EP2414482A1

EP2414482A1 - Europium, yttrium oxide, or gadolinium core/shell composition, phosphor including said composition, and methods for preparing same

Info

Publication number: EP2414482A1
Application number: EP10710053A
Authority: EP
Inventors: Valérie BUISSETTE; Thierry Le-Mercier
Original assignee: Rhodia Operations SAS
Current assignee: Rhodia Operations SAS
Priority date: 2009-03-30
Filing date: 2010-03-25
Publication date: 2012-02-08
Also published as: FR2943682B1; KR101350548B1; JP6038648B2; US9045679B2; KR20110122215A; CN102356142B; JP2012521953A; FR2943682A1; US20120032113A1; WO2010112394A1; CA2752664C; CA2752664A1; CN102356142A

Abstract

The invention relates to a composition, as well as to a phosphor containing said composition, including an inorganic core and a europium and yttrium oxide or gadolinium shell uniformly covering the inorganic core at a thickness greater than or equal to 300 nm. The composition is produced by: forming a suspension including the inorganic core and the pH of which is 8 to 11; adding a solution containing a europium salt and an yttrium or gadolinium salt to the suspension and maintaining the pH of the reaction medium at a constant value; and separating the formed solid and calcining same to a temperature of 1000°C at most.

Description

COMPOSITION BASED ON EUROPIUM, YTTRIUM OXIDE OR

GADOLINIUM, CORE / SHELL TYPE, LUMINOPHORE COMPRISING THIS COMPOSITION AND PROCESSES FOR PREPARING THE SAME

The present invention relates to a composition based on europium, yttrium oxide or gadolinium, core / shell type, a phosphor comprising this composition and their methods of preparation.

The yttrium or gadolinium oxides doped with europium are well known for their luminescence properties. They emit a red light when irradiated by UV radiation. Phosphors exploiting this property and based on these oxides are commonly used on an industrial scale, for example in trichromatic fluorescent lamps.

These phosphors contain rare earths that are expensive and also subject to significant fluctuations. Reducing the cost of these luminophores is therefore an important issue.

Moreover the rarity of some rare earths such as europium leads to seek to reduce the amount of these in phosphors.

There is therefore a significant need for reduced-quantity rare-earthed phosphors, it being of course understood that this reduction in the quantity must not be to the detriment of the luminescence properties of the luminophores, these properties having to be of the same order as those current phosphors.

The object of the invention is a phosphor meeting this need. For this purpose, the invention relates to a composition which is characterized in that it comprises:

- a mineral heart; and

a shell based on europium oxide and an yttrium or gadolinium oxide and homogeneously covering the mineral core to a thickness equal to or greater than 300 nm.

The invention also relates to a phosphor which is characterized in that it comprises a composition of the type described above.

Given the specific presence of the mineral core, which occupies a large part of the volume of the phosphor, the amount of rare earth used is reduced compared to currently known solid phosphors, which reduces its cost. Nevertheless, apart from this difference in internal structure, the phosphors of the invention have properties similar to those of currently known phosphors.

In particular, it turns out that the presence of the non-phosphor core has no significant influence on the luminescent properties of the phosphors of the invention. In most cases, luminescence properties similar to those obtained with products of the prior art are obtained.

Other features, details and advantages of the invention will appear even more fully on reading the description which follows, as well as various concrete but non-limiting examples intended to illustrate it.

It is also specified for the remainder of the description that, unless otherwise indicated, in all ranges or limits of values that are given, the values at the terminals are included, the ranges or limits of values thus defined thus covering any value at least equal to and greater than the lower bound and / or at most equal to or less than the upper bound.

For rare earth is meant for the remainder of the description the elements of the group consisting of yttrium and the elements of the periodic classification of atomic number inclusive between 57 and 71.

By specific surface is meant the specific surface B. AND. determined by adsorption of krypton. The surface measurements given in the present description were carried out on an ASAP2010 device after degassing the powder for 8 hours, at 200 ^° C.

As has been seen above, the invention relates to two types of products: compositions which may be called also in the remainder of this description "precursors", and phosphors obtained from these compositions or precursors. The luminophores themselves have sufficient luminescence properties to make them directly usable in the desired applications. The precursors have no luminescence properties or possibly luminescence properties that are too low for use in these same applications.

These two types of products will now be described more precisely.

Compositions or precursors

The compositions of the invention are characterized essentially by their specific structure of the core / shell type which will be described in detail below.

The mineral core is based on a material that can be in particular an oxide or a phosphate. Among the oxides, mention may in particular be made of oxides of zirconium, zinc, titanium, magnesium, aluminum (alumina) and the oxides of one or more rare earths, one of which may optionally act as a dopant.

As the rare earth oxide, gadolinium oxide, ytthium oxide and cerium oxide may be mentioned more particularly.

Yttrium oxide, gadolinium oxide and alumina may be chosen preferably. Alumina may be chosen even more preferably because it has the particular advantage of allowing a calcination at a higher temperature during the passage of the precursor to the phosphor without there being a diffusion of the dopant in the heart. This thus makes it possible to obtain a product whose luminescence properties are optimal because of a better crystallization of the shell, a consequence of the higher calcination temperature.

Among the phosphates, mention may be made of orthophosphates of one or more rare earths, one of which may optionally act as a dopant, such as lanthanum (LaPO ₄ ), lanthanum and cerium ((LaCe) PO ₄ ), yttrium (YPO ₄ ), polyphosphates of rare earths or aluminum.

According to a particular embodiment, the core material is a lanthanum orthophosphate, a gadolinium orthophosphate or an yttrium orthophosphate.

Mention may also be made of alkaline earth phosphates such as Ca ₂ P ₂ O ₇ , zirconium phosphate ZrP ₂ O ₇ , and alkaline earth hydroxylapatites.

Furthermore, other mineral compounds such as vanadates, in particular rare earth (YVO ₄ ), germanates, silicates, in particular zinc or zirconium silicate, tungstates, molybdates, sulphates (BaSO _{4) are} suitable. ), borates (YBO ₃ , GdBO ₃ ), carbonates and titanates (such as BaTiO ₃ ), zirconates, alkaline earth metal aluminates, optionally doped with a rare earth, such as barium aluminates and / or magnesium, such as MgAl ₂ O ₄ , BaAl ₂ O ₄ , or BaMgAl 10O17.

Finally, compounds derived from the above compounds, such as mixed oxides, in particular rare earth oxides, for example mixed oxides of zirconium and cerium, mixed phosphates, in particular rare earths, and phosphovanadates, may be suitable. In particular, the core material may have particular optical properties, including reflective properties of UV radiation. By the expression "the mineral core is based on" is meant a set comprising at least 50%, preferably at least 70%, and more preferably at least 80% or 90% by weight of the material in question. According to a particular embodiment, the core may consist essentially of said material (ie at a content of at least 95% by weight, for example at least 98%, or even at least 99% by weight) or entirely constituted by this material. material.

Several interesting variants of the invention will now be described below. According to a first variant, the core is made of a dense material which corresponds in fact to a generally well-crystallized material or to a material whose surface area is low.

By low specific surface area is meant a specific surface area of at most 5 m ² / g, more particularly at most 2 m ² / g, even more particularly at most 1 m ² / g, and especially from plus 0.6 m ² / g.

According to another variant, the core is based on a temperature-stable material. By this is meant a material whose melting point is at an elevated temperature, which does not degrade to a troublesome by-product for application as a luminophore at the same temperature and which remains crystallized and therefore does not become a material. amorphous still at this same temperature. The high temperature referred to herein is a temperature at least greater than 900 ^° C., preferably at least greater than 1000 ^° C. and even more preferably at least 1200 ° C.

The third variant consists in using for the core a material which combines the characteristics of the two previous variants, thus a material with a low specific surface area and temperature stability.

The use of a heart according to at least one of the variants described above has several advantages. Firstly, the core / shell structure of the precursor is particularly well preserved in the resulting phosphor which provides a maximum cost advantage.

Furthermore, it has been found that the phosphors obtained from the precursors of the invention in the manufacture of which a core has been used according to at least one of the abovementioned variants, have photoluminescence yields which are not only identical but in some cases higher than those of a phosphor of the same composition but which does not have the core-shell structure.

The core materials can be densified, in particular by using the known technique of molten salts. This technique involves wearing the material to be densified at a high temperature, for example at least 900 ^° C., optionally under a reducing atmosphere, for example an argon / hydrogen mixture, in the presence of a melting agent which may be chosen from chlorides (sodium chloride, for example), fluorides (lithium fluoride for example), borates (lithium borate), carbonates or boric acid.

The core may have a mean diameter of in particular between 1 and 10 microns.

These diameter values can be determined by scanning electron microscopy (SEM) by random counting of at least 150 particles.

The dimensions of the core, as well as the thickness of the shell which will be described later, can be measured in particular in transmission electron microscopy photographs of sections of the compositions / precursors of the invention.

The other structure characteristic of the compositions / precursors of the invention is the shell.

This shell homogeneously covers the core to a thickness that is equal to or greater than 300 nm. By "homogeneous" is meant a continuous layer, completely covering the core and whose thickness is preferably never less than 300 nm. This homogeneity is particularly visible on scanning electron microscopy photographs. X-ray diffraction (XRD) measurements also reveal the presence of two distinct compositions between the heart and the shell. The thickness of the layer may be more particularly at least

500 nm and even more particularly at least 700 nm. It may be equal to or less than 2000 nm (2 μm), more particularly equal to or less than 1500 nm and be between 750 nm and 1500 nm.

The shell is based on or consists of the product with the desired luminescence properties (red phosphor) or capable of giving such properties, that is to say that it is based on or consists of europium oxide and a ytthum (Y ₂ O ₃ ) or gadolinium (Gd ₂ θ ₃ ) oxide or a mixed yttrium-gadolinium oxide ((Y, Gd) ₂ O ₃ ), it being understood that the invention It applies well to the case of the mixed oxide even if the description for the sake of simplification refers thereafter only to an yttrium oxide or a gadolinium oxide. The europium plays the role of dopant for the oxide Ln ₂ O ₃ (Ln denoting yttrium and / or gadolinium) that is to say that it confers luminescence properties to this oxide. The amount of europium is generally between 0.01% and 20% by weight of europium oxide relative to the oxide mass Ln ₂ θ3. Depending in particular on the applications of the compositions, this amount may be between 4% and 15%, more particularly between 4% and 7%, especially in the case of applications in the trichromatic lamps.

In a manner known per se, the europium doped Ln ₂ θ ₃ oxide may optionally contain other additional elements chosen from rare earths other than europium, gadolinium and yttrium. These include terbium, samarium or lanthanum. These other elements may be present as dopants in addition to europium, this may be the case in particular terbium or samarium. In such a case, the amount of this additional element is generally at most 1% by mass of oxide of the element relative to the oxide mass Ln ₂ θ 3. These other elements may also be present in substitution of gadolinium and / or yttrium. This may be for example the case of lanthanum. In such a case, the amount of this additional element is generally at most 40%, more particularly at most 10%, by weight of oxide of the element relative to the weight of Ln ₂ O ₃ oxide. It is important to note here that the dopant or substituent functions attributed to the above-mentioned elements have been purely indicative and not limiting and that the invention covers all cases where the shell comprises an oxide Ln ₂ O ₃ with any of these additional elements regardless of its role or function in this oxide. The particles comprising a core and a shell which constitute the compositions / precursors of the invention generally have a mean diameter which is preferably between 1, 6 microns and 15 microns. This diameter may more particularly be between 3 μm and 10 μm and even more particularly between 4 μm and 8 μm. The average diameter referred to is the volume average of the diameters of a particle population.

The granulometry values given here and for the remainder of the description are measured by the laser granulometry technique, for example by means of a Malvern laser particle size analyzer, on a sample of particles dispersed in ultrasonic water (130 W ) for 1 minute 30 seconds. Moreover, the particles preferably have a low dispersion index, typically at most 0.7, more particularly at most 0.6 and even more particularly at most 0.5.

By "dispersion index" of a population of particles is meant, for the purposes of this description, the ratio I as defined below: where: 0 ₈₄ is the particle diameter for which 84% of the particles have a diameter less than 0s ₄ ;

016 is the particle diameter for which 16% of the particles have a diameter less than _0-1 ; and

050 is the average diameter of the particles, diameter for which 50% of the particles have a diameter less than 0so.

Although the compositions / precursors according to the invention may optionally have luminescence properties after exposure to certain wavelengths, it is possible and even necessary to further improve these luminescence properties by treating these products with post-treatments. , and this in order to obtain real luminophores directly usable as such in the desired application.

It is understood that the boundary between a precursor and a real phosphor remains arbitrary, and depends on the single luminescence threshold from which it is considered that a product can be directly implemented in a manner acceptable to a user.

In the present case, and in a rather general manner, it is possible to consider and identify as precursors of luminophores compositions according to the invention which have not been subjected to heat treatments greater than approximately 1000 ^° C., since such products generally exhibit luminescence properties which can be judged as not satisfying the minimum brightness criterion of commercial phosphors which can be used directly and as such, without any subsequent transformation. Conversely, one can qualify as luminophores, the products which, possibly after being subjected to appropriate treatments, develop suitable glosses, and sufficient to be used directly by an applicator, for example in lamps.

The description of the phosphors according to the invention will be made below.

The luminophores

The luminophores of the invention consist of, or include, the compositions of the invention as described above. As a result, all that has been previously described with regard to these compositions is likewise applicable here for the description of the phosphors according to the invention. These include all the characteristics given above on the structure constituted by the mineral core and the homogeneous shell, on the nature of the mineral core, on that of the shell and in particular the compositions of the oxide Ln ₂ θ 3 as well as granulomethane characteristics.

As will be seen below, the luminophores of the invention are obtained from the compositions / precursors by a heat treatment which has the consequence of not substantially modifying the characteristics of these compositions as mentioned above. However, this treatment leads to better crystallization of the Ln ₂ θ ₃ oxide of the shell, which has the essential consequence of improving the luminescence properties.

The description of the methods for preparing the compositions and phosphors of the invention will be made below.

Preparation processes

The process for preparing a composition according to the invention is characterized in that it comprises the following steps:

(a) forming a suspension comprising the inorganic core and having a pH of between 8 and 11;

(b) adding to said suspension a solution containing a europium salt and a ytthum or gadolinium salt, with, if appropriate, a salt of a rare earth other than yttrium, europium or gadolinium, by maintaining the pH of the reaction medium at a substantially constant value; (c) separating the solid from the medium formed in the preceding step;

(d) the solid thus obtained is calcined at a temperature of at most 1000 ^° C.

The first step is therefore a step in which a suspension is formed which is generally an aqueous suspension which comprises the mineral core as described above. We choose a heart having a particle size adapted to that of the composition that is to be prepared. Thus, it is possible to use in particular a core having a mean diameter of in particular between 1 and 10 μm and having a dispersion index of at most 0.7 or at most 0.6.

The pH of the suspension should be between 8 and 11, more particularly between 8.5 and 9.5. It is brought to this value by addition to the suspension of a base. Suitable bases include, for example, metal hydroxides (NaOH, KOH, Ca (OH) ₂ , ....) or even ammonium hydroxide. The second step (b) of the process uses a solution containing a europium salt and, depending on the composition that is to be prepared, a yttrium salt and / or gadolinium. In the case where it is sought to prepare a composition in which, as seen above, the oxide Ln ₂ O ₃ of the shell comprises other rare earths, this solution then additionally contains a salt. of the one or more rare earths.

Suitable salts include, for example, chlorides, nitrates, sulphates or acetates. Nitrates can be used more particularly.

The solution is added to the suspension formed in the previous step (a).

This addition is preferably progressively, for example over a period which may range from 30 minutes to 1 hour, more particularly from 30 minutes to 2 hours, continuous and with stirring. On the other hand, according to another important characteristic of the process of the invention, which makes it possible in particular to obtain a homogeneous coating of the particles of the mineral core, this addition is done by maintaining the pH of the reaction medium at a substantially constant value, generally to the pH value of the suspension just before the addition of the salt solution. This pH value can thus be between 8 and 11, more particularly between 8.5 and 9.5.

By pH maintained at a substantially constant value it is meant that the pH of the medium will vary by not more than 0.5 pH units around the fixed set point and more preferably by at most 0.1 pH units around this value. To reach these pH values and ensure the required pH control, a base as described above can be added to the reaction medium.

The addition of the solution to the suspension of the second step (b) is carried out at a temperature which can range from ambient (20-25 ^° C.) to 60 ^° C. or 80 ° C.

Contacting the suspension and the solution results in the formation of a precipitate which comprises particles of a rare earth compound (Y and / or Gd and Eu) and ammonium deposited in a homogeneous layer on the core. Variations in temperature and feed rate conditions during mixing of the solution and the slurry make it possible to vary the dimensions of these particles. The thickness of the layer may vary depending on the respective amounts of salt and heart used.

In a next step (c), the precipitate obtained is separated from the liquid phase of the reaction medium by any solid / liquid separation process such as, for example, filtration, centrifugation, decantation or the like. he can also be subjected to one or more washes, for example, to eliminate soluble salts.

The precipitate can be dried to evaporate unbound water, for example by a heat treatment between 50 ⁰ C and 100 ⁰ C, by drying under reduced pressure or by spraying.

It is possible, after step (b), to ripen the reaction medium. The term "ripening step" means maintaining the medium obtained after the end of the addition of the solution under the same temperature and stirring conditions as those of step (b). The duration of this maturing step may be for example between 15 minutes and 3 hours, these values being given for purely indicative and not limiting.

It is also possible to carry out the curing under pressure, for example in an autoclave, at a temperature which can then be between 100 ° C. and 200 ^° C., more particularly between 100 ° C. and 150 ° C. The last step of the process is a calcination step. This step makes it possible to transform the compound of rare earths (Y and / or Gd and Eu) and ammonium of the layer into an oxide of these same rare earths. This calcination is generally done under air.

The duration of calcination can be determined conventionally, for example by controlling the constant weight. For information only, the duration of calcination can vary between 30 minutes and 6 hours.

At the end of this treatment, a composition or a precursor according to the invention is obtained.

The luminophores of the invention are obtained by calcination at a temperature of at least 1200 ^° C. of the compositions / precursors as described above or of the compositions / precursors obtained by the process which has also been described above. This temperature may be between 1200 ° C and 1650 ° C, more particularly between 1300 ⁰ C and 1500 ° C.

By this treatment, the precursors are converted into effective phosphors.

Although, as mentioned above, the precursors themselves may have intrinsic luminescence properties, these properties are generally insufficient for the intended applications and are greatly improved by the calcination treatment. Calcination is usually done under air.

In a known manner, the calcination can be carried out in the presence of a flux or fluxing agent such as, for example, lithium fluoride, lithium tetraborate, lithium chloride, sodium chloride or barium chloride, lithium, sodium or barium, lithium phosphate, ammonium chloride, boric oxide and boric acid and ammonium phosphates, and mixtures thereof.

It is also possible to carry out the calcination in the absence of any flux, thus without prior mixing of the melting agent with the precursor, which simplifies the process and contributes to reducing the level of impurities present in the phosphor. In addition, it avoids the use of products whose implementation must be made in strict safety standards given their potential toxicity which is the case of a large number of melting agents mentioned above.

After treatment, the calcined product is advantageously washed, so as to obtain a phosphor as pure as possible and in a deagglomerated or weakly agglomerated state. In the latter case, it is possible to deagglomerate the phosphor by subjecting it to deagglomeration treatment under mild conditions.

The aforementioned heat treatments make it possible to obtain luminophores which retain a core / shell structure and a particle size distribution very close to those of the precursor particles.

In addition, the heat treatment can be conducted without inducing sensitive phenomena of europium diffusion from the outer phosphor layer to the core.

It will be noted that according to one conceivable variant of the invention, it is possible to conduct in a single step the successive calcinations of the two processes which have been described, that is to say to obtain the luminophore directly without stopping at precursor.

Furthermore, the invention covers the phosphor obtainable by the preparation method described above.

By virtue of their properties, the phosphors of the invention can be used in lighting or visualization systems having an excitation source in the UV range (200-280 nm), for example around

254 nm. Of particular note are mercury vapor trichromatic lamps, backlighting lamps for liquid crystal systems, in tubular or planar form (LCD Back Lighting). They exhibit a high gloss under UV excitation, and a lack of luminescence loss as a result of thermal post-treatment. Their luminescence is in particular stable under UV at temperatures ranging from ambient to 300 ^° C.

The luminophores of the invention are also good candidates as red phosphors for VUV excitation systems (or "plasma"), such as plasma screens and trichromatic lamps without mercury, including Xenon excitation lamps (tubular or planar). The phosphors of the invention have a strong red emission under VUV excitation (for example, around 147 nm and 172 nm). The phosphors are stable under VUV excitation.

The phosphors of the invention can also be used as red phosphors in LED devices. They can be used especially in systems excitable in the near UV. They can also be used in UV excitation labeling systems.

They can also be dispersed in organic matrices (for example, plastic matrices or transparent polymers under UV ...), mineral (for example, silica) or organo-mineral hybrids. The phosphors of the invention can be used in cathode excitation systems of CRT (cathode ray tubes) or FED (field emission display) or SED (surface emission display) type.

The luminophores of the invention can be used in the manufacture of the devices mentioned above by well-known techniques, for example by screen printing, by spraying, by electrophoresis or sedimentation.

The invention also relates, in another aspect, to luminescent devices, comprising, as a source of red luminescence, the phosphors as described above or the phosphors obtained from the process also described above.

Examples will now be given.

In the examples which follow, the particles prepared have been characterized in terms of granulometry, morphology and composition by the following methods. Quranometric measurements

Particle diameters were determined using a laser granulometer (Malvern 2000) on a sample of particles dispersed in ultrasonic water (130 W) for 1 min 30 seconds.

Electron microscopy Transmission electron microscopy is performed on a section (microtomy) of the particles, using a JEOL 2010 FEG high resolution TEM microscope. The spatial resolution of the apparatus for the chemical composition measurements by EDS (scattering spectroscopy in energy) is <2 nm. The correlation of observed morphologies and measured chemical compositions makes it possible to highlight the core-shell structure, and to measure the thickness of the shell on the plates.

The measurements of chemical composition can also be carried out by EDS on plates made by STEM HAADF. The measurement corresponds to an average performed on at least two spectra.

X-ray diffraction

X diffractograms were made using CuK _α as with copper anti-cathode according to the Bragg-Brendano method. The resolution is chosen to be sufficient to separate the lines of the different compounds, preferably it is Δ (2Θ) <0.02 °.

EXAMPLE 1 COMPARATIVE

An oxide of formula (Yo, 95 Eu 0, os) 2 O 3 is prepared in the following manner. A suspension containing a neutral oxalate of ytthum and europium

(0.095 mole of ytthum for 0.05 mole of europium) is brought to 85 ° C. A solution of 0.255 M ammonium oxalate is added to this suspension in order to obtain molar ratios oxalate / (Y + Eu) of 2 and ammonia / (Y + Eu) of 2. The reaction medium is stirred for 1h, the precipitate obtained is recovered by filtration, then washed and dried at 100 ^° C. Finally, it is calcined at 900 ^° C. under air for 2 hours. It has a mean diameter measured in Malvern granulometry of 6.5 microns.

EXAMPLE 2 This example relates to a core / shell precursor according to the invention, the core of which is a lanthanum phosphate.

Synthesis of the heart LaPO ₄

500 ml of a phosphoric acid solution H ₃ PO ₄ (1. 725 mol / l), previously brought to pH 1, 9 by addition of ammonia and heated to 60 ° C., are added in one hour to 500 ml. a solution of lanthanum nitrate (1, 5 mol / l). The pH during the precipitation is regulated to 1.9 by the addition of ammonia. At the end of the precipitation step, the reaction medium is further maintained for 1 h at 60 ^° C. The precipitate is then easily recovered by filtration, washed with water and then dried at 60 ° C. in air. The powder obtained is then subjected to a heat treatment at 900 ° C. in air.

The product obtained is densified by calcining it for 2 hours in the presence of 1% by weight of LiF at 1100 ^° C. under a reducing atmosphere (Ar / H ₂ ). We then obtain a rare earth phosphate monazite phase. The average particle size, measured by a statistic of 150 particles on microscope slides (SEM), is 3.2 μm.

Synthesis of the precursor

A solution A is prepared by mixing 275.5 ml of 3.45 mol / l solution of ytthum nitrate and 24.9 ml of a solution of europium nitrate at 2.01 mol / l and then completing at 1 I.

22 g of the previously prepared core are dispersed in 650 ml of deionized water, with stirring at 400 rpm, at room temperature. The pH is raised to 8.5 by the addition of 6N ammonia. 340 ml of solution A, which is still under stirring, are then added, over a period of 1 h 30, and under pH regulation, at a pH of 8.5, with 6N ammonia. At the end of the addition, a maturation of 1 h is carried out at the same temperature. The product obtained is then filtered, washed with 2 volumes of water, then dried at 100 ^° C. for 5 hours and then calcined at 900 ^° C. for 2 hours.

The core / shell precursor is then obtained, the X-ray diffraction showing the presence of two distinct crystalline phases (Y, Eu) 2θ3 and LaPO ₄ . The product has a mean particle size (D50) equal to 8.3 μm.

A TEM plate is made on the resin-coated product, prepared by ultramicrotomy (thickness -100 nm) and placed on a perforated membrane.

The particles are seen in section. In this photograph, we see a section of particle whose heart is spherical and is surrounded by a shell of average thickness of 800 nm.

EXAMPLE 3

This example relates to a phosphor according to the invention.

The precursor product obtained in Example 2 is calcined for 4 hours at 1300 ° C. in air. A luminophore emitting in the red under UV excitation is obtained.

This phosphor exhibits a photoluminescence (PL) efficiency of 98% relative to the luminophore obtained by calcination under identical conditions of the precursor oxide of the comparative example. This efficiency is determined by the integration of the emission spectrum under excitation of 254 nm, measured with a spectrofluorimeter in the wavelength range of 500 nm to 750 nm. It is 100% normalized for the comparative phosphor. The slight decrease in PL (2%) relative to the comparative phosphor obtained from the precursor of Example 1 and after calcination under conditions identical to that of the present example is entirely acceptable, given the saving of 30%. made in europium.

EXAMPLE 4

This example relates to a core / shell precursor according to the invention, the core of which is an yttrium oxide.

Synthesis of the heart Y? Os

An oxide of formula Y2O3 is prepared according to a procedure similar to that described in Comparative Example 1. Thus, a suspension containing a neutral oxalate of yttrium is brought to 45 ° C. A solution of 0.255 M ammonium oxalate is added to this suspension in order to obtain oxalate / Y molar ratios of 2.5 and ammonia / Y of 5. The reaction medium is stirred for 15 minutes, the precipitate obtained is recovered by filtration, then washed and dried at 100 ^° C. Finally, it is calcined at 900 ^° C. under air for 2 hours. It has a mean diameter measured in Malvern granulometry of 4 microns.

The product obtained is then calcined at 1450 ° C. in the presence of 1% by weight of boric acid. It is then deagglomerated with the ball mill for 2 hours, then washed and dried.

Synthesis of the precursor

A solution A is prepared by mixing 275.5 ml of 3.45 mol / l yttrium nitrate solution and 24.9 ml of a solution of europium nitrate at 2.01 mol / l, and then completing at 1 I.

21 g of the previously prepared core are dispersed in 650 ml of deionized water, with stirring at 400 rpm, at room temperature. The pH is raised to 8.5 by the addition of 6N ammonia. 340 ml of solution A, which is still under stirring, are then added, over a period of 1 h 30, and under pH regulation, at a pH of 8.5, with 6N ammonia. At the end of the addition, a maturation of 1 h is carried out at the same temperature.

The product obtained is then filtered, washed with 2 volumes of water, then dried at 100 ^° C. for 5 hours and then calcined at 900 ^° C. for 2 hours. The precursor is then obtained, the X-ray diffraction showing the presence of two distinct crystalline phases (Y, Eu) 2θ3 and Y2O3. The product has a mean particle size (D50) equal to 7.5 microns. On a TEM shot made as described in Example 2, there is a particle section whose core is spherical and is surrounded by a shell of average thickness of 800 nm.

EXAMPLE 5

This example relates to a phosphor according to the invention. The precursor product obtained in Example 4 is calcined for 4 hours at 1500 ^° C. under air and then deagglomerated in order to obtain a mean size (D50) of 7.5 μm. A luminophore emitting in the red under UV excitation is obtained. This phosphor has a PL of 100%. The yield is therefore identical to that of the comparative phosphor obtained from the precursor of Example 1 and after calcination under conditions identical to that of the present example for a saving of 30% in europium.

EXAMPLE 6

This example relates to a precursor heart / shell according to the invention whose core is alumina.

A solution A is prepared by mixing 350 ml of solution of ytthum nitrate and europium at 2 mol / l (0.665 mol of ytthum and 0.005 mol of europium). 15.3 g of a laser granulometry powder D ₅ o = 3 μm, with vigorous stirring, are dispersed in 650 ml of deionized water at room temperature. The pH is raised to 8.3 by the addition of 6N ammonia.

Then, in 1 h 30, solution A is added, still under stirring, and under pH regulation (pH at 8.3). At the end of the addition, a maturation of 1 h is carried out at the same temperature.

The product obtained is then filtered, washed with 2 volumes of water, then dried at 100 ^° C. for 5 hours and then calcined at 900 ° C. for 2 hours.

The precursor sought is then obtained. X-ray diffraction reveals the presence of two distinct crystalline phases (Y, Eu) ₂ O ₃ and Al ₂ O ₃ .

The product has a mean particle size (D ₅ o) equal to 6.5 microns.

EXAMPLE 7

This example relates to a luminophore according to the invention obtained from the precursor of Example 6.

The precursor of Example 6 is calcined for 4 hours at 1550 ^° C. under air. A luminophore emitting in the red under UV excitation is obtained. The Luminescence efficiency is measured by integrating the photoluminescence spectrum between 550 nm and 780 nm.

This phosphor exhibits a luminescence efficiency of 100% relative to the luminophore obtained by calcination under equivalent conditions of the precursor of Example 1, for a granulometry (D ₅ o) equal to 7.2 μm.

Claims

1- Composition characterized in that it comprises: - a mineral heart; and

2- Composition according to claim 1, characterized in that the mineral core is based on a material whose specific surface area is at most 5 m ² / g, more particularly at most 1 m ² / g.

3- Composition according to claim 1 or 2, characterized in that the mineral core is based on a lanthanum orthophosphate, a gadolinium orthophosphate or an orthophosphate of yttrium or an oxide selected from yttrium oxide, gadolinium oxide, zirconium oxide and alumina.

4- Composition according to one of the preceding claims, characterized in that the shell further comprises, as a dopant of yttrium oxide or gadolinium or substituent of the yttrium or gadolinium element, at least one earth rare other than yttrium, europium or gadolinium, which may be more particularly terbium, samahum or lanthanum.

5. Composition according to one of the preceding claims, characterized in that the shell covers the core to a thickness equal to or less than 2000 nm, more particularly between 750 nm and 1500 nm.

6. Composition according to one of the preceding claims, characterized in that it consists of particles having a mean diameter of between 1, 6 and 15 microns.

7- phosphor, characterized in that it comprises a composition according to one of the preceding claims.

8- Process for preparing a composition according to one of claims 1 to 6, characterized in that it comprises the following steps: (a) forming a suspension comprising the inorganic core and having a pH of between 8 and 11;

(b) adding to said suspension a solution containing a europium salt and a ytthum or gadolinium salt, with, if appropriate, a salt of a rare earth other than yttrium, europium or gadolinium, by maintaining the pH of the reaction medium at a substantially constant value;

(c) separating the solid from the medium formed in the preceding step;

(d) the solid thus obtained is calcined at a temperature of at most 1000 ^° C.

9- Method according to claim 8, characterized in that the suspension formed in the first step (a) has a pH between 8.5 and 9.5.

10- Method according to claim 8 or 9, characterized in that it comprises a step of maturing the reaction medium at the end of step (b).

11 - Method according to one of claims 8 to 10, characterized in that during step (b) the pH of the reaction medium is maintained at a constant value between 8 and 11, more particularly between 8.5 and 9 5.

12- Method according to one of claims 8 to 11, characterized in that during step (a) the addition of the solution containing the salts to the suspension is done gradually and with stirring.

13- A process for preparing a phosphor according to claim 7, characterized in that calcined at a temperature of at least 1200 ⁰ C a composition according to one of claims 1 to 6 or a composition obtained by the method according to one of claims 8 to 12.

14- Method according to claim 13, characterized in that the calcination takes place in the presence of a fluxing agent.

15- luminescent device, characterized in that it comprises or in that it is manufactured using a phosphor according to claim 7 or a phosphor obtained by the method according to one of claims 13 or 14.