HUP0200603A2 - Lanthanum, lutetium, yttrium or gadolinium borate comprising two doping agents and its precursor, use in plasma or x-ray systems - Google Patents

Description

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LANTHANUM, LUTECIUM, YTTRIUM AND GADOLINIUM BORATE DERIVATIVES CONTAINING TWO DOMINANTS AND PRECURSOR THEREOF, AS WELL AS THEIR APPLICATION IN PLASMA AND X-RAY SYSTEMS

The invention relates to a derivative of lanthanum, lutetium, yttrium or gadolinium borate containing two additives, a precursor thereof and its use in plasma or X-ray systems.

Plasma systems (screens and lamps) are new processes developed for display and lighting purposes. One specific application is the replacement of current television screens with lighter and smaller flat screens, which is made possible by the use of plasma panels.

In plasma systems, a gas is introduced into a chamber and ionized by an electrical discharge. This process produces high-energy electromagnetic radiation. The photons are directed at a luminescent material.

Furthermore, in systems based on the use of X-rays, photons excite a luminescent material. To achieve this, the material must have luminophore properties that absorb in the emission range of plasma or X-rays and emit with good efficiency in the visible range.

Rare earth borates are known to be such materials. However, research is needed into products with even better lighting efficiency.

Therefore, the aim of our invention is to research materials with good lighting efficiency.

94079-3035T FO/te

The object of our invention is achieved by a rare earth borate of the general formula LnBCh, in which Ln is one or more rare earth metals, namely lanthanum, lutetium, yttrium and/or gadolinium, which comprises at least one first element, namely a trivalent element suitable for collecting electrons, and at least one second element, namely a trivalent element suitable for collecting holes and at least one of europium(II).

The invention also extends to the preparation of this rare earth borate, in which a rare earth carbonate or hydroxycarbonate and the said elements are reacted with boric acid in a reaction medium in the form of an aqueous solution, and the reaction product is calcined.

Further features, details and advantages of the solution according to our invention can be learned from the following parts of the description, as well as from specific examples, which are included for the purpose of illustrating our invention. We do not intend to limit our invention to what is contained therein.

The borate of our invention contains one or more rare earth metals of the general formula Ln, which may form the matrix of the product together with the borate. Ln means at least one rare earth metal, namely lanthanum, lutetium, yttrium and/or gadolinium or a combination of at least two of these rare earth metals. In the case of a combination, the ratio of the different rare earth metals can be adjusted to the desired value. According to one embodiment of our invention, yttrium and gadolinium are used in the combination.

The borat according to our invention also contains a further first and second element, these elements may act as additives.

As mentioned above, the first element is a trivalent element that is good at collecting electrons. This is an element whose section is better at collecting electrons than holes.

More preferably, europium, samarium, thulium and ytterbium can be mentioned as the first element, these elements may be present alone or in combination.

The europium and/or samarium content is preferably between 2% and 25%, more preferably between 4% and 15%. This is expressed in moles of europium and/or samarium based on the moles of all rare earth metals and further elements or the borate additives mentioned. This ratio is therefore [Di]/[Z(Ln + Di + D2)], where Di denotes the first element or all first elements (in this case europium and/or samarium) and Dj denotes the second element or all second elements. In this case, europium (111) is used as the additive.

The thulium content is preferably between 0.1% and 5% and the ytterbium content is preferably between 1% and 2.5%. The thulium and ytterbium content is expressed in the same way as given above for europium and samarium.

The borate of the present invention comprises, in combination with the first element or additive, at least one second element, namely europium(II) and at least one of a trivalent element capable of collecting holes. This is an element whose section is more effective in collecting holes than in collecting electrons.

This second element may in particular be cerium, terbium, praseodymium, bismuth, lead, antimony, chromium or iron, and these elements may be present alone or in combination.

In a further embodiment of our invention, the first element may be europium and the second element may be terbium. A preferred embodiment is a combined borate derivative of yttrium and gadolinium, which also contains europium and terbium. Another preferred embodiment is a borate derivative of lanthanum, which contains thulium as the first element and terbium as the second element. A third preferred embodiment of our invention is a solution where the first element is thulium and the second element is cerium, in which case the borate is preferably gadolinium borate.

The total second element content is between 5 ppm and 1000 ppm, which can also be expressed in moles based on the moles of all rare earth metals and borate derivatives of said elements. This ratio is therefore [D2]/[E(Ln+Di+D2)]. Its value is more preferably between 10 ppm and 500 ppm, more preferably between 30 ppm and 100 ppm. In the case of this third embodiment, i.e. in the borates containing thulium and cerium, the cerium content is preferably between 0.1% and 15%, preferably between 0.5% and 10%, more preferably between 5% and 10%. The thulium content is in particular between 5 ppm and 5000 ppm, more preferably between 50 ppm and 5000 ppm.

The borate may be present in the form of particles of varying sizes within wide limits. However, the volume median diameter of the particles is generally not more than 10 μm, more preferably not more than 5 μm, and even more preferably between 0.5 μm and 5 μm.

In the description, the median meter and dispersion indices, this index will be mentioned later, are determined using the laser diffraction method using a Coulter LS 230 type granulometer.

In a specific embodiment of the invention, the borate of the invention may be in the form of particles with a specific morphology, i.e., cubic, parallelepiped (plate) or spherical. The shape of the particles depends on the properties of the rare earth metal forming the borate. The borate is in the form of a parallelepiped or plate in the case of lanthanum and lutetium, spherical in the case of yttrium and cubic in the case of rare earth metal combinations of yttrium and gadolinium, and the dispersion index of the borate is at most 0.8. This index is more preferably at most 0.7 and even more preferably at most 0.6. In the solution of the invention, products with a dispersion index of 0.4 or 0.5 can also be produced.

The dispersion index is defined by the following ratio: σ/m = (dgo-d io)2dso where dgo is the equivalent diameter at which 90% of the material passes;

nut means equivalent diameter at which 10% of the material passes;

dso is the mean diameter of the particles.

The borates of the present invention can be prepared by any known process. Examples include solid reaction processes in which precursors of the elements in the borate composition, such as oxides or carbonates, are mixed and melted.

The process is described in more detail below. This process relates to the preparation of borate of the above-mentioned morphology (parallelepiped or plate, sphere, cube) having a dispersion index of up to 0.8.

In the process, a rare earth metal or earth metal precursor or additive or a rare earth metal carbonate or hydroxycarbonate or additive is used as the element used in the borate composition.

We can start from a mixture of rare earth carbonates or hydrocarbonates and various additives or from a mixture of mixed rare earth carbonates and hydroxycarbonates and additives.

Rare earth carbonates and hydroxycarbonates are products which are known per se and can be obtained, for example, by precipitation of one or more rare earth salts with ammonium carbonate or bicarbonate. These carbonates or hydroxycarbonates can be used directly after precipitation or, if appropriate, after washing and/or drying.

The starting carbonate or hydroxycarbonate is reacted with boric acid. The reaction is preferably carried out with heating, for example at a temperature between 40°C and 90°C.

A further feature of the process is that an aqueous solution is used as the reaction medium. This means that water is present in the reaction medium in an amount necessary to achieve a weight ratio of water/boric acid+carbonate of at least 300%, more preferably at least 600%. This ratio is even more preferably at least 1000%.

We can also work in excess boric acid. In this case, the excess is, for example, between 1 mol% and 250 mol% (i.e., the value [B]/[E(Ln+Di+D2)] = between 1.01 and 5).

The reaction is preferably carried out by removing the carbon dioxide formed in the process. The removal can be carried out by bubbling an inert gas, such as nitrogen, through the reaction medium. This variant allows a product with a finer particle size to be obtained.

At the end of the reaction, a precipitate is obtained.

According to one embodiment of our invention, the precipitate is aged in the reaction medium, preferably by maintaining the medium at the same temperature as the reaction temperature. During the aging, the pH of the aging medium is maintained at a constant pH. This pH can be fixed at a specific value, which is, for example, the pH of the reaction mixture at the end of the reaction, i.e. after the addition of boric acid. The pH can be maintained by adding a base, for example an ammonia solution. The aging can last for a period of between 1 hour and 3 hours.

The precipitate is then separated from the reaction medium by known methods, for example by filtration. The separated precipitate is optionally washed and dried. One advantage of the process described is that it provides a precipitate that is easy to filter and wash. After drying, it is further washed with a dilute acid, for example nitric acid, in order to remove traces of carbonate that have not completely reacted.

The precipitate is then calcined.

Calcination results in the formation of borate. This is generally carried out at a temperature of between 500 °C and 1400 °C, more preferably between 500 °C and 1100 °C. The calcination may be carried out in air. A reducing atmosphere (e.g. hydrogen) or an inert atmosphere (argon) or of course a mixture of these may be used during the calcination.

Calcination is suitable for developing the luminescent properties of the product. Calcination is carried out at a higher temperature if the luminescent properties are to be enhanced.

According to one variant, the calcination step can be carried out as follows:

The dried precipitate is mixed with, for example, 5% to 15% boric acid. The calcination is then carried out, for example, at a temperature of between 900°C and 1100°C. In this variant, a borate with good luminescent properties is obtained at a relatively slightly elevated calcination temperature.

Our invention also extends to the borate precursor composition, which is described below.

This composition comprises leather and at least one rare earth metal, namely lanthanum, lutetium, yttrium and/or gadolinium, and at least one first element, namely a trivalent element capable of collecting electrons, and at least one second element, namely europium(II) and at least one of the trivalent elements capable of collecting holes, which, after calcination, forms a rare earth borate derivative of the general formula LnBOa, where Ln is a rare earth metal or earth metals, which borate also comprises at least one first element and at least one second element. The first element may be europium, samarium, thulium or ytterbium and the second element may be cerium, terbium, praseodymium, bismuth, lead, antimony, chromium or iron.

The composition can be prepared by mixing compounds of boron, rare earth metals and other elements, which are compounds that, after calcination, form the desired borate. The compounds formed by these rare earth metals and other elements are, for example, oxides or carbonates.

The composition can also be prepared by the more specific process described above for borates. This process is characterized in that the carbonate or hydroxycarbonate of at least one of the above-mentioned rare earth metals, at least one of the first element and at least one of the second element is reacted with boric acid in a reaction medium in the form of an aqueous solution, and the reaction product is optionally washed and/or dried. Everything stated above in connection with the said process, including the preparation, separation and optionally washing and/or drying of the precipitate, also applies to this case. The precipitate obtained is the hydroxycarbonate of the said rare earth metals or one of them with the general formula LnB(OH)4CO3 or the hydroxycarbonate of at least one of the first element and the hydroxycarbonate of at least one of the second element.

The morphological properties (cubic, parallelepiped (plate-like) or spherical particles) and particle size (particle size and in particular dispersion index not greater than 0.8) of such hydroxycarbonate-based compositions may be identical to those described above in relation to borate.

The calcination of the precursor composition is carried out at a temperature necessary to convert the composition to borate, which is, for example, at least 500°C as described above.

Our invention extends to the use of the borate described above as a luminophore in any display and lighting using plasma excitation. By this we mean processes in which conditions conventional in plasma systems are used, i.e. a gas is used which, after ionization, emits radiation corresponding to a wavelength of at least about 100 nm to about 200 nm, more preferably between 140 nm and 200 nm.

Our invention also extends to systems suitable for this type of display and lighting which include a borate according to our invention as a luminophore.

Among these types of display and lighting systems, plasma screens and lamps can be mentioned.

The borate of our invention can be used in any system based on the use of X-rays. X-rays are radiation consisting of photons with energies between about 10 Kev and about 100 Kev.

Our invention also extends to X-ray imaging systems and imaging systems, of which medical imaging is particularly noteworthy.

In the production of plasma or X-ray based systems, the borate is treated by well known methods, such as serigraphy, electrophoresis and sedimentation.

Our invention is illustrated in more detail in the following examples.

Example 1-4

The final formulation is prepared as follows, the additive content is expressed as follows:

Example 1 (Comparative): Yo,726Gdo, ₂ i8Euo,o56B03

Example 2: A product made from the same composition as in Example 1, but containing 70 ppm terbium.

Example 3 (Comparative): YojgGdo.isEuo.oeBOa

Example 4: A product made from the same composition as in Example 3, but containing 50 ppm praseodymium.

Production:

The carbonate suspension of all rare earth metals (Ln) in the preparation is prepared in one hour at a temperature of 80 °C as follows: 1.31 L of 1.3 mol/l ammonium bicarbonate solution is added in portions with proper stirring to 1.50 L of 0.7 mol/l rare earth nitrate mixture solution until a molar ratio of 1.6 COa/Ln is achieved.

Then, a 0.8 mol/l boric acid solution is added to the same medium, while stirring and heating are continued for 30 minutes at a molar ratio of 1.5 B/Ln. The resulting suspension is then allowed to mature, maintaining the temperature at 60 °C, and the pH is adjusted to at least 4.6 with 6N ammonia solution.

After this 3-hour maturation phase, a white precipitate is obtained, which is separated by filtration and then washed with demineralized water.

The wet product is then dried overnight at 60 °C and then calcined in a jacketed furnace (in air) at 1100 °C for 2 hours in the presence of 15% H3BO3 to produce the rare earth borate.

The particle size of the product of Example 2 was determined by the aforementioned Coulter method. It is noted that the measurement was carried out on a 0.1% by weight aqueous dispersion of the calcined product in sodium hexametaphosphate, which had previously been treated for 3 minutes with an ultrasonic probe (probe tip diameter 12 mm, 450 W). The average diameter of the product of Example 2 was 2.3 μηι and the dispersion index was 0.48.

Properties:

The intensity of the emission peak at 597 nm of the borates prepared in powder form under X-ray excitation (XR:30KV) is determined as a function of the amount of the second element (terbium or praseodinium). The results are presented in Table 1 below.

Table 1

Example Second element content (ppm) Emission intensity at 597 nm (arbitrary unit) 1. 0 100 2. 70 130 3. 0 100 4. 50 125

The intensity of the emission peak at 150 nm of the powder product of Examples 1 and 2 upon plasma excitation is .1- · ♦.**.<'* ·< * ·Μ *

The results are shown in Table 2 below.

Table 2

Example Second element content (ppm) Emission intensity at 150 nm (arbitrary unit) 1. 0 4,200 2. 70 5,100

Examples 5 and 6

The following two products from the composition are prepared as given in the previous examples:

Example 5 (Comparative): Gdo,9sCeo,o2B03

6. Example: Gdo.ggCeo.oosTMo.oosBOa

The intensity of the emission peak at 597 nm of the borates prepared in powder form upon X-ray excitation (XR:30KV) is determined. The results are presented in Table 3 below.

Table 3

Example Intensity 5. 0.186 6. 0.196

According to the results, the emission intensity of the compound obtained by adding 0.5% cerium and 0.5% thulium is higher than that of the compound obtained with 2% cedam.

Claims (20)

Patent claims 1. A rare earth borate derivative of the general formula LnBO ₃ , characterized in that Ln represents one or more rare earth metals, namely lanthanum, lutetium, yttrium or gadolinium, and further comprises at least one first element, namely a trivalent element suitable for collecting electrons, and at least one second element, namely europium (I l) or a trivalent element suitable for collecting holes. 2. The borate derivative according to claim 1, characterized in that the first element is europium, samarium, thulium or ytterbium and the second element is cerium, terbium, praseodymium, bismuth, lead, antimony, chromium or iron. 3. The borate derivative according to claim 2, characterized in that it contains thulium as the first element and cerium as the second element. 4. A borate derivative according to claim 1 or 2, characterized in that it contains, as a first element, europium or samarium in an amount of between 2 mol% and 25 mol%, based on the moles of all rare earth metals and said elements present in the borate. 5. The borate according to claim 2, characterized in that it contains between 0.1 mol% and 5 mol% of thulium, based on the moles of all rare earth metals and said elements present in the borate. 6. The borate according to claim 2, characterized in that it contains ytterbium in an amount of between 1 mol% and 2.5 mol%, based on the moles of all rare earth metals and said elements present in the borate. 7. A borate derivative according to any one of claims 1, 2 or 4-6, characterized in that it contains a second element in an amount of between 5 ppm and 1000 ppm, preferably between 10 ppm and 500 ppm, based on the moles of all rare earth metals and said elements. 8. A borate derivative according to any one of claims 1-7, characterized in that Ln is a combination of yttrium and gadolinium. 9. A borate derivative according to any one of claims 1-8, characterized in that it contains europium as the first element and terbium or praseodymium as the second element. 10. A borate derivative according to any one of claims 1-9, characterized in that Ln is lanthanum, thulium as the first element and terbium as the second element. 11. The borate derivative according to any one of claims 1-10, characterized in that it is in the form of cubic, parallelepiped or spherical particles and has a dispersion index of at most 0.8. 12. A precursor composition, characterized in that it contains leather and one or more rare earth metals, namely lanthanum, lutetium, yttrium and/or gadolinium, and further comprises as a first element at least one trivalent element suitable for electron collection and as a second element europium(11) and at least one trivalent element suitable for hole collection, and further, after calcination, it is converted into a rare earth borate of the general formula LnBO ₃ , in which formula Ln means a rare earth metal or earth metals, wherein the borate also contains at least one first element and at least one second element. 13. A precursor composition according to claim 12, characterized in that it comprises europium, samarium, thulium and/or ytterbium as a first element and cerium, terbium, praseodymium, bismuth, lead, antimony, chromium and/or iron as a second element. 14. A precursor composition according to claim 12 or 13, characterized in that said at least one rare earth metal is in the form of hydroxyborocarbonate, said at least one first element is in the form of hydroxyborocarbonate and said at least one second element is in the form of hydroxyborocarbonate. 15. A process for preparing a rare earth borate according to any one of claims 1-11, characterized in that a rare earth carbonate or hydroxycarbonate and said elements are reacted with boric acid in a reaction medium in the form of an aqueous solution, and the reaction product is calcined. 16. The method according to claim 15, characterized in that the reaction medium is prepared using an amount of water necessary to achieve a weight ratio of at least 300%, more preferably at least 600%, of water/boric acid+carbonate or hydroxycarbonate. 17. The process according to claim 15 or 16, characterized in that the reaction is carried out by removing carbon dioxide formed in the process. 18. A method for preparing a precursor composition according to claim 12 or 13, characterized in that the carbonate or hydroxycarbonate of said at least one rare earth metal, said at least one first element and said at least one second element is reacted with boric acid in a reaction medium in the form of an aqueous solution, and the reaction product is optionally washed and/or dried. 19- A display method based on plasma excitation or X-ray application, characterized in that a borate according to any one of claims 1-11 is used as the luminophore. 20. A display system based on plasma excitation or X-ray application, characterized in that it contains a borate according to any one of claims 1-11 as a luminophore. Λ& .½ ./(,<. Tü ‘ The authorized representative: Dr. F Tlóra érVári, patent attorney