Use of a compound based on a rare-earth phosphate as a luminophor in plasma systems

Abstract

The invention describes the use of phosphate compounds based on yttrium, lanthanum, gadolinium or lutetium phosphate, doped with one or more rare earth metals selected from terbium, praseodymium or cerium, as light-emitting materials in plasma systems. à

Description

The invention relates to the use of rare metal phosphate-containing compounds as light-emitting materials in plasma systems. Plasma systems (screens and lamps) are part of a new visualization and lighting process that is evolving. One concrete example is the replacement of current television screens with flat screens that are not as heavy and larger, a replacement that has almost been achieved with the use of plasma panels. In plasma systems, a gas introduced into a zone is ionized by an electrical discharge. During the process, high-energy electromagnetic rays are generated. The photons are directed towards a luminescent material. To be effective, this material must be a light-emitting material that absorbs in the emission range of the plasma and emits in the visible range, with the highest possible yield and in the appropriate color. The object of the invention is to produce such a light-emitting material. The invention therefore relates to the use of a phosphate compound based on yttrium, lanthanum, gadolinium or lutetium phosphate, doped with one or more rare earth metals selected from terbium, praseodymium or cerium, as a light-emitting material in plasma systems. Other features, details and advantages of the invention can be learned from the description. The invention relates to the use of the above-mentioned compound as a light-emitting material under conditions characteristic of plasma systems. This invention is understood to mean any system in which a gas is present which, after ionization, emits radiation corresponding to a wavelength of at least 10 to 200 nm, i.e. the far ultraviolet range. Among these types of systems, we can mention plasma mice and lamps. According to the invention, a compound is used which consists of a matrix of the yttrium, lanthanum, gadolinium or lutetium phosphate type. The phosphates that can be used can be of different types. They can be orthophosphates with the formula LnPO4, where Ln is one of the elements mentioned above. We can also use metaphosphates with the formula LnP3O9 or pentaphosphates with the formula LnP5O14. To the matrix of the type described above, one or more rare earth metals are also added, which may be terbium, praseodymium or cerium. The rare earth metal is chosen depending on the color of emission we want. The added rare earth content is expressed in atomic percentage relative to the total content of rare earths in the phosphate (added rare earth / added rare earth+Ln), and is generally between 10% and 50%, preferably between 20% and 45%. According to a preferred embodiment of the invention, the phosphate used is lanthanum phosphate. In another preferred embodiment, a combination of terbium and cerium is used as the rare earth additive; terbium is present in a greater amount than cerium, more specifically the Tb / (Ce+Tb) atomic ratio is at least 80%. An example of a product with the following composition can be mentioned: La0 76Tb0 22Ce0 02PO4 According to another embodiment of the invention, it may be advantageous to use a phosphate of a certain particle size. The average particle size of the phosphates is therefore between 1 and 20 pm, preferably between 2 and 6 pm. Furthermore, they may have a dispersion index of <0.5, preferably <0.4. The average diameter of the particles is measured using a CILAS type laser granulometer (CILAS HR 850). The dispersion index I is determined by the following formula: Φ84-Φ16 1=2 <D50 ahol - Φ84 is the particle diameter at which 84% of the particles have a diameter smaller than this Φ84; - Φ16 is the particle diameter at which 16% of the particles have a diameter smaller than Φ16; and - Φ50 is the average diameter of the particles. The phosphates used, especially orthophosphates, may occur in monoclinic crystal form. The phosphates used in the present invention may be prepared by any known method. A specific method is described below. The process consists of carrying out direct precipitation at controlled pH by reacting a first solution containing soluble salts of rare earth metals, in which these elements are present in the stoichiometric ratio necessary to obtain the product of the desired formula, with a second solution containing phosphate ions. The solution containing the rare earth soluble salts is added gradually and continuously to the solution containing the phosphate ions. The initial pH of the solution containing phosphate ions is <2, preferably between 1 and 2. The pH of the precipitation medium is then adjusted to a pH value of less than 2, preferably between 1 and 2. By pH adjustment is meant that the pH of the precipitation reaction medium is kept at a certain value, which value is constant or substantially constant, by adding alkaline compounds or buffer solutions to the solution containing phosphate ions, simultaneously with the addition of the solution containing soluble salts of rare earth metals. The pH of the medium thus deviates from the fixed value by no more than 0.5 pH units, preferably by no more than 0.1 pH units in either direction. The fixed pH value preferably corresponds to the initial pH of the solution containing phosphate ions, which is <2. This pH maintenance is preferably achieved by the addition of an alkaline compound, as will be explained in more detail below. EN 221 257 B1 The precipitation is preferably carried out in an aqueous medium, the temperature is not critical, preferably between room temperature (i.e. a temperature between 15-25°C) and 100°C. The reaction mixture is stirred during the precipitation. The rare earth salt concentration in the first solution can vary within wide limits. Thus, the total rare earth metal concentration can be between 0.01 mol / 1 and 3 mol / 1. The rare earth metals which can be used are those which are soluble in aqueous media, such as nitrates, chlorides, acetates, carboxylates, or mixtures thereof. Nitrate salts are preferred according to the invention. The phosphate ions, which react with the solution of the rare earth salts, can be introduced in the form of pure compounds or in the form of solutions, for example as phosphoric acid, alkali or other metal phosphates, which form a soluble compound with the anions containing the rare earth metals. According to a preferred embodiment, the phosphate ions are added in the form of ammonium phosphate, since the ammonium cation decomposes during subsequent calcination, and thus a high purity rare earth phosphate can be produced. Among the ammonium phosphates, diammonium phosphate and monoammonium phosphate are preferred for the process according to the invention. The phosphate ions are present in such an amount that the PO4 / Ln molar ratio between the two solutions is >1, preferably between 1.1 and 3. As mentioned above, the solution containing phosphate ions should initially (i.e. before the addition of the rare earth salt solution) have a pH of less than 2, preferably between 1 and 2. If the solution used does not naturally have such a pH, then the pH of the solution must be adjusted to the appropriate desired value, this can be done by adding a base (e.g. ammonium hydroxide in the case of an initial phosphoric acid solution) or an acid (e.g. nitric acid in the case of an initial diammonium phosphate solution). Subsequently, during the addition of the solution containing one or more rare earth salts, the pH of the medium in which the precipitate is formed gradually decreases; in order to maintain the pH of the reaction medium at the desired value, i.e. below 2, preferably between 1 and 2, a base is simultaneously added to the medium. Examples of suitable basic compounds include metal hydroxides (NaOH, KOH, Ca(OH)2, etc.), or ammonium hydroxide, or any other basic compound whose components, when added to the reaction medium, do not form any precipitate with any component otherwise present in the reaction medium and allow the pH of the precipitation medium to be controlled. Immediately after the precipitation step, a phosphate precipitate is obtained which can be recovered by any known method, preferably by simple filtration. The recovered product is then washed, for example with water, in order to remove any impurities, such as adsorbed nitrate and / or ammonium groups. The resulting precipitate is then subjected to heat treatment, generally at temperatures above 600 °C, preferably at a temperature between 900 and 1200 °C. This heat treatment, or in other words calcination, allows a product to be obtained that has luminescent properties. The calcination can be carried out in a reducing atmosphere (e.g. hydrogen) or in a neutral atmosphere (e.g. argon), or in a mixture thereof, or in a non-reducing atmosphere, such as an oxidizing atmosphere, such as air. The phosphates used in the invention can also be prepared by chamotte calcination. In this case, the procedure is to mix the different rare earth oxides or to take a mixed rare earth oxide and to phosphatize this mixture or this mixed oxide by calcination in the presence of diammonium phosphate. To develop luminescent properties, phosphates can also be subjected to heat treatment in the presence of a treating salt. Examples of suitable treating salts include lithium fluoride, lithium chloride, potassium chloride, ammonium chloride, boric oxide and ammonium phosphates, the list is of course by no means exhaustive. The treating salt is mixed with the mixed phosphate to be treated and the mixture is then heated to a temperature above 1000°C, preferably between 1000 and 1200°C, in an atmosphere which is appropriate for the quality of the rare earth metal and which is reducing, for example in the case of cerium and terbium. After the treatment, the product is washed and then rinsed, thus obtaining the light-emitting material in a state of being disintegrated into particles. As mentioned above, the disclosed phosphate-containing compounds exhibit luminescent properties upon electromagnetic excitation in the wavelength range used in plasma systems. Therefore, these compounds can be used as light-emitting materials in such systems, and more generally in their preparation. The application of light-emitting materials can be carried out by known methods, for example by imprinting, electrophoresis or deposition. Our invention will be illustrated by an example below. Example 1 Production of phosphates The preparation of the product with the formula La055Ce030Te0I5PO4 is given below. The other phosphates with the formula Lao88Tb012PO4, Lao83Tb0|7PO4, Lao70Tb030PO4 and Lao78Tb022PO4 are prepared in the same way using the appropriate proportions of the reagents. To 500 ml of H3PO4 phosphoric acid solution, the pH of which was previously adjusted to 1.4 with ammonium hydroxide, 500 ml of rare earth nitrate solution, with a total concentration of 1.5 mol / l, is added at 60 °C over 1 hour, the concentration distribution being as follows: 0.825 mol / l La(NO3)3; 0.45 mol / l Ce(NO3)3 and 0.225 mol / l Tb(NO3)3. The phosphate / rare earth molar ratio is 1.15. The pH is maintained at 1.4 during precipitation by adding ammonium hydroxide. EN 221 257 B1 After the precipitation step, the reaction mixture is kept at 60°C for another hour. The precipitate is then recovered by filtration, washed with water and air-dried at 60 °C. The product then consists of white powder (dusting powder appearance) and particles (agglomerates) of 3-6 pm size, which particles themselves consist of compact aggregates of approximately 250 nm size resulting from the aggregation of elementary crystallites of 30 and 150 nm size. The powder is then heat-treated in air at 1150 °C. X-ray analysis shows that the product is orthophosphate with a monoclinic crystal structure. It consists of particles of about 250 nm in the form of spherical particles with a size of 3-6 pm. According to the CILAS particle size determination, which was carried out after ultrasonic particle disintegration, the Φ50 is 4.5 microns, and the particle size distribution is very narrow, since the dispersion index is <0.4. Properties of phosphates The properties of phosphates are determined in two ways. A) The powder is manually compacted into cylindrical cavities with a diameter of 8 mm, which are then placed in a vacuum zone of 10-6 Torr at room temperature. The excitation source is electromagnetic emission originating from a synchrotron and having a wavelength between 150 and 350 nm. The yields correspond to the number of photons emitted in the visible range relative to the number of random photons appearing at 160 nm and 200 nm, respectively. Products Yield in % at 200 nm Yield in % at 160 nm Lao.eeTüo.nPOí 60 72 La<),83Tb0 J7PO4 65 76 LaoojsTbo^Pfü 70 85 La^C^Tb^PO, 70 85 B) The products are evaluated in a plasma cell containing helium-neon type gas. The light output is measured with a photometer and compared with the values ​​obtained with manganese-supplemented zinc silicate, the latter being arbitrarily taken as 100. Product Yield LaogjTbo 17PO4 125 La07gTb0 22PO4 134 LaojoTbo 3ttPO4 120 It can be seen that the yields are higher than the values ​​obtained with previously known, supplemented zinc silicate.

Claims

1. Use of a phosphate compound based on yttrium, lanthanum, gadolinium or lutetium phosphate, doped with one or more rare earth metals selected from terbium, praseodymium or cerium, as a light-emitting material in plasma systems.

2. Use according to claim 1, characterized in that the phosphate is a phosphate of the formula LnPO4, LnP3O9 or LnP5O14, where Ln is yttrium, lanthanum, gadolinium or lutetium.

3. Use according to claim 1 or 2, characterized in that the phosphate is lanthanum phosphate.

4. Use according to any one of claims 1-3, characterized in that the additional rare earth metal content in the phosphate, expressed in atomic percentage, is between 10% and 50%, preferably between 20% and 45%, relative to the total rare earth metal content in the phosphate.

5. Use according to any one of claims 1-4, characterized in that the additive is a combination of terbium and cerium, in which the amount of terbium is preferably greater than that of cerium.

6. Use according to claim 5, characterized in that the atomic ratio of terbium Tb / (Ce+Tb) is at least 80%.

7. Use according to any one of claims 1-6, characterized in that the phosphate has an average particle size of between 1 and 20 µm and a dispersion index of <0.5.