WO2023237801A1

WO2023237801A1 - Glazed ceramic piece

Info

Publication number: WO2023237801A1
Application number: PCT/ES2023/070379
Authority: WO
Inventors: Sonia MARÍN CORTÉS; Esther ENRÍQUEZ PÉREZ; José Francisco FERNÁNDEZ LOZANO; Víctor FUERTES DE LA LLAVE; Miguel Ángel GARCÍA GARCÍA-TUÑÓN; Luis GUAITA DELGADO
Original assignee: Consejo Superior De Investigaciones Científicas (Csic); Keraben Grupo Sau
Priority date: 2022-06-10
Filing date: 2023-06-08
Publication date: 2023-12-14
Also published as: GB2618667A; ES2928896B2; GB2618667A8; ES2928896A1; BE1030334B1; BE1030334A1

Abstract

The invention relates to a glazed ceramic piece comprising: a) a ceramic substrate, b) a final layer of ceramic glaze, c) a luminescent composition integrated in a discreet area, or several discreet areas of the final layer of glaze, such that the luminescent composition comprises crystallisations with a feldspar structure, a vitreous phase, and crystalline particles comprising one or more lanthanide oxides.

Description

Glazed ceramic piece

FIELD OF THE INVENTION

The present invention is related to the technology of markers for ceramic products, both to prevent fraud and counterfeiting and for advertising purposes or to obtain decorative effects.

BACKGROUND

In the field of ceramic materials that are useful in construction, among others, it is important to be able to track the origin of a ceramic piece and be able to determine its authenticity, as well as its age.

To determine the authenticity of a product and avoid fraud, luminescent compositions have been used, which generally emit light in the region of the visible spectrum when illuminated with a certain type of light, for example, ultraviolet light. Even at the end of the product's useful life, for example, years after a building has been constructed and at the time of demolition, it is important to be able to identify, for example, the year of manufacture and/or the manufacturer and/or the plant from the manufacturer of the ceramic material used in the construction of the building.

In most ceramic materials for construction, a thermal treatment is necessary, which is associated with the formation of a reactive liquid phase at high temperature, which degrades the response of a luminescent composition and makes its integration into the ceramic material difficult. Given this difficulty, the procedures for obtaining ceramic materials for construction do not incorporate luminescent compositions.

The use of luminescent compounds or pigments to verify the authenticity of a valuable document has been known for some time, for example, a document printed on paper with at least one element of authenticity in the form of a document is disclosed in DE 198 04 032. luminescent substance based on a grid, doped with at least one metal from the rare earth group, which absorbs essentially in the visible. Holmium is used as a rare earth element. Document LIS2013193346 (A1) discloses a method of identifying an object, wherein the object comprises a security element containing one or more inorganic luminescent pigments, the method comprises generating an emission spectrum of an inorganic luminescent pigment; and compare the spectrum obtained with the predetermined spectrum for the inorganic luminescent pigment.

This document discloses a pigment, CaAhShC^Eu.Pr, for materials identification, such that said pigment is a plagioclase-type feldspar doped with a rare earth.

Furthermore, the purpose of the invention according to this document is to identify materials, among which ceramic materials are mentioned. The luminescent pigment is said to be introduced into a matrix transparent to UV light. Although transparent ceramic matrices usually have UV absorption, the meaning of “transparent to UV light” in this document US2013193346 is relative, since many ceramic enamel matrices, according to this, would be semi-transparent. However, it does not disclose whether or not the luminescent material is integrated into the crystalline structure of the luminescent pigment, nor how to obtain that ceramic material with the luminescent composition in its structure.

No details of obtaining the “security element” are disclosed, it is only said that it can be included immediately in the object to be marked, for example, it can be mixed with the starting materials, or it can be applied by impression.

According to the present invention, a pigment with a feldspar structure that includes the rare earth is not generated, but rather a lanthanide oxide is included in a matrix that generates feldspar crystals and the luminescence comes from the lanthanide oxide, not from the doped feldspar composition. , because lanthanide oxide is not incorporated into the feldspar crystal lattice.

EP3685864 discloses a luminescent composition comprising albite or anorthite and a doping substance. The dopant can be an oxide of the lanthanide element, for example, europium oxide in a proportion between 1 and 30%. According to EP3685864, the items marked with the luminescent composition are not ceramic pieces, but mentions valuable items, including documents on paper media such as passports. Therefore, it does not disclose or suggest the possibility of marking ceramic pieces.

US10203282B2 relates to a mineral wool product comprising mineral fibers, in particular mineral wool adapted for use as a construction material. Substances active against UV or IR light are used in which the coloration can only be seen with the naked eye under ambient conditions when irradiated with a source of UV or IR radiation. The weight percentage of the UV or IR active component, based on the total mineral wool product, or alternatively the mineral wool component (for composite structural elements), may be less than 1.0% by weight.

The purpose of this composition is to follow the trace of the construction material to know its origin.

However, it does not disclose a ceramic piece marked with luminescent compositions and, therefore, it does not solve the specific problems posed by these ceramic materials to include the luminescent composition in them and achieve a sufficiently high luminescence with a small amount of luminescent material.

The UV or IR active component may be distributed throughout the mineral fiber product, for example, homogeneously between the mineral fibers and is preferably applied to the mineral fibers prior to curing of any binder of the mineral fiber product.

The active component according to this document is distributed throughout the material that is intended to be marked.

The article entitled Preparation of Eu ²⁺ and Dy ³ * co-activated CaA S Os-based phosphor and its optical properties, by Wang, YH; Wang, ZY; Qian, GD, Oct 2004, Material Letters, 58 (26), pp.3308-3311 discloses anorthite doped with a lanthanide, such as dysprosium or Eu ²⁺ and its use as a luminescent material. It does not disclose or suggest the specific use as a luminescent material for marking ceramic materials, nor the obtaining of said marked ceramic materials.

The article titled Persistent luminescence in CaAÍ2SÍ20a:Eu ²⁺ , R ³⁺ (R = Pr, Nd, Dy, Ho and Er), by Guifang Ju, Yihua Hu, Li Chen, Xiaojuan Wang, Zhongfei Mu; Feb 2014, Journal of Luminescence, 146, pp.102-108, refers to the persistent luminescence in europium-doped CaAhShOs after co-doping with rare earth auxiliary ions (Pr ⁺ⁱ Nd ⁺ , Dy ³⁺ , Ho ^{3 +} and Er ³⁺ ). This article discloses anorthite doped with rare earths and its use as a luminescent material, but it does not disclose ceramic pieces that include said luminescent materials, nor the use as a luminescent material for marking ceramic materials, so it does not describe how to obtain said marked ceramic materials.

CN108467727A, also discloses a material that is albite doped with rare earths, such as Sm ³⁺ and its use as a luminescent material. It does not mention the use as a luminescent material for marking ceramic materials, as it refers particularly to its use in LEDs. CN 107578254 (A) discloses Zr disclosed as a doping element of a luminescent composition. It does not mention the use as a luminescent material for marking ceramic materials, as it refers particularly to its use in LEDs.

CN110105950, also discloses a material that is albite doped with rare earths and its use as a luminescent material. It alludes to the use of albite in the enamel industry, although it does not mention its use as a luminescent material for marking ceramic materials.

CN111423118 discloses a ceramic piece comprising: 40-50 parts albite powder, 10-13 parts lithium/iron mica powder, 14-16 parts kaolin, 25-32 parts quartz powder, burnt talcum powders 7-8.5 parts, 5-6 parts titanium oxide, 3-3.6 parts sodium silicate. It may include 4-5 parts of rare earth oxides, preferably lanthanum and scandium. However, it does not disclose several fundamental characteristics of the present invention that require:

- that a luminescent composition is present in the ceramic piece (therefore distinguishable from the rest of the piece)

- that the luminescent composition is only found in one or several discrete areas of the final layer of ceramic enamel,

- that the lanthanide oxides are present only in certain crystalline particles of the luminescent composition, (as a crystalline phase therefore differentiable from the rest of the luminescent composition)

- that the luminescent composition has phases differentiated from each other.

According to CN111423118, the rare earth compound is not located in a discrete area, but rather distributed throughout the base mass of the piece (which would be the support) and/or throughout the enamel layer.

EP3685864 discloses a security marker, which comprises: a glass matrix comprising at least the elements silicon and oxygen; and a first crystalline phase formed by crystalline particles embedded in said matrix; where said particles are feldspars or feldspathoids; wherein the average size of said particles is less than 500 nm; and where there is an interface between the crystalline particles and the glass matrix

And depending on the production procedure, a lanthanide oxide can be included in the mixture of step (i).

However, it is not indicated that lanthanide oxide is located in crystalline particles other than feldspar crystalline formations, nor is it located in the glass phase of the composition, as is the case of the present invention. Furthermore, in all cases it refers to materials in particle form and, therefore, does not integrate a luminescent region in glazes on ceramic tiles.

In the state of the art, the dopant element or element that causes luminescence is incorporated into a crystalline structure that will act as a luminescent pigment or is incorporated in the frit production stage so that it is subsequently incorporated as a dopant in the crystalline phase. of feldspar to form the luminescent pigment.

According to the state of the art, the rare earth or element that causes luminescence is always incorporated as a “dopant” in a crystalline structure or in a glass structure.

The objective pursued in the state of the art is to be able to determine the presence of the doping element using equipment (a fluorometer) in order to be able to use very low proportions that are undetectable by other means.

Unlike what is indicated according to the state of the art, according to the present invention, crystalline particles that comprise lanthanide oxide in the luminescent composition are integrated into the glaze of the ceramic piece and said particles that comprise lanthanide oxide are maintained (at minus a significant proportion of it) as a marker signal that gives a very high luminescent signal response and therefore visible to the human eye.

The feldspar crystalline structure obtained according to the present invention is due to the crystals generated in the luminescent composition and in the event that the feldspar crystalline particles include lanthanide cations in solid solution, that is, they incorporate lanthanide cations as dopant , the luminescent signal is not relevant and is not visible to the human eye.

In the present invention, lanthanide oxide is incorporated as such into the mixture of components that produces, after thermal treatment during firing of the ceramic material, the luminescent composition. Said luminescent composition is integrated into the glaze of the ceramic material.

This precursor mixture of the luminescent composition is heat treated and ground to then make an ink. This heat treatment and grinding are steps that have not been carried out until now in the state of the art.

A mixture of frit, kaolin and lanthanide oxide is the precursor of the luminescent composition, described later in this report. The luminescent composition comprises regions of crystals with a feldspar structure and a minority phase without long-range structure, that is, a glass phase.

The luminescent composition integrated into the glaze is only present once the ceramic piece has been shaped into green and after the ceramic piece has been heat treated.

The main difference of the present invention with the state of the art is that in the state of the art the lanthanide element or combinations of lanthanide elements are included in the crystalline or glass structure of the luminescent composition. Consequently, the luminescence is very low and the use of a high concentration of dopant is not possible since the intensity decreases with increasing concentration. This effect is dominant if the excitation energy is transferred between many cations in the lattice in the time necessary for radiative decay, such as occurs when the concentration of rare earth cations in a crystalline or amorphous matrix increases.

In most prior art documents, luminescent properties are reported in terms of arbitrary units, so they are not data that can be compared with others. If the number of luminescent photons is very low, they are only detected by fluorimeter-type equipment and the human eye is not capable of detecting them. Determining the authenticity of a document is generally performed by determining the characteristic rare earth emission at a specific wavelength using optical equipment. In fact, this limitation in the detection processes of the luminescent marker is enhanced to avoid the detection of the presence of said marker and thus hide it from possible falsification attempts.

The problem that the present invention solves is to achieve a luminescent signal high enough for observation by the human eye. To this end, a process has been developed that allows the particles that comprise the lanthanide oxide to be maintained in dispersed or agglomerated form in the glass phase region. In this way, the lanthanide oxide does not form a solid solution with the feldspar crystalline phase, nor does it form a solid solution with the glassy phase of the luminescent composition. Even if it were incorporated in solid solution in the crystalline phase of the feldspar or in the glass phase, it would do so in a small proportion, so that the luminescence signal would not be affected by that proportion of lanthanide oxide in solid solution.

An essential advantage achieved in the present invention is related to the increase in luminescence performance with respect to pure lanthanide oxide, by using a maximum of 10% by weight of said lanthanide oxide or a maximum of 10% by weight. of a combination of lanthanide oxides, with respect to the weight of the composition, because said lanthanide oxide is dispersed in the region of the glass phase of the luminescent composition, outside the regions of feldspar crystals, in the form of an independent oxide of said crystalline network, that is, it is not integrated into it, or if it is, it is in such a low proportion that the decrease in luminescent signal is not relevant.

However, despite all the research activity in the technology of ceramic construction materials, the problems that these pose when it comes to being able to integrate into them a brand or identifying or decorative element that is revealed only under certain conditions have not been resolved. lighting conditions and that it is durable and stable for years so that it can also be used, not only for decorative effects, but also to know the life time of a material, or of a construction in which that material has been used. And in particular, it is necessary to achieve these aforementioned objectives with a small amount of luminescent material so that the production costs associated with raw materials such as lanthanide elements do not prevent their use in construction materials.

Description of the invention

The present invention relates to a glazed ceramic piece comprising a luminescent composition, such that the composition is invisible in the visible, or IR, spectrum, but visible by illumination with UV light in the visible range, or in the near infrared range, and such that said luminescent composition comprises a doping agent that is one or more lanthanide oxides.

The present invention refers, more specifically, to a glazed ceramic piece that comprises: a) a ceramic support b) a final layer of ceramic glaze, c) a luminescent composition integrated in a discrete area, or several discrete areas, of the final layer of enamel, such that said luminescent composition comprises:

- crystallizations with a feldspar structure;

- a glassy phase; and

- crystalline particles comprising one or more lanthanide oxides.

Ceramic support” and “support layer” are used interchangeably with the same meaning. The term "lanthanide oxide" herein may refer to a lanthanide oxide or a combination of lanthanide oxides.

The crystalline particles comprising lanthanide oxide are present in the composition so that they are partially part of the crystallizations with a feldspar structure and partially dispersed in the glass phase, or so that they are not part of the crystallizations with a feldspar structure, but which are completely dispersed in the glass phase.

The crystalline particles comprising lanthanide oxide are preferably present in the composition, so that they are not part of the crystallizations with a feldspar structure, but are mainly dispersed in the glass phase.

The term “dispersed” means that the crystalline particles are distributed in the glass phase, forming particles independent of said glass phase. The crystalline particles comprising lanthanide oxide may be in the form of individual particles separated from each other or forming agglomerates that are separated from each other so as not to form a percolating continuum.

Crystalline particles comprising lanthanide oxide may comprise other cations, such as Si4 ⁺ , Al3 ⁺ , Ca2 ⁺ , Sr2 ⁺ , Zn2 ⁺ , K ⁺ and/or Na ⁺ .

The crystalline particles comprising lanthanide oxide are arranged in the luminescent composition in a proportion of lanthanide oxide, with respect to the total lanthanide oxide dosed in step a) of the production process, such that when exposed to UV light the composition luminescent produces a luminescence signal of an intensity of at least 10 ⁶ , preferably at least 10 ⁷ , and more preferably at least 10 ⁸ , for a surface area of 2 cm ² , (measured in arbitrary units). Said luminescence values are comparable with the luminescence signal obtained for the amount of lanthanide oxide (100% by weight of lanthanide oxide) of the untreated starting material. These luminescence measurements are carried out using the same experimental measurement conditions, and the lanthanide oxide of partic acid gives a luminescence signal of an intensity of 10 ⁷ . That is, the amount of lanthanide oxide present in the luminescent composition is an amount such that the luminescence signal obtained is at least 10 ⁶ in arbitrary units, with respect to the luminescence signal obtained for the case in which 100% of that amount was pure lanthanide oxide. The amount of lanthanide oxide in the luminescent composition can also be defined indirectly, with respect to the amount of lanthanide oxide added in step a) and once it has been heat treated. The crystalline particles comprising lanthanide oxide are present in the luminescent composition in a proportion of lanthanide oxide, with respect to the total lanthanide oxide dosed in step a) of the production process, in a proportion such that upon exposure to light UV the composition produces a luminescence signal of an intensity of at least 10 ⁶ , preferably at least 10 ⁷ , and more preferably at least 10 ⁸ , for a surface area of 2 cm ² , (measured in arbitrary units), compared with the measurement made for the lanthanide oxide powder (100% by weight of lanthanide oxide) starting with heat treatment at 1220°C 6 minutes similar to that of a ceramic tile which, using the same experimental measurement conditions, gives a luminescence signal with an intensity of 10 ⁷ . That is, the response of the luminescent compositions of the present invention in terms of intensity of the luminescence signal (measured in arbitrary units) and with respect to the lanthanide oxide used is at least 10 ² times greater per percentage unit of lanthanide oxide.

According to preferred embodiments, these crystalline particles are arranged in the luminescent composition so that they do not form a solid solution with the feldspar crystalline phase, nor do they form a solid solution with the glassy phase of the luminescent composition. It can be verified that they do not form any of these solid solutions by measuring the luminescence as explained in the previous paragraph.

According to particular embodiments in the glazed ceramic piece, at least 80% of the lanthanide oxide in the form of crystalline particles, present in the composition, is in the region of the glass phase, preferably 90%, and more preferably 100%. Emission wavelength will depend on the specific lanthanide oxide, or oxides, used.

It is not possible to obtain a direct measurement of the amount of lanthanide oxide dispersed in the glass phase (in which it is mostly found) or included in the crystallizations with a feldspar structure, of the luminescent composition, but it can be known. indirect by the luminescence signal obtained.

The thermal treatment mentioned in the luminescence measurement consists of a treatment in an air atmosphere at 1220°C with a stay at maximum temperature. of 6 minutes and following a characteristic temperature profile for obtaining ceramic tile firing with a total duration of less than 60 minutes. The comparison between the signal of the luminescent composition comprising a maximum proportion of 10% by weight of the lanthanide oxide and the 100% lanthanide oxide itself reveals an increase in the efficiency in the use of lanthanide oxide for the purposes of the property. of luminescence. Comparison of the luminescent composition with heat-treated lanthanide oxide shows that lanthanide oxides suffer a decrease in their luminescent signal due to rapid heat treatments. The present invention requires heat treatments to consolidate the ceramic glaze on the surface of the ceramic tile and thus obtain the mechanical and resistance properties required for these construction materials.

The lanthanide elements whose oxides can be used in the luminescent composition of the present invention can be selected from Er, Gd, Ho, Sm, Ce, Nd, Yb, Tb, Pr, Tm and Eu and combinations thereof.

The luminescent composition comprises a glass phase and at least two crystalline phases, one of the crystalline phases is composed of crystallizations with a feldspar structure and the second is composed of crystalline particles that comprise lanthanide oxide. The crystalline particles comprising lanthanide oxide are preferentially dispersed in the region of the glass phase.

The expression “active substance” or “active particles” refers to the substance or substances responsible for luminescence.

Herein the expression “crystalline particles based on” or “comprising” lanthanide oxide is used to define the active particles or active substances that are present in the luminescent composition and that are microstructurally distinguished from feldspar crystalline particles. and the glass phase. The crystalline particles comprising lanthanide oxide have been generated from the lanthanide oxide particles added to the precursor mixture of the luminescent composition and during the heat treatment of the ceramic piece they are susceptible to incorporating cations in solid solution. These cations are mainly cations from the glass phase such as S¡ ⁴⁺ , Al ³⁺ , Ca ²⁺ , Sr ²⁺ , Zn ²⁺ , K ⁺ and/or Na ⁺ . The authors of the present invention have verified that the luminescence of said crystalline particles of lanthanide oxide and other cations is higher than that corresponding to the starting lanthanide oxide thermally treated at the same temperature. Surprisingly according to the present invention for lanthanide oxide dosages less than 10% by weight (compared to the precursor mixture, which does not lose more than 2% by weight during heat treatment), the authors have observed a greater intensity in the comparative luminescence value of the luminescent composition. Likewise, according to the present invention it has been observed that dosages of lanthanide oxide greater than 5% by weight produce the formation of a new crystalline phase based on silicates that correlates with a reduction in the comparative value of the luminescent signal.

The crystalline particles comprising lanthanide oxide can form crystalline agglomerates. The crystalline particles or these agglomerates that they can form are mainly dispersed in the glass phase of the luminescent composition.

Herein the term “lanthanide oxide” refers to a chemical compound that contains the element lanthanide in any of the oxidation states described in the state of the art, including a mixture of different oxidation states. Lanthanide elements are called “rare earth” elements because they are found in the form of oxides.

Herein the expression “feldspar crystalline structure” and “feldspar crystals” are used interchangeably with the same meaning. In all cases it means that the component referred to has a feldspar crystalline structure. Apart from these crystalline regions with a feldspar structure, the glass phase may comprise cations in a proportion equivalent to that of the chemical composition of a feldspar.

The glassy phase is present in the luminescent composition in a proportion by weight equal to or less than 30%, and preferably equal to or less than 20%, with respect to the corresponding weight of all the crystalline phases in the luminescent composition, that is, after stage h) of the procedure.

The term “glass phase” refers to inorganic materials or compounds that do not exhibit long-range crystalline order in their atomic structure.

The percentage by mass of crystalline phases of the luminescent composition is equal to or greater than 70% by weight, preferably equal to or greater than 80% by weight or with particular preference greater than 85% by weight with respect to the total weight of the luminescent composition.

The weight of the luminescent composition with respect to the ceramic piece is defined in terms of weight, g/m ² . According to particular embodiments, in glazed ceramics, the luminescent composition has a grammage of between 10 and 90 grams/m ² , preferably between 20 and 80 grams/m ² , and more preferably, 30 and 60 grams/m ² .

The term “integrated” means that they form an inseparable whole.

The expression “discrete zone” means that the luminescent composition is not mixed with the enamel (as seen in Figure 4), nor distributed throughout the final enamel layer, but in one or several specific areas of the final enamel layer. . These discrete areas do not occupy more than 50% of the surface of the enamel layer, preferably less than 30% and more preferably, they occupy a surface equal to or less than 10%.

Preferably, the luminescent composition is arranged in at least one or more discrete areas of the enamel layer that covers the ceramic piece, or final enamel layer.

The term “final layer” means that it is the one that constitutes the external surface of the piece, and therefore, visible.

The ceramic piece comprises at least one support layer and a final layer of glaze on the support layer.

Optionally, the ceramic piece comprises between the support layer and the final glaze layer: a layer of ceramic slip as an opacifier for the ceramic support.

Optionally, the ceramic piece comprises, in addition to the slip layer, a first layer of ceramic glaze between the support layer and the slip layer.

The luminescent composition is always in the final layer of the enamel and can be on the ceramic support or on a layer of enamel. In all cases it is covered with a final layer of enamel so that it is integrated into the glaze of the ceramic piece.

The amount of lanthanide oxides in the luminescent composition is between 0.1% and 10% by weight, including both limits, with respect to the total weight of the luminescent composition, preferably, between 0.1% and 5% by weight and more preferably between 0.1% and 2% by weight, including both limits, with respect to the total weight of the luminescent composition. The precursor mixture of the luminescent composition gives rise to an ink, and said ink is deposited on a green glazed ceramic piece, that is, before its heat treatment. After the heat treatment process, the ink has become the luminescent composition integrated into the glazed ceramic piece.

The precursor mixture of the luminescent composition of the invention, with which an ink is obtained, is thermally treated during the process of obtaining the ceramic piece, but since the only losses that occur due to calcination, fundamentally, are those of dehydroxylation of kaolin that have a value between 8-12% by weight of kaolin itself. The proportion of kaolin used in the precursor mixture of the luminescent composition is between 5 and 10% by weight. The weight loss experienced by the precursor mixture after heat treatment is thus between 0.4 and 1.2% by weight. It can be confirmed that the amount of lanthanide oxides in the luminescent composition and in the ink is within the same range.

The luminescent composition, as defined above, is obtained once the ink has been applied to a ceramic piece and it has been heat treated.

The term "ink", "digital ink" or "ink-jet ink" or "ceramic ink" refers to the ink that comprises the precursor mixture of the "luminescent composition" of the invention. Printing is one of the ways in which ink can be applied to a ceramic piece.

The ink is prepared with the precursors of the luminescent composition that have been heat treated and ground for incorporation into the mixture that gives rise to the ink.

Since the ink already includes lanthanide oxide, its luminescent signal in terms of the emission spectrum is similar in “green” and in firing, that is, when the luminescent composition has already been obtained. The difference is that in green it is alterable and in fired it is an integrated part of the ceramic piece. And this feature is a sign of the preservation of the crystalline structure of the particles comprising lanthanide oxide. Likewise, the luminescent signal alters in intensity as a consequence of the thermal treatment.

The luminescent composition, according to particular embodiments, comprises an amorphous phase or glassy phase and the following crystalline phases:

- a crystalline phase in the form of particles with a feldspar structure, which we have called “crystallizations with a feldspar structure”; and - a crystalline phase comprising one or more oxides of the elements: Er, Gd, Ho, Sm, Ce, Nd, Yb, Tb, Pr, La, Tm and Eu, or combinations thereof.

The crystalline particles comprising the lanthanide oxides are the luminescent active particles of the luminescent composition.

The luminescent composition according to preferred embodiments comprises crystalline particles comprising europium oxide as a luminescent active substance.

“Europium oxide” refers to an oxide having europium in any of its oxidation states.

Crystallizations with a feldspar structure are characterized by presenting a dual morphology with particles of size in the range of micrometers and particles of size in the range of nanometers. Microcrystalline particles with a feldspar structure are characterized by being surrounded by regions of nanoparticles with a feldspar structure and glass phase. The microcrystalline particles with a feldspar structure have a size range between 1 and 12 pm, preferably between 2 and 8 pm, and more preferably between 3 and 6 pm. Nanostructured particles or nanoparticles with a feldspar structure have particle sizes <500 nm, preferably <350 nm, and more preferably <200 nm.

It is very common for nanoparticles to give only the upper limit, since it is implicit that when the size is less than 1 nm, the nanoparticles would stop being nanometric and become sub-nanometric. On the other hand, the sizes of the crystal lattice are usually subnanometric; if the size of one unit cell is reached, there is no longer a long-range crystalline structure and they are no longer crystalline particles.

In the present invention, the crystalline particles comprising lanthanide oxide in the luminescent composition are preferably located in the region where the glass phase and the nanostructured particles with a feldspar structure are located. That is, they are found among the microcñstalline particles with a feldspar structure, they coexist with the microparticles with a feldspar structure, the nanoparticles with a feldspar structure and the glass phase. The particles comprising lanthanide oxide in the luminescent composition are the active particles and are characterized by having particle sizes <2 pm, preferably <1.5 pm, and preferably <1 pm. In a particular embodiment, the crystalline particles comprising lanthanide oxide are crystalline particles comprising europium oxide.

The glaze that is part of the ceramic piece may include, among the usual crystalline phases used in the ceramic glaze sector, tertiary crystalline phases, such as phases of: SiC>2 type quartz or cristobalite; zircon, ZrSiC; zirconia, ZrO ₂ ; wollastonite, CaSiCh; diopside, MgCaS¡ ₂ 0 ₆ ; mullite, AleSkO ; spinel, MgAhC; gahnite, ZnAhC ; cordierite, (Mg.Fe^AUSisOis; sphene, CaTiSiOs; and its solid solutions. The role of the tertiary crystalline phases defined in ceramic glazes is to modify the mechanical properties of the glaze and/or its optical response. The presence of these phases does not contribute to the active luminescent response of the luminescent composition. Its presence is related to the usual processes and uses in ceramic glazes, specifically for tiles.

The crystallizations with a feldspar structure that are part of the luminescent composition preferably have a plagioclase type structure. Plagioclases are a group of minerals that belong to the feldspar group and crystallize in the triclinic system. They belong to the group of tectosilicates, which include sodium and calcium in their composition; preferably they have a structure of albite, oligoclase, andesine, labradorite, banalsite, bytownite, anorthite or combinations of them.

Said luminescent composition arranged in one or several discrete areas in the final enamel layer of the ceramic piece can have various shapes so that it allows information about the ceramic piece to be associated with it for recognition.

According to particular embodiments, the glazed ceramic piece comprises the luminescent composition arranged in such a way that a recognizable pattern can be seen when illuminated with ultraviolet light. That is, the luminescent composition has a certain shape that is obtained at the time of decorating the ceramic piece.

The luminescent composition is visible in the visible or near-infrared spectrum, under ultraviolet illumination, generating on the surface of the piece when illuminated with said light, an image of a recognizable pattern, such as an image of a code.

For example, according to particular embodiments, the luminescent composition has a form of a recognizable pattern selected from, for example, codes such as a QR code, a barcode, a nomenclature code, a letter code, texts, symbols, names, logos, brands, signatures, numbers, link to a web page, a decorative figure, a graphic and a texture.

An image recognition technique can be used, but is not essential. According to the invention, a recognizable element can be generated, for example, a letter code such as XL, which means “Extra Large”.

The luminescent composition is not distinguished from the rest of the surface of the enameled piece by its color or texture. Since the luminescent composition is integrated, that is, within the final enamel layer, the differences in brightness generated by differences in the light diffraction index and by the roughness of the surface of the luminescent composition with respect to the ceramic support or the enamel, are not noticeable by the human eye, even for individuals trained in examining the surfaces of ceramic pieces.

The advantage of incorporating the luminescent composition in a ceramic piece corresponds to its integration into the final layer of the glaze of the piece, so that its presence is not perceptible under visible lighting, while through exposure to a UV light source The drawing or pattern represented by the luminescent composition is shown.

The advantages produced by the detection of this composition under UV light illumination make it possible to generate: double decoration effects only visible with UV light for nightclubs or public spaces of different types, etc. texts or symbols, name, logos, brands and/or identity of manufacturing companies or clients, QR codes or bar codes as a link to an information web page containing production batch number, product characteristics or certificate of materials raw materials used, artist's signature, serial number or author's mark of a collection of ceramic pieces.

In particular, a traceability system for ceramic pieces includes:

- a code formed by the luminescent composition in the ceramic piece that has been sintered with the ceramic piece itself

- a system for reading said code - the information corresponding to the ceramic piece, stored in a computer system.

According to particular embodiments, the ceramic piece comprises a luminescent composition that is a code, which:

- it is not visible when illuminated with white light and therefore does not interfere with the decoration of the ceramic piece,

- is visible with UV light illumination in the visible range or in the near infrared range (not being visible to the human eye),

- is indelible and permanent

- is associated with product information.

Preferably, the ceramic piece of the invention is a tile or a tile.

The printing process is the preferred process to integrate the luminescent composition into the glaze of the ceramic piece. To generate, for example, a code with the luminescent composition, printing is the best solution in terms of resolution and costs. Other techniques can be used, but they have less resolution and a higher cost.

The use of a luminescent composition integrated into the glaze of a ceramic piece allows preserving the aesthetic qualities of the ceramic under visible light and the technical characteristics of said ceramic such as its easy cleaning, resistance to stains, resistance to chemical agents, resistance mechanics and resistance to light exposure.

In the present invention, the ceramic piece comprises a support or support layer that can be made of any material known in the common art in the ceramic industry, in particular, it can be a support selected from porcelain stoneware, red stoneware, white body, terracotta or clinker. These supports make up the four most common product types in the ceramic tile industry.

The present invention also refers to a procedure for obtaining the ceramic piece defined above. Obtaining a glazed ceramic piece that comprises the integrated luminescent composition defined above, comprises: a) Homogenization of a precursor mixture of the luminescent composition by wet grinding; b) Drying of the homogenized composition obtained in step a); c) Sifting of the product obtained in stage b); d) Thermal treatment in an air atmosphere of the product obtained in stage c); obtaining a precursor luminescent composition; e) Grinding to condition the particle size of the precursor luminescent composition obtained in step d); f) formation of a ceramic ink with the precursor luminescent composition resulting from step e), g) decoration of a ceramic piece in green with the ceramic ink from step f) and h) firing of the ceramic piece that comprises the luminescent composition giving place to the enameled piece.

The steps d) and e) described are not carried out in the procedures according to the state of the art.

The precursor mixture of the luminescent composition, in step a), comprises a frit, kaolin, water, dispersant and lanthanide oxide.

The materials that can be used for the frits and dispersant are common in the art. In particular, all the materials described in document WO2016155909 and in the proportions indicated in WO2016155909 can be used in this invention. They are frits corresponding to vitro-ceramic enamels that present thermal warmth. The materials used and the procedures followed in WO2016155909 have been followed to obtain the frits used in the present invention.

The kaolin can be any, and preferably contain at least 35% by equivalent weight of AI2O3.

A glaze is obtained by thermally treating a mixture of a frit, kaolin, water and dispersant. In the state of the art, the term enamel is also used. liquid or enamel slip or enamel powder to refer to the precursor mixture that, once heat treated, gives rise to ceramic enamel.

As the enamel and the precursor mixture of the luminescent composition have very similar matrices, that is, their behavior in terms of firing presents almost equal values of softening, melting and stretching temperatures, as well as thermal expansion coefficients that do not differ in more than 5% in their values, said similarity produces the effective integration of the resulting luminescent composition and the glaze of the ceramic piece.

The size of the particles of the starting material of the lanthanide oxide(s) used in the precursor luminescent composition (in step a)) has a dso value of between 1±0.5 pm and 12±1 pm, preferably, between 2±1 pm and 8±1 pm, and even more preferably between 3±1 and 6±1 pm.

The drying of the homogenized composition obtained in step a) is carried out in a conventional manner in the glaze processing technology for ceramic pieces, such as tiles, for example, using a forced air oven at 60°C for 4 hours. The sieving of the product obtained in step b) is carried out in a conventional manner in the technology of processing glazes for ceramic pieces, such as tiles, for example, using a vibrosieve that uses a 150 mesh stainless steel mesh that is equivalent to a mesh light of 100 pm.

The thermal treatment in an air atmosphere of the product obtained in step c) is carried out in a conventional manner in the glaze processing technology for ceramic pieces such as ceramic tiles, for example, using an electric oven or a gas oven equipped with thermal regulation. to follow a programmed thermal cycle.

The precursor luminescent composition obtained by steps a) to d) of the procedure defined above comprises:

- fried;

- kaolin; and

- crystalline particles of one or more lanthanide oxides.

The precursor luminescent composition is not a product that is isolated, but is only the product resulting from step d). Therefore, it cannot be defined by the components that are used in stage a) of the procedure, because they have undergone various transformations so that they can only be defined through these stages of the procedure.

Stage g) of the procedure includes:

- deposit the ceramic ink obtained in step f) of the procedure in green:

- directly on the green support layer of the ceramic piece, or

- on a first layer of enamel deposited on the support layer in green or

- on a layer of enamel deposited on a layer of slip deposited on the green support layer.

In the event that the ink is deposited on a first layer of enamel, which is in turn deposited on the green support layer, then a final layer of enamel will be applied over it.

If the ink is deposited directly on the green support layer, then a layer of enamel will be applied over it.

The luminescent composition is integrated into the glaze of the ceramic piece in step g) through, for example, digital decoration on the piece in green, that is, before its heat treatment in step h). The ceramic piece must be subjected in step h) to a subsequent synthesizing heat treatment to obtain the final ceramic piece.

The ink application can be carried out with any of the known printing methods: planographic printing, gravure printing, digital printing.

The ceramic ink necessarily has to have the usual properties of the inks used in the sector. The essential thing is the precursor luminescent composition that it includes.

In step f) obtaining the ink is achieved through a dispersion process in a liquid medium by mechanical stirring of a composition that comprises:

- a precursor mixture of the luminescent composition resulting from step e);

- a solvent; and

- a dispersant. Obtaining inks is carried out by a conventional procedure. Both the solvents that can be used according to the present invention and the dispersants are any commonly used in the sector.

The term “solvent” refers to at least one chemical substance that dissolves a solute (solid, liquid or gas, chemically different). Generally the solvent is a liquid at the working temperature. In this invention, the term solvent is also used to prepare a suspension of at least one solid with low solubility in a liquid for processing. The terms solvent, solvent or vehicle are used as equivalents in this invention.

A preferred procedure for incorporating the luminescent composition into the green ceramic piece consists of a digital decoration process using a digital ceramic ink or ink-jet ink, obtained in step f) of the procedure. This ink comprises at least one lanthanide oxide - which has been included in step a) of the procedure - and a solvent, which is included in step f), and, preferably the solvent is an organic chemical compound, or is base aqueous or a mixture of both. The ink also includes a dispersing agent that acts by increasing the stability of the ink and its solids content. The decoration process is carried out on the green piece (which is then fired) by printing, for example, digital printing or ink-jet printing.

Stage h) of the procedure includes:

- carry out a heat treatment of the piece resulting from step g) above, through which the luminescent composition is consolidated and becomes a part integral with the ceramic piece, as it is integrated into it.

In the present invention, the heat treatment of step h) is carried out at a cooking temperature between 1100 and 1240°C, preferably between 1120°C and 1230°C, and more preferably between 1130°C and 1225°C. The duration of the heat treatment requires a thermal cycle with a total duration of between 45 minutes to 120 minutes with a stay time at the maximum treatment temperature between 6 minutes and 60 minutes. The heat treatment is carried out in standard heat treatment ovens for the ceramic pieces industry, such as tiles, such as gas tunnel ovens or electric ovens. The composition of the invention can also be used for the authentication or anti-counterfeiting of security documents, security articles and valuables.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1. Micrographs obtained by scanning electron microscopy of the precursor mixtures of the luminescent compositions thermally treated at different temperatures in step d) and of the precursor mixtures after grinding in step e). Micrographs corresponding to: A) CL1 1100°C; B) CL1 1220°C; C) CL2 1100°C; and D) CL2 1220°C. The indicated temperature refers to the treatment temperature in d) and the reference to the luminescent composition is the reference to the composition of the precursor mixture.

Figure 2. Micrographs obtained by scanning electron microscopy of the CL6 luminescent compositions after step h). Micrographs corresponding to the luminescent compositions polished and chemically etched using a 5% HF solution for 2 seconds. The micrographs correspond to different magnifications. In the micrographs, the darker regions correspond to crystalline particles of the feldspar phase, 1, and the lighter regions to crystalline particles that comprise lanthanide oxides, 2. The glassy phase is not observed in the micrograph because to highlight the microstructure A chemical attack is carried out on the polished surface that eliminates said glassy phase. Thus the gaps between the crystalline particles constitute the regions where the glass phase is located.

Figure 3. Glazed porcelain stoneware ceramic tile that presents rectangle-shaped drawings using T3 ink once fired and under UV lighting.

Figure 4. Scheme of some options for integrating a luminescent composition 5 into a ceramic piece. The integration of a luminescent composition 5 in a ceramic enamel 4 on a ceramic support 3 is carried out by depositing the luminescent composition 5 either on the ceramic support 3, or on a layer of enamel 6 that is covering the ceramic support 3, or on a layer of enamel 6 that is covering a layer of slip 7 covering the ceramic support 3.

Figure 5. Glazed porcelain stoneware ceramic tile integrating a CL1 luminescent composition formulated with T3 ink for digital printing forming QR code type drawings and company logo after stage h). The photographs correspond to the tile under A) visible lighting and B) UV lighting. Figure 6. Representation of the luminescent composition according to the invention, showing schematically: micrometric crystalline particles with feldspar structure, 1; crystalline particles comprising lanthanide oxide, 2; glass phase, 8; nanometric crystalline particles with feldspar structure, 9.

EXAMPLES:

In the following examples CL1 and CL2 designate precursor compositions of luminescent compositions that are called by the same term. Simply depending on the stage of the procedure that is being referred to is what determines whether it is the precursor composition or the final luminescent composition.

In the accompanying examples, the materials used are described in WO2016155909 and the procedures are those followed in WO2016155909 to obtain the frits used.

EXAMPLE 1. Preparation of a luminescent composition with a plagioclase structure incorporating EU2O3 particles.

The luminescent compositions CL1 and CL2 were prepared by a process that includes steps a) to d) described above, followed by f) Formation of a ceramic ink for digital decoration with the product obtained in step e).

And then, to obtain the final ceramic piece, steps g) and h are carried out: g) Digital decoration of a ceramic piece in green; h) Firing of the ceramic piece.

The precursor mixture of the luminescent composition CL1 was prepared by mixing in step a) the following components:

180 grams of a frit A (shown in Table 1)

19.67 grams of a kaolin, preferably containing at least 35% by equivalent weight of AI2O3

2 grams of EU2O3

135 grams of deionized water. The expression “equivalent weight” is commonly used in technology, and is the way of expressing the content of a certain cation in the material. It serves to define the oxidation state of the cation.

In the formulation of the precursor mixture, the weight loss due to calcination of the kaolin used was considered, which was 14% by weight, for a treatment of 600°C and 6 minutes. The dosage of kaolin in the formula once this loss was considered corresponded to 17.25 grams. Glass frit is a material melted at 1500°C and suddenly cooled in water and is characterized by being glassy in nature and does not present short-range crystalline order.

The size of the EU2O3 starting particles used has a dso value of 4±1 pm.

Frit A has the composition shown in table 1

The luminescent composition CL2 was prepared in a similar way using frit B, the composition of which is shown in Table 1.

The composition of frit A, frit B and kaolin expressed in terms of equivalent oxides is shown in Table 1.

Table 1. Chemical composition expressed in terms of equivalent oxides (% by weight with respect to the total of each material, that is, in each row of the table it will be with respect to Frit A; Frit B or kaolin).

(*) In other compounds, minor oxides are considered such as B2O3, P2O5, TÍO2, Fe2Ü3, MgO, among other minor oxides, where the percentage by weight of each of them is less than 1% by weight with respect to the total. (**) the weight loss of kaolin by calcination at 540°C is 14% by weight corresponding to the loss of hydroxyls from the network. The chemical analysis was carried out on the sample calcined at 600°C for 6 minutes.

In stage a) a process of manual weighing and dosing of the components was used in a porcelain jar mill containing 10 mm diameter alumina balls. The weight of the ceramic balls used is 500 grams. The ball mill used is from Heramika S.A. The grinding duration was 20 minutes. After step a), a homogeneous suspension is obtained that was extracted from the porcelain grinding jar using a 100 pm sieve. The homogeneous suspension or slip is deposited in a glass tray and dried in a forced air oven until the humidity is less than 1% by weight. In stage b) an oven at 60°C was used for a time of 24 hours.

The dry product obtained in stage b) was dry sieved using a 100 pm sieve in stage c), achieving a particle size (D50) of this material with a monomodal distribution of 6±3 pm. The determination of the particle size distribution was carried out using a laser system particle analyzer (Mastersizer S, Malvern). The dried and sieved product was placed in an alumina crucible for thermal treatment in step d).

In stage d) an electric oven was used in an air atmosphere at temperatures of 600, 700, 800, 900, 1000, 1100 and 1220°C for 6 minutes with a heating speed of 15°C/min and cooling according to the oven. , so that different samples were obtained, depending on the heat treatment received.

The material resulting from step d) was conditioned in a step e) to obtain a particle size suitable for the formulation of a digital decoration ink (common sizes in digital printing technology), it was first ground in a ball mill. of alumina in an aqueous medium similar to step a) and was subsequently ground in a laboratory attrition mill that reproduces the conditions of the Miniseries attrition mill with Minipur chamber from NETZSCH, for 1 hour using zirconia microballs stabilized with yttrium of 1mm diameter. This microball grinding process (in a mill analogous to the Miniseries attrition mill with Minipur chamber from NETZSCH) is a simplification of the standard process for preparing a ceramic ink for digital decoration that usually requires 6 hours of grinding in different types of microbeads to obtain a particle size less than 2 pm. All the precursors of the prepared luminescent compositions, that is, CL1 and CL2, thermally treated in stage d) at different temperatures and conditioned in stage e) were pressed into disc-type specimens of 2 cm in diameter and 2 mm thick. . In order to evaluate the properties of the material, these pressed specimens replace processes f) and g) to obtain a standard material and evaluate the luminescence of the composition after step h). The specimens were treated in stage h) with a thermal cycle with a maximum temperature of 1220°C maintained for 6 minutes and with a total cycle duration of 50 minutes in an electric laboratory oven from PIROMETROL that simulates a sintering cycle. of industrial ceramic tile for a porcelain stoneware material. Comparatively, pressed disk samples of the precursor luminescent compositions after heat treatment in step d) and, for comparative purposes, of the precursor luminescent compositions after step d) were prepared.

The luminescent compositions CL1 and CL2 after step h) present a crystalline structure determined by X-ray powder diffraction (XRD D8 Advance, Bruker) that corresponds to an albite-type plagioclase for composition CL1 and anorthite-type for composition CL2. The percentage of crystalline phase of the compositions determined from the X-ray diffraction diagrams corresponds to values greater than 92% in both luminescent compositions. Using XRD, the presence of crystalline phases of europium oxide is not detected, because its percentage is below the detection limit of the technique.

The average particle size (dso) of the luminescent compositions determined by laser diffraction (Mastersizer S, Malvern) after step e) of particle size conditioning showed a value <2 pm for compositions CL1 and CL2 treated in a thermal cycle up to 1000°C 6 minutes in step d). Treatments at higher temperatures generate average particle sizes >10 pm. It is therefore more advantageous to use temperatures lower than 1000 °C for 6 minutes and obtain particle sizes smaller than 10 pm, which result in greater efficiency of the subsequent stage e). Additionally, it was examined by scanning electron microscopy (FE-SEM S- 4700, Hitachi) to analyze the size and distribution of the particles in more detail. Figure 1 shows the micrographs of the samples based on the luminescent compositions CL1 and after the pretreatment of step d) at temperatures at 1100°C and 1220°C for 6 minutes and after step e). In the microstructural characterization, particles with irregular morphologies were observed characterized by the presence of multiple fracture edges with crystal sizes preferably less than 2 pm and more rounded particles with sizes less than 1 pm and with abundant presence of particles of sizes less than 0.5 pm. The particles have a tendency to form agglomerates where irregular particles are surrounded by more rounded particles. The particle sizes observed by scanning electron microscopy were smaller than those previously reported by the laser diffraction method, indicating that the laser diffraction technique is measuring the size of aggregates rather than particles.

The authors of the present invention have found that in the luminescent compositions there are no relevant differences in terms of particle sizes or luminescent response for thermal treatments in step d) whose duration is 1 hour or even 2 or 4 hours compared to the 6 minute long treatments. Therefore, 6-minute treatments are preferable as they represent an advantage over longer conventional synthesis heat treatments.

For the formation of inks in step f), according to the state of the art, a particle size of less than 2 pm is required. This size is obtained through intensive microgrinding. The particle size obtained in the present invention presents an advantage from the point of view of processing a ceramic ink for decoration by printing, for example digital, since step e) of the procedure allows obtaining a suitable particle size, that is < 2 pm, for stage f) from a microgrinding process that involves a lower energy cost with a reduction in grinding times of at least 80%.

The luminescent response of the luminescent compositions was determined using a fluorimeter (FluoroLog-3 Modular Spectrofluorometer, HORIBA Scientific). The fluorometer is equipped with an IR detector (IGA1.9010L) covering a wide wavelength range from 800 to 2100 nm, and a low dark current PMT detector (R2658) (1 nA at 1250 V) for measurements in the range of 185 to 1010 nm. The experimental measurements were carried out using different methods depending on the format of the sample: tablet in the case of stage d) and h) and powder in stage e). Both methods comprise an excitation wavelength of 393 nm, vaing the entrance and exit slits. These were respectively 0.5nm and 10 nm in the case of measurements carried out in powder, with an integration time of 1 s, using a glass cuvette and 1 nm and 5 nm in the case of samples in pellet format, with 0.1 s integration. The luminescent intensity values recorded for the maximum of the emission band centered 610-625 nm of the different compositions in the different treatment stages are collected in a comparative way with the reference sample: the oxide of europium, EU2O3, measured under the same experimental conditions and corresponding thermal treatment of each stage. All luminescent compositions presented the two characteristic bands of Eu ³⁺ around 590-592 nm and 610-625 nm, respectively, using an excitation wavelength of 393 nm. These emission bands correspond to the orange spectral range and are attributed to the ff transitions characteristic of Eu ³⁺ , ⁵ D ₀ ^ ⁷ Fi (590-592 nm) and ⁵ D ₀ ^ ⁷ F ₂ (610-612 nm), respectively.

Table 2. Maximum values of the luminescent emission band centered between 610-612 nm for an excitation wavelength of 393 nm of the different luminescent compositions, in different process stages. The luminescent emission was characterized comparatively using samples of the same surface area and thickness under the same experimental conditions, specifically, for discs of 2 cm in diameter and 2 mm thick in the case of the stage d)+ h) and in powder measured in bucket for the case of step e).

The luminescent compositions of the present invention have an advantage over the reference pure EU2O3 material in step d) since the luminescent intensity values are higher per unit of weight percentage of EU2O3 in the composition of only 1% in weight of EU2O3. The reference pure EU2O3 composition would present a luminescent signal of 5.0x10 ⁷ which per percentage unit of EU2O3 corresponds to a value of 5.0x10 ⁵ , this value for CL1 being >5.0x10 ⁸ in all thermal treatments of the stage d). Thus, greater efficiency is achieved in the use of lanthanide oxide in terms of luminescent emission, which is at least 10 ² per percentage unit of lanthanide oxide.

For comparative purposes, a frit C was prepared with a chemical composition similar to the chemical composition of Frit A where a proportion of EU2O3 was incorporated prior to the melting or frying stage at 1500°C in a ratio of components of the fñta/oxide of europium of 180/2. Frit C had a chemical composition such that when formulating in step a), 182 grams of frit C, 19.67 grams of kaolin and 135 grams of water were added. This formulation is chemically analogous to that of composition CL1, that is, it contains 1% by weight of EU2O3. From the precursor mixture of the luminescent composition CL3, following the same preparation process of the luminescent composition CL1 in steps b) to h) and as a result, a material with a luminescent band centered between 610-625 nm (determined in the same experimental conditions described for the luminescent composition CL1) whose intensity is less than 10 ³ . In this way, it was found that the incorporation of the Eu ³⁺ cations in the feldspar crystalline network or in the glass network inhibits the ff transitions characteristic of Eu ³⁺ . The fundamental difference between composition CL1 and luminescent composition CL3 is that the lanthanide oxide in composition CL3 is incorporated during the fritting process of the material, which reaches higher temperatures (1500°C) favoring the decomposition of the oxide, and causing the Eu ³⁺ cation to become embedded in the glass phase of frit C, that is, as a dopant of said glass network. Therefore, crystalline particles comprising europium oxide are not added to the precursor mixture of the luminescent composition in step a). After step h), in the luminescent composition CL3, the Eu ³⁺ cations are found as a dopant distributed either in the crystal lattice of the feldspar crystals or in the glass phase. It has not been possible to determine the location of the preferred one as a dopant, although the effect as a dopant of said cations is a luminescence value only detectable by a fluorometer and not detectable by the eye. In this way, the presence of crystalline particles comprising europium oxide in the luminescent composition CL3 is not obtained. The luminescent composition CL1 exhibits a luminescence that is >5 orders of magnitude higher than the luminescence of CL3.

EXAMPLE 2. Preparation of a luminescent composition with a plagioclase structure incorporating different percentages of EU2O3.

In an additional study, the same treatment procedure for the CL1 compositions was followed, but instead of adding 1% by weight of EU2O3 in step a) of the process, amounts of 2%, 5% and 10% by weight of EU2O3 to frit A to generate the compositions CL4, CL5 and CL6, respectively.

The luminescent response of the luminescent compositions was determined using the same fluorimeter as in the previous case. The experimental measurements were carried out using the method developed for samples in pellet format of 2 cm in diameter and 2 mm in thickness. A 17.4 mm x 6.2 mm rectangular mask was used. The luminescent intensity values recorded for the maximum of the emission band centered 610-625 nm of the different compositions in the different treatment stages are collected in a comparative way with the reference sample: europium oxide, measured under the same conditions. experimental and corresponding thermal treatment of each stage. All luminescent compositions presented the two characteristic bands of Eu ³⁺ around 590-592 nm and 610-625 nm, respectively, using an excitation wavelength of 395 nm. These emission bands correspond to the orange spectral range and are attributed to the ff transitions characteristic of Eu ³⁺ , ⁵ D ₀ ^ ⁷ Fi (590-592 nm) and ⁵ D ₀ ^ ⁷ F ₂ (610-612 nm), respectively. All the prepared luminescent compositions presented values of particle sizes and luminescent properties similar to those of the CL1 compositions. The most relevant differences observed were that the CL4 luminescent composition presented a value of the maximum of the luminescence band centered at 610-625 nm that is up to 30% greater in intensity compared to the CL1 luminescent compositions. The luminescent compositions CL5 and CL6 had a value of the maximum of the luminescence band centered at 610-625 nm that is up to 15% and 50% lower in intensity compared to the luminescent composition CL1, respectively. In this way, compositions with proportions between 1% and 2% of EU2O3 have an adequate luminescent response.

Table 3. Maximum values of the luminescent emission band centered between 610-612 nm for an excitation wavelength of 395 nm of the different luminescent compositions, in different process stages (steps e) and h)). The luminescent emission was characterized comparatively using samples of the same surface and thickness under the same experimental conditions, specifically discs of 2 cm in diameter and 2 mm in thickness)

The microstructure of the luminescent composition CL6 Figure 2, showed the presence of crystalline particles comprising europium oxide in coexistence with feldspar microparticles. The crystalline particles comprising europium oxide in Figure 2 correspond to the lighter regions in the image that are grouped in clusters of grains. The sizes of the crystalline particles comprising europium oxide have grain sizes between 100 and 500 nm. The microstructure shows an abundance of plagioclase-type feldspar microcstals. Between the plagioclase microcrystals and the regions with crystalline particles that comprise europium oxide, voids are observed corresponding to the glassy phase that has been eliminated by immersion of the surface in the acid solution for chemical attack.

EXAMPLE 3. Preparation of luminescent plagioclase compositions of different emission wavelengths.

The process described in Example 1 for the luminescent composition CL1 was followed. 1% by weight of lanthanide oxides was added. The lanthanide oxides used were crystalline oxides of the following elements: Er, Eu, Gd, Ho, Sm, Ce, Nd, Yb, Tb, Pr, La and Tm. After stages a), b) and c), in stage d), a ceramic disc of each composition was directly pressed, which was thermally treated at a single temperature of 1220°C for 6 min following a standard industrial cycle for a ceramic tile. .

Table 3 shows the different excitation wavelengths along with the emission wavelengths for each of the rare earths in the luminescent compositions CL7 to CL17.

Table 3. Wavelength values for the luminescent emissions of luminescent compositions CL7 to CL17 with different rare earths in 1% by weight dosage of the rare earth oxide and the corresponding excitation wavelengths associated with said luminescent emissions. The luminescent emission was characterized comparatively using samples of the same surface area and thickness under the same experimental conditions as those carried out for the CL1 composition.

Luminescent compositions CL7 to CL17 have the advantage of emitting at different wavelengths under UV excitation, that is, they allow emitting in different colors. The luminescent compositions CL7, CL11, CL13 and CL15 have an additional advantage by emitting in the near infrared for excitation in the visible area. Since infrared emissions are not detectable by the human eye, the presence of these compositions allows obtaining a luminescent composition detectable only by equipment with optoelectronic detectors. EXAMPLE 4. Integration of a luminescent composition in a glazed ceramic tile.

The process of integrating a luminescent composition requires the preparation of the decoration ink in step f). The characteristics of luminescent compositions obtained in examples 1 to 3 were suitable with respect to their particle size for the formulation of a ceramic digital ink or ink-jet ink. This ink was formulated in step f) through a dispersion process in a liquid medium by mechanical stirring of a composition that comprises:

- A precursor mixture of the luminescent composition resulting from step e);

- A solvent; and

- A dispersant.

The process of obtaining inks is known.

A specific example of the preparation of an ink with one of the luminescent compositions obtained, CL1, is described below.

A solvent-based ink, T1, was prepared in the laboratory by adding 40 grams of CL1 treated at 600°C in step e) of the luminescent composition, in 100 grams of a mixture of tri(propylene glycol) butyl. ether and poly(propylene glycol) monobutyl ether in a 50/50 ratio as solvents. To homogenize the ink, a T25 high shear agitator from IKA was used, working at 10,000 rpm for 10 minutes. Similarly, an aqueous-based ink, T2, was prepared in the laboratory by adding 33 grams of CL1 treated at 600°C in step e) of the luminescent composition, in 45 grams of deionized water and 20 grams of ethylene glycol as solvents and the addition of 0.6 grams of sodium salt of polycarboxylic acid as dispersant.

The laboratory inks T1 and T2 had parameters suitable for use as digital decoration inks on ceramics with zeta potential values <-45 mV (determined with a Nanolab, Micromeñtics equipment) and viscosities at 200 s ^-1 of 25 mPa. s (determined by a rotational viscometer from HAAKE).

From these results, 1500 grams of the luminescent composition CL1 were prepared, thermally treated at 600°C in step d) of the preparation of said luminescent composition. Said CL1 composition was used for the preparation of an industrial ceramic ink T3 in the company VIDRES SA (Villareal, Castellón). The formulation of ink T3 was similar to the formulations of inks T1 and T2, but for its formulation an industrial microball mill with a ceramic chamber was used where steps e) and f) were carried out simultaneously. Using this T3 ink, the Vidres company printed different drawings on two types of inkjet technology on a Cretaprint® digital printer. ceramic supports: porcelain stoneware and white body. These ceramic supports make up the two most common product types in the ceramic tile industry. Porcelain stoneware requires a firing temperature between 1200-1220°C and generates a product with a water absorption of less than 0.5% by weight. Porcelain stoneware ceramic tiles are preferably used as ceramic flooring both indoors and outdoors. The white paste support requires a lower firing temperature, 1135 °C, and has a greater water absorption capacity, between 6 to 10%. White-body ceramic tile is used as wall covering. A layer of matte glaze for porcelain stoneware obtained from frit B by combining it with kaolin was deposited on the green support layers of porcelain stoneware following what is described in WO2016155909. A layer of slip suitable for white body was deposited on the green support layer of white body. On said support layers in green, porcelain with enamel and white paste with slip, defined areas of the precursor mixture of the luminescent composition were deposited in the form of digital ink obtained in step f). The deposition of said defined areas of the digital ink was carried out by digital printing with different grammages corresponding to 20, 40, 60 and 80 g/m ² , obtaining different ceramic pieces with thicknesses of different luminescent compositions. The glazes and slips used in these applications are standard glazes and slips used in the ceramic tile industry.

Pieces were prepared with different ceramic supports, with different glazes and with different slips, and for the different glazes and/or slips tested, a linearity was observed in the intensity of the luminescent signal with the grammage value. Specifically, thickness values of the area of the luminescent composition lower than 20 pm were obtained for all the weights used. The increase in weight corresponded to an increase in the thickness of the area or pattern of the luminescent composition on the ceramic tile. Because the standard thicknesses of glazes on ceramic tiles are between 50 and 200 pm, the grammage values for the luminescent composition are between 40 and 10% of the thickness of the standard glazes. The range of weights corresponding to standard enamel is between 5 and 500 grams/m ² , which is equivalent to thicknesses of 4 to 400 pm. Figure 2 corresponds to a fired ceramic piece representative of the use of T3 ink deposited on a porcelain stoneware tile coated with a matte glaze, and observed under UV lighting. Table 5 shows the luminescence values for the different weights of ink applied and the thicknesses of the drawing of the luminescent composition obtained with the T3 ink after firing. The values obtained for the intensity of the luminescent emission are lower than those of the bulk material obtained for 2 mm thick discs. No However, these values are sufficient to be appreciated by the human eye as they notably exceed the phototopic and scotopic limits. This aspect represents an advantage of luminescent compositions since it allows obtaining a thickness value such that it can be integrated into the interior of a standard enamel layer in the ceramic tile industry while maintaining luminescence values suitable for observation.

An advantage of the present invention is that, given the reduced thickness of the integrated luminescent composition, the luminescent composition had a grammage of between 10 and 90 grams/m ² , more preferably between 20 and 80 grams/m ² . In this way, the proportion of europium oxide required per m ² of ceramic tile is between 0.2 and 8 g/m ² . This amount will be reduced proportionally when integrating the luminescent composition with a certain shape, that is, without covering the total surface of the ceramic tile. A non-limiting calculation indicates that the cost incurred per m ² of ceramic tile from the incorporation of a given form of a luminescent composition corresponds to units of euro cents. The passed-on cost of incorporating an integrated luminescent zone of the present invention is therefore an advantage for products intended as construction materials whose production cost per m ² is in the region of euro units.

Figure 3 shows a glazed porcelain stoneware ceramic tile that presents rectangle-shaped drawings using T3 ink once fired and under UV lighting.

Table 5. Value of the maximum luminescent intensity for luminescent emission in the 610-625 nm region of regions where T3 ink was deposited on glazed porcelain stoneware ceramic tile once fired. The luminescent emission was determined for a circular region of 1 cm in diameter under an excitation wavelength of 393 nm, the experimental conditions being similar to those carried out in examples 1, 2 and 3 except with regard to the thickness of the composition. luminescent and, therefore, the intensity units are comparable

A relevant advantage of luminescent compositions for their integration into a glazed ceramic piece corresponds to the values of chromatic coordinates and expansion coefficients. The luminescent compositions CL1 and CL2 present thermal expansion coefficient values (determined in the temperature range of 30 to 500°C in an 801 L dilatometer, Bahr) of 53±1x10 ^-7 and 65±1x10 ^-7 °C' ¹ , which are suitable for adjustment with ceramic tile components that have values in the range of 50. 10' ⁷ to 75. 10' ⁷ °C' ¹ . The whiteness coefficient (L*) determined by colorimetry (Konica Minolta colorimeter, Spectra Magic NX, with Color Data Software CM-S100w) were 91 ±1 and 92 ±1 for CL1 and CL2 respectively. These values are suitable for white enamel values used in industry that require L* values greater than 87.

Another advantage of luminescent compositions is related to the presence of Zr ⁴ * cations in their composition. The luminescent compositions comprise said cations which, expressed in terms of their equivalent oxide, ZrÜ2, correspond to 1.1% for the luminescent composition CL1 and 6.5% for the luminescent composition CL2. The luminescent composition CL1 is transparent to visible light while the luminescent composition CL2 is opaque.

The presence of Zr ⁴ * cations in luminescent compositions is advantageous for regulating transparency, but what matters is that the luminescent composition is visible under UV light. Whether it is opaque or not is something that has to do with whether the region can be seen in visibility or whether it remains camouflaged.

The luminescent compositions CL4 to CL17 presented values in the referred properties similar to those described and included in the range of those determined between the values of CL1 and CL2 in terms of the physical properties of thermal expansion coefficient, whiteness coefficient and transparency.

Taken together, physical properties and luminescent properties are an advantage for the integration of luminescent compositions in glazed ceramic pieces decorated by digital decoration processes, preferably in ceramic tiles. Figure 5 shows an example of integrating a CL1 luminescent composition into a porcelain ges tile to generate a QR code or a company logo.

The examples demonstrate that the incorporation in the fusion process of the frit that would later incorporate the feldspar, gives a very low signal of 5 orders of magnitude smaller, compared to the result obtained with the procedure of the invention.

Claims

1. A glazed ceramic piece comprising: a) a ceramic support b) a final layer of ceramic glaze, c) a luminescent composition integrated into a discrete area, or several discrete areas, of the final layer of glaze, such that said composition luminescent includes:

- crystallizations with a feldspar structure;

- a glassy phase; and

- crystalline particles comprising one or more lanthanide oxides.

2. A glazed ceramic piece, according to claim 1, in which the crystalline particles comprising lanthanide oxide are arranged in the luminescent composition outside the network of crystallizations with a feldspar structure, in a proportion of lanthanide oxide with respect to the total of lanthanide oxide such that on exposure to UV light the composition produces a luminescence signal of an intensity of at least 10 ⁶ , preferably of at least 10 ⁷ , and more preferably of at least 10 ⁸ for a surface of 2 cm ² , with respect to 100% by weight of the reference lanthanide oxide, which has a luminescent signal of 10 ⁷ -measured in arbitrary units and with the same measurement conditions.

3. A glazed ceramic piece, according to claim 1, wherein the crystalline particles comprising lanthanide oxide are arranged in the luminescent composition so that said particles do not form a solid solution with the feldspar crystalline phase, nor do they form a solid solution with the glass phase of the luminescent composition.

4. A glazed ceramic piece, according to one of claims 1 to 3, in which the glass phase is present in the luminescent composition in a proportion by weight equal to or less than 30%, and preferably equal to or less than 20%, with respect to to the weight of all the crystalline phases in the luminescent composition.

5. A glazed ceramic piece, according to one of claims 1 to 3, wherein the amount of lanthanide oxides in the luminescent composition is between 0.1% and 10% by weight with respect to the total weight of the luminescent composition, preferably between 0.1% and 5% by weight and more preferably between 0.1% and 2% by weight.

6. A glazed ceramic piece, according to any one of the preceding claims, in which at least 80% of the lanthanide oxide in the form of crystalline particles, present in the composition, is in the region of the glass phase.

7. A glazed ceramic piece according to any one of the preceding claims, wherein the particles comprising lanthanide oxide have a size of <2 pm, preferably <1.5 pm, and preferably <1 pm.

8. A glazed ceramic piece, according to one of the previous claims, comprising:

- a ceramic support,

- a final layer of enamel in which the luminescent composition is integrated.

9. A glazed ceramic piece, according to claim 8, which comprises between the ceramic support and the final layer of glaze: a layer of ceramic slip as an opacifier of the ceramic support.

10. A glazed ceramic piece, according to the preceding claim, comprising, in addition to the slip layer, a first layer of ceramic glaze between the ceramic support and the slip layer.

11. A glazed ceramic piece, according to any one of the preceding claims, wherein the luminescent composition comprises

- an amorphous phase,

- a crystalline phase in the form of particles with a feldspar structure, and

- a crystalline phase comprising one or more oxides of the elements: Er, Gd, Ho, Sm, Ce, Nd, Yb, Tb, Pr, La, Tm and Eu, or combinations thereof.

12. A glazed ceramic piece, according to any one of the preceding claims, wherein the luminescent composition comprises crystalline particles comprising europium oxide.

13. A glazed ceramic piece, according to any one of the preceding claims, wherein the crystallizations with a feldspar structure have a dual morphology with particles of size between 1 and 12 pm, preferably between 2 and 8 pm, and more preferably between 3 and 6 pm and particles of a size of the order of nanometers, with a limit of <500 nm, preferably <350 nm, and more preferably <200 nm.

14. A glazed ceramic piece, according to any one of the preceding claims, in which the crystallizations with a feldspar structure have a feldspar structure. plagioclases, preferably have a crystalline structure of albite, oligoclase, andesine, labradorite, banalsite, bytownite, anorthite or combinations of them.

15. A glazed ceramic piece, according to any one of the preceding claims, wherein the luminescent composition is arranged in the ceramic piece in such a way that it has the shape of a recognizable pattern when illuminated with ultraviolet light.

16. A glazed ceramic piece, according to the previous claim, in which the recognizable pattern is selected from codes, texts, symbols, names, logos, brands, signatures, numbers, link to a web page, a decorative figure, a graphic and a texture.

17. A glazed ceramic piece according to any one of the preceding claims, wherein the luminescent composition has a weight of between 10 and 90 grams/m ² , preferably between 20 and 80 grams/m ² , and more preferably, 30 and 60 grams/m ² .

18. A glazed ceramic piece, according to the previous claim, selected between a tile and a tile.

19. A procedure to obtain the enameled ceramic piece, defined in any one of the preceding claims, which comprises: a) homogenization of a precursor mixture of the luminescent composition by wet grinding; b) drying the homogenized composition obtained in step a); c) sieving of the product obtained in step b); d) thermal treatment in an air atmosphere of the product obtained in step c); obtaining a precursor luminescent composition e) grinding to condition the particle size of the precursor luminescent composition obtained in step d); f) formation of a ceramic ink with the precursor luminescent composition resulting from step e), g) decoration of a ceramic piece in green with the ceramic ink from step f); and h) firing the ceramic piece that comprises the luminescent composition, giving rise to the enameled piece.

20. A process according to claim 19, wherein in step a) particles of lanthanide oxide starting material with a size such that the dso value is between 1 ± 0.5 pm and 12 ± 1 are used. pm, preferably between 2±1 pm and 8±1 pm, and even more preferably between 3±1 and 6±1 pm.

21. A procedure according to claim 19, wherein step g) comprises:

- deposit the ceramic ink obtained in step f) of the procedure in green:

- directly on the green ceramic support of the ceramic piece, or

- on a first layer of enamel deposited on the ceramic support in green or

- on a layer of enamel deposited on a layer of slip deposited on the green ceramic support.

22. A method according to any one of claims 19 to 22, in which step g) is carried out by a method selected from planographic printing, gravure printing and digital printing.

23. A procedure according to any one of the preceding claims 19 to 22, wherein step h) comprises:

- carry out a heat treatment of the piece resulting from step g), at a firing temperature between 1100 and 1240 °C, preferably between 1120 °C and 1230 °C, and more preferably between 1130 °C and 1225 °C.

24. A procedure, according to any one of the preceding claims 19 to 23, in which step h) comprises performing a thermal treatment that comprises performing a thermal cycle with a total duration of between 45 minutes to 120 minutes with a residence time at the maximum treatment temperature between 6 minutes and 6 minutes.

25. A precursor luminescent composition obtained by the procedure comprising steps a) to d) of the procedure defined in claim 19, comprising:

- fried;

- kaolin; and

- crystalline particles of one or more lanthanide oxides.

26. A precursor luminescent composition according to claim 25, wherein the crystalline lanthanide oxide particles are arranged in the luminescent composition outside the network of crystallizations with a feldspar structure in a ratio of lanthanide oxide to total lanthanide oxide. lanthanide such that under exposure to UV light the composition produces a luminescence signal of an intensity of at least 10 ⁴ , preferably at least 10 ⁵ , and more preferably at least 10 ⁷ for a surface area of 2 cm ² , compared to the luminescent signal of the starting lanthanide oxide powder - measured in arbitrary units and with the same measurement conditions -

27. A precursor luminescent composition according to claim 25 or 26, obtained through steps a) to d) of the procedure defined in claim 19.

28. An ink comprising the precursor luminescent composition defined in one of claims 25 to 27.

29. Use of the luminescent composition obtained in step h) of claim 19, for the authentication or fight against falsification of security documents, security articles and valuable objects.

30. Use of the luminescent composition to generate: double decoration effects only visible with UV light for public spaces, texts or symbols, names, logos, brands and/or company identity. QR codes or barcodes, artist signature, serial number or author's mark of a collection of ceramic pieces.