GB2168337A

GB2168337A - Ceramic foam

Info

Publication number: GB2168337A
Application number: GB08527212A
Authority: GB
Inventors: Yoshihisa Kato; Masashi Fujimoto
Original assignee: Toshiba Ceramics Co Ltd
Current assignee: Coorstek KK
Priority date: 1984-12-12
Filing date: 1985-11-05
Publication date: 1986-06-18
Also published as: JPS61141682A; DE3540449C2; GB2168337B; DE3540449A1; GB8527212D0; JPH058148B2

Abstract

A ceramic foam has a three-dimensionally reticulated ceramic structure in that a bulk density thereof is between 0.4 and 2.0. An apparent porosity of many ceramic strands is 5% or less. Each of the strands has plural ceramic layers. A method for manufacturing a ceramic foam includes the steps of impregnating a basic urethane foam having a three-dimensionally reticulated structure with a slurry or slip for ceramic manufacturing purpose, drying them plural times so as to increase a thickness of the ceramic strands, and firing them thereby to obtain a ceramic body.

Description

SPECIFICATION A ceramic foam and a method for making the same This invention relates to a ceramic foam having a three-dimensionally reticulated ceramic structure which can be used as a filter for removing slugs contained in a molten metal, a heater for a burner or the like, and a method for manufacturing the same.

In order to remove slugs and other obstacles contained in a molten metal, a ceramic foam filter is used. The conventional ceramic foams used as a filter are made of cordierite, alumina or silicon carbide and have a void ratio of 75-95%, a bulk density of 0.2-03, an apparent porosity of interconnected ceramic strands ranging from 10% to 20%.

In a conventional ceramic foam, a soft urethane foam having a three-dimensionally reticulated structure is used as a starting foam and infused or impregnated with a ceramic slurry and then fired. Crushing strength is between 10 and 30 Kgf/cm2 or less. Bending strength is 5 Kgf/cm2 or less. Since the strengths are so weak, the ceramic foams are sometimes broken prior to use or in use.

The object of the invention is to provide a ceramic foam and a method for manufacturing the same in which strengths of the ceramic foam are improved so that it is not easily broken prior to use or in use.

The present invention is a ceramic foam having a three-dimensionally reticulated ceramic structure composed of many interconnected ceramic strands characterized in that a bulk density of the ceramic foam is between 0.4 and 2.0 and each of the strands has plural ceramic layers.

The present invention is also a method for manufacturing a ceramic foam comprising the steps of mixing a starting ceramic material with water and a binder thereby to form a slurry or slip, impregnating a starting foam having a three-dimensionally reticulated structure with the slurry or slip, drying them, and firing them thereby to obtain a ceramic body having a three-dimensionally reticulated ceramic structure composed of many interconnected ceramic strands characterized in that the impregnating and drying steps are repeated plural times thereby to form plural layers so as to adjust a thickness of the strands.

Even if a bulk density of the ceramic foam is slightly adjusted, strengths of the ceramic foam can be remarkably increased. A void ratio thereof is not so changed. Thus a molten metal can pass effectively through the ceramic foam.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawing in which Figures 1A, 1B, 1C, 1D and 1E are cross-sectional views showing a series of steps of a method for manufacturing a ceramic foam and particularly a strand thereof according to this invention.

Embodiment 1 First, a ceramic slip is prepared. A starting ceramic material including AT203 98% by weight is composed of small particles, sizes of which are between 0.1 and 44 micrometers, and a mean particle diameter of which is between 5 and 0.5 micrometers. The starting ceramic material is mixed with water and a rapid drying binder such as PVA by means of a ball mill so as to obtain a ceramic slurry or slip having a viscosity of 2-15 poise.

Also, a starting foam is prepared. A soft urethane foam is suitable thereas which has a three-dimensionally reticulated urethane structure composed of many interconnected strands. Figure 1A shows a cross-section of one strand 1 thereof.

As Figure 1B shows, a first ceramic layer 2 is formed on the urethane strand 1 by impregnating or infusing the strand 1 of the urethane foam with the slip. Surplus slip is removed. It is dried to be hardened at a temperature of 50-100 C.

Next, as shown in Figure 1C, a second ceramic layer 3 is formed on the first layer 2 by impregnating the first layer 2 with the slip. At this time, also, surplus slip is removed. It is again dried to be hardened as in the first layer 2.

In addition, as shown in Figure 1D, a third layer 4 is formed on the second layer 3 by infusing it with the slip. Surplus slip is removed. It is dried to be hardened in the same way.

Finally, in Figure 1E, the strands 5 consisting of the starting foam 1 and the ceramic layers 2,3,4 thereon are fired at a high temperature of 15000C or more. As a result, the ceramic layers 2, 3, 4 become a porcelain ceramic multi-layer as a ceramic strand 5'. Many of such a ceramic strand 5' are interconnected to constitute a three-dimensionally reticulated ceramic structure as in the starting foam. At the same time, the urethane strands 1 disappear on firing thereby to leave a long opening 6 in a central portion of each strand 5'.

A bulk density of the ceramic foam may be set so as to have a value of 0.4-2.0 by adjusting a thickness of the ceramic multi-layer of the strand 5.

In Table 1, a bulk density of the ceramic foam is between 0.4 and 2.0. Crushing strength is between 50 and 100 Kgf/cm2 and therefore remarkably increased in comparison with the prior art crushing strength of 10-30 Kgf/cm2 or less. Bending strength is between 10 and 35 Kgf/cm2, which is remarkably increased in comparison with the prior art of 5 Kgf/cm2 or less. Void ratio thereof is 85-90%, which is merely very slightly changed in comparison with the void ratios of the prior art. An apparent porosity of the ceramic strands 5' is 5% or less.

Embodiment 2 Although in the embodiment 1 the soft urethane foam 1 is impregnated with a common slip for the purpose of forming the first, second and third layers 2, 3, 4, a plurality of different slips are used for each layer according to the embodiment 2.

In the embodiment 2, a starting ceramic material is of AI203 98% by weight. The slip of the embodiment 1 may be used for a first layer 2, but other different slips of different sizes and blend ratios are used for a second layer 3 and a third layer 4.

For instance, for the first layer 2, a starting ceramic material is composed of small particles having particle sizes of 10-0.1 micrometers and an average particle diameter of 5-0-5 micrometers. A porosity of the first layer 2 of the ceramic strands 5' is to become 0%.

For the second layer 3, a starting ceramic material is composed of two groups of small particles: one having 70-90% by weight of particle sizes between 10 and 0.1 micrometers; and the other having 10-30% by weight of particle sizes between 44 and 10 micrometers. They are blended with water and a binder thereby to become a slip. A porosity of the second layer 3 of the ceramic strands 5' is to become 5-15%.

For the third layer 4, two groups of small particles are used, one of which has 50-70% by weight of particle sizes between 10 and 0.1 micrometers, and the other of which has 30-50% by weight of particle sizes between 44 and 10 micrometers. A porosity of the third layer 4 of the strands 5' is to become 1525%.

The impregnating, drying and firing steps in the embodiment 2 are substantially the same as in the embodiment 1.

A bulk density of the ceramic foam is between 0.4 and 2.0. Crushing strength and bending strength are substantially the same as those of the embodiment 1.

It is observed in the embodiment 2 that many fine pores having pore diameters of 50-1 micrometers are located in the ceramic strands 5'. Therefore, filtering efficiency is excellent.

Incidentally, the above-stated ceramic foam can have a catalytic function by holding active AI2O3("- AI2Os) or PT catalyst on the third layer 4. Holding power of the active Al2O3 or the like can be improved so as to increase lifetime due to the microporosity in the surface layer thereof.

Embodiment 3 In the embodiment 3, the chemical composition of a ceramic foam is Al2O3-ZrO2 unlike in the embodiments 1 and 2. In this embodiment, each ceramic strand 5' has an increased thickness in the same way as in the embodiments 1 and 2.

In the embodiment 3, a bulk density of the ceramic foam is 0.4-2.0 like in the embodiments 1 and 2.

The crushing strength and bending strength thereof have the same value as those of the embodiment 1.

Also, the void ratio and the pore diameter have the same as those of the embodiment 1. A thermal shock resistance of the embodiment 3 is better than that of the embodiment 1.

This invention is not limited to the embodiments as above-mentioned. For instance, as far as a bulk density of the ceramic foam is between 0;4 and 2.0, any number of layers can be formed. For example, only a first and second layers can be formed. If desired, a fourth layer and any other layer may be added.

Also, a starting ceramic material can be selected from SIC, Si3N4, Al203-SiO2, cordierite, and ZrO2.

If a bulk density of the ceramic foam is more than 2.0, void ratio is decreased too much so that a molten metal cannot easily pass through the continuous pores of the ceramic foam. Thus, filtering function is decreased.

TABLE 1 Items Embodiment 1 Embodiment 3 Chemical Composition 98% Al2O3 AI2O#-ZrO2 Bulk Density 0.4 - 2.0 0.4 - 2.0 Crushing Strength (Kgf/cm2) 50-100 50- 100 Bending Strength (Kgf/cm2) 10 - 35 10 - 35 Void Ratio 85 - 90 85- 90 Pore Diameter (mm) 1 - 2.5 1 - 2.5 Heating Resisting Temperature ( C) 1700 or more 1700 or more Apparent Porosity of Strands 5% or less 5% or less

Claims

1. A ceramic foam having a three-dimensionally reticulated ceramic structure composed of many interconnected ceramic strands characterized in that a bulk density of the ceramic foam is between 0.4 and 2.0 and each of the strands has plural ceramic layers.

2. The ceramic foam of Claim 1, wherein an apparent porosity of the strands is 5% or less.

3. The ceramic foam of Claim 1, wherein the chemical composition of the strands in AI2Os.

4. The ceramic foam of Claim 1, wherein the chemical composition of the strands is AI2O#-ZrO2.

5. The ceramic foam of Claim 1, wherein the chemical composition of the strands is selected from SiC, Si3N4, Al203-SiO2, cordierite, and ZrO2.

6. The ceramic foam of Claim 1, wherein void ratio of the ceramic foam is between 85% and 90%.

7. The ceramic foam of Claim 1, wherein the ceramic foam has pores having pore diameters between 1 mm and 2.5 mm.

8. The ceramic foam of Claim 1, wherein the strands have a large number of fine pores having sizes between 1 micrometer and 50 micrometers.

9. The ceramic foam of Claim 1, wherein the strands are composed of plural layers made of a common composition.

10. The ceramic foam of Claim 1, wherein the strands are composed of plural layers made of different compositions.

11. The ceramic foam of Claim 10, wherein the different compositions have different sizes and blend ratios thereof.

12. The ceramic foam of Claim 10, wherein the plural layers have different porosities, respectively.

13. The ceramic foam of Claim 2, wherein the plural layers include a first layer having a porosity of 0%, a second layer having a porosity of 5-15%, and a third layer having a porosity of 15-25%.

14. A method for manufacturing a ceramic foam comprising the steps of mixing a starting ceramic material with water and a binder thereby to form a slurry or slip, impregnating a starting foam having a three-dimensionally reticulated structure with the slurry or slip, drying them, and firing them thereby to obtain a ceramic body having a three-dimensionally reticulated ceramic structure composed of many interconnected ceramic strands characterized in that the impregnating and drying steps are repeated plural times thereby to form plural layers so as to adjust a thickness of the strands.

15. The method of Claim 14, wherein a common starting ceramic material is used for the plural layers.

16. The method of Claim 14, wherein the starting ceramic material is made of AI2O3 98% by weight and composed of small particles, sizes of which are between 0.1 and 44 micrometers, and a mean particle diameter of which is between 5 and 0.5 micrometers.

17. The method of Claim 14, wherein plural different starting ceramic materials are used for the plural layers.

18. The method of Claim 14, wherein the plural layers have a first, second and third layers, and wherein for the first layer the starting ceramic material is composed of small particles having particle sizes of 10-0.1 micrometers and an average particle diameter of 5-0.5 micrometers, and wherein for the second layer the starting ceramic material is composed of two groups of small particles: one having 7090% by weight of particle sizes between 10 and 0.1 micrometers; and the other having 10-30% by weight of particle sizes between 44 and 10 micrometers, and wherein for the third layer two groups of small particles are used, one of which has 50-70% by weight of particle sizes between 10 and 0.1 micrometers, and the other of which 30-50% by weight of particle sizes between 44 and 10 micrometers.

19. The method of Claim 14, wherein the slurry or slip has a viscosity of 2-15 poise.

20. A ceramic foam substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

21. A method of making ceramic foam substantially as hereinbefore described with reference to the accompanying drawing.