GB2101579A

GB2101579A - Ceramic structure

Info

Publication number: GB2101579A
Application number: GB08118710A
Authority: GB
Inventors: Douglas Cecil Gifford
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1981-06-18
Filing date: 1981-06-18
Publication date: 1983-01-19

Abstract

A ceramic structure suitable for use as a liquid metal filter consists of multi-plicity of layers of parallel ceramic extrusions which are extruded in separate layer's at 90 DEG to each other and in the same plane, the resulting extruded structure then being fired to sinter the ceramic. <IMAGE>

Description

SPECIFICATION Ceramic structure This invention relates to ceramic structures and particularly to such structures suitable for use as filters useful for the filtration of liquid metal.

The benefits of liquid metal filtration are widely known to those skilled in the art. Presence of inclusions in castings is known to reduce their performance by impairing their fabricability and machinability and by reducing the mechanical properties of the metal.

It has been well known in the past to filter liquid metal through a variety of types of filter, all of which have suffered disadvantages. Most such filters consist of either screens or perforate plates, a loosely packed bed of solids, a bonded porous medium or particular types of ceramic foam structure.

Screen filters have been used extensively, however they suffer the disadvantage that the size of the particles which may be filtered from the melt are limited to the relatively large apertures formed within the screen or plate.

Lcosely packed bodies of solids are also well known and usually consist of loosely packed beds of refractory balls, however such beds suffer a major disadvantage known as channelling. Since the bed is loosely packed aggregate or granules, there is a strong tendency for the liquid metal to push the granules aside and form low resistance paths for the metal to flow through. Once a channel is formed it usually continues to become larger, and large amounts of inclusion then pass through the channel with the liquid metal.

Bonded porous filters have usually consisted of refractory particles sintered together by a suitable bonding agent for example clay or enamel. The main disadvantage of this type of filter is the difficulty in maintaining consistency of porosity in producting. Furthermore it is almost impossible to manufacture such filters having a porosity of more than 30% by volume. It is therefore necessary to have a relatively large head of liquid metal to ensure passage of the metal through the filter at a reasonable rate.

Porous ceramic foam filters are also well known. Such filters are usually manufactured using a flexible organic foam as a precursor material which is coated with ceramic slurry. The foam is subsequently dried and then fired to burn out the organic foam and sinter the ceramic thereby producing a porous reticulated ceramic foam.

Filters of this type have been used for liquid metal filtration, however they all suffer the disadvantages that they are relatively weak, are liable to shed loose ceramic particles into the liquid metal and it is difficult to control the porosity of the foam.

An object of the present invention is to provide a ceramic structure particularly suitable for use as a filter in which the aforementioned disadvantages are substantially eliminated.

According to the present invention a ceramic structure consists of a multiplicity of layers of ceramic extrusions, each layer consisting of a plurality of ceramic extrusions which are extruded from extrusion nozzles such that the extrusions are arranged parallel to each other, and the extrusions in each successive layer being arranged in the same plane but at substantially 900 to the extrusions in the preceding layer.

Preferably the ceramic material is supplied to the extrusion nozzles as an extremely stiff aqueous slurry.

Preferably after extruding the first layer of ceramic extrusions the extrusion nozzles may be indexed through 900 prior to extruding the next successive layer.

Alternatively after extruding the first layer of ceramic extrusions the layer may be indexed through 900 prior to extruding the next successive layer.

The cross-sectional shape of each respective extrusion may be round or rectangular or alternatively corrugated to increase its surface area.

Furthermore during extrusion of the successive layers a supply of heated air may be provided to the ceramic such but is at least partially rigidfied such that the successive layers of ceramic extrusion do not distort or crush the lower layers.

After the requisite number of successive layers have been extruded the assembly is dried and then fired within a furnace.

Preferably such a structure is particularly suitable for use as a filter for the filtration of liquid metal.

For better understanding of the present invention an embodiment thereof will be more particularly described by way of example only and with reference to the accompanying drawings in which Figure 1 shows an enlarged pictorial view of a portion of a ceramic structure made in accordance with the present invention Figure 2 and 3 both show alternative cross section shapes of the ceramic extrusion as an alternative to the circular ones shown in figure 1.

Figure 4 shows a pictorial view of a machine used to make an example of the structure as shown at figure 1.

Referring to the drawings the ceramic structure consists of a plurality of ceramic extrusions 1 Oa 1 Ob and 1 0c layed down in layers 12, 13, 14, 15, and 16. The ceramic material consists of an aqueous sul of ceramic and for best results the following composition has been found useful.

TabularAlumina less than 350 mesh 4.5 Kg Fused Alumina less than 5 micron 1.5 Kg, Ball Clay (English Clays Ltd HYMCO-PRIMA) 0.9 Kg, Nepheline Syenite less than 300 mesh 0.45 Kg, Dextrin 0.45 Kg, Demineralised Water 1.9 Kg.

Panacide EDH 0.5 grams and Viscalex HV30 trace.

The ceramic slurry is loaded into a hopper20 and pumped by means of pump 21 through tubes 22 to the extrusion head 24 from which it is extruded through a plurality of extrusion nozzle to form a plurality of ceramic extrusions25 upon the support member26. The support member26 is arranged to be slidably mounted upon a base member27 such that the support member26 can be moved with respect of the extrusion head 24 to lay down the ceramic extrusions in their desired locations upon the support member21.

If it is required to manufacture a ceramic structure which is wider than the extrusion head 24 the head may be moved within its supporting frame24a to the location shown in broken lines at 28. During the extrusion process it has been found desirable to blow a supply of heated air at the extruded ceramic to partially dry it off such to ensure that the ceramic does not become distorted or deformed as successive layers are laid down.

After laying of the first layer of extrusions is completed, the support member26 is indexed through 900 and the extrusion head 24 is raised by an amount corresponding to the thickness of the first laid layer of ceramic extrusions to maintain the optimum distance between the extrusion nozzles and the already laid down ceramic extrusions. The second layer is then laid down in the same manner as the first layer. It will be appreciated that any number of layers of ceramic extrusions may be laid down in a similar manner until the required structure is made.

When the extrusion of all the respective layers is completed the ceramic structure is heat dried or allowed to completely dry in air and is then subsequently fired in a furnace. For optimum results it has been found preferable to use the following firing sequence: 4.5 hours heating from 200C to 1 48O0C, 4.0 hours heating held at 1 4800C, 8 to 12 hours cooling from 14800C to 4000C and hereafter removing the completed ceramic structure from the furnace.

It will be appreciated by those skilled in the art that whilst the particularly described apparatus may be used for the manufacture of a ceramic structure made in accordance with the present invention. This specification is not in any way restricted to such an elementary type of apparatus as it is envisaged considerable modification will become necessary when high speed production of such a type of extruded ceramic is required.

Furthermore it will be appreciated that by present use of the present invention it is possible to manufacture a ceramic structure useful as a liquid metal filter in which the designed filter porosity can be easily selected and maintained by suitable choice of ceramic extrusion thickness, the distance at which the extrusion are laid apart from each other, and also the number of layers in the structure.

Although the embodiment of the present invention has been particularly directed towards a ceramic structure suitable for use as a liquid metal filter it will be appreciated that such a structure may be used for a variety of purposes such as for example a gas filter, or a substrate for use in supporting a catalyst of the type used in engine exhaust systems. Alternatively the ceramic structure may be used as a support member or refractory brick or for that matter in any situation in which its advantages of high strength, light weight and reproductivity etc may be best used to their best advantage.

Claims

1. A method of making a ceramic structure consisting of a multiplicity of layers of ceramic extrusions, each layer consisting of a plurality of ceramic extrusions which are extruded from extrusion nozzles such that the extrusions are arranged parallel to each other, and the extrusions in each successive layer being arranged in the same plane but at substantially 900 to the extrusions in the preceding layer.

2. A method as claimed in claim 1 in which the ceramic material is supplied to the extrusion nozzles as an extremely stiff aqueous slurry.

3. A method as claimed in claim 1 in which after extruding the first layer ot ceramic extrusions the extrusion nozzles may be indexed through 900 prior to extruding the next successive layer.

4. A method as claimed in claim 1 in which the cross-sectional shape of each respective extrusion may be round or rectangular or alternatively corrugated to increase its surface area.

5. A method as claimed in claims 1,2 and 3 in which during extrusion of the successive layers a supply of heated air may be provided to the ceramic such but is at least partially rigidfied such that the successive layers of ceramic extrusion do not distort or crush the lower layers.

6. A method as claimed in claims 1,2,3 and 5 in which after the requisite number of successive layers have been extruded the assembly is dried and then fired within a furnace.

7. A ceramic structure is particularly suitable for use as a filter for the filteration of liquid metal.

8. A ceramic structure as claimed in any preceding claim substantially as hereinbefore described by way of example only with reference to the accompanying drawing.