EP1957407A1

EP1957407A1 - Sub-micron rare earth borate, a method for the production thereof and the use in the form of luminophor

Info

Publication number: EP1957407A1
Application number: EP06820165A
Authority: EP
Inventors: Valérie BUISSETTE; Thierry Le-Mercier; Yvan Montardi; Laurent Thiers
Original assignee: Rhodia Operations SAS
Current assignee: Rhodia Operations SAS
Priority date: 2005-10-13
Filing date: 2006-10-09
Publication date: 2008-08-20
Also published as: CN101287680A; JP2009511694A; JP2015214480A; JP5791865B2; FR2892113B1; US20100065780A1; US8980130B2; KR20120044385A; KR101214720B1; CN101287680B; JP6095726B2; FR2892113A1; WO2007042653A1; KR20080059390A; KR101214729B1; KR20100082868A

Abstract

The inventive rare earth borate is embodied in the form of a liquid phase suspension of substantially monocrystalline particles whose mean size ranges from 100 to 400 nm. Said borate is produced according a method consisting in roasting a rare earth borocarbonate or hydroxyborocarbonate with a temperature which is sufficient for forming a borate and obtaining a product whose specific surface area is equal to or greater than 3 m<SUP>2</SUP>/g and in carrying out the humid grinding of the roasted product. The inventive borate can be used in the form of luminophor, in particular, for producing a luminescent transparent material.

Description

RARE SUBMICRONIC ROUND BORATE, PROCESS FOR PREPARING THE SAME AND USE THEREOF AS LUMINOPHORE

The present invention relates to a submicron rare earth borate, its method of preparation and the use of this borate as a luminophore.

The fields of luminescence and electronics are currently undergoing important developments. An example of these developments is the development of plasma systems (screens and lamps) for new visualization and lighting techniques. A concrete application is the replacement of current television screens with flat screens. These new applications require phosphor materials with further improved properties. Thus, in addition to their luminescence property, these materials are required to have specific characteristics of morphology or particle size in order, in particular, to facilitate their use in the desired applications.

More specifically, it is required to have luminophores in the form of particles as possible as individualized and very thin, submicron, especially less than 500 nm. Furthermore and always in the development in the fields of luminescence and electronics, it is sought to obtain materials in the form of films, transparent and luminescent.

The main object of the invention is to provide products having such granulometric characteristics. A second object of the invention is to obtain a luminescent material of the above type.

For this purpose, the rare earth borate of the invention is characterized in that it is in the form of a suspension in a liquid phase of substantially monocrystalline particles of average size between 100 and 400 nm.

The invention also relates to a method for preparing a borate as defined above which is characterized in that it comprises the following steps:

- a rare earth borocarbonate or hydroxyborocarbonate is calcined at a temperature sufficient to form a borate and to obtain a product having a specific surface area of at least 3 m ² / g;

a wet grinding of the product resulting from the calcination is carried out. Other features, details and advantages of the invention will appear even more completely on reading the description which follows, as well as various concrete but nonlimiting examples intended to illustrate it and the appended drawing in which:

FIG. 1 is a TEM photo of a suspension according to the invention. In the present description, rare earth is understood to mean the elements of the group consisting of yttrium and the elements of the Periodic Table with an atomic number inclusive of between 57 and 71.

By specific surface is meant the specific surface B. AND. determined by nitrogen adsorption according to ASTM D 3663-78 established from the BRUNAUER-EMMETT-TELLER method described in the journal "The Journal of the American Chemical Society, 60, 309 (1938)".

The rare earth borate of the invention is preferably of the orthoborate type, of formula LnBOβ, Ln representing at least one rare earth.

However, the invention also applies to the rare earth oxyborate borate of formula Ln ₃ BO ₆ . Of course, the borate of the invention may be phasically pure but it may also be in the form of a mixture of phases, the majority phase being the orthoborate or oxyborate phase, minority phases may also be present. It is emphasized here that the invention applies to borates of one or more rare earths. Therefore, throughout the description, all that is described about rare earth borate, rare earth carbonate or hydroxycarbonate and their preparation processes must be 'hear as applying also to the case where several rare earths are present.

The rare earth constitutive of the borate of the invention, that is to say the one that forms with boron the matrix of the product generally belongs to the group of rare earths which have no property of luminescence. Thus, this constitutive rare earth borate can be chosen, alone or in combination, in the group comprising yttrium, gadolinium, lanthanum, lutetium and scandium. It may be more particularly yttrium and / or gadolinium.

The borate may further comprise one or more dopants. In a manner known per se, the dopants are used in combination with the matrix to give it luminescence properties. These dopants can be chosen from antimony, bismuth and rare earths. In the latter case, the rare earth or rare earths used as dopant are chosen from the group of rare earths with luminescence properties and they are different from the rare earth constitutive of the borate. As doping rare earth, mention may be made of cerium, terbium, europium, dysprosium, holmium, ytterbium, neodymium, thulium, erbium and praseodymium. Terbium, thulium, cerium and europium are more particularly used. Content Dopant is usually at most 50 mol% relative to the rare earth borate matrix (ratio [dopant] / [ΣLn]), ΣLn representing the rare earth and dopant set in the borate.

The borate of the invention consists of particles which have the essential characteristic of being submicron and monocrystalline.

More specifically, these particles have an average size (d ₅ o) of between 100 and 400 nm, more particularly between 100 and 300 nm and even more particularly between 100 and 200 nm. For certain applications of the borate of the invention, for example for the manufacture of a transparent material, as described below, it is possible to use a borate whose particles have an even smaller size, between 100 nm and 150 nm.

Moreover, these particles may have a narrow particle size dispersion; more precisely their dispersion index may be at most 1, preferably at most 0.7 and even more preferably at most 0.5.

For the whole of the description, the average size and the dispersion index are the values obtained by implementing the laser diffraction technique using a laser granulometer (mass distribution).

By dispersion index is meant the ratio: σ / m = (d ₈₄ -d ₁₆ ) / 2d ₅₀ in which:

d ₈₄ is the particle diameter for which 84% of the particles have a diameter of less than ₈₄ ;

- die is the particle diameter for which 16% of the particles have a diameter less than di ₆ ;

d ₅₀ is the average diameter of the particles.

It is specified here that the measurements of average size are made on suspensions which have undergone a passage to the probe-with ultrasounds' according to the well-known methods implemented for this type of measurements. The other characteristic of the constitutive particles of the borate of the invention is their monocrystallinity. Indeed, for the most part, that is to say for at least 90% of them and, preferably for all of them, these particles consist of a single crystal.

This monocrystalline aspect of the particles can be demonstrated by the transmission electron microscopy (TEM) analysis technique.

For suspensions whose particles are within a size range of at most about 250 nm) the monocrystalline appearance of the particles can also be demonstrated by comparing the average particle size measured by the laser diffraction technique mentioned above with the value of measuring crystal size or coherent domain obtained from X-ray diffraction analysis (XRD). It is specified here that the value measured in XRD corresponds to the size of the coherent domain calculated from the width of the diffraction line [1, 0.2]. The two values: average laser diffraction size and XRD have indeed the same order of magnitude, ie they are in a ratio of at most 2, more particularly at most 1, 5.

As a consequence of their monocrystalline character, the particles of the borate of the invention are in well separated and individualized form. There are no or few agglomerates of particles. This good individualization of the particles can be demonstrated by comparing the dδo measured by the laser diffraction technique with that measured from an image obtained by transmission electron microscopy (TEM). Here again, the values obtained by these two techniques have the same order of magnitude, in the proportions given above.

The borate of the invention is generally in the form of a suspension in a liquid phase of the particles which have just been described. This suspension can sediment over time and this sedimentation can cause agglomeration of the particles together. However, and this is an important property of the suspension of the invention, a simple agitation using a very low mechanical energy, in particular an ultrasonic treatment, for example with a power of 120 W for 3 minutes, makes it possible to disaggregate these particles and therefore to return to a suspension whose particles have all the characteristics that have been given above.

The liquid phase of the suspensions according to the invention may be water or a mixture of water / solvent miscible with water or an organic solvent.

The organic solvent may be very particularly a solvent miscible with water. There may be mentioned, for example, alcohols such as methanol or ethanol, glycols such as ethylene glycol, acetate derivatives of glycols such as ethylene glycol monoacetate, glycol ethers, polyols or ketones.

This liquid phase may also include a dispersant. This dispersant may be chosen from known dispersants, for example from polyphosphates (M _{n + 2} P _n O _{3n + I} ) or metaphosphates ([MPO ₃ ] _π ) which are alkaline (M denotes an alkaline such as sodium), especially as sodium hexametaphosphate. It can be chosen also from silicates alkali (sodium silicate), amino-alcohols, phosphonates, citric acid and its salts, derivatives of phosphosuccinic acid ((HOOC) _n -R-PO ₃ H ₂ where R is an alkyl radical), polyacrylic, polymethacrylic, polystyrene sulfonic acids and their salts. Citric acid and metaphosphates are particularly preferred.

The amount of dispersant may be between 1% and 10%, more particularly between 2.5% and 5%, this amount being expressed as mass of dispersant relative to the mass of solid in the dispersion.

The concentration of the suspension can vary over a wide range. By way of example, it may be between about 10 g / l and about 200 g / l, more particularly between 40 g / l and 100 g / l.

The invention also relates to a borate which is in solid form, that is to say a powder which has the characteristic of being able to lead to the borate in suspension form described above. In other words, when this powder is redispersed in a liquid phase, after simple stirring, without the need to apply a large mechanical energy, including, again, by simple ultrasonic treatment, for example with a With a power of approximately 450 W, a suspension of the borate having the characteristics given above is obtained. Of course, all that has been previously described concerning the nature and composition of the borate: nature of the crystallographic phase (orthoborate), nature and quantity of the rare earth and the dopant, applies identically for the borate under solid form.

The process for preparing the borate of the invention in the form of suspension will now be described.

In this process, rare earth borocarbonate or hydroxyborocarbonate (LnB (CO ₃ ) ₃ or LnB (OH) ₄ CO ₃ respectively, in the case of the preparation of an orthoborate) is used as a rare starting material.

This borocarbonate or this hydroxyborocarbonate is calcined at a temperature sufficient to form a borate and to obtain a product having a specific surface area of at least 3 m ² / g.

This surface may be more particularly between 3 m ² / g and 10 m ² / g and even more particularly between 5 m ² / g and 8 m ² / g.

The temperature sufficient to obtain the borate phase is generally at least 450 ^° C., more particularly at least 500 ^° C. and it may for example be between 450 ° and 700 ^° C. The duration of the calcination is a function of temperature and it is usually all the lower as the temperature is high. For example, calcination at 500 ^° C. for two hours makes it possible to obtain this phase.

The calcination must also make it possible to obtain the specific surface values which have been given above. These values are generally obtained for calcination at a temperature of between about 800 ^° C. and about 900 ° C., more particularly between 825 ° C. and 875 ° C. Again, the duration of calcination is even lower than the temperature is high. It can thus be understood, for example, between 10 minutes and 5 hours.

These specific temperature and surface conditions make it possible to obtain a product having the characteristics which are best adapted to obtain, after wet grinding, the rare earth borate suspension of the invention.

It will be noted that the calcination described above can be done either in two steps or two distinct times, or in a single step, that is to say with a gradual rise in temperature such that the product which undergoes the calcination is subjected to a temperature and duration of calcination sufficient to obtain the borate phase. For example, a plateau at 500 ° C. can be observed for a period of two hours and then the calcination temperature can be increased again to reach a higher value, for example between 800 ^° C. and 900 ° C.

The calcination that has been described above can be done under air. It is not necessary to calcine in a reducing atmosphere but it would not go beyond the frame of the present invention by implementing, at least in a second part of this calcination, reducing atmospheres (hydrogen for example) or neutral (argon ) or mixtures thereof.

The last step of the process consists in grinding the product resulting from the calcination.

Wet grinding is carried out in water or in a water / solvent mixture or in an organic solvent of the same type as the solvents which have been described above for the liquid phase constituting the suspension.

In a known manner can be used during grinding a dispersant of the type described above.

At the end of the wet grinding, the borate of the invention is obtained in the form of a suspension. Note that in the case of a suspension in a water / solvent mixture or in an organic solvent, this suspension can be prepared from an aqueous suspension as obtained by the process which has just been described and adding the organic solvent to this aqueous suspension and if necessary distillation to remove water.

The borocarbonate or rare earth hydroxyborocarbonate used as starting material can be prepared by various methods. A first method will be described below in which starting from a carbonate or a rare earth carbonate or a mixture of carbonates or hydroxycarbonates of different rare earths or carbonates or hydroxycarbonates mixed rare earths in the case of the preparation of borates of several rare earths. The rare earth carbonates or hydroxycarbonates are known products per se and can be obtained for example by precipitation of one or more rare earth salts with carbonate or ammonium bicarbonate.

The first step of the process involves reacting the starting carbonate or hydroxycarbonate with boric acid. According to one characteristic of this process, the starting reaction medium is in the form of an aqueous solution. This means that the amount of water present in the reaction medium is such that the weight ratio water / boric acid + carbonate is at least 300%, more particularly at least 1000%. This ratio can be even more particularly at least 1500%.

Preferably, the reaction is carried out hot, for example at a temperature of between 40 ^° C. and 90 ^° C.

You can work with an excess of boric acid. This excess may be for example between 5% and 100% by mole ([B] / [Ln] = 1.05 to 2, Ln = Rare Earth).

It may be advantageous to carry out the reaction by removing the CO2 formed during the reaction. This elimination can be done for example by sweeping the reaction medium with a neutral gas such as nitrogen. This variant makes it possible to obtain products of finer granulometry. According to another variant, the reaction is carried out by attacking the rare earth carbonate or hydroxycarbonate with the boric acid in the precipitating mother liquors thereof. It is advantageous to carry out this attack on freshly prepared carbonate or hydroxycarbonate.

At the end of the reaction, a precipitate is obtained which is separated from the reaction medium by any known means, for example by filtration and which, optionally, is washed and then dried. After drying, it is also possible to carry out an additional washing with a dilute acid, for example nitric acid, in order to eliminate the possible traces of carbonate which has not completely reacted. A second method can also be implemented for the preparation of a rare earth borocarbonate or hydroxyborocarbonate. This second method comprises the following steps:

boric acid and a rare earth salt are mixed;

The mixture thus obtained is reacted with a carbonate or a bicarbonate;

the precipitate thus obtained is recovered.

The rare earth salt may be an inorganic or organic salt. Water-soluble salts are preferably used. As the rare earth salt, mention may be made more particularly of nitrate. The starting mixture may additionally contain, if desired, a salt of the doping element and what has been said for the rare earth salts also applies here. Boric acid can be used as a solution or, preferably, in solid form.

The mixture can be carried out at room temperature or by heating.

Since the mixture thus obtained is acidic, it can be neutralized to a pH value of 4 or about 4, for example by adding an ammonia solution.

The second step of the process consists in reacting the mixture obtained in the preceding step with a carbonate or a bicarbonate.

As carbonate or bicarbonate, ammonium carbonate or bicarbonate may be used more particularly.

According to one variant, the reaction is carried out in the presence of a base. As a useful base, mention may be made of alkali or alkaline earth hydroxides, ammonia, secondary, tertiary or quaternary amines. 25. Ammonia is preferably used.

It should be noted that, in the case of the preparation of a borate compound comprising a dopant or substituent, the salt of the dopant or substituent can also be introduced during the reaction if this has not been done in the previous step. .

According to a preferred embodiment, the reaction is carried out by regulating the pH. By this is meant that the pH of the reaction medium is adjusted to a fixed value and admitting a variation of at most 0.5 pH units around this set target value. This regulation can be done by adjusting the amount of base used for the reaction. This fixed value is preferably between 4 and 6.

The reaction can be carried out at ambient temperature or at a warm temperature. A ripening step can then be carried out optionally. This step consists in maintaining the reaction medium at a temperature given, preferably hot, at a constant pH and at the value described above, optionally in a controlled atmosphere. The duration of this ripening is generally at least 15 minutes and not more than 8 hours.

At the end of the reaction, a precipitate of borocarbonate or hydroxyborocarbonate is obtained which is separated from the reaction medium by any known means, for example by filtration, and which, optionally, is washed and then dried.

It will be noted here that the processes described above make it possible to obtain either an orthoborate or an oxyborate depending on the stoichiometry of the starting reagents or on the Ln / B ratio in the starting reaction medium. The description that has just been made concerns the preparation of the borate in the form of a suspension. In order to obtain the borate of the invention in the form of a powder, this suspension is used and the solid product is separated from the liquid phase using any known separation technique, for example by filtration. The solid product thus obtained may be dried optionally and then resuspended in a liquid phase of the same type as that described above.

By virtue of its properties and the nature of the dopant, Eu, Ce, Tb and Tm, for example, the borates of the invention are understood to mean the borates in the form of a suspension or the borates in solid form, can be used as phosphors. These borates exhibit luminescence properties under electromagnetic excitation in the wavelength range used in plasma systems (screens and lamps where the excitation is created by a rare gas or a mixture of noble gases such as xenon and / or and neon) and in mercury vapor lamps in the case of borates doped with cerium and terbium in combination. Therefore, they can be used as phosphors in plasma systems (display screen or lighting system) or in mercury vapor lamps.

The invention therefore also relates to luminescent devices comprising the borate described above or as obtained by the method described above or manufactured using this same borate. Similarly, the invention relates to plasma systems or mercury vapor lamps, in the manufacture of which the borate can enter, or comprising the same borate. The use of phosphors in the manufacture of plasma systems is done according to well-known techniques, for example by screen printing, electrophoresis or sedimentation.

The particle size properties of the borates of the invention allow them to be used as markers in semi-transparent inks using the up-conversion mechanisms in the IR-Visible or luminescence in I ¹ IR, for example for carrying out a marking by an invisible bar code system. In this case, the pair of dopants will preferably be Yb and Er.

The borates of the invention can also be used as markers in a material such as paper, cardboard, textile, glass or a macromolecular material. This can be of different types: elastomeric, thermoplastic, thermosetting.

On the other hand, the particular properties of these borates, when they are not doped, in the visible range and UV (no absorption), make them suitable for use as a reflecting barrier in system lighting lamps. mercury vapor.

The invention also relates to a luminescent material which comprises, or may be manufactured using, at least one borate according to the invention or a borate obtained by the process as described above. According to a preferred embodiment, this luminescent material may be furthermore transparent. In this case, the borate entering into its composition or in its manufacture is a borate according to the invention and of average size between 100 nm and 200 nm, preferably between 100 nm and 150 nm.

It should be noted that this material may comprise, or be manufactured using, in addition to the borate of the invention, other borates, or more generally, other luminophores, in the form of submicron or nanometric particles.

This material can be in two forms, that is to say either in a mass form, the whole of the material having the properties of transparency and luminescence is in a composite form, that is to say in this case in the form of a substrate and a layer on this substrate, the layer then only having these properties of transparency and luminescence. In this case, the borate of the invention is contained in said layer. The substrate of the material is a substrate which may be silicon, silicone-based or quartz-based. It can also be a glass or a polymer such as polycarbonate. The substrate, for example the polymer, may be in a rigid form and a sheet or plate a few millimeters thick. It can also be in the form of a film of a few tens of microns or even a few microns to a few tenths of a millimeter thick.

For the purposes of the invention, transparent material is understood to mean a material which has a haze of at most 60% and a total transmission of from minus 60% and, preferably, a haze of at most 40% and a total transmission of at least 80%. The total transmission is the amount of total light that passes through the layer, relative to the amount of incident light. The haze corresponds to the ratio of the diffuse transmission of the layer to its total transmission.

These two quantities are measured under the following conditions: the layer of material with a thickness of between 0.2 μm and 1 μm is deposited on a standard glass substrate, 0.5 mm thick. The mass fraction of borate particles in the material is at least 20%. The total transmission and diffuse transmission measurements are made through the material and substrate layer, using a standard procedure on a Perkin Elmer Lamda 900 spectrometer, equipped with an integrating sphere, for a wavelength of 550 nm.

The material, and more particularly the aforementioned layer, may comprise, besides a borate according to the invention, binders or fillers of the polymer (polycarbonate, methacrylate), silicate, silica ball, phosphate, titanium oxide or other mineral fillers type. to improve in particular the mechanical and optical properties of the material.

The mass fraction of borate particles in the material may be between 20% and 99%.

The thickness of the layer may be between 30 nm and 10 μm, preferably between 100 nm and 3 μm and even more preferably between 100 nm and 1 μm.

The material, in its composite form, can be obtained by depositing on the substrate, optionally previously washed for example with a sulfo-chromic mixture, a borate suspension of the invention. It is also possible to add at the time of this deposit, binders or charges mentioned above. This deposit can be achieved by a spraying technique, "spin-coating" or "dip-coating". After deposition of the layer, the substrate is dried in air and it can optionally subsequently undergo a heat treatment. The heat treatment is carried out by heating at a temperature which is generally at least 200 ^° C. and the higher value of which is fixed in particular taking into account the compatibility of the layer with the substrate so as to avoid interfering reactions in particular. The drying and the heat treatment can be conducted under air, under an inert atmosphere, under vacuum or under hydrogen.

It has been seen above that the material may comprise binders or fillers. It is possible in this case to use suspensions that comprise themselves at least one of these binders or these fillers or precursors thereof.

The material in the mass form can be obtained by incorporating the borate particles in a polymer type matrix for example, such as polycarbonate, polymethacrylate or silicone.

Finally, the invention relates to a luminescent system which comprises a material of the type described above and, in addition, an excitation source which may be a source of UV photons, such as a UV diode or an excitation of the Hg gas type. rare or X-rays. The system can be used as a transparent wall-mounted lighting device.

Examples will now be given. In these examples the particle size was determined according to the laser diffraction technique mentioned above. It is furthermore specified that the measurement was carried out with a Coulter-type apparatus on suspensions diluted at a concentration of between 1 g / l and 10 g / l and which have previously undergone a passage to the ultrasound probe (450 W probe). for 2 minutes 30 seconds.

EXAMPLE 1 This example relates to the preparation of a suspension of an yttrium, gadolinium and europium borate according to the invention.

A solution consists of a mixture of nitrates of yttrium, gadolinium and europium, of following composition (in atomic%):

Y: 72% Gd: 23%

Eu: 5%

In a reactor containing deionized water, crystallized boric acid is mixed with the solution of rare earth nitrates (Ln) in such proportions that the molar ratio B / Ln is equal to 1.5. The mixture thus formed is then neutralized to pH 4.4 with 6N ammonia and the concentration of the mixture is adjusted to 0.6 mol / liter in Ln elements by addition of water.

On 2.8 liters of the preceding solution, stirred and heated at 60 ^° C., a precipitating solution of ammonium bicarbonate at a concentration of 1.34 mol / l of NH ₄ HCO ₃ and ammonia 0.7 mole / liter. During this addition, the pH is maintained at a value of at least 4.6 by addition of 6N ammonia. The addition of the precipitating solution is stopped as soon as the pH of the mixture reaches the value of 5, the rare earth concentration of the mixture obtained is 0.39 mol / liter.

During the entire reaction, the temperature of the reaction medium is kept constant and equal to 60 ^° C.

This mixture is then heated and stirred for 40 minutes.

The precipitate is then filtered on Bϋchner and then washed with 2 liters of boric acid solution at 2 g / liter (0.03 mol / liter).

The solid obtained is dried at 60 ^° C. overnight and then calcined at 85 ^° C. for 1 hour 15 minutes.

The powder obtained after slight deagglomeration corresponds, by X-ray diffraction analysis, to a pure rare earth orthoborate, YBO ₃ type, with particles of spherical shape.

The resulting powder is wet milled in a Molinex ball mill with 0.4-0.8 mm ZrO ₂ -SiO ₂ beads. The bead occupancy rate in the grinding chamber is 65%, and the rotational speed of the rotor is 1000 rpm. The concentration of the suspension is 20% by mass of solid, and a dispersant, sodium hexametaphosphate (HMP), is added at a level of 0.05 g HMP / g borate powder (ie 5% by mass). The grinding lasts 90 minutes.

The analysis by laser granulometry gives the following results:

The analysis of the sample thus obtained by X-ray diffraction reveals a YBO ₃ phase, with a coherent domain size in the [1, 0.2] direction of 104 nm.

It is noted that the value of the d50 (laser) and that of the size of the coherent domain have the same order of magnitude, which confirms the monocrystalline nature of the particles.

EXAMPLE 2

This example relates to the preparation of a suspension of yttrium borate, gadolinium and terbium according to the invention.

The synthesis is identical to that of Example 1, replacing the europium with terbium, with the following composition (in atomic%): Y: 58%

Gd: 33%

Tb: 9%

The powder obtained after drying and calcination at 85O ⁰ C has the same morphological characteristics as in Example 1.

The wet grinding of this powder is carried out in ethanol, in a Netzch Labstar ball mill (grinding module coated with polyurethane - maximum permissible temperature of 60 ^° C. - sieve of 0.1 mm - volume of the chamber of 920 ml) . The beads used are 0.4-0.8 mm ZrO ₂ -SiO ₂ balls, with a volume ratio occupied by the beads of 70%. The concentration of the suspension is 20% by mass of solid. The mill is used in recirculation, with a rotation speed of 3000 rpm.

The granulometry characteristics are then as follows:

The attached figure is a photo obtained by MET of the suspension resulting from grinding. This photo shows the monocrystalline character of the particles.

EXAMPLE 3

This example also relates to the preparation of a borate according to the invention in powder form.

The synthesis is identical to Example 2, and the pulp resulting from the wet milling is dried for 24 hours at room temperature.

The powdery product thus obtained is resuspended in water to give a suspension.

The granulometry characteristics of the suspension are then as follows:

EXAMPLE 4

This example concerns the preparation of a transparent, luminescent and emitting material in the red. The suspension of Example 1 (3 ml at 40 g / l) is mixed with a solution of sodium hexametaphosphate at 20 g / l in solution in water, in proportions such that the polyphosphate / borate ratio is 10% by mass. The mixture is deposited on a previously hydrophilized glass substrate (plasma treatment of 30 seconds) by spin-coating (1900 rpm for 65 seconds). The film is then dried for 1 h at 120 ^° C. in an oven. Two successive deposits are made. The thickness of the layer after deposition is about 300 nm.

A film transparent and luminescent to the eye under UV excitation is obtained.

The film has a total transmission of 86% and haze of 18% at 550 nm (values measured under the conditions described above). The film luminesce in the red under UV excitation (230 nm) and VUV (172 nm). The brightness and the transparency of the film are not altered after thermal aftertreatment (at 450 ^° C. for 1 hour), as well as under UV irradiation (24h at 230 nm).

Claims

1- rare earth borate, characterized in that it is in the form of a suspension in a liquid phase of substantially monocrystalline particles of average size between 100 and 400 nm.

2- Borate according to claim 1, characterized in that the particles have a mean size of between 100 and 300 nm.

3- Borate according to one of the preceding claims, characterized in that the particles have a dispersion index of at most 0.7.

4- Borate according to one of the preceding claims, characterized in that the rare earth (rare earth constitutive borate) belongs to the group comprising yttrium, gadolinium, lanthanum, lutetium and scandium.

5. Borate according to one of the preceding claims, characterized in that it further comprises, as dopant, at least one element selected from antimony, bismuth and rare earths other than that constituting the borate, the rare earth dopant which can be more particularly cerium, terbium, europium, thulium, erbium and praseodymium.

6. Borate according to one of the preceding claims, characterized in that it has a doping element content of at most 50 mol%.

7- rare earth borate, characterized in that it is in the form of a powder capable of giving after redispersion in a liquid phase the borate in the form of suspension according to one of the preceding claims.

8- Process for preparing a borate according to one of claims 1 to 6, characterized in that it comprises the following steps:

- a rare earth borocarbonate or hydroxyborocarbonate is calcined at a temperature sufficient to form a borate and to obtain a product having a specific surface area of at least 3 m ² / g; a wet grinding of the product resulting from the calcination is carried out.

9- Method according to. claim 8, characterized in that a rare earth borocarbonate or hydroxyborocarbonate obtained by reacting a rare earth carbonate or hydroxycarbonate with boric acid, the starting reaction medium being in the form of an aqueous solution.

10- Method according to claim 8, characterized in that a borocarbonate or a rare earth hydroxyborocarbonate is used which has been obtained by a process which comprises the following steps:

boric acid and a rare earth salt are mixed;

the mixture thus obtained is reacted with a carbonate or a bicarbonate; the precipitate thus obtained is recovered.

11- The method of claim 10, characterized in that the above mixture is reacted with a carbonate or a bicarbonate in the presence of a base, preferably by adjusting the pH of the reaction medium to a fixed value, in particular between 4 and 6.

12- Process for preparing a rare earth borate in the form of a powder, characterized in that the solid product is separated from the liquid phase from the suspension obtained at the end of the wet grinding step method according to one of claims 8 to 11.

13- luminescent device, characterized in that it comprises, or in that it is manufactured using a borate according to one of claims 1 to 7 or a borate obtained by the process according to one of claims 8 to 12 .

14- plasma system, characterized in that it comprises, or in that it is manufactured using a borate according to one of claims 1 to 7 or a borate obtained by the method according to one of claims 8 to 12.

15- mercury vapor lamp, characterized in that it comprises, or in that it is manufactured using a borate according to one of claims 1 to 7 or a borate obtained by the process according to one of claims 8 to 12.

16- luminescent material, characterized in that it comprises, or in that it is manufactured using a borate according to one of claims 1 to 7 or a borate obtained by the process according to one of claims 8 to 12 . 17. The material as claimed in claim 16, characterized in that it is transparent and in that the borate mentioned above has a mean size of between 100 nm and 200 nm.

18- luminescent system, characterized in that it comprises a material according to claim 16 or 17 and further an excitation source.