FOAM CERAMICS, FOAM CERAMICS FILTERS, METHOD FOR THE PRODUCTION AND USE THEREOF

Abstract

The invention generally relates to foam ceramics (3) and to filters comprising such a foam ceramic, and to a method for producing foam ceramics and filters comprising or made of such a foam ceramic. Another aspect relates to the use of the foam ceramic (3) and of a filter comprising or made of such a foam ceramic.

Claims

1. A foam ceramic, comprising a base material comprising Al.sub.2O.sub.3 and preferably Li.sub.2O; and a matrix comprising SiO.sub.2 and/or B.sub.2O.sub.3 and/or P.sub.2O.sub.5 and/or Li.sub.2O and/or CaO; wherein the coefficients of thermal expansion of the base material preferably differ from the coefficients of thermal expansion of the matrix by at most 6*10.sup.−6/K.

2. A foam ceramic, comprising a base material comprising Al.sub.2O.sub.3; and a matrix comprising SiO.sub.2; in particular a foam ceramic according to claim 1, wherein the foam ceramic comprises more than 15 wt % of SiO.sub.2, and at most 25 wt % of SiO.sub.2.

3. A foam ceramic, comprising a base material comprising Al.sub.2O.sub.3; and a matrix comprising SiO.sub.2; in particular a foam ceramic according to claim 1; wherein the foam ceramic has a content of B.sub.2O.sub.3 of at most 500 ppm by weight.

4. The foam ceramic according to claim 1, wherein the foam ceramic comprises Li.sub.2O, wherein the Li.sub.2O content of the foam ceramic is at least 0.3 wt % and at most 5 wt %.

5. The foam ceramic according to claim 1, comprising at least 0.1 wt % of CaO and at most 20 wt % of CaO.

6. The foam ceramic according to claim 1, comprising at least 67 wt % of Al.sub.2O.sub.3 and at most 95 wt % of Al.sub.2O.sub.3.

7. The foam ceramic according to claim 1, comprising at least 75 wt % of Al.sub.2O.sub.3 and at most 95 wt % of Al.sub.2O.sub.3.

8. The foam ceramic according to claim 1, comprising at least 5 wt % of SiO.sub.2 and at most 25 wt % of SiO.sub.2.

9. The foam ceramic according to claim 1, comprising between at least 0.1 wt % of B.sub.2O.sub.3 and at most 5 wt % of B.sub.2O.sub.3.

10. The foam ceramic according to claim 1, wherein the foam ceramic is free of P.sub.2O.sub.5, apart from unavoidably traces; or wherein the foam ceramic is in the form of a phosphate-bonded foam ceramic, with a content of P.sub.2O.sub.5 in the foam ceramic of at most 10 wt % and preferably at least 5 wt %; and wherein the foam ceramic preferably comprises Li.sub.2O as a constituent of the matrix.

11. The foam ceramic according to claim 1, wherein the foam ceramic comprises at least 0.1 wt % of CaO and at most 20 wt % of CaO.

12. The foam ceramic according to claim 1, wherein the base material comprises α-Al.sub.2O.sub.3.

13. The foam ceramic according to claim 1, wherein the matrix is at least partially glassy.

14. The foam ceramic according to claim 1, wherein the base material is present in particulate form.

15. The foam ceramic according to claim 1, wherein the matrix comprises Li.sub.2O, preferably a lithium-containing silicate glass and/or a lithium-containing borate glass.

16. The foam ceramic according to claim 1, comprising the following constituents, in wt %: Al.sub.2O.sub.3 67 to 95 Li.sub.2O 0 to 5 SiO.sub.2 0 to 25 B.sub.2O.sub.3 0 to 5, and/or with a content of B.sub.2O.sub.3 of at most 500 ppm by weight CaO 0 to 20 P.sub.2O.sub.5 0 to 10.

17. The foam ceramic according to claim 1, comprising the following constituents, in vol %, based on the solids content: α-Al.sub.2O.sub.3 (corundum) 85 to 95 Quartz 0.8 to 2 Cristobalite 0 to 2.

18. The foam ceramic according to claim 1, having a coefficient of linear thermal expansion of at least 7*10.sup.−6/K and at most 9*10.sup.−6/K.

19. A method for producing a foam ceramic, in particular a foam ceramic according to claim 1, comprising the steps of providing a preferably aqueous slip comprising a starting material comprising Al.sub.2O 3 and a starting material comprising SiO.sub.2 and/or B.sub.2O.sub.3 and/or P.sub.2O.sub.5 and/or Li.sub.2O and/or CaO; soaking an open-cell foam, in particular an open-cell polymer foam, with the slip, so as to obtain a foam coated with the slip; drying the foam so as to obtain a green body of a foam ceramic; preferably coating the dried green filter, spraying viscous sprayable slip onto the dried green filter; preferably burning out the polymer foam; and sintering the green body to obtain a foam ceramic.

20. The method according to claim 19, wherein the slip comprises a silicate glass frit and/or a borate glass frit, and wherein the glass frit preferably comprises Li.sub.2O as a constituent.

21. The method according to claim 19, wherein the slip comprises a lithium-containing starting material, wherein the glass is a silicate glass.

22. A filter for filtering melts of non-ferrous metals, in particular melts of light metals, wherein the filter comprises a foam ceramic according to claim 1.

Description

DESCRIPTION OF THE DRAWINGS

[0123] The invention will now be explained in more detail with reference to the figures, wherein:

[0124] FIGS. 1 to 3 show XRD plots of different foam ceramics, and

[0125] FIG. 4 shows dilatometer curves of different foam ceramics.

[0126] FIG. 5 is a schematic cross-sectional view of a foam ceramic.

[0127] FIG. 1 shows a first XRD plot of a first conventional phosphate-bonded foam ceramic. Quantitative evaluation of this diffractogram reveals a phase content of 90.5 vol % of α-Al.sub.2O.sub.3 (corundum), 6.4 vol % of SiO.sub.2 (cristobalite), 2.8 vol % of AlPO.sub.4, and 0.2 vol % of SiO.sub.2 (quartz). The phase content of amorphous material, which is estimated from the formation of an elevated background of reflected X-rays in the 20 angle range up to about 30°, is very low here.

[0128] FIG. 2 shows an XRD plot of a commercially available SiO.sub.2-bonded filter material. Evaluation reveals that this material comprises only 5.8 vol % of Al.sub.2O.sub.3 in the form of corundum and 0.4 vol % of SiO.sub.2 in the form of cristobalite. Since this is not a phosphate-bonded filter material, no AlPO.sub.4 can be detected. The content of SiO.sub.2 in the form of quartz is 6.7 vol % here, which is higher than in phosphate-bonded foam ceramics. Furthermore, the foam ceramic comprises 34.2 vol % of Al.sub.2SiO.sub.5 in the form of kyanite, and 52.9 vol % of boromullite with an assumed composition of Al.sub.4.5Si.sub.0.9B.sub.0.6O.sub.9.4. What is noticeable, in addition to the very different phase content compared to conventional phosphate-bonded foam ceramics and the occurrence of phases that are not present in phosphate-bonded foam ceramics, is a significantly increased content of amorphous material (visible as an “amorphous hump” in particular in the 2θ angle range between 16° and

[0129] FIG. 3 shows an XRD plot of a foam ceramic according to an embodiment, which corresponds to example 2 according to the present application. This is a foam ceramic which comprises P.sub.2O.sub.5 and Li.sub.2O. Evaluation of the diffractogram reveals a phase content of 90.4 vol % of Al.sub.2O.sub.3, 1.6 vol % of SiO.sub.2 (cristobalite), and 6.9 vol % of AlPO.sub.4, and 1.1 vol % of SiO.sub.2 (quartz). The background of the diffractogram in the 20 angle range up to 30° is slightly higher than in the diffractogram of FIG. 1. Hence, the amorphous phase content is somewhat higher here than in a conventional phosphate-bonded foam ceramic. However, shifts in phase content are apparent in the form that the material according to an embodiment of the present disclosure includes less cristobalite, but slightly more quartz, and significantly more crystalline AlPO.sub.4. Surprisingly, no crystalline phase comprising Li.sub.2O is detectable in the diffractogram. The inventors assume that Li.sub.2O is present as a constituent of the amorphous phase which is increased in comparison with conventional phosphate-bonded foam ceramics. Such a filter material exhibits particularly good strength, in particular snowing is further reduced in comparison to a conventional phosphate-bonded foam ceramic, for example.

[0130] It is also surprising that this material, despite the crystallographically detectable greater phase content of AlPO.sub.4, nevertheless exhibits a smaller jump in volume during production than the conventional phosphate-bonded filter material. This is particularly surprising since this jump in volume of usually about 2-3% is attributable to the transformation of berlinite or AlPO.sub.4 at about 200° C. FIG. 4 shows dilatometer curves (obtained according to DIN 51045-1:2005-08 and DIN 51045-2:2009-04, although, other than in the standard, the heating rate was 10 K/min here) for a conventional phosphate-bonded foam ceramic denoted by 1.) here (corresponding to the foam ceramic characterized in FIG. 1), and for a foam ceramic according to an embodiment denoted by 2.) here, which corresponds to the foam ceramic characterized in FIG. 3 in terms of its phase content. As can be seen, the jump in volume at about 200° C., which indicates a phase transformation at 200° C., is significantly reduced in the foam ceramic 2.) according to an embodiment.

[0131] The reason for this is not fully understood. However, the inventors assume that this could possibly be due to the chemical composition of the matrix in particular, possibly also due to the fact that the matrix for foam ceramics according to the present disclosure has a greater content in amorphous phase than a conventional phosphate-bonded foam ceramic. However, not only the presence of an amorphous phase alone seems to be of importance, but also a suitable chemical composition. This is because the lower jump in volume for a foam ceramic according to embodiments leads to improved strength, which is also reflected in less chalking of the foam ceramic, among other things. It is true that the non-phosphate-bonded foam ceramic shown in FIG. 2 also has an amorphous phase, in particular also with a larger proportion than the foam ceramic characterized in FIG. 3 or curve 2 of FIG. 4 according to an embodiment. However, such a foam ceramic is characterized by a rather low strength, which is also apparent from a strong particle release. It is precisely the combination of a suitable composition of the foam ceramic, in particular also of the matrix, and the generation of suitable crystalline phases that the advantageous properties of foam ceramics according to embodiments result.

[0132] FIG. 5 shows a schematic cross-sectional view of a foam ceramic 3 according to an exemplary embodiment. The foam ceramic 3 comprises a solid phase 4 and pores 5.