GB2290604A

GB2290604A - Apparatus and method for firing ceramic sheets

Info

Publication number: GB2290604A
Application number: GB9411905A
Authority: GB
Inventors: Fiona Catherine Ruth Wroe; William Jones
Original assignee: EA Technology Ltd
Current assignee: EA Technology Ltd
Priority date: 1994-06-14
Filing date: 1994-06-14
Publication date: 1996-01-03
Also published as: GB9411905D0; WO1995034795A1

Abstract

A refractory holder for retaining one or more ceramic sheets during the firing from a green state to a sintered state comprises a first member 12 having a first setting surface 28 and a second member 22 having a second setting surface 32. The second setting surface is opposed and spaced parallel to the first setting surface so as to define a volume for the receipt of one or more ceramic sheets. Spacing means 30 is provided to retain the first and second setting surfaces in their spaced relationship. Holding means 36 is provided to hold the first and second members against any forces applied by the or each of the sheets during firing. A method of firing one or more ceramic sheets is also described. <IMAGE>

Description

APPARATUS AND METHOD FOR USE IN THE FIRING OF CERAMIC SHEETS The present invention relates to an apparatus and method for use in the firing of one or more ceramic sheets from a green state to a sintered state.

Ceramic sheets which contain a ploymer as a binder in the green (unfired) state and which comprise, for example, a polymer/stabilized Zirconia powder mixture or a polymer/Lanthanum Chromate powder mixture find many uses in modern industry once they have been sintered and the binder removed. For example, thin polymer based ceramic sheets of this type may be used in the manufacture of solid oxide fuel cells during which they might typically undergo a silk screen printing process before then being co-fired to form a single cell having an interconnect. To this end it is desirable for the ceramic sheets to exhibit a high degree of flatness, excellent surface contact and a good mechanical strength.However, because of the difficulty in obtaining and maintaining a uniform temperature distribution across the ceramic sheets while they are being fired, it has been found that their edges tend to curl, detracting from their flatness, and thermal stresses are induced, reducing their mechanical strength.

As a result, the rejection rate is very high. Clearly this must be improved in order to optimise the manufacturing process.

The problem associated with the generation of thermal stresses within ceramic materials during firing is addressed in the applicant's co-pending UK patent application no. 9316616.3, the contents of which are incorporated herein by reference. The problem remains however as to what can be done to prevent the curling of the edges of the ceramic sheets when they are fired from a green state to a sintered state.

In the past ceramic sheets have typically been fired in the manner illustrated in Figure 1. The sheet 10 is placed in its green state on a bottom setter 12 and both the bottom setter and the ceramic sheet are then placed within a furnace 14. The furnace 14 may include one or more radiant heating elements 16 or else may be coupled to a microwave source. In some arrangements, such as that shown in Figure 1, the furnace 14 may comprise both radiant heating elements 16 and a microwave source 18. As stated previously, in this arrangement there is a tendency for the edges of the sheet 10 to curl throughout the heating and cooling cycles of the firing process and for the edges to remain curled when the sheet assumes its final sintered state. Clearly, for the reasons already stated such a distorted sheet must be rejected.

In order to overcome the problem associated with the curling of the edges of the ceramic sheet 10, there is described within the prior art a firing apparatus such as that illustrated in Figure 2. In this arrangement the sheet 10 is retained in a holder 20 comprising a bottom setter 12 and a top setter 22. As before, the holder 20 is then placed within a furnace 14 and the ceramic sheet subjected to a conventional firing process. By providing a two piece holder 20, it was hoped that the weight of the top setter 22 would prevent the curling of the edges of the ceramic sheet 10. Unfortunately however, this proved not to be the case. Furthermore, as the sheet 10 shrank during the firing process, it was found that the weight of the top setter 22 restricted the sheet's freedom of movement and resulted in the sheet tearing along additional lines of stress.Accordingly, there is still a requirement for an apparatus and method for use in the firing of ceramic sheets which addresses the problem associated with the curling of the sheets around their edges.

According to a first aspect of the present invention there is provided a refractory holder for use in retaining one or more ceramic sheets during the firing of the sheets from a green state to a sintered state, the holder comprising a first member having a first setting surface; a second member having a second setting surface, the second setting surface being opposed and spaced parallel to the first setting surface so as to define therebetween a volume for the receipt of said one or more ceramic sheets; spacing means to retain the first and second setting surfaces in said spaced relationship; and holding means to hold the first and second members against any forces applied by the or each of the sheets during firing.

Advantageously, the spacing means may be formed integrally with one or both of said first and second members. Preferably, one or both of the first and second setting surfaces may be recessed within their respective members so as to thereby define an upstanding peripheral wall portion, the or each peripheral wall portion serving to define said spacing means.

Advantageously, the volume defined between said first and second setting surfaces for the receipt of said one or more ceramic sheets may be fully enclosed.

Advantageously, the volume defined between said first and second setting surfaces for the receipt of said one or more ceramic sheets may be oversized with respect to the or each of the sheets to be fired in the plane of that sheet.

Advantageously, the distance between the first and second setting surfaces may be substantially equal to the thickness of the sheet or, where more then one sheet is retained within the holder, the combined thickness of all of the sheets and their associated separators, when the or each of the sheets is in its green state.

Advantageously, one or both of the first and second members may comprise a first portion having a first surface flatness and a second portion having a second surface flatness, said second portion having a greater surface flatness than said first portion and defining the setting surface of said respective first or second member. Preferably the second portion may be formed of high density alumina.

Advantageously, the holder may be adapted for use in a furnace capable of generating a radiant heat flux and the first and second setting surfaces may be arranged so as to support the or each of the sheets substantially perpendicularly to said flux.

Advantageously, the first and second setting surfaces may be disposed substantially horizontally and said holding means may comprise a weight applied to the upper of the two members. Alternatively, said holding means may comprise a clamp.

Advantageously, the holder may be adapted to retain two or more ceramic sheets and to support the sheets at a predetermined angle to the vertical. Preferably said predetermined angle may be selected such that a constant pressure is applied across the thickness of at least one of the sheets, said constant pressure including a component due to the weight of a sheet adjacent said at least one of the sheets and being such as to allow sufficient freedom of movement for said at least one of the sheets to shrink during firing without tearing.

Advantageously, the holder may comprise third and fourth setting surfaces arranged transversely of said first and second setting surfaces.

According to a second aspect to the present invention there is provided a method of firing one or more ceramic sheets from a green state to a sintered state comprising the steps of locating the or each of the sheets between two opposed, parallel spaced setting surfaces which are retained in said spaced relationship by spacing means, supplying heating energy to the or each of the sheets in order to fire said sheets, and holding said setting surfaces against any forces applied by the or each of the sheets during the firing process.

A number of embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 3 is a cross-sectional schematic view of a holder in accordance with a first embodiment of the present invention; Figure 4 is a cross-sectional schematic view of a holder in accordance with a second embodiment of the present invention; Figure 5 is a cross-sectional schematic view of a holder in accordance with a third embodiment of the present invention; Figure 6 is a cross-sectional schematic view of a holder in accordance with a fourth embodiment of the present invention; Figure 7 is a cross-sectional schematic view of a holder in accordance with a fifth embodiment of the present invention; Figure 8 is a cross-sectional schematic view of a holder in accordance with a sixth embodiment of the present invention; and Figure 9 is a cross-sectional schematic view of a holder in accordance with a seventh embodiment of the present invention; Referring to Figure 3, there is shown a refractory holder 20 comprising a bottom setter 12 and a top setter 22. However, in contrast to the prior art, the bottom setter 12 is provided in an upper surface 24 with a recessed portion 26. The recessed portion 26 is located centrally of the bottom setter 12 and so defines both a first setting surface 28 and an upstanding peripheral wall 30. The top setter 22 is supported by the peripheral wall 30 such that a lower surface of the top setter 32 defines with the recessed portion 26 an enclosed volume 34 for the receipt of a ceramic sheet 10.Preferably the height of the peripheral wall 30 is such that the distance between the first setting surface 28 and the second setting surface defined by the lower surface 32 of the top setter 22 is substantially equal to the thickness of the ceramic sheet 10 when the sheet is in its green state. In this way the holder 20 can exert a slight pressure across the thickness of the sheet 10 at the start of the firing process although clearly, as the firing process continues and the sheet shrinks this pressure will be reduced until such time as the thickness of the sheet is less than the height of the peripheral wall 30 whereupon the pressure will have been reduced to zero. What this initial pressure does enable however, is for the ceramic sheet 10 to take up the flatness of the first and second setting surfaces 28 and 32.For this reason these two surfaces are preferably machined to a flatness of 40 microns or less.

The limited distance between the first and second setting surfaces 28 and 32 also serves to restrain the ceramic sheet 10 from buckling, curling or corrugating out of the plane of the sheet. In order to ensure that any forces generated by the attempted curling of the sheet 10 are insufficient to separate the two setters 12, 22 a holding weight 36 may be provided on an upper surface 38 of the top setter 22.

Although at the start of the firing process the holder 20 exerts a slight pressure across the thickness of the ceramic sheet 10, the recessed portion 26 is preferably oversized with respect to the ceramic sheet in the plane of the sheet. In this way the holder 20 provides the ceramic sheet 10 with sufficient freedom of movement to contract during the firing process in the plane of the sheet without giving rise to additional stresses along which the sheet may subsequently tear.

The holder 20 may find use in a variety of furnaces.

In the arrangement shown in Figure 3 the holder 20 is placed within a furnace 14 which includes both a plurality of radiant heating elements 16 and a microwave source 18.

Clearly however, the furnace 14 may equally be provided with only one of these heating means or else be provided with convective heating means. Preferably the furnace 14 is adapted so as to raise the temperature of the ceramic sheet 10 to a minimum sintering temperature consistent with achieving the required density and grain size.

If the holder 20 is to be used in conjunction with a furnace 14 which incorporates a microwave source 18, the two setters 12 and 22 are preferably formed of a material which is transparent to microwave energy. Alternatively, the two setters 12 and 22 may be formed of a material which is susceptible to microwaves and which is capable of then radiating heat to the ceramic sheet 10. What is important however, is to avoid the generation of hot spots within either the ceramic sheet 10 or the two setters 12 and 22 which might lead to the start of a thermal runaway effect and the eventual destruction of the sheet.

Likewise, if the holder 20 is to be used in conjunction with a furnace which incorporates one or more radiant heating elements 16, the two setters 12 and 22 are preferably formed of a material which is capable of absorbing heat energy and transferring that heat energy to the ceramic sheet 10 in such a way as to produce an even temperature distribution across the sheet. In this way, and particularly by providing a volume 34 for the receipt of the ceramic sheet 10 which is totally enclosed, the sheet is protected during the firing process against localised regions of focused radiant heat energy that might otherwise lead to the generation of thermal stresses within the ceramic material.Another advantage of providing a holder which defines a totally enclosed volume for the receipt of the ceramic sheet 10 is that where the furnace employs gas radiant or convection heating, the sheet is protected from the products of combustion.

Other factors which affect the choice of the material from which the two setters 12 and 22 are formed are that the material must be capable of withstanding the range of temperatures that are likely to be experienced within the furnace 14, whilst at the same time being capable of being machined to a high degree of flatness. In addition the setters 12 and 22 should also be formed of materials which minimise contamination and other adverse reactions between the setters and the ceramic sheet 10. Two of the currently preferred materials for forming the two setters 12 and 22 and which satisfy the above citeria are Alumina thermal insulation board and Zircar thermal insulation board of varying densities.

Turning now to Figure 4, there is shown a holder in accordance with a second embodiment of the present invention. This embodiment includes many features which are common to that described with reference to Figure 3 and which have therefore been denoted by the same reference numerals and which will not now be further described.

The main difference between the embodiments of Figures 3 and 4, is that in Figure 4 the two setters 12 and 22 are disposed vertically and not horizontally. This of course means that the holding weight 36 of the previous embodiment needs to be replaced by an alternative holding means in the form of a ceramic clamp 40. As before, the ceramic clamp 40 serves to hold the two setters 12 and 22 in mutual engagement against any separating forces that might be applied to the first and second setting surfaces 28 and 32 by the ceramic sheet 10 during the firing process.

The advantage of disposing the two setters 12 and 22 vertically rather than horizontally arises from the fact that in the illustrated furnace 14 the radiant heating elements 16 are also disposed vertically. By so arranging the holder 20 such that the ceramic sheet 10 is disposed generally perpendicularly to the radiant heat flux, it is possible to provide the sheet with a more uniform temperature distribution since there is no one part of the sheet which is closer to a radiant heating element than any other part of the sheet. This in turn leads to a reduction in the thermal stresses generated within the sheet and to a diminishing of the tendency of the sheet to curl around its edges.

Turning now to Figures 5 and 6, there are shown two further holders each in accordance with a further embodiment of the present invention. The two holders share many features in common with those described in relation to Figures 3 and 4 and which are therefore denoted by the same reference numerals. Thus, looking more closely at the embodiment shown in Figure 5 and comparing it with that shown in Figure 3, it will be seen that the main difference between the two lies in the greater depth of the recessed portion 26 in the embodiment of Figure 5 and the consequently increased height of the upstanding peripheral wall 30. This increased height allows for the provision of a first ceramic plate 42 within the recessed portion 26.In this way the first setting surface 28 may be defined by an upper surface of the first ceramic plate 42 which, being separately formed from the rest of the bottom setter 12 may be machined to a higher degree of flatness. For example, the first ceramic plate 42 may be formed of high density alumina and have a flatness of the order of 5 microns.

In order to provide the top setter 22 with a similarly flat second setting surface 32, a second ceramic plate 44 may be interposed between the upper surface of the bottom setter 24 and the lower surface of the top setter 32. In this way, the second setting surface, instead of being defined by the lower surface 32 of the top setter 22, is defined by a lower surface 46 of the second ceramic plate 44. Thus when, during the firing process, the ceramic sheet 10 takes up the flatness of the first and second setting surfaces 28 and 46 it does so with the improved surface flatness of the first and second ceramic plates 42 and 44.

The embodiment shown in Figure 6 is similar to that described in relation to Figure 5 except that, as with the embodiment of Figure 4, the two setters 12 and 14 are disposed generally vertically. In this way the embodiment of Figure 6 may enjoy not only the increased surface flatness of the embodiment of Figure 5, but also the improved temperature distribution of the embodiment of Figure 4.

Turning now to Figure 7, there is shown a holder in accordance with yet a further embodiment of the present invention. Again, the holder has many features in common with those that have been previously described and which are therefore denoted by common reference numerals.

The embodiment of Figure 7 is similar to that illustrated in Figure 3, except that the embodiment in Figure 7 has been adapted so as to accommodate a plurality of ceramic sheets 10 which may be co-fired simultaneously. To this end the recessed portion 26 within the bottom setter 12 is made deeper thereby raising the height of the upstanding peripheral wall 30. In common with the embodiment of Figure 5, the embodiment of Figure 7 is again provided with a first setting surface 28 defined by an upper surface of a first ceramic plate 42 received within the recessed portion 26 and a second setting surface 46 defined by a lower surface of a second ceramic plate 44 interposed between the upper surface 24 of the bottom setter 12 and the lower surface 32 of the top setter 22.Nevertheless, even allowing for the thickness of the first ceramic plate 42, the depth of the recessed portion 26 is such as to accommodate a stack of ceramic sheets 10 each of which is spaced from its neighbour by a separator 48 in the form of a plate of for example high density alumina or a layer of ceramic paper.

Although the embodiment of Figure 7 has the advantage that it enables a plurality of ceramic sheets 10 to be co-fired simultaneously, it suffers from the disadvantage that the initial pressure applied across the thickness of the ceramic sheets is not the same for each of the sheets within the stack. This arises because the applied pressure is due not only to the combined weight of the top setter 22 and the holding weight 36, but also to the weight of each of the ceramic sheets 10 and separators 48 disposed above the ceramic sheet in question. Thus, the pressure applied across the thickness of the bottom sheet within the stack is very much greater than that applied across the thickness of the top sheet.Accordingly, the lower sheets within the stack are more restricted in their freedom of movement and so when fired tend to suffer from additional stresses along which they can subsequently tear.

This problem is alleviated by the embodiment shown in Figure 8 which is similar to that described in relation to Figure 7 except that, as with Figure 4 and 6, the two setters 12 and 22 are disposed substantially vertically.

Not only does this arrangement give rise to the improved temperature distribution of the embodiment of Figure 4, but it also ensures that the pressure applied across the thickness of the ceramic sheets 10 is the same for each of the sheets within the stack.

One disadvantage of the embodiment of Figure 8 and which is common to each of the holders adapted for firing a single ceramic sheet is that once the sheet or sheets have achieved a certain degree of shrinkage, the pressure which was initially applied across their thickness is reduced to zero. It is nevertheless desirable to maintain a slight but positive pressure across the thickness of the sheet or sheets throughout the firing process since in this way each sheet can be restrained from curling around its edges. At the same time however, this pressure must not exceed certain limits otherwise, as with the sheets in the lower part of the stack illustrated in Figure 7, the freedom of the sheet to shrink during the firing process is unduly restricted which leads to the generation of stresses along which the sheet may subsequently tear.One solution to this problem would be to provide a holding means similar to the ceramic clamp 40 illustrated in Figure 8 and which is capable of either adjusting or maintaining the pressure applied across the thickness of the sheets throughout the firing process.

Another, and perhaps more elegant solution, is shown in Figure 9 in which the holder 20 is again adapted to contain a plurality of ceramic sheets 10 each of which is spaced from its neighbours by a separator 48. In contrast to the other holders previously described, the holder of Figure 9 is provided with a pair of side setters 50 and 52 in addition to the top and bottom setters 22 and 12. The two side setters 50 and 52 are each provided with a respective setting surface 54 and 56 which, like the first and second setting surfaces previously described, are mutually opposed and extend substantially parallel to each other. However, unlike the previously described setting surfaces, the setting surfaces 54 and 56 are both inclined to the vertical. As a result the ceramic sheets 10 contained within the holder 20 are also inclined to the vertical. Thus, unlike the embodiment shown in Figure 8, the pressure applied across the thickness of each of the sheets 10 includes a component which arises from the weight of its neighbours. Unlike the pressure applied by the ceramic clamp 40 in the embodiment of Figure 8, this component of pressure is present throughout the firing process. At the same time, it will be apparent that the size of the component is dependent upon the angle at which the setting surfaces 54 and 56 are inclined to the vertical. Thus by careful selection of this angle a sufficient pressure may be applied to the ceramic sheets 10 throughout the firing process to prevent the sheets from curling around their edges whilst at the same time providing sufficient freedom of movement to allow the sheets to shrink without generating the additional stresses along which the sheets might subsequently tear.

Although the embodiments of Figure 3 to 8 have been described with reference to a holder in which only one of the setters includes a recessed portion, it will be apparent to those skilled in the art that either one or both of the setters may be recessed and that if so desired, the first and second setting surfaces may be spaced apart by the mutual abutment of peripheral wall portions provided on each of the setters. Likewise, although the first and second setting surfaces have been described as being spaced apart by means of an upstanding peripheral wall portion formed integrally with one of the setters, it will again be apparent to those skilled in the art that the first and second setting surfaces may be held apart in a mutually opposed spaced relationship by any suitable spacing means.

Claims

CLAIMS:

1. A refractory holder for use in retaining one or more ceramic sheets during the firing of the sheets from a green state to a sintered state, the holder comprising a first member having a first setting surface; a second member having a second setting surface, the second setting surface being opposed and spaced parallel to the first setting surface so as to define therebetween a volume for the receipt of said one or more ceramic sheets; spacing means to retain the first and second setting surfaces in said spaced relationship; and holding means to hold the first and second members against any forces applied by the or each of the sheets during firing.

2. A refractory holder in accordance with claim 1, wherein said spacing means is formed integrally with one or both of said first and second members.

3. A refractory holder in accordance with claim 1 or claim 2, wherein one or both of said first and second setting surfaces are recessed within their respective members so as to thereby define an upstanding peripheral wall portion, the or each peripheral wall portion serving to define said spacing means.

4. A refractory holder in accordance with any preceding claim, wherein the volume defined between said first and second setting surfaces for the receipt of said one or more ceramic sheets is fully enclosed.

5. A refractory holder in accordance with any preceding claim, wherein the volume defined between said first and second setting surfaces for the receipt of said one or more ceramic sheets is oversized with respect to the or each of the sheets to be fired in the plane of that sheet.

6. A refractory holder in accordance with any preceding claim, wherein the distance between the first and second setting surfaces is substantially equal to the thickness of the sheet or, where more than one sheet is retained within the holder, the combined thickness of all of the sheets and their associated separators, when the or each of the sheets is in its green state.

7. A refractory holder in accordance with any preceding claim, wherein one or both of said first and second members comprises a first portion having a first surface flatness and second portion having a second surface flatness, said second portion having a greater surface flatness than said first portion and defining the setting surface of said respective first or second member.

8. A refractory holder in accordance with claim 7, wherein said second portion is formed of high density alumina.

9. A refractory holder in accordance with any preceding claim, wherein the holder is adapted for use in a furnace capable of generating a radiant heat flux and the first and second setting surfaces are arranged so as to support the or each of the sheets substantially perpendicularly to said flux.

10. A refractory holder in accordance with any preceding claim, wherein said first and second setting surfaces are disposed substantially horizontally and said holding means comprises a weight applied to the upper of the two members.

11. A refractory holder in accordance any of claims 1 to 9, wherein said holding means comprises a clamp.

12. A refractory holder in accordance with any preceding claim, wherein the holder is adapted to retain two or more ceramic sheets and support the sheets at a predetermined angle to the vertical.

13. A refractory holder in accordance any of claims 12, wherein said predetermined angle is selected such that a constant pressure is applied across the thickness of at least one of the sheets, said constant pressure including a component due to the weight of a sheet adjacent said at least one of the sheets and being such as to allow sufficient freedom of movement for said at least one of the sheets to shrink during firing without tearing.

14. A refractory holder in accordance with any preceding claim, and comprising third and fourth setting surfaces arranged transversely of said first and second setting surfaces.

15. A refractory holder substantially as herein described with reference to any of Figures 3 to 9 of the accompanying drawings.

16. A method of firing one or more ceramic sheets from a green state to a sintered state comprising the steps of locating the or each of the sheets between two opposed, parallel spaced setting surfaces which are retained in said spaced relationship by spacing means, supplying heating energy to the or each of the sheets in order to fire said sheets, and holding said setting surfaces against any forces applied by the or each of the sheets during the firing process.

17. A method of firing one or more ceramic sheets from a green state to a sintered state substantially as herein described with reference to any of Figures 3 to 9 of the accompanying drawings.