularly desirable if the emissions in the NIR range also form or contain machine-readable security codes that could be used for authenticity verification, nominal value coding or, if necessary, also for sorting, for example, different banknote denominations or value products.

The invention is thus based on the technical problem of providing a security feature which has a high level of protection against forgery or a high level of security and which at the same time can be verified with relatively little technical effort.

The technical object is achieved according to the invention by a security feature for a security or value document according to claim 1 and a security or value document according to claim 16.

Advantageous refinements of the invention result from the subjects according to the subclaims. One aspect of the invention relates to a security feature for a security or value document, with at least one luminescent substance which can be excited in the ultraviolet spectral range and which emits essentially in the infrared spectral range after being excited. In addition, another aspect of the invention relates to a security or value document with a security feature according to the invention.

A preferred embodiment of the invention relates to a security feature in which the selected phosphors emit in the near infrared spectral range at wavelengths from 780 nm to 3 mm and preferably in the wavelength range from 800 nm to 950 nm, and where the at least one phosphor is preferably doped with trivalent neodymium activator Ions (Nd ³⁺ ) and trivalent chromium sensitizer ions (Cr ³⁺ ).

Basically there are only very few ions which, due to their electronic structure, can be expected to act as activators in suitable basic lattices, after excitation with a higher-energy UV radiation, emissions in the preferred NIR spectral range, in particular between 800 to 950 nm. The possible activators include the trivalent neodymium ions (Nd ³⁺ ), although these ions are characterized by the fact that they only have 4f-4f transitions in almost all basic lattices in the specified energy range for excitation, which is forbidden by quantum theory, which leads to that the efficiency of the spectral excitability and thus also that of the luminescences resulting from the excitation is extremely low. For this reason, it is preferred to find suitable sensitizer ions, which in turn are able to effectively absorb the excitation radiation and then transfer it to the Nd ³⁺ activator ions with a likewise high efficiency.

In view of the inventive task, numerous experimental investigations by the inventors have shown that in particular Cr ³⁺ ions in selected basic phosphor lattices are able to exert this sensitizing effect with regard to the desired luminescence of the Nd ³⁺ activator ions.

An advantageous embodiment of the invention therefore comprises a security feature that is based on the use of novel, specially configured Phosphors based in the UV-A range (for example in the UV-C range between 200 nm and 280 nm and / or in the UV-B wavelength range between 280 nm and 315 nm and / or preferably in the UV-A spectral range between 315 and 380 nm) and which have at least two emission peaks that are spectrally closely spaced in the NIR range, preferably in the range between 800 and 950 nm.

The preferred luminescence properties could be realized in particular through the combination of Nd ³⁺ activator and Cr ³⁺ sensitizer ions and through the selection of suitable basic lattices. The surprisingly high efficiency of the energy transfer between the Cr ³⁺ and Nd ³⁺ ions results in luminescence intensities that ensure reliable detection.

The phosphor classes particularly suitable for generating UV-excitable emissions in the NIR range between 800 and 950 nm include, in particular, Cr ³⁺ - and Nd ³⁺ - codoped basic lattice materials with a garnet structure, or correspondingly co-activated aluminates or gallates, which have a magnetoplumbite Structure and special tungstates, which are also referred to as double perovskites with regard to their structural classification in specialist literature.

It should be noted that the basic lattice materials used for the phosphor configuration are known in principle from different types of technical applications. For example, the magnetoplumbites of the form SrGa ₁₂ O ₁₉ , SrAI ₂ O ₁₉ , LaMgAl ₁₁ O ₁₉ (LMA) were described as fluorescent host lattices as early as 1972 and 1974 in the work presented by JMPJ Versiegen (Versiegen 1972, Verstegen 1974), with these being used for doping in the case of excitation with 254 nm radiation, pigments which finally emit in the visible range were primarily Mn ²⁺ , but also Eu ²⁺ - TI ⁺ Ce ³⁺ and Tb ³⁺ ions, or combinations of these activators. Garnet-based phosphors which are doped differently and luminescent under different excitation conditions are also known.

Important examples of individual inventive phosphors that can be assigned to these phosphor classes are the phosphors Y ₃ Al ₅ O ₁₂ : Cr ³⁺ , Nd ³⁺ (garnet), SrAI ₁₂ O ₁₉ : Cr ³⁺ , Nd ^{3 +} , LaMgAlnO ₁₉ : Cr ³⁺ , Nd ³⁺ and SrGa ₁₂ 0i ₉ : Cr ³⁺ , Nd ³⁺ (each magnetoplumbite) and Ca ₂ MgWO ₆ : Cr ³⁺ , Nd ³⁺ (double perovskite). Special designs

According to an alternative notation, which also allows information on the concentrations of the activator and coactivator (sensitizer) ions and on the basic lattice substituents that can be used, the preferred phosphor classes can be identified by the general formulas listed below:

A preferred embodiment of the invention relates to a security feature in which the at least one phosphor has a garnet-phosphor basic grid and can in particular be described by the general formula (Ln _1-x Ndx) 3 (M _1-y Cr _y ) 5 _O12 . Ln here preferably stands for one of the two rare earth elements yttrium (Y) and / or lutetium (Lu), although these elements can also be completely or partially replaced by the elements lanthanum (La) and / or gadolinium (Gd), while M takes precedence denotes the elements aluminum (AI) and / or gallium (Ga), which can also be at least partially substituted by scandium (Sc). For the Nd and Cr concentrations in the case of the garnet phosphors according to the invention, the relations 0 <x <0.2 and 0.005 <y <0.4 or preferably the relations 0.01 <x <0.1 and 0 apply, 01 <y <0.06.

Another preferred embodiment of the invention relates to a

Security _feature in which the at least one phosphor represents a magnetopium bit of the general formula (EA _1-x Nd _x ) (M _{1-yx / 12} Cr _y / \ / _{x / 12} ) i ₂ 0i ₉ . In this case, the designation EA is used for the elements calcium (Ca), strontium (Sr), barium (Ba) and lead (Pb), which are wholly or partly as

Basic lattice components can act. The letter M here again primarily symbolizes the elements Al and / or Ga, while N preferably stands for the elements magnesium (Mg) and / or zinc (Zn), which are inserted into the grid for the purpose of charge compensation. The advantageous concentrations of the activator and sensitizer elements can be determined by the relationships 0 <x <0.2 and 0.005 <y <0.1 or preferably by the relationships 0.02 <x <0.1 and 0.02 <y <0.06 can be described.

Another advantageous embodiment of the invention relates to a

Security feature in which the at least one phosphor comprises a magnetopium bit of the type (Ln _1-x Nd _x ) N (M _1-y Cr _y ) ₁₁ O ₁₉ . Here, Ln preferably denotes one of the rare earth elements La and / or Gd, and N again primarily denotes the elements Mg and / or Zn and M are in turn used primarily to identify the elements Al and / or Ga. In the case of this type of magnetopium bit phosphors, too, the concentrations of the activator and sensitizer elements are in the range from 0 <x <0.2 or from 0.005 <y <0.1 and preferably in the range from 0.02 <x < 0.1 and 0.02 <y <0.06 are set.

Yet another advantageous embodiment of the invention relates to a security _feature in which the at least one phosphor has a double perovskite structure and is given by the general formula (EA _1-3X Nd _x Na _x2 ) ₂ (Mg1. _3y Cr _y Li _2y ) W0 ₆ , is marked. EA is used again to identify the elements Ca and / or Sr and / or Ba, the elements sodium (Na) and lithium (Li) act as charge compensators and the indices to identify the advantageous concentration ranges for the activator and sensitizer element values meet the conditions 0.001 <x <0.1 and 0.001 <y <0.1 or, in a particularly preferred manner, the conditions 0.005 <x <0.03 and 0.005 <y <0.04.

In addition, tungstates of the form AI2W3O12, LiLuW ₂ 0 ₈ and Ba ₆ Lu ₂ W ₃ 0i ₈ can also be selected as further preferred basic lattices for the formation of Cr ³⁺ and Nd ³⁺ codoped phosphors. In general, it should be pointed out at this point that the list of possible host lattices for the provision of Cr ³⁺ - and Nd ³⁺ - co-activated luminophores that can be excited in the ultraviolet spectral range and emit effectively in the NIR range does not represent a restriction. Rather, further classes of material or individual compounds can also be included in the selection of the inorganic solid-state compounds suitable as the basic phosphor lattice.

Another preferred embodiment of the invention relates to a security feature that not only contains a single inventive phosphor, but also comprises multiple phosphors in the form of phosphor combinations, these multiple phosphors having different basic grids and wherein all the phosphors forming the security feature and the corresponding phosphor combinations can be excited in the ultraviolet spectral range and emit after excitation in the infrared spectral range, in particular in the near infrared spectral range (NIR) in a wavelength range from 780 nm to 3 mm, preferably in the wavelength range from 800 nm to 950 nm. Another preferred exemplary embodiment of the invention relates to a security feature in which the several inventive phosphors which have been put together to form phosphor combinations and which have different basic lattices are characterized by an essentially identical or similar aging resistance. The same or similar aging resistance is particularly advantageous because on this basis it can be achieved that the emission properties of the security features equipped with such phosphor combinations do not change over a longer period of use, which means that the emission spectrum and thus the ratios of the intensities of the individual emission lines and / or emission bands of such a security feature are essentially retained over the life cycle of the corresponding security and value documents.

An advantageous embodiment of the invention relates to a security feature in which the at least one phosphor or the multiple phosphors combined to form phosphor combinations in one or more ultraviolet wavelength range (s), in particular in the wavelength range between 200 nm and 280 nm (UV-C range) and / or in the wavelength range between 280 nm and 315 nm (UV-B) and / or preferably in the UV-A spectral range between 315 and 380 nm and wherein the resulting infrared emission spectra include several individual emission lines and / or emission bands, the maxima of which are preferred are only a few nanometers apart. In particular, they are at a distance of less than 10 nm, particularly preferably less than 5 nm, very particularly preferably less than 3 nm. In a particularly advantageous embodiment of the invention, luminescence codes can be assigned to the infrared emission spectrum of the at least one inventive phosphor or to the emission spectrum of the combination of several phosphors, which are formed by the spectral sequence of selected emission lines and / or emission bands. These security codes, which can preferably be read out by machine, can be used for authenticity verification, nominal value coding or also for sorting, for example of different banknote denominations or value products. An alternative or supplementary embodiment of the invention relates to a security feature in which the at least one phosphor or the phosphors combined to form phosphor combinations have mean grain sizes of approximately 5 nanometers to approximately 15 micrometers.

Luminescent security features can preferably be used on, on or in different security or value documents (for example bank notes, ID cards, passports, driver's licenses, etc.) or in product protection.

The selected Cr ³⁺ and Nd ³⁺ codoped phosphors forming the security feature can be applied or attached, for example, with the help of conventional printing technologies (gravure, flexographic, offset or screen printing processes, etc.) or by using different types of coating processes The materials to be coated can consist of paper, different plastics or also of other organic or inorganic substances.

It should be emphasized once again that important criteria for the selection of the phosphors that can be excited in the UV range and emit linearly in the NIR range between 800 and 950 nm are, for example, the highest possible luminescence yield, sufficiently high stability and aging resistance to environmental influences, as well as a selected printing and application processes are the particle size distribution of the luminescent pigments. These properties are, for example, also of great importance for the manner in which the corresponding security features are applied on or in the respective security and value documents as well as for secure verifiability over the entire service life or service life of the security or value document.

Yet another preferred exemplary embodiment of the invention relates to a security feature that can be used, for example, to secure the authenticity of banknotes, ID cards, passports, driver's licenses, etc., and further information, for example about the position and / or shape of the security feature, is assigned to the security feature. In particular, the security feature can have a specific contour, for example in the form of a symbol, a number or a pictogram. The invention is explained in more detail below on the basis of preferred exemplary embodiments with reference to the figures named below.

Here show:

Fig. 1: the emission spectra of preferred exemplary embodiments for three Cr ³⁺ -

Nd ³⁺ codoped magnetopium bit phosphors after 365 nm excitation,

Fig. 2: Excerpts from the emission spectra according to the

Embodiments 1 to 3 obtained Cr ³⁺ - Nd ³⁺ codoped phosphors with a magnetoplumbite structure after UV excitation with a 365 nm UV radiation source,

Fig. 3: a comparative representation of the excitation spectra of the NIR

Luminescence of the inventive phosphors provided on the basis of the exemplary embodiments 1 to 3,

4: the excitation spectrum of the 896 nm emission of the phosphor according to embodiment 1 in comparison with a phosphor activated exclusively with Nd ³⁺ ions with an otherwise similar composition,

5: the excitation spectrum for the 879 nm emission of a Cr ³⁺ - Nd ³⁺ - codoped tungsten atomic phosphor according to embodiment 4 in comparison to a phosphor of comparable composition activated exclusively with Nd ³⁺ ions, and FIG

Fig. 6: Excerpts from the emission spectrum of the Cr ³⁺ - Nd ³⁺ - codoped

Phosphor according to embodiment 4 after 365 nm UV excitation.

Fig. 1 shows the emission spectrum 11 of a preferred embodiment 1, which relates to a magnetoplumbite phosphor of the type (EA _1-X Ndx) (M _{1-yx / 12} Cr _y N _X12 ) ₁₂ O ₁₉ , the specific composition of the exemplary Strontium gallate phosphor can be described by the formula Sr _0.96 Nd _0.04 Ga _{11, 36} Cr0 ₆₀ Mg _0.04 O ₁₉ . For its preparation, 0.1417 g (0.960 mmol) of SrC0 ₃ , 0.0067 g (0.020 mmol) of Nd ₂ 0 ₃ , 1.0646 g (5.680 mmol) of Ga ₂ 0 ₃ , 0.0456 g (0.300 mmol) of Cr ₂ 0 ₃ and 0.0025 g (0.040 mmol) MgF ₂ completely homogenized by pestle with the addition of acetone. Once the solvent has evaporated, the dry one becomes Powder mixture transferred to a corundum crucible. The sample is first precalcined at 1000 ° C. for 2 hours in an air atmosphere in order to decompose the strontium carbonate used, and then re-calcined in the presence of air at 1400 ° C. for 6 hours in order to obtain the end product. Between the two heating steps and after the thermal treatment, the sample is ground and then sieved.

In addition, FIG. 1 shows a further emission spectrum 12 for a further exemplary embodiment 2 of a Cr ³⁺ - Nd ³⁺ - codoped strontium aluminate phosphor with a comparable magnetopium bit structure. For this purpose, 0.1417 g (0.960 mmol) SrC0 ₃ , 0.0067 g (0.020 mmol) Nd ₂ 0 ₃ , 0.5852 g (5.740 mmol) Al ₂ O ₃ , 0.0365 g (0.240 mmol) Cr ₂ O ₃ and 0.0025 g (0.040 mmol) MgF ₂ mixed and homogenized with the addition of acetone. The annealing process again comprises two stages, first the batch mixture is annealed for two hours in the presence of atmospheric oxygen at 1000 ° C., followed by a further thermal treatment at 1600 ° C. for 8 h under otherwise identical annealing conditions. The usual process steps for sample preparation and grain size adjustment (intermediate and final grinding, sieving processes) correspond to those which were also used in the case of the first exemplary embodiment. After completion of the production procedure, the phosphor has the composition Sr _0, Nd 0, Al _11.48 Cr ₀ , ₄₈ Mg ₀ , ₀₄ .

The method steps of preparation, thermal treatment and post-treatment of the sample described in the above-mentioned embodiments 1 and 2 can in principle also be used in the case of the production of magnetopium bit phosphors of the general form (Ln _1-x Nd _x ) N (M _1-y Cr _y ) ₁₁ O ₁₉ can be applied. However, the batch mixture used for the synthesis of a corresponding Cr ³⁺ - Nd ³⁺ - codoped lanthanum magnesium aluminate phosphor according to embodiment 3, which contains 0.1564 g (0.480 mmol) La ₂ 0 ₃ , 0.0067 g (0.020 mmol) Nd ₂ 0 ₃ , 0.5327 g (5.225 mmol) Al ₂ 0 ₃ , 0.0418 g (0.275 mmol) Cr ₂ 0 ₃ , 0.0387 g (0.960 mmol) MgO and 0.025 g (0.040 mmol) MgF ₂ contains, after a first two-hour calcination at 1000 ° C, then annealed three times for 8 hours at 1600 ° C. The annealing material is ground intensively between the individual heating steps. The resulting phosphor is characterized by the formula La _0.96 Nd _0.04 MgAl _10.45 Cr _0.55 O ₁₉ . 1 shows the emission spectra 11, 21 and 31 of the Cr ³⁺ -Nd ³⁺ codoped described in the three exemplary embodiments 1, 2 and 3

Magnetoplumbite phosphors when excited by a 365 nm UV radiation source.

As can be seen from FIG. 1, these emission spectra have intense luminescence in the NIR range between 840 and 940 nm and between 1020 and 1120 nm. As far as the spectral range between 800 and 950 nm mentioned first and preferred according to the invention is concerned, the exemplary inventive phosphors are primarily characterized by two narrow-band emissions between 850 and 880 nm and between 880 and 920 nm. Depending on the specific phosphor composition in each case and the associated differently strong crystal fields, characteristic differences are recorded for the spectral emission maxima and the intensity ratios between the individual emission lines or bands of the various phosphors.

This can be seen in particular from FIG. 2, in which extracts of the emission spectra of the three exemplary embodiments (wavelength range 850 to 930 nm) have been compiled in one illustration. The spectral maxima of the individual emission lines were marked in FIG.

In comparison to fluorescent grids activated exclusively with Nd ³⁺ ions, the selected magnetopium bit and additionally equipped according to the invention with Cr ³⁺ sensitizers, as well as, for example, the corresponding codoped garnet or other preferably used phosphors in the spectral range from 250 to 700 nm have a very high absorption strength. This is illustrated in FIG. 3, in which the excitation spectra 12, 22 and 32 for the NIR emissions occurring in the spectral range between 890 and 910 nm of the Cr ³⁺ - Nd ³⁺ -codoped magnetoplumbit- obtained according to embodiments 1 to 3 Phosphors are shown in summary.

As FIG. 3 shows, these excitation spectra essentially consist of three distinguishable excitation bands. Two of these bands with their maxima between 400 and 500 nm and between 500 and 600 nm are primarily positioned in the visible spectral range, with the first mentioned excitation band extending into the near UV range. These gangs can electronic transitions from the ⁴ A ₂ ground state of Cr ³⁺ into the excited states ⁴ T ₁ ( ⁴ F) or ⁴ T ₂ ( ⁴ F) can be assigned. Another excitation band comprises the UV range 230-350 nm responsible for the absorption in this spectral electronic transition is generally in the literature as ⁴ A ₂ -. ⁴ indicates T1 ⁽⁴ P) junction. Even in phosphors doped exclusively with Cr ³⁺ ions, this absorption or excitation band is often only extremely weak. This also applies, for example, to the phosphor Al ₂ 0 ₃ : Cr ³⁺ (ruby laser) used in numerous technical applications and preferably as a laser material. In contrast, the specially configured Cr ³⁺ - Nd ³⁺ - codoped phosphors, which are preferably selected and specially configured for the purpose of realizing the inventive idea, are fundamentally distinguished by comparatively high absorption strengths in the ultraviolet spectral range. The efficiency of the spectral excitability can be adjusted and maximized by optimizing the specific phosphor composition.

The surprisingly high efficiency of the sensitizing effect emanating from the Cr ³⁺ ions additionally built into the selected Nd ³⁺ -activated phosphors is illustrated once again in FIG. 4. In FIG. 4, the excitation spectrum 12 for the linear 896 nm luminescence of a Cr ³⁺ - Nd ³⁺ - codoped strontium gallate phosphor according to embodiment 1 is the excitation spectrum 13 of a basic lattice material activated exclusively with Nd ³⁺ ions with otherwise compared with the same composition. Both excitation spectra were recorded under identical measurement conditions.

A particularly high absorption strength in the UV range was also achieved, for example, in the case of the preferred Cr ³⁺ - Nd ³⁺ - codoped tungstate phosphors of the general formula (EA _1-3 xNd _x Na _2x ) ₂ (Mg _1-3y Cr _y Li _2y ) W0 ₆ , can be determined.

FIG. 5 shows the excitation spectrum 42 of a corresponding phosphor configured in accordance with exemplary embodiment 4.

0.1912 g (1.910 mmol) CaCO ₃ , 0.0050 g (0.015 mmol) Nd ₂ O ₃ , 0.0032 g (0.030 mmol) Na ₂ CO ₃ , 0.0385 g (0.955 mmol) are used to produce this phosphor mmol) MgO, 0.001 1 g (0.0075 mmol) Cr ₂ 0 ₃ , 0.001 1 g (0.015 mmol) Li ₂ CO ₃ and 0.2318 g (1,000 mmol) WO ₃ mixed intensively and with the addition of acetone completely homogenized. The dried powder mixture is then transferred to a corundum crucible and initially precalcined at 600 ° C. for 2 hours in air. After regrinding, a second annealing procedure also takes place in the presence of an air atmosphere at 1300 ° C. for 5 h. The synthesis procedure according T his phosphor obtained is characterized by a composition _o by the formula Ca _{1, 91} _{Nd, o3} Na _0.06 Mg _0.955 Cr _0.015 Li can be _0.03 WO _6.

As shown in FIG. 5, the Cr ³⁺ - Nd ³⁺ - codoped tungstate phosphor produced in accordance with exemplary embodiment 5 has a broad excitation band in the UV range between 230 and 380 nm, the intensities of which are those of the excitation bands that are also present in the visible Significantly exceeds the spectral range. In comparison to the situation described in the case of the Cr ³⁺ - Nd ³⁺ - codoped magnetoplumbite materials, this means practically a reversal of the intensity relationships of the excitation bands found in the UV or in the visible spectral range. This characteristic also applies to other representatives of this class of phosphor with a double perovskite structure.

Preferred tungstate phosphors with a particularly high spectral excitability in the UV range also have a tendency to develop two excitation maxima. From this fact it can be concluded that in these - in the context of the invention - particularly advantageous Cr ³⁺ - Nd ³⁺ - codoped luminophores, in addition to electronic transitions of the form ⁴ A ₂ - ⁴ T- ₁ ( ⁴ P), so-called band-band -Transitions are responsible for absorption in the ultraviolet spectral range.

The excitation spectrum 43, likewise recorded in FIG. 5, of a tungstate phosphor synthesized according to exemplary embodiment 4 but activated exclusively with Nd ³⁺ ions illustrates this finding. In addition to the comparatively weak absorption lines that are characteristic of the Nd ³⁺ activator ions and are based on 4f-4f transitions, the corresponding excitation spectrum also has an absorption band with a maximum at around 290 nm, which could possibly be assigned to such a band-to-band transition .

Another peculiarity of the preferred Cr ³⁺ - Nd ³⁺ - co-activated tungstate phosphors can be seen in the fact that they, to a significantly greater extent than in the case of the garnet and magnetopium bit luminophores described, after UV excitation, especially in the spectral range between 860 and 940 nm, have line groups with numerous closely spaced emission lines.

The extract 41 shown in FIG. 6 from the emission spectrum of an exemplary tungstate phosphor according to embodiment 4 shows that the spectral distances between the maxima of the individual lines are mostly only a few nanometers. These facts result in promising starting points for assigning machine-readable luminescence codes to the emission spectra in the NIR range in a further embodiment of the invention, these codes being formed by the spectral sequence of the emission lines and / or emission bands of at least these phosphors. Another possibility would be to mix different individual phosphors of the materials identified as particularly suitable phosphor classes in order to provide the inventive security features in order to generate highly complex NIR emission spectra in this way.

The excerpts shown in FIG. 2 from the emission spectra 11, 21 and 31 of the three exemplary Cr ³⁺ - Nd ³⁺ - co-activated magnetoplumbite phosphors illustrate this possibility.

Reference list

11 Emission spectrum of a Cr ³⁺ -Nd ³⁺ codoped phosphor with

Magnetoplumbite structure according to embodiment 1 after 365 nm

Suggestion

21 Emission spectrum of a Cr ³⁺ -Nd ³⁺ codoped phosphor with

Magnetoplumbite structure according to embodiment 2 after 365 nm

Suggestion

31 Emission spectrum of a Cr ³⁺ -Nd ³⁺ codoped phosphor with

Magnetoplumbite structure according to embodiment 3 after 365 nm

Suggestion

41 Emission spectrum of a Cr ³⁺ -Nd ³⁺ -codoped phosphor with magnetoplumbite structure according to embodiment 4 after 365 nm excitation

12 Excitation spectrum of the NIR emission of a Cr ³⁺ -Nd ³⁺ codoped

Phosphor according to embodiment 1

22 Excitation spectrum of the NIR emission of a Cr ³⁺ -Nd ³⁺ codoped

Phosphor according to embodiment 2

32 excitation spectrum of the NIR emission of a Cr ³⁺ -Nd ³⁺ -codoped phosphor according to embodiment 3

42 excitation spectrum of the NIR emission of a Cr ³⁺ -Nd ³⁺ -codoped phosphor according to embodiment 4

13 Excitation spectrum of the NIR emission of a reference phosphor doped exclusively with Nd ³⁺ ions with regard to embodiment 1

43 Excitation spectrum of the NIR emission of a reference phosphor doped exclusively with Nd ³⁺ ions with regard to embodiment 4

Claims

Expectations

1 . Security feature for a security or value document, with at least one fluorescent substance which can be excited in the ultraviolet spectral range and which, after being excited, emits essentially in the infrared spectral range.

2. Security feature according to claim 1, characterized in that the emission in the near infrared spectral range (NIR) takes place in a wavelength range from 780 nm to 3 mm, preferably in the wavelength range from 800 nm to 950 nm.

3. Security feature according to claim 1 or 2, characterized in that the at least one phosphor comprises trivalent neodymium activator ions (Nd ³⁺ ) and trivalent chromium sensitizer ions (Cr ³⁺ ).

4. Security feature according to at least one of the preceding claims, characterized in that the at least one phosphor is a garnet basic lattice of the general formula (Ln _1-x Nd _x ) ₃ (M _1.y Cr _y ) ₅ O ₁₂ and / or a magnetoplumbite - Basic lattice of the type (EA _1.x Ndx) (M _{1.yx / 12} Cr _y N _{x / 12} ) ₁₂ O ₁₉ and / or a magnetoplumbite basic lattice of the general formula (Ln _1-x Nd _x ) N (M _{- y} Cr _y ) O ₁₉ .

5. Security feature according to at least one of the preceding claims, characterized in that the at least one phosphor has a tungstate basic lattice with a double perovskite structure of the general formula (EA _1- _3x Nd _x Na _2x ) ₂ (Mg _1-3y Cr _y Li _2y ) WO ₆ .

6. Security feature according to at least one of the preceding claims, characterized in that the at least one phosphor has a Y3AI5O ₁₂ basic lattice, a SrAI ₁₂ O ₁₉ basic lattice, a LaMgAI-n O-ig basic lattice, a SrGa-i ₂ 0i ₉ - Has a basic grid and / or a Ca ₂ MgW0 ₆ basic grid.

7. Security feature according to at least one of the preceding claims, characterized in that the at least one phosphor has an Al ₂ W ₃ O ₁₂ basic grid, a LiLuW ₂ 0 ₈ basic grid and / or a Ba ₆ Lu ₂ W ₃ 0i ₈ basic grid .

8. Security feature according to at least one of claims 4 to 7, characterized in that the security feature comprises several phosphors and / or phosphor combinations with different basic grids.

9. Security feature according to claim 8, characterized in that the plurality of phosphors and / or combinations of phosphors with different basic grids have essentially the same or similar aging resistance.

10. Security feature according to at least one of the preceding claims, characterized in that the at least one phosphor in one or more ultraviolet wavelength range (s), in particular in the wavelength range between 200 nm and 280 nm (UV-C range) and / or in the wavelength range between 280 nm and 315 nm (UV-B) and / or preferably in the UV-A spectral range between 315 and 380 nm can be excited.

11. Security feature according to at least one of the preceding claims, characterized in that the at least one phosphor is characterized by an infrared emission spectrum with several individual emission lines and / or emission bands.

12. Security feature according to claim 1 1, characterized in that the infrared emission spectrum of the at least one phosphor or the plurality of phosphors and / or phosphor combinations with different basic grids is assigned a code, the code being determined by the spectral sequence of the emission lines and / or emission bands of the at least these phosphors is formed.

13. Security feature according to claim 11 and / or 12, characterized in that the maxima of the emission lines and / or emission bands of the infrared emission spectrum are a few nanometers apart, in particular a distance of less than 10 nm, particularly preferably less than 5 nm, entirely particularly preferably of less than 3 nm.

14. Security feature according to at least one of the preceding claims, characterized in that the at least one phosphor has an average grain size of about 5 nanometers to about 15 micrometers.

15. Security feature according to at least one of the preceding claims, characterized in that further information about the manner of the arrangement of the security feature, for example about the position or a shape of the security feature, for example in the form of a symbol, a number or a pictogram, is formed is.

16. Security or value document with a security feature according to at least one of the preceding claims.