Strontium Aluminate Mixed Oxide and Method for Producing Same

Abstract

The invention relates to a strontium aluminate mixed oxide precursor and a method for producing same, as well as to a strontium aluminate mixed oxide and method for producing same. The strontium aluminate mixed oxide precursor can be transformed into a strontium aluminate mixed oxide at relatively low temperature. The strontium aluminate mixed oxide is characterized by substantially spherically-shaped particles with a spongy- or porous bone-like microstructure. A luminescent material including a strontium aluminate mixed oxide is also provided.

Claims

1. A method of preparing a strontium aluminate mixed oxide precursor, the method comprising: i) providing a boehmite suspension, wherein the boehmite in the boehmite suspension has a crystallite size (021 and/or 020 reflex) of between 3 nm and 50 nm; ii) adding a strontium compound to the boehmite suspension to form a Sr—Al suspension; iii) hydrothermally treating i) the boehmite suspension, ii) the Sr—Al suspension or iii) both the boehmite suspension and the Sr—Al suspension to obtain a hydrothermally treated boehmite suspension, a hydrothermally treated Sr—Al suspension, or both a hydrothermally treated boehmite suspension and a hydrothermally treated Sr—Al suspension; and iv) drying the hydrothermally treated Sr—Al suspension or the Sr—Al suspension to produce the strontium aluminate mixed oxide precursor.

2. A method of preparing a strontium aluminate mixed oxide, the method comprising: i) providing a boehmite suspension, wherein the boehmite in the boehmite suspension has a crystallite size (021 and/or 020 reflex) of between 3 nm and 50 nm; ii) adding a strontium compound to the boehmite suspension to form a Sr—Al suspension; iii) hydrothermally treating i) the boehmite suspension, ii) the Sr—Al suspension or iii) both the boehmite suspension and the Sr—Al suspension to obtain a hydrothermally treated boehmite suspension, a hydrothermally treated Sr—Al suspension, or both a hydrothermally treated boehmite suspension and a hydrothermally treated Sr—Al suspension; iv) drying the hydrothermally treated Sr—Al suspension or Sr—Al suspension to produce the strontium aluminate mixed oxide precursor; and v) calcining the strontium aluminate mixed oxide precursor to produce the strontium aluminate mixed oxide.

3. The method according to claim 1 comprising: a) according to a first method: i) providing a boehmite suspension, wherein the boehmite in the boehmite suspension has a crystallite size (021 and/or 020 reflex) of between 3 nm and 50 nm; ii) hydrothermally treating the suspension to obtain a hydrothermally treated boehmite suspension; iii) adding a strontium compound to the hydrothermally treated boehmite suspension to form a strontium alumina suspension (Sr—Al suspension); and iv) drying the Sr—Al suspension to produce the strontium aluminate mixed oxide precursor; or b) according to a second method: i) providing a boehmite suspension, wherein the boehmite in the boehmite suspension has a crystallite size (021 and/or 020 reflex) of between 3 nm and 50 nm; ii) adding a strontium compound to the boehmite suspension to form a strontium alumina suspension (Sr—Al suspension); iii) hydrothermally treating the Sr—Al suspension to obtain a hydrothermally treated Sr—Al suspension; and iv) drying the hydrothermally treated Sr—Al suspension to produce the strontium aluminate mixed oxide precursor; or c) according to a third method: i) providing a boehmite suspension, wherein the boehmite in the boehmite suspension has a crystallite size (021 and/or 020 reflex) of between 3 nm and 50 nm; ii) hydrothermally treating the suspension to obtain a hydrothermally treated boehmite suspension; iii) adding a strontium compound to the hydrothermally treated boehmite suspension to form a strontium alumina suspension (Sr—Al suspension); iv) hydrothermally treating the Sr—Al suspension to obtain a hydrothermally treated Sr—Al suspension; and v) drying the hydrothermally treated Sr—Al suspension to produce the strontium aluminate mixed oxide precursor.

4. The method according to claim 1 which the hydrothermal treatment is carried out by heating the boehmite suspension, the Sr—Al suspension or both to a temperature in the range of from 100° C. to 250° C. for a duration in the range of from 0.5 h to 14 h in the presence of water.

5. The method according to claim 1, wherein the boehmite in the boehmite suspension has a crystallite size (021 and/or 020 reflex) of between 3 nm and 45 nm.

6. The method according to claim 1, wherein the strontium compound is a strontium salt or a mixture of strontium salts.

7. The method according to claim 2, wherein calcining the strontium aluminate mixed oxide precursor to produce the strontium aluminate mixed oxide is carried out at a temperature in the range of from 900° C. to 1100° C., for a duration in the range of from 0.5 h to 5 h.

8. The method according to claim 1, wherein the boehmite suspension is further characterized by one or more of: a) the boehmite suspension is an aqueous suspension; b) the boehmite suspension has a solids content in the range of from 1 wt % to 30 wt %; and/or c) the boehmite in the boehmite suspension has a mean particle size (d.sub.50) in the range of from 0.01 μm and 100 μm.

9. A strontium aluminate mixed oxide, wherein the strontium aluminate mixed oxide has particles with a spongy- or porous bone-like microstructure, which is characterized by being built up from a non-porous primary framework of particles that are interconnected by thin, fragile sinter necks to a three-dimensional secondary structure, with a pronounced macro pore structure, and at least the following properties: i) a pore volume of less than 0.05 ml/g for pore radii in the range of from 15 Å to 500 Å, measured by Hg intrusion; and ii) a pore volume in the range of from 0.7 to 1.5 ml/g for pore radii in the range of from 300 Å to 5000 Å, measured by Hg intrusion.

10. The strontium aluminate mixed oxide according to claim 9 having a particle size in the range of from 1 to 150 μm measured by laser diffraction methods.

11. The strontium aluminate mixed oxide according to claim 9 having a substantially spherically-shaped particles with a volume based sphericity of the particles between 0.90 and 1.00 determined by dynamic image analysis according to ISO 13322-2 (2006).

12. The strontium aluminate mixed oxide according to claim 9 having a BET surface area of less than 20 m.sup.2/g.

13. The strontium aluminate mixed oxide according to claim 9, wherein the strontium aluminate mixed oxide is SrAl.sub.2O.sub.4, SrAl.sub.4O.sub.7, Sr.sub.3Al.sub.2O.sub.6, SrAl.sub.12O.sub.19, Sr.sub.4Al.sub.14O.sub.25 or mixtures thereof.

14. The strontium aluminate mixed oxide according to claim 9, wherein the strontium aluminate mixed oxide is doped with an activator.

15. A strontium aluminate mixed oxide obtainable by the method of claim 2, wherein the strontium aluminate mixed oxide has particles with a spongy- or porous bone-like microstructure, which is characterized by being built up from a non-porous primary framework of particles that are interconnected by thin, fragile sinter necks to a three-dimensional secondary structure, with a pronounced macro pore structure, and at least the following properties: i) a pore volume of less than 0.05 ml/g for pore radii in the range of from 15 Å to 500 Å, measured by Hg intrusion; and ii) a pore volume in the range of from 0.7 to 1.5 ml/g for pore radii in the range of from 300 Å to 5000 Å, measured by Hg intrusion.

16. The method according to claim 2 comprising: a) according to a first method: i) providing a boehmite suspension, wherein the boehmite in the boehmite suspension has a crystallite size (021 and/or 020 reflex) of between 3 nm and 50 nm; ii) hydrothermally treating the suspension to obtain a hydrothermally treated boehmite suspension; iii) adding a strontium compound to the hydrothermally treated boehmite suspension to form a strontium alumina suspension (Sr—Al suspension); and iv) drying the Sr—Al suspension to produce the strontium aluminate mixed oxide precursor; or b) according to a second method: i) providing a boehmite suspension, wherein the boehmite in the boehmite suspension has a crystallite size (021 and/or 020 reflex) of between 3 nm and 50 nm; ii) adding a strontium compound to the boehmite suspension to form a strontium alumina suspension (Sr—Al suspension); iii) hydrothermally treating the Sr—Al suspension to obtain a hydrothermally treated Sr—Al suspension; and iv) drying the hydrothermally treated Sr—Al suspension to produce the strontium aluminate mixed oxide precursor; or c) according to a third method: i) providing a boehmite suspension, wherein the boehmite in the boehmite suspension has a crystallite size (021 and/or 020 reflex) of between 3 nm and 50 nm; ii) hydrothermally treating the suspension to obtain a hydrothermally treated boehmite suspension; iii) adding a strontium compound to the hydrothermally treated boehmite suspension to form a strontium alumina suspension (Sr—Al suspension); iv) hydrothermally treating the Sr—Al suspension to obtain a hydrothermally treated Sr—Al suspension; and v) drying the hydrothermally treated Sr—Al suspension to produce the strontium alum inate mixed oxide precursor.

17. The method according to claim 2 in which the hydrothermal treatment is carried out by heating the boehmite suspension, the Sr—Al suspension or both to a temperature in the range of from 100° C. to 250° C. for a duration in the range of from 0.5 h to 14 h in the presence of water.

18. The method according to claim 2, wherein the boehmite in the boehmite suspension has a crystallite size (021 and/or 020 reflex) of between 3 nm and 45 nm.

19. The method according to claim 2, wherein the strontium compound is a strontium salt or a mixture of strontium salts.

20. The method according to claim 2, wherein the boehmite suspension is further characterized by one or more of: a) the boehmite suspension is an aqueous suspension; b) the boehmite suspension has a solids content in the range of from 1 wt % to 30 wt %; and/or c) the boehmite in the boehmite suspension has a mean particle size (d.sub.50) in the range of from 0.01 μm and 100 μm.

Description

EXAMPLES

[0122] The invention will now be described with reference to the following non-limiting examples and Figures in which:

[0123] FIG. 1 is an SEM image of the strontium alum inate mixed oxide produced according to Example 1;

[0124] FIG. 2 is an SEM image of the strontium aluminate mixed oxide produced according to Comparative Example 1;

[0125] FIG. 3 is an SEM image of the strontium aluminate mixed oxide produced according to Comparative Example 2;

[0126] FIG. 4 is graph comparing the pore volume and pore sizes of Example 1 and Comparative Example 1;

[0127] FIG. 5 is an XRD of the phase composition of the strontium aluminate mixed oxide produced according to Example 1;

[0128] FIG. 6 is an XRD showing a mixture of monoclinic SrAl.sub.2O.sub.4 together with SrAl.sub.12O.sub.19 and Sr.sub.3Al.sub.2O.sub.6 as side products of the product according to comparative Example 1; and

[0129] FIG. 7 is a graph comparing particle size (measured by Malvern Mastersizer) and milling times of Example 1 and Comparative Example 1.

EXAMPLE 1

[0130] Strontium aluminate mixed oxide was produced according to the method of the invention. 208.8g of a boehmite-containing suspension with 7.1% solid content was mixed with a solution comprising 16.3 g strontium acetate and 17.2 g SrCO.sub.3 in 200 g distilled water. After hydrothermal treatment at 210° C. for 5 h, at a pH of 8.5 the suspension was spray dried to obtain a strontium aluminate mixed oxide precursor. The strontium aluminate mixed oxide precursor was calcined at 1000° C. for 3 h to form a strontium aluminate mixed oxide. The strontium aluminate mixed oxide was analyzed by SEM and X-ray diffraction (XRD).

[0131] FIG. 1 clearly shows the spongy-like or porous bone-like microstructure of the strontium aluminate mixed oxide.

[0132] As shown in FIG. 4, the pore volume of the strontium alum inate mixed oxide for pore sizes between 300 Å and 5000 Å, determined by Hg intrusion, is 1.02 ml/g. The pore volume of the strontium alum inate mixed oxide for pore sizes between 15 Å and 500 Å, determined by Hg intrusion, is 0.02 ml/g.

[0133] FIG. 5 shows the presence of SrAl.sub.2O.sub.4 in the phase composition of the strontium aluminate mixed oxide produced according the method of the invention.

Comparative Example 1

[0134] A stoichiometric mixture of SrCO.sub.3 and γ-Al.sub.2O.sub.3 (commercially available under the trade name PURALOX SBa-150 with a mean particle size (d50) of 30 μm) was mixed in a mortar and calcined for 3 h at 1500° C., according to a conventional ceramic method.

[0135] FIG. 2 shows the dense, highly sintered nature of the product prepared by the conventional ceramic method. The highly sintered nature of the material is confirmed by the measured pore volumes. As shown in FIG. 4, the pore volume for pore sizes provided as pore radius between 300 Å and 5000 Å determined by Hg intrusion is smaller than 0.01 ml/g and the pore volume for pore sizes between 15 Å and 500 Å determined by Hg intrusion, is also smaller than 0.01 ml/g.

[0136] FIG. 6 shows a mixture of monoclinic SrAl.sub.2O.sub.4 together with SrAl.sub.12O.sub.19 and Sr.sub.3A.sub.2O.sub.6 as side products, indicating that the solid state reaction is still not fully completed at 1500° C.

Milling Tests

[0137] Material from Example 1 and Comparative Example 1 were each suspended in water up to a 5 wt % solids content. Each suspension was treated in an ULTRA-TUR-RAX high shear mixer, operated at 600 rpm.

[0138] The particle size (d.sub.50) of the solids in each suspension was measured by laser diffraction after 5, 10 and 15 minutes. For comparative purposes, the measured d.sub.50 value after 10 minutes and 15 minutes is referenced to the d.sub.50 value as determined after 5 minutes.

[0139] As shown in Table 1 and FIG. 7, it is clear that the inventive material of Example 1 easily breaks up during milling, whereas no particle reduction is observed with the materials obtained from Comparative Example 1 under these milling conditions.

TABLE-US-00001 TABLE 1 5 min 10 min 15 min Example 1 relative d.sub.50 100%  64% 59% Comparative relative d.sub.50 100% 102% 98% Example 1

Comparative Example 2

[0140] The Sr-aluminate was prepared according to Kutty et. al (Mat. Res. Bull. 25, (1990), 1355.

[0141] A solution of 25 wt. % NH.sub.3 in water was added to an aqueous solution of Al-sulfate at 60° C. to precipitate an Al.sub.2O.sub.3*xH.sub.2O gel. The gel was washed free of sulfate with water. The X-ray powder diffraction analysis of this gel revealed a pronounced pseudoboehmite character with crystallite size measured at the of 1.5 nm.

The gel was mixed with SrO that was freshly prepared from calcination of SrCO.sub.3 in a molar ratio of Al:Sr of 2:1. The mixture was hydrothermally aged at 240° C. for 6 h. The solid was separated by filtration, washed with water, acetone-dried and calcined at 1000° C. for 3 h.

[0142] The obtained pore volume is included in Table 2

TABLE-US-00002 TABLE 2 Comparative Comparative Example 1 Example 2 Example 1 Pore volume (Hg) <0.01 0.026 0.02 15-500 Å Pore volume (Hg) <0.01 0.65 1.02 300-5000 Å

[0143] As can be seen in FIG. 3 the shape of the particles is not spherical and the microstructure deviates from the spongy-bone character.

[0144] Therefore, it is shown that the material properties as described in the present invention cannot be obtained by the process described by Kutty et al.