FR2782995A1 - LANTHANE, LUTECIUM, YTTRIUM OR GADOLINIUM BORATE COMPRISING TWO DOPANTS AND ITS PRECURSOR, USE IN PLASMA OR X-RAY SYSTEMS - Google Patents

Abstract

The invention concerns a lanthanum, lutetium, yttrium or gadolinium borate comprising two doping agents, said borate precursor, and the use of said borate in plasma or X-ray systems. Said rare earth borate corresponds to the formula LnBO3, wherein Ln represents one or several rare earths selected among lanthanum, lutetium, yttrium and gadolinium, and it further comprises at least a first element selected among trivalent elements capable of capturing electrons such as europium, samarium, thulium and ytterbium and at least a second element selected among europium II and the trivalent elements capable of capturing defect electrons, such as cerium, terbium, praseodymium, bismuth, lead, antimony, chromium and iron.

Description

BORATE OF LANTHANE. OF LUTECIUM. YTTRIUM OR GADOLINIUM

  COMPRISING TWO DOPANTS AND ITS PRECURSOR. USE IN

PLASMA OR X-RAY SYSTEMS

RHODIA CHEMISTRY

  The present invention relates to a lanthanum, lutetium, yttrium or gadolinium borate comprising two dopants, its precursor and its use in plasma or X-ray systems. Plasma systems (screens and lamps) are among the new visualization and lighting techniques that are developing. A concrete example is that of replacing current television screens with flat screens, lighter and larger, replacement which is about to be solved

  by the use of plasma panels.

  In plasma systems, a gas introduced into an enclosure is ionized under the effect of an electric discharge. During this process, high energy electromagnetic radiation is emitted. The photons are directed towards a

luminescent material.

  Likewise, in systems using X-rays, the photons will

excite luminescent material.

  To be effective, this material must be a phosphor absorbing in the emission domain of plasma or X-rays, emitting in the visible range

with good yield.

  Rare earth borates are known as suitable materials of this type.

  However, it appears necessary to find products with a yield

bright further increased.

  The object of the invention is therefore to provide materials with improved light output. For this purpose, the rare earth borate of the invention corresponds to the formula LnBO3 and it is characterized in that Ln represents one or more rare earths chosen from lanthanum, lutetium, yttrium and gadolinium, and in this that it further comprises at least a first element chosen from trivalent elements capable of picking up electrons and at least a second element chosen from trivalent elements

likely to pick up holes.

  The invention also relates to a process for the preparation of such a rare earth borate which is characterized in that a carbonate or a hydroxycarbonate of rare earth and the abovementioned elements is reacted with boric acid, the reaction medium presenting

  in the form of an aqueous solution, and the reaction product is calcined.

  Other characteristics, details and advantages of the invention will also appear

  more fully on reading the description which follows, as well as the various

  concrete but non-limiting examples intended to illustrate it. The borate of the invention is based on one or more rare earths Ln which can form with the boron the matrix of the product. Ln represents at least one rare earth chosen from lanthanum, lutetium, yttrium and gadolinium or a combination of at least two of these rare earths. In the case of a combination, the respective proportions of the different rare earths can be arbitrary. According to one embodiment

  particular of the invention, yttrium and gadolinium are present in combination.

  The borate of the invention further comprises a first and a second element

  additional, these elements can play the role of dopants.

  As indicated above, the first element is chosen from trivalent elements capable of capturing electrons. It is an element whose cross section of electron capture is greater than the cross section of hole capture. As the first element, mention may be made more particularly of europium, samarium, thulium and ytterbium, these elements being able to be taken alone or in combination

combination.

  The content of europium and / or samarium can be between 2% and 25% and more particularly between 4 and 15%. This content is expressed in moles of europium and / or samarium relative to the moles of all of the rare earths and of the abovementioned additional or doping elements of the borate, ie the ratio [D1] / [; (Ln + Di + D2)], D1 designating the first element or the sum of the first elements (in this case europium and / or the samarium) and D2 designating the second element or the sum of the other second elements. The europium used

  as dopant here is europium III.

  The thulium content may be between 0.1% and 5% and that of ytterbium may be between 1% and 2.5%. The contents of thulium and ytterbium are expressed in the same way as that given above for europium and samarium. The borate of the invention comprises, in combination with the first element or dopant, at least one second element chosen from the trivalent elements capable of picking up holes. This is an element whose cross section of hole capture

  is greater than the electron capture cross section.

  This second element can in particular be chosen from cerium, terbium, praseodymium, bismuth, lead, antimony, chromium and iron, these elements being able to

be taken alone or in combination.

  The overall content of the second element may be between 5 ppm and 1000 ppm in moles of the second element, which content is also expressed in moles relative to the moles of all the rare earths and of the abovementioned elements of the borate, that is to say by the ratio [D2] / [(Ln + D1 + D2)]. This content can be more particularly between 10 ppm and 500 ppm and even more particularly between 30 ppm and

1 00ppm.

  According to a particular embodiment of the invention, the first aforementioned element can be europium and the second terbium. Mention may more particularly be made of yttrium and gadolinium borate in combination, further comprising europium and terbium. According to another particular embodiment, mention may be made of lanthanum borate comprising thulium as the first element and terbium as the second

element.

  The borate can be in the form of particles, the sizes of which can vary within wide limits. However, the volume median diameter of the particles is generally at most 10 μm, more particularly at most 5 μm and,

  even more particularly, it can be between 0.5 and 5 pm.

  For the whole of the description, the values of median diameter and of index of

  dispersion, index which will be mentioned later, are the values obtained by implementing the laser diffraction technique using a granulometer of the Coulter LS 230 type. According to a particular embodiment of the invention, the borate of the invention can appear in a specific morphology, that is to say in the form of cubic, parallelepipedic (platelet) or spherical particles. The shape of the particles depends on the nature of the rare earth that makes up the borate. In the case of lanthanum and lutetium, borate is rather in the form of a parallelepiped or in the form of a plate, in the case of yttrium in spherical form and in cubic form for the combination of the rare earths yttrium and gadolinium. Furthermore, the borate can have a dispersion index of at most 0.8. This index can more particularly be at most 0.7 and even more particularly at most 0.6. It is possible, within the framework of the present invention, to obtain products having a dispersion index of 0.4 or 0.5. By dispersion index is meant the ratio: a / m = (d9o-dlo) / 2d5o in which: d9o is the equivalent diameter for which the loop is equal to 90%; do10 is the equivalent diameter for which the loop is 10%;

  - d5o is the median diameter of the particles.

  The borates of the invention can be obtained by any known method. Mention may be made of the processes using a solid / solid reaction by mixing and melting the precursors of the elements included in the composition of the borate, by

  example of oxides or carbonates.

  A more specific method will be described below. This process applies more particularly to the preparation of a borate which has the morphologies described more

  high (parallelepiped or plate, sphere, cube) and a dispersion index of at most 0.8.

  The method uses as precursors of the rare earth (s) or doping elements included in the composition of the borate, a carbonate or a hydroxycarbonate

of these rare earths or dopants.

  It is possible to start with a mixture of carbonates or hydroxycarbonates of rare earths and of different dopants or of carbonates or hydroxycarbonates of mixed

rare earths and dopants.

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

  after a possible washing and / or drying.

  The starting carbonate or hydroxycarbonate is reacted with boric acid. Preferably, the reaction is carried out hot, for example at a temperature

between 40 C and 90 C.

  According to another characteristic of this process, the 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 water / boric acid + carbonate mass ratio is at least 300%, more particularly at least 600%. This report may still be

  more particularly at least 1000%.

  We can work with an excess of boric acid. This excess can for example be

  between 1% and 250% by mole (ie [B] / [:( Ln + D1 + D2] = 1.01 to 5).

  It may be advantageous to carry out the reaction by eliminating the CO2 formed during it. 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 grain size.

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

  According to a variant of the invention, the precipitate can be ripened in the reaction medium, preferably keeping the medium at the same temperature as that at which the reaction took place. It is also possible during this ripening to keep the pH of the ripening medium constant. This pH can be fixed at a given value which can be, for example, the pH value presented by the reaction medium just at the end of the reaction, that is to say at the end of the introduction of boric acid. . This maintenance of the pH can be done by adding a base, for example by adding an ammonia solution. The ripening can last between 1 and 3 hours. The precipitate is then separated from the reaction medium by any known means, for example by filtration. The separated precipitate is optionally washed and then dried. One of the advantages of the process described is that it makes it possible to obtain a precipitate which can easily be filtered and washed. After drying, an additional wash can also be carried out with a dilute acid, for example nitric acid to remove possible traces of

  carbonate that has not fully reacted.

  The precipitate is then calcined.

  This calcination makes it possible to obtain the borate. It is generally carried out at a temperature between 500 and 1400 C, more particularly between 500 and 1100 C. It is quite possible to carry out this calcination in air. Of course, reductive atmospheres can be used for this calcination (hydrogen by

  example) or neutral (argon) or mixtures thereof.

  Calcination also makes it possible to develop the luminescence properties of the product. We will calcine at a temperature all the higher as we want

  high luminescence properties.

  A variant of the calcination step can be mentioned. This variant consists in mixing the dried precipitate with boric acid, in a proportion of acid which can be of the order of 5% to 15% by mass for example. The calcination is then carried out at a temperature which can be between 900 ° C. and 1100 ° C. for example. This variant makes it possible to have a borate with good performance of

  luminescence at a relatively low calcination temperature.

  The invention also relates to a borate precursor composition which has just been described above. This composition is characterized in that it is based on boron, on one or more rare earths chosen from lanthanum, lutetium, yttrium and gadolinium, of at least one first element chosen from trivalent elements capable of capturing electrons and at least one second element chosen from trivalent elements capable of capturing holes and in that it is capable of giving, after calcination, a rare earth borate of formula LnBO3, Ln representing the earth (s) rare aforementioned, said borate further comprising at least a first element mentioned above and at least a second element mentioned above. The first element can be chosen from europium, samarium, thulium and ytterbium and the second element from cerium, terbium, praseodymium, bismuth, lead, antimony, chromium and iron. This composition can be prepared by mixing boron compounds, rare earths and the aforementioned other elements, these compounds being chosen from those which allow, after calcination, to obtain the desired borate. These rare earth compounds and other elements can be oxides or carbonates by

example.

  The composition can also be prepared by a more specific method of the type described above for the borate. This process is characterized in that a carbonate or a hydroxycarbonate of at least one aforementioned rare earth, at least one aforementioned first element and at least one aforementioned second element is reacted with boric acid, the reaction medium in the form of an aqueous solution and, optionally, the reaction product is washed and / or dried. All that has been said above for this process, until obtaining, separation, possibly washing and / or drying, also applies here. The precipitate as obtained is based on a hydroxyborocarbonate of one or more of the aforementioned rare earths, which may be represented by the formula LnB (OH) 4CO3, of a hydroxyborocarbonate of at least one first element

  above and a hydroxyborocarbonate of at least one second element above.

  This hydroxyborocarbonate-based composition may have the same morphology (cubic, parallelepipedic (platelet) or spherical) and particle size characteristics (particle size and dispersion index of at most

  0.8 in particular) which have been described above concerning borate.

  The calcination of the precursor composition is carried out at a temperature sufficient, for example, as described above, of at least 500 ° C., to transform this

borate composition.

  The invention also relates to the use of the borate described above as a phosphor in any display and lighting method using plasma excitation. By this is meant the processes which implement conditions which are those of plasma systems, that is to say using a gas emitting after ionization a radiation corresponding at least to wavelengths situated between approximately 100 nm and approximately 200 nm, more particularly between 140nm and

200nm.

  The invention also covers display and lighting systems of this type which

  contain a borate according to the invention as a phosphor.

  As display and lighting systems of this type, mention may be made of:

plasma screens and lamps.

  The borate of the invention can also be used in any system using X-rays. The term X-rays means those made up of photons

  whose energy is between about 10 and about 100 Kev.

  The invention also covers X-ray display systems, it is possible to

  mention imaging systems, especially medical imaging.

  The use of borate in the manufacture of plasma or X-ray systems is carried out according to well known techniques, for example by screen printing, electrophoresis or sedimentation.

  Examples will now be given.

EXAMPLES

  Products are prepared whose final composition is as follows, the contents of doping elements being expressed in the manner given above: Example 1 (comparative): Y0.726Gd0.218Eu0.056BO3 Example 2: product of the same composition as the example 1 but with 70ppm of terbium. Example 3 (comparative): Y0.79Gd0.15Eu0.06BO3 Example 4: product of the same composition as Example 3 but with 50 ppm of

praseodymium.

  Preparation: A hot carbonate suspension of all the rare earths (Ln) used in the composition of the products is prepared hot (60 ° C.) by adding, in 1 hour, 1.311 of a 1.3M / I solution. ammonium bicarbonate on 1.501 solution of a mixture of rare earth nitrates, suitably stirred, concentration 0.7 M / l,

  in proportions such that the C03 / Ln molar ratio is equal to 1.6.

  Then, in the same medium, still with stirring and hot, a 0.8M / I boric acid solution is added over 30 minutes so that the molar ratio B / Ln is equal to 1.5. The suspension obtained is then left to mature while maintaining the

  temperature at 60 C and pH at minimum 4.6 by adding a 6N ammonia solution.

  After this 3 hour ripening phase, a white precipitate is obtained

  which is separated by filtration, then a washing operation is carried out with demineralized water.

  The wet product is then dried at 60 ° C. overnight, then calcined in a muffle furnace (in air) at 1100 ° C. for 2 hours, in the presence of 15% H3BO3,

  in order to obtain the rare earth borate.

  The particle size was determined for the product of Example 2 according to the abovementioned Coulter technique. It is further specified that the measurement was carried out on a dispersion of the calcined product in an aqueous solution at 0.1% by weight of sodium hexametaphosphate and which has previously undergone a passage through the ultrasonic probe (probe with tip 12mm diameter, 450W) for 3 minutes. The product of Example 2 has a median diameter of 2.3 μm and a dispersion index

0.48.

  Properties: The intensity of the emission peak is measured at 597nm under X excitation (RX: 30KV) of the borates prepared, in powder form, as a function of the content of second element.

  (terbium or praseodymium). The results are given in Table 1 below.

Table 1

  Example Second content Element emission intensity (ppm) 597nm (arbitrary unit)

1 0 100

2 70 130

3 0 100

4 50 125

  The intensity of the emission peak at 150 nm under plasma excitation is measured with a VUV spectrometer of the products of Examples 1 and 2 in powder form. The results

  are given in table 2 below.

Table 2

  Example Second content Element emission intensity (ppm) 150nm (arbitrary unit)

1 0 4200

2 70 5100

Claims (16)

  1- Rare earth borate of formula LnBO3, characterized in that Ln represents one or more rare earths chosen from lanthanum, lutetium, yttrium and gadolinium, and in that it also comprises at least one first element chosen from trivalent elements capable of capturing electrons and at least one second element chosen   among the trivalent elements likely to capture holes.   2- Borate according to claim 1, characterized in that the first element is chosen from europium, samarium, thulium and ytterbium and the second element is chosen from cerium, terbium, praseodymium, bismuth, lead, antimony, chromium and iron.   3- Borate according to claim 2, characterized in that the content of europium or samarium is between 2% and 25% by mole of europium or samarium by   compared to the moles of all the rare earths and of the abovementioned elements of borate.   4- Borate according to claim 2, characterized in that the thulium content is between 0.1% and 5% by mole of thulium relative to the moles of all   rare earths and the aforesaid elements of borate.   - Borate according to claim 2, characterized in that the ytterbium content is between 1% and 2.5% by mole of ytterbium relative to the moles of the whole   rare earths and the aforementioned elements of borate.   6- Borate according to one of the preceding claims, characterized in that the content   second element mentioned above is between 5 ppm and 1000 ppm in mole of second element relative to the moles of all the rare earths and of the aforementioned elements   and more particularly between 1 Oppm and 500ppm.   7- Borate according to one of the preceding claims, characterized in that Ln   represents yttrium and gadolinium in combination.   8- Borate according to one of the preceding claims, characterized in that the first   above-mentioned element is europium and the second aforementioned element is terbium.   9- Borate according to one of the preceding claims, characterized in that Ln   represents lanthanum, the first element mentioned above is thulium and the second element mentioned above is terbium.   10- Borate according to one of the preceding claims, characterized in that it   present in the form of cubic, parallelepipedic or spherical particles and in this   that it has a dispersion index of at most 0.8.   11- Precursor composition characterized in that it is based on boron, on one or more rare earths chosen from lanthanum, lutetium, yttrium and gadolinium, of at least one first element chosen from among the elements trivalents capable of capturing electrons and at least one second element chosen from trivalent elements capable of capturing holes and in that it is capable of giving, after calcination, a rare earth borate of formula LnBO3, Ln representing the aforementioned rare earths, said borate further comprising at least a first aforementioned element   and at least one second element mentioned above.   12- precursor composition according to claim 1 1, characterized in that the first element is chosen from europium, samarium, thulium and ytterbium and the second element is chosen from cerium, terbium, praseodymium, bismuth, lead, Antimony, chromium and iron.   13- A precursor composition according to claim 11 or 12, characterized in that it is based on a hydroxyborocarbonate of one or more of the aforementioned rare earths, of a hydroxyborocarbonate of at least one aforementioned first element and of a   hydroxyborocarbonate of at least one second element mentioned above.   14- Process for the preparation of a rare earth borate according to one of claims 1 to   , characterized in that a rare earth carbonate or hydroxycarbonate and the abovementioned elements are reacted with boric acid, the reaction medium being   in the form of an aqueous solution, and the reaction product is calcined.   - Method according to claim 14, characterized in that the reaction medium is formed using water in a mass ratio water / boric acid + carbonate   or hydroxycarbonate of at least 300%, more particularly at least 600%.   16- Method according to claim 14 or 15, characterized in that one conducts the   reaction by eliminating the CO2 formed during it.   17- A method of preparing a precursor composition according to claim 1 1 or 12, characterized in that reacts a carbonate or a hydroxycarbonate of at least one aforementioned rare earth, of at least one first element mentioned above and d 'At least one second element mentioned above with boric acid, the reaction medium being in the form of an aqueous solution and, optionally, the reaction product is washed and / or dried. 18- Visualization method using plasma excitation or X-ray, characterized in that a borate is used as the phosphor according to one of claims 1 to 10.   19- Display system using plasma excitation or X-ray radiation, characterized in that it comprises, as a phosphor, a borate according to one of claims 1 to 10.