KR20050010513A - Method for manipulating a rare earth chloride or bromide or iodide in a crucible comprising carbon - Google Patents

Abstract

The present invention relates, in particular, to a process for the composition of said composition in the environment of crystal growth from a composition comprising a rare earth halide, wherein said crystal is represented by the formula A _e Ln _f X _{(3f + e)} , where Ln E represents one or more rare earths, X represents one or more halogen atoms selected from Cl, Br or I, and A represents one or more alkali metals such as K, Li, Na, Rb or Cs so that e which may be zero is 2f or less And f is 1 or more. In this way it is possible to grow single crystals that exhibit excellent scintillation properties.

Description

METHODS FOR MANIPULATING A RARE EARTH CHLORIDE OR BROMIDE OR IODIDE IN A CRUCIBLE COMPRISING CARBON}

Blended rare earths melt at very high temperatures of 200-500 ° C., higher than other halides, and they are corrosive to oxides such as silica and to precious metals (platinum), so that they can only be adjusted in carbon crucibles for crystal growth. With regard to the adjustment of fluorides, the following documents may be mentioned: Japanese Patent No. 03285898, Blistanov et al., J. Crystal Growth 237-239 (2002), 899-903, Duffy et al. , J. Crystal Growth 203 (1999), 405-411, US Pat. No. 3 959 442, and Korczak et al., J. Crystal Growth vol 61 (1983), No 3, (XP-002233142).

However, adjusting fluoride in a carbon crucible often produces dark spots in the crystals. In addition, for those skilled in the art, the reaction between carbon and the melt is inherently more fluoride than in the case of other halides because chloride, bromide or iodide is much more hygroscopic than fluoride. weak. In fact, it is known that water or derivatives thereof present in natural materials attack graphite to produce dark spots in the final crystals. This is because chlorides, bromide or iodides other than fluoride are used where non-carbon materials are used, followed by other materials such as silica or platinum. This is because chlorides, bromine, have already been observed when those skilled in the art have already observed such sunspots when growing BaF ₂ or LaF ₃ crystals (the two crystals are made in a very similar manner and similar for these preparations). It can be expected that many sunspots are observed when adjusting natural material halides in the form of cargoes or iodides. However, Applicants have found that carbon-containing crucibles for adjusting chlorinated, brominated or iodide rare earths, which have less disadvantages in silica crucibles, can be used.

When rare earth halides in the form of chloride, bromide or iodide are especially doped with cerium, these, in particular cerium-doped LnBr ₃ and cerium-doped LnCl _3, are particularly useful for nuclear imaging ( Positron electorn tomography or PET, gamma cameras, etc., have very useful scintillation properties. In order to explore these properties, it is necessary to obtain compounds in the form of large crystals (particularly single crystals) that are transparent at 320 to 500 nm where Ce ^III emission occurs while maintaining trivalent (reducing) valence. In addition, the high chemical reactivity of rare earth halides severely limits the choice of materials that can be used as crucibles. Crystals of this type are generally made by pulling in equipment that includes induction-heated crucibles or crucibles heated by radiation of resistive elements. The natural materials used can cause impurities that appear undesired in the final crystals.

The invention in particular the chloride, bromide or adjustment of the composition to grow the crystal from a composition comprising a rare-earth halide of the iodide form of method, the determination is generally the formula A _e Ln _f X _{(3f + e),} and , Wherein Ln represents at least one rare earth, X represents at least one halogen atom selected from Cl, Br or I, A represents at least one alkali metal selected from K, Li, Na, Rb, or Cs, e and f relates to a process for the adjustment of a composition wherein e may be 0 or less and f is 1 or more.

Related rare earths (in the form of halides) are described in Group 3 of the Periodic Table ( Handbook of Chemistry and Physics 1994-1995, 75 ^th edition) containing lanthanide compounds of Sc, Y, La, and Ce to Lu According to the new notation). More specifically, it relates to halides Y, La, Gd and Lu, and in particular to compounds doped with Ce or Pr (the term "dopant" refers herein to a small number of rare earths that substitute for one or more of rare earths). , Decimal and more or less values are included as subscripts of the symbol Ln). Rare earth halides in connection with the present application may be represented by LnX ₃ , where Ln represents rare earth and X represents a halogen atom, which may be Cl, Br or I.

More specifically said compositions comprising the rare earth halides of interest are in particular compounds:

ALn ₂ X ₇ in which Ln represents at least one rare earth, X represents at least one halogen atom selected from Cl, Br or I and A represents an alkali metal such as Rb or Cs;

- in particular a possibly doped with 0.1 to 50% by weight of CeCl ₃ LaCl _3;

- in particular a possibly doped with 0.1 to 50% by weight CeBr ₃ LnBr _3;

- in particular a possibly doped with 0.1 to 50% by weight CeBr ₃ LaBr _3;

- in particular a possibly doped with 0.1 to 50% by weight CeBr ₃ GdBr _3;

La _x Ln _(1- , in particular possibly doped with 0.1 to 50% by weight CeX ₃ , x possibly in the range from 0 to 1, Ln being a rare earth different from La, and X being halogen as described above _x) X ₃ ;

In particular La _x Gd _(1-x) Br ₃ , possibly doped with 0.1 to 50% by weight CeBr ₃ , with x possibly in the range from 0 to 1;

La _x Lu _(1-x) Br ₃ , possibly doped with 0.1 to 50% by weight of CeBr ₃ , with x possibly in the range from 0 to 1;

Ln 'and Ln "are two rare earths different from the Ln type, X' and X" are two halogens different from the X type, in particular Cl and Br, x is possibly in the range 0 to 1 , y is Ln ' _x Ln " _(1-x) X _{3 (1-y)} X" _3y , possibly in the range of 0-1;

In particular RbGd ₂ Br ₇ possibly doped with 0.1 to 50% by weight CeBr ₃ ;

In particular RbLn ₂ Cl ₇ possibly doped with 0.1 to 50% by weight of CeCl ₃ ;

In particular RbLn ₂ Br ₇ possibly doped with 0.1 to 50% by weight CeBr ₃ ;

Especially CsLn ₂ Cl ₇ possibly doped with 0.1 to 50% by weight CeCl ₃ ;

Especially CsLn ₂ Br ₇ possibly doped with 0.1 to 50% by weight CeBr ₃ ; And

In particular K ₂ LaCl ₅ possibly doped with 0.1 to 50% by weight of CeCl ₃ .

The composition generally does not contain fluorides. Thus, any rare earth halide in the composition is in its chloride, bromide or iodide form (and this includes mixed halides of this type). Thus, the present invention is in particular LaCl ₃ , LaBr ₃ , GdBr ₃ and / or La _x Gd _(1-x) Br ₃ , where x is in the range of 0 to 1, CeCl ₃ and / or CeBr ₃ are suitable. It relates to a method for adjusting the composition comprising a. The composition also includes ammonium halides. The composition generally comprises at least 10% by weight rare earth, more generally at least 20% by weight rare earth. Rare earth halide-rich compositions generally comprise at least 80% by weight, more generally at least 90% by weight and even at least 99% by weight of the halides of the aforementioned formula A _e Ln _f X _{(3f + e)} .

The object of the present invention relates in particular to the use of crucibles consisting of graphite or amorphous carbon, in particular with the thermally decomposable coating for the growth of crystals comprising rare earth halides, especially halides which are one of the compounds already mentioned for the formula. It relates to the use of coated graphite crucibles. Such use may be under conditions which oxidize, preferably under vacuum or in the presence of an inert gas such as nitrogen or argon, preferably under an oxygen + water partial pressure of less than 10 millibars.

To grow rare earth halides, it is common to use sealed silica (SiO ₂ ) vessels or platinum crucibles. However, rare earth halides tend to adhere to these materials, making it difficult to extract them from the crucible. Fractures may occur in rare earth halides or crucibles when trying to extract halides upon cooling (in the solid state). However, when reheating the surfaces of halides to extract halides, there is also a risk of thermal shock, which is undesirably oxidized due to the high temperatures required (above 800 ° C.) and crushing them. In addition, platinum is very expensive. Silica crucibles, however, have low thermal conductivity, but are permeable to thermal radiation, making it difficult to control the temperature and consequently the crystal growth. In order for heat transfer to be fully controlled, thermally conductive radiopaque crucibles are preferred.

When graphite crucibles are used to grow CaF ₂ , BaF ₂ or TI: NaI crystals, small amounts of graphite particles are observed, they are contained within the crystals produced, and the small amount of particles in particular penetrate in near ultraviolet light. Very disadvantageous for the optical properties of the crystals. This shows that it is difficult to inherently predict the applicability of materials such as crucibles.

WO 01/60944 and WO 01/60945 teach the use of sealed SiO ₂ containers to contain rare earth halides.

U.S. Patent No. 5 911 824 and U.S. Patent No. 6 093 245 teach the use of graphite crucibles for growing NaI single crystals. However, growing such NaI crystals in graphite crucibles results in poor quality for use as scintillation crystals because they contain graphite inclusions.

The present invention relates in particular to the use of a material comprising at least 20% by weight of carbon for contacting a composition comprising rare earth halides at temperatures above 500 ° C., in particular for growing single crystals comprising rare earth halides. The present invention also relates to a process for adjusting a composition comprising rare earth halides with a material comprising at least 20% by weight of carbon, wherein the composition is in molten contact with the material at a temperature above 500 ° C.

Within the context of this application, "carbon rich material" is understood to mean an opaque material comprising at least 20% by weight of carbon. For example, such materials may be graphite, amorphous carbon (or glassy carbon), graphite coated with pyrolytic carbon, graphite coated with silicon carbide, graphite coated with boron nitride (possibly pyrolytic boron nitride). Can be configured.

As in the case of the crucible, the term "adjusting" should be used in its general sense as it covers the simple contact between the material and the composition and the simple fact that the material is a container for a composition comprising a halide. Accordingly, the present invention also relates to a method of using a carbon-rich material for contacting a composition comprising a rare earth halide and a carbon-rich material for forming a portion of the crucible. The material may on the one hand comprise a graphite substrate and on the other hand may comprise a coating possibly composed of pyrolytic carbon, silicon carbide or boron nitride (possibly pyrolytic boron nitride). If such a coating is present, the carbon-rich material is in contact with the composition through the coating. The coating serves to block any pores, especially in graphite.

The composition may be in the molten state or in the solid state as occurs during the growth of the rare earth halide single crystal. This is because such a composition comprising rare earth halides for this growth is first melted in the crucible and then the crystallization process is carried out to grow single crystals. In such a process, the crucible is contacted with a composition comprising molten rare earth halide and in contact with the rare earth halide single crystal.

The present invention utilizes a carbon-rich material as a crucible to grow single crystals containing rare earth halides, thereby easily recovering the crystals upon cooling without melting their surface, wherein the crystals cause contamination in their mass. This problem is solved because it has been found that there are no dark spots that can be. This is not even observed at low pull rates in the case of NaI, ie even below 5 mm / h at growth rates. Surprisingly, the inventors also observed that, unlike the growth of the present invention, growth in a silica crucible by the use of the same natural material in the same furnace at the same tensile rate produces black spots in mass. Unless the present description limits the scope of the present invention, the particles, in particular graphite particles, are such that the graphite heating component or even natural is more readily adsorbed to the walls of the crucible when the natural material comprises a composition comprising rare earth halides. Can be derived from a material.

It has been found that it is easy to remove crystals from the crucible, especially when the carbon-rich material is used as a crucible for Bridgeman growth. This shows that the crystal is hardly stressed during cooling, and therefore there is little risk of it being broken. It may be mentioned that carbon-rich materials, such as graphite, are at an affordable price and can be easily formed in any form. In addition, since graphite is a reducing agent, the graphite protects the bath from oxidation when substantially contacting the composition with graphite, which, in the case of a doped product, has a doped rare earth (e.g. It is possible to stabilize Ce) which is stabilized at 3. Crucibles consisting of a material comprising at least 20% by weight of carbon can be used several times, ie at least five, even at least ten times, to contact the composition comprising rare earth halides, each time the crucible is returned to room temperature. Thus, the use of such crucibles in particular for the adjustment of compositions comprising rare earth glides for the growth of rare earth halide single crystals from molten compositions comprising rare earth halides is particularly advantageous. The invention also relates to the use of the same crucible at least five times or even at least ten times for carrying out the method according to the invention.

The term " material comprising at least 20% by weight carbon " refers to a total of at least 20% by weight even when carbon is not uniformly distributed in any material (eg, a material comprising a graphite substrate and a boron nitride coating). Represents any material comprising carbon. It is also possible to use materials which are themselves in contact with the composition comprising rare earth halides, also comprising at least 20% by weight of carbon, for example the following materials:

A material comprising a graphite substrate and a pyrolytic carbon coating;

A material comprising a graphite substrate and a silicon carbide coating; And

A material consisting entirely of graphite (no coating).

Preferably, the entire object whose surface is in contact with the composition comprising the rare earth halide also contains at least 20% by weight of carbon at any point in its composition.

The surface in contact with the carbon-rich material may comprise at least 90 weight percent carbon. The entire object having its surface in contact with the composition comprising the rare earth halide may comprise at least 90% by weight carbon.

The carbon-rich material may, for example, consist entirely of graphite. Preferably, it may be graphite coated with the thermally decomposable carbon layer intended to contact the composition comprising the rare earth halide due to the low surface state and the flat surface finish provided by the thermally decomposable carbon layer. . It may be graphite coated with a silicon carbide layer intended to contact a composition comprising a rare earth halide.

If uncoated graphite is used, it is preferable to use graphite which is as concentrated as possible, i.e. graphite having as low open porosity as possible, for example less than 10% by volume open porosity. It is preferably used for contacting the composition comprising the rare earth halide under nitrogen or under vacuum.

Within the context of the method according to the invention, the contact can occur at 20 to 1,000 ° C. The present invention is more particularly advantageous at temperatures of 500 to 1,000, particularly when the composition comprising the rare earth halide is in the molten state.

Growth of such crystals can be performed using known techniques such as Bridgman growth, Kyropoulos, Czochralski growth, growth using a gradient freezing method. Preferably, the crystals are pulled at a controlled growth rate of less than 5 mm / hour. The inherent advantages of the present invention are particularly useful in growing only the bridge. The crystals can be pooled in conventional equipment, i.e., in which the crucible is heated by heating a component consisting of graphite. This seems to be the crucible used in the context of the present invention to attract graphite particles, although these graphite heating components also exhibit one possible contamination factor. The invention therefore also relates in particular to a method wherein the opaque material of the crucible type is heated by the graphite heating component.

Example 1: Anhydrous LaCl ₃

50 g of CeCl ₃ was incorporated into 500 g of a charge of dry LaCl ₃ in powder form with an average particle size of about 500 μm. The combination was placed in a graphite furnace (graphite heating component). The system was increased to 300 ° C. under high vacuum (pressure less than 1 × 10 ⁻⁴ millibars) and then a stream of nitrogen was established. The product was melted in a graphite crucible. Bridgeman growth was performed at a tensile rate of 1 to 3 mm / h. The crystals obtained were transparent (no black spots) and did not adhere to the crucible.

Example 2: Anhydrous LaBr ₃

15 g of NH ₄ Br were incorporated into 500 g of anhydrous LaBr ₃ charge in powder form with an average particle size of about 500 μm. The system was increased to 170 ° C. under vacuum, and then a stream of nitrogen was established. The product was melted in a graphite crucible. Only the bridges performed growth. The crystals obtained were transparent (no black spots) and did not adhere to the crucible.

Example 3 (comparative) LaCl ₃ Silica crucible

50 g of LaCl _{3 in} powder form were placed in a silica crucible placed in a sealed furnace. The system underwent a vacuum purge and then established a stream of nitrogen. The temperature is increased until the charge is completely melted (about 900 ° C.). After cooling, the LaCl ₃ crystals and the silica crucible bind together and the combination bursts.

Example 4 (comparative) LaCl ₃ Platinum crucible

40 g of LaCl _{3 in} powder form were placed in a silica crucible placed in a sealed furnace. The system underwent a vacuum purge and then established a stream of nitrogen. The temperature is increased until the charge is completely melted (ie, about 900 ° C.). After cooling, the LaCl ₃ crystals and the platinum crucible bind together. Crystals could not be removed without cooling rupture.

Example 5 (comparative) BaF under vacuum ₂ / Graphite crucible

1,000 g of BaF ₂ containing about 800% oxygen by weight was placed in a graphite furnace. The system was increased to 1,360 ° C. under high vacuum until the charge melted. The product was melted in a graphite crucible. Only the bridges performed growth. The crystals obtained were clear but contained dark spots and gas bubbles. Many sunspots were observed at the heel of the crystal. The crystals adhered weakly to the crucible, which is the result of the presence of trace amounts of graphite on the surface of the crystal in contact with the graphite. The transmittance of these crystals is severely degraded in the ultraviolet.

Example 6: LaCl Coated with Pyrolytic Carbon ₃ / Graphite crucible

55 g of CeCl ₃ were incorporated into 504 g of a charge of dry LaCl ₃ in powder form. The combination was placed in a graphite furnace. The system was increased to 300 ° C. under high vacuum (pressure less than 1 × 10 ⁻⁴ millibars) and then a stream of nitrogen was established. The product was melted in a crucible consisting of graphite coated with a pyrolytic carbon coating. Only the bridges performed growth. The single crystal obtained was transparent without any dark spots and did not adhere to the crucible. When excited by 622keV γ radiation ( ¹³⁷ Cs), the emitted light intensity was 118% of the light intensity of the TI: NaI crystal (reference scintillation crystal). This reflects the good crystalline properties of lanthanum halides, especially in terms of stabilizing trivalent cerium (active ions for flashing).

Example 7: LaBr Coated with Pyrolytic Carbon ₃ / Graphite crucible

0.5% by weight of CeBr ₃ was incorporated into 911 g of a charge of anhydrous LaBr ₃ . The combination was placed in a graphite furnace. The system was increased to 600 ° C. under vacuum, and then a stream of nitrogen was established. The product was melted at about 820 ° C. in a crucible consisting of graphite coated with pyrolytic carbon. Only the bridges performed growth. The obtained crystal had no black spots and did not adhere to the crucible.

Example 8 (comparative) LaBr ₃ Silica crucible

0.5% by weight CeBr ₃ was incorporated into 743 g of a charge of anhydrous LaBr ₃ (derived from the same batch as in the case of Example 7). The material was the same as in the previous experiment. The combination was placed in a graphite furnace. The system was increased to 820 ° C. under vacuum. The product was melted at about 820 ° C. in a clear silica crucible. Only the bridges performed growth. The crystals obtained (unlike the previous example) contained many sunspots in the central zone. The interface with the crucible was highly ruptured.

Example 9: LaCl Coated with Pyrolytic BN ₃ / Graphite crucible

50 g of dry LaCl _{3 in} powder form was placed in a crucible consisting of graphite coated with pyrolytic BN. A vacuum purge operation was performed and a stream of nitrogen was established. The temperature was increased until the charge melted completely (about 900 ° C.). The crystals did not adhere to the crucible. After cooling, the cast block without black spots was easily removed from the crucible.

Example 10 LaBr Coated with Pyrolytic BN ₃ / Graphite crucible

50 g of dry LaBr _{3 in} powder form was placed in a crucible consisting of graphite coated with pyrolytic BN and then in a sealed furnace. A vacuum purge operation was performed and a stream of nitrogen was established. The temperature was increased until the charge melted completely (about 900 ° C.). After cooling, the cast blockage without black spots was easily removed from the crucible. The crystals did not adhere to the crucible.

Example 10 (comparative): NaI / graphite crucible

A 100 kg charge of NaI doped with 1,000 ppm by weight of platinum iodide was placed in a crucible consisting of graphite coated with a pyrolytic carbon coating. This crucible was then placed in the bridge only furnace (normally a platinum crucible was used) and only the standard NaI bridge was applied to the growth cycle. The crystals were annealed in intu . The obtained crystals were easily removed from the crucible upon cooling. The end of crystallization (referred to as end of crystal) was gray and contained a small amount of carbon. The crystal had a cloudy content and the energy resolution on the 2-inch component was greater than 8.5% at 622 keV, insufficient for standard NaI crystals. The flashing properties of these crystals (light yield per γ photons and energy resolution) are substantially degraded compared to the crystals obtained in platinum crucibles by the same natural material.

Example 12 LaCl ₃ / Graphite crucible; KIROPLUS GROWTH

155 g of anhydrous CeCl ₃ were incorporated into 1504 g of a charge of anhydrous LaCl ₃ in powder form. The combination was placed in a graphite furnace. The system was increased to 300 ° C. under vacuum (pressure less than 1.0 millibar) and then a stream of nitrogen was established. The product was melted in a graphite crucible. Kiroplus growth was performed (by quenching LaCl ₃ seeds in a molten bath). The crystals obtained were clear without any dark spots. The residue of the molten bath did not adhere to the crucible, which allowed the crucible to be easily cleaned.

Claims (22)

A method of adjusting a composition comprising rare earth halides in the form of rare earth chlorides, bromide or iodide by means of an opaque material, the material comprising at least 20% by weight of carbon, the composition having a temperature above 500 ° C. And contacting said material in a molten state. The method of claim 1, wherein the material forms part of the crucible. 3. The method of claim 1, wherein the material comprises graphite or amorphous carbon. 4. 4. The method of claim 3, wherein the material comprises a graphite substrate and a coating, wherein the coating is intended to be in contact with the composition comprising rare earth halides. 5. The method of claim 4, wherein the coating is pyrolytic carbon. 5. The method of claim 4, wherein the coating consists of boron nitride. 5. The method of claim 4, wherein the coating consists of silicon carbide. 4. The method of claim 3, wherein the material consists entirely of graphite or amorphous carbon. The composition of any one of claims 1 to 8, wherein the contact is made at 500 to 1,000 ° C. 10. The method of any one of the preceding claims, wherein the composition comprises at least 10% by weight of rare earths. The composition of any of claims 1 to 10, wherein the composition comprises at least 20% by weight of rare earths. The composition of any one of the preceding claims, characterized in that the adjustment is related to the growth from the composition of single crystals comprising rare earth halides. 13. A method according to claim 12, wherein said growth is carried out at a growth rate of less than 5 mm / h. The composition of claim 12, wherein the growth is in the form of Bridgeman. The method according to any one of claims 12 to 14, wherein the single crystal is of formula A _e Ln _f X _{(3f + e)} , wherein Ln represents one or more rare earths, and X is one or more selected from Cl, Br or I A halogen atom, and A represents one or more alkali metals such as K, Li, Na, Rb, or Cs, wherein e, which may be 0, is 2f or less and f is one or more. The compound of claim 15, wherein the single crystal is of Formula A _e Ln _f X _{(3f + e)} , wherein Ln represents at least one rare earth, X represents at least one halogen atom selected from Cl, Br or I, and A is Rb or Cs, characterized in that the composition of the composition. The composition of claim 15, wherein the composition comprises LaCl ₃ , and / or LaBr ₃ , and / or GdBr ₃ and / or La _x Gd _(1-x) Br ₃ , wherein x is 0 to 1. Characterized in that the composition method of the composition. 18. The method of claim 17, wherein the composition also comprises CeCl ₃ and / or CeBr ₃ . The composition of any one of claims 1 to 18, wherein the opaque material is heated by a graphite component. 20. A method according to any one of the preceding claims, characterized in that the crucible is used at least five times, with the crucible returning to room temperature each time. 21. The method of any one of claims 1 to 20, wherein the crucible is used at least ten times in the same crucible, each time being returned to room temperature. A crystalline material prepared by the method of any one of the method claims according to claim 1.